Nonpathogenic xanthomonads: a new aspect in Xanthomonas phylogeny

Xanthomonas is traditionally known as a genus of bacterial plant pathogens. International quarantaine regulations are implemented for several Xanthomonas species and pathovars. While Xanthomonas plant pathogens are well characterized, little is understood about the ecology of nonpathogenic xanthomonads living in close association with plants but causing no apparent disease symptoms on the host. In the past, these strains were largely overlooked and their relatedness to described Xanthomonas species was often unknown [1]. Today, it is possible to classify unknown Xanthomonas strains in a phylogenetic tree of Xanthomonas based on comparison of gyrB sequences [2]. Our aim was to define the phylogenetic position of environmental and previously unclassified xanthomonads, including nonpathogenic strains and poorly characterized pathovars, based on comparison of gyrB sequences and MLSA of six housekeeping genes. In gyrB sequence analysis, which was concordant with the MLSA results, these strains showed a wide phylogenetic diversity. Some strains were allocated to existing groups, others fitted within previously identified new lineages [2], and still others formed new entities. The nonpathogenic Xanthomonas strains were distinct from the pathogenic species or pathovars that cause diseases on the plants from which they were isolated. Many of them were identified as belonging to X. arboricola, X. pisi, X. hortorum, or X. sacchari but came from different hosts and were not concordant with the known pathogenicity of those species. Specifically X. arboricola is a not well-defined species harboring a variety of non-pathogenic xanthomonads from diverse origins [3]. Still other strains, isolated from a range of hosts, grouped together forming new clades that may represent new species or subspecies. Plant health and seed certifications are mandatory to prevent the introduction, spread and establishment of quarantine organisms. Misidentification of nonpathogenic xanthomonads as regulated organisms can have serious economic consequences. In the past, the association of nonpathogenic xanthomonads with plant material may have resulted in unnecessary destruction of economically valuable crops. We suggest that in all cases when xanthomonads are found, a gyrB sequence is determined and verified with available Xanthomonas barcode sequences in databases such as Q-bank (www.q-bank.eu), and particularly the diverse nonpathogenic xanthomonads need the attention of plant protection organizations, plant quarantine bureaus and seed health units.