The receptor-like cytoplasmic kinase RIPK regulates broad-spectrum ROS signaling in multiple layers of plant immune system.

Production of reactive oxygen species (ROS) via the activity of respiratory burst oxidase homologs (RBOHs) plays a vital role in multiple layers of the plant immune system, including pathogen-associated molecular pattern-triggered immunity (PTI), damage-associated molecular pattern-triggered immunity (DTI), effector-triggered immunity (ETI), and systemic acquired resistance (SAR). It is generally established that RBOHD is activated by different receptor-like cytoplasmic kinases (RLCKs) in response to various immune elicitors. In this study, we showed that RPM1-INDUCED PROTEIN KINASE (RIPK), an RLCK VII subfamily member, contributes to ROS production in multiple layers of plant immune system. The ripk mutants showed reduced ROS production in response to treatment with all examined immune elicitors that trigger PTI, DTI, ETI, and SAR. We found that RIPK can directly phosphorylate the N-terminal region of RBOHD in vitro, and the levels of phosphorylated S343/S347 residues of RBOHD are sigfniciantly lower in ripk mutants compared with the wild type upon treatment with all tested immune elicitors. We further demonstrated that phosphorylation of RIPK is required for its function in regulating RBOHD-mediated ROS production. Similar to rbohd, ripk mutants showed reduced stomatal closure and impaired SAR, and were susceptible to the necrotrophic bacterium Pectobacterium carotovorum. Collectively, our results indicate that RIPK regulates broad-spectrum RBOHD-mediated ROS signaling during PTI, DTI, ETI, and SAR, leading to subsequent RBOHD-dependent immune responses.