BLA1 Affects Leaf Angles by Altering Brassinosteroid Biosynthesis in Rice (<i>Oryza sativa</i> L.)

Brassinosteroids (BRs) are crucial plant hormones influencing diverse developmental processes in rice. While several enzymes in BR biosynthesis have been identified, their regulatory mechanisms remain largely unknown. This study highlights a novel regulatory pathway wherein the CHD3 chromatin remodeler, <i>BLA1,</i> epigenetically modulates the expression of key BR biosynthesis genes, <i>BRD1</i> and <i>D2.</i> Phenotypic analysis of <i>bla1</i> mutants revealed significant alterations, such as increased leaf angles and longer mesocotyls, which were alleviated by BR synthesis inhibitors. Moreover, the <i>bla1</i> mutants showed elevated BR levels that correlated with the significant upregulation of the expression levels of <i>BRD1</i> and <i>D2</i>, particularly at the lamina joint sites. Mechanistically, the yeast one-hybrid and chromatin immunoprecipitation assays revealed specific binding of BLA1 to the promoter regions of <i>BRD1</i> and <i>D2,</i> accompanied by a marked enrichment of the transcriptionally active histone modification, H3K4me3, on these loci in the <i>bla1</i> mutant. Functional assessments of the <i>brd1</i> and <i>d2</i> mutants confirmed their reduced sensitivity to BR, further underscoring their critical regulatory roles in BR-mediated developmental processes. Our findings uncovered an epigenetic mechanism that governs BR biosynthesis and orchestrates the expression of <i>BRD1</i> and <i>D2</i> to modulate BR levels and influence rice growth and development.