The Kinase OsCPK4 Regulates a Buffering Mechanism That Fine-Tunes Innate Immunity

The calcium-dependent protein kinase OsCPK4 has been demonstrated to play important roles in salt and drought tolerance, plant growth, and development in rice ( Oryza sativa ). However, little is known about molecular mechanisms underlying OsCPK4 function in rice immunity. In this study, we demonstrated that the generation of oxidative burst and pathogenesis-related gene expression triggered by microbe-associated molecular patterns were significantly enhanced in the oscpk4 mutants. These mutant lines are more resistant to bacterial blight and fungal blast diseases than the wild-type plants, indicating that OsCPK4 negatively regulates innate immunity in rice. OsCPK4 was further identified to interact with a receptor-like cytoplasmic kinase OsRLCK176. OsRLCK176 accumulation is negatively regulated by OsCPK4. Interestingly, the kinase-dead OsCPK4 promotes OsRLCK176 degradation more strongly than the wild-type protein. OsCPK4 and OsRLCK176 mutually phosphorylate each other and form a feedback loop. Moreover, the kinase activity and phosphorylation of OsCPK4 and OsRLCK176 contribute to the stability of OsRLCK176. These findings indicate that the kinase-inactive OsCPK4 promotes OsRLCK176 degradation and restricts plant defenses, whereas the activation of OsCPK4-OsRLCK176 phosphorylation circuit invalidates the OsRLCK176 degradation machinery, thus enhancing plant immunity. Collectively, the study proposes a novel defense buffering mechanism mediated by OsCPK4, which fine-tunes microbe-associated molecular pattern-triggered immunity in rice.