Two anion transporters AtClCa and AtClCe fulfil interconnecting but not redundant roles in nitrate assimilation pathways

Functional analyses of the NRT2 family of nitrate transporters in Arabidopsis

Nitrogen is an essential macronutrient for plant growth and development. Nitrate is the major form of nitrogen acquired by most crops and also serves as a vital signaling molecule. Nitrate is absorbed from the soil into root cells usually by the low-affinity NRT1 NO3 - transporters and high-affinity NRT2 NO3 - transporters, with NRT2s serving to absorb NO3 - under NO3 -limiting conditions. Seven NRT2 members have been identified in Arabidopsis, and they have been shown to be involved in various biological processes. In this review, we summarize the spatiotemporal expression patterns, localization, and biotic and abiotic responses of these transporters with a focus on recent advances in the current understanding of the functions of the seven AtNRT2 genes. This review offers beneficial insight into the mechanisms by which plants adapt to changing environmental conditions and provides a theoretical basis for crop research in the near future.

Nitrogen transport and assimilation in tea plant (Camellia sinensis): a review

Nitrogen is one of the most important nutrients for tea plants, as it contributes significantly to tea yield and serves as the component of amino acids, which in turn affects the quality of tea produced. To achieve higher yields, excessive amounts of N fertilizers mainly in the form of urea have been applied in tea plantations where N fertilizer is prone to convert to nitrate and be lost by leaching in the acid soils. This usually results in elevated costs and environmental pollution. A comprehensive understanding of N metabolism in tea plants and the underlying mechanisms is necessary to identify the key regulators, characterize the functional phenotypes, and finally improve nitrogen use efficiency (NUE). Tea plants absorb and utilize ammonium as the preferred N source, thus a large amount of nitrate remains activated in soils. The improvement of nitrate utilization by tea plants is going to be an alternative aspect for NUE with great potentiality. In the process of N assimilation, nitrate is reduced to ammonium and subsequently derived to the GS-GOGAT pathway, involving the participation of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH). Additionally, theanine, a unique amino acid responsible for umami taste, is biosynthesized by the catalysis of theanine synthetase (TS). In this review, we summarize what is known about the regulation and functioning of the enzymes and transporters implicated in N acquisition and metabolism in tea plants and the current methods for assessing NUE in this species. The challenges and prospects to expand our knowledge on N metabolism and related molecular mechanisms in tea plants which could be a model for woody perennial plant used for vegetative harvest are also discussed to provide the theoretical basis for future research to assess NUE traits more precisely among the vast germplasm resources, thus achieving NUE improvement.

Proton exchange by the vacuolar nitrate transporter CLCa is required for plant growth and nitrogen use efficiency

Nitrate is a major nutrient and osmoticum for plants. To deal with fluctuating nitrate availability in soils, plants store this nutrient in their vacuoles. Chloride channel a (CLCa), a 2NO3-/1H+ exchanger localized to the vacuole in Arabidopsis (Arabidopsis thaliana), ensures this storage process. CLCa belongs to the CLC family, which includes anion/proton exchangers and anion channels. A mutation in a glutamate residue conserved across CLC exchangers is likely responsible for the conversion of exchangers to channels. Here, we show that CLCa with a mutation in glutamate 203 (E203) behaves as an anion channel in its native membrane. We introduced the CLCaE203A point mutation to investigate its physiological importance into the Arabidopsis clca knockout mutant. These CLCaE203A mutants displayed a growth deficit linked to the disruption of water homeostasis. Additionally, CLCaE203A expression failed to complement the defect in nitrate accumulation of clca and favored higher N-assimilation at the vegetative stage. Further analyses at the post-flowering stages indicated that CLCaE203A expression results in an increase in N uptake allocation to seeds, leading to a higher nitrogen use efficiency compared to the wild-type. Altogether, these results point to the critical function of the CLCa exchanger on the vacuole for plant metabolism and development.

Cryo-EM structure of the plant nitrate transporter AtCLCa reveals characteristics of the anion-binding site and the ATP-binding pocket

Nitrate is one of the major nitrogen sources for most plants. Chloride channel (CLC) proteins mediate the transport and vacuole storage of nitrate in plants, but the structural basis of nitrate transport by plant CLC proteins remains unknown. Here, we solved the cryo-EM structure of ATP-bound Arabidopsis thaliana CLCa (AtCLCa) at 2.8 Å resolution. Structural comparison between nitrate-selective AtCLCa and chloride-selective CLC-7 reveals key differences in the central anion-binding site. We observed that the central nitrate is shifted by ∼1.4 Å from chloride, which is likely caused by a weaker interaction between the anion and Pro160; the side chains of aromatic residues around the central binding site are rearranged to accommodate the larger nitrate. Additionally, we identified the ATP-binding pocket of AtCLCa to be located between the cytosolic cystathionine β-synthase domains and the N-terminus. The N-terminus may mediate the ATP inhibition of AtCLCa by interacting with both ATP and the pore-forming transmembrane helix. Together, our studies provide insights into the nitrate selectivity and ATP regulation of plant CLCs.

The mechanisms of chromogranin B-regulated Cl- homeostasis

Chloride is the most abundant inorganic anions in almost all cells and in human circulation systems. Its homeostasis is therefore important for systems physiology and normal cellular activities. This topic has been extensively studied with chloride loaders and extruders expressed in both cell surfaces and intracellular membranes. With the newly discovered, large-conductance, highly selective Cl- channel formed by membrane-bound chromogranin B (CHGB), which differs from all other known anion channels of conventional transmembrane topology, and is distributed in plasma membranes, endomembrane systems, endosomal, and endolysosomal compartments in cells expressing it, we will discuss the potential physiological importance of the CHGB channels to Cl- homeostasis, cellular excitability and volume control, and cation uptake or release at the cellular and subcellular levels. These considerations and CHGB's association with human diseases make the CHGB channel a possible druggable target for future molecular therapeutics.

Genome-Wide Identification and Functional Characterization of the Chloride Channel TaCLC Gene Family in Wheat (Triticum aestivum L.)

In plants, chloride channels (CLC) are involved in a series of specific functions, such as regulation of nutrient transport and stress tolerance. Members of the wheat Triticum aestivum L. CLC (TaCLC) gene family have been proposed to encode anion channels/transporters that may be related to nitrogen transportation. To better understand their roles, TaCLC family was screened and 23 TaCLC gene sequences were identified using a Hidden Markov Model in conjunction with wheat genome database. Gene structure, chromosome location, conserved motif, and expression pattern of the resulting family members were then analyzed. Phylogenetic analysis showed that the TaCLC family can be divided into two subclasses (I and II) and seven clusters (-a, -c1, -c2, -e, -f1, -f2, and -g2). Using a wheat RNA-seq database, the expression pattern of TaCLC family members was determined to be an inducible expression type. In addition, seven genes from seven different clusters were selected for quantitative real-time PCR (qRT-PCR) analysis under low nitrogen stress or salt stress conditions, respectively. The results indicated that the gene expression levels of this family were up-regulated under low nitrogen stress and salt stress, except the genes of TaCLC-c2 cluster which were from subfamily -c. The yeast complementary experiments illustrated that TaCLC-a-6AS-1, TaCLC-c1-3AS, and TaCLC-e-3AL all had anion transport functions for NO₃⁻ or Cl⁻, and compensated the hypersensitivity of yeast GEF1 mutant strain YJR040w (Δgef1) in restoring anion-sensitive phenotype. This study establishes a theoretical foundation for further functional characterization of TaCLC genes and provides an initial reference for better understanding nitrate nitrogen transportation in wheat.

