The transcription factor OsbHLH55 regulates leaf inclination through the OsMAPK6–OsWRKY53 signaling pathway in rice

Leaf inclination, an essential characteristic of ideal plant architecture, plays a crucial role in determining crop yield by influencing leaf area index and planting density. Brassinosteroid (BR) serves as the major phytohormone involved in leaf inclination regulation, while the mechanism of BR signaling regulation in rice still needs exploration when compared to Arabidopsis. Here, a bHLH transcription factor OsbHLH55 was highly expressed in the lamina joint, the osbhlh55 gene editing mutants exhibited large leaf inclination resulted from increased size of parenchyma cells on the adaxial side of the lamina joint, while OsbHLH55 overexpression lines showed small leaf inclination. In addition, the osbhlh55 mutants were hypersensitive to BR and the expression was inhibited by BR, suggesting that OsbHLH55 was negatively regulated by BR. Genetic experiments verify that OsbHLH55 can directly bind to the promoter of OsWRKY53 to repress its expression, which is involved in the BR signaling pathway. Moreover, OsbHLH55 can interact with and be phosphorylated by the OsMAPK6 in vitro. Collectively, our investigation revealed OsbHLH55 as a negative regulator of rice BR signaling to function in leaf inclination regulation, and shedding light on the intricate interplay between BR and MAPK signaling pathways in rice.