The plasma membrane transport systems and adaptation to salinity

Hydrogen sulfide priming enhanced salinity tolerance in sunflower by modulating ion hemostasis, cellular redox balance, and gene expression

BACKGROUND

The salinity threat represents an environmental challenge that drastically affects plant growth and yield. Besides salinity stress, the escalating world population will greatly influence the world's food security in the future. Therefore, searching for effective strategies to improve crop salinity resilience and sustain agricultural productivity under high salinity is a must. Seed priming is a reliable, simple, low-risk, and low-cost technique. Therefore, this work aimed to evaluate the impact of seed priming with 0.5 mM NaHS, as a donor of H2S, in mitigating salinity effects on sunflower seedlings. Primed and nonprime seeds were established in nonsaline soil irrigated with tape water for 14 d, and then exposed to 150 mM NaCl for 7 d.

RESULTS

Salinity stress significantly reduced the seedling growth, biomass accumulation, K+, Ca2+, and salinity tolerance index while elevating Na+ uptake and translocation. Salinity-induced adverse effects were significantly alleviated by H2S priming. Upregulation in gene expression (HaSOS2, HaGST) under NaCl stress was further enhanced by H2S priming. Also, H2S reduced lipid peroxidation, electrolyte leakage, and H2O2 content, but elevated the antioxidant defense system. NaCl-induced levels of ascorbate, glutathione, and α tocopherol, as well as the activities of AsA-GSH cycle enzymes: ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and glutathione S-transferase, were further enhanced by H2S priming. Increased level of H2S and total thiol by NaCl was also further stimulated by H2S priming.

CONCLUSION

H2S priming has proved to be an efficient strategy to improve sunflower seedlings' salinity tolerance by retaining ion homeostasis, detoxifying oxidative damage, modulating gene expression involved in ion homeostasis and ROS scavenging, and boosting endogenous H2S. These findings suggested that H2S acts as a regulatory molecule activating the functional processes responsible for sunflower adaptive mechanisms and could be adopted as a crucial crop management strategy to combat saline conditions. However, it would be of great interest to conduct further studies in the natural saline field to broaden our understanding of crop adaptive mechanisms and to support our claims.

Deciphering the Genetic Mechanisms of Salt Tolerance in Sorghum bicolor L.: Key Genes and SNP Associations from Comparative Transcriptomic Analyses

Sorghum bicolor L. is a vital cereal crop for global food security. Its adaptability to diverse climates make it economically, socially, and environmentally valuable. However, soil salinization caused by climate extremes poses a threat to sorghum. This study aimed to identify candidate salt-tolerant genes and single nucleotide polymorphisms (SNPs) by performing a comparative transcriptome analysis on a mutant sorghum line and its wild type. The mutant line was generated through gamma ray exposure and selection for salt tolerance. Phenotypic measurements were taken, followed by mRNA sequencing and variant calling. In this study, potential genes and non-synonymous SNPs associated with salt tolerance were inferred, including LOC8071970, LOC8067721, LOC110430887, LOC8070256, and LOC8056880. These genes demonstrated notable differences in nsSNPs in comparison to the wild type, suggesting their potential roles in salt tolerance. Additionally, LOC8060874 (cyanohydrin beta-glucosyltransferase) was suggested as a key gene involved in salt tolerance due to its possible role in dhurrin biosynthesis under salt stress. In upcoming research, additional reverse genetics studies will be necessary in order to verify the function of those candidate genes in relation to salt stress. In conclusion, this study underscores the significance of investigating salt tolerance mechanisms and the potential key genes associated with salt tolerance in sorghum. Our findings may provide insights for future breeding strategies aimed at enhancing salinity tolerance and crop productivity.

Combined Proteomic and Metabolomic Analysis of the Molecular Mechanism Underlying the Response to Salt Stress during Seed Germination in Barley

Salt stress is a major abiotic stress factor affecting crop production, and understanding of the response mechanisms of seed germination to salt stress can help to improve crop tolerance and yield. The differences in regulatory pathways during germination in different salt-tolerant barley seeds are not clear. Therefore, this study investigated the responses of different salt-tolerant barley seeds during germination to salt stress at the proteomic and metabolic levels. To do so, the proteomics and metabolomics of two barley seeds with different salt tolerances were comprehensively examined. Through comparative proteomic analysis, 778 differentially expressed proteins were identified, of which 335 were upregulated and 443 were downregulated. These proteins, were mainly involved in signal transduction, propanoate metabolism, phenylpropanoid biosynthesis, plant hormones and cell wall stress. In addition, a total of 187 salt-regulated metabolites were identified in this research, which were mainly related to ABC transporters, amino acid metabolism, carbohydrate metabolism and lipid metabolism; 72 were increased and 112 were decreased. Compared with salt-sensitive materials, salt-tolerant materials responded more positively to salt stress at the protein and metabolic levels. Taken together, these results suggest that salt-tolerant germplasm may enhance resilience by repairing intracellular structures, promoting lipid metabolism and increasing osmotic metabolites. These data not only provide new ideas for how seeds respond to salt stress but also provide new directions for studying the molecular mechanisms and the metabolic homeostasis of seeds in the early stages of germination under abiotic stresses.

