Ion exchangers NHX1 and NHX2 mediate active potassium uptake into vacuoles to regulate cell turgor and stomatal function in Arabidopsis

Burkholderia phytofirmans PsJN induces long-term metabolic and transcriptional changes involved in Arabidopsis thaliana salt tolerance

Salinity is one of the major limitations for food production worldwide. Improvement of plant salt-stress tolerance using plant-growth promoting rhizobacteria (PGPR) has arisen as a promising strategy to help overcome this limitation. However, the molecular and biochemical mechanisms controlling PGPR/plant interactions under salt-stress remain unclear. The main objective of this study was to obtain new insights into the mechanisms underlying salt-stress tolerance enhancement in the salt-sensitive Arabidopsis thaliana Col-0 plants, when inoculated with the well-known PGPR strain Burkholderia phytofirmans PsJN. To tackle this, different life history traits, together with the spatiotemporal accumulation patterns for key metabolites and salt-stress related transcripts, were analyzed in inoculated plants under short and long-term salt-stress. Inoculated plants displayed faster recovery and increased tolerance after sustained salt-stress. PsJN treatment accelerated the accumulation of proline and transcription of genes related to abscisic acid signaling (Relative to Dessication, RD29A and RD29B), ROS scavenging (Ascorbate Peroxidase 2), and detoxification (Glyoxalase I 7), and down-regulated the expression of Lipoxygenase 2 (related to jasmonic acid biosynthesis). Among the general transcriptional effects of this bacterium, the expression pattern of important ion-homeostasis related genes was altered after short and long-term stress (Arabidopsis K(+) Transporter 1, High-Affinity K(+) Transporter 1, Sodium Hydrogen Exchanger 2, and Arabidopsis Salt Overly Sensitive 1). In all, the faster and stronger molecular changes induced by the inoculation suggest a PsJN-priming effect, which may explain the observed tolerance after short-term and sustained salt-stress in plants. This study provides novel information about possible mechanisms involved in salt-stress tolerance induced by PGPR in plants, showing that certain changes are maintained over time. This opens up new venues to study these relevant biological associations, as well as new approaches to a better understanding of the spatiotemporal mechanisms involved in stress tolerance in plants.

The Arabidopsis Vacuolar Sorting Receptor1 is required for osmotic stress-induced abscisic acid biosynthesis

Osmotic stress activates the biosynthesis of the phytohormone abscisic acid (ABA) through a pathway that is rate limited by the carotenoid cleavage enzyme 9-cis-epoxycarotenoid dioxygenase (NCED). To understand the signal transduction mechanism underlying the activation of ABA biosynthesis, we performed a forward genetic screen to isolate mutants defective in osmotic stress regulation of the NCED3 gene. Here, we identified the Arabidopsis (Arabidopsis thaliana) Vacuolar Sorting Receptor1 (VSR1) as a unique regulator of ABA biosynthesis. The vsr1 mutant not only shows increased sensitivity to osmotic stress, but also is defective in the feedback regulation of ABA biosynthesis by ABA. Further analysis revealed that vacuolar trafficking mediated by VSR1 is required for osmotic stress-responsive ABA biosynthesis and osmotic stress tolerance. Moreover, under osmotic stress conditions, the membrane potential, calcium flux, and vacuolar pH changes in the vsr1 mutant differ from those in the wild type. Given that manipulation of the intracellular pH is sufficient to modulate the expression of ABA biosynthesis genes, including NCED3, and ABA accumulation, we propose that intracellular pH changes caused by osmotic stress may play a signaling role in regulating ABA biosynthesis and that this regulation is dependent on functional VSR1.

New insights into plant salt acclimation: the roles of vesicle trafficking and reactive oxygen species signalling in mitochondria and the endomembrane system

In this study, we investigated the cellular and molecular mechanisms that regulate salt acclimation. The main objective was to obtain new insights into the molecular mechanisms that control salt acclimation. Therefore, we carried out a multidisciplinary study using proteomic, transcriptomic, subcellular and physiological techniques. We obtained a Nicotiana tabacum BY-2 cell line acclimated to be grown at 258 mM NaCl as a model for this study. The proteomic and transcriptomic data indicate that the molecular response to stress (chaperones, defence proteins, etc.) is highly induced in these salt-acclimated cells. The subcellular results show that salt induces sodium compartmentalization in the cell vacuoles and seems to be mediated by vesicle trafficking in tobacco salt-acclimated cells. Our results demonstrate that abscisic acid (ABA) and proline metabolism are crucial in the cellular signalling of salt acclimation, probably regulating reactive oxygen species (ROS) production in the mitochondria. ROS may act as a retrograde signal, regulating the cell response. The network of endoplasmic reticulum and Golgi apparatus is highly altered in salt-acclimated cells. The molecular and subcellular analysis suggests that the unfolded protein response is induced in salt-acclimated cells. Finally, we propose that this mechanism may mediate cell death in salt-acclimated cells.

A vacuolar antiporter is differentially regulated in leaves and roots of the halophytic wild rice Porteresia coarctata (Roxb.) Tateoka

Vacuolar NHX-type antiporters play a role in Na(+)/K(+) uptake that contributes to growth, nutrition and development. Under salt/osmotic stress they mediate the vacuolar compartmentalization of K(+)/Na(+), thereby preventing toxic Na(+)K(+) ratios in the cytosol. Porteresia coarctata (Roxb.) Tateoka, a mangrove associate, is a distant wild relative of cultivated rice and is saline as well as submergence tolerant. A vacuolar NHX homolog isolated from a P. coarctata cDNA library (PcNHX1) shows 96 % identity (nucleotide level) to OsNHX1. Diurnal PcNHX1 expression in leaves was found to be largely unaltered, though damped by salinity. PcNHX1 promoter directed GUS expression is phloem-specific in leaves, stem and roots of transgenic plants in the absence of stress. Under NaCl stress, GUS expression was also seen in the epidermal and sub-epidermal layers (mesophyll, guard cells and trichomes) of leaves, root tip. The salinity in the rhizosphere of P. coarctata varies considerably due to diurnal/semi-diurnal tidal inundation. The diurnal expression of PcNHX1 in leaves and salinity induced expression in roots may have evolved in response to dynamic changes in salinity of in the P. coarctata rhizosphere. Despite high sequence conservation between OsNHX1 and PcNHX1, the distinctive expression pattern of PcNHX1 exemplifies how variation in expression is fine tuned to suit the halophytic growth habitat of a plant.

