Pectobacterium aroidearum as a Causative Agent of Potato Soft Rot in China: Virulence Characterization and a Type 1 Fimbriae Cluster Contributing to Pathogenicity.
Soft rot Pectobacteriaceae (SRP) are destructive pathogens of potato crops, posing a global threat to food security. SRP populations exhibit significant genetic heterogeneity, with the prevalence of potato-infecting species shifting over time. In this study, we identified Pectobacterium aroidearum as a novel agent causing severe tuber soft rot in winter-planted potatoes in Guangdong Province, where it co-occurred with P. brasiliense and P. carotovorum. However, the pathogenic potential of P. aroidearum has been largely uncharacterized. Here, comparative virulence assays demonstrated that P. aroidearum is more aggressive than its counterparts. This heightened virulence was linked to enhanced pathogenic traits, including plant cell wall-degrading enzymes (PCWDEs) production, swimming motility, exopolysaccharide (EPS) production, and air-liquid (AL) biofilm formation. Genomic analysis revealed that while core virulence genes are conserved, a type 1 fimbriae-encoding fim cluster is unique to P. aroidearum. Deletion of fimA, fimC, fimD, and fimH impaired bacterial adhesion, EPS production, AL biofilm formation, and full virulence, functionally characterizing this distinctive factor. Genes within the bacterial cellulose synthesis operon were significantly down-regulated across fim mutants. Accordingly, the mutant colonies showed a marked decrease in calcofluor binding compared with the wild-type strain. Genes with significant up-regulation in the mutants were primarily involved in responses to extracellular stimuli. These findings indicate that the fim cluster underpins the threat posed by this emerging pathogen, which warrants increased attention in disease management. The considerable genetic diversity and frequent gene flow among P. aroidearum isolates from various hosts further highlight its epidemiological risk.