A unified nomenclature of NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER family members in plants

Members of the plant NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER (NRT1/PTR) family display protein sequence homology with the SLC15/PepT/PTR/POT family of peptide transporters in animals. In comparison to their animal and bacterial counterparts, these plant proteins transport a wide variety of substrates: nitrate, peptides, amino acids, dicarboxylates, glucosinolates, IAA, and ABA. The phylogenetic relationship of the members of the NRT1/PTR family in 31 fully sequenced plant genomes allowed the identification of unambiguous clades, defining eight subfamilies. The phylogenetic tree was used to determine a unified nomenclature of this family named NPF, for NRT1/PTR FAMILY. We propose that the members should be named accordingly: NPFX.Y, where X denotes the subfamily and Y the individual member within the species.

A novel morphological response of maize (Zea mays) adult roots to heterogeneous nitrate supply revealed by a split-root experiment

Approximately 35-55% of total nitrogen (N) in maize plants is taken up by the root at the reproductive stage. Little is known about how the root of an adult plant responds to heterogeneous nutrient supply. In this study, root morphological and physiological adaptations to nitrate-rich and nitrate-poor patches and corresponding gene expression of ZmNrt2.1 and ZmNrt2.2 of maize seedlings and adult plants were characterized. Local high nitrate (LoHN) supply increased both lateral root length (LRL) and density of the treated nodal roots of adult maize plants, but only increased LRL of the treated primary roots of seedlings. LoHN also increased plant total N acquisition but not N influx rate of the treated roots, when expressed as per unit of root length. Furthermore, LoHN markedly increased specific root length (m g(-1)) of the treated roots but significantly inhibited the growth of the lateral roots outside of the nitrate-rich patches, suggesting a systemic carbon saving strategy within a whole root system. Surprisingly, local low nitrate (LoLN) supply stimulated nodal root growth of adult plants although LoLN inhibited growth of primary roots of seedlings. LoLN inhibited the N influx rate of the treated roots and did not change plant total N content. The gene expression of ZmNrt2.1 and ZmNrt2.2 of the treated roots of seedlings and adult plants was inhibited by LoHN but enhanced by LoLN. In conclusion, maize adult roots responded to nitrate-rich and nitrate-poor patches by adaptive morphological alterations and displayed carbon saving strategies in response to heterogeneous nitrate supply.

Members of the NPF3 transporter subfamily encode pathogen-inducible nitrate/nitrite transporters in grapevine and Arabidopsis

Vitis vinifera, the major grapevine species cultivated for wine production, is very susceptible to Erysiphe necator, the causal agent of powdery mildew (PM). This obligate biotrophic fungal pathogen attacks both leaf and berry, greatly affecting yield and quality. To investigate possible mechanisms of nutrient acquisition by successful biotrophs, we characterized a candidate NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER FAMILY (NPF, formerly NRT1/PTR) member, grapevine NFP3.2, that was up-regulated in E. necator-inoculated susceptible V. vinifera Cabernet Sauvignon leaves, but not in resistant V. aestivalis Norton. Expression in Xenopus laevis oocytes and two-electrode voltage clamp measurements showed that VvNPF3.2 is a low-affinity transporter for both nitrate and nitrite and displays characteristics of NPF members from other plants. We also cloned the Arabidopsis ortholog, AtNPF3.1, and showed that AtNPF3.1 similarly transported nitrate and nitrite with low affinity. With an Arabidopsis triple mutant that is susceptible to E. necator, we found that AtNPF3.1 is up-regulated in the leaves of infected Arabidopsis similarly to VvNPF3.2 in susceptible grapevine leaves. Expression of the reporter β-glucuronidase (GUS) driven by the promoter of VvNPF3.2 or AtNPF3.1 in Arabidopsis indicated that both transporters are expressed in vascular tissue, with expression in major and minor veins, respectively. Interestingly, the promoter of VvNPF3.2 allowed induced expression of GUS in minor veins in PM-infected leaves. Our experiments lay the groundwork for investigating the manipulation of host nutrient distribution by biotrophic pathogens and characterizing physiological variables in the pathogenesis of this difficult to study grapevine disease.