The ins and outs of intracellular ion homeostasis: NHX-type cation/H(+) transporters

The biochemical characterization of cation/H(+) exchange has been known since 1985 [1], yet only recently have we begun to understand the contribution of individual exchangers to ion homeostasis in plants. One particularly important class of exchangers is the NHX-type that is associated with Na(+) transport and therefore salinity tolerance. New evidence suggests that under normal growth conditions NHXs are critical regulators of K(+) and pH homeostasis and have important roles, depending on their cellular localization, in the generation of turgor as well as in vesicular trafficking. Recent advances highlight novel and exciting functions of intracellular NHXs in growth and development, stress adaptation and osmotic adjustment. Here, we elaborate on new and emerging cellular and physiological functions of this group of H(+)-coupled cation exchangers.

pH in the plant endomembrane system-an import and export business

pH homeostasis is an essential process in all plant cells and the maintenance of correct luminal pH in the compartments of the endomembrane system is important not only for secondary active transport but also for a variety of cellular functions including protein modification, sorting, and trafficking. Due to their electrogenicity primary H(+)-pumps cannot establish and control the often large proton-gradients single-handedly but require the co-action of other ion transporters that serve as either shunt conductances or proton-leaks. Here, I will thus focus on recent results that highlight the interplay of proton-pumps and proton-coupled transporters in controlling pH in the compartments of the plant endomembrane system.

A stress responsive gene of Fortunella crassifolia FcSISP functions in salt stress resistance

Exploration of genes functioning in salt tolerance is crucial for generating transgenic plants with enhanced salt tolerance. In this study, we report the isolation and functional characterization of a stress-responsive gene FcSISP from Meiwa kumquat (Fortunella crassifolia). FcSISP encodes a putative protein of 47 amino acids, with a calculated molecular mass of 4.94 kDa and theoretical isoelectric point of 3.76, and was localized in the nucleus. Transcript levels of FcSISP were induced by dehydration, cold, salt and bacterium causing citrus canker, and hormones (salicylic acid and abscisic acid), with the greatest induction under salt treatment. Overexpression of FcSISP in tobacco (Nicotiana nudicaulis) conferred enhanced salt tolerance. The transgenic lines accumulated lower Na(+) contents, leading to reduced Na/K ratio, but accumulated more proline than the wild type (WT). Steady state mRNA levels of genes involved in Na(+) exchange (three SOS genes and three NHX genes) and proline synthesis (P5CS and P5CR) were higher in the transgenic lines in comparison with WT. Moreover, overexpression of FcSISP in trifoliate orange [Poncirus trifoliata (L.) Raf.], a widely-used and salt-sensitive citrus rootstock, led to elevated salt tolerance. Taken together, the data demonstrate that FcSISP plays a positive role in salt tolerance and that it holds a great potential for engineering salt tolerance in crops.

Regulation of Na(+) fluxes in plants

When exposed to salt, every plant takes up Na(+) from the environment. Once in the symplast, Na(+) is distributed within cells and between different tissues and organs. There it can help to lower the cellular water potential but also exert potentially toxic effects. Control of Na(+) fluxes is therefore crucial and indeed, research shows that the divergence between salt tolerant and salt sensitive plants is not due to a variation in transporter types but rather originates in the control of uptake and internal Na(+) fluxes. A number of regulatory mechanisms has been identified based on signaling of Ca(2+), cyclic nucleotides, reactive oxygen species, hormones, or on transcriptional and post translational changes of gene and protein expression. This review will give an overview of intra- and intercellular movement of Na(+) in plants and will summarize our current ideas of how these fluxes are controlled and regulated in the early stages of salt stress.

Vacuolar Na⁺/H⁺ NHX-Type Antiporters Are Required for Cellular K⁺ Homeostasis, Microtubule Organization and Directional Root Growth

Na⁺/H⁺ antiporters (NHXs) are integral membrane transporters that catalyze the electroneutral exchange of K⁺ or Na⁺ for H⁺ and are implicated in cell expansion, development, pH and ion homeostasis and salt tolerance. Arabidopsis contains four vacuolar NHX isoforms (NHX1-NHX4), but only the functional roles for NHX1 and NHX2 have been assessed thus far. Colocalization studies indicated that NHX3 and NHX4 colocalize to the tonoplast. To investigate the role of all vacuolar NHX isoforms, a quadruple knockout nhx1nhx2nhx3nhx4, lacking all vacuolar NHXs, was generated. Seedlings of nhx1nhx2nhx3nhx4 displayed significantly reduced growth, with markedly shorter hypocotyls. Under high K⁺, but not Na⁺, pronounced root skewing occurred in nhx1nhx2nhx3nhx4, suggesting that the organization of the cytoskeleton might be perturbed. Whole mount immunolabeling of cortical microtubules indicated that high K⁺ caused significant microtubule reorganization in nhx1nhx2nhx3nhx4 root cells of the elongation zone. Using microtubule stabilizing (Taxol) and destabilizing (propyzamide) drugs, we found that the effect of K⁺ on nhx1nhx2nhx3nhx4 root growth was antagonistic to that of Taxol, whereas elevated K⁺ exacerbated the endogenous effect of propyzamide on root skewing. Collectively, our results suggest that altered K⁺ homeostasis leads to an increase in the dynamics of cortical microtubule reorganization in nhx1nhx2nhx3nhx4 root epidermal cells of the elongation zone.

Functional analysis of the Na+,K+/H+ antiporter PeNHX3 from the tree halophyte Populus euphratica in yeast by model-guided mutagenesis

Na+,K+/H+ antiporters are H+-coupled cotransporters that are crucial for cellular homeostasis. Populus euphratica, a well-known tree halophyte, contains six Na+/H+ antiporter genes (PeNHX1-6) that have been shown to function in salt tolerance. However, the catalytic mechanisms governing their ion transport remain largely unknown. Using the crystal structure of the Na+/H+ antiporter from the Escherichia coli (EcNhaA) as a template, we built the three-dimensional structure of PeNHX3 from P. euphratica. The PeNHX3 model displays the typical TM4-TM11 assembly that is critical for ion binding and translocation. The PeNHX3 structure follows the 'positive-inside' rule and exhibits a typical physicochemical property of the transporter proteins. Four conserved residues, including Tyr149, Asn187, Asp188, and Arg356, are indentified in the TM4-TM11 assembly region of PeNHX3. Mutagenesis analysis showed that these reserved residues were essential for the function of PeNHX3: Asn187 and Asp188 (forming a ND motif) controlled ion binding and translocation, and Tyr149 and Arg356 compensated helix dipoles in the TM4-TM11 assembly. PeNHX3 mediated Na+, K+ and Li+ transport in a yeast growth assay. Domain-switch analysis shows that TM11 is crucial to Li+ transport. The novel features of PeNHX3 in ion binding and translocation are discussed.

