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

The Halophyte Species Solanum chilense Dun. Maintains Its Reproduction despite Sodium Accumulation in Its Floral Organs

Salinity is a growing global concern that affects the yield of crop species, including tomato (Solanum lycopersicum). Its wild relative Solanum chilense was reported to have halophyte properties. We compared salt resistance of both species during the reproductive phase, with a special focus on sodium localization in the flowers. Plants were exposed to NaCl from the seedling stage. Salinity decreased the number of inflorescences in both species but the number of flowers per inflorescence and sepal length only in S. lycopersicum. External salt supply decreased the stamen length in S. chilense, and it was associated with a decrease in pollen production and an increase in pollen viability. Although the fruit set was not affected by salinity, fruit weight and size decreased in S. lycopersicum. Concentrations and localization of Na, K, Mg, and Ca differed in reproductive structures of both species. Inflorescences and fruits of S. chilense accumulated more Na than S. lycopersicum. Sodium was mainly located in male floral organs of S. chilense but in non-reproductive floral organs in S. lycopersicum. The expression of Na transporter genes differed in flowers of both species. Overall, our results indicated that S. chilense was more salt-resistant than S. lycopersicum during the reproductive phase and that differences could be partly related to dissimilarities in element distribution and transport in flowers.

Identification and characterisation of monovalent cation/proton antiporters (CPAs) in Phyllostachys edulis and the functional analysis of PheNHX2 in Arabidopsis thaliana

Plant monovalent cation/proton antiporters (CPAs), types of transmembrane transporters, play important roles in resistance to salt stress. In this study, 37 CPA genes from moso bamboo (Phyllostachys edulis) were identified and characterised. The expression profiles of 10 CPA1 genes (PheNHXs) of moso bamboo were detected by qRT-PCR, which showed that they were specifically expressed in six tissues. In addition, the expression of 10 PheNHXs in leaves and roots changed significantly under 150/200 mM NaCl and 100 μM ABA treatments. In particular, the expression of PheNHX2 in leaves and roots was significantly upregulated under NaCl treatment, thus, we cloned PheNHX2 and analysed its function. Subcellular localisation analysis showed that PheNHX2 was located on the vacuolar membrane. Overexpression of PheNHX2 reduced seed germination and root growth of Arabidopsis thaliana under salt stress, as well as severely affecting cellular Na⁺ and K⁺ content, which in turn reduced the salt tolerance of transgenic Arabidopsis. Measurements of physiological indicators, including chlorophyll content, malondialdehyde content, peroxidase and catalase enzyme activities and relative electrical conductivity, all supported this conclusion. Under salt stress, PheNHX2 also inhibited the expression of some stress-related and ion transport-related genes in transgenic Arabidopsis. Overall, these results indicate that overexpression of PheNHX2 reduces the salt tolerance of transgenic Arabidopsis. This investigation establishes a foundation for subsequent functional studies of moso bamboo CPA genes, and it provides a deeper understanding of PheNHX2 regulation in relation to the salt tolerance of moso bamboo.

Alterations in stomatal response to fluctuating light increase biomass and yield of rice under drought conditions

The acceleration of stomatal closure upon high to low light transition could improve plant water use efficiency and drought tolerance. Herein, using genome-wide association study, we showed that the genetic variation in OsNHX1 was strongly associated with the changes in τ_cl , the time constant of stomatal closure, in 206 rice accessions. OsNHX1 overexpression in rice resulted in a decrease in τ_cl , and an increase in biomass, grain yield under drought. Conversely, OsNHX1 knockout by CRISPR/CAS9 shows opposite trends for these traits. We further found three haplotypes spanning the OsNHX1 promoter and CDS regions. Two among them, HapII and HapIII, were found to be associated with a high and low τ_cl , respectively. A near-isogenic line (NIL, S464) was developed through replacing the genomic region harboring HapII (~10 kb) from MH63 (recipient) rice cultivar by the same sized genomic region containing Hap III from 02428 (donor). Compared with MH63, S464 shows a reduction by 35% in τ_cl and an increase by 40% in the grain yield under drought. However, under normal conditions, S464 maintains closely similar grain yield as MH63. The global distribution of the two OsNHX1 haplotypes is associated with the local precipitation. Taken together, the natural variation in OsNHX1 could be utilized to manipulate the stomatal dynamics for an improved rice drought tolerance.

