Agriculture Reference
In-Depth Information
4 Tolerance Mechanism of Plant against Cd
Plants have evolved a complex array of mechanisms to maintain optimal metal
levels and avoid the detrimental effects of excessively high concentrations (Cle-
mens
2001
). When these homeostatic mechanisms are overwhelmed, plants suffer
metal-induced damage and pro-oxidant conditions within cells. However, higher
plants are very well equipped with antioxidant mechanisms (Mittler et al.
2004
;
Gill and Tuteja
2010
). Plant cells display an antioxidant network, including numer-
ous soluble and membrane compounds, particularly in mitochondria and in chlo-
roplasts where respiratory and photosynthetic electron transfer chainstake place,
respectively. Antioxidant enzymes are considered as those that either catalyze such
reactions, or are involved in the direct processing of ROS (Gill and Tuteja
2010
).
Plants possess very efficient enzymatic (SOD; CAT; APX; GR; MDHAR; DHAR;
GOPX and GST) and non-enzymatic (AsA; GSH; phenolic compounds, alkaloids,
non-protein amino acids and α-tocopherols) antioxidant defense systems.
Ascorbate peroxidase (EC 1.11.1.11) is a heme protein, and its primary function
is the rapid removal of H
2
O
2
at the site of generation (Asada
1992
). APX isozymes
are distributed in at least four distinct cell compartments, the stroma (sAPX), thyla-
koid membrane (tAPX), the mitochondria (mAPX), and the cytosol (cAPX) (Ishi-
kawa et al.
1998
; Asada
1992
). The various isoforms of APX respond differentially
to metabolic and environmental signals (Kuboi et al.
1987
). Thylakoid membrane-
bound APX is a limiting factor of antioxidant systems under photoxidative stress in
chloroplasts and the enhanced tAPX activity maintains the redox status of ascorbate
under stress conditions (Yabuta et al.
2002
). Chloroplasts contain APX in two iso-
forms, thylakoid-bound and soluble stromal enzymes. At least one-half of the chlo-
roplastic APX is tAPX, but the ratio of tAPX/sAPX varies according to the plant
species, possibly, leaf age, but the biosynthetic ratio of the two APXs is controlled
by alternative splicing (Asada
2006
). The tAPX binds with the stroma thylakoids
where the PSI complex is located, while sAPX is thought to be localized in the
stroma (Asada
2006
). Plants also contain the cytosolic isoforms of APX (cAPX),
which has a different amino acid sequence in comparison to chloroplastic APXs, but
participate in the scavenging of H
2
O
2
in compartments other than chloroplasts. The
cAPX is a homodimer and its electron donor is not so specific for ascorbate, unlike
tAPX and sAPX (Asada
2006
).
Ascorbate peroxidase has an important role in the scavenging of H
2
O
2
un-
der stressed conditions but its activity depends on the Cd concentration applied.
Increased leaf APX activity under Cd stress has been reported in
Ceratophyllum de-
mersum
(Arvind and Prasad
2003
),
Brassica juncea
(Mobin and Khan
2007
),
Pisum
sativum
(Romero-Puertas et al.
1999
),
Phaseolus aureus
(Shaw
1995
),
Phaseolus
vulgaris
(Chaoui et al.
1997b
),
Zea mays
(Krantev et al.
2008
),
Triticum aestivum
(Khan et al.
2007
),
Vigna mungo
(Singh et al.
2008
) and
Brassica campestris
(An-
jum et al.
2008
), however, in
Hordeum vulgare
roots the APX activity was reduced
at high concentration of Cd (Hegedus et al.
2001
). Balestrasse et al. (
2001
) reported
that low Cd levels led to an increased APX activity in
Glycine max
roots and nod-
