The Na+ transporter AtHKT1;1 controls retrieval of Na+ from the xylem in Arabidopsis

Identification of candidate genes related to salt tolerance of the secretohalophyte Atriplex canescens by transcriptomic analysis

BACKGROUND

Atriplex canescens is a typical C₄ secretohalophyte with salt bladders on the leaves. Accumulating excessive Na⁺ in tissues and salt bladders, maintaining intracellular K⁺ homeostasis and increasing leaf organic solutes are crucial for A. canescens survival in harsh saline environments, and enhanced photosynthetic activity and water balance promote its adaptation to salt. However, the molecular basis for these physiological mechanisms is poorly understood. Four-week-old A. canescens seedlings were treated with 100 mM NaCl for 6 and 24 h, and differentially expressed genes in leaves and roots were identified, respectively, with Illumina sequencing.

RESULTS

In A. canescens treated with 100 mM NaCl, the transcripts of genes encoding transporters/channels for important nutrient elements, which affect growth under salinity, significantly increased, and genes involved in exclusion, uptake and vacuolar compartmentalization of Na⁺ in leaves might play vital roles in Na⁺ accumulation in salt bladders. Moreover, NaCl treatment upregulated the transcripts of key genes related to leaf organic osmolytes synthesis, which are conducive to osmotic adjustment. Correspondingly, aquaporin-encoding genes in leaves showed increased transcripts under NaCl treatment, which might facilitate water balance maintenance of A. canescens seedlings in a low water potential condition. Additionally, the transcripts of many genes involved in photosynthetic electron transport and the C₄ pathway was rapidly induced, while other genes related to chlorophyll biosynthesis, electron transport and C₃ carbon fixation were later upregulated by 100 mM NaCl.

CONCLUSIONS

We identified many important candidate genes involved in the primary physiological mechanisms of A. canescens salt tolerance. This study provides excellent gene resources for genetic improvement of salt tolerance of important crops and forages.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na+ extrusion to the soil solution, K+ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na+:K+ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

The Effect of AtHKT1;1 or AtSOS1 Mutation on the Expressions of Na⁺ or K⁺ Transporter Genes and Ion Homeostasis in Arabidopsis thaliana under Salt Stress

HKT1 and SOS1 are two key Na⁺ transporters that modulate salt tolerance in plants. Although much is known about the respective functions of HKT1 and SOS1 under salt conditions, few studies have examined the effects of HKT1 and SOS1 mutations on the expression of other important Na⁺ and K⁺ transporter genes. This study investigated the physiological parameters and expression profiles of AtHKT1;1, AtSOS1, AtHAK5, AtAKT1, AtSKOR, AtNHX1, and AtAVP1 in wild-type (WT) and athkt1;1 and atsos1 mutants of Arabidopsis thaliana under 25 mM NaCl. We found that AtSOS1 mutation induced a significant decrease in transcripts of AtHKT1;1 (by 56⁻62% at 6⁻24 h), AtSKOR (by 36⁻78% at 6⁻24 h), and AtAKT1 (by 31⁻53% at 6⁻24 h) in the roots compared with WT. This led to an increase in Na⁺ accumulation in the roots, a decrease in K⁺ uptake and transportation, and finally resulted in suppression of plant growth. AtHKT1;1 loss induced a 39⁻76% (6⁻24 h) decrease and a 27⁻32% (6⁻24 h) increase in transcripts of AtSKOR and AtHAK5, respectively, in the roots compared with WT. At the same time, 25 mM NaCl decreased the net selective transport capacity for K⁺ over Na⁺ by 92% in the athkt1;1 roots compared with the WT roots. Consequently, Na⁺ was loaded into the xylem and delivered to the shoots, whereas K⁺ transport was restricted. The results indicate that AtHKT1;1 and AtSOS1 not only mediate Na⁺ transport but also control ion uptake and the spatial distribution of Na⁺ and K⁺ by cooperatively regulating the expression levels of relevant Na⁺ and K⁺ transporter genes, ultimately regulating plant growth under salt stress.

Phylogenetically diverse endophytic bacteria from desert plants induce transcriptional changes of tissue-specific ion transporters and salinity stress in Arabidopsis thaliana

Salinity severely hampers crop productivity worldwide and plant growth promoting bacteria could serve as a sustainable solution to improve plant growth under salt stress. However, the molecular mechanisms underlying salt stress tolerance promotion by beneficial bacteria remain unclear. In this work, six bacterial isolates from four different desert plant species were screened for their biochemical plant growth promoting traits and salinity stress tolerance promotion of the unknown host plant Arabidopsis thaliana. Five of the isolates induced variable root phenotypes but could all increase plant shoot and root weight under salinity stress. Inoculation of Arabidopsis with five isolates under salinity stress resulted in tissue-specific transcriptional changes of ion transporters and reduced Na⁺/K⁺ shoot ratios. The work provides first insights into the possible mechanisms and the commonality by which phylogenetically diverse bacteria from different desert plants induce salinity stress tolerance in Arabidopsis. The bacterial isolates provide new tools for studying abiotic stress tolerance mechanisms in plants and a promising agricultural solution for increasing crop yields in semi-arid regions.

Role and Functional Differences of HKT1-Type Transporters in Plants under Salt Stress

Abiotic stresses generally cause a series of morphological, biochemical and molecular changes that unfavorably affect plant growth and productivity. Among these stresses, soil salinity is a major threat that can seriously impair crop yield. To cope with the effects of high salinity on plants, it is important to understand the mechanisms that plants use to deal with it, including those activated in response to disturbed Na⁺ and K⁺ homeostasis at cellular and molecular levels. HKT1-type transporters are key determinants of Na⁺ and K⁺ homeostasis under salt stress and they contribute to reduce Na⁺-specific toxicity in plants. In this review, we provide a brief overview of the function of HKT1-type transporters and their importance in different plant species under salt stress. Comparison between HKT1 homologs in different plant species will shed light on different approaches plants may use to cope with salinity.

Lateral Transport of Organic and Inorganic Solutes

Organic (e.g., sugars and amino acids) and inorganic (e.g., K⁺, Na⁺, PO₄2-, and SO₄2-) solutes are transported long-distance throughout plants. Lateral movement of these compounds between the xylem and the phloem, and vice versa, has also been reported in several plant species since the 1930s, and is believed to be important in the overall resource allocation. Studies of Arabidopsis thaliana have provided us with a better knowledge of the anatomical framework in which the lateral transport takes place, and have highlighted the role of specialized vascular and perivascular cells as an interface for solute exchanges. Important breakthroughs have also been made, mainly in Arabidopsis, in identifying some of the proteins involved in the cell-to-cell translocation of solutes, most notably a range of plasma membrane transporters that act in different cell types. Finally, in the future, state-of-art imaging techniques should help to better characterize the lateral transport of these compounds on a cellular level. This review brings the lateral transport of sugars and inorganic solutes back into focus and highlights its importance in terms of our overall understanding of plant resource allocation.

Functional Analysis of Ion Transport Properties and Salt Tolerance Mechanisms of RtHKT1 from the Recretohalophyte Reaumuria trigyna

Reaumuria trigyna is an endangered recretohalophyte and a small archaic feral shrub that is endemic to arid and semi-arid plateau regions of Inner Mongolia, China. Based on transcriptomic data, we isolated a high-affinity potassium transporter gene (RtHKT1) from R. trigyna, which encoded a plasma membrane-localized protein. RtHKT1 was rapidly up-regulated by high Na+ or low K+ and exhibited different tissue-specific expression patterns before and after stress treatment. Transgenic yeast showed tolerance to high Na+ or low K+, while transgenic Arabidopsis exhibited tolerance to high Na+ and sensitivity to high K+, or high Na+-low K+, confirming that Na+ tolerance in transgenic Arabidopsis depends on a sufficient external K+ concentration. Under external high Na+, high K+ and low K+ conditions, transgenic yeast accumulated more Na+-K+, Na+ and K+, while transgenic Arabidopsis accumulated less Na+-more K+, more Na+ and more Na+-K+, respectively, indicating that the ion transport properties of RtHKT1 depend on the external Na+-K+ environment. Salt stress induced up-regulation of some ion transporter genes (AtSOS1/AtHAK5/AtKUP5-6), as well as down-regulation of some genes (AtNHX1/AtAVP1/AtKUP9-12), revealing that multi-ion-transporter synergism maintains Na+/K+ homeostasis under salt stress in transgenic Arabidopsis. Overexpression of RtHKT1 enhanced K+ accumulation and prevented Na+ transport from roots to shoots, improved biomass accumulation and Chl content in salt-stressed transgenic Arabidopsis. The proline content and relative water content increased significantly, and some proline biosynthesis genes (AtP5CS1 and AtP5CS2) were also up-regulated in salt-stressed transgenic plants. These results suggest that RtHKT1 confers salt tolerance on transgenic Arabidopsis by maintaining Na+/K+ homeostasis and osmotic homeostasis.

