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Yonekura-Sakakibara and Hanada 2011 ). Among these UGTs, UGT71B6 which
belongs to the subfamily E displays an activity toward naturally occurring ( + )-
ABA for glucose conjugation in vitro with an optimum pH between 6.5 and 7.0.
Indeed, UGT71B6 overexpression in plants leads to high-level accumulation of
ABA-GE. Interestingly, in these transgenic plants, the levels of ABA catabolites,
such as PA, DPA and neo-PA, were significantly reduced, which can be explained
by the fact that these two pathways are involved in the inactivation of ABA in a
competitive manner. Moreover, these results suggest a coordination of the two cat-
abolic pathways in lowering cellular ABA levels. Interestingly, the free ABA level
was not affected in these transgenic plants, suggesting that there is a compensation
mechanism to adjust the cellular ABA levels in plant cells. However, transgenic
plants overexpressing UGT71B6 were slightly less sensitive to exogenous ABA
during germination and showed a slightly higher water loss rate from detached
leaves. In addition, UGT71B6 overexpressers displayed hyposensitivity to 6 %
glucose at post-germination-growth stages. In contrast, UGT71B6 loss-of-function
mutants did not display any noticeable phenotypes, which might be due to the
functional redundancy (Priest et al. 2006 ). In fact, in Arabidopsis , the subfamily E
contains additional UGTs: At3g21790 and At3g21800 that are highly homologous
to UGT71B6 and are located immediately upstream and downstream of UGT71B6 ,
respectively (Fig. 5.1 a). However, their physiological role and biochemical activi-
ties have not been addressed at the molecular and biochemical levels.
An expression study revealed that UGT71B6 is strongly induced under various
abiotic stress conditions (Priest et al. 2006 ). Interestingly, UGT71B6 was induced
as early as 30 min after the start of the abiotic stress treatment. Thus, under abi-
otic stress conditions, the induction of UGT71B6 is as rapid as NCED3, which
is involved in the de novo ABA biosynthetic pathway (Endo et al. 2008 ). It is
expected that UGT71B6 is induced at the later time points because it is involved
in lowering ABA levels after abiotic stress responses. However, the rapid induc-
tion of UGT71B6 upon abiotic stress treatments raises the intriguing possibility
that UGT71B6 might also be involved in abiotic stress responses. One possible
role of UGT71B6 under abiotic stress conditions is to fine-tune cellular ABA lev-
els to prevent the excessive accumulation of ABA under abiotic stress conditions.
However, this idea needs to be further tested in future studies.
5.3 ʲ -Glucosidase Homologs in the Hydrolysis of ABA-GE
Whether ABA-GE has any physiological role has been controversial for a long
time. Considering that (i) the amount of free ABA under water stress conditions
is greater than that of endogenous ABA-GE under normal conditions (Neil et al.
1983 ); (ii) ABA-GE was not able to be hydrolyzed under water stress condi-
tions (Milborrow 1978 ); and (iii) the activity of ABA-GE splitting enzymes was
not increased (Lehmann and Vlasov 1988 ), it has been generally considered that
ABA-GE is a simple by-product, rather than a stored form of ABA produced
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