External NO3 Research Articles

Disentangling external loadings, hydrodynamics and biogeochemical controls on the fate of nitrate in a coastal embayment

Nitrogen, as an essential nutrient, largely contributes to the coastal eutrophication. However, the accurate depiction and evaluation of how external loadings, hydrodynamics, and biogeochemical reactions mediate the occurrence, transport, and transformation of nitrate (NO3-) within coastal embayment still pose ongoing challenges to date. In this study, we took advantage of dual isotopes of NO3- to track external NO3- loadings, radium and dual isotopes of H2O to characterize the influences of hydrodynamic on NO3- transport, δ18O-NO3- and δ18O-H2O along with microbial analysis to explore major NO3- biogeochemical reactions in Tolo Harbour, Hong Kong. The multiple isotopic evidence showed that NO3- in surface harbour water was predominantly contributed by precipitation in wet season and its impact was strengthened by stratification. In dry season, NO3- in the surface harbour water became largely influenced by benthic input and biogeochemical reactions due to intensified vertical mixing. Based on NO3- mass balance model, biogeochemical reaction, especially nitrification, was found to be the major process to secure the closure of NO3- budget and increase NO3- inventory from wet to dry season. Hydrodynamics redistributed the external NO3- loadings and mediated nitrogen biogeochemical reactions, both of which further synergistically regulated the fate of NO3- in the embayment.

Physio-molecular traits of contrasting bread wheat genotypes associated with 15N influx exhibiting homeolog expression bias in nitrate transporter genes under different external nitrate concentrations.

The high affinity nitrate transport system is a potential target for improving nitrogen use efficiency of bread wheat growing either under optimal or limiting nitrate concentration. Nitrate uptake is one of the most important traits to take into account to improve nitrogen use efficiency in wheat (Triticum aestivum L.). In this study, we aimed to gain an insight into the regulation of NO3- -uptake and translocation systems in two contrasting wheat genotypes [K9107(K9) vs. Choti Lerma (CL)]. Different conditions, such as NO3--uptake rates, soil-types, N-free solid external media, and external NO3- levels at the seedling stage, were considered. We also studied the contribution of homeolog expression of five genes encoding two nitrate transporters in the root tissue, along with their overall transcript expression levels relative to specific external nitrate availability. We observed that K9107 had a higher 15N influx than Choti Lerma under both limiting as well as optimum external N conditions in vermiculite-perlite (i.e., N-free solid) medium, with the improved translocation efficiency in Choti Lerma. However, in different soil types, different levels of 15N-enrichment in both the genotypes were found. Our results also demonstrated that the partitioning of dry matter in root and shoot was different under these growing conditions. Moreover, K9107 showed significantly higher relative expression of TaNRT2.1 at the lowest and TaNPF6.1 and TaNPF6.2 at the highest external nitrate concentrations. We also observed genotype-specific and nitrate starvation-dependent homeolog expression bias in all five nitrate transporter genes. Our data suggest that K9107 had a higher NO3- influx capacity, involving different nitrate transporters, than Choti Lerma at the seedling stage.

MtNPF6.5 mediates chloride uptake and nitrate preference in Medicago roots.

The preference for nitrate over chloride through regulation of transporters is a fundamental feature of plant ion homeostasis. We show that Medicago truncatula MtNPF6.5, an ortholog of Arabidopsis thaliana AtNPF6.3/NRT1.1, can mediate nitrate and chloride uptake in Xenopus oocytes but is chloride selective and that its close homologue, MtNPF6.7, can transport nitrate and chloride but is nitrate selective. The MtNPF6.5 mutant showed greatly reduced chloride content relative to wild type, and MtNPF6.5 expression was repressed by high chloride, indicating a primary role for MtNPF6.5 in root chloride uptake. MtNPF6.5 and MtNPF6.7 were repressed and induced by nitrate, respectively, and these responses required the transcription factor MtNLP1. Moreover, loss of MtNLP1 prevented the rapid switch from chloride to nitrate as the main anion in nitrate-starved plants after nitrate provision, providing insight into the underlying mechanism for nitrate preference. Sequence analysis revealed three sub-types of AtNPF6.3 orthologs based on their predicted substrate-binding residues: A (chloride selective), B (nitrate selective), and C (legume specific). The absence of B-type AtNPF6.3 homologues in early diverged plant lineages suggests that they evolved from a chloride-selective MtNPF6.5-like protein.

