Patent Publication Number: US-8536407-B2

Title: Heat resistant plants and plant tissues comprising a variant adenosine diphosphate glucose pyrophosphorylase small subunit protein and methods of use thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application Number PCT/US2009/001903, filed Mar. 26, 2009, which is a continuation of U.S. application Ser. No. 12/082,339, filed Apr. 9, 2008, each of which is hereby incorporated by reference herein in its entirety, including any figures, tables, nucleic acid sequences, amino acid sequences, or drawings. 
    
    
     GOVERNMENT SUPPORT 
     The subject matter of this application has been supported by a research grant from the National Science Foundation under grant number IOS-0444031. Accordingly, the government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Heat stress leads to decreased maize yield (Peters et al., 1971; Thompson, 1975; Chang, 1981; Christy and Williamson, 1985). This can be attributed to reduced photosynthate availability and transportation from source to sink tissues, poor pollination, reduced cell and granule size and number, early seed abortion and/or reduced grain filling period. Growth of endosperm starts with a lag phase in which cells actively divide and continues with a linear phase in which cells increase in size and starch synthesis occurs. Elevated temperature during lag phase resulted in reduced yield (Jones et al., 1984). These investigators suggested that reduced yield was due to reduced cell and granule number and size as well as seed abortion. Additionally, elevated temperatures during the linear phase resulted in shorter grain filling period and subsequently smaller kernels (Jones et al., 1984). Similar results were found by Hunter et al. (1977) and Tollenaar and Bruulsema (1988). 
     Records from five states that traditionally produce more than 50% of the US corn showed that average daily temperature was 23.6° C., around 2° C. higher than optimum during grain filling (Singletary et al., 1994). Photosynthate availability during grain filling is not reduced at high temperatures, at least in barley and wheat. Indeed, sucrose content in barley and wheat seeds was either unchanged or elevated at high temperatures (Bhullar and Jenner, 1986; Wallwork et al., 1998). Also photosynthesis in maize increases up to 32° C. (Duncan and Hesketh, 1968; Hofstra and Hesketh, 1969; Christy et al., 1985). Moreover, Cheikhn and Jones (1995) studied the ability of maize kernels to fix  14 C sucrose and hexoses at different temperatures. They found that these sugars increased in the seed at elevated-temperatures. The evidence above suggests that limited sugar availability and transport into the kernel during grain filling are not the cause of temperature-induced yield decreases. 
     There have been extensive efforts to identify biochemical pathways that impact grain filling during elevated temperatures. Singletary et al. (1993; 1994) assayed starch biosynthetic enzymes in maize kernels grown in vitro at elevated temperatures (22° C. to 36° C.). They found that ADP-glucose pyrophosphorylase (AGPase) and soluble starch synthase (SSS) were more heat labile compared to other enzymes participating in starch synthesis. They suggested that heat lability of AGPase and SSS contributes to grain filling cessation. Duke and Doehlert (1996) found that transcripts of several genes encoding enzymes of the starch synthesis pathway, including those encoding AGPase, were decreased at 35° C. compared to 25° C. However, enzyme assays showed that only AGPase activity was strikingly lower. They suggested that this could be due to a higher turnover rate of AGPase compared to other enzymes. Finally, Wilhelm et al. (1999), through Q 10  analysis, showed that AGPase had the most pronounced reduction in activity compared to several other enzymes. Maize AGPase indeed lost 96% of its activity when heated at 57° C. for 5 min (Hannah et al., 1980). 
     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. 1998; 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. A detailed characterization of the maize transgenic events is under way (Greene and Hannah, in preparation). 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 resistance to heat conditions and/or increased starch biosynthesis. Increased resistance of a plant or plant tissue to heat conditions provides for decreased yield losses as compared to the yield losses generally observed at elevated temperatures. One aspect of the invention concerns polynucleotides that encode a mutant plant small subunit of AGPase. In one embodiment, a polynucleotide of the invention encodes a plant AGPase small subunit having an amino acid mutation wherein the threonine amino acid corresponding to amino acid position 462 of wild type maize AGPase small subunit is substituted with an amino acid that confers increased heat stability. In another embodiment, a polynucleotide encodes a chimeric plant AGPase small subunit composed of sequences from two different plants (as described in U.S. Pat. No. 7,173,165) and comprising an amino acid mutation of the invention wherein the threonine amino acid corresponding to amino acid position 462 of wild type maize AGPase small subunit is substituted with an amino acid that confers increased heat stability. The mutation in the chimeric AGPase synergistically enhances heat stability. The subject invention also comprises a mutant plant small subunit of AGPase encoded by a polynucleotide of the invention. Characterization of heat stability as well as 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 drawings will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  shows glycogen produced by  E. coli  cells expressing BT2, BT2-TI, MP, MP-TI along with SH2. Glycogen from cells expressing only SH2, BT2, BT2-TI, and MP alone. Glycogen is measured in glucose units. The error bars indicate standard deviation (N=3). 
         FIGS. 2A and 2B  show dot blots of crude extracts from  E. coli  cells expressing BT2, BT2-TI, MP, MP-TI with the complementary subunit SH2. AGPase was visualized by using a monoclonal antibody against BT2. The density of the spots was estimated by using ImageJ. 
         FIG. 3  shows specific activity of AGPase variants in crude and partially purified protein extracts from non-induced  E. coli  cells. Activity was measured in the reverse direction. n.d.=not detectable. The error bars indicate standard deviation (N=3). 
         FIG. 4  shows purification of AGPase. SDS-PAGE of purified recombinant BT2/SH2, TI/SH2, MP/SH2, and MP-TI/SH2. Precision Plus Protein All Blue Standard from Biorad was used as a marker. The upper arrow on the left points to the large subunit. The lower arrow on the left points to the small subunit. 
         FIGS. 5A and 5B  show heat stability of purified BT2/SH2, BT2-TI/SH2, and MP/SH2. The half-life (T 1/2 ) of each AGPase is expressed as mean±standard error. The p-values are estimated by an F-test implemented by Prizm (Graph pad, San Diego Calif.). In  FIG. 5A , the assay was conducted in the forward direction. In  FIG. 5B , the assay was conducted in the reverse direction. 
         FIGS. 6A and 6B  show heat stability of purified MP/SH2 and MP-TI/SH2. The half-life (T 1/2 ) of each AGPase is expressed as mean±standard error. The p-values are estimated by an F-test implemented by Prizm (Graph pad, San Diego Calif.). In  FIG. 6A , the assay was conducted in the forward direction. In  FIG. 6B , the assay was conducted in the reverse direction. 
         FIGS. 7A-7C  show 3D modeling of BT2 and TI.  FIG. 7A  is the predicted 3D structure of BT2 monomer. The TI change is marked by a red circle. The areas of BT2 that are directly involved in subunit-subunit interactions are highlighted by yellow boxes.  FIG. 7B  shows the distances of carbon atoms of Thr462 (1,2) from those of Pro60 (4,5) and Leu61 (3).  FIG. 7C  shows the distances of carbon atoms of Ile462 (1,2,3,4) from those of Pro60 (5,6) and Leu61 (7,8). The Thr462 and Ile462 contacting residues were determined by using FirstGlance Jmol. Dark gray spheres indicate carbon atoms of Thr462 and Ile462. Light gray spheres indicate carbon atoms of contacting residues. Oxygen and nitrogen atoms are indicated by red and blue color respectively. 
         FIG. 8  show strength of AGPase subunit-subunit interactions. SH2 was used as a bait and BT2, TI, and MP as a prey in a yeast two hybrid system. A quantitative β-galactosidase assay was used to quantify the interactions between the bait and the prey. The error bars indicate 2× standard error (N=4). 
     
