OxyR, a regulator of the hydrogen peroxide stress response in Ralstonia solanacearum is necessary for full virulence on tomato plants. Z. FLORES-CRUZ (1), C. Allen (1)
(1) University of Wisconsin-Madison, Madison, WI, USA
Phytopathology 98:S54

Abstract: In response to pathogen attack, plants produce an oxidative burst of reactive oxygen species (ROS) that signal defenses and may have antimicrobial effects. However, little is known of the effect of oxidative stress on pathogens. The bacterial wilt pathogen Ralstonia solanacearum expresses likely oxidative stress response genes during tomato pathogenesis. Analysis of the available genomes of R. solanacearum suggests this bacterium is exposed to oxidative stress since each encodes at least 16 predicted ROS detoxification enzymes. We hypothesize that the oxidative stress response of R. solanacearum contributes to pathogen survival in the host and to disease development. To test this we created an oxyR mutant in R. solanacearum; oxyR is a positive regulator of the hydrogen peroxide stress response. The oxyR mutant was catalase negative and did not grow well on plates unless catalase was added exogenously. Growth in the presence of hydrogen peroxide was significantly slower than the parental strain; however, the oxyR mutant grew faster than wild-type in the presence of the superoxide generator Paraquat. In addition, the oxyR mutant was significantly reduced in virulence on tomato plants in a naturalistic soil soak assay. These results support the hypothesis that plant ROS have direct antimicrobial effects, and indicate that R. solanacearum confronts a stressful oxidative environment during growth inside compatible hosts.