CLCNt2 Mediates Nitrate Content in Tobacco Leaf, Impacting the Production of Tobacco-Specific Nitrosamines in Cured Leaves

Nitrate accumulation in tobacco (Nicotiana tabacum L.) leaf, particularly in the burley (BU) type, is a reservoir for the generation of nitrosating agents responsible for the formation of tobacco-specific nitrosamines (TSNAs). TSNAs are mainly produced via the nitrosation of alkaloids occurring during the curing of tobacco leaves. Additional formation of TSNAs may also occur during tobacco storage, leaf processing and in some circumstances via pyrosynthesis during combustion. Two TSNA species, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonornicotine (NNN) are found in the tobacco products and have been documented to be animal carcinogens. A previous study showed that decreasing the accumulation of nitrate in tobacco leaf via the overexpression of a deregulated form of nitrate reductase is efficient to reduce the production of TSNAs. We pursue in finding another molecular genetic target to lower nitrate in BU tobacco. Suppressing expression or knocking-out CLCNt2 has a direct impact on leaf nitrate and TSNA reduction in cured leaves without altering biomass. This study provides now a straight path toward the development of new commercial tobacco varieties with reduced TSNA levels by breeding of variants deficient in active CLCNt2 copies.

ClCd and ClCf act redundantly at the trans-Golgi network/early endosome and prevent acidification of the Golgi stack

The trans-Golgi network/early endosome (TGN/EE) serves as the central hub in which exocytic and endocytic trafficking pathways converge and specificity of cargo routing needs to be achieved. Acidification is a hallmark of the TGN/EE and is maintained by the vacuolar H+-ATPase (V-ATPase) with support of proton-coupled antiporters. We show here that ClCd and ClCf, two distantly related members of the Arabidopsis Cl- channel (ClC) family, colocalize in the TGN/EE, where they act redundantly, and are essential for male gametophyte development. Combining an inducible knockdown approach and in vivo pH measurements, we show here that reduced ClC activity does not affect pH in the TGN/EE but causes hyperacidification of trans-Golgi cisternae. Taken together, our results show that ClC-mediated anion transport into the TGN/EE is essential and affects spatiotemporal aspects of TGN/EE maturation as well as its functional separation from the Golgi stack.

Potential Networks of Nitrogen-Phosphorus-Potassium Channels and Transporters in Arabidopsis Roots at a Single Cell Resolution

Nitrogen (N), phosphorus (P), and potassium (K) are three major macronutrients essential for plant life. These nutrients are acquired and transported by several large families of transporters expressed in plant roots. However, it remains largely unknown how these transporters are distributed in different cell-types that work together to transfer the nutrients from the soil to different layers of root cells and eventually reach vasculature for massive flow. Using the single cell transcriptomics data from Arabidopsis roots, we profiled the transcriptional patterns of putative nutrient transporters in different root cell-types. Such analyses identified a number of uncharacterized NPK transporters expressed in the root epidermis to mediate NPK uptake and distribution to the adjacent cells. Some transport genes showed cortex- and endodermis-specific expression to direct the nutrient flow toward the vasculature. For long-distance transport, a variety of transporters were shown to express and potentially function in the xylem and phloem. In the context of subcellular distribution of mineral nutrients, the NPK transporters at subcellular compartments were often found to show ubiquitous expression patterns, which suggests function in house-keeping processes. Overall, these single cell transcriptomic analyses provide working models of nutrient transport from the epidermis across the cortex to the vasculature, which can be further tested experimentally in the future.

Adaptation of cucumber seedlings to low temperature stress by reducing nitrate to ammonium during it's transportation

BACKGROUND

Low temperature severely depresses the uptake, translocation from the root to the shoot, and metabolism of nitrate and ammonium in thermophilic plants such as cucumber (Cucumis sativus). Plant growth is inhibited accordingly. However, the availability of information on the effects of low temperature on nitrogen transport remains limited.

RESULTS

Using non-invasive micro-test technology, the net nitrate (NO3-) and ammonium (NH4+) fluxes in the root hair zone and vascular bundles of the primary root, stem, petiole, midrib, lateral vein, and shoot tip of cucumber seedlings under normal temperature (NT; 26 °C) and low temperature (LT; 8 °C) treatment were analyzed. Under LT treatment, the net NO3- flux rate in the root hair zone and vascular bundles of cucumber seedlings decreased, whereas the net NH4+ flux rate in vascular bundles of the midrib, lateral vein, and shoot tip increased. Accordingly, the relative expression of CsNRT1.4a in the petiole and midrib was down-regulated, whereas the expression of CsAMT1.2a-1.2c in the midrib was up-regulated. The results of 15N isotope tracing showed that NO3--N and NH4+-N uptake of the seedlings under LT treatment decreased significantly compared with that under NT treatment, and the concentration and proportion of both NO3--N and NH4+-N distributed in the shoot decreased. Under LT treatment, the actual nitrate reductase activity (NRAact) in the root did not change significantly, whereas NRAact in the stem and petiole increased by 113.2 and 96.2%, respectively.

CONCLUSIONS

The higher net NH4+ flux rate in leaves and young tissues may reflect the higher NRAact in the stem and petiole, which may result in a higher proportion of NO3- being reduced to NH4+ during the upward transportation of NO3-. The results contribute to an improved understanding of the mechanism of changes in nitrate transportation in plants in response to low-temperature stress.

The chloride channels: Silently serving the plants

Chloride channels (CLCs), member of anion transporting proteins, are present ubiquitously in all life forms. Diverging from its name, the CLC family includes both channel and exchanger (proton-coupled) proteins; nevertheless, they share conserved structural organization. They are engaged in diverse indispensable functions such as acid and fluoride tolerance in prokaryotes to muscle stabilization, transepithelial transport, and neuronal development in mammals. Mutations in genes encoding CLCs lead to several physiological disorders in different organisms, including severe diseases in humans. Even in plants, loss of CLC protein function severely impairs various cellular processes critical for normal growth and development. These proteins sequester Cl^- into the vacuole, thus, making them an attractive target for improving salinity tolerance in plants caused by high abundance of salts, primarily NaCl. Besides, some CLCs are involved in NO₃ ^- transport and storage function in plants, thus, influencing their nitrogen use efficiency. However, despite their high significance, not many studies have been carried out in plants. Here, we have attempted to concisely highlight the basic structure of CLC proteins and critical residues essential for their function and classification. We also present the diverse functions of CLCs in plants from their first cloning back in 1996 to the knowledge acquired as of now. We stress the need for carrying out more in-depth studies on CLCs in plants, for they may have future applications towards crop improvement.

Dynamic measurement of cytosolic pH and [NO3 -] uncovers the role of the vacuolar transporter AtCLCa in cytosolic pH homeostasis

Ion transporters are key players of cellular processes. The mechanistic properties of ion transporters have been well elucidated by biophysical methods. Meanwhile, the understanding of their exact functions in cellular homeostasis is limited by the difficulty of monitoring their activity in vivo. The development of biosensors to track subtle changes in intracellular parameters provides invaluable tools to tackle this challenging issue. AtCLCa (Arabidopsis thaliana Chloride Channel a) is a vacuolar NO₃ ^-/H⁺ exchanger regulating stomata aperture in A thaliana Here, we used a genetically encoded biosensor, ClopHensor, reporting the dynamics of cytosolic anion concentration and pH to monitor the activity of AtCLCa in vivo in Arabidopsis guard cells. We first found that ClopHensor is not only a Cl^- but also, an NO₃ ^- sensor. We were then able to quantify the variations of NO₃ ^- and pH in the cytosol. Our data showed that AtCLCa activity modifies cytosolic pH and NO₃ ^- In an AtCLCa loss of function mutant, the cytosolic acidification triggered by extracellular NO₃ ^- and the recovery of pH upon treatment with fusicoccin (a fungal toxin that activates the plasma membrane proton pump) are impaired, demonstrating that the transport activity of this vacuolar exchanger has a profound impact on cytosolic homeostasis. This opens a perspective on the function of intracellular transporters of the Chloride Channel (CLC) family in eukaryotes: not only controlling the intraorganelle lumen but also, actively modifying cytosolic conditions.

