Plant NHX cation/proton antiporters

Protein architecture and core residues in unwound α-helices provide insights to the transport function of plant AtCHX17

Using Arabidopsis thaliana AtCHX17 as an example, we combine structural modeling and mutagenesis to provide insights on its protein architecture and transport function which is poorly characterized. This approach is based on the observation that protein structures are significantly more conserved in evolution than linear sequences, and mechanistic similarities among diverse transporters are emerging. Two homology models of AtCHX17 were obtained that show a protein fold similar to known structures of bacterial Na(+)/H(+) antiporters, EcNhaA and TtNapA. The distinct secondary and tertiary structure models highlighted residues at positions potentially important for CHX17 activity. Mutagenesis showed that asparagine-N200 and aspartate-D201 inside transmembrane5 (TM5), and lysine-K355 inside TM10 are critical for AtCHX17 activity. We reveal previously unrecognized threonine-T170 and lysine-K383 as key residues at unwound regions in the middle of TM4 and TM11 α-helices, respectively. Mutation of glutamate-E111 located near the membrane surface inhibited AtCHX17 activity, suggesting a role in pH sensing. The long carboxylic tail of unknown purpose has an alternating β-sheet and α-helix secondary structure that is conserved in prokaryote universal stress proteins. These results support the overall architecture of AtCHX17 and identify D201, N200 and novel residues T170 and K383 at the functional core which likely participates in ion recognition, coordination and/or translocation, similar to characterized cation/H(+) exchangers. The core of AtCHX17 models according to EcNhaA and TtNapA templates faces inward and outward, respectively, which may reflect two conformational states of the alternating access transport mode for proteins belonging to the plant CHX family.

Plant endosomal NHX antiporters: Activity and function

The Arabidopsis NHX antiporter family contains eight members that are divided into three subclasses: vacuolar, endosomal, and plasma membrane. While the plasma membrane and vacuolar NHXs have been studied extensively, the activity and function of the endosomal NHXs are beginning to be discovered. AtNHX5 and AtNHX6 are endosomal Na(+),K(+)/H(+) antiporters that share high sequence similarity. They are localized in the Golgi, trans-Golgi network (TGN), and prevacuolear compartment (PVC). Studies have shown that AtNHX5 and AtNHX6 mediate K(+) and Na(+) transport, and regulate cellular pH homeostasis. Sequence alignment has found that AtNHX5 and AtNHX6 contain four conserved acidic amino acid residues in transmembrane domains that align with yeast and human NHXs. Three of these conserved acidic residues are critical for K(+) transport and seedling growth in Arabidopsis. Moreover, studies have shown that the precursors of the seed storage proteins are missorted to the apoplast in the nhx5 nhx6 knockout mutant, suggesting that AtNHX5 and AtNHX6 regulate protein transport into the vacuole. Further analysis found that AtNHX5 and AtNHX6 regulated the binding of VSR to its cargoes. Taken together, AtNHX5 and AtNHX6 play an important role in cellular ion and pH homeostasis, and are essential for protein transport into the vacuole.

Molecular characterization and expression analysis of the mulberry Na(+)/H(+) exchanger gene family

Na(+)/H(+) exchangers (NHXs) have important roles in cellular pH, and Na(+) and K(+) homeostasis in plants. Mulberry is not only an important traditional economic woody plant known for its leaves, which are the exclusive food source of the silkworm Bombyx mori, but it can also adapt to many different adverse conditions, including saline environments. However, little is known about the NHXs in this important perennial tree. In this study, we identified and cloned seven putative NHX gene family members from Morus atropurpurea based on a genome-wide analysis of the Morus genome database. A phylogenetic analysis and genomic organization of mulberry NHXs suggested that the mulberry NHX family forms three distinct subgroups. Transcriptome data and real-time PCR of different mulberry varieties under normal culture conditions revealed that the mulberry NHX family has a different tissue-specific pattern in the two mulberry species. The MaNHX genes' expression analyses under different stresses (salt and drought) and signal molecules (abscisic acid, salicylic acid, hydrogen peroxide and methyl jasmonate) revealed that MaNHXs not only could be induced by salt, drought and abscisic acid as describe in the literature, but were also induced by other signal molecules, which indicated that MaNHX members exhibited diverse and complicated expression patterns in different mulberry tissues under various abiotic stresses, phytohormones and plant signaling molecules. Our results provide some insights into new and emerging cellular and physiological functions of this group of H(+)-coupled cation exchangers, beyond their function in salt tolerance, and also provide the basis for further characterizations of MaNHXs' physiological functions.

Enhanced Salt Tolerance Conferred by the Complete 2.3 kb cDNA of the Rice Vacuolar Na(+)/H(+) Antiporter Gene Compared to 1.9 kb Coding Region with 5' UTR in Transgenic Lines of Rice

Soil salinity is one of the most challenging problems that restricts the normal growth and production of rice worldwide. It has therefore become very important to produce more saline tolerant rice varieties. This study shows constitutive over-expression of the vacuolar Na(+)/H(+) antiporter gene (OsNHX1) from the rice landrace (Pokkali) and attainment of enhanced level of salinity tolerance in transgenic rice plants. It also shows that inclusion of the complete un-translated regions (UTRs) of the alternatively spliced OsNHX1 gene provides a higher level of tolerance to the transgenic rice. Two separate transformation events of the OsNHX1 gene, one with 1.9 kb region containing the 5' UTR with CDS and the other of 2.3 kb, including 5' UTR, CDS, and the 3' UTR regions were performed. The transgenic plants with these two different constructs were advanced to the T3 generation and physiological and molecular screening of homozygous plants was conducted at seedling and reproductive stages under salinity (NaCl) stress. Both transgenic lines were observed to be tolerant compared to WT plants at both physiological stages. However, the transgenic lines containing the CDS with both the 5' and 3' UTR were significantly more tolerant compared to the transgenic lines containing OsNHX1 gene without the 3' UTR. At the seedling stage at 12 dS/m stress, the chlorophyll content was significantly higher (P < 0.05) and the electrolyte leakage significantly lower (P < 0.05) in the order 2.3 kb > 1.9 kb > and WT lines. Yield in g/plant in the best line from the 2.3 kb plants was significantly more (P < 0.01) compared, respectively, to the best 1.9 kb line and WT plants at stress of 6 dS/m. Transformation with the complete transcripts rather than the CDS may therefore provide more durable level of tolerance.

