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PubMed1986

Cloning and nitrate induction of nitrate reductase mRNA.

Nitrate is the major source of nitrogen taken from the soil by higher plants but requires reduction to ammonia prior to incorporation into amino acids. The first enzyme in the reducing pathway is a nitrate-inducible enzyme, nitrate reductase (EC 1.6.6.1). A specific polyclonal antiserum raised against purified barley nitrate reductase has been used to immunoprecipitate in vivo labeled protein and in vitro translation products, demonstrating that nitrate induction increases nitrate reductase protein and translatable mRNA. A partial cDNA clone for barley nitrate reductase has been isolated and identified by hybrid-selected translation. RNA blot-hybridization analysis shows that nitrate induction also causes a marked increase in the steady-state level of nitrate reductase mRNA.

Journal Article↗
PubMed1993

Insertion of transposon Tn5 into a structural gene of the membrane-bound nitrate reductase of Thiosphaera pantotropha results in anaerobic overexpression of periplasmic nitrate reductase activity.

Chlorate-resistant mutants of the denitrifying bacterium Thiosphaera pantotropha were generated by transposon Tn5 mutagenesis. One class was deficient in membrane-bound nitrate reductase activity but retained a periplasmic nitrate reductase activity. Using transposon marker rescue it was shown that in one such mutant, M-6, the transposon was inserted in the membrane-bound nitrate reductase beta subunit structural gene (termed narH in order to be consistent with the nomenclature of the Escherichia coli major nitrate reductase operon). The translated sequence (total of 106 amino acids) from around the point of transposon insertion showed approximately 90% amino acid identity with the beta subunits of the E. coli nitrate reductases. Under anaerobic growth conditions M-6 overproduced the periplasmic nitrate reductase activity allowing anaerobic growth with nitrate as electron acceptor. A regulatory link was inferred between the presence of the membrane-bound nitrate reductase and expression of the periplasmic nitrate reductase. This is the first demonstration of full denitrification in an organism possessing only a periplasmic nitrate reductase.

Amino Acid Sequence↗
PubMed1997

Spectroscopic and kinetic characterization of the recombinant cytochrome c reductase fragment of nitrate reductase. Identification of the rate-limiting catalytic step.

The recombinant NADH-cytochrome c reductase fragment of spinach NADH-nitrate reductase (EC 1.6.6.1), consisting of the contiguous heme-containing cytochrome b domain and flavin-containing NADH-cytochrome b reductase fragment, has been characterized spectroscopically and kinetically. Reductive titration with sodium dithionite indicates heme reduction takes place prior to flavin reduction, which correlates well with the reduction potentials for enzyme-bound heme (15 mV) and FAD (-280 mV). Reductive titration with NADH also indicates that the reduced enzyme forms a charge-transfer complex with NAD+. The circular dichroism spectrum of the oxidized fragment is primarily due to the flavin, whereas the ferrous heme dominates the circular dichroism spectrum of reduced enzyme. Three kinetic phases are observed in the course of the reaction of the enzyme with NADH, each with a distinct spectral signature. The fast phase represents flavin reduction, concomitant with the formation of a charge-transfer complex between reduced flavin and NAD+, and exhibits hyperbolic dependence on NADH concentration with a Kd of 3 microM and a limiting rate constant of 560 s-1. Electron transfer from reduced flavin to heme with a rate constant of 12 s-1 is the intermediate phase, which is rate-limited by breakdown of the charge-transfer complex between NAD+ and reduced flavin. The slow phase is dismutation of a pair of molecules of two-electron reduced enzyme (generated at the end of the second phase of the reaction) to give one molecule each of one- and three- electron reduced enzyme, with a second order rate constant of 2 x 10(6) M-1 s-1. In the presence of excess NADH, this dismutation reaction is followed by the rapid reaction of the one-electron reduced enzyme with a second equivalent of NADH to generate fully reduced enzyme. On the basis of this work, it appears that dissociation of NAD+ from the reduced flavin site rate limits electron transfer to the cytochrome and likely represents the overall rate-limiting step of catalysis.

Binding Sites↗
PubMed1989

Evidence for a plasma-membrane-bound nitrate reductase involved in nitrate uptake of Chlorella sorokiniana.

