Evidence for cotransport of nitrate and protons in maize roots : I. Effects of nitrate on the membrane potential

Exogenous auxin alters the polycyclic aromatic hydrocarbons apoplastic and symplastic uptake by wheat seedling roots

Polycyclic aromatic hydrocarbons (PAHs) are a category of organic pollutants known for their high carcinogenicity. Our previous research has illustrated that plant roots actively absorb PAHs through a co-transport mechanism with H+ ions. Because auxin can increase the H+-ATPase activity, the wheat roots were exposed to PAHs with/without auxins to study whether auxins facilitate the uptake of PAHs by plant roots and to gain insights into the underlying mechanisms of this process. In our study, indole acetic acid (100 μM) and α-naphthaleneacetic acid (10 μM) significantly increased the PAHs concentrations in apoplast and symplast, and the treating time and concentrations were positively correlated with PAHs accumulations. The time-dependent kinetics for 36 h followed the Elovich equation, and the concentration-dependent kinetics of apoplastic and symplastic uptake for 4 h could be described with the Freundlich and Michaelis-Menten equations, respectively. The proportion of PAHs accumulated in apoplast could be enhanced by auxins in most treatments. Our findings offer novel insights into the mechanisms of PAH uptake by plant roots under auxin exposure. Additionally, this research aids in refining strategies for ensuring crop safety and improving phytoremediation of PAH-contaminated soil and water.

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.

Copper toxicity compromises root acquisition of nitrate in the high affinity range

The application of copper (Cu)-based fungicides for crop protection plans has led to a high accumulation of Cu in soils, especially in vineyards. Copper is indeed an essential micronutrient for plants, but relatively high concentrations in soil or other growth substrates may cause toxicity phenomena, such as alteration of the plant's growth and disturbance in the acquisition of mineral nutrients. This last aspect might be particularly relevant in the case of nitrate ( NO 3 - ) , whose acquisition in plants is finely regulated through the transcriptional regulation of NO 3 - transporters and plasma membrane H⁺-ATPase in response to the available concentration of the nutrient. In this study, cucumber plants were grown hydroponically and exposed to increasing concentrations of Cu (i.e., 0.2, 5, 20, 30, and 50 µM) to investigate their ability to respond to and acquire NO 3 - . To this end, the kinetics of substrate uptake and the transcriptional modulation of the molecular entities involved in the process have been assessed. Results showed that the inducibility of the high-affinity transport system was significantly affected by increasing Cu concentrations; at Cu levels higher than 20 µM, plants demonstrated either strongly reduced or abolished NO 3 - uptake activity. Nevertheless, the transcriptional modulation of both the nitrate transporter CsNRT2.1 and the accessory protein CsNRT3.1 was not coherent with the hindered NO 3 - uptake activity. On the contrary, CsHA2 was downregulated, thus suggesting that a possible impairment in the generation of the proton gradient across the root PM could be the cause of the abolishment of NO 3 - uptake.

Nitrate transporter MdNRT2.4 interacts with rhizosphere bacteria to enhance nitrate uptake in apple rootstocks

Plants have developed complex mechanisms to adapt to changing nitrate (NO3-) concentrations and can recruit microbes to boost nitrogen absorption. However, little is known about the relationship between functional genes and the rhizosphere microbiome in NO3- uptake of apple rootstocks. Here, we found that variation in Malus domestica NO3- transporter (MdNRT2.4) expression contributes to nitrate uptake divergence between two apple rootstocks. Overexpression of MdNRT2.4 in apple seedlings significantly improved tolerance to low nitrogen via increasing net NO3- influx at the root surface. However, inhibiting the root plasma membrane H+-ATPase activity abolished NO3- uptake and led to NO3- release, suggesting that MdNRT2.4 encodes an H+-coupled nitrate transporter. Surprisingly, the nitrogen concentration of MdNRT2.4-overexpressing apple seedlings in unsterilized nitrogen-poor soil was higher than that in sterilized nitrogen-poor soil. Using 16S ribosomal RNA gene profiling to characterize the rhizosphere microbiota, we found that MdNRT2.4-overexpressing apple seedlings recruited more bacterial taxa with nitrogen metabolic functions, especially Rhizobiaceae. We isolated a bacterial isolate ARR11 from the apple rhizosphere soil and identified it as Rhizobium. Inoculation with ARR11 improved apple seedling growth in nitrogen-poor soils, compared with uninoculated seedlings. Together, our results highlight the interaction of host plant genes with the rhizosphere microbiota for host plant nutrient uptake.

Plasma Promotes Fungal Cellulase Production by Regulating the Levels of Intracellular NO and Ca2

For the industrial-scale production of useful enzymes by microorganisms, technological development is required for overcoming a technical bottleneck represented by poor efficiency in the induction of enzyme gene expression and secretion. In this study, we evaluated the potential of a non-thermal atmospheric pressure plasma jet to improve the production efficiency of cellulolytic enzymes in Neurospora crassa, a filamentous fungus. The total activity of cellulolytic enzymes and protein concentration were significantly increased (1.1~1.2 times) in media containing Avicel 24-72 h after 2 and 5 min of plasma treatment. The mRNA levels of four cellulolytic enzymes in fungal hyphae grown in media with Avicel were significantly increased (1.3~17 times) 2-4 h after a 5 min of plasma treatment. The levels of intracellular NO and Ca2+ were increased in plasma-treated fungal hyphae grown in Avicel media after 48 h, and the removal of intracellular NO decreased the activity of cellulolytic enzymes in media and the level of vesicles in fungal hyphae. Our data suggest that plasma treatment can promote the transcription and secretion of cellulolytic enzymes into the culture media in the presence of Avicel (induction condition) by enhancing the intracellular level of NO and Ca2+.

Identification and characterization of apple MdNLP7 transcription factor in the nitrate response

Nitrogen is an essential nutrient for plant growth and development. Low utilization of nitrogen fertilizer during agricultural production causes a series of environmental problems, such as water eutrophication, soil acidity, and air pollution. Investigating the patterns and mechanisms of crop NO₃^- absorption and utilization therefore key to fully improving crop nitrogen utilization rates and promoting sustainable agricultural development. Apple is one of the most important horticultural crops in the world. Its nitrogen demand by apple during the growth period is very high, but few studies have been performed on apple genes, that regulate the NO₃^- response. Here, we found that the apple transcription factor MdNLP7 promoted nitrogen absorption and assimilation by activating the expression of MdNIA2 and MdNRT1.1. MdNLP7 also regulated H₂O₂ content by increasing catalase activity, which may also influence nitrate utilization. Our findings provide insight into the mechanisms by which MdNLP7 controls nitrate utilization in apple.

Rhizosphere pH and cation-anion balance determine the exudation of nitrification inhibitor 3-epi-brachialactone suggesting release via secondary transport

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH₄ ⁺ nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH₄ ⁺ by K⁺ revealed that the NH₄ ⁺ effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH₄ ⁺ uptake (R² = 0.92 for high ~6.8 and R² = 0.84 for low ~4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH₄ ⁺ availability, but merely as the result of associated changes in ΔP.

