ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 35

2008 ; Sauter et al. 2002 ). Glucosylation at the carboxyl group of ABA is catalyzed
by glucosyltransferase (Lim et al. 2005 ; Xu et al. 2002 ). Eight ABA glucosyltrans-
ferases have been reported in Arabidopsis , although only one (UGT71B6) selec-
tively recognizes ( + )-ABA, but not ( − )-R-ABA (Lim et al. 2005 ) (Table 2.1 ). The
single knockout mutant of UGT71B6 does not show a significant difference in the
levels of ABA and its metabolites compared with the wild type (Priest et al. 2006 ).
In contrast, overexpression of UGT71B6 (UGT71B6OX) causes the accumulation
of a large amount of ABA-GE (Priest et al. 2006 ). Nevertheless, UGT71B6OX
displays only minor changes in ABA levels (Priest et al. 2006 ). This might be
because the ABA 8′-hydroxylation pathway is predominantly involved in homeo-
static regulation of ABA.
ʲ -glucosidase hydrolyzes ABA-GE to form active ABA. There are two homolo-
gous ʲ -glucosidases in Arabidopsis , AtBG1 and AtBG2, which localize to ER and
vacuole, respectively (Lee et al. 2006 ; Xu et al. 2012 ) (Table 2.1 ). The enzymatic
activity of AtBG1 is enhanced by dehydration-induced polymerization (Lee et al.
2006 ). It was reported that the purine metabolite allantoin promotes polymeriza-
tion of AtBG1 and increases endogenous ABA levels (Watanabe et al. 2014 ). In
contrast, the AtBG2 protein exists in high molecular weight complexes through
polymerization under non-stress conditions and is protected from degradation
under dehydration stress (Xu et al. 2012 ). However, the details of how allantoin
regulates the polymerization of AtBG1 and how protein degradation of vacuole-
localized AtBG2 is inhibited under stress remain unknown. The atbg2 mutant does
not affect endogenous ABA levels, whereas the atbg1 mutant shows markedly
reduced ABA levels in the extracellular space, but not in the intracellular space
or xylem sap (Lee et al. 2006 ; Xu et al. 2012 ). Additionally, the atbg1 atbg2 dou-
ble mutant shows a more sensitive phenotype to drought stress (Xu et al. 2012 ).
Therefore, ʲ -glucosidase-mediated ABA production from ABA-GE is also consid-
ered a key pathway for regulating the local ABA concentration in response to envi-
ronmental stimuli.
2.6 Chemical Inhibitors of ABA Metabolism
Inhibitors of ABA metabolism have been utilized in studies of the physiological
roles of ABA in plants. They are especially useful for analyzing ABA physiology
in plant species that are difficult to perform genetic analyses on.
Many researchers have used fluridone and norflurazon as inhibitors of ABA
biosynthesis (Fig. 2.4 ). These inhibitors target phytoene desaturase, which func-
tions in carotenoid biosynthesis, and cause concurrent bleaching of plants due to
the destruction of chlorophyll resulting in loss of ABA biosynthesis (Gamble and
Mullet 1986 ). This approach cannot distinguish the effects of ABA from the con-
sequences of depleting most carotenoids and their derivatives (Taylor et al. 2005 ).
As mentioned above, carotenoid deficiency causes a pleiotropic phenotype includ-
ing ABA deficiency. Therefore, we need to use these inhibitors carefully.