Functional diversification of the GALA type III effector family contributes to Ralstonia solanacearum adaptation on different plant hosts.

Type III effectors from phytopathogenic bacteria exhibit a high degree of functional redundancy, hampering the evaluation of their precise contribution to pathogenicity. This is illustrated by the GALA type III effectors from Ralstonia solanacearum, which have been shown to be collectively, but not individually, required for disease on Arabidopsis thaliana and tomato. We investigated evolution, redundancy and diversification of this family in order to understand the individual contribution of the GALA effectors to pathogenicity. From sequences available, we reconstructed GALA phylogeny and performed selection studies. We then focused on the GALAs from the reference strain GMI1000 to examine their ability to suppress plant defense responses and contribution to pathogenicity on three different host plants: A. thaliana, tomato (Lycopersicum esculentum) and eggplant (Solanum melongena). The GALA family is well conserved within R. solanacearum species. Patterns of selection detected on some GALA family members, together with experimental results, show that GALAs underwent functional diversification. We conclude that functional divergence of the GALA family likely accounts for its remarkable conservation during R. solanacearum evolution and could contribute to R. solanacearum's adaptation on several host plants.