A Comprehensive Biophysical Model of Ion and Water Transport in Plant Roots. II. Clarifying the Roles of SOS1 in the Salt-Stress Response in Arabidopsis

SOS1 transporters play an essential role in plant salt tolerance. Although SOS1 is known to encode a plasma membrane Na⁺/H⁺ antiporter, the transport mechanisms by which these transporters contribute to salt tolerance at the level of the whole root are unclear. Gene expression and flux measurements have provided conflicting evidence for the location of SOS1 transporter activity, making it difficult to determine their function. Whether SOS1 transporters load or unload Na⁺ from the root xylem transpiration stream is also disputed. To address these areas of contention, we applied a mathematical model to answer the question: what is the function of SOS1 transporters in salt-stressed Arabidopsis roots? We used our biophysical model of ion and water transport in a salt-stressed root to simulate a wide range of SOS1 transporter locations in a model Arabidopsis root, providing a level of detail that cannot currently be achieved by experimentation. We compared our simulations with available experimental data to find reasonable parameters for the model and to determine likely locations of SOS1 transporter activity. We found that SOS1 transporters are likely to be operating in at least one tissue of the outer mature root, in the mature stele, and in the epidermis of the root apex. SOS1 transporter activity in the mature outer root cells is essential to maintain low cytosolic Na⁺ levels in the root and also restricts the uptake of Na⁺ to the shoot. SOS1 transporters in the stele actively load Na⁺ into the xylem transpiration stream, enhancing the transport of Na⁺ and water to the shoot. SOS1 transporters acting in the apex restrict cytosolic Na⁺ concentrations in the apex but are unable to maintain low cytosolic Na⁺ levels in the mature root. Our findings suggest that targeted, tissue-specific overexpression or knockout of SOS1 may lead to greater salt tolerance than has been achieved with constitutive gene changes. Tissue-specific changes to the expression of SOS1 could be used to identify the appropriate balance between limiting Na⁺ uptake to the shoot while maintaining water uptake, potentially leading to enhancements in salt tolerance.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na⁺ extrusion to the soil solution, K⁺ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na⁺:K⁺ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

High-Affinity K+ Transporters from a Halophyte, Sporobolus virginicus, Mediate Both K+ and Na+ Transport in Transgenic Arabidopsis, X. laevis Oocytes and Yeast

Class II high-affinity potassium transporters (HKTs) have been proposed to mediate Na+-K+ co-transport in plants, as well as Na+ and K+ homeostasis under K+-starved and saline environments. We identified class II HKTs, namely SvHKT2;1 and SvHKT2;2 (SvHKTs), from the halophytic turf grass, Sporobolus virginicus. SvHKT2;2 expression in S. virginicus was up-regulated by NaCl treatment, while SvHKT2;1 expression was assumed to be up-regulated by K+ starvation and down-regulated by NaCl treatment. Localization analysis revealed SvHKTs predominantly targeted the plasma membrane. SvHKTs complemented K+ uptake deficiency in mutant yeast, and showed both inward and outward K+ and Na+ transport activity in Xenopus laevis oocytes. When constitutively expressed in Arabidopsis, SvHKTs mediated K+ and Na+ accumulation in shoots under K+-starved conditions, and the K+ concentration in xylem saps of transformants was also higher than in those of wild-type plants. These results suggest transporter-enhanced K+ and Na+ uploading to the xylem from xylem parenchyma cells. Together, our data demonstrate that SvHKTs mediate both outward and inward K+ and Na+ transport in X. laevis oocytes, and possibly in plant and yeast cells, depending on the ionic conditions.

Salinity tolerance in Australian wild Oryza species varies widely and matches that observed in O. sativa

BACKGROUND

Soil salinity is widespread in rice-producing areas globally, restricting both vegetative growth and grain yield. Attempts to improve the salt tolerance of Asian rice, Oryza sativa-the most salt sensitive of the major cereal crops-have met with limited success, due to the complexity of the trait and finite variation in salt responses among O. sativa lines. Naturally occurring variation among the more than 20 wild species of the Oryza genus has great potential to provide breeders with novel genes to improve resistance to salt. Here, through two distinct screening experiments, we investigated variation in salinity tolerance among accessions of two wild rice species endemic to Australia, O. meridionalis and O. australiensis, with O. sativa cultivars Pokkali and IR29 providing salt-tolerant and sensitive controls, respectively.

RESULTS

Rice plants were grown on soil supplemented with field-relevant concentrations of NaCl (0, 40, 80, and 100 mM) for 30 d, a period sufficient to reveal differences in growth and physiological traits. Two complementary screening approaches were used: destructive phenotyping and high-throughput image-based phenotyping. All genotypes displayed clear responses to salt treatment. In the first experiment, both salt-tolerant Pokkali and an O. australiensis accession (Oa-VR) showed the least reduction in biomass accumulation, SES score and chlorophyll content in response to salinity. Average shoot Na+/K+ values of these plants were the lowest among the genotypes tested. In the second experiment, plant responses to different levels of salt stress were quantified over time based on projected shoot area calculated from visible red-green-blue (RGB) and fluorescence images. Pokkali grew significantly faster than the other genotypes. Pokkali and Oa-VR plants displayed the same absolute growth rate under 80 and 100 mM, while Oa-D grew significantly slower with the same treatments. Oa-VR showed substantially less inhibition of growth in response to salinity when compared with Oa-D. Senescence was seen in Oa-D after 30 d treatment with 40 mM NaCl, while the putatively salt-tolerant Oa-VR had only minor leaf damage, even at higher salt treatments, with less than a 40% increase in relative senescence at 100 mM NaCl compared to 120% for Oa-VR.

CONCLUSION

The combination of our two screening experiments uncovered striking levels of salt tolerance diversity among the Australian wild rice accessions tested and enabled analysis of their growth responses to a range of salt levels. Our results validate image-based phenotyping as a valuable tool for quantitative measurement of plant responses to abiotic stresses. They also highlight the potential of exotic germplasm to provide new genetic variation for salinity tolerance in rice.

Transcriptome analysis reveals complex response of the medicinal/ornamental halophyte Iris halophila Pall. to high environmental salinity

The remediation and subsequent use of saline-alkaline land are of great significance to ecological environment construction and sustainable agricultural development. Iris halophila Pall. is a salt-tolerant medicinal and ornamental plant, which has good application prospects in the ecological construction of saline-alkaline land; therefore, study of the molecular mechanisms of salt tolerance in I. halophila has important theoretical and practical value. To evaluate the molecular mechanism of the response of I. halophila to salt toxicity, I. halophila seedlings were treated with salt (300 mM NaCl) and subjected to deep RNA sequencing. The clean reads were obtained and assembled into 297,188 unigenes. Among them, 1120 and 100 salt-responsive genes were identified in I. halophila shoots and roots, respectively. Among them, the key flavonoid and lignin biosynthetic genes, hormone signaling genes, sodium/potassium ion transporter genes, and transcription factors were analyzed and summarized. Quantitative reverse-transcription PCR analysis strengthened the reliability of the RNA sequencing results. This work provides an overview of the transcriptomic responses to salt toxicity in I. halophila and identifies the responsive genes that may contribute to its reduced salt toxicity. These results lay an important foundation for further study of the molecular mechanisms of salt tolerance in I. halophila and related species.

Fluctuating selection on migrant adaptive sodium transporter alleles in coastal Arabidopsis thaliana

Stressors such as soil salinity and dehydration are major constraints on plant growth, causing worldwide crop losses. Compounding these insults, increasing climate volatility requires adaptation to fluctuating conditions. Salinity stress responses are relatively well understood in Arabidopsis thaliana, making this system suited for the rapid molecular dissection of evolutionary mechanisms. In a large-scale genomic analysis of Catalonian A. thaliana, we resequenced 77 individuals from multiple salinity gradients along the coast and integrated these data with 1,135 worldwide A. thaliana genomes for a detailed understanding of the demographic and evolutionary dynamics of naturally evolved salinity tolerance. This revealed that Catalonian varieties adapted to highly fluctuating soil salinity are not Iberian relicts but instead have immigrated to this region more recently. De novo genome assembly of three allelic variants of the high-affinity K⁺ transporter (HKT1;1) locus resolved structural variation between functionally distinct alleles undergoing fluctuating selection in response to seasonal changes in soil salinity. Plants harboring alleles responsible for low root expression of HKT1;1 and consequently high leaf sodium (HKT1;1 ^HLS ) were migrants that have moved specifically into areas where soil sodium levels fluctuate widely due to geography and rainfall variation. We demonstrate that the proportion of plants harboring HKT1;1 ^HLS alleles correlates with soil sodium level over time, HKT1;1 ^HLS -harboring plants are better adapted to intermediate levels of salinity, and the HKT1;1 ^HLS allele clusters with high-sodium accumulator accessions worldwide. Together, our evidence suggests that HKT1;1 is under fluctuating selection in response to climate volatility and is a worldwide determinant in adaptation to saline conditions.