Two phloem nitrate transporters, NRT1.11 and NRT1.12, are important for redistributing xylem-borne nitrate to enhance plant growth

This study of the Arabidopsis (Arabidopsis thaliana) nitrate transporters NRT1.11 and NRT1.12 reveals how the interplay between xylem and phloem transport of nitrate ensures optimal nitrate distribution in leaves for plant growth. Functional analysis in Xenopus laevis oocytes showed that both NRT1.11 and NRT1.12 are low-affinity nitrate transporters. Quantitative reverse transcription-polymerase chain reaction and immunoblot analysis showed higher expression of these two genes in larger expanded leaves. Green fluorescent protein and β-glucuronidase reporter analyses indicated that NRT1.11 and NRT1.12 are plasma membrane transporters expressed in the companion cells of the major vein. In nrt1.11 nrt1.12 double mutants, more root-fed (15)NO3(-) was translocated to mature and larger expanded leaves but less to the youngest tissues, suggesting that NRT1.11 and NRT1.12 are required for transferring root-derived nitrate into phloem in the major veins of mature and larger expanded leaves for redistributing to the youngest tissues. Distinct from the wild type, nrt1.11 nrt1.12 double mutants show no increase of plant growth at high nitrate supply. These data suggested that NRT1.11 and NRT1.12 are involved in xylem-to-phloem transfer for redistributing nitrate into developing leaves, and such nitrate redistribution is a critical step for optimal plant growth enhanced by increasing external nitrate.

The nitrate transporter (NRT) gene family in poplar

Nitrate is an important nutrient required for plant growth. It also acts as a signal regulating plant development. Nitrate is actively taken up and transported by nitrate transporters (NRT), which form a large family with many members and distinct functions. In contrast to Arabidopsis and rice there is little information about the NRT family in woody plants such as Populus. In this study, a comprehensive analysis of the Populus NRT family was performed. Sixty-eight PtNRT1/PTR, 6 PtNRT2, and 5 PtNRT3 genes were identified in the P. trichocarpa genome. Phylogenetic analysis confirmed that the genes of the NRT family are divided into three clades: NRT1/PTR with four subclades, NRT2, and NRT3. Topological analysis indicated that all members of PtNRT1/PTR and PtNRT2 have 8 to 12 trans-membrane domains, whereas the PtNRT3 proteins have no or up to two trans-membrane domains. Four PtNRT3 members were predicted as secreted proteins. Microarray analyses revealed tissue-specific expression patterns of PtNRT genes with distinct clusters of NRTs for roots, for the elongation zone of the apical stem segment and the developing xylem and a further cluster for leaves, bark and wood. A comparison of different poplar species (P. trichocarpa, P. tremula, P. euphratica, P. fremontii x P. angustifolia, and P. x canescens) showed that the tissue-specific patterns of the NRT genes varied to some extent with species. Bioinformatic analysis of putative cis-regulatory elements in the promoter regions of PtNRT family retrieved motifs suggesting the regulation of the NRT genes by N metabolism, by energy and carbon metabolism, and by phytohormones and stress. Multivariate analysis suggested that the combination and abundance of motifs in distinct promoters may lead to tissue-specificity. Our genome wide analysis of the PtNRT genes provides a valuable basis for functional analysis towards understanding the role of nitrate transporters for tree growth.

