Hyperpolarisation-activated calcium currents found only in cells from the elongation zone of Arabidopsis thaliana roots

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Physiological and TMT-based quantitative proteomic responses of barley to aluminium stress under phosphorus-Piriformospora indica interaction

Barley (Hordeum vulgare L.) is one of the most important cereal crop in the world, and is also the one being seriously affected by heavy metals, particularly aluminium (Al). Keeping in view the utility of barley as food, fodder and raw material for traditional beer brewing, the top-notch quality and higher production of this crop must be sustained. Phosphorus (P) has a quintessential role in plant growth with a potential to relieve symptoms caused by Al poisoning. Displaying a phytopromotive and stress alleviatory potential, Piriformospora indica (P. indica) can improve the stress tolerance in crops. Several studies have been conducted to evaluate the mechanism of Al translocation in a variety of crops including barley, however, the bio-remediative studies related to detoxification and/or sequestration of metals are scarce. Therefore, the current study was carried out to elucidate the tolerance mechanism of an Al-sensitive barley cultivar ZU9 following the colonization with P. indica and exogenous P supply by physio-biochemical, elemental, leaf ultrastructural and root proteome analyses. When compared to the Al alone treated counterparts, the Al + P + P.i treated plants exhibited 4.1-, 1.38-, 2.7 and 1.35-fold improved root and shoot fresh and dry weights, respectively. With the provision of additional phosphorus, the content of P in the root and shoot for Al + P + P.i group was reportedly higher (71.6% and 49.5%, respectively) as compared to the control group. Moreover, inoculation of P. indica combined with P improved barley leaves' cell arrangement and also maintained normal cell wall shape. The root protemics experiment was divided into three groups: Al, Al + P.i and Al + P + P.i. In total, 28, 598, and 823 differentially expressed proteins were found in Al + P.i vs. Al and Al + P + P.i vs. Al, and phenylpropanoid biosynthesis was the most prominently enriched pathway, which contributed significantly to the recuperating effects of P-P. indica interaction. Conslusively, it was found that the percentage of protein related to peroxidase was 70/359 (Al + P + P.i vs. Al) and 92/447 (Al + P + P.i vs. Al + P.i), respectively, which indicated that P. indica in combination with P might be involved in the regulation of peroxidases, increasing the adaptability of barley plants by enhanced reactive oxygen species (ROS) scavenging mechansism.

Ca alleviated Cd-induced toxicity in Salix matsudana by affecting Cd absorption, translocation, subcellular distribution, and chemical forms

Cadmium (Cd), a ubiquitous and highly toxic heavy metal pollutant, is toxic to animals and plants. Calcium (Ca) is an essential component for plant growth and reduces plant Cd absorption by competing with Cd. To gain deeper insight into the effects of Ca on Cd absorption, translocation, subcellular distribution, and chemical forms in S. matsudana seedlings under Cd stress, an investigation was conducted on these properties. Adding Ca alleviated Cd physiological toxicity in S. matsudana, reduced Cd absorption, increased the translocation from roots to shoots, lead to subcellular redistribution of Cd by increasing the proportion of Cd in soluble fractions but decreasing Cd in the cell wall and changed the chemical forms of Cd from 0.6 mol/L HCl- and 2% HAc-extracted Cd to 1 mol/L NaCl-extracted Cd. The energy dispersive X-ray analyses (EDXA) results revealed that after adding Ca, Cd was transferred through the root epidermis, cortex, endodermis, and vascular cylinder, transported to the shoots, and was highly accumulated in leaf epidermal and mesophyll cells, but less in leaf vein and guard cells. The genes involved in Cd uptake and xylem loading included NRAMP1, ZIP8, HMA2, and HMA4, which were up-regulated significantly (P < 0.05) in the Cd and Cd + Ca treatments compared to the control. The findings of this study provide new insight into the mechanism that Ca alleviates Cd toxicity in woody tree species, as well as propose an important prospect of Ca addition for improving the phytoremediation of Cd contamination.

ROOT HAIR DEFECTIVE 2 vesicular delivery to the apical plasma membrane domain during Arabidopsis root hair development

Arabidopsis (Arabidopsis thaliana) root hairs develop as long tubular extensions from the rootward pole of trichoblasts and exert polarized tip growth. The establishment and maintenance of root hair polarity is a complex process involving the local apical production of reactive oxygen species generated by A. thaliana nicotinamide adenine dinucleotide phosphate (NADPH) oxidase respiratory burst oxidase homolog protein C/ROOT HAIR-DEFECTIVE 2 (AtRBOHC/RHD2). Loss-of-function root hair defective 2 (rhd2) mutants have short root hairs that are unable to elongate by tip growth, and this phenotype is fully complemented by GREEN FLUORESCENT PROTEIN (GFP)-RHD2 expressed under the RHD2 promoter. However, the spatiotemporal mechanism of AtRBOHC/RHD2 subcellular redistribution and delivery to the plasma membrane (PM) during root hair initiation and tip growth are still unclear. Here, we used advanced microscopy for detailed qualitative and quantitative analysis of vesicular compartments containing GFP-RHD2 and characterization of their movements in developing bulges and growing root hairs. These compartments, identified by an independent molecular marker mCherry-VTI12 as the trans-Golgi network (TGN), deliver GFP-RHD2 to the apical PM domain, the extent of which corresponds with the stage of root hair formation. Movements of TGN/early endosomes, but not late endosomes, were affected in the bulging domains of the rhd2-1 mutant. Finally, we revealed that structural sterols might be involved in the accumulation, docking, and incorporation of TGN compartments containing GFP-RHD2 to the apical PM of root hairs. These results help in clarifying the mechanism of polarized AtRBOHC/RHD2 targeting, maintenance, and recycling at the apical PM domain, coordinated with different developmental stages of root hair initiation and growth.

Calcium spikes, waves and oscillations in plant development and biotic interactions

The calcium ion (Ca²⁺) is a universal signal in all eukaryotic cells. A fundamental question is how Ca²⁺, a simple cation, encodes complex information with high specificity. Extensive research has established a two-step process (encoding and decoding) that governs the specificity of Ca²⁺ signals. While the encoding mechanism entails a complex array of channels and transporters, the decoding process features a number of Ca²⁺ sensors and effectors that convert Ca²⁺ signals into cellular effects. Along this general paradigm, some signalling components may be highly conserved, but others are divergent among different organisms. In plant cells, Ca²⁺ participates in numerous signalling processes, and here we focus on the latest discoveries on Ca²⁺-encoding mechanisms in development and biotic interactions. In particular, we use examples such as polarized cell growth of pollen tube and root hair in which tip-focused Ca²⁺ oscillations specify the signalling events for rapid cell elongation. In plant-microbe interactions, Ca²⁺ spiking and oscillations hold the key to signalling specificity: while pathogens elicit cytoplasmic spiking, symbiotic microorganisms trigger nuclear Ca²⁺ oscillations. Herbivore attacks or mechanical wounding can trigger Ca²⁺ waves traveling a long distance to transmit and convert the local signal to a systemic defence program in the whole plant. What channels and transporters work together to carve out the spatial and temporal patterns of the Ca²⁺ fluctuations? This question has remained enigmatic for decades until recent studies uncovered Ca²⁺ channels that orchestrate specific Ca²⁺ signatures in each of these processes. Future work will further expand the toolkit for Ca²⁺-encoding mechanisms and place Ca²⁺ signalling steps into larger signalling networks.

