Evidence for a Universal Pathway of Abscisic Acid Biosynthesis in Higher Plants from 18O Incorporation Patterns

Abstract

Previous labeling studies of abscisic acid (ABA) with (18)O(2) have been mainly conducted with water-stressed leaves. In this study, (18)O incorporation into ABA of stressed leaves of various species was compared with (18)O labeling of ABA of turgid leaves and of fruit tissue in different stages of ripening. In stressed leaves of all six species investigated, avocado (Persea americana), barley (Hordeum vulgare), bean (Phaseolus vulgaris), cocklebur (Xanthium strumarium), spinach (Spinacia oleracea), and tobacco (Nicotiana tabacum), (18)O was most abundant in the carboxyl group, whereas incorporation of a second and third (18)O in the oxygen atoms on the ring of ABA was much less prominent after 24 h in (18)O(2). ABA from turgid bean leaves showed significant (18)O incorporation, again with highest (18)O enrichment in the carboxyl group. The (18)O-labeling pattern of ABA from unripe avocado mesocarp was similar to that of stressed leaves, but in ripe fruits there was, besides high (18)O enrichment in the carboxyl group, also much additional (18)O incorporation in the ring. In ripening apple fruit tissue (Malus domestica), singly labeled ABA was most abundant with more (18)O incorporated in the tertiary hydroxyl group than in the carboxyl group of ABA. Smaller quantities of this monolabeled product (C-1'-(18)OH) were also detected in the stressed leaves of barley, bean, and tobacco, and in avocado fruits. It is postulated that a large precursor molecule yields an aldehyde cleavage product that is, in some tissues, rapidly converted to ABA with retention of (18)O in the carboxyl group, whereas in ripening fruits and in the stressed leaves of some species the biosynthesis of ABA occurs at a slower rate, allowing this intermediate to exchange (18)O with water. On the basis of (18)O-labeling patterns observed in ABA from different tissues it is concluded that, despite variations in precursor pool sizes and intermediate turnover rates, there is a universal pathway of ABA biosynthesis in higher plants which involves cleavage of a larger precursor molecule, presumably an oxygenated carotenoid.