Rice endosperm starch regulator1 controls starch granule formation and amylopectin synthesis in rice endosperm.

Starch is the most abundant component of the rice endosperm and plays a vital role in determining grain yield and quality. However, the mechanisms underlying starch granule formation remains poorly understood. In this study, we identified a plastid coiled-coil protein specifically expressed in rice developing seeds. Functional loss of this gene led to increased seed chalkiness and reduced amylopectin synthesis, leading us to designate it as RICE ENDOSPERM STARCH REGULATOR1 (OsRESR1). Amyloplasts in wild-type rice endosperm cells typically contain more than seven granules per amyloplast. Strikingly, osresr1 mutants exhibited a higher proportion of small amyloplasts (≤ 6 starch granules per amyloplast) in central endosperm, along with a notable decrease in starch granule number within pericarp amyloplasts. Furthermore, OsRESR1 interacted with starch synthesis-related enzymes Pho1, BE2a, and BE2b. Zymogram analysis assays revealed a significant reduction in the synthetic activities of Pho1 in osresr1 mutants, while the activities of BE isoforms remain unchanged. In vitro enzyme activity assays and accumulated phosphogluconate accumulation analysis further verified that OsRESR1 regulates starch granule formation and amylopectin synthesis through the synthetic activity of Pho1. Above all, this study elucidates the critical role of OsRESR1 in starch granule formation and provides mechanistic insights into its regulation of starch synthesis pathways mediated via Pho1.