Genome-Wide Association to Study the Host-Specificity Determinants of Xanthomonas perforans.

Abstract

Xanthomonas perforans and X. euvesicatoria are the causal agents of bacterial spot disease of tomato and pepper, endemic to the Southeastern United States. Although very closely related, the two bacterial species differ in host-specificity, where X. perforans is the dominant pathogen of tomato and X. euvesicatoria of pepper. This is in part due to the activity of avirulence proteins that are secreted by X. perforans strains and elicit an effector-triggered immunity (ETI) in pepper leaves, thereby restricting pathogen growth. In recent years, the emergence of several pepper-pathogenic X. perforans lineages has revealed variability within the bacterial species to multiply and cause disease in pepper, even in the absence of avirulence gene activity. Here, we investigated the basal evolutionary processes underlying the host-range of this species using multiple genome-wide association analyses. Surprisingly, we identified two novel gene-candidates that were significantly associated with pepper-pathogenic X. perforans and X. euvesicatoria. Both candidates were predicted to be involved in the transport/acquisition of nutrients common to plant-cell wall or apoplast and included a TonB-dependent receptor, which was disrupted through independent mutations within the X. perforans lineage. The other included a symporter of protons/glutamate, gltP, enriched with pepper-associated mutations near the promoter and start codon of the gene. Functional analysis of these candidates revealed that only the TonB-dependent receptor had a minor effect on the symptom development and growth of X. perforans in pepper leaves, indicating that pathogenicity to this host may have evolved independently within the bacterial species and is likely a complex, multigenic trait.