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et al.
1997
) and hence K
+
channels play important role in modulation of Ca
2+
signaling
system. Excessive hyperpolarization or depolarization may result in membrane
damage and affect signal transduction (Maathuis et al.
1997
). When excessive
hyperpolarization occurs, K
+
in
channels exhibit a strongly increasing tendency to
open and K
+
uptake through these channels prevents the membrane voltage from
becoming too negative. When signals induce rapid and excess membrane depolar-
ization, K
+
out
channels will open, and the resulting effl ux of K
+
will tend to limit the
extent of the depolarization. In this way, K
+
channels will modulate the membrane
potential and thereby activate Ca
2+
signaling system.
4.9
K
+
/H
+
Exchange Response in Ca
2+
Signaling System
K
+
/H
+
exchange response is an important component in the defense signaling system
(Orlandi et al.
1992
). The elicitors have been shown to trigger apoplastic alkalini-
zation combined with cytosolic acidifi cation in plant cells (Sakano
2001
; Felle et al.
2004
). Transient shifts of intracellular and apoplastic pH have been reported to be
essential steps in several signal transduction processes (Felle et al.
2004
). NAD
kinase is an important enzyme involved in Ca
2+
signaling and in ROS signaling
systems. NAD kinase is activated over a pH range of 7.1-6.8. The cytosolic pH in
unstimulated tobacco cells was 7.5, which changed to lower level after being stimu-
lated. At the acidic pH, NAD kinase was activated resulting in HR-related defense
responses (Karita et al.
2004
).
The enzyme H
+
/K
+
-ATPase is a proton pump which is responsible for the acidi-
fi cation of cytoplasm. The enzyme is a member of the P-type ATPase superfamily, a
large family of related proteins that transport ions across cell membranes. As an ion
pump, the H
+
/K
+
-ATPase is able to transport ions against a concentration gradient
using energy derived from the hydrolysis of ATP. A phosphate group is transferred
from adenosine triphosphate (ATP) to the H
+
/K
+
-ATPase during the transport cycle.
This phosphate transfer powers a conformational change in the enzyme that helps
drive ion transport. The enzyme transports one H
+
in exchange of one K
+
(Kuhlbrandt
2004
). The K
+
/H
+
exchange response may be mediated by Ca
2+
infl ux (Atkinson
et al.
1990
). The Ca
2+
-dependent protein kinase, activated by increased cytosolic
Ca
2+
, may evoke ion fl uxes that lead to extracellular alkalinization and depolarization
of the plasma membrane (Schaller and Oecking
1999
).
4.10
PAMPs and DAMPs May Trigger Calcium Ion
Infl ux/Effl ux Through Different Ca
2+
Channels
It has been shown that DND1 (a cyclic nucleotide-gated ion channel, CNGC) is
important for cytosolic Ca
2+
elevation in response to bacterial PAMP lipopoly-
saccharides (LPS) and the damage-associated molecular pattern (DAMP)/
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