Functional assignment to positively selected sites in the core type III effector RipG 7 from R alstonia solanacearum

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Abstract
Summary: The soil‐borne pathogen R alstonia 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 ( T3E s), translocated into the host cells. Of the conserved T3E s among R . solanacearum strains, the F box protein RipG7 is required for R . solanacearum pathogenesis on M edicago truncatula . In this work, we describe the natural rip G 7 variability existing in the R . solanacearum species complex. We show that eight representative rip G 7 orthologues have different contributions to pathogenicity on M . truncatula : only rip G 7 from Asian or African strains can complement the absence of rip G 7 in GMI 1000 ( A sian reference strain). Nonetheless, RipG 7 proteins from American and Indonesian strains can still interact with M . truncatula SKP 1‐like/ MSKa protein, essential for the function of RipG 7 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 RipG 7. We identified 11 sites under positive selection in the LRR domains of RipG 7. By studying the functional impact of these 11 sites, we show the contribution of five positively selected sites for the function of RipG 7 CMR15 in M . truncatula colonization. This work reveals the genetic and functional variation of the essential core T3E RipG 7 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.
Funding Information
  • LABEX ‘TULIP’ (ANR-07-JCJC-0133, ANR-10-LABX-41)
  • China Scholarship Council

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