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

Poplar calcineurin B-like proteins PtCBL10A and PtCBL10B regulate shoot salt tolerance through interaction with PtSOS2 in the vacuolar membrane

The calcineurin B-like protein (CBL) family represents a unique group of calcium sensors in plants. In Arabidopsis, CBL10 functions as a shoot-specific regulator in salt tolerance. We have identified two CBL10 homologs, PtCBL10A and PtCBL10B, from the poplar (Populus trichocarpa) genome. While PtCBL10A was ubiquitously expressed at low levels, PtCBL10B was preferentially expressed in the green-aerial tissues of poplar. Both PtCBL10A and PtCBL10B were targeted to the tonoplast and expression of either one in the Arabidopsis cbl10 mutant could rescue its shoot salt-sensitive phenotype. Like PtSOS3, both PtCBL10s physically interacted with the salt-tolerance component PtSOS2. But in contrast to the SOS3-SOS2 complex at the plasma membrane, the PtCBL10-SOS2 interaction was primarily associated with vacuolar compartments. Furthermore, overexpression of either PtCBL10A or PtCBL10B conferred salt tolerance on transgenic poplar plants by maintaining ion homeostasis in shoot tissues under salinity stress. These results not only suggest a crucial role of PtCBL10s in shoot responses to salt toxicity in poplar, but also provide a molecular basis for genetic engineering of salt-tolerant tree species.

Mitogen-activated protein kinase 6 controls root growth in Arabidopsis by modulating Ca2+ -based Na+ flux in root cell under salt stress

Little is known about the role of mitogen-activated protein kinase 6 (MPK6) in Na(+) toxicity and inhibition of root growth in Arabidopsis under NaCl stress. In this study, we found that root elongation in seedlings of the loss-of-function mutants mpk6-2 and mpk6-3 was less sensitive to NaCl or Na-glutamate, but not to KCl or mannitol, as compared with that of wild-type (WT) seedlings. The less sensitive characteristic was eliminated by adding the Ca(2+) chelator EGTA or the Ca(2+) channel inhibitor LaCl3, but not the Ca(2+) ionophore A23187. This suggested that the tolerance of mpk6 to Na(+) toxicity was Ca(2+)-dependent. We measured plasma membrane (PM) Na(+)-conducted currents (NCCs) in root cells. Increased concentrations of NaCl increased the inward NCCs while decreased the outward NCCs in WT root cells, attended by a positive shift in membrane potential. In mpk6 root cells, NaCl significantly increased outward but not inward NCCs, accompanied by a negative shift in membrane potential. That is, mpk6 decreased NaCl-induced the Na(+) accumulation by modifying PM Na(+) flux in root cells. Observations of aequorin luminescence revealed a NaCl-induced increase of cytosolic Ca(2+) in mpk6 root cells, resulting from PM Ca(2+) influx. An increase of cytosolic Ca(2+) was required to alleviate the NaCl-increased Na(+) content and Na(+)/K(+) ratio in mpk6 roots. Together, these results show that mpk6 accumulated less Na(+) in response to NaCl because of the increased cytosolic Ca(2+) level in root cells; thus, its root elongation was less inhibited than that of WT by NaCl.

Deciphering early events involved in hyperosmotic stress-induced programmed cell death in tobacco BY-2 cells

Hyperosmotic stresses represent one of the major constraints that adversely affect plants growth, development, and productivity. In this study, the focus was on early responses to hyperosmotic stress- (NaCl and sorbitol) induced reactive oxygen species (ROS) generation, cytosolic Ca(2+) concentration ([Ca(2+)]cyt) increase, ion fluxes, and mitochondrial potential variations, and on their links in pathways leading to programmed cell death (PCD). By using BY-2 tobacco cells, it was shown that both NaCl- and sorbitol-induced PCD seemed to be dependent on superoxide anion (O2·(-)) generation by NADPH-oxidase. In the case of NaCl, an early influx of sodium through non-selective cation channels participates in the development of PCD through mitochondrial dysfunction and NADPH-oxidase-dependent O2·(-) generation. This supports the hypothesis of different pathways in NaCl- and sorbitol-induced cell death. Surprisingly, other shared early responses, such as [Ca(2+)]cyt increase and singlet oxygen production, do not seem to be involved in PCD.

Cyclic mononucleotides modulate potassium and calcium flux responses to H2O2 in Arabidopsis roots

Cyclic mononucleotides are messengers in plant stress responses. Here we show that hydrogen peroxide (H2O2) induces rapid net K(+)-efflux and Ca(2+)-influx in Arabidopsis roots. Pre-treatment with either 10μM cAMP or cGMP for 1 or 24h does significantly reduce net K(+)-leakage and Ca(2+)-influx, and in the case of the K(+)-fluxes, the cell permeant cyclic mononucleotides are more effective. We also examined the effect of 10μM of the cell permeant 8-Br-cGMP on the Arabidopsis microsomal proteome and noted a specific increase in proteins with a role in stress responses and ion transport, suggesting that cGMP is sufficient to directly and/or indirectly induce complex adaptive changes to cellular stresses induced by H2O2.

Apoplastic calmodulin promotes self-incompatibility pollen tube growth by enhancing calcium influx and reactive oxygen species concentration in Pyrus pyrifolia

Calmodulin (CaM) has been associated with various physiological and developmental processes in plants, including pollen tube growth. In this study, we showed that CaM regulated the pear pollen tube growth in a concentration-dependent bi-phasic response. Using a whole-cell patch-clamp configuration, we showed that apoplastic CaM induced a hyperpolarization-activated calcium ion (Ca²⁺) current, and anti-CaM largely inhibited this type of Ca²⁺ current. Moreover, upon anti-CaM treatment, the reactive oxygen species (ROS) concentration decreased and actin filaments depolymerized in the pollen tube. Interestingly, CaM could partially rescue the inhibition of self-incompatible pear pollen tube growth. This phenotype could be mediated by CaM-enhanced pollen plasma membrane Ca²⁺ current, tip-localized ROS concentration and stabilized actin filaments. These data indicated that Ca²⁺, ROS and actin filaments were involved with CaM in regulating pollen tube growth and provide a potential way for overcoming pear self-incompatibility.

