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

Heterologous expression of the durum wheat TdHKT1;4-1 partially complements the mutant athkt1 in Arabidopsis thaliana under severe salt stress

High-affinity K+ (HKT) transporters which mediate Na+-specific transport or Na+-K+ co-transport play a key role in plant salt tolerance. In our previous functional study in Xenopus oocytes, we demonstrated that the durum wheat TdHKT1;4-1 acts as a Na+-selective transporter. Here, we investigated the function of TdHKT1;4-1 and its contribution in salt stress tolerance in the Arabidopsis athkt1 mutant background. Our results revealed that TdHKT1;4-1 partially complements the salt sensitivity phenotype of the athkt1 transgenic lines. Comparative physiological analyses and oxidative stress status under moderate salt stress (50 mM NaCl) showed that both transgenic lines SH3 and SH5 restored the salt stress tolerance comparable to the level observed in Wt plants. Whereas, under severe salt stress treatment (100 mM NaCl), the athkt1 transgenic lines exhibited an intermediate salt stress tolerance between Wt and athkt1 mutant. Moreover, TdHKT1;4-1 was highly expressed in leaves under moderate and severe salt stress, while in roots, it was largely expressed only under severe salt stress. In addition, antioxidant enzymes such as catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were significantly expressed in SH3 and SH5 lines compared to athkt1 and Wt under moderate stress. Therefore, TdHKT1;4-1 seems to differ from its Arabidopsis homologous counterpart, as it contributes to salt stress tolerance up to a specific threshold, above which the TdHKT1;4-1 expression may lead to higher root Na+ influx, hence increasing its toxicity during salt stress.

Functional analysis of TmHKT1;4-A2 promoter through deletion analysis provides new insight into the regulatory mechanism underlying abiotic stress adaptation

Bioinformatic, molecular, and biochemical analysis were performed to get more insight into the regulatory mechanism by which TmHKT1;4-A2 is regulated. HKT transporters from different plant species have been shown to play important role in plant response to salt. In previous work, TmHKT1;4-A2 gene from Triticum monococcum has been characterized as a major gene for Nax1 QTL (Tounsi et al. Plant Cell Physiol 57:2047-2057, 2016). So far, little is known about its regulatory mechanism. In this study, the promoter region of TmHKT1;4-A2 (1400 bp) was isolated and considered as the full-length promoter (PA2-1400). In silico analysis revealed the presence of important cis-acting elements related to abiotic stresses and phytohormones. Interestingly, our real-time RT-PCR analysis provided evidence that TmHKT1;4-A2 is regulated not only by salt stress but also by osmotic, heavy metal, oxidative, and hormones stresses. In transgenic Arabidopsis plants, TmHKT1;4-A2 is strongly active in vascular tissues of roots and leaves. Through 5'-end deletion analysis, we showed that PA2-1400 promoter is able to drive strong GUS activity under normal conditions and in response to different stresses compared to PA2-824 and PA2-366 promoters. These findings provide new information on the regulatory mechanism of TmHKT1;4-A2 and shed more light on its role under different stresses.

High affinity Na+ transport by wheat HKT1;5 is blocked by K. PLANT DIRECT 2020;4:e00275. [PMID: 33103046 PMCID: PMC7576878 DOI: 10.1002/pld3.275]

The wheat sodium transporters TmHKT1;5-A and TaHKT1;5-D are encoded by genes underlying the major shoot Na+ exclusion loci Nax2 and Kna1 from Triticum monococcum (Tm) and Triticum aestivum (Ta), respectively. In contrast to HKT2 transporters that have been shown to exhibit high affinity K+-dependent Na+ transport, HKT1 proteins have, with one exception, only been shown to catalyze low affinity Na+ transport and no K+ transport. Here, using heterologous expression in Xenopus laevis oocytes we uncover a novel property of HKT1 proteins, that both TmHKT1;5-A and TaHKT1;5-D encode dual (high and low) affinity Na+-transporters with the high-affinity component being abolished when external K+ is in excess of external Na+. Three-dimensional structural modeling suggested that, compared to Na+, K+ is bound more tightly in the selectivity filter region by means of additional van der Waals forces, which is likely to explain the K+ block at the molecular level. The low-affinity component for Na+ transport of TmHKT1;5-A had a lower K m than that of TaHKT1;5-D and was less sensitive to external K+. We propose that these properties contribute towards the improvements in shoot Na+-exclusion and crop plant salt tolerance following the introgression of TmHKT1;5-A into diverse wheat backgrounds.

