Isolation and Characterization of Three Genes Negatively Regulated by Phytochrome Action in Lemna gibba

We have isolated three distinct cDNA clones from Lemna gibba representing mRNAs that increase in abundance during dark treatment. All three mRNAs showed reduced expression in response to red or white light. These mRNAs range from approximately 680 to 800 nucleotides in length and thus encode relatively small proteins (maximum relative molecular weight 17,000 to 19,000). The genes corresponding to these dark-abundant mRNAs are designated NPR (negatively phytochrome regulated) 1, 2, and 3. Differences in the rapidity of mRNA accumulation during dark treatment were observed for each of the genes in both mature green plants and in etiolated plants. Differences in accumulation pattern were also observed in etiolated plants, depending on whether the plants received a far-red light treatment prior to darkness. Transcription of all three genes, assayed in nuclei isolated from either green or etiolated plants, increased during dark treatment. In etiolated plants, a single 2 minute red light treatment caused a detectable decrease in the transcription of the genes after the dark treatment, and 10 minutes of far-red light given immediately after the red light resulted in a reversal of the effect of red light. Additionally, treatment of the plants with far-red light prior to darkness resulted in greater rates of transcription of the NPR genes. Therefore, we conclude that phytochrome action results in decreased transcription of these NPR genes. Each of the NPR mRNAs are encoded by one to two genes.