Methods and materials for increasing starch biosynthesis in plants

Abstract

The subject invention concerns materials and methods for providing plants or plant tissue with increased starch biosynthesis. Increased starch biosynthesis provides for increased yield. One aspect of the invention concerns polynucleotides that encode a mutant plant large subunit of AGPase. In one embodiment, a polynucleotide of the invention encodes a plant AGPase large subunit having an amino acid mutation wherein the cysteine amino acid corresponding to amino acid position 424 of wild type maize AGPase large subunit is substituted with an amino acid that results in increased AGPase activity when provided in an AGPase enzyme. In a specific embodiment, the cysteine corresponding to amino acid position 424 of wild type maize AGPase large subunit is substituted with a valine amino acid. The subject invention also comprises a mutant plant large subunit of AGPase encoded by a polynucleotide of the invention. Characterization of kinetic and allosteric properties indicates increased starch yield is provided when the polynucleotides of the invention are expressed in plants such as monocot endosperms.

Claims

1. A polynucleotide encoding a mutant maize endosperm AGPase large subunit protein, or a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein, wherein said protein comprises an amino acid mutation of the cysteine at position 424 of the wild type maize endosperm AGPase large subunit protein, wherein said cysteine is replaced by an amino acid that confers increased enzymatic activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme, or wherein said protein comprises an amino acid mutation of the serine amino acid at position 163 of the wild type maize endosperm AGPase large subunit protein, wherein said serine is replaced by an amino acid that confers increased heat stability of AGPase activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme; or said protein comprises both of said amino acid mutations; and wherein said functional fragment comprises one or both of said amino acid mutations. 2. The polynucleotide according to claim 1 , wherein said replacement amino acid that confers increased enzymatic activity is a valine or said replacement amino acid that confers increase heat stability is a phenylalanine. 3. The polynucleotide according to claim 1 , wherein said mutant maize endosperm AGPase large subunit protein encoded by said polynucleotide comprises the amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 18, or SEQ ID NO: 47, or a functional fragment thereof that exhibits the same activity as the full-length AGPase large subunit protein. 4. The polynucleotide according to claim 1 , wherein said polynucleotide comprises the nucleotide sequence shown in SEQ ID NO: 1, SEQ ID NO: 17, or SEQ ID NO: 47, or a fragment thereof that encodes a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein. 5. The polynucleotide according to claim 1 , wherein said polynucleotide is provided in an expression construct. 6. A transgenic plant or plant tissue comprising a polynucleotide encoding a mutant maize endosperm AGPase large subunit protein, or a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein, wherein said protein comprises an amino acid mutation of the cysteine at position 424 of the wild type maize endosperm AGPase large subunit protein, wherein said cysteine is replaced by an amino acid that confers increased enzymatic activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme, or wherein said protein comprises an amino acid mutation of the serine amino acid at position 163 of the wild type maize endosperm AGPase large subunit protein, wherein said serine is replaced by an amino acid that confers increased heat stability of AGPase activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme; or said protein comprises both of said amino acid mutations; and wherein said functional fragment comprises one or both of said amino acid mutations. 7. The plant or plant tissue according to claim 6 , wherein the plant also expresses a small subunit protein of AGPase. 8. The plant or plant tissue according to claim 6 , wherein said mutant maize endosperm AGPase large subunit protein comprises a second mutation that confers increased heat stability and/or said mutant maize endosperm AGPase large subunit protein further comprises a mutation that confers increased seed weight. 9. The plant or plant tissue according to claim 6 , wherein said plant or plant tissue is monocotyledonous. 10. The plant or plant tissue according to claim 9 , wherein said monocotyledonous plant or plant tissue is selected from the group consisting of rice, wheat, barley, oat, sorghum, maize, lily, and millet. 11. The plant or plant tissue according to claim 6 , wherein said plant is Zea mays or said plant tissue is from Zea mays . 12. The plant or plant tissue according to claim 6 , wherein said plant tissue is a seed. 13. The plant or plant tissue according to claim 6 , wherein said replacement amino acid that confers increased enzymatic activity is a valine or said replacement amino acid that confers increased heat stability is a phenylalanine. 14. The plant or plant tissue according to claim 6 , wherein said mutant maize endosperm AGPase large subunit protein encoded by said polynucleotide comprises the amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 18, or SEQ ID NO: 47, or a functional fragment thereof that exhibits the same activity as the full-length AGPase large subunit protein. 15. The plant or plant tissue according to claim 6 , wherein said polynucleotide comprises the nucleotide sequence shown in SEQ ID NO: 1, SEQ ID NO: 17, or SEQ ID NO: 47, or a fragment thereof that encodes a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein. 16. A method of increasing starch biosynthesis and/or heat resistance of a plant, said method comprising incorporating a polynucleotide into the genome of a plant and expressing the protein encoded by said polynucleotide, wherein said polynucleotide encodes a mutant maize endosperm AGPase large subunit protein, or a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein, wherein said protein comprises an amino acid mutation of the cysteine at position 424 of the wild type maize endosperm AGPase large subunit protein, wherein said cysteine is replaced by an amino acid that confers increased enzymatic activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme, or wherein said protein comprises an amino acid mutation of the serine amino acid at position 163 of the wild type maize endosperm AGPase large subunit protein, wherein said serine is replaced by an amino acid that confers increased heat stability of AGPase activity when said mutant AGPase large subunit protein is expressed to form an AGPase enzyme; or said mutant plant AGPase large subunit protein comprises both of said amino acid mutations; and wherein said functional fragment comprises one or both of said amino acid mutations. 17. The method according to claim 16 , wherein said replacement amino acid that confers increased enzymatic activity is a valine or said replacement amino acid that confers increased heat stability is a phenylalanine. 18. The method according to claim 16 , wherein said mutant plant maize endosperm AGPase large subunit protein encoded by said polynucleotide comprises the amino acid sequence shown in SEQ ID NO: 2, SEQ ID NO: 18, or SEQ ID NO: 47, or a functional fragment thereof that exhibits the same activity as the full-length AGPase large subunit protein. 19. The method according to claim 16 , wherein said polynucleotide comprises the nucleotide sequence shown in SEQ ID NO: 1, SEQ ID NO: 17, or SEQ ID NO: 47, or a fragment thereof that encodes a functional fragment of said protein that exhibits the same activity as the full-length AGPase large subunit protein. 20. The method according to claim 16 , wherein said mutant maize endosperm AGPase large subunit protein comprises a second mutation that confers increased heat stability and/or said mutant maize endosperm AGPase large subunit protein further comprises a mutation that confers increased seed weight. 21. The plant or plant tissue according to claim 6 , wherein said plant or plant tissue is dicotyledonous. 22. The plant or plant tissue according to claim 21 , wherein said dicotyledonous plant or plant tissue is selected from the group consisting of pea, alfalfa, chickpea, chicory, clover, kale, lentil, soybean, tobacco, potato, sweet potato, radish, cabbage, rape, apple tree, and lettuce. 23. The polynucleotide according to claim 1 , wherein said mutant maize endosperm AGPase large subunit protein comprises a second mutation that confers increased heat stability and/or said mutant maize endosperm AGPase large subunit protein further comprises a mutation that confers increased seed weight.
