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How to name those to blame: A practical taxonomical study of the Xanthomonas causing bacterial leaf spot of lettuce has major consequences for the boundaries delimitation of X. hortorum

Abstract : Assessment of the taxonomy and diversity of the Xanthomonas strains causing bacterial leaf spot of lettuce (BLSL), commonly referred to as X. campestris pv. vitians, has been a long-lasting issue which held back the global efforts made to understand this pathogen. In order to provide a sound basis essential to its study, we conducted a polyphasic approach on a panel of strains obtained through sampling campaigns or acquired from collections. Results of a multilocus sequence analysis combined with phenotypic assays revealed that the pathotype strain of X. campestris pv. vitians does not match the description of the nomenspecies provided by Brown in 1918 [1]. However, LMG 938 = CFBP 8686 does fit this description. Therefore, we propose that LMG 938 = CFBP 8686 replaces LMG 937 = CFBP 2538 as pathotype strain of X. campestris pv. vitians. Then, overall genome relatedness indices (OGRIs) calculations and crossed-pathogenicity assays on lettuce allowed to validate the nomenclature X. hortorum pv. vitians comb. nov. originally proposed by Vauterin et al. in 1995 [2]. Interestingly, genome-based phylogenies exhibited an evolutionary continuum between closely related species X. hortorum and X. cynarae, confirmed by multiple OGRIs calculations between all the taxonomically relevant members of these two species. Phenotypic profiles characterized with Biolog GEN III microplates did not reveal any stable diagnostic traits legitimizing the distinction into two different species but a strong stable core phenotype. Thus, to be in accordance with nowadays standards in bacterial taxonomy and with respect to the global taxonomical framework of the genus Xanthomonas [3, 4], we propose the new combination of X. hortorum Vauterin et al. 1995 and X. cynarae (Trébaol 2000) Timilsina et al. 2018 into a new X. hortorum comb. nov. This new stable species would allow to integrate new uncharacterized members of X. hortorum isolated over the years on a wide range of host plants, making it a promising model for the study of genomic and ecologic determinants underlying adaptation to host plants. 1. Brown NA. Some bacterial diseases of lettuce. J Agric Res 1918;13:367. 2. Vauterin L, Hoste B, Kersters K, Swings J. Reclassification of Xanthomonas. Int J Syst Evol Microbiol 1995;45:472-489. 3. Chun J, Oren A, Ventosa A, Christensen H, Arahal DR, et al. Proposed minimal standards for the use of genome data for the taxonomy of prokaryotes. Int J Syst Evol Microbiol 2018;68:461-466. 4. Rosselló-Móra R, Amann R. Past and future species definitions for Bacteria and Archaea. Syst Appl Microbiol 2015;38:209-216.
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Contributor : Lucas Morinière <>
Submitted on : Thursday, May 7, 2020 - 12:27:24 PM
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Lucas Morinière, Alexandre Burlet, Emma Rosenthal, Xavier Nesme, Perrine Portier, et al.. How to name those to blame: A practical taxonomical study of the Xanthomonas causing bacterial leaf spot of lettuce has major consequences for the boundaries delimitation of X. hortorum. 14èmes Rencontres Plantes-Bactéries, Jan 2020, Aussois, France. ⟨hal-02561970⟩

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