Anti-nitrate-reductase (NR) immunoglobulin-G (IgG) fragments inhibited nitrate uptake into Chlorella cells but had no affect on nitrate uptake. Intact anti-NR serum and preimmune IgG fragments had no affect on nitrate uptake. Membrane-associated NR was detected in plasma-membrane (PM) fractions isolated by aqueous two-phase partitioning. The PM-associated NR was not removed by sonicating PM vesicles in 500 mM NaCl and 1 mM ethylenediaminetetraacetic acid and represented up to 0.8% of the total Chlorella NR activity. The PM NR was solubilized by Triton X-100 and inactivated by Chlorella NR antiserum. Plasma-membrane NR was present in ammonium-grown Chlorella cells that completely lacked soluble NR activity. The subunit sizes of the PM and soluble NRs were 60 and 95 kDa, respectively, as determined by sodium-dodecyl-sulfate electrophoresis and western blotting.

Biological Transport↗
PubMed1988

Influence of nar (nitrate reductase) genes on nitrate inhibition of formate-hydrogen lyase and fumarate reductase gene expression in Escherichia coli K-12.

In Escherichia coli, aerobiosis inhibits the synthesis of enzymes for anaerobic respiration (e.g., nitrate reductase and fumarate reductase) and for fermentation (e.g., formate-hydrogen lyase). Anaerobically, nitrate induces nitrate reductase synthesis and inhibits the formation of both fumarate reductase and formate-hydrogen lyase. Previous work has shown that narL+ is required for the effects of nitrate on synthesis of both nitrate reductase and fumarate reductase. Another gene, narK (whose function is unknown), has no observable effect on formation of these enzymes. We report here our studies on the role of nar genes in fumarate reductase and formate-hydrogen lyase gene expression. We observed that insertions in narX (also of unknown function) significantly relieved nitrate inhibition of fumarate reductase gene expression. This phenotype was distinct from that of narL insertions, which abolished this nitrate effect under certain growth conditions. In contrast, insertion mutations in narK and narGHJI (the structural genes for the nitrate reductase enzyme complex) significantly relieved nitrate inhibition of formate-hydrogen lyase gene expression. Insertions in narL had a lesser effect, and insertions in narX had no effect. We conclude that nitrate affects formate-hydrogen lyase synthesis by a pathway distinct from that for nitrate reductase and fumarate reductase.

Aldehyde Oxidoreductases↗
PubMed2006

Functional complementation of a nitrate reductase defective mutant of a green alga Dunaliella viridis by introducing the nitrate reductase gene.

Nitrate reductase (NR) catalyzes NAD (P) H dependent reduction of nitrate to nitrite. Transformation systems have been established in several species of green algae by nitrate reductase gene functional complementation. In this report, an endogenous NR cDNA (3.4 kb) and a genomic fragment (14.6 kb) containing the NR gene (DvNIA1) were isolated from the D. viridis cDNA and genomic libraries respectively. Southern blot and Northern blot analyses showed that this gene exists as a single copy in D. viridis and is induced by nitrate. To obtain a NR defective mutant as a recipient strain, D. viridis cells were treated with a chemical mutagen and then cultured on a chlorate-containing plate to enrich chlorate tolerant mutants. Southern analysis showed that one isolate, B14, had a deletion in the DvNIA1 gene region. Using electroporation conditions determined in this laboratory, plasmid pDVNR containing the intact DvNIA1 gene has been electroporated into the defective mutant B14. Strains retaining a nitrate assimilation phenotype were obtained from nitrate plates after spreading the electroporated cells. In some individual strains, transcription of the introduced gene was detected. NR activity in these strains was slightly higher than that in the defective B14 cell, but excretion of nitrite into culture media was almost as high as that of the wild-type cell. Possible episomal presence of the introduced DNA in D. viridis is discussed.

Amino Acid Sequence↗
PubMed1997

Tobacco mutants with a decreased number of functional nia genes compensate by modifying the diurnal regulation of transcription, post-translational modification and turnover of nitrate reductase.