Initial short-term nitrate uptake in juvenile, cultivated Saccharina latissima (Phaeophyceae) of variable nutritional state

Abstract To reach the goal of large-scale seaweed cultivation in Norway, new knowledge concerning commercially important species like the kelp Saccharina latissima is essential. This includes fundamental understanding of physiological mechanisms like nutrient uptake kinetics to better understand its ecological niche and nutritional requirements. The initial short-term nitrate (NO3−) uptake kinetics in cultivated S. latissima juvenile sporophytes were evaluated under nitrogen-saturated and nitrogen-limited conditions. The uptake was measured for concentrations in a gradient from 2 to 18 μM NO3− which is representative of Norwegian coastal waters in the main growth season of winter and early spring. Preconditioning treatments led to internal nitrogen pools (total tissue nitrogen and intracellular nitrate) that were significantly lower for the N-limited than for the N-saturated sporophytes prior to the experiment. Nitrate uptake rates, related to biomass (V) and intracellular nitrogen content (U), were linearly related to the substrate concentrations for both N-limited and N-saturated sporophytes, indicating that S. latissima requires high ambient nitrate concentrations to maintain rapid growth. The sporophytes with deficient internal nitrogen pools exhibited higher uptake rates of NO3− than sporophytes with higher internal pools of nitrogen. Mathematical modelling was used to investigate the temporal development of total tissue nitrogen (QN) based on the nitrogen-specific uptake rates (U) and revealed a near linear response of U to changes in QN. The model also found that a maximum estimated value for QN was only approached after more than 20 days at external NO3- concentrations of 8 μM. These results expand the physiological understanding of cultivated S. latissima and are important for a sustainable upscaling of seaweed farm production.

Knock-Down of CsNRT2.1, a Cucumber Nitrate Transporter, Reduces Nitrate Uptake, Root length, and Lateral Root Number at Low External Nitrate Concentration.

Nitrogen (N) is a macronutrient that plays a crucial role in plant growth and development. Nitrate () is the most abundant N source in aerobic soils. Plants have evolved two adaptive mechanisms such as up-regulation of the high-affinity transport system (HATS) and alteration of the root system architecture (RSA), allowing them to cope with the temporal and spatial variation of . However, little information is available regarding the nitrate transporter in cucumber, one of the most important fruit vegetables in the world. In this study we isolated a nitrate transporter named CsNRT2.1 from cucumber. Analysis of the expression profile of the CsNRT2.1 showed that CsNRT2.1 is a high affinity nitrate transporter which mainly located in mature roots. Subcellular localization analysis revealed that CsNRT2.1 is a plasma membrane transporter. In N-starved CsNRT2.1 knock-down plants, both of the constitutive HATS (cHATS) and inducible HATS (iHATS) were impaired under low external concentration. Furthermore, the CsNRT2.1 knock-down plants showed reduced root length and lateral root numbers. Together, our results demonstrated that CsNRT2.1 played a dual role in regulating the HATS and RSA to acquire effectively under N limitation.

Tracing Nitrogen Biogeochemistry During the Beginning of a Spring Phytoplankton Bloom in the Yellow Sea Using Coupled Nitrate Nitrogen and Oxygen Isotope Ratios

AbstractNitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.

