Nitrate Influx Research Articles

Engineering nitrogen use efficiency with alanine aminotransferase

Nitrogen (N) is the most important factor limiting crop productivity worldwide. The ability of plants to acquire N from applied fertilizers is one of the critical steps limiting the efficient use of nitrogen. To improve N use efficiency, genetically modified plants that overexpress alanine aminotransferase (AlaAT) were engineered by introducing a barley AlaAT cDNA driven by a canola root specific promoter (btg26). Compared with wild-type canola, transgenic plants had increased biomass and seed yield both in the laboratory and field under low N conditions, whereas no differences were observed under high N. The transgenics also had increased nitrate influx. These changes resulted in a 40% decrease in the amount of applied nitrogen fertilizer required under field conditions to achieve yields equivalent to wild-type plants.

Dissection of the AtNRT2.1:AtNRT2.2 Inducible High-Affinity Nitrate Transporter Gene Cluster

Using a new Arabidopsis (Arabidopsis thaliana) mutant (Atnrt2.1-nrt2.2) we confirm that concomitant disruption of NRT2.1 and NRT2.2 reduces inducible high-affinity transport system (IHATS) by up to 80%, whereas the constitutive high-affinity transport system (CHATS) was reduced by 30%. Nitrate influx via the low-affinity transport system (LATS) was unaffected. Shoot-to-root ratios were significantly reduced compared to wild-type plants, the major effect being upon shoot growth. In another mutant uniquely disrupted in NRT2.1 (Atnrt2.1), IHATS was reduced by up to 72%, whereas neither the CHATS nor the LATS fluxes were significantly reduced. Disruption of NRT2.1 in Atnrt2.1 caused a consistent and significant reduction of shoot-to-root ratios. IHATS influx and shoot-to-root ratios were restored to wild-type values when Atnrt2.1-nrt2.2 was transformed with a NRT2.1 cDNA isolated from Arabidopsis. Disruption of NRT2.2 in Atnrt2.2 reduced IHATS by 19% and this reduction was statistically significant only at 6 h after resupply of nitrate to nitrogen-deprived plants. Atnrt2.2 showed no significant reduction of CHATS, LATS, or shoot-to-root ratios. These results define NRT2.1 as the major contributor to IHATS. Nevertheless, when maintained on agar containing 0.25 mm KNO(3) as the sole nitrogen source, Atnrt2.1-nrt2.2 consistently exhibited greater stress and growth reduction than Atnrt2.1. Evidence from real-time PCR revealed that NRT2.2 transcript abundance was increased almost 3-fold in Atnrt2.1. These findings suggest that NRT2.2 normally makes only a small contribution to IHATS, but when NRT2.1 is lost, this contribution increases, resulting in a partial compensation.

Nitrate influx kinetic parameters of five potato cultivars during vegetative growth

Information on nitrate uptake kinetic parameters, and their variation among cultivars, is lacking for potato. A growth room solution culture experiment with five potato cultivars (Atlantic, Chieftain, Red Pontiac, Russet Norkotah and Shepody) at three sampling dates [10, 17 and 22 days after transplanting (DAT)] and three replicates was conducted to investigate the variation in nitrate uptake kinetic parameters among cultivars and with plant age during vegetative growth. Potato plantlets from tissue culture were grown in a re-circulating hydroponic system. On each sampling date, depletion of nitrate concentration in 3 L of nutrient solution by a single plant was measured over a 360-min period. Nitrate influx parameters were calculated from depletion curves. Root length (RL), average diameter and root/shoot ratio were determined and relative growth rate and plant N demand per unit of RL were calculated. The maximum influx (I max) ranged from 1.30 to 3.65 pmol cm−1 s−1 and was significantly different among cultivars. Russet Norkotah and Atlantic had the highest values of I max at 22 DAT. Values of the Michaelis constant (K m) and minimum concentration (C min) were not significantly different among cultivars except for Red Pontiac which had the highest values of K m and C min. The I max of all cultivars decreased over time. There was a strong negative correlation between I max and RL. The plant N demand per unit of RL explained more than 50% of variation in I max among the cultivars at 17 and 22 DAT. The results suggest that I max in combination with root morphology may be important in controlling differences in N uptake efficiency among potato cultivars.

