A Receptor-Like Cytoplasmic Kinase is Involved in the Activation of a Plant Innate Immune Receptor

Plants have evolved sophisticated surveillance systems to recognize pathogen effectors delivered into host cells. It remains largely unknown how resistant proteins perceive virulent effectors. RPM1 is an NB-LRR immune receptor that recognizes the Pseudomonas syringae effectors AvrB and AvrRpm1. Both effectors associate with RIN4, an immune regulator, and it has been hypothesized that RPM1 recognizes the posttranslational modification of RIN4. Using co-immunoprecipitation approach, we identified a set of RIN4 interacting proteins. RIPK, one of the interacting proteins, can directly interact with AvrB and RIN4. RIPK can directly phosphorylate RIN4 at amino acids T21, S160, and T166. RIN4 phosphorylation at T166 is specifically induced by AvrB and AvrRpm1, leading to the activation of the RPM1 immune receptor. Furthermore, ripk knockout lines display reduced RPM1-mediated defense responses. These data indicate that AvrB may specifically target RLCK complexes or mimic the activity of host kinases to induce RIN4 phosphorylation, leading to the activation of immune responses. Additional results analyzing the importance of this kinase and other closely related kinases during immune signaling will be reported.