AtNHX5 and AtNHX6 Control Cellular K+ and pH Homeostasis in Arabidopsis: Three Conserved Acidic Residues Are Essential for K+ Transport

AtNHX5 and AtNHX6, the endosomal Na+,K+/H+ antiporters in Arabidopsis, play an important role in plant growth and development. However, their function in K+ and pH homeostasis remains unclear. In this report, we characterized the function of AtNHX5 and AtNHX6 in K+ and H+ homeostasis in Arabidopsis. Using a yeast expression system, we found that AtNHX5 and AtNHX6 recovered tolerance to high K+ or salt. We further found that AtNHX5 and AtNHX6 functioned at high K+ at acidic pH while AtCHXs at low K+ under alkaline conditions. In addition, we showed that the nhx5 nhx6 double mutant contained less K+ and was sensitive to low K+ treatment. Overexpression of AtNHX5 or AtNHX6 gene in nhx5 nhx6 recovered root growth to the wild-type level. Three conserved acidic residues, D164, E188, and D193 in AtNHX5 and D165, E189, and D194 in AtNHX6, were essential for K+ homeostasis and plant growth. nhx5 nhx6 had a reduced vacuolar and cellular pH as measured with the fluorescent pH indicator BCECF or semimicroelectrode. We further show that AtNHX5 and AtNHX6 are localized to Golgi and TGN. Taken together, AtNHX5 and AtNHX6 play an important role in K+ and pH homeostasis in Arabidopsis. Three conserved acidic residues are essential for K+ transport.

Salinity tolerance in plants. Quantitative approach to ion transport starting from halophytes and stepping to genetic and protein engineering for manipulating ion fluxes

Ion transport is the fundamental factor determining salinity tolerance in plants. The Review starts from differences in ion transport between salt tolerant halophytes and salt-sensitive plants with an emphasis on transport of potassium and sodium via plasma membranes. The comparison provides introductory information for increasing salinity tolerance. Effects of salt stress on ion transport properties of membranes show huge opportunities for manipulating ion fluxes. Further steps require knowledge about mechanisms of ion transport and individual genes of ion transport proteins. Initially, the Review describes methods to measure ion fluxes, the independent set of techniques ensures robust and reliable basement for quantitative approach. The Review briefly summarizes current data concerning Na(+) and K(+) concentrations in cells, refers to primary thermodynamics of ion transport and gives special attention to individual ion channels and transporters. Simplified scheme of a plant cell with known transport systems at the plasma membrane and tonoplast helps to imagine the complexity of ion transport and allows choosing specific transporters for modulating ion transport. The complexity is enhanced by the influence of cell size and cell wall on ion transport. Special attention is given to ion transporters and to potassium and sodium transport by HKT, HAK, NHX, and SOS1 proteins. Comparison between non-selective cation channels and ion transporters reveals potential importance of ion transporters and the balance between the two pathways of ion transport. Further on the Review describes in detail several successful attempts to overexpress or knockout ion transporters for changing salinity tolerance. Future perspectives are questioned with more attention given to promising candidate ion channels and transporters for altered expression. Potential direction of increasing salinity tolerance by modifying ion channels and transporters using single point mutations is discussed and questioned. An alternative approach from synthetic biology is to create new regulation networks using novel transport proteins with desired properties for transforming agricultural crops. The approach had not been widely used earlier; it leads also to theoretical and pure scientific aspects of protein chemistry, structure-function relations of membrane proteins, systems biology and physiology of stress and ion homeostasis. Summarizing, several potential ways are aimed at required increase in salinity tolerance of plants of interest.

SpAHA1 and SpSOS1 Coordinate in Transgenic Yeast to Improve Salt Tolerance

In plant cells, the plasma membrane Na+/H+ antiporter SOS1 (salt overly sensitive 1) mediates Na+ extrusion using the proton gradient generated by plasma membrane H+-ATPases, and these two proteins are key plant halotolerance factors. In the present study, two genes from Sesuvium portulacastrum, encoding plasma membrane Na+/H+ antiporter (SpSOS1) and H+-ATPase (SpAHA1), were cloned. Localization of each protein was studied in tobacco cells, and their functions were analyzed in yeast cells. Both SpSOS1 and SpAHA1 are plasma membrane-bound proteins. Real-time polymerase chain reaction (PCR) analyses showed that SpSOS1 and SpAHA1 were induced by salinity, and their expression patterns in roots under salinity were similar. Compared with untransformed yeast cells, SpSOS1 increased the salt tolerance of transgenic yeast by decreasing the Na+ content. The Na+/H+ exchange activity at plasma membrane vesicles was higher in SpSOS1-transgenic yeast than in the untransformed strain. No change was observed in the salt tolerance of yeast cells expressing SpAHA1 alone; however, in yeast transformed with both SpSOS1 and SpAHA1, SpAHA1 generated an increased proton gradient that stimulated the Na+/H+ exchange activity of SpSOS1. In this scenario, more Na+ ions were transported out of cells, and the yeast cells co-expressing SpSOS1 and SpAHA1 grew better than the cells transformed with only SpSOS1 or SpAHA1. These findings demonstrate that the plasma membrane Na+/H+ antiporter SpSOS1 and H+-ATPase SpAHA1 can function in coordination. These results provide a reference for developing more salt-tolerant crops via co-transformation with the plasma membrane Na+/H+ antiporter and H+-ATPase.

Co-expression of vacuolar Na(+)/H(+) antiporter and H(+)-pyrophosphatase with an IRES-mediated dicistronic vector improves salinity tolerance and enhances potassium biofortification of tomato

Potassium (K) deficiency is a worldwide problem. Thus, the K biofortification of crops is needed to enhance human nutrition. Tomato represents an ideal candidate for such biofortification programs thanks to its widespread distribution and its easy growth on a commercial scale. However, although tomato is moderately tolerant to abiotic stresses, the crop losses due to salinity can be severe. In this study, we generated transgenic tomato plants over-expressing a Na(+)-K(+)/H(+) exchanger gene (TNHXS1), singly or with H(+)-pyrophosphatase (H(+)-PPiase) gene using a bicistronic construct. Transgenic tomato lines co-expressing both genes (LNV) significantly showed higher salinity tolerance than the wild-type (WT) plans or those expressing the TNHXS1 gene alone (LN). Indeed, under salt stress conditions, double transgenic plants produced higher biomass and retained more chlorophyll and catalase (CAT) activity. In addition, they showed earlier flowering and produced more fruits. To address K deficiencies in humans, an increase of 50% in K content of vegetable products was proposed. In this study, ion content analysis revealed that, under salt stress, fruits from double transgenic plants accumulated 5 times more potassium and 9 times less sodium than WT counterparts. Interestingly, the ionomic analysis of tomato fruits also revealed that LNV had a distinct profile compared to WT and to LN plants. Indeed, LNV fruits accumulated less Fe(2+), Ca(2+), Mg(2+) and Zn(2+), but more Mn(2+). This study demonstrates the effectiveness of bicistronic constructs as an important tool for the enhancement of biofortification and salt stress tolerance in crops.