Genome-wide identification and expression analysis of the CLC superfamily genes in tea plants (Camellia sinensis)

The voltage-gated chloride channel (CLC) superfamily is one of the most important anion channels that is widely distributed in bacteria and plants. CLC is involved in transporting various anions such as chloride (Cl^-) and fluoride (F^-) in and out of cells. Although Camellia sinensis is a hyper-accumulated F plant, there is no studies on the CLC gene superfamily in the tea plant. Here, 8 CLC genes were identified from C. sinensis and they were named CsCLC1-8. The structure of CsCLC genes and the proteins were not conserved; the number of exons varied from 3 to 24, and the number of transmembrane domains contained 2 to 10. Furthermore, phylogenetic analysis revealed that CsCLC4-8 in subclass I contained the typical conserved domains GxGIPE (I), GKxGPxxH (II) and PxxGxLF (III), and CsCLC1-3 in subclass II did not contain any of the three conserved residues. We measured the expression levels of CsCLCs in roots, stems and leaves to assess the responses to different concentrations of Cl^- and F^-. The result indicated that CsCLCs participated in subfunctionalization in response to Cl^- and F^-, and CsCLC1-3 was more sensitive to F^- treatments than CsCLC4-8, CsCLC6 and CsCLC7 may participate in absorption and long-distance transport of Cl^-.

Reducing the Nitrate Content in Vegetables Through Joint Regulation of Short-Distance Distribution and Long-Distance Transport

As an important nitrogen source, nitrate (NO₃ ^-) absorbed by plants is carried throughout the plant via short-distance distribution (cytoplasm to vacuole) and long-distance transportation (root to shoot), the two pathways that jointly regulate the content of NO₃ ^- in plants. NO₃ ^- accumulation within the vacuole depends on the activities of both tonoplast proton pumps and chloride channel (CLC) proteins, and less NO₃ ^- is stored in vacuoles when the activities of these proteins are reduced. The ratio of the distribution of NO₃ ^- in the cytoplasm and vacuole affects the long-distance transport of NO₃ ^-, which is regulated by the proteins NPF7.3 and NPF7.2 that play opposite but complementary roles. NPF7.3 is responsible for loading NO₃ ^- from the root cytoplasm into the xylem, whereas NPF7.2 regulates the unloading of NO₃ ^- from the xylem, thereby facilitating the long-distance transport of NO₃ ^- through the roots to the shoots. Vegetables, valued for their nutrient content, are consumed in large quantities; however, a high content of NO₃ ^- can detrimentally affect the quality of these plants. NO₃ ^- that is not assimilated and utilized in plant tissues is converted via enzyme-catalyzed reactions to nitrite (NO₂ ^-), which is toxic to plants and harmful to human health. In this review, we describe the mechanisms underlying NO₃ ^- distribution and transport in plants, a knowledge of which will contribute to breeding leafy vegetables with lower NO₃ ^- contents and thus be of considerable significance from the perspectives of environmental protection and food safety.

Molecular cloning and characterisation of SaCLCa1, a novel protein of the chloride channel (CLC) family from the halophyte Suaeda altissima (L.) Pall

The SaCLCa1 gene, a putative orthologue of AtCLCa, the Arabidopsis thaliana gene encoding a NO₃-/H⁺ antiporter, was cloned from the halophyte Suaeda altissima. It belonged to the CLC family, comprising anionic channels and anion/H⁺ antiporters. SaCLCa1 ion specificity was studied by heterologous expression of this gene in Saccharomyces cerevisiae GEF1 disrupted strain, Δgef1, where GEF1 encoded the only CLC family protein, the Cl- transporter Gef1p, in undisrupted strains of this organism. For comparison, the function of another recently identified S. altissima CLC family gene, SaCLCc1, was also characterised. Expression of SaCLCc1 in Δgef1 cells restored their ability to grow on selective media. This supported the chloride specificity of this transporter. By contrast, expression of SaCLCa1 did not complement the growth defect phenotype of Δgef1 cells. However, growth of the Δgef1 mutant on the selective media was partially restored when it was transformed with SaCLCa1(C562 T), encoding the modified protein SaCLCa1(P188S), in which proline responsible for NO₃- selectivity in selective filter was replaced by serine providing chloride selectivity. Quantitative real-time polymerase chain reactions (qRT-PCR) showed that significant induction of SaCLCa1 occurred in the roots of S. altissima when plants were grown on nitrate-deficient medium, while SaCLCc1 activation was observed in S. altissima leaves of plants grown in increasing Cl- concentrations of nutrient solution. These results suggested that SaCLCa1 and SaCLCc1 function as anionic transporters with nitrate and chloride specificities, respectively.

Role of Nitrate Reductase in NO Production in Photosynthetic Eukaryotes

Nitric oxide is a gaseous secondary messenger that is critical for proper cell signaling and plant survival when exposed to stress. Nitric oxide (NO) synthesis in plants, under standard phototrophic oxygenic conditions, has long been a very controversial issue. A few algal strains contain NO synthase (NOS), which appears to be absent in all other algae and land plants. The experimental data have led to the hypothesis that molybdoenzyme nitrate reductase (NR) is the main enzyme responsible for NO production in most plants. Recently, NR was found to be a necessary partner in a dual system that also includes another molybdoenzyme, which was renamed NO-forming nitrite reductase (NOFNiR). This enzyme produces NO independently of the molybdenum center of NR and depends on the NR electron transport chain from NAD(P)H to heme. Under the circumstances in which NR is not present or active, the existence of another NO-forming system that is similar to the NOS system would account for NO production and NO effects. PII protein, which senses and integrates the signals of the C–N balance in the cell, likely has an important role in organizing cell responses. Here, we critically analyze these topics.

Roles of nitrogen and cytokinin signals in root and shoot communications in maximizing of plant productivity and their agronomic applications

Nitrogen is an essential, often limiting, factor in plant growth and development. To regulate growth under limited nitrogen supply, plants sense the internal and external nitrogen status, and coordinate various metabolic processes and developmental programs accordingly. This coordination requires the transmission of various signaling molecules that move across the entire plant. Cytokinins, phytohormones derived from adenine and synthesized in various parts of the plant, are considered major local and long-distance messengers. Cytokinin metabolism and signaling are closely associated with nitrogen availability. They are systemically transported via the vasculature from plant roots to shoots, and vice versa, thereby coordinating shoot and root development. Tight linkage exists between the nitrogen signaling network and cytokinins during diverse developmental and physiological processes. However, the cytokinin-nitrogen interactions and the communication systems involved in sensing rhizospheric nitrogen status and in regulating canopy development remain obscure. We review current knowledge on cytokinin biosynthesis, transport and signaling, nitrogen acquisition, metabolism and signaling, and their interactive roles in regulating root-shoot morphological and physiological characteristics. We also discuss the role of spatio-temporal regulation of cytokinins in enhancing beneficial crop traits of yield and nitrogen use efficiency.

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.