GOVERNMENT SUPPORT The subject matter of this application has been supported by a research grant from the National Science Foundation under grant numbers IOS-0444031 and IOS-0815104. Accordingly, the government has certain rights in this invention. BACKGROUND OF THE INVENTION AGPase catalyzes the first committed step in starch (plants) and glycogen (bacteria) synthesis. It involves the conversion of glucose-1-P (G-1-P) and ATP to ADP-glucose and pyrophosphate (PPi). AGPase is a heterotetramer in plants consisting of two identical small and two identical large subunits. The large and the small subunits are encoded by shrunken-2 (Sh2) and brittle-2 (Bt2) respectively in maize endosperm. AGPase is allosterically regulated by small effector molecules that are indicative of the energy status of the cell. AGPase is activated by 3-PGA, the first carbon assimilatory product, and inhibited/deactivated by inorganic phosphate (Pi) in cyanobacteria, green algae and angiosperms. The importance of maize endosperm AGPase in starch synthesis has been shown by the kernel phenotype of mutants in either subunit of the enzyme. Indeed, such mutants result in shrunken kernels and a large reduction in endosperm starch content (Tsai and Nelson, 1966; Hannah and Nelson, 1976). There is also evidence that AGPase catalyses a rate-limiting step in starch synthesis (Stark et al. 1992; Giroux et al. 1996; Greene et al 1998b; Sakulsingharoja et al. 2004; Obana et al. 2006; Wang et al. 2007). Greene and Hannah (1998a) isolated a mutant form of maize AGPase with a single amino acid change in the large subunit termed HS33. They showed that the altered enzyme was more heat-stable and that stability was due to stronger subunit-subunit interactions. When wheat and rice were transformed with a Sh2 variant that contains the HS33 change along with a change that affects the allosteric properties of AGPase (Rev6)(Giroux et al., 1996), yield was increased by 38% and 23% respectively (Smidansky et al., 2002; 2003). Remarkably, the increase was due to an increase in seed number rather than individual seed weight. Transformation of maize with the Sh2 variant containing the Rev6 and HS33 changes also gives rise to enhanced seed number. Seed yield/ear can be increased up to 68% in maize. Enhanced seed number cannot be explained by Rev6 since, when expressed alone in maize, it increases only seed weight (Hannah, unpublished). The above studies show the importance of AGPase heat stability in cereal yield. Cross et al. (2004) generated a mosaic small subunit (MP) consisting of the first 200 amino acids of BT2 and the last 275 amino acids of the potato tuber small subunit. MP in a complex with SH2 had several features that could lead to agronomic gain (Cross et al., 2004; Boehlein et al., 2005). Some of those features were increased activity in the absence of the activator 3-PGA, increased affinity for 3-PGA and elevated heat stability compared to wildtype maize endosperm AGPase (BT2/SH2). Preliminary data show that maize plants with transgenic MP containing AGPase variant expressed in maize endosperm provides for a starch yield increase (Hannah, unpublished data). BRIEF SUMMARY OF THE INVENTION The subject invention concerns materials and methods for providing plants or plant tissue with increased starch biosynthesis. Increased starch biosynthesis provides for increased yield. One aspect of the invention concerns polynucleotides that encode a mutant plant large subunit of AGPase. In one embodiment, a polynucleotide of the invention encodes a plant AGPase large subunit having an amino acid mutation wherein the cysteine amino acid corresponding to amino acid position 424 of wild type maize AGPase large subunit is substituted with an amino acid that results in increased AGPase activity when provided in an AGPase enzyme. In a specific embodiment, the cysteine corresponding to amino acid position 424 of wild type maize AGPase large subunit is substituted with a valine amino acid. The subject invention also comprises a mutant plant large subunit of AGPase encoded by a polynucleotide of the invention. Characterization of kinetic and allosteric properties indicates increased starch yield is provided when the polynucleotides of the invention are expressed in plants such as monocot endosperms. BRIEF DESCRIPTION OF THE DRAWINGS The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIG. 1 shows an amino acid tree of AGPase large subunits in angiosperms. The tree was constructed by Georgelis et al. (2008). Boxes indicate duplication events. Positively selected amino acid sites detected in thick branches (Georgelis et al., 2008) were utilized as candidate sites for functional divergence. Bootstrap values >50% of the branches leading to the large subunit groups are shown. FIG. 2 shows an amino acid alignment between maize endosperm (SH2) and potato tuber large subunit. Red boxes indicate sites that make direct contact with the small subunit as determined by Tuncel et al. (2008). Blue and red arrows indicate type-II and positively selected sites respectively. FIGS. 3A and 3B show AGPase subunit interactions. The white structures correspond to the resolved structure of potato tuber small subunit homodimers (PDB #: 1yp2). Cyan and magenta modeled structures of the small (BT2) and the large subunit (SH2) of maize endosperm, respectively, are superimposed on the structure of potato tuber small subunit homodimers. Red circles indicate the candidate Pi binding sites. Head-to-head ( FIG. 3A ) and Tail-to-tail ( FIG. 3B ) subunit interactions. FIG. 4 shows superimposition of maize endosperm large subunit (SH2) modeled structure (magenta) on the potato tuber large subunit modeled structure (white). Red areas indicate sites in the potato tuber large subunit that are proposed to make direct contact with the small subunit (Tuncel et al., 2008). FIGS. 5A and 5B show placement of all type-II and positively selected sites on the subunit interfaces of maize endosperm large subunit (SH2). Type-I sites 149 and 361, that were changed by site-directed mutagenesis, are also placed on the structure of SH2. SH2 modeled structure (green) was superimposed on potato tuber large subunit modeled structure (white). Red areas indicate sites in the potato tuber large subunit that are proposed to make direct contact with the small subunit (Tuncel et al., 2008). Type-I, type-II and positively selected sites detected by Georgelis et al. (2008) are shown in yellow. FIG. 5A shows areas that participate in tail-to-tail interactions. FIG. 5B shows areas that participate in head-to-head interactions. FIG. 6 shows glycogen quantitation produced by SH2 wild type and variants expressed in E. coli along with BT2. * indicates significant difference compared to wild type BT2/SH2 at p=0.05 (Student t-test) (N=4). FIG. 7 shows heat stability of SH2 wild type and variants in a complex with BT2. * indicates significant difference compared to wild type BT2/SH2 at p=0.05 (Student t-test) (N=6). FIG. 8 shows Western blot of protein extracts from E. coli cells expressing SH2, V502T, and A508S along with BT2. FIG. 9 shows placement of SH2 sites 213, 424, 502 and 508 on the modeled SH2 structure. SH2 modeled structure (magenta) was superimposed on potato tuber large subunit modeled structure (white). Red areas indicate sites in the potato tuber large subunit that are proposed to make direct contact with the small subunit (Tuncel et al., 2008). Red circles indicate the candidate Pi binding sites. BRIEF DESCRIPTION OF THE SEQUENCES SEQ ID NO:1 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant plant large subunit AGPase polypeptide of the invention. SEQ ID NO:2 is an amino acid sequence of a mutant polypeptide of the invention encoded by SEQ ID NO:1. SEQ ID NOs:3, 4, and 5 are polynucleotide sequences encoding a mutant plant AGPase small subunit that can be used according to the present invention. SEQ ID NOs:6, 7, and 8 are amino acid sequences of a mutant plant AGPase small subunit that can be used according to the present invention and that is encoded by SEQ ID NOs: 3, 4, and 5, respectively. SEQ ID NO:9 is a polynucleotide sequence encoding a mutant plant AGPase small subunit that can be used according to the present invention. SEQ ID NO:10 is an amino acid sequence of a mutant plant AGPase small subunit that can be used according to the present invention. SEQ ID NO:11 is an amino acid sequence of a mutant plant AGPase small subunit that can be used according to the present invention. SEQ ID NO:12 is a polynucleotide sequence encoding a mutant plant AGPase small subunit that can be used according to the present invention. SEQ ID NO:13 is an amino acid sequence of a mutant plant AGPase small subunit that can be used according to the present invention and that is encoded by SEQ ID NO:12. SEQ ID NO:14 is an amino acid sequence of a mutant plant AGPase large subunit polypeptide of the invention. SEQ ID NO:15 is an amino acid sequence of a mutant plant AGPase large subunit polypeptide of the invention. SEQ ID NO:16 is an amino acid sequence of a mutant plant AGPase large subunit polypeptide of the invention. SEQ ID NO:17 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant plant large subunit AGPase polypeptide of the invention. SEQ ID NO:18 is an amino acid sequence of a mutant plant AGPase large subunit polypeptide of the invention encoded by SEQ ID NO:17. SEQ ID NOs:19-46 are oligonucleotides that can be used according to the present invention. SEQ ID NO:47 is a polynucleotide sequence comprising a nucleotide sequence encoding the mutant plant large subunit AGPase polypeptide of the invention. SEQ ID NO:48 is a polynucleotide sequence comprising a nucleotide sequence encoding a wild type plant large subunit AGPase polypeptide. SEQ ID NO:49 is an amino acid sequence of a wild type maize endosperm large subunit AGPase polypeptide. SEQ ID NO:50 is an amino acid sequence of a wild type potato tuber large subunit AGPase polypeptide. DETAILED DESCRIPTION OF THE INVENTION The subject invention concerns materials and methods for providing plants with increased starch production. One aspect of the invention concerns polynucleotides that encode a mutant plant large subunit of AGPase. In one embodiment, a polynucleotide of the invention encodes a plant AGPase large subunit having an amino acid mutation wherein the cysteine amino acid corresponding to amino acid position 424 of wild type maize endosperm AGPase large subunit is substituted with an amino acid that results in increased AGPase activity when provided in an AGPase enzyme. In a specific embodiment, the amino acid substituted for the cysteine is a valine. In an exemplified embodiment, the mutant plant AGPase large subunit encoded by the polynucleotide comprises the amino acid sequence shown in SEQ ID NO:2, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, or a fragment or variant thereof. Polynucleotides of the invention encoding a mutant plant large subunit of AGPase can also optionally comprise any one of the mutations described in any of U.S. Pat. Nos. 5,589,618; 5,650,557; 5,872,216; 6,069,300; 6,184,438; 6,403,863; 6,809,235; 7,173,165; 7,312,378; and 6,969,783. In one embodiment, a mutant large subunit of the invention comprises a Rev6 mutation as described in U.S. Pat. Nos. 5,872,216; 5,650,557; and 5,589,618, and International published application number WO 98/10082, and/or one or more