Starch Synthase 3 isoforms are essential for normal starch granule initiation in wheat endosperm

Wheat grains have two distinct types of starch granules. Large, lenticular A-type granules are formed from a single initiation per amyloplast during early grain development, while numerous small B-type granules are initiated during later grain development. Here, we demonstrate that the two isoforms of Starch Synthase 3 in wheat (SS3a and SS3b) are essential for normal starch granule initiation in the endosperm. The ss3a mutant of durum wheat had deformed A-type granules, while the ss3b mutant had no detectable differences in starch granule morphology. However, the ss3a ss3b double mutant had an enhanced phenotype compared to ss3a - with more aberrant A-type granules, a stronger reduction in granule sizes, and an increased relative volume of B-type granules. Interestingly, the A-type granules with defective morphology in the ss3a and ss3a ss3b mutants were 'semicompound', resulting from multiple initiation points. SS3a and SS3b proteins interacted with each other, and both isoforms also interacted with BGC1, a known component of granule initiation. Importantly, ss3a ss3b had almost double the amount of resistant starch compared to ss3a, suggesting overlapping roles between both isoforms in polymer biosynthesis. SS3a and SS3b are useful targets for altering starch granule morphology and increasing resistant starch in wheat.