Xanthomonas arboricola pv. fragariae: what's in a name?

Authors


E-mail: martine.maes@ilvo.vlaanderen.be

Abstract

Polyphasic analysis exposed important heterogeneity between bacterial strains catalogued as Xanthomonas arboricola pv. fragariae (Xaf) from different culture collections. Two draft whole-genome sequences revealed pathogenicity related genes of the type-three secretion system in strain LMG 19146, whereas none were found in the Xaf pathotype strain LMG 19145. Also, considerable sequence divergence was observed in the phylogenetic marker genes gyrB, rpoD, dnaK and fyuA. Further study of 16 Xaf culture-collection strains showed that co-classification is not justified. Partial 16S rRNA gene and gyrB sequencing demonstrated that 12 strains belonged to X. arboricola, but that they did not form one homogeneous group within the species. The four remaining strains were identified as Xanthomonas fragariae and Xanthomonas sp. All sequence-based identifications were confirmed by MALDI-TOF MS fingerprinting. Also, the pathogenicity genes hrcQ and avrBs2 were detected in only three of the 12 analysed X. arboricola strains. The X. arboricola and Xanthomonas sp. strains showed pectolytic activity, and upon inoculation in strawberry none of the strains reproduced the leaf blight symptoms reported for Xaf. This study demonstrates that (i) no clear criteria exist for the identification of strains as Xaf, (ii) the name Xaf is currently used for a genetically diverse assortment of strains, and (iii) the species X. arboricola holds many undetermined plant-associated bacteria besides the described pathovars.

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