Basic HTML Version
Plant Gene and Trait, 2013, Vol.4, No.20, 109
-
123
http://pgt.sophiapublisher.com
115
stress tolerance and reproduction (reviewed in Noctor
et al., 2012; Zhang, 2013). As a stress tolerance
mechanisms plants usually up-regulate the both of
these antioxidant level. They are the integral
component of AsA-GSH cycle present in different
cellular organelles responsible for elimination of
excessive ROS species in plants; however, both of the
components have the capacity to scavenge ROS
directly. Plenty of research studies in plant have
demonstrated that the level of AsA and GSH, and the
alteration of AsA/AsA+DHA and GSH/GSSG ratios
positively correlated with abiotic oxidative stress
tolerance (Hossain et al., 2010, 2011b, 2013a, 2013b;
Wang et al., 2010b; Mostofa and Fujita, 2013). In our
present experiment, we found that both AsA and GSH
level increased in response to drought stress, however,
greater increase was observed in the GSH contents.
The increases in AsA content in response to drought
stress have also observed in our previous studies
(Hossain et al., 2013a, 2013b) also correlate well with
other research groups where they found an increase in
AsA and GSH level (Liu et al., 2010; Chugh et al.,
2013). The ascorbate content in the H
2
O
2
pre-treated
seedling near to control plants. Our results is
inconsistent with the results of Liu et al. (2010) who
found that both AsA and ascorbate redox state
increased in H
2
O
2
pre-treated drought stressed
seedlings. One possible explanation of lower AsA
content in the H
2
O
2
pre-treated seedlings is probably
due to the higher APX activity that utilizes AsA as a
reducing equivalent or the lower rates of its synthesis.
In addition with its involvement with AsA-GSH cycle
the GSH also involved with other GSH related
enzyme like GST, GPX and Gly I. In the present
experiment, although the GSH level showed robust
increase in drought stressed seedlings, however, we
observed severe oxidative stress in leaf tissues. In
contrast to our present experiment results, increased
level of GSH and its redox state were observed in
H
2
O
2
pre-treated drought stress seedlings (Liu et al.,
2010). Yu et al. (2002) also stated that only GSH level
is not the sole factor in enhancing oxidative stress
tolerance. Importantly, the lower GSH content in
H
2
O
2
pre-treated seedlings as compared to the
seedlings subjected to drought stress alone. The
increase in GST activity is probable main reason for
behind this situation because GST utilizes GSH to
convert toxic compound into non-toxic forms and
sequestrated into the vacuole (Dixon et al., 2010).
The ROS detoxifying antioxidant enzymes and
antioxidants studied on our present experiment is the
major-hydrogen scavenging species (Figure 2). The
APX, CAT and GPX are among the most three
important enzymes associated with the detoxification
of H
2
O
2
in plant cells and also for regulating the
appropriate level of H
2
O
2
to perform its signalling
functions. Ascorbate peroxidase utilized AsA as a
reducing equivalent during conversion of H
2
O
2
into
water, whereas GSH is used by GPX. Importantly,
CAT converted H
2
O
2
to H
2
O without any reducing
equivalent and primarily associated with peroxisome
where the maximum H
2
O
2
produced during
photorespiration, a typical situation during drought
stress. The present experiment showed that the activity
of APX and CAT decreased in response to drought
stress whereas GPX activity showed a significant
increase. The findings of the present experiments well
coherent with our previous studies where we observed
that drought and salt stress led to a significant
decrease in APX and CAT activity (Hossain et al.,
2011b, 2013a, 2013b). Such a decrease in CAT
activity could indicate its inactivation by the
accumulated H
2
O
2
induced by drought stress and
could be partly explained by photoinactivation of
enzymes (Hertwig et al., 1992; Zhang and Kirkham,
1994). Conversely, significant increase in APX and
CAT activity was observed in H
2
O
2
pre-treated
seedlings. The increase in APX and CAT activity in
H
2
O
2
pre-treated drought, salt, chilling and heavy
metal-stressed seedlings was also reported (He et al.,
2009; Liu et al., 2010; Wang et al., 2010b). The
increase in GPX activity was also observed in plant
under drought stress conditions (Liu et al., 2010;
Haluskova et al., 2009) because H
2
O
2
was found to
activate the GPX promoter (Avsian et al., 2004).
However, we don’t find any significant increase in
GPX activity in H
2
O
2
pre-treated drought stressed
seedlings although its level was found to increase in
response to drought stress.
Oxidation of AsA through ROS or by APX during
H
2
O
2
detoxification produces short lived
monodehydroascorbate (MDHA), which can rapidly
disproportionate to dehydroascorbate (DHA). Plants
can restore AsA from MDHA and DHA by using two
enzymes of AsA-GSH cyle, one is NADPH dependent