An introduction to antioxidants and their roles in plant stress tolerance

Abstract

Various abiotic stresses lead to the formation of reactive oxygen species (ROS) in plants which are highly reactive and toxic in plant cell. The ROS comprises both free radical (superoxide radicals, O2-; hydroxyl radical, OH-; perhydroxyl radical, HO2-; and alkoxy radicals, RO-) and non-radical (molecular) forms such as hydrogen peroxide (H2O2) and singlet oxygen (O2). Chloroplasts and mitochondria are the major sites for the generation of O2-. Plant's abiotic stress tolerance requires a number of physiological and biochemical mechanisms which includes enzymatic (superoxide dismutase, SOD; catalase, CAT; ascorbate peroxidase, APX; glutathione reductase, GR; monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR; glutathione peroxidase, GPX; guaiacol peroxidase, GPOX; and glutathione-S-transferase, GST) and non-enzymatic (ascorbic acid, ASH; glutathione, GSH; phenolic compounds; alkaloids; flavonoids; carotenoids; non-protein amino acids; and a-tocopherols). Increased environmental stresses imbalance the production of reactive oxygen species and thereby quench the activity of antioxidants and thus resulting in oxidative damage. ROS can cause damage to cell structures, nucleic acids, lipids and proteins. Certain ROS like OH- ions are said to react with all components of DNA and damage the purines and pyrimidines. The increased production of antioxidants thus helps the plant to withstand the environmental stress. This chapter focuses on the description of antioxidant defence system under abiotic stress in plants and its involvement in the removal of reactive oxygen species. Hence various types of antioxidants, their types and role will be discussed in detail.