RAR1 is Involved in an Effector-Induced Protein Phosphorylation in Arabidopsis

Plants possess various elegant surveillance systems to recognize pathogens and to activate immune responses. In order to counteract plant disease resistance, bacterial pathogen Pseudomonas syringae delivers dozens of type III effectors into host cells to interfere with immune responses. One of the effectors AvrB targets a host protein RIN4 and induces RIN4 phosphorylation in Arabidopsis. Phosphorylated RIN4 in turn triggers an immune receptor RPM1 to mount defense. Previously we have shown that AvrB-induced RIN4 phosphorylation depends on RIPK, a receptor like cytoplasmic kinase (RLCK). In addition to RIPK, here we found at least five additional RLCKs were induced by AvrB, and all of them were able to phosphorylate RIN4 in vitro. Knockout mutants of these kinases displayed compromised RPM1 activation as revealed by decreased disease resistance to P. syringae harboring AvrB and AvrRpm1, suggesting that they are functional redundant to RIPK. Notably, AvrB-induced RLCK expression requires RAR1, a central regulator in plant innate immunity. However, phosphomimetic RIN4-triggered RPM1 activation does not need RAR1, implying RAR1 acts in early RPM1-mediated resistance pathway. We propose that RAR1 functions in RPM1-mediated immune responses by regulating AvrB-interacting kinase expression.