A bacterial effector protein prevents MAPK-mediated phosphorylation of SGT1 to suppress plant immunity

Author summary Plant immune receptors are subjected to a tight regulation in order to avoid auto-immune responses, but must be promptly activated upon perception of pathogen threats. Plant intracellular immune receptors can perceive effector proteins injected inside plant cells; for most of these receptors, the SUPPRESSOR OF G2 ALLELE OFskp1(SGT1) constitutes an essential regulator that controls their stability and activation. In this work, we found that phosphorylation mediated by MAPKs regulates SGT1, and is required for the activation of intracellular immune receptors. Supporting the biological relevance of this finding, we found that a bacterial pathogen,Ralstonia solanacearum, injects an effector protein that inhibits the interaction between MAPKs and SGT1, thereby suppressing SGT1 phosphorylation and the associated effector-triggered immune responses. These results suggest the existence of a phosphorylation-mediated feedback loop between MAPKs and SGT1, which leads to the activation of immune responses, and can be targeted by bacterial virulence activities. Nucleotide-binding domain and leucine-rich repeat-containing (NLR) proteins function as sensors that perceive pathogen molecules and activate immunity. In plants, the accumulation and activation of NLRs is regulated by SUPPRESSOR OF G2 ALLELE OFskp1(SGT1). In this work, we found that an effector protein named RipAC, secreted by the plant pathogenRalstonia solanacearum, associates with SGT1 to suppress NLR-mediated SGT1-dependent immune responses, including those triggered by anotherR.solanacearumeffector, RipE1. RipAC does not affect the accumulation of SGT1 or NLRs, or their interaction. However, RipAC inhibits the interaction between SGT1 and MAP kinases, and the phosphorylation of a MAPK target motif in the C-terminal domain of SGT1. Such phosphorylation is enhanced upon activation of immune signaling and contributes to the activation of immune responses mediated by the NLR RPS2. Additionally, SGT1 phosphorylation contributes to resistance againstR.solanacearum. Our results shed light onto the mechanism of activation of NLR-mediated immunity, and suggest a positive feedback loop between MAPK activation and SGT1-dependent NLR activation.