Vellosillo T, Vicente J, Kulasekaran S, Hamberg M & Castresana C.
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.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). 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.
Oxidative burst involving production of reactive oxygen species (ROS) is a nearly ubiquitous response of plants to pathogen attack and has a key role in both MTI and ETI signaling. ROS activation is likely a primary consequence of the damage produced during the course of infection. However, whereas overaccumulation of ROS might enhance plant susceptibility or cause an uncontrolled defense with spreading cell death lesions that can kill the plant, a tight regulation over ROS production and elimination through enzymatic and nonenzymatic antioxidants has allowed plants to use these reactive compounds as a critical feature of MTI and ETI. Reported defense responses associated with the production of ROS include direct killing of pathogens, activation of host cell death (HR), and contribution to cell wall strengthening. Moreover, emerging data highlight the role of ROS as signals in MTI and ETI as well as their contribution to provide an appropriate redox environment needed to activate defense. The importance of ROS in plant defense will be discussed here with a focus on the production of distinct types of ROS and on the cellular compartments involved in their production.