Genome of the halotolerant green alga Picochlorum sp. reveals strategies for thriving under fluctuating environmental conditions

An expected outcome of climate change is intensification of the global water cycle, which magnifies surface water fluxes, and consequently alters salinity patterns. It is therefore important to understand the adaptations and limits of microalgae to survive changing salinities. To this end, we sequenced the 13.5 Mbp genome of the halotolerant green alga Picochlorum SENEW3 (SE3) that was isolated from a brackish water pond subject to large seasonal salinity fluctuations. Picochlorum SE3 encodes 7367 genes, making it one of the smallest and most gene dense eukaryotic genomes known. Comparison with the pico-prasinophyte Ostreococcus tauri, a species with a limited range of salt tolerance, reveals the enrichment of transporters putatively involved in the salt stress response in Picochlorum SE3. Analysis of cultures and the protein complement highlight the metabolic flexibility of Picochlorum SE3 that encodes genes involved in urea metabolism, acetate assimilation and fermentation, acetoin production and glucose uptake, many of which form functional gene clusters. Twenty-four cases of horizontal gene transfer from bacterial sources were found in Picochlorum SE3 with these genes involved in stress adaptation including osmolyte production and growth promotion. Our results identify Picochlorum SE3 as a model for understanding microalgal adaptation to stressful, fluctuating environments.

Phosphorylation of the vacuolar anion exchanger AtCLCa is required for the stomatal response to abscisic acid

Eukaryotic anion/proton exchangers of the CLC (chloride channel) family mediate anion fluxes across intracellular membranes. The Arabidopsis thaliana anion/proton exchanger AtCLCa is involved in vacuolar accumulation of nitrate. We investigated the role of AtCLCa in leaf guard cells, a specialized plant epidermal cell that controls gas exchange and water loss through pores called stomata. We showed that AtCLCa not only fulfilled the expected role of accumulating anions in the vacuole during stomatal opening but also mediated anion release during stomatal closure in response to the stress hormone abscisic acid (ABA). We found that this dual role resulted from a phosphorylation-dependent change in the activity of AtCLCa. The protein kinase OST1 (also known as SnRK2.6) is a key signaling player and central regulator in guard cells in response to ABA. Phosphorylation of Thr(38) in the amino-terminal cytoplasmic domain of AtCLCa by OST1 increased the outward anion fluxes across the vacuolar membrane, which are essential for stomatal closure. We provide evidence that bidirectional activities of an intracellular CLC exchanger are physiologically relevant and that phosphorylation regulates the transport mode of this exchanger.

Regulation of potassium transport in plants under hostile conditions: implications for abiotic and biotic stress tolerance

Intracellular potassium homeostasis is a prerequisite for the optimal operation of plant metabolic machinery and plant's overall performance. It is controlled by K(+) uptake, efflux and intracellular and long-distance relocation, mediated by a large number of K(+) -selective and non-selective channels and transporters located at both plasma and vacuolar membranes. All abiotic and biotic stresses result in a significant disturbance to intracellular potassium homeostasis. In this work, we discuss molecular mechanisms and messengers mediating potassium transport and homeostasis focusing on four major environmental stresses: salinity, drought, flooding and biotic factors. We argue that cytosolic K(+) content may be considered as one of the 'master switches' enabling plant transition from the normal metabolism to 'hibernated state' during first hours after the stress exposure and then to a recovery phase. We show that all these stresses trigger substantial disturbance to K(+) homeostasis and provoke a feedback control on K(+) channels and transporters expression and post-translational regulation of their activity, optimizing K(+) absorption and usage, and, at the extreme end, assisting the programmed cell death. We discuss specific modes of regulation of the activity of K(+) channels and transporters by membrane voltage, intracellular Ca(2+) , reactive oxygen species, polyamines, phytohormones and gasotransmitters, and link this regulation with plant-adaptive responses to hostile environments.

Closing gaps: linking elements that control stomatal movement

Stomata are an attractive experimental system in plant biology, because the responses of guard cells to environmental signals can be directly linked to changes in the aperture of stomatal pores. In this review, the mechanics of stomatal movement are discussed in relation to ion transport in guard cells. Emphasis is placed on the ion pumps, transporters, and channels in the plasma membrane, as well as in the vacuolar membrane. The biophysical properties of transport proteins for H(+), K(+), Ca(2+), and anions are discussed and related to their function in guard cells during stomatal movements. Guard cell signaling pathways for ABA, CO2, ozone, microbe-associated molecular patterns (MAMPs) and blue light are presented. Special attention is given to the regulation of the slow anion channel (SLAC) and SLAC homolog (SLAH)-type anion channels by the ABA signalosome. Over the last decade, several knowledge gaps in the regulation of ion transport in guard cells have been closed. The current state of knowledge is an excellent starting point for tackling important open questions concerning stress tolerance in plants.

Plant salt-tolerance mechanisms

Crop performance is severely affected by high salt concentrations in soils. To engineer more salt-tolerant plants it is crucial to unravel the key components of the plant salt-tolerance network. Here we review our understanding of the core salt-tolerance mechanisms in plants. Recent studies have shown that stress sensing and signaling components can play important roles in regulating the plant salinity stress response. We also review key Na+ transport and detoxification pathways and the impact of epigenetic chromatin modifications on salinity tolerance. In addition, we discuss the progress that has been made towards engineering salt tolerance in crops, including marker-assisted selection and gene stacking techniques. We also identify key open questions that remain to be addressed in the future.

Cellular and tissue distribution of potassium: physiological relevance, mechanisms and regulation

Potassium (K(+)) is the most important cationic nutrient for all living organisms. Its cellular levels are significant (typically around 100mM) and are highly regulated. In plants K(+) affects multiple aspects such as growth, tolerance to biotic and abiotic stress and movement of plant organs. These processes occur at the cell, organ and whole plant level and not surprisingly, plants have evolved sophisticated mechanisms for the uptake, efflux and distribution of K(+) both within cells and between organs. Great progress has been made in the last decades regarding the molecular mechanisms of K(+) uptake and efflux, particularly at the cellular level. For long distance K(+) transport our knowledge is less complete but the principles behind the overall processes are largely understood. In this chapter we will discuss how both long distance transport between different organs and intracellular transport between organelles works in general and in particular for K(+). Where possible, we will provide examples of specific genes and proteins that are responsible for these phenomena.

Plastidial transporters KEA1, -2, and -3 are essential for chloroplast osmoregulation, integrity, and pH regulation in Arabidopsis

Multiple K(+) transporters and channels and the corresponding mutants have been described and studied in the plasma membrane and organelle membranes of plant cells. However, knowledge about the molecular identity of chloroplast K(+) transporters is limited. Potassium transport and a well-balanced K(+) homeostasis were suggested to play important roles in chloroplast function. Because no loss-of-function mutants have been identified, the importance of K(+) transporters for chloroplast function and photosynthesis remains to be determined. Here, we report single and higher-order loss-of-function mutants in members of the cation/proton antiporters-2 antiporter superfamily KEA1, KEA2, and KEA3. KEA1 and KEA2 proteins are targeted to the inner envelope membrane of chloroplasts, whereas KEA3 is targeted to the thylakoid membrane. Higher-order but not single mutants showed increasingly impaired photosynthesis along with pale green leaves and severely stunted growth. The pH component of the proton motive force across the thylakoid membrane was significantly decreased in the kea1kea2 mutants, but increased in the kea3 mutant, indicating an altered chloroplast pH homeostasis. Electron microscopy of kea1kea2 leaf cells revealed dramatically swollen chloroplasts with disrupted envelope membranes and reduced thylakoid membrane density. Unexpectedly, exogenous NaCl application reversed the observed phenotypes. Furthermore, the kea1kea2 background enables genetic analyses of the functional significance of other chloroplast transporters as exemplified here in kea1kea2Na(+)/H(+) antiporter1 (nhd1) triple mutants. Taken together, the presented data demonstrate a fundamental role of inner envelope KEA1 and KEA2 and thylakoid KEA3 transporters in chloroplast osmoregulation, integrity, and ion and pH homeostasis.

