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 AGP ase. The subject invention also comprises a mutant plant small subunit of AGP ase encoded by a polynucleotide of the invention. The subject invention also concerns plants comprising a polynucleotide of the invention and method for making the plants.