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Plant Gene and Trait, 2013, Vol.4, No.20, 109
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GR activity under drought stress have also been
reported in sensitive cultivar (Sánchez-Rodríguez et
al., 2011; Chaugh et al., 2013). Importantly, H
2
O
2
pre-treated drought-stressed seedlings had higher GR
activity. Increased GR activity in the H
2
O
2
pre-treated
drought-stressed seedlings contributes to the
maintenance of higher GSH/GSSG ratio.
MG is an inevitable product of abiotic stresses which
can again aggravate ROS production.Upon exposure
of plants to abiotic or biotic stresses a rapid
accumulation of endogenous MG has been observed
and its detoxification is one of the potential
mechanisms for inducing tolerance to biotic and
abiotic stresses (Yadav et al., 2005a; Hossain et al.,
2009; Hossain and Fujita, 2009; Hossain et al., 2011a;
Banu et al., 2010; Upadhyaya et al., 2011). MG is
toxic to plant cells, causing inhibition of cell
proliferation, protein inactivation and inhibition of
ROS detoxification systems and as a consequence
disrupts cellular functions (Matins et al., 2001; Hoque
et al., 2010, 2012a), however signalling roles for MG
in inducing abiotic stress tolerance have also been
reported (Hoque et al., 2012b; 2012c). The glyoxalase
system is the most important detoxification pathway
of MG in plants. The glyoxalase system is comprised
of two enzymes: Gly I and Gly II that convert MG to
less toxic hydroxyacids. Glyoxalase I convert MG to
S-D-lactoylglutathione (SLG) by utilizing GSH, while
Gly II converts SLG to D-lactic acid, and this reaction
GSH is regenerated. In addition with its (glyoxalase
system) prime function to detoxify highly reactive MG,
the system also played an important role in recycling
trapped GSH in plant antioxidant defence system and
to maintain higher redox state (Creighton et al., 1988).
Like MG, the SLG produced by Gly I was also found
to toxic at high cellular concentration (Thornalley,
1996). Plants showed tolerance reaction against
abiotic or biotic stress by limiting over-accumulation
MG levels through the upregulation of Gly I and Gly
II activities and also by modulating the GSH-based
detoxification systems that ultimately lowered the
level of lipid peroxidation (Yadav et al., 2005a;
Singla-Pareek et al., 2006; Hossain et al., 2013a).
Recent genetic and proteomic studies have shown the
glyoxalase pathway has a profound effect on stress
tolerance. The transcript abundance and activities of
Gly I and Gly II are induced by various abiotic and
biotic stresses (Espartero et al., 1995; Yadav et al.,
2005b; Singla-Pareek et al., 2003, 2006; Hossain et al.,
2009; Lin et al., 2010; Mustafiz et al., 2011).
Wild-type stress tolerant studies and gain-of-function
studies have shown that the antioxidative and
glyoxalase defence systems are closely linked and that
the glyoxalase system has a direct influence on the
ROS detoxification (Yadav et al., 2005a, 2005b;
El-Shabrawi et al., 2010; Upadhyaya et al., 2011) and
plants over-expressing either Gly I or Gly II gene
enhances plant abiotic stress tolerance (Singla-Pareek
et al., 2003, 2006; Lin et al., 2010; Wu et al., 2012;
Viveros et al., 2013). Recently, Upadhayaya et al.
(2011) showed that
GalUR
gene over-expressing
transgenic potato plants had greater salinity tolerance
and showed stimulated activities of the antioxidant
enzymes APX, DHAR, GR, GST, and GPX, and the
glyoxalase system enzymes (Gly I and Gly II), as well
as by enhanced GSH:GSSG ratios. Greater
accumulation of AsA was found in the transgenic
plants with a restricted increase in MG levels under
salt stress. Additionally, a relatively higher GSH:
GSSG ratio is also maintained in these transgenic
plants which could also help to protect them from
salinity induced oxidative stress. Induction of both the
ROS and MG detoxification capacity and the
favourable changes in the GSH and AsA redox state
and of these transgenic plants were thought to be the
main reasons for enhanced salinity tolerance. Our
results showed that drought stress showed profound
increase in both GSH and GSSG, whereas the
GSH/GSSG ratio decreased significantly. The increase
in higher GSSG content indicates the higher oxidative
load in the drought stressed seedlings which are also
evidenced by sharp decrease in GSH/GSSG ratio.
Importantly, the higher level of GSH and GSSG ratio
in H
2
O
2
pre-treated drought stressed seedlings indicate
that H
2
O
2
pre-treatment cannot increase in GSSG
probably due to higher GR and Gly II activity.
Therefore, efficient recycling of GSH through
glyoxalase system and GR activity seems to be an
important determinant in plant stress tolerance.
These findings of the present experiment greatly
coherent with our previous studies in different crop
species under various abiotic stress situations
(Hossain et al., 2013a; 2013b; Mostofa and Fujita,
2013). The results of this study and our previous
findings (Hossain et al., 2010; 2011b, Hossain et al.,
2013a; 2013b) affirmed that simultaneous induction