Effect of Saline–Alkali Stress on Sugar Metabolism of Jujube Fruit

Sugar, an osmoregulatory substance used by plants to adapt to abiotic stresses such as drought and salinity, is one of the most important indexes of fruit quality. In this study, 0–150 mM saline–alkali solutions (NaCl:NaHCO3 = 3:1) were used to irrigate the roots of 10-year-old “Junzao” fruit trees during the growth period to explore the regulation mechanism of different concentrations of saline–alkali stress on sugar and reactive oxygen metabolism in jujube fruit at maturity. The results showed that under low stress (0~90 mM), the contents of sucrose, glucose, and fructose in the jujube fruit and the activities of sucrose phosphate synthase (SPS), sucrose synthase decomposition direction (SS-I), and sucrose synthase synthesis direction (SS-II) increased with increases in stress concentration, results that were consistent with the relative expression trends of the SPS and SS genes; however, the results were reversed under high concentrations (120 and 150 mM). The soluble acid invertase (S-AI) activity decreased with increases in stress concentration under low stress, and the results were reversed with high stress, which was consistent with the relative expression trends of the ZjcINV3, ZjnINV1, and ZjnINV3. Research regarding the response of antioxidant enzymes in fruits under saline–alkali stress showed that only the differences in peroxidase (POD) activity under saline–alkali stress were consistent with sugar accumulation; the proline (PRO), catalase (CAT) decreased and the malondialdehyde (MDA) superoxide dismutase (SOD) increased with increases in saline–alkali stress. These results indicate that the sugar metabolism and antioxidase jointly promote and regulate sugar accumulation in jujube fruits in a low saline–alkali environment.

Identification and Transcriptome Analysis of Genes Related to Membrane Lipid Regulation in Sweet Sorghum under Salt Stress

Sweet sorghum has strong stress resistance and is considered a promising energy crop. In the present study, the effects of salt on the membrane lipid metabolism of two sweet sorghum inbred lines (salt-tolerant M-81E and salt-sensitive Roma) were analyzed. After treatment with 150 mM NaCl, higher levels of fresh weight and chlorophyll fluorescence, as well as lower levels of malondialdehyde (MDA) were found in salt-tolerant M-81E. Concomitantly, 702 and 1339 differentially expression genes (DEGs) in M-81E and Roma were identified in response to salt stress. We determined that most DEGs were related to glycerophospholipid metabolism, glycerolipid metabolism, and other membrane lipid metabolisms. Under NaCl treatment, the expression of the membrane-associated phospholipase A1 was down-regulated at the transcriptional level, along with an increased content of phosphatidylcholine (PC) in both cultivars. The inhibition of triacylglycerol (TAG) mobilization in M-81E delayed salt-induced leaf senescence. Furthermore, enhanced levels of glycerol-3-phosphate acyltransferase (GPAT) expression contributed to improved salt resistance in M-81E. The results of this study demonstrate membrane the role of lipid regulation in mediating salt-defensive responses in sweet sorghum and expand our understanding of the relationship between changes in membrane lipid content and salt resistance.

How salt stress-responsive proteins regulate plant adaptation to saline conditions

An overview is presented of recent advances in our knowledge of candidate proteins that regulate various physiological and biochemical processes underpinning plant adaptation to saline conditions. Salt stress is one of the environmental constraints that restrict plant distribution, growth and yield in many parts of the world. Increased world population surely elevates food demands all over the globe, which anticipates to add a great challenge to humanity. These concerns have necessitated the scientists to understand and unmask the puzzle of plant salt tolerance mechanisms in order to utilize various strategies to develop salt tolerant crop plants. Salt tolerance is a complex trait involving alterations in physiological, biochemical, and molecular processes. These alterations are a result of genomic and proteomic complement readjustments that lead to tolerance mechanisms. Proteomics is a crucial molecular tool that indicates proteins expressed by the genome, and also identifies the functions of proteins accumulated in response to salt stress. Recently, proteomic studies have shed more light on a range of promising candidate proteins that regulate various processes rendering salt tolerance to plants. These proteins have been shown to be involved in photosynthesis and energy metabolism, ion homeostasis, gene transcription and protein biosynthesis, compatible solute production, hormone modulation, cell wall structure modification, cellular detoxification, membrane stabilization, and signal transduction. These candidate salt responsive proteins can be therefore used in biotechnological approaches to improve tolerance of crop plants to salt conditions. In this review, we provided comprehensive updated information on the proteomic data of plants/genotypes contrasting in salt tolerance in response to salt stress. The roles of salt responsive proteins that are potential determinants for plant salt adaptation are discussed. The relationship between changes in proteome composition and abundance, and alterations observed in physiological and biochemical features associated with salt tolerance are also addressed.

