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

Abstract

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 an effector protein named RipAC, secreted by the plant pathogen Ralstonia solanacearum, associates with SGT1 to suppress NLR-mediated SGT1-dependent immune responses, including those triggered by another R. solanacearum effector, 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 against R. 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.

What the Wild Things Do: Mechanisms of Plant Host Manipulation by Bacterial Type III-Secreted Effector Proteins

“…The strategy of manipulating host protein complexes is also illustrated by the T3SE RipAC from R. solanacearum . This effector inhibits ETI but not cell death caused by general elicitors such as the Phytophthora infestans secreted protein INF1 or the mammalian pro-apoptotic protein Bax, indicating specificity for programmed cell death responses arising from T3SE recognition [ 112 , 113 ]. Interestingly, RipAC contains tandem LRR motifs that are required for ETI suppression, suggesting that it may act as a scaffold for protein–protein interactions.…”

“…Interestingly, RipAC contains tandem LRR motifs that are required for ETI suppression, suggesting that it may act as a scaffold for protein–protein interactions. Indeed, two independent studies found that RipAC interacts with homologs of SUPPRESSOR OF G2 ALLELE OF skp1 (SGT1), a positive regulator of plant immunity [ 112 , 113 ]. A molecular chaperone complex comprising SGT1, RAR1, and Hsp90 is required for NLR stability and function [ 273 ], and RipAC interferes with the SGT1–RAR1 interaction in an LRR-dependent manner [ 113 ].…”

“…A molecular chaperone complex comprising SGT1, RAR1, and Hsp90 is required for NLR stability and function [ 273 ], and RipAC interferes with the SGT1–RAR1 interaction in an LRR-dependent manner [ 113 ]. This T3SE also blocks the association of SGT1 with the MAPKs MPK3 and MPK6, thus inhibiting the phosphorylation of SGT1 that normally follows ETI activation [ 112 ]. The consequent impairment of NLR function underlies the attenuation of ETI by RipAC.…”

The Arabidopsis E3 ubiquitin ligase PUB4 regulates BIK1 homeostasis and is targeted by a bacterial type-III effector

“…To verify whether RipAC affects PTI in Arabidopsis, we characterized PAMPinduced responses in Arabidopsis transgenic lines overexpressing RipAC (Yu et al, 2020a). Two independent RipAC-GFP overexpression lines showed a decreased ROS burst in response to bacterial or fungal PAMPs (flg22 Pto , elf18 Rsol , and chitin; Figures 1A-C).…”

“…R. solanacearum relies on a type-III secretion system to deliver over 70 Type-III effector proteins (T3Es) into the host cytoplasm (Sabbagh et al, 2019). One of these T3Es, RipAC, is able to suppress effector-triggered immunity (ETI) by targeting SGT1, and is required for full virulence of R. solanacearum in tomato and Arabidopsis (Yu et al, 2020a).…”