Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 146

A signaling pathway from the Arabidopsis PYR/PYL/RCAR family receptors
for ABA to ABF transcription factor or SLAC1 anion channel activation through
SnRK2.2/2.3/2.6 has been described recently. Similar to the ABA-signaling pathway
via CPK3/6/21/23 in guard cells, the PYR/PYL/RCAR receptors bind to ABA and
inhibit the activity of phosphatase A-type PP2Cs which dephosphorylate and inac-
tivate SnRK2s in the absence of ABA and thus activate downstream ABF transcrip-
tion factors or SLAC1 anion channel to induce ABA-responsive gene expression or
stomatal closure (Ma et al. 2009 ; Park et al. 2009 ; Fujii et al. 2009 ; Umezawa et al.
2009 ; Lee et al. 2009 , 2013 ; Cutler et al. 2010 ; Geiger et al. 2010 , 2011 ; Brandt
et al. 2012 . See Fig. 8.2 and Chap. 6 of this topic). Most recently, SnRK2.2 and
SnRK2.3 were shown to be substrats of the Arabidopsis glycogen synthase kinase
3 (GSK3)-like kinase, BIN2, which, together with its homologs BIN2-like kinases
BILs, regulates negatively brassinosteroid signaling (He et al. 2002 ; Li and Nam
2002 ), but is positively involved in ABA signaling (Cai et al. 2014 ). BIN2 kinase
activates SnRK2.2 and SnRK2.3 through phosphorylation, and BIN2/BILs function
downstream of PYR1/PYL/RCAR receptors and PP2Cs (Cai et al. 2014 ). This sug-
gests a reversible phosphorylation process of SnRK2s where phosphorylation and
dephosphorylation may be catalyzed by BIN2/BIL kinases and PP2Cs, respectively.

8.3.4 SnRK3S/CIPKs: Mediators of Ca 2 + -Dependent ABA

Signaling

The CBL Ca 2 + sensor interacts with its target CIPK effector to form a Ca 2 + sen-
sor-effector complex to sense Ca 2 + signal and trigger downstream signaling
(Halford and Hey 2009 ; Luan 2009 ; Stefan and Kudla 2009 ; Coello et al. 2011 ).
SnRK3s/CIPKs, together with corresponding CBLs, have been suggested to regu-
late Ca 2 + -dependent ABA signaling in concert with CDPKs. In Arabidopsis , it was
reported that CIPK15/SnRK3.1/PKS3 interacts with CBL1/ScaBP5, likely acting as
a CBL1-CIPK15 calcium sensor-kinase complex, functions to negatively regulate
ABA responses (Guo et al. 2002 ). The CBL1 closely related calcium sensor CBL9 and
CIPK3/SnRK3.17, likely forming a CBL9-CIPK3 complex, were also shown to func-
tion as important, negative regulators of ABA signaling: Genetic evidence reveals that
the cipk3 and cbl9 loss-of-function mutants are hypersensitive to ABA (Kim et al. 2003 ;
Pandey et al. 2004 , 2008 ). Additionally, disruption of the CBL1 gene renders plants'
hypersensitive to drought and salt stress, suggesting that CBL1 functions as an impor-
tant integrator of abiotic stress responses (Albrecht et al. 2003 ; Cheong et al. 2003 );
however, in these studies, the cbl1 loss-of-function mutant showed no ABA-related phe-
notypes (Albrecht et al. 2003 ; Cheong et al. 2003 ). So it remains controversial whether
CBL1 mediates ABA responses. A further study identified CIPK1/SnRK3.16 as a
negative regulator of ABA signaling and suggests that alternative complex formation
of the kinase CIPK1 with either CBL9 or CBL1 mediates ABA-dependent and ABA-
independent signaling responses (D'Angelo et al. 2006 ). Thus, additional studies will be
needed to clarify the role of CBL1 in ABA signaling.