The Catabolism of (±)-Abscisic Acid by Excised Leaves of Hordeum vulgare L. cv Dyan and Its Modification by Chemical and Environmental Factors

Excised light-grown leaves and etiolated leaves of Hordeum vulgare L. cv Dyan catabolized applied (+/-)-[2-(14)C]abscisic acid ([+/-]-[2-(14)C]ABA) to phaseic acid (PA), dihydrophaseic acid (DPA), and 2'-hydroxymethyl ABA (2'-HMABA). Identification of these catabolites was made by microchemical methods and by combined capillary gas chromatographymass spectrometry (GC-MS) following high dose feeds of nonlabeled substrate to leaves. Circular dichroism analysis revealed that 2'-HMABA was derived from the (-) enantiomer of ABA. By selecting tissue samples in which endogenous catabolites were undetectable by gas chromatography, it was possible to identify unequivocally ABA catabolites by GC-MS without the need to employ deuteriated substrate to distinguish the (+/-)-ABA catabolites from the same endogenous compounds. Refeeding studies were used to confirm the catabolic route. The methyl ester of (+/-)-[2(14)C]-ABA was hydrolyzed efficiently by light-grown leaves of H. vulgare. Leaf age played a significant role in (+/-)-ABA catabolism, with younger leaves being less able than their older counterparts to catabolize this compound. The catabolism of (+/-)-ABA was inhibited markedly in water-stressed Hordeum leaves which was characterized by a decreased incorporation of label into 2'-HMABA, DPA, and conjugates. The specific, mixed function oxidase inhibitor, ancymidol, did not inhibit, dramatically, (+/-)-ABA catabolism in light-grown leaves of Hordeum whereas the 80s ribosome, translational inhibitor, cycloheximide, inhibited this process markedly. The 70s ribosome translational inhibitors, lincomycin and chloramphenicol, were less effective than cycloheximide in inhibiting (+/-)-ABA catabolism, implying that cytoplasmic protein synthesis is necessary for the catabolism of (+/-)-ABA in Hordeum leaves whereas chloroplast protein synthesis plays only a minor role. This further suggests that the enzymes involved in (+/-)-ABA catabolism in this plant are cytoplasmically synthesized and are ;turned-over' rapidly, although the enzyme responsible for glycosylating (+/-)-ABA itself appeared to be stable.