Ecologically Significant Genetic Loci of P. allii : Contributions to Pathogenesis and Competition

Pantoea allii , one of four Pantoea species known to cause onion center rot, is infrequently isolated from onion compared to its closely related onion-pathogenic sister taxa. To better understand the genomic diversity and genetic determinants of pathogenicity in this species, we analyzed a collection of 38 P. allii strains isolated from two primary ecological niches, plants and water, across three continents using comparative genomics and phylogenetic approaches. Core-genome phylogeny, average nucleotide identity (ANI), and gene presence–absence analyses revealed three genetically distinct lineages. All strains harbored conserved biosynthetic gene clusters (BGCs) for quorum sensing, carotenoid production, siderophores, and thiopeptides. In contrast, two phosphonate BGCs, key determinants of onion pathogenicity, exhibited lineage-specific distributions. Onion-associated strains from Lineages 1 and 2 carried the Halophos BGC associated with onion tissue necrosis, and onion isolates encoded the alt gene cluster conferring thiosulfinate tolerance. Lineage 3 strains, isolated from both onion and rainwater, either lacked a phosphonate BGC loci or carried the HiVir phosphonate BGC. In addition, Lineage 3 strains lacked the alt cluster altogether. The localization of these virulence genes in the genome varied, with Halophos integrated in the chromosome, HiVir encoded on the conserved Large Pantoea Plasmid, and alt located on a small, variable plasmids (plasmid B). The Type IV and Type VI secretion systems showed variable genomic architectures, with plasmid-borne T4SSs and two chromosomal T6SS loci differing in conservation and gene content. Additionally, conserved Pantailocin phage islands were detected in most genomes. Overall, this study reveals that while core metabolic and competitive traits are conserved across P. allii , virulence-associated loci display lineage-specific partitioning, reflecting ecological differentiation and evolutionary plasticity within the species.