Nitrate transport in plants: which gene and which control?

Overexpression of Phytoglobin1 in Rice Leads to Enhanced Nitrogen Use Efficiency via Modulation of Nitric Oxide

Nitric oxide (NO) is one of the byproducts of nitrogen metabolism. Excess amount of NO is scavenged by phytoglobins. The role of phytoglobin mediated NO homoeostasis in modulation of nitrate transporters was investigated using NO scavenger cPTIO, phytoglobin overexpressing rice and Arabidopsis. Growing plants under low nitrate leads to generation of reduced levels of NO accompanied by elevated expression of high affinity transporters (HATs) such as NRT2.1, NRT2.3 and NRT2.4. Scavenging of NO by cPTIO under optimal nitrate caused enhanced HATs expression. Phytoglobin overexpressing Arabidopsis showed improved growth and enhanced expression of HATs under low nitrogen in comparison to WT. Pretreatment of optimal nitrate grown plants with NO scavenger cPTIO enhanced HATs expression and shifting of these primed plants from optimal to low nitrate leads to further elevation of HATs expression accompanied by enhanced nitrogen uptake and its accumulation with positive effect on growth. Phytoglobin overexpression in rice leads to enhanced HATs expression, improved growth, nitrogen accumulation under low nitrate. Pgb OE lines showed enhanced accumulation of amino acids. Taken together our results suggest an important role of phytoglobins in nitrogen uptake and assimilation.

Plant growth and nitrate absorption and assimilation of two sweet potato cultivars with different N tolerances in response to nitrate supply

In sweet potato, rational nitrogen (N) assimilation and distribution are conducive to inhibiting vine overgrowth. Nitrate (NO3-) is the main N form absorbed by roots, and cultivar is an important factor affecting N utilization. Herein, a hydroponic experiment was conducted that included four NO3- concentrations of 0 (N0), 4 (N1), 8 (N2) and 16 (N3) mmol L-1 with two cultivars of Jishu26 (J26, N-sensitive) and Xushu32 (X32, N-tolerant). For J26, with increasing NO3- concentrations, the root length and root surface area significantly decreased. However, no significant differences were observed in these parameters for X32. Higher NO3- concentrations upregulated the expression levels of the genes that encode nitrate reductase (NR2), nitrite reductase (NiR2) and nitrate transporter (NRT1.1) in roots for both cultivars. The trends in the activities of NR and NiR were subject to regulation of NR2 and NiR2 transcription, respectively. For both cultivars, N2 increased the N accumulated in leaves, growth points and roots. For J26, N3 further increased the N accumulation in these organs. Under higher NO3- nutrition, compared with X32, J26 exhibited higher expression levels of the NiR2, NR2 and NRT1.1 genes, a higher influx NO3- rate in roots, and higher activities of NR and NiR in leaves and roots. Conclusively, the regulated effects of NO3- supplies on root growth and NO3- utilization were more significant for J26. Under high NO3- conditions, J26 exhibited higher capacities of NO3- absorption and distributed more N in leaves and in growth points, which may contribute to higher growth potential in shoots and more easily cause vine overgrowth.

Glutamine as sole nitrogen source prevents induction of nitrate transporter gene NRT2.4 and affects amino acid metabolism in Arabidopsis

Plants assimilate inorganic nitrogen (N) to glutamine. Glutamine is the most abundant amino acid in most plant species, the N-supplying precursor of all N-containing compounds in the cell and the first organic nitrogen molecule formed from inorganic nitrogen taken up by the roots. In addition to its role in plant nutrition, glutamine most likely also has a function as a signaling molecule in the regulation of nitrogen metabolism. We investigated whether glutamine influences the high-affinity transporter system for nitrate uptake. Therefore, we analyzed the expression of the nitrate transporter NRT2.4, which is inducible by N deficiency, in Arabidopsis thaliana grown under different nitrogen starvation scenarios, comparing nitrate or glutamine as the sole nitrogen source. Using the reporter line ProNRT2.4:GFP and two independent knockout lines, nrt2.4-1 and nrt2.4-2, we analyzed gene expression and amino acid profiles. We showed that the regulation of NRT2.4 expression depends on available nitrogen in general, for example on glutamine as a nitrogen source, and not specifically on nitrate. In contrast to high nitrate concentrations, amino acid profiles changed to an accumulation of amino acids containing more than one nitrogen during growth in high glutamine concentrations, indicating a switch to nitrogen storage metabolism. Furthermore, we demonstrated that the nrt2.4-2 line shows unexpected effects on NRT2.5 gene expression and the amino acids profile in shoots under high glutamine supply conditions compared to Arabidopsis wild type and nrt2.4-1, suggesting non-NRT2.4-related metabolic consequences in this knockout line.

A biostimulant prepared from red seaweed Kappaphycus alvarezii induces flowering and improves the growth of Pisum sativum grown under optimum and nitrogen-limited conditions

Nitrogen (N) is one of the critical elements required by plants and is therefore one of the important limiting factors for growth and yield. To increase agricultural productivity, farmers are using excessive N fertilizers to the soil, which poses a threat to the ecosystem, as most of the applied nitrogen fertilizer is not taken up by crops, and runoff to aquatic bodies and the environment causes eutrophication, pollution, and greenhouse gas emissions. In this study, we used LBS6, a Kappaphycus alvarezii-based biostimulant as a sustainable alternative to improve the growth of plants under different NO3 - fertigation. A root drench treatment of 1 ml/L LBS6 significantly improved the growth of Pisum sativum plants grown under optimum and deficient N conditions. No significant difference was observed in the growth of LBS6-treated plants grown with excessive N. The application of LBS6 induced flowering under optimum and deficient N conditions. The total nitrogen, nitrate and ammonia contents of tissues were found to be higher in treated plants grown under N deficient conditions. The LBS6 treatments had significantly higher chlorophyll content in those plants grown under N-deficient conditions. The root drench application of LBS6 also regulated photosynthetic efficiency by modulating electron and proton transport-related processes of leaves in the light-adapted state. The rate of linear electron flux, proton conductivity and steady-state proton flux across the thylakoid membrane were found to be higher in LBS6-treated plants. Additionally, LBS6 also reduced nitrogen starvation-induced, reactive oxygen species accumulation by reduction in lipid peroxidation in treated plants. Gene expression analysis showed differential regulation of expression of those genes involved in N uptake, transport, assimilation, and remobilization in LBS6-treated plants. Taken together, LBS6 improved growth of those treated plants under optimum and nitrogen-limited condition by positively modulating their biochemical, molecular, and physiological processes.