The membrane proteome of stroma thylakoids from Arabidopsis thaliana studied by successive in-solution and in-gel digestion

From individual localization and large-scale proteomic studies, we know that stroma-exposed thylakoid membranes harbor part of the machinery performing the light-dependent photosynthetic reactions. The minor components of the stroma thylakoid proteome, regulating and maintaining the photosynthetic machinery, are in the process of being unraveled. In this study, we developed in-solution and in-gel proteolytic digestion methods, and used them to identify minor membrane proteins, e.g. transporters, in stroma thylakoids prepared from Arabidopsis thaliana (L.) Heynh Columbia-0 leaves. In-solution digestion with chymotrypsin yielded the largest number of peptides, but in combination with methanol extraction resulted in identification of the largest number of membrane proteins. Although less efficient in extracting peptides, in-gel digestion with trypsin and chymotrypsin led to identification of additional proteins. We identified a total of 58 proteins including 44 membrane proteins. Almost half are known thylakoid proteins with roles in photosynthetic light reactions, proteolysis and import. The other half, including many transporters, are not known as chloroplast proteins, because they have been either curated (manually assigned) to other cellular compartments or not curated at all at the plastid protein databases. Transporters include ATP-binding cassette (ABC) proteins, transporters for K(+) and other cations. Other proteins either have a role in processes probably linked to photosynthesis, namely translation, metabolism, stress and signaling or are contaminants. Our results indicate that all these proteins are present in stroma thylakoids; however, individual studies are required to validate their location and putative roles. This study also provides strategies complementary to traditional methods for identification of membrane proteins from other cellular compartments.

pH Regulation by NHX-Type Antiporters Is Required for Receptor-Mediated Protein Trafficking to the Vacuole in Arabidopsis

Protein trafficking requires proper ion and pH homeostasis of the endomembrane system. The NHX-type Na(+)/H(+) antiporters NHX5 and NHX6 localize to the Golgi, trans-Golgi network, and prevacuolar compartments and are required for growth and trafficking to the vacuole. In the nhx5 nhx6 T-DNA insertional knockouts, the precursors of the 2S albumin and 12S globulin storage proteins accumulated and were missorted to the apoplast. Immunoelectron microscopy revealed the presence of vesicle clusters containing storage protein precursors and vacuolar sorting receptors (VSRs). Isolation and identification of complexes of VSRs with unprocessed 12S globulin by 2D blue-native PAGE/SDS-PAGE indicated that the nhx5 nhx6 knockouts showed compromised receptor-cargo association. In vivo interaction studies using bimolecular fluorescence complementation between VSR2;1, aleurain, and 12S globulin suggested that nhx5 nhx6 knockouts showed a significant reduction of VSR binding to both cargoes. In vivo pH measurements indicated that the lumens of VSR compartments containing aleurain, as well as the trans-Golgi network and prevacuolar compartments, were significantly more acidic in nhx5 nhx6 knockouts. This work demonstrates the importance of NHX5 and NHX6 in maintaining endomembrane luminal pH and supports the notion that proper vacuolar trafficking and proteolytic processing of storage proteins require endomembrane pH homeostasis.

The wheat NHX antiporter gene TaNHX2 confers salt tolerance in transgenic alfalfa by increasing the retention capacity of intracellular potassium

Previous studies have shown that TaNHX2 transgenic alfalfa (Medicago sativa L.) accumulated more K(+) and less Na(+) in leaves than did the wild-type plants. To investigate whether the increased K(+) accumulation in transgenic plants is attributed to TaNHX2 gene expression and whether the compartmentalization of Na(+) into vacuoles or the intracellular compartmentalization of potassium is the critical mechanism for TaNHX2-dependent salt tolerance in transgenic alfalfa, aerated hydroponic culture was performed under three different stress conditions: control condition (0.1 mM Na(+) and 6 mM K(+) inside culture solution), K(+)-sufficient salt stress (100 mM NaCl and 6 mM K(+)) and K(+)-insufficient salt stress (100 mM NaCl and 0.1 mM K(+)). The transgenic alfalfa plants had lower K(+) efflux through specific K(+) channels and higher K(+) absorption through high-affinity K(+) transporters than did the wild-type plants. Therefore, the transgenic plants had greater K(+) contents and [K(+)]/[Na(+)] ratios in leaf tissue and cell sap. The intracellular compartmentalization of potassium is critical for TaNHX2-induced salt tolerance in transgenic alfalfa.

ZxNHX controls Na⁺ and K⁺ homeostasis at the whole-plant level in Zygophyllum xanthoxylum through feedback regulation of the expression of genes involved in their transport

BACKGROUND AND AIMS

In order to cope with arid environments, the xerohalophyte Zygophyllum xanthoxylum efficiently compartmentalizes Na(+) into vacuoles, mediated by ZxNHX, and maintains stability of K(+) in its leaves. However, the function of ZxNHX in controlling Na(+) and K(+) homeostasis at the whole-plant level remains unclear. In this study, the role of ZxNHX in regulating the expression of genes involved in Na(+) and K(+) transport and spatial distribution was investigated.

METHODS

The role of ZxNHX in maintaining Na(+) and K(+) homeostasis in Z. xanthoxylum was studied using post-transcriptional gene silencing via  Agrobacterium-mediated transformation. Transformed plants were grown with or without 50 mm NaCl, and expression levels and physiological parameters were measured.

KEY RESULTS

It was found that 50 mm NaCl induced a 620 % increase in transcripts of ZxSOS1 but only an 80 % increase in transcripts of ZxHKT1;1 in roots of wild-type (WT) plants. Consequently, the ability of ZxSOS1 to transport Na(+) exceeded that of ZxHKT1;1, and Na(+) was loaded into the xylem by ZxSOS1 and delivered to the shoots. However, in a ZxNHX-silenced line (L7), the capacity to sequester Na(+) into vacuoles of leaves was weakened, which in turn regulated long-distance Na(+) transport from roots to shoots. In roots of L7, NaCl (50 mm) increased transcripts of ZxSOS1 by only 10 %, whereas transcripts of ZxHKT1;1 increased by 53 %. Thus, in L7, the transport ability of ZxHKT1;1 for Na(+) outweighed that of ZxSOS1. Na(+) was unloaded from the xylem stream, consequently reducing Na(+) accumulation and relative distribution in leaves, but increasing the relative distribution of Na(+) in roots and the net selective transport capacity for K(+) over Na(+) from roots to shoots compared with the WT. Silencing of ZxNHX also triggered a downregulation of  ZxAKT1 and ZxSKOR in roots, resulting in a significant decrease in K(+) accumulation in all the tissues in plants grown in 50 mm NaCl. These changes led to a significant reduction in osmotic adjustment, and thus an inhibition of growth in ZxNHX-silenced lines.

