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during ABA inactivation (Zeevart 1983 ; Piotrowska and Bajguz 2011 ). However,
there have been multiple lines of evidence that ABA-GE may have a physiologi-
cal role such as being a stored form of ABA. Johnson and Ferrell ( 1982 ) reported
that in needles of Pseudotsugamenziesii, the level of conjugated ABA decreases
and concomitantly the level of free ABA increases. In addition, it has been sug-
gested that ʲ -glucosidase homologs located at either intracellular or extracellular
spaces harbor the activity that hydrolyzes glucose conjugated, biologically inac-
tive ABA-GE to produce active ABA in plants. Indeed, in Arabidopsis , a large
number of genes encode the putative ʲ -glucosidases (Fig. 5.1 b) and more than 30
ʲ -glucosidases were predicted to be secreted to the apoplasmic space (Sauter et al.
2002 ). Consistent with this notion, Dietz et al. ( 2000 ) showed that intercellular
washing fluid of barely primary leaves harbors the activity to release active ABA
from physiologically inactive ABA-GE. The ABA liberated by this ʲ -glucosidase
can be taken up directly by the cells or loaded into the xylem for long-distance
transportation. Based on this observation, they suggested that ABA-GE is a stored
form of ABA. Moreover, considering the impermeability of lipid membranes
toward ABA-GE, it has been suggested that ABA-GE is an ideal compound for
long-distance transportation of ABA. Furthermore, because of this property, it has
been postulated that ABA-GE may function as a long-distance signal transmitted
from roots to shoots under dehydration stress conditions.
Following up on this long speculation, the most compelling evidence on the pos-
sible role of ABA-GE as a stored form of ABA was provided by recent publications
on two ʲ -glucosidases in Arabidopsis, AtBG1 and AtBG2, which can hydrolyze
ABA-GE to ABA (Lee et al. 2006 ; Xu et al. 2012 ). A genetic approach showed
that atbg1 and atbg2 mutants display increased sensitivity to dehydration and salin-
ity stresses. Moreover, atbg1 atbg2 double-knockout mutants showed the additive
effect on dehydration stress sensitivity, suggesting that they act redundantly. By
contrast, the ectopic expression of AtBG1 and AtBG2 displayed an enhanced resist-
ance to osmotic stress. Moreover, atbg1 mutants showed many ABA-deficient phe-
notypes. Consistent with these phenotypes, atbg1 mutants have lower ABA levels,
in particular, greatly reduced extracellular pools of ABA, compared to the wild-type
plants. Intriguingly, AtBG1 and AtBG2 localize to the ER and vacuole, respectively,
indicating that ABA is produced in the ER and vacuole in plant cells in addition to
the cytosol through the de novo biosynthetic pathway. These findings raise funda-
mentally important questions such as what is the physiological meaning of ABA
production in multiple compartments and how these multiple ABA biosynthetic
pathways in different organelles are coordinated to maintain the active ABA pool at
a specific level depending on cellular and environmental conditions. Another inter-
esting aspect of these proteins is the regulation of their activity according to the
environmental conditions. In the case of AtBG1, it undergoes polymerization into
high-molecular weight forms to increase the enzymatic activity under dehydration
stress conditions (Lee et al. 2006 ; Watanabe et al. 2014 ), whereas AtBG2 existing
as high-molecular weight forms at all conditions accumulates to higher levels under
dehydration stress to increase ABA production (Xu et al. 2012 ).
In regulating ABA production at the whole plant level, one mechanism would
be to differentially regulate expression of genes involved in ABA production.
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