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

Current advances in plant-microbe communication via volatile organic compounds as an innovative strategy to improve plant growth

Volatile organic compounds (VOCs) emitted by microorganisms have demonstrated an important role to improve growth and tolerance against abiotic stress on plants. Most studies have used Arabidopsis thaliana as a model plant, extending to other plants of commercial interest in the last years. Interestingly, the microbial VOCs are characterized by its biodegradable structure, quick action, absence of toxic substances, and acts at lower concentration to regulate plant physiological changes. These compounds modulate plant physiological processes such as phytohormone pathways, photosynthesis, nutrient acquisition, and metabolisms. Besides, the regulation of gene expression associated with cell components, biological processes, and molecular function are triggered by microbial VOCs. Otherwise, few studies have reported the important role of VOCs for confer plant tolerance to abiotic stress, such as drought and salinity. Although VOCs have shown an efficient action to enhance the plant growth under controlled conditions, there are still great challenges for their greenhouse or field application. Therefore, in this review, we summarize the current knowledge about the technical procedures, study cases, and physiological mechanisms triggered by microbial VOCs to finally discuss the challenges of its application in agriculture.

Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation

HKT channels are a plant protein family involved in sodium (Na⁺) and potassium (K⁺) uptake and Na⁺-K⁺ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K⁺ shortage by allowing increased Na⁺ uptake under K⁺ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na⁺/K⁺ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.

Na+ and/or Cl- Toxicities Determine Salt Sensitivity in Soybean (Glycine max (L.) Merr.), Mungbean (Vigna radiata (L.) R. Wilczek), Cowpea (Vigna unguiculata (L.) Walp.), and Common Bean (Phaseolus vulgaris L.)

Grain legumes are important crops, but they are salt sensitive. This research dissected the responses of four (sub)tropical grain legumes to ionic components (Na+ and/or Cl-) of salt stress. Soybean, mungbean, cowpea, and common bean were subjected to NaCl, Na+ salts (without Cl-), Cl- salts (without Na+), and a "high cation" negative control for 57 days. Growth, leaf gas exchange, and tissue ion concentrations were assessed at different growing stages. For soybean, NaCl and Na+ salts impaired seed dry mass (30% of control), more so than Cl- salts (60% of control). All treatments impaired mungbean growth, with NaCl and Cl- salt treatments affecting seed dry mass the most (2% of control). For cowpea, NaCl had the greatest adverse impact on seed dry mass (20% of control), while Na+ salts and Cl- salts had similar intermediate effects (~45% of control). For common bean, NaCl had the greatest adverse effect on seed dry mass (4% of control), while Na+ salts and Cl- salts impaired seed dry mass to a lesser extent (~45% of control). NaCl and Na+ salts (without Cl-) affected the photosynthesis (Pn) of soybean more than Cl- salts (without Na+) (50% of control), while the reverse was true for mungbean. Na+ salts (without Cl-), Cl- salts (without Na+), and NaCl had similar adverse effects on Pn of cowpea and common bean (~70% of control). In conclusion, salt sensitivity is predominantly determined by Na+ toxicity in soybean, Cl- toxicity in mungbean, and both Na+ and Cl- toxicity in cowpea and common bean.

Coconut genome assembly enables evolutionary analysis of palms and highlights signaling pathways involved in salt tolerance

Coconut (Cocos nucifera) is the emblematic palm of tropical coastal areas all around the globe. It provides vital resources to millions of farmers. In an effort to better understand its evolutionary history and to develop genomic tools for its improvement, a sequence draft was recently released. Here, we present a dense linkage map (8402 SNPs) aiming to assemble the large genome of coconut (2.42 Gbp, 2n = 32) into 16 pseudomolecules. As a result, 47% of the sequences (representing 77% of the genes) were assigned to 16 linkage groups and ordered. We observed segregation distortion in chromosome Cn15, which is a signature of strong selection among pollen grains, favouring the maternal allele. Comparing our results with the genome of the oil palm Elaeis guineensis allowed us to identify major events in the evolutionary history of palms. We find that coconut underwent a massive transposable element invasion in the last million years, which could be related to the fluctuations of sea level during the glaciations at Pleistocene that would have triggered a population bottleneck. Finally, to better understand the facultative halophyte trait of coconut, we conducted an RNA-seq experiment on leaves to identify key players of signaling pathways involved in salt stress response. Altogether, our findings represent a valuable resource for the coconut breeding community.

Plant mineral transport systems and the potential for crop improvement

Nutrient transporter genes could be a potential candidate for improving crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. The world's food supply is nearing a crisis in meeting the demands of an ever-growing global population, and an increase in both yield and nutrient value of major crops is vitally necessary to meet the increased population demand. Nutrients play an important role in plant metabolism as well as growth and development, and nutrient deficiency results in retarded plant growth and leads to reduced crop yield. A variety of cellular processes govern crop plant nutrient absorption from the soil. Among these, nutrient membrane transporters play an important role in the acquisition of nutrients from soil and transport of these nutrients to their target sites. In addition, as excess nutrient delivery has toxic effects on plant growth, these membrane transporters also play a significant role in the removal of excess nutrients in the crop plant. The key function provided by membrane transporters is the ability to supply the crop plant with an adequate level of tolerance against environmental stresses, such as soil acidity, alkalinity, salinity, drought, and pathogen attack. Membrane transporter genes have been utilized for the improvement of crop plants, with enhanced nutrient uptake leading to increased crop yield by providing tolerance against different biotic and abiotic stresses. Further understanding of the basic mechanisms of nutrient transport in crop plants could facilitate the advanced design of engineered plant crops to achieve increased yield and improve nutrient quality through the use of genetic technologies as well as molecular breeding. This review is focused on nutrient toxicity and tolerance mechanisms in crop plants to aid in understanding and addressing the anticipated global food demand.

Agro-Physiologic Responses and Stress-Related Gene Expression of Four Doubled Haploid Wheat Lines under Salinity Stress Conditions

Simple Summary

Productivity of wheat can be enhanced using salt-tolerant genotypes. However, the assessment of salt tolerance potential in wheat through agro-physiological traits and stress-related gene expression analysis could potentially minimize the cost of breeding programs and be a powerful way for the selection of the most salt-tolerant genotype. The study evaluated the salt tolerance potential of four doubled haploid lines of wheat and compared them with the check cultivar Sakha-93 using an extensive set of agro-physiologic parameters and salt-stress-related gene expressions. The results indicated that the five genotypes tested displayed reduction in all traits evaluated except the canopy temperature and electrical conductivity, which had the greatest decline occurring in the check cultivar and the least decline in DHL2. The genotypes DHL21 and DHL5 exhibited increased expression rate of salt-stress-related genes under salt stress conditions. The multiple linear regression model and path coefficient analysis showed a coefficient of determination of 0.93. Concluding, the number of spikelets, and/or number of kernels were identified to be unbiased traits for assessing wheat DHLs under salinity conditions, given their contribution and direct impact on the grain yield. Moreover, the two most salt-tolerant genotypes DHL2 and DHL21 can be useful as genetic resources for future breeding programs.

Abstract

Salinity majorly hinders horizontal and vertical expansion in worldwide wheat production. Productivity can be enhanced using salt-tolerant wheat genotypes. However, the assessment of salt tolerance potential in bread wheat doubled haploid lines (DHL) through agro-physiological traits and stress-related gene expression analysis could potentially minimize the cost of breeding programs and be a powerful way for the selection of the most salt-tolerant genotype. We used an extensive set of agro-physiologic parameters and salt-stress-related gene expressions. Multivariate analysis was used to detect phenotypic and genetic variations of wheat genotypes more closely under salinity stress, and we analyzed how these strategies effectively balance each other. Four doubled haploid lines (DHLs) and the check cultivar (Sakha93) were evaluated in two salinity levels (without and 150 mM NaCl) until harvest. The five genotypes showed reduced growth under 150 mM NaCl; however, the check cultivar (Sakha93) died at the beginning of the flowering stage. Salt stress induced reduction traits, except the canopy temperature and initial electrical conductivity, which was found in each of the five genotypes, with the greatest decline occurring in the check cultivar (Sakha-93) and the least decline in DHL2. The genotypes DHL21 and DHL5 exhibited increased expression rate of salt-stress-related genes (TaNHX1, TaHKT1, and TaCAT1) compared with DHL2 and Sakha93 under salt stress conditions. Principle component analysis detection of the first two components explains 70.78% of the overall variation of all traits (28 out of 32 traits). A multiple linear regression model and path coefficient analysis showed a coefficient of determination (R²) of 0.93. The models identified two interpretive variables, number of spikelets, and/or number of kernels, which can be unbiased traits for assessing wheat DHLs under salinity stress conditions, given their contribution and direct impact on the grain yield.

