Understanding and Improving Salt Tolerance in Plants

NaCl-responsive ROS scavenging and energy supply in alkaligrass callus revealed from proteomic analysis

BACKGROUND

Salinity has obvious effects on plant growth and crop productivity. The salinity-responsive mechanisms have been well-studied in differentiated organs (e.g., leaves, roots and stems), but not in unorganized cells such as callus. High-throughput quantitative proteomics approaches have been used to investigate callus development, somatic embryogenesis, organogenesis, and stress response in numbers of plant species. However, they have not been applied to callus from monocotyledonous halophyte alkaligrass (Puccinellia tenuifora).

RESULTS

The alkaligrass callus growth, viability and membrane integrity were perturbed by 50 mM and 150 mM NaCl treatments. Callus cells accumulated the proline, soluble sugar and glycine betaine for the maintenance of osmotic homeostasis. Importantly, the activities of ROS scavenging enzymes (e.g., SOD, APX, POD, GPX, MDHAR and GR) and antioxidants (e.g., ASA, DHA and GSH) were induced by salinity. The abundance patterns of 55 salt-responsive proteins indicate that salt signal transduction, cytoskeleton, ROS scavenging, energy supply, gene expression, protein synthesis and processing, as well as other basic metabolic processes were altered in callus to cope with the stress.

CONCLUSIONS

The undifferentiated callus exhibited unique salinity-responsive mechanisms for ROS scavenging and energy supply. Activation of the POD pathway and AsA-GSH cycle was universal in callus and differentiated organs, but salinity-induced SOD pathway and salinity-reduced CAT pathway in callus were different from those in leaves and roots. To cope with salinity, callus mainly relied on glycolysis, but not the TCA cycle, for energy supply.

Characterization and expression analysis of P5CS (Δ1-pyrroline-5-carboxylate synthase) gene in two distinct populations of the Atlantic Forest native species Eugenia uniflora L

Eugenia uniflora is an Atlantic Forest native species, occurring in contrasting edaphoclimatic environments. The identification of genes involved in response to abiotic factors is very relevant to help in understanding the processes of local adaptation. 1-Pyrroline-5-carboxylate synthetase (P5CS) is one interesting gene to study in this species since it encodes a key enzyme of proline biosynthesis, which is an osmoprotectant during abiotic stress. Applying in silico analysis, we identified one P5CS gene sequence of E. uniflora (EuniP5CS). Phylogenetic analysis, as well as, gene and protein structure investigation, revealed that EuniP5CS is a member of P5CS gene family. Plants of E. uniflora from two distinct environments (restinga and riparian forest) presented differences in the proline accumulation and P5CS expression levels under growth-controlled conditions. Both proline accumulation and gene expression level of EuniP5CS were higher in the genotypes from riparian forest than those from restinga. When these plants were submitted to drought stress, EuniP5CS gene was up-regulated in the plants from restinga, but not in those from riparian forest. These results demonstrated that EuniP5CS is involved in proline biosynthesis in this species and suggest that P5CS gene may be an interesting candidate gene in future studies to understand the processes of local adaptation in E. uniflora.

Isolation and identification of salt-tolerant plant-growth-promoting rhizobacteria and their application for rice cultivation under salt stress

Growth and productivity of rice are negatively affected by soil salinity. However, some salt-tolerant rhizosphere-inhabiting bacteria can improve salt resistance of plants, thereby augmenting plant growth and production. Here, we isolated a total of 53 plant-growth-promoting rhizobacteria (PGPR) from saline and non-saline areas in Bangladesh where electrical conductivity was measured as >7.45 and <1.80 dS/m, respectively. Bacteria isolated from saline areas were able to grow in a salt concentration of up to 2.60 mol/L, contrary to the isolates collected from non-saline areas that did not survive beyond 854 mmol/L. Among the salt-tolerant isolates, Bacillus aryabhattai, Achromobacter denitrificans, and Ochrobactrum intermedium, identified by comparing respective sequences of 16S rRNA using the NCBI GenBank, exhibited a higher amount of atmospheric nitrogen fixation, phosphate solubilization, and indoleacetic acid production at 200 mmol/L salt stress. Salt-tolerant isolates exhibited greater resistance to heavy metals and antibiotics, which could be due to the production of an exopolysaccharide layer outside the cell surface. Oryza sativa L. fertilized with B. aryabhattai MS3 and grown under 200 mmol/L salt stress was found to be favoured by enhanced expression of a set of at least four salt-responsive plant genes: BZ8, SOS1, GIG, and NHX1. Fertilization of rice with osmoprotectant-producing PGPR, therefore, could be a climate-change-preparedness strategy for coastal agriculture.