Phylogenetic Diversity and Physiological Roles of Plant Monovalent Cation/H+ Antiporters

The processes of plant nutrition, stress tolerance, plant growth, and development are strongly dependent on transport of mineral nutrients across cellular membranes. Plant membrane transporters are key components of these processes. Among various membrane transport proteins, the monovalent cation proton antiporter (CPA) superfamily mediates a broad range of physiological and developmental processes such as ion and pH homeostasis, development of reproductive organs, chloroplast operation, and plant adaptation to drought and salt stresses. CPA family includes plasma membrane-bound Na⁺/H⁺ exchanger (NhaP) and intracellular Na⁺/H⁺ exchanger NHE (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins. In this review, we have completed the phylogenetic inventory of CPA transporters and undertaken a comprehensive evolutionary analysis of their development. Compared with previous studies, we have significantly extended the range of plant species, including green and red algae and Acrogymnospermae into phylogenetic analysis. Our data suggest that the multiplication and complexation of CPA isoforms during evolution is related to land colonisation by higher plants and associated with an increase of different tissue types and development of reproductive organs. The new data extended the number of clades for all groups of CPAs, including those for NhaP/SOS, NHE/NHX, KEA, and CHX. We also critically evaluate the latest findings on the biological role, physiological functions and regulation of CPA transporters in relation to their structure and phylogenetic position. In addition, the role of CPA members in plant tolerance to various abiotic stresses is summarized, and the future priority directions for CPA studies in plants are discussed.

Auxin Homeostasis and Distribution of the Auxin Efflux Carrier PIN2 Require Vacuolar NHX-Type Cation/H+ Antiporter Activity

The Arabidopsis vacuolar Na+/H+ transporters (NHXs) are important regulators of intracellular pH, Na+ and K+ homeostasis and necessary for normal plant growth, development, and stress acclimation. Arabidopsis contains four vacuolar NHX isoforms known as AtNHX1 to AtNHX4. The quadruple knockout nhx1nhx2nhx3nhx4, lacking any vacuolar NHX-type antiporter activity, displayed auxin-related phenotypes including loss of apical dominance, reduced root growth, impaired gravitropism and less sensitivity to exogenous IAA and NAA, but not to 2,4-D. In nhx1nhx2nhx3nhx4, the abundance of the auxin efflux carrier PIN2, but not PIN1, was drastically reduced at the plasma membrane and was concomitant with an increase in PIN2 labeled intracellular vesicles. Intracellular trafficking to the vacuole was also delayed in the mutant. Measurements of free IAA content and imaging of the auxin sensor DII-Venus, suggest that auxin accumulates in root tips of nhx1nhx2nhx3nhx4. Collectively, our results indicate that vacuolar NHX dependent cation/H+ antiport activity is needed for proper auxin homeostasis, likely by affecting intracellular trafficking and distribution of the PIN2 efflux carrier.

Genetic localization of the SPC gene controlling pod coiling direction in Medicago truncatula

BACKGROUND

Handedness in plants introduced by helical growth of organs is frequently observed, and it has fascinated plant scientists for decades. However, the genetic control of natural handedness has not been revealed. In the model legume Medicago truncatula, pods can be coiled in a clockwise or anti-clockwise manner, providing a model for genetic analysis of plant handedness.

OBJECTIVE

We aimed to localize the Sense of Pod Coiling (SPC) gene controlling pod coiling direction in M. truncatula.