Transcriptome Profiling Provides Insights into the Early Development of Tiller Buds in High- and Low-Tillering Orchardgrass Genotypes

Orchardgrass (Dactylis glomerata L.) is among the most economically important perennial cool-season grasses, and is considered an excellent hay, pasture, and silage crop in temperate regions worldwide. Tillering is a vital feature that dominates orchardgrass regeneration and biomass yield. However, transcriptional dynamics underlying early-stage bud development in high- and low-tillering orchardgrass genotypes are unclear. Thus, this study assessed the photosynthetic parameters, the partially essential intermediate biomolecular substances, and the transcriptome to elaborate the early-stage profiles of tiller development. Photosynthetic efficiency and morphological development significantly differed between high- (AKZ-NRGR667) and low-tillering genotypes (D20170203) at the early stage after tiller formation. The 206.41 Gb of high-quality reads revealed stage-specific differentially expressed genes (DEGs), demonstrating that signal transduction and energy-related metabolism pathways, especially photosynthetic-related processes, influence tiller induction and development. Moreover, weighted correlation network analysis (WGCNA) and functional enrichment identified distinctively co-expressed gene clusters and four main regulatory pathways, including chlorophyll, lutein, nitrogen, and gibberellic acid (GA) metabolism pathways. Therefore, photosynthesis, carbohydrate synthesis, nitrogen efficient utilization, and phytohormone signaling pathways are closely and intrinsically linked at the transcriptional level. These findings enhance our understanding of tillering in orchardgrass and perennial grasses, providing a new breeding strategy for improving forage biomass yield.

Genetic analysis of the variation for mineral accumulation in the leaves and seeds of natural germplasm of Brassica rapa L. (AA) and the its derived forms extracted from an allotetraploid B.juncea L.(AABB)

Brassica rapa L. (2n = 20; AA) is a vegetable and oilseed crop that is grown all over the world. Its leaves, shoots, and seeds store significant amounts of minerals. We used inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the concentrations of eleven minerals in the leaves and seeds of 195 advanced generation inbred lines, of which 92 represented natural (NR) B. rapa and the remaining 103 were derived (DR) from a set of mother genotypes originally extracted from an allotetraploid B. juncea (2n = 36; AABB). The inbred lines differed for the composition of leaf and seed minerals. Leaf concentrations of N, K, Zn, and Se were higher in the DR subpanel as compared to NR subpanel, along with high seed accumulations of K and Se. DArT genotyping and genome wide association mapping led to the identification of SNPs associated with leaf and seed mineral compositions. Chromosomes A03, A05, and A10 harboured the most associated loci. Annotations of the regions adjacent to respective GWAS peaks allowed prediction of genes known for acquisition, transport, and accumulation of minerals and heavy metal detoxification. Transcriptome analysis revealed differential expression patterns of the predicted candidates, with most genes either down-regulated in derived genotypes relative to natural forms or their expression being comparable between the two. General downregulation may be a consequence of extracting B. rapa from allotetraploid B. juncea through genome resection. Some of the identified SNPs may be used as DNA markers for breeding programmes designed to modify the leaf and seed mineral compositions.

A mechanistic mathematical model for describing and predicting the dynamics of high-affinity nitrate intake into roots of maize and other plant species

A fully mechanistic dynamical model for plant nitrate uptake is presented. Based on physiological and regulatory pathways and based on physical laws, we form a dynamic system mathematically described by seven differential equations. The model evidences the presence of a short-term positive feedback on the high-affinity nitrate uptake, triggered by the presence of nitrate around the roots, which induces its intaking. In the long run, this positive feedback is overridden by two long-term negative feedback loops which drastically reduces the nitrate uptake capacity. These two negative feedbacks are due to the generation of ammonium and amino acids, respectively, and inhibit the synthesis and the activity of high-affinity nitrate transporters. This model faithfully predicts the typical spiking behavior of the nitrate uptake, in which an initial strong increase of nitrate absorption capacity is followed by a drop, which regulates the absorption down to the initial value. The model outcome was compared with experimental data and they fit quite nicely. The model predicts that after the initial exposure of the roots with nitrate, the absorption of the anion strongly increases and that, on the contrary, the intensity of the absorption is limited in presence of ammonium around the roots.

Balancing nitrate acquisition strategies in symbiotic legumes

MAIN CONCLUSION

Legumes manage both symbiotic (indirect) and non-symbiotic (direct) nitrogen acquisition pathways. Understanding and optimising the direct pathway for nitrate uptake will support greater legume growth and seed yields. Legumes have multiple pathways to acquire reduced nitrogen to grow and set seed. Apart from the symbiotic N2-fixation pathway involving soil-borne rhizobia bacteria, the acquisition of nitrate and ammonia from the soil can also be an important secondary nitrogen source to meet plant N demand. The balance in N delivery between symbiotic N (indirect) and inorganic N uptake (direct) remains less clear over the growing cycle and with the type of legume under cultivation. In fertile, pH balanced agricultural soils, NO3- is often the predominant form of reduced N available to crop plants and will be a major contributor to whole plant N supply if provided at sufficient levels. The transport processes for NO3- uptake into legume root cells and its transport between root and shoot tissues involves both high and low-affinity transport systems called HATS and LATS, respectively. These proteins are regulated by external NO3- availability and by the N status of the cell. Other proteins also play a role in NO3- transport, including the voltage dependent chloride/nitrate channel family (CLC) and the S-type anion channels of the SLAC/SLAH family. CLC's are linked to NO3- transport across the tonoplast of vacuoles and the SLAC/SLAH's with NO3- efflux across the plasma membrane and out of the cell. An important step in managing the N requirements of a plant are the mechanisms involved in root N uptake and the subsequent cellular distribution within the plant. In this review, we will present the current knowledge of these proteins and what is understood on how they function in key model legumes (Lotus japonicus, Medicago truncatula and Glycine sp.). The review will examine their regulation and role in N signalling, discuss how post-translational modification affects NO3- transport in roots and aerial tissues and its translocation to vegetative tissues and storage/remobilization in reproductive tissues. Lastly, we will present how NO3-influences the autoregulation of nodulation and nitrogen fixation and its role in mitigating salt and other abiotic stresses.

Identification and Functional Characterization of MdNRT1.1 in Nitrogen Utilization and Abiotic Stress Tolerance in Malus domestica

Nitrate is one of the main sources of nitrogen for plant growth. Nitrate transporters (NRTs) participate in nitrate uptake and transport, and they are involved in abiotic stress tolerance. Previous studies have shown that NRT1.1 has a dual role in nitrate uptake and utilization; however, little is known about the function of MdNRT1.1 in regulating apple growth and nitrate uptake. In this study, apple MdNRT1.1, a homolog of Arabidopsis NRT1.1, was cloned and functionally identified. Nitrate treatment induced an increased transcript level of MdNRT1.1, and overexpression of MdNRT1.1 promoted root development and nitrogen utilization. Ectopic expression of MdNRT1.1 in Arabidopsis repressed tolerance to drought, salt, and ABA stresses. Overall, this study identified a nitrate transporter, MdNRT1.1, in apples and revealed how MdNRT1.1 regulates nitrate utilization and abiotic stress tolerance.

