Oxidative stress and antioxidant defence systems in response to pesticide stress - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 111

coniferyl alcohol. NADH utilized in the production of
H 2 O 2 is solely provided by malate dehydrogenase (Gross,
1977). The hypersensitive response (HR) caused by the
bacterium Xanthomonas campestris pv. malvacearum in
cotton plant, and potassium (K) deficiency stress in
Arabidopsis results in the generation of ROS by cell-wall-
located peroxidases (Martinez et al., 1998; Kim et al.,
2010). Diamine oxidases are also found to be involved
in the production of activated oxygen in the cell wall
(Elstner, 1991).

activated in plant cells in reponse to ROS accumulation
caused by abiotic stresses like temperature, salinity and
osmotic stresses. This signalling pathway includes differ-
ent zinc finger proteins and WRKY transcription factors.
The H 2 O 2 produced as a result of abscisic acid (ABA) is
an important signal in regulating stomatal closure to
minimize water loss through the activation of calcium-
permeable channels in the plasma membrane (Pel et al.,
2000). ABA-induced increases in H 2 O 2 levels play an
important role in ABA-induced stomatal closure up to a
certain level; however, further increases in H 2 O 2 levels
do not favour stomata closure (Jannat et al., 2011). Joo
et al. (2001) demonstrated a role for ROS as a second
messenger in mediating root gravitropism by inducing
asymmetric movement of auxin within 60 min.
Antioxidants like N -acetylcysteine, ascorbic acid and
Trolox (a vitamin E analogue) that participate in ROS
scavenging inhibited root gravitropism (Joo et al., 2001).
ROS signalling pathways interact with GAs in hormone-
regulated programmed cell death in barley aluerone
cells, stimulated by GA that induces ROS accumulation,
whereas ABA maintains low ROS concentrations
through activation of the alternative oxidase pathway
and ROS scavenging system (Fath et al., 2002).. An
exogenous supply of H 2 O 2 results in low GA signalling,
which hampers germination; however, ABA signalling
remains unaffected (Bahin et al., 2011). Plants have
developed a very complex signalling network to mediate
both biotic and abiotic stress responses due to ROS syn-
thesis. Excessive production of ROS, called oxidative
burst, during plant-pathogen interactions plays an
important role in signal transduction and thus ROS acts
as a second messenger for the signal transmission
(Klessig et al., 2000; Nanda et al., 2010). ROS production
also plays an important role in the expression of
defensive genes in tomato plants in response to wound-
ing (Orozco-Cárdenas et al., 2001).
Lignin is a protective polymer of plant cell walls that
plays a significant role in protecting plants from various
environmental stresses. According to Denness et al. (2011)
ROS production causing cell wall damage regulates lignin
biosynthesis in plants. ROS also play an important role in
signal transduction pathways involved in responses to
osmotic stress, low temperatures and heavy metals (Yuasa
et al., 2001; Xiong et al., 2002; Yeh et al., 2007). ROS also
play an important role in drought conditions in plants, and
it has been suggested that ROS are the signals by which
plants can sense drought conditions (Yeh et al., 2007).

7.4.2.7 Apoplast
Enzymes located in the cell wall have been proved to
be responsible for ROS production (Apel & Hirt,
2004; Heyno et al., 2011). Oxalate oxidase, a cell-wall-
associated enzyme (also known as germin), is responsible
for H 2 O 2 release. CO 2 from oxalic acid in the apoplast is
also involved in H 2 O 2 accumulation during interactions
between different cereal species and fungi (Wojtaszek,
1997; Lane, 2002). Amine oxidase-like enzymes may
provide defence responses under stress conditions in
plants via production of H 2 O 2 (Cona et al., 2006). Heyno
et al. (2011)observedthatapoplasticOH•generation
depends totally on peroxidase localized in the cell wall.

7.4.3 role of rOS as messengers
ROS function as second messengers at very low or
moderate concentrations and mediate several plant
responses including stomatal closure (Neill et al., 2002;
Kwak et al., , 2003; Yan et al., 2007), programmed cell
death (Bethke & Jones, 2001; Mittler, 2002) and gravit-
ropism (Joo et al., 2001), and provide tolerance to both
biotic and abiotic stresses (Torres et al., 2002; Miller et al.,
2008). Plants can sense, transduce and translate ROS
signals into appropriate cellular responses via redox-
sensitive enzymes, calcium mobilization, protein
phosphorylation and gene expression. Xiong et al.
(2002) provided an extensive review of ROS signalling
via tyrosine phosphatase formed as result of oxidation
of conserved cysteine residues. ROS can coordinate with
other components such as protein phosphatases, pro-
tein kinases and transcription factors (Cheng & Song,
2006) and communicate with other signal molecules
resulting in a signalling network cascade (Neill et al.,
2002). The balance between oxidant production and
removal by the antioxidant determines the strength,
lifetime and size of the ROS signalling pool. Miller et al.
(2008) identified a signalling pathway that becomes