LF1 mediates brassinosteroid signaling to regulate leaf angle development in rice.

Leaf angle is a key determinant of plant architecture and grain yield in rice, yet the complex genetic regulatory network remains unclear. Here, we reveal that the HD-ZIP III transcription factor LF1 mediates brassinosteroid (BR) signaling to modulate leaf angle. A miRNA165/166-resistant, gain-of-function lf1 mutant displays an enlarged leaf angle driven by elongated adaxial parenchymal cells and diminished abaxial sclerenchyma. At the transcriptional level, the BR-activated transcription factor OsBZR1 directly induces LF1, which in turn upregulates OsOFP8, forming an OsBZR1-LF1-OsOFP8 hierarchical transcriptional cascade that mediates BR signaling to regulate leaf angle development. At the post-translational level, LF1 protein stability is fine-tuned by reciprocal post-translational modifications, wherein OsMAPK6-mediated phosphorylation at threonine residue at position 148 promotes LF1 accumulation and transcriptional activity, directly counteracting its ubiquitination and degradation by the APC/CTAD1 complex. Genetic analyses demonstrate that LF1, OsMAPK6, and TAD1 function in a common pathway to regulate leaf angle development. Overall, our study establishes LF1 as a central hub coupling transcriptional and post-translational mechanisms to modulate BR-mediated leaf angle, providing targets for plant architecture improvement.