A Model for the Circadian Oscillations in Expression and Activity of Nitrate Reductase in Higher Plants

Nitrate is the major nitrogen source for many plants. The first step of the nitrate assimilation pathway is the reduction of nitrate to nitrite, catalysed by nitrate reductase (NR). Circadian oscillations in expression and activity of NR have been demonstrated in many plant species. The pathway by which this circadian behaviour is regulated remains to be elucidated. In this study, based on recent experimental observations, a mathematical model is proposed to explain the origin of diurnal and circadian oscillations in NR gene expression and enzyme activity. The dynamic model is based on the feedback interconnections between NR and its substrate, nitrate. In the model, NR activity is regulated at the transcriptional level, in response to the balance between nitrate influx and reduction, and at the post-translational levels in response to signals from carbon assimilation. Conditions for the model system to generate self-sustained circadian oscillations are investigated by numerical simulations. Under light/dark cycles, the simulation results are in agreement with the observed diurnal pattern of changes in leaf nitrate concentration, NR transcript level and NR activity. Within a range of kinetic parameter values, circadian oscillation behaviour persists even under constant light, with periods of approx. 24 h. These simulation results suggest that sustained circadian oscillations can originate from the feedback interactions between NR and its substrate, nitrate, without the need to postulate the existence of an endogenous 'circadian clock'.

Modified Isotope Pairing Technique to Study N Transformations in Polluted Aquatic Systems: Theory

Denitrification of nitrate in sediments of polluted aquatic systems has the potential to release considerable nitrogen amounts into the atmosphere in the form of dinitrogen (N2) and/or nitrous oxide (N2O). Nitrate may diffuse into the anoxic sediment layer either from the oxic sediment layer (after being produced there by nitrifiers) or directly from the overlying water. Currently, the most common technique that explicitly distinguishes between the two main nitrate sources is the Isotope Pairing Technique (IPT). 15N-labeled nitrate is added to the water column, and formation of 15N labeled N2 molecules is subsequently monitored. The main shortcoming of the IPT is that the formation of N20 is ignored, thus resulting in an underestimation of sediment denitrification. Another limitation is the inability to account for a possible influx of nitrate through the hyporheic zone (e.g., nitrate-polluted groundwater) into the anoxic sediment layer. We have further developed and analyzed the theoretical basis of the original IPT. The two important factors that we took into account are the isotopic composition of N20 and the input of an external source of nitrate. We also examined the option of adding 15N-labeled ammonium to the water column to specifically adjustthe technique to field studies. The presented modified technique allows us to (i) improve the estimation of sediment denitrification capacity, (ii) gain an insight into the N20 formation mechanism(s) and fluxes, and (iii) assess inputs of nitrate-polluted water through the hyporheic zone.