Nitrate supply decreases uptake and accumulation of ciprofloxacin in Brassica parachinensis

How nitrate (NO₃^-) fertilization influences ciprofloxacin (CIP) uptake by crops remains unsolved. Here, two Brassica parachinensis cultivars differing in CIP accumulation were cultivated to investigate the effects of NO₃^- supply on CIP uptake and the underlying mechanism. The results showed that NO₃^- supply effectively reduced CIP toxicity and accumulation in the two cultivars, especially in the low CIP cultivar. Moreover, NO₃^- supply induced different mechanisms of coping with CIP stress in the two cultivars through influencing subcellular distribution of CIP. The uptake of CIP by root was demonstrated to be a carrier-mediated, energy-consuming, and proton motive force-dependent influx process. Consequently, a mechanism of nitrate supply decreasing CIP uptake was proposed that uptake of CIP and NO₃- into root cell would compete for the proton motive force and share a common energy source provided by plasma membrane H⁺-ATPase. Besides, regulating the concentration balances of cytoplasmic NO₃- and proton by inhibiting the activities of NRase and two tonoplast proton pumps (V-ATPase and V-PPase) led to opposite effect on CIP uptake, further supporting this inference. Our results provide a novel insight into CIP uptake by plant roots, and improve the strategy of minimizing CIP accumulation in crops for food safety by fertilization management.

The expression patterns and putative function of nitrate transporter 2.5 in plants

Nitrate transporter 2.5 (NRT2.5) was originally characterized as the transporter for nitrogen (N) limitation. In Arabidopsis, NRT2.5 is expressed mainly under extremely low NO₃^- and N starvation conditions, and this must work in conjunction with NAR2.1. NRT2.5 is expressed both in the roots and leaves in Arabidopsis, poplars, tea trees and cassava. This is also expressed in the seeds of Arabidopsis and wheat. In wheat, NRT2.5 is expressed in the embryo and shell and plays a role in the accumulation of NO₃^- in the seeds. In maize, this is also expressed in silk, cobs and tassel husk leaves. In rice, OsNRT2.5 (also known as OsNRT2.3a) may help the species to remove NO₃^- from the roots to shoots. In addition, NRT2.5 may interact with TGA3, MYC1, LBD37, LBD38, TaNAC2 and other transcription factors and participate in the transmission of NO₃^- signals. The present review summarizes the functions of NRT2.5 in different plant species, which may help plant breeders and molecular biologists to improve crop yield. Abbreviations: NRT, Nitrate transporter; NUE, nitrogen use efficiency; PTR, peptide transporter; NPF, nitrate peptide transporter family; CLC, chloride channel; LAC1/SLAH, slow anion channel-associated 1 homolog 3; LATS, low-affinity transporter systems; HATS, high-affinity transport systems; NNP, nitrate-nitrite-porter; MFS, major facilitator superfamily.

Plant nitrogen uptake and assimilation: regulation of cellular pH homeostasis

The enzymatic controlled metabolic processes in cells occur at their optimized pH ranges, therefore cellular pH homeostasis is fundamental for life. In plants, the nitrogen (N) source for uptake and assimilation, mainly in the forms of nitrate (NO3-) and ammonium (NH4+) quantitatively dominates the anion and cation equilibrium and the pH balance in cells. Here we review ionic and pH homeostasis in plant cells and regulation by N source from the rhizosphere to extra- and intracellular pH regulation for short- and long-distance N distribution and during N assimilation. In the process of N transport across membranes for uptake and compartmentation, both proton pumps and proton-coupled N transporters are essential, and their proton-binding sites may sense changes of apoplastic or intracellular pH. In addition, during N assimilation, carbon skeletons are required to synthesize amino acids, thus the combination of NO3- or NH4+ transport and assimilation results in different net charge and numbers of protons in plant cells. Efficient maintenance of N-controlled cellular pH homeostasis may improve N uptake and use efficiency, as well as enhance the resistance to abiotic stresses.

Regulatory mechanisms of nitrogen (N) on cadmium (Cd) uptake and accumulation in plants: A review

Cadmium (Cd) is a heavy metal that is toxic to plants and animals. Nitrogen (N), the most significant macro-nutrient and a common input for crop production, is often excessively applied than plants' demands by farmers to obtain more economic benefits. Understanding the regulatory mechanisms of N that control Cd uptake, translocation, and accumulation may enable the development of solutions regarding Cd pollution in the trophic chain, a major and global threat to agricultural sustainability and human health. In this review, we clarified that an increased amount of N, regardless of its form, enhances Cd uptake, translocation, and accumulation in plants, and nitrate promotes Cd uptake more than any other N form. We also described that N fertilizer alters the Cd exchange capacity and the bio-available Cd content in soil; regulates nitric oxide induced divalent cation gene expression of Nramp1, HMA2, and IRT1; and changes cell wall isolation, chelation capacity, and oxidative resistance to regulate Cd accumulation in plants. By revealing the integrated interaction effects between Cd accumulation and N fertiliser use, we propose new challenges to investigate the functions and mechanisms of N in Cd-contaminated croplands and develop suitable N-fertilisation protocols to practically reduce food health risks in agricultural food production.

Does nitrogen source influence cadmium distribution in Arabidopsis plants?

The purpose of the present work was to study the effect of the nitrogen source (NO₃^- vs NH₄⁺) on cadmium (Cd) uptake, translocation and partition and its associated toxicity in hydroponically-grown Arabidopsis plants. After a short growth period on a complete Hoagland nutrient solution, Arabidopsis seedlings continued in the same growth medium (NA) or were switched to NO₃^- (N) or NH₄⁺ (A) as sole N sources and supplied with 2.5 μM Cd. Unrelated to the nitrogen source, Cd reached higher levels in roots than in leaves. However, when ammonium was the source of nitrogen, Cd accumulation in roots was lower than in N or NA medium and the metal translocation to the aerial part was restricted, reaching values 25%-35% below the levels observed in plants grown with N or NA. Cadmium negatively affected chlorophyll content and PSII quantum yield, independently of the nitrogen source, with the highest decrease (35%) under NA treatment. Proline content increased, either with NA, N or A supplied in the presence of Cd, whereas a rise in total anthocyanin content was clearly favored when ammonium was the source of nitrogen, with or without Cd. In leaves, while NIA1 and NIA2 expression was markedly reduced by Cd in the presence of N or NA, ammonium source slightly reduced NIA1 expression but greatly upregulated NIA2 expression upon Cd exposure. The decay in NR activity was independent of the nitrogen source when Cd was applied and this decay was accompanied by a great increase in NH₄⁺ levels either with nitrates or ammonium in the medium in the presence of Cd. Only NIA1 was detected in roots and its expression, together with NR activity and nitrates levels, was the highest in N medium devoid of Cd. The possibility of reducing Cd health risks through nitrogen fertilization practices is discussed.