Ability of leaf mesophyll to retain potassium correlates with salinity tolerance in wheat and barley

This work investigated the importance of the ability of leaf mesophyll cells to control K(+) flux across the plasma membrane as a trait conferring tissue tolerance mechanism in plants grown under saline conditions. Four wheat (Triticum aestivum and Triticum turgidum) and four barley (Hordeum vulgare) genotypes contrasting in their salinity tolerance were grown under glasshouse conditions. Seven to 10-day-old leaves were excised, and net K(+) and H(+) fluxes were measured from either epidermal or mesophyll cells upon acute 100 mM treatment (mimicking plant failure to restrict Na(+) delivery to the shoot) using non-invasive microelectrode ion flux estimation (the MIFE) system. To enable net ion flux measurements from leaf epidermal cells, removal of epicuticular waxes was trialed with organic solvents. A series of methodological experiments was conducted to test the efficiency of different methods of wax removal, and the impact of experimental procedures on cell viability, in order to optimize the method. A strong positive correlation was found between plants' ability to retain K(+) in salt-treated leaves and their salinity tolerance, in both wheat and especially barley. The observed effects were related to the ionic but not osmotic component of salt stress. Pharmacological experiments have suggested that voltage-gated K(+) -permeable channels mediate K(+) retention in leaf mesophyll upon elevated NaCl levels in the apoplast. It is concluded that MIFE measurements of NaCl-induced K(+) fluxes from leaf mesophyll may be used as an efficient screening tool for breeding in cereals for salinity tissue tolerance.

Involvement of the glutamate receptor AtGLR3.3 in plant defense signaling and resistance to Hyaloperonospora arabidopsidis

Like their animal counterparts, plant glutamate receptor-like (GLR) homologs are intimately associated with Ca(2+) influx through plasma membrane and participate in various physiological processes. In pathogen-associated molecular patterns (PAMP)-/elicitor-mediated resistance, Ca(2+) fluxes are necessary for activating downstream signaling events related to plant defense. In this study, oligogalacturonides (OGs), which are endogenous elicitors derived from cell wall degradation, were used to investigate the role of Arabidopsis GLRs in defense signaling. Pharmacological investigations indicated that GLRs are partly involved in free cytosolic [Ca(2+)] ([Ca(2+)]cyt) variations, nitric oxide (NO) production, reactive oxygen species (ROS) production and expression of defense-related genes by OGs. In addition, wild-type Col-0 plants treated with the glutamate-receptor antagonist 6,7-dinitriquinoxaline-2,3-dione (DNQX) had a compromised resistance to Botrytis cinerea and Hyaloperonospora arabidopsidis. Moreover, we provide genetic evidence that AtGLR3.3 is a key component of resistance against H. arabidopsidis. In addition, some OGs-triggered immune events such as defense gene expression, NO and ROS production are also to different extents dependent on AtGLR3.3. Taken together, these data provide evidence for the involvement of GLRs in elicitor/pathogen-mediated plant defense signaling pathways in Arabidopsis thaliana.

Insights into genomics of salt stress response in rice

Plants, as sessile organisms experience various abiotic stresses, which pose serious threat to crop production. Plants adapt to environmental stress by modulating their growth and development along with the various physiological and biochemical changes. This phenotypic plasticity is driven by the activation of specific genes encoding signal transduction, transcriptional regulation, ion transporters and metabolic pathways. Rice is an important staple food crop of nearly half of the world population and is well known to be a salt sensitive crop. The completion and enhanced annotations of rice genome sequence has provided the opportunity to study functional genomics of rice. Functional genomics aids in understanding the molecular and physiological basis to improve the salinity tolerance for sustainable rice production. Salt tolerant transgenic rice plants have been produced by incorporating various genes into rice. In this review we present the findings and investigations in the field of rice functional genomics that includes supporting genes and networks (ABA dependent and independent), osmoprotectants (proline, glycine betaine, trehalose, myo-inositol, and fructans), signaling molecules (Ca2+, abscisic acid, jasmonic acid, brassinosteroids) and transporters, regulating salt stress response in rice.

An estuarine species of the alga Vaucheria (Xanthophyceae) displays an increased capacity for turgor regulation when compared to a freshwater species

Turgor regulation is the process by which walled organisms alter their internal osmotic potential to adapt to osmotic changes in the environment. Apart from a few studies on freshwater oomycetes, the ability of stramenopiles to turgor regulate has not been investigated. In this study, turgor regulation and growth were compared in two species of the stramenopile alga Vaucheria, Vaucheria erythrospora isolated from an estuarine habitat, and Vaucheria repens isolated from a freshwater habitat. Species were identified using their rbcL sequences and respective morphologies. Using a single cell pressure probe to directly measure turgor in Vaucheria after hyperosmotic shock, V. erythrospora was found to recover turgor after a larger shock than V. repens. Threshold shock values for this ability were >0.5 MPa for V. erythrospora and <0.5 MPa for V. repens. Recovery was more rapid in V. erythrospora than V. repens after comparable shocks. Turgor recovery in V. erythrospora was inhibited by Gd(3+) and TEA, suggesting a role for mechanosensitive channels, nonselective cation channels, and K(+) channels in the process. Growth studies showed that V. erythrospora was able to grow over a wider range of NaCl concentrations. These responses may underlie the ability of V. erythrospora to survive in an estuarine habitat and restrict V. repens to freshwater. The fact that both species can turgor regulate may indicate a fundamental difference between members of the Stramenopila, as research to date on oomycetes suggests they are unable to turgor regulate.

Plant calcium-permeable channels

Experimental and modeling breakthroughs will help establish the genetic identities of plant calcium channels.

Arbuscular mycorrhizal fungi native from a Mediterranean saline area enhance maize tolerance to salinity through improved ion homeostasis

Soil salinity restricts plant growth and productivity. Na(+) represents the major ion causing toxicity because it competes with K(+) for binding sites at the plasma membrane. Inoculation with arbuscular mycorrhizal fungi (AMF) can alleviate salt stress in the host plant through several mechanisms. These may include ion selection during the fungal uptake of nutrients from the soil or during transfer to the host plant. AM benefits could be enhanced when native AMF isolates are used. Thus, we investigated whether native AMF isolated from an area with problems of salinity and desertification can help maize plants to overcome the negative effects of salinity stress better than non-AM plants or plants inoculated with non-native AMF. Results showed that plants inoculated with two out the three native AMF had the highest shoot dry biomass at all salinity levels. Plants inoculated with the three native AMF showed significant increase of K(+) and reduced Na(+) accumulation as compared to non-mycorrhizal plants, concomitantly with higher K(+) /Na(+) ratios in their tissues. For the first time, these effects have been correlated with regulation of ZmAKT2, ZmSOS1 and ZmSKOR genes expression in the roots of maize, contributing to K(+) and Na(+) homeostasis in plants colonized by native AMF.