    
    
     BRIEF DESCRIPTION OF THE SEQUENCES 
     SEQ ID NO:1 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:2 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:3 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (MP-TI) of the invention. 
     SEQ ID NO:4 is an amino acid sequence of a mutant polypeptide (MP-TI) of the invention. 
     SEQ ID NO:5 is an amino acid sequence of a mutant polypeptide (TI+YC) of the invention. 
     SEQ ID NO:6 is an amino acid sequence of a mutant polypeptide (TI+QTCL) of the invention. 
     SEQ ID NO:7 is an amino acid sequence of a mutant polypeptide (TI+ETCL) of the invention. 
     SEQ ID NO:8 is an amino acid sequence of a mutant polypeptide (MP-TI+YC) of the invention. 
     SEQ ID NO:9 is an amino acid sequence of a mutant polypeptide (MP-TI+QTCL) of the invention. 
     SEQ ID NO:10 is an amino acid sequence of a mutant polypeptide (MP-TI+ETCL) of the invention. 
     SEQ ID NO:11 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:5) of the invention. 
     SEQ ID NO:12 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:6) of the invention. 
     SEQ ID NO:13 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:7) of the invention. 
     SEQ ID NO:14 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:8) of the invention. 
     SEQ ID NO:15 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:9) of the invention. 
     SEQ ID NO:16 is a polynucleotide sequence comprising a nucleotide sequence encoding a mutant polypeptide (SEQ ID NO:10) of the invention. 
     SEQ ID NO:17 is an oligonucleotide that can be used according to the present invention. 
     SEQ ID NO:18 is an oligonucleotide that can be used according to the present invention. 
     SEQ ID NO:19 is an oligonucleotide that can be used according to the present invention. 
     SEQ ID NO:20 is an oligonucleotide that can be used according to the present invention. 
     SEQ ID NO:21 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:22 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:23 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:24 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:25 is an amino acid sequence of a mutant polypeptide (TI) of the invention. 
     SEQ ID NO:26 is an amino acid sequence of potato tuber AGPase small subunit protein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The subject invention concerns materials and methods for providing plants with increased resistance to heat conditions and/or increased starch biosynthesis. Increased resistance of a plant to heat conditions provides for decreased yield losses as compared to yield losses generally observed at elevated temperatures. 
     One aspect of the invention concerns polynucleotides that encode a mutant plant small subunit of AGPase. In one embodiment, a polynucleotide of the invention encodes a plant AGPase small subunit having 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, the amino acid substituted is an isoleucine. In one embodiment, the mutant plant AGPase small subunit is maize endosperm AGPase small subunit. In an exemplified embodiment, the mutant plant AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:2, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, or a fragment or variant thereof. In another embodiment, the mutant plant AGPase small subunit is barley, wheat, sorghum, potato, or rice. In a specific embodiment, the mutant barley, wheat, sorghum, rice, or potato AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, respectively. In another embodiment, the polynucleotide encodes a mutant plant AGPase small subunit that can additionally 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 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:5, SEQ ID NO:6, or SEQ ID NO:7, or a fragment or variant thereof. In specific embodiments, the polynucleotide comprises the nucleotide sequences shown in SEQ ID NO:11, SEQ ID NO:12, or SEQ ID NO:13, or a fragment or variant thereof. 
     In another embodiment, a polynucleotide encodes a chimeric plant AGPase small subunit composed of sequences from two different plants (as described in U.S. Pat. No. 7,173,165) and also comprising an amino acid mutation of the invention 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, the amino acid substituted is an isoleucine. In one embodiment, the chimeric AGPase small subunit comprises a C-terminal portion from one plant and an N-terminal portion from another plant. In one embodiment, a chimeric protein 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 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 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 protein 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 and include the mutation corresponding to amino acid position 462 of the present invention. In a specific embodiment, the C-terminus region of a chimeric protein of the present invention comprises the terminal 276 amino acids of the AGPase small subunit of potato tuber. In an exemplified embodiment, the chimeric protein 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 protein 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 corresponding to amino acid position 462 of the present invention. In an exemplified embodiment, the plant chimeric AGPase small subunit comprises the amino acid sequence shown in SEQ ID NO:4, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:3, or a fragment or variant thereof. In another embodiment, the polynucleotide encodes a mutant plant AGPase small subunit that can additionally comprise an amino acid mutation described in published International patent application WO 2005/019425 (Hannah and Linebarger). In a further embodiment, the mutant AGPase small subunit encoded by the polynucleotide also comprises an amino acid mutation wherein a tyrosine at position 36 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 at positions 34 and 35, respectively. In specific embodiments, the amino acid inserted between position 34 and 35 of the mutant 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:8, SEQ ID NO:9, or SEQ ID NO:10, or a fragment or variant thereof. In specific embodiments, the polynucleotide comprises the nucleotide sequences shown in SEQ ID NO:14, SEQ ID NO:15, or SEQ ID NO:16, or a fragment or variant thereof. 