Three CNGC Family Members, CNGC5, CNGC6, and CNGC9, Are Required for Constitutive Growth of Arabidopsis Root Hairs as Ca2+-Permeable Channels

The genetic identities of Ca²⁺ channels in root hair (RH) tips essential for constitutive RH growth have remained elusive for decades. Here, we report the identification and characterization of three cyclic nucleotide-gated channel (CNGC) family members, CNGC5, CNGC6, and CNGC9, as Ca²⁺ channels essential for constitutive RH growth in Arabidopsis. We found that the cngc5-1cngc6-2cngc9-1 triple mutant (designated shrh1) showed significantly shorter and branching RH phenotypes as compared with the wild type. The defective RH growth phenotype of shrh1 could be rescued by either the expression of CNGC5, CNGC6, or CNGC9 single gene or by the supply of high external Ca²⁺, but could not be rescued by external K⁺ supply. Cytosolic Ca²⁺ imaging and patch-clamp data in HEK293T cells showed that these three CNGCs all function as Ca²⁺-permeable channels. Cytosolic Ca²⁺ imaging in growing RHs further showed that the Ca²⁺ gradients and their oscillation in RH tips were dramatically attenuated in shrh1 compared with those in the wild type. Phenotypic analysis revealed that these three CNGCs are Ca²⁺ channels essential for constitutive RH growth, with different roles in RHs from the conditional player CNGC14. Moreover, we found that these three CNGCs are involved in auxin signaling in RHs. Taken together, our study identified CNGC5, CNGC6, and CNGC9 as three key Ca²⁺ channels essential for constitutive RH growth and auxin signaling in Arabidopsis.

Chloride as a Beneficial Macronutrient in Higher Plants: New Roles and Regulation

Chloride (Cl-) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3-), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl- has gone from being considered a harmful ion, accidentally absorbed through NO3- transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl- determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl- and NO3- molecules, the NO3-/Cl- plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl- influx (ZmNPF6.4 and ZmNPF6.6), Cl- efflux (AtSLAH3 and AtSLAH1), and Cl- compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl-, as well as for cell turgor regulation and stress tolerance in plants.

The SOS2-SCaBP8 Complex Generates and Fine-Tunes an AtANN4-Dependent Calcium Signature under Salt Stress

Calcium signals act as universal second messengers that trigger many cellular processes in animals and plants, but how specific calcium signals are generated is not well understood. In this study, we determined that AtANN4, a putative calcium-permeable transporter, and its interacting proteins, SCaBP8 and SOS2, generate a calcium signal under salt stress, which initially activates the SOS pathway, a conserved mechanism that modulates ion homeostasis in plants under salt stress. After activation, SCaBP8 promotes the interaction of protein kinase SOS2 with AtANN4, which enhances its phosphorylation by SOS2. This phosphorylation of AtANN4 further increases its interaction with SCaBP8. Both the interaction with and phosphorylation of AtANN4 repress its activity and alter calcium transients and signatures in HEK cells and plants. Our results reveal how downstream targets are required to create a specific calcium signal via a negative feedback regulatory loop, thereby enhancing our understanding of the regulation of calcium signaling.

Genetic engineering of the biosynthesis of glycinebetaine leads to alleviate salt-induced potassium efflux and enhances salt tolerance in tomato plants

Tomato (Solanum lycopersicum cv. 'Moneymaker') was transformed with the choline oxidase gene codA from Arthrobacter globiformis, which was modified to allow for targeting to both chloroplasts and the cytosol. Glycine betaine (GB) was accumulated in transformed plants, while no detectable GB was found in wild-type (WT) plants. Compared to WT plants, transgenic lines showed significantly higher photosynthetic rates (Pn) and antioxidant enzyme activities and lower reactive oxygen species (ROS) accumulation in the leaves when exposed to salt stress. Furthermore, compared with WT plants, K⁺ efflux decreased and Na⁺ efflux increased in roots of transgenic plants under salt stress; resulted in lower Na⁺/K⁺ ratios in transgenic lines. The exogenous application of GB also significantly reduced NaCl-induced K⁺ efflux and increased Na⁺ efflux in WT plants. A qRT-PCR assay indicated that GB enhanced NaCl-induced expression of genes encoding the K⁺ transporter, Na⁺/H⁺ antiporter, and H⁺-ATPase. These results suggest that the enhanced salt tolerance conferred by codA in transgenic tomato plants might be due to the regulation of ion channel and transporters by GB, which would allow high potassium levels and low sodium levels to be maintained in transgenic plants under salt stress condition.

SLAH1, a homologue of the slow type anion channel SLAC1, modulates shoot Cl- accumulation and salt tolerance in Arabidopsis thaliana

Salinity tolerance is correlated with shoot chloride (Cl(-)) exclusion in multiple crops, but the molecular mechanisms of long-distance Cl(-) transport are poorly defined. Here, we characterize the in planta role of AtSLAH1 (a homologue of the slow type anion channel-associated 1 (SLAC1)). This protein, localized to the plasma membrane of root stelar cells, has its expression reduced by salt or ABA, which are key predictions for a protein involved with loading Cl(-) into the root xylem. Artificial microRNA knockdown mutants of AtSLAH1 had significantly reduced shoot Cl(-) accumulation when grown under low Cl(-), whereas shoot Cl(-) increased and the shoot nitrate/chloride ratio decreased following AtSLAH1 constitutive or stelar-specific overexpression when grown in high Cl(-) In both sets of overexpression lines a significant reduction in shoot biomass over the null segregants was observed under high Cl(-) supply, but not low Cl(-) supply. Further in planta data showed AtSLAH3 overexpression increased the shoot nitrate/chloride ratio, consistent with AtSLAH3 favouring nitrate transport. Heterologous expression of AtSLAH1 in Xenopus laevis oocytes led to no detectible transport, suggesting the need for post-translational modifications for AtSLAH1 to be active. Our in planta data are consistent with AtSLAH1 having a role in controlling root-to-shoot Cl(-) transport.