A RootNav analysis of morphological changes in Brassica napus L. roots in response to different nitrogen forms

Root architecture impacts the capacity of plants to access nutrients and water. By using the RootNav package, comprehensive morphological changes in root system architecture in response to different nitrogen concentrations and sources were investigated in Brassica napus. Ten-day-old seedlings showed a decrease in total root length, lateral root (LR) density and root exploration area with increasing external NO3 − concentrations from 0.5 to 10 mM. The application of 0.5 mM NO3 − induced more branching in the root system relative to the treatments with higher NO3 − concentrations. The allocation of biomass into the root was greater in the low NO3 − treatment relative to the high NO3 − treatments, reflecting the fact that plants invested more resources in their roots when N uptake from the environment was limited. High NH4 + concentration (10 mM) led to a stunted root system in B. napus, which showed a decrease in primary root (PR) length, total root length, LR branching zone, LR density and root exploration area. Increasing l-glutamate concentration from 0.01 to 0.1 mM suppressed PR length and total root length, whilst the LR branching zone did not change in the different l-glutamate treatments, suggesting that l-glutamate even at micromolar level could arrest PR growth and LR growth in B. napus. In conclusion, external N availability induced changes in the overall root morphology, with root elongation decreasing under an extremely N impoverished environment, but increasing under relatively mild N deficiency (for both NO3 − and NH4 +).

Mining a ‘Georgia Jet’ sweetpotato root transcriptome dataset for nutrient-responsive genes related to root system architecture variability and storage root formation

Sweetpotato is a globally important food crop that plays a crucial role in food security in developing countries where it is often grown under marginal conditions. Sweetpotato yield is determined by the number of adventitious roots (ARs) that forms storage roots (SRs). Recent information indicates that sweetpotato root system architecture (RSA) is related to the competency of an AR to undergo SR formation (SRF). It has also been found that local nitrogen presence influenced sweetpotato RSA development which in turn influenced the uptake of mobile and non-mobile nutrients from the growth substrate. We considered this information in examining a LSQUOGeorgia JetRSQUO root transcriptome dataset to search for genes that are related to SRF, on one hand, and are responsive to nutrient availability, on the other. Using this approach, we found that putative nitrate (NO3-) and ammonium transporters were down-regulated in initiating SRs, while a putative potassium transporter was up-regulated. In addition, we found that a homologue of the Arabidopsis ANR1 gene, a component of the signalling pathway that links external NO3- to RSA development, shows up-regulated expression in initiating SRs. The same gene sequence exhibits homology to IbMADS1, shown previously to act as positive regulators of cell proliferation in root vascular tissue leading to SRF. Taken together, the evidence suggests that RSA provides an important link between environmental signals (nutrient availability) and gene networks previously associated with SRF in sweetpotatoes. This information can be useful for efficiently identifying gene-based markers in breeding programs, especially for low-input environments.

Knockdown of the partner protein OsNAR2.1 for high-affinity nitrate transport represses lateral root formation in a nitrate-dependent manner.

The morphological plasticity of root systems is critical for plant survival, and understanding the mechanisms underlying root adaptation to nitrogen (N) fluctuation is critical for sustainable agriculture; however, the molecular mechanism of N-dependent root growth in rice remains unclear. This study aimed to identify the role of the complementary high-affinity NO3− transport protein OsNAR2.1 in NO3−-regulated rice root growth. Comparisons with wild-type (WT) plants showed that knockdown of OsNAR2.1 inhibited lateral root (LR) formation under low NO3− concentrations, but not under low NH4+ concentrations. 15N-labelling NO3− supplies (provided at concentrations of 0–10 mM) demonstrated that (i) defects in LR formation in mutants subjected to low external NO3− concentrations resulted from impaired NO3− uptake, and (ii) the mutants had significantly fewer LRs than the WT plants when root N contents were similar between genotypes. LR formation in osnar2.1 mutants was less sensitive to localised NO3− supply than LR formation in WT plants, suggesting that OsNAR2.1 may be involved in a NO3−-signalling pathway that controls LR formation. Knockdown of OsNAR2.1 inhibited LR formation by decreasing auxin transport from shoots to roots. Thus, OsNAR2.1 probably functions in both NO3− uptake and NO3−-signalling.