Homology Modeling Identifies Crucial Amino-Acid Residues That Confer Higher Na+ Transport Capacity of OcHKT1;5 from Oryza coarctata Roxb

HKT1;5 loci/alleles are important determinants of crop salinity tolerance. HKT1;5s encode plasmalemma-localized Na+ transporters, which move xylem Na+ into xylem parenchyma cells, reducing shoot Na+ accumulation. Allelic variation in rice OsHKT1;5 sequence in specific landraces (Nona Bokra OsHKT1;5-NB/Nipponbare OsHKT1;5-Ni) correlates with variation in salt tolerance. Oryza coarctata, a halophytic wild rice, grows in fluctuating salinity at the seawater-estuarine interface in Indian and Bangladeshi coastal regions. The distinct transport characteristics of the shoots and roots expressing the O. coarctata OcHKT1;5 transporter are reported vis-à-vis OsHKT1;5-Ni. Yeast sodium extrusion-deficient cells expressing OcHKT1;5 are sensitive to increasing Na+ (10-100 mM). Electrophysiological measurements in Xenopus oocytes expressing O. coarctata or rice HKT1;5 transporters indicate that OcHKT1;5, like OsHKT1;5-Ni, is a Na+-selective transporter, but displays 16-fold lower affinity for Na+ and 3.5-fold higher maximal conductance than OsHKT1;5-Ni. For Na+ concentrations >10 mM, OcHKT1;5 conductance is higher than that of OsHKT1;5-Ni, indicating the potential of OcHKT1;5 for increasing domesticated rice salt tolerance. Homology modeling/simulation suggests that four key amino-acid changes in OcHKT1;5 (in loops on the extracellular side; E239K, G207R, G214R, L363V) account for its lower affinity and higher Na+ conductance vis-à-vis OsHKT1;5-Ni. Of these, E239K in OcHKT1;5 confers lower affinity for Na+ transport, as evidenced by Na+ transport assays of reciprocal site-directed mutants for both transporters (OcHKT1;5-K239E, OsHKT1;5-Ni-E270K) in Xenopus oocytes. Both transporters have likely analogous roles in xylem sap desalinization, and differences in xylem sap Na+ concentrations in both species are attributed to differences in Na+ transport affinity/conductance between the transporters.

Supportive role of the Na+ transporter CmHKT1;1 from Cucumis melo in transgenic Arabidopsis salt tolerance through improved K+/Na+ balance

KEY MESSAGE

CmHKT1;1 selectively exports Na+ from plant cells. Upon NaCl stress, its expression increased in a salt-tolerant melon cultivar. Overexpression of CmHKT1;1 increased transgenic Arabidopsis salt tolerance through improved K+/Na+ balance. High-affinity K+ transporters (HKTs) are thought to be involved in reducing Na+ in plant shoots under salt stress and modulating salt tolerance, but their function in a moderately salt-tolerant species of melon (Cucumis melo L.) remains unclear. In this study, a Na+ transporter gene, CmHKT1;1 (GenBank accession number: MK986658), was isolated from melons based on genome data. The transcript of CmHKT1;1 was relatively more abundant in roots than in stems or leaves from melon seedlings. The tobacco transient expression system showed that CmHKT1;1 was plasma-membrane localized. Upon salt stress, CmHKT1;1 expression was more strongly upregulated in a salt-tolerant melon cultivar, 'Bingxuecui' (BXC) compared with a salt-sensitive cultivar, 'Yulu' (YL). Electrophysiological evidence demonstrated that CmHKT1;1 only transported Na+, rather than K+, when expressed in Xenopus laevis oocytes. Overexpression of CmHKT1;1 increased salt sensitivity in Saccharomyces cerevisiae and salt tolerance in Arabidopsis thaliana. Under NaCl treatments, transgenic Arabidopsis plants accumulated significantly lower concentrations of Na+ in shoots than wild type plants and showed a better K+/Na+ balance, leading to better Fv/Fm, root length, biomass, and enhanced plant growth. The CmHKT1;1 gene may serve as a useful candidate for improving crop salt tolerance.

