Brassinosteroids (BRs) are a class of steroid hormones regulating multiple aspects of plant growth, development, and adaptation. Compared with extensive studies in Arabidopsis (), the mechanism of BR signaling in rice () is less understood. Here, we identified OsWRKY53, a transcription factor involved in defense responses, as an important regulator of rice BR signaling. Phenotypic analyses showed that overexpression led to enlarged leaf angles and increased grain size, in contrast to the erect leaves and smaller seeds in mutant. In addition, the exhibited decreased BR sensitivity, whereas overexpression plants were hypersensitive to BR, suggesting that OsWRKY53 positively regulates rice BR signaling. Moreover, we show that OsWRKY53 can interact with and be phosphorylated by the OsMAPKK4-OsMAPK6 cascade, and the phosphorylation is required for the biological function of OsWRKY53 in regulating BR responses. Furthermore, we found that BR promotes OsWRKY53 protein accumulation but represses transcript level. Taken together, this study revealed the novel role of OsWRKY53 as a regulator of rice BR signaling and also suggested a potential role of OsWRKY53 in mediating the cross talk between the hormone and other signaling pathways.

In rice (Oryza sativa) and other plants, plant architecture and seed size are closely related to yield. Brassinosteroid (BR) signaling and the mitogen-activated protein kinase (MAPK) pathway (MAPKKK10–MAPKK4–MAPK6) are two major regulatory pathways that control rice architecture and seed size. However, their possible relationship and crosstalk remain elusive. Here, we show that WRKY53 mediated the crosstalk between BR signaling and the MAPK pathway. Biochemical and genetic assays demonstrated that GSK2 phosphorylates WRKY53 and lowers its stability, indicating that WRKY53 is a substrate of GSK2 in BR signaling. WRKY53 interacted with BZR1; they function synergistically to regulate BR-related developmental processes. We also provide genetic evidence showing that WRKY53 functions in a common pathway with the MAPKKK10–MAPKK4–MAPK6 cascade in leaf angle and seed size control, suggesting that WRKY53 is a direct substrate of this pathway. Moreover, GSK2 phosphorylated MAPKK4 to suppress MAPK6 activity, suggesting that GSK2-mediated BR signaling might also regulated MAPK pathway. Together, our results revealed a critical role for WRKY53 and uncovered sophisticated levels of interplay between BR signaling and the MAPK pathway in regulating rice architecture and seed size.