Responses of Hydroponically Grown Common Bean Fed with Nitrogen-free Nutrient Solution to Root Inoculation with N2-fixing Bacteria

To date, few attempts have been made to assess the impact of Rhizobium inoculation on N2 fixation and plant yield in soilless cultivations of common bean. In the present study, common bean (P. vulgaris L.) grown on an inert medium (pumice) was inoculated with either Rhizobium tropici CIAT899 or a commercial product containing a mix of N2-fixing bacteria, specifically rhizobia, and Azotobacter sp. The plants treated with both inoculants were supplied with nitrogen (N)-free (0% N) nutrient solution (NS) throughout the cropping period. A third treatment with non-inoculated plants, which were supplied with a standard (100% N) NS was applied as a control. Inoculation with R. tropici significantly increased the total number of root nodules (80 nodules per plant on average) in comparison with the other two treatments (nine nodules per plant on average). The supply of N-free NS restricted markedly both total plant biomass and pod yield, whereas the inoculation with R. tropici mitigated this effect. The aboveground tissues of plants fed with N-free NS contained appreciably less N than those fed with standard solution when they were inoculated with the commercial inoculant (1.7 vs. 29 mg·g−1 dry weight, respectively). The shoot total N concentration 45, 65, and 90 days after transplanting (32, 31, and 29 mg·g−1 dry weight, respectively) was not reduced by the supply of N-free NS when the plants were inoculated with R. tropici. This finding indicates that, at least from the first sampling date onward, the tissue N level was not a limiting factor for growth and yield in plants inoculated with R. tropici. The supply of N-free NS restricted appreciably the potassium (K), magnesium (Mg), and zinc (Zn) levels in the aboveground plant biomass, regardless of inoculation treatment. The impaired growth and yield in plants fed with N-free NS and inoculated with R. tropici is ascribed to both a N shortage at early growth stages and a reduced K+ uptake aimed at electrochemically balancing the anion-to-cation uptake ratio under conditions of no external NO3– supply.

Signal interactions in the regulation of root nitrate uptake.

In most aerobic soils, nitrate (NO3(-)) is the main nitrogen source for plants and is often limiting for plant growth and development. To adapt to a changing environment, plants have developed complex regulatory mechanisms that involve short and long-range signalling pathways in response to both NO3(-) availability in the soil and other physiological processes like growth or nitrogen (N) and carbon (C) metabolisms. Over the past decade, transcriptomic approaches largely contributed to the identification of molecular elements involved in these regulatory mechanisms, especially at the level of root NO3(-)uptake. Most strikingly, the data obtained revealed the high level of interaction between N and both hormone and C signalling pathways, suggesting a strong dependence on growth, development, and C metabolism to adapt root NO3(-) uptake to both external NO3(-) availability and the N status of the plant. However, the signalling mechanisms involved in the cross-talk between N, C, and hormones for the regulation of root NO3(-) uptake remain largely obscure. The aim of this review is to discuss the recent advances concerning the regulatory pathways controlling NO3(-) uptake in response to N signalling, hormones, and C in the model plant Arabidopsis thaliana. Then, to further characterize the level of interaction between these signalling pathways we built on publicly available transcriptome data to determine how hormones and C treatments modify the gene network connecting root NO3(-) transporters and their regulators.

Variation in nitrate uptake and denitrification rates across a salinity gradient in Mediterranean semiarid streams