     The subject invention also concerns methods for increasing heat stability and/or increasing starch biosynthesis, 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 certain embodiments, the polynucleotides introduced into the plant encode one or more polypeptides comprising the amino acid sequence shown in any of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, or SEQ ID NO:3, or a fragment or variant thereof. In further specific embodiments, the polynucleotide comprises the nucleotide sequences shown in SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, or SEQ ID NO:16, 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 concerns mutant AGPase small subunit polypeptides encoded by the polynucleotides of the invention. In one embodiment, the polypeptide comprises the amino acid sequence shown in SEQ ID NO:2, or a fragment or variant thereof. In other embodiments, the polypeptide comprises the amino acid sequence shown in SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, or a fragment or variant thereof. In another embodiment, the polypeptide comprises the amino acid sequence shown in SEQ ID NO:4, or a fragment or variant thereof. In still a further embodiment, the polypeptide comprises the amino acid sequence shown in any of SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10, or a fragment or variant thereof. 
     The subject invention also concerns mutant plant AGPase enzymes comprising one or more mutant AGPase small subunit polypeptides of the invention. The mutant plant AGPase can also comprise one or more wild type AGPase large subunit polypeptides. In specific embodiments, a mutant plant AGPase enzyme comprises one or more mutant AGPase small subunit polypeptides any of which can comprise the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, or a fragment or variant of any such sequence, wherein the mutant AGPase enzyme exhibits increased heat stability relative to a wild type AGPase enzyme. In one embodiment, the mutant plant enzyme comprises two mutant AGPase small 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 AGPase small subunit polypeptides of the invention and one or more mutant AGPase large subunit polypeptides. In one embodiment, the mutant AGPase large subunit polypeptide can be any of those 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. In one embodiment, a mutant AGPase large subunit polypeptide comprises a Rev6 mutation. In another embodiment, a mutant AGPase large subunit polypeptide 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 AGPase small subunit polypeptides of the invention, wherein the mutant AGPase 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 AGPase large subunit polypeptides wherein the mutant AGPase large subunit polypeptides can have the same mutation(s) or can have different mutation(s). In a further embodiment, the mutant plant AGPase enzyme comprises two mutant AGPase small subunit polypeptides of the invention and two mutant SH2 polypeptides, wherein the mutant AGPase small subunit polypeptides and the mutant AGPase 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 heat stability relative to wild type plant AGPase. In one embodiment, the method comprises incorporating or providing one or more mutant AGPase small subunit polypeptides of the present invention with wild type or mutant AGPase large 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 polypeptide of the present invention. In one embodiment, the AGPase enzyme also comprises a mutant AGPase large subunit polypeptide subunit, such as a mutant large subunit comprising a Rev6 and/or a heat stability mutation, such as HS33. 
     The subject invention also concerns plants, plant tissue, and plant cells of the invention 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. 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, lettuce, and hop. 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 is a transgenic plant, plant tissue, or plant cell. In another embodiment, a plant, plant tissue, or plant cell 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-1a 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, MEG1 (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 wild type or mutant polypeptide 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 wild type or mutant polypeptide and/or AGPase enzyme 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 AGPase small subunit 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, ornithine, 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 wild type or mutant AGPase small or large subunit 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 AGPase small subunit polypeptide of the present invention and/or a wild type or mutant AGPase large subunit polypeptide 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 polypeptide having the substitution still retains substantially the same functional activity (e.g., enzymatic and/or increased heat stability of an AGPase enzyme) as the polypeptide that does not have the substitution. Polynucleotides encoding a mutant AGPase small subunit polypeptide and/or a wild type or mutant AGPase large subunit 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 wild type or mutant AGPase small or large subunit 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&#39;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 heat stability relative to a wild type plant, wherein a polynucleotide encoding a mutant AGPase small subunit polypeptide of the present invention is introduced into a plant cell and the polypeptide(s) encoded by the polynucleotide(s) is expressed. In one embodiment, the plant cell comprises non-mutant genes encoding wild type AGPase large subunit polypeptide. In another embodiment, the plant cell comprises at least one polynucleotide encoding a mutant AGPase large subunit polypeptide. In a further embodiment, a polynucleotide encoding a mutant AGPase large subunit polypeptide is also introduced into a plant cell along with the polynucleotide encoding the mutant AGPase small 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 AGPase small 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:3, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, or SEQ ID NO:16, or the complement thereof. 