AtHKT1 gene regulating K+ state in whole plant improves salt tolerance in transgenic tobacco plants

The status of K+ is important for plant health. However, little is known about if high-affinity potassium transporter HKTs may help K+ retention under salt stress. Here, we determined the effect of Arabidopsis thaliana transporter gene (AtHKT1) on the K+ status, Na+-induced toxicity, and salt tolerance in tobacco (Nicotiana tabacum L.). Six AtHKT1 transformed tobacco lines (T1, T2, … T6) were contrasted with a non-transgenic plantlet at the whole-plant and molecule levels. AtHKT1 gene was expressed in the xylems of stem, root and leaf vein in the transgenic tobacco, with the line T3 having highest expression. At Day 15, in the 200 mmol L-1 NaCl stress treatment, the transgenic plants remained a healthy K+ status, while the control plants decreased K+ content by 70% and Na+ contents in leaves and stems were 1.7 times that in the transgenic line. The AtHKT1 expression enhanced the activities of SOD, CAT and POD, raised chlorophyll and soluble sugar contents and root activity, and decreased MDA and proline contents and electrolyte leakage destruction. The constitutive over-expression of AtHKT1 that helps maintain a healthy K+ status while reducing Na+ toxicity may serve as a possible mechanism in maximizing productivity of tobacco under salt stress.

A Sodium Transporter HvHKT1;1 Confers Salt Tolerance in Barley via Regulating Tissue and Cell Ion Homeostasis

Our previous studies showed that high salt tolerance in Tibetan wild barley accessions was associated with HvHKT1;1, a member of the high-affinity potassium transporter family. However, molecular mechanisms of HvHKT1;1 for salt tolerance and its roles in K+/Na+ homeostasis remain to be elucidated. Functional characterization of HvHKT1;1 was conducted in the present study. NaCl-induced transcripts of HvHKT1;1 were significantly higher in the roots of Tibetan wild barley XZ16 relative to other genotypes, being closely associated with its higher biomass and lower tissue Na+ content under salt stress. Heterologous expression of HvHKT1;1 in Saccharomyces cerevisiae (yeast) and Xenopus laevis oocytes showed that HvHKT1;1 had higher selectivity for Na+ over K+ and other monovalent cations. HvHKT1;1 was found to be localized at the cell plasma membrane of root stele and epidermis. Knock-down of HvHKT1;1 in barley led to higher Na+ accumulation in both roots and leaves, while overexpression of HvHKT1;1 in salt-sensitive Arabidopsis hkt1-4 and sos1-12 loss-of-function lines resulted in significantly less shoot and root Na+ accumulation. Additionally, microelectrode ion flux measurements and root elongation assay revealed that the transgenic Arabidopsis plants exhibited a remarkable capacity for regulation of Na+, K+, Ca2+ and H+ homeostasis under salt stress. These results indicate that HvHKT1;1 is critical in radial root Na+ transport, which eventually reduces shoot Na+ accumulation. Additionally, HvHKT1;1 may be indirectly involved in retention of K+ and Ca2+ in root cells, which also improves plant salt tolerance.

Pumpkin CmHKT1;1 Controls Shoot Na⁺ Accumulation via Limiting Na⁺ Transport from Rootstock to Scion in Grafted Cucumber

Soil salinity adversely affects the growth and yield of crops, including cucumber, one of the most important vegetables in the world. Grafting with salt-tolerant pumpkin as the rootstock effectively improves the growth of cucumber under different salt conditions by limiting Na⁺ transport from the pumpkin rootstock to the cucumber scion. High-affinity potassium transporters (HKTs) are crucial for the long distance transport of Na⁺ in plants, but the function of pumpkin HKTs in this process of grafted cucumber plants remains unclear. In this work, we have characterized CmHKT1;1 as a member of the HKT gene family in Cucurbita moschata and observed an obvious upregulation of CmHKT1;1 in roots under NaCl stress conditions. Heterologous expression analyses in yeast mutants indicated that CmHKT1;1 is a Na⁺-selective transporter. The transient expression in tobacco epidermal cells and in situ hybridization showed CmHKT1;1 localization at plasma membrane, and preferential expression in root stele. Moreover, ectopic expression of CmHKT1;1 in cucumber decreased the Na⁺ accumulation in the plants shoots. Finally, the CmHKT1;1 transgenic line as the rootstock decreased the Na⁺ content in the wild type shoots. These findings suggest that CmHKT1;1 plays a key role in the salt tolerance of grafted cucumber by limiting Na⁺ transport from the rootstock to the scion and can further be useful for engineering salt tolerance in cucurbit crops.

Na+ extrusion from the cytosol and tissue-specific Na+ sequestration in roots confer differential salt stress tolerance between durum and bread wheat

The progress in plant breeding for salinity stress tolerance is handicapped by the lack of understanding of the specificity of salt stress signalling and adaptation at the cellular and tissue levels. In this study, we used electrophysiological, fluorescence imaging, and real-time quantitative PCR tools to elucidate the essentiality of the cytosolic Na+ extrusion in functionally different root zones (elongation, meristem, and mature) in a large number of bread and durum wheat accessions. We show that the difference in the root's ability for vacuolar Na+ sequestration in the mature zone may explain differential salinity stress tolerance between salt-sensitive durum and salt-tolerant bread wheat species. Bread wheat genotypes also had on average 30% higher capacity for net Na+ efflux from the root elongation zone, providing the first direct evidence for the essentiality of the root salt exclusion trait at the cellular level. At the same time, cytosolic Na+ accumulation in the root meristem was significantly higher in bread wheat, leading to the suggestion that this tissue may harbour a putative salt sensor. This hypothesis was then tested by investigating patterns of Na+ distribution and the relative expression level of several key genes related to Na+ transport in leaves in plants with intact roots and in those in which the root meristems were removed. We show that tampering with this sensing mechanism has resulted in a salt-sensitive phenotype, largely due to compromising the plant's ability to sequester Na+ in mesophyll cell vacuoles. The implications of these findings for plant breeding for salinity stress tolerance are discussed.

NtLTP4, a lipid transfer protein that enhances salt and drought stresses tolerance in Nicotiana tabacum

Lipid transfer proteins (LTPs), a class of small, ubiquitous proteins, play critical roles in various environmental stresses. However, their precise biological functions remain unknown. Here we isolated an extracellular matrix-localised LTP, NtLTP4, from Nicotiana tabacum. The overexpression of NtLTP4 in N. tabacum enhanced resistance to salt and drought stresses. Upon exposure to high salinity, NtLTP4-overexpressing lines (OE lines) accumulated low Na+ levels. Salt-responsive genes, including Na+/H+ exchangers (NHX1) and high-affinity K+ transporter1 (HKT1), were dramatically higher in OE lines than in wild-type lines. NtLTP4 might regulate transcription levels of NHX1 and HKT1 to alleviate the toxicity of Na+. Interestingly, OE lines enhanced the tolerance of N. tabacum to drought stress by reducing the transpiration rate. Moreover, NtLTP4 could increase reactive oxygen species (ROS)-scavenging enzyme activity and expression levels to scavenge excess ROS under drought and high salinity conditions. We used a two-hybrid yeast system and screened seven putative proteins that interact with NtLTP4 in tobacco. An MAPK member, wound-induced protein kinase, was confirmed to interact with NtLTP4 via co-immunoprecipitation and a firefly luciferase complementation imaging assay. Taken together, this is the first functional analysis of NtLTP4, and proves that NtLTP4 positively regulates salt and drought stresses in N. tabacum.

