Regulation of nitrate assimilation in the cyanobacterium Phormidium laminosum

The intracellular ratio of 2-oxoglutarate to glutamine has been analyzed under nutritional conditions leading to different activity levels of nitrate-assimilating enzymes in Phormidium laminosum (Agardh) Gom. This non-N₂-fixing cyanobacterium adapted to the available nitrogen source by modifying its nitrate reductase (NR; EC 1.7.7.2), nitrite reductase (NiR; EC 1.7.7.1) and glutamine synthetase (GS; EC 6.3.1.2) activities. The 2-oxoglutarate/glutamine ratio was similar in cells adapted to grow with nitrate or ammonium. However, metabolic conditions that increased this ratio [i.e., nitrogen starvation or l-methionine-d,l-sulfoximine (MSX) treatment] corresponded to high activity levels of NR, NiR, GS (except in MSX-treated cells) and glutamate synthase (GOGAT; EC 1.4.7.1). By contrast, metabolic conditions that diminished this ratio (i.e., addition of ammonium to nitrate-growing cells or addition of nitrate or ammonium to nitrogen-starved cells) resulted in low activity levels. The variation in the 2-oxoglutarate/glutamine ratio preceded the changes in enzyme activities. These results suggest that changes in the 2-oxoglutarate/glutamine ratio could be the signal that triggers the adaptation of P. laminosum cells to variations in the available nitrogen source, as occurs in enterobacteria.Abbreviations Chl chlorophyll - GOGAT ferredoxin-dependent glutamate synthase (EC 1.4.7.1) - GS glutamine synthetase (EC 6.3.1.2) - MSX l-methionine-d,l-sulfoximine - NiR nitrite reductase (EC 1.7.7.1) - NR nitrate reductase (EC 1.7.7.2) - TP total protein This work has been partially supported by grants from the Spanish Ministry of Education and Science (DGICYT PB88-0300 and PB92-0464) and the University of the Basque Country (042.310-EC203/94). M.I.T. was the recipient of a fellowship from the Basque Government.     

Induction of nitrate reductase, nitrite reductase, and glutamine synthetase isoforms in sunflower cotyledons as affected by nitrate, light, and plastid integrity

Summary We investigated the inducibility of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), and glutamine synthetase (GS; EC 6.3.1.2) isoforms in cotyledons of 7-day-old seedlings of sunflower (Helianthus annuus L.) in relation to light, nitrogen source (NO ₃ ^– , NO ₂ ^– or NH ₄ ⁺ ), and the involvement of plastids. Nitrate was absolutely (and specifically) required for NR induction, and stimulated more effectively than NO ₂ ^– or NH ₄ ⁺ the synthesis of NiR and chloroplastic GS (GS₂) over the constitutive levels present in N-free-grown seedlings. In vivo inhibition of NR activity by tungsten application to seedlings and measurements of tissue NO ₃ ^– concentration indicate that NO ₃ ^– -dependent enzyme induction is elicited by NO ₃ ^– per se and not by a product of its assimilatory reduction, e.g., NO ₂ ^– or NH ₄ ⁺ . In the presence of NO ₃ ^– , light remarkably enhanced the appearance of NR, NiR, and GS₂, while the activity of the cytosolic GS isoform (GS₁) was adversely affected. Cycloheximide suppressed much more efficiently than chloramphenicol the light- and NO ₃ ^– -dependent increase of GS₂ activity, indicating that sunflower chloroplastic GS is synthesized on cytoplasmic 80S ribosomes. When the plastids were damaged by photooxidation in cotyledons made carotenoid-free by application of norflurazon, the positive action of light and NO ₃ ^– on the appearance of NR, NiR, and GS₂ isoform was greatly abolished. Therefore, it is suggested that intact chloroplasts are required for the inductive effect of light and NO ₃ ^– and/or for the accumulation of newly formed enzymes in the organelle.Abbreviations CAP chloramphenicol - CHX cycloheximide - GS glutamine synthetase - GS₁ cytosolic GS - GS₂ plastidic (chloroplastic) GS - NF norflurazon - NiR nitrite reductase - NR nitrate reductase     