Nitrate Uptake and Use Efficiency: Pros and Cons of Chloride Interference in the Vegetable Crops

Over the past five decades, nitrogen (N) fertilization has been an essential tool for boosting crop productivity in agricultural systems. To avoid N pollution while preserving the crop yields and profit margins for farmers, the scientific community is searching for eco-sustainable strategies aimed at increasing plants' nitrogen use efficiency (NUE). The present article provides a refined definition of the NUE based on the two important physiological factors (N-uptake and N-utilization efficiency). The diverse molecular and physiological mechanisms underlying the processes of N assimilation, translocation, transport, accumulation, and reallocation are revisited and critically discussed. The review concludes by examining the N uptake and NUE in tandem with chloride stress and eustress, the latter being a new approach toward enhancing productivity and functional quality of the horticultural crops, particularly facilitated by soilless cultivation.

Genome-wide identification and characterization of high-affinity nitrate transporter 2 (NRT2) gene family in tomato (Solanum lycopersicum) and their transcriptional responses to drought and salinity stresses

The high-affinity nitrate transporter 2 (NRT2) proteins play vital roles in both nitrate (NO₃^-) uptake and translocation in plants. Although the gene families coding the NRT2 proteins have been identified and functionally characterized in many plant species, the systematic identification of NRT2 gene family members has not previously been reported in tomato (Solanum lycopersicum). Moreover, little is known about their expression profiles in response to different environmental stresses. The present study sought to identify the NRT2 gene family members within the tomato genome, and then to characterize them in detail by means of bioinformatics, physiological and expression analyses. Four novel NRT2 genes were identified in the tomato genome, all of which contained the same domain belonging to the major facilitator superfamily (PF07690). The co-expression network of the SlNRT2 genes revealed that they were co-expressed with several other genes in a number of different molecular pathways, including the transport, photosynthesis, fatty acid metabolism and amino acid catabolism pathways. Several phosphorylation sites were predicted in the NRT2 proteins. The SlNRT2 genes interact with many other genes that perform various functions in many crucial pathways within the tomato genome. The sequence variations observed at the gene and protein levels indicate the dynamic regulation of the SlNRT2 gene family members in relation to cell metabolism, particularly with regard to the nitrogen assimilation pathway. The responses of the SlNRT2 genes to drought and salinity stresses are diverse, and they are neither stress- nor tissue-specific. The findings of this study should provide a useful scientific basis for future studies concerning the roles of the NRT2 gene family in plants.

Nitrogen nutrition modifies the susceptibility of Arabidopsis thaliana to the necrotrophic fungus, Alternaria brassicicola

The impact of the form of nitrogen (N) source (nitrate versus ammonium) on the susceptibility to Alternaria brassicicola, a necrotrophic fungus, has been examined in Arabidopsis thaliana at the rosette stage. Nitrate nutrition was found to increase fungal lesions considerably. There was a similar induction of defence gene expression following infection under both N nutritions, except for the phytoalexin deficient 3 gene, which was overexpressed with nitrate. Nitrate also led to a greater nitric oxide production occurring in planta during the saprophytic growth and lower nitrate reductase (NIA1) expression 7 days after inoculation. This suggests that nitrate reductase-dependent nitric oxide production had a dual role, whereby, despite its known role in the generic response to pathogens, it affected plant metabolism, and this facilitated fungal infection. In ammonium-grown plants, infection with A. brassicicola induced a stronger gene expression of ammonium transporters and significantly reduced the initially high ammonium content in the leaves. There was a significant interaction between N source and inoculation (presence versus absence of the fungus) on the total amino acid content, while N nutrition reconfigured the spectrum of major amino acids. Typically, a higher content of total amino acid, mainly due to a stronger increase in asparagine and glutamine, is observed under ammonium nutrition while, in nitrate-fed plants, glutamate was the only amino acid which content increased significantly after fungal inoculation. N nutrition thus appears to control fungal infection via a complex set of signalling and nutritional events, shedding light on how nitrate availability can modulate disease susceptibility.

Physiological and Molecular Investigation of Urea Uptake Dynamics in Cucumis sativus L. Plants Fertilized With Urea-Doped Amorphous Calcium Phosphate Nanoparticles

At present, the quest for innovative and sustainable fertilization approaches aiming to improve agricultural productivity represents one of the major challenges for research. In this context, nanoparticle-based fertilizers can indeed offer an interesting alternative with respect to traditional bulk fertilizers. Several pieces of evidence have already addressed the effectiveness of amorphous calcium phosphate-based nanoparticles as carriers for macronutrients, such as nitrogen (N), demonstrating increase in crop productivity and improvement in quality. Nevertheless, despite N being a fundamental nutrient for crop growth and productivity, very little research has been carried out to understand the physiological and molecular mechanisms underpinning N-based fertilizers supplied to plants via nanocarriers. For these reasons, this study aimed to investigate the responses of Cucumis sativus L. to amorphous calcium phosphate nanoparticles doped with urea (U-ACP). Urea uptake dynamics at root level have been investigated by monitoring both the urea acquisition rates and the modulation of urea transporter CsDUR3, whereas growth parameters, the accumulation of N in both root and shoots, and the general ionomic profile of both tissues have been determined to assess the potentiality of U-ACP as innovative fertilizers. The slow release of urea from nanoparticles and/or their chemical composition contributed to the upregulation of the urea uptake system for a longer period (up to 24 h after treatment) as compared to plants treated with bulk urea. This prolonged activation was mirrored by a higher accumulation of N in nanoparticle-treated plants (approximately threefold increase in the shoot of NP-treated plants compared to controls), even when the concentration of urea conveyed through nanoparticles was halved. In addition, besides impacting N nutrition, U-ACP also enhanced Ca and P concentration in cucumber tissues, thus having possible effects on plant growth and yield, and on the nutritional value of agricultural products.

New insights into N-utilization efficiency in tomato (Solanum lycopersicum L.) under N limiting condition

Understanding Nitrogen Use Efficiency (NUE) physiological and molecular mechanisms in high N demanding crops has become decisive for improving NUE in sustainable cropping systems. How the Nitrogen Utilization Efficiency (NUtE) component contributes to the NUE enhancement under nitrate limiting conditions in tomato remains to be elucidated. This study deals with the changes in several important nitrate metabolism related gene expressions (nitrate assimilation, transport, remobilization and storage/sequestration) engendered by short and long-term limiting nitrate exposure in two selected NUE-contrasting genotypes, Regina Ostuni (RO) and UC82, efficient and inefficient, respectively. At short-term, nitrate limiting supply triggered higher SlCLCa and SlNRT1.7 expressions in RO root and shoot, respectively, suggesting a higher nitrate storage and remobilization compared to UC82, explaining how RO withstood the nitrate deficiency better than UC82. At long-term, nitrate reductase (SlNR) and nitrite reductase (SlNIR) expression were not significantly different between nitrate treatments in RO, while significantly down-regulated under nitrate limiting treatment in UC82. In addition, SlCLCa and SlNRT1.8 transcript levels were significantly lower in RO, while those of SlNRT1.5 and SlNR appeared significantly higher. This suggested that the efficient genotype stored less nitrate compared to UC82, which was allocated and assimilated to the shoot. More interestingly, the expression of SlNRT2.7 was significantly higher in RO shoot compared to UC82 and strongly correlated to RO higher growth as well as to NUE and NUtE component. Our findings underlined the differential regulation of N-metabolism genes that may confer to NUtE component a pivotal role in NUE enhancement in tomato.

