Agriculture Reference
In-Depth Information
Glutathione
oxidized (GSSG)
Ascorbate (AsA)
H
2
O
2
NADP'
NADPH
Ascorbate
peroxidase
(APX)
Monodehydroascorbate
reductase
(MDHAR)
Dehyaroascorbate
reductase
(DHAR)
Glutathione
reductase
(GR)
MDHA
H
2
O
NADPH
NADP'
Glutathione
reduced (GSH)
Dehydroascorbate
(DHA)
Fig. 14.4
The ascorbate-glutathione (Asc-GSH) cycle or Asada-Halliwell pathway. H
2
O
2
is
removed by APX and Asc is regenerated by the Asc-GSH cycle, involving MDHAR, DHAR and
GR. Asc is first oxidized to MDHA. If MDHA is not rapidly reduced again to Asc by MDHAR, it
will spontaneously disintegrate into Asc and DHA. DHAR recycles Asc from DHA using GSH that
is regenerated through the action of GR in a NADPH-dependent reaction. The dotted line indicates
non-enzymatic reactions
the ascorbic acid (AA) to monodehydroascorbate (MDHA), which is then reduced
by NADPH in a reaction catalysed by monodehydroascorbate reductase (MDHAR).
MDHA also undergoes spontaneous disproportionation reaction to AA and dehy-
droascorbate (DHA). The latter is reduced to AA by GSH in a reaction catalyzed by
dehydroascorbate reductase (DHAR), producing glutathione disulphide (GSSG),
which in turn is reduced by glutathione reductase (GR) to GSH in the presence of
NADPH (Clijsters et al.
1999
). Inadequate regulation of ROS generation potentially
leads to oxidative damage. The first line of defense against ROS-mediated toxicity
is achieved by SOD that catalyzes the dismutation of superoxide radicals to H
2
O
2
and O
2
. Enhanced SOD activity in both the plants suggested that chromium caused
oxidative stress (Diwan et al.
2008
; Diwan et al.
2010a
; Jabeen et al.
2010
). SOD,
the first enzyme in detoxifying process, converts O
2
−
radicals to H
2
O
2
. Chromium
mediated enhancement in activity of SOD has been observed in several studies,
which may be due to either direct effect of this metal on the SOD gene or to an
indirect effect mediated via an increase in the level of O
2
−
radicals (Chongpradit-
num et al.
1992
). Enhanced SOD activity has been associated with stress tolerance
in plants because it neutralizes the reactivity of O
2
−
, which is overproduced under
oxidative stress. It has been well documented that SOD activity has a protective
role in heavy metal plants. At higher levels of chromium, however, the increase
in SOD activity was not comparable to that evident at lower chromium levels as
there might be inactivation of the enzyme by H
2
O
2
(Yamaguchi et al.
1995
). On the
other hand, the up-regulation in the SOD activity in chromium-sensitive plants in
response to chromium stress was not strong enough to detoxify the superoxide radi-
cals completely, thus reflecting lesser tolerance towards chromium stress, which
also indicated that O
2
scavenging function of SOD was impaired with duration and
levels of chromium treatments (Diwan et al.
2010a
; Labra et al.
2006
; Zhang et al.
2005
; Zhang et al.
2007
). Peroxidases are known to play a significant role under
oxidative stress conditions and it has been shown that peroxidase activity can be