Transcriptome skimming of lentil (Lens culinaris Medikus) cultivars with contrast reaction to salt stress

Extensive transcriptomic skimming was conducted to decipher molecular, morphological, physiological, and biochemical responses in salt-tolerant (PDL-1) and salt-sensitive (L-4076) cultivars under control (0 mM NaCl) and salinity stress (120 mM NaCl) conditions at seedling stage. Morphological, physiological, and biochemical studies revealed that PDL-1 exhibited no salt injury and had higher K⁺/Na⁺ ratio, relative water content (RWC), chlorophyll, glycine betaine, and soluble sugars in leaves while lower H₂O₂ induced fluorescence signals in roots as compared to L-4076. Transcriptomic profile revealed a total of 17,433 significant differentially expressed genes (DEGs) under different treatments and cultivar combinations that include 2557 upregulated and 1533 downregulated transcripts between contrasting cultivars under salt stress. Accuracy of transcriptomic analysis was validated through quantification of 10 DEGs via quantitative real-time polymerase chain reaction (qRT-PCR). DEGs were functionally characterized by Gene Ontology (GO) analysis and assigned to various metabolic pathways using MapMan. DEGs were found to be significantly associated with phytohormone-mediated signal transduction, cellular redox homoeostasis, secondary metabolism, nitrogen metabolism, and cellular stress signaling. The present study revealed putative molecular mechanism of salinity tolerance in lentil together with identification of 5643 simple sequence repeats (SSRs) and 176,433 single nucleotide polymorphisms (SNPs) which can be utilized to enhance linkage maps density along with detection of quantitative trait loci (QTLs) associated with traits of interests. Stress-related pathways identified in this study divulged plant functioning that can be targeted to improve salinity stress tolerance in crop species.

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.

Comparative transcriptome analysis reveals the regulatory effects of acetylcholine on salt tolerance of Nicotiana benthamiana

Salinity is a major cause of crop losses worldwide. Acetylcholine (ACh) can ameliorate the adverse effects of abiotic stresses on plant growth, including salinity stress; however, the underlying molecular mechanisms of this process are unclear. Here, seedlings of Nicotiana benthamiana grown under normal conditions or exposed to 150 mmol L-1 NaCl salinity stress were then treated with a root application of 10 μM ACh. Exogenous ACh application resulted in the downregulation of the activity of the antioxidant enzymes, ascorbate peroxidase, and catalase. ACh-treated plants had lower levels of reactive oxygen species, including the superoxide anion radical and hydrogen peroxide. Transcriptome analysis indicated that ACh treatment under salt stress promoted the differential expression of 658 genes in leaves of N. benthamiana (527 were upregulated and 131 were downregulated). Gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that exogenous ACh application was associated with a substantial increase in the transcripts of genes related to cell wall peroxidases, xyloglucan endotransglucosylases or hydrolases, and expansins, indicating that ACh activates cell wall biosynthesis in salt-stressed plants. ACh also enhanced the expression of genes associated with the auxin, gibberellin, brassinosteroid, and salicylic acid signalling pathways, indicating that ACh induces the activation of these pathways under salt stress. Collectively, these findings indicate that ACh-induced salt tolerance in N. benthamiana seedlings is mediated by the inhibition of antioxidant enzymes, activation of cell wall biosynthesis, and hormone signalling pathways. Stress-induced genes involved in osmotic regulation and oxidation resistance were induced by ACh under salt stress. The genes whose transcript levels were elevated by ACh treatment in salt-stressed N. benthamiana could be used as molecular markers of the physiological status of plants under salt stress.

Plant Proton Pumps and Cytosolic pH-Homeostasis

Proton pumps create a proton motif force and thus, energize secondary active transport at the plasma nmembrane and endomembranes of the secretory pathway. In the plant cell, the dominant proton pumps are the plasma membrane ATPase, the vacuolar pyrophosphatase (V-PPase), and the vacuolar-type ATPase (V-ATPase). All these pumps act on the cytosolic pH by pumping protons into the lumen of compartments or into the apoplast. To maintain the typical pH and thus, the functionality of the cytosol, the activity of the pumps needs to be coordinated and adjusted to the actual needs. The cellular toolbox for a coordinated regulation comprises 14-3-3 proteins, phosphorylation events, ion concentrations, and redox-conditions. This review combines the knowledge on regulation of the different proton pumps and highlights possible coordination mechanisms.

Linking diverse salinity responses of 14 almond rootstocks with physiological, biochemical, and genetic determinants

Fourteen commercial almond rootstocks were tested under five types of irrigation waters to understand the genetic, physiological, and biochemical bases of salt-tolerance mechanisms. Treatments included control (T1) and four saline water treatments dominant in sodium-sulfate (T2), sodium-chloride (T3), sodium-chloride/sulfate (T4), and calcium/magnesium-chloride/sulfate (T5). T3 caused the highest reduction in survival rate and trunk diameter, followed by T4 and T2, indicating that Na and, to a lesser extent, Cl were the most toxic ions to almond rootstocks. Peach hybrid (Empyrean 1) and peach-almond hybrids (Cornerstone, Bright’s Hybrid 5, and BB 106) were the most tolerant to salinity. Rootstock’s performance under salinity correlated highly with its leaf Na and Cl concentrations, indicating that Na⁺ and Cl^- exclusion is crucial for salinity tolerance in Prunus. Photosynthetic rate correlated with trunk diameter and proline leaf ratio (T3/T1) significantly correlated with the exclusion of Na⁺ and Cl^-, which directly affected the survival rate. Expression analyses of 23 genes involved in salinity stress revealed that the expression differences among genotypes were closely associated with their performance under salinity. Our genetic, molecular, and biochemical analyses allowed us to characterize rootstocks based on component traits of the salt-tolerance mechanisms, which may facilitate the development of highly salt-tolerant rootstocks.

Biochemical and molecular characterisations of salt tolerance components in rice varieties tolerant and sensitive to NaCl: the relevance of Na+ exclusion in salt tolerance in the species

Soil salinisation is a major abiotic stress in agriculture, and is especially a concern for rice production because among cereal crops, rice is the most salt-sensitive. However, the production of rice must be increased substantially by the year 2050 to meet the demand of the ever growing population. Hence, understanding the biochemical events determining salt tolerance in rice is highly desirable so that the trait can be introduced in cultivars of interest through biotechnological intervention. In this context, an initial study on NaCl response in four Indica rice varieties showed a lower uptake of Na+ in the salt-tolerant Nona Bokra and Pokkali than in the salt-sensitive IR64 and IR29, indicating Na+ exclusion as a primary requirement of salt tolerance in the species. This was also supported by the following features in the salt-tolerant, but not in the -sensitive varieties: (1) highly significant NaCl-induced increase in the activity of PM-H+ATPase, (2) a high constitutive level and NaCl-induced threonine phosphorylation of PM-H+ATPase, necessary to promote its activity, (3) a high constitutive expression of 14-3-3 protein that makes PM-H+ATPase active by binding with the phosphorylated threonine at the C-terminal end, (4) a high constitutive and NaCl-induced expression of SOS1 in roots, and (5) significant NaCl-induced expression of OsCIPK 24, a SOS2 that phosphorylates SOS1. The vacuolar sequestration of Na+ in seedlings was not reflected from the expression pattern of NHX1/NHX1 in response to NaCl. NaCl-induced downregulation of expression of HKTs in roots of Nona Bokra, but upregulation in Pokkali also indicates that their role in salt tolerance in rice could be cultivar specific. The study indicates that consideration of increasing exclusion of Na+ by enhancing the efficiency of SOS1/PM-H+ATPase Na+ exclusion module could be an important aspect in attempting to increase salt tolerance in the rice varieties or cultivars of interest.

