ABA Regulation of Stomatal Movement - Abscisic Acid: Metabolism, Transport and Signaling - page 281

genetic components identified in the last decades, mainly including cell surface
receptors, secreted small signaling peptides, a series of transcription factors, MAK
kinase modules, and microRNA (Nadeau 2009 ; Dong and Bergmann 2010 ). The
up- and down-regulation of the associated genes of the regulatory components
result in the changes of stomata index compared to wild-type plants. In addition,
emerging evidences show that stomatal development is also regulated by environ-
mental factors, including CO 2 , light density, and water stress. High carbon diox-
ide (HIC) was identified as a positive regulator of CO 2 signaling for stomatal
development; the hic mutant growing in elevated CO 2 showed an increased sto-
mata density and index (Gray et al. 2000 ; Rae et al. 2006 ). On the other side, the
stomata density of some plants decreased significantly compared to their ancients
living in hundreds of years ago, and the elevated atmosphere CO 2 resulted from
the human activity and industrial development was believed to be the major cause.
This manner of stomata regulation by atmosphere CO 2 takes hundreds of years
and therefore is “supper slow” compared to CO 2 -induced stomatal movement, but
accumulating and significant. Stomata development is also regulated by light. An
increase in light intensity results in an increase in stomatal index in some plants.
The light signal is perceived by mature leaves, and stomata patterning changes
in young developing leaves (Casson and Gray 2008 ). How the signal is transited
from mature leaves to young leaves is still unknown. Water condition is a main
factor regulating stomata movement, and it is well known that ABA functions as
the main signal to trigger stomatal closure in response to drought stress. It has
been reported a long time ago that a reduced soil-watering condition resulted in a
reduction in stomata index in Caltha palustris and wheat (Quarrie and Jones 1977 ;
Salisbury 1928 ; Casson and Gray 2008 ). But whether drought stress/ABA func-
tions as a general regulator for stomatal development and patterning in plant king-
dom is still unknown. Nevertheless, stomata development is an important aspect of
gas exchange for plants, in addition to the stomatal movement.
15.3 ABA Triggering Stomatal Closure
Comes Mainly from Roots
Root is the organ of plants sensing water loss of soil firstly. Then, ABA is synthe-
sized in roots, and the ABA stored in roots as inactive ABA-glucose ester (ABA-GE)
can also be released at the same time (Lee et al. 2006 ; Schroeder and Nambara
2006 ; Xie et al. 2006 ). Free ABA synthesized in roots is released into xylem from
parenchyma cells and transported through xylem into leaves to close stomata.
Glucose-conjugated ABA released by roots can be transported through xylem into
leaves as well, and it is believed that conjugated ABA needs to be released as a free
ABA by ʲ -glucosidases in leaves to be active. Leaves have the ability to synthesize
ABA when leaf turgor pressure is very low because of water loss. However, ABA
synthesis begins in root when soil is just beginning to loss water, and leaf turgor
pressure is still unaffected obviously at that time (Hartung et al. 2002 ). Therefore,