Modulation of ethylene biosynthesis by ACC and AIB reveals a structural and functional relationship between the K15NO3 uptake rate and root absorbing surfaces

The modification of root traits in relation to nitrate uptake represents a source for improvement of nitrogen uptake efficiency. Because ethylene signalling modulates growth of exploratory and root hair systems more rapidly (minutes to hours) than nitrate signalling (days to weeks), a pharmacological approach was used to decipher the relationships between root elongation and N uptake. Rape seedlings were grown on agar plates supplied with 1mM K(15)NO3 and treated with different concentrations of either the ethylene precursor, ACC (0.1, 1, and 10 μM) or an inhibitor of ethylene biosynthesis, AIB (0.5 and 1 μM). The results showed that rapid modulation of root elongation (up to 8-fold) is more dependent on the ethylene than the nitrate signal. Indeed, ACC treatment induced a partial compensatory increase in (15)N uptake associated with overexpression of the BnNRT2.1 and BnNRT1.1 genes. Likewise, daily root elongation between treatments was not associated with daily nitrate uptake but was correlated with N status. This suggested that a part of the daily root response was modulated by cross talks between ethylene signalling and N and C metabolisms. This was confirmed by the reduction in C allocation to the roots induced by ACC treatment and the correlations of changes in the root length and shoot surface area with the aspartate content. The observed effects of ethylene signalling in the root elongation and NRT gene expression are discussed in the context of the putative role of NRT2.1 and NRT1.1 transporters as nitrate sensors.

ABA transport and transporters

Abscisic acid (ABA) metabolism, perception, and transport form a triptych allowing higher plants to use ABA as a signaling molecule. The molecular bases of ABA metabolism are now well described and, over the past few years, several ABA receptors have been discovered. Although ABA transport has long been demonstrated in planta, the first breakthroughs in identifying plasma membrane-localized ABA transporters came in 2010, with the identification of two ATP-binding cassette (ABC) proteins. More recently, two ABA transporters in the nitrate transporter 1/peptide transporter (NRT1/PTR) family have been identified. In this review, we discuss the role of these different ABA transporters and examine the scientific impact of their identification. Given that the NRT1/PTR family is involved in the transport of nitrogen (N) compounds, further work should determine whether an interaction between ABA and N signaling or nutrition occurs.

LbNrt RNA silencing in the mycorrhizal symbiont Laccaria bicolor reveals a nitrate-independent regulatory role for a eukaryotic NRT2-type nitrate transporter

Fungal nitrogen metabolism plays a fundamental role in function of mycorrhizal symbiosis and consequently in nutrient cycling of terrestrial ecosystems. Despite its global ecological relevance the information on control and molecular regulation of nitrogen utilization in mycorrhizal fungi is very limited. We have extended the nitrate utilization RNA silencing studies of the model mycorrhizal basidiomycete, Laccaria bicolor, by altering the expression of LbNrt, the sole nitrate transporter-encoding gene of the fungus. Here we report the first nutrient transporter mutants for mycorrhizal fungi. Silencing of LbNrt results in fungal strains with minimal detectable LbNrt transcript levels, significantly reduced growth capacity on nitrate and altered symbiotic interaction with poplar. Transporter silencing also creates marked co-downregulation of whole Laccaria fHANT-AC (fungal high-affinity nitrate assimilation cluster). Most importantly, this effect on the nitrate utilization pathway appears independent of extracellular nitrate or nitrogen status of the fungus. Our results indicate a novel and central nitrate uptake-independent regulatory role for a eukaryotic nitrate transporter. The possible cellular mechanisms behind this regulation mode are discussed in the light of current knowledge on NRT2-type nitrate transporters in different eukaryotes.

The response of the maize nitrate transport system to nitrogen demand and supply across the lifecycle

