Investigating glutamate receptor-like gene co-expression in Arabidopsis thaliana

Metabolite analyses of single cells

Single-cell analysis is a promising method for understanding not only cellular physiology but also biological mechanisms of multicellular organisms. Although neighboring cells in multicellular organisms originate from the same genomic information, different circumstances around cells or epigenetic differences have different influences on each cell, leading to differing expression of genes, and thus differing levels and dynamics of metabolites, in single cells. However, single-cell analysis is a tough challenge, even with recent technologies, because of the small size of single cells. Unlike genes, metabolites cannot be amplified, and therefore metabolite analysis is another issue. To analyze such a tiny quantity of metabolites in a single cell, various techniques have been tried and developed. Especially in mass spectrometry, marked improvements in both detection sensitivity and ionization techniques have opened up the challenge for the analysis of metabolites in single cells. In this review, we discuss the method for metabolite detection at the level of single cells and recent advancements in technology.

Wheat grain yield on saline soils is improved by an ancestral Na⁺ transporter gene

The ability of wheat to maintain a low sodium concentration ([Na(+)]) in leaves correlates with improved growth under saline conditions. This trait, termed Na(+) exclusion, contributes to the greater salt tolerance of bread wheat relative to durum wheat. To improve the salt tolerance of durum wheat, we explored natural diversity in shoot Na(+) exclusion within ancestral wheat germplasm. Previously, we showed that crossing of Nax2, a gene locus in the wheat relative Triticum monococcum into a commercial durum wheat (Triticum turgidum ssp. durum var. Tamaroi) reduced its leaf [Na(+)] (ref. 5). Here we show that a gene in the Nax2 locus, TmHKT1;5-A, encodes a Na(+)-selective transporter located on the plasma membrane of root cells surrounding xylem vessels, which is therefore ideally localized to withdraw Na(+) from the xylem and reduce transport of Na(+) to leaves. Field trials on saline soils demonstrate that the presence of TmHKT1;5-A significantly reduces leaf [Na(+)] and increases durum wheat grain yield by 25% compared to near-isogenic lines without the Nax2 locus.

Glutamate receptor homologs in plants: functions and evolutionary origins

The plant glutamate-like receptor homologs (GLRs) are homologs of mammalian ionotropic glutamate receptors (iGluRs) which were discovered more than 10 years ago, and are hypothesized to be potential amino acid sensors in plants. Although initial progress on this gene family has been hampered by gene redundancy and technical issues such as gene toxicity; genetic, pharmacological, and electrophysiological approaches are starting to uncover the functions of this protein family. In parallel, there has been tremendous progress in elucidating the structure of animal glutamate receptors (iGluRs), which in turn will help understanding of the molecular mechanisms of plant GLR functions. In this review, we will summarize recent progress on the plant GLRs. Emerging evidence implicates plant GLRs in various biological processes in and beyond N sensing, and implies that there is some overlap in the signaling mechanisms of amino acids between plants and animals. Phylogenetic analysis using iGluRs from metazoans, plants, and bacteria showed that the plant GLRs are no more closely related to metazoan iGluRs as they are to bacterial iGluRs, indicating the separation of plant, other eukaryotic, and bacterial GLRs might have happened as early on as the last universal common ancestor. Structural similarities and differences with animal iGluRs, and the implication thereof, are also discussed.

The ins and outs of cellular Ca(2+) transport

The cytoplasmic Ca(2+) signals that participate in nearly all aspects of plant growth and development encode information as binary switches or information-rich signatures. They are the result of influx (thermodynamically passive) and efflux (thermodynamically active) activities mediated by membrane transport proteins. On the influx side, confirming the molecular identities of Ca(2+)-permeable channels is still a major research topic. Cyclic nucleotide-gated channels and glutamate receptor-like channels are candidates well supported by evidence. On the efflux side, CAX antiporters and P-type ATPase pumps are the principal molecular entities. Both of these active transporters load Ca(2+) into specific compartments and have the potential to reduce the magnitude and duration of a Ca(2+) transient. Recent studies indicate calmodulin-activated Ca(2+) pumps in endomembrane systems can dampen the magnitude and duration of a Ca(2+) transient that could otherwise grow into a Ca(2+) cell death signature. An important challenge following molecular characterization of the influx and efflux pathways is to understand how they are coordinately regulated to produce a Ca(2+) switch or encode specific information into a Ca(2+) signature.

