Agriculture Reference
In-Depth Information
GSH levels and allows increased γ-ECS activity under stressed conditions (Noctor
and Foyer
1998
).
Environmental stresses trigger an increase in ROS levels in plants and the re-
sponse of glutathione can be crucial for adaptive responses. Antioxidant activity
in the leaves and chloroplast of
Phragmites australis
Trin. (cav.) ex Steudel was
associated with a large pool of GSH, protecting the activity of many photosynthetic
enzymes against the thiophilic bursting of Cd exerting a direct important protective
role in the presence of Cd (Pietrini et al.
2003
). Increased concentration of GSH
has been observed with the increasing Cd concentration in
Pisum sativum
(Gupta
et al.
2002
), romaine lettuce (Maier et al.
2003
),
Phragmites australis
(Pietrini et al.
2003
),
Brassica juncea
(Qadir et al.
2004
),
Pisum sativum
(Metwally et al.
2005
),
Sedum alfredii
(Sun et al.
2007
),
Oryza sativa
(Hassan et al.
2008
). However, decay
in GSH content in
Glycine max
roots (Balestrasse et al.
2001
),
Helianthus annuus
leaves (Gallego et al.
1996b
),
Zea mays
seedlings (Rauser
1990
),
Pisum sativum
(Ruegsegger et al.
1990
),
Pinus sylvestris
roots (Schutzendubel et al.
2001
),
Cucu-
mis sativus
chloroplast (Zhang et al.
2003
),
Populus
×
Canescens
roots (Schutzen-
dubel et al.
2002
) and
Oryza sativa
leaves (Hsu and Kao
2004
) has been reported
under Cd stress. Furthermore, unaltered GSH content was observed in the nodules
of
Glycine max
(Balestrasse et al.
2001
). Cadmium-induced depletion of GSH has
been mainly attributed to phytochelatin synthesis (Grill et al.
1985
). PC-heavy met-
al complexes have been reported to be accumulated in the vacuole of tobacco leaves
and in
Avena sativa
. These complexes have been shown to be transported across the
tonoplast (Salt and Rauser
1995
). The decline in the levels of GSH might also be
attributed to an increased utilization for ascorbate synthesis or for a direct interac-
tion with Cd (Pietrini et al.
2003
). The variety of response to Cd-induced oxidative
stress is probably related not only to the levels of Cd supplied, but also to the plant
species, the age of the plant and duration of the treatment.
All plants can synthesize ascorbate, which can accumulate to millimolar con-
centrations in both photosynthetic and non-photosynthetic tissues (Foyer et al.
1983
). Ascorbate is one of the most powerful antioxidants (Noctor and Foyer
1998
;
Smirnoff et al.
2001
), which reacts directly with hydroxyl radicals, superoxide and
singlet oxygen, and reduces H
2
O
2
to water
via
ascorbate peroxide reaction (Noctor
and Foyer
1998
). Ascorbate also acts as an electron donor in the regeneration of
α-tocopherol. Under physiological conditions, it exists mostly in reduced form in
leaves and chloroplast and its intracellular concentration can build up to millimolar
range (
viz
. 20 mM in the cytosol and 20-300 mM in the chloroplast stroma) (Foy-
er and Lelandais
1996
). The ability to donate electrons in a wide range of enzy-
matic and non-enzymatic reactions makes ascorbate the main ROS-detoxifying
compound in the aqueous phase. In addition to the importance of ascorbate in the
ascorbate-glutathione cycle, it plays a role in preserving the activities of enzymes
that contain prosthetic transition metal ions (Noctor and Foyer
1998
). The ascor-
bate redox system consists of L-ascorbic acid, MDHA and DHA. Both oxidized
forms of ascorbate are relatively unstable in aqueous environments while DHA
can be chemically reduced by GSH to ascorbate (Foyer and Halliwell
1976
). Evi-
dence to support the actual role of DHAR, GSH and GR in maintaining the foliar
