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, <i>OsMYBR1</i>, that regulates starch biosynthesis in rice. <i>OsMYBR1</i> is highly expressed during endosperm development. Mutations of <i>OsMYBR1</i> result in reduced grain thickness and a decrease in 1000-grain weight. The endosperm of <i>osmybr1</i> 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 <i>OsGBSSⅠ</i>, <i>OsAGPL1</i>, <i>OsAGPL2</i>, <i>OsBEⅡb</i>, <i>OsISA1</i>, <i>PHOL</i>, and <i>OsSSⅢa</i>, is altered in <i>osmybr1</i> mutants. Further experiments indicated that OsMYBR1 directly binds to the promoters of <i>OsGBSSⅠ</i>, <i>OsAGPL1</i>, <i>OsAGPL2</i>, <i>OsISA1</i>, <i>OsBEⅡb</i>, and <i>PHOL</i>, resulting in an increase in the expression of <i>OsGBSSⅠ</i> but a decrease in the expression of <i>OsAGPL2</i>, <i>OsISA1</i>, and <i>OsSSⅢa</i>. In contrast, <i>OsMYBR1</i>-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 <i>OsMYBR1</i> could be a promising strategy for improving rice ECQ.