Gene regulation at transcriptional and post-transcriptional levels to combat salt stress in plants

Soil salinity is a major challenge that will be faced more and more by human population in the near future. Higher salt concentrations in the soil limit the growth and production of crops, which poses serious threats to global food production. Various plant breeding approaches have been followed in the past which are reported to reduce the effect of salt stress by inducing the level of protective metabolites like osmolytes and antioxidants. Conventional breeding approaches are time-consuming and not cost-effective. In recent times, genetic engineering has been largely followed to confer salt tolerance through introgressions of single transgenes or stacking multiple transgenes. However, most of such works are limited only at the laboratory level and field trials are still awaited to prove the long-term efficacy of such transgenics. In this review, we attempt to present a broad overview of the current strategies undertaken to develop halophytic and salt-tolerant crops. The salt-induced damages in the plants are highlighted, followed by representing the novel traits, associated with salt stress, which can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of transcriptional and epigenetic regulation in plants for amelioration of salt-induced damages has been reviewed. The role of post-transcriptional mechanisms such as microRNA regulation, genome editing and alternative splicing, during salt stress, and their implications in the development of salt-tolerant crops are also discussed. Finally, we present a short overview about the role of ion transporters and rhizobacteria in the engineering of salt tolerance in crop species.

Genetic, Epigenetic, Genomic and Microbial Approaches to Enhance Salt Tolerance of Plants: A Comprehensive Review

Simple Summary

Globally, soil salinity, which refers to salt-affected soils, is increasing due to various environmental factors and human activities. Soil salinity poses one of the most serious challenges in the field of agriculture as it significantly reduces the growth and yield of crop plants, both quantitatively and qualitatively. Over the last few decades, several studies have been carried out to understand plant biology in response to soil salinity stress with a major emphasis on genetic and other hereditary components. Based on the outcome of these studies, several approaches are being followed to enhance plants’ ability to tolerate salt stress while still maintaining reasonable levels of crop yields. In this manuscript, we comprehensively list and discuss various biological approaches being followed and, based on the recent advances in the field of molecular biology, we propose some new approaches to improve salinity tolerance of crop plants. The global scientific community can make use of this information for the betterment of crop plants. This review also highlights the importance of maintaining global soil health to prevent several crop plant losses.

Abstract

Globally, soil salinity has been on the rise owing to various factors that are both human and environmental. The abiotic stress caused by soil salinity has become one of the most damaging abiotic stresses faced by crop plants, resulting in significant yield losses. Salt stress induces physiological and morphological modifications in plants as a result of significant changes in gene expression patterns and signal transduction cascades. In this comprehensive review, with a major focus on recent advances in the field of plant molecular biology, we discuss several approaches to enhance salinity tolerance in plants comprising various classical and advanced genetic and genetic engineering approaches, genomics and genome editing technologies, and plant growth-promoting rhizobacteria (PGPR)-based approaches. Furthermore, based on recent advances in the field of epigenetics, we propose novel approaches to create and exploit heritable genome-wide epigenetic variation in crop plants to enhance salinity tolerance. Specifically, we describe the concepts and the underlying principles of epigenetic recombinant inbred lines (epiRILs) and other epigenetic variants and methods to generate them. The proposed epigenetic approaches also have the potential to create additional genetic variation by modulating meiotic crossover frequency.

Post-translational regulation of the membrane transporters contributing to salt tolerance in plants

This review article summarises the role of membrane transporters and their regulatory kinases in minimising the toxicity of Na+ in the plant under salt stress. The salt-tolerant plants keep their cytosolic level of Na+ up to 10-50mM. The first line of action in this context is the generation of proton motive force by the plasma membrane H+-ATPase. The generated proton motive force repolarises the membrane that gets depolarised due to passive uptake of Na+ under salt stress. The proton motive force generated also drives the plasma membrane Na+/H+ antiporter, SOS1 that effluxes the cytosolic Na+ back into the environment. At the intracellular level, Na+ is sequestered by the vacuole. Vacuolar Na+ uptake is mediated by Na+/H+ antiporter, NHX, driven by the electrochemical gradient for H+, generated by tonoplast H+ pumps, both H+ATPase and PPase. However, it is the expression of the regulatory kinases that make these transporters active through post-translational modification enabling them to effectively manage the cytosolic level of Na+, which is essential for tolerance to salinity in plants. Yet our knowledge of the expression and functioning of the regulatory kinases in plant species differing in tolerance to salinity is scant. Bioinformatics-based identification of the kinases like OsCIPK24 in crop plants, which are mostly salt-sensitive, may enable biotechnological intervention in making the crop cultivar more salt-tolerant, and effectively increasing its annual yield.

Sorghum under saline conditions: responses, tolerance mechanisms, and management strategies

An overview is presented of recent advances in our knowledge of responses and mechanisms rendering adaptation to saline conditions in sorghum. Different strategies deployed to enhance salinity stress tolerance in sorghum are also pointed out. Salinity stress is a growing problem worldwide. Sorghum is the fifth key crop among cereals. Understanding responses and tolerance strategies in sorghum would be therefore helpful effort for providing biomarkers for designing greatest salinity-tolerant sorghum genotypes. When sorghum exposed to salinity, salinity-tolerant genotypes most probably reprogram their gene expression to activate adaptive biochemical and physiological responses for survival. The review thus discusses the possible physiological and biochemical responses that confer salinity tolerance to sorghum under saline conditions. Although it is not characterized in sorghum, salinity perceiving and transmitting signals to downstream responses via signaling transduction pathways most likely are essential strategy for sorghum adaptation to salinity stress. Sorghum has also shown to withstand moderate saline environments and retain the germination, growth, and photosynthetic activities. Salinity-tolerant sorghum genotypes show the ability to exclude excessive Na⁺ from reaching shoots and induce ion homeostasis. Osmotic homeostasis and ROS detoxification are also evident as salinity tolerance strategies in sorghum. These above mechanisms lead to re-establishment of cellular ionic, osmotic, and redox homeostasis as well as photosynthesis efficiency. It is noteworthy that these mechanisms act individually or co-operatively to minimize the salinity hazards and enhance acclimation in sorghum. We conclude, however, that although these responses contribute to sorghum tolerance to salinity stress, they seem to be not adequate at higher concentrations of salinity, which agrees with sorghum ranking as moderately salinity-tolerant crop. Also, some of these tolerance strategies reported in other crops are not well studied and documented in sorghum, but most probably have roles in sorghum. Further improvement in sorghum salinity tolerance using different approaches is definitely necessary to meet the requirements of its harsh production environments, and therefore, these approaches are addressed.