Control of the diffusive boundary layer on benthic fluxes: a model study

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 292:61-74 (2005) - doi:10.3354/meps292061 Control of the diffusive boundary layer on benthic fluxes: a model study B. A. Kelly-Gerreyn*, D. J. Hydes, J. J. Waniek National Oceanography Centre, Southampton, University of Southampton, Waterfront Campus, European Way,Southampton SO14 3ZH, UK *Email: bag@noc.soton.ac.uk ABSTRACT : A simple, steady state, reaction-diffusion diagenesis model is used to quantify the possible error associated with benthic flux measurements which neglect the presence of the diffusive boundary layer (DBL). Model application is restricted to non-bioturbated, fine-grained sediments in which oxygen consumption is dominated (~65% of the consumption budget) by organic carbon degradation, oxygen penetration depths are low (<0.5 cm) and solute exchange across the sediment-water interface (SWI) is diffusive. The effect of different thicknesses of the DBL is tested on sediments with different organic carbon reactivities (k = 1, 5, 10, 20, 40 yr–1). When imposing the range of DBL thicknesses observed in nature (0.01 to 0.1 cm) on the model, the model simulates lower oxygen fluxes (by up to 22% for k = 40 yr–1) across the SWI compared to fluxes simulated in the absence of a DBL. Greater reactivity increases the impact of the DBL by lowering the oxygen penetration depth. Changes in the DBL directly influence oxygen fluxes and aerobic mineralisation by changing the diffusion path length to a relatively thin oxic sediment layer. The changes in anaerobic processes are small (<8% for denitrification and <3% for sulphate reduction) and, together with the associated solute fluxes (nitrate, sulphate, ammonium) across the SWI, occur in response to changes in porewater oxygen concentrations induced by the DBL, rather than by direct interactions with the DBL. Changes included a 380% increase in nitrate influxes and a 90% reduction in nitrate effluxes. Rates of nitrification decreased by up to 18%. Thicker DBLs also decreased the organic carbon degradation rate by a maximum of 22%, implicating the DBL as a factor in organic carbon preservation for highly reactive sediments. Measurements of near-bed currents in a macro-tidal estuary (Southampton Water, UK) suggest that the observed range in DBL thicknesses can exist for up to 31% of the time (sampling period: 3 to 4 mo). The presence of these DBL thicknesses in such a dynamic environment, makes it is reasonable to assume that the establishment of the DBL may be widespread. Consequently, it is only reasonable to neglect the DBL over sediments in which aerobic mineralisation is dominant, when organic reactivity is low. KEY WORDS: Diffusive boundary layer · Benthic fluxes · Organic carbon reactivity · Earlydiagenesis model Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 292. Online publication date: May 12, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Two perfectly conserved arginine residues are required for substrate binding in a high-affinity nitrate transporter.

This study represents the first attempt to investigate the molecular mechanisms by which nitrate, an anion of significant ecological, agricultural, and medical importance, is transported into cells by high-affinity nitrate transporters. Two charged residues, R87 and R368, located within hydrophobic transmembrane domains 2 and 8, respectively, are conserved in all 52 high-affinity nitrate transporters sequenced thus far. Site-directed replacements of either of R87 or R368 residues by lysine were found to be tolerated, but such residue changes increased the K(m) for nitrate influx from micromolar to millimolar values. Seven other amino acid substitutions of R87 or R368 all led to loss of function and lack of growth on nitrate. No evidence was obtained of R87 or R368 forming a salt-bridge with conserved acidic residues. Remarkably, the phenotype of loss-of-function mutant R87T was found to be alleviated by an alteration to lysine of N459, present in the second copy of the nitrate signature (transmembrane domain 11), suggesting a structural or functional interplay between residues R87 and N459 in the three-dimensional NrtA protein structure. Failure of the potential reciprocal second site suppressor N168K (in the first nitrate signature copy of transmembrane domain 5) to revert R368T was observed. Taken with recent structural studies of other major facilitator superfamily proteins, the results suggest that R87 and R368 are involved in substrate binding and probably located in a region of the protein close to N459.

Effect on marine sediment nitrogen fluxes caused by organic matter enrichment with varying organic carbon structure and nitrogen content

Enrichments with five types of organic carbon, differing in nitrogen content and type of organic carbon, were made to a marine sediment in order to study effects on nitrogen fluxes. The enrichments used were Ulva lactuca, Ascophyllum nodosum, Zostera marina, Ceratium spp., lignin, and mixtures of U. lactuca and A. nodosum. Fluxes of ammonium, nitrate, and phosphate were measured in short-term (48 h) microcosm experiments, using a carbon enrichment loading similar to that at the sampling site. Changes in microbial community growth and structure due to three types enrichments were also detected using molecular methods in a microcosm experiment run for 4 days to allow for detection at DNA-level.Ammonium fluxes changed from an efflux in the control to an influx for all enrichments apart from U. lactuca. The change was significantly related to the added material's C:N ratio when Ceratium spp. was excluded. All enrichments induced an influx of nitrate suggesting the formation of anoxic micro zones, but there was no relationship with C:N ratio. Instead, the magnitude of the nitrate influx is suggested to be related to the structure of the organic carbon, where enrichments containing a large pool of cellulose and lignin (Z. marina and lignin) gave rise to a lower influx, compared to algal material with more easily degraded organic matter like lipids and starch, which induced the highest influx of nitrate (Ceratium spp., U. lactuca, and A. nodosum). The occurrence of an ammonium influx together with increased nitrate influx and a lower efflux of phosphate in the enrichments suggests a growth of heterotrophic bacteria. This was also confirmed using molecular methods (PCR-DGGE) where the relative abundance of bacterial species in the enrichments increased.The sum of ammonium and nitrate fluxes showed that total nitrogen removal was enhanced in all enrichments compared to the nonenriched control sediment, but in different ways. Ceratium spp. induced the highest removal followed by A. nodosum, U. lactuca, Z. marina, and lastly, by lignin. The same pattern was observed also in a second experiment. These results indicate that easily degradable organic carbon, together with a lower C:N ratio of the added material, will remove nitrogen from the water phase, making the recycling of nitrogen to the overlying water mass smaller.

