Résumé
The genus Xanthomonas and pseudomonads of the “syringae” group encompass most of the plant pathogenic bacteria, among which several quarantine organisms registered by the European Council Directive 2000/29/CE and bioterror agents registered in dual-use goods regulations. However the taxonomy of these groups is still controversial or even out-of-date in the case of “syringae” group, which makes difficult the drafting and the revision of the statutory texts. Sequencing of protein-coding genes is more and more used to resolve phylogenetic relationships between highly related species, and within species. Multilocus sequence analysis (MLSA) is a potential alternative to DNA-DNA hybridizations to define bacterial species provided that a sufficient degree of congruence betweenboth methods could be demonstrated. In this study, we sequenced fragments of four housekeeping genes (atpD, dnaK, efP and gyrB) for a collection representative of Xanthomonas genus and gyrB and rpoD genes for more than one hundred strains representative of the nine genomospecies of the “syringae” group. Within these two taxa, species and genomospecies are supported by extensive DNA-DNA similarity values which allowed comparison of the methods. Our results showed that MLSA is as resolutive as DNA-DNA hybridizations to circumscribe species or genomospecies, and phylogenetic trees are in agreement with previous DNA fingerprints results. Within “syringae” group, each genomospecies is represented by a discrete cluster supported by bootstrap values above 98%. The only incongruence is represented by genomospecies 3 and 8 which form a unique cluster in MLSA. Additional hybridization experiments are needed to solve this point. Partial sequencing of only one gene is sufficient to assign unkown isolates to a genomospecies but unsufficient to resolve the phylogenetic relationships between the genomospecies. Within Xanthomonas, MLSA data evidenced the distance of X. albilineans, X. hyacinthi and X. translucens from the core genus, and the relationships between previously and newly described species. In both genera, MLSA resolves phylogenetic relationships between highly related species more accurately than rrs trees. These data will strongly support the taxonomic studies that are needed in these two major groups of plant pathogenic bacteria.