Magnesium alleviates plant toxicity of aluminium and heavy metals

Magnesium (Mg) is an essential nutrient that can alleviate soilborne toxicity of many ions. This review paper critically assesses the literature on interactions and mechanisms influencing Mg alleviation of aluminium (Al) and heavy metal toxicity. Hydrated radii of Mg2+ and Al3+ are similar; therefore, these two ions compete for binding to ion transporters and other important biological molecules. In monocotyledonous species such as rice and wheat, millimolar concentrations of Mg alleviate Al toxicity, mainly by decreasing Al saturation and activity at cell wall and plasma membrane binding sites. In dicotyledonous legume species such as soybean (Glycine max), rice bean (Vigna umbellata) and broad bean (Vicia faba), micromolar concentrations of Mg may enhance biosynthesis of organic ligands and thus underpin alleviation of Al toxicity. Resistance to Al may be enhanced by increased expression of the genes coding for Mg transporters, as well as by upregulation of activity of Mg-transport proteins; intracellular Mg2+ activity may thus be increased under Al stress, which may increase the activity of H+-ATPases. In Vicia faba, Mg-related enhancement in the activity of plasma membrane H+-ATPase under Al stress was found to be due to post-translational modification (increased phosphorylation of the penultimate threonine as well as association with regulatory 14-3-3 proteins), resulting in increased resistance to Al stress. Magnesium can alleviate heavy metal stress by decreasing negative electrical potential and thus metal ion activities at the plasma membrane surface (physico-chemical competition), by enhancing activities of enzymes involved in biosynthesis of organic ligands, and by increasing vacuolar sequestration of heavy metals via increasing H+-pumping activity at the tonoplast. Future work should concentrate on characterising the role of intracellular Mg2+ homeostasis and Mg transporters in alleviating metal stress as well as in transcriptional, translational and post-translational regulation of H+-pumps and enzymes involved in biosynthesis and exudation of organic ligands.

Mn-euvering manganese: the role of transporter gene family members in manganese uptake and mobilization in plants

Manganese (Mn), an essential trace element, is important for plant health. In plants, Mn serves as a cofactor in essential processes such as photosynthesis, lipid biosynthesis and oxidative stress. Mn deficient plants exhibit decreased growth and yield and are more susceptible to pathogens and damage at freezing temperatures. Mn deficiency is most prominent on alkaline soils with approximately one third of the world's soils being too alkaline for optimal crop production. Despite the importance of Mn in plant development, relatively little is known about how it traffics between plant tissues and into and out of organelles. Several gene transporter families have been implicated in Mn transport in plants. These transporter families include NRAMP (natural resistance associated macrophage protein), YSL (yellow stripe-like), ZIP (zinc regulated transporter/iron-regulated transporter [ZRT/IRT1]-related protein), CAX (cation exchanger), CCX (calcium cation exchangers), CDF/MTP (cation diffusion facilitator/metal tolerance protein), P-type ATPases and VIT (vacuolar iron transporter). A combination of techniques including mutant analysis and Synchrotron X-ray Fluorescence Spectroscopy can assist in identifying essential transporters of Mn. Such knowledge would vastly improve our understanding of plant Mn homeostasis.

Low-pH and aluminum resistance in arabidopsis correlates with high cytosolic magnesium content and increased magnesium uptake by plant roots

Low-pH stress and Al(3+) toxicity affect root growth in acid soils. It was hypothesized that the capacity of genotypes to maintain Mg(2+) uptake in acidic environments may contribute to low-pH and Al resistance, but explicit evidence is lacking. In this work, an Al-resistant alr104 mutant and two Al-sensitive mutants (als5 and als3) of Arabidopsis thaliana were compared with the wild type (Col-0) for Mg(2+) uptake and intracellular Mg(2+) concentration under low-pH and combined low-pH/Al stresses. Magnesium accumulation in roots was measured in long-term (7 d) experiments. The Mg(2+) fluxes were measured using ion-sensitive microelectrodes at the distal elongation and the mature root zones in short-term (0-60 min) experiments. Intracellular Mg(2+) concentrations were measured in intact root cells at the distal elongation zone using magnesium-specific fluorescent dye and fluorescent lifetime imaging (FLIM) analysis. Under low-pH stress, Arabidopsis mutants als5 and alr104 maintained a higher Mg concentration in roots, and had greater Mg(2+) influx than the wild type and the als3 mutant. Under combined low-pH/Al treatment, Al-resistant genotypes (wild type and alr104) maintained a higher Mg(2+) accumulation, and had a higher Mg(2+) influx and higher intracellular Mg(2+) concentration than Al-sensitive genotypes (als3 and als5). Overall, these results show that increased Mg(2+) uptake correlates with an enhanced capacity of Arabidopsis genotypes to cope with low-pH and combined low-pH/Al stresses.

Developmental distribution of the plasma membrane-enriched proteome in the maize primary root growth zone

Within the growth zone of the maize primary root, there are well-defined patterns of spatial and temporal organization of cell division and elongation. However, the processes underlying this organization remain poorly understood. To gain additional insights into the differences amongst the defined regions, we performed a proteomic analysis focusing on fractions enriched for plasma membrane (PM) proteins. The PM is the interface between the plant cell and the apoplast and/or extracellular space. As such, it is a key structure involved in the exchange of nutrients and other molecules as well as in the integration of signals that regulate growth and development. Despite the important functions of PM-localized proteins in mediating these processes, a full understanding of dynamic changes in PM proteomes is often impeded by low relative concentrations relative to total proteins. Using a relatively simple strategy of treating microsomal fractions with Brij-58 detergent to enrich for PM proteins, we compared the developmental distribution of proteins within the root growth zone which revealed a number of previously known as well as novel proteins with interesting patterns of abundance. For instance, the quantitative proteomic analysis detected a gradient of PM aquaporin proteins similar to that previously reported using immunoblot analyses, confirming the veracity of this strategy. Cellulose synthases increased in abundance with increasing distance from the root apex, consistent with expected locations of cell wall deposition. The similar distribution pattern for Brittle-stalk-2-like protein implicates that this protein may also have cell wall related functions. These results show that the simplified PM enrichment method previously demonstrated in Arabidopsis can be successfully applied to completely unrelated plant tissues and provide insights into differences in the PM proteome throughout growth and development zones of the maize primary root.

Anion channels in plant cells

Plant anion channels allow the efflux of anions from cells. They are involved in turgor pressure control, changes in membrane potential, organic acid excretion, tolerance to salinity and inorganic anion nutrition. The recent molecular identification of anion channel genes in guard cells and in roots allows a better understanding of their function and of the mechanisms that control their activation.