METHODS

Linkage analysis was used with a biparental population for fine mapping of the SPC gene. The genome sequence of M. truncatula Mt4.0 was used for marker identification and physical mapping. Single nucleotide polymorphisms (SNPs) between the parental lines were converted to CAPS (cleaved amplified polymorphic sequences) markers. Genetic map was constructed using the software JoinMap version 3.0. Gene predication and annotation provided by the M. truncatula genome database (http://www.medicagogenome.org) was confirmed with the programs of FGENESH and Pfam 32.0, respectively. Quantitative reverse transcription PCR (qRT-PCR) was used to analyze the relative expression levels of candidate genes.

RESULTS

The genetic analysis indicated that the anti-clockwise coiling is dominant to clockwise and is controlled by the single gene, SPC. The SPC gene was delimited to a 250 kb-region on Chromosome 7. Total of 15 protein-coding genes were identified in the SPC locus through gene annotation and sequence analysis. Of those, two genes, potentially encoding a receptor-like kinase and a vacuolar cation/proton exchanger respectively, were selected as candidates for the SPC gene.

CONCLUSIONS

The result presented here lay a foundation for gene cloning of SPC, which will help us to understand the molecular mechanisms underlying helical growth in plant organs.

Function of NHX-type transporters in improving rice tolerance to aluminum stress and soil acidity

In this study, we show that ectopic expression of either HtNHX1 or HtNHX2, from Helianthus tuberosus plant (located at vacuolar and endosome membranes, respectively), in rice plants could enhance its tolerance to aluminum (Al³⁺) stress and soil acidity. Plant sodium (potassium)/proton (Na⁺(K⁺)/H⁺ antiporters of the NHX family have been extensively characterized as they are related to the enhancement of salt tolerance. However, no previous study has reported NHX transporter functions in plant tolerance to Al³⁺ toxicity. In this study, we demonstrate their role as a component of the Al³⁺ stress tolerance mechanism. We show that the ectopic expression of either HtNHX1 or HtNHX2 , from Helianthus tuberosus plant, in rice (located at vacuole and endosome, respectively) could also enhance rice tolerance to Al³⁺ stress and soil acidity. Expression of either HtNHX1 or HtNHX2 reduced the inhibitory effect of Al³⁺ on the rice root elongation rate; both genes were reported to be equally effective in improvement of stress conditions. Expression of HtNHX1 enhanced Al³⁺-trigged-secretion of citrate acids, rhizosphere acidification, and also reduced K⁺ efflux from root tissues. In contrast, expression of HtNHX2 prevented Al³⁺-trigged-decrease of H⁺ influx into root tissues. Al³⁺-induced damage of the cell wall extensibility at the root tips was impaired by either HtNHX1 or HtNHX2. Co-expression of HtNHX1 and HtNHX2 further improved rice growth, particularly under the Al³⁺ stress conditions. The results demonstrate that HtNHX1 and HtNHX2 improved rice tolerance to Al³⁺ via different mechanisms by altering the K⁺ and H⁺ fluxes and the cell wall structure.

OsNHAD is a chloroplast membrane-located transporter required for resistance to salt stress in rice (Oryza sativa)

Salt stress is one of the major environmental factors limiting crop productivity. Although physiological and molecular characterization of salt stress response in plants has been the focus for many years, research on transporters for sodium ion (Na⁺) uptake, translocation and accumulation in plants, particularly in food crops like rice is limited. In this study, we functionally identified an uncharacterized sodium ion transporter named OsNHAD which encodes a putative Na⁺ ⁄ H⁺ antiporter in rice. Homology search shows its close relation to the Arabidopsis Na⁺/H⁺ antiporter AtNHD1 with 72.74% identity of amino acids. OsNHAD transcripts mainly express in leaves and are induced by Na⁺ stress. Confocal laser scanning microscopy analysis of OsNHAD::GFP fusion in tobacco leaves shows that OsNHAD resides in the chloroplast envelop. Knock-down of OsNHAD by RNA interference led to increased rice sensitivity to Na⁺, manifested by stunted plant growth, enhanced cellular damage, reduced PSII activity and changed chloroplast morphology. Mutation of OsNHAD also resulted in accumulation of more Na⁺ in chloroplasts and in shoots as well, suggesting that OsNHAD is involved in mediating efflux and detoxification of Na⁺ but does not affect K⁺ accumulation in plant cells. Complementation test reveals that OsNHAD was able to functionally restore the Arabidopsis mutant atnhd1-1 growth phenotype. These results suggest that OsNHAD possibly mediates homeostasis of sodium ions in the subcellular compartments and tissues of the plants when challenged to salt stress.

