(4) Glutathione Reductase (GR)When acting as an antioxidant by participating in enzymic as well as nonenzymic oxidation-reduction cycles, GSH is oxidized to GSSG. In AsA-GSH cycle, GSH is oxidized in a reaction catalyzed by DHAR. Glutathione reductase (GR, EC 1.6.4.2), a NAD(P)H-dependent enzyme catalyzes the reduction of GSSG to GSH and, thus, maintains high cellular GSH/GSSG ratio. GR belongs to a group of flavoenzymes and contains an essential disulfide group [226]. The catalytic mechanism involves two steps: first the flavin moiety is reduced by NADPH, the flavin is oxidized and a redox active disulfide bridge is reduced to produce a thiolate anion and a cysteine. The second step involves the reduction of GSSG via thiol-disulfide interchange reactions [226]. If the reduced enzyme is not reoxidized by GSSG, it can suffer a reversible inactivation. Although it is located in the chloroplasts, cytosol, mitochondria, and peroxisomes, around 80% of GR activity in photosynthetic tissues is accounted for by chloroplastic isoforms [227]. In chloroplast, GSH and GR are involved in detoxification of H2O2 generated by Mehler reaction.Several authors have reported increased activity of GR under environmental stresses [8, 11, 15, 125, 223]. Pastori and Trippi [228] observed correlation between the oxidative stress resistance and activity of GR and suggested that oxidative stress caused by paraquat or H2O2 could stimulate GR de novo synthesis, probably at the level of translation by preexisting mRNA. Antisense-mediated depletion of tomato chloroplast GR has been shown to enhance susceptibility to chilling stress [229]. Overexpression of GR in N. tabacum and Populus plants leads to higher foliar AsA contents and improved tolerance to oxidative stress [138, 230].Due to the complexity of ROS detoxification system, overexpressing one component of antioxidative defense system may or may not change the capacity of the pathway as a whole [231, 232]. Several studies have shown that overexpression of combinations of antioxidant enzymes in transgenic plants has synergistic effect on stress tolerance [233, 234]. Kwon et al. [234] demonstrated that simultaneous expression of Cu/Zn-SOD and APX genes in tobacco chloroplasts enhanced tolerance to methyl viologen (MV) stress compared to expression of either of these genes alone. Similarly, enhanced tolerance to multiple environmental stresses has been developed by simultaneous overexpression of the genes of SOD and APX in the chloroplasts [235, 236], SOD and CAT in cytosol [231] and SOD and GR in cytosol [233]. Further, simultaneous expression of multiple antioxidant enzymes, such as Cu/Zn-SOD, APX, and DHAR, in chloroplasts has shown to be more effective than single or double expression for developing transgenic plants with enhanced tolerance to multiple environmental stresses [26]. Therefore, in order to achieve tolerance to multiple environmental stresses, increased emphasis is now given to produce transgenic plants overexpressing multiple antioxidants.
Reactions And Reagents O.p Agarwal Pdf 198
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