Biochemical pH clamp: the forgotten resource in membrane bioenergetics

Solute uptake and release by plant cells are frequently energized by coupling to H⁺ influx supported by the proton motive force (pmf). The pmf results from a stable pH difference between the apoplast and the cytosol, with bulk values ranging from 4.9 to 5.8 and from 7.1 to 7.5, respectively, in combination with a negative electrical membrane potential. The P-type H⁺ ATPases pumping H⁺ from the cytosol into the apoplast at the expense of ATP hydrolysis are generally viewed as the only pmf source, exclusively linking membrane transport to energy metabolism. However, recent evidence suggests that pump activity may be insufficient to energize transport, particularly under stress conditions. Indeed, cytosolic H⁺ scavenging and apoplastic H⁺ generation by metabolism (denoted as 'active' buffering in contrast to the readily exhausted 'passive' matrix buffering) also stabilize the pH gradient. In the cytosol, H⁺ scavenging is mainly associated with malate decarboxylation catalyzed by malic enzyme, and via the GABA shunt of the tricarboxylic acid (TCA) cycle involving glutamate decarboxylation. In the apoplast, formation of bicarbonate from CO₂ , the end-product of respiration, generates H⁺ at pH ≥ 6. Membrane potential is stabilized by K⁺ release and/or by anion uptake via ion channels. Finally, thermodynamic aspects of active buffering are discussed.

Effects of antibiotics on nitrogen uptake of four wetland plant species grown under hydroponic culture

To investigate the effects of antibiotics on nitrogen removal and uptake by wetland plants, four typical macrophyte species, Cyperus alternifolius L., Typha angustifolia L., Lythrum salicaria L., and Acorus calamus L., were grown in hydroponic cultivation systems and fed wastewater polluted with 10 μg L^-1 Ofloxacin (OFL) and Tetracycline (TET). Biomass production, nitrogen mass concentration, chlorophyll content, root exudates, and nitrogen removal efficiency of hydroponic cultivation were investigated. The results indicated that in all hydroponic systems, NH₄⁺-N was entirely removed from the hydroponic substrate within 1 day and plant nitrogen accumulation was the main role of the removed NO₃^-. OFL and TET stimulated the accumulation of biomass and nitrogen of A. calamus but significantly inhibited the NO₃^--N removal ability of L. salicaria (98.6 to 76.2%) and T. augustifolia (84.3 to 40.2%). This indicates that A. calamus may be a good choice for nitrogen uptake in wetlands contaminated with antibiotics. OFL and TET improved the concentrations of total organic carbon (TOC), total nitrogen (TN), organic acid, and soluble sugars in root exudates, especially for oxalic acid. Considering the significant correlation between TOC of root exudates and nitrogen removal efficiency, the TOC of root exudates may be an important index for choosing macrophytes to maintain nitrogen removal ability in wetlands contaminated with antibiotics.

Azospirillum brasilense inoculation counteracts the induction of nitrate uptake in maize plants

Nitrogen (N) represents one of the limiting factors for crop growth and productivity and to date has been widely supplied via external application of fertilizers. However, the use of plant growth-promoting rhizobacteria (PGPR) might represent a valuable tool to further improve plant nutrition. This study examines the influence of Azospirillum brasilense strain Cd on nitrate uptake in maize (Zea mays) plants, focusing on the high-affinity transport system (HATS). Plants were induced with nitrate (500 µM) and either inoculated or not with Azospirillum. Inoculation decreased the nitrate uptake rate in induced plants, suggesting that Azospirillum may negatively affect HATS in the short term. The expression dynamics of ZmNF-YA and ZmLBD37 suggested that Azospirillum affected the N balance in the plants, most probably by supplying them with reduced N, i.e. NH4+. This was further corroborated by measurements of total N and the expression of ammonium transporter genes. Overall, our data demonstrate that Azospirillum can counteract the plant response to nitrate induction, albeit without compromising N nutrition. This suggests that the agricultural application of microbial inoculants requires fine-tuning of external fertilizer inputs.

Coumarin enhances nitrate uptake in maize roots through modulation of plasma membrane H+ -ATPase activity

Coumarin is one of the simplest plant secondary metabolites, widely distributed in the plant kingdom, affecting root form and function, including anatomy, morphology and nutrient uptake. Although, some plant responses to coumarin have been described, comprehensive knowledge of the physiological and molecular mechanisms is lacking. Maize seedlings exposed to different coumarin concentrations, alone or in combination with 200 μm nitrate (NO₃^- ), were analysed, through a physiological and molecular approach, to elucidate action of coumarin on net NO₃^- uptake rate (NNUR). In detail, the time course of NNUR, plasma membrane (PM) H⁺ -ATPase activity, proton pumping and related gene expression (ZmNPF6.3, ZmNRT2.1, ZmNAR2.1, ZmHA3 and ZmHA4) were evaluated. Coumarin alone did not affect nitrate uptake, PM H⁺ -ATPase activity or transcript levels of ZmNRT2.1 and ZmHA3. In contrast, coumarin alone increased ZmNPF6.3, ZmNAR2.1 and ZmHA4 expression in response to abiotic stress. When coumarin and NO₃^- were concurrently added to the nutrient solution, a significant increase in the NNUR, PM H⁺ -ATPase activity, together with ZmNAR2.1:ZmNRT2.1 and ZmHA4 expression was observed, suggesting that coumarin affected the inducible component of the high affinity transport system (iHATS), and this effect appeared to be mediated by nitrate. Moreover, results with vanadate, an inhibitor of the PM H⁺ -ATPase, suggested that this enzyme could be the main target of coumarin. Surprisingly, coumarin did not affect PM H⁺ -ATPase activity by direct contact with plasma membrane vesicles isolated from maize roots, indicating its possible elicitor role in gene transcription.

The Integration of Electrical Signals Originating in the Root of Vascular Plants

Plants have developed different signaling systems allowing for the integration of environmental cues to coordinate molecular processes associated to both early development and the physiology of the adult plant. Research on systemic signaling in plants has traditionally focused on the role of phytohormones as long-distance signaling molecules, and more recently the importance of peptides and miRNAs in building up this communication process has also been described. However, it is well-known that plants have the ability to generate different types of long-range electrical signals in response to different stimuli such as light, temperature variations, wounding, salt stress, or gravitropic stimulation. Presently, it is unclear whether short or long-distance electrical communication in plants is linked to nutrient uptake. This review deals with aspects of sensory input in plant roots and the propagation of discrete signals to the plant body. We discuss the physiological role of electrical signaling in nutrient uptake and how nutrient variations may become an electrical signal propagating along the plant.

