Light regulation of nitrate reductase in higher plants: Which photoreceptors are involved?

The regulation by light of nitrate reductase (NR) in higher plants is very complex. NR is firstly regulated by transcriptional control. In etiolated plants, phytochrome is the main photoreceptor involved and its low fluence response (LFR) is the common response mode. The effect of the very low fluence response (VLFR) has been reported for one of the isoforms in Arabidopsis thaliana, NIA2. The existence of a far-red light-mediated high irradiance response (HIR) has been demonstrated in phytochrome A over-expressing tobacco, confirming classical photophysiological experiments. For Lemna aequinoctialis, the existence of a red light-mediated high irradiance response has been assumed. In green plants, the effect of phytochrome on the level of NR protein is very modest, and the effect of light is mediated via photosynthesis, most probably through biochemical signals. The molecular basis of this switch from the action of phytochromes in etiolated tissue to effects of photosynthesis in green tissue is not known. The influence of light on NR is also regulated by fast post-translational modification. This mechanism is based on the phosphorylation/dephosphorylation of a serine residue in the hinge 1 region of NR and the subsequent Mg2+/polyamine-dependent binding of the phosphorylated form to a 14-3-3 protein. Several kinases phosphorylate NR, and phosphatases not only act on NR but also dephosphorylate NR kinases. No influence of phytochrome is detectable. Instead, post-translational modification depends on the effect of photosynthesis. Photosynthesis may exert its effect on this process via biochemical products of photosynthesis (glucose, sugar phosphates), via substrates (nitrate, NADH) and/or by influencing Ca2+ flux.