The subject invention concerns materials and methods for providing plants or plant tissue with increased resistance to heat conditions. Increased resistance of a plant or plant tissue to heat conditions provides for decreased yield losses generally observed at elevated temperatures. One aspect of the invention concerns polynucleotides that encode a mutant plant small subunit of AGPase. The subject invention also comprises a mutant plant small subunit of AGPase 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.