Newberry, E. | Search bioRxiv

bioRxiv2023

Genetic and functional diversity help explain pathogenic, weakly pathogenic, and commensal lifestyles in the genus Xanthomonas

The genus Xanthomonas has been primarily studied for pathogenic interactions with plants. However, besides host and tissue specific pathogenic strains, this genus also comprises nonpathogenic strains isolated from a broad range of hosts, sometimes in association with pathogenic strains, and other environments, including rainwater. Based on their incapacity or limited capacity to cause symptoms on the host of isolation, nonpathogenic xanthomonads can be further characterized as commensal and weakly pathogenic. This study aimed to understand the diversity and evolution of nonpathogenic xanthomonads compared to their pathogenic counterparts based on their co-occurrence and phylogenetic relationship and to identify genomic traits that form the basis of a life-history framework that groups xanthomonads by ecological strategies. We sequenced genomes of 83 strains spanning the genus phylogeny and identified eight novel species, indicating unexplored diversity. While some nonpathogenic species have experienced a recent loss of a type III secretion system, specifically, the hrp2 cluster, we observed an apparent lack of association of the hrp2 cluster with lifestyles of diverse species. We gathered evidence for gene flow among co-occurring pathogenic and nonpathogenic strains, suggesting the potential of nonpathogenic strains to act as a reservoir of adaptive traits for pathogenic strains and vice versa. We further identified traits enriched in nonpathogens that suggest a strategy of stress tolerance, rather than avoidance, during their association with a broad range of host plants.

ecology↗

bioRxiv2019

Recently emerged and diverse lineages of Xanthomonas perforans have independently evolved through plasmid acquisition and homologous recombination originating from multiple Xanthomonas species

Xanthomonas perforans is the predominant pathogen responsible for bacterial leaf spot of tomato and X. euvesicatoria of pepper in the southeast United States. Previous studies have indicated significant changes in the X. perforans population collected from Florida tomato fields over the span of two decades including a shift in race, diversification into three genetic groups, and host range expansion to pepper. Recombination originating from X. euvesicatoria was identified as the primary factor driving the diversification of X. perforans in Florida. The aim of this study was to genetically characterize X. perforans strains that were isolated from tomato and pepper plants grown in Alabama and compare them to the previously published genomes available from GenBank. Surprisingly, a maximum likelihood phylogeny coupled with a Bayesian analysis of population structure revealed the presence of two novel genetic groups in Alabama, which each harbored a different transcription activation-like effector (TALE). While one TALE, avrHah1, was associated with adaptation of X. perforans to pepper, the other was identified as a new class within the avrBs3 family, designated here as pthXp1. Examination of patterns of homologous recombination between X. perforans and other closely related Xanthomonas spp. indicated that the lineages identified here emerged in part through recent recombination events originating from xanthomonads associated with diverse hosts of isolation. Our results also suggest that the evolution of pathogenicity to pepper has likely emerged independently within X. perforans and in one lineage, was associated with the recombination-mediated remodeling of the Xps type II secretion and TonB transduction systems.\n\nImportanceThe emergence of novel pathogen lineages has important implications in the sustainability of genetic resistance as a disease management tool in agricultural ecosystems. In this study, we identified two novel lineages of X. perforans in Alabama. While one lineage was isolated from symptomatic pepper plants, confirming the host range expansion of X. perforans, the other lineage was isolated from tomato and acquired a novel transcription activation-like effector, pthXp1. Unlike AvrBs4, PthXp1overcomes Bs4-mediated resistance in tomato, indicating the evolution of this novel lineage towards fitness on this host. Our findings also show that different phylogenetic groups of the pathogen have experienced independent recombination events originating from multiple Xanthomonas species. This suggests a continuous gene flux between related xanthomonads associated with diverse plant hosts which results in the emergence of novel pathogen lineages and associated phenotypes, including host range expansion.

genomics↗