Targeting granule initiation and amyloplast structure to create giant starch granules in wheat

Abstract

Starch granule size influences the functional, digestive, and processing qualities of starch, but its genetic control in plants is poorly understood. Here, we demonstrate that both space and substrate constraints limit starch granule size in wheat and provide an approach to achieve substantial increases in granule size. Wheat starch typically contains large A-type and small B-type granules. To increase A-type granule size, we explored the effect of mutations in the plastid division component PARALOG OF ARC 6 (PARC6), which increases amyloplast size and therefore the space available for granule growth, and in B-GRANULE CONTENT 1 (BGC1), which reduces the number of granule initiations and thus competition between growing granules for space and substrates. While parc6 and bgc1 single mutants had only modest increases in A-type granule size, the parc6 bgc1 double mutant produced striking giant granules that were more than double the size of typical A-type granules. The increase in granule size in parc6 bgc1 was reproducible in both the glasshouse and field, and had no detectable effect on plant growth, grain size and starch composition or content. We demonstrate that the size increase affects a range of functional properties, including viscosity and pasting temperature. Overall, by targeting both constraints, we created a new class of giant cereal starch that has not been previously observed in nature, with altered physicochemical properties that can be used in food and industrial applications. Significance statementWe provide a major advance in understanding the factors determining starch granule size in plants - a trait that strongly influences starch functionality. Starch granules from cereals are typically smaller than those of most root/tuber crops, rarely exceeding 30 {micro}m. We created a novel cereal starch in wheat that exceeds this size range. By simultaneously increasing amyloplast size and reducing granule initiations, we increased space for granule growth and reduced competition between growing granules for substrates. This resulted in a new class of cereal starch where nearly 50% of the starch volume were in granules greater than 30 {micro}m, and some exceeded 50 {micro}m. These giant granules had altered physicochemical properties that could find novel application in food and industry.