CONCLUSIONS

The results suggest that ZxNHX is essential for controlling Na(+), K(+) uptake, long-distance transport and their homeostasis at whole-plant level via feedback regulation of the expression of genes involved in Na(+), K(+) transport. The net result is the maintenance of the characteristic salt accumulation observed in Z. xanthoxylum and the regulation of its normal growth. A model is proposed for the role of ZxNHX in regulating the Na(+) transport system in Z. xanthoxylum under saline conditions.

Mechanisms of salt tolerance in habanero pepper plants (Capsicum chinense Jacq.): Proline accumulation, ions dynamics and sodium root-shoot partition and compartmentation

Despite its economic relevance, little is known about salt tolerance mechanisms in pepper plants. To address this question, we compared differences in responses to NaCl in two Capsicum chinense varieties: Rex (tolerant) and Chichen-Itza (sensitive). Under salt stress (150 mM NaCl over 7 days) roots of Rex variety accumulated 50 times more compatible solutes such as proline compared to Chichen-Itza. Mineral analysis indicated that Na(+) is restricted to roots by preventing its transport to leaves. Fluorescence analysis suggested an efficient Na(+) compartmentalization in vacuole-like structures and in small intracellular compartments in roots of Rex variety. At the same time, Na(+) in Chichen-Itza plants was compartmentalized in the apoplast, suggesting substantial Na(+) extrusion. Rex variety was found to retain more K(+) in its roots under salt stress according to a mineral analysis and microelectrode ion flux estimation (MIFE). Vanadate-sensitive H(+) efflux was higher in Chichen-Itza variety plants, suggesting a higher activity of the plasma membrane H(+)-ATPase, which fuels the extrusion of Na(+), and, possibly, also the re-uptake of K(+). Our results suggest a combination of stress tolerance mechanisms, in order to alleviate the salt-induced injury. Furthermore, Na(+) extrusion to apoplast does not appear to be an efficient strategy for salt tolerance in pepper plants.

Cloning and functional characterization of a vacuolar Na+/H+ antiporter gene from mungbean (VrNHX1) and its ectopic expression enhanced salt tolerance in Arabidopsis thaliana

Plant vacuolar NHX exchangers play a significant role in adaption to salt stress by compartmentalizing excess cytosolic Na⁺ into vacuoles and maintaining cellular homeostasis and ionic equilibrium. We cloned an orthologue of the vacuolar Na⁺/H⁺ antiporter gene, VrNHX1 from mungbean (Vigna radiata), an important Asiatic grain legume. The VrNHX1 (Genbank Accession number JN656211.1) contains 2095 nucleotides with an open reading frame of 1629 nucleotides encoding a predicted protein of 542 amino acids with a deduced molecular mass of 59.6 kDa. The consensus amiloride binding motif (⁸⁴LFFIYLLPPI⁹³) was observed in the third putative transmembrane domain of VrNHX1. Bioinformatic and phylogenetic analysis clearly suggested that VrNHX1 had high similarity to those of orthologs belonging to Class-I clade of plant NHX exchangers in leguminous crops. VrNHX1 could be strongly induced by salt stress in mungbean as the expression in roots significantly increased in presence of 200 mM NaCl with concomitant accumulation of total [Na⁺]. Induction of VrNHX1 was also observed under cold and dehydration stress, indicating a possible cross talk between various abiotic stresses. Heterologous expression in salt sensitive yeast mutant AXT3 complemented for the loss of yeast vacuolar NHX1 under NaCl, KCl and LiCl stress indicating that VrNHX1 was the orthologue of ScNHX1. Further, AXT3 cells expressing VrNHX1 survived under low pH environment and displayed vacuolar alkalinization analyzed using pH sensitive fluorescent dye BCECF-AM. The constitutive and stress inducible expression of VrNHX1 resulted in enhanced salt tolerance in transgenic Arabidopsis thaliana lines. Our work suggested that VrNHX1 was a salt tolerance determinant in mungbean.

Genome-wide survey on genomic variation, expression divergence, and evolution in two contrasting rice genotypes under high salinity stress

Expression profiling is one of the most important tools for dissecting biological functions of genes and the upregulation or downregulation of gene expression is sufficient for recreating phenotypic differences. Expression divergence of genes significantly contributes to phenotypic variations. However, little is known on the molecular basis of expression divergence and evolution among rice genotypes with contrasting phenotypes. In this study, we have implemented an integrative approach using bioinformatics and experimental analyses to provide insights into genomic variation, expression divergence, and evolution between salinity-sensitive rice variety Nipponbare and tolerant rice line Pokkali under normal and high salinity stress conditions. We have detected thousands of differentially expressed genes between these two genotypes and thousands of up- or downregulated genes under high salinity stress. Many genes were first detected with expression evidence using custom microarray analysis. Some gene families were preferentially regulated by high salinity stress and might play key roles in stress-responsive biological processes. Genomic variations in promoter regions resulted from single nucleotide polymorphisms, indels (1–10 bp of insertion/deletion), and structural variations significantly contributed to the expression divergence and regulation. Our data also showed that tandem and segmental duplication, CACTA and hAT elements played roles in the evolution of gene expression divergence and regulation between these two contrasting genotypes under normal or high salinity stress conditions.

Strategies for improving potassium use efficiency in plants

Potassium is a macronutrient that is crucial for healthy plant growth. Potassium availability, however, is often limited in agricultural fields and thus crop yields and quality are reduced. Therefore, improving the efficiency of potassium uptake and transport, as well as its utilization, in plants is important for agricultural sustainability. This review summarizes the current knowledge on the molecular mechanisms involved in potassium uptake and transport in plants, and the molecular response of plants to different levels of potassium availability. Based on this information, four strategies for improving potassium use efficiency in plants are proposed; 1) increased root volume, 2) increasing efficiency of potassium uptake from the soil and translocation in planta, 3) increasing mobility of potassium in soil, and 4) molecular breeding new varieties with greater potassium efficiency through marker assisted selection which will require identification and utilization of potassium associated quantitative trait loci.

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.

