The receptor-like cytoplasmic kinase OsSTRK1 regulates brassinosteroid signaling by phosphorylating OsGSK2

In the BR signaling pathway, GSK3-like kinases are crucial negative regulators, and inactivation of GSK3-like kinases occurs through dephosphorylation by the phosphatase BSU1. However, the identity of the kinases that can phosphorylate GSK3-like kinases in BR signaling remains unknown. In this study, we identify that OsSTRK1 interacts with and phosphorylates OsGSK2 in rice to stabilize OsGSK2, preventing its interaction with the E3 ubiquitin ligase OsTUD1. Overexpression of OsSTRK1 leads to a BR-repressed phenotype and decreased sensitivity to BR, while RNAi of OsSTRK1 induces enhanced sensitivity to BR. We identify that Tyr-223 of OsGSK2 as a critical phosphorylation site that is essential for the role of OsSTRK1 in modulating BR signaling, thus influencing the growth and development of rice. Overall, our findings uncover the essential role of OsSTRK1 in the phosphorylation and activation of OsGSK2, thereby completing the assembly of the core components of the BR signaling pathway.