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factors to regulate gene expression, supporting the point of view that a single
CDPK member may have multiple substrates, possibly integrating different sign-
aling pathways. As mentioned earlier, the HSP1 was identified as a functionally
interacting partner of AtCPK10 in ABA signaling of guard cells (Zou et al.
2010
),
suggesting that different CDPK members may share a common substrate or the
same signaling pathway and that AtCPK11 may function similarly to AtCPK10
through the common substrate to regulate guard cell signaling in response to ABA.
Recently, two closet homologous
Arabidopsis
CDPK members, AtCPK21 and
AtCPK23 from subgroup II (Hrabak et al.
2003
; Boudsocq and Sheen
2013
),
together with AtCPK3 and AtCPK6, were shown to interact with and phospho-
rylate the guard cell anion channels SLAC1 and SLAH3 and to be involved in a
Ca
2
+
-dependent signaling pathway of guard cells in response to ABA through
regulation of these two direct targets (Geiger et al.
2010
,
2011
; Brandt et al.
2012
;
Scherzer et al.
2012
; Demir et al.
2013
). We will describe in the next section this
Ca
2
+
-dependent ABA-signaling network in the
Arabidopsis
guard cells.
8.2.3 CDPKs: Key Regulators of Ca
2
+
-Dependent ABA
Signaling in Guard Cells
It has been known that, during stomatal opening, inward K
+
channels mediate
K
+
uptake for net solute accumulation that drives water influx, guard cell swell-
ing, and pore opening, while during stomatal closure, outward K
+
channels play
essential roles in solute removal from guard cells. Efflux of anions through anion
channels is also essential for stomatal closure (Pandey et al.
2007
; Roelfsema
et al.
2012
). In response to drought stress, ABA induces stomatal closure. It has
been known that ABA regulates ion fluxes involved in stomatal movements in a
Ca
2
+
-dependent (Hetherington and Woodward
2003
; Hetherington and Brownlee
2004
) and Ca
2
+
-independent manner (Levchenko et al.
2005
; Marten et al.
2007
).
Recent studies in
Arabidopsis
showed that the Ca
2
+
-independent activation of
ion fluxes across the guard cell membranes is mainly mediated by the SNF1-
related protein kinase (SnRK) open stomata 1 (OST1) (described in the next sec-
tion), while Ca
2
+
-dependent activation of such ion fluxes is directly associated
with the CDPK-dependent phosphorylation of ion channels. The ABA activation
of slow-type anionic channels and Ca
2
+
permeable channels were reported to
be impaired in the
cpk3 cpk6
double mutant that displays ABA-insensitive phe-
notypes in ABA-induced stomatal closure (Mori et al.
2006
). Despite some dis-
crepancies, AtCPK3 and AtCPK6 were shown to phosphorylate the major guard
cell anion channel SLAC1 (Negi et al.
2008
) and its related homolog SLAH3
anion channel to activate these channels in
Xenopus
(
Xenopus laevis
) oocytes in
a Ca
2
+
-dependent fashion and in response to ABA, suggesting that AtCPK3 and
AtCPK6 play an important role in the ABA-controlled, Ca
2
+
-dependent activa-
tion of SLAC-type anion currents in guard cells (Brandt et al.
2012
; Scherzer et al.
2012
). The SLAC1 anion channel and its homolog SLAH3 were also shown to
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