Differences in the influx of glutamine and nitrate into Viscum album from the xylem sap of its hosts

The flux of inorganic and organic nitrogen into the mistletoe Viscum album L. from the xylem sap of a deciduous ( Populus × euamericana) and a coniferous host ( Abies alba Mill.) was analyzed. For this purpose, a perfusion system was developed in which the xylem sap of the host was replaced by an artificial perfusion solution. With this system flux rates into the mistletoe were determined in feeding experiments either with the organic nitrogen source [1,2- 13C 2]glutamine at high and the inorganic nitrogen source 15NO 3 – at low concentration or vice versa. Glutamine influx was already saturated at the low concentration in the xylem sap and was—different from nitrate—not enhanced, when a 250-fold higher concentration was applied. Nitrate influx matched glutamine influx only at high inorganic/organic nitrogen ratios in the perfusion solution. This result indicates a preferential influx of glutamine over nitrate from the host xylem into the mistletoe at the concentrations found in the xylem sap of trees. Surprisingly, a high percentage of both N sources were accumulated in the mistletoe stem, indicating excessive N nutrition of the mistletoe leaves. Since 13C isotope signature was significantly reduced in the outflowing perfusion solution, either an upload of organic compounds from the phloem into the xylem, or an efflux of organic compounds from haustorium of mistletoe into the xylem has to be assumed. 15N isotope signatures enriched in the outflowing perfusion solution support the idea of a nitrate uptake system at the host xylem–haustorium interface, which favors the light N isotope of nitrate.

Nitrate Reductase Activity Is Required for Nitrate Uptake into Fungal but Not Plant Cells

The ability to transport net nitrate was conferred upon transformant cells of the non-nitrate-assimilating yeast Pichia pastoris after the introduction of two genes, one encoding nitrate reductase and the other nitrate transport. It was observed that cells of this lower eukaryote transformed with the nitrate transporter gene alone failed to display net nitrate transport despite having the ability to produce the protein. In addition, loss-of-function nitrate reductase mutants isolated from several nitrate-assimilating fungi appeared to be unable to accumulate nitrate. Uptake assays using the tracer (13)NO(3)(-) showed that nitrate influx is negligible in cells of a nitrate reductase null mutant. In parallel studies using a higher eukaryotic plant, Arabidopsis thaliana, loss-of-function nitrate reductase strains homozygous for both NIA1 insertion and NIA2 deletion were found to have no detectable nitrate reductase mRNA or nitrate reductase activity but retained the ability to transport nitrate. The reasons for these fundamental differences in nitrate transport into the cells of representative members of these two eukaryotic kingdoms are discussed.

Putative role of γ ‐aminobutyric acid (GABA) as a long‐distance signal in up‐regulation of nitrate uptake in Brassica napus L.

ABSTRACTThe relationship between nitrate influx, BnNrt2 nitrate transporter gene expression and amino acid composition of phloem exudate was investigated during N‐deprivation (short‐term experiment) and over a growth cycle (long‐term experiment) in Brassica napus L. The data showed a positive correlation between γ‐aminobutyric acid (GABA) in phloem exudate and nitrate uptake in the short‐ and the long‐term experiments. The hypothesis that this non‐protein amino acid could up‐regulate nitrate uptake via a long‐distance signalling pathway was tested by providing an exogenous GABA supply to the roots. The effect of GABA was compared with the effects of Gln, Glu and Asn, each known to be inhibitors of nitrate uptake. The results showed that GABA treatment induced a significant increase of BnNrt2 mRNA expression, but had less effect on nitrate influx. By contrast, Gln, Glu and Asn significantly reduced nitrate influx and BnNrt2 mRNA expression compared with the control plants. This study provides the first evidence that GABA may act as a putative long‐distance inter‐organ signal molecule in plants in conjunction with negative control exerted by Gln. The up‐regulation effect of GABA on nitrate uptake is discussed in the context of its role in N metabolism, nutritional stress and the recent discovery of a putative role of GABA as a signal molecule in plant development.

