ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 95

the intensity of ABA insensitivity of the SOAR1 -overexpression lines is compara-
ble to that of the triple loss-of-function mutant of the SnRK2 members, uncover-
ing that SOAR1 is a hub of ABA signaling pathways.
Further genetic evidence reveals that SOAR1 functions downstream of ABAR
and upstream of the ABA-responsive transcription factor ABI5 (Mei et al. 2014 ),
suggesting a possible ABAR-SOAR1-ABI5 linked signaling cascades. Given
that PPR proteins are a class of RNA-binding proteins involved in many aspects
of RNA processing (Meierhoff et al. 2003 ; Williams and Barkan 2003 ; Lurin
et al. 2004 ), the ABI5 mRNA may be a target of the SOAR1 protein (Mei et al.
2014 ). The alteration in SOAR1 expression changes significantly the expres-
sion of a subset of genes, of which the encoded proteins have been identified to
be directly involved in the PYR/PYL/RCAR-mediated ABA signaling, suggest-
ing the PYR/PYL/RCAR-mediated and ABAR-mediated signaling pathways may
crosstalk to regulate ABA signaling through SOAR1. Together, the discovery
of SOAR1 as a crucial ABA signaling component suggests a central role of the
ABAR-mediated signaling pathway in the highly complicated ABA signaling net-
work, and exploration of the mechanisms of the SOAR1 protein that functions in
the nuclear events will be of particular importance to understand the mechanisms
of ABA signaling (Fig. 6.1 ).
6.4 G Protein-coupled Receptors (GPCRs): Receptors for
ABA at the Cell Surface
As described earlier, previous experiments suggested that there are plasma mem-
brane-localized ABA receptors to perceive the extracellular ABA signal (Gilroy
and Jones 1992 ; Anderson et al. 1994 ; Assmann 1994 ; Jeannette et al. 1999 ;
Finkelstein et al. 2002 ; Verslues and Zhu 2007 ). G protein-coupled receptors
(GPCRs) are a class of plasma membrane proteins, which constitute, together with
their G protein partners, one of the most elaborate receptor-effector-linked signal-
ing cascades and play a vital role in perceiving extracellular signal in eukaryotic
cells (Pierce et al. 2002 ; Offermanns 2003 ). The core components of the G pro-
tein signaling are comprised of G ʱ , G ʲ , and G ʳ subunits, and G protein-coupled
receptors (GPCRs), among which the three G protein subunit form heterotrimeric
G proteins (Pierce et al. 2002 ; Offermanns 2003 ). The catalytically active protein
of the complex, G ʱ subunit, which has both GTP-binding and GTPase activity,
acts as a bimodal molecular switch, typically with a GDP-bound off mode and a
GTP-bound on mode. GDP-bound G ʱ remains associated with G ʲʳ (GDP-G ʱʲʳ )
and represents the inactive signaling status. Signal perception of GPCRs results in
their conformation changes, which leads to exchange of GDP for GTP at the G ʱ
subunit and dissociation of the heterotrimer into free GTP-G ʱ and G ʲʳ dimers,
both of which can relay signal to trigger downstream signaling events. The G pro-
tein signaling is terminated by regeneration of the GDP-bound G ʱ subunit and
re-association of the G protein heterotrimer, thus completing one signaling cycle