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CaM-regulated NAD kinase (Harding et al.
1997
). The indirectly induced H
2
O
2
production would have activated the Ca
2+
infl ux.
The NADPH oxidase-derived ROS stimulates a Ca
2+
infl ux into the cytoplasm.
The rise in Ca
2+
level in turn activates NADPH oxidase to produce ROS (Takeda
et al.
2008
), suggesting a positive feedback regulation of Ca
2+
infl ux - ROS signal-
ing system. Protein phosphorylation has been shown to be a prerequisite for the
Ca
2+
-dependent activation of
Arabidopsis
NADPH oxidases (Kimura et al.
2012
). In
Arabidopsis
, the respiratory oxidase homologue F (
AtrbohF
) encodes NADPH oxidase.
AtRbohF exhibited ROS-producing activity that was synergistically activated by
protein phosphorylation and Ca
2+
. The two EF-hand motifs of AtRbohF in the ter-
minal cytosolic region were found to be crucial for its Ca
2+
-dependent activation. A
protein kinase inhibitor inhibited the Ca
2+
-dependent activation of AtRbohD in a
dose-dependent manner, suggesting that protein phosphorylation is a prerequisite
for the Ca
2+
-dependent activation of RbohF (Kimura et al.
2012
). These results sug-
gest a positive feedback regulation of Ca
2+
and ROS in the Ca
2+
infl ux signaling
system (Fig.
4.1
).
4.3
Ca
2+
Infl ux Channels in Plant Cell Plasma Membrane
4.3.1
Voltage-Gated Ca
2+
-Permeable Channels
Several Ca
2+
-permeable channels have been found in plant plasma membranes and
they have been implicated in immune response signaling systems (Gaxiola et al.
2007
; Hamilton et al.
2000
; White and Broadley
2003
; Ma et al.
2009b
; Qi et al.
2010
; Kwaaitaal et al.
2011
; Michard et al.
2011
; Vatsa et al.
2011
). Calcium ion
channels are integral membrane proteins that are involved in transport of solutes
across the cell membrane in plants (Maathuis et al.
1997
). Diffusion of ions through
the channel is mostly due to membrane voltage (Miedema et al.
2008
). Ion channels
remain in “closed” conformational state and the ion channels “open” in response to
ligands or to a change in membrane voltage (Maathuis et al.
1997
). Some channels
open in hyperpolarizing conditions (i.e. at rather negative membrane voltages,
inward current) (Schroeder et al.
1994
). Another class of channels open in depolar-
izing conditions (at relatively positive voltage, outward current) (Tester
1990
).
Channels are increasingly activated at more negative and more positive membrane
voltage (Maathuis et al.
1997
). Changes in membrane potential are associated with
the initiation of a number of signal transduction pathways (Ward et al.
1995
). Most
of these Ca
2+
channels are not strictly selective for Ca
2+
, and they also facilitate the
transport of other cations (Very and Sentenac
2002
).
Depolarization-activated Ca
2+
-permeable channels are common in plasma mem-
brane of plant cells. They activate signifi cantly at voltages more positive than about
−150 to −100 mV. These voltage-gated Ca
2+
channels mediate Ca
2+
infl ux across the
plasma membrane of cells (Huang et al.
1994
).
Ca
2+
channel opens upon
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