AtHKT1;1 mediates nernstian sodium channel transport properties in Arabidopsis root stelar cells

The Arabidopsis AtHKT1;1 protein was identified as a sodium (Na⁺) transporter by heterologous expression in Xenopus laevis oocytes and Saccharomyces cerevisiae. However, direct comparative in vivo electrophysiological analyses of a plant HKT transporter in wild-type and hkt loss-of-function mutants has not yet been reported and it has been recently argued that heterologous expression systems may alter properties of plant transporters, including HKT transporters. In this report, we analyze several key functions of AtHKT1;1-mediated ion currents in their native root stelar cells, including Na⁺ and K⁺ conductances, AtHKT1;1-mediated outward currents, and shifts in reversal potentials in the presence of defined intracellular and extracellular salt concentrations. Enhancer trap Arabidopsis plants with GFP-labeled root stelar cells were used to investigate AtHKT1;1-dependent ion transport properties using patch clamp electrophysiology in wild-type and athkt1;1 mutant plants. AtHKT1;1-dependent currents were carried by sodium ions and these currents were not observed in athkt1;1 mutant stelar cells. However, K⁺ currents in wild-type and athkt1;1 root stelar cell protoplasts were indistinguishable correlating with the Na⁺ over K⁺ selectivity of AtHKT1;1-mediated transport. Moreover, AtHKT1;1-mediated currents did not show a strong voltage dependence in vivo. Unexpectedly, removal of extracellular Na⁺ caused a reduction in AtHKT1;1-mediated outward currents in Columbia root stelar cells and Xenopus oocytes, indicating a role for external Na⁺ in regulation of AtHKT1;1 activity. Shifting the NaCl gradient in root stelar cells showed a Nernstian shift in the reversal potential providing biophysical evidence for the model that AtHKT1;1 mediates passive Na⁺ channel transport properties.

(Questions)(n) on phloem biology. 1. Electropotential waves, Ca2+ fluxes and cellular cascades along the propagation pathway

This review explores the relationships between electrical long-distance signalling, Ca(2+) influx coincident with propagation of electropotential waves, and cellular responses to Ca(2+) influx including the consequences for sieve-tube conductivity and mass flow. Ca(2+) influx is inherent to electropotential waves and appears to constitute the key link between rapid physical signals and resultant chemical cascades in sieve tubes and adjacent cells. Members of several channel groups are likely involved the regulation of Ca(2+) levels in sieve elements. Among them are hyperpolarization-activated, depolarization-activated, and mechanosensitive Ca(2+) channels located in the plasma membrane and Ca(2+) dependent Ca(2+) channels that reside in ER-membranes of sieve elements. These channels collectively determine intracellular Ca(2+) levels in sieve elements and their neighbour cells. The latter cells react to Ca(2+) elevation by inducing diverse functional responses dependent on the cell type. If the Ca(2+) concentration in sieve elements surpasses a threshold level, dual sieve-plate occlusion by proteins and callose deposition is triggered. Occlusion is reversed when Ca(2+) levels subside. Electrical messages may regulate the degree of sieve plate hydraulic conductivity in intact plants by partial sieve-plate occlusion that has a major impact on volume flow through sieve tubes. Furthermore, complete but temporary occlusion of sieve tubes may modify mass flow patterns in intact plants.

A bicistronic, Ubiquitin-10 promoter-based vector cassette for transient transformation and functional analysis of membrane transport demonstrates the utility of quantitative voltage clamp studies on intact Arabidopsis root epidermis

To date the use of fluorescent reporter constructs in analysing membrane transport has been limited primarily to cell lines expressing stably either the tagged transporter protein(s) or markers to identify lineages of interest. Strategies for transient expression have yet to be exploited in transport analysis, despite their wide application in cellular imaging studies. Here we describe a Gateway-compatible, bicistronic vector, incorporating the constitutive Ubiqutin-10 gene promoter of Arabidopsis that gives prolonged expression after transient transformation and enables fluorescence marking of cells without a fusion construct. We show that Arabidopsis root epidermal cells are readily transformed by co-cultivation with Agrobacterium and are tractable for quantitative electrophysiological analysis. As a proof of principle, we transiently transformed Arabidopsis with the bicistronic vector carrying GFP as the fluorescent marker and, separately, the integral plasma membrane protein SYP121 essential for the inward K+ channel current. We demonstrate that transient expression of SYP121 in syp121 mutant plants is sufficient to rescue the K+ current in vivo. The combination of transient expression and use of the bicistronic vector promises significant advantages for studies of membrane transport and nutrient acquisition in roots.

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.

Role of magnesium in alleviation of aluminium toxicity in plants

Magnesium is pivotal for activating a large number of enzymes; hence, magnesium plays an important role in numerous physiological and biochemical processes affecting plant growth and development. Magnesium can also ameliorate aluminium phytotoxicity, but literature reports on the dynamics of magnesium homeostasis upon exposure to aluminium are rare. Herein existing knowledge on the magnesium transport mechanisms and homeostasis maintenance in plant cells is critically reviewed. Even though overexpression of magnesium transporters can alleviate aluminium toxicity in plants, the mechanisms governing such alleviation remain obscure. Possible magnesium-dependent mechanisms include (i) better carbon partitioning from shoots to roots; (ii) increased synthesis and exudation of organic acid anions; (iii) enhanced acid phosphatase activity; (iv) maintenance of proton-ATPase activity and cytoplasmic pH regulation; (v) protection against an aluminium-induced cytosolic calcium increase; and (vi) protection against reactive oxygen species. Future research should concentrate on assessing aluminium toxicity and tolerance in plants with overexpressed or antisense magnesium transporters to increase understanding of the aluminium-magnesium interaction.

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.

R type anion channel: a multifunctional channel seeking its molecular identity

Plant genomes code for channels involved in the transport of cations, anions and uncharged molecules through membranes. Although the molecular identity of channels for cations and uncharged molecules has progressed rapidly in the recent years, the molecular identity of anion channels has lagged behind. Electrophysiological studies have identified S-type (slow) and R-type (rapid) anion channels. In this brief review, we summarize the proposed functions of the R-type anion channels which, like the S-type, were first characterized by electrophysiology over 20 years ago, but unlike the S-type, have still yet to be cloned. We show that the R-type channel can play multiple roles.

Transmembrane potential measurements on plant cells using the voltage-sensitive dye ANNINE-6

The charging of the plasma membrane is a necessary condition for the generation of an electric-field-induced permeability increase of the plasmalemma, which is usually explained by the creation and the growth of aqueous pores. For cells suspended in physiological buffers, the time domain of membrane charging is in the submicrosecond range. Systematic measurements using Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) protoplasts stained with the fast voltage-sensitive fluorescence dye ANNINE-6 have been performed using a pulsed laser fluorescence microscopy setup with a time resolution of 5 ns. A clear saturation of the membrane voltage could be measured, caused by a strong membrane permeability increase, commonly explained by enhanced pore formation, which prevents further membrane charging by external electric field exposure. The field strength dependence of the protoplast's transmembrane potential V (M) shows strong asymmetric saturation characteristics due to the high resting potential of the plants plasmalemma. At the pole of the hyperpolarized hemisphere of the cell, saturation starts at an external field strength of 0.3 kV/cm, resulting in a measured transmembrane voltage shift of ∆V(M) = -150 mV, while on the cathodic (depolarized) cell pole, the threshold for enhanced pore formation is reached at a field strength of approximately 1.0 kV/cm and ∆V(M) = 450 mV, respectively. From this asymmetry of the measured maximum membrane voltage shifts, the resting potential of BY-2 protoplasts at the given experimental conditions can be determined to V(R) = -150 mV. Consequently, a strong membrane permeability increase occurs when the membrane voltage diverges |V(M)| = 300 mV from the resting potential of the protoplast. The largest membrane voltage change at a given external electric field occurs at the cell poles. The azimuthal dependence of the transmembrane potential, measured in angular intervals of 10° along the circumference of the cell, shows a flattening and a slight decrease at higher fields at the pole region due to enhanced pore formation. Additionally, at the hyperpolarized cell pole, a polarization reversal could be observed at an external field range around 1.0 kV/cm. This behavior might be attributed to a fast charge transfer through the membrane at the hyperpolarized pole, e.g., by voltage-gated channels.

