AUTHOR=Rosenthal Emma , Potnis Neha , Bull Carolee T. 

TITLE=Comparative Genomic Analysis of the Lettuce Bacterial Leaf Spot Pathogen, Xanthomonas hortorum pv. vitians, to Investigate Race Specificity

JOURNAL=Frontiers in Microbiology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2022.840311

DOI=10.3389/fmicb.2022.840311

ISSN=1664-302X

ABSTRACT=<p>Bacterial leaf spot (BLS) of lettuce caused by <italic>Xanthomonas hortorum</italic> pv. <italic>vitians</italic> (<italic>Xhv</italic>) was first described over 100 years ago and remains a significant threat to lettuce cultivation today. This study investigated the genetic relatedness of the <italic>Xhv</italic> strains and the possible genetic sources of this race-specific pathogenicity. Whole genome sequences of eighteen <italic>Xhv</italic> strains representing the three races, along with eight related <italic>Xanthomonas</italic> strains, were included in the analysis. A maximum likelihood phylogeny based on concatenated whole genome SNPs confirmed previous results describing two major lineages of <italic>Xhv</italic> strains. Gene clusters encoding secretion systems, secondary metabolites, and bacteriocins were assessed to identify putative virulence factors that distinguish the <italic>Xhv</italic> races. Genome sequences were mined for effector genes, which have been shown to be involved in race specificity in other systems. Two effectors identified in this study, <italic>xopAQ</italic> and the novel variant <italic>xopAF2</italic>, were revealed as possible mediators of a gene-for-gene interaction between <italic>Xhv</italic> race 1 and 3 strains and wild lettuce <italic>Lactuca serriola</italic> ARM-09-161-10-1. Transposase sequence identified downstream of <italic>xopAF2</italic> and prophage sequence found nearby within <italic>Xhv</italic> race 1 and 3 insertion sequences suggest that this gene may have been acquired through phage-mediated gene transfer. No other factors were identified from these analyses that distinguish the <italic>Xhv</italic> races.</p>