Factors involved in the coordinate appearance of nitrite reductase and glutamine synthetase in the mustard (Sinapis alba L.) seedling

The extent to which the appearances of nitrite reductase (NIR; EC 1.7.7.1) and glutamine synthetase (GS; EC 6.3.1.2) are coordinated was studied in mustard (Sinapis alba L.) seedlings. It was established by immunotitration that the increased activities of NIR and GS in the presence of light and nitrate can be attributed to the de-novo synthesis of enzyme protein. The bulk of the NIR and GS was found in the developing cotyledons. In the absence of nitrate in the growth medium there was no coordinate appearance of NIR and GS. While light strongly stimulated the appearance of GS, the level of NIR was hardly affected and remained low. On the other hand, in the presence of nitrate in the medium the appearances of NIR and GS were strictly coordinated, the GS level being considerably above that of NIR. It is argued that phytochrome-controlled synthesis of GS in the absence of nitrate is part of the mechanism to reassimilate ammonium liberated during proteolysis of storage protein and metabolism of the resulting amino acids, whereas the strictly coordinated synthesis in the presence of light and nitrate indicates the dominance of nitrate assimilation under these circumstances. The fact that the level of GS was always considerably above that of NIR appears to be a safety measure to prevent ammonium accumulation.Abbreviations FR standardized far-red light (3.5 W·m^–2), to drive the high-irradiance reaction of phytochrome - GS glutamine synthetase, EC 6.3.1.2 - NIR nitrite reductase, EC 1.7.7.1 This work was supported by Heidelberger Akademie der Wissenschaften (Forschungsstelle Nitratassimilation).     

Development of nitrogen-assimilating enzymes in sunflower cotyledons during germination as affected by the exogenous nitrogen source

Activities of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.3) were measured in cotyledons of sunflower (Helianthus annuus L. cv Peredovic) seedlings during germination and early growth under various external nitrogen sources. The presence of NO ₃ ^- in the medium promoted a gradual increase in the levels of NR and NiR activities during the first 7 d of germination. Neither NR nor NiR activities were increased in a nitrogen-free medium or in media with either NH ₄ ⁺ or urea as nitrogen sources. Moreover, the presence of NH ₄ ⁺ did not abolish the NO ₃ ^- -dependent appearance of NR and NiR activities. The increase of NR activity was impaired both by cycloheximide and chloramphenicol, which indicates that both cytoplasmic 80S and plastidic 70S ribosomes are involved in the synthesis of the NR molecule. By contrast, the appearance of NiR activity was only inhibited by cycloheximide, indicating that NiR seems to be exclusively synthesized on the cytoplasmic 80S ribosomes. Glutamine-synthetase activity was also strongly increased by external NO ₃ ^- but not by NH ₄ ⁺ or urea. The appearance of GS activity was more efficiently suppressed by cycloheximide than chloramphenicol. This indicates that GS is mostly synthesized in the cytoplasm. The cotyledons of the dry seed contain high levels of GDH activity which decline during germination independently of the presence or absence of a nitrogen source. Cycloheximide, but not chloramphenicol, greatly prevented the decrease of GDH activity.Abbreviations GDH glutamate dehydrogenase - GS glutamine synthetase - NiR nitrite reductase - NR nitrate reductase     

Glutamine synthetase activity,ammonia assimilation and control of nitrate reduction in the unicellular red algaCyanidium caldarium