Antarctic Extremophiles: Biotechnological Alternative to Crop Productivity in Saline Soils

Salinization of soils is one of the main sources of soil degradation worldwide, particularly in arid and semiarid ecosystems. High salinity results in osmotic stress and it can negatively impact plant grow and survival. Some plant species, however, can tolerate salinity by accumulating osmolytes like proline and maintaining low Na+ concentrations inside the cells. Another mechanism of saline stress tolerance is the association with symbiotic microorganism, an alternative that can be used as a biotechnological tool in susceptible crops. From the immense diversity of plant symbionts, those found in extreme environments such as Antarctica seems to be the ones with most potential since they (and their host) evolved in harsh and stressful conditions. We evaluated the effect of the inoculation with a consortium of plant growth-promoting rhizobacteria (PGPB) and endosymbiotic fungi isolated from an Antarctic plant on saline stress tolerance in different crops. To test this we established 4 treatments: (i) uninoculated plants with no saline stress, (ii) uninoculated plants subjected to saline stress (200 mM NaCl), (iii) plants inoculated with the microorganism consortium with no saline stress, and (iv) inoculated plants subjected to saline stress. First, we assessed the effect of symbiont consortium on survival of four different crops (cayenne, lettuce, onion, and tomato) in order to obtain a more generalized response of this biological interaction. Second, in order to deeply the mechanisms involved in salt tolerance, in lettuce plants we measured the ecophysiological performance (Fv/Fm) and lipid peroxidation to estimate the impact of saline stress on plants. We also measured proline accumulation and NHX1 antiporter gene expression (involved in Na+ detoxification) to search for possible mechanism of stress tolerance. Additionally, root, shoot, and total biomass was also obtained as an indicator of productivity. Overall, plants inoculated with microorganisms from Antarctica increased the fitness related traits in several crops. In fact, three of four crops selected to assess the general response increased its survival under salt conditions compared with those uninoculated plants. On the other hand, saline stress negatively impacted all measured trait, but inoculated plants were significantly less affected. In control osmotic conditions, there were no differences in proline accumulation and lipid peroxidation between inoculation treatments. Interestingly, even in control salinity, Fv/Fm was higher in inoculated plants after 30 and 60 days. Under osmotic stress, Fv/Fm, proline accumulation and NHX1 expression was significantly higher and lipid peroxidation lower in inoculated plants compared to uninoculated individuals. Moreover, inoculated plants exposed to saline stress had a similar final biomass (whole plant) compared to individuals under no stress. We conclude that Antarctic extremophiles can effectively reduce the physiological impact of saline stress in a salt-susceptible crops and also highlight extreme environments such as Antarctica as a key source of microorganism with high biotechnological potential.

Cation Specificity of Vacuolar NHX-Type Cation/H+ Antiporters

Cation/H⁺ (NHX-type) antiporters are important regulators of intracellular ion homeostasis and are critical for cell expansion and plant stress acclimation. In Arabidopsis (Arabidopsis thaliana), four distinct NHX isoforms, named AtNHX1 to AtNHX4, locate to the tonoplast. To determine the concerted roles of all tonoplast NHXs on vacuolar ion and pH homeostasis, we examined multiple knockout mutants lacking all but one of the four vacuolar isoforms and quadruple knockout plants lacking any vacuolar NHX activity. The nhx triple and quadruple knockouts displayed reduced growth phenotypes. Exposure to sodium chloride improved growth while potassium chloride was deleterious to some knockouts. Kinetic analysis of K⁺ and Na⁺ transport indicated that AtNHX1 and AtNHX2 are the main contributors to both vacuolar pH and K⁺ and Na⁺ uptake, while AtNHX3 and AtNHX4 differ in Na⁺/K⁺ selectivity. The lack of any vacuolar NHX activity resulted in no K⁺ uptake, highly acidic vacuoles, and reduced but not abolished vacuolar Na⁺ uptake. Additional K⁺/H⁺ and Na⁺/H⁺ exchange activity assays in the quadruple knockout indicated Na⁺ uptake that was not H⁺ coupled, suggesting the existence of an alternative, cation/H⁺-independent, Na⁺ conductive pathway in vacuoles. These results highlight the importance of NHX-type cation/H⁺ antiporters in the maintenance of cellular cation homeostasis and in growth and development.

