Genetic variation in the activity of nitrate reductase and nitrite reductase in Lolium
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Nitrogen Assimilation
To gain an insight into the diurnal changes of nitrogen assimilation in roots the in vitro activities of cytosolic and plasma membrane‐bound nitrate reductase (EC 1.6.6.1), nitrite reductase (EC 1.7.7.1) and cytosolic and plastidic glutamine synthetase (EC 6.3.1.2) were studied. Simultaneously, changes in the contents of total protein, nitrate, nitrite, and ammonium were followed. Roots of intact tobacco plants (Nicotiana tabacum cv. Samsun) were extracted every 3 h during a diurnal cycle. Nitrate reductase, nitrite reductase and glutamine synthetase were active throughout the day–night cycle. Two temporarily distinct peaks of nitrate reductase were detected: during the day a peak of soluble nitrate reductase in the cytosol, in the dark phase a peak of plasma membrane‐bound nitrate reductase in the apoplast. The total activities of nitrate reduction were similar by day and night. High activities of nitrite reductase prevented the accumulation of toxic amounts of nitrite throughout the entire diurnal cycle. The resulting ammonium was assimilated by cytosolic glutamine synthetase whose two activity peaks, one in the light period and one in the dark, closely followed those of nitrate reductase. The contribution of plastidic glutamine synthetase was negligible. These results strongly indicate that nitrate assimilation in roots takes place at similar rates day and night and is thus differently regulated from that in leaves.
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Aspergillus nidulans
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Total activities of nitrate and nitrite reductases were higher in 4 to 20 day old maize plants in the leaves than in the roots. The ratio of activities found in the leaves and in the roots respectively was much higher in the case of nitrate reductase than in the case of nitrite reductase. On the other hand higher glutamate dehydrogenase activity in the roots than in the leaves clearly indicates that the roots play a more important role in the assimilation of ammonium than in the assimilation of nitrate. When comparing the distribution of seminal and nodal adventitious roots of maize seedlings with the assimilation of inorganic nitrogen on the basis of enzyme levels, it could be deduced that during the first 20 days of seedling growth seminal roots were more involved in the assimilation of nitrate whereas nodal adventitious roots were more active in ammonium assimilation.
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1. Possible mechanisms regulating the activities of three enzymes involved in nitrate assimilation, nitrate reductase, nitrite reductase and glutamate dehydrogenase, were studied in radish cotyledons. 2. Nitrate-reductase and nitrite-reductase activities are low in nitrogen-deficient cotyledons, and are induced by their substrates. 3. Glutamate dehydrogenase is present regardless of the nitrogen status, and the enzyme can be increased only slightly by long-term growth on ammonia. 4. Although nitrate is the best inducer of nitrate reductase, lower levels of induction are also obtained with nitrite and ammonia. The experiments did not distinguish between direct or indirect induction by these two molecules. 5. Nitrite reductase is induced by nitrite and only indirectly by nitrate. 6. The induction of both nitrate reductase and nitrite reductase is prevented by the inhibitors actinomycin D, puromycin and cycloheximide, indicating a requirement for the synthesis of RNA and protein. 7. The decay of nitrate reductase, determined after inhibition of protein synthesis, is slower than the synthesis of the enzyme. Nitrite reductase is much more stable than nitrate reductase. 8. The synthesis of nitrate reductase is not repressed by ammonia, but is repressed by growth on a nitrite medium. 9. There is no inhibition of nitrate reductase, nitrite reductase or glutamate dehydrogenase by the normal end products of assimilation, but cyanate is a fairly specific inhibitor of nitrate reductase.
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Plants take up nitrogen in the form of nitrate and ammonia through nitrate assimilation pathway.Nitrate assimilation is highly regulated process by two successive enzymes,nitrate reductase (NR) and nitrite reductase (NiR),which mainly controll the primary nitrogen assimilation.In higher plants,various external and endogenous factors influence the regulation of NR and NiR gene expression transcriptionally as well as post-transcriptionally.Post-translational modulation is an important mechanism to eliminate the accumulation of nitrite.Development of molecular biology methods will contribute to the research of regulatory networks of NR and NiR by creating mutants and transgenic lines.
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To gain an insight into the diurnal changes of nitrogen assimilation in roots the in vitro activities of cytosolic and plasma membrane‐bound nitrate reductase (EC 1.6.6.1), nitrite reductase (EC 1.7.7.1) and cytosolic and plastidic glutamine synthetase (EC 6.3.1.2) were studied. Simultaneously, changes in the contents of total protein, nitrate, nitrite, and ammonium were followed. Roots of intact tobacco plants ( Nicotiana tabacum cv. Samsun) were extracted every 3 h during a diurnal cycle. Nitrate reductase, nitrite reductase and glutamine synthetase were active throughout the day–night cycle. Two temporarily distinct peaks of nitrate reductase were detected: during the day a peak of soluble nitrate reductase in the cytosol, in the dark phase a peak of plasma membrane‐bound nitrate reductase in the apoplast. The total activities of nitrate reduction were similar by day and night. High activities of nitrite reductase prevented the accumulation of toxic amounts of nitrite throughout the entire diurnal cycle. The resulting ammonium was assimilated by cytosolic glutamine synthetase whose two activity peaks, one in the light period and one in the dark, closely followed those of nitrate reductase. The contribution of plastidic glutamine synthetase was negligible. These results strongly indicate that nitrate assimilation in roots takes place at similar rates day and night and is thus differently regulated from that in leaves.
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