In higher plants aldehyde oxidases (AO) appear to be homo- and heterodimeric assemblies of AO subunits with probably different physiological functions. AO-delta may be involved in the last step of abscisic acid biosynthesis, at least in leaves and seeds. In vitro, AO-delta oxidizes abscisic aldehyde to abscisic acid (ABA) (PubMed:10972874). In vitro, AO-delta also uses 1-naphthaldehyde, indole-3-aldehyde (IAld), benzaldehyde and cinnamaldehyde as substrate; the AAO2-AAO3 dimer also uses abscisic aldehyde as substrate. 2-cis-(+)-abscisic aldehyde + H2O + O2 = 2-cis-(+)-abscisate + H(+) + H2O2 1-naphthaldehyde + H2O + O2 = 1-naphthoate + H(+) + H2O2 H2O + indole-3-acetaldehyde + O2 = (indol-3-yl)acetate + H(+) + H2O2 Binds 2 [2Fe-2S] clusters. Binds 1 Mo-molybdopterin (Mo-MPT) cofactor per subunit. All these kinetic values were obtained with AO-delta dimer. Aldehyde oxidases (AO) are homodimers and heterodimers of AO subunits. AO-delta is a AAO3 homodimer. AAO3 also forms a dimer with AAO2. Interacts with PUB44, and this interaction probably results in targeting of this protein to the proteasome. Expressed in vascular tissues of all organs, particularly in phloem companion cells and xylem parenchymatic cells. Highly expressed in roots and rosettes, and to lower extent in seedlings, stems and flowers. Expressed at very low levels in siliques and dry seeds. Also detected in root dividing cells (tips and primordia), in mesophyll cells and inside the guard cells. Transcripts are induced by dehydration, in rosettes but not in roots. Induction by cold, ABA, sodium chloride (NaCl) and polyethylene glycol (PEG) is dependent of the zeaxanthin epoxidase ABA1 protein (ZEP). Induction by glucose requires the short chain alcohol dehydrogenase ABA2 protein. Repressed by mannitol. Impaired abscisic acid (ABA) biosynthesis. In vitro, cannot discriminate between (+) and (-) enantiomers of abscisic acid and leads respectively to (+) and (-) cis-ABA. Belongs to the xanthine dehydrogenase family.