     The subject invention also concerns isolated mutant AGPase small subunit polypeptides. In one embodiment, the mutant AGPase small subunit polypeptide is an AGPase small subunit polypeptide of  Zea mays . In a specific embodiment, an AGPase small subunit polypeptide of the invention has an amino acid sequence as shown in SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, or functional fragment or variant thereof. An AGPase small subunit polypeptide or enzyme of the invention can be purified using standard techniques known in the art. In one embodiment, a polynucleotide of the invention encoding an AGPase small subunit polypeptide is incorporated into a microorganism, such as  E. coli , and the AGPase small subunit polypeptide expressed in the microorganism and then isolated therefrom. 
     In certain embodiments, 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, or 475 amino acids of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10. 
     In certain embodiments, 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 full-length sequence, such as those shown in SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10. Thus, for SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10, 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, and/or 475. 
     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, such as those of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 are included in the present invention. Thus, using SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 as an example, a 25 consecutive amino acid fragment could correspond to amino acids of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 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, and/or 451-475. 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:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 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:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 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, and 474 (or 475) consecutive amino acids of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10 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., 475-24=451 for SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, or SEQ ID NO:8; 476-24=452 for SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, or SEQ ID NO:10). “c” is an integer between 25 and the number of amino acids of the full length polypeptide sequence (475 for SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, or SEQ ID NO:8; 476 for SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:9, or SEQ ID NO:10) and “n” is an integer smaller than “c” by at least 24. Therefore, for SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, or SEQ ID NO:10, “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, and 451 (or 452); 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, and 475 (or 476) 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 AGPase small subunit polypeptide of the invention or an AGPase large subunit polypeptide, 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 an AGPase large subunit polypeptide or a fragment of a mutant AGPase small subunit 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:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25. Fragments of a mutant large or small subunit AGPase polypeptide of the invention also contemplated herein include fragments of the polypeptide wherein all or a part of a transit or signal sequence of the polypeptide is removed. 
     The subject invention also concerns cells transformed with a polynucleotide of the present invention encoding a mutant AGPase small subunit 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:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ′ID NO:24, or SEQ ID NO:25, 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:3, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, or SEQ ID NO:16, or a sequence encoding a functional fragment or variant of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25. In one embodiment, a cell is also transformed with a polynucleotide encoding a mutant AGPase large subunit polypeptide as described herein. 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 certain embodiments, the polynucleotides introduced into the plant encode one or more polypeptides comprising the amino acid sequence shown in any of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, or SEQ ID NO:25, or a fragment or variant thereof. In a specific embodiment, the polynucleotide comprises the nucleotide sequence shown in SEQ ID NO:1, or SEQ ID NO:3, or a fragment or variant thereof. In further specific embodiments, the polynucleotide comprises the nucleotide sequences shown in SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, or SEQ ID NO:16, 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. 
     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 
     Random Mutagenesis 
     Mutations were introduced into Sh2 and Bt2 by PCR random mutagenesis (GeneMorph II EZClone Domain Mutagenesis Kit, Stratagene). A mixture of non-biased, error-prone DNA polymerases was used to introduce point mutations. Wildtype Sh2 and Bt2 coding sequences in pMONcSh2 and pMONcBt2 (Giroux et al., 1996) respectively were used as templates for PCR. Two pairs of primers (Sh2: 5′-GAAGGAGATATATCCATGG-3′ (SEQ ID NO:17), 5′-GGATCCCCGGGTACCGAGCTC-3′ (SEQ ID NO:18) Bt2: 5′-GAAGGAGATATATCCATGG-3′ (SEQ ID NO:19), 5′-GTTGATATCTGAATTCGAGCTC-3′ (SEQ ID NO:20)) flanking Sh2 and Bt2 were used for error-prone PCR. Mutant Sh2 clones produced by PCR were subcloned into vector pMONcSh2 according to Stratagene protocols. pMONcSh2 was then used to transform  E. coli  strain AC70R1-504 that contained wildtype Bt2 in the compatible vector pMONcBt2. Mutant Bt2 clones produced by PCR were subcloned into vector pMONcBt2. pMONcBt2 was then used to transform  E. coli  strain AC70R1-504 that contained wildtype Sh2 in the compatible vector pMONcSh2. 
     Bacterial Expression System 
     A bacterial expression system (Iglesias et al., 1993) allowed us to randomly mutagenize maize endosperm AGPase genes and score AGPase activity in a fast and efficient way by exposing plates to iodine vapors as described below. The  E. coli  system is ideal for studying plant AGPases for a number of reasons discussed in Georgelis et al. (2007). 
     Glycogen Detection 
     Glycogen synthesis was detected by production of brown staining colonies following exposure to iodine vapors.  E. coli  cells were grown on Kornberg media in the presence of 75 μg/mL spectinomycin, 50 μg/mL kanamycin and 1% w/v glucose for 16 h at 37° C. (Govons et al., 1969). The colonies were exposed to iodine vapors for 1 min. Colonies with inactive AGPase produced no color following exposure to iodine vapors while active AGPase produced glycogen and, in turn, brown staining with iodine. AGPase variants staining darker than wildtype were selected for further study. 