Disruption of the nitrate transporter genes AtNRT2.1 and AtNRT2.2 restricts growth at low external nitrate concentration.

The high-affinity transport systems in Arabidopsis thaliana (L.) Heynh. involve potentially seven genes. Among these, the AtNRT2.1 and/or AtNRT2.2 genes have been shown to play a major role in the inducible component of this transport system. The physiological impact of a disruption of AtNRT2.1 and AtNRT2.2 on plant growth and N-metabolism was investigated. The reduced nitrate uptake in the mutant under a limiting N-regime was found to correlate with a significant difference in shoot/root ratio between wild type and mutant and a drastically reduced nitrate level in the shoot of the mutant. Carbohydrate analyses of plants under a low nitrate supply revealed a slight increase in glucose and fructose in the mutant shoots as well as an increase in sucrose and starch contents in mutant shoots. Interestingly, the AtNRT2.4 and AtNRT2.5 genes were over-expressed in the mutant growing in reduced N-conditions, without any compensation by root nitrate influx. These results are discussed in the context of the putative role of the different NRT2 genes.

Effect of low molecular size humic substances on nitrate uptake and expression of genes involved in nitrate transport in maize (Zea mays L.).

In this study, a detailed characterization of earthworm low molecular size humic substances (LMS) was performed and these substances were used to study their effect on the nitrate influx in roots, tissue nitrate content, and expression of maize genes putatively involved in nitrate uptake in maize (Zea mays L.). The results show that the humic fraction with low molecular size used in this study is endowed with the characteristic structural network described for most humic substances so far isolated and confirm the presence of IAA in this fraction. The results also show that the LMS fraction of humic substances stimulates the uptake of nitrate by roots and the accumulation of the anion at the leaf level. Moreover, the analysis of the expression of genes encoding two putative maize nitrate transporters (ZmNrt2.1 and ZmNrt1.1) and of two maize H(+)-ATPase isoforms (Mha1 and Mha2) show that these substances may exert direct effects on gene transcription in roots, as shown for the Mha2 gene, and long-distance effects in shoots, as observed for the ZmNrt2.1 gene.

Inhibition of nitrate influx by glutamine in Lolium perenne depends upon the contribution of the HATS to the total influx.

Plants of Lolium perenne L. were grown in sterile solution culture supplied with 2 mol m(-3) nitrogen as either nitrate or ammonium. Glutamine at 5 mol m(-3) was added to the nutrient solution of half the plants for 24 h. Root nitrate influx (at external nitrate concentrations 0-2000 mmol m(-3)) and amino acid concentrations were determined. In a second experiment the concentration of the added glutamine was varied from 0-5 mol m(-3) and nitrate influx determined at 250 and 2000 mmol m(-3). The maximum rate of influx attributed to the high affinity transport system (HATS) was reduced by 66% by the presence of glutamine achieved through an 84% reduction in its constitutive component and a 59% reduction in its inducible component. Influx attributed to LATS was unaffected by the addition of glutamine. The inhibition of total nitrate influx by glutamine was positively related to the contribution of HATS to the total influx. In both nitrate- and ammonium-grown plants, the concentration of glutamine required to inhibit nitrate influx significantly was lower when influx was determined at 250 mmol m(-3) compared with 2000 mmol m(-3) nitrate. The addition of glutamine increased its concentrations in root tissue. However, the results cannot be attributed to changes in glutamine alone as its addition also resulted in increased concentrations of other amino acids. Implications for plants growing under field conditions are discussed.

Effects of NaCl salinity on N-15-nitrate fluxes and specific root length in the halophyte Plantago maritima L.

