The Bacterial Effector AvrB-Induced RIN4 Hyperphosphorylation Is Mediated by a Receptor-Like Cytoplasmic Kinase Complex in Arabidopsis.

Bacterial pathogen Pseudomonas syringae delivers diverse type III effectors into host cells to interfere with their immune responses. One of the effectors, AvrB, targets a host guardee protein RIN4 and induces RIN4 phosphorylation in Arabidopsis. Phosphorylated RIN4 activates the immune receptor RPM1 to mount defense. AvrB-induced RIN4 phosphorylation depends on RIPK, a receptor-like cytoplasmic kinase (RLCK). In this study, we found several other RLCKs that were also able to phosphorylate RIN4. We demonstrated that these RLCKs formed a complex with RIPK and were functionally redundant to RIPK. We also found that unphosphorylated RIN4 was epistatic to phosphorylated RIN4 in terms of RPM1 activation. AvrB-induced RLCK gene expression and phosphorylated RIN4-triggered RPM1 activation required RAR1, a central regulator in plant innate immunity. Our results unravel a mechanism in which plants employ multiple kinases to hyperphosphorylate the guardee protein RIN4 to ensure immune activation during pathogen invasion.