The transcriptional rates of four different genes in shoots of barley grown under different light regimes were quantified by monitoring nuclear RNA transcripts using gene-specific hybridization probes. Isolated nuclei were pulse-labelled with [cx-~'P]UTP and the relative rates of light-harvesting chlorophyll a/b protein (LHCP) mRNA, NADPH : protochlorophyllide oxidoreductase mRNA, B1 hordein mRNA, and 26-S rRNA synthesis were measured. Irradiation of dark-grown plants with a red light pulse increased the rate of LHCP mRNA synthesis tenfold within 3 h, and the rate of rRNA synthesis more than twofold within 9 h. The relative rate of synthesis of the oxidoreductase mRNA decreased following a red light pulse reaching a minimum after 3 -6 h. As a direct proof of phytochrome involvement in the light-induced stimulation of LHCP and the repression of the oxidoreductase transcripts for both responses, red/far-red reversibility could be demonstrated. We conclude that phytochrome is able both to increase the transcription of certain nuclear genes and decrease the transcription of others.The light-dependent control of plant development is one of the central themes of current plant biology [I]. Light has been shown to induce an increase in a number of mRNA species encoding, for instance, several plastid-specific proteins like the light-harvesting chlorophyll a/b protein and the small subunit of the ribulose-I ,5-bisphosphate carboxylase [2-131. At the same time the concentration of other transcripts, e.g. those coding for the NADPH : protochlorophyllide oxidoreductase (reductase) and phytochrome, is drastically reduced upon illumination of darkgrown plants [14-171. In both instances the reciprocal effect of light on the level of these two groups of mRNAs is mediated by the same photoreceptor phytochrome [3, 5, 6, 8 -1 1,13 -171. These light effects could occur by two possible mechanisms : a phytochrome-controlled change in the rate of transcription initiation and/or a phytochrome-induced change in the stability of transcripts. In the present study we have concentrated on elucidating the first of these two mechanisms. Isolated nuclei of barley have been pulse-labelled and the newly synthesized transcripts analyzed. Available evidence suggests that the majority of RNA synthesis in isolated nuclei represents elongation of already initiated RNA chains and that the rates of transcription in pulse-labelled isolated nuclei can be monitored accurately without the complications of RNA processing or stabilization which occurs within the intact cell [18]. In previous studies it could be demonstrated [I91 that phytochrome controls the overall transcription rate in isolated Avena nuclei. Studies on the synthesis of specific transcripts in isolated nuclei of pea and duckweed, a lightinduced increase in the rate of transcription of genes encoding the light-harvesting chlorophyll a/b protein and the small subunit of the ribulose-l,5-bisphosphate carboxylase has been described [12, 131. This light effect was mediated by phytochrome [13]. As...