ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 34

OH

OH OH

O

COOH COOH

O O

9 -hydroxy-ABA Neophaseic acid

HO

OH OH OH

O

COOH COOH COOH

CYP707A

O O O

ABA 8 -hydroxy-ABA Phaseic acid

-glucosidase ABA-GT

OH OH

O

OH

R 1

OH COOH COOH

CO 2 -Glc

O

O

R 2

ABA glucose ester 7 -hydroxy-ABA

Dihydrophaseic acid

R 1 =OH R 2 =H

epi -dihydrophaseic acid

R 1 =H R 2 =OH

Fig. 2.3 ABA catabolic pathways in higher plants. In hydroxylation pathways, ABA 8′-hydroxy-
lation is thought to be a major ABA catabolic route. ABA is also inactivated to ABA glucosyl
ester (ABA-GE) by ABA-glucosyltransferase (ABA-GT), but ABA-GE is converted to free ABA
by ʲ -glucosidase

of ABA. In addition to the ABA 8′-hydroxylation pathway, ABA is also hydroxy-
lated at C-7′ and C-9′ to form 7′- and 9′-hydroxy-ABA, respectively (Hampson
et al. 1992 ; Zhou et al. 2004 ). These hydroxylated ABA catabolites have substan-
tial biological activity (Hill et al. 1995 ; Zhou et al. 2004 ). Unstable 9′-hydroxy-
ABA is spontaneously isomerized to form neophaseic acid (NeoPA) like PA (Zhou
et al. 2004 ). NeoPA is a minor catabolite, but exists in a wide range of plant spe-
cies (Zhou et al. 2004 ). The levels of neoPA accumulate substantially in restricted
tissues such as developing seeds of Arabidopsis thaliana and Brassica napus
(Kanno et al. 2010 ; Zhou et al. 2004 ). Arabidopsis CYP707As have minor activ-
ity for the 9′-hydroxylation of ABA in vitro (Okamoto et al. 2011 ). Additionally,
endogenous neoPA levels in the Arabidopsis cyp707a mutant are lower than in the
wild type, as with PA (Okamoto et al. 2011 ). Therefore, neoPA is thought to be
produced by CYP707A as a side reaction. On the other hand, the gene encoding
7′-hydroxylase has not been identified yet. Hydroxylated catabolites of ABA are
further conjugated with glucose (Cutler and Krochko 1999 ; Nambara and Marion-
Poll 2005 ).
Among several conjugated catabolites, ABA glucosyl ester (ABA-GE) is the
most widespread (Hartung et al. 2002 ). ABA-GE has been thought to be an inac-
tive pool of ABA and accumulate in the vacuole or apoplast. However, ABA-GE
is considered a storage or long-distance transport form of ABA that functions as a
mobile stress signal from roots to shoots (Hartung et al. 2002 ; Jiang and Hartung