Streams are significant locations for nitrate (NO3 −) processing within landscapes. This is especially important in dry climates given the limited water availability for biological processes elsewhere. In arid and semiarid regions, many streams are naturally saline. Elevated salinity can constrain the structure and function of aquatic organisms, which is expected to increase worldwide being associated to global warming. We investigated whole-reach NO3 − uptake and denitrification in nine semiarid streams of variable water salinity (i.e. from freshwater to hyposaline) to test if NO3 − processing would decrease with increasing salinity. We used pulse additions and Tracer Addition for Spiraling Curve Characterization (TASCC) to measure whole-reach uptake of added NO3 −, and the acetylene block technique to measure sediment denitrification. TASCC results showed that only five of nine streams were able to retain added NO3 −. Of these five retentive streams, four were saline; however, salinity did not control significantly the variation in whole-reach NO3 − uptake observed across streams. Other measured environmental variables such as streambed NH4 + and organic carbon availability were better at explaining this variation. Denitrification was detected in all streams except one and its variation across streams was also independent of salinity. Although denitrification rates tended to be high, their contribution to whole-reach NO3 − uptake was insignificant (≤2.16 %). Alternative pathways, heterotrophic assimilation and/or dissimilatory NO3 − reduction to NH4 +, were probably responsible for most whole-reach NO3 − uptake. Together, our results highlight that the function of streams in controlling external NO3 − inputs is highly variable and salinity does not apparently constrain this role.

Nitrate uptake along the maize primary root: an integrated physiological and molecular approach

The spatial variability of the nitrate (NO₃-) uptake along the maize primary root axis was investigated at physiological and molecular levels. Net NO₃- uptake rate (NNUR) and its kinetic parameters, together with the gene expression of a high-affinity NO₃- transporter (NRT2.1), were evaluated. The activity and the expression of plasma membrane H⁺ -ATPase (PM H⁺ -ATPase), key enzyme in plant nutrition, were also analysed. The NNUR showed a heterogeneous spatial pattern along the root, where the regions closer to the root tip early exhibited higher capacity to absorb NO₃- than the basal regions, because of a higher maximum NNUR and faster induction of the inducible high-affinity transport system (iHATS), the presence of the high-affinity transport system (HATS) also at external NO₃- concentrations >100 µm and an improved NO₃- transport because of lower K(m) values. ZmNRT2.1 transcript abundances were not spatially correlated with NNUR, suggesting that post-translational effects or NAR2 protein co-expression could be involved. By contrast, PM H⁺ -ATPase displayed a similar spatial-temporal pattern as that of nitrate uptake, resulting in higher activity in the root tip than in the basal regions. Increased activities of the enzyme after nitrate supply resulted in enhanced expression of MAH3 and MAH4, PM H⁺ -ATPase subfamily II genes, while MAH1 was not expressed.

Denitrification Enzyme Activity as an Indicator of Nitrate Movement through a Diversion Wetland

The Davis Pond freshwater diversion is intended to help restore Louisiana's coastal wetlands by reintroducing Mississippi River water to Barataria Basin. We hypothesized that the high NO3− concentration (2.0 mg NO3–N L−1) of the Mississippi River water would control the rate of denitrification in the receiving marsh given that the soils are saturated, anaerobic, and contain high C. Therefore, areas of high denitrification enzyme activity (DEA) in the marsh would represent soils exposed to river NO3− and actively involved in denitrification. Data from 88 soil samples (0–10 cm) collected throughout the marsh revealed significantly higher rates of DEA in a 715‐ha area adjacent to the diversion inflow. This area of generally high DEA contained >80% of all DEA observed while representing only 19% of the total marsh area at the low discharge rate of 39.5 m3 s−1 The area of high DEA coincided with the highest surface water NO3− and indicated that the marsh has a greater aerial capacity for NO3− removal than is utilized. A laboratory experiment suggested that soils loaded with external NO3− typically had higher DEA rates than soils receiving no added NO3− The DEA was strongly dependent on soil depth (92% of DEA occurred at 0–5 cm) and internal N cycling was substantial in this wetland soils. This study demonstrates the applicability of using soil DEA to map where denitrification activity is greatest, the aerial extent of soils involved in denitrification, and the general flow path of introduced nutrients in large wetlands where NO3− is the limiting factor for denitrification.

