Abscisic Acid Biosynthesis in Isolated Embryos of Zea mays L.

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Abstract
Previous labeling experiments with 18O2 have supported the hypothesis that stress-induced abscisic acid (ABA) is synthesized through an indirect pathway involving an oxygenated carotenoid (xanthophyll) as a precursor. To investigate ABA formation under nonstress conditions, an 18O2 labeling experiment was conducted with isolated embryos from in vitro grown maize (Zea mays L.) kernels. Of the ABA produced during the incubation in 18O2, three-fourths contained a single 18O atom located in the carboxyl group. Approximately one-fourth of the ABA synthesized during the experiment contained two 18O atoms. These results suggest that ABA synthesized in maize embryos under nonstress conditions also proceeds via the indirect pathway, requiring a xanthophyll precursor. It was also found that the newly synthesized ABA was preferentially released into the surrounding medium.