The involvement of wheat U-box E3 ubiquitin ligase TaPUB1 in salt stress tolerance

U-box E3 ubiquitin ligases play important roles in the ubiquitin/26S proteasome machinery and in abiotic stress responses. TaPUB1-overexpressing wheat (Triticum aestivum L.) were generated to evaluate its function in salt tolerance. These plants had more salt stress tolerance during seedling and flowering stages, whereas the TaPUB1-RNA interference (RNAi)-mediated knock-down transgenic wheat showed more salt stress sensitivity than the wild type (WT). TaPUB1 overexpression upregulated the expression of genes related to ion channels and increased the net root Na⁺ efflux, but decreased the net K⁺ efflux and H⁺ influx, thereby maintaining a low cytosolic Na⁺ /K⁺ ratio, compared with the WT. However, RNAi-mediated knock-down plants showed the opposite response to salt stress. TaPUB1 could induce the expression of some genes that improved the antioxidant capacity of plants under salt stress. TaPUB1 also interacted with TaMP (Triticum aestivum α-mannosidase protein), a regulator playing an important role in salt response in yeast and in plants. Thus, low cytosolic Na⁺ /K⁺ ratios and better antioxidant enzyme activities could be maintained in wheat with overexpression of TaPUB1 under salt stress. Therefore, we conclude that the U-box E3 ubiquitin ligase TaPUB1 positively regulates salt stress tolerance in wheat.

Constitutive Contribution by the Rice OsHKT1;4 Na+ Transporter to Xylem Sap Desalinization and Low Na+ Accumulation in Young Leaves Under Low as High External Na+ Conditions

HKT Na⁺ transporters correspond to major salt tolerance QTLs in different plant species and are targets of great interest for breeders. In rice, the HKT family is composed of seven or eight functional genes depending on cultivars. Three rice HKT genes, OsHKT1;1, OsHKT1;4 and OsHKT1;5, are known to contribute to salt tolerance by reducing Na⁺ accumulation in shoots upon salt stress. Here, we further investigate the mechanisms by which OsHKT1;4 contributes to this process and extend this analysis to the role of this transporter in plants in presence of low Na⁺ concentrations. By analyzing transgenic rice plants expressing a GUS reporter gene construct, we observed that OsHKT1;4 is mainly expressed in xylem parenchyma in both roots and leaves. Using mutant lines expressing artificial microRNA that selectively reduced OsHKT1;4 expression, the involvement of OsHKT1;4 in retrieving Na⁺ from the xylem sap in the roots upon salt stress was evidenced. Since OsHKT1;4 was found to be also well expressed in the roots in absence of salt stress, we extended the analysis of its role when plants were subjected to non-toxic Na⁺ conditions (0.5 and 5 mM). Our finding that the transporter, expressed in Xenopus oocytes, displayed a relatively high affinity for Na⁺, just above 1 mM, provided first support to the hypothesis that OsHKT1;4 could have a physiological role at low Na⁺ concentrations. We observed that progressive desalinization of the xylem sap along its ascent to the leaf blades still occurred in plants grown at submillimolar Na⁺ concentration, and that OsHKT1;4 was involved in reducing xylem sap Na⁺ concentration in roots in these conditions too. Its contribution to tissue desalinization from roots to young mature leaf blades appeared to be rather similar in the whole range of explored external Na⁺ concentrations, from submillimolar to salt stress conditions. Our data therefore indicate that HKT transporters can be involved in controlling Na⁺ translocation from roots to shoots in a much wider range of Na⁺ concentrations than previously thought. This asks questions about the roles of such a transporter-mediated maintaining of tissue Na⁺ content gradients in non-toxic conditions.

Structural variations in wheat HKT1;5 underpin differences in Na+ transport capacity

An important trait associated with the salt tolerance of wheat is the exclusion of sodium ions (Na+) from the shoot. We have previously shown that the sodium transporters TmHKT1;5-A and TaHKT1;5-D, from Triticum monoccocum (Tm) and Triticum aestivum (Ta), are encoded by genes underlying the major shoot Na+-exclusion loci Nax1 and Kna1, respectively. Here, using heterologous expression, we show that the affinity (K m) for the Na+ transport of TmHKT1;5-A, at 2.66 mM, is higher than that of TaHKT1;5-D at 7.50 mM. Through 3D structural modelling, we identify residues D471/a gap and D474/G473 that contribute to this property. We identify four additional mutations in amino acid residues that inhibit the transport activity of TmHKT1;5-A, which are predicted to be the result of an occlusion of the pore. We propose that the underlying transport properties of TmHKT1;5-A and TaHKT1;5-D contribute to their unique ability to improve Na+ exclusion in wheat that leads to an improved salinity tolerance in the field.

