Emerging Complexity in Reactive Oxygen Species Production and Signaling during the Response of Plants to Pathogens

Plants have evolved a complex immune system to perceive microbial pathogens and respond by producing defense compounds preventing infection. Defense hormones such as salicylic acid (SA), jasmonates, and ethylene are key signals regulating the production of antimicrobial defenses. Moreover, other hormone pathways have critical actions by controlling responses to pathogen attack such as distribution of resources, cell death, water stress, or plant architecture. A fine-tuning regulation of these pathways through complex regulatory networks is necessary to achieve resistance against different pathogen classes (López et al., 2008; Grant and Jones, 2009). Plants activate two forms of immunity by recognition of distinct pathogen molecules. A first and rapid response, known as basal resistance (microbe-associated molecular pattern-triggered immunity or MTI), is triggered after recognition of conserved microbial molecules (microbe-associated molecular patterns) by extracellular plant receptors (pattern recognition receptors; Boller and Felix, 2009). Second, effector-triggered immunity (ETI) is activated by resistance (R)-gene products (largely inside the cell) after recognition of specific effectors molecules (delivered into the plant cell by pathogens) and is commonly accompanied by a hypersensitive reaction (HR) involving localized host cell death at the point of infection (Jones and Dangl, 2006).