Characterisation of 13 glutamate receptor-like genes encoded in the tomato genome by structure, phylogeny and expression profiles

Glutamate receptor-like genes (GLRs) are intimately associated with plant development, defence responses and signalling pathways. Structural and expression analyses of SlGLRs were performed to better characterise their roles in fruit development and metabolism. Utilising recently released tomato genomic sequence data, 15 GLRs were identified in the tomato genome (SlGLRs). Thirteen of these genes were represented by full-length sequences. Phylogenetic analysis of the SlGLRs and the AtGLRs indicates the occurrence of a tomato-specific clade (Clade I) that may have diverged prior to the evolving of other clades. Among the Clade II genes, five (SlGLR2.1, SlGLR2.2, SlGLR2.3, SlGLR2.4, and SlGLR2.5) were located proximally on chromosome 6, indicating possible gene duplication events. The expression level of four of these genes was low in all analysed samples. However, SlGLR2.2 expression level was notably higher, indicating that this gene may be functionally important. The results of this study may provide clues to the functions of the SlGLRs and enable future detailed characterisations of each gene.

Calcium regulation of tip growth: new genes for old mechanisms

We review the recent advances on Ca(2+) in tip-growing cells, with a special focus on pollen tubes. New genes for Ca(2+) pumps, channels and sensing proteins have been recently described, with special emphasis on cyclic nucleotide gated channels (CNGCs) and glutamate receptor-like channels (GLRs). We also review the current state of knowledge in what concerns Ca(2+) sensor and relay proteins, where the knowledge of the cell models is less advanced. While these newly described genes offer promise to a better understanding of the spatial and temporal patterns of Ca(2+) signalling that may be relevant for the formation of the phenotype, we discuss the necessity to investigate further links in the network downstream of the Ca(2+) signature, with a special need for mechanisms of feed-back that might render functional feed-back loops approachable by modelling and genetics. Given the available literature, we conclude on the need to investigate more on the role of two specific classes of proteins, the calcium binding protein kinases (CPKs) and the Calcineurin B-like proteins (CBLs) and their regulatory relationships to ion channels (summarized in Figure 3b).

Cell-specific compartmentation of mineral nutrients is an essential mechanism for optimal plant productivity--another role for TPC1?

Vacuoles of different leaf cell-types vary in their capacity to store specific mineral elements. In Arabidopsis thaliana potassium (K) accumulates preferentially in epidermal and bundle sheath cells whereas calcium (Ca) and magnesium (Mg) are stored at high concentrations only in mesophyll cells. Accumulation of these elements in a particular vacuole can be reciprocal, i.e. as [K]vac increases [Ca]vac decreases. Mesophyll-specific Ca-storage involves CAX1 (a Ca2+/H+ antiporter) and Mg-storage involves MRS2-1/MGT2 and MRS2-5/MGT3 (both Mg2+-transporters), all of which are preferentially expressed in the mesophyll and encode tonoplast-localised proteins. However, what controls leaf-cell [K]vac is less well understood. TPC1 encodes the two-pore Ca2+ channel protein responsible for the tonoplast-localised SV cation conductance, and is highly expressed in cell-types that not preferentially accumulate Ca. Here, we evaluate evidence that TPC1 has a role in maintaining differential K and Ca storage across the leaf, and propose a function for TPC1 in releasing Ca2+ from epidermal and bundle sheath cell vacuoles to maintain low [Ca]vac. Mesophyll-specific Ca storage is essential to maintain apoplastic free Ca concentration at a level that does not perturb a range of physiological parameters including leaf gas exchange, cell wall extensibility and growth. When plants are grown under serpentine conditions (high Mg/Ca ratio), MGT2/MRS2-1 and MGT3/MRS2-5 are required to sequester additional Mg2+ in vacuoles to replace Ca2+ as an osmoticum to maintain growth. An updated model of Ca2+ and Mg2+ transport in leaves is presented as a reference for future interrogation of nutritional flows and elemental storage in plant leaves.

