Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Photooxidative Stress
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ton (Payton et al., 2001). Drought-induced photooxidative stress caused an oxidation of
the glutathione pool in barley leaves (Smirnoff, 1993). It is imperative to believe that the
tissue concentration of glutathione, redox states and glutathione-related enzymes confirm
the central role of glutathione metabolism in plant responses to photooxidative
stress. Further one might reasonably expect that the in vivo concentrations of glutathione
influence the plant antioxidant capacity, the steady state levels and turnover
of AsA.
5.1.4. Other Compounds as Antioxidants
Like carotenoids, tocopherols (vitamin E) are also known to involve in energy uptake
and dissipation, a property determined by the presence of ring-closed conjugated double
bonds in the benzene moiety of their molecule (Burton et al., 1982; Fryer, 1992; Halliwell
and Guttiridge, 1999). Sugar alcohols such as mannitol can also serve as more efficient
carbon sink for light reaction products and may therefore alleviate photooxidative stress
(Smirnoff and Cumbus, 1989)
5.2. Enzymatic ROS Scavenging System in Plants
The ROS scavenging enzymes are located in different compartments of plant cells as
already indicated in Figure 2. The ROS scavenging systems in plant cells consist of
superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX),
monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), glutathione
peroxidase (GPX) and glutathione reductase (GR) (Foyer, 2002; Reddy et al.,
2004).
5.2.1. Superoxide Dismutase (SOD)
The SOD is one of the fastest enzymes (Vmax = 2 x 10 9 M -1 s -1 ) with an optimum close to
the diffusion rate of O 2?¯ (McCord and Fridovich, 1969). The conversion of O 2?¯ to
H 2
O 2
is the first link in the enzymatic scavenging of ROS and SOD is considered as the
primary defense against oxygen radicals. Three different types of SOD have been
found in plants containing either Mn, Fe or Cu and Zn as prosthetic metals (Asada,
1999). Different isoforms of SOD in plants are differentially expressed and localized in
different compartments in the plant cell (Bowler et al., 1994; Wingle and Karpinski, 1996;
Schinkel et al., 1998). Accordingly plant SODs are classified as MnSODs, FeSODs and
Cu/ZnSODs and distinct isozymes have been identified in the cytosol, mitochondria
and chloroplast (Bowler et al., 1994). Plant SODs are also reported in peroxisomes
(Scandalios, 1997) and glyoxysomes (Bueno and del Rio, 1992). All plant SODs are
encoded by the nuclear genome and organelle isozymes are transported post
translationally to the appropriate cellular compartment (Perl-Treves et al., 1990). In
maize, ten different SOD isozymes have been reported; four cytosolic Cu/Zn SODs,