An Arabidopsis soil-salinity-tolerance mutation confers ethylene-mediated enhancement of sodium/potassium homeostasis

High soil Na concentrations damage plants by increasing cellular Na accumulation and K loss. Excess soil Na stimulates ethylene-induced soil-salinity tolerance, the mechanism of which we here define via characterization of an Arabidopsis thaliana mutant displaying transpiration-dependent soil-salinity tolerance. This phenotype is conferred by a loss-of-function allele of ethylene overproducer1 (ETO1; mutant alleles of which cause increased production of ethylene). We show that lack of ETO1 function confers soil-salinity tolerance through improved shoot Na/K homeostasis, effected via the ethylene resistant1-constitutive triple response1 ethylene signaling pathway. Under transpiring conditions, lack of ETO1 function reduces root Na influx and both stelar and xylem sap Na concentrations, thereby restricting root-to-shoot delivery of Na. These effects are associated with increased accumulation of respiratory burst oxidase homolog F (RBOHF)-dependent reactive oxygen species in the root stele. Additionally, lack of ETO1 function leads to significant enhancement of tissue K status by an RBOHF-independent mechanism associated with elevated high-affinity K(+) TRANSPORTER5 transcript levels. We conclude that ethylene promotes soil-salinity tolerance via improved Na/K homeostasis mediated by RBOHF-dependent regulation of Na accumulation and RBOHF-independent regulation of K accumulation.

Clues for regulatory processes in fungal uptake and transfer of minerals to the basidiospore

Several fungal species are notorious for the preferential acquisition of toxicants such as AsCdHgPbU in their wild-grown basidiomes, but it is not known how, or whether at all, mineral uptake is regulated. In this study, basidiomes of Kuehneromyces mutabilis, Pleurotus ostreatus, and Hypholoma fasciculare were grown on Fagus sylvatica logs embedded in sand, uranium-overburden soil, and garden soil (SIO) at a lab scale to raise the accessible mineral resources 30 to >1,000 times over those available in the timber alone. Non-embedded logs and a field culture established on SIO served as controls. Concentrations of 22 minerals were determined by inductively coupled plasma mass spectrometry from microwave-digested samples of timber, soils, whole and dissected mushrooms, and basidiospores. It was the goal to determine whether mineral uptake rates vary simply with their concentration in the substrate or undergo selections which indicate the ability of metal sensing and optimizing/delimiting the quantity of (essential) elements on their passage from a substrate via basidiome to the basidiospores. It is shown that an underrepresented substrate mineral is up-concentrated to a more or less regulated and physiologically compatible mean, whereas a rising external mineral supply leads to uptake blockage by downregulation of the bioconcentration rate in the vicinity of an apparent mycelial saturation point. The resulting concentrations in whole K. mutabilis basidiomes of the essential metals, CaCoCuFeMgMn(Sr)Zn corresponded surprisingly with those in wheat grains which share the main metabolic pathways with fungi and whose metallome is believed to be out-regulated for an optimum and stress-free development. Concentrations of nonessential metals, too, fitted the range of those common crops, whereas KP reached the higher typical level of fungi. Minerals entering the lower stipe of the K. mutabilis basidiome were specifically enriched/diluted on a passage to the gills and once more abruptly up/down-concentrated at the basidium/sterigma/spore interface. Mineral concentrations of spores corresponded then again with those in wheat grains, with the metalloenzyme-linked CdCoCuFeMnNa(Ni) appearing moderately higher. It is concluded that the substrate/fungal interface may be the major site of metal sensing/selecting and uptake regulation. Concentration shifts obtained during the mineral transfer through the basidiome are then subject to ultimate corrections at the gill/spore interface.

Post-transcriptional regulation of GORK channels by superoxide anion contributes to increases in outward-rectifying K⁺ currents

· Ion fluxes are ubiquitous processes in the plant and animal kingdoms, controlled by fine-tuned regulations of ion channel activity. Yet the mechanism that cells employ to achieve the modification of ion homeostasis at the molecular level still remains unclear. This is especially true when it comes to the mechanisms that lead to cell death. · In this study, Arabidopsis thaliana cells were exposed to ozone (O₃). Ion flux variations were analyzed by electrophysiological measurements and their transcriptional regulation by RT-PCR. Reactive oxygen species (ROS) generation was quantified by luminescence techniques and caspase-like activities were investigated by laser confocal microscopy. · We highlighted the delayed activation of K(+) outward-rectifying currents after an O₃ -induced oxidative stress leading to programmed cell death (PCD). Caspase-like activities are detected under O₃ exposure and could be decreased by K(+) channel blocker. Molecular experiments revealed that the sustained activation of K(+) outward current could be the result of an unexpected O₂ ·⁻ post-transcriptional regulation of the guard cell outward-rectifying K(+) (GORK) channels. · This consists of a likely new mode of regulating the processing of the GORK mRNA, in a ROS-dependent manner, to allow sustained K(+) effluxes during PCD. These data provide new mechanistic insights into K(+) channel regulation during an oxidative stress response.

Salicylic acid improves salinity tolerance in Arabidopsis by restoring membrane potential and preventing salt-induced K+ loss via a GORK channel

Despite numerous reports implicating salicylic acid (SA) in plant salinity responses, the specific ionic mechanisms of SA-mediated adaptation to salt stress remain elusive. To address this issue, a non-invasive microelectrode ion flux estimation technique was used to study kinetics of NaCl-induced net ion fluxes in Arabidopsis thaliana in response to various SA concentrations and incubation times. NaCl-induced K(+) efflux and H(+) influx from the mature root zone were both significantly decreased in roots pretreated with 10-500 μM SA, with strongest effect being observed in the 10-50 μM SA range. Considering temporal dynamics (0-8-h SA pretreatment), the 1-h pretreatment was most effective in enhancing K(+) retention in the cytosol. The pharmacological, membrane potential, and shoot K(+) and Na(+) accumulation data were all consistent with the model in which the SA pretreatment enhanced activity of H(+)-ATPase, decreased NaCl-induced membrane depolarization, and minimized NaCl-induced K(+) leakage from the cell within the first hour of salt stress. In long-term treatments, SA increased shoot K(+) and decreased shoot Na(+) accumulation. The short-term NaCl-induced K(+) efflux was smallest in the gork1-1 mutant, followed by the rbohD mutant, and was highest in the wild type. Most significantly, the SA pretreatment decreased the NaCl-induced K(+) efflux from rbohD and the wild type to the level of gork1-1, whereas no effect was observed in gork1-1. These data provide the first direct evidence that the SA pretreatment ameliorates salinity stress by counteracting NaCl-induced membrane depolarization and by decreasing K(+) efflux via GORK channels.