Addition ofl-methionine-dl-sulphoximine to cells ofCyanidium caldarium brings about a loss of glutamine synthetase activity. Concomitantly ammonia assimilation is prevented.Under physiological conditions nitrate reductase [NAD(P)H: nitrate oxidoreductase EC 1.6.6.2] is reversibly converted into an inactive enzyme upon addition of ammonia. In the presence of methionine sulphoximine, when glutamine synthetase activity is lost, nitrate reductase is no longer inactivated by ammonia. It is suggested that ammonia itself is not the actual effector of nitrate reductase inactivation.Concomitantly with the failure of nitrate reductase to undergo ammonia-inactivation, in the presence of methionine sulphoximine nitrate reduction is an uncontrolled process, thus, in media with nitrate ammonia continues to be produced and excreted into the external medium at a constant rate.Abbreviations NR Nitrate reductase - GS Glutamine synthetase - GOGAT Glutamate syntase - MSX l-methionine-dl-sulphoximine     

Bottle gourd rootstock-grafting affects nitrogen metabolism in NaCl-stressed watermelon leaves and enhances short-term salt tolerance

The plant growth, nitrogen absorption, and assimilation in watermelon (Citrullus lanatus [Thunb.] Mansf.) were investigated in self-grafted and grafted seedlings using the salt-tolerant bottle gourd rootstock Chaofeng Kangshengwang (Lagenaria siceraria Standl.) exposed to 100 mM NaCl for 3 d. The biomass and NO₃⁻ uptake rate were significantly increased by rootstock while these values were remarkably decreased by salt stress. However, compared with self-grafted plants, rootstock-grafted plants showed higher salt tolerance with higher biomass and NO₃⁻ uptake rate under salt stress. Salinity induced strong accumulation of nitrate, ammonium and protein contents and a significant decrease of nitrogen content and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT) in leaves of self-grafted seedlings. In contrast, salt stress caused a remarkable decrease in nitrate content and the activities of GS and GOGAT, and a significant increase of ammonium, protein, and nitrogen contents and NR activity, in leaves of rootstock-grafted seedlings. Compared with that of self-grafted seedlings, the ammonium content in leaves of rootstock-grafted seedlings was much lower under salt stress. Glutamate dehydrogenase (GDH) activity was notably enhanced in leaves of rootstock-grafted seedlings, whereas it was significantly inhibited in leaves of self-grafted seedlings, under salinity stress. Three GDH isozymes were isolated by native gel electrophoresis and their expressions were greatly enhanced in leaves of rootstock-grafted seedlings than those of self-grafted seedlings under both normal and salt-stress conditions. These results indicated that the salt tolerance of rootstock-grafted seedlings might (be enhanced) owing to the higher nitrogen absorption and the higher activities of enzymes for nitrogen assimilation induced by the rootstock. Furthermore, the detoxification of ammonium by GDH when the GS/GOGAT pathway was inhibited under salt stress might play an important role in the release of salt stress in rootstock-grafted seedlings.     

Effect of applied benzyladenine on endogenous cytokinin content during the early stages of bud development of kiwifruit

The filamentous non-N₂-fixing cyanobacterium Phormidium laminosum (strain OH-1-p.Cl₁) was able to utilize glutamine as the sole nitrogen source. The addition to ammonium-grown cultures of the irreversible inhibitor of glutamine synthetase activity L-methionine-D, L-sulfoximine (MSX) inhibited cell growth. Supplying glutamine to the culture restored cell growth. This re-established growth was not due to interference by glutamine of MSX uptake by the cells, since glutamine synthetase (GS, EC 6.3.1.2) activity remained completely inhibited by MSX even when glutamine was simultaneously present. Both glutamine and ammonium exerted a negative effect on nitrate reductase (NR. EC 1.7.7.2) and nitrite reductase (NiR, EC 1.7.7.1) in vivo. This negative effect was reversed by MSX. When glutamine was added to MSX-treated cells, intracellular glutamine level was high, but the activity of both reductases remained at a high level. These results suggest that the presence of the active form of glutamine synthetase is required for the in vivo prevention of nitrate assimilation caused by ammonium and glutamine.     