Decreased capacity for sodium export out of Arabidopsis chloroplasts impairs salt tolerance, photosynthesis and plant performance

Salt stress is a widespread phenomenon, limiting plant performance in large areas around the world. Although various types of plant sodium/proton antiporters have been characterized, the physiological function of NHD1 from Arabidopsis thaliana has not been elucidated in detail so far. Here we report that the NHD1-GFP fusion protein localizes to the chloroplast envelope. Heterologous expression of AtNHD1 was sufficient to complement a salt-sensitive Escherichia coli mutant lacking its endogenous sodium/proton exchangers. Transport competence of NHD1 was confirmed using recombinant, highly purified carrier protein reconstituted into proteoliposomes, proving Na(+) /H(+) antiport. In planta NHD1 expression was found to be highest in mature and senescent leaves but was not induced by sodium chloride application. When compared to wild-type controls, nhd1 T-DNA insertion mutants showed decreased biomasses and lower chlorophyll levels after sodium feeding. Interestingly, if grown on sand and supplemented with high sodium chloride, nhd1 mutants exhibited leaf tissue Na(+) levels similar to those of wild-type plants, but the Na(+) content of chloroplasts increased significantly. These high sodium levels in mutant chloroplasts resulted in markedly impaired photosynthetic performance as revealed by a lower quantum yield of photosystem II and increased non-photochemical quenching. Moreover, high Na(+) levels might hamper activity of the plastidic bile acid/sodium symporter family protein 2 (BASS2). The resulting pyruvate deficiency might cause the observed decreased phenylalanine levels in the nhd1 mutants due to lack of precursors.

Control of vacuolar dynamics and regulation of stomatal aperture by tonoplast potassium uptake

Stomatal movements rely on alterations in guard cell turgor. This requires massive K(+) bidirectional fluxes across the plasma and tonoplast membranes. Surprisingly, given their physiological importance, the transporters mediating the energetically uphill transport of K(+) into the vacuole remain to be identified. Here, we report that, in Arabidopsis guard cells, the tonoplast-localized K(+)/H(+) exchangers NHX1 and NHX2 are pivotal in the vacuolar accumulation of K(+) and that nhx1 nhx2 mutant lines are dysfunctional in stomatal regulation. Hypomorphic and complete-loss-of-function double mutants exhibited significantly impaired stomatal opening and closure responses. Disruption of K(+) accumulation in guard cells correlated with more acidic vacuoles and the disappearance of the highly dynamic remodelling of vacuolar structure associated with stomatal movements. Our results show that guard cell vacuolar accumulation of K(+) is a requirement for stomatal opening and a critical component in the overall K(+) homeostasis essential for stomatal closure, and suggest that vacuolar K(+) fluxes are also of decisive importance in the regulation of vacuolar dynamics and luminal pH that underlie stomatal movements.

Evaluating contribution of ionic, osmotic and oxidative stress components towards salinity tolerance in barley

BACKGROUND

Salinity tolerance is a physiologically multi-faceted trait attributed to multiple mechanisms. Three barley (Hordeum vulgare) varieties contrasting in their salinity tolerance were used to assess the relative contribution of ionic, osmotic and oxidative stress components towards overall salinity stress tolerance in this species, both at the whole-plant and cellular levels. In addition, transcriptional changes in the gene expression profile were studied for key genes mediating plant ionic and oxidative homeostasis (NHX; RBOH; SOD; AHA and GORK), to compare a contribution of transcriptional and post-translational factors towards the specific components of salinity tolerance.

RESULTS

Our major findings are two-fold. First, plant tissue tolerance was a dominating component that has determined the overall plant responses to salinity, with root K(+) retention ability and reduced sensitivity to stress-induced hydroxyl radical production being the main contributing tolerance mechanisms. Second, it was not possible to infer which cultivars were salinity tolerant based solely on expression profiling of candidate genes at one specific time point. For the genes studied and the time point selected that transcriptional changes in the expression of these specific genes had a small role for barley's adaptive responses to salinity.

CONCLUSIONS

For better tissue tolerance, sodium sequestration, K(+) retention and resistance to oxidative stress all appeared to be crucial. Because these traits are highly interrelated, it is suggested that a major progress in crop breeding for salinity tolerance can be achieved only if these complementary traits are targeted at the same time. This study also highlights the essentiality of post translational modifications in plant adaptive responses to salinity.

Mechanism of salinity tolerance in plants: physiological, biochemical, and molecular characterization

Salinity is a major abiotic stress limiting growth and productivity of plants in many areas of the world due to increasing use of poor quality of water for irrigation and soil salinization. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, and molecular or gene networks. A comprehensive understanding on how plants respond to salinity stress at different levels and an integrated approach of combining molecular tools with physiological and biochemical techniques are imperative for the development of salt-tolerant varieties of plants in salt-affected areas. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress.

Genome structures and transcriptomes signify niche adaptation for the multiple-ion-tolerant extremophyte Schrenkiella parvula

Schrenkiella parvula (formerly Thellungiella parvula), a close relative of Arabidopsis (Arabidopsis thaliana) and Brassica crop species, thrives on the shores of Lake Tuz, Turkey, where soils accumulate high concentrations of multiple-ion salts. Despite the stark differences in adaptations to extreme salt stresses, the genomes of S. parvula and Arabidopsis show extensive synteny. S. parvula completes its life cycle in the presence of Na⁺, K⁺, Mg²⁺, Li⁺, and borate at soil concentrations lethal to Arabidopsis. Genome structural variations, including tandem duplications and translocations of genes, interrupt the colinearity observed throughout the S. parvula and Arabidopsis genomes. Structural variations distinguish homologous gene pairs characterized by divergent promoter sequences and basal-level expression strengths. Comparative RNA sequencing reveals the enrichment of ion-transport functions among genes with higher expression in S. parvula, while pathogen defense-related genes show higher expression in Arabidopsis. Key stress-related ion transporter genes in S. parvula showed increased copy number, higher transcript dosage, and evidence for subfunctionalization. This extremophyte offers a framework to identify the requisite adjustments of genomic architecture and expression control for a set of genes found in most plants in a way to support distinct niche adaptation and lifestyles.