Source and sink mechanisms of nitrogen transport and use

Contents Summary 35 I. Introduction 35 II. Nitrogen acquisition and assimilation 36 III. Root-to-shoot transport of nitrogen 38 IV. Nitrogen storage pools in vegetative tissues 39 V. Nitrogen transport from source leaf to sink 40 VI. Nitrogen import into sinks 42 VII. Relationship between source and sink nitrogen transport processes and metabolism 43 VIII. Regulation of nitrogen transport 43 IX. Strategies for crop improvement 44 X. Conclusions 46 Acknowledgements 47 References 47 SUMMARY: Nitrogen is an essential nutrient for plant growth. World-wide, large quantities of nitrogenous fertilizer are applied to ensure maximum crop productivity. However, nitrogen fertilizer application is expensive and negatively affects the environment, and subsequently human health. A strategy to address this problem is the development of crops that are efficient in acquiring and using nitrogen and that can achieve high seed yields with reduced nitrogen input. This review integrates the current knowledge regarding inorganic and organic nitrogen management at the whole-plant level, spanning from nitrogen uptake to remobilization and utilization in source and sink organs. Plant partitioning and transient storage of inorganic and organic nitrogen forms are evaluated, as is how they affect nitrogen availability, metabolism and mobilization. Essential functions of nitrogen transporters in source and sink organs and their importance in regulating nitrogen movement in support of metabolism, and vegetative and reproductive growth are assessed. Finally, we discuss recent advances in plant engineering, demonstrating that nitrogen transporters are effective targets to improve crop productivity and nitrogen use efficiency. While inorganic and organic nitrogen transporters were examined separately in these studies, they provide valuable clues about how to successfully combine approaches for future crop engineering.

The Arabidopsis CPSF30-L gene plays an essential role in nitrate signaling and regulates the nitrate transceptor gene NRT1.1

Plants have evolved sophisticated mechanisms to adapt to fluctuating environmental nitrogen availability. However, more underlying genes regulating the response to nitrate have yet to be characterized. We report here the identification of a nitrate regulatory mutant whose mutation mapped to the Cleavage and Polyadenylation Specificity Factor 30 gene (CPSF30-L). In the mutant, induction of nitrate-responsive genes was inhibited independent of the ammonium conditions and was restored by expression of the wild-type 65 kDa encoded by CPSF30-L. Molecular and genetic evidence suggests that CPSF30-L works upstream of NRT1.1 and independently of NLP7 in response to nitrate. Analysis of the 3'-UTR of NRT1.1 showed that the pattern of polyadenylation sites was altered in the cpsf30 mutant. Transcriptome analysis revealed that four nitrogen-related clusters were enriched in the differentially expressed genes of the cpsf30 mutant. Nitrate uptake was decreased in the mutant along with reduced expression of the nitrate transporter/sensor gene NRT1.1, while nitrate reduction and amino acid content were enhanced in roots along with increased expression of several nitrate assimilatory genes. These findings indicate that the 65 kDa protein encoded by CPSF30-L mediates nitrate signaling in part by regulating NRT1.1 expression, thus adding an important component to the nitrate signaling network.

An update on the regulation of photosynthesis by thylakoid ion channels and transporters in Arabidopsis

In natural, variable environments, plants rapidly adjust photosynthesis for optimal balance between light absorption and utilization. There is increasing evidence suggesting that ion fluxes across the chloroplast thylakoid membrane play an important role in this regulation by affecting the proton motive force and consequently photosynthesis and thylakoid membrane ultrastructure. This article presents an update on the thylakoid ion channels and transporters characterized in Arabidopsis thaliana as being involved in these processes, as well as an outlook at the evolutionary conservation of their functions in other photosynthetic organisms. This is a contribution to shed light on the thylakoid network of ion fluxes and how they help plants to adjust photosynthesis in variable light environments.

Impact of the ion transportome of chloroplasts on the optimization of photosynthesis

Ions play fundamental roles in all living cells, and their gradients are often essential to fuel transport, regulate enzyme activities, and transduce energy within cells. Regulation of their homeostasis is essential for cell metabolism. Recent results indicate that modulation of ion fluxes might also represent a useful strategy to regulate one of the most important physiological processes taking place in chloroplasts, photosynthesis. Photosynthesis is highly regulated, due to its unique role as a cellular engine for growth in the light. Controlling the balance between ATP and NADPH synthesis is a critical task, and availability of these molecules can limit the overall photosynthetic yield. Photosynthetic organisms optimize photosynthesis in low light, where excitation energy limits CO2 fixation, and minimize photo-oxidative damage in high light by dissipating excess photons. Despite extensive studies of these phenomena, the mechanism governing light utilization in plants is still poorly understood. In this review, we provide an update of the recently identified chloroplast-located ion channels and transporters whose function impacts photosynthetic efficiency in plants.

Plant nitrate transporters: from gene function to application

We summarize nitrate transporters and discuss their potential in breeding for improved nitrogen use efficiency and yield.

How Chlamydomonas handles nitrate and the nitric oxide cycle

The green alga Chlamydomonas is a valuable model system capable of assimilating different forms of nitrogen (N). Nitrate (NO3-) has a relevant role in plant-like organisms, first as a nitrogen source for growth and second as a signalling molecule. Several modules are necessary for Chlamydomonas to handle nitrate, including transporters, nitrate reductase (NR), nitrite reductase (NiR), GS/GOGAT enzymes for ammonium assimilation, and regulatory protein(s). Transporters provide a first step for influx/efflux, homeostasis, and sensing of nitrate; and NIT2 is the key transcription factor (RWP-RK) for mediating the nitrate-dependent activation of a number of genes. Here, we review how NR participates in the cycle NO3- →NO2- →NO →NO3-. NR uses the partner protein amidoxime-reducing component/nitric oxide-forming nitrite reductase (ARC/NOFNiR) for the conversion of nitrite (NO2-) into nitric oxide (NO). It also uses the truncated haemoglobin THB1 in the conversion of nitric oxide to nitrate. Nitric oxide is a negative signal for nitrate assimilation; it inhibits the activity and expression of high-affinity nitrate/nitrite transporters and NR. During this cycle, the positive signal of nitrate is transformed into the negative signal of nitric oxide, which can then be converted back into nitrate. Thus, NR is back in the spotlight as a strategic regulator of the nitric oxide cycle and the nitrate assimilation pathway.

Contribution of Cyclic and Pseudo-cyclic Electron Transport to the Formation of Proton Motive Force in Chloroplasts

Photosynthetic electron transport is coupled to proton translocation across the thylakoid membrane, resulting in the formation of a trans-thylakoid proton gradient (ΔpH) and membrane potential (Δψ). Ion transporters and channels localized to the thylakoid membrane regulate the contribution of each component to the proton motive force (pmf). Although both ΔpH and Δψ contribute to ATP synthesis as pmf, only ΔpH downregulates photosynthetic electron transport via the acidification of the thylakoid lumen by inducing thermal dissipation of excessive absorbed light energy from photosystem II antennae and slowing down of the electron transport through the cytochrome b₆f complex. To optimize the tradeoff between efficient light energy utilization and protection of both photosystems against photodamage, plants have to regulate the pmf amplitude and its components, ΔpH and Δψ. Cyclic electron transport around photosystem I (PSI) is a major regulator of the pmf amplitude by generating pmf independently of the net production of NADPH by linear electron transport. Chloroplast ATP synthase relaxes pmf for ATP synthesis, and its activity should be finely tuned for maintaining the size of the pmf during steady-state photosynthesis. Pseudo-cyclic electron transport mediated by flavodiiron protein (Flv) forms a large electron sink, which is essential for PSI photoprotection in fluctuating light in cyanobacteria. Flv is conserved from cyanobacteria to gymnosperms but not in angiosperms. The Arabidopsis proton gradient regulation 5 (pgr5) mutant is defective in the main pathway of PSI cyclic electron transport. By introducing Physcomitrella patens genes encoding Flvs, the function of PSI cyclic electron transport was substituted by that of Flv-dependent pseudo-cyclic electron transport. In transgenic plants, the size of the pmf was complemented to the wild-type level but the contribution of ΔpH to the total pmf was lower than that in the wild type. In the pgr5 mutant, the size of the pmf was drastically lowered by the absence of PSI cyclic electron transport. In the mutant, ΔpH occupied the majority of pmf, suggesting the presence of a mechanism for the homeostasis of luminal pH in the light. To avoid damage to photosynthetic electron transport by periods of excess solar energy, plants employ an intricate regulatory network involving alternative electron transport pathways, ion transporters/channels, and pH-dependent mechanisms for downregulating photosynthetic electron transport.

