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NADPH oxidase
Peroxidase
Xanthine oxidase
ROS generation
Amine oxidase
Oxalate oxidase
Glycollate oxidase
Lipoxygenase
Fig. 5.1
Enzymes involved in generation and accumulation of ROS in plant cells
diphenyleneiodonium (DPI) but not by cyanide or azide, and cell wall peroxidases,
which are inhibited by cyanide or azide but not by DPI (Grant et al.
2000b
;
Bolwell et al.
2002
) are the two important groups of enzymes involved in ROS
production (Suzuki et al.
2011
; Daudi et al.
2012
; Lehtonen et al.
2012
; O'Brein
et al.
2012
). The NADPH oxidases or cell wall peroxidases have been impli-
cated in the ROS production in different plant systems. In rose cells H
2
O
2
is
produced by a plasma membrane NADPH oxidase, whereas in bean cells H
2
O
2
is derived directly from cell wall peroxidases (Bolwell et al.
1998
).
An additional source of ROS may emanate intracellularly from xanthine oxidase
activity (Allan and Fluhr
1997
). Xanthine oxidase is a reductase supplying electrons
to NAD
+
to produce NADH (Halliwell and Gutteridge
1989
) and produces O
2
−
(Montalbini
1992
). Amine oxidases can induce ROS production by acting on amines
as substrates for the enzymes. These are a ubiquitous group of plant enzymes and
catalyze the oxidation of a variety of monoamines, diamines, and polyamines to the
corresponding aldehyde and release H
2
O
2
(Tipping and McPherson
1995
). Oxalate
oxidase is also a H
2
O
2
-generating enzyme (Zhou et al.
1998
). Some enzymes, such
as glycolate oxidase (Rojas and Mysore
2012
; Rojas et al.
2012
) and urate oxidase
(Halliwell and Gutteridge
1989
), can produce H
2
O
2
(Halliwell and Gutteridge
1989
). Lipoxygenase catalyzes the direct oxygenation of polyunsaturated fatty
acids and produces O
2
−
(Thompson et al.
1987
).
Different elicitors may induce ROS production by different types of enzymes.
The cryptogein-induced ROS burst was insensitive to the NO synthase inhibitor
L-NMMA, whereas L-arginine-induced ROS was sensitive to this inhibitor.
Cryptogein-induced ROS was signifi cantly inhibited by DPI, whereas L-arginine-
induced ROS burst remained unaffected (Allan and Fluhr
1997
). These studies
indicated that cryptogein-induced ROS burst is due to the action of NADPH oxi-
dase-type enzymes or xanthine oxidase. The L-arginine-induced ROS may be due
to the action of peroxidase- or amine oxidase-type enzymes (Allan and Fluhr
1997
).
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