Calcium-permeable channels in plant cells

Calcium signal transduction is a central mechanism by which plants sense and respond to endogenous and environmental stimuli. Cytosolic Ca(2+) elevation is achieved via two cellular pathways, Ca(2+) influx through Ca(2+) channels in the plasma membrane and Ca(2+) release from intracellular Ca(2+) stores. Because of the significance of Ca(2+) channels in cellular signaling, interaction with the environment and developmental processes in plants, a great deal of effort has been invested in recent years with regard to these important membrane proteins. Because of limited space, in this review we focus on recent findings giving insight into both the molecular identity and physiological function of channels that have been suggested to be responsible for the elevation in cytosolic Ca(2+) level, including cyclic nucleotide gated channels, glutamate receptor homologs, two-pore channels and mechanosensitive Ca(2+) -permeable channels. We provide an overview of the regulation of these Ca(2+) channels and their physiological roles and discuss remaining questions.

Magnesium transporters, MGT2/MRS2-1 and MGT3/MRS2-5, are important for magnesium partitioning within Arabidopsis thaliana mesophyll vacuoles

• Magnesium accumulates at high concentrations in dicotyledonous leaves but it is not known in which leaf cell types it accumulates, by what mechanism this occurs and the role it plays when stored in the vacuoles of these cell types. • Cell-specific vacuolar elemental profiles from Arabidopsis thaliana (Arabidopsis) leaves were analysed by X-ray microanalysis under standard and serpentine hydroponic growth conditions and correlated with the cell-specific complement of magnesium transporters identified through microarray analysis and quantitative polymerase chain reaction (qPCR). • Mesophyll cells accumulate the highest vacuolar concentration of magnesium in Arabidopsis leaves and are enriched for members of the MGT/MRS2 family of magnesium transporters. Specifically, AtMGT2/AtMRS2-1 and AtMGT3/AtMRS2-5 were shown to be targeted to the tonoplast and corresponding T-DNA insertion lines had perturbed mesophyll-specific vacuolar magnesium accumulation under serpentine conditions. Furthermore, transcript abundance of these genes was correlated with the accumulation of magnesium under serpentine conditions, in a low calcium-accumulating mutant and across 23 Arabidopsis ecotypes varying in their leaf magnesium concentrations. • We implicate magnesium as a key osmoticum required to maintain growth in low calcium concentrations in Arabidopsis. Furthermore, two tonoplast-targeted members of the MGT/MRS2 family are shown to contribute to this mechanism under serpentine conditions.

Glutamate receptor-like genes form Ca2+ channels in pollen tubes and are regulated by pistil D-serine

Elevations in cytosolic free calcium concentration ([Ca(2+)](cyt)) constitute a fundamental signal transduction mechanism in eukaryotic cells, but the molecular identity of Ca(2+) channels initiating this signal in plants is still under debate. Here, we show by pharmacology and loss-of-function mutants that in tobacco and Arabidopsis, glutamate receptor-like channels (GLRs) facilitate Ca(2+) influx across the plasma membrane, modulate apical [Ca(2+)](cyt) gradient, and consequently affect pollen tube growth and morphogenesis. Additionally, wild-type pollen tubes grown in pistils of knock-out mutants for serine-racemase (SR1) displayed growth defects consistent with a decrease in GLR activity. Our findings reveal a novel plant signaling mechanism between male gametophyte and pistil tissue similar to amino acid-mediated communication commonly observed in animal nervous systems.

