The type III effector RipBB from Ralstonia pseudosolanacearum inhibits plant immunity responses and contributes to virulence on peanut

Abstract

Ralstonia solanacearum species complex (RSSC) is a serious soilborne phytopathogen affecting over 310 plant species. R. pseudosolanacearum is one clade of RSSC, which infects the important oil crop peanut. A variety of virulence factors are employed by RSSC to promote disease, among which type III effectors (T3Es) are prominent. How T3Es manipulate the interaction between R. pseudosolanacearum and peanut is unclear. A T3E RipBB was previously found specifically in a more virulent peanut R. pseudosolanacearum PeaFJ1 strain. In the present study, the function of RipBB was analysed. Loss of RipBB from PeaFJ1 strain resulted in attenuated pathogenicity to peanut, and complementation with RipBB recovered the virulence of the mutant strain. Transient expression of RipBB induced cell death and inhibited flg22‐triggered reactive oxygen species (ROS) burst in the leaves of Nicotiana benthamiana. The expression of pattern‐triggered immunity (PTI)‐related genes were also suppressed by RipBB transient expression. Among the available sequenced 639 RSSC strains, RipBB is an infrequent T3E that is only present in eight strains. Two ankyrin (ANK) repeats were identified in RipBB, which play an important role in localizing the protein to the cytomembrane and nucleus. Altogether, we verified that RipBB contributes to infecting peanut by acting as a virulence T3E, and causes cell death and suppresses immunity in N. benthamiana. These results enhance the study of ANK‐containing effectors. Further elucidation of the molecular mechanisms underlying RipBB effect on immunity may reveal ANK‐containing effector functions in host cells, helping to understanding the mechanism of R. pseudosolanacearum–peanut interaction.