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Mol Plant Pathol. 2016 May;17(4):553-64. doi: 10.1111/mpp.12302. Epub 2015 Dec 2.

Functional assignment to positively selected sites in the core type III effector RipG7 from Ralstonia solanacearum.

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INRA, Laboratoire des Interactions Plantes Micro-organismes (LIPM), UMR441, CS52627 Chemin de Borde Rouge, 31326, Castanet-Tolosan, France.
CNRS, Laboratoire des Interactions Plantes Micro-organismes (LIPM), UMR2594, CS52627 Chemin de Borde Rouge, 31326, Castanet-Tolosan, France.
Institute of Applied Simulations, School of Life Sciences and Facility Management, Zürich University of Applied Sciences, Gruenalstrasse 14, 8820, Wädesnwil, Switzerland.
Centre de Recherche de Biochimie Macromoléculaire, CNRS, UMR5237, 1919 Route de Mende, 34000, Montpellier, France.
Institute of Plant Biology, National Taiwan University, Taipei, 11529, Taiwan, R.O.C.
Université de Toulouse, INP, ENSAT, 18 Chemin de Borde Rouge, Castanet-Tolosan, 31326, France.


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. Of 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 natural ripG7 variability existing in the R. solanacearum species complex. We show that eight representative ripG7 orthologues 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 a result of the variability in the leucine-rich repeat (LRR) domain of RipG7. We identified 11 sites under positive selection in the LRR domains of RipG7. By studying the functional impact of these 11 sites, we show the contribution of five positively selected sites for the function of RipG7CMR15 in M. 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 T3E, and sheds light on the co-evolutionary arms race between the bacterium and its hosts.


LRR; Medicago truncatula; Ralstonia solanacearum; positive selection; type III effector; virulence function

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