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difference between wild-type and
abh1
mutant, suggesting that ABH1 is related
to ABA signaling transduction in guard cells, but not in ABA level regulation
(Hugouvieux et al.
2001
). Patch clamping analysis showed that K
in
currents were
down-regulated, but anion currents were up-regulated in
abh1
mutant guard cells
(Hugouvieux et al.
2002
), supporting an important roles of ABH1 in stomatal
movement. ABH1 is a nuclear mRNA cap-binding protein, which may function in
guard cells by regulating the transcript level of some components of ABA signal-
ing pathway. Further research found that the expression of
PP2CA
in
abh1
can
partially suppress the ABA hypersensitive phenotype of
abh1
(Kuhn et al.
2006
),
suggesting a connection between ABH1 and PP2CA in stomatal movement regula-
tion. Using
abh1
mutant as a background line, a new screening of EMS mutagen-
ized
Arabidopsis
seeds was recently conducted and leads to the isolation of two
new mutants called
soa2
(
suppressor of abh1 hypersensitivity to ABA 2
) and
soa3
(Daszkowska-Golec et al.
2013
). Both
soa2
and
soa3
showed drought-tolerant
phenotypes, suggesting a function of these two proteins in stomatal movement
(Daszkowska-Golec et al.
2013
). But the genetic identification of SOA2 and SOA3
is still unknown currently.
15.5.2 Second Messengers in Guard Cells
Several types of small molecules function as second messengers and regulators
for ABA-induced stomatal movement, including free Ca
2
+
, ROS, NO, CO, cyclic
nucleotides (cAMP and cGMP), and cADP ribose.
Ca
2
+
is an important second messenger for signaling transduction in both
mammalian and plant cells, and the changes/oscillation of cytosolic free Ca
2
+
concentration encode signals to mediate the signaling from diverse upstream
stimuli to downstream targets. In guard cells, ABA can trigger Ca
2
+
increases
and oscillation, for which Ca
2
+
are from both external Ca
2
+
influx mediated by
hyperpolarization-activated plasma membrane Ca
2
+
channels and Ca
2
+
release
from intracellular Ca
2
+
stores in an IP
3
-dependent manner. The imposed external
Ca
2
+
-induced cytosolic Ca
2
+
changes can close stomata in
Arabidopsis
(Allen
et al.
2001
), and ABA can induce the production of IP
3
(Lee et al.
1996
), which
involves in Ca
2
+
oscillation by inducing intracellular Ca
2
+
release in guard cells.
Therefore, the Ca
2
+
oscillation patterns encode the ABA signaling in guard cells.
Cytosolic Ca
2
+
has diverse functions as a second messenger and can mediate the
signaling from many different stimuli, including ABA, CO
2
, and ozone for stoma-
tal movement (Young et al.
2006
; Vahisalu et al.
2008
). Several core components
of ABA signaling network, including PP2Cs and CPKs as well as SnRKs, con-
tain Ca
2
+
-binding domain and may be regulated by Ca
2
+
. Patch clamping analy-
sis showed that the activation of both anion channel SLAC1 and Ca
2
+
channels
in
cpk3cpk6
double mutant is impaired, suggesting a Ca
2
+
-dependent activation
of SLAC1 by CPK3 and CPK6 (Mori et al.
2006
). The direct protein interaction
between CPK6 and SLAC1 as well as the activation of SLAC1 by CPK6 was
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