Augmenting salt tolerance in rice by regulating uptake and tissue specific accumulation of Na+ - through Ca2+ -induced alteration of biochemical events

The protective effect of Ca²⁺ against NaCl toxicity was investigated in two rice varieties with contrasting for salt tolerance to understand the mechanistic details of the antagonism to address adverse effects of salinity on agriculture. The study primarily examined the influence of Ca²⁺ on expression/activity of the effectors and regulators involved in Na⁺ translocation. Calcium reduced uptake of Na⁺ concomitant with higher tissue K⁺ /Na⁺ in seedlings, comparatively more in salt-tolerant Nona Bokra than in salt-sensitive IR-64, together with a significant increase in root PM H⁺ ATPase in the former, but not in the latter. Increased antagonism in Nona Bokra could be the result of Ca²⁺ signalling-mediated phosphorylation of PM H⁺ ATPase in roots caused by a significant Ca²⁺ -dependent increase in expression of OsCIPK24, which did not occur in IR-64. Furthermore, significant Ca²⁺ -mediated NaCl-induced increase in transcription of 14-3-3 protein in Nona Bokra, but not in IR-64, might also lead to a greater protective effect of Ca²⁺ in the former, as 14-3-3 protein is essential for activating PM H⁺ ATPase. Thus, efficient functioning of PM H⁺ ATPase could be key in determining resistance of plants to salinity, implying that identification of the Ca²⁺ -dependent kinase phosphorylating the PM H⁺ ATPase threonine residue and manipulation of its expression, together with expression of 14-3-3 proteins could be an important strategy to improve salt tolerance of crops and their cultivation in salt-affected lands.

Progress in understanding salt stress response in plants using biotechnological tools

Salinization is a worldwide environmental problem, which is negatively impacting crop yield and thus posing a threat to the world's food security. Considering the rising threat of salinity, it is need of time, to understand the salt tolerant mechanism in plants and find avenues for the development of salinity resistant plants. Several plants tolerate salinity in a different manner, thereby halophytes and glycophytes evolved altered mechanisms to counter the stress. Therefore, in this review article, physiological, metabolic, and molecular aspects of the plant adaptation to salt stress have been discussed. The conventional breeding techniques for developing salt tolerant plants has not been much successful, due to its multigenic trait. The inflow of data from plant sequencing projects and annotation of genes led to the identification of many putative genes having a role in salt stress. The bioinformatics tools provided preliminary information and were helpful for making salt stress-specific databases. The microRNA identification and characterization led to unraveling the finer intricacies of the network. The transgenic approach finally paved a way for overexpressing some important genes viz. DREB, MYB, COMT, SOS, PKE, NHX, etc. conferred salt stress tolerance. In this review, we tried to show the effect of salinity on plants, considering ion homeostasis, antioxidant defense response, proteins involved, possible utilization of transgenic plants, and bioinformatics for coping with this stress factor. An overview of previous studies related to salt stress is presented in order to assist researchers in providing a potential solution for this increasing environmental threat.

Aquaporins and cation transporters are differentially regulated by two arbuscular mycorrhizal fungi strains in lettuce cultivars growing under salinity conditions

The aim was to identify the effects of AM symbiosis on the expression patterns of genes associated with K+ and Na+ compartmentalization and translocation and on K+/Na+ homeostasis in some lettuce (Lactuca sativa) cultivars as well as the effects of the relative abundance of plant AQPs on plant water status. Two AM fungi species (Funneliformis mosseae and Claroideoglomus lamellosum) isolated from the hyper-arid Atacama Desert (northern Chile) were inoculated to two lettuce cultivars (Grand Rapids and Lollo Bionda), and watered with 0 and 60 mM NaCl. At 60 days of plant growth, the AM symbiotic development, biomass production, nutrient content (Pi, Na+, K+), physiological parameters, gene expressions of ion channels and transporters (NHX and HKT1), and aquaporins proteins abundance (phosphorylated and non-phosphorylated) were evaluated. Salinity increased the AM root colonization by both inocula. AM lettuce plants showed an improved growth, increased relative water content and improved of K/Na ratio in root. In Grand Rapids cultivar, the high efficiency of photosystem II was higher than Lollo Bionda cultivar; on the contrary, stomatal conductance was higher in Lollo Bionda. Nevertheless, both parameters were increased by AM colonization. In the same way, LsaHKT1;1, LsaHKT1;6, LsaNHX2, LsaNHX4, LsaNHX6 and LsaNHX8 genes and aquaporins PIP2 were up-regulated differentially by both AM fungi. The improved plant growth was closely related to a higher water status due to increased PIP2 abundance, as well as to the upregulation of LsaNHX gene expression, which concomitantly improved plant nutrition and K+/Na+ homeostasis maintenance.