Plant mineral transport systems and the potential for crop improvement

Nutrient transporter genes could be a potential candidate for improving crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. The world's food supply is nearing a crisis in meeting the demands of an ever-growing global population, and an increase in both yield and nutrient value of major crops is vitally necessary to meet the increased population demand. Nutrients play an important role in plant metabolism as well as growth and development, and nutrient deficiency results in retarded plant growth and leads to reduced crop yield. A variety of cellular processes govern crop plant nutrient absorption from the soil. Among these, nutrient membrane transporters play an important role in the acquisition of nutrients from soil and transport of these nutrients to their target sites. In addition, as excess nutrient delivery has toxic effects on plant growth, these membrane transporters also play a significant role in the removal of excess nutrients in the crop plant. The key function provided by membrane transporters is the ability to supply the crop plant with an adequate level of tolerance against environmental stresses, such as soil acidity, alkalinity, salinity, drought, and pathogen attack. Membrane transporter genes have been utilized for the improvement of crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. Further understanding of the basic mechanisms of nutrient transport in crop plants could facilitate the advanced design of engineered plant crops to achieve increased yield and improve nutrient quality through the use of genetic technologies as well as molecular breeding. This review is focused on nutrient toxicity and tolerance mechanisms in crop plants to aid in understanding and addressing the anticipated global food demand.

The Photosynthetic Bacterium Rhodopseudomonas palustris Strain PS3 Exerts Plant Growth-Promoting Effects by Stimulating Nitrogen Uptake and Elevating Auxin Levels in Expanding Leaves

Rhodopseudomonas palustris strain PS3, a phototrophic bacterium, was originally isolated from a paddy field located in Taipei city, Taiwan, and showed positive effects on the growth of leafy vegetables. The aim of this study was to clarify the mechanism of the beneficial effects exerted by PS3 on plants. An ineffective R. palustris strain, YSC3, isolated from a paddy field located in Yilan County, was used as the negative control for comparative analyses. We cultivated non-heading Chinese cabbage (Brassica rapa var. chinensis) in 1/2 strength Hoagland hydroponic solution, in which nitrate is the main nitrogen source. We evaluated various plant physiological responses to inoculation with different bacterial inoculants. The N use efficiency (NUE) of PS3-inoculated plants was dramatically higher than that of YSC3-inoculated plants. The nitrate uptake efficiency (NUpE) was significantly elevated in plants treated with PS3; however, no excess nitrate accumulation was observed in leaves. We also noticed that the endogenous indole-3-acetic acid (IAA) levels as well as the cell division rate in the leaves of PS3-inoculated plants were significantly higher than those in the leaves of YSC3-inoculated plants. We examined the bacterial transcription of some genes during root colonization, and found that the expression level of IAA synthesis related gene MAO was almost the same between these two strains. It suggests that the elevated endogenous IAA in the PS3-inoculated plants was not directly derived from the exogenous IAA produced by this bacterium. Taken together, we deduced that PS3 inoculation could promote plant growth by enhancing nitrate uptake and stimulating the accumulation of endogenous auxin in young expanding leaves to increase the proliferation of leaf cells during leaf development.

Improving coordination of plant growth and nitrogen metabolism for sustainable agriculture

The agricultural green revolution of the 1960s boosted cereal crop yield was in part due to cultivation of semi-dwarf green revolution varieties. The semi-dwarf plants resist lodging and require high nitrogen (N) fertilizer inputs to maximize yield. To produce higher grain yield, inorganic fertilizer has been overused by Chinese farmers in intensive crop production. With the ongoing increase in the food demand of global population and the environmental pollution, improving crop productivity with reduced N supply is a pressing challenge. Despite a great deal of research efforts, to date only a few genes that improve N use efficiency (NUE) have been identified. The molecular mechanisms underlying the coordination of plant growth, carbon (C) and N assimilation is still not fully understood, thus preventing significant improvement. Recent advances have shed light on how explore NUE within an overall plant biology system that considered the co-regulation of plant growth, C and N metabolisms as a whole, rather than focusing specifically on N uptake and assimilation. There are several potential approaches to improve NUE discussed in this review. Increasing knowledge of how plants sense and respond to changes in N availability, as well as identifying new targets for breeding strategies to simultaneously improve NUE and grain yield, could usher in a new green revolution.

The Role of Gibberellins in Regulation of Nitrogen Uptake and Physiological Traits in Maize Responding to Nitrogen Availability

Modified gibberellin (GA) signaling leads to semi-dwarfism with low nitrogen (N) use efficiency (NUE) in crops. An understanding of GA-mediated N uptake is essential for the development of crops with improved NUE. The function of GA in modulating N uptake capacity and nitrate (NO3-) transporters (NRTs) was analyzed in the GA synthesis-deficient mutant zmga3ox grown under low (LN) and sufficient (SN) N conditions. LN significantly suppressed the production of GA1, GA3, and GA4, and the zmga3ox plants showed more sensitivity in shoots as well as LN stress. Moreover, the higher anthocyanin accumulation and the decrease of chlorophyll content were also recorded. The net NO3- fluxes and 15N content were decreased in zmga3ox plants under both LN and SN conditions. Exogenous GA3 could restore the NO3- uptake in zmga3ox plants, but uniconazole repressed NO3- uptake. Moreover, the transcript levels of ZmNRT2.1/2.2 were downregulated in zmga3ox plants, while the GA3 application enhanced the expression level. Furthermore, the RNA-seq analyses identified several transcription factors that are involved in the GA-mediated transcriptional operation of NRTs related genes. These findings revealed that GAs influenced N uptake involved in the transcriptional regulation of NRTs and physiological responses in maize responding to nitrogen supply.