ABA control of plant macroelement membrane transport systems in response to water deficit and high salinity

Plant growth and productivity are adversely affected by various abiotic stressors and plants develop a wide range of adaptive mechanisms to cope with these adverse conditions, including adjustment of growth and development brought about by changes in stomatal activity. Membrane ion transport systems are involved in the maintenance of cellular homeostasis during exposure to stress and ion transport activity is regulated by phosphorylation/dephosphorylation networks that respond to stress conditions. The phytohormone abscisic acid (ABA), which is produced rapidly in response to drought and salinity stress, plays a critical role in the regulation of stress responses and induces a series of signaling cascades. ABA signaling involves an ABA receptor complex, consisting of an ABA receptor family, phosphatases and kinases: these proteins play a central role in regulating a variety of diverse responses to drought stress, including the activities of membrane-localized factors, such as ion transporters. In this review, recent research on signal transduction networks that regulate the function ofmembrane transport systems in response to stress, especially water deficit and high salinity, is summarized and discussed. The signal transduction networks covered in this review have central roles in mitigating the effect of stress by maintaining plant homeostasis through the control of membrane transport systems.

Sodium in plants: perception, signalling, and regulation of sodium fluxes

Although not essential for most plants, sodium (Na(+)) can be beneficial to plants in many conditions, particularly when potassium (K(+)) is deficient. As such it can be regarded a 'non-essential' or 'functional' nutrient. By contrast, the many salinized areas around the globe force plants to deal with toxicity from high levels of Na(+) in the environment and within tissues. Progress has been made in identifying the relevant membrane transporters involved in the uptake and distribution of Na(+). The latter is important in the context of mitigating salinity stress but also for the optimization of Na(+) as an abundantly available functional nutrient. In both cases plants are likely to require mechanism(s) to monitor Na(+) concentration, possibly in multiple compartments, to regulate gene expression and transport activities. Extremely little is known about whether such mechanisms are present and if so, how they operate, either at the cellular or the tissue level. This paper gives an overview of the regulatory and potential sensing mechanisms that pertain to Na(+), in both the context of salt stress and Na(+) as a nutrient.

Transport, signaling, and homeostasis of potassium and sodium in plants

Potassium (K⁺) is an essential macronutrient in plants and a lack of K⁺ significantly reduces the potential for plant growth and development. By contrast, sodium (Na⁺), while beneficial to some extent, at high concentrations it disturbs and inhibits various physiological processes and plant growth. Due to their chemical similarities, some functions of K⁺ can be undertaken by Na⁺ but K⁺ homeostasis is severely affected by salt stress, on the other hand. Recent advances have highlighted the fascinating regulatory mechanisms of K⁺ and Na⁺ transport and signaling in plants. This review summarizes three major topics: (i) the transport mechanisms of K⁺ and Na⁺ from the soil to the shoot and to the cellular compartments; (ii) the mechanisms through which plants sense and respond to K⁺ and Na⁺ availability; and (iii) the components involved in maintenance of K⁺/Na⁺ homeostasis in plants under salt stress.

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.

The K+/H+ antiporter LeNHX2 increases salt tolerance by improving K+ homeostasis in transgenic tomato

The endosomal LeNHX2 ion transporter exchanges H(+) with K(+) and, to lesser extent, Na(+) . Here, we investigated the response to NaCl supply and K(+) deprivation in transgenic tomato (Solanum lycopersicum L.) overexpressing LeNHX2 and show that transformed tomato plants grew better in saline conditions than untransformed controls, whereas in the absence of K(+) the opposite was found. Analysis of mineral composition showed a higher K(+) content in roots, shoots and xylem sap of transgenic plants and no differences in Na(+) content between transgenic and untransformed plants grown either in the presence or the absence of 120 mm NaCl. Transgenic plants showed higher Na(+)/H(+) and, above all, K(+)/H(+) transport activity in root intracellular membrane vesicles. Under K(+) limiting conditions, transgenic plants enhanced root expression of the high-affinity K(+) uptake system HAK5 compared to untransformed controls. Furthermore, tomato overexpressing LeNHX2 showed twofold higher K(+) depletion rates and half cytosolic K(+) activity than untransformed controls. Under NaCl stress, transgenic plants showed higher uptake velocity for K(+) and lower cytosolic K(+) activity than untransformed plants. These results indicate the fundamental role of K(+) homeostasis in the better performance of LeNHX2 overexpressing tomato under NaCl stress.

A novel AtKEA gene family, homolog of bacterial K+/H+ antiporters, plays potential roles in K+ homeostasis and osmotic adjustment in Arabidopsis

AtKEAs, homologs of bacterial KefB/KefC, are predicted to encode K⁺/H⁺ antiporters in Arabidopsis. The AtKEA family contains six genes forming two subgroups in the cladogram: AtKEA1-3 and AtKEA4-6. AtKEA1 and AtKEA2 have a long N-terminal domain; the full-length AtKEA1 was inactive in yeast. The transport activity was analyzed by expressing the AtKEA genes in yeast mutants lacking multiple ion carriers. AtKEAs conferred resistance to high K⁺ and hygromycin B but not to salt and Li⁺ stress. AtKEAs expressed in both the shoot and root of Arabidopsis. The expression of AtKEA1, -3 and -4 was enhanced under low K⁺ stress, whereas AtKEA2 and AtKEA5 were induced by sorbitol and ABA treatments. However, osmotic induction of AtKEA2 and AtKEA5 was not observed in aba2-3 mutants, suggesting an ABA regulated mechanism for their osmotic response. AtKEAs’ expression may not be regulated by the SOS pathway since their expression was not affected in sos mutants. The GFP tagging analysis showed that AtKEAs distributed diversely in yeast. The Golgi localization of AtKEA3 was demonstrated by both the stably transformed seedlings and the transient expression in protoplasts. Overall, AtKEAs expressed and localized diversely, and may play roles in K⁺ homeostasis and osmotic adjustment in Arabidopsis.

Functional characterization of a wheat NHX antiporter gene TaNHX2 that encodes a K(+)/H(+) exchanger

The subcellular localization of a wheat NHX antiporter, TaNHX2, was studied in Arabidopsis protoplasts, and its function was evaluated using Saccharomyces cerevisiae as a heterologous expression system. Fluorescence patterns of TaNHX2-GFP fusion protein in Arabidopsis cells indicated that TaNHX2 localized at endomembranes. TaNHX2 has significant sequence homology to NHX sodium exchangers from Arabidopsis, is abundant in roots and leaves and is induced by salt or dehydration treatments. Western blot analysis showed that TaNHX2 could be expressed in transgenic yeast cells. Expressed TaNHX2 protein suppressed the salt sensitivity of a yeast mutant strain by increasing its K⁺ content when exposed to salt stress. TaNHX2 also increased the tolerance of the strain to potassium stress. However, the expression of TaNHX2 did not affect the sodium concentration in transgenic cells. Western blot analysis for tonoplast proteins indicated that the TaNHX2 protein localized at the tonoplast of transgenic yeast cells. The tonoplast vesicles from transgenic yeast cells displayed enhanced K⁺/H⁺ exchange activity but very little Na^+/H⁺ exchange compared with controls transformed with the empty vector; Na⁺/H⁺ exchange was not detected with concentrations of less than 37.5 mM Na⁺ in the reaction medium. Our data suggest that TaNHX2 is a endomembrane-bound protein and may primarily function as a K⁺/H⁺ antiporter, which is involved in cellular pH regulation and potassium nutrition under normal conditions. Under saline conditions, the protein mediates resistance to salt stress through the intracellular compartmentalization of potassium to regulate cellular pH and K⁺ homeostasis.