Inheritance of nitrite reductase and regulation of nitrate reductase, nitrite reductase, and glutamine synthetase isozymes

Banding patterns of nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) from leaves of diploid barley (Hordeum vulgare), tetraploid wheat (Triticum durum), hexaploid wheat (Triticum aestivum), and tetraploid wild oats (Avena barbata) were compared following starch gel electrophoresis. Two NR isozymes, which appeared to be under different regulatory control, were observed in each of the three species. The activity of the more slowly migrating nitrate reductase isozyme (NR1) was induced by NO3- in green seedlings and cycloheximide inhibited induction. However, the activity of the faster NR isozyme (NR2) was unaffected by addition of KNO3, and it was not affected by treatments of cycloheximide or chloramphenicol. Only a single isozyme of nitrite reductase was detected in surveys of three tetraploid and 18 hexaploid wheat, and 48 barley accessions; however, three isozymes associated with different ecotypes were detected in the wild oats. Inheritance patterns showed that two of the wild oat isozymes were governed by a single Mendelian locus with two codominant alleles; however, no variation was detected for the third isozyme. Treatment of excised barely and wild oat seedlings with cycloheximide and chloramphenicol showed that induction of NiR activity was greatly inhibited by cycloheximide, but only slightly by chloramphenicol. Only a single GS isozyme was detected in extracts of green leaves of wheat, barley, and wild oat seedlings. No electrophoretic variation was observed within or among any of these three species. Thus, this enzyme appears to be the most structurally conserved of the three enzymes.     

Effect of glutamine on the induction of nitrate reductase

Nitrate reductase (NR. EC 1.6.6.1/2) is a substrate inducible enzyme that could be repressed by its end product glutamine or amino acids. To test this hypothesis, 6-day-old maize seedlings ( Zea mays cv. W64A × W182E) were grown hydroponically in a 1/10 strength Hougland's salt solution modified to contain no nitrogen. Previous experiments had established that after a 24-h induction with NO₃⁻ (5 mM KNO₃⁻) the level of NR activity and protein had reached a constant level. In the present experiments when glutamine (5 mM) was included together with NO₃⁻, there was a significant reduction in NR activity (34% of the control values). NR protein and NR mRNA accumulation in the root. In the shoot, on the other hand, glutamine additions had little or no effect on the levels of either NR activity (81% of control) or NR protein. Inhibition of glutamine synthetase by methionine sulfoximine (MSX) resulted in reduced levels of glutamine in both root and shoot tissues. Contrary in our prediction, however, it had no effect on NR activity and mRNA content in roots. In the shoot, on the other hand, there was a marked reduction of NR activity (34% of the control value) and NR protein, but no apparent effect on NR mRNA. When detached shoots were treated with MSX and other inhibitors of glutamine synthetase (tabtoxinine-β-lactam or phosphinothricin) the induction of NR activity by NO₃⁻ was also inhibited. Glutamine additions 15 or 50 mM to detached shoots had essentially no effect on the induction of NR activity (90% of control). These results demonstrate that the influence of glutamine and MSX on the induction of NR in maize root and shoot tissues, respectively, is very different.     

Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 m m and 2.0 m m B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR_L), concentration of B, nitrate (NO₃⁻), ammonium (NH₄⁺), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR_L, organic N, soluble proteins, and NR and NiR activities. The lowest NO₃⁻ and NH₄⁺ concentration in leaves was recorded when plants were supplied with 2.0 m m B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO₃⁻ reduction and increases NH₄⁺ assimilation in tomato plants.     