Adaptation of euhalophyte Suaeda salsa to nitrogen starvation under salinity

Suaeda salsa L. (S. salsa) is an annual euhalophyte with high salt tolerance. The NO₃^- content in soils where S. salsa populations occur are very low, especially in intertidal habitat. However, it remains unclear how S. salsa populations adapt to low nitrogen environments. Plants of two S. salsa populations were pre-cultured with nitrate nitrogen (1 mM of NO₃^--N) for 30 days. Then, the seedlings were cultured with 1 mM of NO₃^--N and N-free solution (N starvation) at 200 mM of NaCl for an additional 14 days. The expression of two genes in S. salsa, nitrate transporter 1.7 (SsNRT1.7) and nitrate transporter 2.5 (SsNRT2.5) in old and mature leaves, was markedly upregulated during N starvation in the intertidal population, when compared to the inland population, but this was not the case in young leaves. After N starvation, the decrease in NO₃^- and chlorophyll content, net photosynthetic rate in young leaves, and shoot dry weight in the intertidal population were lower than those in the inland population. In conclusion, SsNRT1.7 and SsNRT2.5 may play a role in NO₃^- remobilization, especially in the intertidal population, during N starvation. This trait may benefit the intertidal population for adapting to low nitrogen environments.

More Transporters, More Substrates: The Arabidopsis Major Facilitator Superfamily Revisited

The Major Facilitator Superfamily (MFS) is ubiquitous in living organisms and represents the largest group of secondary active membrane transporters. In plants, significant research efforts have focused on the role of specific families within the MFS, particularly those transporting macronutrients (C, N, and P) that constitute the vast majority of the members of this superfamily. Other MFS families remain less explored, although a plethora of additional substrates and physiological functions have been uncovered. Nevertheless, the lack of a systematic approach to analyzing the MFS as a whole has obscured the high diversity and versatility of these transporters. Here, we present a phylogenetic analysis of all annotated MFS domain-containing proteins encoded in the Arabidopsis thaliana genome and propose that this superfamily of transporters consists of 218 members, clustered in 22 families. In reviewing the available information regarding the diversity in biological functions and substrates of Arabidopsis MFS members, we provide arguments for intensified research on these membrane transporters to unveil the breadth of their physiological relevance, disclose the molecular mechanisms underlying their mode of action, and explore their biotechnological potential.

Trading on the arbuscular mycorrhiza market: from arbuscules to common mycorrhizal networks

Arbuscular mycorrhiza (AM) symbiosis occurs between obligate biotrophic fungi of the phylum Glomeromycota and most land plants. The exchange of nutrients between host plants and AM fungi (AMF) is presumed to be the main benefit for the two symbiotic partners. In this review article, we outline the current concepts of nutrient exchanges within this symbiosis (mechanisms and regulation). First, we focus on phosphorus and nitrogen transfer from the fungal partner to the host plant, and on the reciprocal transfer of carbon compounds, with a highlight on a possible interplay between nitrogen and phosphorus nutrition during AM symbiosis. We further discuss potential mechanisms of regulation of these nutrient exchanges linked to membrane dynamics. The review finally addresses the common mycorrhizal networks formed AMF, which interconnect plants from similar and/or different species. Finally the best way to integrate this knowledge and the ensuing potential benefits of AM into sustainable agriculture is discussed.

Physiological responses and small RNAs changes in maize under nitrogen deficiency and resupply

BACKGROUND

Maize is an important crop in the world, nitrogen stress severely reduces maize yield. Although a large number of studies have identified the expression changes of microRNAs (miRNAs) under N stress in several species, the miRNAs expression patterns of N-deficient plants under N resupply remain unclear.

OBJECTIVE

The primary objective of this study was to identify miRNAs in response to nitrogen stress and understand relevant physiological changes in nitrogen-deficient maize after nitrogen resupply.

METHODS

Physiological parameters were measured to study relevant physiological changes under different nitrogen conditions. Small RNA sequencing and qRT-PCR analysis were performed to understand the response of miRNAs under different nitrogen conditions.

RESULTS

The content of chlorophyll, soluble protein and nitrate nitrogen decreased than CK by 0.52, 0.49 and 0.82 times after N deficiency treatment and increased than ND by 0.52, 1.36 and 0.65 times after N resupply, respectively. Conversely, the activity of superoxide dismutase (SOD) and peroxidase (POD) increased by 0.67 and 1.64 times than CK after N deficiency, respectively, and decreased by 0.09 and 0.35 times than ND after N resupply. A total of 226 known miRNAs were identified by sRNA sequencing; 106 miRNAs were differentially expressed between the control and N-deficient groups, and 103 were differentially expressed between the N-deficient and N-resupply groups (P < 0.05). Real-time quantitative PCR (qPCR) was used to further validate and analyze the expression of the identified miRNAs. A total of 1609 target genes were identified by target prediction, and some differentially expressed miRNAs were predicted to target transcription factors and functional proteins. Gene Ontology (GO) analysis was used to determine the biological function of these targets and revealed that some miRNAs, such as miR169, miR1214, miR2199, miR398, miR408 and miR827 might be involved in nitrogen metabolism regulation.

CONCLUSION

Our study comprehensively provides important information on miRNA functions and molecular mechanisms in response to N stress. These findings may assist to improve nitrogen availability in plants.

Chemometric analysis of Amaranthus retroflexus in relation to livestock toxicity in southern Australia

Amaranthus retroflexus L., an introduced invasive weed in southern Australia, has been associated with acute renal failure and/or mortality in a number of livestock species. While its leaves, flowers and stems are generally reported to contain high levels of nitrogen, few studies have fully characterised the chemical composition of A. retroflexus foliage with respect to mammalian toxicity. We performed extensive metabolic profiling of stems, leaves, roots and inflorescence tissues of A. retroflexus collected from three spatially and/or temporally distinct toxicity outbreaks, and report on the 1) composition of primary and secondary metabolites in methanolic extracts of A. retroflexus tissues using HPLC and HPLC-MS QToF and 2) chemometric analysis of A. retroflexus extracts in relation to the associated toxin(s). All tissues of A. retroflexus possessed an abundance of N-containing metabolites, particularly quaternary ammonium compounds which were identified as betaines, two of which (valine betaine and isoleucine betaine) are rarely encountered in plants. Cytotoxicity to murine fibroblasts was highest in extracts of leaf tissue and was associated with a single, a small modified peptide with high similarity to N-acetyl-L-α-aspartyl-L-alanyl-L-α-aspartyl-L-α-glutamyl-O-(carboxymethyl)-L-tyrosyl-L-leucinamide, a synthetic phosphotyrosyl mimic involved in cell signaling processes. One possible mode of action leading to acute renal failure in grazing livestock by a modified peptide such as this is proposed.

