Housekeeping Gene Sequencing and Multilocus Variable-Number Tandem-Repeat Analysis To Identify Subpopulations within Pseudomonas syringae pv. maculicola and Pseudomonas syringae pv. tomato That Correlate with Host Specificity

Abstract

ABSTRACTPseudomonas syringaepv. maculicola causes bacterial spot onBrassicaceaeworldwide, and for the last 10 years severe outbreaks have been reported in the Loire Valley, France.P. syringaepv. maculicola resemblesP. syringaepv. tomato in that it is also pathogenic for tomato and causes the same types of symptoms. We used a collection of 106 strains ofP. syringaeto characterize the relationships betweenP. syringaepv. maculicola and related pathovars, paying special attention toP. syringaepv. tomato. Phylogenetic analysis ofgyrBandrpoDgene sequences showed thatP. syringaepv. maculicola, which causes diseases inBrassicaceae, forms six genetic lineages within genomospecies 3 ofP. syringaestrains as defined by L. Gardan et al. (Int. J. Syst. Bacteriol. 49[Pt 2]:469–478, 1999), whereasP. syringaepv. tomato forms two distinct genetic lineages. A multilocus variable-number tandem-repeat (VNTR) analysis (MLVA) conducted with eight minisatellite loci confirmed the genetic structure obtained withrpoDandgyrBsequence analyses. These results provide promising tools for fine-scale epidemiological studies on diseases caused byP. syringaepv. maculicola andP. syringaepv. tomato. The two pathovars had distinct host ranges; onlyP. syringaepv. maculicola strains were pathogenic forBrassicaceae. A subpopulation ofP. syringaepv. maculicola strains that are pathogenic for Pto-expressing tomato plants were shown to lackavrPto1andavrPtoBor to contain a disruptedavrPtoBhomolog. Taking phylogenetic and pathological features into account, our data suggest that the DC3000 strain belongs toP. syringaepv. maculicola. This study shows thatP. syringaepv. maculicola andP. syringaepv. tomato appear multiclonal, as they did not diverge from a single common ancestral group within the ancestralP. syringaegenomospecies 3, and suggests that pathovar specificity withinP. syringaemay be due to independent genetic events.