Effect of salt stress on growth, Na+ accumulation and proline metabolism in potato (Solanum tuberosum) cultivars

Potato (Solanum tuberosum) is a major crop world-wide and the productivity of currently used cultivars is strongly reduced at high soil salt levels. We compared the response of six potato cultivars to increased root NaCl concentrations. Cuttings were grown hydroponically and treated with 0 mM, 60 mM and 180 mM NaCl for one week. Growth reduction on salt was strongest for the cultivars Mozart and Mona Lisa with a severe senescence response at 180 mM NaCl and Mozart barely survived the treatment. The cultivars Desiree and Russett Burbank were more tolerant showing no senescence after salt treatment. A clear difference in Na(+) homeostasis was observed between sensitive and tolerant cultivars. The salt sensitive cultivar Mozart combined low Na(+) levels in root and stem with the highest leaf Na(+) concentration of all cultivars, resulting in a high Na(+) shoot distribution index (SDI) for Mozart as compared to Desiree. Overall, a positive correlation between salt tolerance and stem Na(+) accumulation was found and the SDI for Na(+) points to a role of stem Na(+) accumulation in tolerance. In stem tissue, Mozart accumulated more H2O2 and less proline compared to the tolerant cultivars. Analysis of the expression of proline biosynthesis genes in Mozart and Desiree showed a clear reduction in proline dehydrogenase (PDH) expression in both cultivars and an increase in pyrroline-5-carboxylate synthetase 1 (P5CS1) gene expression in Desiree, but not in Mozart. Taken together, current day commercial cultivars show promising differences in salt tolerance and the results suggest that mechanisms of tolerance reside in the capacity of Na(+) accumulation in stem tissue, resulting in reduced Na(+) transport to the leaves.

Proteomics-based investigation of salt-responsive mechanisms in plant roots

Salinity is one of the major abiotic stresses that limits agricultural productivity worldwide. Plant roots function as the primary site of salinity perception. Salt responses in roots are essential for maintaining root functionality, as well as for transmitting the salt signal to shoot for proper salt response and adaptation in the entire plant. Therefore, a thorough understanding of signaling and metabolic mechanisms of salt response in roots is critical for improving plant salt tolerance. Current proteomic studies have provided salt-responsive expression patterns of 905 proteins in 14 plant species. Through integrative analysis of salt-responsive proteins and previous physiological and molecular findings, this review summarizes current understanding of salt responses in roots and highlights proteomic findings on the molecular mechanisms in the fine-tuned salt-responsive networks. At the proteome level, the following processes become dominant in root salt response: (i) salt signal perception and transduction; (ii) detoxification of reactive oxygen species (ROS); (iii) salt uptake/exclusion and compartmentalization; (iv) protein translation and/or turnover dynamics; (v) cytoskeleton/cell wall dynamics; (vi) carbohydrate and energy metabolism; and (vii) other salt-responsive metabolisms. These processes work together to gain cellular homeostasis in roots and determine the overall phenotype of plant growth and development under salt stress.

Negative short-term salt effects on the soybean-Bradyrhizobium japonicum interaction and partial reversion by calcium addition

The short-term (2h) effects of salt stress (50 and 150mM NaCl) on early events of soybean- Bradyrhizobium japonicum (rhizobia) interaction were analysed, determining the following parameters in root hair with or without calcium addition: deformation, apoplastic superoxide radical production (O2⚫-), root hair death and sodium/potassium ion content. We also analysed whether this short-term salt stress influenced later formation of crown and noncrown nodules, determining the number and weight of nodules. The negative effect of salt stress on these characters was attenuated by the addition of 5mM CaCl2. We also analysed the expression of pathogenesis-related proteins (PRP) genes PR-1, PR-2, PR-3, and four isoforms of PR-5. The expression of PR-2 increased under saline conditions and decreased in osmotic treatment and saline treatment supplemented with calcium in the presence of the symbiont. The changes in PR-2 expression levels, together with the death of root hairs provide a possible mechanism for the inhibition of infection by the symbiont under salinity, and suggests a possible overlap with responses to plant pathogens.

Roles and functions of plant mineral nutrients

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

Potassium transport and signaling in higher plants

As one of the most important mineral nutrient elements, potassium (K(+)) participates in many plant physiological processes and determines the yield and quality of crop production. In this review, we summarize K(+) signaling processes and K(+) transport regulation in higher plants, especially in plant responses to K(+)-deficiency stress. Plants perceive external K(+) fluctuations and generate the initial K(+) signal in root cells. This signal is transduced into the cytoplasm and encoded as Ca(2+) and reactive oxygen species signaling. K(+)-deficiency-induced signals are subsequently decoded by cytoplasmic sensors, which regulate the downstream transcriptional and posttranslational responses. Eventually, plants produce a series of adaptive events in both physiological and morphological alterations that help them survive K(+) deficiency.

Cadmium impairs ion homeostasis by altering K+ and Ca2+ channel activities in rice root hair cells

Cadmium (Cd2+) interferes with the uptake, transport and utilization of several macro- and micronutrients, which accounts, at least in part, for Cd2+ toxicity in plants. However, the mechanisms underlying Cd2+ interference of ionic homeostasis is not understood. Using biophysical techniques including membrane potential measurements, scanning ion-selective electrode technique for non-invasive ion flux assays and patch clamp, we monitored the effect of Cd2+ on calcium (Ca2+) and potassium (K+) transport in root hair cells of rice. Our results showed that K+ and Ca2+ contents in both roots and shoots were significantly reduced when treated with exogenous Cd2+. Further studies revealed that three cellular processes may be affected by Cd2+, leading to changes in ionic homeostasis. First, Cd2+ -induced depolarization of the membrane potential was observed in root hair cells, attenuating the driving force for cation uptake. Second, the inward conductance of Ca2+ and K+ was partially blocked by Cd2+, decreasing uptake of K+ and Ca2+ . Third, the outward K+ conductance was Cd2+ -inducible, decreasing the net content of K+ in roots. These results provide direct evidence that Cd2+ impairs uptake of Ca2+ and K+, thereby disturbing ion homeostasis in plants.

Patch-clamp protocols to study cell ionic homeostasis under saline conditions

The patch-clamp technique was designed to measure any electrogenic transport across the whole cell and organelle (vacuolar) membranes and excised membrane patches. Here, we describe preparation of protoplasts and vacuoles, as well as patch-clamp assays, to detect the functional expression of K(+) and cation channels of plasma membrane and tonoplast, as well as plasma membrane anion channels and vacuolar and plasma membrane H(+) pumps. All of these contribute to the intracellular ionic homeostasis under saline conditions.