Nitrate transporters: an overview in legumes

The nitrate transporters, belonging to NPF and NRT2 families, play critical roles in nitrate signaling, root growth and nodule development in legumes. Nitrate plays an essential role during plant development as nutrient and also as signal molecule, in both cases working via the activity of nitrate transporters. To date, few studies on NRT2 or NPF nitrate transporters in legumes have been reported, and most of those concern Lotus japonicus and Medicago truncatula. A molecular characterization led to the identification of 4 putative LjNRT2 and 37 putative LjNPF gene sequences in L. japonicus. In M. truncatula, the NRT2 family is composed of 3 putative members. Using the new genome annotation of M. truncatula (Mt4.0), we identified, for this review, 97 putative MtNPF sequences, including 32 new sequences relative to previous studies. Functional characterization has been published for only two MtNPF genes, encoding nitrate transporters of M. truncatula. Both transporters have a role in root system development via abscisic acid signaling: MtNPF6.8 acts as a nitrate sensor during the cell elongation of the primary root, while MtNPF1.7 contributes to the cellular organization of the root tip and nodule formation. An in silico expression study of MtNPF genes confirmed that NPF genes are expressed in nodules, as previously shown for L. japonicus, suggesting a role for the corresponding proteins in nitrate transport, or signal perception in nodules. This review summarizes our knowledge of legume nitrate transporters and discusses new roles for these proteins based on recent discoveries.

Post-translational regulation of nitrogen transporters in plants and microorganisms

For microorganisms and plants, nitrate and ammonium are the main nitrogen sources and they are also important signaling molecules controlling several aspects of metabolism and development. Over the past decade, numerous studies revealed that nitrogen transporters are strongly regulated at the transcriptional level. However, more and more reports are now showing that nitrate and ammonium transporters are also subjected to post-translational regulations in response to nitrogen availability. Phosphorylation is so far the most well studied post-translational modification for these transporters and it affects both the regulation of nitrogen uptake and nitrogen sensing. For example, in Arabidopsis thaliana, phosphorylation was shown to activate the sensing function of the root nitrate transporter NRT1.1 and to switch the transport affinity. Also, for ammonium transporters, a phosphorylation-dependent activation/inactivation mechanism was elucidated in recent years in both plants and microorganisms. However, despite the fact that these regulatory mechanisms are starting to be thoroughly described, the signaling pathways involved and their action on nitrogen transporters remain largely unknown. In this review, we highlight the inorganic nitrogen transporters regulated at the post-translational level and we compare the known mechanisms in plants and microorganisms. We then discuss how these mechanisms could contribute to the regulation of nitrogen uptake and/or nitrogen sensing.

Root-shoot interactions explain the reduction of leaf mineral content in Arabidopsis plants grown under elevated [CO2 ] conditions

Although shoot N depletion in plants exposed to elevated [CO2 ] has already been reported on several occasions, some uncertainty remains about the mechanisms involved. This study illustrates (1) the importance of characterizing root-shoot interactions and (2) the physiological, biochemical and gene expression mechanisms adopted by nitrate-fed Arabidopsis thaliana plants grown under elevated [CO2 ]. Elevated [CO2 ] increases biomass and photosynthetic rates; nevertheless, the decline in total soluble protein, Rubisco and leaf N concentrations revealed a general decrease in leaf N availability. A transcriptomic approach (conducted at the root and shoot level) revealed that exposure to 800 ppm [CO2 ] induced the expression of genes involved in the transport of nitrate and mineral elements. Leaf N and mineral status revealed that N assimilation into proteins was constrained under elevated [CO2 ]. Moreover, this study also highlights how elevated [CO2 ] induced the reorganization of nitrate assimilation between tissues; root nitrogen assimilation was favored over leaf assimilation to offset the decline in nitrogen metabolism in the leaves of plants exposed to elevated [CO2 ].

Large-Scale Public Transcriptomic Data Mining Reveals a Tight Connection between the Transport of Nitrogen and Other Transport Processes in Arabidopsis

Movement of nitrogen to the plant tissues where it is needed for growth is an important contribution to nitrogen use efficiency. However, we have very limited knowledge about the mechanisms of nitrogen transport. Loading of nitrogen into the xylem and/or phloem by transporter proteins is likely important, but there are several families of genes that encode transporters of nitrogenous molecules (collectively referred to as N transporters here), each comprised of many gene members. In this study, we leveraged publicly available microarray data of Arabidopsis to investigate the gene networks of N transporters to elucidate their possible biological roles. First, we showed that tissue-specificity of nitrogen (N) transporters was well reflected among the public microarray data. Then, we built coexpression networks of N transporters, which showed relationships between N transporters and particular aspects of plant metabolism, such as phenylpropanoid biosynthesis and carbohydrate metabolism. Furthermore, genes associated with several biological pathways were found to be tightly coexpressed with N transporters in different tissues. Our coexpression networks provide information at the systems-level that will serve as a resource for future investigation of nitrogen transport systems in plants, including candidate gene clusters that may work together in related biological roles.

NPKS uptake, sensing, and signaling and miRNAs in plant nutrient stress

Sessile nature of higher plants consequently makes it highly adaptable for nutrient absorption and acquisition from soil. Plants require 17 essential elements for their growth and development which include 14 minerals (macronutrients: N, P, K, Mg, Ca, S; micronutrients: Cl, Fe, B, Mn, Zn, Cu, Ni, Mo) and 3 non-mineral (C, H, O) elements. The roots of higher plants must acquire these macronutrients and micronutrients from rhizosphere and further allocate to other plant parts for completing their life cycle. Plants evolved an intricate series of signaling and sensing cascades to maintain nutrient homeostasis and to cope with nutrient stress/availability. The specific receptors for nutrients in root, root system architecture, and internal signaling pathways help to develop plasticity in response to the nutrient starvation. Nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) are essential for various metabolic processes, and their deficiency negatively effects the plant growth and yield. Genes coding for transporters and receptors for nutrients as well as some small non-coding RNAs have been implicated in nutrient uptake and signaling. This review summarizes the N, P, K, and S uptake, sensing and signaling events in nutrient stress condition especially in model plant Arabidopsis thaliana and involvement of microRNAs in nutrient deficiency. This article also provides a framework of uptake, sensing, signaling and to highlight the microRNA as an emerging major players in nutrient stress condition. Nutrient-plant-miRNA cross talk may help plant to cope up nutrient stress, and understanding their precise mechanism(s) will be necessary to develop high yielding smart crop with low nutrient input.

A Lotus japonicus mutant defective in nitrate uptake is also affected in the nitrate response to nodulation

A chlorate-resistant mutant (Ljclo1) of the model legume Lotus japonicus was identified that showed normal levels of nitrate reductase enzyme activity but had decreased uptake of nitrate, as determined from nitrate depletion and electrophysiological measurements. The data suggest that the mutant could be affected specifically in the low-affinity but not in the high-affinity nitrate transport system, and also showed decreased uptake of chlorate. Back-crosses of the mutant plant to the wild type indicated that it is affected in a single Mendelian recessive trait. Thus, the mutation produced in Ljclo1 may be related to some of the low-affinity nitrate transporters or to a regulatory mechanism associated with nitrate/chlorate uptake. Both size and chlorophyll content in young leaves of the mutant plants were significantly reduced compared to the wild type. In addition, nodulation performance of the mutant plants was similar to the wild type in the absence of any exogenous nitrate. However, the nodule:root biomass ratio in mutant plants was considerably reduced in the presence of 1-2 mm nitrate. The levels of several transcripts for nitrate transport and assimilation genes were determined for the wild type and mutant plants and were slightly different. The results suggest interdependence between nitrate uptake, plant growth and nodulation in Ljclo1 mutant plants.

