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Evidence Against a Direct Antimicrobial Role of H₂O₂ in the Infection of Plants by Erwinia chrysanthemi

We have investigated the role of bacterial resistance to oxidative stress in pathogenesis. The oxyR gene from the pathogenic bacterium Erwinia chrysanthemi has been characterized. It is closely related to that found in Es-cherichia coli (88% overall amino acid identity). An E. chrysanthemi oxyR mutant strain was constructed by marker exchange. After induction with a sublethal dose of H₂O₂, this mutant was more sensitive to H₂O₂ and showed reduced levels of catalase and glutathione reductase activities, compared with the wild type. The oxyR mutant was unable to form individual colonies on agar plates unless catalase was added exogenously. However, it retained full virulence in potato tubers and tobacco leaves. These results suggest that the host-produced H₂O₂ has no direct antimicrobial effect on the interaction of E. chrysanthemi with the two plant species.