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Adaptation of genetically monomorphic bacteria: evolution of copper resistance through multiple horizontal gene transfers of complex and versatile mobile genetic elements

Richard D., Ravigné V., Rieux A., Facon B., Boyer C., Boyer K., Grygiel P., Javegny S., Terville M.A., Canteros B.I., Robene I., Vernière C., Chabirand A., Pruvost O., Lefeuvre P.. 2018. In : 13èmes Rencontres Plantes-Bactéries - Book of Abstracts. Paris : SFP, p. 21-21. Rencontres Plantes-Bactéries. 13, 2018-01-29/2018-02-02, Aussois (France).

Copper-based compounds are widely used in integrated pest management (1PM) programs aiming at controlling agriculturally important plant bacterial pathogens and the latter have adapted in response to this selective pressure. Copper resistance of Xanthomonas citri pv. citri (Xcc), a major citrus pathogen worldwide causing Asialie citrus canker, was first observed in Argentina two decades ago and subsequently reported as a copLAB-based, plasmid-encoded system. The emergence of resistant strains has since been reported in Réunion (South West lndian Ocean) and Martinique (Eastern Caribbean Sea). Disease severity was found markedly increased in groves established with susceptible cultivars and infected with copper-resistant Xcc. Using tandem repeat-based genotyping and copLAB PCR, we demonstrated that the genetic structure of the copperresistant strains from these three regions was made up of two distant clusters and varied for the detection of copLAB amplicons. ln order to investigate this pattern more closely, we sequenced six copper-resistant Xcc strains from Argentina, Martinique and Réunion, together with reference copper-resistant Xanthomonas and Stenotrophomonas strains using long-read sequencing technology. Genes involved in copper resistance were found to be strain-dependent with the nove! identification in Xcc of copABCD and a eus heavy metal efflux resistance-nodulation-division system. The genes providing the adaptive trait were part of a mobile genetic element similar to Tn3-like transposons and included in a conjugative plasmid. The mining of ail bacterial genomes available from public databases suggested that the mobile elements containing copper resistance genes and their plasmid environments were primarily detected in the Xanthomonadaceae family.

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