Arabidopsis thaliana Trihelix Transcription Factor AST1 Mediates Salt and Osmotic Stress Tolerance by Binding to a Novel AGAG-Box and Some GT Motifs

Trihelix transcription factors are characterized by containing a conserved trihelix (helix-loop-helix-loop-helix) domain that binds to GT elements required for light response, and they play roles in light stress and in abiotic stress responses. However, only a few of them have been functionally characterized. In the present study, we characterized the function of AST1 (Arabidopsis SIP1 clade Trihelix1) in response to salt and osmotic stress. AST1 shows transcriptional activation activity, and its expression is induced by osmotic and salt stress. A conserved sequence highly present in the promoters of genes regulated by AST1 was identified, which was bound by AST1, and termed the AGAG-box with the sequence [A/G][G/A][A/T]GAGAG. Additionally, AST1 also binds to some GT motifs including the sequence of GGTAATT, TACAGT, GGTAAAT and GGTAAA, but failed in binding to the sequence of GTTAC and GGTTAA. Chromatin immunoprecipitation combined with quantitative real-time reverse transcription-PCR analysis suggested that AST1 binds to the AGAG-box and/or some GT motifs to regulate the expression of stress tolerance genes, resulting in reduced reactive oxygen species, Na+ accumulation, stomatal apertures, lipid peroxidation, cell death and water loss rate, and increased proline content and reactive oxygen species scavenging capability. These physiological changes affected by AST1 finally improve salt and osmotic tolerance.

Mechanisms of Sodium Transport in Plants-Progresses and Challenges

Understanding the mechanisms of sodium (Na⁺) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na⁺ accumulation and assure the maintenance of low Na⁺ concentration in the cytosol and, hence, plant tolerance to salt stress. Na⁺ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na⁺ is compartmentalized into vacuoles by Na⁺/H⁺ exchangers (NHXs). Na⁺ efflux from the plant roots is mediated by the activity of Na⁺/H⁺ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na⁺, K⁺-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na⁺, K⁺-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na⁺ influx, compartmentalization, and efflux in higher plants in response to salt stress.

Salt tolerance of two perennial grass Brachypodium sylvaticum accessions

We studied the salt stress tolerance of two accessions isolated from different areas of the world (Norway and Tunisia) and characterized the mechanism(s) regulating salt stress in Brachypodium sylvaticum Osl1 and Ain1. Perennial grasses are widely grown in different parts of the world as an important feedstock for renewable energy. Their perennial nature that reduces management practices and use of energy and agrochemicals give these biomass crops advantages when dealing with modern agriculture challenges such as soil erosion, increase in salinized marginal lands and the runoff of nutrients. Brachypodium sylvaticum is a perennial grass that was recently suggested as a suitable model for the study of biomass plant production and renewable energy. However, its plasticity to abiotic stress is not yet clear. We studied the salt stress tolerance of two accessions isolated from different areas of the world and characterized the mechanism(s) regulating salt stress in B. sylvaticum Osl1, originated from Oslo, Norway and Ain1, originated from Ain-Durham, Tunisia. Osl1 limited sodium transport from root to shoot, maintaining a better K/Na homeostasis and preventing toxicity damage in the shoot. This was accompanied by higher expression of HKT8 and SOS1 transporters in Osl1 as compared to Ain1. In addition, Osl1 salt tolerance was accompanied by higher abundance of the vacuolar proton pump pyrophosphatase and Na⁺/H⁺ antiporters (NHXs) leading to a better vacuolar pH homeostasis, efficient compartmentation of Na⁺ in the root vacuoles and salt tolerance. Although preliminary, our results further support previous results highlighting the role of Na⁺ transport systems in plant salt tolerance. The identification of salt tolerant and sensitive B. sylvaticum accessions can provide an experimental system for the study of the mechanisms and regulatory networks associated with stress tolerance in perennials grass.

Functional differences in transport properties of natural HKT1;1 variants influence shoot Na+ exclusion in grapevine rootstocks

Under salinity, Vitis spp. rootstocks can mediate salt (NaCl) exclusion from grafted V. vinifera scions enabling higher grapevine yields and production of superior wines with lower salt content. Until now, the genetic and mechanistic elements controlling sodium (Na⁺ ) exclusion in grapevine were unknown. Using a cross between two Vitis interspecific hybrid rootstocks, we mapped a dominant quantitative trait locus (QTL) associated with leaf Na⁺ exclusion (NaE) under salinity stress. The NaE locus encodes six high-affinity potassium transporters (HKT). Transcript profiling and functional characterization in heterologous systems identified VisHKT1;1 as the best candidate gene for controlling leaf Na⁺ exclusion. We characterized four proteins encoded by unique VisHKT1;1 alleles from the parents, and revealed that the dominant HKT variants exhibit greater Na⁺ conductance with less rectification than the recessive variants. Mutagenesis of VisHKT1;1 and TaHKT1.5-D from bread wheat, demonstrated that charged amino acid residues in the eighth predicted transmembrane domain of HKT proteins reduces inward Na⁺ conductance, and causes inward rectification of Na⁺ transport. The origin of the recessive VisHKT1;1 alleles was traced to V. champinii and V. rupestris. We propose that the genetic and functional data presented here will assist with breeding Na⁺ -tolerant grapevine rootstocks.

T-DNA Tagging-Based Gain-of-Function of OsHKT1;4 Reinforces Na Exclusion from Leaves and Stems but Triggers Na Toxicity in Roots of Rice Under Salt Stress

The high affinity K⁺ transporter 1;4 (HKT1;4) in rice (Oryza sativa), which shows Na⁺ selective transport with little K⁺ transport activity, has been suggested to be involved in reducing Na in leaves and stems under salt stress. However, detailed physiological roles of OsHKT1;4 remain unknown. Here, we have characterized a transfer DNA (T-DNA) insertion mutant line of rice, which overexpresses OsHKT1;4, owing to enhancer elements in the T-DNA, to gain an insight into the impact of OsHKT1;4 on salt tolerance of rice. The homozygous mutant (the O/E line) accumulated significantly lower concentrations of Na in young leaves, stems, and seeds than the sibling WT line under salt stress. Interestingly, however, the mutation rendered the O/E plants more salt sensitive than WT plants. Together with the evaluation of biomass of rice lines, rhizosphere acidification assays using a pH indicator bromocresol purple and ²²NaCl tracer experiments have led to an assumption that roots of O/E plants suffered heavier damages from Na which excessively accumulated in the root due to increased activity of Na⁺ uptake and Na⁺ exclusion in the vasculature. Implications toward the application of the HKT1-mediated Na⁺ exclusion system to the breeding of salt tolerant crop cultivars will be discussed.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Humic Acid Confers HIGH-AFFINITY K+ TRANSPORTER 1-Mediated Salinity Stress Tolerance in Arabidopsis

Excessive salt disrupts intracellular ion homeostasis and inhibits plant growth, which poses a serious threat to global food security. Plants have adapted various strategies to survive in unfavorable saline soil conditions. Here, we show that humic acid (HA) is a good soil amendment that can be used to help overcome salinity stress because it markedly reduces the adverse effects of salinity on Arabidopsis thaliana seedlings. To identify the molecular mechanisms of HA-induced salt stress tolerance in Arabidopsis, we examined possible roles of a sodium influx transporter HIGH-AFFINITY K+ TRANSPORTER 1 (HKT1). Salt-induced root growth inhibition in HKT1 overexpressor transgenic plants (HKT1-OX) was rescued by application of HA, but not in wild-type and other plants. Moreover, salt-induced degradation of HKT1 protein was blocked by HA treatment. In addition, the application of HA to HKT1-OX seedlings led to increased distribution of Na+ in roots up to the elongation zone and caused the reabsorption of Na+ by xylem and parenchyma cells. Both the influx of the secondary messenger calcium and its cytosolic release appear to function in the destabilization of HKT1 protein under salt stress. Taken together, these results suggest that HA could be applied to the field to enhance plant growth and salt stress tolerance via post-transcriptional control of the HKT1 transporter gene under saline conditions.