Complex phylogeny and gene expression patterns of members of the NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER family (NPF) in wheat

NPF (formerly referred to as low-affinity NRT1) and 'high-affinity' NRT2 nitrate transporter genes are involved in nitrate uptake by the root, and transport and distribution of nitrate within the plant. The NPF gene family consists of 53 members in Arabidopsis thaliana, however only 11 of these have been functionally characterized. Although homologous genes have been identified in genomes of different plant species including some cereals, there is little information available for wheat (Triticum aestivum). Sixteen genes were identified in wheat homologous to characterized Arabidopsis low-affinity nitrate transporter NPF genes, suggesting a complex wheat NPF gene family. The regulation of wheat NFP genes by plant N-status indicated involvement of these transporters in substrate transport in relation to N-metabolism. The complex expression pattern in relation to tissue specificity, nitrate availability and senescence may be associated with the complex growth patterns of wheat depending on sink/source demands, as well as remobilization during grain filling.

Isolation and functional characterization of a high affinity urea transporter from roots of Zea mays

BACKGROUND

Despite its extensive use as a nitrogen fertilizer, the role of urea as a directly accessible nitrogen source for crop plants is still poorly understood. So far, the physiological and molecular aspects of urea acquisition have been investigated only in few plant species highlighting the importance of a high-affinity transport system. With respect to maize, a worldwide-cultivated crop requiring high amounts of nitrogen fertilizer, the mechanisms involved in the transport of urea have not yet been identified. The aim of the present work was to characterize the high-affinity urea transport system in maize roots and to identify the high affinity urea transporter.

RESULTS

Kinetic characterization of urea uptake (<300 μM) demonstrated the presence in maize roots of a high-affinity and saturable transport system; this system is inducible by urea itself showing higher Vmax and Km upon induction. At molecular level, the ORF sequence coding for the urea transporter, ZmDUR3, was isolated and functionally characterized using different heterologous systems: a dur3 yeast mutant strain, tobacco protoplasts and a dur3 Arabidopsis mutant. The expression of the isolated sequence, ZmDUR3-ORF, in dur3 yeast mutant demonstrated the ability of the encoded protein to mediate urea uptake into cells. The subcellular targeting of DUR3/GFP fusion proteins in tobacco protoplasts gave results comparable to the localization of the orthologous transporters of Arabidopsis and rice, suggesting a partial localization at the plasma membrane. Moreover, the overexpression of ZmDUR3 in the atdur3-3 Arabidopsis mutant showed to complement the phenotype, since different ZmDUR3-overexpressing lines showed either comparable or enhanced 15[N]-urea influx than wild-type plants. These data provide a clear evidence in planta for a role of ZmDUR3 in urea acquisition from an extra-radical solution.

CONCLUSIONS

This work highlights the capability of maize plants to take up urea via an inducible and high-affinity transport system. ZmDUR3 is a high-affinity urea transporter mediating the uptake of this molecule into roots. Data may provide a key to better understand the mechanisms involved in urea acquisition and contribute to deepen the knowledge on the overall nitrogen-use efficiency in crop plants.

New insight into the strategy for nitrogen metabolism in plant cells

Nitrogen (N) is one of the most important mineral nutrients required by higher plants. Primary N absorbed by higher plants includes nitrate (NO3(-)), ammonium (NH4(+)), and organic N. Plants have developed several mechanisms for regulating their N metabolism in response to N availability and environmental conditions. Numerous transporters have been characterized and the mode of N movement within plants has been demonstrated. For further assimilation of N, various enzymes are involved in the key processes of NO3(-) or NH4(+) assimilation. N and carbon (C) metabolism are tightly coordinated in the fundamental biochemical pathway that permits plant growth. As N and C metabolism are the fundamental constituents of plant life, understanding N regulation is essential for growing plants and improving crop production. Regulation of N metabolism at the transcriptional and posttranscriptional levels provides important perceptions in the complex regulatory network of plants to adapt to changing N availability. In this chapter, recent advances in elucidating molecular mechanisms of N metabolism processes and regulation strategy, as well as interactions between C and N, are discussed. This review provides new insights into the strategy for studying N metabolism at the cellular level for optimum plant growth in different environments.

Nitrogen transporter and assimilation genes exhibit developmental stage-selective expression in maize (Zea mays L.) associated with distinct cis-acting promoter motifs

Nitrogen is considered the most limiting nutrient for maize (Zea mays L.), but there is limited understanding of the regulation of nitrogen-related genes during maize development. An Affymetrix 82K maize array was used to analyze the expression of ≤ 46 unique nitrogen uptake and assimilation probes in 50 maize tissues from seedling emergence to 31 d after pollination. Four nitrogen-related expression clusters were identified in roots and shoots corresponding to, or overlapping, juvenile, adult, and reproductive phases of development. Quantitative real time PCR data was consistent with the existence of these distinct expression clusters. Promoters corresponding to each cluster were screened for over-represented cis-acting elements. The 8-bp distal motif of the Arabidopsis 43-bp nitrogen response element (NRE) was over-represented in nitrogen-related maize gene promoters. This conserved motif, referred to here as NRE43-d8, was previously shown to be critical for nitrate-activated transcription of nitrate reductase (NIA1) and nitrite reductase (NIR1) by the NIN-LIKE PROTEIN 6 (NLP6) in Arabidopsis. Here, NRE43-d8 was over-represented in the promoters of maize nitrate and ammonium transporter genes, specifically those that showed peak expression during early-stage vegetative development. This result predicts an expansion of the NRE-NLP6 regulon and suggests that it may have a developmental component in maize. We also report leaf expression of putative orthologs of nitrite transporters (NiTR1), a transporter not previously reported in maize. We conclude by discussing how each of the four transcriptional modules may be responsible for the different nitrogen uptake and assimilation requirements of leaves and roots at different stages of maize development.

The effects of rising atmospheric carbon dioxide on shoot-root nitrogen and water signaling

Terrestrial higher plants are composed of roots and shoots, distinct organs that conduct complementary functions in dissimilar environments. For example, roots are responsible for acquiring water and nutrients such as inorganic nitrogen from the soil, yet shoots consume the majority of these resources. The success of such a relationship depends on excellent root-shoot communications. Increased net photosynthesis and decreased shoot nitrogen and water use at elevated CO2 fundamentally alter these source-sink relations. Lower than predicted productivity gains at elevated CO2 under nitrogen or water stress may indicate shoot-root signaling lacks plasticity to respond to rising atmospheric CO2 concentrations. The following presents recent research results on shoot-root nitrogen and water signaling, emphasizing the influence that rising atmospheric carbon dioxide levels are having on these source-sink interactions.

