The Arabidopsis aldehyde oxidase 3 (AA03) gene product catalyzes the finalstep in abscisic acid biosynthesis in leaves

Abscisic acid (ABA) is a plant hormone involved in seed development and ger mination and in responses to various environmental stresses. The last step of ABA biosynthesis involves oxidation of abscisic aldehyde, and aldehyde o xidase (EC 1.2.3.1) is thought to catalyze this reaction. An aldehyde oxida se isoform, AO delta, encoded by AAO3, one of four Arabidopsis aldehyde oxi dase genes (AAO1, AAO2, AAO3, and AAO4), is the most likely candidate for t he enzyme, because it can efficiently catalyze the oxidation of abscisic al dehyde to ABA. Here, we report the isolation and characterization of an ABA -deficient Arabidopsis mutant that maps at the AAO3 locus. The mutant exhib its a wilty phenotype in rosette leaves, but seed dormancy is not affected. ABA levels were significantly reduced in the mutant leaves, explaining the wilty phenotype in rosettes, whereas the level in the mutant seeds was les s reduced. No AO delta activity could be detected in the rosette leaves of the mutant. Sequence data showed that the mutant contains a G to A substitu tion in the AAO3 gene. The mutation causes incorrect splicing of the ninth intron of AAO3 mRNA. We thus conclude that the ABA-deficient mutant is impa ired in the AAO3 gene and that the gene product, AO delta, is an aldehyde o xidase that catalyzes the last step of ABA biosynthesis in Arabidopsis, spe cifically in rosette leaves. Other aldehyde oxidases may be involved in ABA biosynthesis in other organs.