AtHKT1 drives adaptation of Arabidopsis thaliana to salinity by reducing floral sodium content

Arabidopsis thaliana high-affinity potassium transporter 1 (AtHKT1) limits the root-to-shoot sodium transportation and is believed to be essential for salt tolerance in A. thaliana. Nevertheless, natural accessions with 'weak allele' of AtHKT1, e.g. Tsu-1, are mainly distributed in saline areas and are more tolerant to salinity. These findings challenge the role of AtHKT1 in salt tolerance and call into question the involvement of AtHKT1 in salinity adaptation in A. thaliana. Here, we report that AtHKT1 indeed drives natural variation in the salt tolerance of A. thaliana and the coastal AtHKT1, so-called weak allele, is actually hyper-functional in reducing flowers sodium content upon salt stress. Our data showed that AtHKT1 positively contributes to saline adaptation in a linear manner. Forward and reverse genetics analysis established that the single AtHKT1 locus is responsible for the variation in the salinity adaptation between Col-0 and Tsu-1. Reciprocal grafting experiments revealed that shoot AtHKT1 determines the salt tolerance of Tsu-1, whereas root AtHKT1 primarily drives the salt tolerance of Col-0. Furthermore, evidence indicated that Tsu-1 AtHKT1 is highly expressed in stems and is more effective compared to Col-0 AtHKT1 at limiting sodium flow to the flowers. Such efficient retrieval of sodium to the reproductive organ endows Tsu-1 with stronger fertility compared to Col-0 upon salt stress, thus improving Tsu-1 adaptation to a coastal environment. To conclude, our data not only confirm the role of AtHKT1 in saline adaptation, but also sheds light on our understanding of the salt tolerance mechanisms in plants.

Arbuscular Mycorrhizal Symbiosis Alleviates Salt Stress in Black Locust through Improved Photosynthesis, Water Status, and K+/Na+ Homeostasis

Soil salinization and the associated land degradation are major and growing ecological problems. Excess salt in soil impedes plant photosynthetic processes and root uptake of water and nutrients such as K+. Arbuscular mycorrhizal (AM) fungi can mitigate salt stress in host plants. Although, numerous studies demonstrate that photosynthesis and water status are improved by mycorrhizae, the molecular mechanisms involved have received little research attention. In the present study, we analyzed the effects of AM symbiosis and salt stress on photosynthesis, water status, concentrations of Na+ and K+, and the expression of several genes associated with photosynthesis (RppsbA, RppsbD, RprbcL, and RprbcS) and genes coding for aquaporins or membrane transport proteins involved in K+ and/or Na+ uptake, translocation, or compartmentalization homeostasis (RpSOS1, RpHKT1, RpNHX1, and RpSKOR) in black locust. The results showed that salinity reduced the net photosynthetic rate, stomatal conductance, and relative water content in both non-mycorrhizal (NM) and AM plants; the reductions of these three parameters were less in AM plants compared with NM plants. Under saline conditions, AM fungi significantly improved the net photosynthetic rate, quantum efficiency of photosystem II photochemistry, and K+ content in plants, but evidently reduced the Na+ content. AM plants also displayed a significant increase in the relative water content and an evident decrease in the shoot/root ratio of Na+ in the presence of 200 mM NaCl compared with NM plants. Additionally, mycorrhizal colonization upregulated the expression of three chloroplast genes (RppsbA, RppsbD, and RprbcL) in leaves, and three genes (RpSOS1, RpHKT1, and RpSKOR) encoding membrane transport proteins involved in K+/Na+ homeostasis in roots. Expression of several aquaporin genes was regulated by AM symbiosis in both leaves and roots depending on soil salinity. This study suggests that the beneficial effects of AM symbiosis on the photosynthetic capacity, water status, and K+/Na+ homeostasis lead to the improved growth performance and salt tolerance of black locust exposed to salt stress.

Engineering crop nutrient efficiency for sustainable agriculture

Increasing crop yields can provide food, animal feed, bioenergy feedstocks and biomaterials to meet increasing global demand; however, the methods used to increase yield can negatively affect sustainability. For example, application of excess fertilizer can generate and maintain high yields but also increases input costs and contributes to environmental damage through eutrophication, soil acidification and air pollution. Improving crop nutrient efficiency can improve agricultural sustainability by increasing yield while decreasing input costs and harmful environmental effects. Here, we review the mechanisms of nutrient efficiency (primarily for nitrogen, phosphorus, potassium and iron) and breeding strategies for improving this trait, along with the role of regulation of gene expression in enhancing crop nutrient efficiency to increase yields. We focus on the importance of root system architecture to improve nutrient acquisition efficiency, as well as the contributions of mineral translocation, remobilization and metabolic efficiency to nutrient utilization efficiency.

The endophytic fungus Piriformospora indica enhances Arabidopsis thaliana growth and modulates Na+/K+ homeostasis under salt stress conditions

The mutualistic, endophytic fungus Piriformospora indica has been shown to confer biotic and abiotic stress tolerance to host plants. In this study, we investigated the impact of P. indica on the growth of Arabidopsis plants under normal and salt stress conditions. Our results demonstrate that P. indica colonization increases plant biomass, lateral roots density, and chlorophyll content under both conditions. Colonization with P. indica under salt stress was accompanied by a lower Na⁺/K⁺ ratio and less pronounced accumulation of anthocyanin, compared to control plants. Moreover, P. indica colonized roots under salt stress showed enhanced transcript levels of the genes encoding the high Affinity Potassium Transporter 1 (HKT1) and the inward-rectifying K⁺ channels KAT1 and KAT2, which play key roles in regulating Na⁺ and K⁺ homeostasis. The effect of P. indica colonization on AtHKT1;1 expression was also confirmed in the Arabidopsis line gl1-HKT:AtHKT1;1 that expresses an additional AtHKT1;1 copy driven by the native promoter. Colonization of the gl1-HKT:AtHKT1;1 by P. indica also increased lateral roots density and led to a better Na⁺/K⁺ ratio, which may be attributed to the observed increase in KAT1 and KAT2 transcript levels. Our findings demonstrate that P. indica colonization promotes Arabidopsis growth under salt stress conditions and that this effect is likely caused by modulation of the expression levels of the major Na⁺ and K⁺ ion channels, which allows establishing a balanced ion homeostasis of Na⁺/K⁺ under salt stress conditions.

OsHKT1;5 mediates Na+ exclusion in the vasculature to protect leaf blades and reproductive tissues from salt toxicity in rice

Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC1 locus, which is involved in a higher K⁺ /Na⁺ ratio in shoots, corresponds to the OsHKT1;5 gene encoding a Na⁺ -selective transporter. However, physiological roles of OsHKT1;5 in rice exposed to salt stress remain elusive, and no OsHKT1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T-DNA insertional OsHKT1;5 mutants. Measurements of ion contents in tissues and ²² Na⁺ tracer imaging experiments showed that loss-of-function of OsHKT1;5 in salt-stressed rice roots triggers massive Na⁺ accumulation in shoots. Salt stress-induced increases in the OsHKT1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno-staining using an anti-OsHKT1;5 peptide antibody indicated that OsHKT1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of ²² Na⁺ tracer to leaf sheaths also demonstrated the involvement of OsHKT1;5 in xylem Na⁺ unloading in leaf sheaths. Furthermore, OsHKT1;5 was indicated to be present in the plasma membrane and found to localize also in the phloem of diffuse vascular bundles in basal nodes. Together with the characteristic ²² Na⁺ allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of OsHKT1;5 in mediating Na⁺ exclusion in the phloem to prevent Na⁺ transfer to young leaf blades. Our findings further demonstrate that the function of OsHKT1;5 is crucial over growth stages of rice, including the protection of the next generation seeds as well as of vital leaf blades under salt stress.

A cocktail of volatile compounds emitted from Alcaligenes faecalis JBCS1294 induces salt tolerance in Arabidopsis thaliana by modulating hormonal pathways and ion transporters

In our previous study we showed that volatile organic compounds (VOCs) from Alcaligenes faecalis JBCS1294 (JBCS1294) induced tolerance to salt stress in Arabidopsis thaliana by influencing the auxin and gibberellin pathways and upregulating the expression of key ion transporters. The aim of this study was to evaluate the contribution of each VOC and blends of the VOCs on the induction of salt tolerance and signaling pathways. The key VOCs emitted from JBCS1294 were dissolved in lanolin and applied to one side of bipartite I-plates that contained Arabidopsis seeds on Murashige and Skoog (MS) media supplemented with NaCl on the other side. Changes in plant growth were investigated using Arabidopsis mutant lines and hormone inhibitors, and gene expression was assessed by real-time PCR (qPCR). Among the VOCs, butyric acid conferred salt tolerance over a concentration range of 5.6μM (10ng)-56mM (100μg), whereas propionic and benzoic acid were effective at micromolar doses. Intriguingly, the optimized cocktail of the three VOCs increased fresh weight of Arabidopsis under salt stress compared to that achieved with each single compound. However, Arabidopsis growth was not promoted by the VOCs without salt stress. Exogenous indole-3-acetic acid (IAA) application arrested salt tolerance or growth promotion of Arabidopsis induced by volatiles from propionic acid, but not from butyric acid and an optimized volatile mixture of butyric acid, propionic acid, and benzoic acid (1PBB). High and intense auxin-responsive DR5:GUS activity was observed in the roots of Arabidopsis grown on media without salt via 1PBB, butyric acid, and benzoic acid. Growth promotion by the cocktail was inhibited in the eir1 mutant and in Col-0 plants treated with inhibitors of auxin and gibberellin. The present study clearly demonstrated the effects of individual VOCs and blends of VOCs from a rhizobacterial strain on the induction of salt stress. The results with the blend of VOCs, which mimics bacterial emissions in nature, may lead to a deeper understanding of the interaction between rhizobacteria and plants.

