Nevertheless, other types of SOD have been shown to be important in some plant–pathogen interactions, as the soft-rot pathogen Dickeya dadantii (Erwinia chrysanthemi) 3937 has been shown to require Mn-SOD activity for the successful maceration of Saintpaulia ionantha leaves, although interestingly the Mn-SOD mutant retained the ability

to macerate potato tubers (Santos et al., 2001). It seems likely that the relative importance of different antioxidant enzymes varies according to environmental factors such as pH and metal ion availability. Possession of multiple antioxidant enzymes that vary in terms of substrate, cofactor and optimal environmental conditions enables plant pathogenic Pseudomonas to colonize a range of different environments and to adapt to the changing environment present in healthy and diseased plant tissue. One environment that is less frequently considered in the context of plant pathogenesis is the environment encountered buy PD0325901 during

dispersal. P. syringae and related pathogens are commonly dispersed in aerosols, which carry an inherent risk of dessication and subsequent accumulation of ROS within the cell (Cox, 1989). By demonstrating that exogenous catalase can significantly enhance the ‘resuscitation’ HM781-36B mouse of airborne bacteria cells, including P. syringae cells, Marthi et al. (1991) have shown that antioxidant enzymes are likely to be important not only during pathogenesis many but also during the dispersal of pathogenic bacteria. Another

important factor in a bacterial pathogen’s ability to withstand the oxidative burst is its coating of extracellular polysaccharides (EPS), which act to protect the bacterium against oxidative stress. Examples of EPS found in Pseudomonas species include alginate and levan (Fett & Dunn, 1989; Fett et al., 1989; Chang et al., 2007). EPS can be very complex and can differ greatly between related pathogens, which may be related to their role in bacteria–host interactions, and the pathogen’s need to escape detection (de Pinto et al., 2003; Silipo et al., 2010). In P. syringae pv. syringae, EPS has been shown to have a role in leaf colonization and symptom development (Yu et al., 1999); the EPS of P. syringae and P. aeruginosa are known to be upregulated by exposure to ROS (Keith & Bender, 1999). Keith et al. (2003) studied the expression of the algD gene, involved in alginate production, in planta, and found evidence that this gene is upregulated in response to ROS produced by the plant and that this induction of alginate production occurs in both compatible and incompatible plant–pathogen interactions (Keith et al., 2003). In P. syringae pv. syringae B728a, EPS production has been shown to be regulated via quorum sensing (Quiñones et al., 2005). Mutants impaired in quorum sensing lack alginate and have increased sensitivity to ROS, providing further evidence for the importance of EPS in withstanding oxidative stress (Quiñones et al., 2005).