An understanding of nitrate (NO3-) uptake throughout the lifecycle of plants, and how this process responds to nitrogen (N) availability, is an important step towards the development of plants with improved nitrogen use efficiency (NUE). NO3- uptake capacity and transcript levels of putative high- and low-affinity NO3- transporters (NRTs) were profiled across the lifecycle of dwarf maize (Zea mays) plants grown at reduced and adequate NO3-. Plants showed major changes in high-affinity NO3- uptake capacity across the lifecycle, which varied with changing relative growth rates of roots and shoots. Transcript abundances of putative high-affinity NRTs (predominantly ZmNRT2.1 and ZmNRT2.2) were correlated with two distinct peaks in high-affinity root NO3- uptake capacity and also N availability. The reduction in NO3- supply during the lifecycle led to a dramatic increase in NO3- uptake capacity, which preceded changes in transcript levels of NRTs, suggesting a model with short-term post-translational regulation and longer term transcriptional regulation of NO3- uptake capacity. These observations offer new insight into the control of NO3- uptake by both plant developmental processes and N availability, and identify key control points that may be targeted by future plant improvement programmes to enhance N uptake relative to availability and/or demand.

Enhanced proton translocating pyrophosphatase activity improves nitrogen use efficiency in Romaine lettuce

Plant nitrate (NO3(-)) acquisition depends on the combined activities of root high- and low-affinity NO3(-) transporters and the proton gradient generated by the plasma membrane H(+)-ATPase. These processes are coordinated with photosynthesis and the carbon status of the plant. Here, we present the characterization of romaine lettuce (Lactuca sativa 'Conquistador') plants engineered to overexpress an intragenic gain-of-function allele of the type I proton translocating pyrophosphatase (H(+)-PPase) of Arabidopsis (Arabidopsis thaliana). The proton-pumping and inorganic pyrophosphate hydrolytic activities of these plants are augmented compared with control plants. Immunohistochemical data show a conspicuous increase in H(+)-PPase protein abundance at the vasculature of the transgenic plants. Transgenic plants displayed an enhanced rhizosphere acidification capacity consistent with the augmented plasma membrane H(+)-ATPase proton transport values, and ATP hydrolytic capacities evaluated in vitro. These transgenic lines outperform control plants when challenged with NO3(-) limitations in laboratory, greenhouse, and field scenarios. Furthermore, we report the characterization of a lettuce LsNRT2.1 gene that is constitutive up-regulated in the transgenic plants. Of note, the expression of the LsNRT2.1 gene in control plants is regulated by NO3(-) and sugars. Enhanced accumulation of (15)N-labeled fertilizer by transgenic lettuce compared with control plants was observed in greenhouse experiments. A negative correlation between the level of root soluble sugars and biomass is consistent with the strong root growth that characterizes these transgenic plants.

Knockdown of a rice stelar nitrate transporter alters long-distance translocation but not root influx

Root nitrate uptake is well known to adjust to the plant's nitrogen demand for growth. Long-distance transport and/or root storage pools are thought to provide negative feedback signals regulating root uptake. We have characterized a vascular specific nitrate transporter belonging to the high-affinity Nitrate Transporter2 (NRT2) family, OsNRT2.3a, in rice (Oryza sativa ssp. japonica 'Nipponbare'). Localization analyses using protoplast expression, in planta promoter-β-glucuronidase assay, and in situ hybridization showed that OsNRT2.3a was located in the plasma membrane and mainly expressed in xylem parenchyma cells of the stele of nitrate-supplied roots. Knockdown expression of OsNRT2.3a by RNA interference (RNAi) had impaired xylem loading of nitrate and decreased plant growth at low (0.5 mm) nitrate supply. In comparison with the wild type, the RNAi lines contained both nitrate and total nitrogen significantly higher in the roots and lower in the shoots. The short-term [(15)N]NO(3)(-) influx (5 min) in entire roots and NO(3)(-) fluxes in root surfaces showed that the knockdown of OsNRT2.3a in comparison with the wild type did not affect nitrate uptake by roots. The RNAi plants showed no significant changes in the expression of some root nitrate transporters (OsNRT2.3b, OsNRT2.4, and OsNAR2.1), but transcripts for nia1 (nitrate reductase) had increased and OsNRT2.1 and OsNRT2.2 had decreased when the plants were supplied with nitrate. Taken together, the data demonstrate that OsNRT2.3a plays a key role in long-distance nitrate transport from root to shoot at low nitrate supply level in rice.

