The catabolism of (±)-abscisic acid by excised leaves of Hordeum vulgare L. cv Dyan and its modification by chemical and environmental factors

Abstract

Excised light-grown leaves and etiolated leaves of Hordeum vulgare L. cv Dyan catabolized applied (±)-[2-14C]abscisic acid (1±]-[2-14C]ABA) to phaseic acid (PA), dihydrophaseic acid (DPA), and 2ʹ-hydroxymethyl ABA (2ʹ-HMABA). Identification of these catabolites was made by microchemical methods and by combined capillary ps chromatographymass spectrometry (GC-MS) following high dose feeds of nonlabeled substrate to leaves. Circular dichroism analysis revealed that 2ʹ-HMABA was derived from the (-) enantiomer of ABA. By selectn tissue samples in which endogenous catabolites were undetectable by gas chromatography, it was possible to identify unequivocally ABA catabolites by GCMS without the need to employ deuteriated substrate to distinguish the (±)-ABA catabolites from the same endogenous compounds. Refeedi studies were used to confirm the catabolic route. The methyl ester of(±)- [214C]-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 2ʹ-HMABA, DPA, and conjugates. The specific, mixed function oxidase inhibitor, ancymidol, did not inhibit, dramatically, (±)-ABA catabolism in light-grown leaves of Hordm 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.