Sodium transport system in plant cells

Since sodium, Na, is a non-essential element for the plant growth, the molecular mechanism of Na(+) transport system in plants has remained elusive for the last two decades. The accumulation of Na(+) in soil through irrigation for sustainable agricultural crop production, particularly in arid land, and by changes in environmental and climate conditions leads to the buildup of toxic level of salts in the soil. Since the latter half of the twentieth century, extensive molecular research has identified several classes of Na(+) transporters that play major roles in the alleviation of ionic stress by excluding toxic Na(+) from the cytosol or preventing Na(+) transport to the photosynthetic organs, and also in osmotic stress by modulating intra/extracellular osmotic balance. In this review, we summarize the current knowledge of three major Na(+) transporters, namely NHX, SOS1, and HKT transporters, including recently revealed characteristics of these transporters.

Enhanced salt resistance in apple plants overexpressing a Malus vacuolar Na+/H+ antiporter gene is associated with differences in stomatal behavior and photosynthesis

High salinity is a major abiotic factor that limits crop production. The dwarfing apple rootstock M.26 is sensitive to such stress. To obtain an apple that is adaptable to saline soils, we transformed this rootstock with a vacuolar Na(+)/H(+) antiporter, MdNHX1. Differences in salt tolerance between transgenic and wild-type (WT) rootstocks were examined under field conditions. We also compared differences when 'Naganofuji No. 2' apple was grafted onto these transgenic or WT rootstocks. Plants on the transgenic rootstocks grew well during 60 d of mild stress (100 mM NaCl) while the WT exhibited chlorosis, inhibited growth and even death. Compared with the untreated control, the stomatal density was greater in both non-grafted and grafted WT plants exposed to 200 mM NaCl. In contrast, that density was significantly decreased in leaves from grafted transgenic plants. At 200 mM NaCl, net photosynthesis, stomatal conductance, intercellular CO2 concentration, and chlorophyll contents were markedly reduced in the WT, whereas the declines in those values were only minor in similarly stressed transgenic plants. Therefore, we conclude that overexpressing plants utilize a better protective mechanism for retaining higher photosynthetic capacity. Furthermore, this contrast in tolerance and adaptability to stress is linked to differences in stomatal behavior and photosynthetic rates.

Endocytic regulation of alkali metal transport proteins in mammals, yeast and plants

The relative concentrations of ions and solutes inside cells are actively maintained by several classes of transport proteins, in many cases against their concentration gradient. These transport processes, which consume a large portion of cellular energy, must be constantly regulated. Many structurally distinct families of channels, carriers, and pumps have been characterized in considerable detail during the past decades and defects in the function of some of these proteins have been linked to a growing list of human diseases. The dynamic regulation of the transport proteins present at the cell surface is vital for both normal cellular function and for the successful adaptation to changing environments. The composition of proteins present at the cell surface is controlled on both the transcriptional and post-translational level. Post-translational regulation involves highly conserved mechanisms of phosphorylation- and ubiquitylation-dependent signal transduction routes used to modify the cohort of receptors and transport proteins present under any given circumstances. In this review, we will summarize what is currently known about one facet of this regulatory process: the endocytic regulation of alkali metal transport proteins. The physiological relevance, major contributors, parallels and missing pieces of the puzzle in mammals, yeast and plants will be discussed.

Developing transgenic Jatropha using the SbNHX1 gene from an extreme halophyte for cultivation in saline wasteland

Jatropha is an important second-generation biofuel plant. Salinity is a major factor adversely impacting the growth and yield of several plants including Jatropha. SbNHX1 is a vacuolar Na⁺/H⁺ antiporter gene that compartmentalises excess Na⁺ ions into the vacuole and maintains ion homeostasis. We have previously cloned and characterised the SbNHX1 gene from an extreme halophyte, Salicornia brachiata. Transgenic plants of Jatropha curcas with the SbNHX1 gene were developed using microprojectile bombardment mediated transformation. Integration of the transgene was confirmed by PCR and Rt-PCR and the copy number was determined by real time qPCR. The present study of engineering salt tolerance in Jatropha is the first report to date. Salt tolerance of the transgenic lines JL2, JL8 and JL19 was confirmed by leaf senescence assay, chlorophyll estimation, plant growth, ion content, electrolyte leakage and malondialdehyde (MDA) content analysis. Transgenic lines showed better salt tolerance than WT up to 200 mM NaCl. Imparting salt tolerance to Jatropha using the SbNHX1 gene may open up the possibility of cultivating it in marginal salty land, releasing arable land presently under Jatropha cultivation for agriculture purposes. Apart from this, transgenic Jatropha can be cultivated with brackish water, opening up the possibility of sustainable cultivation of this biofuel plant in salty coastal areas.

Role of HKT1 in Thellungiella salsuginea, a model extremophile plant

Maintenance of the cytosolic Na(+)/K(+) ratio under saline conditions is crucial for plants. HKT-type Na(+) transporters play a key role in keeping low cytosolic Na(+) concentrations thus retaining a low Na(+)/K(+) ratio, that reduces Na(+) toxicity and causing high salinity stress tolerance. Two HKT-type transporters, AtHKT1 from Arabidopsis and TsHKT1;2 from Thellungiella salsuginea, that share high DNA and protein sequence identities, are distinguished by fundamentally different ion selection and salinity stress behavior. On the level of transcription, TsHKT1;2 is dramatically induced upon salt stress, whereas AtHKT1 is downregulated. TsHKT1;2-RNAi lines show severe potassium deficiency and are also sensitive to high [Na(+)]. We have validated the ability of the TsHKT1;2 protein to act as an efficient K(+) transporter in the presence of high [Na(+)] by expression in yeast cells. K(+) specificity is based on amino acid differences in the pore of the transporter protein relative to AtHKT1.

Different evolutionary histories of two cation/proton exchanger gene families in plants

BACKGROUND

Gene duplication events have been proposed to be involved in the adaptation of plants to stress conditions; precisely how is unclear. To address this question, we studied the evolution of two families of antiporters. Cation/proton exchangers are important for normal cell function and in plants, Na+,K+/H+ antiporters have also been implicated in salt tolerance. Two well-known plant cation/proton antiporters are NHX1 and SOS1, which perform Na+ and K+ compartmentalization into the vacuole and Na+ efflux from the cell, respectively. However, our knowledge about the evolution of NHX and SOS1 stress responsive gene families is still limited.