Time-Resolved Investigation of Molecular Components Involved in the Induction of [Formula: see text] High Affinity Transport System in Maize Roots

The induction, i.e., the rapid increase of nitrate ([Formula: see text]) uptake following the exposure of roots to the anion, was studied integrating physiological and molecular levels in maize roots. Responses to [Formula: see text] treatment were characterized in terms of changes in [Formula: see text] uptake rate and plasma membrane (PM) H+-ATPase activity and related to transcriptional and protein profiles of NRT2, NRT3, and PM H+-ATPase gene families. The behavior of transcripts and proteins of ZmNRT2s and ZmNRT3s suggested that the regulation of the activity of inducible high-affinity transport system (iHATS) is mainly based on the transcriptional/translational modulation of the accessory protein ZmNRT3.1A. Furthermore, ZmNRT2.1 and ZmNRT3.1A appear to be associated in a ∼150 kDa oligomer. The expression trend during the induction of the 11 identified PM H+-ATPase transcripts indicates that those mainly involved in the response to [Formula: see text] treatment are ZmHA2 and ZmHA4. Yet, partial correlation between the gene expression, protein levels and enzyme activity suggests an involvement of post-transcriptional and post-translational mechanisms of regulation. A non-denaturing Deriphat-PAGE approach allowed demonstrating for the first time that PM H+-ATPase can occur in vivo as hexameric complex together with the already described monomeric and dimeric forms.

Cadmium accumulation is enhanced by ammonium compared to nitrate in two hyperaccumulators, without affecting speciation

Nitrogen fertilization could improve the efficiency of Cd phytoextraction in contaminated soil and thus shorten the remediation time. However, limited information is available on the effect of N form on Cd phytoextraction and associated mechanisms in plants. This study examined the effect of N form on Cd accumulation, translocation, and speciation in Carpobrotus rossii and Solanum nigrum Plants were grown in nutrient solution with 5-15 μM Cd in the presence of 1000 µM NH4 (+) or NO3 (-) Plant growth and Cd uptake were measured, and Cd speciation was analyzed using synchrotron-based X-ray absorption spectroscopy. Shoot Cd accumulation was 30% greater with NH4 (+) than NO3 (-) supply. Carpobrotus rossii accumulated three times more Cd than S. nigrum. However, Cd speciation in the plants was not influenced by N form, but it did vary with species and tissues. In C. rossii, up to 91% of Cd was bound to S-containing ligands in all tissues except the xylem sap where 87-95% were Cd-OH complexes. Furthermore, the proportion of Cd-S in shoots was substantially lower in S. nigrum (44-69%) than in C. rossii (60-91%). It is concluded that the application of NH4 (+) (instead of NO3 (-)) increased shoot Cd accumulation by increasing uptake and translocation, rather than changing Cd speciation, and is potentially an effective approach for increasing Cd phytoextraction.

NAR2.1/NRT2.1 functional interaction with NO3(-) and H(+) fluxes in high-affinity nitrate transport in maize root regions

Spatial and temporal fluctuations in nitrate (NO3(-)) availability are very common in agricultural soils. Therefore, understanding the molecular and physiological mechanisms involved in regulating NO3(-) uptake in regions along the primary root is important for improving the NO3(-) uptake efficiency (NUpE) in crops. Different regions of maize primary root, named R1, R2 and R3, NO3(-) starved for 3 days, were exposed to 50 μM NO3(-). Electrophysiological measurements (membrane potential and H(+) and NO3(-) fluxes) and NPF6.3, NRT2.1, NAR2.1, MHA1, MHA3 and MHA4 gene expression analyses were carried out. The results confirmed variable spatial and temporal patterns in both NO3(-) and H(+) fluxes and gene expression along the primary maize root. A significant correlation (P = 0.0023) between nitrate influx and gene transcript levels was observed only when NAR2.1 and NRT2.1 co-expression were considered together, showing for the first time the NRT2.1/NAR2.1 functional interaction in nitrate uptake along the root axis. Taken together these results suggest differing roles among the primary root regions, in which the apical part seem to be involved to sensing and signaling in contrast with the basal root which appears to be implicate in nitrate acquisition.

Induction of high-affinity NO3- uptake in grapevine roots is an active process correlated to the expression of specific members of the NRT2 and plasma membrane H+-ATPase gene families

The phenomenon of NO3- induction in plant roots has been characterised both in herbaceous and woody plants. Grapevine (Vitis vinifera L.) plants, hydroponically grown, showed an increase in NO3- uptake rate in response to anion treatment for different periods in the nutrient solution after 1 week of NO3- deprivation. The expression profile of the two high-affinity NO3- transporters VvNRT2.4A and VvNRT2.4B, and the gene encoding the accessory protein VvNAR2.2 exhibits a similar trend to that of the anion uptake. The induction, also involving the increase in activity and protein levels of plasma membrane H+-ATPase, is correlated with the expression profile of two (VvHA2 and VvHA4) out of eight putative plasma membrane H+-ATPase genes identified in grapevine genome.

Interaction of phenanthrene and potassium uptake by wheat roots: a mechanistic model

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are potentially carcinogenic, mutagenic and toxic to both human and non-human organisms. Dietary intake of PAHs is a dominant route of exposure for the general population where food crops are a major source of dietary PAHs. Over 20% of main food crops contain PAHs that exceed the control limits in China. However, the mechanisms on PAH accumulation in crops are not well understood.

RESULTS

Here we report the physiological mechanism of potassium (K+)-stimulated uptake of phenanthrene (PHE, a model PAH) in wheat. PHE uptake is stimulated by the external K+. The addition of blockers (tetraethlyammonium and barium) for K+ channels does not suppress the process, suggesting that K+ channels are not involved. The introduction of PHE and K+ elicits a much greater depolarization in root cell membrane potential than that of either PHE or K+. K+ activates the plasma membrane proton (H+)-ATPase in a K+-dependent manner. The pattern is quite similar to that in PHE uptake in the presence of K+. The external medium pH treated with PHE and K+ is higher than that with K+, and lower than that with PHE, indicating that H+ pump involves in the interaction between PHE and K+ uptake.

CONCLUSIONS

Therefore, it is concluded that a K+ influx/H+ efflux reaction is coupled with the transport of PHE into wheat root cells. Our results provide a novel insight into the PHE uptake by crop roots.

Ecological significance and complexity of N-source preference in plants

BACKGROUND

Plants can utilize two major forms of inorganic N: NO3(-) (nitrate) and NH4(+) (ammonium). In some cases, the preference of one form over another (denoted as β) can appear to be quite pronounced for a plant species, and can be an important determinant and predictor of its distribution and interactions with other species. In many other cases, however, assignment of preference is not so straightforward and must take into account a wide array of complex physiological and environmental features, which interact in ways that are still not well understood.

SCOPE

This Viewpoint presents a discussion of the key, and often co-occurring, factors that join to produce the complex phenotypic composite referred to by the deceptively simple term 'N-source preference'.

CONCLUSIONS

N-source preference is much more complex a biological phenomenon than is often assumed, and general models predicting how it will influence ecological processes will need to be much more sophisticated than those that have been so far developed.