Overexpression of MdMIPS1 enhances salt tolerance by improving osmosis, ion balance, and antioxidant activity in transgenic apple

Myo-inositol and its derivatives play vital roles in plant stress tolerance. Myo-inositol-1-phosphate synthase (MIPS) is the rate-limiting enzyme of myo-inositol biosynthesis. However, the role of apple MIPS-mediated myo-inositol biosynthesis in stress tolerance remains elusive. In this study, we found that ectopic expression of MdMIPS1 from apple increased myo-inositol content and enhanced tolerance to salt and osmotic stresses in transgenic Arabidopsis lines. In transgenic apple lines over-expressing MdMIPS1, the increased myo-inositol levels could promote accumulation of other osmoprotectants such as glucose, sucrose, galactose, and fructose, to alleviate salinity-induced osmotic stress. Also, it was shown that overexpression of MdMIPS1 enhanced salinity tolerance by improving the antioxidant system to scavenge ROS, as well as Na⁺ and K⁺ homeostasis. Taken together, our results revealed a protective role of MdMIPS1-mediated myo-inositol biosynthesis in salt tolerance by improving osmotic balance, antioxidant defense system, and ion homeostasis in apple.

Comprehensive dissection into morpho-physiologic responses, ionomic homeostasis, and transcriptomic profiling reveals the systematic resistance of allotetraploid rapeseed to salinity

BACKGROUND

Salinity severely inhibit crop growth, yield, and quality worldwide. Allotetraploid rapeseed (Brassica napus L.), a major glycophyte oil crop, is susceptible to salinity. Understanding the physiological and molecular strategies of rapeseed salinity resistance is a promising and cost-effective strategy for developing highly resistant cultivars.

RESULTS

First, early leaf senescence was identified and root system growth was inhibited in rapeseed plants under severe salinity conditions. Electron microscopic analysis revealed that 200 mM NaCl induced fewer leaf trichomes and stoma, cell plasmolysis, and chloroplast degradation. Primary and secondary metabolite assays showed that salinity led to an obviously increased anthocyanin, osmoregulatory substances, abscisic acid, jasmonic acid, pectin, cellulose, reactive oxygen species, and antioxidant activity, and resulted in markedly decreased photosynthetic pigments, indoleacetic acid, cytokinin, gibberellin, and lignin. ICP-MS assisted ionomics showed that salinity significantly constrained the absorption of essential elements, including the nitrogen, phosphorus, potassium, calcium, magnesium, iron, mangnese, copper, zinc, and boron nutrients, and induced the increase in the sodium/potassium ratio. Genome-wide transcriptomics revealed that the differentially expressed genes were involved mainly in photosynthesis, stimulus response, hormone signal biosynthesis/transduction, and nutrient transport under salinity.

CONCLUSIONS

The high-resolution salt-responsive gene expression profiling helped the efficient characterization of central members regulating plant salinity resistance. These findings might enhance integrated comprehensive understanding of the morpho-physiologic and molecular responses to salinity and provide elite genetic resources for the genetic modification of salinity-resistant crop species.

Plant NHX Antiporters: From Function to Biotechnological Application, with Case Study

Salt stress is one of the major abiotic stresses that negatively affect crops worldwide. Plants have evolved a series of mechanisms to cope with the limitations imposed by salinity. Molecular mechanisms, including the upregulation of cation transporters such as the Na+/H+ antiporters, are one of the processes adopted by plants to survive in saline environments. NHX antiporters are involved in salt tolerance, development, cell expansion, growth performance and disease resistance of plants. They are integral membrane proteins belonging to the widely distributed CPA1 sub-group of monovalent cation/H+ antiporters and provide an important strategy for ionic homeostasis in plants under saline conditions. These antiporters are known to regulate the exchange of sodium and hydrogen ions across the membrane and are ubiquitous to all eukaryotic organisms. With the genomic approach, previous studies reported that a large number of proteins encoding Na+/H+ antiporter genes have been identified in many plant species and successfully introduced into desired species to create transgenic crops with enhanced tolerance to multiple stresses. In this review, we focus on plant antiporters and all the aspects from their structure, classification, function to their in silico analysis. On the other hand, we performed a genome-wide search to identify the predicted NHX genes in Argania spinosa L. We highlighted for the first time the presence of four putative NHX (AsNHX1-4) from the Argan tree genome, whose phylogenetic analysis revealed their classification in one distinct vacuolar cluster. The essential information of the four putative NHXs, such as gene structure, subcellular localization and transmembrane domains was analyzed.

Chitosan regulates metabolic balance, polyamine accumulation, and Na+ transport contributing to salt tolerance in creeping bentgrass

BACKGROUND

Chitosan (CTS), a natural polysaccharide, exhibits multiple functions of stress adaptation regulation in plants. However, effects and mechanism of CTS on alleviating salt stress damage are still not fully understood. Objectives of this study were to investigate the function of CTS on improving salt tolerance associated with metabolic balance, polyamine (PAs) accumulation, and Na+ transport in creeping bentgrass (Agrostis stolonifera).

RESULTS

CTS pretreatment significantly alleviated declines in relative water content, photosynthesis, photochemical efficiency, and water use efficiency in leaves under salt stress. Exogenous CTS increased endogenous PAs accumulation, antioxidant enzyme (SOD, POD, and CAT) activities, and sucrose accumulation and metabolism through the activation of sucrose synthase and pyruvate kinase activities, and inhibition of invertase activity. The CTS also improved total amino acids, glutamic acid, and γ-aminobutyric acid (GABA) accumulation. In addition, CTS-pretreated plants exhibited significantly higher Na+ content in roots and lower Na+ accumulation in leaves then untreated plants in response to salt stress. However, CTS had no significant effects on K+/Na+ ratio. Importantly, CTS enhanced salt overly sensitive (SOS) pathways and also up-regulated the expression of AsHKT1 and genes (AsNHX4, AsNHX5, and AsNHX6) encoding Na+/H+ exchangers under salt stress.

CONCLUSIONS

The application of CTS increased antioxidant enzyme activities, thereby reducing oxidative damage to roots and leaves. CTS-induced increases in sucrose and GABA accumulation and metabolism played important roles in osmotic adjustment and energy metabolism during salt stress. The CTS also enhanced SOS pathway associated with Na+ excretion from cytosol into rhizosphere, increased AsHKT1 expression inhibiting Na+ transport to the photosynthetic tissues, and also up-regulated the expression of AsNHX4, AsNHX5, and AsNHX6 promoting the capacity of Na+ compartmentalization in roots and leaves under salt stress. In addition, CTS-induced PAs accumulation could be an important regulatory mechanism contributing to enhanced salt tolerance. These findings reveal new functions of CTS on regulating Na+ transport, enhancing sugars and amino acids metabolism for osmotic adjustment and energy supply, and increasing PAs accumulation when creeping bentgrass responds to salt stress.

Silver Nanoparticle Regulates Salt Tolerance in Wheat Through Changes in ABA Concentration, Ion Homeostasis, and Defense Systems

Salinity is major abiotic stress affecting crop yield, productivity and reduces the land-usage area for agricultural practices. The purpose of this study is to analyze the effect of green-synthesized silver nanoparticle (AgNP) on physiological traits of wheat (Triticum aestivum) under salinity stress. Using augmented and high-throughput characterization of synthesized AgNPs, this study investigated the proximity of AgNPs-induced coping effects under stressful cues by measuring the germination efficiency, oxidative-biomarkers, enzymatic and non-enzymatic antioxidants, proline and nitrogen metabolism, stomatal dynamics, and ABA content. Taken together, the study shows a promising approach in salt tolerance and suggests that mechanisms of inducing the salt tolerance depend on proline metabolism, ions accumulation, and defense mechanisms. This study ascertains the queries regarding the correlation between nanoparticles use and traditional agriculture methodology; also significantly facilitates to reach the goal of sustainable developments for increasing crop productivity via much safer and greener approachability.

Comprehensive transcriptome profiling of Caragana microphylla in response to salt condition using de novo assembly

OBJECTIVE

To investigate the response of Caragana microphylla in salt condition, transcriptome analysis, differentially expressed genes (DEGs) comparison with Arabidopsis thaliana, and the chlorophyll content analysis were performed.

RESULTS

Gene Ontology (GO) term, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of DEGs indicated that salt condition affected photosynthesis and chlorophyll in C. microphylla. The DEGs compared with salt responsive genes of A. thaliana indicated that C. microphylla's responses to salt differed greatly from those of the model plant and that the results also indicated up-regulated genes related to photosynthesis and chlorophyll in C. microphylla. Moreover, we confirmed that salt-treated C. microphylla increased chlorophyll content, and the genes of protoporphyrin IX downstream in chlorophyll biosynthesis were induced in the heatmap analysis.

