BRASSINOSTEROID‐SIGNALING KINASE 1 modulates OPEN STOMATA 1 phosphorylation and contributes to stomatal closure and plant immunity

SUMMARY Stomatal movement plays a critical role in plant immunity by limiting the entry of pathogens. OPEN STOMATA 1 (OST1) is a key component that mediates stomatal closure in plants, however, how OST1 functions in response to pathogens is not well understood. RECEPTOR‐LIKE KINASE 902 (RLK902) phosphorylates BRASSINOSTEROID‐SIGNALING KINASE 1 (BSK1) and positively modulates plant resistance. In this study, by a genome‐wide phosphorylation analysis, we found that the phosphorylation of BSK1 and OST1 was missing in the rlk902 mutant compared with the wild‐type plants, indicating a potential connection between the RLK902‐BSK1 module and OST1‐mediated stomatal closure. We showed that RLK902 and BSK1 contribute to stomatal immunity, as the stomatal closure induced by the bacterial pathogen Pto DC3000 was impaired in rlk902 and bsk1‐1 mutants. Stomatal immunity mediated by RLK902 was dependent on BSK1 phosphorylation at Ser230, a key phosphorylation site for BSK1 functions. Several phosphorylation sites of OST1 were important for RLK902‐ and BSK1‐mediated stomatal immunity. Interestingly, the phosphorylation of Ser171 and Ser175 in OST1 contributed to the stomatal immunity mediated by RLK902 but not by BSK1, while phosphorylation of OST1 at Ser29 and Thr176 residues was critical for BSK1‐mediated stomatal immunity. Taken together, these results indicate that RLK902 and BSK1 contribute to disease resistance via OST1‐mediated stomatal closure. This work revealed a new function of BSK1 in activating stomatal immunity, and the role of RLK902‐BSK1 and OST1 module in regulating pathogen‐induced stomatal movement. Significance Statement BSK1 functions in stomatal immunity by modulating the phosphorylation of OST1 at the Ser29 and Thr176 sites. The phosphorylation of Ser171, Ser175, Ser29, and Thr176 in OST1 contributes to the stomatal immunity mediated by RLK902. This work uncovers a molecular mechanism of the RLK902‐BSK1 and OST1 module in stomatal immunity.