Agriculture Reference
In-Depth Information
in cytoplasm, nuclei and mitochondria and is the major soluble antioxidant in these
cell compartments. GSH has been associated with several growth and development
related events in plants, including cell differentiation, cell death and senescence,
pathogen resistance and enzymatic regulation (Ogawa
2005
; Rausch and Wachter
2005
) and its content is affected by S nutrition (Blake-Kalff et al.
2000
). GSH is the
major reservoir of non-protein sulphur. It is the major redox buffer in most aerobic
cells, and plays an important role in physiological functions, including redox regu-
lation, conjugation of metabolites, detoxification of xenobiotics and homeostasis
and cellular signaling that triggers adaptive responses (Noctor et al.
2002
; Kopriva
and Koprivova
2005
). It also plays an indirect role in protecting membranes by
maintaining α-tocopherol and zeaxanthin in the reduced state. It can also function
directly as a free radical scavenger by reacting with superoxide, singlet oxygen
and hydroxyl radicals. GSH protects proteins against denaturation caused by the
oxidation of protein thiol groups under stress. In addition, GSH is a substrate for
glutathione peroxidase (GPX) and glutathione-S-transferases (GST), which are also
involved in the removal of ROS (Noctor et al.
2002
). GSH is a precursor of PCs,
which are crucial in controlling cellular heavy metal concentrations. GSH and its
oxidized form, GSSG maintains a redox balance in the cellular compartments. This
property of glutathione is of great biological importance since it allows fine-tuning
of the cellular redox environment under normal conditions and upon onset of stress,
and provides the basis for GSH stress signaling. A central nucleophilic cysteine
(Cys) residue is responsible for higher reductive potential of GSH. It scavenges
cytotoxic H
2
O
2
, and reacts non-enzymatically with other ROS, i.e., O
2
·ˉ, OH· and
1
O
2
(Larson
1988
). The central role of GSH in the antioxidative defense system
is due to its ability to regenerate another water soluble antioxidant, ascorbate, in
ascorbate-glutathione cycle (Foyer and Halliwell
1976
; Noctor and Foyer
1998
).
The role of GSH in the antioxidant defense system provides a strong basis for its
use as a stress marker. However, the concentration of cellular GSH has a major ef-
fect on its antioxidant function and it varies considerably under Cd stress. Further-
more, strong evidence has indicated that an elevated GSH concentration is corre-
lated with the ability of plants to withstand metal-induced oxidative stress (Freeman
et al.
2004
). Xiang et al. (
2001
) observed that plants with low levels of glutathione
were highly sensitive to even low levels of Cd
2 +
due to limited PC synthesis. The
increased demand for GSH can be met by the activation of pathways involved in
sulphur assimilation and cysteine biosynthesis. Its concentration is controlled by
a complex homeostatic mechanism where the availability of sulphur seems to be
required (May et al.
1998
). Manipulation of GSH biosynthesis increases resistance
to oxidative stress (Youssefian et al.
2001
; Sirko et al.
2004
). It has been observed
that upon Cd exposure, one of the main responses observed was the induction of
genes involved in sulphur assimilation-reduction and glutathione metabolism in the
roots of
Arabidopsis
(Herbette et al.
2006
).
Feedback inhibition of γ-glutamylcysteine synthase (γ-ECS) by GSH has been
considered as a fundamental central point for GSH synthesis. In vitro studies with
the enzymes from tobacco and parsley cells showed that the plant γ-ECS was in-
hibited by GSH (Noctor and Foyer
1998
). Oxidation of GSH to GSSG decreases