Expressing class I wheat NHX (TaNHX2) gene in eggplant (Solanum melongena L.) improves plant performance under saline condition

Brinjal or eggplant (Solanum melongena L.) is an important solanaceous edible crop, and salt stress adversely affects its growth, development, and overall productivity. To cope with excess salinity, vacuolar Na⁺/H⁺ antiporters provide the best mechanism for ionic homeostasis in plants under salt stress. We generated transgenic eggplants by introducing wheat TaNHX2 gene that encodes a vacuolar Na⁺/H⁺ antiporter in to the eggplant genome via Agrobacterium-mediated transformation using pBin438 vector that harbors double35S:TaNHX2 to confer salinity tolerance. Polymerase chain reaction and southern hybridization confirmed the presence and integration of TaNHX2 gene in T₁ transgenic plants. Southern positive transgenic eggplants showed varied levels of TaNHX2 transcripts as evident by RT-PCR and qRT-PCR. Stress-inducible expression of TaNHX2 significantly improved growth performance and Na⁺ and K⁺ contents from leaf and roots tissues of T₂ transgenic eggplants under salt stress, compared to non-transformed plants. Furthermore, T₂ transgenic eggplants displayed the stable leaf relative water content and chlorophyll content, proline accumulation, improved photosynthetic efficiency, transpiration rate, and stomatal conductivity than the non-transformed plants under salinity stress (200 mM NaCl). Data showed that the T₂ transgenic lines revealed that reduction in MDA content, hydrogen peroxide, and oxygen radical production associated with the significant increase of antioxidant enzyme activity in transgenic eggplants than non-transformed plants under salt stress (200 mM NaCl). This study suggested that the TaNHX2 gene plays an important regulatory role in conferring salinity tolerance of transgenic eggplant and thus may serve as a useful candidate gene for improving salinity tolerance in other vegetable crops.

K+ Efflux Antiporters 4, 5, and 6 Mediate pH and K+ Homeostasis in Endomembrane Compartments

KEA4, KEA5, and KEA6 are members of the Arabidopsis (Arabidopsis thaliana) K⁺ efflux antiporter (KEA) family that share high sequence similarity but whose function remains unknown. Here, we show their gene expression pattern, subcellular localization, and physiological function in Arabidopsis. KEA4, KEA5, and KEA6 had similar tissue expression patterns, and the three KEA proteins localized to the Golgi, the trans-Golgi network, and the prevacuolar compartment/multivesicular bodies, suggesting overlapping roles of these proteins in the endomembrane system. Phenotypic analyses of single, double, and triple mutants confirmed functional redundancy. The triple mutant kea4 kea5 kea6 had small rosettes, short seedlings, and was sensitive to low K⁺ availability and to the sodicity imposed by high salinity. Also, the kea4 kea5 kea6 mutant plants had a reduced luminal pH in the Golgi, trans-Golgi network, prevacuolar compartment, and vacuole, in accordance with the K/H exchange activity of KEA proteins. Genetic analysis indicated that KEA4, KEA5, and KEA6 as well as endosomal Na⁺/H⁺exchanger5 (NHX5) and NHX6 acted coordinately to facilitate endosomal pH homeostasis and salt tolerance. Neither cancelling nor overexpressing the vacuolar antiporters NHX1 and NHX2 in the kea4 kea5 kea6 mutant background altered the salt-sensitive phenotype. The NHX1 and NHX2 proteins in the kea4 kea5 kea6 mutant background could not suppress the acidity of the endomembrane system but brought the vacuolar pH close to wild-type values. Together, these data signify that KEA4, KEA5, and KEA6 are endosomal K⁺ transporters functioning in maintaining pH and ion homeostasis in the endomembrane network.

