The OsBUL1–OsbHLH186–OsBC1 transcriptional complex regulates rice grain size and leaf angle through the brassinosteroid signaling pathway

Grain size and leaf angle are closely related to the final yields of rice (Oryza sativa). Brassinosteroids (BRs) are plant-specific steroid hormones that play a crucial role in regulating grain size and leaf angle; however, the underlying molecular mechanisms require further investigation. Here, we report on OsbHLH186, which encodes an atypical bHLH transcription factor. OsbHLH186 influences grain size and leaf angle by affecting cell expansion. The cr-osbhlh186 mutants exhibit smaller grains and erect leaves, whereas overexpressed OX-OsbHLH186 plants display larger grains and increased leaf angle. OsbHLH186 acts as a positive regulator in response to BR signaling, with the cr-osbhlh186 mutant being insensitive to exogenous BR treatment, while OX-OsbHLH186 plants are hypersensitive. Biochemical and genetic analyses demonstrate that OsbHLH186 interacts with BRASSINOSTEROID UPREGULATED 1-LIKE1 (OsBUL1) and OsBC1, functioning within a common pathway. Further transient expression assays indicate that OsbHLH186 and OsBUL1 mediate the transcriptional activity of OsBC1. Overall, these findings suggest that OsbHLH186 is associated with a potential transcriptional complex that mediates BR signaling and rice development, indicating that OsbHLH186 could serve as a promising target for improving plant architecture and grain shape in rice.