Cloning and characterization of Na+/H+ antiporter (LfNHX1) gene from a halophyte grass Leptochloa fusca for drought and salt tolerance

Abiotic stresses such as salinity and drought have adverse effects on plants. In the present study, a Na(+)/H(+) antiporter gene homologue (LfNHX1) has been cloned from a local halophyte grass (Leptochloa fusca). The LfNHX1 cDNA contains an open reading frame of 1,623 bp that encodes a polypeptide chain of 540 amino acid residues. LfNHX1 protein sequence showed high similarity with NHX1 homologs reported from other halophyte plants. Amino acid and nucleotide sequence similarity, protein topology modeling and the presence of conserved functional domains in the LfNHX1 protein sequence classified it as a vacuolar NHX1 homolog. The overexpression of LfNHX1 gene under CaMV35S promoter conferred salt and drought tolerance in tobacco plants. Under drought stress, transgenic plants showed higher relative water contents, photosynthetic rate, stomatal conductance and membrane stability index as compared to wild type plants. More negative value of leaf osmotic potential was also observed in transgenic plants when compared with wild type control plants. Transgenic plants showed better germination and root growth at 2 mg L(-1) Basta herbicide and three levels (100, 200 and 250 mM) of sodium chloride. These results showed that LfNHX1 is a potential candidate gene for enhancing drought and salt tolerance in crops.

Functional characterization in Xenopus oocytes of Na+ transport systems from durum wheat reveals diversity among two HKT1;4 transporters

Plant tolerance to salinity constraint involves complex and integrated functions including control of Na(+) uptake, translocation, and compartmentalization. Several members of the high-affinity K(+) transporter (HKT) family, which comprises plasma-membrane transporters permeable to K(+) and Na(+) or to Na(+) only, have been shown to play major roles in plant Na(+) and K(+) homeostasis. Among them, HKT1;4 has been identified as corresponding to a quantitative trait locus (QTL) of salt tolerance in wheat but was not functionally characterized. Here, we isolated two HKT1;4-type cDNAs from a salt-tolerant durum wheat (Triticum turgidum L. subsp. durum) cultivar, Om Rabia3, and investigated the functional properties of the encoded transporters using a two-electrode voltage-clamp technique, after expression in Xenopus oocytes. Both transporters displayed high selectivity for Na(+), their permeability to other monovalent cations (K(+), Li(+), Cs(+), and Rb(+)) being ten times lower than that to Na(+). Both TdHKT1;4-1 and TdHKT1;4-2 transported Na(+) with low affinity, although the half-saturation of the conductance was observed at a Na(+) concentration four times lower in TdHKT1;4-1 than in TdHKT1;4-2. External K(+) did not inhibit Na(+) transport through these transporters. Quinine slightly inhibited TdHKT1;4-2 but not TdHKT1;4-1. Overall, these data identified TdHKT1;4 transporters as new Na(+)-selective transporters within the HKT family, displaying their own functional features. Furthermore, they showed that important differences in affinity exist among durum wheat HKT1;4 transporters. This suggests that the salt tolerance QTL involving HKT1;4 may be at least in part explained by functional variability among wheat HKT1;4-type transporters.

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.

Bioengineering for salinity tolerance in plants: state of the art

Genetic engineering of plants for abiotic stress tolerance is a challenging task because of its multifarious nature. Comprehensive studies for developing abiotic stress tolerance are in progress, involving genes from different pathways including osmolyte synthesis, ion homeostasis, antioxidative pathways, and regulatory genes. In the last decade, several attempts have been made to substantiate the role of "single-function" gene(s) as well as transcription factor(s) for abiotic stress tolerance. Since, the abiotic stress tolerance is multigenic in nature, therefore, the recent trend is shifting towards genetic transformation of multiple genes or transcription factors. A large number of crop plants are being engineered by abiotic stress tolerant genes and have shown the stress tolerance mostly at laboratory level. This review presents a mechanistic view of different pathways and emphasizes the function of different genes in conferring salt tolerance by genetic engineering approach. It also highlights the details of successes achieved in developing salt tolerance in plants thus far.

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.

Global transcriptome profiling of Salicornia europaea L. shoots under NaCl treatment

BACKGROUND

Soil salinity is a major abiotic stress that limits agriculture productivity worldwide. Salicornia europaea is well adapted to extreme saline environments with more than 1,000 mM NaCl in the soil, so it could serve as an important model species for studying halophilic mechanisms in euhalophytes. To obtain insights into the molecular basis of salt tolerance, we present here the first extensive transcriptome analysis of this species using the Illumina HiSeq™ 2000.

PRINCIPAL FINDINGS

A total of 41 and 39 million clean reads from the salt-treated (Se200S) and salt-free (SeCKS) tissues of S. europaea shoots were obtained, and de novo assembly produced 97,865 and 101,751 unigenes, respectively. Upon further assembly with EST data from both Se200S and SeCKS, 109,712 high-quality non-redundant unigenes were generated with a mean unigene size of 639 bp. Additionally, a total of 3,979 differentially expressed genes (DEGs) were detected between the Se200S and SeCKS libraries, with 348 unigenes solely expressed in Se200S and 460 unigenes solely expressed in SeCKS. Furthermore, we identified a large number of genes that are involved in ion homeostasis and osmotic adjustment, including cation transporters and proteins for the synthesis of low-molecular compounds. All unigenes were functionally annotated within the COG, GO and KEGG pathways, and 10 genes were validated by qRT-PCR.

CONCLUSION

Our data contains the extensive sequencing and gene-annotation analysis of S. europaea. This genetic knowledge will be very useful for future studies on the molecular adaptation to abiotic stress in euhalophytes and will facilitate the genetic manipulation of other economically important crops.

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.

Reduced tonoplast fast-activating and slow-activating channel activity is essential for conferring salinity tolerance in a facultative halophyte, quinoa

Halophyte species implement a "salt-including" strategy, sequestering significant amounts of Na(+) to cell vacuoles. This requires a reduction of passive Na(+) leak from the vacuole. In this work, we used quinoa (Chenopodium quinoa) to investigate the ability of halophytes to regulate Na(+)-permeable slow-activating (SV) and fast-activating (FV) tonoplast channels, linking it with Na(+) accumulation in mesophyll cells and salt bladders as well as leaf photosynthetic efficiency under salt stress. Our data indicate that young leaves rely on Na(+) exclusion to salt bladders, whereas old ones, possessing far fewer salt bladders, depend almost exclusively on Na(+) sequestration to mesophyll vacuoles. Moreover, although old leaves accumulate more Na(+), this does not compromise their leaf photochemistry. FV and SV channels are slightly more permeable for K(+) than for Na(+), and vacuoles in young leaves express less FV current and with a density unchanged in plants subjected to high (400 mm NaCl) salinity. In old leaves, with an intrinsically lower density of the FV current, FV channel density decreases about 2-fold in plants grown under high salinity. In contrast, intrinsic activity of SV channels in vacuoles from young leaves is unchanged under salt stress. In vacuoles of old leaves, however, it is 2- and 7-fold lower in older compared with young leaves in control- and salt-grown plants, respectively. We conclude that the negative control of SV and FV tonoplast channel activity in old leaves reduces Na(+) leak, thus enabling efficient sequestration of Na(+) to their vacuoles. This enables optimal photosynthetic performance, conferring salinity tolerance in quinoa species.

