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

Photooxidative Stress
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(Zeng et al., 2005). GSTs are also induced in the transport of toxic secondary products
and cell signaling during stress responses (Agarwal et al., 2002; Loyall et al., 2002).
Also, free radical-induced toxic compounds including lipid peroxidases (4-hydroxyalkenals)
and products of oxidative DNA damage are congjugated to GSH by GST and
detoxified or sequestered into vacuole for further degradation (Coleman et al., 1997).
The GST is thus considered as a key enzyme to maintain glutathione homeostasis.
5.2.5. Catalase (CAT)
Catalase (CAT) effectively scavenges H 2
O 2
. In plants tissues, CAT is localized in peroxisomes
to scavenge H 2
O 2
produced by glycolate oxidase in photorespiratory cycle.
The activities of CAT were also reported from mitochondria in plants but not in chloroplasts
(Scandalios et al., 1980). Transgenic tobacco plants, deficient in CAT isozymes,
developed necrotic lesions under high light which indicated that CAT is necessary for
protection of plant cells against photooxidative stress (Chamnogpol et al., 1998). Catalase
protein synthesis has been linked to photosynthetic and photorespiratory pathways
(Schmidt et al., 2002).
5.2.6. Antioxidant Enzymes in C 4
Plants
Antioxidant enzymes are known to be distributed among all photosynthetic cells in
higher plants. The distribution of antioxidant enzymes between mesophyll and bundle
sheath cells in C 4
plants have been recently reported (Foyer, 2002, Sundar et al., 2004).
GR and DHAR were exclusively localized in mesophyll cells whereas most of the SOD
and APX were reported in the extracts from both mesophyll and bundle sheath cells.
CAT and MDHR were approximately equally distributed between mesophyll and bundle
sheath tissues. H 2
O 2
was found to accumulate only in mesophyll cells. The localization
studies on the antioxidant enzymes in C 4
plants are very interesting because the enzymes
of PCR cycle, which are sensitive to H 2
O 2
are located safely in bundle sheath
cells. Kingston-Smith and Foyer (2000) suggested that oxidative damage under stressful
conditions, if any, in C 4
plants will be restricted to bundle sheath tissue because of
inadequate antioxidant protection in this tissue. However, very little mechanistic information
is available on photooxidative stress-induced antioxidative metabolism between
the two cell types in C 4
plants.
5.2.7. Other Scavenging Proteins
Thioredoxin is a small ubiquitous protein that plays a redox-regulatory role in plants.
H 2
O 2
is a potent oxidant of enzymes thiol groups and its inhibitory effects on CO 2
fixation is due to the inactivation of thiol regulated enzymes. In illuminated chloroplasts,
an electron flux may occur via the thioredoxin system (Polle, 1997). Several
stromal enzymes are light regulated via this system, which transfers reducing equiva-