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channels (Laanemets et al.
2013
). Together, these results show that ABA may
regulate inward K
+
channel currents in different levels, including transcriptional
level of gene expression, protein trafficking, K
+
channel activity, and coordination
of different types of channels in guard cells. These regulating mechanisms work
together to facilitate a fast and efficient stomatal movement regulation.
15.6.3 Plasma Membrane Anion Channels
Anion efflux causes the depolarization of the plasma membrane of guard cells,
further opens the outward-rectifying K
+
channel, and so stomata can be closed.
Therefore, anion channels in the plasma membrane of guard cells play core roles
for stomatal closure. Over 20 years ago, the activity of anion channel currents
was recorded using patch clamping technique in
Vicia faba
guard cell protoplast
(Schroeder and Hagiwara
1989
). Based on the electrophysiological analysis, two
different types of depolarization-activated anion channel currents were dissected
electrophysiologically in
Vicia faba
guard cells (Schroeder and Keller
1992
). One
was activated by depolarization voltage and rapidly deactivated by hyperpolarization
voltage, and so was named as R-type (rapid) anion current (Schroeder and Keller
1992
). The second depolarization-activated current is activated and deactivated very
slowly, and so was named as S-type (slow) anion currents (Schroeder and Keller
1992
). Further study revealed that S-type anion channels in
Arabidopsis
guard cells
can be activated by cytosolic Ca
2
+
and external ABA (Pei et al.
1997
). However, the
genetic identities of the two types of anion channels remained unknown for many
years. Recent work from different groups identified slow anion channel-associated 1
(SLAC1) as an important component for stomatal closure in
Arabidopsis
(Negi et al.
2008
; Vahisalu et al.
2008
).
SLAC1
mainly expresses in guard cells, occasionally in
vascular tissue (Negi et al.
2008
; Vahisalu et al.
2008
). SLAC1 belongs to a small
family with five members in
Arabidopsis
, and the other four members were named
as SLAC1 homologs (SLAHs), which mainly express in transmitting tissue in root
and stem in
Arabidopsis
(Negi et al.
2008
). SLAC1 has ten TM domains, a hydro-
philic N-terminal tail and a hydrophilic C-terminal tail, and these structural charac-
ters suggest that SLAC1 is an ion channel. The loss-of-function mutants of
SLAC1
show strong insensitive phenotypes in stomatal closure in response to ABA, ozone,
ROS, darkness, CO
2
elevation, and decrease of relative air humidity (Negi et al.
2008
; Vahisalu et al.
2008
), indicating that SLAC1 is a key downstream component
for a variety of signals (including ABA) in guard cells. The S-type anion channel
currents activated by ABA and cytosolic Ca
2
+
in guard cells were completely abol-
ished in
slac1
mutants (Vahisalu et al.
2008
), and the fused SLAC1 with GFP can
light up the plasma membrane of
Arabidopsis
guard cells and onion epidermal cells
(Negi et al.
2008
; Vahisalu et al.
2008
), suggesting SLAC1 is the long-sought S-type
anion channel in guard cells. Coexpression of the genes of SLAC1 and protein
kinases OST1 or CPKs heterologously in
Xenopus
oocytes results in S-type anion
currents, and coexpression of
SLAC1
and
OST1
with the negative regulators of ABA
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