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A Competitive Index Assay Identifies Several Ralstonia solanacearum Type III Effector Mutant Strains with Reduced Fitness in Host Plants

Abstract : Ralstonia solanacearum, the causal agent of bacterial wilt, is a soil bacterium which can naturally infect a wide range of host plants through the root system. Pathogenicity relies on a type III secretion system which delivers a large set of approximately 75 type III effectors (T3E) into plant cells. On several plants, pathogenicity assays based on quantifi-cation of wilting symptoms failed to detect a significant contribution of R. solanacearum T3E in this process, thus revealing the collective effect of T3E in pathogenesis. We developed a mixed infection-based method with R. solana-cearum to monitor bacterial fitness in plant leaf tissues as a virulence assay. This accurate and sensitive assay provides evidence that growth defects can be detected for T3E mutants: we identified 12 genes contributing to bacterial fitness in eggplant leaves and 3 of them were also implicated in bacterial fitness on two other hosts, tomato and bean. Contribution to fitness of several T3E appears to be host specific, and we show that some known avirulence determinants such as popP2 or avrA do provide competitive advantages on some susceptible host plants. In addition, this assay revealed that the efe gene, which directs the production of ethylene by bacteria in plant tissues, and hdfB, involved in the biosynthesis of the secondary metabolite 3-hydroxy-oxindole, are also required for optimal growth in plant leaf tissues. Ralstonia solanacearum, the causative agent of bacterial wilt of solanaceous plants, is responsible for some of the most devastating bacterial plant diseases in the world. The high economic and social impact of this organism results from its wide geographical distribution in all warm and tropical countries of the globe. This impact also results from the very wide host range of R. solanacearum, which comprises over 200 plant species representing over 50 botanical families and covering both monocots and dicots (Allen et al. 2005; Denny 2006). As a soilborne plant pathogen, R. solanacearum naturally infects plants via the roots. Bacteria infect tomato plants through root tips and lateral root cracks, and rapidly develop within intercellular spaces of the inner cortex. Intercellular infection extends to the vascular parenchyma to finally invade protoxy
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Alberto Macho, Alice Guidot, Patrick Barberis, Carmen Beuzón, Stéphane Génin. A Competitive Index Assay Identifies Several Ralstonia solanacearum Type III Effector Mutant Strains with Reduced Fitness in Host Plants. Molecular Plant-Microbe Interactions, American Phytopathological Society, 2010, 23 (9), pp.1197-1205. ⟨10.1094/MPMI-23-9-1197⟩. ⟨hal-02936826⟩

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