     Glycogen Quantitation 
     Glycogen quantitation was performed by phenol reaction (Hanson and Phillips, 1981). In brief, glycogen was extracted from 1.6 ml of  E. coli  cells (OD 600 =2.0) grown in LB containing 2% w/v glucose by boiling for 3 hours in 50% w/v KOH. Glycogen was then precipitated by adding ethanol to 70% v/v and centrifuging at 10000×g for 10 min. After pellet drying, 200 μl de-ionized water, 200 μl of 5% w/v phenol and 1 mL of concentrated sulfuric acid were added. Glycogen was estimated by the absorbance at 488 nm. 
     DNA Sequencing 
     Sh2 and Bt2 mutants that produced enhanced glycogen were double-pass sequenced by the Genome Sequencing Services Laboratory (GSSL) of the Interdisciplinary Center for Biotechnology Research at the University of Florida. Data analysis was performed by Bioedit software (Hall, 1999). 
     Purification of Maize Endosperm AGPase from AC70R1-504  E. coli  Cells 
     AC70R1-504  E. coli  cells expressing maize endosperm AGPase were grown in 2 L of Luria-Broth (LB) medium in the presence of 75 μg/mL spectinomycin, 50 μg/mL kanamycin and 2% w/v glucose for 16 h at 37° C. with shaking. At OD600=0.6, 0.2 mM isopropyl-beta-D-thiogalactoside (IPTG) and 0.02 mg/mL nalidixic acid were added to induce protein expression. The cultures were immediately moved to room temperature and grown for 4 h with shaking. The following steps were conducted at 4° C. Cells were harvested by centrifuging at 3000×g and the pellet was resuspended in 16 mL of buffer A (50 mM KH 2 PO 4  pH 7.0, 5 mM MgCl 2 , 0.5 mM EDTA) and protease inhibitors (1 μg/mL pepstatin, 0.1 mM PMSF, 10 μg/mL chymostatin, and 1 mM benzamidine). The cells were lysed with a French press and centrifuged at 26000×g. The protein concentration of the supernatant was adjusted to 30 mg/mL by adding buffer A. Three tenths of volume of 1% protamine sulfate were added and the mixture stirred on ice for 20 min and then centrifuged at 26000×g for 20 min. The supernatant was brought to 45% saturation with ammonium sulfate, stirred on ice for 20 min and centrifuged at 26000×g for 20 min. The pellet was re-suspended in 2-2.5 mL of buffer A. The mixture was passed through a strong anion exchange column (macro-prep High Q support, Biorad), and an Econo-pac hydroxyapatite cartridge (Biorad) as described by Boehlein et al. (2005). AGPase was desalted by using Zeba Micro Desalt Spin Columns (Pierce) before assaying. AGPase was exchanged into 50 mM HEPES, 5 mM MgCl 2 , 0.5 mM EDTA and 0.5 mg/mL BSA (for stability). 
     Kinetic Characterization of AGPase 
     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 and Pi. More specifically, 0.04-0.06 μg of purified AGPase was assayed for specific activity in a total volume of 200 μl of 50 mM HEPES pH 7.4, 15 mM MgCl 2 , 1.0 mM ATP, and 2.0 mM G-1-P at 37° C. for 10 min. For determining K m s for ATP and G-1-P and K a  for 3-PGA, varying amounts of ATP, G-1-P and 3-PGA respectively. K i  for Pi was estimated by adding various amounts of Pi, 1 mM ATP, 2 mM G-1-P, and 2.5 mM 3-PGA were used. The reaction was stopped by boiling for 2 min. PPi was estimated by coupling the reaction to a reduction in NADH concentration using 300 μl of coupling reagent. The coupling reagent contained 25 mM imidazole pH 7.4, 4 mM MgCl 2 , 1 mM EDTA, 0.2 mM NADH, 0.725U aldolase, 0.4U triose phosphate isomerase, 0.6U glycerophosphate dehydrogenase, 1 mM fructose 6-phosphate and 0.8 μg of pyrophosphate dependent phosphofructokinase (PPi-PFK). All the enzymes were purchased from Sigma except for PPi-PFK which was produced and purified according to Deng et al. 1999 with some modifications (Boehlein and Hannah, unpublished data). NADH concentration was estimated by absorbance at 340 nm. PPi concentration was calculated by a standard curve developed by using various amounts of PPi instead of AGPase. The amount of PPi produced by AGPase was linear with time and enzyme concentration. The kinetic constants of AGPase were calculated by Prism 4.0 (Graph Pad, San Diego Calif.). 
     Measuring AGPase Specific Activity from Crude or Partially Purified Protein Extracts 
     AC70R1-504  E. coli  cells expressing maize endosperm AGPase were grown in 2 L of Luria-Broth (LB) medium in the presence of 75 μg/mL spectinomycin, 50 μg/mL kanamycin and 2% w/v glucose at 37° C. with shaking until OD600=2.0. Gene expression was not induced. The following steps were conducted at 4° C. Cells were harvested by centrifuging at 3000×g and the pellet was resuspended in 16 mL of buffer A (50 mM KH 2 PO 4  pH 7.0, 5 mM MgCl 2 , 0.5 mM EDTA) and protease inhibitors (1 μg/mL pepstatin, 0.1 mM PMSF, 10 μg/mL chymostatin, and 1 mM benzamidine). The cells were lysed with a French press and centrifuged at 26000×g. AGPase activity of the crude extract was measured from the supernatant stored at −80° C. The rest of the supernatant was partially purified through protamine sulfate and ammonium sulfate as described above. Protein extracts were desalted as described by Boehlein et al. (2005) before assay and were exchanged into 50 mM HEPES, 5 mM MgCl 2 , and 0.5 mM EDTA. AGPase specific activity was monitored in the reverse direction (ADP-glucose+PPi→G-1-P+ATP) using saturating amounts of substrates and activator as described by Boehlein et al. (2005). 
     Determining Heat Stability of AGPase 
     AGPase was purified as described above. AGPase was further diluted 1/100 (v/v) in 50 mM HEPES, 5 mM MgCl 2 , 0.5 mM EDTA and 0.5 mg/mL BSA and heat treated at 42 or 53° C. for various times, and then cooled on ice. The activity remaining after heat treatment was monitored in the forward and reverse direction by using saturating amounts of ATP, G-1-P and 3-PGA. The data were plotted as log of percentage of remaining activity versus time of heat treatment. The inactivation constant t 1/2  was calculated from the formula t 1/2 =0.693/(−2.3*slope). 
     Qualitative Determination of the AGPase Purity 
     The purity of AGPase was monitored in the following way. Six μg of AGPase were diluted 1:1 in denaturing solution (100 mM Tris-Cl pH 6.8, 4% SDS, and 8 mM DTT), heated at 95° C. for 5 min, electrophoresed on a 5% SDS polyacrylamide gel at 150V for 1 h and visualized by staining with Coomassie Brilliant Blue (Laemmli, 1970). 
     Protein Blot Analysis of Crude Extracts 