Mapping of HKT1;5 Gene in Barley Using GWAS Approach and Its Implication in Salt Tolerance Mechanism

Sodium (Na⁺) accumulation in the cytosol will result in ion homeostasis imbalance and toxicity of transpiring leaves. Studies of salinity tolerance in the diploid wheat ancestor Triticum monococcum showed that HKT1;5-like gene was a major gene in the QTL for salt tolerance, named Nax2. In the present study, we were interested in investigating the molecular mechanisms underpinning the role of the HKT1;5 gene in salt tolerance in barley (Hordeum vulgare). A USDA mini-core collection of 2,671 barley lines, part of a field trial was screened for salinity tolerance, and a Genome Wide Association Study (GWAS) was performed. Our results showed important SNPs that are correlated with salt tolerance that mapped to a region where HKT1;5 ion transporter located on chromosome four. Furthermore, sodium (Na⁺) and potassium (K⁺) content analysis revealed that tolerant lines accumulate more sodium in roots and leaf sheaths, than in the sensitive ones. In contrast, sodium concentration was reduced in leaf blades of the tolerant lines under salt stress. In the absence of NaCl, the concentration of Na⁺ and K⁺ were the same in the roots, leaf sheaths and leaf blades between the tolerant and the sensitive lines. In order to study the molecular mechanism behind that, alleles of the HKT1;5 gene from five tolerant and five sensitive barley lines were cloned and sequenced. Sequence analysis did not show the presence of any polymorphism that distinguishes between the tolerant and sensitive alleles. Our real-time RT-PCR experiments, showed that the expression of HKT1;5 gene in roots of the tolerant line was significantly induced after challenging the plants with salt stress. In contrast, in leaf sheaths the expression was decreased after salt treatment. In sensitive lines, there was no difference in the expression of HKT1;5 gene in leaf sheath under control and saline conditions, while a slight increase in the expression was observed in roots after salt treatment. These results provide stronger evidence that HKT1;5 gene in barley play a key role in withdrawing Na⁺ from the xylem and therefore reducing its transport to leaves. Given all that, these data support the hypothesis that HKT1;5 gene is responsible for Na⁺ unloading to the xylem and controlling its distribution in the shoots, which provide new insight into the understanding of this QTL for salinity tolerance in barley.

Salt stress reveals differential physiological, biochemical and molecular responses in T. monococcum and T. durum wheat genotypes

Salt stress responses implicate a complex mechanism and differ from plant species to another. In this study, we analyzed the physiological, biochemical and molecular responses to salt stress of the diploid wheat (T. monococcum) and compared to the tetraploid wheat (T. durum). Our results showed that the diploid wheat cultivar (cv. Turkey) is relatively tolerant to different salt stress conditions than the tetraploid wheat cultivar (cv. Om Rabia3). This tolerance was manifested by significant germination, plant growth and uptake of water generating cell turgor and development. Moreover, total chlorophyll content was higher in the diploid wheat than that in the tetraploid wheat. The Na⁺ content in leaf blade of the cv. Om Rabia3 was significantly higher than that of the cv. Turkey, suggesting that the diploid cultivar accumulates less toxic sodium in the photosynthetic tissues. This mechanism could be explained by the recirculation of the toxic ions Na⁺ into the xylem sap by SOS1 protein, which coordinates with HKT-like proteins to reduce the accumulation of Na⁺ ions in leaf blade. Interestingly, the expression of the three genes SOS1, HKT and NHX was enhanced under salinity especially in leaf blade of the cv. Turkey. Moreover, this wheat cultivar induced the antioxidative enzymes CAT and SOD activity more efficiently than the other cultivar.

