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regulated through ROS production as observed for SA, abscisic acid (ABA),
jasmonic acid (JA) and ethylene (ET) (Gaupels et al.
2011
). Such ROS-mediated
hormonal regulations play key roles in the crosstalk between biotic and abiotic
stress signaling (Kawano
2003
; Ströher and Dietz
2006
; Mori and Schroeder
2004
). Although many components of the ROS signaling networks have recently
been identified, the mechanisms for orchestrated controls of the diversified ROS
production mechanisms at different cellular sites through fine tuning of ROS
feedback control to meet the physiological requirements such as plant growth,
development, stress adaptation and programmed cell death (PCD) are now actively
studied (Coll et al.
2011
).
2.2 Calcium Signaling Events Downstream and Upstream
of ROS
Land plants often show a variety of plasticity and behavior to adopt and conceive
the environmental stresses such as drought, salinity and attacks by pathogens or
insects. When plant cells are exposed to environmental stresses or perceive sig-
naling molecules involved in growth and development, ion channels are transiently
activated to convert these stimuli into intracellular events (Furuichi et al.
2007
).
When ion channels are activated by given stimuli, transient changes in cytosolic
concentration of the specific ion(s) and membrane potential are induced by rapid
activation of ion channels coupled to receptors, or themselves behaving as specific
receptors. Since the activities of some sort of the channels are known to be reg-
ulated by the membrane potential, the initial activation of ion channels may further
cause the sequential activation of other channels to relay and enhance the signals.
As above, the regulation of ion channel is inevitable for the signal transductions
and determination of plant behaviors at the end.
Among the ions taken up by the plant cells, calcium ion (Ca
2+
) plays an essential
role as an intracellular secondary messenger in plants and thereby the cytoplasmic
free Ca
2+
concentration ([Ca
2+
]
c
) is strictly regulated (Muto
1993
; Knight et al.
1991
; Sanders et al.
2002
). In the steady state, [Ca
2+
]
c
is constantly maintained to be
at low levels by homeostatic mechanisms involving continuous export of Ca
2+
by
Ca
2+
-ATPase and H
+
/Ca
2+
antiporter. When Ca
2+
-permeable channels are activated
in response to a variety of biotic and abiotic stimuli, the induced pulse of channel-
mediated Ca
2+
influx causes a drastic increase in the [Ca
2+
]
c
.
Since Muto and Miyachi (
1977
) discovered the presence of calmodulin (CaM)
from pea seedlings, the first calcium-binding protein found in plants, a growing
number of CaM-related proteins have been identified in plants. Recent studies
have confirmed that CaM members are likely involved in SA-mediated plant
defense signaling (Du et al.
2009
). After the action of calcium channels, Ca
2+
entered into the cells may bind and interact with Ca
2+
-regulated signaling proteins
such as CaMs and calcium-dependent protein kinases (CDPKs) which constitute a
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