Comparative transcriptome analysis of genes involved in Na+ transport in the leaves of halophyte Halogeton glomeratus

Compartmentalization of Na⁺ into vacuoles is considered to be the most critical aspect of salt tolerance in H. glomeratus, an annual, succulent halophyte. Previous analysis of transcriptome involved in the H. glomeratus salt stress response relied on next-generation sequencing technologies that limit the capture of accurately spliced, full-length isoforms. To gain deeper insights into its salt stress response, we used the H. glomeratus Iso-Seq transcriptome database as a reference, and subsequent next-generation sequencing was subjected to various NaCl concentrations of leaves from plants revealed 115 upregulated and 87 downregulated differentially expressed isoforms (core DEIs). The majority of the core DEIs were involved in carbohydrate metabolism and energy production and conversion. In contrast, levels of known isoforms encoding Na⁺ transporters did not change significantly under salt stress. However, 16 core DEIs of unknown function were predicted to possess transmembrane domains, suggesting that these candidate isoforms could be involved in Na⁺ transport in H. glomeratus. These results suggest a potential means for identification of novel Na⁺ transporters, in addition to providing a foundation for further investigation of Na⁺ transport networks in halophytes.

Auxin Contributes to the Intraorgan Regulation of Gene Expression in Response to Shade

Plants sense and respond to light via multiple photoreceptors including phytochrome. The decreased ratio of red to far-red light that occurs under a canopy triggers shade-avoidance responses, which allow plants to compete with neighboring plants. The leaf acts as a photoperceptive organ in this response. In this study, we investigated how the shade stimulus is spatially processed within the cotyledon. We performed transcriptome analysis on microtissue samples collected from vascular and nonvascular regions of Arabidopsis (Arabidopsis thaliana) cotyledons. In addition, we mechanically isolated and analyzed the vascular tissue. More genes were up-regulated by the shade stimulus in vascular tissues than in mesophyll and epidermal tissues. The genes up-regulated in the vasculature were functionally divergent and included many auxin-responsive genes, suggesting that various physiological/developmental processes might be controlled by shade stimulus in the vasculature. We then investigated the spatial regulation of these genes in the vascular tissues. A small vascular region within a cotyledon was irradiated with far-red light, and the response was compared with that when the whole seedling was irradiated with far-red light. Most of the auxin-responsive genes were not fully induced by the local irradiation, suggesting that perception of the shade stimulus requires that a wider area be exposed to far-red light or that a certain position in the mesophyll and epidermis of the cotyledon be irradiated. This result was consistent with a previous report that auxin synthesis genes are up-regulated in the periphery of the cotyledon. Hence, auxin acts as an important intraorgan signaling factor that controls the vascular shade response within the cotyledon.

Transcriptomic analysis reveals vacuolar Na+ (K+)/H+ antiporter gene contributing to growth, development, and defense in switchgrass (Panicum virgatum L.)

BACKGROUND

Intracellular Na+ (K+)/H+ antiporters (NHXs) have pivotal functions in regulating plant growth, development, and resistance to a range of stresses. To gain insight into the molecular events underlying their actions in switchgrass (Panicum virgatum L.), we analyzed transcriptomic changes between PvNHX1-overexpression transgenic lines and wild-type (WT) plants using RNA sequencing (RNA-seq) technology.

RESULTS

The comparison of transcriptomic data from the WT and transgenic plants revealed a large number of differentially expressed genes (DEGs) in the latter. Gene ontology (GO) and KEGG pathway analyses showed that these DEGs were associated with a wide range of functions, and participated in many biological processes. For example, we found that PvNHX1 had an important role in plant growth through its regulation of photosynthetic activity and cell expansion. In addition, PvNHX1 regulated K+ homeostasis, cell expansion and pollen development, indicating that it has unique and specific roles in flower development. We also found that transgenic switchgrass exhibited a higher level of transcription of defense-related genes, especially those involved in disease resistance.