Allelic variants of OsHKT1;1 underlie the divergence between indica and japonica subspecies of rice (Oryza sativa) for root sodium content

Salinity is a major factor limiting crop productivity. Rice (Oryza sativa), a staple crop for the majority of the world, is highly sensitive to salinity stress. To discover novel sources of genetic variation for salt tolerance-related traits in rice, we screened 390 diverse accessions under 14 days of moderate (9 dS·m-1) salinity. In this study, shoot growth responses to moderate levels of salinity were independent of tissue Na+ content. A significant difference in root Na+ content was observed between the major subpopulations of rice, with indica accessions displaying higher root Na+ and japonica accessions exhibiting lower root Na+ content. The genetic basis of the observed variation in phenotypes was elucidated through genome-wide association (GWA). The strongest associations were identified for root Na+:K+ ratio and root Na+ content in a region spanning ~575 Kb on chromosome 4, named Root Na+ Content 4 (RNC4). Two Na+ transporters, HKT1;1 and HKT1;4 were identified as candidates for RNC4. Reduced expression of both HKT1;1 and HKT1;4 through RNA interference indicated that HKT1;1 regulates shoot and root Na+ content, and is likely the causal gene underlying RNC4. Three non-synonymous mutations within HKT1;1 were present at higher frequency in the indica subpopulation. When expressed in Xenopus oocytes the indica-predominant isoform exhibited higher inward (negative) currents and a less negative voltage threshold of inward rectifying current activation compared to the japonica-predominant isoform. The introduction of a 4.5kb fragment containing the HKT1;1 promoter and CDS from an indica variety into a japonica background, resulted in a phenotype similar to the indica subpopulation, with higher root Na+ and Na+:K+. This study provides evidence that HKT1;1 regulates root Na+ content, and underlies the divergence in root Na+ content between the two major subspecies in rice.

Characterization of Two HKT1;4 Transporters from Triticum monococcum to Elucidate the Determinants of the Wheat Salt Tolerance Nax1 QTL

TmHKT1;4-A1 and TmHKT1;4-A2 are two Na⁺ transporter genes that have been identified as associated with the salt tolerance Nax1 locus found in a durum wheat (Triticum turgidum L. subsp. durum) line issued from a cross with T. monococcum. In the present study, we were interested in getting clues on the molecular mechanisms underpinning this salt tolerance quantitative trait locus (QTL). By analyzing the phylogenetic relationships between wheat and T. monococcum HKT1;4-type genes, we found that durum and bread wheat genomes possess a close homolog of TmHKT1;4-A1, but no functional close homolog of TmHKT1;4-A2. Furthermore, performing real-time reverse transcription-PCR experiments, we showed that TmHKT1;4-A1 and TmHKT1;4-A2 are similarly expressed in the leaves but that TmHKT1;4-A2 is more strongly expressed in the roots, which would enable it to contribute more to the prevention of Na⁺ transfer to the shoots upon salt stress. We also functionally characterized the TmHKT1;4-A1 and TmHKT1;4-A2 transporters by expressing them in Xenopus oocytes. The two transporters displayed close functional properties (high Na⁺/K⁺ selectivity, low affinity for Na⁺, stimulation by external K⁺ of Na⁺ transport), but differed in some quantitative parameters: Na⁺ affinity was 3-fold lower and the maximal inward conductance was 3-fold higher in TmHKT1;4-A2 than in TmHKT1;4-A1. The conductance of TmHKT1;4-A2 at high Na⁺ concentration (>10 mM) was also shown to be higher than that of the two durum wheat HKT1;4-type transporters so far characterized. Altogether, these data support the hypothesis that TmHKT1;4-A2 is responsible for the Nax1 trait and provide new insight into the understanding of this QTL.

Synthetic strategies for plant signalling studies: molecular toolbox and orthogonal platforms

Plants deploy a wide array of signalling networks integrating environmental cues with growth, defence and developmental responses. The high level of complexity, redundancy and connection between several pathways hampers a comprehensive understanding of involved functional and regulatory mechanisms. The implementation of synthetic biology approaches is revolutionizing experimental biology in prokaryotes, yeasts and animal systems and can likewise contribute to a new era in plant biology. This review gives an overview on synthetic biology approaches for the development and implementation of synthetic molecular tools and techniques to interrogate, understand and control signalling events in plants, ranging from strategies for the targeted manipulation of plant genomes up to the spatiotemporally resolved control of gene expression using optogenetic approaches. We also describe strategies based on the partial reconstruction of signalling pathways in orthogonal platforms, like yeast, animal and in vitro systems. This allows a targeted analysis of individual signalling hubs devoid of interconnectivity with endogenous interacting components. Implementation of the interdisciplinary synthetic biology tools and strategies is not exempt of challenges and hardships but simultaneously most rewarding in terms of the advances in basic and applied research. As witnessed in other areas, these original theoretical-experimental avenues will lead to a breakthrough in the ability to study and comprehend plant signalling networks.

