TRANSCRIPTIONAL REGULATION OF NITRATE REDUCTASE MESSENGER-RNA LEVELS BY CYTOKININ-ABSCISIC ACID INTERACTIONS IN ETIOLATED BARLEY LEAVES

To investigate the mechanisms by which the expression of a specific gene may be modulated by multiple hormones, the regulation of nitrate reductase (NR) mRNA levels by cytokinin and abscisic acid (ABA) was studied in etiolated barley (Hordeum vulgare L. cv Robust) leaves using a barley NR cDNA as a probe. Northern blot analyses of the levels of NR mRNA indicate that accumulation of the mRNA was enhanced by the cytokinin, benzyladenine (BA), and suppressed by ABA and that the hormonal effects on NR gene expression were responsive to dose concentrations (10(-7) to 10(-4) molar). The NR mRNA levels were influenced by the BA/ABA concentration ratios, and the inhibition of the NR transcript levels by ABA was not totally reversed by addition of equal concentrations (10(-7) to 10(-4) molar) of BA. Nuclear runoff transcription studies showed that the level of transcription was suppressed by ABA, and addition of an equal concentration of BA partially reversed the ABA action. Thus, the interaction of cytokinin and ABA on NR gene expression is, at least in part, at the level of transcription.