Oxidative stress and antioxidant defence systems in response to pesticide stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 115

function is rapid removal of H 2 O 2 at the site of generation
(Asada, 1992). APX isoenzymes are distributed in at least
four distinct cell compartments: the stroma (sAPX), thy-
lakoid membrane (tAPX), mitochondria (mAPX) and
cytosol (cAPX) (Asada, 1992; Miyake & Asada, 1992;
Ishikawa et al., 1998). Isolation and characterization of
cDNA encoding the different isoforms of APX has been
done (Ishikawa et al., 1998). The two chloroplastic APX
(Chl APX) isoenzymes are encoded by a single gene
( ApxII ) and the mRNA undergoes alterative splicing of
its two 3′ terminal exons (Yoshimura et al., 1999).
Diverse isoforms of APX exhibit differential responses
under varied stress. The enzyme has two cytosolic forms
with a purely defensive role and a membrane-bound
(27 kDa) form having a functional role in addition to
hydrogen peroxide scavenging.
Ascorbate can be regenerated from MDHA by the
reaction catalysed by MDHAR. MDHAR are flavin nucle-
otide-containing enzymes found in chloroplasts and in
cytosol, as well as in mitochondria and peroxisomes.
They catalyse the reduction of MDHA to ascorbate by
NAD(P)H 2 . MDHA radical can also be reduced to ascor-
bate by photoreduced ferredoxin in the chloroplast PSI.
Alternatively it can spontaneously dissociate to ascorbate
and DHA, which can subsequently be reduced by
another enzyme, DHA reductase, which regenerates
ascorbate. DHA reductase, present in chloroplast stroma,
reduces DHA to ascorbate by the ubiquitous cellular pep-
tide, glutathione (GSH). Usually APX operates in a cycle
with glutathione reductase (GR). GR uses reducing
equivalents derived from glucose through the pentose
phosphate pathway and NADPH to generate the reduced
from of glutathione (GSH) from the oxidized disulphide
form (GSSG) resulting from the action of APX.
Glutathione reductases are mainly found in chloro-
plast, cytoplasm and mitochondria of the plants. They
range in size from around 90 to 140 kDa and usually con-
tain two protein subunits, each with a flavin dinucleotide
(FAD) at its active site. It appears that NADPH reduces
the flavin nucleotide, which then transfers its electrons
onto a disulphide bridge (-S-S-) in the enzyme. The two
sulphydryl groups (-SH) furnished interact with GSSG
and reduce it to GSH. The activity of GR suggests that the
GSH/GSSG ratio in normal cells is kept high. The utiliza-
tion of NADPH acts as an energy sink, which may affect
indirectly the efficiency of the electron transport system.
It also causes the production of a trans-thylakoidal
proton gradient that is involved in control of electron

transport (Foyer et al., 1994). Despite the replacement of
most of the old organochlorine insecticides by pyre-
throids, little is known about the ecological side effects
of these pesticides on the soils, overall plant growth,
plant yields and the associated soil microbes in the
agro-ecosystem.

7.4.6 antioxidative defence mechanism
against pesticides
Increased ROS production is a vigorous and sensitive
response of plants to environmental stimuli (Jiang &
Yang, 2009; Jan et al., 2012). Herbicides are known to
generate activated oxygen species, which are likely to
contribute to the toxic effects of these herbicides (Asada
& Takahashi, 1987; Halliwell, 1987). The oxidative stress
induced by synthetic pyrethroid insecticide toxicity is
well documented (Sayeed et al., 2003; Parvez &
Raisuddin, 2005). Increased lipid peroxidation rate is
regarded as a general response to many stresses like
heavy metals (Chaoui et al., 1997; Lozano-Rodriguez
et al., 1997; Vanaja et al., 2000), high salinity (Gharsally &
Cherif, 1984; Hernandez et al., 2000) and low tempera-
ture (Rodionov et al., 1973). The increased concentration
of TBARS in Glycine max at 0.20% treatment with alpha-
methrin and deltamethrin and 0.25% treatment with
lambda-cyhalothrin insecticide suggests that these plants
are highly susceptible to insecticidal stress.
Synthetic pyrethroid insecticide-treated seedlings
demonstrated a significant enhancement in SOD activity,
especially during the flowering stage. The relatively low
activity post-flowering could be because old leaves gener-
ally contain low concentrations of antioxidants, making
them more prone to enhanced oxidative injury than
young leaves (Polle, 1997). It has been demonstrated
that SOD plays an important role in protecting against
oxidative damage in plants (Bowler et al., 1992). Bashir
et al. (2007) observed that activities of antioxidative
enzymes like APX, SOD, GR, CAT and glutathione
increase till the flowering stage followed by a decreasing
trend during senescence of the plant. Similar observa-
tions have been recorded in the roots of Japanese radish
and pea plant by Morimura et al. (1999) and Donahue
et al. (1997), respectively. They found high activities of
SOD (H 2 O 2 -generating enzyme), MDHAR, DHAR and
GR (ascorbic acid (AsA)-regenerating enzymes) were
associated with an increase in APX during root growth.
These results suggest that the AsA-APX system may play
an important role in the protection of root tissues