CONCLUSIONS

These results showed a similar pattern to some halophytes plants with increased chlorophyll at a certain salt concentration, and we assumed that C. microphylla also has a mechanism to adapt or tolerate moderate salt conditions.

Multidimensional Evaluation for Detecting Salt Tolerance of Bread Wheat Genotypes Under Actual Saline Field Growing Conditions

Field-based trials and genotype evaluation until yielding stage are two important steps in improving the salt tolerance of crop genotypes and identifying what parameters can be strong candidates for the better understanding of salt tolerance mechanisms in different genotypes. In this study, the salt tolerance of 18 bread wheat genotypes was evaluated under natural saline field conditions and at three saline irrigation levels (5.25, 8.35, and 11.12 dS m-1) extracted from wells. Multidimensional evaluation for salt tolerance of these genotypes was done using a set of agronomic and physio-biochemical attributes. Based on yield index under three salinity levels, the genotypes were classified into four groups ranging from salt-tolerant to salt-sensitive genotypes. The salt-tolerant genotypes exhibited values of total chlorophyll, gas exchange (net photosynthetic rate, transpiration rate, and stomatal conductance), water relation (relative water content and membrane stability index), nonenzymatic osmolytes (soluble sugar, free proline, and ascorbic acid), antioxidant enzyme activities (superoxide dismutase, catalase, and peroxidase), K+ content, and K+/Na+ ratio that were greater than those of salt-sensitive genotypes. Additionally, the salt-tolerant genotypes consistently exhibited good control of Na+ and Cl- levels and maintained lower contents of malondialdehyde and electrolyte leakage under high salinity level, compared with the salt-sensitive genotypes. Several physio-biochemical parameters showed highly positive associations with grain yield and its components, whereas negative association was observed in other parameters. Accordingly, these physio-biochemical parameters can be used as individual or complementary screening criteria for evaluating salt tolerance and improvement of bread wheat genotypes under natural saline field conditions.

Exogenous Silicon Enhanced Salt Resistance by Maintaining K+/Na+ Homeostasis and Antioxidant Performance in Alfalfa Leaves

Silicon (Si) has been known to enhance salt resistance in plants. In this experiment, 4-weeks-old alfalfa seedlings were exposed to different NaCl concentrations (0-200 mM) with or without 2 mM Si for two weeks. The results showed that NaCl-stressed alfalfa seedlings showed a decrease in growth performance, such as stem extension rate, predawn leaf water potential (LWP) and the chlorophyll content, potassium (K⁺) concentration, as well as the ratio of potassium/sodium ion (K⁺/Na⁺). In contrast, NaCl-stressed alfalfa seedlings increased leaf Na⁺ concentration and the malondialdehyde (MDA) level, as well as the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in alfalfa leaves. Besides, exogenous Si application enhanced photosynthetic parameters of NaCl-stressed alfalfa seedlings, which was accompanied by the improvement in predawn LWP, level of chlorophyll content, and water use efficiency (WUE). The Si-treated plants enhanced salinity tolerance by limiting Na⁺ accumulation while maintaining K⁺ concentration in leaves. It also established K⁺/Na⁺ homeostasis by increasing K⁺/Na⁺ radio to protect the leaves from Na⁺ toxicity and thereby maintained higher chlorophyll retention. Simultaneously, Si-treated plants showed higher antioxidant activities and decreased MDA content under NaCl stress. Our study concluded that Si application enhanced salt tolerance of alfalfa through improving the leaves photosynthesis, enhancing antioxidant performance and maintaining K⁺/Na⁺ homeostasis in leaves. Our data further indicated exogenous Si application could be effectively manipulated for improving salt resistance of alfalfa grown in saline soil.

Induced in vitro adaptation for salt tolerance in date palm (Phoenix dactylifera L.) cultivar Khalas

BACKGROUND

Soil salinity causes huge economic losses to agriculture productivity in arid and semiarid areas worldwide. The affected plants face disturbances in osmotic adjustment, nutrient transport, ionic toxicity and reduced photosynthesis. Conventional breeding approaches produce little success in combating various stresses in plants. However, non-conventional approaches, such as in vitro tissue culturing, produce genetic variability in the development of salt-tolerant plants, particularly in woody trees.

RESULTS

Embryogenic callus cultures of the date palm cultivar Khalas were subjected to various salt levels ranging from 0 to 300 mM in eight subcultures. The regenerants obtained from the salt-treated cultures were regenerated and evaluated using the same concentration of NaCl with which the calli were treated. All the salt-adapted (SA) regenerants showed improved growth characteristics, physiological performance, ion concentrations and K+/Na+ ratios than the salt non-adapted (SNA) regenerants and the control. Regression between the leaf Na+ concentration and net photosynthesis revealed an inverse nonlinear correlation in the SNA regenerants. Leaf K+ contents and stomatal conductance showed a strong linear relationship in SA regenerants compared with the inverse linear correlation, and a very poor coefficient of determination in SNA regenerants. The genetic fidelity of the selected SA regenerants was also tested using 36 random amplified polymorphic DNA (RAPD) primers, of which 26 produced scorable bands. The primers generated 1-10 bands, with an average of 5.4 bands per RAPD primer; there was no variation between SA regenerants and the negative control.

CONCLUSION

This is the first report of the variants generated from salt-stressed cultures and their potential adaptation to salinity in date palm cv. Khalas. The massive production of salt stress-adapted date palm plants may be much easier using the salt adaptation approach. Such plants can perform better during exposure to salt stress compared to the non-treated date palm plants.

Na+ Transporter SvHKT1;1 from a Halophytic Turf Grass Is Specifically Upregulated by High Na+ Concentration and Regulates Shoot Na+ Concentration

We characterized an Na⁺ transporter SvHKT1;1 from a halophytic turf grass, Sporobolus virginicus. SvHKT1;1 mediated inward and outward Na⁺ transport in Xenopus laevis oocytes and did not complement K⁺ transporter-defective mutant yeast. SvHKT1;1 did not complement athkt1;1 mutant Arabidopsis, suggesting its distinguishable function from other typical HKT1 transporters. The transcript was abundant in the shoots compared with the roots in S. virginicus and was upregulated by severe salt stress (500 mM NaCl), but not by lower stress. SvHKT1;1-expressing Arabidopsis lines showed higher shoot Na⁺ concentrations and lower salt tolerance than wild type (WT) plants under nonstress and salt stress conditions and showed higher Na⁺ uptake rate in roots at the early stage of salt treatment. These results suggested that constitutive expression of SvHKT1;1 enhanced Na⁺ uptake in root epidermal cells, followed by increased Na⁺ transport to shoots, which led to reduced salt tolerance. However, Na⁺ concentrations in phloem sap of the SvHKT1;1 lines were higher than those in WT plants under salt stress. Based on this result, together with the induction of the SvHKT1;1 transcription under high salinity stress, it was suggested that SvHKT1;1 plays a role in preventing excess shoot Na⁺ accumulation in S. virginicus.

Emerging crosstalk between two signaling pathways coordinates K+ and Na+ homeostasis in the halophyte Hordeum brevisubulatum

K+/Na+ homeostasis is the primary core response for plant to tolerate salinity. Halophytes have evolved novel regulatory mechanisms to maintain a suitable K+/Na+ ratio during long-term adaptation. The wild halophyte Hordeum brevisubulatum can adopt efficient strategies to achieve synergistic levels of K+ and Na+ under high salt stress. However, little is known about its molecular mechanism. Our previous study indicated that HbCIPK2 contributed to prevention of Na+ accumulation and K+ reduction. Here, we further identified the HbCIPK2-interacting proteins including upstream Ca2+ sensors, HbCBL1, HbCBL4, and HbCBL10, and downstream phosphorylated targets, the voltage-gated K+ channel HbVGKC1 and SOS1-like transporter HbSOS1L. HbCBL1 combined with HbCIPK2 could activate HbVGKC1 to absorb K+, while the HbCBL4/10-HbCIPK2 complex modulated HbSOS1L to exclude Na+. This discovery suggested that crosstalk between the sodium response and the potassium uptake signaling pathways indeed exists for HbCIPK2 as the signal hub, and paved the way for understanding the novel mechanism of K+/Na+ homeostasis which has evolved in the halophytic grass.