Nitrate facilitates cadmium uptake, transport and accumulation in the hyperaccumulator Sedum plumbizincicola

The aims of this study are to investigate whether and how the nitrogen form (nitrate (NO3 (-)) versus ammonium (NH4 (+))) influences cadmium (Cd) uptake and translocation and subsequent Cd phytoextraction by the hyperaccumulator species Sedum plumbizincicola. Plants were grown hydroponically with N supplied as either NO3 (-) or NH4 (+). Short-term (36 h) Cd uptake and translocation were determined innovatively and quantitatively using a positron-emitting (107)Cd tracer and positron-emitting tracer imaging system. The results show that the rates of Cd uptake by roots and transport to the shoots in the NO3 (-) treatment were more rapid than in the NH4 (+) treatment. After uptake for 36 h, 5.6 (0.056 μM) and 29.0 % (0.290 μM) of total Cd in the solution was non-absorbable in the NO3 (-) and NH4 (+) treatments, respectively. The local velocity of Cd transport was approximately 1.5-fold higher in roots (3.30 cm h(-1)) and 3.7-fold higher in shoots (10.10 cm h(-1)) of NO3 (-)- than NH4 (+)-fed plants. Autoradiographic analysis of (109)Cd reveals that NO3 (-) nutrition enhanced Cd transportation from the main stem to branches and young leaves. Moreover, NO3 (-) treatment increased Cd, Ca and K concentrations but inhibited Fe and P in the xylem sap. In a 21-day hydroponic culture, shoot biomass and Cd concentration were 1.51 and 2.63 times higher in NO3 (-)- than in NH4 (+)-fed plants. We conclude that compared with NH4 (+), NO3 (-) promoted the major steps in the transport route followed by Cd from solution to shoots in S. plumbizincicola, namely its uptake by roots, xylem loading, root-to-shoot translocation in the xylem and uploading to the leaves. S. plumbizincicola prefers NO3 (-) nutrition to NH4 (+) for Cd phytoextraction.

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.

Cadmium inhibits the induction of high-affinity nitrate uptake in maize (Zea mays L.) roots

Cadmium (Cd) detoxification involves glutathione and phytochelatins biosynthesis: the higher need of nitrogen should require increased nitrate (NO(3)(-)) uptake and metabolism. We investigated inducible high-affinity NO(3)(-) uptake across the plasma membrane (PM) in maize seedlings roots upon short exposure (10 min to 24 h) to low Cd concentrations (0, 1 or 10 μM): the activity and gene transcript abundance of high-affinity NO(3)(-) transporters, NO(3)(-) reductases and PM H(+)-ATPases were analyzed. Exposure to 1 mM NO(3)(-) led to a peak in high-affinity (0.2 mM) NO(3)(-) uptake rate (induction), which was markedly lowered in Cd-treated roots. Plasma membrane H(+)-ATPase activity was also strongly limited, while internal NO(3)(-) accumulation and NO(3)(-) reductase activity in extracts of Cd treated roots were only slightly lowered. Kinetics of high- and low-affinity NO(3)(-) uptake showed that Cd rapidly (10 min) blocked the inducible high-affinity transport system; the constitutive high-affinity transport system appeared not vulnerable to Cd and the low-affinity transport system appeared to be less affected and only after a prolonged exposure (12 h). Cd-treatment also modified transcript levels of genes encoding high-affinity NO(3)(-) transporters (ZmNTR2.1, ZmNRT2.2), PM H(+)-ATPases (ZmMHA3, ZmMHA4) and NO(3)(-) reductases (ZmNR1, ZmNADH:NR). Despite an expectable increase in NO(3)(-) demand, a negative effect of Cd on NO(3)(-) nutrition is reported. Cd effect results in alterations at the physiological and transcriptional levels of NO(3)(-) uptake from the external solution and it is particularly severe on the inducible high-affinity anion transport system. Furthermore, Cd would limit the capacity of the plant to respond to changes in NO(3) (-) availability.

Early Zn2+-induced effects on membrane potential account for primary heavy metal susceptibility in tolerant and sensitive Arabidopsis species

BACKGROUND AND AIMS

Uptake of heavy metals by plant root cells depends on electro-physiological parameters of the plasma membrane. In this study, responses of the plasma membrane in root cells were analysed where early reactions to the metal ion-induced stress are localized. Three different Arabidopsis species with diverse strategies of their adaptation to heavy metals were compared: sensitive Arabidopsis thaliana and tolerant A. halleri and A. arenosa.

METHODS

Plants of A. thaliana Col-0 ecotype and plants of A. arenosa and A. halleri originating from natural metallicolous populations were exposed to high concentrations of Zn(2+). Plants were tested for root growth rate, cellular tolerance, plant morphology and cell death in the root apex. In addition, the membrane potential (E(M)) of mature cortical root cells and changes in the pH of the liquid culture media were measured.

KEY RESULTS

Primary roots of A. halleri and A. arenosa plants grew significantly better at increased Zn(2+) concentrations than A. thaliana plants. Elevated Zn(2+) concentrations in the culture medium induced rapid changes in E(M). The reaction was species-specific and concentration-dependent. Arabidopsis halleri revealed the highest insensitivity of the plasma membrane and the highest survival rate under prolonged treatment with extra-high concentrations. Plants were able to effectively adjust the pH in the control, but much less at Zn(2+)-induced lower pH.

CONCLUSIONS

The results indicate a similar mode of early reaction to Zn(2+), but with different extent in tolerant and sensitive species of Arabidopsis. The sensitivity of A. thaliana and a high tolerance of A. halleri and A. arenosa were demonstrated. Plasma membrane depolarization was lowest in the hyperaccumulator A. halleri and highest in A. thaliana. This indicates that rapid membrane voltage changes are an excellent tool to monitor the effects of heavy metals.

H(+)/phenanthrene symporter and aquaglyceroporin are implicated in phenanthrene uptake by wheat (Triticum aestivum L.) roots

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic pollutants that are toxic to human and nonhuman organisms. Dietary intake of PAHs is a dominant route of exposure for the general population because food crops are a major source of dietary PAHs. The mechanism for crop root uptake of PAHs remains unclear. Here we reveal that wheat root uptake of PAHs involves active and passive processes. The passive uptake is mercury and glycerol dependent. Mercury and glycerol inhibit uptake, indicating that aquaglyceroporins sensitive to mercury contribute to passive uptake. Active uptake is mediated by a phenanthrene/H symporter. The electrical response of wheat roots triggered by phenanthrene consists of two sequential phases: depolarization followed by repolarization. The depolarization is phenanthrene concentration dependent, with saturation kinetics that have an apparent of K(m) 10.8 μmol L(-1). As uptake proceeds, external solution pH increase is noticed. Lower pH favors the uptake. Vanadate and 2,4-dinitrophenol suppress the electrical response to phenanthrene and phenanthrene uptake, suggesting that plasma membrane H(+)-ATPase is involved in the establishment of an electrochemical proton gradient acting as a driving force for active uptake. Therefore, it is suggested that aquaglyceroporin and phenanthrene/H symporter are implicated in phenanthrene uptake. Our results provide insight into PAH uptake mechanism in wheat roots that is relevant to strategies for reducing PAH accumulation in wheat for food safety, improving phytoremediation of PAH-contaminated soils or water by agronomic practices and genetic modification to target remedial plants for higher PAH uptake capacity.

