ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 282

it has been proposed that ABA in leaves triggering stomata closure in response to
dehydration of soil is mainly imported from roots, not synthesized locally in leaves.
Indeed both free ABA and conjugated ABA were detected in xylem flow (Hartung
et al. 2002 ). On the contrary, ABA level can increase significantly in response to a
reduced atmospheric relative humidity while roots are not stressed by drought (Xie
et al. 2006 ), suggesting that ABA can be produced or released in leaves in response
to dry atmosphere. Further study found that ʲ -glucosidase plays essential roles in
leaves in catalyzing ABA release from ABA-GE, which is originally imported from
roots, and this can explain at least partially the increase of ABA level in leaves in
response to dry atmosphere. The free/active ABA can be degraded quickly by P450
protein in leaves after acting as a signal to close stomata, and does not deposit and
accumulate in guard cells after plants are rescued from drought stress (Jiang and
Hartung 2008 ; Schroeder and Nambara 2006 ; Peuke et al. 2002 ). Therefore, ABA
signaling intensity for stomatal movement regulation is regulated by ABA synthe-
sis, long-distance transportation of ABA and conjugated ABA, release of active ABA
from ABA-GE, and ABA breakdown.
15.4 ABA Receptors, the Key Proteins from Where
ABA Signaling Initiates for Stomatal Closure
Numerous studies have been done regarding ABA signaling transduction for sto-
matal movement. The simple and easiness of stomata movement assay and the
identification of many genetic mutants (mainly in Arabidopsis ) are the main rea-
sons. For stomata, ABA perceiving is the first step for ABA signaling transduc-
tion, and ABA receptors are the key proteins sensing ABA stimulus for guard
cells. Despite the identification and isolation of a large number of downstream
components of ABA signaling pathway in guard cells in the last few decades,
ABA receptors were completely unknown until recent few years. Using bio-
chemical approach, several proteins binding to ABA in vitro were identified and
suggested as ABA receptors, including CHLH (H subunit of Mg-chelatase) and
three G protein-coupled receptors GCR2, GTG1, and GTG2. CHLH (H subunit
of Mg-chelatase) plays important roles in chlorophyll biosynthesis and plastid-to-
nucleus signaling (Shen et al. 2006 ). GCR2, GTG1, and GTG2 were suggested
as ABA receptors by interacting with GPA1, the ʱ subunit of heterotrimeric G
protein, in guard cells (Pandey et al. 2009 ; Liu et al. 2007 ). The functions of het-
erotrimeric G protein in ABA-induced stomatal closure have been reported repeat-
edly (Fan et al. 2008 ; Wang et al. 2001 ), supporting the conclusion of that the
three G protein-coupled receptors are ABA receptors. As ABA receptors should
be where ABA signaling start in guard cells, a strong ABA-insensitive phenotype
of stomatal movement was expected in the ABA receptor mutants. However, the
phenotype of ABA-induced stomatal closure is controversial for gcr2 (Gao et al.
2007 ; Liu et al. 2007 ) and partial in gtg1/gtg2 (Pandey et al. 2009 ). gun5 mutant
with a point mutation in the CHLH gene seems to show wild-type phenotypes