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

Photooxidative Stress
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known to protect plant leaves from photooxidation and suggest that this photoprotective
function involves the maintenance of a low level of free chlorophyll under high light
conditions to minimize the formation of 1 O 2
. ELIPs are thus known as scavengers of free
chlorophyll molecules released during the rapid turnover of the photosynthetic complexes
and the reorganization of photosynthetic machinery in high light (Adamska,
1997; Hutin et al., 2003). ELIPs are also considered as plant defense systems in light
stress conditions, which have the potential to become new selection markers for the
identification and development of transgenic crop plants, tolerant to photooxidative
stress conditions.
The APX is a regulatory enzyme in ascorbate-glutathione cycle. Furthermore,
this cycle is essential for the continuous regeneration of ascorbate and glutathione
which are known to be rapidly depleted under photooxidative stress. Recently, it was
demonstrated that chloroplastic APXs (chl APXs) were inactivated as a result of the
change of redox status of AsA under photooxidative stress (Yoshimura, 2000). Chl
APXs were more strongly inactivated than thiol-modulated enzymes in the Calvin cycle,
which are believed to be the most sensitive enzymes to H 2
O 2
. Yabuta et al. (2002)
demonstrated that transgenic tobacco plants (TpTAP-12) overexpressing APX (37-fold
high activity than the wild-type plants) showed increased tolerance to photooxidative
stress as well as to chilling stress with high light intensity. Nevertheless, photooxidative
tress may not have detrimental effects if scavenging of ROS is triggered in the
proper cell compartment. Targeting of APX and SOD to chloroplasts resulted in increased
stress tolerance in tobacco to high light (Kwon et al., 2002; Yabuta et al., 2002).
Chloroplastic overexpression of GR can increase the leaf GSH/GSSG ratio and mitigate
the damage due to photooxidative stress (Foyer and Noctor, 2001). Increased defense
against photooxidative stress was also conferred by targeting bacterial CAT to tobacco
chloroplasts (Mohamed et al., 2003). Induction of glutathione peroxidases and 2-cysteine
peroxiredoxins was also suggested to be crucial in controlling chain type reaction
that follows the initiation of lipid peroxidation by 1 O 2
and OH ? in plant cell membranes
(Mullineaux et al., 1998; Bair and Dietz, 1999).
Transgenic plants have been produced which overaccumulated ROS-scavenging
metabolites or overexpressed ROS-scavenging enzymes (SOD, CAT, APX, GR)
for improved oxidative stress tolerance which also showed enhanced yield and survival
under environmental stress conditions (Edreva, 2005a). Overexpression of SOD in alfalfa
conferred tolerance to high light (Alscher et al., 2002). Miyagawa et al (2000)
showed that CAT from E.coli with higher affinity for H 2
O 2
than plant CATs, was
overexpressed in tobacco thus conferring protection to high light stress. A deeper
understanding of such submolecular bases of ROS-related processes may constitute a
rationale for developing transgenic plants for tolerance to photooxidative stress. Murchie
et al. (2005) showed an up-regulation of genes involved photoprotection and photooxidative
stress when rice plants were treated with high irradiance. A significant increase
in the level of expression of MDHAR was observed. High light intensities also upregulated
the activities of APX (Karpinski et al., 1999; Rossel et al., 2002). In Arabidopsis,