CONCLUSION

We showed that PvNHX1 had an important role in plant growth and development through its regulation of photosynthetic activity, cell expansion, K+ homeostasis, and pollen development. Additionally, PvNHX1 overexpression activated a complex signal transduction network in response to various biotic and abiotic stresses. In relation to plant growth, development, and defense responses, PvNHX1 also had a vital regulatory role in the formation of a series of plant hormones and transcription factors (TFs). The reliability of the RNA-seq data was confirmed by quantitative real-time PCR. Our data provide a valuable foundation for further research into the molecular mechanisms and physiological roles of NHXs in plants.

Two NHX-type transporters from Helianthus tuberosus improve the tolerance of rice to salinity and nutrient deficiency stress

The NHX-type cation/H⁺ transporters in plants have been shown to mediate Na⁺ (K⁺ )/H⁺ exchange for salinity tolerance and K⁺ homoeostasis. In this study, we identified and characterized two NHX homologues, HtNHX1 and HtNHX2 from an infertile and salinity tolerant species Helianthus tuberosus (cv. Nanyu No. 1). HtNHX1 and HtNHX2 share identical 5'- and 3'-UTR and coding regions, except for a 342-bp segment encoding 114 amino acids (L₂₇₂ to Q₃₈₅ ) which is absent in HtNHX2. Both hydroponics and soil culture experiments showed that the expression of HtNHX1 or HtNHX2 improved the rice tolerance to salinity. Expression of HtNHX2, but not HtNHX1, increased rice grain yield, harvest index, total nutrient uptake under K⁺ -limited salt-stress or general nutrient deficiency conditions. The results provide a novel insight into NHX function in plant mineral nutrition.

Transcriptome sequencing and comparative analysis of differentially-expressed isoforms in the roots of Halogeton glomeratus under salt stress

Although Halogeton glomeratus (H. glomeratus) has been confirmed to have a unique mechanism to regulate Na⁺ efflux from the cytoplasm and compartmentalize Na⁺ into leaf vacuoles, little is known about the salt tolerance mechanisms of roots under salinity stress. In the present study, transcripts were sequenced using the BGISEQ-500 sequencing platform (BGI, Wuhan, China). After quality control, approximately 24.08 million clean reads were obtained and the average mapping ratio to the reference gene was 70.00%. When comparing salt-treated samples with the control, a total of 550, 590, 1411 and 2063 DEIs were identified at 2, 6, 24 and 72h, respectively. Numerous differentially-expressed isoforms that play important roles in response and adaptation to salt condition are related to metabolic processes, cellular processes, single-organism processes, localization, biological regulation, responses to stimulus, binding, catalytic activity and transporter activity. Fifty-eight salt-induced isoforms were common to different stages of salt stress; most of these DEIs were related to signal transduction and transporters, which maybe the core isoforms regulating Na⁺ uptake and transport in the roots of H. glomeratus. The expression patterns of 18 DEIs that were detected by quantitative real-time polymerase chain reaction were consistent with their respective changes in transcript abundance as identified by RNA-Seq technology. The present study thoroughly explored potential isoforms involved in salt tolerance on H. glomeratus roots at five time points. Our results may serve as an important resource for the H. glomeratus research community, improving our understanding of salt tolerance in halophyte survival under high salinity stress.