Physiological and molecular mechanisms of plant salt tolerance

Salt tolerance is an important economic trait for crops growing in both irrigated fields and marginal lands. The plant kingdom contains plant species that possess highly distinctive capacities for salt tolerance as a result of evolutionary adaptation to their environments. Yet, the cellular mechanisms contributing to salt tolerance seem to be conserved to some extent in plants although some highly salt-tolerant plants have unique structures that can actively excrete salts. In this review, we begin by summarizing the research in Arabidopsis with a focus on the findings of three membrane transporters that are important for salt tolerance: SOS1, AtHKT1, and AtNHX1. We then review the recent studies in salt tolerance in crops and halophytes. Molecular and physiological mechanisms of salt tolerance in plants revealed by the studies in the model plant, crops, and halophytes are emphasized. Utilization of the Na(+) transporters to improve salt tolerance in plants is also summarized. Perspectives are provided at the end of this review.

Physiological and molecular mechanisms of plant salt tolerance

Salt tolerance is an important economic trait for crops growing in both irrigated fields and marginal lands. The plant kingdom contains plant species that possess highly distinctive capacities for salt tolerance as a result of evolutionary adaptation to their environments. Yet, the cellular mechanisms contributing to salt tolerance seem to be conserved to some extent in plants although some highly salt-tolerant plants have unique structures that can actively excrete salts. In this review, we begin by summarizing the research in Arabidopsis with a focus on the findings of three membrane transporters that are important for salt tolerance: SOS1, AtHKT1, and AtNHX1. We then review the recent studies in salt tolerance in crops and halophytes. Molecular and physiological mechanisms of salt tolerance in plants revealed by the studies in the model plant, crops, and halophytes are emphasized. Utilization of the Na(+) transporters to improve salt tolerance in plants is also summarized. Perspectives are provided at the end of this review.

Redox-dependent functional switching of plant proteins accompanying with their structural changes

Reactive oxygen species (ROS) can be generated during the course of normal aerobic metabolism or when an organism is exposed to a variety of stress conditions. It can cause a widespread damage to intracellular macromolecules and play a causal role in many degenerative diseases. Like other aerobic organisms plants are also equipped with a wide range of antioxidant redox proteins, such as superoxide dismutase, catalase, glutaredoxin, thioredoxin (Trx), Trx reductase, protein disulfide reductase, and other kinds of peroxidases that are usually significant in preventing harmful effects of ROS. To defend plant cells in response to stimuli, a part of redox proteins have shown to play multiple functions through the post-translational modification with a redox-dependent manner. For the alternative switching of their cellular functions, the redox proteins change their protein structures from low molecular weight to high molecular weight (HMW) protein complexes depending on the external stress. The HMW proteins are reported to act as molecular chaperone, which enable the plants to enhance their stress tolerance. In addition, some transcription factors and co-activators have function responding to environmental stresses by redox-dependent structural changes. This review describes the molecular mechanism and physiological significance of the redox proteins, transcription factors and co-activators to protect the plants from environmental stresses through the redox-dependent structural and functional switching of the plant redox proteins.

Salinity tolerance mechanisms in glycophytes: An overview with the central focus on rice plants

Elevated Na(+) levels in agricultural lands are increasingly becoming a serious threat to the world agriculture. Plants suffer osmotic and ionic stress under high salinity due to the salts accumulated at the outside of roots and those accumulated at the inside of the plant cells, respectively. Mechanisms of salinity tolerance in plants have been extensively studied and in the recent years these studies focus on the function of key enzymes and plant morphological traits. Here, we provide an updated overview of salt tolerant mechanisms in glycophytes with a particular interest in rice (Oryza sativa) plants. Protective mechanisms that prevent water loss due to the increased osmotic pressure, the development of Na(+) toxicity on essential cellular metabolisms, and the movement of ions via the apoplastic pathway (i.e. apoplastic barriers) are described here in detail.

A Na+/Ca2+ exchanger-like protein (AtNCL) involved in salt stress in Arabidopsis

Calcium ions (Ca(2+)) play a crucial role in many key physiological processes; thus, the maintenance of Ca(2+) homeostasis is of primary importance. Na(+)/Ca(2+) exchangers (NCXs) play an important role in Ca(2+) homeostasis in animal excitable cells. Bioinformatic analysis of the Arabidopsis genome suggested the existence of a putative NCX gene, Arabidopsis NCX-like (AtNCL), encoding a protein with an NCX-like structure and different from Ca(2+)/H(+) exchangers and Na(+)/H(+) exchangers previously identified in plant. AtNCL was identified to localize in the Arabidopsis cell membrane fraction, have the ability of binding Ca(2+), and possess NCX-like activity in a heterologous expression system of cultured mammalian CHO-K1 cells. AtNCL is broadly expressed in Arabidopsis, and abiotic stresses stimulated its transcript expression. Loss-of-function atncl mutants were less sensitive to salt stress than wild-type or AtNCL transgenic overexpression lines. In addition, the total calcium content in whole atncl mutant seedlings was higher than that in wild type by atomic absorption spectroscopy. The level of free Ca(2+) in the cytosol and Ca(2+) flux at the root tips of atncl mutant plants, as detected using transgenic aequorin and a scanning ion-selective electrode, required a longer recovery time following NaCl stress compared with that in wild type. All of these data suggest that AtNCL encodes a Na(+)/Ca(2+) exchanger-like protein that participates in the maintenance of Ca(2+) homeostasis in Arabidopsis. AtNCL may represent a new type of Ca(2+) transporter in higher plants.

Proton channels in algae: reasons to be excited

A fundamental requirement of all eukaryotes is the ability to translocate protons across membranes. This is critical in bioenergetics, for compartmentalized metabolism, and to regulate intracellular pH (pH(i)) within a range that is compatible with cellular metabolism. Plants, animals, and algae utilize specialized transport machinery for membrane energization and pH homeostasis that reflects the prevailing ionic conditions in which they evolved. The recent characterization of H(+)-permeable channels in marine and freshwater algae has led to the discovery of novel functions for these transport proteins in both cellular pH homeostasis and sensory biology. Here we review the potential implications for understanding the origins and evolution of membrane excitability and the phytoplankton-based marine ecosystem responses to ocean acidification.

