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

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.

Aggressive Emerging Pathovars of Xanthomonas arboricola Represent Widespread Epidemic Clones Distinct from Poorly Pathogenic Strains, as Revealed by Multilocus Sequence Typing

“…It should also be observed that when symptoms were recorded, a long time (more than 1 month) was necessary from inoculation to symptom observation, indicating low aggressiveness, if any (20,71). Our results support the hypothesis of Vandroemme et al (21) that strains of pathovar fragariae represent "common plant residents," as should be the case for strains isolated from diverse hosts for which no pathovar affiliation is available.…”

“…5). This situation is similar to the one described above for pathovar fragariae (21) and to the one reported by Sawada et al (62) for grapevine isolates classified in X. arboricola. These authors concluded that "the causal bacterium was a genetically heterogeneous complex of opportunistic pathogens with weak pathogenicity" and did not propose a new pathovar name for grapevine isolates.…”

“…fragariae was previously shown using multilocus sequencing and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) profiling (21). Furthermore, the authors were unable to reproduce symptoms for the 13 strains of the pathovar, although eight of them were used in the original description of the pathovar (20).…”

“…Strains of the pathovars corylina, juglandis, and pruni are classified as quarantine pests in many countries. In contrast, strains of pathovars fragariae and populi are considered saprophytic strains or opportunistic pathogens (21,59). X. arboricola also includes strains that are not classified as pathovars.…”

“…fragariae, causing leaf blight of strawberry. Based on pathogenicity tests, virulence gene content, and genetic heterogeneity, the pathovar status of X. arboricola strains from strawberry was recently questioned (21). A phylogenetic analysis of the genus Xanthomonas, based on partial sequencing of the housekeeping gene gyrB, showed that the pathotype strains of X. campestris pv.…”