Although nitrate reductase (NR. EC 1.6.6.1) is thought to control the rate of nitrate assimilation, mutants with 40-45% of wildtype (WT) NR activity (NRA) grow as fast as the WT. We have investigated how tobacco (Nicotiana tabacum L. cv. Gatersleben) mutants with one or two instead of four functional nia genes compensate. (i) The nia transcript was higher in the leaves of the mutants. However, the diurnal rhythm was retained in the mutants, with a maximum at the end of the night and a strong decline during the photoperiod. (ii) Nitrate reductase protein and NRA rose to a maximum after 3-4 h light in WT leaves, and then decreased by 50-60% during the second part of the photoperiod and the first part of the night. Leaves of mutants contained 40-60% less NR protein and NRA after 3-4 h illumination, but NR did not decrease during the photoperiod. At the end of the photoperiod the WT and the mutants contained similar levels of NR protein and NRA. (iii) Darkening led to a rapid inactivation of NR in the WT and the mutants. However, in the mutants, this inactivation was reversed after 1-3 h darkness. Calyculin A prevented this reversal. When magnesium was included in the assay to distinguish between the active and inactive forms of NR, mutants contained 50% more activity than the WT during the night. Conversion of [15N]-nitrate to organic compounds in leaves in the first 6 h of the night was 60% faster in the mutants than in the WT. (iv) Growth of WT plants in enhanced carbon dioxide prevented the decline of NRA during the second part of the photoperiod, and led to reactivation of NR in the dark. (v) Increased stability of NR in the light and reversal of dark-inactivation correlated with decreased levels of glutamine in the leaves. When glutamine was supplied to detached leaves it accelerated the breakdown of NR, and led to inactivation of NR, even in the light. (vi) Diurnal changes were also investigated in roots. In the WT, the amount of nia transcript rose to a maximum after 4 h illumination and then gradually decreased. The amplitude of the changes in transcript amount was smaller in roots than in leaves, and there were no diurnal changes in NRA. In mutants, nia transcript levels were high through the photoperiod and the first part of the night. The NRA was 50% lower during the day but rose during the night to an activity almost as high as in the WT. The rate of [15N]-nitrate assimilation in the roots of the mutants resembled that in the WT during the first 6 h of the night. (vii) Diurnal changes were also compared in Nia30(145) transformants with very low NRA, and in nitrate-deficient WT plants. Both sets of plants had similar low growth rates. Nitrate reductase did not show a diurnal rhythm in leaves or roots of Nia30(145), the leaves contained very low glutamine, and NR did not inactivate in the dark. Nitrate-deficient WT plants were watered each day with 0.2 mM nitrate. After watering, there was a small peak of nia transcript NR protein and NRA and, slightly later, a transient increase of glutamine and other amino acids in the leaves. During the night glutamine was low, and NR did not inactivate. In the roots, there was a very marked increase of nitrate, nia transcript and NRA 2-3 h after the daily watering with 0.2 mM nitrate. (viii) It is concluded that WT plants have excess capacity for nitrate assimilation. They only utilise this potential capacity for a short time each day, and then down-regulate nitrate assimilation in response, depending on the conditions, to accumulation of the products of nitrate assimilation or exhaustion of external nitrate. Genotypes with a lower capacity for nitrate assimilation compensate by increasing expression of NR and weakening the feedback regulation, to allow assimilation to continue for a longer period each day.

Circadian Rhythm↗
PubMed2003

Sequential injection analysis of nitrites and nitrates in human serum using nitrate reductase.

BACKGROUND: Nitrates and nitrites, as products of nitric oxide in human serum, are currently used as markers in many diseases. A sequential injection analysis (SIA) flow system, for their determination in serum, is presented. The exact timing of fluidic manipulations and the small volumes used in the automated SIA systems provide exquisite control of the reaction conditions and an economy in the biological fluid and enzymes used. METHODS: For nitrite determinations, 150 microl of sample and 50 microl of Griess reagent were sequentially aspirated to the system and sent to the detector. Nitrates were determined as nitrites after reduction through 0.09 U of nitrate reductase and 75 microl of NADPH. RESULTS: Nitrates and nitrites were determined at concentrations up to 4 and 3 mg/l, respectively, with relative standard deviation (rsd) <5%. Detection limits of 0.14 and 0.03 mg/l, respectively, were calculated. Statistical evaluation showed good agreement between the results obtained, for 15 deproteinized serum samples, with both the SIA system and the comparison batch procedure. CONCLUSIONS: The automatic developed method seems to be a good alternative for routine implementation since it is four times faster, and it requires one third of sample and one half of nitrate reductase than the comparison batch procedure.

Data Interpretation, Statistical↗
PubMed2004

Tuning a nitrate reductase for function. The first spectropotentiometric characterization of a bacterial assimilatory nitrate reductase reveals novel redox properties.

