Abscisic Acid-induced Changes in Nucleotide Composition of Rapidly Labeled Ribonucleic Acid Species of Lentil Root: Reversal by Kinetin.

An antagonism between cytokinins and growth inhibitors has been shown in germination (4, 5, 9, 12). Cytokinin-inhibitor antagonism has also been observed at the level of enzyme synthesis (6, 13). Based on a number of studies in which exogenous hormones were used to control germination, it has been hypothesized that in the regulation of germination, gibberellin is the primary stimulus and that the action of inhibitors and cytokinins present in the seed is essentially "preventive" and "permissive," respectively (5, 6, 10). On the basis of this premise gibberellininduced germinative process(es) would be inhibited by growth inhibitor(s) and reversed by cytokinin(s). Other reports show that an antagonism between cytokinins and growth inhibitors is not limited to germination, but is found in shoot and root growth as well, suggesting the possibility that such hormonal interaction may be an essential feature in the control of plant growth and development (6, 12). The molecular basis for cytokinin-inhibitor antagonism is not known. Recent work from our laboratories showed that abscisic acid (ABA) induced pronounced changes in the nucleotide composition of rapidly labeled RNA species of pear embryos. This change was largely due to an increase in the UMP content and a decrease in the GMP content (7). This communication will show that similar changes are induced by ABA3 in rapidly labeled RNA species of excised lentil (Lens culinaris) roots, and also that the ABA effect is reversed by kinetin. In addition, it will be shown that in lentil roots ABA promotes 32P labeling of RNA species, whereas kinetin inhibits such labeling. Excised lentil root tips were selected for the present studies because they are extremely sensitive to kinetin treatment (2).