Basic HTML Version
Plant Gene and Trait, 2013, Vol.4, No.20, 109
-
123
http://pgt.sophiapublisher.com
116
MDHAR and the other is GSH dependent DHAR.
Differential regulation of these enzymes in ascorbate
recycle was observed in response to various stresses
(reviewed in Gallie, 2013). Genetically engineered
plants over-expressing MDHAR and DHAR genes
showed greater protection against abiotic oxidative
stress and higher level of AsA content in the leaf
tissues and other plant organs (Eltayeb et al., 2006,
2007; Wang et al., 2010b; Yin et al., 2010). In the
present study, we found no significant variation in
MDAHR activity in H
2
O
2
pre-treated and non-treated
drought stressed seedlings. However, DAHR activity
was found to increase in response to drought stress
indicating that under drought stress conditions AsA is
regenerated via GSH dependent DHAR because
MDHAR activity is limited due to the unavailability
of NADPH under oxidative conditions (Asada and
Takahashi, 1987). Similar increases in DHAR activity
under drought stress have also been reported in a
range of plant species (Liu et al., 2010;
Sánchez-Rodríguez et al., 2011). Surprisingly, we
observed no significant variation in DHAR activity in
H
2
O
2
pre-treated drought stressed seedlings although
in our previous studies observed a significant increase
in DHAR activity in cold or heat pre-treated drought
stressed seedlings (Hossain et al., 2013a, 2013b). The
findings of our present experiment is inconsistent with
the results of Liu et al. (2010) who observed that both
MDHAR and DHAR activities was higher in H
2
O
2
pre-treated seedlings under drought stress. This
variation is probably due to differences in plant species,
treatment methods, and duration of stress treatment.
Glutathione S-transferase, a superfamily of
multifunctional enzymes, plays a central role in
GSH-mediated detoxification in plant cells (Dixon et
al., 2010). This is one of the heavily studied enzymes
in plants due to its multiple roles plant detoxification
processes. Glutathione S-transferase catalyses the
binding of various xenobiotics with GSH to produce
less toxic and more water-soluble conjugates
(Edwards et al., 2000). Besides catalyzing the
conjugation of electrophilic metabolites to GSH, GST
isozymes also have peroxidase activity (Foyer and
Noctor, 2011). Furthermore, GSTs may act as binding
proteins that sequestrate flavonoids (e.g. anthocyanins)
in the vacuole for protection against environmental
stresses (Tahkokorpi et al., 2007). In our present
experiment, we observed non-significant increase in
GST activity under drought stress. The present
research results correlate well with our previous
experimental results (Hossain et al., 2013a, 2013b).
However, significant increase in GST activity under
drought stress has been reported (Liu et al. 2010). The
augmentation of GST activity under the condition of
drought stress is insufficient to protect cells from
drought-induced oxidative damage. Importantly, H
2
O
2
pre-treatment favorably modulates the GST activity
suppressed the production of H
2
O
2
and MDA level
denoting that GST play important role in reducing
drought-induced oxidative damage.
Glutathione reductase is crucial for the operation of
AsA-GSH pathway and which is primarily responsible
for the regeneration of GSH to GSSG using NADPH
as a reducing equivalent. This enzyme plays an
important role by maintaining the reduced status of
GSH and AsA pools and proper GSH/GSSG ratio that
is more decisive in determining plant resistance to
abiotic and biotic stresses than in the actual GSH
content (Wang et al., 2010b; Hossain et al., 2012b).
The elevated level of GR might be able to increase the
ratio of NADP
+
to NADPH and thereby make sure the
availability of NADP
+
to accept electrons from the
photosynthetic electron transport chain. Under this
circumstance, the rate of electron flow to O
2
is
reduced and thereby reduces the formation of O
2
•−
metal catalyzed formation of •OH through the
Haber-Weiss reaction (Yang et al., 2008; Hossain et al.,
2013a). Importantly, the higher GSSG levels in the
drought stressed seedlings which were attributed to
significant decrease in GSH/GSSG ratio. Decrease in
GR activity in response to drought stress was also
observed in the sensitive cultivar (Sánchez-Rodríguez
et al., 2011). Usually, tolerant plants tend to have high
activities of GR as compared to the sensitive plants
(Selote and Khanna-Chopra, 2004; Sekmen et al.,
2007; Aghaei et al., 2009; Sánchez-Rodríguez et al.,
2011). We observed a slight decrease in GR activity as
well as higher GSSG content in the seedlings under
drought stress conditions. Reduction on GR activity in
mustard seedlings under drought stress is one of most
important factors for its susceptibility to drought stress.
We, therefore, speculated that the inhibition or
insufficient of GR was the major factors for rapid
increase in the GSSG in drought-stressed seedlings
which was attributed to significant decrease in
GSH/GSSG ratio. Similar to our results decrease in