The effect of salinity on nitrate influx, efflux, nitrate net uptake rate and net nitrogen translocation to the shoot was assessed in a 15N steady state labelling experiment in the halophyte Plantago maritima L. raised for 14 days on solution supplied with 50, 100 and 200 mol m−3 sodium chloride or without sodium chloride. Additionally, salinity induced changes in root morphology were determined. Specific root length increased upon exposure to elevated sodium chloride concentrations due to variations in biomass allocation and length growth of the tap root. Changes in root morphology, however, had a minor effect on nitrate fluxes when expressed on a root fresh weight basis. The decreased rate of nitrate net uptake in plants grown on elevated levels of sodium chloride was almost entirely due to a decrease in nitrate influx. Expressed as a proportion of influx, nitrate efflux remained unchanged and was even lower at the highest salinity level. At all sodium chloride concentrations applied the initial rate of nitrogen net translocation to the shoot decreased relative to the rate of nitrate net uptake. It is concluded that under steady state conditions the negative effect of sodium chloride on the rate of nitrate net uptake at non growth-limiting salinity levels was due to the interaction between sodium chloride and nitrate transporters in the root plasma membrane and/or processes mediating the translocation of nitrogen compounds, possibly nitrate, to the shoot.

Regulation of the nitrate transporter gene AtNRT2.1 in Arabidopsis thaliana: responses to nitrate, amino acids and developmental stage.

The NR72.1 gene codes for a high-affinity nitrate transporter in Arabidopsis thaliana. To examine the regulation of NRT2.1 gene expression, we used a promoter-beta-glucuronidase (GUS) fusion and found that the NRT2.1 promoter directs expression to the epidermal, cortical and endodermal cell layers of mature root parts. The gene appeared to be expressed essentially in roots, but was also present in the leaf hydathodes. Investigation of NRT2.1 expression pattern during the plant developmental cycle showed that it increased rapidly during early vegetative growth, peaked prior to floral stem emergence, and decreased to very low levels in flowering and silique-bearing plants. Experiments with various nitrogen supply regimes demonstrated the induction of NRT2.1 expression by nitrate and repression by amino acids. Amino acid analysis showed that this repression was specifically related to increased internal glutamine, suggesting a role for this particular amino acid in nitrogen signalling responsible for nitrate uptake regulation. Taken together, our results support the hypothesis that the NRT2.1 gene codes for a major component of the inducible high-affinity transport system for nitrate, which is spatially and developmentally controlled at the transcriptional level. Surprisingly, NRT2.1 was not expressed in younger root parts, although a similar rate of nitrate influx was observed in both young and old root samples. This lack of correlation between nitrate influx and NRT2.1 expression suggests that another high-affinity nitrate transporter operates in root tips.

Prospects for improving nitrogen use efficiency: Insights given by 15N-labelling experiments

In higher plants, recent advances in plant molecular genetics, combined with modern physiological and biochemical studies, have expanded our understanding of the regulatory mechanisms controlling the primary steps of inorganic nitrogen assimilation and the subsequent biochemical pathways involved in nitrogen supply and recycling for higher plant metabolism, growth and development. In this presentation, we describe improvements in our understanding of the molecular controls of nitrogen assimilation through the use of transgenic plants and the study of genetic variability in model and crop species. To illustrate this research programme, the physiological impact of modified gene expression, using either transgenic plants or different genotypes, was studied using 15N-labelling experiments in order to monitor the influx of nitrate or ammonia and its subsequent incorporation into amino acids.

Impact of the Spring 2000 phytoplankton bloom in Chesapeake Bay on optical properties and light penetration in the Rhode River, Maryland

