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In-Depth Information
The oxidative burst induced in French bean cultured cells by a fungal elicitor,
involves an apoplastic peroxidase (Bolwell et al.
1995
). The O
2
-heme complex of
peroxidase is reduced to compound III by reductants exported from the cell. Under
elevated pH conditions, the complex is effectively hydrolyzed to release H
2
O
2
. In
this model the source of electrons has not been identifi ed, but the release of a reduc-
tant from elicited cells has been observed (Bolwell et al.
1995
). Bolwell et al. (
1998
)
showed that in bean cells treated with a fungal elicitor, H
2
O
2
was derived directly
from cell wall peroxidases following extracellular alkalinization and the appearance
of a reductant.
Production of apoplastic ROS by chitin treatment in
Physcomitrella patens
has
been shown to require peroxidase. The fungal elicitor chitin caused an immediate
oxidative burst in wild-type
P
.
patens
but not in the
Prx34
mutants lacking the
chitin-responsive secreted class III peroxidase (Prx34), suggesting the requirement
of peroxidase for the production of ROS (Lehtonen et al.
2012
).
∆
5.3
ROS-Scavenging Systems May Be Involved
in Fine-Tuning Accumulation of ROS
Various ROS-scavenging systems, including catalases, ascorbate peroxidases,
glutathione, superoxide dismutases are involved in increases in ROS in the plant
cell (Mittler et al.
2004
) and in activation of plant defense responses (Mittler et al.
1999
; Klessig et al.
2000
). It is widely reported that inhibition of catalase leads to
accumulation of H
2
O
2
(Takahashi et al.
1997
). Salicylic acid (SA), which inhibits
catalase, increases accumulation of H
2
O
2
in elicited cells (Delaney et al.
1994
;
Willekens et al.
1994
). Xanthine oxidase and peroxidase also reduce the level of
catalase and hence increase the production of H
2
O
2
(Milosevic and Slusarenko
1996
). Compartmentalization of both ROS production and activation of ROS-
scavenging systems contribute to fi ne-tuning of ROS levels and their signaling
properties (Torres et al.
2006
).
5.4
Site of Production of ROS
Production of ROS induced by various signals has been detected in the apoplast and
also within cells. Plasma membranes and organelles, such as mitochondria, peroxi-
somes, and chloroplasts have been shown to act as ROS generators (Grant and
Loake
2000
; Asada
2006
; Torres et al.
2006
; Astamker et al.
2007
). It has been
shown that isolated nuclei can also generate H
2
O
2
in response to calcium addition
(Astamker et al.
2007
). ROS accumulated within the tobacco cells more rapidly
than the response outside the cell (Astamker et al.
2007
). Sang et al. (
2012
) showed
that the PAMP harpin activated NADP oxidase located in the plasma membrane and
induced generation of ROS in the apoplast. H
2
O
2
was generated in apoplasts in a
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