Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 141

networks via protein phosphorylation to coordinate the dynamic plant cellular pro-
cesses. Among these characteristics, functional redundancy, mainly due to multi-
ple members of the CDPK family, may provide plants with robust responsiveness
to ABA and adaptability to stressful environment, and substrate specificity may be
a key feature to confer functional specificity of different CDPK members in ABA
and stress signaling (Fig. 8.2 ).
The ABA-responsive transcription factors ABFs were identified as substrates of
several CDPKs in Arabidopsis , which may be key downstream players of CDPKs
to regulate ABA-mediated gene expression. Other substrates of diverse iden-
tity, including transcription factors, heat-shock protein, and ion channels, were
also identified as regulators to relay CDPK-mediated ABA signaling. AtCPK4
and AtCPK11 function redundantly to regulate major ABA response, possibly by
phosphorylating two ABA-responsive transcription factors ABF1 and ABF4, and
AtCPK23, phosphorylating ABF4, may function in concert with AtCPK4 and
AtCPK11. AtCPK11 may also interact with a nuclear zinc finger transcription fac-
tor AtDi19-1 and a heat-shock protein HSP1 to regulate ABA signaling. AtCPK10
use the same HSP1 as a functionally interacting partner in ABA signaling of guard
cells. Four other CDPK members, AtCPK3, AtCPK6, AtCPK21, and AtCPK23,
may function redundantly in guard cell signaling in response to ABA, likely by
targeting the common substrates anion channels SLAC1 and SLAH3 (Fig. 8.2 ).
Thus, a single CDPK member may have multiple substrates, possibly integrating
different signaling pathways, and different CDPK members may also share a com-
mon substrate or the same pathway to regulate ABA signaling.
8.3 SNF1-Related Protein Kinases (SnRKs) Involved in
ABA Signaling
8.3.1 SnRK Family: SnRK1, SnRK2, and SnRK3
Plants contain a large group of protein kinase related to the classical sucrose
non-fermenting-1 (SNF1)-type kinases from yeast where they function as global
regulators of carbon metabolism; these kinases are named SNF1-related protein
kinases (SnRKs) (Halford and Hardie 1998 ; Hrabak et al. 2003 ). SnRKs are a
class of Ser/Thr kinases like CDPKs (Hardie 2000 ; Hrabak et al. 2003 ). Based on
the analysis of their evolutionary origins (sequence similarity and domain struc-
ture), SnRKs are recognized as a family member of the CDPK-SnRK superfam-
ily, which can be subdivided into three subfamilies: SnRK1, SnRK2, and SnRK3
(Hrabak et al. 2003 ). The Arabidopsis genome contains 38 SnRKs, of which 3
(SnRK1.1 to SnRK1.3) are SnRK1s; 10 (SnRK2.1 to SnRK2.10) are SnRK2s;
and 25 (SnRK3.1 to SnRK3.25) are SnRK3s (Hrabak et al. 2003 ). The SnRKs of
the three subfamilies share strong sequence identity in their kinase domains but
have divergent C-terminal domains which are likely to regulate their activities in
response to a variety of metabolic and/or stress signaling pathways (Hrabak et al.
2003 ; Halford and Hey 2009 ).