Systemic regulation of soybean nodulation by acidic growth conditions

Mechanisms inhibiting legume nodulation by low soil pH, although highly prevalent and economically significant, are poorly understood. We addressed this in soybean (Glycine max) using a combination of physiological and genetic approaches. Split-root and grafting studies using an autoregulation-of-nodulation-deficient mutant line, altered in the autoregulation-of-nodulation receptor kinase GmNARK, determined that a systemic, shoot-controlled, and GmNARK-dependent mechanism was critical for facilitating the inhibitory effect. Acid inhibition was independent of aluminum ion concentration and occurred early in nodule development, between 12 and 96 h post inoculation with Bradyrhizobium japonicum. Biological effects were confirmed by measuring transcript numbers of known early nodulation genes. Transcripts decreased on both sides of split-root systems, where only one side was subjected to low-pH conditions. Our findings enhance the present understanding of the innate mechanisms regulating legume nodulation control under acidic conditions, which could benefit future attempts in agriculture to improve nodule development and biological nitrogen fixation in acid-stressed soils.

Functional assessment of the Medicago truncatula NIP/LATD protein demonstrates that it is a high-affinity nitrate transporter

The Medicago truncatula NIP/LATD (for Numerous Infections and Polyphenolics/Lateral root-organ Defective) gene encodes a protein found in a clade of nitrate transporters within the large NRT1(PTR) family that also encodes transporters of dipeptides and tripeptides, dicarboxylates, auxin, and abscisic acid. Of the NRT1(PTR) members known to transport nitrate, most are low-affinity transporters. Here, we show that M. truncatula nip/latd mutants are more defective in their lateral root responses to nitrate provided at low (250 μm) concentrations than at higher (5 mm) concentrations; however, nitrate uptake experiments showed no discernible differences in uptake in the mutants. Heterologous expression experiments showed that MtNIP/LATD encodes a nitrate transporter: expression in Xenopus laevis oocytes conferred upon the oocytes the ability to take up nitrate from the medium with high affinity, and expression of MtNIP/LATD in an Arabidopsis chl1(nrt1.1) mutant rescued the chlorate susceptibility phenotype. X. laevis oocytes expressing mutant Mtnip-1 and Mtlatd were unable to take up nitrate from the medium, but oocytes expressing the less severe Mtnip-3 allele were proficient in nitrate transport. M. truncatula nip/latd mutants have pleiotropic defects in nodulation and root architecture. Expression of the Arabidopsis NRT1.1 gene in mutant Mtnip-1 roots partially rescued Mtnip-1 for root architecture defects but not for nodulation defects. This suggests that the spectrum of activities inherent in AtNRT1.1 is different from that possessed by MtNIP/LATD, but it could also reflect stability differences of each protein in M. truncatula. Collectively, the data show that MtNIP/LATD is a high-affinity nitrate transporter and suggest that it could have another function.

Molecular characterization of the Lotus japonicus NRT1(PTR) and NRT2 families

Nitrate is an essential element for plant growth, both as a primary nutrient in the nitrogen assimilation pathway and as an important signal for plant development. Low- and high-affinity transport systems are involved in the nitrate uptake from the soil and its distribution between different plant tissues. By an in silico search, we identified putative members of both systems in the model legume Lotus japonicus. We investigated, by a time course analysis, the transcripts abundance in root tissues of nine and four genes encoding putative low-affinity (NRT1) and high-affinity (NRT2) nitrate transporters, respectively. The genes were sub-classified as inducible, repressible and constitutive on the basis of their responses to provision of nitrate, auxin or cytokinin. Furthermore, the analysis of the pattern of expression in root and nodule tissues after Mesorhizobium loti inoculation permitted the identification of sequences significantly regulated during the symbiotic interaction. The interpretation of the global regulative networks obtained allowed to postulate roles for nitrate transporters as possible actors in the cross-talks between different signalling pathways triggered by biotic and abiotic factors.