heat stable (HS) mutations as described in U.S. Pat. Nos. 7,312,378; 6,809,235; 6,403,863; 6,069,300; and 6,969,783, and International published application numbers WO 99/58698; WO 2003/0070901; WO 98/22601; and WO 02/072784, such as the HS33 mutation. In a specific embodiment, the polynucleotide encodes a mutant plant AGPase large subunit having the amino acid sequence shown in SEQ ID NO: 14 (Rev6+c424v), SEQ ID NO:15 (HS33+c424v); or SEQ ID NO:16 (Rev6+HS33+c424v). In another embodiment, a composition of the invention or a polynucleotide of the invention encoding a mutant plant large AGPase subunit can also comprise a polynucleotide that encodes a mutant plant AGPase small subunit that can comprise an amino acid mutation as described in published International patent application WO 2005/019425 (Hannah and Linebarger). In one embodiment, the mutant AGPase small subunit encoded by the polynucleotide comprises an amino acid mutation wherein the tyrosine corresponding to amino acid position 36 of wild type small subunit maize endosperm AGPase is substituted with a cysteine. The mutant AGPase small subunit can also optionally comprise an amino acid inserted between the serine and threonine amino acids corresponding to amino acid positions 34 and 35 of wild type maize endosperm AGPase, respectively. In specific embodiments, the amino acid inserted between amino acids at position 34 and 35 of the AGPase small subunit is a glutamic acid or glutamine. In exemplified embodiments, the mutant plant AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:6, SEQ ID NO:7, or SEQ ID NO:8, or a fragment or variant thereof. In specific embodiments, the polynucleotide comprises the nucleotide sequences shown in SEQ ID NO:3, SEQ ID NO:4, or SEQ ID NO:5, or a fragment or variant thereof. In another embodiment, a composition of the invention or a polynucleotide of the invention encoding a mutant large subunit AGPase can also comprise a polynucleotide that encodes a chimeric plant AGPase small subunit comprising sequences from two different plants (as described in U.S. Pat. No. 7,173,165) and/or a polynucleotide that encodes a plant AGPase small subunit that comprises an amino acid mutation wherein the threonine amino acid corresponding to amino acid position 462 of wild type maize endosperm AGPase small subunit is substituted with an amino acid that confers increased heat stability. In a specific embodiment of the latter, the amino acid substituted for threonine is an isoleucine. In one embodiment, the mutant plant AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:13, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:12, or a fragment or variant thereof. A chimeric AGPase small subunit that can be used in the present invention can comprise a C-terminal portion from one plant and an N-terminal portion from another plant. In one embodiment, a chimeric AGPase small subunit of the present invention comprises an N-terminus sequence having approximately the first 150 to 250 amino acids of the N-terminus of a first plant AGPase small subunit and a C-terminus sequence comprising approximately the terminal 300 residues or less of the C-terminus of a second plant AGPase small subunit. Thus, the C-terminus of the chimeric small subunit can comprise the terminal 300, or 299, or 298, or 297, or 296, or 295, and so forth, residues of the C-terminus of the second plant. The small subunit sequences can be from an AGPase of a monocot or dicot plant, or both a monocot and a dicot. Monocotyledonous plants, such as, for example, rice, wheat, barley, oats, sorghum, maize, lilies, and millet are included within the scope of the invention. Dicot plants can include, for example, tobacco, soybean, potato, sweet potato, radish, cabbage, rape, apple tree, and lettuce. In one embodiment, the first 200 or so amino acids of the N-terminus of the chimeric AGPase small subunit are from the N-terminus of maize endosperm AGPase small subunit and the C-terminus amino acids are from the C-terminus of potato tuber AGPase small subunit plus the mutation corresponding to amino acid position 462 of the present invention. In a specific embodiment, the C-terminus region of a chimeric AGPase small subunit of the present invention comprises the terminal 276 amino acids of the AGPase small subunit of potato tuber. In an exemplified embodiment, the chimeric AGPase small subunit comprises a portion of the small subunit of maize endosperm AGPase and a portion of the small subunit of potato tuber AGPase. In a specific embodiment, the chimeric AGPase small subunit contains a) the first 199 amino acids (i.e., amino acids 1 through 199) from the small subunit of maize endosperm AGPase and the carboxyl terminal end of the small subunit of potato tuber AGPase, starting at amino acid 246 (i.e., amino acids 246 through 521) using the amino acid sequence shown for the protein deposited as Genbank accession number X61186 (or, alternatively, starting at amino acid 175 using the numbering system for the potato AGPase subunit as in Hannah et al., 2001) and b) the mutation wherein the threonine amino acid corresponding to amino acid position 462 of wild type maize endosperm AGPase small subunit is substituted with an amino acid that confers increased heat stability, such as an isoleucine. In an exemplified embodiment, the plant chimeric AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:10 or SEQ ID NO:11, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:9, or a fragment or variant thereof. The subject invention concerns materials and methods for providing plants with increased resistance to heat conditions. Increased resistance of a plant to heat conditions provides for decreasing the yield losses that are generally observed at elevated temperatures. In one embodiment, a polynucleotide of the invention encodes a plant AGPase large subunit having an amino acid mutation wherein the serine amino acid corresponding to amino acid position 163 of wild type maize endosperm AGPase large subunit is substituted with an amino acid that results in increased heat stability of AGPase activity when provided in an AGPase enzyme. In a specific embodiment, the amino acid substituted for the serine is a phenylalanine. In an exemplified embodiment, the mutant plant AGPase large subunit comprises the amino acid sequence shown in SEQ ID NO:18, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ IDNO: 17, or a fragment or variant thereof. In addition to comprising a mutation providing increased resistance to heat conditions, the mutant plant AGPase large subunit can also comprise a mutation of the invention providing for increased starch production. In one embodiment, the mutant plant AGPase large subunit comprises the amino acid sequence shown in SEQ ID NO:47, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:47, or a fragment or variant thereof. The subject invention also comprises methods for increasing starch biosynthesis and/or resistance to heat conditions and increasing crop yield of a plant or plant tissue. In one embodiment, a method of the invention comprises introducing one or more polynucleotides of the present invention into a plant. In an exemplified embodiment, the mutant plant AGPase large subunit encoded by the polynucleotide comprises the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, SEQ ID NO:17, or SEQ ID NO:47, or a fragment or variant thereof. In one embodiment, the polynucleotide is stably incorporated into the genome of the plant or plant tissue. The polynucleotide can comprise regulatory elements, such as a promoter and/or enhancer sequences, that provide for increased expression of the polynucleotide and/or the polypeptide encoded thereby. In a specific embodiment, the promoter sequence is one that provides for constitutive or tissue-specific (e.g., endosperm) expression. Plants or plant tissues containing the polynucleotide, or progeny of the plants, optionally can be screened for increased expression of a polynucleotide or polypeptide of the invention. In one embodiment, multiple copies of one or more polynucleotides of the invention are introduced into a plant or plant tissue and stably incorporated into the genome of the plant. In one embodiment, a polynucleotide of the invention is provided in an expression construct as described herein. The subject invention also comprises mutant large subunit AGPase polypeptides encoded by the polynucleotides of the invention. In one embodiment, the polypeptide comprises the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. The subject invention also concerns mutant plant AGPase enzymes comprising one or more mutant polypeptides of the invention. In specific embodiments, a mutant plant AGPase enzyme comprises one or more mutant polypeptides any of which can comprise the amino acid sequence of SEQ ID NO:2, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:18, or SEQ ID NO:47, or a fragment or variant of any such sequence, wherein the mutant AGPase enzyme exhibits increased enzymatic activity relative to a wild type AGPase enzyme. In one embodiment, the mutant plant enzyme comprises two mutant AGPase large subunits of the invention, wherein the mutant polypeptides can have the same mutation(s) or can have different mutation(s). The subject invention also concerns mutant plant AGPase enzymes comprising one or more mutant large subunit polypeptides of the invention and one or more mutant small subunit polypeptides. In one embodiment, the mutant large subunit polypeptide can also comprise any of the mutations described in any of U.S. Pat. Nos. 5,589,618; 5,650,557; 5,872,216; 6,069,300; 6,184,438; 6,403,863; 6,809,235; 7,173,165; 7,312,378; and 6,969,783. In one embodiment, a mutant large subunit polypeptide of the invention comprises a Rev6 mutation. In another embodiment, a mutant large subunit AGPase polypeptide of the invention also comprises one or more heat stable (HS) mutations as described in U.S. Pat. Nos. 6,069,300; 6,403,863; 6,809,235; 7,312,378; and 6,969,783, and published International patent application nos. WO 99/58698; WO 2003/0070901; WO 98/22601; and WO 02/072784, such as, for example, the HS33 mutation. In one embodiment, the mutant plant AGPase enzyme comprises two mutant small subunit polypeptides, wherein the mutant small subunit polypeptides can have the same mutation(s) or can have different mutation(s), as described herein. In another embodiment, the mutant plant AGPase enzyme comprises two mutant large subunit AGPase polypeptides wherein the mutant polypeptides can have the same mutation(s) or can have different mutation(s), as described herein. In a further embodiment, the mutant plant AGPase enzyme comprises two mutant small subunit AGPase polypeptides and two mutant large subunit AGPase polypeptides, wherein the mutant small subunit polypeptides and the mutant large subunit polypeptides can have the same mutation(s) or can have different mutation(s), as described herein. The subject invention also concerns methods for providing for a mutant plant AGPase enzyme having increased enzymatic activity relative to wild type plant AGPase. In one embodiment, the method comprises incorporating or providing one or more mutant AGPase large subunit polypeptides of the present invention with wild type or mutant AGPase small subunits in an AGPase enzyme. In one embodiment, the AGPase enzyme comprises a tetramer of polypeptide