Tissue Tolerance Coupled With Ionic Discrimination Can Potentially Minimize the Energy Cost of Salinity Tolerance in Rice

Salinity is one of the major constraints in rice production. To date, development of salt-tolerant rice cultivar is primarily focused on salt-exclusion strategies, which incur greater energy cost. The present study aimed to evaluate a balancing strategy of ionic discrimination vis-à-vis tissue tolerance, which could potentially minimize the energy cost of salt tolerance in rice. Four rice genotypes, viz., FL478, IR29, Kamini, and AC847, were grown hydroponically and subjected to salt stress equivalent to 12 dS m-1 at early vegetative stage. Different physiological observations (leaf chlorophyll content, chlorophyll fluorescence traits, and tissue Na+ and K+ content) and visual scoring suggested a superior Na+-partitioning strategy operating in FL478. A very low tissue Na+/K+ ratio in the leaves of FL478 after 7 days of stress hinted the existence of selective ion transport mechanism in this genotype. On the contrary, Kamini, an equally salt-tolerant genotype, was found to possess a higher leaf Na+/K+ ratio than does FL478 under similar stress condition. Salt-induced expression of different Na+ and K+ transporters indicated significant upregulation of SOS, HKT, NHX, and HAK groups of transporters in both leaves and roots of FL478, followed by Kamini. The expression of plasma membrane and vacuolar H+ pumps (OsAHA1, OsAHA7, and OsV-ATPase) were also upregulated in these two genotypes. On the other hand, IR29 and AC847 showed greater salt susceptibility owing to excess upward transport of Na+ and eventually died within a few days of stress imposition. But in the "leaf clip" assay, it was found that both IR29 and Kamini had high tissue-tolerance and chlorophyll-retention abilities. On the contrary, FL478, although having higher ionic-discrimination ability, showed the least degree of tissue tolerance as evident from the LC50 score (amount of Na+ required to reduce the initial chlorophyll content to half) of 336 mmol g-1 as against 459 and 424 mmol g-1 for IR29 and Kamini, respectively. Overall, the present study indicated that two components (ionic selectivity and tissue tolerance) of salt tolerance mechanism are distinct in rice. Unique genotypes like Kamini could effectively balance both of these strategies to achieve considerable salt tolerance, perhaps with lesser energy cost.

Engineering salinity tolerance in plants: progress and prospects

There is a need to integrate conceptual framework based on the current understanding of salt stress responses with different approaches for manipulating and improving salt tolerance in crop plants. Soil salinity exerts significant constraints on global crop production, posing a serious challenge for plant breeders and biotechnologists. The classical transgenic approach for enhancing salinity tolerance in plants revolves by boosting endogenous defence mechanisms, often via a single-gene approach, and usually involves the enhanced synthesis of compatible osmolytes, antioxidants, polyamines, maintenance of hormone homeostasis, modification of transporters and/or regulatory proteins, including transcription factors and alternative splicing events. Occasionally, genetic manipulation of regulatory proteins or phytohormone levels confers salinity tolerance, but all these may cause undesired reduction in plant growth and/or yields. In this review, we present and evaluate novel and cutting-edge approaches for engineering salt tolerance in crop plants. First, we cover recent findings regarding the importance of regulatory proteins and transporters, and how they can be used to enhance salt tolerance in crop plants. We also evaluate the importance of halobiomes as a reservoir of genes that can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of microRNAs as critical post-transcriptional regulators in plant adaptive responses to salt stress is reviewed and their use for engineering salt-tolerant crop plants is critically assessed. The potentials of alternative splicing mechanisms and targeted gene-editing technologies in understanding plant salt stress responses and developing salt-tolerant crop plants are also discussed.

Multi-dimensional evaluation of response to salt stress in wheat

Soil salinity is a major threat to crop production worldwide. The global climate change is further accelerating the process of soil salinization, particularly in dry areas of the world. Increasing genetic variability of currently used wheat varieties by introgression of exotic alleles/genes from related progenitors' species in breeding programs is an efficient approach to overcome limitations due to the absence of valuable genetic diversity in elite cultivars. Synthetic hexaploid wheat (SHW) is widely regarded as donor of favourable exotic alleles to improve tolerance against biotic and abiotic stresses such as salinity stress. In this study, synthetic backcross lines (SBLs) winter wheat population "Z86", derived from crosses involving synthetic hexaploid wheat Syn86L with German elite winter wheat cultivar Zentos, was evaluated for salinity tolerance at different developmental stages under controlled and field conditions in three growing seasons. High genetic variability was detected across the SBLs and their parents at various growth stages under controlled as well as under salt stress field trials. Greater performance of Zentos over Syn86L was detected at germination stage across all salt treatments and with respect to shoot dry weight (SDW) and root dry weight (RDW) at seedling stage. Whereas for the root length (RL) and the shoot length (SL) Syn86L surpassed the elite cultivar and most of the progenies. Our experiments revealed for almost all traits that some genotypes among the SBLs showed higher performance than their parents. Furthermore, positive transgressive segregations were detected among the SBLs for germination at high salinity levels, as well as for RDW and SDW at seedling stage. Therefore, the studied Z86 population is a suitable population for assessment of salinity stress on morphological and physiological traits at different plant growth stages. The identified SBLs provide a valuable source for genetic gain through recombination of superior alleles that can be directly applied in breeding programs for efficiently breeding cultivars with improved salinity tolerance and desired agronomic traits.