Spatial and temporal patterns of net nitrate uptake regulation and kinetics along the tap root of Citrus aurantium

Spatial–temporal variation of the regulation and the kinetics of net nitrate (NO3−) uptake rate (NNUR) along the tap root of Citrus aurantium L. were analysed. Suberin incrustation in the peripheral cell layers and plasma membrane (PM) H+-ATPase localisation, anatomical and physiological factors involved in NO3− uptake were also investigated. The results clearly indicated a spatially uniform distribution of the regulation process, accompanied by a temporal heterogeneous pattern of the kinetics of NO3− uptake along citrus tap root. In particular, kinetic analysis had a biphasic pattern, saturating (high affinity transport system) and linear (low affinity transport system), in response to increasing external NO3− concentrations in each root region, where 200 μM NO3− represented the threshold separating these two systems. Kinetic parameters, Km and Vmax, clearly indicated that apical segments reached the maximum value of induction before basal segments. Hence, the apical root zones, early exhibiting the maximum of potential capacity to absorb the NO3−, could be considered more efficient than basal root segments for acquiring NO3− from external solution. Suberin incrustations in the hypodermal cell layer, characterised by uniform fluorescence intensity among the root segments, could be responsible for the unchanged NNUR, while the PM H+-ATPase could explain the temporal pattern of NNUR.

Conversion of the 2 Cl−/1 H+ antiporter ClC-5 in a NO3−/H+ antiporter by a single point mutation

Several members of the CLC family are secondary active anion/proton exchangers, and not passive chloride channels. Among the exchangers, the endosomal ClC-5 protein that is mutated in Dent's disease shows an extreme outward rectification that precludes a precise determination of its transport stoichiometry from measurements of the reversal potential. We developed a novel imaging method to determine the absolute proton flux in Xenopus oocytes from the extracellular proton gradient. We determined a transport stoichiometry of 2 Cl(-)/1 H+. Nitrate uncoupled proton transport but mutating the highly conserved serine 168 to proline, as found in the plant NO3(-)/H+ antiporter atClCa, led to coupled NO3(-)/H+ exchange. Among several amino acids tested at position 168, S168P was unique in mediating highly coupled NO3(-)/H+ exchange. We further found that ClC-5 is strongly stimulated by intracellular protons in an allosteric manner with an apparent pK of approximately 7.2. A 2:1 stoichiometry appears to be a general property of CLC anion/proton exchangers. Serine 168 has an important function in determining anionic specificity of the exchange mechanism.

Nitrate-dependent control of root architecture and N nutrition are altered by a plant growth-promoting Phyllobacterium sp

Both root architecture and plant N nutrition are altered by inoculation with the plant growth-promoting rhizobacteria (PGPR) Phyllobacterium strain STM196. It is known that NO3- and N metabolites can act as regulatory signals on root development and N transporters. In this study, we investigate the possible interrelated effects on root development and N transport. We show that the inhibition of Arabidopsis lateral root growth by high external NO3- is overridden by Phyllobacterium inoculation. However, the leaf NO3- pool remained unchanged in inoculated plants. By contrast, the Gln root pool was reduced in inoculated plants. Unexpectedly, NO3- influx and the expression levels of AtNRT1.1 and AtNRT2.1 genes coding for root NO3- transporters were also decreased after 8 days of Phyllobacterium inoculation. Although the mechanisms by which PGPR exert their positive effects remain unknown, our data show that they can optimize plant development independently from N supply, thus alleviating the regulatory mechanisms that operate in axenic conditions. In addition, we found that Phyllobacterium sp. elicited a very strong induction of AtNRT2.5 and AtNRT2.6, both genes preferentially expressed in the shoots whose functions are unknown.

A comparison of nitrate transport in four different rice (Oryza sativa L.) cultivars

