Agriculture Reference
In-Depth Information
tivum
plants by Dixit et al. (
2001
) and in roots of
Oryza sativa
and
Phragmites aus-
tralis
plants (Moons
2003
; Iannelli et al.
2002
). H
2
O
2
can be metabolized by another
plant peroxidase-scavenging enzyme called glutathione peroxidase (EC 1.11.1.9)
(Noctor et al.
2002
). Millar et al. (
2003
) identified a family of seven related proteins
in cytosol, chloroplast, mitochondria and endoplasmic reticulum, named AtGPX1-
AtGPX7 in
Arabidopsis
. Stress increases GPX activity in cultivars of
Capsicum
annuum
plants (Leon et al. 2002) but decreases in roots and causes no significant
change in the leaves of Cd-exposed
Pisum sativum
plants (Dixit et al.
2001
).
Superoxide dismutase (EC 1.15.1.1) was first isolated by Mann and Kleilin
(
1938
) and was thought to be a copper-storage protein. Subsequently, it was identi-
fied by different names, erythrocuprein, indophenol oxidase, and tetrazolium oxi-
dase until its catalytic function was discovered by McCord and Fridovich (
1969
).
SOD catalyzes the disproportionate superoxide O
2
·ˉ to hydrogen peroxide and
molecular oxygen. It removes O
2
·ˉ and hence decreases the risk of hydroxyl radical
formation from O
2
·ˉ via the metal catalyzed Haber-Weiss-type reaction. There are
three distinct types of SOD isozymes, classified on the basis of the metal cofactor
the copper/zinc (Cu/Zn-SOD), the manganese (Mn-SOD) and the iron (Fe-SOD),
that have been reported in various plant species (Bannister et al.
1987
; Alscher et al.
2002
). These isozymes can be separated by native polyacrylamide gel electropho-
resis. Their activity is detected by negative staining and identified on the basis of
their sensitivity to KCN and H
2
O
2
. The Mn-SOD is resistant to both inhibitors,
Cu/Zn-SOD is sensitive to both inhibitors, whereas Fe-SOD is resistant to KCN and
sensitive to H
2
O
2
. The subcellular distribution of these isozymes is also distinctive.
The Mn-SOD is found in the mitochondria of eukaryotic cells and in peroxisomes
(del Rio et al.
2003
); some Cu/Zn-SOD isozymes are found in the cytosolic frac-
tions, and also in chloroplasts of higher plants (del Rio et al.
2002
). The Fe-SOD
isozymes, often not detected in plants (Ferreira et al.
2002
) are usually associated
with the chloroplast compartment when present (Bowler et al.
1992
; Alscher et al.
2002
). The prokaryotic Mn-SOD and Fe-SOD, and the eukaryotic Cu/Zn-SOD
enzymes are dimers, whereas Mn-SOD of mitochondria is tetramers. Peroxisomes
and glyoxysomes of watermelons (
Citrillus vulgaris
) have been shown to contain
both Cu/Zn- and Mn-SOD activity, but there are no reports of extracellular SOD
enzymes in plants. All forms of SOD are nuclear-encoded and targeted to their
respective subcellular compartments by an amino terminal targeting sequence. Sev-
eral forms of SOD have been cloned from a variety of plants (Bowler et al.
1992
).
The response of SOD to heavy metal stress varies considerably depending upon
plant species, stage of the plant development, metal in the experiment and the ex-
posure time. SOD activity in leaves exhibited increases in activity in response to
Cd, whereas there was no significant variation in its activity in roots (Vitoria et al.
2001
). Activity staining for SOD in
Glycine max
revealed seven isozymes in leaves
and eight in roots, corresponding to Mn-SOD and Cu/Zn SOD isozymes. Although a
clear effect of Cd on plant growth was observed, the activities of the SOD isozymes
were unaltered (Ferreira et al.
2002
). In
Saccharum officinarum
seedlings, several
isozymes have been observed, but growth in the presence of Cd did not result in
any significant alteration in SOD activity (Fornazier et al.
2002
). In pea plants, a
