Oxidative Stress and Antioxidative Defense System in Plants Growing under Abiotic Stresses

Due to their sessile lifestyle, plants are exposed to drastic environmental conditions. The production of reactive oxygen species (ROS), an unavoidable product of aerobic metabolism, increases in plant cells facing stressful environmental conditions. ROS include highly reactive free radicals and non-radical molecules, e.g., superoxide anion (O₂ ^•−), hydroxyl radical (^•OH), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂). The reduction of molecular oxygen (O₂) in stepwise fashion or exposure to high energy causes the production of ROS in several organelles of plant cells (Figure 7.1). During abiotic stresses, ROS are produced due to the disruption of various metabolic pathways (metabolic ROS), such as respiration and photosynthesis, and as part of the abiotic stress-response signal transduction network in plants (Suzuki et al., 2012; Choudhury et al., 2016). Metabolic ROS and signaling ROS can directly modify the redox state and hence, the function of regulatory proteins, and alter transcription and translation (Miller et al., 2010; Foyer and Noctor, 2016). Signaling ROS have been implicated in reducing the level of metabolic ROS by activating an acclimation response that reduces the effect of stresses on metabolic pathways. At low concentrations, ROS serve as important regulators of growth and defense pathways (Kärkönen, 2015). ROS also play an important role in lignification and other cross-linking processes in the cell wall (Bradley et al., 1992; Ogawa et al., 1997; 94 Desikin et al., 2001; Mittler, 2002). The production and signaling of ROS in association with the action of brassinosteroids, auxin, gibberellins, ethylene, abscisic acid, strigolactones, jasmonic acid, and salicylic acid (SA) have been shown to play an important role in the coordinated regulation of growth and stress tolerance. Multiple points of reciprocal control and integration nodes involving Ca²⁺-dependent processes and mitogen-activated protein kinase phosphorylation cascades have been identified in both local and systemic cross-talk of ROS and hormone signaling pathways (Xia et al., 2015). Figure 7.1 Formation of reactive oxygen species in plants. Activation of O₂ occurs by two different mechanisms. Stepwise monovalent reduction of O₂ leads to the formation of O₂ ^•−, H₂O₂, and ^•OH, whereas energy transfer to O₂ leads to the formation of ¹O₂. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781351104609/8c6b5c1f-baa4-4b36-8a42-cd1ff505ae8c/content/fig7_1_B.tif"/>