subunits, wherein one, two, or more of the subunits is a mutant large subunit polypeptide of the present invention. In one embodiment, the AGPase enzyme comprises a mutant large subunit of the invention which also comprises additional mutations, for example, as described in any of U.S. Pat. Nos. 5,589,618; 5,650,557; 5,872,216; 6,069,300; 6,184,438; 6,403,863; 6,809,235; 7,173,165; 7,312,378; and 6,969,783, such as the Rev6 mutation and/or a heat stability mutation, such as HS33. In one embodiment, the AGPase enzyme also comprises a further mutant large subunit polypeptide subunit, such as a large subunit comprising a Rev6 mutation (as described in U.S. Pat. Nos. 5,872,216; 5,650,557; and 5,589,618, and International published application number WO 98/10082) and/or a heat stability mutation (as described in U.S. Pat. Nos. 7,312,378; 6,809,235; 6,403,863; 6,069,300; and 6,969,783, and International published application numbers WO 99/58698; WO 2003/0070901; WO 98/22601; and WO 02/072784), such as HS33. In one embodiment, a mutant plant AGPase enzyme of the invention also comprises one or more mutant AGPase small subunits as described herein (see, for example, U.S. Pat. No. 7,173,165 and International published application number WO 2003/047527). The subject invention also concerns plants, plant tissue, and plant cells that comprise a polynucleotide or the protein encoded by the polynucleotide of the invention, or that express a mutant polypeptide of the invention, or a fragment or variant thereof, or that comprise or express a mutant plant AGP enzyme of the present invention. In an exemplified embodiment, the mutant plant AGPase large subunit encoded by the polynucleotide comprises the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, SEQ ID NO:17, or SEQ ID NO:47, or a fragment or variant thereof. Plant tissue includes, but is not limited to, seed, scion, and rootstock. Plants within the scope of the present invention include monocotyledonous plants, such as, for example, rice, wheat, barley, oats, rye, sorghum, maize, sugarcane, pineapple, onion, bananas, coconut, lilies, turfgrasses, and millet. Plants within the scope of the present invention also include dicotyledonous plants, such as, for example, tomato, cucumber, squash, peas, alfalfa, melon, chickpea, chicory, clover, kale, lentil, soybean, beans, tobacco, potato, sweet potato, yams, cassava, radish, broccoli, spinach, cabbage, rape, apple trees, citrus (including oranges, mandarins, grapefruit, lemons, limes and the like), grape, cotton, sunflower, strawberry, and lettuce. Herb plants containing a polynucleotide of the invention are also contemplated within the scope of the invention. Herb plants include parsley, sage, rosemary, thyme, and the like. In one embodiment, the plant, plant tissue, or plant cell is Zea mays. In one embodiment, a plant, plant tissue, or plant cell of the invention is a transgenic plant, plant tissue, or plant cell. In another embodiment, a plant, plant tissue, or plant cell of the invention is one that has been obtained through a breeding program. Polynucleotides useful in the present invention can be provided in an expression construct. Expression constructs of the invention generally include regulatory elements that are functional in the intended host cell in which the expression construct is to be expressed. Thus, a person of ordinary skill in the art can select regulatory elements for use in bacterial host cells, yeast host cells, plant host cells, insect host cells, mammalian host cells, and human host cells. Regulatory elements include promoters, transcription termination sequences, translation termination sequences, enhancers, and polyadenylation elements. As used herein, the term “expression construct” refers to a combination of nucleic acid sequences that provides for transcription of an operably linked nucleic acid sequence. As used herein, the term “operably linked” refers to a juxtaposition of the components described wherein the components are in a relationship that permits them to function in their intended manner. In general, operably linked components are in contiguous relation. An expression construct of the invention can comprise a promoter sequence operably linked to a polynucleotide sequence encoding a mutant polypeptide of the invention. Promoters can be incorporated into a polynucleotide using standard techniques known in the art. Multiple copies of promoters or multiple promoters can be used in an expression construct of the invention. In a preferred embodiment, a promoter can be positioned about the same distance from the transcription start site in the expression construct as it is from the transcription start site in its natural genetic environment. Some variation in this distance is permitted without substantial decrease in promoter activity. A transcription start site is typically included in the expression construct. If the expression construct is to be provided in or introduced into a plant cell, then plant viral promoters, such as, for example, a cauliflower mosaic virus (CaMV) 35S (including the enhanced CaMV 35S promoter (see, for example U.S. Pat. No. 5,106,739)) or a CaMV 19S promoter or a cassava vein mosaic can be used. Other promoters that can be used for expression constructs in plants include, for example, prolifera promoter, Ap3 promoter, heat shock promoters, T-DNA 1′- or 2′-promoter of A. tumefaciens , polygalacturonase promoter, chalcone synthase A (CHS-A) promoter from petunia, tobacco PR-la promoter, ubiquitin promoter, actin promoter, alcA gene promoter, pin2 promoter (Xu et al., 1993), maize WipI promoter, maize trpA gene promoter (U.S. Pat. No. 5,625,136), maize CDPK gene promoter, and RUBISCO SSU promoter (U.S. Pat. No. 5,034,322) can also be used. Tissue-specific promoters, for example fruit-specific promoters, such as the E8 promoter of tomato (accession number: AF515784; Good et al. (1994)) can be used. Fruit-specific promoters such as flower organ-specific promoters can be used with an expression construct of the present invention for expressing a polynucleotide of the invention in the flower organ of a plant. Examples of flower organ-specific promoters include any of the promoter sequences described in U.S. Pat. Nos. 6,462,185; 5,639,948; and 5,589,610. Seed-specific promoters such as the promoter from a β-phaseolin gene (for example, of kidney bean) or a glycinin gene (for example, of soybean), and others, can also be used. Endosperm-specific promoters include, but are not limited to, MEGI (EPO application No. EP1528104) and those described by Wu et al. (1998), Furtado et al. (2001), and Hwang et al. (2002). Root-specific promoters, such as any of the promoter sequences described in U.S. Pat. No. 6,455,760 or U.S. Pat. No. 6,696,623, or in published U.S. patent application Nos. 20040078841; 20040067506; 20040019934; 20030177536; 20030084486; or 20040123349, can be used with an expression construct of the invention. Constitutive promoters (such as the CaMV, ubiquitin, actin, or NOS promoter), developmentally-regulated promoters, and inducible promoters (such as those promoters than can be induced by heat, light, hormones, or chemicals) are also contemplated for use with polynucleotide expression constructs of the invention. Expression constructs of the invention may optionally contain a transcription termination sequence, a translation termination sequence, a sequence encoding a signal peptide, and/or enhancer elements. Transcription termination regions can typically be obtained from the 3′ untranslated region of a eukaryotic or viral gene sequence. Transcription termination sequences can be positioned downstream of a coding sequence to provide for efficient termination. A signal peptide sequence is a short amino acid sequence typically present at the amino terminus of a protein that is responsible for the relocation of an operably linked mature polypeptide to a wide range of post-translational cellular destinations, ranging from a specific organelle compartment to sites of protein action and the extracellular environment. Targeting gene products to an intended cellular and/or extracellular destination through the use of an operably linked signal peptide sequence is contemplated for use with the polypeptides of the invention. Classical enhancers are cis-acting elements that increase gene transcription and can also be included in the expression construct. Classical enhancer elements are known in the art, and include, but are not limited to, the CaMV 35S enhancer element, cytomegalovirus (CMV) early promoter enhancer element, and the SV40 enhancer element. Intron-mediated enhancer elements that enhance gene expression are also known in the art. These elements must be present within the transcribed region and are orientation dependent. Examples include the maize shrunken-1 enhancer element (Clancy and Hannah, 2002). DNA sequences which direct polyadenylation of mRNA transcribed from the expression construct can also be included in the expression construct, and include, but are not limited to, an octopine synthase or nopaline synthase signal. The expression constructs of the invention can also include a polynucleotide sequence that directs transposition of other genes, i.e., a transposon. Polynucleotides of the present invention can be composed of either RNA or DNA. Preferably, the polynucleotides are composed of DNA. The subject invention also encompasses those polynucleotides that are complementary in sequence to the polynucleotides disclosed herein. Polynucleotides and polypeptides of the invention can be provided in purified or isolated form. Because of the degeneracy of the genetic code, a variety of different polynucleotide sequences can encode mutant polypeptides of the present invention. A table showing all possible triplet codons (and where U also stands for T) and the amino acid encoded by each codon is described in Lewin (1985). In addition, it is well within the skill of a person trained in the art to create alternative polynucleotide sequences encoding the same, or essentially the same, mutant polypeptides of the subject invention. These variant or alternative polynucleotide sequences are within the scope of the subject invention. As used herein, references to “essentially the same” sequence refers to sequences which encode amino acid substitutions, deletions, additions, or insertions which do not materially alter the functional activity of the polypeptide encoded by the polynucleotides of the present invention. Allelic variants of the nucleotide sequences encoding a large subunit AGPase of the invention are also encompassed within the scope of the invention. Substitution of amino acids other than those specifically exemplified or naturally present in a mutant polypeptide of the invention are also contemplated within the scope of the present invention. For example, non-natural amino acids can be substituted for the amino acids of a mutant large subunit AGPase polypeptide, so long as the mutant polypeptide having the substituted amino acids retains substantially the same functional activity as the mutant polypeptide in which amino acids have not been substituted. Examples of non-natural amino acids include, but are not limited to, omithine, citrulline, hydroxyproline, homoserine, phenylglycine, taurine, iodotyrosine, 2,4-diaminobutyric acid, a-amino isobutyric acid, 4-aminobutyric acid, 