Bacterial cytoplasmic assimilatory nitrate reductases are the least well characterized of all of the subgroups of nitrate reductases. In the present study the ferredoxin-dependent nitrate reductase NarB of the cyanobacterium Synechococcus sp. PCC 7942 was analyzed by spectropotentiometry and protein film voltammetry. Metal and acid-labile sulfide analysis revealed nearest integer values of 4:4:1 (iron/sulfur/molybdenum)/molecule of NarB. Analysis of dithionite-reduced enzyme by low temperature EPR revealed at 10 K the presence of a signal that is characteristic of a [4Fe-4S](1+) cluster. EPR-monitored potentiometric titration of NarB revealed that this cluster titrated as an n = 1 Nernstian component with a midpoint redox potential (E(m)) of -190 mV. EPR spectra collected at 60 K revealed a Mo(V) signal termed "very high g" with g(av) = 2.0047 in air-oxidized enzyme that accounted for only 10-20% of the total molybdenum. This signal disappeared upon reduction with dithionite, and a new "high g" species (g(av) = 1.9897) was observed. In potentiometric titrations the high g Mo(V) signal developed over the potential range of -100 to -350 mV (E(m) Mo(6+/5+) = -150 mV), and when fully developed, it accounted for 1 mol of Mo(V)/mol of enzyme. Protein film voltammetry of NarB revealed that activity is turned on at potentials below -200 mV, where the cofactors are predominantly [4Fe-4S](1+) and Mo(5+). The data suggests that during the catalytic cycle nitrate will bind to the Mo(5+) state of NarB in which the enzyme is minimally two-electron-reduced. Comparison of the spectral properties of NarB with those of the membrane-bound and periplasmic respiratory nitrate reductases reveals that it is closely related to the periplasmic enzyme, but the potential of the molybdenum center of NarB is tuned to operate at lower potentials, consistent with the coupling of NarB to low potential ferredoxins in the cell cytoplasm.

Amino Acid Sequence↗
PubMed1996

Isolation of periplasmic nitrate reductase genes from Rhodobacter sphaeroides DSM 158: structural and functional differences among prokaryotic nitrate reductases.

The phototrophic bacterium Rhodobacter sphaeroides DSM 158 has a periplasmic nitrate reductase which is induced by nitrate and it is not repressed by ammonium or oxygen. In a Tn5 mutant lacking nitrate reductase activity, transposon insertion is localized in a 1.2 kb EcoRI fragment. A 0.6 kb BamHI-EcoRI segment of this region was used as a probe to isolate, from the wild-type strain, a 6.8 kb PstI fragment carrying the putative genes coding for the periplasmic nitrate reductase. In vivo protein expression and DNA sequence analysis reveal the presence in this region of three genes, napABC, probably organized in an operon. These genes are required for nitrate reduction, as deduced by mutational and complementation studies. The napA gene codes for a protein with a high homology to the periplasmic nitrate reductase from Alcaligenes eutrophus and, to a lesser extent, to other prokaryotic nitrate reductases and molybdenum-containing enzymes. The napB gene product has two haem c-binding sites and shows a high homology with the cytochrome c-type subunit of the periplasmic nitrate reductase from A. eutrophus. NAPA and NAPB proteins appear to be translated with signal peptides of 29 and 24 residues, respectively, indicating that mature proteins are located in the periplasm. The napC gene codes for a 25 kDa protein with a transmembrane sequence of 17 hydrophobic residues. NAPC has four haem c-binding sites and is homologous to the membrane-bound c-type cytochromes encoded by Pseudomonas stutzeri nirT and Escherichia coli torC genes. The phenotypes of defined insertion mutants constructed for each gene also indicate that periplasmic nitrate reductase from R. sphaeroides DSM 158 is a dimeric complex of a 90 kDa catalytic subunit (NAPA) and a 15 kDa cytochrome c (NAPB), which receives electrons from a membrane-anchored tetrahaem protein (NAPC), thus allowing electron flow between membrane and periplasm. This nitrate-reducing system differs from the assimilatory and respiratory bacterial nitrate reductases at the level of cellular localization, regulatory properties, biochemical characteristics and gene organization.

Amino Acid Sequence↗
PubMed1990

Nitrate reductases of Escherichia coli: sequence of the second nitrate reductase and comparison with that encoded by the narGHJI operon.

