Going to the Basics: Comparative Assessment of Onion Virulence in Diverse <i>Pantoea</i> Species

Center rot of onion is caused by diverse <i>Pantoea</i> species, including <i>P. ananatis, P. agglomerans, P. allii,</i> and <i>P. stewartii</i> ssp. <i>indologenes</i>. While the roles of swimming motility, exopolysaccharide (EPS) production, and quorum sensing (QS) have been explored in <i>P. ananatis</i> in the context of onion disease, those strains were not onion-derived; similar studies in onion-derived <i>Pantoea</i> strains, particularly in <i>P. agglomerans</i> and <i>P. stewartii</i> ssp. <i>indologenes</i> are lacking. In this study, we examined the roles of QS (<i>luxI, luxR</i>), EPS production (<i>epsG</i>), and swimming motility (<i>motB</i>) genes on the virulence of strains from three <i>Pantoea</i> species: <i>P. ananatis</i> PNA 97-1^R, <i>P. agglomerans</i> AR1a^R, and <i>P. stewartii</i> subsp. <i>indologenes</i> PNA 03-3^R, through gene deletions and virulence assays. Mutant characterization confirmed that <i>motB</i> was required for swimming motility, <i>epsG</i> for EPS production, and <i>luxI</i> for acyl-homoserine lactone production in strains from all three <i>Pantoea</i> species. Deletion of <i>motB</i> or <i>epsG</i> did not affect symptom development by any strains, indicating that motility and EPS production were dispensable for onion infection, whereas <i>luxI</i> was essential for virulence in <i>P. ananatis</i> PNA 97-1^R and <i>P. stewartii</i> subsp. <i>indologenes</i> PNA 03-3^R, but not in <i>P. agglomerans</i> AR1a^R, which retained complete virulence despite the <i>luxI</i> deletion. In <i>P. ananatis</i>, while Δ<i>luxI</i> produced no symptoms in onion, Δ<i>luxR</i> and Δ<i>luxIR</i> were virulent, suggesting a regulatory role of <i>luxR</i> as well. These findings indicate that QS-mediated regulation of virulence varies among onion-associated <i>Pantoea</i> strains, whereas motility and EPS production were dispensable for pathogenicity in the strains evaluated.