1. A second set of polymers that have been shown to affect oxidative

stress tolerance in Pseudomonas are polyesters, such as poly(3-hydroxyalkanoate) (PHA). PHAs are accumulated as discrete granules and are believed to play a role in carbon storage PI3K Inhibitor Library mouse and stress tolerance (Madison & Huisman, 1999; Castro-Sowinski et al., 2010). Pseudomonads do not generally produce the most widely studied PHA, poly(3-hydroxybutryate) but do produce a variety of medium chain length PHAs (Huisman et al., 1989; Kessler & Palleroni, 2000). PHA synthesis has been shown to enhance the tolerance of pseudomonads to a range of different stresses, including cold and oxidative stress (Ayub et al., 2009; Castro-Sowinski et al., 2010), although the molecular mechanisms underpinning the positive association between PHA accumulation and oxidative stress tolerance are not yet fully understood. Thus far, this review has focussed on the concept of bacteria defending themselves against the plant host’s ROS production. However, pathogenic pseudomonads are also capable of utilizing ROS for their own

ends. For example, several pathovars of P. syringae produce a phytotoxin known as coronatine, which is known to be necessary for full virulence of this pathogen (Bender et al., 1987; Uppalapati et al., 2008). Coronatine has a number of functions in planta, including acting as a mimic of the plant hormone methyl jasmonate to antagonistically suppress salicylate-based defences (Zhao et al., 2003). It is also known Selumetinib manufacturer to be involved in symptom development, causing a chlorotic halo around the infection site, owing to a loss of chlorophyll a and b contents (Ishiga et al., 2009). Loss of chlorophyll is correlated with a large reduction in the efficiency of photosytem II, owing to a coronatine-induced downregulation of genes involved in chlorophyll synthesis, photosystem proteins, oxygen-evolving ifoxetine complex proteins and the Calvin cycle, as well as the induction of chlorophyllase (Ishiga et al., 2008). It has recently been found that this loss of photosynthetic ability is associated with the light-dependent generation

of ROS and downregulation of thylakoid Cu-Zn SOD activity. This ROS generation appears to be necessary for the development of the necrotic lesions that characterize the bacterial speck disease caused by this pathogen (Ishiga et al., 2008). In conjunction with this, coronatine induces many genes involved in tomato ROS homeostasis and suppresses SOD at the thylakoids, increasing the amount of ROS accumulated (Uppalapati et al., 2008). Meanwhile, coronatine upregulates SOD in the cytosol, probably reducing the pathogen’s own exposure to ROS (Ishiga et al., 2008). Similarly, both coronatine-producing and nonproducing strains of P. syringae have been shown to induce production of the plant hormone ABA and to increase plant sensitivity to ABA (de Torres-Zabala et al., 2007; Goel et al., 2008; Rico et al., 2010).