Functional Assignment to Positively Selected Sites in the Core Type III Effector RipG7 from Ralstonia solanacearum

; ; ; ; ; ; ; ; ; ; (). Functional Assignment to Positively Selected Sites in the Core Type III Effector RipG7 from Ralstonia solanacearum. Molecular Plant Pathology DOI: 10.1111/mpp.12302. Peer reviewed.

The soil-borne pathogen Ralstonia solanacearum causes bacterial wilt in a broad range of plants. The main virulence determinants of R. solanacearum are the Type III Secretion System (T3SS) and its associated Type III Effectors (T3Es), translocated into the host cells. Among the conserved T3Es among R. solanacearum strains, The Fbox protein RipG7 is required for R. solanacearum pathogenesis on Medicago truncatula.

In this work we describe the existing natural ripG7 variability existing in the R. solanacearum species complex. We show that eight representative ripG7 orthologs have different contributions to pathogenicity on M. truncatula: only ripG7 from Asian or African strains can complement the absence of ripG7 in GMI1000 (Asian reference strain). Nonetheless, RipG7 proteins from American and Indonesian strains can still interact with M. truncatula SKP1-like/MSKa protein, essential for the function of RipG7 in virulence. This indicates that the absence of complementation is most likely due to variability in the leucine-rich repeat domain (LRR) of RipG7. We identified eleven sites under positive selection in the LRR domains of RipG7. By studying the functional impact of those 11 sites, we show the contribution of 5 positively selected sites for the function of RipG7CMR15 in Medicago truncatula colonization.

This work reveals the genetic and functional variation of the essential core T3E RipG7 from R. solanacearum. This analysis is the first of its kind on an essential disease-controlling type III effector and sheds light on the co-evolutionary arms race between the bacterium and its hosts.