Plant nitrogen assimilation and use efficiency

Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture. Plant NUE is inherently complex, as each step-including N uptake, translocation, assimilation, and remobilization-is governed by multiple interacting genetic and environmental factors. The limiting factors in plant metabolism for maximizing NUE are different at high and low N supplies, indicating great potential for improving the NUE of current cultivars, which were bred in well-fertilized soil. Decreasing environmental losses and increasing the productivity of crop-acquired N requires the coordination of carbohydrate and N metabolism to give high yields. Increasing both the grain and N harvest index to drive N acquisition and utilization are important approaches for breeding future high-NUE cultivars.

Plant nitrogen assimilation and use efficiency

Crop productivity relies heavily on nitrogen (N) fertilization. Production and application of N fertilizers consume huge amounts of energy, and excess is detrimental to the environment; therefore, increasing plant N use efficiency (NUE) is essential for the development of sustainable agriculture. Plant NUE is inherently complex, as each step-including N uptake, translocation, assimilation, and remobilization-is governed by multiple interacting genetic and environmental factors. The limiting factors in plant metabolism for maximizing NUE are different at high and low N supplies, indicating great potential for improving the NUE of current cultivars, which were bred in well-fertilized soil. Decreasing environmental losses and increasing the productivity of crop-acquired N requires the coordination of carbohydrate and N metabolism to give high yields. Increasing both the grain and N harvest index to drive N acquisition and utilization are important approaches for breeding future high-NUE cultivars.

The Arabidopsis nitrate transporter NRT2.4 plays a double role in roots and shoots of nitrogen-starved plants

Plants have evolved a variety of mechanisms to adapt to N starvation. NITRATE TRANSPORTER2.4 (NRT2.4) is one of seven NRT2 family genes in Arabidopsis thaliana, and NRT2.4 expression is induced under N starvation. Green fluorescent protein and β-glucuronidase reporter analyses revealed that NRT2.4 is a plasma membrane transporter expressed in the epidermis of lateral roots and in or close to the shoot phloem. The spatiotemporal expression pattern of NRT2.4 in roots is complementary with that of the major high-affinity nitrate transporter NTR2.1. Functional analysis in Xenopus laevis oocytes and in planta showed that NRT2.4 is a nitrate transporter functioning in the high-affinity range. In N-starved nrt2.4 mutants, nitrate uptake under low external supply and nitrate content in shoot phloem exudates was decreased. In the absence of NRT2.1 and NRT2.2, loss of function of NRT2.4 (triple mutants) has an impact on biomass production under low nitrate supply. Together, our results demonstrate that NRT2.4 is a nitrate transporter that has a role in both roots and shoots under N starvation.

Characterization of a dual-affinity nitrate transporter MtNRT1.3 in the model legume Medicago truncatula

Primary root growth in the absence or presence of exogenous NO(3)(-) was studied by a quantitative genetic approach in a recombinant inbred line (RIL) population of Medicago truncatula. A quantitative trait locus (QTL) on chromosome 5 appeared to be particularly relevant because it was seen in both N-free medium (LOD score 5.7; R(2)=13.7) and medium supplied with NO(3)(-) (LOD score, 9.5; R(2)=21.1) which indicates that it would be independent of the general nutritional status. Due to its localization exactly at the peak of this QTL, the putative NRT1-NO(3)(-) transporter (Medtr5g093170.1), closely related to Arabidopsis AtNRT1.3, a putative low-affinity nitrate transporter, appeared to be a significant candidate involved in the control of primary root growth and NO(3)(-) sensing. Functional characterization in Xenopus oocytes using both electrophysiological and (15)NO(3)(-) uptake approaches showed that Medtr5g093170.1, named MtNRT1.3, encodes a dual-affinity NO(3)(-) transporter similar to the AtNRT1.1 'transceptor' in Arabidopsis. MtNRT1.3 expression is developmentally regulated in roots, with increasing expression after completion of germination in N-free medium. In contrast to members of the NRT1 superfamily characterized so far, MtNRT1.3 is environmentally up-regulated by the absence of NO(3)(-) and down-regulated by the addition of the ion to the roots. Split-root experiments showed that the increased expression stimulated by the absence of NO(3)(-) was not the result of a systemic signalling of plant N status. The results suggest that MtNRT1.3 is involved in the response to N limitation, which increases the ability of the plant to acquire NO(3)(-) under N-limiting conditions.