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.

Nickel: an overview of uptake, essentiality and toxicity in plants

Nickel even though recognized as a trace element, its metabolism is very decisive for certain enzyme activities, maintaining proper cellular redox state and various other biochemical, physiological and growth responses. Study of the aspects related with uptake, transport and distributive localization of Ni is very important in various cellular metabolic processes particularly under increased nitrogen metabolism. This review article, in core, encompasses the dual behavior of Ni in plants emphasizing its systemic partitioning, essentiality and ill effects. However, the core mechanism of molecules involved and the successive physiological conditions required starting from the soil absorption, neutralization and toxicity generated is still elusive, and varies among the plants.

Accumulation of phenanthrene by roots of intact wheat (Triticum acstivnm L.) seedlings: passive or active uptake?

BACKGROUND

Polycyclic aromatic hydrocarbons (PAHs) are of particular concern due to their hydrophobic, recalcitrant, persistent, potentially carcinogenic, mutagenic and toxic properties, and their ubiquitous occurrence in the environment. Most of the PAHs in the environment are present in surface soil. Plants grown in PAH-contaminated soils or water can become contaminated with PAHs because of their uptake. Therefore, they may threaten human and animal health. However, the mechanism for PAHs uptake by crop roots is little understood. It is important to understand exactly how PAHs are transported into the plant root system and into the human food chain, since it is beneficial in governing crop contamination by PAHs, remedying soils or waters polluted by PAHs with plants, and modeling potential uptake for risk assessment.

RESULTS

The possibility that plant roots may take up phenanthrene (PHE), a representative of PAHs, via active process was investigated using intact wheat (Triticum acstivnm L.) seedlings in a series of hydroponic experiments. The time course for PHE uptake into wheat roots grown in Hoagland solution containing 5.62 microM PHE for 36 h could be separated into two periods: a fast uptake process during the initial 2 h and a slow uptake component thereafter. Concentration-dependent PHE uptake was characterized by a smooth, saturable curve with an apparent Km of 23.7 microM and a Vmax of 208 nmol g(-1) fresh weight h(-1), suggesting a carrier-mediated uptake system. Competition between PHE and naphthalene for their uptake by the roots further supported the carrier-mediated uptake system. Low temperature and 2,4-dinitrophenol (DNP) could inhibit PHE uptake equally, indicating that metabolism plays a role in PHE uptake. The inhibitions by low temperature and DNP were strengthened with increasing concentration of PHE in external solution within PHE water solubility (7.3 muM). The contribution of active uptake to total absorption was almost 40% within PHE water solubility. PHE uptake by wheat roots caused an increase in external solution pH, implying that wheat roots take up PHE via a PHE/nH+ symport system.

CONCLUSION

It is concluded that an active, carrier-mediated and energy-consuming influx process is involved in the uptake of PHE by plant roots.

Evaluation of protein pattern changes in roots and leaves of Zea mays plants in response to nitrate availability by two-dimensional gel electrophoresis analysis

BACKGROUND

Nitrogen nutrition is one of the major factors that limit growth and production of crop plants. It affects many processes, such as development, architecture, flowering, senescence and photosynthesis. Although the improvement in technologies for protein study and the widening of gene sequences have made possible the study of the plant proteomes, only limited information on proteome changes occurring in response to nitrogen amount are available up to now. In this work, two-dimensional gel electrophoresis (2-DE) has been used to investigate the protein changes induced by NO3- concentration in both roots and leaves of maize (Zea mays L.) plants. Moreover, in order to better evaluate the proteomic results, some biochemical and physiological parameters were measured.

RESULTS

Through 2-DE analysis, 20 and 18 spots that significantly changed their amount at least two folds in response to nitrate addition to the growth medium of starved maize plants were found in roots and leaves, respectively. Most of these spots were identified by Liquid Chromatography Electrospray Ionization Tandem Mass Spectrometry (LC-ESI-MS/MS). In roots, many of these changes were referred to enzymes involved in nitrate assimilation and in metabolic pathways implicated in the balance of the energy and redox status of the cell, among which the pentose phosphate pathway. In leaves, most of the characterized proteins were related to regulation of photosynthesis. Moreover, the up-accumulation of lipoxygenase 10 indicated that the leaf response to a high availability of nitrate may also involve a modification in lipid metabolism.Finally, this proteomic approach suggested that the nutritional status of the plant may affect two different post-translational modifications of phosphoenolpyruvate carboxylase (PEPCase) consisting in monoubiquitination and phosphorylation in roots and leaves, respectively.

CONCLUSION

This work provides a first characterization of the proteome changes that occur in response to nitrate availability in leaves and roots of maize plants. According to previous studies, the work confirms the relationship between nitrogen and carbon metabolisms and it rises some intriguing questions, concerning the possible role of NO and lipoxygenase 10 in roots and leaves, respectively. Although further studies will be necessary, this proteomic analysis underlines the central role of post-translational events in modulating pivotal enzymes, such as PEPCase.

Gene structure and expression of the high-affinity nitrate transport system in rice roots

Rice has a preference for uptake of ammonium over nitrate and can use ammonium-N efficiently. Consequently, transporters mediating ammonium uptake have been extensively studied, but nitrate transporters have been largely ignored. Recently, some reports have shown that rice also has high capacity to acquire nitrate from growth medium, so understanding the nitrate transport system in rice roots is very important for improving N use efficiency in rice. The present study identified four putative NRT2 and two putative NAR2 genes that encode components of the high-affinity nitrate transport system (HATS) in the rice (Oryza sativa L. subsp. japonica cv. Nipponbare) genome. OsNRT2.1 and OsNRT2.2 share an identical coding region sequence, and their deduced proteins are closely related to those from mono-cotyledonous plants. The two NAR2 proteins are closely related to those from mono-cotyledonous plants as well. However, OsNRT2.3 and OsNRT2.4 are more closely related to Arabidopsis NRT2 proteins. Relative quantitative reverse transcription-polymerase chain reaction analysis showed that all of the six genes were rapidly upregulated and then downregulated in the roots of N-starved rice plants after they were re-supplied with 0.2 mM nitrate, but the response to nitrate differed among gene members. The results from phylogenetic tree, gene structure and expression analysis implied the divergent roles for the individual members of the rice NRT2 and NAR2 families. High-affinity nitrate influx rates associated with nitrate induction in rice roots were investigated and were found to be regulated by external pH. Compared with the nitrate influx rates at pH 6.5, alkaline pH (pH 8.0) inhibited nitrate influx, and acidic pH (pH 5.0) enhanced the nitrate influx in 1 h nitrate induced roots, but did not significantly affect that in 4 to 8 h nitrate induced roots.

