In this review, we will primarily delve deeper into the domains of ROS, the antioxidant machinery and how they synergistically counteract the effects of environmental stress. The omnipresent nature of both arms of the antioxidant machinery underlies the necessity of detoxification of ROS for cellular survival ( Gill et al., 2011). Redox homeostasis in plants during stressful conditions, is maintained by two arms of the antioxidant machinery-the enzymatic components comprising of the superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione-S-transferase (GST), and catalase (CAT), and the non-enzymatic low molecular compounds like ascorbic acid (AA), reduced glutathione (GSH), α-tocopherol, carotenoids, phenolics, flavonoids, and proline ( Gill and Tuteja, 2010 Miller et al., 2010 Gill et al., 2011). These are very lethal and causes extensive damage to protein, DNA and lipids and thereby affects normal cellular functioning ( Apel and Hirt, 2004 Foyer and Noctor, 2005). The ROS mainly comprise of 1O 2, H 2O 2, O Estimates show that only 1–2% of the O 2 consumption by plant tissues, leads to the formation of ROS. The survival of the plants, therefore depends on many important factors like change in growth conditions, severity and duration of stress conditions and the capacity of the plants to quickly adapt to changing energy equation ( Miller et al., 2010). The delicate balance between ROS generation and ROS scavenging is disturbed by the different types of stress factors like salinity, drought, extreme temperatures, heavy metals, pollution, high irradiance, pathogen infection, etc (Figure 1). However, they are unable to cause damage, as they are being scavenged by different antioxidant mechanisms ( Foyer and Noctor, 2005). Under favorable conditions, ROS is constantly being generated at basal levels. Recent findings also shed light on the role of apoplast as a site for ROS generation ( Jubany-Marí et al., 2009 Roychoudhury and Basu, 2012). Aerobic metabolism constantly generates ROS which are confined to the different plant cellular compartments, like the chloroplast, mitochondria and peroxisomes. Molecular oxygen was introduced to the early reducing atmosphere of the Earth about 2.7 billion years ago by O 2- evolving photosynthetic organisms, causing the advent of the reactive oxygen species (ROS) as unwanted byproducts ( Halliwell, 2006). Such a comprehensive knowledge of ROS action and their regulation on antioxidants will enable us to develop strategies to genetically engineer stress-tolerant plants. In this review, we emphasize on the different types of ROS, their cellular production sites, their targets, and their scavenging mechanism mediated by both the branches of the antioxidant systems, highlighting the potential role of antioxidants in abiotic stress tolerance and cellular survival. These two components work hand in hand to scavenge ROS. To ensure survival, plants have developed efficient antioxidant machinery having two arms, (i) enzymatic components like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR) (ii) non-enzymatic antioxidants like ascorbic acid (AA), reduced glutathione (GSH), α-tocopherol, carotenoids, flavonoids, and the osmolyte proline. The cellular damages are manifested in the form of degradation of biomolecules like pigments, proteins, lipids, carbohydrates, and DNA, which ultimately amalgamate in plant cellular death. The role of the ROS family is that of a double edged sword while they act as secondary messengers in various key physiological phenomena, they also induce oxidative damages under several environmental stress conditions like salinity, drought, cold, heavy metals, UV irradiation etc., when the delicate balance between ROS production and elimination, necessary for normal cellular homeostasis, is disturbed. There are secondary sites as well like the endoplasmic reticulum, cell membrane, cell wall and the apoplast. The ROS production in plants is mainly localized in the chloroplast, mitochondria and peroxisomes. and non-radicals like H 2O 2 and 1O 2. #Scavengers animals freeThe major members of the ROS family include free radicals like O In recent years, it has become apparent that ROS plays an important signaling role in plants, controlling processes such as growth, development and especially response to biotic and abiotic environmental stimuli. Reactive oxygen species (ROS) were initially recognized as toxic by-products of aerobic metabolism. Xavier's College (Autonomous), Kolkata, India Post Graduate Department of Biotechnology, St.
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