Rice OsNAR2.1 interacts with OsNRT2.1, OsNRT2.2 and OsNRT2.3a nitrate transporters to provide uptake over high and low concentration ranges

Plants take up both nitrate and ammonium as main nitrogen (N) sources. Although ammonium is the predominant form in anaerobic-flooded paddy soil, it has been proposed that rice and other wetland plants may take up significant amounts of nitrate formed by nitrification of ammonium in the rhizosphere. A two-component system for nitrate transport including NRT2s with a partner protein (NAR2 or NRT3.1) has been identified in Arabidopsis. We report the physiological function of another member of the NAR2 family, OsNAR2.1 in rice (Oryza sativa, ssp. Japonica, cv. Nipponbare). OsNAR2.1 was mainly expressed in roots and induced by nitrate and suppressed by ammonium and some amino acids. Knockdown of OsNAR2.1 by RNA interference synchronously suppressed expression of OsNRT2.1, OsNRT2.2 and OsNRT2.3a in the osnar2.1mutants. Both high- and low-affinity nitrate transports were greatly impaired by OsNAR2.1 knockdown. Yeast two hybridization showed that OsNAR2.1 not only interacted with OsNRT2.1/OsNRT2.2, but also with OsNRT2.3a. Taken together, the data demonstrate that OsNAR2.1 plays a key role in enabling the plant to cope with variable nitrate supply.

Arabidopsis nitrate transporter NRT1.9 is important in phloem nitrate transport

This study of the Arabidopsis thaliana nitrate transporter NRT1.9 reveals an important function for a NRT1 family member in phloem nitrate transport. Functional analysis in Xenopus laevis oocytes showed that NRT1.9 is a low-affinity nitrate transporter. Green fluorescent protein and β-glucuronidase reporter analyses indicated that NRT1.9 is a plasma membrane transporter expressed in the companion cells of root phloem. In nrt1.9 mutants, nitrate content in root phloem exudates was decreased, and downward nitrate transport was reduced, suggesting that NRT1.9 may facilitate loading of nitrate into the root phloem and enhance downward nitrate transport in roots. Under high nitrate conditions, the nrt1.9 mutant showed enhanced root-to-shoot nitrate transport and plant growth. We conclude that phloem nitrate transport is facilitated by expression of NRT1.9 in root companion cells. In addition, enhanced root-to-shoot xylem transport of nitrate in nrt1.9 mutants points to a negative correlation between xylem and phloem nitrate transport.

Spatial expression and regulation of rice high-affinity nitrate transporters by nitrogen and carbon status

The high affinity nitrate transport system (HATS) plays an important role in rice nitrogen acquisition because, even under flooded anaerobic cultivation when NH(4)(+) dominates, significant nitrification occurs on the root surface. In the rice genome, four NRT2 and two NAR2 genes encoding HATS components have been identified. One gene OsNRT2.3 was mRNA spliced into OsNRT2.3a and OsNRT2.3b and OsNAR2.1 interacts with OsNRT2.1/2.2 and OsNRT2.3a to provide nitrate uptake. Using promoter-GUS reporter plants and semi-quantitative RT-PCR analyses, it was observed that OsNAR2.1 was expressed mainly in the root epidermal cells, differently from the five OsNRT2 genes. OsNAR2.1, OsNRT2.1, OsNRT2.2, and OsNRT2.3a were up-regulated by nitrate and suppressed by NH(4)(+) and high root temperature (37 °C). Expression of all these genes was increased by light or external sugar supply. Root transcripts of OsNRT2.3b and OsNRT2.4 were much less abundant and not affected by temperature. Expression of OsNRT2.3b was insensitive to the form of N supply. Expression of OsNRT2.4 responded to changes in auxin supply unlike all the other NRT2 genes. A region from position -311 to -1, relative to the translation start site in the promoter region of OsNAR2.1, was found to contain the cis-element(s) necessary for the nitrate-, but not light- and sugar-dependent activation. However, it was difficult to define a conserved cis-element in the promoters of the nitrate-regulated OsNRT2/OsNAR2 genes. The results imply distinct physiological functions for each OsNRT2 transporter, and differential regulation pathways by N and C status.