Dual localization of plant glutamate receptor AtGLR3.4 to plastids and plasmamembrane

Bioinformatic approaches have allowed the identification in Arabidopsis thaliana of twenty genes encoding for homologues of animal ionotropic glutamate receptors (iGLRs). Some of these putative receptor proteins, grouped into three subfamilies, have been located to the plasmamembrane, but their possible location in organelles has not been investigated so far. In the present work we provide multiple evidence for the plastid localization of a glutamate receptor, AtGLR3.4, in Arabidopsis and tobacco. Biochemical analysis was performed using an antibody shown to specifically recognize both the native protein in Arabidopsis and the recombinant AtGLR3.4 fused to YFP expressed in tobacco. Western blots indicate the presence of AtGLR3.4 in both the plasmamembrane and in chloroplasts. In agreement, in transformed Arabidopsis cultured cells as well as in agroinfiltrated tobacco leaves, AtGLR3.4::YFP is detected both at the plasmamembrane and at the plastid level by confocal microscopy. The photosynthetic phenotype of mutant plants lacking AtGLR3.4 was also investigated. These results identify for the first time a dual localization of a glutamate receptor, revealing its presence in plastids and chloroplasts and opening the way to functional studies.

Cell-specific vacuolar calcium storage mediated by CAX1 regulates apoplastic calcium concentration, gas exchange, and plant productivity in Arabidopsis

The physiological role and mechanism of nutrient storage within vacuoles of specific cell types is poorly understood. Transcript profiles from Arabidopsis thaliana leaf cells differing in calcium concentration ([Ca], epidermis <10 mM versus mesophyll >60 mM) were compared using a microarray screen and single-cell quantitative PCR. Three tonoplast-localized Ca(2+) transporters, CAX1 (Ca(2+)/H(+)-antiporter), ACA4, and ACA11 (Ca(2+)-ATPases), were identified as preferentially expressed in Ca-rich mesophyll. Analysis of respective loss-of-function mutants demonstrated that only a mutant that lacked expression of both CAX1 and CAX3, a gene ectopically expressed in leaves upon knockout of CAX1, had reduced mesophyll [Ca]. Reduced capacity for mesophyll Ca accumulation resulted in reduced cell wall extensibility, stomatal aperture, transpiration, CO(2) assimilation, and leaf growth rate; increased transcript abundance of other Ca(2+) transporter genes; altered expression of cell wall-modifying proteins, including members of the pectinmethylesterase, expansin, cellulose synthase, and polygalacturonase families; and higher pectin concentrations and thicker cell walls. We demonstrate that these phenotypes result from altered apoplastic free [Ca(2+)], which is threefold greater in cax1/cax3 than in wild-type plants. We establish CAX1 as a key regulator of apoplastic [Ca(2+)] through compartmentation into mesophyll vacuoles, a mechanism essential for optimal plant function and productivity.

Sodium transport in plants: a critical review

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

Calcium storage in plants and the implications for calcium biofortification

Calcium (Ca) is an essential nutrient for plants and animals, with key structural and signalling roles, and its deficiency in plants can result in poor biotic and abiotic stress tolerance, reduced crop quality and yield. Likewise, low Ca intake in humans has been linked to various diseases (e.g. rickets, osteoporosis, hypertension and colorectal cancer) which can threaten quality of life and have major economic costs. Biofortification of various food crops with Ca has been suggested as a good method to enhance human intake of Ca and is advocated as an economically and environmentally advantageous strategy. Efforts to enhance Ca content of crops via transgenic means have had promising results. Overall Ca content of transgenic plants has been increased but in some cases adverse affects on plant function have been observed. This suggests that a better understanding of how Ca ions (Ca(2+)) are stored and transported through plants is required to maximise the effectiveness of future approaches.

