Control of Nitrate Reductase and Nitrite Reductase Gene Expression by Light, Nitrate and a Plastidic Factor

Abstract

AbstractIn the present review we consider regulation of the build‐up of the nitrate reducing apparatus (NO−3 → NH+4) in the course of light‐mediated plastidogenesis. In particular, the regulation of the appearance of nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.7.7.1) will be treated. While both enzymes are nuclear‐encoded, NR is a cytosolic and NIR a plastidic protein. Substrate induction by nitrate, control by light (phytochrome) and dependence on a ‘plastidic factor’ are the main features of the control system.In mustard (Sinapis alba L.) seedling cotyledons the NR‐mRNA level was determined by a strong synergistic coaction of nitrate and light. Synthesis of NR (a cytosolic protein with two isoforms) correlated with the level of NR mRNA. A positive control of NR gene expression by the plastidic factor was observed. It appeared that the plant cell regulates NR gene expression as if NR would be a plastidic protein.Control of gene expression in the case of plastidic NIR in the same mustard cotyledons was conspicuously different. Whereas, in case of NR, the transcript level was determined by a synergism between light and nitrate, in the case of NIR the transcript level was controlled by light alone, with nitrate being ineffective. However, in both cases actual enzyme synthesis was controlled by nitrate, and the dependence on positive control by the plastidic factor was the same. Thus, the difference between NR and NIR gene expression appears to be a matter of transcriptional control. From a teleonomic viewpoint, the control system to establish the nitrate‐reducing apparatus appears (almost) perfect.In spinach (Spinacia oleracea L.) seedling cotyledons the NIR transcript level was unaffected by light but determined by nitrate. The strong action of light on NIR synthesis was multiplicatively superimposed on the action of nitrate, consistent with the conclusion that nitrate affects the transcript level while light controls enzyme synthesis.In tobacco (Nicotiana tabacum L., cv. Coker 176) seedlings the NIR transcript level was determined by a synergistic action between nitrate and light, i.e. both factors coact at the level of transcription. There was no effect of light on NIR synthesis in the absence of nitrate while a strong action of light on protein synthesis was observed in the presence of nitrate. The specific effect of blue light on enzyme appearance, i.e. an effect which cannot be attributed to phytochrome, was not seen at the transcript level.Thus, in tobacco a coaction of nitrate and light (phytochrome) is required to bring about a high NIR transcript level, while in mustard the NIR transcript level was determined by phytochrome alone and in spinach by nitrate alone. It is obvious that in different plants phytochrome and nitrate control NIR gene expression differently. Only the dependence on the plastidic factor appears to be the same in all cases studied so far.The conspicuous regulatory differences between spinach and tobacco offer a chance to address the crucial question of whether the NIR‐promoter from spinach fused to a reporter gene and introduced in tobacco, responds to nitrate and phytochrome in accordance with spinach (donor) or in accordance with tobacco (host). Experiments are in progress.