2-trans-abscisic acid biosynthesis and metabolism of ABA-aldehyde and xanthoxin in wild type and the aba mutant of arabidopsis thaliana

Abscisic acid (ABA) and 2-trans-ABA (t-ABA) biosynthesis were studied in wild type Landsberg erecta and the three allelic aba mutants of Arabidopsis thaliana (L.) Heynh., which are impaired in epoxy-carotenoid biosynthesis. Labelling experiments with ¹⁸O₂and mass spectrometric analysis of [¹⁸O]ABA and its catabolites ABA-glucose ester (ABA-GE) and phaseic acid (PA), and t- ABA and t-ABA-GE, showed that t-ABA biosynthesis was less affected than ABA biosynthesis by mutations at the ABA locus. The aba-4 allele caused the most severe impairment of ABA biosynthesis compared with the other two mutant alleles aba-1 and aba-3, yet aba-4 plants synthesized as much t-ABA as wild type Landsberg erecta plants. Feeding experiments with RS- [²H₆]ABA-aldehyde isomers and unlabelled xanthoxin isomers suggest that t-xanthoxin and t-ABA-aldehyde are precursors to ABA and t-ABA in Arabidopsis