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

Photooxidative Stress
167
5.1.2. Ascorbic Acid (AsA)
L-Ascorbic acid (AsA) is an abundant metabolite in plant cells, some times reaching
levels upto 10% of plant cell carbohydrate content (Smirnoff and Pallanca, 1995). AsA
plays an important role in stress physiology of plants as well as plant growth and
development. One of the most important activity of AsA is protection of plant cell
against photooxidative stress (Davey et al., 2002). The biosynthesis of AsA and its
involvement in protection against photooxidative stress suggest link between photosynthesis,
light and AsA pool size. Leaf AsA content was markedly correlated with
light intensity at the leaf surface (Foyer, 1993), Barley and Arabidopsis leaves accumulate
significantly more AsA under high light compared to low light conditions (Conklin
et al., 1997) of late, there is an increasing body of evidence confirming the role of AsA
in the detoxification of ROS in the plants. AsA as the capacity to directly eliminate
several different ROS, including 1 O 2
, O 2-
, OH ? (Padh, 1990). AsA is also known to maintain
the membrane bound antioxidant á-tocopherol in the reduced state (Liebler et al.,
1986). In addition AsA plays a major role in photoprotection as a cofactor in the xanthophyll
cycle (Conklin, 2001). The conversion of violaxanthin to zeaxanthin across the
thylakoid membrane is thought to be involved in non-photochemical quenching of
excess light energy in PSII (Deming-Adams and Adams, 1996). AA was shown to be
required as a cofactor for the enzyme violoxanthine-de-epoxidase (Hager, 1969). AsAdeficient
Arabidopsis mutants had lower levels of non-photochemical quenching due
to a decrease in this de-epoxidation reaction (Conklin, 2001). However, the regeneration
of AA in the plant cells is always associated with glutathione cycle as discussed below.
5.1.3. Glutathione
The non-protein, water-soluble and low molecular weight tripeptide thiol glutathione
(GSH; ã-glutamyl cysteinyl glycine) plays a pivotal role in minimizing cellular disfunction,
arising through stress induced redox perturbation (Vernoux et al., 2002). Interest in
the benefits of genetically engineered cellular GSH concentrations in higher plants was
prompted by the observations that elevated GSH levels correlated with stress tolerance.
Successive oxidation and reduction of ascorbate, glutathione and NADPH would
perform the potential scavenging of H 2
O 2
generated through photooxidative stress in
the chloroplast. These reactions are collectively referred as ascorbate-glutathione cycle
(Figure 4). Later this pathway has been identified in other sub-cellular compartments
including mitochondria, peroxisomes as well as in roots, endosperm, root nodules and
petals (Bielawski and Jay, 1986; Klapheek et al., 1990; Mullineaux et al., 1996). Glutathione
content in the plant cells is now used as a marker of oxidative stress in higher
plants (Grill et al., 2001; Sopory, 2003; Tausz et al., 2004). Glutathione has also been
shown as an antioxidant in mitochondria, cytosol, peroxisomes and nucleus (Noctor et
al., 2002; Muller et al., 2002).