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SOD, the first enzyme in the
detoxifying process, converts superoxide anion radicals  (O2-°) to hydrogen
peroxide (H2O2), and APX reduces H2O2
to water using ascorbic acid as a specific electron donor (Asada, 1992;
Foyer et al., 1994; Asada, 1999). Chloroplasts,
the major component of photosynthetic tissue, is highly sensitive to damage by
reactive oxygen species, which are frequently generated by the reaction of
chloroplast O2 and electrons that escape from the photosynthetic
electron transfer system (Foyer et
al., 1994).

Location of SOD in plant cells

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Copper, zinc superoxide dismutase
(Cu, Zn SOD; EC catalyzes a two-step dismutation of the toxic
superoxide radical (O2-) to molecular oxygen and hydrogen
peroxide through alternate reduction and oxidation of copper ion at
diffusion-limited catalytic rate (Liochev and Fridovich, 2000).

Superoxide dismutase (E.C1.15.1.1) an
ubiquitous antioxidant enzyme catalyzes the dismutation of the super oxide
radical into hydrogen peroxide and molecular oxygen and protects
oxygen-metabolizing cells against harmful effects of superoxide free-radicals (Chaudhary, Chand and Kaur, 2013).

            2O2- + 2H? SOD
O2 + H2O2

Cu, Zn SOD forms the first line of
defense in all aerobic organisms against reactive oxygen radicals and is vital
to the survival of cells (Halliwell and Gutteridge, 1990). Therefore,
SOD is an important enzyme for aerobic organisms including plants and humans
and also in those applications which require scavenging of O2-
(Bafana et al., 2011).

SOD (EC, the first enzyme
in the detoxifying process, converts O2- radicals to H2O2
In plants, Cu/Zn SOD isoforms are found primarily in chloroplasts and in the
cytosol, and Mn SODs are located primarily in mitochondria (Rabinowitch and
Fridovich, 1983). In addition, peroxisomal localization of Mn SODs has been
reported in pea (Sandalio et al,.
1987). Tobacco (Nicotiana tabacum) plants also contain
chloroplast-localized Fe SOD (Van Camp et
al., 1990). In chloroplasts, H2O2 is reduced by
APX (EC using ascorbate as an electron donor (Bowler et
al., 1992).

Cu-Zn SODs are found throughout the
plant cell. There are two different groups of Cu-Zn SODs. The first group
consists of cytoplasmic and periplasmic forms, which are homodimeric. The
second group comprises the chloroplastic and extracellular Cu-Zn SODs, which
are homotetrameric (Bordo et al.,

Efforts to fortify antioxidant
defenses in chloroplasts or in the cytosol include overexpression of CuZnSOD or
APX, which provides enhanced tolerance to oxidative stress in plants (Gupta et al., 1993a, b; Perl et al., 1993; Mittler et al., 1999; Murgia et al. 2004). However, it is noted that
in some cases overexpression of a single antioxidative enzyme did not provide
protection against oxidative or abiotic stresses (Tepperman and Dunsmuir,
1990; Pitcher et al., 1991;
Torsethaugen et al., 1997), signifying
that overexpression of one enzyme may not change the function of the entire
anti-oxidant pathway.

The plant SOD isoenzymes also differ
in their subcellular location. Typically, FeSOD MnSOD is mitochondrial, is
plastidic, and CuZnSOD can be plastidic or cytosolic (Bowler et al., 1992). There are also
reports of peroxisomal and extracellular SODs (Streller and Wingsle, 1994;
Bueno et al., 1995).

Numerous attempts have been made to
enhance stress tolerance in plants by modifying the production of SOD enzymes.
Ectopic production of cytosolic Cu/Zn SOD improved stress tolerance in tobacco (Faize
et al., 2011), potato (Perl et al.,
1993), sugar beet (Tertivanidis et al.,
2004) and plum (Diaz-Vivancos et al.,

Antioxidants such as ?avonoids, carotenoids,
vitamins, phenols and dietary glutathiones are capable of acting as free
radical scavengers, peroxide decomposers, singlet and triplet oxygen quenchers,
enzyme inhibitors and synergists (Singh et al., 2000). The
antioxidant activity of phenolic compounds are attributed due to their redox
properties, which can play an important role in adsorbing and quenching singlet,
neutralizing free radicals and triplet oxygen or decomposing peroxides and have
health functional properties that may protect humans from various diseases (Larson
al., 1988; Heinonen et al., 1998).

Plants have developed various
mechanisms such as modulating the expression of stress tolerance genes and synthesizing
compatible solutes (Ahmad et al., 2010) to cope with the oxidative
stresses caused by unfavorable environments. Antioxidant defense systems are
well known for scavenging reactive oxygen species (Ros) produced in different
stressful conditions, such as activation of the antioxidant enzymes superoxide
dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and peroxidase
(POD) (Allen et al., 1997; Kwon et al., 2002).

Plants have both enzymatic and
nonenzymatic antioxidant systems to prevent or alleviate the damage from Ros.
Several enzymes can efficiently detoxify Ros. Superoxide radicals are
disproportionately detoxified by superoxide dismutase (SOD), and hydrogen
peroxide is destroyed by catalase (CAT) and different kinds of peroxidases such
as guaiacol peroxidase (GPX), etc. A major hydrogen peroxide-detoxifying system
in plants is the ascorbate-glutathione cycle that includes ascorbate peroxidase
(APX) and glutathione reductase (GR) (Asada et al., 1994).

Abiotic stresses are known to act as
a catalyst in producing free radical reactions resulting in oxidative stress in
various plants where reactive oxygen species (Ros) such as superoxide radical
(O2-), hydroxyl radical (OH), hydrogen peroxide (H2O2)
and alkoxyl radical (RO.) The first reaction in the
“ascorbate-glutathione” cycle is transformation of two molecules of
superoxide anion radical (O2-°), with two protons (H+),
into one molecule of hydrogen peroxide (H2O2) and one
molecule of oxygen (O2). The enzyme that catalyses this reaction is
superoxide dismutase (SOD):

+ O2-° + 2H+ SOD H2O2 + O2
(Goran, Z. et al., 2008)

Free radicals (such as the superoxide
anion) are chemical species containing one or more unpaired electrons (O2
+ e = O2-°), and therefore are extremely reactive. Free
radicals act on several cell components producing damage and modifying cell
functions. The targets for these dangerous molecules are basically
polyunsaturated fatty acids, some proteins and genetic material. 

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