     Samples were vacuum blotted onto a PVDF membrane (Biorad) by using Hybri-Dot blot apparatus (Life Technologies). The PVDF membrane had been pre-soaked in methanol for 5 min and then in transfer buffer [20% (v/v) methanol, 0.303% (w/v) Tris and 1.44% (w/v) glycine] for 10 min. The membrane was incubated with blocking buffer [0.8% (w/v) NaCl, 0.02% (w/v) KCl, 0.144% (w/v) Na 2 HPO 4 , 0.024% (w/v) KH 2 PO 4 , 5% (w/v) bovine serum albumin (BSA), and 0.05% (v/v) Tween-20] for 1 h with constant shaking. The blot was incubated with blocking buffer containing 1:10000 (v/v) of monoclonal antibody against BT2 (kindly provided by Sue Boehlein) for 1 h with shaking. Then, the blot was washed 3×10 min with washing buffer (blocking buffer—BSA) with constant shaking. The blot was then incubated with a 1:60000 dilution of goat anti-mouse secondary antibody conjugated with horseradish peroxidase (Pierce) for 45 min. Finally the blot was washed 3×10 min. Proteins were visualized using an enhanced chemiluminescent substrate kit (Pierce). 
     3D Modeling 
     BT2 monomer structure was modeled after the potato small unit in the recently-published three dimensional structure of the potato tuber homotetrameric AGPase (RCSB Protein Data Bank #:1YP2). Homology modeling was done by using SWISS MODEL (Peitsch, 1995; Guex and Peitsch, 1997; Schwede et al. 2003; Kopp and Schwede, 2004; Arnold et al, 2006). Amino acid 462 (Thr or Ile) contacting residues were determined by using Jmol, an open-source Java viewer for chemical structures in 3D. 
     Yeast Two Hybrid 
     Yeast transformations and a β-galactosidase assay were conducted as described by Greene and Hannah (1998b). The only modification was the use of pGBKT7 and pGADT7 as vectors for the bait and the prey respectively. pGBKT7-53 and pGADT7-T plasmids were used as a positive control. 
     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 
     A mutant Bt2 library was created by error-prone PCR. The mutational load was ˜2 non-synonymous mutations per clone (Georgelis et al. 2007). The mutants were expressed in  E. coli  along with a wildtype Sh2 gene. Approximately 50,000 colonies were screened for glycogen production. Ten dark staining colonies were picked. The two darkest staining Bt2 mutants were sequenced. Both had the same non-synonymous mutation resulting in a change of amino acid 462 from threonine to isoleucine (TI). The threonine in that position is absolutely conserved among the higher plant small subunits (data not shown). BT2-TI/SH2 (BT2 comprising the TI mutation and complexed with SH2) produced more glycogen than did BT2/SH2 ( FIG. 1 ). Cells expressing BT2-TI and BT2, as homotetramers, did not produce detectable amounts of glycogen ( FIG. 1 ). This indicates that the amount of  E. coli -synthesized glycogen depends exclusively on the complex of BT2-TI or BT2 with SH2. 
     A dot-blot of the crude extracts from cells expressing BT2/SH2 and BT2-TI/SH2 indicated that BT2-TI is found in higher amounts in  E. coli  ( FIGS. 2A-2B ). While AGPase activity levels of crude extracts from non-induced cells expressing BT2/SH2 and BT2-TI/SH2 were too low to detect, the partially purified extract from BT2-TI/SH2 had 20 times more activity than did the partially purified extract from BT2/SH2 ( FIG. 3 ). The possibility that BT2-TI/SH2 produced more protein and activity because of more efficient transcription/translation is unlikely since the codons ACA (T) to ATA (I) are used with the same frequency in  E. coli  (6.1 and 5.0% respectively) (Nakamura et al., 2000). This suggests that the higher amount of protein and activity in BT2-TI/SH2 cells is due to increased stability of the AGPase. 
     To determine the kinetic properties and heat stability of BT2-TI/SH2 and decipher the cause of enhanced glycogen synthesis in  E. coli , recombinant BT2-TI/SH2 and BT2/SH2 AGPases were purified ( FIG. 4 ). The kinetic properties of BT2-TI/SH2, as summarized in Table 3, show that the K m  for G-1-P and ATP, K a  for 3-PGA and K i  for Pi were indistinguishable from BT2/SH2. Surprisingly, the k cat  of BT2-TI is 30% lower than the k cat  of BT2/SH2. These kinetic properties then cannot account for the darker staining of BT2-TI/SH2 in  E. coli . However, BT2-TI/SH2 is clearly more heat-stable than BT2/SH2 ( FIGS. 5A-5B ). These results strongly suggest that the high heat stability of BT2-TI/SH2 accounts for the enhanced amount of glycogen in  E. coli.    
     MP is a small subunit variant that can lead to agronomic gain. Some of its features include increased activity in the absence of the activator 3-PGA, increased affinity for 3-PGA, decreased affinity for Pi (Table 3) and elevated heat stability compared to BT2/SH2 ( FIG. 3 ) (Cross et al., 2004; Boehlein et al., 2005). Since BT2-TI/SH2 was not as heat-stable as MP/SH2 ( FIG. 3 ), the amino acid change of TI was introduced into MP in an effort to further increase the heat stability of MP (MP-TI). 
     Cells expressing MP-TI/SH2 (MP having the TI mutation and complexed with SH2) produced the same amount of glycogen as cells expressing MP/SH2 ( FIG. 1 ). However, greater amounts of the MP-TI protein relative to BT2 were found in crude extracts of  E. coli  expressing the two proteins with SH2 ( FIGS. 2A-2B ). The activity of the crude extracts and the partially purified extracts from MP-TI/SH2 was 2-3 fold higher than from MP/SH2 ( FIG. 3 ). MP-TI/SH2, in its pure form ( FIG. 4 ), maintained the favorable kinetic properties of MP/SH2 (Table 3) except that its k cat  was reduced ˜30% compared to MP/SH2. Additionally, MP-TI/SH2 exhibits greater heat stability than does MP/SH2 ( FIGS. 6A-6B ). 
     The crystal structure of maize endosperm AGPase has not been resolved. The only relevant structure is a potato tuber small subunit homotetramer (Jin et al., 2005). The potato tuber small subunit shows 88% identity and 96% similarity to BT2. BT2 monomer structure was modeled after the resolved structure of the potato tuber small subunit ( FIG. 7A ). The residue mutated in TI is part of a β-helix and it makes hydrophobic contact with two residues (Pro, Leu) of the N-terminus of the small subunit ( FIG. 7B ). The amino acid change from Thr to Ile in TI shortens the distance from the Pro and Leu mentioned above ( FIG. 7C ). It is tempting to speculate that the TI mutation strengthens the hydrophobic interaction between the C- and the N-terminus of the small subunit and results in greater stability. Unlike MP, whose heat stability is attributed to residues at/near the subunit-subunit interfaces, TI may not directly affect subunit-subunit interactions since it is far from the subunit-subunit interfaces ( FIG. 7A ). 
