Phosphorylation-Regulated Activation of the Arabidopsis RRS1-R/RPS4 Immune Receptor Complex Reveals Two Distinct Effector Recognition Mechanisms

The Arabidopsis immune receptors RPS4 and RRS1 interact to co-confer responsiveness to bacterial effectors. The RRS1-R allele, with RPS4, recognizes AvrRps4 and PopP2, while RRS1-S responds only to AvrRps4. Here we show that the C-terminus of RRS1-R, but not RRS1-S, is phosphorylated. Phosphorylation at Thr1214 in the WRKY domain maintains RRS1-R in its inactive state. Thr1214 phosphorylation also inhibits acetylation of RRS1-R by PopP2 and PopP2 catalyzes O-acetylation of Thr1214 thereby preventing its phosphorylation. Phosphorylation at five Ser sites is required for PopP2, but not AvrRps4, responsiveness, and facilitates C-terminal interaction with TIRRRS1. De-repression of RRS1-R or RRS1-S involves effector-triggered proximity between their N-terminal TIR domain and their C-termini. This effector-promoted interaction between TIRRRS1 and the RRS1 C-terminus relieves the inhibitory effect of TIRRRS1 on TIRRPS4. Our data reveal effector-triggered and phosphorylation-regulated conformational changes within RRS1 that results in distinct modes of derepression of the complex by PopP2 and AvrRps4.