Studying plant salt tolerance with the voltage clamp technique

Voltage clamp is one of the key techniques for the dissection, identification, and monitoring of ion transporters in plant cells. Voltage clamp-based research work on salinity stress in plants enables the characterization of many plant ATP-dependent pumps, ion channels, and ion-coupled carriers through heterologous expression in Xenopus laevis oocytes and in vivo measurements in salt-tolerant and salt-sensitive giant green algae such as Chara and many plant species. We have modified and developed a reliable set of procedures for voltage clamp analysis in intact guard cells and root epidermal cells from Arabidopsis thaliana with potentially broad applications in the salinity response of plants. These procedures greatly extend the duration of measurements and scope for analysis of the predominant K(+) and anion channels.

Zinc ions block H⁺/OH⁻ channels in Chara australis

Chara australis cells exposed to media of pH 10 and above exhibit high conductance, arising from the opening of H⁺/OH⁻ channels in the plasma membrane. This high conductance can be totally inhibited by 1.0 mm ZnCl₂ and restored by 0.5 mm 2-mercaptoethanol (ME). Important for carbon fixation, H⁺/OH⁻ channels play a key role in cell pH banding. Banding was also shown to be abolished by 1.0 mm ZnCl₂ and restored in some cells by ME. The proton pump is also involved in banding, but was little affected by ZnCl₂ over the periods needed for the inhibition of H⁺/OH⁻ channels. Previously, we postulated that H⁺/OH⁻ channels open transiently at the onset of saline stress in salt-sensitive C. australis, causing membrane potential difference (PD) noise; and remain open in latter stages of saline stress, contributing to cell deterioration. ZnCl₂ totally inhibited the saline noise and the upwardly concave I/V characteristics associated with the putative H⁺/OH⁻ currents. Again, ME reversed both these effects. We discuss the mode of action of zinc ions and ME with reference to animal voltage-gated H⁺ channels and water channels.

Paxillus involutus strains MAJ and NAU mediate K(+)/Na(+) homeostasis in ectomycorrhizal Populus x canescens under sodium chloride stress

Salt-induced fluxes of H(+), Na(+), K(+), and Ca(2+) were investigated in ectomycorrhizal (EM) associations formed by Paxillus involutus (strains MAJ and NAU) with the salt-sensitive poplar hybrid Populus × canescens. A scanning ion-selective electrode technique was used to measure flux profiles in non-EM roots and axenically grown EM cultures of the two P. involutus isolates to identify whether the major alterations detected in EM roots were promoted by the fungal partner. EM plants exhibited a more pronounced ability to maintain K(+)/Na(+) homeostasis under salt stress. The influx of Na(+) was reduced after short-term (50 mm NaCl, 24 h) and long-term (50 mm NaCl, 7 d) exposure to salt stress in mycorrhizal roots, especially in NAU associations. Flux data for P. involutus and susceptibility to Na(+)-transport inhibitors indicated that fungal colonization contributed to active Na(+) extrusion and H(+) uptake in the salinized roots of P. × canescens. Moreover, EM plants retained the ability to reduce the salt-induced K(+) efflux, especially under long-term salinity. Our study suggests that P. involutus assists in maintaining K(+) homeostasis by delivering this nutrient to host plants and slowing the loss of K(+) under salt stress. EM P. × canescens plants exhibited an enhanced Ca(2+) uptake ability, whereas short-term and long-term treatments caused a marked Ca(2+) efflux from mycorrhizal roots, especially from NAU-colonized roots. We suggest that the release of additional Ca(2+) mediated K(+)/Na(+) homeostasis in EM plants under salt stress.

Ion Transporters and Abiotic Stress Tolerance in Plants

Adaptation of plants to salt stress requires cellular ion homeostasis involving net intracellular Na⁺ and Cl⁻ uptake and subsequent vacuolar compartmentalization without toxic ion accumulation in the cytosol. Sodium ions can enter the cell through several low- and high-affinity K⁺ carriers. Some members of the HKT family function as sodium transporter and contribute to Na⁺ removal from the ascending xylem sap and recirculation from the leaves to the roots via the phloem vasculature. Na⁺ sequestration into the vacuole depends on expression and activity of Na⁺/H⁺ antiporter that is driven by electrochemical gradient of protons generated by the vacuolar H⁺-ATPase and the H⁺-pyrophosphatase. Sodium extrusion at the root-soil interface is presumed to be of critical importance for the salt tolerance. Thus, a very rapid efflux of Na⁺ from roots must occur to control net rates of influx. The Na⁺/H⁺ antiporter SOS1 localized to the plasma membrane is the only Na⁺ efflux protein from plants characterized so far. In this paper, we analyze available data related to ion transporters and plant abiotic stress responses in order to enhance our understanding about how salinity and other abiotic stresses affect the most fundamental processes of cellular function which have a substantial impact on plant growth development.

Calcium signatures and signaling in cytosol and organelles of tobacco cells induced by plant defense elicitors

Calcium signatures induced by two elicitors of plant defense reactions, namely cryptogein and oligogalacturonides, were monitored at the subcellular level, using apoaequorin-transformed Nicotiana tabacum var Xanthi cells, in which the apoaequorin calcium sensor was targeted either to cytosol, mitochondria or chloroplasts. Our study showed that both elicitors induced specific Ca(2+) signatures in each compartment, with the most striking difference relying on duration. Common properties also emerged from the analysis of Ca(2+) signatures: both elicitors induced a biphasic cytosolic [Ca(2+)] elevation together with a single mitochondrial [Ca(2+)] elevation concomitant with the first cytosolic [Ca(2+)] peak. In addition, both elicitors induced a chloroplastic [Ca(2+)] elevation peaking later in comparison to cytosolic [Ca(2+)] elevation. In cryptogein-treated cells, pharmacological studies indicated that IP(3) should play an important role in Ca(2+) signaling contrarily to cADPR or nitric oxide, which have limited or no effect on [Ca(2+)] variations. Our data also showed that, depending on [Ca(2+)] fluxes at the plasma membrane, cryptogein triggered a mitochondrial respiration increase and affected excess energy dissipation mechanisms in chloroplasts. Altogether the results indicate that cryptogein profoundly impacted cell functions at many levels, including organelles.

