Biology Reference
In-Depth Information
5.1
Reactive Oxygen Intermediates Involved
in Oxidative Burst
The rapid and transient production of reactive oxygen species (ROS), also called
oxidative burst or respiratory burst, is one of the most rapid defense responses
observed in plants due to pathogen-associated molecular pattern (PAMP)/elicitor
treatment (Yang et al.
1997
; Grant and Loake
2000
; Faize et al.
2004
; Asada
2006
;
Sagi and Fluhr
2006
; Vidhyasekaran,
2007
; Lehtonen et al.
2012
). The ROS,
which are also called as reactive oxygen intermediates (ROI; Grant and Loake
2000
; Pieterse and Van Loon
2004
), include hydrogen peroxide (H
2
O
2
), super-
oxide (O
2
−
), singlet oxygen (
1
O
2
), and hydroxyl radical (OH˚) (Grant and Loake
2000
; Vidhyasekaran
2007
).
O
2
−
is the fi rst ROS induced by elicitor treatment (Haga et al.
1995
; Faize et al.
2004
). The superoxide is only short lived and is thought to be produced in the
outer surface of the cell within a few minutes of elicitor treatment (Sagi and
Fluhr
2006
). The half life of O
2
−
is less than a second and is usually rapidly
dismutated either nonenzymatically or via superoxide dismutase (SOD) to H
2
O
2
,
which is relatively stable (Grant and Loake
2000
). The negatively charged O
2
−
could traverse the plasma membrane as the neutral hydroperoxyl (HO
2
) or being
converted to the membrane-permeable H
2
O
2
(Sagi and Fluhr
2006
). Protonation
of O
2
−
can produce the hydroperoxyl radical HO
2
-
, which can convert fatty acids
to toxic lipid peroxides. Moreover, in the presence of divalent metal ions such as
Fe
2+
, H
2
O
2
can undergo Fenton reaction, producing the hydroxyl radical (OH˚)
(Grant and Loake
2000
). Singlet oxygen (
1
O
2
) is an excited state of molecular
oxygen that can be generated in a number of ways including the spontaneous
dismutation of two O
2
−
radicals (Elstner
1982
; Scandalios
1993
). Among the dif-
ferent ROS, H
2
O
2
is the most attractive candidate for defense signaling because
of its relatively long life and high permeability across membranes (Allan and
Fluhr
1997
).
5.2
Upstream Events in ROS Signaling System
5.2.1
Enzymes Involved in ROS Generation
The oxidative burst is often a very rapid response induced by elicitor, occurring
within seconds in some systems, such as cultured cells of French bean (
Phaseolus
vulgaris
) and soybean (Bolwell et al.
1995
). In other systems, such as rose (
Rosa
damascena
) cultured cells (Arnott and Murphy
1991
), it may be delayed for few
minutes or hours. These observations suggest that the oxidative burst may not
require
de novo
protein synthesis but involves the activation of pre-existing enzymes.
Several enzymes have been implicated in the PAMP/elicitor-induced apo-
plastic ROS production (Fig.
5.1
). NADPH oxidases, which are inhibited by
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