The structural genes for NRZ, the second nitrate reductase of Escherichia coli, have been sequenced. They are organized in a transcription unit, narZYWV, encoding four subunits, NarZ, NarY, NarW and NarV. The transcription unit is homologous (73% identity) to the narGHJI operon which encodes the genes for NRA, the better characterized nitrate reductase of this organism. The level of homology between the corresponding polypeptides ranges from 69% for the NarW/NarJ pair to 86% for the NarV/NarI pair. The NarZ polypeptide contains the five conserved regions present in all other known molybdoproteins of E. coli and their relative order is the same. The NarY polypeptide, which contains the same four cysteine clusters in the same order as NarH, is probably an electron transfer unit of the complex. Upstream of narZ, an open reading frame, ORFA, is present which could encode a product which has homology (73% identity) with the COOH-terminal end of NarK. The ORFA-narZ intergenic region, however, is about 80 nucleotides long and does not contain the cis-acting elements, NarL and Fnr boxes, nor the terC4 terminator sequence present in the 500 nucleotide narK-narG intergenic region. This might explain why the narZYWV and the narGHJI operons are regulated differently. Our results tend to support the hypothesis that a DNA fragment larger than that encompassing the narGHJI genes has been duplicated.

Amino Acid Sequence↗
PubMed1997

Sequences necessary for nitrate-dependent transcription of Arabidopsis nitrate reductase genes.

Nitrate increases the transcription of the two Arabidopsis thaliana nitrate reductase genes. We demonstrated previously that 238 and 330 bp of the 5' flanking regions, designated as NP1 and NP2, of the two nitrate reductase genes NR1 and NR2, respectively, are sufficient for nitrate-dependent transcription (Y. Lin, C.-F. Hwang, J.B. Brown, C.-L. Cheng [1994] Plant Physiol 106: 477-484). Here we identify the cis-acting elements of NP1 and NP2 that are necessary for nitrate-dependent transcription by linker-scanning (LS) analysis. In transgenic plants one LS mutant of NP1 and two LS mutants of NP2 exhibited significantly lower nitrate-induced reporter gene chloramphenicol acetyltransferase activity. To distinguish which of these three mutants lost nitrate inducibility, competitive reverse-transcriptase polymerase chain reaction was used to measure the chloramphenicol acetyltransferase mRNA levels before and after nitrate induction. The single LS mutant in NP1 lost its response to nitrate, whereas the two LS mutants in NP2 partially lost their response to nitrate. A 12-bp sequence is conserved between the NP1 site and the two NP2 sites. This sequence motif is also conserved in the 5' flanking regions of other nitrate-inducible plant genes. Gel mobility shift experiments indicate that these three regions bind to similar proteins. The binding is constitutive with respect to nitrate treatment and was observed in both nonphotosynthetic suspension cells and green leaves.

Arabidopsis↗
PubMed2003

The Bradyrhizobium japonicum napEDABC genes encoding the periplasmic nitrate reductase are essential for nitrate respiration.

The napEDABC gene cluster that encodes the periplasmic nitrate reductase from Bradyrhizobium japonicum USDA110 has been isolated and characterized. napA encodes the catalytic subunit, and the napB and napC gene products are predicted to be a soluble dihaem c and a membrane-anchored tetrahaem c-type cytochrome, respectively. napE encodes a transmembrane protein of unknown function, and the napD gene product is a soluble protein which is assumed to play a role in the maturation of NapA. Western blots of the periplasmic fraction from wild-type cells grown anaerobically with nitrate revealed the presence of a protein band with a molecular size of about 90 kDa corresponding to NapA. A B. japonicum mutant carrying an insertion in the napA gene was unable to grow under nitrate-respiring conditions, lacked nitrate reductase activity, and did not show the 90 kDa protein band. Complementation of the mutant with a plasmid bearing the napEDABC genes restored both nitrate-dependent anaerobic growth of the cells and nitrate reductase activity. A membrane-bound and a periplasmic c-type cytochrome, with molecular masses of 25 kDa and 15 kDa, respectively, were not detected in the napA mutant strain incubated anaerobically with nitrate, which identifies those proteins as the NapC and the NapB components of the B. japonicum periplasmic nitrate reductase enzyme. These results suggest that the periplasmic nitrate reductase is the enzyme responsible for anaerobic growth of B. japonicum under nitrate-respiring conditions. The promoter region of the napEDABC genes has been characterized by primer extension. A major transcript initiates 66.5 bp downstream of the centre of a putative FNR-like binding site.

Amino Acid Sequence↗
PubMed1988

Distinction of cnxH cofactor gene-specified protomers with monoclonal antibodies to Aspergillus nitrate reductase.

