Catalase Activity Contributes to the High Virulence of the Erwinia amylovora TS3128 Strain

One of the earlier responses among plant defenses against phytopathogens is an oxidative burst that rapidly produces reactive oxygen species (ROS). Erwinia amylovora, the causal agent of fire blight in rosaceous plants, induces ROS in host immunity and uses it to invade plant tissues. We hypothesized that the detoxification capacity of H₂O₂ among ROS would vary among E. amylovora strains, affecting their survival and virulence in/on host plants. We identified E. amylovora TS3128 strain as having the highest minimum inhibitory concentration by catalase activity compared with other strains (TS3315, ATCC15580, and LMG2068). This high catalase activity of TS3128 was correlated with the detoxification of exogenous H₂O₂ by high expression levels of catalase genes katA and katG. However, the katA gene was observed to play a major role, as demonstrated by mutants lacking each gene. Finally, this process was confirmed to be related to the higher virulence of this strain compared with other strains in an immature apple fruit assay. We also confirmed that the high virulence of TS3128 was due to higher expression levels of catalase genes, the type III secretion system apparatus, and its effector gene under oxidative stress conditions. Our results reveal significant differences in catalase activity among the E. amylovora strains, correlating with their virulence levels through well-known virulence factors, amylovoran, and type III secretion system effector proteins in E. amylovora.