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

KEY MESSAGE

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

Ecological Stoichiometry Homeostasis of Six Microelements in Leymus chinensis Growing in Soda Saline-Alkali Soil

Soil salinization poses severe threats to grassland ecosystems in various parts of the world, including the Songnen Plain in northeast China. Severe impairment of plant growth in this soil is generally attributed to high soil pH, total alkalinity, and sodium (Na) contents. This paper focuses on the ecological stoichiometry of microelements, which has received much less attention than relations of macroelements, in the soil and plants (specifically Leymus chinensis) growing in it. The results show that the soil’s manganese (Mn), zinc (Zn), iron (Fe), copper (Cu), nickel (Ni) and molybdenum (Mo) contents are lower than average in Chinese soils, but only Mn and Zn are severely deficient in L. chinensis. With increases in soil pH, total alkalinity, and Na, the Mo contents in both soil and L. chinensis slightly increase, while contents of the other microelements decline. Homeostasis indices obtained for the six microelements—and Fe/Zn, Fe/Ni, Fe/Cu, and Cu/Zn ratios—were all between 0.82 and 3.34 (ranging from just below the “plastic” threshold to “weakly homeostatic”). Despite Zn deficiency in the soil, Zn appears to have the highest homeostasis of the six elements in L. chinensis (homeostasis indices of Zn, Cu, Ni, Mn, Fe and Mo were 3.34, 2.54, 1.86, 1.76, 1.52, and 1.33, respectively). In addition, the Cu/Zn ratio had the highest homeostasis index (1.85), followed by Fe/Zn (1.02), Fe/Cu (0.95) and Fe/Ni (0.82). Appropriate application of Mn and Zn fertilizers is recommended to promote the growth and development of L. chinensis in soda saline-alkali soil.

Effects of Drought and Salinity on Two Commercial Varieties of Lavandula angustifolia Mill

Global warming is not only affecting arid and semi-arid regions but also becoming a threat to agriculture in Central and Eastern European countries. The present study analyzes the responses to drought and salinity of two varieties of Lavandula angustifolia cultivated in Romania. Lavender seedlings were subjected to one month of salt stress (100, 200, and 300 mM NaCl) and water deficit (complete withholding of irrigation) treatments. To assess the effects of stress on the plants, several growth parameters and biochemical stress markers (photosynthetic pigments, mono and divalent ions, and different osmolytes) were determined in control and stressed plants after the treatments. Both stress conditions significantly inhibited the growth of the two varieties, but all plants survived the treatments, indicating a relative stress tolerance of the two varieties. The most relevant mechanisms of salt tolerance are based on the maintenance of foliar K⁺ levels and the accumulation of Ca²⁺ and proline as a functional osmolyte in parallel with increasing external salinities. Under water stress, significant increases of Na⁺ and K⁺ concentrations were detected in roots, indicating a possible role of these cations in osmotic adjustment, limiting root dehydration. No significant differences were found when comparing the stress tolerance and stress responses of the two selected lavender varieties.

Effects of Drought and Salinity on Two Commercial Varieties of Lavandula angustifolia Mill

Global warming is not only affecting arid and semi-arid regions but also becoming a threat to agriculture in Central and Eastern European countries. The present study analyzes the responses to drought and salinity of two varieties of Lavandula angustifolia cultivated in Romania. Lavender seedlings were subjected to one month of salt stress (100, 200, and 300 mM NaCl) and water deficit (complete withholding of irrigation) treatments. To assess the effects of stress on the plants, several growth parameters and biochemical stress markers (photosynthetic pigments, mono and divalent ions, and different osmolytes) were determined in control and stressed plants after the treatments. Both stress conditions significantly inhibited the growth of the two varieties, but all plants survived the treatments, indicating a relative stress tolerance of the two varieties. The most relevant mechanisms of salt tolerance are based on the maintenance of foliar K⁺ levels and the accumulation of Ca²⁺ and proline as a functional osmolyte in parallel with increasing external salinities. Under water stress, significant increases of Na⁺ and K⁺ concentrations were detected in roots, indicating a possible role of these cations in osmotic adjustment, limiting root dehydration. No significant differences were found when comparing the stress tolerance and stress responses of the two selected lavender varieties.

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress

Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown. The present study investigated the impact of exogenously applied ACh (10 µm) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150 mm NaCl). Salinity reduced root hydraulic conductivity while ACh-treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin-IX, Mg-photoporphyrin-IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up-regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO2 concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity-induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio. Results of the present study indicate that ACh can be effective in ameliorating NaCl-induced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

Extending thermotolerance to tomato seedlings by inoculation with SA1 isolate of Bacillus cereus and comparison with exogenous humic acid application

Heat stress is one of the major abiotic stresses that impair plant growth and crop productivity. Plant growth-promoting endophytic bacteria (PGPEB) and humic acid (HA) are used as bio-stimulants and ecofriendly approaches to improve agriculture crop production and counteract the negative effects of heat stress. Current study aimed to analyze the effect of thermotolerant SA1 an isolate of Bacillus cereus and HA on tomato seedlings. The results showed that combine application of SA1+HA significantly improved the biomass and chlorophyll fluorescence of tomato plants under normal and heat stress conditions. Heat stress increased abscisic acid (ABA) and reduced salicylic acid (SA) content; however, combined application of SA1+HA markedly reduced ABA and increased SA. Antioxidant enzymes activities revealed that SA1 and HA treated plants exhibited increased levels of ascorbate peroxidase (APX), superoxide dismutase (SOD), and reduced glutathione (GSH). In addition, heat stress markedly reduced the amino acid contents; however, the amino acids were increased with co-application of SA1+HA. Similarly, inductively-coupled plasma mass-spectrometry results showed that plants treated with SA1+HA exhibited significantly higher iron (Fe+), phosphorus (P), and potassium (K+) uptake during heat stress. Heat stress increased the relative expression of SlWRKY33b and autophagy-related (SlATG5) genes, whereas co-application of SA1+HA augmented the heat stress response and reduced SlWRKY33b and SlATG5 expression. The heat stress-responsive transcription factor (SlHsfA1a) and high-affinity potassium transporter (SlHKT1) were upregulated in SA1+HA-treated plants. In conclusion, current findings suggest that co-application with SA1+HA can be used for the mitigation of heat stress damage in tomato plants and can be commercialized as a biofertilizer.

Salt Tolerance Mechanisms of Plants

Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context in order to link them to observed phases in growth rate that occur in response to stress. Recent advances in phenotyping can now functionally or genetically link cellular signaling responses, ion transport, water management, and gene expression to growth, development, and survival. Halophytes, which are naturally salt-tolerant plants, are highlighted as success stories to learn from. We emphasize that (a) filling the major knowledge gaps in salt-induced signaling pathways, (b) increasing the spatial and temporal resolution of our knowledge of salt stress responses, (c) discovering and considering crop-specific responses, and (d) including halophytes in our comparative studies are all essential in order to take our approaches to increasing crop yields in saline soils to the next level.

Responses to Water Deficit and Salt Stress in Silver Fir (Abies alba Mill.) Seedlings

Forest ecosystems are frequently exposed to abiotic stress, which adversely affects their growth, resistance and survival. For silver fir (Abies alba), the physiological and biochemical responses to water and salt stress have not been extensively studied. Responses of one-year-old seedlings to a 30-day water stress (withholding irrigation) or salt stress (100, 200 and 300 mM NaCl) treatments were analysed by determining stress-induced changes in growth parameters and different biochemical markers: accumulation of ions, different osmolytes and malondialdehyde (MDA, an oxidative stress biomarker), in the seedlings, and activation of enzymatic and non-enzymatic antioxidant systems. Both salt and water stress caused growth inhibition. The results obtained indicated that the most relevant responses to drought are based on the accumulation of soluble carbohydrates as osmolytes/osmoprotectants. Responses to high salinity, on the other hand, include the active transport of Na+, Cl− and Ca2+ to the needles, the maintenance of relatively high K+/Na+ ratios and the accumulation of proline and soluble sugars for osmotic balance. Interestingly, relatively high Na+ concentrations were measured in the needles of A. alba seedlings at low external salinity, suggesting that Na+ can contribute to osmotic adjustment as a ‘cheap’ osmoticum, and its accumulation may represent a constitutive mechanism of defence against stress. These responses appear to be efficient enough to avoid the generation of high levels of oxidative stress, in agreement with the small increase in MDA contents and the relatively weak activation of the tested antioxidant systems.