Boron deficiency decreases plasmalemma H+-ATPase expression and nitrate uptake, and promotes ammonium assimilation into asparagine in tobacco roots

The effects of short-term boron deficiency on several aspects (growth, biomass allocation, metabolite concentrations, gene expression, enzyme activities) related with nitrate assimilation were studied in tobacco (Nicotiana tabacum L.) plants in order to know the early changes caused by this mineral deficiency. For this purpose, plants were grown hydroponically in a nutrient solution supplemented with 10 microM boron and then transferred to a boron-free medium for 1-5 days. Nitrate concentration decreased in both leaves and roots under boron deficiency, which was not observed in control plants. This correlated with the lower net nitrate uptake rate found in boron-deficient plants when compared to boron-sufficient ones. Results suggest that boron deficiency decreases net nitrate uptake by declining the activity of nitrate transporters rather than affecting their transcript levels. This is supported by a drop in the levels of root PMA2 transcript during the boron deficient treatment, which could lead to a decrease in the plasma membrane H+-ATPase activity necessary to get protons out of cell for the cotransport with nitrate inwards. In addition, boron deficiency led to an increase in root Asn content and a decline in glutamine synthetase activity when compared to control plants, which suggest that this mineral deficiency may promote ammonium assimilation via asparagine synthetase in tobacco roots.

Estimation of cytoplasmic nitrate and its electrochemical potential in barley roots using 13NO3 and compartmental analysis

13NO3 was used to determine the intracellular compartmentation of NO3 in barley roots (Hordeum vulgare cv. Klondike), followed by a thermodynamic analysis of nitrate transport. Plants were grown in one-tenth Johnson's medium with 1 mol m3 NO3 (NO3-grown plants) or 1 mol m3 NH4NO3 (NH4NO3-grown plants). The cytoplasmic concentrations of NO3 in roots were only approx. 3-6 mol m3 (half-time for exchange approx. 21 s) in both NO3 and NH4NO3 plants. These pool sizes are consistent with published nitrate microelectrode data, but not with previous compartmental analyses. The electrochemical potential gradient for nitrate across the plasmalemma was +26 +/-1 kJ mol1 in both NO3- and NH4NO3-grown plants, indicating active uptake of nitrate. At an external pH of 6, the plasmalemma electrochemical potential for protons would be approx. -29 +/- 4 kJ mol1. If the cytoplasmic pH was 7.3 +/- 0.2, then 2H+/1NO3 cotransport, or a primary ATP-driven pump (2NO3/1ATP), are both thermodynamically possible. NO3 is also actively transported across the tonoplast (approx. +6 to 7 kJ mol1).

A two-component high-affinity nitrate uptake system in barley

The analysis of genome databases for many different plants has identified a group of genes that are related to one part of a two-component nitrate transport system found in algae. Earlier work using mutants and heterologous expression has shown that a high-affinity nitrate transport system from the unicellular green algae, Chlamydomonas reinhardtii required two gene products for function. One gene encoded a typical carrier-type structure with 12 putative trans-membrane (TM) domains and the other gene, nar2 encoded a much smaller protein that had only one TM domain. As both gene families occur in plants we investigated whether this transport model has more general relevance among plants. The screening for nitrate transporter activity was greatly helped by a novel assay using (15)N-enriched nitrate uptake into Xenopus oocytes expressing the proteins. This assay enables many oocytes to be rapidly screened for nitrate transport activity. The functional activity of a barley nitrate transporter, HvNRT2.1, in oocytes required co-injection of a second mRNA. Although three very closely related nar2-like genes were cloned from barley, only one of these was able to give functional nitrate transport when co-injected into oocytes. The nitrate transport performed by this two-gene system was inhibited at more acidic external pH and by acidification of the cytoplasm. This specific requirement for two-gene products to give nitrate transport function has important implications for attempts to genetically manipulate this fundamental process in plants.

Sodium-dependent nitrate transport at the plasma membrane of leaf cells of the marine higher plant Zostera marina L

NO(3)(-) is present at micromolar concentrations in seawater and must be absorbed by marine plants against a steep electrochemical potential difference across the plasma membrane. We studied NO(3)(-) transport in the marine angiosperm Zostera marina L. to address the question of how NO(3)(-) uptake is energized. Electrophysiological studies demonstrated that micromolar concentrations of NO(3)(-) induced depolarizations of the plasma membrane of leaf cells. Depolarizations showed saturation kinetics (K(m) = 2.31 +/- 0.78 microM NO(3)(-)) and were enhanced in alkaline conditions. The addition of NO(3)(-) did not affect the membrane potential in the absence of Na(+), but depolarizations were restored when Na(+) was resupplied. NO(3)(-)-induced depolarizations at increasing Na(+) concentrations showed saturation kinetics (K(m) = 0.72 +/- 0.18 mM Na(+)). Monensin, an ionophore that dissipates the Na(+) electrochemical potential, inhibited NO(3)(-)-evoked depolarizations by 85%, and NO(3)(-) uptake (measured by depletion from the external medium) was stimulated by Na(+) ions and by light. Our results strongly suggest that NO(3)(-) uptake in Z. marina is mediated by a high-affinity Na(+)-symport system, which is described here (for the first time to our knowledge) in an angiosperm. Coupling the uptake of NO(3)(-) to that of Na(+) enables the steep inwardly-directed electrochemical potential for Na(+) to drive net accumulation of NO(3)(-) within leaf cells.

Isolation and characterization of HvNRT2.3 and HvNRT2.4, cDNAs encoding high-affinity nitrate transporters from roots of barley

Two full-length cDNAs, HvNRT2.3 and HvNRT2.4, were isolated from roots of barley (Hordeum vulgare), using reverse transcriptase-PCR and RACE-PCR. The corresponding polypeptides, consisting of 507 amino acids (molecular masses of 54.6 kD), belong to the major facilitator superfamily (MFS), and are closely related (>87% identity) to those encoded by HvNRT2.1 and HvNRT2.2 (formerly BCH1 and BCH2, respectively) from roots of barley. The latter are considered to encode inducible high-affinity NO(3)(-) transporters (Trueman et al., 1996). HvNRT2 transcripts were undetectable in NO(3)(-)-deprived plants. Following exposure to either NO(3)(-) or NO(2)(-), transcript abundance and (13)NO(3)(-) influx increased to a maximum by 6 to 12 h, then declined in HvNRT2.1, HvNRT2.2, and HvNRT2.3. The pattern of HvNRT2.4 transcript abundance was different, remaining high after achieving peak abundance. When external NO(3)(-) concentrations were varied from 0 to 500 microM under steady-state conditions of NO(3)(-) supply, HvNRT2 transcript accumulation and (13)NO(3)(-) influx were highest in 50 microM NO(3)(-) -grown plants. When NH(4)(+) was provided together with NO(3)(-), transcript accumulation during the first 2 h was similar to that due to NO(3)(-) alone, but by 4 h the transcript level was significantly reduced. HvNRT2 transcript was undetectable in leaf tissues.

