ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 33

by an enzyme having activity like Arabidopsis ABA2, AtABA2 did not show con-
version activity from xanthoxic acid to ABA (Cheng et al. 2002 ). While xanthoxic
acid was not effectively converted to ABA in a cell-free conversion assay, it could
be converted to ABA (Sindhu and Walton 1988 ). These results suggest that xanth-
oxic acid could be a precursor in ABA biosynthesis.
The other pathway, via abscisic alcohol, might be activated in some mutants
(Fig. 2.2 ). When ABAld was supplied to flacca or sitiens mutants, it was converted
to abscisic alcohol, showing that ABAld is reduced to abscisic alcohol and then
oxidized to ABA via a shunt pathway (Rock et al. 1991 ; Taylor et al. 1988 ). This
phenomenon was also observed in the Npaba1/CKR1 mutant in Nicotiana plum-
baginifolia (Parry et al. 1991 ). The shunt pathway appears to be a minor source
of ABA in wild-type plants but might play a significant role in mutants defective
in the oxidation of ABAld to generate ABA directly. One might speculate on the
possibility of the minor ABA biosynthetic pathways operating in an organ and/or
developmental stage-dependent manner.
2.5 ABA Catabolism
In contrast to biosynthesis, ABA is catabolized through several pathways in plants.
ABA catabolism is largely categorized into two types of reaction, hydroxylation
and conjugation (Fig. 2.3 ). Among them, ABA 8′-hydroxylation is a key step in
the major ABA catabolic route in several plant species (Nambara and Marion-
Poll 2005 ). Hydroxylation at C-8′ of ABA is catalyzed by cytochrome P-450
type mono-oxygenases, and unstable 8′-hydroxy-ABA is then isomerized spon-
taneously to phaseic acid (PA). Although PA has faint ABA-like activity (Kepka
et al. 2011 ; Walton 1983 ), substantial PA activity is observed in specific tis-
sues such as barley aleurone layers (Hill et al. 1995 ; Todoroki et al. 1995 ). PA
is further metabolized by an unidentified reductase to form dihydrophaseic acid
(DPA) or epi -DPA, which have almost no biological activity (Walton 1983 ).
ABA 8′-hydroxylases are encoded by the CYP707A family (Kushiro et al. 2004 ;
Saito et al. 2004 ) (Table 2.1 ). The ABA 8′-hydroxylation reaction catalyzed by
CYP707A requires both NADPH and P450 reductase (Kushiro et al. 2004 ; Saito
et al. 2004 ). CYP707A selectively catalyzes the naturally occurring ( + )-S-ABA
enantiomer, but not the unnatural type ( − )-R-ABA (Kushiro et al. 2004 ; Saito
et al. 2004 ). Since ABA 8′-hydroxylase activity was observed in the microsomal
fraction of suspension-cultured corn cells and CYP707A-green fluorescent pro-
tein (GFP) fusion protein localizes to the endoplasmic reticulum (ER), the ABA
8′-hydroxylation reaction is thought to take place in the ER (Krochko et al. 1998 ;
Saika et al. 2007 ). Multiple mutants of CYP707A in Arabidopsis accumulate a
large amount of ABA, whereas overexpression of CYP707A effectively reduces
endogenous ABA (Millar et al. 2006 ; Okamoto et al. 2006 , 2010 , 2011 ; Umezawa
et al. 2006 ). Thus, CYP707A plays a major regulatory role in controlling the level