ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 37

respectively (Kitahata et al. 2006 ). Another group developed several kinds of
sesquiterpene-like carotenoid cleavage dioxygenase (SLCCD) inhibitors against
Arabidopsis NCED3, based on the structures of 9- cis -epoxycarotenoid and xanth-
oxin (Boyd et al. 2009 ). Compounds 13 and 17 inhibited the activity of AtNCED3
in vitro with K i values of 93 and 57 ʼ M, respectively (Fig. 2.4 ) (Boyd et al. 2009 ).
Compound 13 had a greater inhibitory effect on ABA biosynthesis in planta than
compound 17 and abamineSG (Boyd et al. 2009 ).
Most reported ABA catabolic inhibitors are chemicals targeting CYP707A.
The structural features of CYP707A inhibitors are categorized into two types,
azole types and ABA analogues (Fig. 2.4 ). Since CYP707A is a P-450 mono-
oxygenase, several P450 inhibitors containing an azole group are considered can-
didate inhibitors of ABA catabolism. Uniconazole impairs the conversion of ABA
to PA and inhibits the activity of the CYP707A enzyme (Kitahata et al. 2005 ;
Saito et al. 2006 ). Although uniconazole effectively confers drought stress toler-
ance to plants, it was originally developed as a gibberellin biosynthesis inhibi-
tor and consequently induces dwarfism (Izumi et al. 1985 ; Saito et al. 2006 ).
Diniconazole functions as a fungicide, and its chemical structure is similar to that
of S-uniconazole (Fletcher et al. 1986 ). Therefore, diniconazole also inhibits the
activity of CYP707A (Kitahata et al. 2005 ). However, these azole P450 inhibitors
arrest plant growth because of the broad inhibition spectrum of azole compounds
for P450 enzymes (Rademacher 2000 ; Yokota et al. 1991 ). The structure of uni-
conazole was modified, and several inhibitors were synthesized to develop a more
specific inhibitor against CYP707A (Okazaki et al. 2012 ; Todoroki et al. 2009a , b ,
2010 ). Unlike uniconazole, abscinazole-E2B selectively inhibits CYP707A, but
not CYP701A, which is involved in gibberellin biosynthesis. Application of absci-
nazole-E2B increases endogenous ABA levels during dehydration and confers
drought tolerance with less growth arrest.
Among non-azole-type ABA catabolic inhibitors, ABA analogues designed
based on the structure of ABA have the potential to function as specific inhibi-
tors. Several ABA 8′- or 9′-derivatives have been synthesized and tested for their
effect on the activity of the CYP707A enzyme, because C-8′ and its neighboring
position in ABA are thought to be the possible influence groups for the hydroxy-
lation reaction by CYP707A. ( + )-8′-methylidyne-ABA, ( + )-9′-acetylene-ABA,
( − )-9′-acetylene-ABA, and ( + )-9′-vinyl-ABA have been reported as suicide sub-
strates that irreversibly inhibit the activity of ABA 8′-hydroxylase (Cutler et al.
2000 ). Among these, ( + )-9′-acetylene-ABA functions as the most effective inhibi-
tor of ABA 8′-hydroxylase (Cutler et al. 2000 ). There is no clear explanation why
9′-derivatives show potent inhibition effects despite the fact that CYP707A mainly
catalyzes hydroxylation at the C-8′ of ABA. It is possible that the catalytic site of
CYP707A might recognize both C-8′ and -9′ methyl groups for the hydroxylation
reaction, since CYP707A catalyzes both 8′- and 9′-hydroxylation of ABA. In addi-
tion to the suicide effect, ABA 8′- or 9′-derivatives are thought to possess ABA
agonist activity. Indeed, ABA-responsive genes regulated by ( + )-8′-methylidyne-
ABA widely overlap with ABA (Huang et al. 2007 ). To overcome the ABA ago-
nist activity of ABA analogues, several specific inhibitors have been synthesized