Starch synthesis in Brachypodium distachyon endosperm occurs in dynamic, connected amyloplast compartments

The morphology of starch granules is a major determinant of the functional and nutritional properties of starch and is highly variable among cereal species. Much of this morphological variation stems from differences in the spatial and temporal patterns of starch granule initiation in amyloplasts during grain development. Simple granules are thought to arise from a single initiation per amyloplast (e.g. in Brachypodium distachyon), whereas compound granules develop from multiple initiations per amyloplast (e.g. in rice). We used live-cell imaging to visualise amyloplasts in the developing endosperm of Brachypodium, using transgenic lines expressing a fluorescent amyloplast reporter. We discovered that the simple-type starch granules in Brachypodium can arise from multiple initiations per amyloplast. The amyloplasts showed dynamic changes in their structure and formed two types of stromules: stable stromules that formed a stromal continuum between amyloplasts, and short-lived stromules that were more dynamic. We also observed actin-dependent movement of amyloplasts within endosperm cells, and movement of starch granules within the amyloplasts. Our results suggest complex and pleiomorphic amyloplast organisation and mobility that could influence granule formation. This goes beyond the existing 'one granule, one amyloplast' model for simple-type granules and advances our understanding of both amyloplast biogenesis and granule formation.