Modeling the light-induced electric potential difference ΔΨ across the thylakoid membrane based on the transition state rate theory

In photosynthesis, electron transport-coupled proton movement initiates the formation of the light-induced electric potential difference, ΔΨ, across the thylakoid membrane (TM). Ions are transported across the TM to counterbalance the charge of protons accumulated in the lumen. The objective of this work is to construct range of mathematical models for simulation of ΔΨ, using the transition state rate theory (TSRT) for description of movement of ions through the channels. The TSRT considers either single-ion (TSRT-SI) or multi-ion occupancy (TSRT-MI) in the channels. Movement of ions through the channel pore is described by means of energy barriers and binding sites; ions move in and out of vacant sites with rate constants that depend on the barrier heights and well depths, as well as on the interionic repulsion in TSRT-MI model. Three energy motifs are used to describe the TSRT-SI model: two-barrier one-site (2B1S), three-barrier two-site (3B2S), and four-barrier three-site (4B3S). The 3B2S energy motif is used for the TSRT-MI model. The accumulation of cations due to the TM surface negative fixed charges is also taken into account. A model employing the electro-diffusion theory instead of the TSRT is constructed for comparison. The dual wavelength transmittance signal (ΔA515-560nm) measuring the electrochromic shift (ECS) provides a proxy for experimental light-induced ΔΨ. The simulated ΔΨ traces qualitatively agree with the measured ECS traces. The models can simulate different channel conducting regimes and assess their impact on ΔΨ. The ionic flux coupling in the TSRT-MI model suggests that an increase in the internal or external K⁺ concentration may block the outward or the inward Mg²⁺ current, respectively.

Fine-tuning of root elongation by ethylene: a tool to study dynamic structure-function relationships between root architecture and nitrate absorption

Background Recently developed genetic and pharmacological approaches have been used to explore NO3-/ethylene signalling interactions and how the modifications in root architecture by pharmacological modulation of ethylene biosynthesis affect nitrate uptake. Key Results Structure-function studies combined with recent approaches to chemical genomics highlight the non-specificity of commonly used inhibitors of ethylene biosynthesis such as AVG (l-aminoethoxyvinylglycine). Indeed, AVG inhibits aminotransferases such as ACC synthase (ACS) and tryptophan aminotransferase (TAA) involved in ethylene and auxin biosynthesis but also some aminotransferases implied in nitrogen (N) metabolism. In this framework, it can be assumed that the products of nitrate assimilation and hormones may interact through a hub in carbon (C) and N metabolism to drive the root morphogenetic programme (RMP). Although ethylene/auxin interactions play a major role in cell division and elongation in root meristems, shaping of the root system depends also on energetic considerations. Based on this finding, the analysis is extended to nutrient ion-hormone interactions assuming a fractal or constructal model for root development. Conclusion Therefore, the tight control of root structure-function in the RMP may explain why over-expressing nitrate transporter genes to decouple structure-function relationships and improve nitrogen use efficiency (NUE) has been unsuccessful.

A bestrophin-like protein modulates the proton motive force across the thylakoid membrane in Arabidopsis

During photosynthesis, photosynthetic electron transport generates a proton motive force (pmf) across the thylakoid membrane, which is used for ATP biosynthesis via ATP synthase in the chloroplast. The pmf is composed of an electric potential (ΔΨ) and an osmotic component (ΔpH). Partitioning between these components in chloroplasts is strictly regulated in response to fluctuating environments. However, our knowledge of the molecular mechanisms that regulate pmf partitioning is limited. Here, we report a bestrophin-like protein (AtBest), which is critical for pmf partitioning. While the ΔpH component was slightly reduced in atbest, the ΔΨ component was much greater in this mutant than in the wild type, resulting in less efficient activation of nonphotochemical quenching (NPQ) upon both illumination and a shift from low light to high light. Although no visible phenotype was observed in the atbest mutant in the greenhouse, this mutant exhibited stronger photoinhibition than the wild type when grown in the field. AtBest belongs to the bestrophin family proteins, which are believed to function as chloride (Cl^- ) channels. Thus, our findings reveal an important Cl^- channel required for ion transport and homeostasis across the thylakoid membrane in higher plants. These processes are essential for fine-tuning photosynthesis under fluctuating environmental conditions.

The Thermodynamic Flow-Force Interpretation of Root Nutrient Uptake Kinetics: A Powerful Formalism for Agronomic and Phytoplanktonic Models

The ion influx isotherms obtained by measuring unidirectional influx across root membranes with radioactive or stable tracers are mostly interpreted by enzyme-substrate-like modeling. However, recent analyses from ion transporter mutants clearly demonstrate the inadequacy of the conventional interpretation of ion isotherms. Many genetically distinct carriers are involved in the root catalytic function. Parameters Vmax and Km deduced from this interpretation cannot therefore be regarded as microscopic parameters of a single transporter, but are instead macroscopic parameters (Vmapp and Kmapp, apparent maximum velocity and affinity constant) that depend on weighted activities of multiple transporters along the root. The flow-force interpretation based on the thermodynamic principle of irreversible processes is an alternative macroscopic modeling approach for ion influx isotherms in which macroscopic parameters L_j (overall conductance of the root system for the substrate j) and π_j (thermodynamic parameter when J_j = 0) have a straightforward meaning with respect to the biological sample studied. They characterize the efficiency of the entire root catalytic structure without deducing molecular characteristics. Here we present the basic principles of this theory and how its use can be tested and improved by changing root pre- and post-wash procedures before influx measurements in order to come as close as possible to equilibrium conditions. In addition, the constant values of Vm and Km in the Michaelis-Menten (MM) formalism of enzyme-substrate interpretation do not reflect variations in response to temperature, nutrient status or nutrient regimes. The linear formalism of the flow-force approach, which integrates temperature effect on nutrient uptake, could usefully replace MM formalism in the 1-3-dimension models of plants and phytoplankton. This formalism offers a simplification of parametrization to help find more realistic analytical expressions and numerical solution for root nutrient uptake.

A voltage-dependent chloride channel fine-tunes photosynthesis in plants

In natural habitats, plants frequently experience rapid changes in the intensity of sunlight. To cope with these changes and maximize growth, plants adjust photosynthetic light utilization in electron transport and photoprotective mechanisms. This involves a proton motive force (PMF) across the thylakoid membrane, postulated to be affected by unknown anion (Cl(-)) channels. Here we report that a bestrophin-like protein from Arabidopsis thaliana functions as a voltage-dependent Cl(-) channel in electrophysiological experiments. AtVCCN1 localizes to the thylakoid membrane, and fine-tunes PMF by anion influx into the lumen during illumination, adjusting electron transport and the photoprotective mechanisms. The activity of AtVCCN1 accelerates the activation of photoprotective mechanisms on sudden shifts to high light. Our results reveal that AtVCCN1, a member of a conserved anion channel family, acts as an early component in the rapid adjustment of photosynthesis in variable light environments.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

The Arabidopsis Thylakoid Chloride Channel AtCLCe Functions in Chloride Homeostasis and Regulation of Photosynthetic Electron Transport

Chloride ions can be translocated across cell membranes through Cl(-) channels or Cl(-)/H(+) exchangers. The thylakoid-located member of the Cl(-) channel CLC family in Arabidopsis thaliana (AtCLCe) was hypothesized to play a role in photosynthetic regulation based on the initial photosynthetic characterization of clce mutant lines. The reduced nitrate content of Arabidopsis clce mutants suggested a role in regulation of plant nitrate homeostasis. In this study, we aimed to further investigate the role of AtCLCe in the regulation of ion homeostasis and photosynthetic processes in the thylakoid membrane. We report that the size and composition of proton motive force were mildly altered in two independent Arabidopsis clce mutant lines. Most pronounced effects in the clce mutants were observed on the photosynthetic electron transport of dark-adapted plants, based on the altered shape and associated parameters of the polyphasic OJIP kinetics of chlorophyll a fluorescence induction. Other alterations were found in the kinetics of state transition and in the macro-organization of photosystem II supercomplexes, as indicated by circular dichroism measurements. Pre-treatment with KCl but not with KNO3 restored the wild-type photosynthetic phenotype. Analyses by transmission electron microscopy revealed a bow-like arrangement of the thylakoid network and a large thylakoid-free stromal region in chloroplast sections from the dark-adapted clce plants. Based on these data, we propose that AtCLCe functions in Cl(-) homeostasis after transition from light to dark, which affects chloroplast ultrastructure and regulation of photosynthetic electron transport.

