Agriculture Reference
In-Depth Information
TheOH•radicalisthemostreactiveproductofpartial
oxygen reduction in biological systems and has a
half-life of 1 nanosecond. As a result, it can react with
and damage any type of molecule that it encounters
within a cell. The important feature of ROS chemistry
derives from the interconversion of one ROS into
another. In this manner, the Fenton and Haber-Weiss
reactions were considered to be the key events in ROS
interconversions. Beside the above-mentioned
reactions, ROS can interact with each other, which leads
to production of a third group of ROS species that are
more toxic than the primordial ones (Dat
et al.,
2000).
For example, peroxynitrite (ONOO
-
) is produced
when
.
O
2
-
reacts with nitric oxide (NO) (Bartosz, 1996;
Beligni & Lamattina, 1999). Peroxynitrous acid
(ONOOH) in its protonated form of ONOO
-
is a strong
oxidizing agent and gives rise to various highly dam-
aging radical and non-radical reactive species (Halliwell,
2006). Different ROS have very diverse properties and
reactivities,
endoplasmic reticulum and cell walls. Reactive oxygen
species are usually formed by the inevitable flow of elec-
trons to O
2
from the electron transport activities of
chloroplasts, mitochondria and plasma membranes, or
as a by-product in various metabolic pathways.
7.4.2.1 Chloroplasts
The electron transport chain in photosystem I (PSI) and
photosystem II (PSII) are the main sources of ROS in
chloroplasts. Conditions like salinity, increased light
intensity, pesticides, temperature stresses and other
conditions limiting CO
2
fixation result in the enhanced
production of ROS in plants. Under normal conditions,
the electrons from the excited photosystem reduce
NADP to NADPH, which then enters the Calvin cycle
and reduces the final electron acceptor, i.e. CO
2
. Electron
leakage from ferredoxin to O
2
results in generation of
•O
2
-
due to decreased NADP supply and overloading of
the electron transfer ch ain (ETC) under stress condi-
tions (Elstner, 1991). This process is called the Mehler
reaction:
their
order
of
reactivity
being:
OH•>O
2
-
>H
2
O
2
(Sweetlove & Moller, 2009).
Glutathione is a low molecular weight thiol and sul-
phur-containing tripeptide. It is also a universal
redox-sensing component used as a marker of oxidative
stress at the cellular level. Moreover, it plays an essential
role in the detoxification of xenobiotics and also seques-
ters heavy metals by acting as the precursor of heavy
metal-binding phytochelatins. Glutathione serves as
antioxidant in cellular compartments including mito-
chondria, cytosol, peroxisomes, nuclei and chloroplasts.
In the ascorbate-glutathione cycle (AGC), dehydroascor-
bate (DHA) is oxidized to ascorbate (ASC), which utilizes
GSH as an electron donor. This pathway is considered to
be the main pathway of free radical removal in the chlo-
roplast stroma by equilibrating the redox status. The
ASC cycle is catalysed by a set of four enzymes: ascorbate
peroxidase (APX), monodehydroascorbate reductase
(MDHAR), glutathione-dependent dehydroascorbate
reductase (DHAR) and glutathione reductase (GR). ASC
and glutathione participate in cyclic electron transfer
from NADPH, which leads to reduction of H
2
O
2
.
Senescence and plant death occur due to the progressive
oxidation and degradation of glutathione and ASC pools.
−
22
Oferredoxin
+
→
2
•
Oferredoxin
+
2
2
2
In PSI, the electron transport chain involves leakage of
electrons to O
2
from 2Fe-2S and 4Fe-4S clusters,
whereas in PSII, the plastoquinones QA and QB are
electron acceptors in the ETC. Leakage of electrons from
this site to O
2
resultsintheproductionof•O
2
-
(Cleland &
Grace, 1999).
The•O
2
-
formed by O
2
reduction is a rate-limiting
step.Onceformed,•O
2
-
results in the generation of
more aggressive ROS. It may then either be converted to
HO
2
on the internal membrane surface or to H
2
O
2
, by
the enzymatic activity of SOD, on the external mem-
brane surface. H
2
O
2
once formed is converted to the
highlydangerousOH•throughtheFentonreaction.
7.4.2.2 Mitochondria
Reactive oxygen species are produced at several sites of
the ETC in mitochondria. Direct reduction of oxygen to
•O
2
-
in mitochondria occurs in complex I, i.e. i.e.
NADH:ubiquinone oxidoreductase (Arora
et al.,
2002).
There is reverse electron flow, i.e. electron transport
from complex II to I, when NAD
+
-linked substrates for
complex I are limited. This reverse electron flow results
in the increased production of ROS at complex I, and
this flow is regulated by hydrolysis of ATP (Turrens,
2003). Complex III, the ubiquinone-cytochrome
c
7.4.2 Sites of production of rOS
The formation of ROS occurs in all cells irrespective of
stress at several locations, including chloroplasts, mito-
chondria, plasma membranes, peroxisomes, apoplast,
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