     To determine whether TI affects the strength of subunit-subunit interactions SH2 was used as a bait and BT2, TI, and MP were used as a prey in a yeast two-hybrid system (Y2H). A quantitative β-galactosidase assay indicated that, in contrast to MP, TI did not increase the strength of subunit-subunit interactions ( FIG. 8 ). 
     Finally, replacements of the original threonine by amino acids with shorter side chains such as serine, alanine and glycine did not affect the heat stability of AGPase even though they did result in 10-fold or more reduction in k cat  (data not shown). This indicates that the original threonine in position 462 is important for AGPase activity but not for heat stability. 
     EXAMPLE 2 
     The subject invention provides for agronomically important plant AGPase variants by using random mutagenesis and a heterologous bacterial expression system. BT2-TI was isolated as a small subunit variant that increased the amount of glycogen produced by  E. coli  cells when expressed along with SH2. Cells expressing BT2-TI/SH2 had 20-fold higher AGPase activity than cells expressing BT2/SH2. A dot-blot indicated that the crude protein extract from cells expressing BT2-TI/SH2 had more detectable BT2 protein compared to cells expressing BT2/SH2. This result could be attributed to more efficient transcription/translation or to greater AGPase stability and/or solubility. As mentioned previously, a more efficient transcription/translation is unlikely based on codon usage. On the other hand, it was showed that the purified form of BT2-TI/SH2 was significantly more heat-stable than the purified form of BT2/SH2. This may render the BT2-TI/SH2 complex less prone to proteolysis and/or aggregation compared to BT2/SH2 in  E. coli.    
     The kinetic and allosteric properties of BT2-TI/SH2 were indistinguishable from BT2/SH2 except for a 30% lower k cat . The 20-fold increase in AGPase activity of BT2-TI/SH2 expressing cells is interpreted as a higher number of active AGPase molecules compared to BT2/SH2 expressing cells. This also means that less than 5% of the potential AGPase molecules actually function in BT2/SH2. 
     It has been reported that the potato tuber small subunit can form a homotetramer that has significant activity when given extremely high amounts of the activator 3-PGA (Ballicora et al., 1995). As shown herein,  E. coli  cells expressing BT2 and BT2-TI as homotetramers do not produce detectable amounts of glycogen. Hence, the increased amounts of glycogen observed in cells expressing BT2-TI/SH2 compared to cells expressing BT2/SH2 is due to the complex of BT2-TI with SH2 rather than the BT2-TI homotetramer. 
     Another small subunit variant that results in increased heat stability in a complex with SH2 is MP. The heat stability conferred by MP has been mapped to residues near or at the subunit-subunit interaction interfaces (Boehlein, Shaw, Stewart, and Hannah, unpublished data). In contrast, the amino acid change of BT2-TI is far from these interfaces. Structure modeling suggests that the TI change strengthens the intra-subunit hydrophobic interactions between the C- and the N-terminus. The results from Y2H support the idea that, in contrast to MP, TI does not strengthen the subunit-subunit interactions. However, the possibility that TI indirectly affects subunit-subunit interactions through a conformational change cannot be dismissed. It is possible that Y2H may not be sensitive enough to reveal a difference in subunit-subunit interactions between TI/SH2 and BT2/SH2 at the yeast growth temperature of 30° C. 
     A comparison of the heat stability of pure BT2-TI/SH2 and MP/SH2 indicated that BT2-TI/SH2 was not as heat-stable as MP/SH2. Additionally, MP/SH2 has several advantages not shared by BT2-TI/SH2, such as activity in the absence of the activator 3-PGA, a higher affinity for 3-PGA, a lower affinity for the inhibitor Pi and a higher k cat  compared to BT2/SH2. It was investigated whether the heat stability of MP/SH2 could be further improved by introducing the TI change into MP. The resulting variant, MP-TI, when expressed with SH2 in  E. coli , yielded an equal amount of glycogen as MP/SH2. This could mean that either MP-TI/SH2 was not more heat-stable than MP/SH2 or that the production of ADP-glucose catalyzed by AGPase was not limiting in  E. coli  anymore. The latter interpretation may be favored since AGPase activity in crude and partially purified extracts of cells expressing MP-TI/SH2 was 2-3 fold higher than cells expressing MP/SH2. MP-TI/SH2 maintained all the kinetic and allosteric properties of MP/SH2 with the exception a 30% lower k cat . Most importantly, MP-TI/SH2 was more heat-stable than MP/SH2. Two phases of heat stability in both MP-TI/SH2 and MP/SH2 were observed. These phases were probably a result of different states of AGPase. This biphasic mode of heat stability has been observed before and it is not specific to MP/SH2 or MP-TI (Boehlein et al. 2008). The first phase shows lower heat stability than does the second one. MP-TI/SH2 is more heat-stable in both phases compared to MP/SH2. However, what exactly the state of AGPase is in each phase remains enigmatic. The biphasic mode of heat stability was not observed in BT2/SH2 and BT2-TI/SH2 because the samples were not heated for long enough time to reach the second phase and they were heated at lower temperature than MP/SH2 and MP-TI/SH2 (42° C. instead of 53° C.). 
     Table 3 shows the kinetic properties of purified recombinant AGPase variants. The kinetic and allosteric properties of AGPase variants were determined in the forward direction. k cat  (s −1 )(G-1-P) was estimated by varying the amount of G-1-P and keeping ATP at saturating amounts (1 mM). k cat  (s −1 ) (ATP) was estimated by varying the amount of ATP and keeping G-1-P at saturating amounts (2 mM). K i &#39;s are expressed as mean (95% confidence interval). All other values are expressed as mean±standard deviation. The specific activity of AGPase in the absence of 3-PGA is expressed as a percentage of the specific activity in the presence of 10 mM of 3-PGA (mean±standard error). 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                   
                 BT2/SH2 
                 BT2-TI/SH2 
                 MP/SH2 
                 MP-TI/SH2 
               
