Stress Resistance in Pseudomonas syringae: Mechanisms and Strategies

Abstract

The growth and survival of Pseudomonas syringae in association with both the external and internal leaf environment is correlated with an ability to tolerate and/or escape environmental and plant-associated stresses. These stress factors include, most notably, desiccation, nutrient deprivation, exposure to ultraviolet radiation (UVR), and oxidative stress encountered both from exposure to UVA radiation and from the plant defence response. Information on particular physiological and genetic responses of P. syringae to the environmental stress associated with leaf colonisation has increased in recent years. The ability to survive on dry leaf surfaces is a critical ecological adaptation of phyllosphere microbes. Several P. syringae traits including exopolysaccharide production, motility, and even methionine biosynthesis, have been shown to increase survival under dry or alternating wet/dry conditions. The UV-B component of solar UVR causes direct DNA damage and cell death; thus, enzymatic mechanisms of DNA repair are essential for the survival of leaf surface-associated P. syringae. The rulAB determinant encodes a mutagenic DNA repair system that confers UVR tolerance to P. syringae and is required for enhanced survival on UVR-exposed leaf surfaces. rulAB is plasmid-encoded and widely distributed among P. syringae pathovars. The bacterial RpoS alternate sigma factor regulates genes in response to oxidative stress including genes encoding superoxide dismutase. Surprisingly, a P. s. pv. syringae B728a sodAsodB mutant exhibited increased UV-A sensitivity but no change in virulence in bean. Increasing knowledge of stress factors and the responses of P. syringae to stress will continue to foster a better understanding of P. syringae biology and the potential for disease management.