Phenotypic and Molecular-Phylogenetic Analyses Reveal Distinct Features of Crown Gall-Associated Xanthomonas Strains

International audienceIn summer 2019, widespread occurrence of crown gall disease caused byAgrobacterium spp. was observed on commercially grown ornamental plants in south-ern Iran. Beside agrobacteria, pale yellow-pigmented Gram-negative strains resemblingthe members of Xanthomonas were also associated with crown gall tissues on weep-ing fig (Ficus benjamina) and Amaranthus sp. plants. The purpose of the present studywas to characterize the crown gall-associated Xanthomonas strains using plant inocula-tion assays, molecular-phylogenetic analyses, and comparative genomics approaches.Pathogenicity tests showed that the Xanthomonas strains did not induce diseasesymptoms on their host of isolation. However, the strains induced hypersensitive reac-tion on tobacco, geranium, melon, squash, and tomato leaves via leaf infiltration.Multilocus sequence analysis suggested that the strains belong to clade IA ofXanthomonas, phylogenetically close to Xanthomonas translucens, X. theicola, and X.hyacinthi. Average nucleotide identity and digital DNA-DNA hybridization valuesbetween the whole-genome sequences of the strains isolated in this study and refer-ence Xanthomonas strains are far below the accepted thresholds for the definition ofprokaryotic species, signifying that these strains could be defined as two new specieswithin clade IA of Xanthomonas. Comparative genomics showed that the strains iso-lated from crown gall tissues are genetically distinct from X. translucens, as almost allthe type III secretion system genes and type III effectors are lacking in the formergroup. The data obtained in this study provide novel insight into the breadth ofgenetic diversity of crown gall-associated bacteria and pave the way for research ongall-associated Xanthomonas-plant interactions