results in damage to DNA, membranes and proteins, an


results in damage to DNA, membranes and proteins, and induction of oxidative stress responses. Bacteria impaired in the ability to tolerate oxidative stress show increased sensitivity to these antibiotics. Similarly, Bizzini et al. (2009) have shown that superoxide dismutase (SOD) mutants of Enterococcus faecalis show increased sensitivity to β-lactams and glycopeptides; selleck compound Gusarov et al. (2009) have shown that SOD mutants of Bacillus subtilis are more sensitive to the Pseudomonas aeruginosa toxin pyocyanin. Gusarov et al. also show that amelioration of oxidative stress in B. subtilis by nitric oxide alleviates antimicrobial activity. ROS tolerance may therefore play a key role not only in pathogen resistance to plant-derived ROS but also in resistance to plant-derived antimicrobial chemicals and other chemical stressors encountered in the plant environment, such as antibiotics produced by plant-associated bacteria and fungi. Thus, the ability to tolerate elevated levels of ROS is likely to be important for all plant pathogenic pseudomonads. As ROS are a common feature of plant defences and bacterial cell death mechanisms, it is likely to be advantageous for any pathogen to be able to resist their effects. Mechanisms for resistance to toxins generally fall into four main categories: exclusion, export, modifications to the Inhibitor Library target site of the

toxin, and enzymic or chemical inactivation of the toxin (Duffy, 2003; Mergeay et al., 2003). In the case of ROS, regulation of the uptake and sequestration of metal ions, particularly Fe(II), can also have a substantial effect on ROS tolerance, as Fe(II) participates in the Fenton reaction that generates the destructive hydroxyl radical (Cornelis et al., 2011). Mutation

of specific Cell press residues, particularly cysteine residues, can affect the sensitivity or regulatory responses of individual proteins to ROS (e.g. Panmanee et al., 2006; Chen et al., 2006, 2008). However, in general, target site modifications and export mechanisms are likely to provide relatively little protection against high concentrations of ROS, which are not specific to a particular target site, but are able to react with numerous sites in proteins, as well as damaging other cellular components (Mehdy, 1994). Therefore, a common first line of defence is the use of antioxidant enzymes. Antioxidant enzymes known to be present in Pseudomonas include superoxide dismutase (SOD), an enzyme capable of producing hydrogen peroxide from the superoxide radical. Three types of SOD exist in bacteria, distinguished by their metal cofactors: Mn/Fe, Cu-Zn and Ni (Kim et al., 1999). Protection from hydrogen peroxide is provided by the hydrogen peroxide-degrading enzyme catalase and also peroxidases (Albert et al. 1986; Hasset & Cohen, 1989). Genome sequence analyses indicate that the plant pathogen P. syringae pv.

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