Genomic evidence for widespread reciprocal recognition and killing among Pseudomonas syringae strains

Abstract

Community assembly dynamics are in part driven by competition between community members. Diverse bacteria antagonize competitors through the production of toxic compounds, such as bacteriophage-derived tailocins. These toxins are highly specific in their targeting, which is determined by interactions between the tailocins tail fiber and competitors lipopolysaccharide O-antigen moieties. Tailocins play a pivotal role in mediating microbial interactions among the economically significant plant pathogens within the Pseudomonas syringae species complex, with the potential to alter community structure and disease progression in host plants. Previous work looking at 45 P. syringae strains has demonstrated that at least two phylogenetic clades of tail fibers are encoded in the conserved tailocin region across the species complex, which roughly corresponds to two clusters of targeting activity. To better understand the full diversity of tail fibers associated with tailocins in the species complex, we screened 2,161 publicly available genomes for their tailocin tail fiber content, predicted protein structures that represent the diversity of fibers, and investigated forces possibly driving the distribution of fibers throughout the species complex. Here we present evidence that while the two previously described tail fiber clades are indeed widespread among virulent P. syringae strains, their distribution is largely uncorrelated with phylogeny. Instead, we found that the presence of one tail fiber or the other is strongly correlated with the allelic diversity of another gene, associated with lipopolysaccharide O-antigen structure, dTDP-4-dehydrorhamnose reductase. Our findings suggest the presence of two reciprocally targeting groups of strains distributed throughout the P. syringae species complex that transcend phylogenetic relationships.