Going to the Basics: Comparative Assessment of Onion Virulence in Diverse Pantoea Species

Center rot of onion is caused by diverse Pantoea species, including P. ananatis, P. agglomerans, P. allii, and P. stewartii subsp. indologenes. Although swimming motility, exopolysaccharide (EPS) production, and quorum sensing (QS) have been examined in Pantoea ananatis in relation to onion disease, these studies were conducted using non-onion-derived strains. Consequently, the roles of these traits in onion-derived Pantoea strains, particularly P. agglomerans and P. stewartii subsp. indologenes, remain largely unexplored. In this study, we examined the roles of QS (luxI, luxR), EPS production (epsG), and swimming motility (motB) genes on the virulence of strains from three Pantoea species-P. ananatis PNA 97-1^R, P. agglomerans AR1a^R, and P. stewartii subsp. indologenes PNA 03-3^R-through gene deletions and virulence assays. Mutant characterization confirmed that motB was required for swimming motility, epsG for EPS production, and luxI for acyl-homoserine lactone production in strains from all three Pantoea species. Deletion of motB or epsG did not affect symptom development in any of the strains, indicating that motility and EPS production were dispensable for onion infection, whereas luxI was essential for virulence in P. ananatis PNA 97-1^R and P. stewartii subsp. indologenes PNA 03-3^R, but not in P. agglomerans AR1a^R, which retained complete virulence despite the luxI deletion. In P. ananatis, although ΔluxI produced no symptoms in onion, ΔluxR and ΔluxIR were virulent, suggesting a regulatory role of luxR as well. These findings indicate that QS-mediated regulation of virulence varies among onion-associated Pantoea strains, whereas motility and EPS production were dispensable for pathogenicity in the strains evaluated.