Necessity of OxyR for the Hydrogen Peroxide Stress Response and Full Virulence in Ralstonia solanacearum

Abstract

The plant pathogen Ralstonia solanacearum, which causes bacterial wilt disease, is exposed to reactive oxygen species (ROS) during tomato infection and expresses diverse oxidative stress response (OSR) genes during midstage disease on tomato. The R. solanacearum genome predicts that the bacterium produces multiple and redundant ROS-scavenging enzymes but only one known oxidative stress response regulator, OxyR. An R. solanacearum oxyR mutant had no detectable catalase activity, did not grow in the presence of 250 M hydrogen peroxide, and grew poorly in the oxidative environment of solid rich media. This phenotype was rescued by the addition of exogenous catalase, suggesting that oxyR is essential for the hydrogen peroxide stress response. Unexpectedly, the oxyR mutant strain grew better than the wild type in the presence of the superoxide generator paraquat. Gene expression studies indicated that katE, kaG, ahpC1, grxC, and oxyR itself were each differentially expressed in the oxyR mutant background and in response to hydrogen peroxide, suggesting that oxyR is necessary for hydrogen peroxideinducible gene expression. Additional OSR genes were differentially regulated in response to hydrogen peroxide alone. The virulence of the oxyR mutant strain was significantly reduced in both tomato and tobacco host plants, demonstrating that R. solanacearum is exposed to inhibitory concentrations of ROS in planta and that OxyRmediated responses to ROS during plant pathogenesis are important for R. solanacearum host adaptation and virulence.Plants produce reactive oxygen species (ROS) in response to invading pathogens, and an effective oxidative stress response (OSR) contributes to the fitness of phytopathogenic bacteria (18,39,40,51). However, research on the role of regulators of the OSR in the virulence of plant-pathogenic bacteria is limited and contradictory. In Agrobacterium tumefaciens, oxyR is necessary for tumorigenesis (34), but Erwinia chrysanthemi does not need oxyR for soft-rot virulence (32). OxyR is a redox-sensing LysR family transcriptional regulator that has been well characterized in several bacteria (23,43,52). In the absence of oxidative stress, OxyR is reduced and acts as a repressor of several genes, including oxyR itself (47, 52). In the presence of hydrogen peroxide, the conserved cysteines of OxyR (C199 and C208, in Escherichia coli) form a disulfide bond that changes its conformation and converts OxyR into a transcriptional activator (52, 53). In E. coli, OxyRregulated genes include catalase (kat), alkyl hydroperoxide reductase (ahp), glutaredoxin (grx), and glutathione reductase (gor) (53).The plant-pathogenic bacterium Ralstonia solanacearum causes bacterial wilt disease on many economically important crops, including tomato (19). Multiple quantitative virulence factors contribute to the disease (11); however, little is known about how R. solanacearum adapts to its host environment, which is a critical prerequisite for pathogen success. An in vivo expression technology (IVET) screen performed on R...

Transcriptional regulation of bacterial virulence gene expression by molecular oxygen and nitric oxide

“…For example, OxyR has been shown to contribute to the virulence of Bacteroides fragilis, E. coli, Francisella novicida, K. pneumoniae, P. aeruginosa, Ralstonia solanacearum, X. campestris, and Y. pestis, but not Mycobacterium marinum or intestinal colonization by S. enterica. [119][120][121][122][123][124][125][126][127][128][129] In addition to its primary role in response to peroxide stress, OxyR is activated by nitrosative stress as a result of S-nitrosylation (Cys-199, S-NO); de-nitrosylation (Cys-199, SH) inactivates OxyR.…”

Loss of glutamate dehydrogenase in Ralstonia solanacearum alters dehydrogenase activity, extracellular polysaccharide production and bacterial virulence

“…While bacteria multiply in intercellular spaces and xylem which are nutrient poor environments [19], R. solanacearum adapts to the apoplastic environment to overcome the unfavorable conditions [6,11,12]. However, the overall process for R. solanacearum adaptation in the apoplastic environment remains unclear [14].…”

A Non-Classical LysR-Type Transcriptional Regulator PA2206 Is Required for an Effective Oxidative Stress Response in Pseudomonas aeruginosa

“…Mutation of oxyR in P. aeruginosa and other pathogens has been shown to result in an inability to grow on LB agar plates, possibly due to the autoproduction of concentrations as low as 1.2 µM H 2 O 2 per minute [39], [58]. In addition, micromolar concentrations of H 2 O 2 were sufficient to arrest the growth of oxyR mutants in several microbial species reinforcing its position as the master regulator of the oxidative stress response [59], [60]. Therefore, PA2206 may represent an important OxyR-independent regulatory link between perception of an exogenous stress and adaptation through realignment of lower metabolic pathways to conserve energy and facilitate survival.…”