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germination, post-germination growth arrest and promotion of stomatal closure,
but single gtg mutant plants show substantially wild-type phenotypes. The gtg1
gtg2 double mutant phenotype could be fully complemented with expression of
either GTG1 or GTG2 gene, which implies that GTG1 and GTG2 function redun-
dantly in ABA signaling (Pandey et al. 2009 ). Further quantitative proteomics-
based analysis supports a significant role of the GTG proteins in regulation of
ABA response in Arabidopsis roots and provides clues to their possible links with
some of the well-established effectors of the ABA signaling pathways (Alvarez
et al. 2013 ), though a report using independently generated mutant alleles of the
GTG genes suggests a normal ABA sensitivity of the gtg1 gtg2 double mutants
(Jaffe et al. 2012 ).
The ABA-binding assays with 3 H-ABA and purified recombinant GTG pro-
teins shows the ABA-binding ability and specificity of the GTG1 and GTG2
proteins, which meets essential criteria of ligand-receptor binding and sup-
ports the ABA receptor nature of GTGs (Pandey et al. 2009 ). The ABA-binding
ability of the GTG proteins was confirmed by an independent study (Kharenko
et al. 2013 ). Interestingly, the GTG proteins associated with GDP are more
favorable to ABA binding than the GTGs-GTP complexes, and inhibition of
GTPase activity of GTGs will abolish ABA binding, revealing that the GTG
proteins bind ABA in the GTG-GDP form rather that GTG-GTP form (Pandey
et al. 2009 ).
Based on the analysis, a working model was proposed to explain the GTG-
mediated ABA signaling, in which G protein (G ʱ subunit GPA1) regulates the G
protein-coupled receptor: GDP-bound GTGs bind ABA, which initiates the ABA
signaling cascade; GPA1 acts as a 'rheostat' on GTG, such that GTP-bound GPA1,
inhibiting formation of the active complex GTG-GDP, downregulates ABA bind-
ing to the GTGs and represses ABA signaling (Pandey et al. 2009 ; Fig. 6.2 ). This
model suggests an unusual type of G protein signaling, which is opposite from
conventional model for signaling mediated by G proteins, where GDP-bound G ʱ
'turns off' the signaling system (Pandey et al. 2009 ; Klingler et al. 2010 ). Further
studies will be needed to identify downstream effectors to elucidate mechanisms
of the GTG-mediated ABA signaling.
Fig. 6.2 A model for mechanism of the GTG-mediated ABA perception [modified from Pandey
et al. ( 2009 )]. GDP-bound GTGs (GTG-GDP) bind ABA. GTP-bound GPA1 (GPA1-GTP)
inhibits the GTG-GDP formation, downregulating ABA binding to the GTGs and thus repress-
ing ABA signaling. See detailed explanation in the text. Arrows denote activation effects and bars
inhibition effects. Question marks indicate aspects of the model inferred from, but not directly
demonstrated by, experimental results to date
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