Reactive Oxygen Species, Oxidative stress and ROS scavenging system in plants

Formation of small and highly reactive oxygen species (ROS) is normal phenomena in living cells. The Reactive Oxygen Species cover free radical such as superoxide (O2 ̅ ), hydroxyl (OH), perhydroxy (HO2 ̅ ) and alkoxy (RO) and non-radicals like, hydrogen peroxide (H2O2) and singlet oxygen ( O2). Their formation and reactivity is well established and found to be tightly regulated. ROS level exceeding the antioxidative capacity of cells lead to oxidative stress which may result in malfunctioning and ultimately death of the cell. Singlet oxygen, Superoxide, hydroxyl and hydrogen peroxide tends to react easily with most biomolecules of the cell, causing their degradation and destruction, contributing to cellular stress. In plant cells, environmental changes and developmental transitions such as seed germination undergoes ROS generation. To avert the oxidative stress and excess of ROS, plant cells are equipped with antioxidative machinery comprised of both enzymatic and non-enzymatic compounds of low molecular weight. While Superoxide dismutase (SOD), Catalase (CAT), Ascorbate peroxidase (APX), Glutathione peroxidase (GPX), Glutathione-STransferase (GST), Monodehydroascorbate reductase (MDHAR) and Dehydroascorbate reductase (DHAR) forms the enzymatic part, Ascorbate, phenolic compounds, carotenoids and tocopherols contribute in the non-enzymatic arm of the antioxidative defense of the cell. The present review focuses upon the sources of ROS, their characteristics and contribution to oxidative stress and means of antioxidative machinery to deal with oxidative stress. KeywordsReactive Oxygen Species, Oxidative stress, enzymatic antioxidants, non-enzymatic antioxidants _____________________________________________________________________________________________