Co-expression of SpSOS1 and SpAHA1 in transgenic Arabidopsis plants improves salinity tolerance

BACKGROUND

Na+ extrusion from cells is important for plant growth in high saline environments. SOS1 (salt overly sensitive 1), an Na+/H+ antiporter located in the plasma membrane (PM), functions in toxic Na+ extrusion from cells using energy from an electrochemical proton gradient produced by a PM-localized H+-ATPase (AHA). Therefore, SOS1 and AHA are involved in plant adaption to salt stress.

RESULTS

In this study, the genes encoding SOS1 and AHA from the halophyte Sesuvium portulacastrum (SpSOS1 and SpAHA1, respectively) were introduced together or singly into Arabidopsis plants. The results indicated that either SpSOS1 or SpAHA1 conferred salt tolerance to transgenic plants and, as expected, Arabidopsis plants expressing both SpSOS1 and SpAHA1 grew better under salt stress than plants expressing only SpSOS1 or SpAHA1. In response to NaCl treatment, Na+ and H+ in the roots of plants transformed with SpSOS1 or SpAHA1 effluxed faster than wild-type (WT) plant roots. Furthermore, roots co-expressing SpSOS1 and SpAHA1 had higher Na+ and H+ efflux rates than single SpSOS1/SpAHA1-expressing transgenic plants, resulting in the former amassing less Na+ than the latter. As seen from comparative analyses of plants exposed to salinity stress, the malondialdehyde (MDA) content was lowest in the co-transgenic SpSOS1 and SpAHA1 plants, but the K+ level was the highest.

CONCLUSION

These results suggest SpSOS1 and SpAHA1 coordinate to alleviate salt toxicity by increasing the efficiency of Na+ extrusion to maintain K+ homeostasis and protect the PM from oxidative damage induced by salt stress.

Over-expression of a plasma membrane H+-ATPase SpAHA1 conferred salt tolerance to transgenic Arabidopsis

The SpAHA1 gene, encoding a plasma membrane (PM) H⁺-ATPase (AHA) in Sesuvium portulacastrum, was transformed into Arabidopsis plants, and its expression increased salinity tolerance of transgenic Arabidopsis plants: seed germination ratio, root growth, and biomass of transgenic plants were greater compared to wild-type plants under NaCl treatment condition. Upon salinity stress, both Na⁺ and H⁺ effluxes in the roots of SpAHA1 expressing plants were faster than those of untransformed plants. Transformed plants with SpAHA1 had lower Na⁺ and higher K⁺ contents relative to wild-type plants when treated with NaCl, resulting in greater K⁺/Na⁺ ratio in transgenic plants than in wild-type plants under salt stress. Extent of oxidative stress increased in both transgenic and wild-type plants exposed to salinity stress, but overexpression of SpAHA1 could alleviate the accumulation of hydrogen peroxide (H₂O₂) induced by NaCl treatment in transgenic plants relative to wild-type plants; the content of malondialdehyde (MDA) was lower in transgenic plants than that in wild-type plants under salinity stress. These results suggest that the higher H⁺-pumping activity generated by SpAHA1 improved the growth of transgenic plants via regulating ion and reactive oxygen species (ROS) homeostasis in plant cells under salinity stress.

Plasma membrane proteome analysis identifies a role of barley membrane steroid binding protein in root architecture response to salinity

Although the physiological consequences of plant growth under saline conditions have been well described, understanding the core mechanisms conferring plant salt adaptation has only started. We target the root plasma membrane proteomes of two barley varieties, cvs. Steptoe and Morex, with contrasting salinity tolerance. In total, 588 plasma membrane proteins were identified by mass spectrometry, of which 182 were either cultivar or salinity stress responsive. Three candidate proteins with increased abundance in the tolerant cv. Morex were involved either in sterol binding (a GTPase-activating protein for the adenosine diphosphate ribosylation factor [ZIGA2], and a membrane steroid binding protein [MSBP]) or in phospholipid synthesis (phosphoethanolamine methyltransferase [PEAMT]). Overexpression of barley MSBP conferred salinity tolerance to yeast cells, whereas the knock-out of the heterologous AtMSBP1 increased salt sensitivity in Arabidopsis. Atmsbp1 plants showed a reduced number of lateral roots under salinity, and root-tip-specific expression of barley MSBP in Atmsbp1 complemented this phenotype. In barley, an increased abundance of MSBP correlates with reduced root length and lateral root formation as well as increased levels of auxin under salinity being stronger in the tolerant cv. Morex. Hence, we concluded the involvement of MSBP in phytohormone-directed adaptation of root architecture in response to salinity.