Nitrate-dependent shoot sodium accumulation and osmotic functions of sodium in Arabidopsis under saline conditions

Improving crop plants to be productive in saline soils or under irrigation with saline water would be an important technological advance in overcoming the food and freshwater crises that threaten the world population. However, even if the transformation of a glycophyte into a plant that thrives under seawater irrigation was biologically feasible, current knowledge about Na⁺ effects would be insufficient to support this technical advance. Intriguingly, crucial details about Na⁺ uptake and its function in the plant have not yet been well established. We here propose that under saline conditions two nitrate-dependent transport systems in series that take up and load Na⁺ into the xylem constitute the major pathway for the accumulation of Na⁺ in Arabidopsis shoots; this pathway can also function with chloride at high concentrations. In nrt1.1 nitrate transport mutants, plant Na⁺ accumulation was partially defective, which suggests that NRT1.1 either partially mediates or modulates the nitrate-dependent Na⁺ transport. Arabidopsis plants exposed to an osmotic potential of -1.0 MPa (400 mOsm) for 24 h showed high water loss and wilting in sorbitol or Na/MES, where Na⁺ could not be accumulated. In contrast, in NaCl the plants that accumulated Na⁺ lost a low amount of water, and only suffered transitory wilting. We discuss that in Arabidopsis plants exposed to high NaCl concentrations, root Na⁺ uptake and tissue accumulation fulfil the primary function of osmotic adjustment, even if these processes lead to long-term toxicity.

Physiological and molecular mechanisms mediating xylem Na+ loading in barley in the context of salinity stress tolerance

Time-dependent kinetics of xylem Na⁺ loading was investigated using a large number of barley genotypes contrasting in their salinity tolerance. Salt-sensitive varieties were less efficient in controlling xylem Na⁺ loading and showed a gradual increase in the xylem Na⁺ content over the time. To understand underlying ionic and molecular mechanisms, net fluxes of Ca²⁺ , K⁺ and Na⁺ were measured from the xylem parenchyma tissue in response to H₂ O₂ and ABA; both of them associated with salinity stress signalling. Our results indicate that NADPH oxidase-mediated apoplastic H₂ O₂ production acts upstream of the xylem Na⁺ loading and is causally related to ROS-inducible Ca²⁺ uptake systems in the root stelar tissue. It was also found that ABA regulates (directly or indirectly) the process of Na⁺ retrieval from the xylem and the significant reduction of Na⁺ and K⁺ fluxes induced by bumetanide are indicative of a major role of chloride cation co-transporter (CCC) on xylem ion loading. Transcript levels of HvHKT1;5_like and HvSOS1_like genes in the root stele were observed to decrease after salt stress, while there was an increase in HvSKOR_like gene, indicating that these ion transporters are involved in primary Na⁺ /K⁺ movement into/out of xylem.

Physiological, Biochemical, Epigenetic and Molecular Analyses of Wheat (Triticum aestivum) Genotypes with Contrasting Salt Tolerance

Abiotic stress exerts significant impact on plant's growth, development, and productivity. Productivity of crop plants under salt stress is lagging behind because of our limited knowledge about physiological, biochemical, epigenetic, and molecular mechanisms of salt tolerance in plants. This study aimed to investigate physio-biochemical, molecular indices and defense responses of selected wheat cultivars to identify the most contrasting salt-responsive genotypes and the mechanisms associated with their differential responses. Physio-biochemical traits specifically membrane stability index, antioxidant potential, osmoprotectants and chlorophyll contents, measured at vegetative stage, were used for multivariate analysis to identify the most contrasting genotypes. Genetic and epigenetic analyses indicated the possible mechanisms associated with differential response of the wheat genotypes under salt stress. Better antioxidant potential, membrane stability, increased accumulation of osmolytes/phytophenolics, and higher K+/Na+ ratio under 200 mM NaCl stress identified Kharchia-65 to be the most salt-tolerant cultivar. By contrast, increased MDA level, reduced soluble sugar, proline, total chlorophyll, total phenolics contents, and lower antioxidant potential in HD-2329 marked it to be sensitive to the stress. Genetic and bioinformatics analyses of HKT1;4 of contrasting genotypes (Kharchia-65 and HD-2329) revealed deletions, transitions, and transversions resulting into altered structure, loss of conserved motifs (Ser-Gly-Gly-Gly and Gly-Arg) and function in salt-sensitive (HD-2329) genotype. Expression analysis of HKTs rationalized the observed responses. Epigenetic variations in cytosine methylation explained tissue- and genotype-specific differential expression of HKT2;1 and HKT2;3.

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.

Physiological and molecular insights into the high salinity tolerance of Pongamia pinnata (L.) pierre, a potential biofuel tree species

Soil salinity is gradually becoming a threat to the global economy by affecting agricultural productivity worldwide. Here, we analyze the salinity tolerance of Pongamia pinnata with an insight into the underlying physiological and molecular responses. Despite a reduction in net photosynthetic rate, P. pinnata efficiently maintained its leaf water potentials even at 500mM NaCl for 15days and displayed no visible stress symptoms. Na⁺ localization analysis using CoroNa-Green AM revealed effective Na⁺ sequestration in the roots when compared to leaves. Elemental analysis demonstrated that roots accumulated more of Na⁺ while K⁺ content was higher in leaves. At the molecular level, salt stress significantly induced the expression levels of salt overly sensitive1 (SOS1), SOS2, SOS3, high affinity K⁺ transporter (HKT1), ABA biosynthetic and receptor genes (NCED and PYL4), guaiacol peroxidase (POD) exclusively in roots while tonoplast localized Na⁺/H⁺ exchanger (NHX1) was significantly enhanced in leaves. Our results clearly demonstrate that leaves and roots of Pongamia exhibit differential responses under salt stress although roots are more efficient in sequestering the Na⁺ ions. The present study provides crucial inputs for understanding salt tolerance in a tree species which can be further utilized for developing salt tolerance in higher plants.

The sodium transporter encoded by the HKT1;2 gene modulates sodium/potassium homeostasis in tomato shoots under salinity

Excessive soil salinity diminishes crop yield and quality. In a previous study in tomato, we identified two closely linked genes encoding HKT1-like transporters, HKT1;1 and HKT1;2, as candidate genes for a major quantitative trait locus (kc7.1) related to shoot Na⁺ /K⁺ homeostasis - a major salt tolerance trait - using two populations of recombinant inbred lines (RILs). Here, we determine the effectiveness of these genes in conferring improved salt tolerance by using two near-isogenic lines (NILs) that were homozygous for either the Solanum lycopersicum allele (NIL17) or for the Solanum cheesmaniae allele (NIL14) at both HKT1 loci; transgenic lines derived from these NILs in which each HKT1;1 and HKT1;2 had been silenced by stable transformation were also used. Silencing of ScHKT1;2 and SlHKT1;2 altered the leaf Na⁺ /K⁺ ratio and caused hypersensitivity to salinity in plants cultivated under transpiring conditions, whereas silencing SlHKT1;1/ScHKT1;1 had a lesser effect. These results indicate that HKT1;2 has the more significant role in Na⁺ homeostasis and salinity tolerance in tomato.

Genomics, Physiology, and Molecular Breeding Approaches for Improving Salt Tolerance

Salt stress reduces land and water productivity and contributes to poverty and food insecurity. Increased salinization caused by human practices and climate change is progressively reducing agriculture productivity despite escalating calls for more food. Plant responses to salt stress are well understood, involving numerous critical processes that are each controlled by multiple genes. Knowledge of the critical mechanisms controlling salt uptake and exclusion from functioning tissues, signaling of salt stress, and the arsenal of protective metabolites is advancing. However, little progress has been made in developing salt-tolerant varieties of crop species using standard (but slow) breeding approaches. The genetic diversity available within cultivated crops and their wild relatives provides rich sources for trait and gene discovery that has yet to be sufficiently utilized. Transforming this knowledge into modern approaches using genomics and molecular tools for precision breeding will accelerate the development of tolerant cultivars and help sustain food production.

