Agriculture Reference
In-Depth Information
10.2.3 Regulation of ROS Detoxification by ABA
In all aerobic organisms, the concentration of ROS is tightly controlled by vari-
ous ROS-scavenging antioxidative defence systems (Foyer and Noctor
2005
).
However, the equilibrium between the generation and scavenging of ROS is chal-
lenged by various stress factors, which disrupt the redox homoeostasis of cells,
leading to oxidative damage (Foyer et al.
1994
). The antioxidant defence system
consists of antioxidant enzymes—such as superoxide dismutase (SOD), catalase
(CAT) and peroxidase (POD)—and antioxidant compounds, such as ascorbate and
reduced tripeptide glutathione (GSH;
ʳ
-glu-cys-gly). The antioxidant system pro-
tects plants against oxidative damage by scavenging ROS; the presence of anti-
oxidant enzymes and compounds in almost all cellular compartments suggests the
importance of ROS detoxification for protection against various stresses.
Enzymatic antioxidant systems
: The enzymatic antioxidant systems include a
variety of scavengers, such as SOD, ascorbate peroxidase (APX), glutathione per-
oxidase (GPX), glutathione-
S
-transferase (GST) and CAT. The discovery of the
enzymatic activity of SOD 45 years ago (McCord and Fridovich
1969
) opened up
the field of ROS biology.
Three classes of SOD are defined by their metal cofactors: copper/zinc (Cu/Zn-
SOD), manganese (Mn-SOD) and iron (Fe-SOD) (Mittler
2002
). SOD enzymes
provide the first line of defence against the toxic effects of elevated levels of ROS.
Overexpression of these SOD enhances plant tolerance to various stresses. For
example,
Arabidopsis
with Mn-SOD overexpression showed increased salt resist-
ance (Wang et al.
2004
). Transgenic tobacco plants that overexpressed Cu/Zn-
SOD were tolerant to multiple stresses, e.g. drought, salt and HL (Badawi et al.
2004
). In addition, Fe-SOD enzymes have been found to function in early chlo-
roplast development in
Arabidopsis
by protecting the chloroplast nucleoids from
ROS (Myouga et al.
2008
).
CAT enzymes dismutate H
2
O
2
into H
2
O and O
2
and are indispensable for
ROS detoxification under stress conditions (Garg and Manchanda
2009
). APX
has a higher affinity for H
2
O
2
(ﾵM range) than CAT and POD (mM range), and
it may have a more crucial role than CAT and POD in scavenging ROS and pro-
tecting cells during stress. The enhanced expression of APX in plants has been
demonstrated under different stress conditions.
APX
overexpression in
Nicotiana
tabacum
increases plant tolerance of either excess salt or water deficit (Badawi
et al.
2004
).
Arabidopsis
plants that overexpress
OsAPXa
or
OsAPXb
also exhib-
ited increased salt tolerance (Lu et al.
2007
). Transgenic plants that overexpress
SbpAPX
, a peroxisomal ascorbate peroxidase gene cloned from
Salicornia bra-
chiata
, were more tolerant to salt and drought stresses than WT plants (Singh
et al.
2013
).
Another example of an enzymatic antioxidant, GPX enzymes comprise a large
family of diverse isozymes, which can reduce H
2
O
2
and organic and lipid hydrop-
eroxides and can protect plant cells from oxidative damage (Noctor et al.
2002
).
Analysis of the physiological electron donor system for the
Arabidopsis
GPX
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