Alleviation of salt stress-induced inhibition of seed germination in cucumber (Cucumis sativus L.) by ethylene and glutamate

Ethylene is an important plant gas hormone, and the amino acid Glu is emerging as a messenger molecule in plants. To evaluate the role of ethylene and Glu in seed germination and radicle growth under salt stress, effects of 1-aminocyclopropane-1-carboxylic acid (ACC), Ethephon and Glu on germination and radicle growth of cucumber (Cucumis sativus L.) seeds in the absence and presence of 200 mM NaCl were investigated. Seed germination was markedly inhibited by salt stress, and this effect was alleviated by ACC and Ethephon. In contrast to seed germination, ACC and Ethephon had little effect on radicle growth under salt stress. In addition to ethylene, we found exogenous supply of Glu was effective in alleviating the salt stress-induced inhibition of seed germination and radicle growth. The effect of Glu on the seed germination and radicle growth was specific to L-Glu, whereas D-Glu and Gln had no effect. There was an increase in ethylene production during seed imbibition, and salt stress suppressed ethylene production. Exogenous L-Glu evoked ethylene evolution from the imbibed seeds and attenuated the reduction in ethylene evolution induced by salt stress. The alleviative effect of L-Glu on seed germination was diminished by antagonists of ethylene synthesis, aminoethoxyvinylglycine (AVG) and CoCl(2), suggesting that L-Glu is likely to exert its effect on seed germination by modulation of ethylene evolution. These findings demonstrate that ethylene is associated with suppression of seed germination under salt stress and that L-Glu interacts with ethylene in regulation of seed germination under salt stress.

Physiology and biophysics of plant ligand-gated ion channels

Small molecules and metabolites often act as intra- or extracellular messengers in signal transduction pathways. Ligand-gated ion channels provide a mean to transduce those biochemical signals at the membrane into electrical events and ion fluxes. In plants, cyclic nucleotides and glutamate represent intra- and extracellular signalling ligands, respectively. While the former have been shown to regulate voltage-dependent ion channels and are supposed to activate cyclic nucleotide gated (CNG) channels, the latter are perceived by ionotropic glutamate receptors (GLRs). This review summarises our current knowledge about CNG channels and glutamate receptors in plants and their proposed roles in plant development and adaptation to biotic and abiotic stresses.

Detection of a gravitropism phenotype in glutamate receptor-like 3.3 mutants of Arabidopsis thaliana using machine vision and computation

Gene disruption frequently produces no phenotype in the model plant Arabidopsis thaliana, complicating studies of gene function. Functional redundancy between gene family members is one common explanation but inadequate detection methods could also be responsible. Here, newly developed methods for automated capture and processing of time series of images, followed by computational analysis employing modified linear discriminant analysis (LDA) and wavelet-based differentiation, were employed in a study of mutants lacking the Glutamate Receptor-Like 3.3 gene. Root gravitropism was selected as the process to study with high spatiotemporal resolution because the ligand-gated Ca(2+)-permeable channel encoded by GLR3.3 may contribute to the ion fluxes associated with gravity signal transduction in roots. Time series of root tip angles were collected from wild type and two different glr3.3 mutants across a grid of seed-size and seedling-age conditions previously found to be important to gravitropism. Statistical tests of average responses detected no significant difference between populations, but LDA separated both mutant alleles from the wild type. After projecting the data onto LDA solution vectors, glr3.3 mutants displayed greater population variance than the wild type in all four conditions. In three conditions the projection means also differed significantly between mutant and wild type. Wavelet analysis of the raw response curves showed that the LDA-detected phenotypes related to an early deceleration and subsequent slower-bending phase in glr3.3 mutants. These statistically significant, heritable, computation-based phenotypes generated insight into functions of GLR3.3 in gravitropism. The methods could be generally applicable to the study of phenotypes and therefore gene function.

