Mode of Action of Abscisic Acid in Barley Aleurone Layers

As a part of our effort to study the mode of action of abscisic acid (ABA) and its metabolites during seed germination, we have investigated the regulation of ABA metabolism in barley (Hordeum vulgare) aleurone layers and a few other plant tissues. The rate of conversion of (3)HABA to (3)Hphaseic acid (PA), the first stable metabolite of ABA, is enhanced by 2- to 5-fold in barley aleurone layers when the tissue is pretreated with ABA. However, the conversion of (3)HPA to (3)H dihydrophaseic acid (DPA), the next metabolite after PA, is not enhanced by pretreatment with either ABA or PA. The ABA enhancement of its own metabolism in barley aleurone layers is detectable with a pretreatment of ABA ranging from 10(-3) to 10(-4) molar. This apparent self-induction of ABA conversion to PA can be observed after the barley aleurone layers have been treated with 10(-5) molar ABA for as short as 2 hours, and is inhibited by the transcription inhibitor, cordycepin (3'-deoxyadenosine), or the translation inhibitor, cycloheximide. The self-induction of ABA conversion to PA also occurs in wheat aleurone layers, but not in other plant tissues that have been investigated, including corn root tips, barley embryos, barley, and soybean leaf discs. It is probably a phenomenon unique to the aleurone layers of some cereal grains. In view of the recent observations that ABA is able to induce new proteins in barley aleurone layers, we suggest that some of these ABA-induced proteins are involved in the conversion from ABA to PA in this tissue.