Studies on ammonium-assimilating enzymes of Platymonas striata Butcher (Prasinophyceae)

Platymonas striata Butcher displays significant levels of glutamate synthase (GS) (EC 2.6.1.53) and glutamine synthetase (GOGAT) (EC 6.3.1.2.), but very low levels of glutamate dehydrogenase (GDH) (EC 1.4.1.4). This suggests that the GS/GOGAT pathway is important for nitrogen assimilation. The in vitro rates of enzyme activity can however only account for about 10% of the in vivo rates of nitrogen assimilation. Nitrogen-starvation reduced GS activity to undetectable levels. On nitrate or ammonium ion refeeding the cellular GS activity was rapidly restored, and reached levels of 56% and 91% greater than the unstarved values 24h after refeeding nitrate or ammonium respectively.Abbreviations NAR nitrate reductase - NIR nitrate reductase     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.     

Glutamine synthetase of Chlamydomonas: its role in the control of nitrate assimilation

Work is described which suggests that glutamine synthetase (GS) could play an important and direct regulatory role in the control of NO₃ assimilation by the alga. In both steady-state cells and ones disturbed physiologically by changes in light or nitrogen supply the assimilation of NO₃ appears to be limited by the activity of GS. Moreover although in normal cells NH₃ can completely inhibit NO₃ uptake, promote the deactivation of nitrate reductase (NR) and repress the synthesis of NR and nitrite reductase (NIR), these controls are relaxed in cells in which GS is deactivated by treatment with L-methionine-DL-sulfoximine (MSO). It is proposed that the reversible deactivation of GS may play an important part in the regulation of NO₃ assimilation although it is still not clear whether the enzyme itself or products of its metabolism are responsible.Abbreviations GS glutamine synthetase - GS_s glutamine synthetase, synthetase activity - GS_t glutamine synthetase, transferase activity - NR nitrate reductase - NIR nitrite reductase - GDH glutamate dehydrogenase - CHX cycloheximide - MSO L-methionine-DL-sulfoximine - FAD flavine adenine dinucleotide     

小麦开花后不同器官中硝酸还原酶和谷氨酰胺合成酶的活性比较

小麦开花后各器官的硝酸还原酶和谷氨酰胺合成酶均具有一定的活性,旗叶中硝酸还原酶和谷氨酰胺合成酶活性最高。开花后旗叶和根系硝酸还原酶和谷氨酰胺合成酶活性逐渐降低,颗壳和籽粒中硝酸还原酶和谷氨酰胺合成酶活性先升高,达到最大值后又降低。     

氮素水平对花生氮素代谢及相关酶活性的影响

 在大田高产条件下研究了氮素水平对花生(Arachis hypogaea)可溶性蛋白质、游离氨基酸含量及氮代谢相关酶活性的影响, 结果表明, 适当提高氮素水平既能增加花生各器官中可溶性蛋白质和游离氨基酸的含量, 又能提高硝酸还原酶、谷氨酰胺合成酶和谷氨酸脱氢酶等氮素同化酶的活性, 使其达到同步增加; 氮素水平过高虽能提高硝酸还原酶和籽仁蛋白质含量, 但谷氨酰胺合成酶(GS)和谷氨酸脱氢酶(GDH)的活性下降; N素施肥水平不改变花生植株各器官中可溶性蛋白质、游离氨基酸含量以及硝酸还原酶(NR)、谷氨酰胺合成酶、谷氨酸脱氢酶活性的变化趋势, 但适量施N (A2和A3处理)使花生各营养器官中GS、GDH活性提高; 氮素水平对花生各叶片和籽仁中GS、GDH活性的高低影响较大, 但对茎和根中GDH活性大小的影响较小。     

Relation of light and nitrogen source to growth, nitrate reductase and glutamine synthetase activity of jack pine seedlings