[Plant adaptation to nitrogen availability]

Nitrogen is an essential macronutrient for plant development and productivity. The adaptation toward changes in nitrogen availability in the soil is crucial for the immobile plant. Nitrate is the primary nitrogen source in temperate climate. Nitrate transport and assimilation are discussed with emphasis on the adaptation to nitrogen starvation. The integration of nitrogen metabolism with primary and secondary metabolism and the homeostasis with other nutrients are discussed. However, nitrate is not only a nutrient, but also a signaling molecule acting on multiple levels. The molecular players involved in the regulatory network are discussed.

Roles and functions of plant mineral nutrients

Plants require macro- and micronutrients, each of which is essential for a plant to complete its life cycle. Adequate provision of nutrients impacts greatly on plant growth and as such is of crucial importance in the context of agriculture. Minerals are taken up by plant roots from the soil solution in ionic form which is mediated by specific transport proteins. Recently, important progress has been achieved in identifying transport and regulatory mechanisms for the uptake and distribution of nutrients. This and the main physiological roles of each nutrient will be discussed in this chapter.

Uptake, allocation and signaling of nitrate

Plants need to acquire nitrogen (N) efficiently from the soil for growth. Nitrate is one of the major N sources for higher plants. Therefore, nitrate uptake and allocation are key factors in efficient N utilization. Membrane-bound transporters are required for nitrate uptake from the soil and for the inter- and intracellular movement of nitrate inside the plants. Four gene families, nitrate transporter 1/peptide transporter (NRT1/PTR), NRT2, chloride channel (CLC), and slow anion channel-associated 1 homolog 3 (SLAC1/SLAH), are involved in nitrate uptake, allocation, and storage in higher plants. Recent studies of these transporters or channels have provided new insights into the molecular mechanisms of nitrate uptake and allocation. Interestingly, several of these transporters also play versatile roles in nitrate sensing, plant development, pathogen defense, and/or stress response.

Nitrate transport in cucumber leaves is an inducible process involving an increase in plasma membrane H⁺-ATPase activity and abundance

BACKGROUND

The mechanisms by which nitrate is transported into the roots have been characterized both at physiological and molecular levels. It has been demonstrated that nitrate is taken up in an energy-dependent way by a four-component uptake machinery involving high- and low- affinity transport systems. In contrast very little is known about the physiology of nitrate transport towards different plant tissues and in particular at the leaf level.

RESULTS

The mechanism of nitrate uptake in leaves of cucumber (Cucumis sativus L. cv. Chinese long) plants was studied and compared with that of the root. Net nitrate uptake by roots of nitrate-depleted cucumber plants proved to be substrate-inducible and biphasic showing a saturable kinetics with a clear linear non saturable component at an anion concentration higher than 2 mM. Nitrate uptake by leaf discs of cucumber plants showed some similarities with that operating in the roots (e.g. electrogenic H+ dependence via involvement of proton pump, a certain degree of induction). However, it did not exhibit typical biphasic kinetics and was characterized by a higher Km with values out of the range usually recorded in roots of several different plant species. The quantity and activity of plasma membrane (PM) H+-ATPase of the vesicles isolated from leaf tissues of nitrate-treated plants for 12 h (peak of nitrate foliar uptake rate) increased with respect to that observed in the vesicles isolated from N-deprived control plants, thus suggesting an involvement of this enzyme in the leaf nitrate uptake process similar to that described in roots. Molecular analyses suggest the involvement of a specific isoform of PM H+-ATPase (CsHA1) and NRT2 transporter (CsNRT2) in root nitrate uptake. At the leaf level, nitrate treatment modulated the expression of CsHA2, highlighting a main putative role of this isogene in the process.

CONCLUSIONS

Obtained results provide for the first time evidence that a saturable and substrate-inducible nitrate uptake mechanism operates in cucumber leaves. Its activity appears to be related to that of PM H+-ATPase activity and in particular to the induction of CsHA2 isoform. However the question about the molecular entity responsible for the transport of nitrate into leaf cells therefore still remains unresolved.

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.

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.

Towards the profiling of the Arabidopsis thaliana plasma membrane transportome by targeted proteomics

Plant membranes bear a variety of transporters belonging to multigene families that are affected by environmental and nutritional conditions. In addition, they often display high-sequence identity, making difficult in-depth investigation by current shot-gun strategies. In this study, we set up a targeted proteomics approach aimed at identifying and quantifying within single experiments the five major proton pumps of the autoinhibited H(+) ATPases (AHA) family, the 13 plasma membrane intrinsic proteins (PIP) water channels (PIPs), and ten members of ammonium transporters (AMTs) and nitrate transporter (NRT) families. Proteotypic peptides were selected and isotopically labeled heavy versions were used for technical optimization and for quantification of the corresponding light version in biological samples. This approach allowed to quantify simultaneously nine PIPs in leaf membranes and 13 PIPs together with three autoinhibited H(+) ATPases, two ammonium transporters, and two NRTs in root membranes. Similarly, it was used to investigate the effect of a salt stress on the expression of these latter 20 transporters in roots. These novel isoform-specific data were compared with published transcriptome information and revealed a close correlation between PIP isoforms and transcripts levels. The obtained resource is reusable and can be expanded to other transporter families for large-scale profiling of membrane transporters.

The Arabidopsis nitrate transporter NRT1.8 functions in nitrate removal from the xylem sap and mediates cadmium tolerance

Long-distance transport of nitrate requires xylem loading and unloading, a successive process that determines nitrate distribution and subsequent assimilation efficiency. Here, we report the functional characterization of NRT1.8, a member of the nitrate transporter (NRT1) family in Arabidopsis thaliana. NRT1.8 is upregulated by nitrate. Histochemical analysis using promoter-beta-glucuronidase fusions, as well as in situ hybridization, showed that NRT1.8 is expressed predominantly in xylem parenchyma cells within the vasculature. Transient expression of the NRT1.8:enhanced green fluorescent protein fusion in onion epidermal cells and Arabidopsis protoplasts indicated that NRT1.8 is plasma membrane localized. Electrophysiological and nitrate uptake analyses using Xenopus laevis oocytes showed that NRT1.8 mediates low-affinity nitrate uptake. Functional disruption of NRT1.8 significantly increased the nitrate concentration in xylem sap. These data together suggest that NRT1.8 functions to remove nitrate from xylem vessels. Interestingly, NRT1.8 was the only nitrate assimilatory pathway gene that was strongly upregulated by cadmium (Cd(2+)) stress in roots, and the nrt1.8-1 mutant showed a nitrate-dependent Cd(2+)-sensitive phenotype. Further analyses showed that Cd(2+) stress increases the proportion of nitrate allocated to wild-type roots compared with the nrt1.8-1 mutant. These data suggest that NRT1.8-regulated nitrate distribution plays an important role in Cd(2+) tolerance.