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

Photooxidative Stress
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Cytosolic and apoplatic-ROS production have also been reported (Hammond-Kosack
and Jones, 1996; Karpinski et al., 1997).
Photorespiratory production of H 2
O 2
in peroxisomes is well known and the
significance of peroxisomes in ROS metabolism is gaining recognition. Peroxisomes are
not only the sites of ROS production by glycolate oxidase but also the site of detoxification
by catalase (CAT). In addition, Corpas et al. (2001) reported that peroxisomes
might be one of the cellular sites for nitric oxide (NO) biosynthesis. However, the role of
NO in ROS metabolism in plants is still not known. 1 O 2
production in plant cells was in
the range of 240 µmol s -1 and a steady state level of H 2
O 2
was in the range of 0.4 to 0.5
µM and photooxidative stress to the plant enhances the 1 O 2
production to the range of
240-720 µM s -1 and a steady state H 2
O 2
level of 5-15 µM (Mittler, 2002). Different sites of
electron leakage and release of O 2?¯ and H 2
O 2
from mitochondria have been reported
(Tiwari et al., 2002). A site-specific release of free radicals has been associated with the
activity of cyanide-insensitive alternative oxidase (McKersie and Leshem, 1994). In
recent years, new sources of ROS have been identified including NADPH oxidases,
amine oxidases and cell wall- bound peroxidases (Gross, 1980, Vianello and Marci, 1991,
Dat et al., 2000). The generation of ROS is usually low under normal growth conditions.
However stressful conditions including high light, drought, desiccation, salinity, low
temperature, heat shock, heavy metals, UV- radiation, nutrient deprivation, pathogen
attack and air pollution are known to disrupt cellular homeostasis through enhanced
production of ROS (Bowler, 1992; Allen, 1995; Allen et al., 1997; Mittler, 2002; Luna et
al., 2005). Increased generation of ROS is known to cause damage to the photosynthetic
system as well as to other cellular components as shown in table 1.
Among these OH¯, being exclusively reactive, interacts with and damages
several molecular species in plant cell (Zhang, 2003). 1 O 2
and O 2?¯ predominantly attack
chlorophylls and unsaturated fatty acids of cell and organelle membranes. D1-D2 proteins,
Calvin cycle enzymes, Fe +2 -containing enzymes and Mn clusters in PS II are
reported to be the targets of H 2
O 2
(Havaux and Niyogi, 1999; Niyogi, 1999). In situations
where 1 O 2
formation rate exceeds the quenching capacity of the plant cell, increased 1 O 2
can migrate outside the chloroplast and affect the unsaturated lipid components. Most
recently, Rontani et al. (2005) reported 1 O 2
-mediated photooxidation of 18-hydroxyoleic
acid yielding 9-hydroperoxy-18-hydroxyoctadec 10(trans)enoic and 10-
hydroperoxy-8-hydroxyoctadec 8-(trans)enoic-acids. These findings are significant as
they clearly indicate the role of 1 O 2
in the photooxidation of the unsaturated of higher
plant lipid components.
5. DEFENSE SYSTEMS AGAINST PHOTOOXIDATIVE STRESS
Photoprotection in plants is a multi-component process in plants to overcome the
potential damage arising from the absorption of excess light energy. This involves the
balancing measure between the absorbed light energy and its utilization. The inevitable
generation of ROS is due to the imbalance between these two processes. There are