Spermidine oxidase-derived H₂O₂ regulates pollen plasma membrane hyperpolarization-activated Ca(2+) -permeable channels and pollen tube growth

Spermidine (Spd) has been correlated with various physiological and developmental processes in plants, including pollen tube growth. In this work, we show that Spd induces an increase in the cytosolic Ca(2+) concentration that accompanies pollen tube growth. Using the whole-cell patch clamp and outside-out single-channel patch clamp configurations, we show that exogenous Spd induces a hyperpolarization-activated Ca(2+) current: the addition of Spd cannot induce the channel open probability increase in excised outside-out patches, indicating that the effect of Spd in the induction of Ca(2+) currents is exerted via a second messenger. This messenger is hydrogen peroxide (H₂O₂), and is generated during Spd oxidation, a reaction mediated by polyamine oxidase (PAO). These reactive oxygen species trigger the opening of the hyperpolarization-activated Ca(2+) -permeable channels in pollen. To provide further evidence that PAO is in fact responsible for the effect of Spd on the Ca(2+) -permeable channels, two Arabidopsis mutants lacking expression of the peroxisomal-encoding AtPAO3 gene, were isolated and characterized. Pollen from these mutants was unable to induce the opening of the Ca(2+) -permeable channels in the presence of Spd, resulting in reduced pollen tube growth and seed number. However, a high Spd concentration triggers a Ca(2+) influx beyond the optimal, which has a deleterious effect. These findings strongly suggest that the Spd-derived H₂O₂ signals Ca(2+) influx, thereby regulating pollen tube growth.

H2O2 and cytosolic Ca2+ signals triggered by the PM H-coupled transport system mediate K+/Na+ homeostasis in NaCl-stressed Populus euphratica cells

Using confocal microscopy, X-ray microanalysis and the scanning ion-selective electrode technique, we investigated the signalling of H(2)O(2), cytosolic Ca(2+) ([Ca(2+)](cyt)) and the PM H(+)-coupled transport system in K(+)/Na(+) homeostasis control in NaCl-stressed calluses of Populus euphratica. An obvious Na(+)/H(+) antiport was seen in salinized cells; however, NaCl stress caused a net K(+) efflux, because of the salt-induced membrane depolarization. H(2)O(2) levels, regulated upwards by salinity, contributed to ionic homeostasis, because H(2)O(2) restrictions by DPI or DMTU caused enhanced K(+) efflux and decreased Na(+)/H(+) antiport activity. NaCl induced a net Ca(2+) influx and a subsequent rise of [Ca(2+)](cyt), which is involved in H(2)O(2)-mediated K(+)/Na(+) homeostasis in salinized P. euphratica cells. When callus cells were pretreated with inhibitors of the Na(+)/H(+) antiport system, the NaCl-induced elevation of H(2)O(2) and [Ca(2+)](cyt) was correspondingly restricted, leading to a greater K(+) efflux and a more pronounced reduction in Na(+)/H(+) antiport activity. Results suggest that the PM H(+)-coupled transport system mediates H(+) translocation and triggers the stress signalling of H(2)O(2) and Ca(2+), which results in a K(+)/Na(+) homeostasis via mediations of K(+) channels and the Na(+)/H(+) antiport system in the PM of NaCl-stressed cells. Accordingly, a salt stress signalling pathway of P. euphratica cells is proposed.

Making sense out of Ca(2+) signals: their role in regulating stomatal movements

Plant cells maintain high Ca(2+) concentration gradients between the cytosol and the extracellular matrix, as well as intracellular compartments. During evolution, the regulatory mechanisms, maintaining low cytosolic free Ca(2+) concentrations, most likely provided the backbone for the development of Ca(2+)-dependent signalling pathways. In this review, the current understanding of molecular mechanisms involved in Ca(2+) homeostasis of plants cells is evaluated. The question is addressed to which extent the mechanisms, controlling the cytosolic Ca(2+) concentration, are linked to Ca(2+)-based signalling. A large number of environmental stimuli can evoke Ca(2+) signals, but the Ca(2+)-induced responses are likely to differ depending on the stimulus applied. Two mechanisms are put forward to explain signal specificity of Ca(2+)-dependent responses. A signal may evoke a specific Ca(2+) signature that is recognized by downstream signalling components. Alternatively, Ca(2+) signals are accompanied by Ca(2+)-independent signalling events that determine the specificity of the response. The existence of such parallel-acting pathways explains why guard cell responses to abscisic acid (ABA) can occur in the absence, as well as in the presence, of Ca(2+) signals. Future research may shed new light on the relation between parallel acting Ca(2+)-dependent and -independent events, and may provide insights in their evolutionary origin.

Fluorescence resonance energy transfer-sensitized emission of yellow cameleon 3.60 reveals root zone-specific calcium signatures in Arabidopsis in response to aluminum and other trivalent cations

Fluorescence resonance energy transfer-sensitized emission of the yellow cameleon 3.60 was used to study the dynamics of cytoplasmic calcium ([Ca(2+)](cyt)) in different zones of living Arabidopsis (Arabidopsis thaliana) roots. Transient elevations of [Ca(2+)](cyt) were observed in response to glutamic acid (Glu), ATP, and aluminum (Al(3+)). Each chemical induced a [Ca(2+)](cyt) signature that differed among the three treatments in regard to the onset, duration, and shape of the response. Glu and ATP triggered patterns of [Ca(2+)](cyt) increases that were similar among the different root zones, whereas Al(3+) evoked [Ca(2+)](cyt) transients that had monophasic and biphasic shapes, most notably in the root transition zone. The Al(3+)-induced [Ca(2+)](cyt) increases generally started in the maturation zone and propagated toward the cap, while the earliest [Ca(2+)](cyt) response after Glu or ATP treatment occurred in an area that encompassed the meristem and elongation zone. The biphasic [Ca(2+)](cyt) signature resulting from Al(3+) treatment originated mostly from cortical cells located at 300 to 500 mu m from the root tip, which could be triggered in part through ligand-gated Glu receptors. Lanthanum and gadolinium, cations commonly used as Ca(2+) channel blockers, elicited [Ca(2+)](cyt) responses similar to those induced by Al(3+). The trivalent ion-induced [Ca(2+)](cyt) signatures in roots of an Al(3+)-resistant and an Al(3+)-sensitive mutant were similar to those of wild-type plants, indicating that the early [Ca(2+)](cyt) changes we report here may not be tightly linked to Al(3+) toxicity but rather to a general response to trivalent cations.