RESULTS

In this study we performed a comprehensive molecular evolutionary analysis of the NHX and SOS1 families. Using available sequences from a total of 33 plant species, we estimated gene family phylogenies and gene duplication histories, as well as examined heterogeneous selection pressure on amino acid sites. Our results show that, while the NHX family expanded and specialized, the SOS1 family remained a low copy gene family that appears to have undergone neofunctionalization during its evolutionary history. Additionally, we found that both families are under purifying selection although SOS1 is less constrained.

CONCLUSIONS

We propose that the different evolution histories are related with the proteins' function and localization, and that the NHX and SOS1 families are examples of two different evolutionary paths through which duplication events may result in adaptive evolution of stress tolerance.

Two closely linked tomato HKT coding genes are positional candidates for the major tomato QTL involved in Na+ /K+ homeostasis

The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na(+) ] and [K(+) ] was previously reported in chromosome 7 using two connected populations of recombinant inbred lines (RILs) of tomato. HKT1;1 and HKT1;2, two tomato Na(+) -selective class I-HKT transporters, were found to be closely linked, where the maximum logarithm of odds (LOD) score for these QTLs located. When a chromosome 7 linkage map based on 278 single-nucleotide polymorphisms (SNPs) was used, the maximum LOD score position was only 35 kb from HKT1;1 and HKT1;2. Their expression patterns and phenotypic effects were further investigated in two near-isogenic lines (NILs): 157-14 (double homozygote for the cheesmaniae alleles) and 157-17 (double homozygote for the lycopersicum alleles). The expression pattern for the HKT1;1 and HKT1;2 alleles was complex, possibly because of differences in their promoter sequences. High salinity had very little effect on root dry and fresh weight and consequently on the plant dry weight of NIL 157-14 in comparison with 157-17. A significant difference between NILs was also found for [K(+) ] and the [Na(+) ]/[K(+) ] ratio in leaf and stem but not for [Na(+) ] arising a disagreement with the corresponding RIL population. Their association with leaf [Na(+) ] and salt tolerance in tomato is also discussed.

Using membrane transporters to improve crops for sustainable food production

With the global population predicted to grow by at least 25 per cent by 2050, the need for sustainable production of nutritious foods is critical for human and environmental health. Recent advances show that specialized plant membrane transporters can be used to enhance yields of staple crops, increase nutrient content and increase resistance to key stresses, including salinity, pathogens and aluminium toxicity, which in turn could expand available arable land.

A dominant-negative form of Arabidopsis AP-3 β-adaptin improves intracellular pH homeostasis

Intracellular pH (pHi ) is a crucial parameter in cellular physiology but its mechanisms of homeostasis are only partially understood. To uncover novel roles and participants of the pHi regulatory system, we have screened an Arabidopsis mutant collection for resistance of seed germination to intracellular acidification induced by weak organic acids (acetic, propionic, sorbic). The phenotypes of one identified mutant, weak acid-tolerant 1-1D (wat1-1D) are due to the expression of a truncated form of AP-3 β-adaptin (encoded by the PAT2 gene) that behaves as a as dominant-negative. During acetic acid treatment the root epidermal cells of the mutant maintain a higher pHi and a more depolarized plasma membrane electrical potential than wild-type cells. Additional phenotypes of wat1-1D roots include increased rates of acetate efflux, K(+) uptake and H(+) efflux, the latter reflecting the in vivo activity of the plasma membrane H(+) -ATPase. The in vitro activity of the enzyme was not increased but, as the H(+) -ATPase is electrogenic, the increased ion permeability would allow a higher rate of H(+) efflux. The AP-3 adaptor complex is involved in traffic from Golgi to vacuoles but its function in plants is not much known. The phenotypes of the wat1-1D mutant can be explained if loss of function of the AP-3 β-adaptin causes activation of channels or transporters for organic anions (acetate) and for K(+) at the plasma membrane, perhaps through miss-localization of tonoplast proteins. This suggests a role of this adaptin in trafficking of ion channels or transporters to the tonoplast.

Protein S-ACYL Transferase10 is critical for development and salt tolerance in Arabidopsis

Protein S-acylation, commonly known as palmitoylation, is a reversible posttranslational modification that catalyzes the addition of a saturated lipid group, often palmitate, to the sulfhydryl group of a Cys. Palmitoylation regulates enzyme activity, protein stability, subcellular localization, and intracellular sorting. Many plant proteins are palmitoylated. However, little is known about protein S-acyl transferases (PATs), which catalyze palmitoylation. Here, we report that the tonoplast-localized PAT10 is critical for development and salt tolerance in Arabidopsis thaliana. PAT10 loss of function resulted in pleiotropic growth defects, including smaller leaves, dwarfism, and sterility. In addition, pat10 mutants are hypersensitive to salt stresses. We further show that PAT10 regulates the tonoplast localization of several calcineurin B-like proteins (CBLs), including CBL2, CBL3, and CBL6, whose membrane association also depends on palmitoylation. Introducing a C192S mutation within the highly conserved catalytic motif of PAT10 failed to complement pat10 mutants, indicating that PAT10 functions through protein palmitoylation. We propose that PAT10-mediated palmitoylation is critical for vacuolar function by regulating membrane association or the activities of tonoplast proteins.

Osmotic stress responses and plant growth controlled by potassium transporters in Arabidopsis

Osmotic adjustment plays a fundamental role in water stress responses and growth in plants; however, the molecular mechanisms governing this process are not fully understood. Here, we demonstrated that the KUP potassium transporter family plays important roles in this process, under the control of abscisic acid (ABA) and auxin. We generated Arabidopsis thaliana multiple mutants for K(+) uptake transporter 6 (KUP6), KUP8, KUP2/SHORT HYPOCOTYL3, and an ABA-responsive potassium efflux channel, guard cell outward rectifying K(+) channel (GORK). The triple mutants, kup268 and kup68 gork, exhibited enhanced cell expansion, suggesting that these KUPs negatively regulate turgor-dependent growth. Potassium uptake experiments using (86)radioactive rubidium ion ((86)Rb(+)) in the mutants indicated that these KUPs might be involved in potassium efflux in Arabidopsis roots. The mutants showed increased auxin responses and decreased sensitivity to an auxin inhibitor (1-N-naphthylphthalamic acid) and ABA in lateral root growth. During water deficit stress, kup68 gork impaired ABA-mediated stomatal closing, and kup268 and kup68 gork decreased survival of drought stress. The protein kinase SNF1-related protein kinases 2E (SRK2E), a key component of ABA signaling, interacted with and phosphorylated KUP6, suggesting that KUP functions are regulated directly via an ABA signaling complex. We propose that the KUP6 subfamily transporters act as key factors in osmotic adjustment by balancing potassium homeostasis in cell growth and drought stress responses.

