Production of ROS

Environmental stresses are responsible for the production of ROS. The production and removal of ROS is thought to be at equilibrium under normal conditions, whereas environmental stress disturbs this equilibrium by enhancing the production of ROS. ROS are very toxic for the organism as they affect the structure and function of the biomolecules. The main source of ROS production in plants is chloroplasts, mitochondria, and peroxisomes (Fig. 1.5).

Mitochondria are responsible for the generation of oxygen radicals and hydrogen peroxide due to the overreduction of the electron transport chain.

Fig. 1.5 Sites of reactive oxygen species (ROS) and the biological consequences leading to a variety of physiological dysfunctions that can lead to cell death (adopted from Ahmad et al. 2008)

Fig. 1.5 Sites of reactive oxygen species (ROS) and the biological consequences leading to a variety of physiological dysfunctions that can lead to cell death (adopted from Ahmad et al. 2008)

Mehler Reaction Chloroplast

Chloroplasts are found to be the major producer of O. and H2O2 (Davletova et al. 2005). This is because the oxygen pressure in the chlo-roplast is higher than in other organelles. Photoreduction of O2 to O2'- during the photosyn-thetic electron transport takes place and is called Mehler reaction. The production of superoxides is due to the reduction of molecular oxygen in the plastoquinone pool. This reduction may be due to the plastosemiquinone, by ferredoxin (Fd) or by sulfur redox centers in the electron transport chain within PSI (Dat et al. 2000) . These superoxides are converted to hydrogen peroxide either spontaneously or by the action of the enzyme SOD. Hydrogen peroxide is also responsible for the production of hydroxyl radicals (OH ').

The major producer of H2O2 in plant cells are peroxisomes. It has been reported that peroxi-somes are also responsible for the production of superoxides (O2 -) . In peroxisomes, the production of O2- occurs in the peroxisomal matrix and the peroxisomal membrane. In the peroxisomal matrix, the oxidation of xanthine and hypoxan-thine to uric acid in the presence of the enzyme xanthine oxidase generates O2- radicals (Halliwell and Gutteridge 2000). Peroxisomes have got two pathways for the production of H2O2. One is the disproportionation of O2- generated in this organelle and the other is a direct pathway. During photorespiration glycolate is catalyzed by glycolate oxidase, yielding H2O2 . Fatty acid b-oxidation, the enzymatic reaction of flavin oxidases, can also produce H2O2 (Baker and Graham 2002; del Rio et al. 2002).

ROS include 1 O2, O2'-, H2O\ H2O2, OH\ RO' organic hydroperoxide (ROOH), excited carbonyl (RO'2 , etc. They cause damage to biomolecules like proteins, lipids, carbohydrates, and DNA, which ultimately results in cell death (Foyer and Noctor 2005). Fortunately, plants are equipped with an antioxidant machinery that scavenges the ROS and helps the plant to tolerate the stress conditions. The antioxidants include enzymatic anti-oxidants, viz., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glu-tathione reductase (GR), etc., and nonenzymatic antioxidants like ascorbic acid (AsA), vitamin E (a-tocopherol), reduced glutathione (GSH), etc.

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