2-amino butyric acid, γ-amino butyric acid, e-amino hexanoic acid, 6-amino hexanoic acid, 2-amino isobutyric acid, 3-amino propionic acid, norleucine, norvaline, sarcosine, homocitrulline, cysteic acid, t-butylglycine, t-butylalanine, phenylglycine, cyclohexylalanine, β-alanine, fluoro-amino acids, designer amino acids such as β-methyl amino acids, C-methyl amino acids, N-methyl amino acids, and amino acid analogues in general. Non-natural amino acids also include amino acids having derivatized side groups. Furthermore, any of the amino acids in the protein can be of the D (dextrorotary) form or L (levorotary) form. Allelic variants of a protein sequence of a mutant large subunit and/or small subunit AGPase polypeptide of the present invention are also encompassed within the scope of the invention. Amino acids can be generally categorized in the following classes: non-polar, uncharged polar, basic, and acidic. Conservative substitutions whereby a mutant polypeptide enzyme of the present invention having an amino acid of one class is replaced with another amino acid of the same class fall within the scope of the subject invention so long as the mutant polypeptide having the substitution still retains substantially the same functional activity (e.g., increased enzymatic and/or increased heat stability of an AGPase enzyme) as the mutant polypeptide that does not have the substitution. Polynucleotides encoding a mutant polypeptide having one or more amino acid substitutions in the sequence are contemplated within the scope of the present invention. Table 1 below provides a listing of examples of amino acids belonging to each class. TABLE 1 Class of Amino Acid Examples of Amino Acids Nonpolar Ala, Val, Leu, Ile, Pro, Met, Phe, Trp Uncharged Polar Gly, Ser, Thr, Cys, Tyr, Asn, Gln Acidic Asp, Glu Basic Lys, Arg, His The subject invention also concerns variants of the polynucleotides of the present invention that encode functional mutant large or small subunit AGPase polypeptides of the invention. Variant sequences include those sequences wherein one or more nucleotides of the sequence have been substituted, deleted, and/or inserted. The nucleotides that can be substituted for natural nucleotides of DNA have a base moiety that can include, but is not limited to, inosine, 5-fluorouracil, 5-bromouracil, hypoxanthine, 1-methylguanine, 5-methylcytosine, and tritylated bases. The sugar moiety of the nucleotide in a sequence can also be modified and includes, but is not limited to, arabinose, xylulose, and hexose. In addition, the adenine, cytosine, guanine, thymine, and uracil bases of the nucleotides can be modified with acetyl, methyl, and/or thio groups. Sequences containing nucleotide substitutions, deletions, and/or insertions can be prepared and tested using standard techniques known in the art. Fragments and variants of a mutant polypeptide of the present invention can be generated as described herein and tested for the presence of enzymatic and heat stable function using standard techniques known in the art. Thus, an ordinarily skilled artisan can readily prepare and test fragments and variants of a mutant polypeptide of the invention and determine whether the fragment or variant retains functional activity relative to full-length or a non-variant mutant polypeptide. Polynucleotides and polypeptides contemplated within the scope of the subject invention can also be defined in terms of more particular identity and/or similarity ranges with those sequences of the invention specifically exemplified herein. The sequence identity will typically be greater than 60%, preferably greater than 75%, more preferably greater than 80%, even more preferably greater than 90%, and can be greater than 95%. The identity and/or similarity of a sequence can be 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99% as compared to a sequence exemplified herein. Unless otherwise specified, as used herein percent sequence identity and/or similarity of two sequences can be determined using the algorithm of Karlin and Altschul (1990), modified as in Karlin and Altschul (1993). Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. (1990). BLAST searches can be performed with the NBLAST program, score=100, wordlength=12, to obtain sequences with the desired percent sequence identity. To obtain gapped alignments for comparison purposes, Gapped BLAST can be used as described in Altschul et al. (1997). When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (NBLAST and XBLAST) can be used. See NCBI/NIH website. The subject invention also contemplates those polynucleotide molecules having sequences which are sufficiently homologous with the polynucleotide sequences exemplified herein so as to permit hybridization with that sequence under standard stringent conditions and standard methods (Maniatis et al., 1982). As used herein, “stringent” conditions for hybridization refers to conditions wherein hybridization is typically carried out overnight at 20-25 C below the melting temperature (Tm) of the DNA hybrid in 6×SSPE, 5×Denhardt's solution, 0.1% SDS, 0.1 mg/ml denatured DNA. The melting temperature, Tm, is described by the following formula (Beltz et al., 1983): Tm=81.5 C+16.6 Log[Na+]+0.41 (% G+C)−0.61 (% formamide)−600/length of duplex in base pairs. Washes are typically carried out as follows: (1) Twice at room temperature for 15 minutes in 1×SSPE, 0.1% SDS (low stringency wash). (2) Once at Tm-20 C for 15 minutes in 0.2×SSPE, 0.1% SDS (moderate stringency wash). As used herein, the terms “nucleic acid” and “polynucleotide” refer to a deoxyribonucleotide, ribonucleotide, or a mixed deoxyribonucleotide and ribonucleotide polymer in either single- or double-stranded form, and unless otherwise limited, would encompass known analogs of natural nucleotides that can function in a similar manner as naturally-occurring nucleotides. The polynucleotide sequences include the DNA strand sequence that is transcribed into RNA and the strand sequence that is complementary to the DNA strand that is transcribed. The polynucleotide sequences also include both full-length sequences as well as shorter sequences derived from the full-length sequences. Allelic variations of the exemplified sequences also fall within the scope of the subject invention. The polynucleotide sequence includes both the sense and antisense strands either as individual strands or in the duplex. Techniques for transforming plant cells with a gene are known in the art and include, for example, Agrobacterium infection, biolistic methods, electroporation, calcium chloride treatment, PEG-mediated transformation, etc. U.S. Pat. No. 5,661,017 teaches methods and materials for transforming an algal cell with a heterologous polynucleotide. Transformed cells can be selected, redifferentiated, and grown into plants that contain and express a polynucleotide of the invention using standard methods known in the art. The seeds and other plant tissue and progeny of any transformed or transgenic plant cells or plants of the invention are also included within the scope of the present invention. The subject invention also concerns methods for producing a plant that exhibits increased starch production and/or increased resistance to heat conditions relative to a wild type plant, wherein a polynucleotide encoding a mutant large subunit AGPase polypeptide of the present invention is introduced into a plant cell and the polypeptide(s) encoded by the polynucleotide(s) is expressed. In an exemplified embodiment, the mutant plant AGPase large subunit encoded by the polynucleotide comprises the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, or a fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, SEQ ID NO:17, or SEQ ID NO:47, or a fragment or variant thereof. In one embodiment, the plant cell comprises non-mutant genes encoding wild type small subunit AGPase polypeptide. In another embodiment, the plant cell comprises at least one polynucleotide encoding a mutant small subunit AGPase polypeptide, such as one of the mutant small subunits described herein. In a further embodiment, a polynucleotide encoding a mutant small subunit AGPase polypeptide is also introduced into a plant cell along with the polynucleotide encoding a mutant large subunit polypeptide. In one embodiment, the polynucleotide or polynucleotides is incorporated into the genome of the plant cell and a plant is grown from the plant cell. In a preferred embodiment, the plant grown from the plant cell stably expresses the incorporated polynucleotide or polynucleotides. The subject invention also concerns oligonucleotide probes and primers, such as polymerase chain reaction (PCR) primers, that can hybridize to a coding or non-coding sequence of a polynucleotide of the present invention. Oligonucleotide probes of the invention can be used in methods for detecting and quantitating nucleic acid sequences encoding a mutant large subunit polypeptide of the invention. Oligonucleotide primers of the invention can be used in PCR methods and other methods involving nucleic acid amplification. In a preferred embodiment, a probe or primer of the invention can hybridize to a polynucleotide of the invention under stringent conditions. Probes and primers of the invention can optionally comprise a detectable label or reporter molecule, such as fluorescent molecules, enzymes, radioactive moiety (e.g., 3 H, 35 S, 125 I, etc.), and the like. Probes and primers of the invention can be of any suitable length for the method or assay in which they are being employed. Typically, probes and primers of the invention will be 10 to 500 or more nucleotides in length. Probes and primers that are 10 to 20, 21 to 30, 31 to 40, 41 to 50, 51 to 60, 61 to 70, 71 to 80, 81 to 90, 91 to 100 or more nucleotides in length are contemplated within the scope of the invention. Probes and primers of the invention can have complete (100%) nucleotide sequence identity with the polynucleotide sequence, or the sequence identity can be less than 100%. For example, sequence identity between a probe or primer and a sequence can be 99%, 98%, 97%, 96%, 95%, 90%, 85%, 80%, 75%, 70% or any other percentage sequence identity so long as the probe or primer can hybridize under stringent conditions to a nucleotide sequence of a polynucleotide of the invention. In one embodiment, a probe or primer of the invention has 70% or greater, 75% or greater, 80% or greater, 85% or greater, 90% or greater, or 95% to 100% sequence identity with a nucleotide sequence of SEQ ID NO:1, SEQ ID NO:17, or SEQ ID NO:47, or the complement thereof. The subject invention also concerns isolated mutant large subunit AGPase polypeptides. In one embodiment, the mutant polypeptide is a polypeptide of Zea mays . In a specific embodiment, a large subunit AGPase polypeptide of the invention has an amino acid sequence as shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, or functional fragment or variant thereof that exhibits substantially the same activity as a full-length amino acid sequence. A mutant large subunit polypeptide of the invention can be purified using standard techniques known in the art. In one embodiment, a polynucleotide of the invention encoding a mutant large subunit polypeptide is incorporated into a microorganism, such as E. coli , and the mutant large subunit polypeptide expressed in the microorganism and then isolated therefrom. The