Effect of salt stress on fatty acid and α-tocopherol metabolism in two desert shrub species

Compared to Artemisia ordosiea Kraschen, a higher content of α-tocopherol in Artemisia sphaerocephala Kraschen under salt stress inhibits the conversion of linoleic acid (C18:2) into linolenic acid (C18:3), maintains cell membrane stability and contributes to higher salt resistance. Artemisia sphaerocephala Kraschen and Artemisia ordosiea Kraschen are widely distributed in the arid and semiarid desert regions of the northwest of China. Under salt stress, it has been known that α-tocopherol (α-T) improves membrane permeability and maintains Na⁺/K⁺ homeostasis; however, the function of α-T in regulating membrane components of fatty acids is unknown. In this study, 100-day-old plants of A. ordosiea and A. sphaerocephala are subjected to various NaCl treatments for 7, 14, and 21 days. Compared to A. ordosiea, A. sphaerocephala has a higher Na⁺ concentration, higher chlorophyll content and dry weight in all NaCl treatments, but lower relative electric conductivity. The stable unsaturated levels of the lipids in A. sphaerocephala may be attributed to higher level of C18:2. Under 200 mM NaCl treatment, α-T and C18:2 contents in A. sphaerocephala increase significantly, while the Na⁺, C18:1, C18:3 and jasmonic acid (JA) contents decrease. Moreover, α-T is positively correlated with C18:2, but negatively correlated with C18:3.

Evaluation of proline functions in saline conditions

More than one third of the world's irrigated lands are affected by salinity, which has great impact on plant growth and yield worldwide. Proline accumulation under salt stress has been indicated to correlate with salt tolerance. Exogenous application as well as genetic engineering of metabolic pathways involved in the metabolism of proline has been successful in improving tolerance to salinity. Correlation between proline accumulation as well as its proposed roles and salt adaptation, however, has not been clearly confirmed in several plant species. In addition, the studies relating proline functions and plant salt tolerance are always carried out in growth chambers, and are not successfully verified in field conditions. Further, plant salt tolerance is a complex trait, and studies based solely on proline accumulation do not adequately explain its functions in salinity tolerance, and thus it is difficult to interpret the discrepancies among different data. Moreover, several reports indicate that Pro role in salt tolerance is a matter of debates, as whether Pro accumulation has adaptive significance or is a consequence of alterations in cellular metabolism induced by salinity. As no consensus is obtained on the exact roles of proline production, proline exact roles in the adaptation to saline environments is therefore still lacking and is even a matter of debates. It is obvious that comprehensive future research is needed to establish the proline exact mechanism by which it enhances plant salt tolerance. We propose, however, that proline might be essential for improving salinity tolerance in some species/cultivars, but may not be relevant in others. Evidence supporting both arguments has been presented in order to reassess the feasibility of the proposed roles of Pro in plant salt tolerance mechanism.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

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

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

The durum wheat plasma membrane Na+/H+ antiporter SOS1 is involved in oxidative stress response

We have shown previously that the durum wheat TdSOS1 excludes Na⁺ and Li⁺ ions outside cells. Moreover, this protein is activated by Arabidopsis kinase SOS2 through phosphorylation. The elimination of both SOS2 phosphorylation sites and the auto-inhibitory domain produces a hyperactive TdSOS1∆972 form, which have a maximal activity independent from the regulatory SOS2/SOS3 complex. We demonstrated that the expression of TdSOS1 enhances salt tolerance of the transgenic Arabidopsis plants. In this study, we analyzed the response to H₂O₂-induced oxidative stress of the transgenic Arabidopsis expressing one of the two TdSOS1 forms. Firstly, we showed that the exogenous H₂O₂ treatment leads to an accumulation of SOS1 transcripts in leaves and roots of the durum wheat and also in the transgenic plants. These transgenic plants showed significant oxidative stress tolerance compared to control plants, especially the plants expressing the hyperactive form. This tolerance was manifested by high proline accumulation and low malonyldialdehyde (MDA), O₂˙^- and H₂O₂ contents. Furthermore, the activities of three essential ROS scavenging enzymes (SOD, CAT, and POD) were higher in the transgenic plants under oxidative stress, as compared to control plants. Taken together, these data suggested that TdSOS1 plays a crucial role in response to oxidative stress.

Continental-level population differentiation and environmental adaptation in the mushroom Suillus brevipes

Recent advancements in sequencing technology allowed researchers to better address the patterns and mechanisms involved in microbial environmental adaptation at large spatial scales. Here we investigated the genomic basis of adaptation to climate at the continental scale in Suillus brevipes, an ectomycorrhizal fungus symbiotically associated with the roots of pine trees. We used genomic data from 55 individuals in seven locations across North America to perform genome scans to detect signatures of positive selection and assess whether temperature and precipitation were associated with genetic differentiation. We found that S. brevipes exhibited overall strong population differentiation, with potential admixture in Canadian populations. This species also displayed genomic signatures of positive selection as well as genomic sites significantly associated with distinct climatic regimes and abiotic environmental parameters. These genomic regions included genes involved in transmembrane transport of substances and helicase activity potentially involved in cold stress response. Our study sheds light on large-scale environmental adaptation in fungi by identifying putative adaptive genes and providing a framework to further investigate the genetic basis of fungal adaptation.

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.