A Single Amino-Acid Substitution in the Sodium Transporter HKT1 Associated with Plant Salt Tolerance

A crucial prerequisite for plant growth and survival is the maintenance of potassium uptake, especially when high sodium surrounds the root zone. The Arabidopsis HIGH-AFFINITY K(+) TRANSPORTER1 (HKT1), and its homologs in other salt-sensitive dicots, contributes to salinity tolerance by removing Na(+) from the transpiration stream. However, TsHKT1;2, one of three HKT1 copies in Thellungiella salsuginea, a halophytic Arabidopsis relative, acts as a K(+) transporter in the presence of Na(+) in yeast (Saccharomyces cerevisiae). Amino-acid sequence comparisons indicated differences between TsHKT1;2 and most other published HKT1 sequences with respect to an Asp residue (D207) in the second pore-loop domain. Two additional T salsuginea and most other HKT1 sequences contain Asn (n) in this position. Wild-type TsHKT1;2 and altered AtHKT1 (AtHKT1(N-D)) complemented K(+)-uptake deficiency of yeast cells. Mutant hkt1-1 plants complemented with both AtHKT1(N) (-) (D) and TsHKT1;2 showed higher tolerance to salt stress than lines complemented by the wild-type AtHKT1 Electrophysiological analysis in Xenopus laevis oocytes confirmed the functional properties of these transporters and the differential selectivity for Na(+) and K(+) based on the n/d variance in the pore region. This change also dictated inward-rectification for Na(+) transport. Thus, the introduction of Asp, replacing Asn, in HKT1-type transporters established altered cation selectivity and uptake dynamics. We describe one way, based on a single change in a crucial protein that enabled some crucifer species to acquire improved salt tolerance, which over evolutionary time may have resulted in further changes that ultimately facilitated colonization of saline habitats.

OsHKT1;4-mediated Na(+) transport in stems contributes to Na(+) exclusion from leaf blades of rice at the reproductive growth stage upon salt stress

BACKGROUND

Na(+) exclusion from leaf blades is one of the key mechanisms for glycophytes to cope with salinity stress. Certain class I transporters of the high-affinity K(+) transporter (HKT) family have been demonstrated to mediate leaf blade-Na(+) exclusion upon salinity stress via Na(+)-selective transport. Multiple HKT1 transporters are known to function in rice (Oryza sativa). However, the ion transport function of OsHKT1;4 and its contribution to the Na(+) exclusion mechanism in rice remain to be elucidated.

RESULTS

Here, we report results of the functional characterization of the OsHKT1;4 transporter in rice. OsHKT1;4 mediated robust Na(+) transport in Saccharomyces cerevisiae and Xenopus laevis oocytes. Electrophysiological experiments demonstrated that OsHKT1;4 shows strong Na(+) selectivity among cations tested, including Li(+), Na(+), K(+), Rb(+), Cs(+), and NH4 (+), in oocytes. A chimeric protein, EGFP-OsHKT1;4, was found to be functional in oocytes and targeted to the plasma membrane of rice protoplasts. The level of OsHKT1;4 transcripts was prominent in leaf sheaths throughout the growth stages. Unexpectedly however, we demonstrate here accumulation of OsHKT1;4 transcripts in the stem including internode II and peduncle in the reproductive growth stage. Moreover, phenotypic analysis of OsHKT1;4 RNAi plants in the vegetative growth stage revealed no profound influence on the growth and ion accumulation in comparison with WT plants upon salinity stress. However, imposition of salinity stress on the RNAi plants in the reproductive growth stage caused significant Na(+) overaccumulation in aerial organs, in particular, leaf blades and sheaths. In addition, (22)Na(+) tracer experiments using peduncles of RNAi and WT plants suggested xylem Na(+) unloading by OsHKT1;4.

CONCLUSIONS

Taken together, our results indicate a newly recognized function of OsHKT1;4 in Na(+) exclusion in stems together with leaf sheaths, thus excluding Na(+) from leaf blades of a japonica rice cultivar in the reproductive growth stage, but the contribution is low when the plants are in the vegetative growth stage.

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.