subject invention also concerns a mutant plant AGPase enzyme comprising one or more mutant plant AGPase subunits of the invention. In one embodiment, the mutant enzyme comprises one or more polypeptides having an amino acid sequence shown in SEQ ID NOs:2, 6, 7, 8, 10, 11, 13, 14, 15, 16, 18, or 47. Polypeptides of the invention, and functional peptide fragments thereof, can be used to generate antibodies that bind specifically to a polypeptide of the invention, and such antibodies are contemplated within the scope of the invention. The antibodies of the invention can be polyclonal or monoclonal and can be produced and isolated using standard methods known in the art. Polypeptide fragments according to the subject invention typically comprise a contiguous span of about or at least 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, or 515 amino acids of SEQ ID NO:2 or SEQ ID NO:18. Polypeptide fragments of the subject invention can be any integer in length from at least about 25 consecutive amino acids to 1 amino acid less than the sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47. Thus, for SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, a polypeptide fragment can be any integer of consecutive amino acids from about 25 to 475 amino acids. The term “integer” is used herein in its mathematical sense and thus representative integers include: 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, and/or 515. Each polypeptide fragment of the subject invention can also be described in terms of its N-terminal and C-terminal positions. For example, combinations of N-terminal to C-terminal fragments of about 25 contiguous amino acids to 1 amino acid less than the full length polypeptide of SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47 are included in the present invention. Thus, using SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47 as an example, a 25 consecutive amino acid fragment could correspond to amino acids of SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, selected from the group consisting of 1-25, 2-26, 3-27, 4-28, 5-29, 6-30, 7-31, 8-32, 9-33, 10-34, 11-35, 12-36, 13-37, 14-38, 15-39, 16-40, 17-41, 18-42, 19-43, 20-44, 21-45, 22-46, 23-47, 24-48, 25-49, 26-50, 27-51, 28-52, 29-53, 30-54, 31-55, 32-56, 33-57, 34-58, 35-59, 36-60, 37-61, 38-62, 39-63, 40-64, 41-65, 42-66, 43-67, 44-68, 45-69, 46-70, 47-71, 48-72, 49-73, 50-74, 51-75, 52-76, 53-77, 54-78, 55-79, 56-80, 57-81, 58-82, 59-83, 60-84, 61-85, 62-86, 63-87, 64-88, 65-89, 66-90, 67-91, 68-92, 69-93, 70-94, 71-95, 72-96, 73-97, 74-98, 75-99, 76-100, 77-101, 78-102, 79-103, 80-104, 81-105, 82-106, 83-107, 84-108, 85-109, 86-110, 87-111, 88, -112, 89-113, 90-114, 91-115, 92-116, 93-117, 94-118, 95-119, 96-120, 97-121, 98-122, 99-123, 100-124, 101-125, 102-126, 103-127, 104-128, 105-129, 106-130, 107-131, 108-132, 109-133, 110-134, 111-135, 112-136, 113-137, 114-138, 115-139, 116-140, 117-141, 118-142, 119-143, 120-144, 121-145, 122-146, 123-147, 124-148, 125-149, 126-150, 127-151, 128-152, 129-153, 130-154, 131-155, 132-156, 133-157, 134-158, 135-159, 136-160, 137-161, 138-162, 139-163, 140-164, 141-165, 142-166, 143-167, 144-168, 145-169, 146-170, 147-171, 148-172, 149-173, 150-174, 151-175, 152-176, 153-177, 154-178, 155-179, 156-180, 157-181, 158-182, 159-183, 160-184, 161-185, 162-186, 163-187, 164-188, 165-189, 166-190, 167-191, 168-192, 169-193, 170-194, 171-195, 172-196, 173-197, 174-198, 175-199, 176-200, 177-201, 178-202, 179-203, 180-204, 181-205, 182-206, 183-207, 184-208, 185-209, 186-210, 187-211, 188-212, 189-213, 190-214, 191-215, 192-216, 193-217, 194-218, 195-219, 196-220, 197-221, 198-222, 199-223, 200-224, 201-225, 202-226, 203-227, 204-228, 205-229, 206-230, 207-231, 208-232, 209-233, 210-234, 211-235, 212-236, 213-237, 214-238, 215-239, 216-240, 217-241, 218-242, 219-243, 220-244, 221-245, 222-246, 223-247, 224-248, 225-249, 226-250, 227-251, 228-252, 229-253, 230-254, 231-255, 232-256, 233-257, 234-258, 235-259, 236-260, 237-261, 238-262, 239-263, 240-264, 241-265, 242-266, 243-267, 244-268, 245-269, 246-270, 247-271, 248-272, 249-273, 250-274, 251-275, 252-276, 253-277, 254-278, 255-279, 256-280, 257-281, 258-282, 259-283, 260-284, 261-285, 262-286, 263-287, 264-288, 265-289, 266-290, 267-291, 268-292, 269-293, 270-294, 271-295, 272-296, 273-297, 274-298, 275-299, 276-300, 277-301, 278-302, 279-303, 280-304, 281-305, 282-306, 283-307, 284-308, 285-309, 286-310, 287-311, 288-312, 289-313, 290-314, 291-315, 292-316, 293-317, 294-318, 295-319, 296-320, 297-321, 298-322, 299-323, 300-324, 301-325, 302-326, 303-327, 304-328, 305-329, 306-330, 307-331, 308-332, 309-333, 310-334, 311-335, 312-336, 313-337, 314-338, 315-339, 316-340, 317-341, 318-342, 319-343, 320-344, 321-345, 322-346, 323-347, 324-348, 325-349, 326-350, 327-351, 328-352, 329-353, 330-354, 331-355, 332-356, 333-357, 334-358, 335-359, 336-360, 337-361, 338-362, 339-363, 340-364, 341-365, 342-366, 343-367, 344-368, 345-369, 346-370, 347-371, 348-372, 349-373, 350-374, 351-375, 352-376, 353-377, 354-378, 355-379, 356-380, 357-381, 358-382, 359-383, 360-384, 361-385, 362-386, 363-387, 364-388, 365-389, 366-390, 367-391, 368-392, 369-393, 370-394, 371-395, 372-396, 373-397, 374-398, 375-399, 376-400, 377-401, 378-402, 379-403, 380-404, 381-405, 382-406, 383-407, 384-408, 385-409, 386-410, 387-411, 388-412, 389-413, 390-414, 391-415, 392-416, 393-417, 394-418, 395-419, 396-420, 397-421, 398-422, 399-423, 400-424, 401-425, 402-426, 403-427, 404-428, 405-429, 406-430, 407-431, 408-432, 409-433, 410-434, 411-435, 412-436, 413-437, 414-438, 415-439, 416-440, 417-441, 418-442, 419-443, 420-444, 421-445, 422-446, 423-447, 424-448, 425-449, 426-450, 427-451, 428-452, 429-453, 430-454, 431-455, 432-456, 433-457, 434-458, 435-459, 436-460, 437-461, 438-462, 439-463, 440-464, 441-465, 442-466, 443-467, 444-468, 445-469, 446-470, 447-471, 448-472, 449-473, 450-474, 451-475, 452-476, 453-477, 454-478, 455-479, 456-480, 457-481, 458-482, 459-483, 460-484, 461-485, 462-486, 463-487, 464-488, 465-489, 466-490, 467-491, 468-492, 469-493, 470-494, 471-495, 472-496, 473-497, 474-498, 475-499, 476-500, 477-501, 478-502, 479-503, 480-504, 481-505, 482-506, 483-507, 484-508, 485-509, 486-510, 487-511, 488-512, 489-513, 490-514, and/or 491-515. Similarly, the amino acids corresponding to all other fragments of sizes between 26 consecutive amino acids and 474 (or 475) consecutive amino acids of SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, are included in the present invention and can also be immediately envisaged based on these examples. Therefore, additional examples, illustrating various fragments of the polypeptides of SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, are not individually listed herein in order to avoid unnecessarily lengthening the specification. Polypeptide fragments comprising: 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, 515 consecutive amino acids of SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47 may alternatively be described by the formula “n to c” (inclusive), where “n” equals the N-terminal amino acid position and “c” equals the C-terminal amino acid position of the polypeptide. In this embodiment of the invention, “n” is an integer having a lower limit of 1 and an upper limit of the total number of amino acids of the full length polypeptide minus 24 (e.g., 516−24=492 for SEQ ID NO:2). “c” is an integer between 25 and the number of amino acids of the full length polypeptide sequence (516 for SEQ ID NO:2) and “n” is an integer smaller than “c” by at least 24. Therefore, for SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, “n” is any integer selected from the list consisting of: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 474, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491; and “c” is any integer selected from the group consisting of: 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 273, 274, 275, 276, 277, 278, 279, 280, 281, 282, 283, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295, 296, 297, 298, 299, 300, 301, 302, 303, 304, 305, 306, 307, 308, 309, 310, 311, 312, 313, 314, 315, 316, 317, 318, 319, 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335, 336, 337, 338, 339, 340, 341, 342, 343, 344, 345, 346, 347, 348, 349, 350, 351, 352, 353, 354, 355, 356, 357, 358, 359, 360, 361, 362, 363, 364, 365, 366, 367, 368, 369, 370, 371, 372, 373, 374, 375, 376, 377, 378, 379, 380, 381, 382, 383, 384, 385, 386, 387, 388, 389, 390, 391, 392, 393, 394, 395, 396, 397, 398, 399, 400, 401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, 414, 415, 416, 417, 418, 419, 420, 421, 422, 423, 424, 425, 426, 427, 428, 429, 430, 431, 432, 433, 434, 435, 436, 437, 438, 439, 440, 441, 442, 443, 444, 445, 446, 447, 448, 449, 450, 451, 452, 453, 454, 455, 456, 457, 458, 459, 460, 461, 462, 463, 464, 465, 466, 467, 468, 469, 470, 471, 472, 473, 475, 476, 477, 478, 479, 480, 481, 482, 483, 484, 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, 497, 498, 499, 500, 501, 502, 503, 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, and 515 provided that “n” is a value less than “c” by at least 24. Every combination of “n” and “c” positions are included as specific embodiments of polypeptide fragments of the invention. All ranges used to describe any polypeptide fragment embodiment of the present invention are inclusive unless specifically set forth otherwise. Fragments of a mutant large or small subunit AGPase polypeptide of the invention, as described herein, can be obtained by cleaving the polypeptides of the invention with a proteolytic enzyme (such as trypsin, chymotrypsin, or collagenase) or with a chemical reagent, such as cyanogen bromide (CNBr). Alternatively, polypeptide fragments can be generated in a highly acidic environment, for example at pH 2.5. Polypeptide fragments can also be prepared by chemical synthesis or using host cells transformed with an expression vector comprising a polynucleotide encoding a fragment of a mutant large or small subunit AGPase polypeptide of the invention, for example, a mutant polypeptide that is a fragment of the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47. Fragments of a mutant large or small subunit AGPase polypeptide of the invention also contemplated herein include fragments of the polypeptides wherein all or a part of a transit or signal sequence of the polypeptide is removed. In a specific embodiment, the present invention includes those polypeptides of SEQ ID NO:2, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:18, or SEQ ID NO:47 wherein amino acids 1 to 45 of the amino terminus are deleted. The subject invention also concerns cells transformed with a polynucleotide of the present invention encoding a mutant large subunit AGPase polypeptide of the invention. In one embodiment, the cell is transformed with a polynucleotide sequence comprising a sequence encoding the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:18, or SEQ ID NO:47, and/or optionally a polynucleotide comprising a sequence encoding the amino acid sequence shown in SEQ ID NO:2, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:18, or SEQ ID NO:47, or a functional fragment or variant thereof. In a specific embodiment, the cell is transformed with a polynucleotide sequence shown in SEQ ID NO:1, SEQ ID NO:17, and/or SEQ ID NO:47, or a sequence encoding