Accelerating eutrophication manifest as increasing frequency and magnitude of phytoplankton blooms threatens living resources in many estuaries. Effects of large blooms can be difficult to document because blooms are often unexpected and do not always coincide with scheduled sampling programs. Here we use continuously monitored salinity distributions and optical properties to study the spring bloom of the red tide dinoflagellate,Prorocentrumminimum, in the Rhode River, Maryland, a tributary embayment of upper Chesapeake Bay. Salinity distributions, together with weekly cruise measurements of nutrient concentrations, indicate that the bloom commenced with an influx of nitrate at the mouth due to the arrival of a freshet from the Susquehanna River. Arrival of this freshet at the mouth set up an unstable, inverse salinity gradient within the Rhode River. Continuously monitored absorption and scattering spectra indicated that increases in chlorophyll within the Rhode River initially were due to the influx of chlorophyll that had developed in the main stem of the bay. After the influx, much higher concentrations and steep spatial gradients developed within the Rhode River, subsequent to reduced mixing that accompanied re-establishment of a normal estuarine salinity gradient. We used the monitored absorption and scattering coefficients to determine the effect of the bloom on light attenuation coefficients in the Rhode River. The bloom resulted in a nearly three-fold increase in attenuation coefficient. Attenuation was dominated by chlorophyll in the early stages of the bloom and by detritus after the termination of the bloom. Although the bloom lasted only 20 d, the elevated attenuation coefficients due to the bloom exceeded values that would permit growth of submersed vascular plants for a period of about 45 d.

Effects of nitrate pulses on BnNRT1 and BnNRT2 genes: mRNA levels and nitrate influx rates in relation to the duration of N deprivation in Brassica napus L.

A de-repression mechanism based on the disappearance of 'signals' down-regulating N transporter activity has been proposed in the literature to explain the transient increase of NO(3)(-) uptake by the roots following N deprivation in higher plants. This hypothesis was investigated at the physiological and molecular levels by measuring NO(3)(-) influx into roots of Brassica napus L. grown under low or high external concentrations of KNO(3) following N deprivation. Parallel measurements were made of endogenous NO(3)(-), amino acid concentrations and abundance of mRNA for BnNRT1 and BnNRT2, genes encoding nitrate-inducible transport proteins. The effect of NO(3)(-) pulsing on NO(3)(-) transport components in N-deprived plants was also investigated by measuring influx of high- and low-affinity transport system (HATS and LATS) and assaying mRNA levels. Influx of NO(3)(-) via HATS and LATS, and transcript levels of BnNRT2 and BnNRT1 decreased with the duration of N deprivation. The results suggested that the absence of de-repression of NO(3)(-) influx and BnNRT2 gene expression following N starvation was related to a high amino acid status. Pulsing with NO(3)(-) induced a large increase in BnNRT2 mRNA level, but a comparatively small increase in NO(3)(-) influx via HATS. The level of BnNRT1 mRNA also increased, but there was no effect on LATS uptake activity. The absence of a strict correlation between the NO(3)(-) transport activity and the mRNA BnNRT1 and BnNRT2 levels is discussed in terms of possible post-transcriptional regulation by the amino acids.

Steps towards an integrated view of nitrogen metabolism.

This article discusses how nitrate assimilation is integrated with nitrate uptake, with ammonium assimilation and amino acid synthesis, with pH regulation, and with the sugar supply in tobacco leaves. During the first part of the light period, nitrate assimilation exceeds nitrate uptake by 2-fold and ammonium assimilation by 50%, leading to rapid depletion of nitrate and accumulation of ammonium, glutamine, glycine and serine. NIA, NII and PPC expression show a shared maximum early in the diurnal cycle to direct carbon towards malate synthesis for pH regulation. Later in the diurnal cycle an orchestrated increase of GLN2, PKc, CS, and ICDH-1 expression re-establishes a balance between nitrate assimilation and ammonium metabolism. Nitrate uptake continues throughout the night, replenishing the leaf nitrate pool. These diurnal changes are attenuated or abolished in mutants with low NIA activity, and modified in wild-type plants growing on different nitrogen sources or elevated [CO(2)]. Comparison across genotypes and conditions reveals that NIA transcript levels are always closely related to the balance between nitrate influx and assimilation, but are unrelated to changes of glutamine or 2-oxoglutarate. In a systematic search for other downstream regulators, a wide range of downstream metabolites was fed to detached leaves and glutamate, cysteine, asparagine, and malate identified as candidates. Low sugars totally inhibit nitrate assimilation, overriding signals from nitrogen metabolism. Moderate changes act post-transcriptionally, and larger changes lead to a collapse of the NIA transcript. Low sugars also lead to a collapse of minor amino acids and a dramatic decrease of phenylpropanoids and nicotine. Consequently, wild-type plants growing in unfavourable light regimes and antisense RBCS transformants are simultaneously carbon- and nitrogen-limited.