High-affinity nitrate uptake by rice (Oryza sativa) coleoptiles

Nitrate uptake by rice coleoptiles was evaluated using ¹⁵N-nitrate in relation to the expression of high-affinity nitrate uptake-related genes, OsNRT2s (OsNRT2.1-2.4) and OsNAR2s (OsNAR2.1 and 2.2). Apparent nitrate uptake by coleoptiles was about one-sixth of that by hydroponically cultured seedling roots. Real-time RT-PCR analysis revealed that OsNRT2.1, a root-specific key gene of inducible high-affinity transport system for nitrate, was most strongly induced in coleoptiles following nitrate supply initiation, while other OsNRT2s and OsNAR2s showed modest induction. These results suggest that rice coleoptiles may have high-affinity transport systems for nitrate similar to roots, and can be model organs for nutrient uptake by submerged plant shoots.

Isoforms of plasma membrane H(+)-ATPase in rice root and shoot are differentially induced by starvation and resupply of NO₃⁻ or NH₄+

The goal of this study was to evaluate the effects of nitrogen starvation and resupply in 10 PM H+-ATPase isoforms and the expression of NO₃⁻ and NH₄+ transporters in rice. The net uptake of both forms of NO₃⁻-N or NH₄+-N was increased with its resupply. Resupply of NO₃⁻ resulted in induction of the following PM H+-ATPase isoforms, OsA1, OsA2, OsA5 and OsA7 in the shoots and OsA2, OsA5, OsA7 and OsA8 in the roots. Resupply of NH₄+ resulted in the induction of the following OsA1, OsA3 and OsA7 isoforms in the roots while OsA1 was induced in the shoots. It was observed that increased PM H+-ATPase activity also resulted in increased net uptake of NO₃⁻ and NH₄+. In the roots, OsNRT2.1 and OsNRT2.2 were induced by NO₃⁻ resupply, while OsAMT1.1 and OsAMT1.2 were induced by NH₄+ deficiency. The results showed that the expression of PM H+-ATPase isoforms is related to NO₃⁻ and NH₄+ transporters as well as in which section of the plant it takes place. PM H+-ATPase isoforms OsA2 and OsA7 displayed the strongest induction in response to N resupply, therefore indicating that these genes could be involved in N uptake in rice.

A holistic view of nitrogen acquisition in plants

Nitrogen (N) is the mineral nutrient required in the greatest amount and its availability is a major factor limiting growth and development of plants. As sessile organisms, plants have evolved different strategies to adapt to changes in the availability and distribution of N in soils. These strategies include mechanisms that act at different levels of biological organization from the molecular to the ecosystem level. At the molecular level, plants can adjust their capacity to acquire different forms of N in a range of concentrations by modulating the expression and function of genes in different N uptake systems. Modulation of plant growth and development, most notably changes in the root system architecture, can also greatly impact plant N acquisition in the soil. At the organism and ecosystem levels, plants establish associations with diverse microorganisms to ensure adequate nutrition and N supply. These different adaptive mechanisms have been traditionally discussed separately in the literature. To understand plant N nutrition in the environment, an integrated view of all pathways contributing to plant N acquisition is required. Towards this goal, in this review the different mechanisms that plants utilize to maintain an adequate N supply are summarized and integrated.