The nitrate reductase (NADPH) (EC 1.6.6.3) from Aspergillus nidulans is influenced directly by mutations in the structural gene (niaD) for the major subunit of the enzyme and indirectly by mutation in any of several molybdenum cofactor loci (cnx). The cnxE-14 and the cnxH-3 mutants have been noted to contain the enzyme in two distinct forms following induction with nitrate. With the cnxH-3 as a prototype cnxH mutant, 10 other cnxH were found to be devoid of the assembled (dimeric) form of the enzyme. Two monoclonal antibodies specific for the native enzyme of the wild type (biA-1) recognized an epitope on the enzyme from the cnxE-14 and cnxH-3 mutants that was common to both and another that was unique to the cnxH gene specified protomer. Another monoclonal antibody recognized an epitope that occurs only in the assembled dimerio form of the enzyme from the wild type or the cnxE-14 mutant. The experiments further substantiate the cnxH phenotype as one involving unassembled protomers of the nitrate reductase in Aspergillus.

Antibodies, Monoclonal↗
PubMed2004

Unusual regulatory nitrate reductase activity in cotyledons of Brassica napus seedlings: enhancement of nitrate reductase activity by ammonium supply.

The effect of supplying either nitrate or ammonium on nitrate reductase activity (NRA) was investigated in Brassica napus seedlings. In roots, nitrate reductase activity (NRA) increased as a function of nitrate content in tissues and decreased when ammonium was the sole nitrogen source. Conversely, in the shoots (comprising the cotyledons and hypocotyl), NRA was shown to be independent of nitrate content. Moreover, when ammonium was supplied as the sole nitrogen source, NRA in the shoots was surprisingly higher than under nitrate supply and increased as a function of the tissue ammonium content. Under 15 mM of exogenous ammonium, the NRA was up to 2.5-fold higher than under nitrate supply after 6 d of culture. The NR mRNA accumulation under ammonium nutrition was 2-fold higher than under nitrate supply. The activation state of NR in shoots was especially high compared with roots: from nearly 80% under nitrate supply it reached 94% under ammonium. This high NR activation state under ammonium supply could be the consequence of the slight acidification observed in the shoot tissue. The effect of ammonium on NRA was only observed in cotyledons and when more than 3 mM ammonium was supplied. No such NRA increase was evident in the roots or in foliar discs. Addition of 1 mM nitrate under ammonium nutrition halved NRA and decreased the ammonium content in shoots. Thus, this unusual NRA was restricted to seedling cotyledons when nitrate was lacking in the nitrogen source.

Brassica napus↗
PubMed1999

The activation state of nitrate reductase is not always correlated with total nitrate reductase activity in leaves

The relation between nitrate reductase (NR; EC 1.6.6.1) activity, activation state and NR protein in leaves of barley (Hordeum vulgare L.) seedlings was investigated. Maximum NR activity (NRA(max)) and NR protein content (Western blotting) were modified by growing plants hydroponically at low (0.3 mM) or high (10 mM) nitrate supply. In addition, plants were kept under short-day (8 h light/16 h dark) or long-day (16 h light/8 h dark) conditions in order to manipulate the concentration of nitrate stored in the leaves during the dark phase, and the concentrations of sugars and amino acids accumulated during the light phase, which are potential signalling compounds. Plants were also grown under phosphate deficiency in order to modify their glucose-6-phosphate content. In high-nitrate/long-day conditions, NRA(max) and NR protein were almost constant during the whole light period. Low-nitrate/long-day plants had only about 30% of the NRA(max) and NR protein of high-nitrate plants. In low-nitrate/long-day plants, NRA(max) and NR protein decreased strongly during the second half of the light phase. The decrease was preceded by a strong decrease in the leaf nitrate content. Short daylength generally led to higher nitrate concentrations in leaves. Under short-day/low-nitrate conditions, NRA(max) was slightly higher than under long-day conditions and remained almost constant during the day. This correlated with maintenance of higher nitrate concentrations during the short light period. The NR activation state in the light was very similar in high-nitrate and low-nitrate plants, but dark inactivation was twice as high in the high-nitrate plants. Thus, the low NRA(max) in low-nitrate/long-day plants was slightly compensated by a higher activation state of NR. Such a partial compensation of a low NR(max) by a higher dark activation state was not observed with phosphate-depleted plants. Total leaf concentrations of sugars, of glutamine and glutamate and of glucose-6-phosphate did not correlate with the NR activation state nor with NRA(max).

Journal Article↗