ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 104

suggests that ABAR regulates a central ABA signaling network. It is intriguing
to uncover the mechanism by which SOAR1 functions downstream of ABAR in
ABA signaling.
GTG1 and GTG2 are a novel class of G proteins, namely GPCR-type G pro-
teins, which may perceive intercellular ABA signal. In the GTG-mediated ABA
signaling, GDP-bound GTGs bind ABA, which initiates the ABA signaling cas-
cade; GTP-bound GPA1 inhibits formation of the active complex GTG-GDP,
downregulates ABA binding to the GTGs, and represses ABA signaling (Fig. 6.2 ).
This model suggests an unusual type of G protein signaling, which is opposite to
the conventional model for signaling mediated by G proteins, where GDP-bound
G ʱ turns off the signaling system. Further studies will be needed to identify down-
stream components to elucidate underlying mechanisms of the GTG-mediated
ABA signaling.
The PYR/PYL/RCAR-mediated signaling pathway is the currently best-charac-
terized ABA signaling pathway. Under basal ABA levels, clade A PP2Cs function
as negative regulators to repress ABA signaling, either through dephosphorylation
of SnRK2s or interaction with other targets. ABA binding to the PYR/PYL ABA
receptors inactivates PP2Cs, leading to the activation of SnRK2s through phospho-
rylation by BIN2/BILs kinases, which subsequently phosphorylate downstream tar-
gets, such as members of the ABF transcription factors or regulatory components
of the stomatal aperture, such as the anion channels SLAC1 and SLAH3. Some
PP2C members such as ABI1, working downstream of PYR/PYL/RCAR recep-
tors, also inhibit the calcium-dependent kinases CPK21 and CPK23 (see Chap. 8 ) ,
which share the anion channels SLAC1 and SLAH3 as substrates with SnRK2s to
control stomatal movement. This core signaling pathway connects ABA percep-
tion, inactivation of clade A PP2Cs, and activation of SnRK2 protein kinases and
their various downstream targets to induce a diversity of ABA responses (Fig. 6.3 ).
However, how to integrate known ABA signal components, especially the ABAR-
and GTGs-mediated signaling pathways, into the PYR/PYL/RCAR-mediated sign-
aling pathway, is a challenging question in the future.
Acknowledgements The authors thank all colleagues who provided unpublished results and
apologize to those whose research was not discussed due to page limitation. The research on
ABAR/CHLH in the authors' laboratory was supported by the grants from the National Key
Basic Research Program of China (2012CB114302), National Natural Science Foundation of
China, and the Ministry of Agriculture of China (grant 2013ZX08009003).
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