Juggling with reactive oxygen species and antioxidant defense system – A coping mechanism under salt stress

• Respiratory burst oxidase homologues (RBOHs) through localized ROS bursts regulate multitude of signal transduction pathways. • ROS level is attuned by specific mechanisms enabling reduction in the oxidative burst and ROS-scavenging systems. • ROS signaling network induces salinity tolerance at the cellular and whole-plant levels. • ROS and NO are involved in enhancement of antioxidant systems and interaction with other signaling molecules, such as MAPK, plant hormones, and calcium. • RNAi-mediated gene regulation and CRISPR/Cas9 system for gene editing provide efficient targeted modification for resistance to different stresses in many crops. Even under optimal conditions many metabolic processes produce ROS like superoxide anion (O 2 . - ), hydrogen peroxide (H 2 O 2 ) and hydroxyl radicals (OH ⋅ ), particularly in chloroplast and mitochondria. The overproduction of ROS (O 2 . - , H 2 O 2, OH . , RCO etc.) results from the exposure to various environmental conditions like dehydration, heat, salinity and biotic stresses. All biomolecules like lipids, proteins and DNA are extensively damaged by the reactive oxygen species which disrupts the cell integrity further leading to its death. Plants possess both enzymic and non-enzymatic mechanism for scavenging ROS. The enzymic mechanisms are designed to minimize the concentration of O 2 and H 2 O 2 . The overproduced enzymes are superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), glutathione reductase (GR) and glutathione-synthesizing enzymes. Several evidences have shown that although oxidative stress is a lethal situation for cell by ROS (especially H 2 O 2 and O 2 ⋅ - , it may be involved in cellular signaling procedure as second messenger to induce a large number of genes and produce proteins and osmoprotectant involved in salt stress defenses. This review gives an insight into the recent advancements on how antioxidant defense machinery, the antioxidant enzymes and the non-antioxidant metabolites work together to alleviate the negative effects of ROS and cross-talk with reactive sulfur nitrogen and carbomyl species which also act as an important signal molecule. This comprehensive knowledge about ROS action, their regulation through antioxidant machinery, interactions with RNS, RSS and RCS and in signal transduction will empower us in the development of salinity tolerant plants.