The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis

The three mutant alleles of the ABA locus of Arabidopsis thaliana result in plants that are deficient in the plant growth regulator abscisic acid (ABA). We have used ¹⁸O₂ to label ABA in water-stressed leaves of mutant and wild-type Arabidopsis. Analysis by selected ion monitoring and tandem mass spectrometry of [¹⁸O]ABA and its catabolites, phaseic acid and ABA-glucose ester (β-D-glucopyranosyl abscisate), indicates that the aba genotypes are impaired in ABA biosynthesis and have a small ABA precursor pool of compounds that contain oxygens on the ring, presumably oxygenated carotenoids (xanthophylls). Quantitation of the carotenoids from mutant and wild-type leaves establishes that the aba alleles cause a deficiency of the epoxy-carotenoids violaxanthin and neoxanthin and an accumulation of their biosynthetic precursor, zeaxanthin. These results provide evidence that ABA is synthesized by oxidative cleavage of epoxy-carotenoids (the "indirect pathway"). Furthermore the carotenoid mutant we describe undergoes normal greening. Thus the aba alleles provide an opportunity to study the physiological roles of epoxy-carotenoids in photosynthesis in a higher plant.