The RNA-seq transcriptomic analysis reveals genes mediating salt tolerance through rapid triggering of ion transporters in a mutant barley

Considering the complex nature of salinity tolerance mechanisms, the use of isogenic lines or mutants possessing the same genetic background albeit different tolerance to salinity is a suitable method for reduction of analytical complexity to study these mechanisms. In the present study, whole transcriptome analysis was evaluated using RNA-seq method between a salt-tolerant mutant line "M4-73-30" and its wild-type "Zarjou" cultivar at seedling stage after six hours of exposure to salt stress (300 mM NaCl). Transcriptome sequencing yielded 20 million reads for each genotype. A total number of 7116 transcripts with differential expression were identified, 1586 and 1479 of which were obtained with significantly increased expression in the mutant and the wild-type, respectively. In addition, the families of WRKY, ERF, AP2/EREBP, NAC, CTR/DRE, AP2/ERF, MAD, MIKC, HSF, and bZIP were identified as the important transcription factors with specific expression in the mutant genotype. The RNA-seq results were confirmed at several time points using qRT-PCR for some important salt-responsive genes. In general, the results revealed that the mutant accumulated higher levels of sodium ion in the root and decreased its transfer to the shoot. Also, the mutant increased the amount of potassium ion leading to the maintenance a high ratio [K+]/[Na+] in the shoot compared to its wild-type via fast stomata closure and consequently transpiration reduction under the salt stress. Moreover, a reduction in photosynthesis and respiration was observed in the mutant, resulting in utilization of the stored energy and the carbon for maintaining the plant tissues, which is considered as a mechanism of salt tolerance in plants. Up-regulation of catalase, peroxidase, and ascorbate peroxidase genes has resulted in higher accumulation of H2O2 in the wild-type compared to the mutant. Therefore, the wild-type initiated rapid ROS signals which led to less oxidative scavenging in comparison with the mutant. The mutant increased expression in the ion transporters and the channels related to the salinity to maintain the ion homeostasis. In overall, the results demonstrated that the mutant responded better to the salt stress under both osmotic and ionic stress phases and lower damage was observed in the mutant compared to its wild-type under the salt stress.

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.

Sunflower (Helianthus annuus L.) biochemical properties and seed components affected by potassium fertilization under drought conditions

The seed yield and healthy oil in sunflower (Helianthus annuus L.), as an important industrial crop, decrease under stress. There is not much investigation, to our knowledge, on the use of potassium fertilization, a regulator of plant water potential, affecting the biochemical properties and seed components of sunflower under drought stress. Accordingly, such parameters were investigated in a split-split plot field experiment, conducted in two different field sites (Natanz (Nt) and Eghlid (Eg), Iran), using potassium fertilization (subplots, 0, 150 and 300 kg/ha) and six drought levels (main plots) in four replicates. Although stress significantly affected sunflower biochemical properties and seed components in the two fields, the effects of stress were more pronounced in the Eg site (significant interaction of field and drought). The plant alleviated the stress by increasing the proline, oleic and linoleic acid concentrations, however, potassium fertilization also increased plant tolerance further under stress by enhancing such components compared with control. Interestingly, the Eg site was more responsive to the potassium fertilization (significant interaction of field and fertilization), as the fertilizer resulted in a higher rate of plant biochemical properties and seed components. The use of potassium fertilization at 300 kg/ha (K3) was the most effective treatment in the alleviation of stress. Interestingly, under drought stress, potassium contributed to the enhanced quantity and quality of sunflower by increasing seed components, and enhancing the biochemical properties of the plant, which can also improve crop physiological mechanisms. The results can further increase our understanding related to the effects of potassium fertilization on the yield and physiology of sunflower under drought stress. Such results are of economic, environmental and health significance.

Effect of elevated magnesium sulfate on two riparian tree species potentially impacted by mine site contamination

Globally, mining activities have been responsible for the contamination of soils, surface water and groundwater. Following mine closure, a key issue is the management of leachate from waste rock accumulated during the lifetime of the mine. At Ranger Uranium Mine in northern Australia, magnesium sulfate (MgSO₄) leaching from waste rock has been identified as a potentially significant surface and groundwater contaminant which may have adverse affects on catchment biota. The primary objective of this study was to determine the effect of elevated levels of MgSO₄ on two riparian trees; Melaleuca viridiflora and Alphitonia excelsa. We found that tolerance to MgSO₄ was species-specific. M. viridiflora was tolerant to high concentrations of MgSO₄ (15,300 mg l^-1), with foliar concentrations of ions suggesting plants regulate uptake. In contrast, A. excelsa was sensitive to elevated concentrations of MgSO₄ (960 mg l^-1), exhibiting reduced plant vigour and growth. This information improves our understanding of the toxicity of MgSO₄ as a mine contaminant and highlights the need for rehabililitation planning to mitigate impacts on some tree species of this region.

Regulation of Ammonium Cellular Levels is An Important Adaptive Trait for the Euhalophytic Behavior of Salicornia europaea

Salinization of agricultural land is a devastating phenomenon which will affect future food security. Understanding how plants survive and thrive in response to salinity is therefore critical to potentiate tolerance traits in crop species. The halophyte Salicornia europaea has been used as model system for this purpose. High salinity causes NH4+ accumulation in plant tissues and consequent toxicity symptoms that may further exacerbate those caused by NaCl. In this experiment we exposed Salicornia plants to five concentrations of NaCl (0, 1, 10, 50 and 200 mM) in combination with two concentrations of NH4Cl (1 and 50 mM). We confirmed the euhalophytic behavior of Salicornia that grew better at 200 vs. 0 mM NaCl in terms of both fresh (+34%) and dry (+46%) weights. Addition of 50 mM NH4Cl to the growth medium caused a general growth reduction, which was likely caused by NH4+ accumulation and toxicity in roots and shoots. When plants were exposed to high NH4Cl, high salinity reduced roots NH4+ concentration (-50%) compared to 0 mM NaCl. This correlates with the activation of the NH4+ assimilation enzymes, glutamine synthetase and glutamate dehydrogenase, and the growth inhibition was partially recovered. We argue that NH4+ detoxification is an important trait under high salinity that may differentiate halophytes from glycophytes and we present a possible model for NH4+ detoxification in response to salinity.

Poplar Autophagy Receptor NBR1 Enhances Salt Stress Tolerance by Regulating Selective Autophagy and Antioxidant System

Salt stress is an adverse environmental factor for plant growth and development. Under salt stress, plants can activate the selective autophagy pathway to alleviate stress. However, the regulatory mechanism of selective autophagy in response to salt stress remains largely unclear. Here, we report that the selective autophagy receptor PagNBR1 (neighbor of BRCA1) is induced by salt stress in Populus. Overexpression of PagNBR1 in poplar enhanced salt stress tolerance. Compared with wild type (WT) plants, the transgenic lines exhibited higher antioxidant enzyme activity, less reactive oxygen species (ROS), and higher net photosynthesis rates under salt stress. Furthermore, co-localization and yeast two-hybrid analysis revealed that PagNBR1 was localized in the autophagosome and could interact with ATG8 (autophagy-related gene). PagNBR1 transgenic poplars formed more autophagosomes and exhibited higher expression of ATG8, resulting in less accumulation of insoluble protein and insoluble ubiquitinated protein compared to WT under salt stress. The accumulation of insoluble protein and insoluble ubiquitinated protein was similar under the treatment of ConA in WT and transgenic lines. In summary, our results imply that PagNBR1 is an important selective autophagy receptor in poplar and confers salt tolerance by accelerating antioxidant system activity and autophagy activity. Moreover, the NBR1 gene is an important potential molecular target for improving stress resistance in trees.