Ion Channels in Native Chloroplast Membranes: Challenges and Potential for Direct Patch-Clamp Studies

Photosynthesis without any doubt depends on the activity of the chloroplast ion channels. The thylakoid ion channels participate in the fine partitioning of the light-generated proton-motive force (p.m.f.). By regulating, therefore, luminal pH, they affect the linear electron flow and non-photochemical quenching. Stromal ion homeostasis and signaling, on the other hand, depend on the activity of both thylakoid and envelope ion channels. Experimentally, intact chloroplasts and swollen thylakoids were proven to be suitable for direct measurements of the ion channels activity via conventional patch-clamp technique; yet, such studies became infrequent, although their potential is far from being exhausted. In this paper we wish to summarize existing challenges for direct patch-clamping of native chloroplast membranes as well as present available results on the activity of thylakoid Cl⁻ (ClC?) and divalent cation-permeable channels, along with their tentative roles in the p.m.f. partitioning, volume regulation, and stromal Ca²⁺ and Mg²⁺ dynamics. Patch-clamping of the intact envelope revealed both large-conductance porin-like channels, likely located in the outer envelope membrane and smaller conductance channels, more compatible with the inner envelope location. Possible equivalent model for the sandwich-like arrangement of the two envelope membranes within the patch electrode will be discussed, along with peculiar properties of the fast-activated cation channel in the context of the stromal pH control.

Breaking conceptual locks in modelling root absorption of nutrients: reopening the thermodynamic viewpoint of ion transport across the root

BACKGROUND

The top-down analysis of nitrate influx isotherms through the Enzyme-Substrate interpretation has not withstood recent molecular and histochemical analyses of nitrate transporters. Indeed, at least four families of nitrate transporters operating at both high and/or low external nitrate concentrations, and which are located in series and/or parallel in the different cellular layers of the mature root, are involved in nitrate uptake. Accordingly, the top-down analysis of the root catalytic structure for ion transport from the Enzyme-Substrate interpretation of nitrate influx isotherms is inadequate. Moreover, the use of the Enzyme-Substrate velocity equation as a single reference in agronomic models is not suitable in its formalism to account for variations in N uptake under fluctuating environmental conditions. Therefore, a conceptual paradigm shift is required to improve the mechanistic modelling of N uptake in agronomic models.

SCOPE

An alternative formalism, the Flow-Force theory, was proposed in the 1970s to describe ion isotherms based upon biophysical 'flows and forces' relationships of non-equilibrium thermodynamics. This interpretation describes, with macroscopic parameters, the patterns of N uptake provided by a biological system such as roots. In contrast to the Enzyme-Substrate interpretation, this approach does not claim to represent molecular characteristics. Here it is shown that it is possible to combine the Flow-Force formalism with polynomial responses of nitrate influx rate induced by climatic and in planta factors in relation to nitrate availability.

CONCLUSIONS

Application of the Flow-Force formalism allows nitrate uptake to be modelled in a more realistic manner, and allows scaling-up in time and space of the regulation of nitrate uptake across the plant growth cycle.

An updated model for nitrate uptake modelling in plants. II. Assessment of active root involvement in nitrate uptake based on integrated root system age: measured versus modelled outputs

Background and Aims An updated version of a mechanistic structural-functional model was developed to predict nitrogen (N) uptake throughout the growth cycle by a crop of winter oilseed rape, Brassica napus, grown under field conditions. Methods The functional component of the model derives from a revisited conceptual framework that combines the thermodynamic Flow-Force interpretation of nitrate uptake isotherms and environmental and in planta effects on nitrate influx. Estimation of the root biomass (structural component) is based upon a combination of root mapping along the soil depth profile in the field and a relationship between the specific root length and external nitrate concentration. The root biomass contributing actively to N uptake was determined by introduction of an integrated root system age that allows assignment of a root absorption capacity at a specific age of the root. Key Results Simulations were well matched to measured data of N taken up under field conditions for three levels of N fertilization. The model outputs indicated that the two topsoil layers (0-30 and 30-60 cm) contained 75-88 % of the total root length and biomass, and accounted for 90-95 % of N taken up at harvest. Conclusions This conceptual framework provides a model of nitrate uptake that is able to respond to external nitrate fluctuations at both functional and structural levels.

An updated model for nitrate uptake modelling in plants. I. Functional component: cross-combination of flow-force interpretation of nitrate uptake isotherms, and environmental and in planta regulation of nitrate influx

BACKGROUND AND AIMS

In spite of major breakthroughs in the last three decades in the identification of root nitrate uptake transporters in plants and the associated regulation of nitrate transport activities, a simplified and operational modelling approach for nitrate uptake is still lacking. This is due mainly to the difficulty in linking the various regulations of nitrate transport that act at different levels of time and on different spatial scales.

METHODS

A cross-combination of a Flow-Force approach applied to nitrate influx isotherms and experimentally determined environmental and in planta regulation is used to model nitrate in oilseed rape, Brassica napus. In contrast to 'Enzyme-Substrate' interpretations, a Flow-Force modelling approach considers the root as a single catalytic structure and does not infer hypothetical cellular processes among nitrate transporter activities across cellular layers in the mature roots. In addition, this approach accounts for the driving force on ion transport based on the gradient of electrochemical potential, which is more appropriate from a thermodynamic viewpoint.

KEY RESULTS AND CONCLUSIONS

Use of a Flow-Force formalism on nitrate influx isotherms leads to the development of a new conceptual mechanistic basis to model more accurately N uptake by a winter oilseed rape crop under field conditions during the whole growth cycle. This forms the functional component of a proposed new structure-function mechanistic model of N uptake.

Overexpression of a proton-coupled vacuolar glucose exporter impairs freezing tolerance and seed germination

Arabidopsis vacuoles harbor, besides sugar transporter of the TMT-type, an early response to dehydration like 6 (ERDL6) protein involved in glucose export into the cytosol. However, the mode of transport of ERDL6 and the plant's feedback to overexpression of its activity on essential properties such as, for example, seed germination or freezing tolerance, remain unexplored. Using patch-clamp studies on vacuoles expressing AtERDL6 we demonstrated directly that this carrier operates as a proton-driven glucose exporter. Overexpression of BvIMP, the closest sugar beet (Beta vulgaris) homolog to AtERDL6, in Arabidopsis leads surprisingly to impaired seed germination under both conditions, sugar application and low environmental temperatures, but not under standard conditions. Upon cold treatment, BvIMP overexpressor plants accumulated lower quantities of monosaccharides than the wild-type, a response in line with the reduced frost tolerance of the transgenic Arabidopsis plants, and the fact that cold temperatures inhibits BvIMP transcription in sugar beet leaves. With these findings we show that the tight control of vacuolar sugar import and export is a key requisite for cold tolerance and seed germination of plants.