Rice cultivars with differing salt tolerance contain similar cation channels in their root cells

Salinity poses a major threat for agriculture worldwide. Rice is one of the major crops where most of the high-yielding cultivars are highly sensitive to salinity. Several studies on the genetic variability across rice cultivars suggest that the activity and composition of root plasma membrane transporters could underlie the observed cultivar-specific salinity tolerance in rice. In the current study, it was found that the salt-tolerant cultivar Pokkali maintains a higher K+/Na+ ratio compared with the salt-sensitive IR20 in roots as well as in shoots. Using Na+ reporter dyes, IR20 root protoplasts showed a much faster Na+ accumulation than Pokkali protoplasts. Membrane potential measurements showed that root cells exposed to Na+ in IR20 depolarized considerably further than those of Pokkali. These results suggest that IR20 has a larger plasma membrane Na+ conductance. To assess whether this could be due to different ion channel properties, root protoplasts from both Pokkali and IR20 rice cultivars were patch-clamped. Voltage-dependent K+ inward rectifiers, K+ outward rectifiers, and voltage-independent, non-selective channels with unitary conductances of around 35, 40, and 10 pS, respectively, were identified. Only the non-selective channel showed significant Na+ permeability. Intriguingly, in both cultivars, the activity of the K+ inward rectifier was drastically down-regulated after plant growth in salt but gating, conductance, and activity of all channel types were very similar for the two cultivars.

Isolation and characterization of maize PMP3 genes involved in salt stress tolerance

Plasma membrane protein 3 (PMP3), a class of small hydrophobic polypeptides with high sequence similarity, is responsible for salt, drought, cold, and abscisic acid. These small hydrophobic ploypeptides play important roles in maintenance of ion homeostasis. In this study, eight ZmPMP3 genes were cloned from maize and responsive to salt, drought, cold and abscisic acid. The eight ZmPMP3s were membrane proteins and their sequences in trans-membrane regions were highly conserved. Phylogenetic analysis showed that they were categorized into three groups. All members of group II were responsive to ABA. Functional complementation showed that with the exception of ZmPMP3-6, all were capable of maintaining membrane potential, which in turn allows for regulation of intracellular ion homeostasis. This process was independent of the presence of Ca(2+). Lastly, over-expression of ZmPMP3-1 enhanced growth of transgenic Arabidopsis under salt condition. Through expression analysis of deduced downstream genes in transgenic plants, expression levels of three ion transporter genes and four important antioxidant genes in ROS scavenging system were increased significantly in transgenic plants during salt stress. This tolerance was likely achieved through diminishing oxidative stress due to the possibility of ZmPMP3-1's involvement in regulation of ion homeostasis, and suggests that the modulation of these conserved small hydrophobic polypeptides could be an effective way to improve salt tolerance in plants.

Polyamines interact with hydroxyl radicals in activating Ca(2+) and K(+) transport across the root epidermal plasma membranes

Reactive oxygen species (ROS) are integral components of the plant adaptive responses to environment. Importantly, ROS affect the intracellular Ca(2+) dynamics by activating a range of nonselective Ca(2+)-permeable channels in plasma membrane (PM). Using patch-clamp and noninvasive microelectrode ion flux measuring techniques, we have characterized ionic currents and net K(+) and Ca(2+) fluxes induced by hydroxyl radicals (OH(•)) in pea (Pisum sativum) roots. OH(•), but not hydrogen peroxide, activated a rapid Ca(2+) efflux and a more slowly developing net Ca(2+) influx concurrent with a net K(+) efflux. In isolated protoplasts, OH(•) evoked a nonselective current, with a time course and a steady-state magnitude similar to those for a K(+) efflux in intact roots. This current displayed a low ionic selectivity and was permeable to Ca(2+). Active OH(•)-induced Ca(2+) efflux in roots was suppressed by the PM Ca(2+) pump inhibitors eosine yellow and erythrosine B. The cation channel blockers gadolinium, nifedipine, and verapamil and the anionic channel blockers 5-nitro-2(3-phenylpropylamino)-benzoate and niflumate inhibited OH(•)-induced ionic currents in root protoplasts and K(+) efflux and Ca(2+) influx in roots. Contrary to expectations, polyamines (PAs) did not inhibit the OH(•)-induced cation fluxes. The net OH(•)-induced Ca(2+) efflux was largely prolonged in the presence of spermine, and all PAs tested (spermine, spermidine, and putrescine) accelerated and augmented the OH(•)-induced net K(+) efflux from roots. The latter effect was also observed in patch-clamp experiments on root protoplasts. We conclude that PAs interact with ROS to alter intracellular Ca(2+) homeostasis by modulating both Ca(2+) influx and efflux transport systems at the root cell PM.

Involvement of putative glutamate receptors in plant defence signaling and NO production

Ionotropic glutamate receptors (iGluRs) are non-selective cation channels permeable to calcium, present in animals and plants. In mammals, glutamate is a well-known neurotransmitter and recently has been recognized as an immunomodulator. As animals and plants share common mechanisms that govern innate immunity with calcium playing a key role in plant defence activation, we have checked the involvement of putative iGluRs in plant defence signaling. Using tobacco cells, we first provide evidence supporting the activity of iGluRs as calcium channels and their involvement in NO production as reported in animals. Thereafter, iGluRs were shown to be activated in response to cryptogein, a well studied elicitor of defence response, and partly responsible for cryptogein-induced NO production. However, other cryptogein-induced calcium-dependent events including anion efflux, H(2)O(2) production, MAPK activation and hypersensitive response (HR) did not depend on iGluRs indicating that different calcium channels regulate different processes at the cell level. We have also demonstrated that cryptogein induces efflux of glutamate in the apoplast by exocytosis. Taken together, our results demonstrate for the first time, an involvement of a putative iGluR in plant defence signaling and NO production, by mechanisms that show homology with glutamate mode of action in mammals.

Potassium channel-oxidative phosphorylation relationship in durum wheat mitochondria from control and hyperosmotic-stressed seedlings

Durum wheat mitochondria (DWM) possess an ATP-inhibited K(+) channel, the plant mitoK(ATP) (PmitoK(ATP) ), which is activated under environmental stress to control mitochondrial ROS production. To do this, PmitoK(ATP) collapses membrane potential (ΔΨ), thus suggesting mitochondrial uncoupling. We tested this point by studying oxidative phosphorylation (OXPHOS) in DWM purified from control seedlings and from seedlings subjected both to severe mannitol and NaCl stress. In severely-stressed DWM, the ATP synthesis via OXPHOS, continuously monitored by a spectrophotometric assay, was about 90% inhibited when the PmitoK(ATP) was activated by KCl. Contrarily, in control DWM, although PmitoK(ATP) collapsed ΔΨ, ATP synthesis, as well as coupling [respiratory control (RC) ratio and ratio between phosphorylated ADP and reduced oxygen (ADP/O)] checked by oxygen uptake experiments, were unaffected. We suggest that PmitoK(ATP) may play an important defensive role at the onset of the environmental/oxidative stress by preserving energy in a crucial moment for cell and mitochondrial bioenergetics. Consistently, under moderate mannitol stress, miming an early stress condition, the channel may efficiently control reactive oxygen species (ROS) generation (about 35-fold from fully open to closed state) without impairing ATP synthesis. Anyway, if the stress significantly proceeds, the PmitoK(ATP) becomes fully activated by decrease of ATP concentration (25-40%) and increase of activators [free fatty acids (FFAs) and superoxide anion], thus impairing ATP synthesis.