               
                   
               
               
                 K m  G-1-P  
                 0.050 
                 0.040 
                 0.079 
                 0.059 
               
               
                 (mM) 
                 (±0.008) 
                 (±0.006) 
                 (±0.007) 
                 (±0.005) 
               
               
                 k cat  (s −1 )  
                 38.170 
                 26.200 
                 62.655 
                 42.880 
               
               
                 (G-1-P) 
                 (±1.323) 
                 (±1.401) 
                 (±2.569) 
                 (±1.880) 
               
               
                 K m  ATP  
                 0.102 
                 0.146 
                 0.133 
                 0.112 
               
               
                 (mM) 
                 (±0.020) 
                 (±0.050) 
                 (±0.021) 
                 (±0.013) 
               
               
                 k cat  (s −1 )  
                 43.321 
                 29.112 
                 69.337 
                 49.031 
               
               
                 (ATP) 
                 (±1.554) 
                 (±1.030) 
                 (±3.276) 
                 (±1.903) 
               
               
                 K a  3-PGA  
                 0.480 
                 0.330 
                 0.100 
                 0.068 
               
               
                 (mM) 
                 (±0.137) 
                 (±0.060) 
                 (±0.010) 
                 (±0.010) 
               
               
                 K i  Pi (mM) 
                 2.320 
                 4.070 
                 6.610 
                 5.870 
               
               
                   
                 (0.530, 
                 (2.120, 
                 (4.530, 
                 (4.410, 
               
               
                   
                 4.100) 
                 6.020) 
                 8.690) 
                 7.330) 
               
               
                   
               
            
           
           
               
            
               
                 V max  − 3-PGA/V max  + 3-PGA (nmol/min/mg) 
               
            
           
           
               
               
               
               
            
               
                 BT2/SH2 
                 BT2-TI/SH2 
                 MP/SH2 
                 MP-TI/SH2 
               
               
                   
               
               
                 280/4000 
                 134/2737 
                 1856/6584 
                 1101/4513 
               
               
                 (7.2 ± 3.2%) 
                 (4.9 ± 1.5%) 
                 (28.2 ± 3.1%) 
                 (24.4 ± 5.3%) 
               
               
                   
               
            
           
         
       
     
     It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and the scope of the appended claims. In addition, any elements or limitations of any invention or embodiment thereof disclosed herein can be combined with any and/or all other elements or limitations (individually or in any combination) or any other invention or embodiment thereof disclosed herein, and all such combinations are contemplated with the scope of the invention without limitation thereto. 
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