Comparative physiological and biochemical evaluation of salt and nickel tolerance mechanisms in two contrasting tomato genotypes

Plant tolerance against a combination of abiotic stresses is a complex phenomenon, which involves various mechanisms. Physiological and biochemical analyses of salinity (NaCl) and nickel (Ni) tolerance in two contrasting tomato genotypes were performed in a hydroponics experiment. The tomato genotypes selected were proved to be tolerant (Naqeeb) and sensitive (Nadir) to both salinity and Ni stress in our previous experiment. The tomato genotypes were exposed to combinations of NaCl (0, 75 and 150 mM) and Ni (0, 15, and 20 mg l^-1 ) for 28 days. The results revealed that the tolerant and sensitive tomato genotypes showed similar response to NaCl and Ni stress; however, the level of response was significantly different in both genotypes. The tolerant tomato genotype showed less reduction in growth than the sensitive genotype against both NaCl and Ni stress. Root and shoot ionic analysis showed a decrease in Na and increase in K concentration by increasing Ni levels in the growth medium. Moreover, accumulation of Na and Ni in tissues showed a decrease in membrane stability index and an increase in malondialdehyde contents. The activity of superoxide dismutase, catalase, peroxidase and glutathione reductase under NaCl and Ni stress was significantly higher in the tolerant compared to the sensitive genotype. Enhanced activity of many antioxidant enzymes in Naqeeb under stress conditions is among the other mechanisms that enabled the genotype to better detoxify reactive oxygen species and therefore Naqeeb tolerated the stresses better than Nadir.

Regulation of AtKUP2 Expression by bHLH and WRKY Transcription Factors Helps to Confer Increased Salt Tolerance to Arabidopsis thaliana Plants

Potassium transporters play an essential role in maintaining cellular ion homeostasis, turgor pressure, and pH, which are critical for adaptation under salt stress. We identified a salt responsive Avicennia officinalis KUP/HAK/KT transporter family gene, AoKUP2, which has high sequence similarity to its Arabidopsis ortholog AtKUP2. These genes were functionally characterized in mutant yeast cells and Arabidopsis plants. Both AoKUP2 and AtKUP2 were induced by salt stress, and AtKUP2 was primarily induced in roots. Subcellular localization revealed that AoKUP2 and AtKUP2 are localized to the plasma membrane and mitochondria. Expression of AtKUP2 and AoKUP2 in Saccharomyces cerevisiae mutant strain (BY4741 trk1Δ::loxP trk2Δ::loxP) helped to rescue the growth defect of the mutant under different NaCl and K⁺ concentrations. Furthermore, constitutive expression of AoKUP2 and AtKUP2 conferred enhanced salt tolerance in Arabidopsis indicated by higher germination rate, better survival, and increased root and shoot length compared to the untreated controls. Analysis of Na⁺ and K⁺ contents in the shoots and roots showed that ectopic expression lines accumulated less Na⁺ and more K⁺ than the WT. Two stress-responsive transcription factors, bHLH122 and WRKY33, were identified as direct regulators of AtKUP2 expression. Our results suggest that AtKUP2 plays a key role in enhancing salt stress tolerance by maintaining cellular ion homeostasis.

Expression of maize calcium-dependent protein kinase (ZmCPK11) improves salt tolerance in transgenic Arabidopsis plants by regulating sodium and potassium homeostasis and stabilizing photosystem II

In plants, CALCIUM-DEPENDENT PROTEIN KINASES (CDPKs/CPKs) are involved in calcium signaling in response to endogenous and environmental stimuli. Here, we report that ZmCPK11, one of maize CDPKs, participates in salt stress response and tolerance. Salt stress induced expression and upregulated the activity of ZmCPK11 in maize roots and leaves. Activation of ZmCPK11 upon salt stress was also observed in roots and leaves of transgenic Arabidopsis plants expressing ZmCPK11. The transgenic plants showed a long-root phenotype under control conditions and a short-root phenotype under NaCl, abscisic acid (ABA) or jasmonic acid (JA) treatment. Analysis of ABA and JA content in roots indicated that ZmCPK11 can mediate root growth by regulating the levels of these phytohormones. Moreover, 4-week-old transgenic plants were more tolerant to salinity than the wild-type plants. Their leaves were less chlorotic and showed weaker symptoms of senescence accompanied by higher chlorophyll content and higher quantum efficiency of photosystem II. The expression of Na⁺ /K⁺ transporters (HKT1, SOS1 and NHX1) and transcription factors (CBF1, CBF2, CBF3, ZAT6 and ZAT10) with known links to salinity tolerance was upregulated in roots of the transgenic plants upon salt stress. Furthermore, the transgenic plants accumulated less Na⁺ in roots and leaves under salinity, and showed a higher K⁺ /Na⁺ ratio in leaves. These results show that the improved salt tolerance in ZmCPK11-transgenic plants could be due to an upregulation of genes involved in the maintenance of intracellular Na⁺ and K⁺ homeostasis and a protection of photosystem II against damage.

Calcium-Regulated Phosphorylation Systems Controlling Uptake and Balance of Plant Nutrients

Essential elements taken up from the soil and distributed throughout the whole plant play diverse roles in different tissues. Cations and anions contribute to maintenance of intracellular osmolarity and the formation of membrane potential, while nitrate, ammonium, and sulfate are incorporated into amino acids and other organic compounds. In contrast to these ion species, calcium concentrations are usually kept low in the cytosol and calcium displays unique behavior as a cytosolic signaling molecule. Various environmental stresses stimulate increases in the cytosolic calcium concentration, leading to activation of calcium-regulated protein kinases and downstream signaling pathways. In this review, we summarize the stress responsive regulation of nutrient uptake and balancing by two types of calcium-regulated phosphorylation systems: CPK and CBL-CIPK. CPK is a family of protein kinases activated by calcium. CBL is a group of calcium sensor proteins that interact with CIPK kinases, which phosphorylate their downstream targets. In Arabidopsis, quite a few ion transport systems are regulated by CPKs or CBL-CIPK complexes, including channels/transporters that mediate transport of potassium (KAT1, KAT2, GORK, AKT1, AKT2, HAK5, SPIK), sodium (SOS1), ammonium (AMT1;1, AMT1;2), nitrate and chloride (SLAC1, SLAH2, SLAH3, NRT1.1, NRT2.4, NRT2.5), and proton (AHA2, V-ATPase). CPKs and CBL-CIPKs also play a role in C/N nutrient response and in acquisition of magnesium and iron. This functional regulation by calcium-dependent phosphorylation systems ensures the growth of plants and enables them to acquire tolerance against various environmental stresses. Calcium serves as the key factor for the regulation of membrane transport systems.

Transcriptional, metabolic and DNA methylation changes underpinning the response of Arundo donax ecotypes to NaCl excess

Arundo donax ecotypes react differently to salinity, partly due to differences in constitutive defences and methylome plasticity. Arundo donax L. is a C3 fast-growing grass that yields high biomass under stress. To elucidate its ability to produce biomass under high salinity, we investigated short/long-term NaCl responses of three ecotypes through transcriptional, metabolic and DNA methylation profiling of leaves and roots. Prolonged salt treatment discriminated the sensitive ecotype 'Cercola' from the tolerant 'Domitiana' and 'Canneto' in terms of biomass. Transcriptional and metabolic responses to NaCl differed between the ecotypes. In roots, constitutive expression of ion transporter and stress-related transcription factors' genes was higher in 'Canneto' and 'Domitiana' than 'Cercola' and 21-day NaCl drove strong up-regulation in all ecotypes. In leaves, unstressed 'Domitiana' confirmed higher expression of the above genes, whose transcription was repressed in 'Domitiana' but induced in 'Cercola' following NaCl treatment. In all ecotypes, salinity increased proline, ABA and leaf antioxidants, paralleled by up-regulation of antioxidant genes in 'Canneto' and 'Cercola' but not in 'Domitiana', which tolerated a higher level of oxidative damage. Changes in DNA methylation patterns highlighted a marked capacity of the tolerant 'Domitiana' ecotype to adjust DNA methylation to salt stress. The reduced salt sensitivity of 'Domitiana' and, to a lesser extent, 'Canneto' appears to rely on a complex set of constitutively activated defences, possibly due to the environmental conditions of the site of origin, and on higher plasticity of the methylome. Our findings provide insights into the mechanisms of adaptability of A. donax ecotypes to salinity, offering new perspectives for the improvement of this species for cultivation in limiting environments.