Plasma membrane-associated proline-rich extensin-like receptor kinase 4, a novel regulator of Ca signalling, is required for abscisic acid responses in Arabidopsis thaliana

Plant roots respond to environmental stresses or the exogenous plant hormone abscisic acid (ABA) by undergoing marked physiological and morphological changes. We show here that PERK4, a gene that encodes a member of the Arabidopsis thaliana proline-rich extensin-like receptor kinase family, plays an important role in ABA responses. Mutation of PERK4 by T-DNA insertion decreased sensitivity to ABA with respect to seed germination, seedling growth and primary root tip growth. The effect on root growth was due to enhanced cell elongation rather than cell division. The cytosolic free calcium concentration and Ca(2+) channel currents were lower in perk4 root cells than in wild-type cells in the presence of ABA. Root growth was similar in wild-type and perk4 plants after the application of a Ca(2+) channel blocker. PERK4 localised to the plasma membrane, and was shown to be an ABA- and Ca(2+)-activated protein kinase. Our data suggest that the receptor-like kinase encoded by PERK4 functions at an early stage of ABA signalling to inhibit root cell elongation by perturbing Ca(2+) homeostasis.

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.

NaCl-induced changes in cytosolic free Ca2+ in Arabidopsis thaliana are heterogeneous and modified by external ionic composition

Increases in cytosolic free Ca(2+) ([Ca(2+)](cyt)) are common to many stress-activated signalling pathways, including the response to saline environments. We have investigated the nature of NaCl-induced [Ca(2+)](cyt) signals in whole Arabidopsis thaliana seedlings using aequorin. We found that NaCl-induced increases in [Ca(2+)](cyt) are heterogeneous and mainly restricted to the root. Both the concentration of NaCl and the composition of the solution bathing the root have profound effects on the magnitude and dynamics of NaCl-induced increases in [Ca(2+)](cyt). Alteration of external K(+) concentration caused changes in the temporal and spatial pattern of [Ca(2+)](cyt) increase, providing evidence for Na(+)-induced Ca(2+) influx across the plasma membrane. The effects of various pharmacological agents on NaCl-induced increases in [Ca(2+)](cyt) indicate that NaCl may induce influx of Ca(2+) through both plasma membrane and intracellular Ca(2+)-permeable channels. Analysis of spatiotemporal [Ca(2+)](cyt) dynamics using photon-counting imaging revealed additional levels of complexity in the [Ca(2+)](cyt) signal that may reflect the oscillatory nature of NaCl-induced changes in single cells.

Two voltage-dependent calcium channels co-exist in the apical plasma membrane of Arabidopsis thaliana root hairs

Calcium (Ca2+)-permeable plasma membrane ion channels are critical to root hair elongation and signalling. Arabidopsis thaliana root hair plasma membrane contains a hyperpolarization-activated Ca2+ channel (HACC) conductance. Here, the co-residence of HACC with a depolarization-activated Ca2+ channel (DACC) conductance has been investigated. Whole-cell patch-clamping of apical plasma membrane has been used to study Ca2+ conductances and reveal the negative slope conductance typical of DACCs. Specific voltage protocols, Ba(2+)-permeation and inhibition by the cation channel blocker Gd3+ have been used to identify the DACC conductance. The Gd3+ sensitive DACC conductance was identified in only a minority of cells. DACC activity was quickly masked by the development of the HACC conductance. However, in the period between the disappearance of the negative slope conductance and the predominance of HACC, DACC activity could still be detected. A DACC conductance coexists with HACC in the root hair apical plasma membrane and could provide Ca2+ influx over a wide voltage range, consistent with a role in signalling.

Characterisation of lanthanum toxicity for root growth of Arabidopsis thaliana from the aspect of natural genetic variation

The mechanism of lanthanum (La³⁺) toxicity on root growth of Arabidopsis was studied by physiological and genetic approaches using Landsberg erecta (Ler) × Columbia (Col) recombinant inbred lines (RILs) and other natural accessions. Quantitative trait locus (QTL) analyses revealed regulation of La³⁺ tolerance of the Ler × Col RILs by multiple genetic factors consisted of three significant QTLs and seven epistatic interacting loci pairs. The La content in the root tip was not correlated with La³⁺ tolerance in the RILs, indicating that the observed La³⁺ rhizotoxicity was not related to direct toxicity of La³⁺ in the symplast. The La³⁺ tolerance of root growth in the RILs was not correlated with Al³⁺ and Cu²⁺ tolerances, but was correlated with tolerances for other rare earth elements, including Gd³⁺, a known Ca²⁺ channel antagonist, and verapamil, a Ca²⁺ channel blocker. The genetic architecture of verapamil tolerance in root growth, which was identified by QTL analysis, was closely related to that of La³⁺ tolerance. La³⁺ tolerance and verapamil tolerance or Gd³⁺ tolerance in natural accessions of Arabidopsis also showed a positive correlation. These results indicate that the major La³⁺ toxicity on the root growth of Arabidopsis may involve its action as a Ca²⁺ channel antagonist.

Spatial variation in H2O2 response of Arabidopsis thaliana root epidermal Ca2+ flux and plasma membrane Ca2+ channels

Hydrogen peroxide is an important regulatory agent in plants. This study demonstrates that exogenous H2O2 application to Arabidopsis thaliana root epidermis results in dose-dependent transient increases in net Ca2+ influx. The magnitude and duration of the transients were greater in the elongation zone than in the mature epidermis. In both regions, treatment with the cation channel blocker Gd3+ prevented H2O2-induced net Ca2+ influx, consistent with application of exogenous H2O2 resulting in the activation of plasma membrane Gd3+-sensitive Ca2+-influx pathways. Application of 10 mm H2O2 to the external plasma membrane face of elongation zone epidermal protoplasts resulted in the appearance of a hyperpolarization-activated Ca2+-permeable conductance. This conductance differed from that previously characterized as being responsive to extracellular hydroxyl radicals. In contrast, in mature epidermal protoplasts a plasma membrane hyperpolarization-activated Ca2+-permeable channel was activated only when H2O2 was present at the intracellular membrane face. Channel open probability increased with intracellular [H2O2] and at hyperpolarized voltages. Unitary conductance decreased thus: Ba2+ > Ca2+ (14.5 pS) > Mg2+ > Zn2+ (20 mM external cation, 1 mM H2O2). Lanthanides and Zn2+ (but not TEA+) suppressed the open probability without affecting current amplitude. The results suggest spatial heterogeneity and differential sensitivity of Ca2+ channel activation by reactive oxygen species in the root that could underpin signalling.

Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development

Nonselective cation channels (NSCCs) catalyse passive fluxes of cations through plant membranes. NSCCs do not, or only to a small extent, select between monovalent cations, and several are also permeable to divalent cations. Although a number of NSCC genes has been identified in plant genomes, a direct correlation between gene products and in vivo observed currents is still largely absent for most NSCCs. In this review, physiological functions and molecular properties of NSCCs are critically discussed. Recent studies have demonstrated that NSCCs are directly involved in a multitude of stress responses, growth and development, uptake of nutrients and calcium signalling. NSCCs can also function in the perception of external stimuli and as signal transducers for reactive oxygen species, pathogen elicitors, cyclic nucleotides, membrane stretch, amino acids and purines.

Ethylene activates a plasma membrane Ca(2+)-permeable channel in tobacco suspension cells

Here, the effects of the ethylene-releasing compound, ethephon, and the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), on ionic currents across plasma membranes and on the cytosolic Ca(2+) activity ([Ca(2+)](c)) of tobacco (Nicotiana tabacum) suspension cells were characterized using a patch-clamp technique and confocal laser scanning microscopy. Exposure of tobacco protoplasts to ethephon and ACC led to activation of a plasma membrane cation channel that was permeable to Ba(2+), Mg(2+) and Ca(2+), and inhibited by La(3+), Gd(3+) and Al(3+). The ethephon- and ACC-induced Ca(2+)-permeable channel was abolished by the antagonist of ethylene perception (1-metycyclopropene) and by the inhibitor of ACC synthase (aminovinylglycin), indicating that activation of the Ca(2+)-permeable channels results from ethylene. Ethephon elicited an increase in the [Ca(2+)](c) of tobacco suspension cells, as visualized by the Ca(2+)-sensitive probe Fluo-3 and confocal microscopy. The ethephon-induced elevation of [Ca(2+)](c) was markedly inhibited by Gd(3+) and BAPTA, suggesting that an influx of Ca(2+) underlies the elevation of [Ca(2+)](c). These results indicate that an elevation of [Ca(2+)](c), resulting from activation of the plasma membrane Ca(2+)-permeable channels by ethylene, is an essential component in ethylene signaling in plants.

The Root Apex of Arabidopsis thaliana Consists of Four Distinct Zones of Growth Activities: Meristematic Zone, Transition Zone, Fast Elongation Zone and Growth Terminating Zone

In the growing apex of Arabidopsis thaliana primary roots, cells proceed through four distinct phases of cellular activities. These zones and their boundaries can be well defined based on their characteristic cellular activities. The meristematic zone comprises, and is limited to, all cells that undergo mitotic divisions. Detailed in vivo analysis of transgenic lines reveals that, in the Columbia-0 ecotype, the meristem stretches up to 200 microm away from the junction between root and root cap (RCJ). In the transition zone, 200 to about 520 microm away from the RCJ, cells undergo physiological changes as they prepare for their fast elongation. Upon entering the transition zone, they progressively develop a central vacuole, polarize the cytoskeleton and remodel their cell walls. Cells grow slowly during this transition: it takes ten hours to triplicate cell length from 8.5 to about 35 microm in the trichoblast cell files. In the fast elongation zone, which covers the zone from 520 to about 850 microm from the RCJ, cell length quadruplicates to about 140 microm in only two hours. This is accompanied by drastic and specific cell wall alterations. Finally, root hairs fully develop in the growth terminating zone, where root cells undergo a minor elongation to reach their mature lengths.

Simultaneous flux and current measurement from single plant protoplasts reveals a strong link between K+ fluxes and current, but no link between Ca2+ fluxes and current

We present a thorough calibration and verification of a combined non-invasive self-referencing microelectrode-based ion-flux measurement and whole-cell patch clamp system as a novel and powerful tool for the study of ion transport. The system is shown to be capable of revealing the movement of multiple ions across the plasma membrane of a single protoplast at multiple voltages and in complex physiologically relevant solutions. Wheat root protoplasts are patch clamped in the whole-cell configuration and current-voltage relations obtained whilst monitoring net K+ and Ca2+ flux adjacent to the membrane with ion-selective electrodes. At each voltage, net ion flux (nmol m(-2) sec(-1)) is converted to an equivalent current density (mA m(-2)) taking into account geometry and electrode efficiency, and compared with the net current density measured with the patch clamp system. Using this technique, it is demonstrated that the K+-permeable outwardly rectifying conductance (KORC) is responsible for net outward K+ movement across the plasma membrane [1:1 flux-to-current ratio (1.21 +/- 0.14 SEM, n = 15)]. Variation in the K+ flux-to-current ratio among single protoplasts suggests a heterogeneous distribution of KORC channels on the membrane surface. As a demonstration of the power of the technique we show that despite a significant Ca2+ permeability being associated with KORC (analysis of tail current reversal potentials), there is no correlation between Ca2+ flux and KORC activity. A very significant observation is that large Ca2+ fluxes are electrically silent and probably tightly coupled to compensatory charge movements. This analysis demonstrates that it is mandatory to measure flux and currents simultaneously to investigate properly Ca2+ transport mechanisms and selectivity of ion channels in general.

Effects of feeding Spodoptera littoralis on lima bean leaves. III. Membrane depolarization and involvement of hydrogen peroxide

In response to herbivore (Spodoptera littoralis) attack, lima bean (Phaseolus lunatus) leaves produced hydrogen peroxide (H(2)O(2)) in concentrations that were higher when compared to mechanically damaged (MD) leaves. Cellular and subcellular localization analyses revealed that H(2)O(2) was mainly localized in MD and herbivore-wounded (HW) zones and spread throughout the veins and tissues. Preferentially, H(2)O(2) was found in cell walls of spongy and mesophyll cells facing intercellular spaces, even though confocal laser scanning microscopy analyses also revealed the presence of H(2)O(2) in mitochondria/peroxisomes. Increased gene and enzyme activations of superoxide dismutase after HW were in agreement with confocal laser scanning microscopy data. After MD, additional application of H(2)O(2) prompted a transient transmembrane potential (V(m)) depolarization, with a V(m) depolarization rate that was higher when compared to HW leaves. In transgenic soybean (Glycine max) suspension cells expressing the Ca(2+)-sensing aequorin system, increasing amounts of added H(2)O(2) correlated with a higher cytosolic calcium ([Ca(2+)](cyt)) concentration. In MD and HW leaves, H(2)O(2) also triggered the increase of [Ca(2+)](cyt), but MD-elicited [Ca(2+)](cyt) increase was more pronounced when compared to HW leaves after addition of exogenous H(2)O(2). The results clearly indicate that V(m) depolarization caused by HW makes the membrane potential more positive and reduces the ability of lima bean leaves to react to signaling molecules.