Salt and heavy metal tolerance and expression levels of candidate tolerance genes among four extremophile Cochlearia species with contrasting habitat preferences

To test the concept of a general "mineral stress tolerance", we compared four extremophile Cochlearia species for salt (NaCl), zinc (Zn) and cadmium (Cd) tolerance and accumulation, and for expression of candidate tolerance genes for salt and Zn tolerance. Salt tolerance decreased in the order C. anglica>C. x hollandica>C. danica>C. pyrenaica, corresponding with the average salinity levels in the species' natural environments. The glycophytic metallophyte, C. pyrenaica, showed a relatively high level of salt tolerance, compared to other glycophytic Brassicaceae. Salt tolerance was positively correlated with HKT1 expression and the K⁺ concentration in roots under salt exposure, but uncorrelated with the Na⁺ concentrations in roots and shoots. All the species accumulated Na⁺ primarily in their leaves, and exhibited a high NHX1 expression in leaves, in comparison with other glycophytic Brassicaceae, suggesting that salt tolerance in Cochlearia is based on an efficient vacuolar sequestration of Na⁺ in leaves. The metallicolous C. pyrenaica population was hypertolerant to Zn, but not to Cd, in comparison with the other Cochlearia species. All the Cochlearia species accumulated Zn and Cd primarily in roots, and showed high levels of Cd and Zn tolerance, with unusually low rates of metal accumulation, in comparison with non-metallophytes, or non-metallicolous metallophyte populations, of species belonging to other genera or families. Although Cochlearia, as a genus, shows relatively high levels of tolerance to both salt and heavy metals, this is most probably not due to a common 'mineral stress tolerance' mechanism.

Salt-Induced Tissue-Specific Cytosine Methylation Downregulates Expression of HKT Genes in Contrasting Wheat (Triticum aestivum L.) Genotypes

Plants have evolved several strategies, including regulation of genes through epigenetic modifications, to cope with environmental stresses. DNA methylation is dynamically regulated through the methylation and demethylation of cytosine in response to environmental perturbations. High-affinity potassium transporters (HKTs) have accounted for the homeostasis of sodium and potassium ions in plants under salt stress. Wheat (Triticum aestivum L.) is sensitive to soil salinity, which impedes its growth and development, resulting in decreased productivity. The differential expression of HKTs has been reported to confer tolerance to salt stress in plants. In this study, we investigated variations in cytosine methylation and their effects on the expression of HKT genes in contrasting wheat genotypes under salt stress. We observed a genotype- and tissue-specific increase in cytosine methylation induced by NaCl stress that downregulated the expression of TaHKT2;1 and TaHKT2;3 in the shoot and root tissues of Kharchia-65, thereby contributing to its improved salt-tolerance ability. Although TaHKT1;4 was expressed only in roots and was downregulated under the stress in salt-tolerant genotypes, it was not regulated through variations in cytosine methylation. Thus, understanding epigenetic regulation and the function of HKTs would enable an improvement in salt tolerance and the development of salt-tolerant crops.

Regulation of Na+ and K+ homeostasis in plants: towards improved salt stress tolerance in crop plants

Soil salinity is a major abiotic stress that results in considerable crop yield losses worldwide. However, some plant genotypes show a high tolerance to soil salinity, as they manage to maintain a high K⁺/Na⁺ ratio in the cytosol, in contrast to salt stress susceptible genotypes. Although, different plant genotypes show different salt tolerance mechanisms, they all rely on the regulation and function of K⁺ and Na⁺ transporters and H⁺ pumps, which generate the driving force for K⁺ and Na⁺ transport. In this review we will introduce salt stress responses in plants and summarize the current knowledge about the most important ion transporters that facilitate intra- and intercellular K⁺ and Na⁺ homeostasis in these organisms. We will describe and discuss the regulation and function of the H⁺-ATPases, H⁺-PPases, SOS1, HKTs, and NHXs, including the specific tissues where they work and their response to salt stress.

Genome-wide expression profiling in leaves and roots of date palm (Phoenix dactylifera L.) exposed to salinity

BACKGROUND

Date palm, as one of the most important fruit crops in North African and West Asian countries including Oman, is facing serious growth problems due to salinity, arising from persistent use of saline water for irrigation. Although date palm is a relatively salt-tolerant plant species, its adaptive mechanisms to salt stress are largely unknown.

RESULTS

In order to get an insight into molecular mechanisms of salt tolerance, RNA was profiled in leaves and roots of date palm seedlings subjected to NaCl for 10 days. Under salt stress, photosynthetic parameters were differentially affected; all gas exchange parameters were decreased but the quantum yield of PSII was unaffected while non-photochemical quenching was increased. Analyses of gene expression profiles revealed 2630 and 4687 genes were differentially expressed in leaves and roots, respectively, under salt stress. Of these, 194 genes were identified as commonly responding in both the tissue sources. Gene ontology (GO) analysis in leaves revealed enrichment of transcripts involved in metabolic pathways including photosynthesis, sucrose and starch metabolism, and oxidative phosphorylation, while in roots genes involved in membrane transport, phenylpropanoid biosynthesis, purine, thiamine, and tryptophan metabolism, and casparian strip development were enriched. Differentially expressed genes (DEGs) common to both tissues included the auxin responsive gene, GH3, a putative potassium transporter 8 and vacuolar membrane proton pump.

CONCLUSIONS

Leaf and root tissues respond differentially to salinity stress and this study has revealed genes and pathways that are associated with responses to elevated NaCl levels and thus may play important roles in salt tolerance providing a foundation for functional characterization of salt stress-responsive genes in the date palm.

Cloning and functional characterization of HKT1 and AKT1 genes of Fragaria spp.-Relationship to plant response to salt stress

Commercial strawberry, Fragaria x ananassa Duch., is a species sensitive to salinity. Under saline conditions, Na⁺ uptake by the plant is increased, while K⁺ uptake is significantly reduced. Maintaining an adequate K⁺/Na⁺ cytosolic ratio determines the ability of the plant to survive in saline environments. The goal of the present work was to clone and functionally characterize the genes AKT1 and HKT1 involved in K⁺ and Na⁺ transport in strawberry and to determine the relationship of these genes with the responses of three Fragaria spp. genotypes having different ecological adaptations to salt stress. FaHKT1 and FcHKT1 proteins from F. x ananassa and F. chiloensis have 98.1% of identity, while FaAKT1 and FcAKT1 identity is 99.7%. FaHKT1 and FaAKT1 from F. x ananassa, were functionally characterized in Xenopus oocytes. FaHKT1, belongs to the group I of HKT transporters and is selective for Na⁺. Expression of FaAKT1 in oocytes showed that the protein is a typical inward-rectifying and highly K⁺-selective channel. The relative expression of Fragaria HKT1 and AKT1 genes was studied in roots of F. x ananassa cv. Camarosa and of F. chiloensis (accessions Bau and Cucao) grown under salt stress. The expression of AKT1 was transiently increased in 'Camarosa', decreased in 'Cucao' and was not affected in 'Bau' upon salt stress. HKT1 expression was significantly increased in roots of 'Cucao' and was not affected in the other two genotypes. The increased relative expression of HKT1 and decreased expression of AKT1 in 'Cucao' roots correlates with the higher tolerance to salinity of this genotype in comparison with 'Camarosa' and 'Bau'.

An Insight into microRNA156 Role in Salinity Stress Responses of Alfalfa

Salinity is one of the major abiotic stresses affecting alfalfa productivity. Developing salinity tolerant alfalfa genotypes could contribute to sustainable crop production. The functions of microRNA156 (miR156) have been investigated in several plant species, but so far, no studies have been published that explore the role of miR156 in alfalfa response to salinity stress. In this work, we studied the role of miR156 in modulating commercially important traits of alfalfa under salinity stress. Our results revealed that overexpression of miR156 increased biomass, number of branches and time to complete growth stages, while it reduced plant height under control and salinity stress conditions. We observed a miR156-related reduction in neutral detergent fiber under non-stress, and acid detergent fiber under mild salinity stress conditions. In addition, enhanced total Kjeldahl nitrogen content was recorded in miR156 overexpressing genotypes under severe salinity stress. Furthermore, alfalfa genotypes overexpressing miR156 exhibited an altered ion homeostasis under salinity conditions. Under severe salinity stress, miR156 downregulated SPL transcription factor family genes, modified expression of other important transcription factors, and downstream salt stress responsive genes. Taken together, our results reveal that miR156 plays a role in mediating physiological and transcriptional responses of alfalfa to salinity stress.