Model of Cation Transportation Mediated by High-Affinity Potassium Transporters (HKTs) in Higher Plants

Trk/Ktr/HKT transporters probably were evolved from simple K⁺ channels KcsA. HKT transporters, which mediate Na⁺-uniport or Na⁺/K⁺-symport, maintain K⁺/Na⁺ homeostasis and increase salinity tolerance, can be classified into three subfamilies in higher plants. In this review, we systematically analyzed the characteristics of amino acids sequences and physiological functions of HKT transporters in higher plant. Furthermore, we depicted the hypothetical models of cations selection and transportation mediated by HKT transporters according to the highly conserved structure for the goal of better understanding the cations transportation processes.

The plasma membrane transport systems and adaptation to salinity

Salt stress represents one of the environmental challenges that drastically affect plant growth and yield. Evidence suggests that glycophytes and halophytes have a salt tolerance mechanisms working at the cellular level, and the plasma membrane (PM) is believed to be one facet of the cellular mechanisms. The responses of the PM transport proteins to salinity in contrasting species/cultivars were discussed. The review provides a comprehensive overview of the recent advances describing the crucial roles that the PM transport systems have in plant adaptation to salt. Several lines of evidence were presented to demonstrate the correlation between the PM transport proteins and adaptation of plants to high salinity. How alterations in these transport systems of the PM allow plants to cope with the salt stress was also addressed. Although inconsistencies exist in some of the information related to the responses of the PM transport proteins to salinity in different species/cultivars, their key roles in adaptation of plants to high salinity is obvious and evident, and cannot be precluded. Despite the promising results, detailed investigations at the cellular/molecular level are needed in some issues of the PM transport systems in response to salinity to further evaluate their implication in salt tolerance.

The Na(+) transporter, TaHKT1;5-D, limits shoot Na(+) accumulation in bread wheat

Bread wheat (Triticum aestivum L.) has a major salt tolerance locus, Kna1, responsible for the maintenance of a high cytosolic K(+) /Na(+) ratio in the leaves of salt stressed plants. The Kna1 locus encompasses a large DNA fragment, the distal 14% of chromosome 4DL. Limited recombination has been observed at this locus making it difficult to map genetically and identify the causal gene. Here, we decipher the function of TaHKT1;5-D, a candidate gene underlying the Kna1 locus. Transport studies using the heterologous expression systems Saccharomyces cerevisiae and Xenopus laevis oocytes indicated that TaHKT1;5-D is a Na(+) -selective transporter. Transient expression in Arabidopsis thaliana mesophyll protoplasts and in situ polymerase chain reaction indicated that TaHKT1;5-D is localised on the plasma membrane in the wheat root stele. RNA interference-induced silencing decreased the expression of TaHKT1;5-D in transgenic bread wheat lines which led to an increase in the Na(+) concentration in the leaves. This indicates that TaHKT1;5-D retrieves Na(+) from the xylem vessels in the root and has an important role in restricting the transport of Na(+) from the root to the leaves in bread wheat. Thus, TaHKT1;5-D confers the essential salinity tolerance mechanism in bread wheat associated with the Kna1 locus via shoot Na(+) exclusion and is critical in maintaining a high K(+) /Na(+) ratio in the leaves. These findings show there is potential to increase the salinity tolerance of bread wheat by manipulation of HKT1;5 genes.

Molecular biology of K+ transport across the plant cell membrane: what do we learn from comparison between plant species?

Cloning and characterizations of plant K(+) transport systems aside from Arabidopsis have been increasing over the past decade, favored by the availability of more and more plant genome sequences. Information now available enables the comparison of some of these systems between species. In this review, we focus on three families of plant K(+) transport systems that are active at the plasma membrane: the Shaker K(+) channel family, comprised of voltage-gated channels that dominate the plasma membrane conductance to K(+) in most environmental conditions, and two families of transporters, the HAK/KUP/KT K(+) transporter family, which includes some high-affinity transporters, and the HKT K(+) and/or Na(+) transporter family, in which K(+)-permeable members seem to be present in monocots only. The three families are briefly described, giving insights into the structure of their members and on functional properties and their roles in Arabidopsis or rice. The structure of the three families is then compared between plant species through phylogenic analyses. Within clusters of ortologues/paralogues, similarities and differences in terms of expression pattern, functional properties and, when known, regulatory interacting partners, are highlighted. The question of the physiological significance of highlighted differences is also addressed.