Roles of Ca2+ and cyclic nucleotide gated channel in plant innate immunity

The increase of cytosolic Ca(2+) is a vital event in plant pathogen signaling cascades. Molecular components linking pathogen signal perception to cytosolic Ca(2+) increase have not been well characterized. Plant cyclic nucleotide gated channels (CNGCs) play important roles in the pathogen signaling cascade, in terms of facilitating Ca(2+) uptake into the cytosol in response to pathogen and pathogen associated molecular pattern (PAMP) signals. Perception of pathogens leads to cyclic nucleotide production and the activation of CNGCs. The Ca(2+) signal is transduced through Ca(2+) sensors (Calmodulin (CaM) and CaM-like proteins (CMLs)), which regulates the production of nitric oxide (NO). In addition, roles of Ca(2+)/CaM interacting proteins such as CaM binding Protein (CBP) and CaM-binding transcription activators (CAMTAs)) have been recently identified in the plant defense signaling cascade as well. Furthermore, Ca(2+)-dependent protein kinases (CDPKs) have been found to function as components in terms of transcriptional activation in response to a pathogen (PAMP) signal. Although evidence shows that Ca(2+) is an essential signaling component upstream from many vital signaling molecules (such as NO), some work also indicates that these downstream signaling components can also regulate Ca(2+) homeostasis. NO can induce cytosolic Ca(2+) increase (through activation of plasma membrane- and intracellular membrane-localized Ca(2+) channels) during pathogen signaling cascades. Thus, much work is needed to further elucidate the complexity of the plant pathogen signaling network in the future.

Involvement of calcium-mediated effects on ROS metabolism in the regulation of growth improvement under salinity

Salinity reduces Ca(2+) availability, transport, and mobility to growing regions of the plant and supplemental Ca(2+) is known to reduce salinity damages. This study was undertaken to unravel some of the ameliorative mechanisms of Ca(2+) on salt stress at the cellular and tissue levels. Zea mays L. plants were grown in nutrient solution containing 1 or 80 mM NaCl with various Ca(2+) levels. Measurements of growth and physiological parameters, such as ion imbalance, indicated that the Ca(2+)-induced alleviation mechanisms differed between plant organs. Under salinity, H(2)O(2) levels increased in the leaf-growing tissue with increasing levels of supplemental Ca(2+) and reached the levels of control plants, whereas superoxide levels remained low at all Ca(2+) levels, indicating that Ca(2+) affected growth by increasing H(2)O(2) but not superoxide levels. Salinity completely abolished apoplastic peroxidase activity. Supplemental Ca(2+) increased its activity only slightly. However, under salinity, polyamine oxidase (PAO) activity was shifted toward the leaf base probably as an adaptive mechanism aimed at restoring normal levels of reactive oxygen species (ROS) at the expansion zone where NADPH oxidase could no longer provide the required ROS for growth. Interestingly, addition of Ca(2+) shifted the PAO-activity peak back to its original location in addition to its enhancement. The increase in PAO activity in conjunction with low levels of apoplastic peroxidase is supportive of cellular growth via nonenzymatic wall loosening derived by the increase in H(2)O(2) and less supportive of the peroxidase-mediated cross-linking of wall material. Thus extracellular Ca(2+) can modulate ROS levels at specific tissue localization and developmental stages thereby affecting cellular extension.

Root K(+) acquisition in plants: the Arabidopsis thaliana model

K(+) is an essential macronutrient required by plants to complete their life cycle. It fulfills important functions and it is widely used as a fertilizer to increase crop production. Thus, the identification of the systems involved in K(+) acquisition by plants has always been a research goal as it may eventually produce molecular tools to enhance crop productivity further. This review is focused on the recent findings on the systems involved in K(+) acquisition. From Epstein's pioneering work >40 years ago, K(+) uptake was considered to consist of a high- and a low-affinity component. The subsequent molecular approaches identified genes encoding K(+) transport systems which could be involved in the first step of K(+) uptake at the plant root. Insights into the regulation of these genes and the proteins that they encode have also been gained in recent studies. A demonstration of the role of the two main K(+) uptake systems at the root, AtHKA5 and AKT1, has been possible with the study of Arabidopsis thaliana T-DNA insertion lines that knock out these genes. AtHAK5 was revealed as the only uptake system at external concentrations <10 μM. Between 10 and 200 μM both AtHAK5 and AKT1 contribute to K(+) acquisition. At external concentrations >500 μM, AtHAK5 is not relevant and AKT1's contribution to K(+) uptake becomes more important. At 10 mM K(+), unidentified systems may provide sufficient K(+) uptake for plant growth.

Uptake and translocation of metformin, ciprofloxacin and narasin in forage- and crop plants

Transfer of bioactive organic compounds from soil to plants might represent animal and human health risks. Sewage sludge and manure are potential sources for bioactive compounds such as human- and veterinary drugs. In the present study, uptake of the anti-diabetic compound, metformin, the antibiotic agent ciprofloxacin and the anti-coccidial narasin in carrot (Daucuscarota ssp. sativus cvs. Napoli) and barley (Hordeumvulgare) were investigated. The pharmaceuticals were selected in order to cover various chemical properties, in addition to their presence in relevant environmental matrixes. The root concentration factors (RCF) found in the present study were higher than the corresponding leaf concentration factors (LCF) for the three test pharmaceuticals. The uptake of metformin was higher compared with ciprofloxacin and narasin for all plant compartments analyzed. Metformin was studied more explicitly with regard to uptake and translocation in meadow fescue (Festucapratense), three other carrot cultivars (D.carota ssp. sativus cvs. Amager, Rothild and Nutri Red), wheat cereal (Triticumaestivum) and turnip rape seed (Brassicacampestris). Uptake of metformin in meadow fescue was comparable with uptake in the four carrot cultivars (RCF 2-10, LCF approximately 1.5), uptake in wheat cereals were comparable with barley cereals (seed concentration factors, SCF, 0.02-0.04) while the accumulation in turnip rape seeds was as high as 1.5. All three pharmaceuticals produced negative effects on growth and development of carrots when grown in soil concentration of 6-10 mg kg(-1) dry weight.

(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.