Two-month-old jack pine ( Pinus banksiana Lamb.) seedlings were placed in a greenhouse where both nitrogen source and light level were varied. After 4 months, whole seedling biomass, leaf biomass and relative growth rate were greatest in seedlings grown with NH⁺₄/NO/NO⁻₃-N and full light (FL) and least in seedlings grown with NO ⁻₃-N and low light (LL). NO ⁻₃-seedlings grown under full light and NH⁺₄/NO⁻₃-seedlings grown under low light were approximately equal. This indicates that the extra carbon costs of assimilating only NO⁻₃-N were similar to the reduction of carbon fixation resulting from a 50% decrease in photon flux density. Percentage and total nitrogen content of needles were greater in seedlings grown under low light independent of nitrogen fertilization. Percentage and total nitrogen content of roots were higher under low light and lower when fertilized with NO⁻₃.
Nitrate reductase (NR) activity was higher in roots than in needles, while glutamine synthetase (GS) activity was higher in needles than in roots. Low light resulted in decreased NR activity (mg N)⁻¹ in needles, but not in roots. However, no nitrate was detected in the needles in any treatment. GS activity, on the other hand, was greater under low light in both needles and roots. GS activity in needles is most likely involved with the reassimilation rather than the initial assimilation of ammonium. Some implications of these shifts in enzymatic activity for ecological phenomena in forests are discussed.     

The influence of cytokinins in nitrate regulation of nitrate reductase activity and expression in barley

The responses of nitrate reductase (NR) activity and levels of NR-mRNA to environmental nitrate and exogenous cytokinins are characterised in roots and shoots of barley ( Hordeum vulgare L., cv. Golf), using a chemostate-like culture system for controlling nitrate nutrition. Experiments were mainly performed with split root cultures where nitrate-N was supplied at a constant relative addition rate of 0.09 day⁻¹, and distributed between the subroots in a ratio of 20%:80%. The subroot NR-mRNA level and NR activity, as well as the endogenous level of zeatin riboside (ZR), increased when the local nitrate supply to one of the subroots was increased 4-fold by reversing the nitrate addition ratio (i.e. from 20%:80% to 80%:20%). Also shoot levels of ZR, NR-mRNA and NR activity increased in response to this treatment, even though the total nitrate supply remained unaltered. External supply of ZR at 0.1 μ M caused an approximately 3-fold increase in root ZR levels within 6 h. which is comparable to the nitrate-induced increase in root ZR. External application of ZR. zeatin. isopentenyl adenine or isopentenyl adenosine at 0.1 μ M caused from insignificant to 25% increases in NR-mRNA and activity in roots and up to 100% stimulation in shoots, whereas adenine or adenosine had no effect. No synergistic effects of perturbed nitrate supply and cytokinin application were detected in either roots or shoots. The translocation of nitrate from the root to the shoot was unaffected by application of ZR or switching the nitrate distribution ratio between subroots. The data give arguments for a physiological role of cytokinins in the response of root and shoot NR to environmental nitrate availability. The nature and limitations of the physiological role of cytokinins are discussed.     

Sulfur starvation and restoration affect nitrate uptake and assimilation in rapeseed

We analyzed the effect of omission of sulfur (S) from the nutrient solution and then restoration of S-source on the uptake and assimilation of nitrate in rapeseed. Incubation in nutrient solution without S for 1–6 days led to decline in uptake of nitrate, activities, and expression levels of nitrate reductase (NR) and glutamine synthetase (GS). The nitrite reductase (NiR) and glutamate synthase (GOGAT) activities were not considerably affected. There was significant enhancement in nitrate content and decline in sulfate content. Evaluation of amino acid profile under S-starvation conditions showed two- to fourfold enhancement in the contents of arginine, asparagine and O-acetyl-l-serine (OAS), whereas the contents of cysteine and methionine were reduced heavily. When the S-starved plants were subjected to restoration of S for 1, 3, 5, and 7 days, activities and expression levels of NR and GS recovered within the fifth and seventh days of restoration, respectively. Exogenous supply of metabolites (arginine, asparagine, cysteine, glutamine, OAS, and methionine) also affected the uptake and assimilation of nitrate, with a maximum for OAS. These results corroborate the tight interconnection of S-nutrition with nitrate assimilation and that OAS plays a major role in this regulation. The study must be helpful in developing a nutrient-management technology for optimization of crop productivity.