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

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 OF skp1(SGT1). In this work, we found that the plant pathogen Ralstonia solanacearumsecretes an effector protein, named RipAC, inside plantcells, and it associates with SGT1 to suppress NLR-mediated SGT1-dependent immune responses. RipAC does not affect the accumulation of SGT1, 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, which is required for the interaction with NLRs. Such phosphorylation releases the interaction between SGT1 and the NLR RPS2, and contributes to the activation of RPS2-mediated responses. Our results shed light onto the mechanism of activation of NLR-mediated immunity, and suggest a positive feedbackloop between MAPK activation and SGT1-dependent NLR activation.