Calcium channels in photosynthetic eukaryotes: implications for evolution of calcium-based signalling

Much of our current knowledge on the mechanisms by which Ca(2+) signals are generated in photosynthetic eukaryotes comes from studies of a relatively small number of model species, particularly green plants and algae, revealing some common features and notable differences between 'plant' and 'animal' systems. Physiological studies from a broad range of algal cell types have revealed the occurrence of animal-like signalling properties, including fast action potentials and fast propagating cytosolic Ca(2+) waves. Genomic studies are beginning to reveal the widespread occurrence of conserved channel types likely to be involved in Ca(2+) signalling. However, certain widespread 'ancient' channel types appear to have been lost by certain groups, such as the embryophytes. More recent channel gene loss is also evident from comparisons of more closely related algal species. The underlying processes that have given rise to the current distributions of Ca(2+) channel types include widespread retention of ancient Ca(2+) channel genes, horizontal gene transfer (including symbiotic gene transfer and acquisition of bacterial genes), gene loss and gene expansion within taxa. The assessment of the roles of Ca(2+) channel genes in diverse physiological, developmental and life history processes represents a major challenge for future studies.

Comparative physiology of elemental distributions in plants

BACKGROUND

Plants contain relatively few cell types, each contributing a specialized role in shaping plant function. With respect to plant nutrition, different cell types accumulate certain elements in varying amounts within their storage vacuole. The role and mechanisms underlying cell-specific distribution of elements in plants is poorly understood.

SCOPE

The phenomenon of cell-specific elemental accumulation has been briefly reviewed previously, but recent technological advances with the potential to probe mechanisms underlying elemental compartmentation have warranted an updated evaluation. We have taken this opportunity to catalogue many of the studies, and techniques used for, recording cell-specific compartmentation of particular elements. More importantly, we use three case-study elements (Ca, Cd and Na) to highlight the basis of such phenomena in terms of their physiological implications and underpinning mechanisms; we also link such distributions to the expression of known ion or solute transporters.

CONCLUSIONS

Element accumulation patterns are clearly defined by expression of key ion or solute transporters. Although the location of element accumulation is fairly robust, alterations in expression of certain solute transporters, through genetic modifications or by growth under stress, result in perturbations to these patterns. However, redundancy or induced pleiotropic expression effects may complicate attempts to characterize the pathways that lead to cell-specific elemental distribution. Accumulation of one element often has consequences on the accumulation of others, which seems to be driven largely to maintain vacuolar and cytoplasmic osmolarity and charge balance, and also serves as a detoxification mechanism. Altered cell-specific transcriptomics can be shown, in part, to explain some of this compensation.

Glutamate induces series of action potentials and a decrease in circumnutation rate in Helianthus annuus

Reports concerning the function of glutamate (Glu) in the electrical and movement phenomena in plants are scarce. Using the method of extracellular measurement, we recorded electrical potential changes in the stem of 3-week-old Helianthus annuus L. plants after injection of Glu solution. Simultaneously, circumnutation movements of the stem were measured with the use of time-lapse images. Injection of Glu solution at millimolar (200, 50, 5 mM) concentrations in the basal part of the stem evoked a series of action potentials (APs). The APs appeared in the site of injection and in different parts of the stem and were propagated acropetally and/or basipetally along the stem. Glu injection also resulted in a transient, approximately 5-h-long decrease in the stem circumnutation rate. The APs initiated and propagating in the sunflower stem after Glu injection testify the existence of a Glu perception system in vascular plants and suggest its involvement in electrical, long-distance signaling. Our experiments also demonstrated that Glu is a factor affecting circumnutation movements.

