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involving in stomatal movement, and the ion uptake from cytoplasm to vacuole
and release from vacuole to cytoplasm drives the changes of guard cell turgor
pressure, which is essential for stomatal closure and opening. Over 90 % osmotic
active solutes released from vacuole to cytoplasm are further released to apoplast
through plasma mambrane channels (MacRobbie
1998
). Therefore, ion channels
and ion pumps are the main functional proteins mediating ion flux through vac-
uole membrane. Using patch clamping technique, three types of Ca
2
+
-regulated
channels were identified electrophysiologically in
Vicia faba
guard cell vacuole,
including vacuolar K
+
(VK) channel, slow vacuolar (SV) channel, and fast vacu-
ole (FV) channel (Allen and Sanders
1996
). But these three types of vacuole chan-
nels are regulated by cytosolic Ca
2
+
differentially (Allen and Sanders
1996
) and
were proposed functioning in stomatal movement in response to different stimu-
lus. TPK1 (twin-pore K
+
channel 1) is a vacuolar K
+
(VK) channel identified
in
Arabidopsis
guard cells. TPK1 is from a family called two-pore-domain K
+
channel family, which has five members in
Arabidopsis
, and shares the structure
topology of four TM domains: two pore domains and two EF hand Ca
2
+
-binding
domains (Ward et al.
2009
). TPK1 can be activated by elevated cytosolic Ca
2
+
and low pH during stomatal closure (Gobert et al.
2007
). The disruption of TPK1
in
Arabidopsis
leads to the lack of VK channel activity and slower stomatal clo-
sure in response to ABA (Gobert et al.
2007
), suggesting a role of TPK1 in ABA-
induced stomatal movement.
SV channel activity is present in multiple tissues in plants, and TPC1 (two
pore channel 1) was identified to be a SV channel (Peiter et al.
2005
). TPC1 has
two pore domains and can be activated by cytosolic Ca
2
+
in a voltage-dependent
manner. TPC1 is a Ca
2
+
-permeable channel with a large conductance and thought
to involve in Ca
2
+
release from vacuole in guard cells. The knockout mutation
of
TPC1
leads to an insensitive phenotype to the inhibition of stomatal opening
induced by external Ca
2
+
without affecting ABA-induced stomatal closure (Peiter
et al.
2005
). High extracellular Ca
2
+
can cause the elevation of cytosolic Ca
2
+
in
guard cells and further close stomata. Calcium sensor 1 (CAS1) is the sensor to
sense the changes of extracellular Ca
2
+
concentration for
Arabidopsis
guard cells
(Han et al.
2003
). The down-regulation of
CAS1
expression level impairs extra-
cellular Ca
2
+
-induced stomatal closure, but leaves ABA response intact in guard
cells (Han et al.
2003
). ABA induces cytosolic Ca
2
+
increases and oscillation, and
Ca
2
+
release from intracellular Ca
2
+
stores involves in cytosolic Ca
2
+
oscillation.
Vacuole is believed to be important for the intracellular Ca
2
+
release and cytosolic
Ca
2
+
oscillation. But the intact ABA response in the guard cells of
tpc1
mutant
suggests that the Ca
2
+
channels mediating intracellular Ca
2
+
release for ABA-
induced Ca
2
+
oscillation differ from TPC1.
FV channel is another type of ion channel located in the membrane of vacu-
ole. FV channel is a nonselective cation channel, which is permeable to cation
K
+
, Ca
2
+
, and Mg
2
+
with a strong ion selectivity over anion (Allen and Sanders
1996
). Mg
2
+
can inhibit FV channel activity, but the presence of Mg
2
+
is required
for the activation of FV channels by cytosolic Ca
2
+
(Pei et al.
1999
). FV chan-
nel is insensitive to cytosolic K
+
, but can be regulated by luminal K
+
. It has been
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