Objective This study aimed to elucidate the pathogenic function of Fe-Mn transporter SitABCD-encoding genes (sitA, sitB, sitC, sitD) in Lonsdalea populi.

Method Based on the genome of L. populi N-5-1 strain, the genes encoding the components of SitABCD system were identified, and for each of the 4 genes (sitA, sitB, sitC and sitD), its knockout mutants, complemented strains and overexpression strains were constructed via homologous recombination. Phenotypes including pathogenicity on stems of Populus × euramericana ‘107’, growth rate, swimming motility, biofilm formation and resistance to oxidative stress were analyzed and compared with N-5-1. RT-qPCR was employed to quantify transcript levels of oxidative-stress defense genes sodA, ahpC and katA in overexpression strains.

Result The corresponding knockout mutants, complemented strains and overexpression strains for each of the 4 genes (sitA, sitB, sitC and sitD) were successfully generated. No significant phenotypic differences were observed between deletion mutants and N-5-1 with respect to pathogenicity, growth, motility, biofilm formation or oxidative-stress tolerance. In contrast, the overexpression strains exhibited marked changes: virulence on stems of Populus × euramericana ‘107’ was significantly attenuated, growth under Fe-Mn-limiting conditions was impaired, biofilm biomass and oxidative-stress resistance were reduced, and transcript levels of sodA, ahpC and katA were down-regulated, whereas swimming motility remained unaffected.

Conclusion It was demonstrated that individual knockout of the genes encoding the SitABCD system in L. populi did not affect pathogenicity, but overexpression significantly impaired it, indicating that the precise expression of the SitABCD system is crucial for the pathogenesis of this bacterium. This study provides new insight into the pathogenic mechanism of Lonsdalea populi.