Polyamine catabolism is involved in response to salt stress in soybean hypocotyls

The possible relationship between polyamine catabolism mediated by copper-containing amine oxidase and the elongation of soybean hypocotyls from plants exposed to NaCl has been studied. Salt treatment reduced values of all hypocotyl growth parameters. In vitro, copper-containing amine oxidase activity was up to 77-fold higher than that of polyamine oxidase. This enzyme preferred cadaverine over putrescine and it was active even under the saline condition. On the other hand, saline stress increased spermine and cadaverine levels, and the in vivo copper-containing amine oxidase activity in the elongation zone of hypocotyls. The last effect was negatively modulated by the addition of the copper-containing amine oxidase inhibitor N,N'-diaminoguanidine. In turn, plants treated with the inhibitor showed a significant reduction of reactive oxygen species in the elongation zone, even in the saline situation. In addition, plants grown in cadaverine-amended culture medium showed increased hypocotyl length either in saline or control conditions and this effect was also abolished by N,N'-diaminoguanidine. Taken together, our results suggest that the activity of the copper-containing amine oxidase may be partially contributing to hypocotyl growth under saline stress, through the production of hydrogen peroxide by polyamine catabolism and reinforce the importance of polyamine catabolism and hydrogen peroxide production in the induction of salt tolerance in plants.

The role of cyclic nucleotide-gated ion channels in plant immunity

Since the first plant cyclic nucleotide-gated ion channel (CNGC), HvCBT1, was identified as a calmodulin binding protein, more than a decade has passed and a substantial amount of work has been done to understand the molecular nature and function of these channel proteins. Based on electrophysiological and heterologous expression analyses, plant CNGCs function as non-selective cation channels and, so far, their biological roles have been reported in defense responses, development, and ion homeostasis. Forward genetic approaches identified four AtCNGCs (AtCNGC2, 4, 11, and 12) to be involved in plant immunity, as null mutants for AtCNGC2, 4, 11, and 12 as well as a gain-of- function mutant for AtCNGC11 and 12 exhibited alterations in defense responses. Since ion flux changes have been reported as one of the early events upon pathogen recognition and also are an essential component for the activation of defense responses, the involvement of CNGCs in these ion flux changes has been suggested. However, the recent detailed characterization of null mutants suggested a more complex involvement of this channel family. In this review, we focus on the discoveries and characterization of these CNGC mutants and discuss possible roles of CNGCs as components in plant immunity.

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.

Self-incompatibility in Papaver rhoeas activates nonspecific cation conductance permeable to Ca2+ and K+

Cellular responses rely on signaling. In plant cells, cytosolic free calcium is a major second messenger, and ion channels play a key role in mediating physiological responses. Self-incompatibility (SI) is an important genetically controlled mechanism to prevent self-fertilization. It uses interaction of matching S-determinants from the pistil and pollen to allow "self" recognition, which triggers rejection of incompatible pollen. In Papaver rhoeas, the S-determinants are PrsS and PrpS. PrsS is a small novel cysteine-rich protein; PrpS is a small novel transmembrane protein. Interaction of PrsS with incompatible pollen stimulates S-specific increases in cytosolic free calcium and alterations in the actin cytoskeleton, resulting in programmed cell death in incompatible but not compatible pollen. Here, we have used whole-cell patch clamping of pollen protoplasts to show that PrsS stimulates SI-specific activation of pollen grain plasma membrane conductance in incompatible but not compatible pollen grain protoplasts. The SI-activated conductance does not require voltage activation, but it is voltage sensitive. It is permeable to divalent cations (Ba(2+) ≥ Ca(2+) > Mg(2+)) and the monovalent ions K(+) and NH(4)(+) and is enhanced at voltages negative to -100 mV. The Ca(2+) conductance is blocked by La(3+) but not by verapamil; the K(+) currents are tetraethylammonium chloride insensitive and do not require Ca(2+). We propose that the SI-stimulated conductance may represent a nonspecific cation channel or possibly two conductances, permeable to monovalent and divalent cations. Our data provide insights into signal-response coupling involving a biologically important response. PrsS provides a rare example of a protein triggering alterations in ion channel activity.

Annexins

Annexins are multifunctional lipid-binding proteins. Plant annexins are expressed throughout the life cycle and are under environmental control. Their association or insertion into membranes may be governed by a range of local conditions (Ca(2+), pH, voltage or lipid identity) and nonclassical sorting motifs. Protein functions include exocytosis, actin binding, peroxidase activity, callose synthase regulation and ion transport. As such, annexins appear capable of linking Ca(2+), redox and lipid signalling to coordinate development with responses to the biotic and abiotic environment. Significant advances in plant annexin research have been made in the past 2 yr. Here, we review the basis of annexin multifunctionality and suggest how these proteins may operate in the life and death of a plant cell.

Activation of the plant mitochondrial potassium channel by free fatty acids and acyl-CoA esters: a possible defence mechanism in the response to hyperosmotic stress

The effect of free fatty acids (FFAs) and acyl-CoA esters on K(+) uptake was studied in mitochondria isolated from durum wheat (Triticum durum Desf.), a species that has adapted well to the semi-arid Mediterranean area and possessing a highly active mitochondrial ATP-sensitive K(+) channel (PmitoK(ATP)), that may confer resistance to environmental stresses. This was made by swelling experiments in KCl solution under experimental conditions in which PmitoK(ATP) activity was monitored. Linoleate and other FFAs (laurate, palmitate, stearate, palmitoleate, oleate, arachidonate, and the non-physiological 1-undecanesulphonate and 5-phenylvalerate), used at a concentration (10 μM) unable to damage membranes of isolated mitochondria, stimulated K(+) uptake by about 2-4-fold. Acyl-CoAs also promoted K(+) transport to a much larger extent with respect to FFAs (about 5-12-fold). In a different experimental system based on safranin O fluorescence measurements, the dissipation of electrical membrane potential induced by K(+) uptake via PmitoK(ATP) was found to increase in the presence of 5-phenylvalerate and palmitoyl-CoA, both unable to elicit the activity of the Plant Uncoupling Protein. This result suggests a direct activation of PmitoK(ATP). Stimulation of K(+) transport by FFAs/acyl-CoAs resulted in a widespread phenomenon in plant mitochondria from different mono/dicotyledonous species (bread wheat, barley, triticale, maize, lentil, pea, and topinambur) and from different organs (root, tuber, leaf, and shoot). Finally, an increase in mitochondrial FFAs up to a content of 50 nmol mg(-1) protein, which was able to activate PmitoK(ATP) strongly, was observed under hyperosmotic stress conditions. Since PmitoK(ATP) may act against environmental/oxidative stress, its activation by FFAs/acyl-CoAs is proposed to represent a physiological defence mechanism.

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.