The Ralstonia solanacearum RipAX family effectors repress the phosphorylation of host MAPKs

Bacterial wilt caused by strains of the Ralstonia solanacearum species complex is a severe disease affecting plants. The type III secretion system delivers type III effectors (T3Es) from the pathogen to the host plant and is one of the main virulence determinants of R. solanacearum strains. However, the functions of most T3Es, including the R. solanacearum effectors RipAX1 and RipAX2, are not completely understood. In this study, we determined that RipAX1 and RipAX2 share a potential but highly conserved M91 metalloenzyme structural domain. Heterologous overexpression of RipAX1 or RipAX2 neither caused nor inhibited the hypersensitive response (HR) of plant tissues, and their expression in Saccharomyces cerevisiae did not inhibit yeast growth. RipAX1 and RipAX2 localized to the cytoplasm of Nicotiana benthamiana leaf epidermal cells and Arabidopsis thaliana protoplasts. The two effectors interacted with specific members of the plant mitogen-activated protein kinase (MAPK) family. However, RipAX1 and RipAX2 did not directly cleave MAPKs, despite their predicted metalloenzyme activity, although RipAX1 inhibited the phosphorylation of plant MAPKs. These results elucidate a novel aspect of the molecular mechanism underlying the repression of host immunity by T3Es from R. solanacearum species complex strains.