Plasma membrane anion channels in higher plants and their putative functions in roots

Recent years have seen considerable progress in identifying anion channel activities in higher plant cells. This review outlines the functional properties of plasma membrane anion channels in plant cells and discusses their likely roles in root function. Plant anion channels can be grouped according to their voltage dependence and kinetics: (1) depolarization-activated anion channels which mediate either anion efflux (R and S types) or anion influx (outwardly rectifying type); (2) hyperpolarization-activated anion channels which mediate anion efflux, and (3) anion channels activated by light or membrane stretch. These types of anion channel are apparent in root cells where they may function in anion homeostasis, membrane stabilization, osmoregulation, boron tolerance and regulation of passive salt loading into the xylem vessels. In addition, roots possess anion channels exhibiting unique properties which are consistent with them having specialized functions in root physiology. Most notable are the organic anion selective channels, which are regulated by extracellular Al3+ or the phosphate status of the plant. Finally, although the molecular identities of plant anion channels remain elusive, the diverse electrophysiological properties of plant anion channels suggest that large and diverse multigene families probably encode these channels.

Regulation of cell length in the Arabidopsis thaliana root by the ethylene precursor 1-aminocyclopropane- 1-carboxylic acid: a matter of apoplastic reactions

Treatment of the Arabidopsis thaliana root with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) immediately imposes a reduced maximal cell length beyond which further elongation is blocked. Here, we investigated possible apoplastic reactions involved in the inhibition of cell elongation. Five-day-old Arabidopsis seedlings were transferred to a growth medium supplemented with ACC and the effect on root cell length was recorded after 3 h of treatment. Altered characteristics in the apoplast of the nonelongating cells in the ACC-treated root, such as 'reactive oxygen species' (ROS) production and callose deposition, were detected using specific fluorochromes. The presence of functional hydroxyproline-rich glycoproteins (HRGPs) and the crosslinking of these cell-wall proteins are essential in limiting cell elongation. The ROS that drive the oxidative crosslinking of HRGPs, accumulate in the apoplast of cells in the zone where cell elongation stops. In the same cells, callose is deposited in the cell wall. The final cell length in the Arabidopsis root treated for a short period with ACC is determined in the zone of fast elongation. Both HRGPs crosslinking by ROS and callose deposition in the cell wall of this zone are suggested as causes for the reduced cell elongation.

Improved transcriptional activators and their use in mis-expression traps in Arabidopsis

The synthetic transcription factor LhG4 has been used in numerous mis-expression studies in plants. We show that the sequence encoding the LhG4 activation domain, derived from Saccharomyces cerevisiae GAL4, contains several cryptic polyadenylation signals in Arabidopsis. The GAL4-derived sequence was modified according to preferred Arabidopsis codon usage, generating LhG4AtO which was faithfully transcribed in Arabidopsis plants. In protoplasts, LhG4AtO achieved maximum transactivation of the pOp promoter with 10-fold less input DNA than LhG4. The same methods were used to compare 10 other LhG4 derivatives that carried alternative natural or synthetic activation domains. Lh214 and Lh314, which contain synthetic activation domains comprising trimers of a core acidic activation domain, directed threefold more GUS expression from the pOp promoter with 20-fold less input DNA than LhG4. In contrast, when expressed from the CaMV 35S promoter in transgenic plants carrying a pOp-GUS reporter, Lh214 and Lh314 yielded transformants with substantially lower GUS activities than other constructs including LhG4AtO and LhG4 which performed similarly. When incorporated into an enhancer-trapping vector, however, LhG4AtO and Lh314 yielded enhancer traps with approximately twice the frequency of LhG4, suggesting that the modified activation domains offer improved performance when expressed from weaker transcription signals. To increase the number of LhG4 patterns available for mis-expression studies, we describe a population of enhancer-trap lines obtained with LhG4AtO in a pOp-GUS background. We show that enhancer-trap lines can transactivate an unlinked pOp-green fluorescent protein (pOp-GFP) reporter in the pattern predicted by staining for GUS activity.

Ca2+ influx into lily pollen grains through a hyperpolarization-activated Ca2+-permeable channel which can be regulated by extracellular CaM

Confocal laser scanning microscopy (CLSM) and whole-cell patch-clamp were used to investigate the role of Ca2+ influx in maintaining the cytosolic Ca2+ concentration ([Ca2+]c) and the features of the Ca2+ influx pathway in germinating pollen grains of Lilium davidii D. [Ca2+]c decreased when Ca2+ influx was inhibited by EGTA or Ca2+ channel blockers. A hyperpolarization-activated Ca2+-permeable channel, which can be suppressed by trivalent cations, verapamil, nifedipine or diltiazem, was identified on the plasma membrane of pollen protoplasts with whole-cell patch-clamp recording. Calmodulin (CaM) antiserum and W7-agarose, both of which are cell-impermeable CaM antagonists, lead to a [Ca2+]c decrease, while exogenous purified CaM triggers a transient increase of [Ca2+]c and also remarkably activated the hyperpolarization-activated Ca2+ conductance on plasma membrane of pollen protoplasts in a dose-dependent manner. Both the increase of [Ca2+]c and the activation of Ca2+ conductance which were induced by exogenous CaM were inhibited by EGTA or Ca2+ channel blockers. This primary evidence showed the presence of a voltage-dependent Ca2+-permeable channel, whose activity may be regulated by extracellular CaM, in pollen cells.

Spatial control of transgene expression in rice (Oryza sativa L.) using the GAL4 enhancer trapping system

We used enhancer trapping with the GAL4 transcriptional activator from yeast to obtain spatial control of transgene expression in all organs of the model monocotyledonous species rice (Oryza sativa L. cv. Nipponbare). Our T-DNA enhancer trapping cassette consisted of two principle components: (1) the minimal promoter-equipped gal4 gene placed adjacent to the right border, and (2) the green fluorescent protein gene (gfp) fused to the upstream activation sequence element (UAS) to which GAL4 binds and activates expression, so that gfp expression corresponds to gal4 expression. Agrobacterium-mediated integration of the cassette into the rice genome often brings the gal4 gene under transcriptional control of local genomic enhancers and promoters, resulting in gal4/gfp expression patterns ranging in specificity from single-cell types to constitutive expression. We produced more than 13 000 enhancer trap lines with this cassette and screened T(0) adult plants (1982 lines), T(1) seed (2684 lines) and T(1) seedlings (2667 lines) for gfp expression. Approximately 30% of the lines produced GFP, and we identified lines with gfp expression in specific cell types of all major organs of the rice plant. Subsequently, using the GUS reporter gene (uidA), we demonstrated that UAS:geneX constructs can be transactivated in specific cell types where gal4 and gfp are expressed, thus providing an excellent system for the manipulation of gene expression and physiological function in specific cell types of rice.