a functional fragment or variant of SEQ ID NO:2, SEQ ID NO:18, and/or SEQ ID NO:47. In one embodiment, a cell is also transformed with a polynucleotide encoding a mutant small subunit AGPase polypeptide as described herein, such as SEQ ID NO:3, 4, 5, 9, or 12. In one embodiment, the polynucleotide sequence is provided in an expression construct of the invention. The transformed cell can be a prokaryotic cell, for example, a bacterial cell such as E. coli or B. subtilis , or the transformed cell can be a eukaryotic cell, for example, a plant cell, including protoplasts, or an animal cell. Plant cells include, but are not limited to, dicotyledonous, monocotyledonous, and conifer cells. In one embodiment, the plant cell is a cell from a Zea mays plant. Animal cells include human cells, mammalian cells, avian cells, and insect cells. Mammalian cells include, but are not limited to, COS, 3T3, and CHO cells. The subject invention also concerns methods for increasing starch synthesis in a plant or plant tissue (such as a plant seed or endosperm tissue). In one embodiment, a method of the invention comprises introducing one or more polynucleotides of the present invention into a plant. In one embodiment, the polynucleotide is stably incorporated into the genome of the plant or plant tissue. The polynucleotide can comprise regulatory elements, such as a promoter and/or enhancer sequences, that provide for increased expression of the polynucleotide and/or the polypeptide encoded thereby. In a specific embodiment, the promoter sequence is one that provides for constitutive or tissue-specific (e.g., endosperm) expression. Plants or plant tissues containing the polynucleotide, or progeny of the plants, optionally can be screened for increased expression of a polynucleotide or polypeptide of the invention. In one embodiment, multiple copies of one or more polynucleotides of the invention are introduced into a plant or plant tissue and stably incorporated into the genome of the plant. In one embodiment, a polynucleotide of the invention is provided in an expression construct as described herein. The subject invention also concerns a method for preparing a plant having an AGPase enzyme that exhibits increased enzymatic activity relative to a wild type AGPase enzyme, said method comprising introducing a polynucleotide into a plant cell and growing a plant from said plant cell, wherein said polynucleotide encodes a mutant plant AGPase large subunit protein, or a functional fragment of said protein, said protein comprising an amino acid mutation wherein the amino acid corresponding to the cysteine amino acid at position 424 of wild type maize endosperm AGPase large subunit protein is replaced by an amino acid that confers increased enzymatic activity when said mutant AGPase large subunit is expressed to form an AGPase enzyme. Single letter amino acid abbreviations are defined in Table 2. TABLE 2 Letter Symbol Amino Acid A Alanine B Asparagine or aspartic acid C Cysteine D Aspartic Acid E Glutamic Acid F Phenylalanine G Glycine H Histidine I Isoleucine K Lysine L Leucine M Methionine N Asparagine P Proline Q Glutamine R Arginine S Serine T Threonine V Valine W Tryptophan Y Tyrosine Z Glutamine or glutamic acid Materials and Methods Protein alignment and amino acid numbering. SH2 (Accession #: P55241), and potato tuber large subunit (Accession #: CAA43490) protein sequence alignment were obtained using the MEGA software (Kumar et al, 2004) with BLOSUM matrix followed by manual inspection. The large subunit amino acid numbers used throughout the report correspond to SH2. Structure modeling. BT2, SH2, and the potato tuber large subunit monomer structures were modeled after the potato small subunit in the recently published 3D structure of the potato tuber homotetrameric AGPase (RCSB Protein Data Bank #: 1YP2c). SWISS MODEL was used for performing homology modeling (Schwede et al., 2003; Arnold et al., 2006). The potato tuber large subunit and SH2 were modeled from amino acid #34 and #94 to the end respectively due to poor alignment of the N′ termini. WHATCHECK (Vriend, 1990) and VERIFY3D (Luthy et al., 1992) were used to structurally evaluate the models. The corresponding WHATCHECK values (z-values for Ramachandran plot, backbone conformation, chi-1/chi-2 angle correlation, bond lengths, and bond angles) were within acceptable range. The high quality of the models was verified by the positive values assigned by VERIFY3D throughout all the structures. Visualization and superimposition of models and structures was done with Chimera (Pettersen et al., 2004). Site-directed mutagenesis. The PCR reactions for site-directed mutagenesis were done with high fidelity Vent polymerase (New England Biolabs) by using pMONcSh2 as a template. The following pairs of primers were used for generating C114A, H149S, S163F, M172T, Q213H, V227R, T361C, D368S, P372A, C382F, C424V, E438Q, V502T and A508S respectively: (SEQ ID NO: 19) (5′-cctgttggaggagcatacaggcttattg-3′, (SEQ ID NO: 20) 5′-caataagcctgtatgctcctccaacagg-3′,) (SEQ ID NO: 21) (5′-cttaaccgccatatttctcgtacataccttg-3′, (SEQ ID NO: 22) 5′-caaggtatgtacgagaaatatggcggttaag-3′,) (SEQ ID NO: 23) (5′-caactttgctgatggatttgtacaggtattagc-3′, (SEQ ID NO: 24) 5′-gctaatacctgtacaaatccatcagcaaagttg-3′,) (SEQ ID NO: 25) (5′-gcggctacacaaacgcctgaagagccag-3′, (SEQ ID NO: 26) 5′-ctggctcttcaggcgtttgtgtagccgc-3′,) (SEQ ID NO: 27) (5′-cttgagtggcgatcatctttatcggatg-3′, (SEQ ID NO: 28) 5′-catccgataaagatgatcgccactcaag-3′,) (SEQ ID NO: 29) (5′-cttgtgcagaaacatcgagaggacgatgctg-3′, (SEQ ID NO: 30) 5′-cagcatcgtcctctcgatgtttctgcacaag-3′,) (SEQ ID NO: 31) (5′-gcaaacttggccctctgtgagcagccttcc-3′, (SEQ ID NO: 32) 5′-ggaaggctgctcacagagggccaagtttgc-3′,) (SEQ ID NO: 33) (5′-gcagccttccaagttttcattttacgatccaaaaacacc-3′, (SEQ ID NO: 34) 5′-ggtgtttttggatcgtaaaatgaaaacttggaaggctgc-3′,) (SEQ ID NO: 35) (5′-gtttgatttttacgatgcgaaaacacctttcttc-3′ (SEQ ID NO: 36) 5′-gaagaaaggtgttttcgcatcgtaaaaatcaaa-3′) (SEQ ID NO: 37) (5′-cttcactgcaccccgattcttgcctccgacgc-3′, (SEQ ID NO: 38) 5′-gcgtcggaggcaagaatcggggtgcagtgaag-3′,) (SEQ ID NO: 39) (5′-cgtgtcagctctggagttgaactcaaggactc-3′, (SEQ ID NO: 40) 5′-gagtccttgagttcaactccagagctgacacg-3′,) (SEQ ID NO: 41) (5′-gcggacatctatcaaactgaagaagaag-3′, (SEQ ID NO: 42) 5′-cttcttcttcagtttgatagatgtccgc-3′) (SEQ ID NO: 43) (5′-ggtctggaatcacggtgatcctgaag-3′, (SEQ ID NO: 44) 5′-cttcaggatcaccgtgattccagacc-3′,) (SEQ ID NO: 45) (5′-gatcctgaagaattcaaccatcaacgatg-3′, (SEQ ID NO: 46) 5′-catcgttgatggttgaattcttcaggatc-3′) Glycogen quantitation. Glycogen quantitation was performed by phenol reaction (Hanson and Phillips, 1981) as described by Georgelis and Hannah (2008). Enzyme expression and purification. The SH2 wild type and variants were expressed along with wild type BT2 in bacterial cells AC70R1-504 cells (Iglesias et al., 1993) and the resulting enzymes were purified as described by Georgelis and Hannah (2008). Enzyme kinetics. The forward direction of the reaction was used (G-1-P+ATP→ADP-glucose+PPi) for estimating K cat , K m for ATP and G-1-P, and affinities for 3-PGA (Ka) and Pi (Ki). More specifically, 0.04-0.12 μg of purified enzyme was assayed, at 37° C. for 10 minutes, in the presence of 50 mM HEPES pH 7.4, 15 mM MgCl 2 , 2.5 mM ATP, and 2.0 mM G-1-P and varying amounts of 3-PGA to determine Ka. Ki was determined in the presence of 15 mM 3-PGA. Kms for G-1-P and ATP were estimating by varying the amount of G-1-P and ATP respectively in the presence of 15 mM 3-PGA. The reaction was terminated by boiling for 2 minutes and PPi was coupled to a reduction in NADH concentration using a coupling reagent as described by Georgelis and Hannah (2008). The kinetic constants were calculated by using Prism 4.0 (Graph Pad, San Diego, Calif.). The Hill coefficients were calculated as described by Cross et al. (2004). The specific activity was linear with time and amount of AGPase for all AGPase variants under all conditions. Heat stability. Heat stability of the SH2 wild type and variants expressed with wild type BT2 was determined as described by Georgelis and Hannah (2008). However, the enzyme was heated at 39° C., instead of 42 or 53° C., due to the high heat lability of the variants. Western detection of SH2 and BT2. A Western blot detection of both BT2 and SH2 in BT2/SH2, BT2/V502T and BT2/A508S variants was performed as described by Georgelis and Hannah (2008). The only modification was that a polyclonal antibody against SH2 (1:2000 (v/v)) was used in addition to a polyclonal antibody against BT2 to detect both SH2 and BT2. All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification. Following are examples that illustrate procedures for practicing the invention. These examples should not be construed as limiting. All percentages are by weight and all solvent mixture proportions are by volume unless otherwise noted. Example 1 Previous Phylogenetic Analysis and Structural Mapping of Type-II and Positively Selected Sites The large subunit of AGPase can be classified into five groups depending on sequence similarity and tissue of expression ( FIG. 1 ) (Georgelis et al., 2007; 2008). Group 4 includes only two sequences whose role has not been studied. Accordingly, we had restricted further evolutionary analysis to only the remaining four groups. We had identified 21 type-II sites (Table 3, FIG. 2 ) by doing an analysis of all the pairwise comparisons between the different large subunit groups shown in FIG. 1 (Georgelis et al., 2008). These amino acid sites have the potential to contribute to the functional divergence among AGPase large subunits. Type-II sites 96 and 106 have been shown to play an important role in enzyme catalysis (Ballicora et al., 2005), while site 506 has been implicated in the allosteric properties of AGPase by work conducted with potato tuber AGPase (Ballicora et al., 1998). We also had detected 18 amino acid sites upon which potential positive selection may have taken place in the tree branches following the gene duplications that led to the creation of the different large subunit groups (Table 3, FIG. 2 ) (Georgelis et al., 2008). These sites could also be important in large subunit specialization since functional diversification among different large subunits could have been beneficial for the fitness of the plant. Positively selected sites 104, 230, 441 and 445 are implicated in the allosteric properties of AGPase (Kavakli et al., 2001a; Ballicora et al., 2005; Jin et al., 2005). Finally, we had identified 91 type-I sites. These sites are apparently important for AGPase function in one group but not in another group and they could contribute towards subfunctionalization or specialization or both among large subunit groups. However, the usefulness of type-I sites in detecting functional divergence has been disputed (Philippe et al., 2003). More specifically, it has been argued that type-I divergence between orthologous and paralogous groups is indistinguishable in some instances (Gribaldo et al., 2003). It would be expected that paralogous sequences have functionally diverged more than orthologous sequences. Hence, a much higher type-I divergence in paralogous than in orthologous sequences would be expected. Therefore, there maybe a considerable number of false-positive sites of type-I divergence that do not necessarily represent functional divergence. TABLE 3 Type-II and positively selected sites in the large subunit of AGPase (Georgelis