Integrative Control Between Proton Pumps and SOS1 Antiporters in Roots is Crucial for Maintaining Low Na+ Accumulation and Salt Tolerance in Ammonium-Supplied Sorghum bicolor

An effective strategy for re-establishing K+ and Na+ homeostasis is a challenge for the improvement of plant performance in saline soil. Specifically, attempts to understand the mechanisms of Na+ extrusion from plant cells, the control of Na+ loading in the xylem and the partitioning of the accumulated Na+ between different plant organs are ongoing. Our goal was to provide insight into how an external nitrogen source affects Na+ accumulation in Sorghum bicolor under saline conditions. The NH4+ supply improved the salt tolerance of the plant by restricting Na+ accumulation and improving the K+/Na+ homeostasis in shoots, which was consistent with the high activity and expression of Na+/H+ antiporters and proton pumps in the plasma membrane and vacuoles in the roots, resulting in low Na+ loading in the xylem. Conversely, although NO3--grown plants had exclusion and sequestration mechanisms for Na+, these responses were not sufficient to reduce Na+ accumulation. In conclusion, NH4+ acts as an efficient signal to activate co-ordinately responses involved in the regulation of Na+ homeostasis in sorghum plants under salt stress, which leads to salt tolerance.

Cold stress increases salt tolerance of the extremophytes Eutrema salsugineum (Thellungiella salsuginea) and Eutrema (Thellungiella) botschantzevii

A comparative study was performed to analyze the effect of cold acclimation on improving the resistance of Arabidopsis thaliana, Eutrema salsugineum and Eutrema botschantzevii plants to salt stress. Shoot FW, sodium and potassium accumulation, metabolite content, expression of proton pump genes VAB1, VAB2,VAB3, VP2, HA3 and genes encoding ion transporters SOS1, HKT1, NHX1, NHX2, NHX5 located in the plasma membrane or tonoplast were determined just after the cold treatment and the onset of the salt stress. In the same cold-acclimated E. botschantzevii plants, the Na⁺ concentration after salt treatment was around 80% lower than in non-acclimated plants, whereas the K⁺ concentration was higher. As a result of cold acclimation, the expression of, VAB3, NHX2, NHX5 genes and of SOS1, VP2, HA3 genes was strongly enhanced in E. botschantzevii and in E. salsugineum plants correspondently. None of the 10 genes analyzed showed any expression change in A. thaliana plants after cold acclimation. Altogether, the results indicate that cold-induced adaptation to subsequent salt stress exists in the extremophytes E. botschantzevii and to a lesser extend in E. salsugineum and is absent in Arabidopsis. This phenomenon may be attributed to the increased expression of ion transporter genes during cold acclimation in the Eutrema species.

Sodium efflux in plant roots: what do we really know?

The efflux of sodium (Na(+)) ions across the plasma membrane of plant root cells into the external medium is surprisingly poorly understood. Nevertheless, Na(+) efflux is widely regarded as a major mechanism by which plants restrain the rise of Na(+) concentrations in the cytosolic compartments of root cells and, thus, achieve a degree of tolerance to saline environments. In this review, several key ideas and bodies of evidence concerning root Na(+) efflux are summarized with a critical eye. Findings from decades past are brought to bear on current thinking, and pivotal studies are discussed, both "purely physiological", and also with regard to the SOS1 protein, the only major Na(+) efflux transporter that has, to date, been genetically characterized. We find that the current model of rapid transmembrane sodium cycling (RTSC), across the plasma membrane of root cells, is not adequately supported by evidence from the majority of efflux studies. An alternative hypothesis cannot be ruled out, that most Na(+) tracer efflux from the root in the salinity range does not proceed across the plasma membrane, but through the apoplast. Support for this idea comes from studies showing that Na(+) efflux, when measured with tracers, is rarely affected by the presence of inhibitors or the ionic composition in saline rooting media. We conclude that the actual efflux of Na(+) across the plasma membrane of root cells may be much more modest than what is often reported in studies using tracers, and may predominantly occur in the root tips, where SOS1 expression has been localized.

Isolation and functional characterization of salt-stress induced RCI2-like genes from Medicago sativa and Medicago truncatula

Salt stress is one of the most significant adverse abiotic factors, causing crop failure worldwide. So far, a number of salt stress-induced genes, and genes improving salt tolerance have been characterized in a range of plants. Here, we report the isolation and characterization of a salt stress-induced Medicago sativa (alfalfa) gene (MsRCI2A), which showed a high similarity to the yeast plasma membrane protein 3 gene (PMP3) and Arabidopsis RCI2A. The sequence comparisons revealed that five genes of MtRCI2(A-E) showed a high similarity to MsRCI2A in the Medicago truncatula genome. MsRCI2A and MtRCI2(A-E) encode small, highly hydrophobic proteins containing two putative transmembrane domains, predominantly localized in the plasma membrane. The transcript analysis results suggest that MsRCI2A and MtRCI2(A-D) genes are highly induced by salt stress. The expression of MsRCI2A and MtRCI2(A-C) in yeast mutants lacking the PMP3 gene can functionally complement the salt sensitivity phenotype resulting from PMP3 deletion. Overexpression of MsRCI2A in Arabidopsis plants showed improved salt tolerance suggesting the important role of MsRCI2A in salt stress tolerance in alfalfa.