Antioxidant Protection Mechanism During Abiotic Stresses

Abstract

Abiotic stresses are the major constraints for plant growth and productivity. Adverse effects of abiotic stresses are primarily by the production of various reactive oxygen species (ROS). Reactive oxygen species are broadly categorized into superoxides (O2•−), hydroxyl radicals (OH•), alkoxyl radicals (RO•), peroxyl radicals (ROO•), singlet oxygen (1O2), lipid hydroperoxides (ROOH), hydrogen peroxides (H2O2), and excited carbonyls (RO*). In plants the major sites of ROS production are peroxisomes and chloroplast. Other sites of ROS production are apoplast, cell membrane, endoplasm reticulum, and cell wall. Production of ROS in excess results in lipid peroxidation of membranes, protein degradation, pigment bleaching, and damage to nucleic acids ultimately resulting in cell death. Plants are protected against oxidative stress by enzymatic and non-enzymatic antioxidant mechanisms. Enzymatic antioxidant system include enzymes such as superoxide dismutase (SOD), peroxidase, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), catalase (CAT), glutathione reductase (GR), ascorbate peroxidase (APX), glutathione peroxidase (GPX), guaiacol peroxidase (GPOX), glutathione S-transferase (GST), and polyphenol oxidase (PPO), whereas non-enzymatic antioxidants include ascorbic acid (AA), α-tocopherol, reduced glutathione (GSH), carotenoids, phenolic acids, anthocyanins, flavonoids, and proline. In addition to creating oxidative stress, ROS also act as signalling molecules leading to the expression of many genes involved in stress tolerance, control of cell cycle, cell elongation, osmolyte accumulation, and other systemic responses. The present review covers the biochemistry of ROS synthesis, their scavenging mechanisms, types of ROS, effect of abiotic stresses on their production, and their impact on horticultural crops.