Nitrate transport and signalling in Arabidopsis

Plants have developed adaptive responses allowing them to cope with nitrogen (N) fluctuation in the soil and maintain growth despite changes in external N availability. Nitrate is the most important N form in temperate soils. Nitrate uptake by roots and its transport at the whole-plant level involves a large panoply of transporters and impacts plant performance. Four families of nitrate-transporting proteins have been identified so far: nitrate transporter 1/peptide transporter family (NPF), nitrate transporter 2 family (NRT2), the chloride channel family (CLC), and slow anion channel-associated homologues (SLAC/SLAH). Nitrate transporters are also involved in the sensing of nitrate. It is now well established that plants are able to sense external nitrate availability, and hence that nitrate also acts as a signal molecule that regulates many aspects of plant intake, metabolism, and gene expression. This review will focus on a global picture of the nitrate transporters so far identified and the recent advances in the molecular knowledge of the so-called primary nitrate response, the rapid regulation of gene expression in response to nitrate. The recent discovery of the NIN-like proteins as master regulators for nitrate signalling has led to a new understanding of the regulation cascade.

Proanthocyanidin oxidation of Arabidopsis seeds is altered in mutant of the high-affinity nitrate transporter NRT2.7

NRT2.7 is a seed-specific high-affinity nitrate transporter controlling nitrate content in Arabidopsis mature seeds. The objective of this work was to analyse further the consequences of the nrt2.7 mutation for the seed metabolism. This work describes a new phenotype for the nrt2.7-2 mutant allele in the Wassilewskija accession, which exhibited a distinctive pale-brown seed coat that is usually associated with a defect in flavonoid oxidation. Indeed, this phenotype resembled those of tt10 mutant seeds defective in the laccase-like enzyme TT10/LAC15, which is involved in the oxidative polymerization of flavonoids such as the proantocyanidins (PAs) (i.e. epicatechin monomers and PA oligomers) and flavonol glycosides. nrt2.7-2 and tt10-2 mutant seeds displayed the same higher accumulation of PAs, but were partially distinct, since flavonol glycoside accumulation was not affected in the nrt2.7-2 seeds. Moreover, measurement of in situ laccase activity excluded a possibility of the nrt2.7-2 mutation affecting the TT10 enzymic activity at the early stage of seed development. Functional complementation of the nrt2.7-2 mutant by overexpression of a full-length NRT2.7 cDNA clearly demonstrated the link between the nrt2.7 mutation and the PA phenotype. However, the PA-related phenotype of nrt2.7-2 seeds was not strictly correlated to the nitrate content of seeds. No correlation was observed when nitrate was lowered in seeds due to limited nitrate nutrition of plants or to lower nitrate storage capacity in leaves of clca mutants deficient in the vacuolar anionic channel CLCa. All together, the results highlight a hitherto-unknown function of NRT2.7 in PA accumulation/oxidation.

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 stress affects the turnover and size of nitrogen pools supplying leaf growth in a grass

The effect of nitrogen (N) stress on the pool system supplying currently assimilated and (re)mobilized N for leaf growth of a grass was explored by dynamic ¹⁵N labeling, assessment of total and labeled N import into leaf growth zones, and compartmental analysis of the label import data. Perennial ryegrass (Lolium perenne) plants, grown with low or high levels of N fertilization, were labeled with ¹⁵NO₃⁻/¹⁴NO₃⁻ from 2 h to more than 20 d. In both treatments, the tracer time course in N imported into the growth zones fitted a two-pool model (r² > 0.99). This consisted of a "substrate pool," which received N from current uptake and supplied the growth zone, and a recycling/mobilizing "store," which exchanged with the substrate pool. N deficiency halved the leaf elongation rate, decreased N import into the growth zone, lengthened the delay between tracer uptake and its arrival in the growth zone (2.2 h versus 0.9 h), slowed the turnover of the substrate pool (half-life of 3.2 h versus 0.6 h), and increased its size (12.4 μg versus 5.9 μg). The store contained the equivalent of approximately 10 times (low N) and approximately five times (high N) the total daily N import into the growth zone. Its turnover agreed with that of protein turnover. Remarkably, the relative contribution of mobilization to leaf growth was large and similar (approximately 45%) in both treatments. We conclude that turnover and size of the substrate pool are related to the sink strength of the growth zone, whereas the contribution of the store is influenced by partitioning between sinks.

Function and evolution of channels and transporters in photosynthetic membranes

Chloroplasts from land plants and algae originated from an endosymbiotic event, most likely involving an ancestral photoautotrophic prokaryote related to cyanobacteria. Both chloroplasts and cyanobacteria have thylakoid membranes, harboring pigment-protein complexes that perform the light-dependent reactions of oxygenic photosynthesis. The composition, function and regulation of these complexes have thus far been the major topics in thylakoid membrane research. For many decades, we have also accumulated biochemical and electrophysiological evidence for the existence of solute transthylakoid transport activities that affect photosynthesis. However, research dedicated to molecular identification of the responsible proteins has only recently emerged with the explosion of genomic information. Here we review the current knowledge about channels and transporters from the thylakoid membrane of Arabidopsis thaliana and of the cyanobacterium Synechocystis sp. PCC 6803. No homologues of these proteins have been characterized in algae, although similar sequences could be recognized in many of the available sequenced genomes. Based on phylogenetic analyses, we hypothesize a host origin for most of the so far identified Arabidopsis thylakoid channels and transporters. Additionally, the shift from a non-thylakoid to a thylakoid location appears to have occurred at different times for different transport proteins. We propose that closer control of and provision for the thylakoid by products of the host genome has been an ongoing process, rather than a one-step event. Some of the proteins recruited to serve in the thylakoid may have been the result of the increased specialization of its pigment-protein composition and organization in green plants.

Regulation of photosynthesis by ion channels in cyanobacteria and higher plants

Photosynthesis converts light energy into chemical energy, and supplies ATP and NADPH for CO2 fixation into carbohydrates and for the synthesis of several compounds which are essential for autotrophic growth. Oxygenic photosynthesis takes place in thylakoid membranes of chloroplasts and photosynthetic prokaryote cyanobacteria. An ancestral photoautotrophic prokaryote related to cyanobacteria has been proposed to give rise to chloroplasts of plants and algae through an endosymbiotic event. Indeed, photosynthetic complexes involved in the electron transport coupled to H(+) translocation and ATP synthesis are similar in higher plants and cyanobacteria. Furthermore, some of the protein and solute/ion conducting machineries also share common structure and function. Electrophysiological and biochemical evidence support the existence of ion channels in the thylakoid membrane in both types of organisms. By allowing specific ion fluxes across thylakoid membranes, ion channels have been hypothesized to either directly or indirectly regulate photosynthesis, by modulating the proton motive force. Recent molecular identification of some of the thylakoid-located channels allowed to obtain genetic proof in favor of such hypothesis. Furthermore, some ion channels of the envelope membrane in chloroplasts have also been shown to impact on this light-driven process. Here we give an overview of thylakoid/chloroplast located ion channels of higher plants and of cyanobacterium Synechocystis sp. PCC 6803. We focus on channels shown to be implicated in the regulation of photosynthesis and discuss the possible mechanisms of action.

