The Arabidopsis aldehyde oxidase 3 ( AAO3 ) gene product catalyzes the final step in abscisic acid biosynthesis in leaves

Abscisic acid (ABA) is a plant hormone involved in seed development and germination and in responses to various environmental stresses. The last step of ABA biosynthesis involves oxidation of abscisic aldehyde, and aldehyde oxidase (EC1.2.3.1 ) is thought to catalyze this reaction. An aldehyde oxidase isoform, AOδ, encoded byAAO3 , one of fourArabidopsis aldehyde oxidase genes (AAO1 ,AAO2 ,AAO3 , andAAO4 ), is the most likely candidate for the enzyme, because it can efficiently catalyze the oxidation of abscisic aldehyde to ABA. Here, we report the isolation and characterization of an ABA-deficientArabidopsis mutant that maps at theAAO3 locus. The mutant exhibits 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 less reduced. No AOδ activity could be detected in the rosette leaves of the mutant. Sequence data showed that the mutant contains a G to A substitution in theAAO3 gene. The mutation causes incorrect splicing of the ninth intron ofAAO3 mRNA. We thus conclude that the ABA-deficient mutant is impaired in theAAO3 gene and that the gene product, AOδ, is an aldehyde oxidase that catalyzes the last step of ABA biosynthesis inArabidopsis , specifically in rosette leaves. Other aldehyde oxidases may be involved in ABA biosynthesis in other organs.