Agriculture Reference
In-Depth Information
oxidative damage. It has been reported that plants can
utilize only 1% of the oxygen consumed in the produc-
tionofROSlikehydroxylradical(OH•),singletoxygen
(
1
O
2
)andsuperoxideradical(•O
2
-
) (Asada & Takahashi,
1987).
Lipids are most prone to oxidative damage in almost
all living organisms (Zhang & Kirkham, 1996; Hung &
Koa, 1997). Lipid peroxidation begins with an external
oxidant, usually an oxygen-centred free radical such as
OH•,•O
2
-
orROO•,attackinganallylicmethylenegroup
and converting it to a new cation-centred free radical:
(CuZn-SOD), the manganese type (Mn-SOD), the iron
type (Fe-SOD) and the nickle type (Ni-SOD). Of these,
CuZn-SOD is present in chloroplasts while Fe-SOD is
occasionally found in plant species (Van-Camp
et al.,
1994). All these isoforms of SOD help in protecting cel-
lular molecules from ROS damage (Fridovich, 1986).
The mechanisms controlling and regulating the expres-
sion and activity of different isoforms of SOD are
regulated by the genes and are highly complex. All
these regulatory genes respond differently to different
environmental signals (Sen-Gupta
et al.,
1993).
Catalases are present in both prokaryotes and eukary-
otes and are responsible for the conversion of H
2
O
2
to
water and oxygen. H
2
O
2
generation occurs in the elec-
tron transport chain of mitochondria, PSII system in
chloroplasts and peroxisomes by the action of cytochrome
P450 , oxidase and dehydrogenase reactions, respec-
tively. Once formed, H
2
O
2
readily penetrates cell
membranes and causes major damage to DNA by gener-
atinghighlyreactivehydroxylradicalsOH•,through
Fenton-type reactions involving the interaction of H
2
O
2
with transition metal ions such as Fe. Removal of H
2
O
2
by catalases (CAT) mainly located in peroxisomes is
highly energy efficient, as it does not consume cellular
reducing equivalents. Different isozymes of CAT are
present in plants, like CAT-1, CAT-2 and CAT-3. Among
those, CAT-1 and CAT-2 are present in peroxisomes,
whereas CAT-3 is present in mitochondria. In photosyn-
thesizing plants, CAT together with SOD, forms the
most efficient antioxidative machinery. Ascorbate per-
oxidase (APX) along with CAT helps in the detoxification
of H
2
O
2
and utilizes ascorbate as the electron donor for
the reduction of H
2
O
2
(Asada & Takahashi, 1987).
Ascorbate is the most important reducing substrate for
H
2
O
2
in plant cells (Alscher, 1989; Smirnoff, 1993;
Mehlhorn
et al.,
1996).
Hydrogen peroxide can be detoxified by the action of
APX, DHAR and GR through the ascorbate-glutathione
cycle. Although it is well established that the ascorbate-
glutathione cycle occurs in chloroplasts, it is now clear
that the enzymes of this cycle are also found in mito-
chondria and peroxisomes and may represent an
important antioxidant protection system against H
2
O
2
generated in these organelles (Jimenez
et al.,
1997).
Ascorbate peroxidase, or scavenging peroxidase, is a
haem protein with protoporphyrin IX as its prosthetic
group and a molecular size similar to the classic plant
peroxidase guaiacol peroxidase (GuPX). Its primary
RCH
=
CHCH
+ →=
OH
RCH
CHCH
+
HO
2
2
Singlet oxygen reacts with unsaturated lipids and
undergoes an 'enzymatic reaction' that leads to incorpo-
ration of oxygen into the lipid chain and migration of a
double bond leading to lipid peroxidation. This ensures
oxidative stress as the defence mechanism in plants
accompanied by the formation of various free radicals
and other by-products (del-Rio
et al.,
1996). Lipid per-
oxidation (LPO) results in membrane damage (Vaughan
et al.,
1982) because the geometry of alkyl chains and
the packing order in the bilayer are disrupted and
altered. Membrane damage is directly proportional to
lipid peroxidation - i.e. by measuring membrane
damage; we can easily determine the level of lipid
destruction. It has been observed that during lipid per-
oxidation various products including small hydrocarbons
such as ketones, malondialdehyde (MDA) and other
related compounds are formed from polyunsaturated
precursors (Bird
et al.,
1983; de-Vos
et al.
, 1993; Weckx &
Clijsters, 1996; Reddy
et al.,
1998). Some of these com-
pounds react with thiobarbituric acid (TBA) to form
coloured products and hence are called thiobarbituric
acid reactive substances (TBARS); they can be measured
by monitoring their absorption in a spectrophotometer
at around 530 nm (Gray, 1978). MDA is usually used to
assess the impact of adverse conditions on the organism
(Bennicelli
et al.,
1998).
7.4.5 antioxidants
Superoxide dismutase (SOD) plays an important role in
protecting photosynthesizing plants against oxidative
damage (Salin, 1988; Bowler
et al.,
1992; Foyer
et al.,
1994). A SOD isoform found in the chloroplast has been
found to act in combination with APX, MDHAR and
GR (Salin & Bridges, 1981; Dalton, 1995). In plants
four kinds of SOD are found: the copper-zinc type
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