ABA Biosynthetic and Catabolic Pathways - Abscisic Acid: Metabolism, Transport and Signaling - page 32

of AAOs revealed that the homodimer of AAO3, could utilize ABAld as a substrate,
and recombinant AAO3 effectively converted ABAld into ABA (Seo et al. 2000a ).
In addition, the transcript level of AAO3 was increased by dehydration but tran-
scripts of the other AAOs were not (Seo et al. 2000a ). The Arabidopsis v11 mutant
impaired in AAO3 could not synthesize ABA under drought stress, resulting in a
wilty phenotype (Seo et al. 2000b ). While AAO3 is the major ABAO it was reported
that, the other members of the AAO family have minor contributions to ABA bio-
synthesis in seeds (Seo et al. 2004 ). In addition, ectopic expression of AAO3 suc-
cessfully rescued the ABA-deficient tomato mutant sitiens (Okamoto et al. 2002 ).
sitiens was supposed to impaired in newly identified AO gene other than previously
reported TAOs (Table 2.1 ) (Harrison et al. 2011 ; Min et al. 2000 ). Recently, three
AO genes were isolated from Pisum sativum (Zdunek-Zastocka 2008 ). Recombinant
PsAO3 protein showed abscisic aldehyde oxidase activity and the levels of PsAO3
transcript were increased by progressive drought stress in leaves and roots (Zdunek-
Zastocka and Sobczak 2013 ).
Aldehyde oxidase (AO), xanthine dehydrogenase (XDH), sulfite oxidase (SO),
and nitrate reductase (NR) are MoCo-containing enzymes in plants (Mendel and
H￤nsch 2002 ). Whereas the activities of SO and NR need a dioxo Mo-center in
the MoCo, AO and XDH require the sulfuration of MoCo to generate a mono-
oxo Mo-center for their activity. The tomato flacca, tobacco Npaba1/CKR1,
and Arabidopsis aba3/los5 mutants were isolated as ABA-deficient phenotypes
because of the loss of activities in AO and XDH but not NR (Leydecker et al.
1995 ; Rousselin et al. 1992 ; Sagi et al. 1999 ; Schwartz et al. 1997a ). The aba3/
los5 and flacca mutants are impaired in the gene encoding the MoCo sulfurase
responsible for sulfuration of MoCo (Table 2.1 ) (Bittner et al. 2001 ; Sagi et al.
2002 ; Xiong et al. 2001 ). This is the reason why the biochemical defects of flacca
and aba3 mutants are similar to those of sitiens and aao3 with respect to their ina-
bility to convert ABAld into ABA.
2.4 Indirect Pathway—Possible Minor Routes
After the Production of Xanthoxin
As mentioned above, Xan is first converted to ABAld, which is then converted to
ABA. On the other hand, Xan has been supposed to be converted to ABA via two
possible minor routes in plants (Cowan 2000 ; Nambara and Marion-Poll 2005 ;
Seo and Koshiba 2002 ).
One pathway might operate via xanthoxic acid (Fig. 2.2 ). In ripening avo-
cado mesocarp, inhibition of AO activity by tungstate, a potent inhibitor of the
molybdo-enzymes in plants, results in the accumulation of xanthoxin, suggesting
that xanthoxin is a substrate of AO (Lee and Milborrow 1997 ). When Xan was
utilized as a substrate of AO, xanthoxic acid might be produced by the oxidation
of xan by AO. Although xanthoxic acid is thought to then be converted into ABA