As rice can use both nitrate (NO3-) and ammonium (NH4+), we have tested the hypothesis that the shift in the pattern of cultivars grown in Jiangsu Province reflects the ability of the plants to exploit NO3- as a nitrogen (N) source. Four rice cultivars were grown in solution culture for comparison of their growth on NO3- and NH4+ nitrogen sources. All four types of rice, Xian You 63 (XY63), Yang Dao 6 (YD), Nong Keng 57 (NK) and Si You 917 (SY917), grew well and produced similar amounts of shoot biomass with 1 mmol/L NH4+ as the only N source. However, the roots of NK were significantly smaller in comparison with the other cultivars. When supplied with 1 mmol/L NO3-, YD produced the greatest biomass; while NK achieved the lowest growth among the four cultivars. Electrophysiological measurements on root rhizodermal cells showed that the NO3- -elicited changes in membrane potential (deltaEm) of these four rice cultivars were significantly different when exposed to low external NO3- (<1 mmol/L); while they were very similar at high external NO3- (10 mmol/L). The root cell membrane potentials of YD and XY63 were more responsive to low external NO3- than those of NK and SY917. The deltaEm values for YD and XY63 rhizodermal cells were almost the same at both 0.1 mmol/L and 1 mmol/L NO3-; while for the NK and SY917 the values became larger as the external NO3- increased. For YD cultivar, deltaEm was measured over a range of NO3- concentrations and a Michaelis-Menten fit to the data gave a Km value of 0.17 mmol/L. Net NO3- uptake depletion kinetics were also compared and for some cultivars (YD and XY63) a single-phase uptake system with first order kinetics best fitted the data; while other cultivars (ND and SY917) showed a better fit to two uptake systems. These uptake systems had two affinity ranges: one had a similar Km in all the cultivars (0.2 mmol/L); the other much higher affinity system (0.03 mmol/L) was only present in NK and SY917. The expression pattern of twelve different NO3- transporter genes was tested using specific primers, but only OsNRT1.1 and OsNRT2.1 expression could be detected showing significant differences between the four rice cultivars. The results from both the physiological and molecular experiments do provide some support for the hypothesis that the more popular rice cultivars grown in Jiangsu Province may be better at using NO3- as an N source.

Nitrate and glutamate sensing by plant roots

The architecture of a root system plays a major role in determining how efficiently a plant can capture water and nutrients from the soil. Growth occurs at the root tips and the process of exploring the soil volume depends on the behaviour of large numbers of individual root tips at different orders of branching. Each root tip is equipped with a battery of sensory mechanisms that enable it to respond to a range of environmental signals, including nutrients, water potential, light, gravity and touch. We have previously identified a MADS (MCM1, agamous, deficiens and SRF) box gene (ANR1) in Arabidopsis thaliana that is involved in modulating the rate of lateral root growth in response to changes in the external NO(3)(-) supply. Transgenic plants have been generated in which a constitutively expressed ANR1 protein can be post-translationally activated by treatment with dexamethasone (DEX). When roots of these lines are treated with DEX, lateral root growth is markedly stimulated but there is no effect on primary root growth, suggesting that one or more components of the regulatory pathway that operate in conjunction with ANR1 in lateral roots may be absent in the primary root tip. We have recently observed some very specific effects of low concentrations of glutamate on root growth, resulting in significant changes in root architecture. Experimental evidence suggests that this response involves the sensing of extracellular glutamate by root tip cells. We are currently investigating the possible role of plant ionotropic glutamate receptors in this sensory mechanism.

Plant growth-promoting bacteria and nitrate availability: impacts on root development and nitrate uptake.

Plant growth-promoting bacteria (PGPB) and NO-3 availability both affect NO-3 uptake and root architecture. The presence of external NO-3 induces the expression of NO-3 transporter genes and elicits lateral root elongation in the part of the root system exposed to the NO-3 supply. By contrast, an increase in NO-3 supply leads to a higher plant N status (low N demand), which represses both the NO-3 transporters and lateral root development. The effects of PGPB on NO-3 uptake and root development are similar to those of low NO-3 availability (concomitant stimulation of NO-3 uptake rate and lateral root development). The mechanisms responsible for the localized and long-distance regulation of NO-3 uptake and root development by NO-3 availability are beginning to be elucidated. By contrast, the signalling and transduction pathways elicited by the rhizobacteria remain totally unknown. This review will compare the effects of NO-3 availability and PGPB on root morphogenesis and NO-3 uptake, in order to determine whether interactions exist between the NO-3-dependent and the PGPB-dependent regulatory pathways.