OsMYBR1, a 1R-MYB Family Transcription Factor Regulates Starch Biosynthesis in Rice Endosperm.

Starch is the primary component of the endosperm and plays a crucial role in rice quality. Although the enzymes involved in starch synthesis have been extensively studied, the transcription factors that regulate these enzymes remain largely unknown. Here, we identified a MYB family transcription factor, OsMYBR1, that regulates starch biosynthesis in rice. OsMYBR1 is highly expressed during endosperm development. Mutations of OsMYBR1 result in reduced grain thickness and a decrease in 1000-grain weight. The endosperm of osmybr1 mutants exhibit rounded and loosely packed starch granules, decreased amylose content, altered fine structure of amylopectin, and modified physicochemical properties. The analysis of RT-qPCR showed that the expression of several starch-synthesis enzyme-coding genes (SSEGs), including OsGBSSâ , OsAGPL1, OsAGPL2, OsBEâ ¡b, OsISA1, PHOL, and OsSSâ ¢a, is altered in osmybr1 mutants. Further experiments indicated that OsMYBR1 directly binds to the promoters of OsGBSSâ , OsAGPL1, OsAGPL2, OsISA1, OsBEâ ¡b, and PHOL, resulting in an increase in the expression of OsGBSSâ but a decrease in the expression of OsAGPL2, OsISA1, and OsSSâ ¢a. In contrast, OsMYBR1-overexpressing endosperm appears normal, with starch granule morphology, increased amylopectin content, and improved alkali spreading value, indicating enhanced rice eating and cooking quality (ECQ). These findings suggest that the overexpression of OsMYBR1 could be a promising strategy for improving rice ECQ.