Roles of the transcriptional regulation mediated by the nitrate-responsive cis-element in higher plants

Nitrate is a major nitrogen source for land plants but also acts as a signaling molecule that regulates gene expression, metabolism and plant growth. The genes for nitrate reductase (NR) and nitrite reductase (NIR), which are enzymes in the nitrate assimilation pathway, are typical nitrate-inducible genes. We previously identified the first authentic nitrate-responsive cis-element (NRE) for nitrate-inducible transcription by the analysis of the NIR gene promoter from Arabidopsis. Here we further characterize NRE-mediated regulation using transgenic Arabidopsis plants. First, NRE-mediated regulation is shown to be a primary response to nitrate that is caused by pre-existing components, because the NRE-mediated nitrate-inducible expression of the GUS reporter gene is unaffected by treatment with the protein synthesis inhibitor cycloheximide. Second, we show that NRE-like sequences are present in various dicotyledonous and monocotyledonous NIR gene promoters at similar positions and that they also drive nitrate-inducible expression in Arabidopsis. This suggests that NRE-mediated regulation might be conserved in higher plants. Finally, the NRE-mediated expression of the GUS reporter gene is shown to produce diurnal expression in transgenic Arabidopsis plants. This expression peaked at the beginning of the day and decreased during the day which is very similar to the reported diurnal pattern of the nitrate content, suggesting a role of NRE-mediated regulation in controlling diurnal expression in response to oscillation of the nitrate content over a day/night cycle. These findings further clarify the roles of the NRE-mediated regulatory system in higher plants.