grass_starch_granule_formation

There is extensive interspecies variation in starch granule morphology in the endosperm of grass species, but the factors underpinning this variation are poorly understood. The two major granule morphology types among species are simple and compound granules, where simple granules arise from a single initiation per amyloplast, and compound granules form from multiple granule initiations. Here, we carried out an extensive survey of seed starch morphology, examining specimens from 80 species within the Pooideae using electron microscopy. This not only expanded our current knowledge of the diversity of starch granule types but also confirmed the homoplasy of simple and compound starch granules. We then selected species representing independent origins of simple (Brachyelytrum japonicum, Phaenosperma globosa and Brachypodium distachyon) and compound granules (Nardus stricta, Melica altissima and Calamagrostis brachytricha) for analysis of starch structure and gene expression. There were no clear patterns in starch content, amylopectin structure or amylose content that could distinguish the two granule types. However, comparative transcriptomics over seed development revealed gene expression patterns associated with granule type, most notably increasing expression of SS3a, SBE1 and LDA between 3- and 9-days post anthesis within compound granules. Mutation of these genes in rice, which natively produces compound granules, resulted in altered granule shape and/or size. Our work highlights differences in gene expression that may contribute to natural variation in granule morphology within the Pooideae, whilst providing important new starch phenotype and gene expression datasets that can be widely used to study endosperm development.