Transcription Factor OsWRKY53 Positively Regulates Brassinosteroid Signaling and Plant Architecture.

Brassinosteroids (BRs) are a class of steroid hormones regulating multiple aspects of plant growth, development, and adaptation. Compared with extensive studies in Arabidopsis (Arabidopsis thaliana), the mechanism of BR signaling in rice (Oryza sativa) 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 OsWRKY53 overexpression led to enlarged leaf angles and increased grain size, in contrast to the erect leaves and smaller seeds in oswrky53 mutant. In addition, the oswrky53 exhibited decreased BR sensitivity, whereas OsWRKY53 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 OsWRKY53 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.