06-08-2013, 05:00 PM
Radical burst in plant defense responses to necrotrophic fungi
ABSTRACT
Plants have evolved an array of defensive tools to resist stresses including those from attack by pathogens. One of the earliest events upon recognition of a pathogen is the accumulation of reactive oxygen species (ROS), represented by hydrogen peroxide (H2O2), superoxide (O2−) and hydroxyl radical (OH). ROS are not only the important regulators of growth and development but are also involved in limiting pathogen ingress, induction of cell death and signal transduction of several defense responses. Biotrophs prefer living while necrotrophs prefer dead cells for nutritional purposes and therefore tissue necrosis caused by reactive oxygen species (ROS) during pathogen infection increases host susceptibility to necrotrophic but resistance to biotrophic pathogens. ROS serve as a second messenger in a systemic signaling network in plant immunity and leading to the expression of defence genes, phytoalexin production, and triggering of a hypersensitive response (HR). Avirulent pathogens often induce a biphasic ROS accumulation with a small first phase followed by a continuous phase of much higher intensity. Nitric acid (NO) signaling also participates in defense responses against necrotrophic pathogens in a manner other than the antioxidant of ROS. A variety of enzyme systems have been implicated in ROS generation following pathogen recognition viz., (NADPH) oxidase and superoxide dismutase. Salicylic acid, Jasmonic acid and ethylene can influence induction of ROS and antioxidants thereby influencing resistance or susceptibility of plants to pathogens.