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induces the accumulation of pathogenesis-related (PR) proteins in plants (Kess-
mann and Ryals
1993
; Malamy et al.
1990
; Métraux et al.
1990
).
2 Roles of Reactive Oxygen Species (ROS) and Calcium
in Plant Defense Biology
2.1 ROS as Signaling Chemicals
The production of ROS members such as superoxide anion radicals (O
•-
), hydrogen
peroxide (H
2
O
2
), and hydroxyl radicals (HO
•
) at the cell surface well known as the
''oxidative burst'' is one of the earliest events detectable during the incompatible
interactions between plants and pathogens. As recently reviewed (Yoshioka et al.
2008
), Doke, a plant pathologist in Nagoya University (Nagoya, Japan) made the
first report on the involvement of ROS in the plant-pathogen interaction ca. 30 years
ago, in which he stated that the infection by Phytophthora infestans (late blight
pathogen of potato) in potato tubers causes the generation of O
•-
at the host cells'
plasma membrane (PM), only in the incompatible interactions (Doke
1983a
). A
series of his works demonstrated that the members of ROS possibly function as the
chemical signals required for induction of hypersensitive response (HR) as typified
by host cell death; now often referred to as plant apoptosis (Coll et al.
2011
; De Pinto
et al.
2012
). Among Russian biologists, there is a popular view that plant apoptosis
can be a model for the concept of genome-programmed cell death referred to as
phenoptosis which is widespread in the kingdoms of bacteria, protozoa, fungus,
plants and animals (Skulachev et al.
2012
).
Apart from photosynthetic or photochemical reactions, above works were the
first examples on the ROS generating activity in plants, which is specifically
responsive to the attacks by pathogenic microorganisms. Doke further reported
that the treatment of potato tuber protoplasts with the cell wall preparation from P.
infestans readily induces the ROS production, suggesting that chemical compo-
nents derived from pathogenic microorganisms (elicitors) trigger the burst of ROS
production in order to stimulate the plant defense mechanisms (Doke
1983b
).
In
1985
, Doke and his coworkers have discovered that the membrane fractions
isolated from the potato tubers inoculated with P. infestans produce the O
•-
in an
NADPH-dependent manner, and thus suggested that the enzyme for the ROS
production is the NADPH oxidase, closely resembling those known to be operated
in the activated neutrophils (Doke
1985
; Doke and Chai
1985
; Doke and Miura
1995
). Plant NADPH oxidases, also known as respiratory burst oxidase homo-
logues (RBOHs), are now considered as the most thoroughly studied enzymatic
ROS-generating systems and thus serving as important molecular 'hubs' during
ROS-mediated signaling in plants (Marino et al.
2012
). Our understanding of the
diversified roles for the ROS producers chiefly RBOHs in various plant processes
covering
the
cell
growth,
plant
development
and
plant
response
to
abiotic
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