Chemistry Reference
In-Depth Information
large multigene family in various plant species, by modifying their activities or
affinities to bind the specific sets of downstream targets (Shang et al.
2005
; Asano
et al.
2012
). Recently, functional analysis using gain-of-function and loss-of-
function mutants has revealed that several CDPKs are essential factors in abiotic
stress tolerance, positively or negatively regulating stress tolerance by lowering
the accumulation of ROS (Asano et al.
2012
).
Above signaling events mediated by transient or spontaneous changes in the
[Ca
2+
]
c
, termed Ca
2+
signaling, are mostly initiated by the activation of Ca
2+
-
permeable channels, and a large efforts in the electrophysiological studies eluci-
dated that plants have several Ca
2+
channels belonging to distinct types differing in
their gating mechanisms, namely ligand-gated, voltage-controlled and stretch-
activated (mechanosensitive) channels, and many corresponding genes have been
isolated (Piñeros and Tester
1997
; Furuichi et al.
2007
). In the last decade, a
variety of plant genes encoding Ca
2+
-permeable channels have been identified by
the efforts of genome sequencing projects for certain plants and some of them are
functionally characterized. They mostly differ from the animal types of channels,
and many members of these plant channels permeate not only Ca
2+
, but also K
+
,
Na
+
and other cations. However, it has been revealed that the regulation of
expression levels of some such genes markedly affect the Ca
2+
homeostasis in
plants, and thus in case of some plant species studied, so-called Ca
2+
-permeable
channels surely play important roles in Ca
2+
-signaling (Furuichi et al.
2007
).
2.3 Early Signaling Models for SA: Oxidative and Calcium
Signaling Events
In early 90's, involvement of ROS (Chen et al.
1993a
,
b
) and intracellular calcium
ion (Raz and Fluhr
1992
; Schneider-Müller et al.
1994
) during SA actions leading
to activation of SAR have been proposed based on the model enzyme demon-
strations and calcium depletion model experiments, respectively. By that time, a
number of studies have indicated that SA is an oxidative signal inducer which is
essentially involved in development of SAR against various pathogens with var-
ious natures. However, knowledge on the oxidative SA signal transduction
mechanism was largely lacked. It has been proposed that SA signal transduction
leading to SAR may be mediated by ROS derived from H
2
O
2
, since SA binds and
inhibits catalase (CAT) in vitro (Chen et al.
1993b
). While the CAT inhibition
model mearly explains a passive mechanism supporting the increases in ROS,
models with active mechanisms involving both POX (Kawano et al.
1998
; Kawano
and Muto
2000
) and RBOHs (Yoshioka et al.
2001
,
2003
) have been reported.
In addition to ROS, Ca
2+
is another possible mediator of SA signals as dis-
cussed above, and certain number of reports indicated that Ca
2+
is essential for the
action of SA during plant defense. For instance, removal of Ca
2+
or blockade of
Ca
2+
uptake inhibits the induction of SA-inducible chitinase accumulation in
Search WWH ::
Custom Search