The Rice High-Affinity K+ Transporter OsHKT2;4 Mediates Mg2+ Homeostasis under High-Mg2+ Conditions in Transgenic Arabidopsis

Rice (Oryza sativa; background Nipponbare) contains nine HKT (high-affinity K⁺ transport)-like genes encoding membrane proteins belonging to the superfamily of Ktr/TRK/HKT. OsHKTs have been proposed to include four selectivity filter-pore-forming domains homologous to the bacterial K⁺ channel KcsA, and are separated into OsHKT1s with Na⁺-selective activity and OsHKT2s with Na⁺-K⁺ symport activity. As a member of the OsHKT2 subfamily, OsHKT2;4 renders Mg²⁺ and Ca²⁺ permeability for yeast cells and Xenopus laevis oocytes, besides K⁺ and Na⁺. However, physiological functions related to Mg²⁺in planta have not yet been identified. Here we report that OsHKT2;4 from rice (O. sativa; background Nipponbare) functions as a low-affinity Mg²⁺ transporter to mediate Mg²⁺ homeostasis in plants under high-Mg²⁺ environments. Using the functional complementation assay in Mg²⁺-uptake deficient Salmonella typhimurium strains MM281 and electrophysiological analysis in X. laevis oocytes, we found that OsHKT2;4 could rescue the growth of MM281 in Mg²⁺-deficient conditions and induced the Mg²⁺ currents in oocytes at millimolar range of Mg²⁺. Additionally, overexpression of OsHKT2;4 to Arabidopsis mutant lines with a knockout of AtMGT6, a gene encoding the transporter protein necessary for Mg²⁺ adaptation in Arabidopsis, caused the Mg²⁺ toxicity to the leaves under the high-Mg²⁺ stress, but not under low-Mg²⁺ environments. Moreover, this Mg²⁺ toxicity symptom resulted from the excessive Mg²⁺ translocation from roots to shoots, and was relieved by the increase in supplemental Ca²⁺. Together, our results demonstrated that OsHKT2;4 is a low-affinity Mg²⁺ transporter responsible for Mg²⁺ transport to aerials in plants under high-Mg²⁺ conditions.

Evaluating physiological responses of plants to salinity stress

BACKGROUND

Because soil salinity is a major abiotic constraint affecting crop yield, much research has been conducted to develop plants with improved salinity tolerance. Salinity stress impacts many aspects of a plant's physiology, making it difficult to study in toto Instead, it is more tractable to dissect the plant's response into traits that are hypothesized to be involved in the overall tolerance of the plant to salinity.

SCOPE AND CONCLUSIONS

We discuss how to quantify the impact of salinity on different traits, such as relative growth rate, water relations, transpiration, transpiration use efficiency, ionic relations, photosynthesis, senescence, yield and yield components. We also suggest some guidelines to assist with the selection of appropriate experimental systems, imposition of salinity stress, and obtaining and analysing relevant physiological data using appropriate indices. We illustrate how these indices can be used to identify relationships amongst the proposed traits to identify which traits are the most important contributors to salinity tolerance. Salinity tolerance is complex and involves many genes, but progress has been made in studying the mechanisms underlying a plant's response to salinity. Nevertheless, several previous studies on salinity tolerance could have benefited from improved experimental design. We hope that this paper will provide pertinent information to researchers on performing proficient assays and interpreting results from salinity tolerance experiments.

Arbuscular Mycorrhizal Symbiosis Alleviates Salt Stress in Black Locust through Improved Photosynthesis, Water Status, and K+/Na+ Homeostasis

Soil salinization and the associated land degradation are major and growing ecological problems. Excess salt in soil impedes plant photosynthetic processes and root uptake of water and nutrients such as K⁺. Arbuscular mycorrhizal (AM) fungi can mitigate salt stress in host plants. Although, numerous studies demonstrate that photosynthesis and water status are improved by mycorrhizae, the molecular mechanisms involved have received little research attention. In the present study, we analyzed the effects of AM symbiosis and salt stress on photosynthesis, water status, concentrations of Na⁺ and K⁺, and the expression of several genes associated with photosynthesis (RppsbA, RppsbD, RprbcL, and RprbcS) and genes coding for aquaporins or membrane transport proteins involved in K⁺ and/or Na⁺ uptake, translocation, or compartmentalization homeostasis (RpSOS1, RpHKT1, RpNHX1, and RpSKOR) in black locust. The results showed that salinity reduced the net photosynthetic rate, stomatal conductance, and relative water content in both non-mycorrhizal (NM) and AM plants; the reductions of these three parameters were less in AM plants compared with NM plants. Under saline conditions, AM fungi significantly improved the net photosynthetic rate, quantum efficiency of photosystem II photochemistry, and K⁺ content in plants, but evidently reduced the Na⁺ content. AM plants also displayed a significant increase in the relative water content and an evident decrease in the shoot/root ratio of Na⁺ in the presence of 200 mM NaCl compared with NM plants. Additionally, mycorrhizal colonization upregulated the expression of three chloroplast genes (RppsbA, RppsbD, and RprbcL) in leaves, and three genes (RpSOS1, RpHKT1, and RpSKOR) encoding membrane transport proteins involved in K⁺/Na⁺ homeostasis in roots. Expression of several aquaporin genes was regulated by AM symbiosis in both leaves and roots depending on soil salinity. This study suggests that the beneficial effects of AM symbiosis on the photosynthetic capacity, water status, and K⁺/Na⁺ homeostasis lead to the improved growth performance and salt tolerance of black locust exposed to salt stress.

Improvement of Salt Tolerance Using Wild Rice Genes

Salt stress causes significant reductions in rice production worldwide; thus, improving salt tolerance is a promising approach to meet the increasing food demand. Wild rice germplasm is considered a valuable genetic resource for improving rice cultivars. However, information regarding the improvement of salt tolerance in cultivated rice using wild rice genes is limited. In this study, we identified a salt-tolerant line Dongxiang/Ningjing 15 (DJ15) under salt-stress field conditions from the population of a salt tolerant Dongxiang wild rice × a cultivated rice variety Ningjing16 (NJ16). Genomic resequencing analysis of NJ16, DJ15 and Dongxiang wild rice revealed that the introgressed genomic fragments were unevenly distributed over the 12 chromosomes (Chr.) and mainly identified on Chr. 6, 7, 10, and 11. Using quantitative trait locus (QTL) mapping, we found 9 QTL for salt tolerance (qST) at the seedling stage located on Chr. 1, 3, 4, 5, 6, 8, and 10. In addition, sequence variant analysis within the QTL regions demonstrated that SKC1/HKT8/HKT1;5 and HAK6 transporters along with numerous transcriptional factors were the candidate genes for the salt tolerant QTL. The DJ15/Koshihikari recombinant inbred lines that contained both qST1.2 and qST6, two QTL with the highest effect for salt tolerance, were more tolerant than the parental lines under salt-stress field conditions. Furthermore, the qST6 near-isogenic lines with IR29 background were more tolerant than IR29, indicating that qST1.2 and qST6 could improve salt tolerance in rice. Overall, our study indicates that wild rice genes could markedly improve the salt tolerance of cultivated rice.

The coordinated regulation of Na+ and K+ in Hordeum brevisubulatum responding to time of salt stress

Hordeum brevisubulatum, called as wild barley, is a useful monocotyledonous halophyte for soil improvement in northern China. Although previously studied, its main salt tolerance mechanism remained controversial. The current work showed that shoot Na⁺ concentration was increased rapidly with stress time and significantly higher than in wheat during 0-168h of 100mM NaCl treatment. Similar results were also found under 25 and 50mM NaCl treatments. Even K⁺ was increased from 0.01 to 50mM in the cultural solution, no significant effect was found on tissue Na⁺ concentrations. Interestingly, shoot growth was improved, and stronger root activity was maintained in H. brevisubulatum compared with wheat after 7days treatment of 100mM NaCl. To investigate the long-term stress impact on tissue Na⁺, 100mM NaCl was prolonged to 60 days. The maximum values of Na⁺ concentrations were observed at 7th in shoot and 14th day in roots, respectively, and then decreased gradually. Micro-electrode ion flux estimation was used and it was found that increasing Na⁺ efflux while maintaining K⁺ influx were the major strategies to reduce the Na⁺ concentration during long-term salt stress. Moreover, leaf Na⁺ secretions showed little contribution to the tissue Na⁺ decrease. Thereby, the physiological mechanism for H. brevisubulatum to survive from long-term salt stress was proposed that rapid Na⁺ accumulation occurred in the shoot to respond the initial salt shock, then Na⁺ efflux was triggered and K⁺ influx was activated to maintain a stable K⁺/Na⁺ ratio in tissues.