ABA Signal Perception and ABA Receptors - Abscisic Acid: Metabolism, Transport and Signaling - page 96

(Pierce et al. 2002 ; Cabrera-Vera et al. 2003 ; Offermanns 2003 ; Pandey et al.
2009 ; Hackenberg et al. 2013 ; Urano et al. 2013 ).
G proteins have been shown to regulate plant growth and development and act
as nodes for integration and amplification of a host of abiotic, biotic, and hormo-
nal signals (Assmann, 2004 ). The Arabidopsis genome encodes one G ʱ (GPA1),
one G ʲ (AGB1), two G ʳ (AGG1 and AGG2) subunits and one regulator of G pro-
tein signaling (RGS) protein, which may form G protein complexes to function
with GPCRs to link ligand perception with downstream effectors (Ma et al. 1990 ;
Ma 1994 ; Mason and Botella 2000 , 2001 ; Jones and Assmann 2004 ). Multiple
lines of evidence have shown that the G protein complexes regulate ABA signaling
(Wang et al. 2001 ; Coursol et al. 2003 ; Pandey and Assmann 2004 ; Pandey et al.
2006 ), suggesting that GPCR-type plasma membrane receptors for ABA function
at the cell surface to perceive ABA signal.
6.4.1 GCR1 and GCR2: Do They Sense ABA
at the Cell Surface?
Pandey and Assmann ( 2004 ) identified a GPCR, named GCR1, which encodes a
protein predicted to possess 7-transmembrane domain structure characteristic of
GPCRs. GCR1 negatively regulates ABA signaling in seedling growth and stoma-
tal movement and interacts with GPA1, acting as a negative regulator of GPA1-
mediated ABA responses in guard cells (Pandey and Assmann 2004 ). However, it
remains unknown whether GCR1 perceives extracellular ABA signal as a receptor
for ABA. GCR2 is the second candidate GPCR reported to be involved in ABA
signaling, which was shown to function as a plasma membrane receptor for ABA
(Liu et al. 2007a ). However, it remains controversial whether GCR2 is a mem-
ber of GPCRs and regulates ABA signaling as an ABA receptor (Johnston et al.
2007 ; Liu et al. 2007b ; Gao et al. 2007 ; Chen and Ellis 2008 ; Guo et al. 2008 ;
Illingworth et al. 2008 ; Muschietti and McCormick 2010 ). So, more supporting
evidence is needed to clarify whether GCR2 is a bona fide ABA receptor.
6.4.2 GTG1 and GTG2: GPCR-type G Proteins
Acting as Cell Surface ABA Receptors
Pandey et al. ( 2009 ) identified two homologous GPCR-type G proteins, named
GPCR-type G proteins 1 and 2 (GTG1 and GTG2), which show homology to a
human orphan vertebrate GPCR (GPR89). GTG1 and GTG2 have classic GTP-
binding and GTPase activity of G proteins and interact with GPA1 that in turn
affects GTPase activity of the GTG proteins. The double gtg1 gtg2 mutant exhib-
its significant ABA hypersensitive phenotypes in ABA-induced inhibition of seed