Rice leaf angle is an important agronomic trait determining plant architecture and crop yield. Brassinosteroids (BRs) play crucial roles in controlling rice leaf angle, thus an increasing number of researches were focused on the BR signaling pathway in rice. However, the orthologs of some important components in Arabidopsis BR signaling have not yet been characterized in rice. In this study, we identified a rice bHLH transcription factor named OsBIM1, as the closest rice homolog of AtBIM1 (BES1-Interacting MYC-like Protein1). Overexpression of OsBIM1 significantly increases rice leaf angles, whereas the T-DNA knock-out mutant osbim1 and wide type (WT) showed similar leaf inclination. OsBIM1 overexpression enhances the sensitivity and response to BR treatment in rice. Gene expression analysis showed that the overexpression of OsBIM1 significantly increased the transcripts of INCREASED LEAF INCLINATION1 (OsILI1) that functions as a key transcription factor promoting BR signaling and response. Meanwhile, OsBIM1 inhibited the expression of DWARF2 (OsD2, a key enzyme in BR biosynthesis pathway). OsBIM1 can bind with OsILI1 promoter and enhance OsILI1 expression in response to BR treatment. The promoting effect of OsBIM1 overexpression on leaf angle can still be observed at harvest stage, but overexpression of OsBIM1 resulted in smaller grain size and reduced yield. These results indicate that OsBIM1 functions as a positive regulator in BR signaling, and its overexpression increases rice lamina inclination by promoting BR sensitivity and response.

水稻叶角是决定植物结构和作物产量的重要农艺性状。油菜素内酯（BRs）在控制水稻叶角方面发挥着至关重要的作用，因此越来越多的研究集中在水稻中的BR信号通路上。然而，拟南芥BR信号中一些重要成分的直系同源物尚未在水稻中得到表征。在这项研究中，我们鉴定了一个名为 OsBIM1 的水稻 bHLH 转录因子，它是 AtBIM1 最接近的水稻同源物（BES1-Interacting MYC-like Protein1）。OsBIM1 的过表达显着增加了水稻叶角，而 T-DNA 敲除突变体osbim1和宽型 (WT) 显示出相似的叶倾角。奥斯BIM1过表达增强了水稻对 BR 处理的敏感性和反应。基因表达分析表明，OsBIM1 的过表达显着增加了INCREASED LEAF INCLINATION1 ( OsILI1 ) 的转录物，该转录物是促进 BR 信号传导和反应的关键转录因子。同时，OsBIM1抑制DWARF2（OsD2，BR生物合成途径中的关键酶）的表达。OsBIM1 可以与OsILI1启动子结合并增强OsILI1响应 BR 治疗的表达。OsBIM1过表达对叶角的促进作用在收获期仍可观察到，但过表达OsBIM1导致籽粒变小，产量降低。这些结果表明，OsBIM1 在 BR 信号传导中起正调节作用，其过表达通过促进 BR 敏感性和响应来增加水稻叶片倾斜度。