Reactive Oxygen Species and Antioxidant Defence System in Plant Stress Tolerance

Reactive oxygen species (ROS) are highly reactive free radicals and molecules involving oxygen. ROS are produced by cellular respiration, photosynthesis and ROS-generating enzymes like NADPH-oxidase. The various types of stresses including water stress, temperature stress, metal stress and other factors also cause the production of ROS. The increase in ROS has damaging effect on DNA, protein, membrane stability and cellular homeostasis. The level of ROS is maintained by the activity of enzymes and molecules of antioxidant defence system. The antioxidant enzymes include ascorbate peroxidase (APX), glutathione peroxidase (GPX), catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR) and dehydroascorbate reductase (DHAR) whereas antioxidant molecules are ascorbic acid (AA), carotenoids, glutathione (GSH), polyamines and amino acids, α-tocopherol and phenolic compounds. Polyphosphate, other than the antioxidants, also defends against the oxidative stress and lowers the damage caused by ROS.