ABA and Its Derivatives: Chemistry and Physiological Functions - Abscisic Acid: Metabolism, Transport and Signaling - page 8

mixtures using recombinant CYP707A (Saito et al. 2004 ). This suggests that
CYP707A catalyses only the hydroxylation reaction, and not the isomerization
reaction to PA.
Is 8′-hydroxy-ABA biologically active? It is difficult to answer this question
because its thermodynamic instability prevents us from determining its true activ-
ity. Zou et al. ( 1995 ) reported that a borate complex of 8′-hydroxy-ABA is active
in lipid and oleosin biosynthesis in microspore-derived embryos of B. napus .
Kepka et al. ( 2011 ) extrapolated the activity of 8′-hydroxy-ABA from that of the
tetralone analogue of 8′-hydroxy-ABA ( 24 ). This analogue cannot isomerize to a
PA-type derivative because the C-2′ is in an aromatic ring (Fig. 1.7 ). Tetralone-
ABA is as effective as ABA in some bioassays using Arabidopsis, whereas the
8′-hydroxylated analogue is much less effective than tetralone-ABA. In PP2C
assays using the Arabidopsis ABA receptors, PYL9 and PYR1, 8′-hydroxy-
lated tetralone-ABA is less effective than tetralone-ABA and ABA by a factor of
100-1000. This suggests that 8′-hydroxy-ABA is essentially inactive, at least in
Arabidopsis. Because some space is evident around the C-8′ of ABA in both the
PYR1-ABA and PYL9-ABA complexes (PDB code: 3K90 and 3OQU, respec-
tively), the 8′-hydroxy group is accommodated sterically. Furthermore, since ABA
stabilizes the gate-closed conformation of PYL proteins by producing a hydropho-
bic network, a protic polar moiety around C-8′ may impair the critical function
of the proteins. In any case, currently there is no experimental evidence to help
resolve the discrepancy in ABA activity between Brassica napus and Arabidopsis.

(a)

HO

OH OH

CO 2 H O CO 2 H

O O

12

13

(b)

HO

OH OH

CO 2 H CO 2 H O

O O

24

Fig. 1.7 Intramolecular Michael-type reaction of 8′-hydroxylated compounds. a 8′-Hydroxy-
ABA ( 12 ) is isomerized to PA ( 13 ), whereas b the 8′-hydroxylated tetralone analogue of ABA is
not converted to the bicyclized compound owing to the stable aromatic ring