Arabidopsis roots and shoots show distinct temporal adaptation patterns toward nitrogen starvation

Nitrogen (N) is an essential macronutrient for plants. N levels in soil vary widely, and plants have developed strategies to cope with N deficiency. However, the regulation of these adaptive responses and the coordinating signals that underlie them are still poorly understood. The aim of this study was to characterize N starvation in adult Arabidopsis (Arabidopsis thaliana) plants in a spatiotemporal manner by an integrative, multilevel global approach analyzing growth, metabolites, enzyme activities, and transcript levels. We determined that the remobilization of N and carbon compounds to the growing roots occurred long before the internal N stores became depleted. A global metabolite analysis by gas chromatography-mass spectrometry revealed organ-specific differences in the metabolic adaptation to complete N starvation, for example, for several tricarboxylic acid cycle intermediates, but also for carbohydrates, secondary products, and phosphate. The activities of central N metabolism enzymes and the capacity for nitrate uptake adapted to N starvation by favoring N remobilization and by increasing the high-affinity nitrate uptake capacity after long-term starvation. Changes in the transcriptome confirmed earlier studies and added a new dimension by revealing specific spatiotemporal patterns and several unknown N starvation-regulated genes, including new predicted small RNA genes. No global correlation between metabolites, enzyme activities, and transcripts was evident. However, this multilevel spatiotemporal global study revealed numerous new patterns of adaptation mechanisms to N starvation. In the context of a sustainable agriculture, this work will give new insight for the production of crops with increased N use efficiency.

Solute transporters in plant thylakoid membranes: Key players during photosynthesis and light stress

Plants utilize sunlight to drive photosynthetic energy conversion in the chloroplast thylakoid membrane. Here are located four major photosynthetic complexes, about which we have great knowledge in terms of structure and function. However, much less we know about auxiliary proteins, such as transporters, ensuring an optimum function and turnover of these complexes. The most prominent thylakoid transporter is the proton-translocating ATP-synthase. Recently, four additional transporters have been identified in the thylakoid membrane of Arabidopsis thaliana, namely one copper-transporting P-ATPase, one chloride channel, one phosphate transporter, and one ATP/ADP carrier. Here, we review the current knowledge on the function and physiological role of these transporters during photosynthesis and light stress in plants. Subsequently, we make a survey on the outlook of thylakoid activities awaiting identification of responsible proteins. Such knowledge is necessary to understand the thylakoid network of transporters, and to design strategies for bioengineering crop plants in the future.

The essential role of anionic transport in plant cells: the pollen tube as a case study

Plasma membrane anion transporters play fundamental roles in plant cell biology, especially in stomatal closure and nutrition. Notwithstanding, a lot is still unknown about the specific function of these transporters, their specific localization, or molecular nature. Here the fundamental roles of anionic transport in plant cells are reviewed. Special attention will be paid to them in the control of pollen tube growth. Pollen tubes are extreme examples of cellular polarity as they grow exclusively in their apical extremity. Their unique cell biology has been extensively exploited for fundamental understanding of cellular growth and morphogenesis. Non-invasive methods have demonstrated that tube growth is governed by different ion fluxes, with different properties and distribution. Not much is known about the nature of the membrane transporters responsible for anionic transport and their regulation in the pollen tube. Recent data indicate the importance of chloride (Cl(-)) transfer across the plasma membrane for pollen germination and pollen tube growth. A general overview is presented of the well-known accumulated data in terms of biophysical and functional characterization, transcriptomics, and genomic description of pollen ionic transport, and the various controversies around the role of anionic fluxes during pollen tube germination, growth, and development. It is concluded that, like all other plant cells so far analysed, pollen tubes depend on anion fluxes for a number of fundamental homeostatic properties.

From the soil to the seeds: the long journey of nitrate in plants

Under temperate climates and in cultivated soils, nitrate is the most important source of nitrogen (N) available for crops and, before its reduction and assimilation into amino acids, must enter the root cells and then move in the whole plant. The aim of this review is to provide an overall picture of the numerous membrane proteins that achieve these processes by being localized in different compartments and in different tissues. Nitrate transporters (NRT) from the NRT1 and NRT2 families ensure the capacity of root cells to take up nitrate, through high- and low-affinity systems (HATS and LATS) depending on nitrate concentrations in the soil solution. Other members of the NRT1 family are involved subsequently in loading and unloading of nitrate to and from the xylem vessels, allowing its distribution to aerial organs or its remobilization from old leaves. Once in the cell, nitrate can be stored in the vacuole by passing through the tonoplast, a step that involves chloride channels (CLC) or a NRT2 member. Finally, with the exception of one NRT1 member, the transport of nitrite towards the chloroplast is still largely unknown. All these fluxes are controlled by key factors, the 'major tour operators' like the internal nutritional status of the plant but also by external abiotic factors.

Anion channels/transporters in plants: from molecular bases to regulatory networks

Anion channels/transporters are key to a wide spectrum of physiological functions in plants, such as osmoregulation, cell signaling, plant nutrition and compartmentalization of metabolites, and metal tolerance. The recent identification of gene families encoding some of these transport systems opened the way for gene expression studies, structure-function analyses of the corresponding proteins, and functional genomics approaches toward further understanding of their integrated roles in planta. This review, based on a few selected examples, illustrates that the members of a given gene family exhibit a diversity of substrate specificity, regulation, and intracellular localization, and are involved in a wide range of physiological functions. It also shows that post-translational modifications of transport proteins play a key role in the regulation of anion transport activity. Key questions arising from the increasing complexity of networks controlling anion transport in plant cells (the existence of redundancy, cross talk, and coordination between various pathways and compartments) are also addressed.

The Arabidopsis vacuolar anion transporter, AtCLCc, is involved in the regulation of stomatal movements and contributes to salt tolerance

In plant cells, anion channels and transporters are essential for key functions such as nutrition, resistance to biotic or abiotic stresses, and ion homeostasis. In Arabidopsis, members of the chloride channel (CLC) family located in intracellular organelles have been shown to be required for nitrate homeostasis or pH adjustment, and previous results indicated that AtCLCc is involved in nitrate accumulation. We investigated new physiological functions of this CLC member in Arabidopsis. Here we report that AtCLCc is strongly expressed in guard cells and pollen and more weakly in roots. Use of an AtCLCc:GFP fusion revealed localization to the tonoplast. Disruption of the AtCLCc gene by a T-DNA insertion in four independent lines affected physiological responses that are directly related to the movement of chloride across the tonoplast membrane. Opening of clcc stomata was reduced in response to light, and ABA treatment failed to induce their closure, whereas application of KNO₃ but not KCl restored stomatal opening. clcc mutant plants were hypersensitive to NaCl treatment when grown on soil, and to NaCl and KCl in vitro, confirming the chloride dependence of the phenotype. These phenotypes were associated with modifications of chloride content in both guard cells and roots. These data demonstrate that AtCLCc is essential for stomatal movement and salt tolerance by regulating chloride homeostasis.

The proline 160 in the selectivity filter of the Arabidopsis NO(3)(-)/H(+) exchanger AtCLCa is essential for nitrate accumulation in planta

Nitrate, the major nitrogen source for plants, can be accumulated in the vacuole. Its transport across the vacuolar membrane is mediated by AtCLCa, an antiporter of the chloride channel (CLC) protein family. In contrast to other CLC family members, AtCLCa transports nitrate coupled to protons. Recently, the different behaviour towards nitrate of CLC proteins has been linked to the presence of a serine or proline in the selectivity filter motif GXGIP. By monitoring AtCLCa activity in its native environment, we show that if proline 160 in AtCLCa is changed to a serine (AtCLCa(P160S) ), the transporter loses its nitrate selectivity, but the anion proton exchange mechanism is unaffected. We also performed in vivo analyses in yeast and Arabidopsis. In contrast to native AtCLCa, expression of AtCLCa(P160S) does not complement either the ΔScCLC yeast mutant grown on nitrate or the nitrate under-accumulation phenotype of clca knockout plants. Our results confirm the significance of this amino acid in the conserved selectivity filter of CLC proteins and highlight the importance of the proline in AtCLCa for nitrate metabolism in Arabidopsis.