Agriculture Reference
In-Depth Information
Like MTs, PCs are also involved in chelating activity. They are a family of en-
zymatically synthesized cysteine-rich peptides. PCs are small metal binding pep-
tides with the structure (γ-Glu-Cys)n-Gly, where value of 'n' varies from 2 to 11,
induced by various metals. PCs are synthesized from GSH by the action of enzyme
γ-glutamylcysteine dipeptidyl transpeptidase, trivially named as phytochelatin syn-
thase. The enzyme is constitutively expressed but may be regulated at transcrip-
tional and translational levels by metals and metalloids. Nutrient culture studies
on
P. vulgaris
have depicted a marked enhancement in uptake, and translocation
of chelated
51
Cr in chromium chelated by DTPA was most effectively translocated
followed by
51
Cr-EDTA and
51
Cr-EDDHA (Athalye et al.
1995
). Similarly, signifi-
cant increases in chromium accumulation from Cr(III)-treated maize plants in the
presence of increasing concentrations of organic acid have been observed (Srivas-
tava et al.
1999a
). Shahandeh and Hossner (
2000
) have reported a high increase
in chromium uptake aided by organic acids. Srivastava et al. (
1999b
) found that
increasing concentrations of organic acids resulted in increased uptake of chromium
without affecting the distribution in plant parts. Induction of PCs against accumu-
lated chromium has been reported in root and leaves of
B. juncea
and
Vigna radiata
(Diwan et al.
2010c
). The induction of PCs in leaves was lesser than those observed
in roots. This may be due to binding of metal to other ligands (Salt et al.
1995
) or
to cell wall (Vecchia et al.
2005
) or due to binding with GSH. In
V. radiata
, there
was lesser induction of PCs at 200 µM chromium than at 50 and 100 µM chromium
treatments. The decrease in the PCs at higher concentration may be due to its trans-
port to shoot or due to the fact that PCs might have got degraded due to excessive
chromium accumulation (Diwan et al.
2010c
). 2DE analysis performed on controls
(water) and samples exposed to 100 and 300 ppm chromium showed a consistent
growth inhibition (30 % and 50 % of IG values at 100 and 300 ppm of potassium
dichromate, respectively) and metabolic changes (metallothionein synthesis and
chromium uptake) were observed. A total of 22 proteins showed significant and
reproducible up-regulation in the 100 as well as 300 ppm potassium dichromate
treated samples (Labra et al.
2006
).
5.3 Induction of Antioxidant Defense System
Chromium-induced production of ROS in plants is regulated by ROS-scavenging
system of the cell (Fig.
14.4
), based on the ascorbate cycle and the scavenging
enzymes, comprising enzymes like superoxide dismutase (SOD), catalase (CAT),
ascorbate peroxidase (APX), glutathione reductase (GR), and small antioxidant
molecules like ascorbic acid, cysteine, glutathione, tocopherol, caretenoids, hydro-
quinones and polyamine. The enzymatic parameters, namely SOD and CAT are
involved in the transformation of superoxides and peroxides to non-toxic species
(Caravaca et al.
2005
). Other enzymes, namely, ascorbate peroxidase and glutathi-
one reductase are involved in the ascorbate glutathione pathway. The reaction of
ascorbic acid with H
2
O
2
can occur directly, or it can be catalysed by APX, oxidizing