et al., 2008). Highlights indicate sites that are already known to have a role in kinetic or allosteric properties of AGPase. Type II sites Positively selected sites 96 104 106 106 114 114 151 131 163 142 172 155 213 160 336 198 372 227 374 230 380 261 382 341 396 364 416 368 425 382 438 424 444 441 502 445 506 507 508 The fact that several type-II and positively selected sites have already been shown to be important for the kinetic and allosteric properties of AGPase strongly suggests that the remaining type-II and positively selected sites may also be important for enzyme function. To gain insight into the potential role of these sites, we placed them on the modeled structure of the SH2. The type-I sites were excluded from this initial analysis since a potential inclusion of a high number of false-positives could confound the results. Although the only crystal structure available is a potato tuber small subunit homotetramer (Jin et al., 2005), the high degree of identity (40-45%) and similarity (55-65%) between the small and the large subunits strongly suggests that the structure of the physiologically relevant AGPase heterotetramer will be very similar or identical to the resolved homotetramer structure. Superimposition of SH2 and BT2 on the known structure agrees with this conjecture ( FIGS. 3A and 3B ). Additionally, the potato tuber AGPase heterotetramer was modeled after the homotetrameric structure and a molecular dynamics study was conducted to determine the most thermodynamically favorable interactions between the large and the small subunit ( FIG. 2 ) (Tuncel et al., 2008). Superimposition of the potato tuber large subunit on SH2 indicates that the two structures are virtually identical ( FIG. 4 ). This enables us to use the potato tuber large subunit modeled structure in order to determine the areas of SH2 that interact with BT2. According to the modeled potato tuber heterotetramer (Tuncel et al., 2008), a SH2 molecule makes direct contacts with one molecule of maize endosperm small subunit (BT2) through its C-terminal domain (tail-to-tail interaction) and another molecule of BT2 through its N-terminal catalytic domain (head-to-head interaction) as shown in FIGS. 3A and 3B . We observed that 17 out of 29 amino acid sites (type-II and positively selected) were at or near the subunit interfaces ( FIGS. 5A and 5B ). The areas of SH2 that participate in subunit interactions do not constitute more than 30% of the SH2 monomer structure. This means that almost 60% of the residues that were selected through evolutionary analysis were located in less than 30% of the SH2 monomer. This preferential localization of the residues at subunit interfaces raises the possibility that subunit interfaces are important for the functional specialization of the AGPase large subunit. Example 2 Site-Directed Mutagenesis To determine the role, if any, of type-II and positively selected amino acid sites, and particularly the ones found at the subunit interfaces, in AGPase function and gain insight into their potential roles in large subunit specialization, we performed site-directed mutagenesis in 12 sites in SH2. We mutagenized 7 SH2 sites (4 type-II, 1 positively selected, and 2 both type-II and positively selected) located at the subunit interfaces and 5 sites (3 type-II, 2 positively selected) located at the rest of the SH2 monomer. In all cases, the residue of SH2, which belongs to group 3b ( FIG. 1 ), was changed to a residue found in other groups. To gain more information about the subunit interfaces, we scanned type-I sites for the ones that are located in the subunit interfaces. We selected type-I site 149 as a target because SH2-containing group 3b, contains a His while other groups contain the physicochemically different Ala or Ser. His was changed to a Ser. We also selected type-I site 361, which is also located in subunit interfaces. This site is invariant in Group 2 but variable in Group 3b. Group 3b can be subdivided in two subgroups, one that contains only endosperm specific large subunits including SH2 and one that includes mostly embryo large subunits. SH2 along with the other members of the former subgroup contain a Thr at site 361 while the latter subgroup contains a Cys. The Thr of SH2 was changed to a Cys, which has different physicochemical properties. Example 3 Glycogen Production The wild type SH2 as well as the 14 SH2 variants created by site-directed mutagenesis were expressed with wild type BT2 in E. coli strain AC70R1-504 (Materials and Methods). Resulting glycogen production of cells expressing the SH2 variants and wild type SH2 was quantified. The majority of the SH2 variants (10/14) resulted in different amounts of glycogen compared to wild type SH2 ( FIG. 6 ). This strongly suggests that the majority of the mutations introduced in SH2 were not neutral, at least when expressed in E. coli , despite the fact that the substituted amino acid residues are present in other large subunit groups. Example 4 Characterization of Kinetic and Allosteric Properties of SH2 Variants Glycogen quantitation suggests that some of the mutants created may alter function at the protein/enzyme level. Therefore, the SH2 variants and wild type SH2 were expressed in E. coli along with wild type BT2 and the resulting heterotetramers were purified (Materials and Methods). The affinity of the SH2/BT2 complexes for the allosteric activator 3-PGA (K a ) was determined in the forward direction (G-1-P+ATP4→ADP-glucose+PPi). Interestingly, 7 out of 14 SH2 variants in a complex with BT2 had a higher Ka compared to wild type SH2/BT2 (Table 4). The overwhelming majority of them (6/7) had an amino acid change in a site at the subunit interfaces. Two changes were in the head-to-head interaction areas (H149S, S163F) while four (T361C, D368S, P372A, C382F) were in the tail-to-tail interaction areas ( FIG. 5 ). One change (Q213H) was in the N-terminal catalytic domain far from the subunit interfaces ( FIG. 9 ). The affinity for the deactivator Pi (K i ) was also determined in the presence of 15 mM of 3-PGA by use of Dixon plots. Higher K a in the variants described above was accompanied by a lower K i (Table 4). It has been proposed that 3-PGA and Pi are competing for binding to AGPase and they may even bind to the same site (Boehlein et al., 2008a). Therefore, the lower affinity for 3-PGA may maximize the efficiency of the Pi effect. TABLE 4 Kinetic and allosteric properties of wild type and variant SH2 in a complex with BT2. The Hill coefficient for Ka and Kms varies from 0.9 to 1.3. (15 mM 3-PGA) K m G-1-P (mM) K cat (S −1 ) K m ATP (mM) K a 3-PGA (mM) K i Pi (mM) BT2/SH2 0.07 (±0.01) 39.17 (±1.23) 0.12 (±0.02) 0.31 (±0.06) 16.80 (±3.84) BT2/C114A 0.07 (±0.01) 37.21 (±1.78) 0.14 (±0.01) 0.39 (±0.07) 13.23 (±3.57) BT2/H149S 0.07 (±0.01) 35.21 (±1.50) 0.19 (±0.03) 2.11 (±0.13)  3.96 (±1.50) BT2/S163F 0.09 (±0.01) 34.65 (±2.07) 0.42 (±0.09) 3.29 (±0.81)  1.83 (±0.96) BT2/M172T 0.06 (±0.01) 38.58 (±1.98) 0.09 (±0.01) 0.29 (±0.08) 17.61 (±3.78) BT2/Q213H 0.07 (±0.01) 29.36 (±1.72) 0.21 (±0.03) 3.01 (±0.52)  3.21 (±1.13) BT2/V227R 0.06 (±0.01) 16.17 (±1.06) 0.14 (±0.01) 0.25 (±0.06)  14.5 (±4.01) BT2/T361C 0.06 (±0.01) 38.12 (±1.62) 0.13 (±0.01) 0.71 (±0.05)  5.34 (±1.30) BT2/D368S 0.07 (±0.01) 23.34 (±1.10) 0.11 (±0.01) 1.11 (±0.11)  4.26 (±1.67) BT2/P372A 0.05 (±0.01) 42.32 (±1.55) 0.19 (±0.04) 1.83 (±0.15)  3.72 (±1.26) BT2/C382F 0.08 (±0.01) 40.75 (±1.56) 0.15 (±0.03) 1.01 (±0.12)  2.28 (±1.12) BT2/C424V 0.09 (±0.01) 59.86 (±3.45) 0.14 (±0.01) 0.37 (±0.03) 18.36 (±4.43) BT2/E438Q 0.05 (±0.01) 32.87 (±2.24) 0.13 (±0.01) 0.35 (±0.07) 15.22 (±3.34) BT2/V502T 0.06 (±0.01) 41.91 (±3.13) 0.15 (±0.02) 0.27 (±0.03) 17.93 (±2.93) BT2/A508S 0.06 (±0.01) 35.77 (±1.80) 0.09 (±0.01) 0.30 (±0.05) 20.43 (±4.25) The K m for G-1-P and ATP was determined for all variants at 15 mM 3-PGA. Except for a 4-fold lower affinity of BT2/S 163F for ATP all the other variants were indistinguishable from wild type BT2/SH2 in terms of Kms for substrates. Similarly, the majority of K cat s was close to wild type BT2/SH2 except for BT2/C424V (˜150%), BT2/V227R (˜40%), and BT2/D368S (˜60%). This indicates that the changes in the allosteric variants affected the affinity for effectors to a much greater extent than the effect or the mechanism of activation. Example 5 Heat Stability The structures of the large and the small subunit are almost identical. It has been shown that the loop connecting the C-terminal β-helix to the N-terminal catalytic domain in the small subunit is implicated in the heat stability of AGPase (Boehlein et al. 2008b). This loop makes contact with the homologous loop in the large subunit suggesting that the respective loop in the large subunit is also important for heat stability. Since 9 out of 14 substitutions in SH2 were in sites located at the subunit interfaces, including the loop described above (from amino acid number 362 to 399), the heat stability of the resulting variants was determined. The variants and wild type BT2/SH2 were heated for various amounts of time at 39° C. and remaining activity was determined by assaying in the forward direction using 20 mM 3-PGA and saturating amounts of substrates. With the exception of BT2/S163F that showed a 3-fold increase in heat stability all the other variants were similar to wild type BT2/SH2 ( FIG. 7 ). These results indicate that the majority of the mutagenized sites at the subunit interfaces have a specific role in the allosteric properties of AGPase. Example 6 Correlation of Kinetic and Heat Stability Data with Glycogen Production In general, the amount of glycogen produced by the variants in E. coli was consistent with the kinetics data. Six out seven allosteric variants produced lower amounts of glycogen compared to wild type BT2/SH2. In the case of BT2/S163F, K a was increased and decreased glycogen production might have been expected. However, the higher heat stability of BT2/S163F may counteract the increase in K a . As a result BT2/S163F produces wild type amounts of glycogen. BT2/M172T, BT2/C114A, and BT2/E438Q had wild type kinetic properties and heat stability. Not surprisingly they produced wild type amounts of glycogen. BT2/V227R and BT2/C424V had lower and higher K cat and glycogen production compared to wild type respectively. BT2/V502T and BT2/A508S showed identical kinetic properties and heat stability to wild type. However, their glycogen production was markedly reduced compared to wild type. One possibility could be that these variants have reduced solubility and/or increased susceptibility to proteases in E. coli . An alternative could be reduced transcription and translation. In all these cases, a reduced amount of SH2 and/or BT2 protein in E. coli cells would be expected. To investigate these possibilities, a Western blot analysis was conducted on total and soluble protein extracts from E. coli expressing wild type BT2/SH2, BT2/V502T and BT2/A508S by using polyclonal anti-SH2 and -BT2. The amount of SH2 and BT2 in both total and soluble protein extracts is indistinguishable between wild type and the two variants ( FIG. 8 ). Therefore, the possible explanations discussed above for the reduced glycogen produced by BT2N502T and BT2/A508S should be excluded. The underlying reason for reduced glycogen production in these variants remains unresolved. 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