Cellular ion homeostasis: emerging roles of intracellular NHX Na+/H+ antiporters in plant growth and development

Recent evidence highlights novel roles for intracellular Na(+)/H(+) antiporters (NHXs) in plants. The availability of knockouts and overexpressors of specific NHX isoforms has provided compelling genetic evidence to support earlier physiological and biochemical data which suggested the involvement of NHX antiporters in ion and pH regulation. Most plants sequenced to date contain multiple NHX members and, based on their sequence identity and localization, can be grouped into three distinct functional classes: plasma membrane, vacuolar, and endosomal associated. Orthologues of each functional class are represented in all sequenced plant genomes, suggesting conserved and fundamental roles across taxa. In this review we seek to highlight recent findings which demonstrate that intracellular NHX antiporters (i.e. vacuolar and endosomal isoforms) play roles in growth and development, including cell expansion, cell volume regulation, ion homeostasis, osmotic adjustment, pH regulation, vesicular trafficking, protein processing, cellular stress responses, as well as flowering. A significant new discovery demonstrated that in addition to the better known vacuolar NHX isoforms, plants also contain endosomal NHX isoforms that regulate protein processing and trafficking of cellular cargo. We draw parallels from close orthologues in yeast and mammals and discuss distinctive NHX functions in plants.

Overexpression of SlSOS2 (SlCIPK24) confers salt tolerance to transgenic tomato

The Ca(2+)-dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na(+) and K(+) under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na(+)/H(+) (SlSOS1) and endosomal-vacuolar K(+), Na(+)/H(+) (LeNHX2 and LeNHX4) antiporters, responsible for Na(+) extrusion out of the root, active loading of Na(+) into the xylem, and Na(+) and K(+) compartmentalization.

Molecular cloning and characterization of plasma membrane- and vacuolar-type Na⁺/H⁺ antiporters of an alkaline-salt-tolerant monocot, Puccinellia tenuiflora

A better understanding of salt tolerance in plants might lead to the genetic engineering of crops that can grow in saline soils. Here we cloned and characterized plasma membrane and vacuolar Na⁺/H⁺ antiporters of a monocotyledonous alkaline-tolerant halophyte, Puccinellia tenuiflora. The predicted amino acid sequence of the transporters were very similar to those of orthologs in monocotyledonous crops. Expression analysis showed that (1) NHA was more strongly induced by NaCl in the roots of P. tenuiflora while in rice it was rather induced in the shoots, suggesting that the role of NHA in salt excretion from the roots partly accounts for the difference in the tolerance of the two species, and that (2) NHXs were specifically induced by NaHCO₃ but not by NaCl in the roots of both species, suggesting that vacuolar-type Na⁺/H⁺ antiporters play roles in pH regulation under alkaline salt conditions. Overexpression of the antiporters resulted in increased tolerance of shoots to NaCl and roots to NaHCO₃. Overexpression lines exhibited a lower Na⁺ content and a higher K⁺ content in shoots under NaCl treatments, leading to a higher Na⁺/H⁺ ratio.

Ectopic expression of a bacterium NhaD-type Na+/H+ antiporter leads to increased tolerance to combined salt/alkali stresses

AaNhaD, a gene isolated from the soda lake alkaliphile Alkalimonas amylolytica, encodes a Na(+) /H(+) antiporter crucial for the bacterium's resistance to salt/alkali stresses. However, it remains unknown whether this type of bacterial gene may be able to increase the tolerance of flowering plants to salt/alkali stresses. To investigate the use of extremophile genetic resources in higher plants, transgenic tobacco BY-2 cells and plants harboring AaNhaD were generated and their stress tolerance was evaluated. Ectopic expression of AaNhaD enhanced the salt tolerance of the transgenic BY-2 cells in a pH-dependent manner. Compared to wild-type controls, the transgenic cells exhibited increased Na(+) concentrations and pH levels in the vacuoles. Subcellular localization analysis indicated that AaNhaD-GFP fusion proteins were primarily localized in the tonoplasts. Similar to the transgenic BY-2 cells, AaNhaD-overexpressing tobacco plants displayed enhanced stress tolerance when grown in saline-alkali soil. These results indicate that AaNhaD functions as a pH-dependent tonoplast Na(+) /H(+) antiporter in plant cells, thus presenting a new avenue for the genetic improvement of salinity/alkalinity tolerance.

Expression of wheat Na(+)/H(+) antiporter TNHXS1 and H(+)- pyrophosphatase TVP1 genes in tobacco from a bicistronic transcriptional unit improves salt tolerance

Abiotic stress tolerance of plants is a very complex trait and involves multiple physiological and biochemical processes. Thus, the improvement of plant stress tolerance should involve pyramiding of multiple genes. In the present study, we report the construction and application of a bicistronic system, involving the internal ribosome entry site (IRES) sequence from the 5'UTR of the heat-shock protein of tobacco gene NtHSF-1, to the improvement of salt tolerance in transgenic tobacco plants. Two genes from wheat encoding two important vacuolar ion transporters, Na(+)/H(+) antiporter (TNHXS1) and H(+)-pyrophosphatase (TVP1), were linked via IRES to generate the bicistronic construct TNHXS1-IRES-TVP1. Molecular analysis of transgenic tobacco plants revealed the correct integration of the TNHXS1-IRES-TVP1construct into tobacco genome and the production of the full-length bicistronic mRNA from the 35S promoter. Ion transport analyses with tonoplast vesicles isolated from transgenic lines confirmed that single-transgenic lines TVP1cl19 and TNHXS1cl7 had greater H(+)-PPiase and Na(+)/H(+) antiport activity, respectively, than the WT. Interestingly, the co-expression of TVP1 and TNHXS1 increased both Na(+)/H(+) antiport and H(+)-PPiase activities and induced the H(+) pumping activity of the endogenous V-ATPase. Transgenic tobacco plants expressing TNHXS1-IRES-TVP1 showed a better performance than either of the single gene-transformed lines and the wild type plants when subjected to salt treatment. In addition, the TNHXS1-IRES-TVP1 transgenic plants accumulated less Na(+) and more K(+) in their leaf tissue than did the wild type and the single gene-transformed lines. These results demonstrate that IRES system, described herein, can co-ordinate the expression of two important abiotic stress-tolerance genes and that this expression system is a valuable tool for obtaining transgenic plants with improved salt tolerance.

