The bZIP transcription factors HY5 and HYH are positive regulators of the main nitrate reductase gene in Arabidopsis leaves, NIA2, but negative regulators of the nitrate uptake gene NRT1.1.

Previously we have shown that nitrate reductase (NR) activity was impaired in Arabidopsis seedlings and rosette stage leaves when the basic leucine zipper transcription factors HY5 and HYH were absent. In the present work, we investigated the influence of hy5 and hyh null mutations on the expression of the NR encoding genes NIA1 and NIA2, as well as genes involved in nitrate uptake and further assimilation of nitrite. Only NIA2, and not NIA1, transcript levels were positively influenced by the presence of HY5 and HYH. In the hy5 hyh mutant, enhancement of NIA2 expression by light was impaired in both seedlings and rosette stage plants. Induction of NIA2 transcription by nitrate was not influenced by HY5 or HYH. Although the peak NIA2 transcript level was much higher in wild type in comparison with the hy5 hyh mutant plant, diurnal variations of NIA2 expression were also observed in the mutant. A dual affinity nitrate transporter gene, NRT1.1, was expressed at a higher level in hy5 hyh than in the wild type, indicating that HY5 and HYH are repressors of NRT1.1. In conclusion, HY5 and HYH were activators of NIA2, but inhibitors of NRT1.1 when tested across various light treatments and tissue types.