Unraveling the genomic complexity of secretion systems in the most virulent Xanthomonas arboricola pathovars

Xanthomonas arboricola pathovars pruni (Xap), juglandis (Xaj), and corylina (Xac) are phytopathogenic bacteria that infect Prunus spp., walnut, and hazelnut trees, respectively. In this study, the understanding of the differences among these pathovars was improved with the aim of elucidating their host range and uncovering distinct virulence mechanisms. A comparative genomic analysis was conducted focusing on secretion system clusters across high-quality genomes from two strains of each pathovar. The results revealed that the RaxABC type I secretion system was absent in all analyzed strains. However, the HlyDB type I secretion system was present in both Xap and Xac, with a putative HlyDB effector identified in each Xac strain. Additionally, Xap strains contained a putative PctAB type I secretion system, while only one of the Xac harbored a putative PctAB. Notably, the genomic region surrounding pctA and pctB lacked pctP, suggesting the presence of a novel type I secretion system rather than the canonical PctAB. In contrast, Xaj lacked all the studied type I secretion systems. While the core components of type II and type III secretion systems were highly conserved across strains, significant variation was observed in their substrates. Interestingly, only Xap carried two pathovar-specific type III effectors. Regarding type V secretion systems, complete homologs of EstA, YapH, and XadA were found in all strains, except for one Xac strain, which contained a frameshifted YapH. Additionally, homologs of the XacFhaB/XacFhaC system were found in both Xap strains. However, both Xaj strains and one Xac strain carried an incomplete XacFhaB subunit, while the other Xac strain lacked this system entirely. Finally, analysis of the genomic regions surrounding these secretion system clusters strongly suggests that horizontal gene transfer has played a crucial role in their acquisition, likely contributing to the diversification, emergence and specialization of distinct X. arboricola pathovars.