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

Intracellular NHX proteins are Na(+),K(+)/H(+) antiporters involved in K(+) homeostasis, endosomal pH regulation, and salt tolerance. Proteins NHX1 and NHX2 are the two major tonoplast-localized NHX isoforms. Here, we show that NHX1 and NHX2 have similar expression patterns and identical biochemical activity, and together they account for a significant amount of the Na(+),K(+)/H(+) antiport activity in tonoplast vesicles. Reverse genetics showed functional redundancy of NHX1 and NHX2 genes. Growth of the double mutant nhx1 nhx2 was severely impaired, and plants were extremely sensitive to external K(+). By contrast, nhx1 nhx2 mutants showed similar sensitivity to salinity stress and even greater rates of Na(+) sequestration than the wild type. Double mutants had reduced ability to create the vacuolar K(+) pool, which in turn provoked greater K(+) retention in the cytosol, impaired osmoregulation, and compromised turgor generation for cell expansion. Genes NHX1 and NHX2 were highly expressed in guard cells, and stomatal function was defective in mutant plants, further compromising their ability to regulate water relations. Together, these results show that tonoplast-localized NHX proteins are essential for active K(+) uptake at the tonoplast, for turgor regulation, and for stomatal function.

The jasmonate pathway mediates salt tolerance in grapevines

Salt stress is a major constraint for many crop plants, such as the moderately salt-sensitive economically important fruit crop grapevine. Plants have evolved different strategies for protection against salinity and drought. Jasmonate signalling is a central element of both biotic and abiotic stress responses. To discriminate stress quality, there must be cross-talk with parallel signal chains. Using two grapevine cell lines differing in salt tolerance, the response of jasmonate ZIM/tify-domain (JAZ/TIFY) proteins (negative regulators of jasmonate signalling), a marker for salt adaptation Na(+)/H(+) EXCHANGER (NHX1), and markers for biotic defence STILBENE SYNTHASE (StSy) and RESVERATROL SYNTHASE (RS) were analysed. It is shown that salt stress signalling shares several events with biotic defence including activity of a gadolinium-sensitive calcium influx channel (monitored by apoplastic alkalinization) and transient induction of JAZ/TIFY transcripts. Exogenous jasmonate can rescue growth in the salt-sensitive cell line. Suppression of jasmonate signalling by phenidone or aspirin blocks the induction of JAZ/TIFY transcripts. The rapid induction of RS and StSy characteristic for biotic defence in grapevine is strongly delayed in response to salt stress. In the salt-tolerant line, NHX1 is induced and the formation of reactive oxygen species, monitored as stress markers in the sensitive cell line, is suppressed. The data are discussed in terms of a model where salt stress signalling acts as a default pathway whose readout is modulated by a parallel signal chain triggered by biotic factors downstream of jasmonate signalling.

Expression of LeNHX isoforms in response to salt stress in salt sensitive and salt tolerant tomato species

In general, wild tomato species are more salt tolerant than cultivated species, a trait that is related to enhanced Na(+) accumulation in aerial parts in the wild species, but the molecular basis for these differences is not known. Plant NHX proteins have been suggested to be important for salt tolerance by promoting accumulation of Na(+) or K(+) inside vacuoles. Therefore, differences in expression or activity of NHX proteins in tomato could be at the basis of the enhanced salt tolerance in wild tomato species. To test this hypothesis, we studied the expression level of four NHX genes in the salt sensitive cultivated species Solanum lycopersicum L. cv. Volgogradskij and the salt tolerant wild species Solanum pimpinelifolium L in response to salt stress. First, we determined that in the absence of salt stress, the RNA abundance of LeNHX2, 3 and 4 was comparable in both species, while more LeNHX1 RNA was detected in the tolerant species. LeNHX2 and LeNHX3 showed comparable expression levels and were present in all tissues, while LeNHX4 was expressed above all in stem and fruit tissues. Next, we confirmed that the wild species was more tolerant and accumulated more Na(+) in aerial parts of the plant. This correlated with the observation that salt stress induced especially the LeNHX3 and LeNHX4 isoforms in the tolerant species. These results support a role of NHX genes as determinants of salt tolerance in tomato, inducing enhanced Na(+) accumulation observed in the wild species when grown in the presence of NaCl.

Plant and yeast NHX antiporters: roles in membrane trafficking

The plant NHX gene family encodes Na(+)/H(+) antiporters which are crucial for salt tolerance, potassium homeostasis and cellular pH regulation. Understanding the role of NHX antiporters in membrane trafficking is becoming an increasingly interesting subject of study. Membrane trafficking is a central cellular process during which proteins, lipids and polysaccharides are continuously exchanged among membrane compartments. Yeast ScNhx1p, a prevacuole/ vacuolar Na(+)/H(+) antiporter, plays an important role in regulating pH to control trafficking out of the endosome. Evidence begins to accumulate that plant NHX antiporters might function in regulating membrane trafficking in plants.

Conserved and diversified gene families of monovalent cation/h(+) antiporters from algae to flowering plants

All organisms have evolved strategies to regulate ion and pH homeostasis in response to developmental and environmental cues. One strategy is mediated by monovalent cation-proton antiporters (CPA) that are classified in two superfamilies. Many CPA1 genes from bacteria, fungi, metazoa, and plants have been functionally characterized; though roles of plant CPA2 genes encoding K(+)-efflux antiporter (KEA) and cation/H(+) exchanger (CHX) families are largely unknown. Phylogenetic analysis showed that three clades of the CPA1 Na(+)-H(+) exchanger (NHX) family have been conserved from single-celled algae to Arabidopsis. These are (i) plasma membrane-bound SOS1/AtNHX7 that share ancestry with prokaryote NhaP, (ii) endosomal AtNHX5/6 that is part of the eukaryote Intracellular-NHE clade, and (iii) a vacuolar NHX clade (AtNHX1-4) specific to plants. Early diversification of KEA genes possibly from an ancestral cyanobacterium gene is suggested by three types seen in all plants. Intriguingly, CHX genes diversified from three to four members in one subclade of early land plants to 28 genes in eight subclades of Arabidopsis. Homologs from Spirogyra or Physcomitrella share high similarity with AtCHX20, suggesting that guard cell-specific AtCHX20 and its closest relatives are founders of the family, and pollen-expressed CHX genes appeared later in monocots and early eudicots. AtCHX proteins mediate K(+) transport and pH homeostasis, and have been localized to intracellular and plasma membrane. Thus KEA genes are conserved from green algae to angiosperms, and their presence in red algae and secondary endosymbionts suggest a role in plastids. In contrast, AtNHX1-4 subtype evolved in plant cells to handle ion homeostasis of vacuoles. The great diversity of CHX genes in land plants compared to metazoa, fungi, or algae would imply a significant role of ion and pH homeostasis at dynamic endomembranes in the vegetative and reproductive success of flowering plants.