Wheat MYOSIN-RESEMBLING CHLOROPLAST PROTEIN controls B-type starch granule initiation timing during endosperm development

Molecular factors that contribute to the diverse spatial and temporal patterns of starch granule initiation between species and organs are poorly understood. Wheat (Triticum sp.) endosperm contains both large A-type granules initiated during early grain development and small B-type granules that initiate about 10 to 15 days later. Here, we identify that the MYOSIN-RESEMBLING CHLOROPLAST PROTEIN (MRC) is required for the correct timing of B-type granule initiation in wheat endosperm during grain development. MRC is expressed in the endosperm exclusively in early grain development, before B-type granule initiation. We isolated three independent TILLING mutants of tetraploid wheat (Triticum turgidum cv. 'Kronos') with premature stop or missense mutations in the A-genome homeolog, which we showed to be the only active homeolog in tetraploid wheat due to a disruption of the B-genome homeolog. The mrc mutants had significantly smaller A-type granules and a higher relative volume of B-type granules in the endosperm than the wild type. Whereas B-type granules initiated 15 to 20 days post-anthesis (dpa) in the wild type, they appeared as early as 10 dpa in the mrc-1 mutant. These results suggest a temporal role for MRC in repressing B-type granule initiation, providing insight into how the distinct biochemical mechanisms that control A- and B-type granule initiation are regulated. This role of MRC in the wheat endosperm is distinct from the previously described role of Arabidopsis (Arabidopsis thaliana) MRC in promoting granule initiation in leaves, providing an example of functional diversification among granule initiation proteins.