NO3- and ClO3- fluxes in the chl1-5 mutant of Arabidopsis thaliana. Does the CHL1-5 gene encode a low-affinity NO3- transporter?

The CHL1 gene is considered to encode a low-affinity transport system (LATS) for NO3- in Arabidopsis thaliana (Y.-F. Tsay, J.I. Schroeder, K.A. Feldmann, N.M. Crawford [1993] Cell 72: 705-713). However, the anticipated reduced NO3- uptake by the LATS associated with loss of CHL1 gene activity in chl1-5 deletion mutants was evident only when plants were grown on NH4NO3. When KNO3 was the sole N source, NO3- accumulation and short-term tracer influx (using 13NO3- and 15NO3-) in leaves and roots of wild-type and mutant plants were essentially identical. Nevertheless, root uptake of 36CIO3- by the LATS and CIO3- accumulation in roots and shoots of mutant plants were significantly lower than in wild-type plants when grown on KNO3. One explanation for these results is that a second LATS is able to compensate for the chl1-5 deficiency in KNO3-grown plants. Growth on NH4NO3 may down-regulate the second LATS enough that the anticipated difference in NO3- uptake becomes apparent.

CHL1 encodes a component of the low-affinity nitrate uptake system in Arabidopsis and shows cell type-specific expression in roots

The Arabidopsis CHL1 (AtNRT1) gene confers sensitivity to the herbicide chlorate and encodes a nitrate-regulated nitrate transporter. However, how CHL1 participates in nitrate uptake in plants is not yet clear. In this study, we examined the in vivo function of CHL1 with in vivo uptake measurements and in situ hybridization experiments. Under most conditions tested, the amount of nitrate uptake by a chl1 deletion mutant was found to be significantly less than that of the wild type. This uptake deficiency was reversed when a CHL1 cDNA clone driven by the cauliflower mosaic virus 35S promoter was expressed in transgenic chl1 plants. Furthermore, tissue-specific expression patterns showed that near the root tip, CHL1 mRNA is found primarily in the epidermis, but further from the root tip, the mRNA is found in the cortex or endodermis. These results are consistent with the involvement of CHL1 in nitrate uptake at different stages of root cell development. A functional analysis in Xenopus oocytes indicated that CHL1 is a low-affinity nitrate transporter with a K(m) value of approximately 8.5 mM for nitrate. This finding is consistent with the chlorate resistance phenotype of chl1 mutants. However, these results do not fit the current model of a single, constitutive component for the low-affinity uptake system. To reconcile this discrepancy and the complex uptake behavior observed, we propose a "two-gene" model for the low-affinity nitrate uptake system of Arabidopsis.

Genetic identification of a gene involved in constitutive, high-affinity nitrate transport in higher plants

Two mutations have been found in a gene (NRT2) of Arabidopsis thaliana that specifically impair constitutive, high-affinity nitrate uptake. These mutants were selected for resistance to 0.1 mM chlorate in the absence of nitrate. Progency from one of the backcrossed mutants showed no constitutive uptake of nitrate below 0.5 mM at pH 7.0 in liquid culture (that is, within 30 min of initial exposure to nitrate). All other uptake activities measured (high-affinity phosphate and sulfate uptake, inducible high-affinity nitrate uptake, and constitutive low-affinity nitrate uptake) were present or nearly normal in the backcrossed mutant. Electrophysiological analysis of individual root cells showed that the nrt2 mutant showed little response to 0.25 mM of nitrate, whereas NRT2 wild-type cells showed an initial depolarization followed by recovery. At 10 mM of nitrate both the mutant and wild-type cells displayed similar, strong electrical responses. These results indicate that NRT2 is a critical and perhaps necessary gene for constitutive, high-affinity nitrate uptake in Arabidopsis, but not for inducible, high-affinity nor constitutive, low-affinity nitrate uptake. Thus, these systems are genetically distinct.

NO3- transport across the plasma membrane of Arabidopsis thaliana root hairs: kinetic control by pH and membrane voltage

High-affinity nitrate transport was examined in intact root hair cells of Arabidopsis thaliana using electrophysiological recordings to characterise the response of the plasma membrane to NO3- challenge and to quantify transport activity. The NO3(-)-associated membrane current was determined using a three-electrode voltage clamp to bring membrane voltage under experimental control and to compensate for current dissipation along the longitudinal cell axis. Nitrate transport was evident in the roots of seedlings grown in the absence of a nitrogen source, but only 4-6 days postgermination. In 6-day-old seedlings, additions of 5-100 microM NO3- to the bathing medium resulted in membrane depolarizations of 8-43 mV, and membrane voltage (Vm) recovered on washing NO3- from the bath. Voltage clamp measurements carried out immediately before and following NO3- additions showed that the NO3(-)-evoked depolarizations were the consequence of an inward-directed current that appeared across the entire range of accessible voltages (-300 to +50 mV). Both membrane depolarizations and NO3(-)-evoked currents recorded at the free-running voltage displayed quasi-Michaelian kinetics, with apparent values for Km of 23 +/- 6 and 44 +/- 11 microM, respectively and, for the current, a maximum of 5.1 +/- 0.9 muA cm-2. The NO3- current showed a pronounced voltage sensitivity within the normal physiological range between -250 and -100 mV, as could be demonstrated under voltage clamp, and increasing the bathing pH from 6.1 to 7.4-8.0 reduced the current and the associated membrane depolarizations 3- to 8-fold. Analyses showed a well-defined interaction between the kinetic variables of membrane voltage, pHo and [NO3-]o. At a constant pHo of 6.1, depolarization from -250 to -150 mV resulted in an approximate 3-fold reduction in the maximum current but a 10% rise in the apparent affinity for NO3-. By contrast, the same depolarization effected an approximate 20% fall in the Km for transport as a function in [H+]o. These, and additional characteristics of the transport current implicate a carrier cycle in which NO3- binding is kinetically isolated from the rate-limiting step of membrane charge transit, and they indicate a charge-coupling stoichiometry of 2(H+) per NO3- anion transported across the membrane. The results concur with previous studies showing a high-affinity NO3- transport system in Arabidopsis that is inducible following a period of nitrogen-limiting growth, but they underline the importance of voltage as a kinetic factor controlling NO3- transport at the plant plasma membrane.

Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization

In mammalian small intestine, a H(+)-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H+/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y.J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F.H., Romero, M.F., Singh, S.K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H+/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H(+)-dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino beta-lactam antibiotics but not free amino acids as substrates. The human H+/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H+/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H(+)-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H+/peptide cotransporter to chromosome 13 q33-->q34.