Fig. 1.

Schematic representation of ROS generation in different organelles of plant cell during abiotic and biotic stress, its scavenging through enzymatic and non-enzymatic pathways to pursue oxidative stress tolerance. ROS generation is inevitable by-products of aerobic cell metabolism and dramatically enhance in adverse environmental conditions. Apoplastic ROS is generated through cell membrane-bound NADPH oxidase, cell wall-associated peroxidases and fenton reaction. Singlet oxygen (¹O₂) and hydrogen peroxide (H₂O₂) are produced in high light intensity as well as during photosynthesis and photorespiration in chloroplast. Mitochondrial ETC generates superoxide anion (O^•−₂) in response to stress, which is ultimately converted to H₂O₂ by superoxide dismutase (SOD). In peroxisomes, H₂O₂ is accumulated during glyoxalate synthesis and SOD-mediated dismutation of O^•−₂, produced by Xanthine oxidase (XO). O^•−₂ is dismutated into the H₂O₂ through SOD enzyme. H₂O₂ is catalysed by various enzymes such as catalase (CAT), peroxiredoxins (PRX), ascorbate peroxidase (APx), glutathione peroxidase (GPx) and dehydroascorbate reductase (DHAR). Hydroxyl radicle (OH^•) is reduced non-enzymatically through lipid peroxidation and sugar oxidation. The ¹O₂ is also non-enzymatically processed by donating electron to β-carotene, tocopherol and plastoquinone and confer oxidative stress tolerance. Black arrow (→) indicates ROS generation; Red arrow (→) indicates ROS scavenging