**6. Control of ROS levels: detoxifying mechanisms**

In plants and animals ROS are deactivated by antioxidants. These antioxidants act as an inhibitor of the process of oxidation, even at relatively small concentration and thus have diverse physiological roles [40]. Antioxidant constituents of plant materials act as radical scavengers, and convert the radicals to less reactive species [81, 82]. Plants have developed an array of defense strategies (antioxidant system) to cope up with oxidative stress. Plant cells are equipped with mechanisms allowing scavenging (in the case of oxidative stresses) or homeostasis of ROS (for cellular signaling) [83]. The antioxidative system includes both enzymatic and nonenzymatic systems. The nonenzymatic system includes ascorbic acid (vitamin C); α-tocopherol, carotenes, etc., and enzymic system include superoxide dismutase (SOD), Superoxide dismutase, which can be mitochondrial (MnSOD), cytosolic (Cu/ ZnSOD) or chloroplastic (CuZnSOD, FeSOD), dismutates superoxide radicals into H2 O2 and oxygen [84]. Hydrogen peroxide is eliminated by the action of catalase (CAT), which is located in glyoxysomes and peroxisomes [51]. The ascorbate-glutathione cycle (also called the Halliwell-Asada cycle) also takes part in H<sup>2</sup> O2 scavenging. Ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) are involved in this cycle (**Figure 1**), and are present in chloroplasts, the cytoplasm, mitochondria, peroxisomes and the apoplast. These enzymes also participate in the regeneration of the powerful antioxidants such as reduced glutathione (GSH), ascorbic acid (vitamin C), and α-tocopherol (vitamin E) (**Figure 1**). Glutathione an

**Figure 1.** Main detoxifying mechanisms in plants. CAT, catalase; SOD, superoxide dismutase; APX, ascorbate peroxidase; MDHAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase; ASA, ascorbate; MDHA, monodehydroascorbate; DHA, dehydroascorbate; GSSG, oxidized glutathione; GSH, reduced glutathione; α-tocH, α-tocopherol; α-toc, α-tocopheryl; LOOH, lipid peroxide; LOO, lipid radical (Halliwell-Asada cycle).

important water-soluble antioxidant and is synthesized from the amino acids glycine, glutamate, and cysteine, which directly scavenges ROS such as lipid peroxides, and also plays a vital role in xenobiotic detoxification [85–88]. Research suggests that glutathione and vitamin C work interactively to quench free radicals and that they have a sparing effect upon each other [85–87]. Glutathione peroxidases (GPX) may also catalyze the reduction of H2 O2 and hydroperoxides [85–88]. Polyphenol oxidase (PPO), the function of this antioxidant system is to scavenge the toxic radicals produced during oxidative stress and thus help the plants to survive through such conditions. Various compounds, such as polyphenols, flavonoids and peroxiredoxins [89] also have a strong antioxidant function.