Osmotic stress responses and plant growth controlled by potassium transporters in Arabidopsis

Osmotic adjustment plays a fundamental role in water stress responses and growth in plants; however, the molecular mechanisms governing this process are not fully understood. Here, we demonstrated that the KUP potassium transporter family plays important roles in this process, under the control of abscisic acid (ABA) and auxin. We generated Arabidopsis thaliana multiple mutants for K(+) uptake transporter 6 (KUP6), KUP8, KUP2/SHORT HYPOCOTYL3, and an ABA-responsive potassium efflux channel, guard cell outward rectifying K(+) channel (GORK). The triple mutants, kup268 and kup68 gork, exhibited enhanced cell expansion, suggesting that these KUPs negatively regulate turgor-dependent growth. Potassium uptake experiments using (86)radioactive rubidium ion ((86)Rb(+)) in the mutants indicated that these KUPs might be involved in potassium efflux in Arabidopsis roots. The mutants showed increased auxin responses and decreased sensitivity to an auxin inhibitor (1-N-naphthylphthalamic acid) and ABA in lateral root growth. During water deficit stress, kup68 gork impaired ABA-mediated stomatal closing, and kup268 and kup68 gork decreased survival of drought stress. The protein kinase SNF1-related protein kinases 2E (SRK2E), a key component of ABA signaling, interacted with and phosphorylated KUP6, suggesting that KUP functions are regulated directly via an ABA signaling complex. We propose that the KUP6 subfamily transporters act as key factors in osmotic adjustment by balancing potassium homeostasis in cell growth and drought stress responses.

New insights into the transport mechanisms in plant vacuoles

The vacuole is the largest compartment in plant cells, often occupying more than 80% of the total cell volume. This organelle accumulates a large variety of endogenous ions, metabolites, and xenobiotics. The compartmentation of divergent substances is relevant for a wide range of biological processes, such as the regulation of stomata movement, defense mechanisms against herbivores, flower coloration, etc. Progress in molecular and cellular biology has revealed that a large number of transporters and channels exist at the tonoplast. In recent years, various biochemical and physiological functions of these proteins have been characterized in detail. Some are involved in maintaining the homeostasis of ions and metabolites, whereas others are related to defense mechanisms against biotic and abiotic stresses. In this review, we provide an updated inventory of vacuolar transport mechanisms and a comprehensive summary of their physiological functions.

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.

Asg1 is a stress-inducible gene which increases stomatal resistance in salt stressed potato

The identification of critical components in plant salt stress adaptation has greatly benefitted, in the last two decades, from fundamental discoveries in Arabidopsis and close model systems. Nevertheless, this approach has also highlighted a non-complete overlap between stress tolerance mechanisms in Arabidopsis and agricultural crops. Within a long-running research program aimed at identifying salt stress genetic determinants in potato by functional screening in Escherichia coli, we isolated Asg1, a stress-related gene with an unknown function. Asg1 is induced by salt stress in both potato and Arabidopsis and by abscisic acid in Arabidopsis. Asg1 is actively transcribed in all plant tissues. Furthermore, Asg1 promoter analysis confirmed its ubiquitous expression, which was remarkable in pollen, a plant tissue that undergoes drastic dehydration/hydration processes. Fusion of Asg1 with green fluorescent protein showed that the encoded protein is localized close to the plasma membrane with a non-continuous pattern of distribution. In addition, Arabidopsis knockout asg1 mutants were insensitive to both NaCl and sugar hyperosmotic environments during seed germination. Transgenic potato plants over-expressing the Asg1 gene revealed a stomatal hypersensitivity to NaCl stress which, however, did not result in a significantly improved tuber yield in stress conditions. Altogether, these data suggest that Asg1 might interfere with components of the stress signaling pathway by promoting stomatal closure and participating in stress adaptation.

Ubiquitin-specific protease16 modulates salt tolerance in Arabidopsis by regulating Na(+)/H(+) antiport activity and serine hydroxymethyltransferase stability

Protein ubiquitination is a reversible process catalyzed by ubiquitin ligases and ubiquitin-specific proteases (UBPs). We report the identification and characterization of UBP16 in Arabidopsis thaliana. UBP16 is a functional ubiquitin-specific protease and its enzyme activity is required for salt tolerance. Plants lacking UBP16 were hypersensitive to salt stress and accumulated more sodium and less potassium. UBP16 positively regulated plasma membrane Na(+)/H(+) antiport activity. Through yeast two-hybrid screening, we identified a putative target of UBP16, SERINE HYDROXYMETHYLTRANSFERASE1 (SHM1), which has previously been reported to be involved in photorespiration and salt tolerance in Arabidopsis. We found that SHM1 is degraded in a 26S proteasome-dependent process, and UBP16 stabilizes SHM1 by removing the conjugated ubiquitin. Ser hydroxymethyltransferase activity is lower in the ubp16 mutant than in the wild type but higher than in the shm1 mutant. During salt stress, UBP16 and SHM1 function in preventing cell death and reducing reactive oxygen species accumulation, activities that are correlated with increasing Na(+)/H(+) antiport activity. Overexpression of SHM1 in the ubp16 mutant partially rescues its salt-sensitive phenotype. Taken together, our results suggest that UBP16 is involved in salt tolerance in Arabidopsis by modulating sodium transport activity and repressing cell death at least partially through modulating SMH1stability and activity.

Tonoplast calcium sensors CBL2 and CBL3 control plant growth and ion homeostasis through regulating V-ATPase activity in Arabidopsis

Plant responses to developmental and environmental cues are often mediated by calcium (Ca(2+)) signals that are transmitted by diverse calcium sensors. The calcineurin B-like (CBL) protein family represents calcium sensors that decode calcium signals through specific interactions with a group of CBL-interacting protein kinases. We report functional analysis of Arabidopsis CBL2 and CBL3, two closely related CBL members that are localized to the vacuolar membrane through the N-terminal tonoplast-targeting sequence. While cbl2 or cbl3 single mutant did not show any phenotypic difference from the wild type, the cbl2 cbl3 double mutant was stunted with leaf tip necrosis, underdeveloped roots, shorter siliques and fewer seeds. These defects were reminiscent of those in the vha-a2 vha-a3 double mutant deficient in vacuolar H(+)-ATPase (V-ATPase). Indeed, the V-ATPase activity was reduced in the cbl2 cbl3 double mutant, connecting tonoplast CBL-type calcium sensors to the regulation of V-ATPase. Furthermore, cbl2 cbl3 double mutant was compromised in ionic tolerance and micronutrient accumulation, consistent with the defect in V-ATPase activity that has been shown to function in ion compartmentalization. Our results suggest that calcium sensors CBL2 and CBL3 serve as molecular links between calcium signaling and V-ATPase, a central regulator of intracellular ion homeostasis.