The F-box Containing Bacterial Effector RipG6 Destabilizes a Receptor-Like Cytoplasmic Kinase Involved in Plant Immune Signaling

Bacterial pathogens employ a large array of type IIIsecreted effectors to manipulate host cell immunity and metabolism. Ralstonia solanacearum species complex, the causal agent of bacterial wilt disease in numerous plant hosts, deploy a conserved subset of RipG effectors containing leucine-rich repeats and an F-box motif that collectively contribute to virulence and host specificity. RipG effectors are proposed to hijack the eukaryotic ubiquitin-proteasome machinery through the recruitment of substrates to host Skp1-cullin-F-box ubiquitinligase complexes via RipG F-box/host Skp1 adaptor interactions. However, only few host proteins have been reported to interact with RipG effectors. Here, using a surrogate type III delivery and a heterologous expression systems, we show that RipG6 can suppress plant pattern-triggered immunity in an F-box-dependent manner. We further identified a tomato receptor-like cytoplasmic kinase (SlRLCK-VIII-6) as an interactor of RipG6 in yeast and plant cells. SlRLCK-VIII-6 stability was reduced when co-expressed with RipG6 but not with the RipG6 variant lacking the F-box motif. Lastly, we provide evidence that Nicotiana benthamiana and Arabidopsis thaliana homologs of SlRLCK-VIII-6 can act as positive regulators of plant immune signaling. Together, our work supports a model where RipG6 destabilizes RLCK-VIII-6 possibly via its recruitment in a host ubiquitin-ligase complex in order to suppress plant immunity. Further research into RLCK-VIII role will enhance our understanding of the manipulation of plant immunity signaling by pathogen effectors.