Enhanced Growth Performance and Salinity Tolerance in Transgenic Switchgrass via Overexpressing Vacuolar Na+ (K+)/H+ Antiporter Gene (PvNHX1)

Switchgrass (Panicum virgatum L.) has been increasingly recognized as one of the most valuable perennial bioenergy crop. To improve its biomass production, especially under salt stress, we isolated a putative vacuolar Na⁺ (K⁺)/H⁺ antiporter gene from switchgrass and designated as PvNHX1. Subcellular localization revealed that this protein was localized mainly on the vacuole membrane. The PvNHX1 was found to be expressed throughout the entire growth period of switchgrass, exhibited preferentially expressed in the leaf tissue, and highly induced by salt stress. Transgenic switchgrass overexpressing PvNHX1 showed obvious advantages with respect to plant height and leaf development compared to the wild-type (WT) and transgenic control (EV, expressing the empty vector only) plants, suggesting PvNHX1 may serve as a promoter in switchgrass growth and development. Moreover, transgenic switchgrass were more tolerant than control plants with better growth-related phenotypes (higher shoot height, larger stem diameter, longer leaf length, and width) and physiological capacities (increased proline accumulation, reduced malondialdehyde production, preserved cell membrane integrity, etc.) under high salinity stress. Furthermore, the genes related to cell growth, flowering, and potassium transporters in transgenic switchgrass exhibited a different expression profiles when compared to the control plants, indicating a pivotal function of PvNHX1 in cell expansion and K⁺ homeostasis. Taken together, PvNHX1 is essential for normal plant growth and development, and play an important role in the response to salt stress by improving K⁺ accumulation. Our data provide a valuable foundation for further researches on the molecular mechanism and physiological roles of NHXs in plants.

Tissue tolerance: an essential but elusive trait for salt-tolerant crops

For a plant to persist in saline soil, osmotic adjustment of all plant cells is essential. The more salt-tolerant species accumulate Na+ and Cl- to concentrations in leaves and roots that are similar to the external solution, thus allowing energy-efficient osmotic adjustment. Adverse effects of Na+ and Cl- on metabolism must be avoided, resulting in a situation known as 'tissue tolerance'. The strategy of sequestering Na+ and Cl- in vacuoles and keeping concentrations low in the cytoplasm is an important contributor to tissue tolerance. Although there are clear differences between species in the ability to accommodate these ions in their leaves, it remains unknown whether there is genetic variation in this ability within a species. This viewpoint considers the concept of tissue tolerance, and how to measure it. Four conclusions are drawn: (1) osmotic adjustment is inseparable from the trait of tissue tolerance; (2) energy-efficient osmotic adjustment should involve ions and only minimal organic solutes; (3) screening methods should focus on measuring tolerance, not injury; and (4) high-throughput protocols that avoid the need for control plants and multiple Na+ or Cl- measurements should be developed. We present guidelines to identify useful genetic variation in tissue tolerance that can be harnessed for plant breeding of salt tolerance.

Root-endophytes improve the ecophysiological performance and production of an agricultural species under drought condition

Throughout many regions of the world, climate change has limited the availability of water for irrigating crops. Indeed, current models of climate change predict that arid and semi-arid zones will be places where precipitation will drastically decrease. In this context, plant root-associated fungi appear as a new strategy to improve ecophysiological performance and yield of crops under abiotic stress. Thus, use of fungal endophytes from ecosystems currently subjected to severe drought conditions could improve the ecophysiological performance and quantum yield of crops exposed to drought. In this study, we evaluated how the inoculation of fungal endophytes isolated from Antarctic plants can improve the net photosynthesis, water use efficiency and production of fresh biomass in a lettuce cultivar, grown under different water availability regimes. In addition, we assessed if the presence of biochemical mechanisms and gene expression related with environmental tolerance are improved in presence of fungal endophytes. Overall, those individuals with presence of endophytes showed higher net photosynthesis and maintained higher water use efficiency in drought conditions, which was correlated with greater fresh and dry biomass production as well as greater root system development. In addition, presence of fungal endophytes was correlated with a higher proline concentration, lower peroxidation of lipids and up-/down-regulation of ion homeostasis. Our results suggest that presence of fungal endophytes could minimize the negative effect of drought by improving drought tolerance through biochemical mechanisms and improving nutritional status. Thus, root-endophytes might be a successful biotechnological tool to maintain high levels of ecophysiological performance and productivity in zones under drought.