G-Proteins as Molecular Switches in Signal Transduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 190

plants overexpressing RGS1 were developed and RGS1 overexpression signifi cantly
stimulated the expression of NCED encoding the 9- cis -epoxycarotenoid dioxygen-
ase (NCED) enzyme, which cleaves 9- cis xanthophylls to xanthoxin. The NCED is
the fi rst committed step for ABA synthesis (Chen et al. 2006 ; Wasilewska et al.
2008 ). The transgenic plants overexpressing RGS1 also showed increased expression
of ABA2 gene (Chen et al. 2006 ), which encodes dehydrogenase/reductase involved
in conversion of xanthoxin to abscisic acid aldehyde (Wasilewska et al. 2008 ).
Abscisic aldehyde oxidase (AAO) catalyses the fi nal step of ABA biosynthesis,
which converts ABA aldehyde to ABA (Iuchi et al. 2001 ). These results indicate that
RGS1 is involved in biosynthesis of ABA (Chen et al. 2006 ).

3.19.2 G-Proteins May Act as Abscisic Acid Receptors

The receptor for G-protein called G protein-coupled receptor (GPCR) has also been
identifi ed as a plasma membrane receptor for the plant hormone abscisic acid (Liu
et al. 2007a , b ). The G-protein receptor, GCR2, has been identifi ed as a plasma
membrane-localized receptor for ABA in Arabidopsis (Liu et al. 2007a ). GCR2 is a
membrane protein with seven transmembrane helices. The GCR2 genetically and
physically interacts with the Arabidopsis G protein subunit GPA1 to mediate all

known ABA responses in Arabidopsis . This receptor binds ABA with high affi nity
at physiological concentration. The GCR2 interacts with the G

α

αβγ complex.

Binding of ABA to GCR2 results in the release of G α and G βγ dimer to activate

downstream ABA signaling events (Liu et al. 2007a ).
Another novel class of GPCR-type G proteins, GTGs has been suggested as
ABA receptors (Pandey et al. 2009 ). GTG proteins contain GTP binding/GTPase
activity. GTGs interact with the heterotrimeric G-protein -subunit, GPA1 and the

GTP-bound form of GPA1 inhibits the GTPase activity of GTG proteins. The GDP-
bound GTGs bind ABA stronger than GTP-bound form, thus it might be the active
form for perceiving and transducing ABA signal (Guo et al. 2011 ).

α

3.19.3 G-Proteins May Regulate Inward K + Channels

and Slow Anion Channels Activated by ABA

In the G protein signaling system, the G subunit (AGB1),

and the G-protein coupled receptor (GCR1) regulate the downstream events in ABA
signaling (Pandey et al. 2006 ). GCR1 may interact with GPA1 and the ligand bind-
ing to GCR1 may regulate heterotrimeric G protein signaling via GPA1 and the

α subunit GPA1, the G β

βγ

dimer (Eckardt 2004 ). GPA1 acts upstream of the small GTPase Rac in inducing
disease resistance response in rice (Suharsono et al. 2002 ).
The Arabidopsis GPA1 has been demonstrated to be involved in the regulation of
inward K + channels and slow anion channels activated by ABA (Wang et al. 2001 ).
Ca 2+ -permeable channels, like inward K + channels and S-type anion channels are