Thermoresponsive Phytotoxin Production in Plant Pathogenic Bacteria

Abstract

Besides an intensively studied type III secretory system, the Hrp (hypersensitive response and pathogenicity) system, which delivers so-called avirulence gene products to the host cell causing pathogenicity, virulence factors such as phytotoxins, exopolysaccharides, plant hormones, and extracellular enzymes significantly contribute to the disease development in pathogenic plant-microbe interactions (Alfano and Collmer, 1996). It remains to be determined how and under which environmental conditions most of these virulence factors affect the disease outcome and lead to an increased fitness of the producing pathogens inside the plant. Opportunistic plant pathogens like Pseudomonas syringae preferably infect their host plants under conditions of high humidity and low temperature. Plants, as poikilothermic organisms do not maintain a given temperature but rather adjust to the environmental temperature regime they are exposed to. This complicates a direct comparison of temperature-dependent processes in plant pathogens with the well-studied thermoregulation of virulence factors in human and animal pathogens (Hurme and Rhen, 1998). While temperatures of 37–41°C signal to animal pathogens their arrival inside the warm-blooded host, numerous virulence factors of phytopathogens like Erwinia amylovora, Agrobacterium tumefaciens, E. chrysanthemi, and P. syringae are preferentially expressed, secreted, or assembled at lower temperatures, such as 18–22°C. It is therefore intriguing to assume that general principles govern thermo-adaptation of plant pathogens that are clearly different form those regulatory circuits and cascades in human and animal pathogens. Little is known about those general principles. Therefore, thermoresponsive synthesis of phytotoxins in P. syringae represents an excellent model to study them.