Heavy metal stress and cellular antioxidant systems of plants: A review

Heavy metal (HM) stress is one of the most important abiotic stresses affecting flora and fauna worldwide due to a rapid global increase in the urbanization and industrialization and the consequent rise of HM concentration in the atmosphere. Typically, plants have distinct mechanisms to cope up with HM stress. These mechanisms rendering tolerance to the plants by detoxifying the HMs. Additionally, a number of physiological and molecular changes occur in plant cells due to their exposure to HMs. This culminates in the generation of reactive oxygen species (ROS) that cause oxidative stress in plants, which significantly affects plant metabolism and disrupts normal vital cellular functions. However, in order to cope with such stresses, plants possess a strong antioxidant defence system to counteract increases in the ROS-induced stress. The enzymatic components of this system include superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR). The non-enzymatic components are ascorbate, glutathione and phenolic compounds along with lipid-soluble molecules such as carotenoids and tocopherols. This review makes an effort to collect and collate the currently available information on metal stress and cellular antioxidant systems of plants, emphasizing upon the role of enzymatic and non-enzymatic mechanisms for detoxification of HM-induced oxidative stress.