Abscisic aldehyde oxidase in leaves of Arabidopsis thaliana

Abscisic acid (ABA) is a plant hormone involved in seed development and res ponses to various environmental stresses. Oxidation of abscisic aldehyde is the last step of ABA biosynthesis and is catalysed by aldehyde oxidase (EC 1.2.3.1). We have reported the occurrence of three isoforms of aldehyde ox idase, AO alpha, AO beta and AO gamma, in Arabidopsis thaliana seedlings, b ut none oxidized abscisic aldehyde. Here we report a new isoform, AO delta, found in rosette leaf extracts, which efficiently oxidizes abscisic aldehy de. AO delta was specifically recognized by antibodies raised against a rec ombinant peptide encoded by AAO3, one of four Arabidopsis aldehyde oxidase genes (AAO1, AAO2, AAO3 and AAO4). Functionally expressed AAO3 protein in t he yeast Pichia pastoris showed a substrate preference very similar to that of rosette AO delta. These results indicate that AO delta is encoded by AA O3. AO delta produced in P. pastoris exhibited a very low K-m value for abs cisic aldehyde (0.51 mu M), and the oxidation product was determined by gas chromatography-mass spectrometry to be ABA. Northern analysis showed that AAO3 mRNA is highly expressed in rosette leaves. When the rosette leaves we re detached and exposed to dehydration, AAO3 mRNA expression increased rapi dly within 3 h of the treatment. These results suggest that AO delta, the A AO3 gene product, acts as an abscisic aldehyde oxidase in Arabidopsis roset te leaves.