Shoot Na+ exclusion and increased salinity tolerance engineered by cell type-specific alteration of Na+ transport in Arabidopsis

Soil salinity affects large areas of cultivated land, causing significant reductions in crop yield globally. The Na+ toxicity of many crop plants is correlated with overaccumulation of Na+ in the shoot. We have previously suggested that the engineering of Na+ exclusion from the shoot could be achieved through an alteration of plasma membrane Na+ transport processes in the root, if these alterations were cell type specific. Here, it is shown that expression of the Na+ transporter HKT1;1 in the mature root stele of Arabidopsis thaliana decreases Na+ accumulation in the shoot by 37 to 64%. The expression of HKT1;1 specifically in the mature root stele is achieved using an enhancer trap expression system for specific and strong overexpression. The effect in the shoot is caused by the increased influx, mediated by HKT1;1, of Na+ into stelar root cells, which is demonstrated in planta and leads to a reduction of root-to-shoot transfer of Na+. Plants with reduced shoot Na+ also have increased salinity tolerance. By contrast, plants constitutively expressing HKT1;1 driven by the cauliflower mosaic virus 35S promoter accumulated high shoot Na+ and grew poorly. Our results demonstrate that the modification of a specific Na+ transport process in specific cell types can reduce shoot Na+ accumulation, an important component of salinity tolerance of many higher plants.

Moving cationic minerals to edible tissues: potassium, magnesium, calcium

The principal dietary source to humans of the essential cationic mineral elements potassium, magnesium and calcium is through edible plants. The accumulation of these elements in edible portions is the product of selective transport processes catalysing their short-distance and long-distance movement within a plant. In this article we review recent work describing the identification and characterisation of the molecular mechanisms catalysing the uptake and distribution of potassium, magnesium and calcium between organs, cell types and subcellular compartments. Although potassium and magnesium are redistributed effectively within the plant, calcium concentrations in phloem-fed tissues, such as fruits, seeds and tubers, are generally low. However, limitations to the redistribution of mineral elements within the plant, and its consequences for the biofortification of edible crops, can be overcome by appropriate mineral fertilisation and plant breeding strategies. The techniques of ionomics can help identify better genotypes.

Arabidopsis thaliana glutamate receptor ion channel function demonstrated by ion pore transplantation

Ionotropic glutamate receptors (iGluRs) are ligand-gated cation channels that mediate fast excitatory neurotransmission in the mammalian central nervous system. In the model plant Arabidopsis thaliana, a large family of 20 genes encoding proteins that share similarities with animal iGluRs in sequence and predicted secondary structure has been discovered. Members of this family, termed AtGLRs (A. thaliana glutamate receptors), have been implicated in root development, ion transport, and several metabolic and signalling pathways. However, there is still no direct proof of ligand-gated ion channel function of any AtGLR subunit. We used a domain transplantation technique to directly test whether the putative ion pore domains of AtGLRs can conduct ions. To this end, we transplanted the ion pore domains of 17 AtGLR subunits into rat alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (GluR1) and kainate (GluR6) receptor subunits and tested the resulting chimaeras for ion channel function in the Xenopus oocyte expression system. We show that AtGLR1.1 and AtGLR1.4 have functional Na(+)-, K(+)-, and Ca(2+)-permeable ion pore domains. The properties of currents through the AtGLR1.1 ion pore match those of glutamate-activated currents, depolarisations, and glutamate-triggered Ca(2+) influxes observed in plant cells. We conclude that some AtGLRs have functional non-selective cation pores.

Plant cell types: reporting and sampling with new technologies

Plants have relatively few cell types, but their specialized functions and their interactions are essential for physiology, development, and defense. The contributions of individual cells have been distinguished by methods including in situ reporting, cell sampling, and cell separation, thus far mostly limited to measurement of single transcripts, proteins, or metabolites. Advances in transcriptomics, proteomics, metabolomics, and activity assays with small samples and in the modeling of these data into networks of expression, regulation, interaction, and metabolism make it possible to evaluate the roles of cell types at system levels. Recent analyses include cell types of developing roots, bundle sheath and mesophyll cells of C4-type leaves, xylem and phloem cells of vascular systems, and specialized regions of embryos and shoot apices.