Abscisic acid antagonism

Abscisic acid is a negative regulator in that it primarily antagonizes the action of cytokinins, auxins, and in particular, gibberellins. Abscisic acid decreased the activity of polymerase in radishes (52), peas (53), maize coleoptiles (54), and pear embryos (55). More detailed studies are needed before the question of ABA-induced "modification" of RNA polymerase (54) or "alterations" in the number of sites for template activity (56) can be answered. In barley aleurone cells, ABA-induced suppression of GA-induced <-amylase formation was presumed to involve the continuous synthesis of a short-lived RNA (57). 

Much like the phenolic acids, early work with scopoletin showed it inhibited oxidation of IAA and thus could affect growth in this manner. Inhibition of several other enzymes by scopoletin and coumarin has been shown. Coumarin was reported to induce ethylene synthesis.47 Also, it is one of several phenolic compounds that antagonize abscisic acid-induced inhibition of growth and stomatal closure.52 Undoubtedly, these and possibly other interactions with hormones are part of the physiological action of the coumarins. 

The related substance xanthoxin (4.88) clearly plays a fundamental part in normal plant regulation by antagonizing indolylacetic acid, gibberellic acid, and kinetin. It is readily oxidized to abscisic acid, of which it is probably the natural precursor and with which it shares the trans,trans configuration (Taylor and Burden, 1972). 

X 10" M sharply decreased auxin levels. In seedlings of Amaranthus caudatus the inhibition of light-induced beta-cyanin synthesis by abscisic acid was antagonized by several phenols including coumarin (maximal effect at 10 M). ... 

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