Silicon and Salinity: Crosstalk in Crop-Mediated Stress Tolerance Mechanisms

Salinity stress hinders the growth potential and productivity of crop plants by influencing photosynthesis, disturbing the osmotic and ionic concentrations, producing excessive oxidants and radicals, regulating endogenous phytohormonal functions, counteracting essential metabolic pathways, and manipulating the patterns of gene expression. In response, plants adopt counter mechanistic cascades of physio-biochemical and molecular signaling to overcome salinity stress; however, continued exposure can overwhelm the defense system, resulting in cell death and the collapse of essential apparatuses. Improving plant vigor and defense responses can thus increase plant stress tolerance and productivity. Alternatively, the quasi-essential element silicon (Si)-the second-most abundant element in the Earth's crust-is utilized by plants and applied exogenously to combat salinity stress and improve plant growth by enhancing physiological, metabolomic, and molecular responses. In the present review, we elucidate the potential role of Si in ameliorating salinity stress in crops and the possible mechanisms underlying Si-associated stress tolerance in plants. This review also underlines the need for future research to evaluate the role of Si in salinity stress in plants and the identification of gaps in the understanding of this process as a whole at a broader field level.

A Glycine max sodium/hydrogen exchanger enhances salt tolerance through maintaining higher Na+ efflux rate and K+/Na+ ratio in Arabidopsis

BACKGROUND

Soybean (Glycine max (L.)) is one the most important oil-yielding cash crops. However, the soybean production has been seriously restricted by salinization. It is therefore crucial to identify salt tolerance-related genes and reveal molecular mechanisms underlying salt tolerance in soybean crops. A better understanding of how plants resist salt stress provides insights in improving existing soybean varieties as well as cultivating novel salt tolerant varieties. In this study, the biological function of GmNHX1, a NHX-like gene, and the molecular basis underlying GmNHX1-mediated salt stress resistance have been revealed.

RESULTS

We found that the transcription level of GmNHX1 was up-regulated under salt stress condition in soybean, reaching its peak at 24 h after salt treatment. By employing the virus-induced gene silencing technique (VIGS), we also found that soybean plants became more susceptible to salt stress after silencing GmNHX1 than wild-type and more silenced plants wilted than wild-type under salt treatment. Furthermore, Arabidopsis thaliana expressing GmNHX1 grew taller and generated more rosette leaves under salt stress condition compared to wild-type. Exogenous expression of GmNHX1 resulted in an increase of Na+ transportation to leaves along with a reduction of Na+ absorption in roots, and the consequent maintenance of a high K+/Na+ ratio under salt stress condition. GmNHX1-GFP-transformed onion bulb endothelium cells showed fluorescent pattern in which GFP fluorescence signals enriched in vacuolar membranes. Using the non-invasive micro-test technique (NMT), we found that the Na+ efflux rate of both wild-type and transformed plants after salt treatment were significantly higher than that of before salt treatment. Additionally, the Na+ efflux rate of transformed plants after salt treatment were significantly higher than that of wild-type. Meanwhile, the transcription levels of three osmotic stress-related genes, SKOR, SOS1 and AKT1 were all up-regulated in GmNHX1-expressing plants under salt stress condition.

CONCLUSION

Vacuolar membrane-localized GmNHX1 enhances plant salt tolerance through maintaining a high K+/Na+ ratio along with inducing the expression of SKOR, SOS1 and AKT1. Our findings provide molecular insights on the roles of GmNHX1 and similar sodium/hydrogen exchangers in regulating salt tolerance.

HKT1;5 Transporter Gene Expression and Association of Amino Acid Substitutions With Salt Tolerance Across Rice Genotypes

Plants need to maintain a low Na+/K+ ratio for their survival and growth when there is high sodium concentration in soil. Under these circumstances, the high affinity K+ transporter (HKT) and its homologs are known to perform a critical role with HKT1;5 as a major player in maintaining Na+ concentration. Preferential expression of HKT1;5 in roots compared to shoots was observed in rice and rice-like genotypes from real time PCR, microarray, and RNAseq experiments and data. Its expression trend was generally higher under increasing salt stress in sensitive IR29, tolerant Pokkali, both glycophytes; as well as the distant wild rice halophyte, Porteresia coarctata, indicative of its importance during salt stress. These results were supported by a low Na+/K+ ratio in Pokkali, but a much lower one in P. coarctata. HKT1;5 has functional variability among salt sensitive and tolerant varieties and multiple sequence alignment of sequences of HKT1;5 from Oryza species and P. coarctata showed 4 major amino acid substitutions (140 P/A/T/I, 184 H/R, D332H, V395L), with similarity amongst the tolerant genotypes and the halophyte but in variance with sensitive ones. The best predicted 3D structure of HKT1;5 was generated using Ktrab potassium transporter as template. Among the four substitutions, conserved presence of aspartate (332) and valine (395) in opposite faces of the membrane along the Na+/K+ channel was observed only for the tolerant and halophytic genotypes. A model based on above, as well as molecular dynamics simulation study showed that valine is unable to generate strong hydrophobic network with its surroundings in comparison to leucine due to reduced side chain length. The resultant alteration in pore rigidity increases the likelihood of Na+ transport from xylem sap to parenchyma and further to soil. The model also proposes that the presence of aspartate at the 332 position possibly leads to frequent polar interactions with the extracellular loop polar residues which may shift the loop away from the opening of the constriction at the pore and therefore permit easy efflux of the Na+. These two substitutions of the HKT1;5 transporter probably help tolerant varieties maintain better Na+/K+ ratio for survival under salt stress.

An RNA-binding protein MUG13.4 interacts with AtAGO2 to modulate salinity tolerance in Arabidopsis

Salt stress is a major constraint to plant growth and development, and plants have developed sophisticated mechanisms to cope with it. AtAGO2, an argonaute protein, is known to play an important role in plant adaptation to salt stress; however, the molecular mechanism of this phenomenon remains essentially unexplored. Here, we performed the yeast two-hybrid assay and found an R3H-domain containing protein, designated as MUG13.4, interacting with AtAGO2. Further bimolecular fluorescence complement (BiFC), glutathione-S-transferase (GST) pull-down, and co-immunoprecipitation (Co-IP) assays confirmed that MUG13.4 interacted with AtAGO2, and MUG13.4 could affect the slicing activity of AtAGO2 associated with miR173. MUG13.4 and AtAGO2 were both predominantly expressed in seeds and roots. Phenotypic analyses of the single and double mutants under salt stress confirmed involvement of MUG13.4-AtAGO2 complex as a component of the salt tolerance mechanism. The mug13.4×ago2-1 double mutant displayed retarded growth and hypersensitivity to salt stress that was more pronounced than in mug13.4 or atago2-1 single mutants. TAS1c-tasiRNA generating system in Nicotiana benthamiana revealed that MUG13.4 could influence the slicing activity of AtAGO2. We also found that MUG13.4 increasingly changed the phenotype of slicer-defected mutants of AtAGO2 in response to salt stress. These findings suggested that the function of AtAGO2 upon salt stress was dependent on MUG13.4. Further investigation suggested that AtAGO2 improved Arabidopsis tolerance to salt stress by affecting operation of the SOS signaling cascade at the transcript level. Taken together, these findings reveal a novel function of MUG13.4 in adjusting Arabidopsis adaptation to salt stress.

Bread Wheat With High Salinity and Sodicity Tolerance

Soil salinity and sodicity are major constraints to global cereal production, but breeding for tolerance has been slow. Narrow gene pools, over-emphasis on the sodium (Na+) exclusion mechanism, little attention to osmotic stress/tissue tolerance mechanism(s) in which accumulation of inorganic ions such as Na+ is implicated, and lack of a suitable screening method have impaired progress. The aims of this study were to discover novel genes for Na+ accumulation using genome-wide association studies, compare growth responses to salinity and sodicity in low-Na+ bread Westonia with Nax1 and Nax2 genes and high-Na+ bread wheat Baart-46, and evaluate growth responses to salinity and sodicity in bread wheats with varying leaf Na+ concentrations. The novel high-Na+ bread wheat germplasm, MW#293, had higher grain yield under salinity and sodicity, in absolute and relative terms, than the other bread wheat entries tested. Genes associated with high Na+ accumulation in bread wheat were identified, which may be involved in tissue tolerance/osmotic adjustment. As most modern bread wheats are efficient at excluding Na+, further reduction in plant Na+ is unlikely to provide agronomic benefit. The salinity and sodicity tolerant germplasm MW#293 provides an opportunity for the development of future salinity/sodicity tolerant bread wheat.