Morphological and metabolic responses to salt stress of rice (Oryza sativa L.) cultivars which differ in salinity tolerance

Salinization is one of the most important abiotic stressors for crop growth and productivity. Rice (Oryza sativa L.), as the major food source around the world, is very sensitive to salt, especially at seedling stage. In order to examine how salt stress influences the metabolism of rice, we compared the levels of a range of sugars and organic acids in three rice cultivars with different tolerance under salt stress over time. According to the morphological result, the shoot length and root fresh weight were only affected by salinity in the salt sensitive cultivar (Nipponbare). The responses of metabolites to salinity were time-, tissue- and cultivar-dependent. Shikimate and quinate, involved in the shikimate pathway, were dramatically decreased in the leaves of all three cultivars, which was regarded as a response to salinity. Many sugars in the leaves of the salt tolerant cultivar (Dendang and Fatmawati) showed earlier increases to salt stress compared to Nipponbare leaves. Moreover, only in the leaves of tolerant cultivars (Dendang and Fatimawati), malate was significantly decreased while sucrose was significantly increased. In Dendang roots, mannitol levels were significantly higher than in Nipponbare roots after 14 days of salt treatment, which may be attributed to its higher salt tolerance. It is proposed that these responses in the more tolerant cultivars are involved in their resistance to high salt stress which may lay the foundation for breeding tolerant rice cultivars.

Morphological, physiological and biochemical aspects of salt tolerance of halophyte Petrosimonia triandra grown in natural habitat

Salt tolerance mechanisms of halophyte Petrosimonia triandra, growing in its natural habitat in Cluj County, Romania, were investigated via biomass, growth parameters, water status, ion content, photosynthetic and antioxidative system efficiency, proline accumulation and lipid degradation. Two sampling sites with different soil electrical conductivities were selected: site 1: 3.14 dS m-1 and site 2: 4.45 dS m-1. Higher salinity proved to have a positive effect on growth. The relative water content did not decline severely, Na+ and K+ content of the roots, stem and leaves was more, and the functions of the photosynthetic apparatus and photosynthetic pigment contents were not altered. The efficiency of the antioxidative defence system was found to be assured by coordination of several reactive oxygen species scavengers. The presence of higher salinity led to accumulation of the osmolyte proline, while degradation of membrane lipids was reduced. As a whole, P. triandra evolved different adaptational strategies to counteract soil salinity, including morphological and physiological adaptations, preservation of photosynthetic activity, development of an efficient antioxidative system and accumulation of the osmotic compound, proline.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Host Plant Salinity Stress Affects the Development and Population Parameters of Nilaparvata lugens (Hemiptera: Delphacidae)

Salinization is one of the most critical abiotic stress factors for crops and a rising setback in agro-ecosystems. Changes in weather, land usage, and the salinization of irrigation water are increasing soil salinity of many farmlands. Increased soil salinity alters the plant quality, which subsequently may trigger bottom-up effects on herbivorous insect. We examined the bottom-up effect of salinity stress on population parameters of the brown planthopper (BPH), Nilaparvata lugens through rice (Oryza sativa L.) plant. The results revealed that salinity interfered with egg hatching of BPH. The nymphal development period, adult longevity, and oviposition were also influenced by salinity. Notable differences appeared in the intrinsic growth rate (r), the finite increase rate (λ) and the net reproduction rate (R0) of BPH, and a concentration-dependent effect was detected. Although salinity adversely affected BPH development, population projection predicted a successful growth of the BPH population in a relatively short time under the treatment of low and medium levels of salinity (6, 8, and 10 dS/m of NaCl), whereas higher salt concentrations (12 and 14 dS/m) lead to significant fitness costs in BPH populations. Our study predicts that BPH could become a problem in areas with lower and medium salinity and that those planthoppers may exacerbate the negative effects of salinity for rice production. This study will provide valuable information for understanding the field abundance and distribution of BPH on saline rice field, thus contributing to the development of eco-friendly strategies to manage this pest in saline ecosystems.

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.

Calcium signaling and salt tolerance are diversely entwined in plants

In plants dehydration imposed by salinity can invoke physical changes at the interface of the plasma membrane and cell wall. Changes in hydrostatic pressure activate ion channels and cause depolarization of the plasma membrane due to disturbance in ion transport. During the initial phases of salinity stress, the relatively high osmotic potential of the rhizosphere enforces the plant to use a diverse spectrum of strategies to optimize water and nutrient uptake. Signals of salt stress are recognized by specific root receptors that activate an osmosensing network. Plant response to hyperosmotic tension is closely linked to the calcium (Ca2+) channels and interacting proteins such as calmodulin. A rapid rise in cytosolic Ca2+ levels occurs within seconds of exposure to salt stress. Plants employ multiple sensors and signaling components to sense and respond to salinity stress, of which most are closely related to Ca2+ sensing and signaling. Several tolerance strategies such as osmoprotectant accumulation, antioxidant boosting, polyaminses and nitric oxide (NO) machineries are also coordinated by Ca2+ signaling. Substantial research has been done to discover the salt stress pathway and tolerance mechanism in plants, resulting in new insights into the perception of salt stress and the downstream signaling that happens in response. Nevertheless, the role of multifunctional components such as Ca2+ has not been sufficiently addressed in the context of salt stress. In this review, we elaborate that the salt tolerance signaling pathway converges with Ca2+ signaling in diverse pathways. We summarize knowledge related to different dimensions of salt stress signaling pathways in the cell by emphasizing the administrative role of Ca2+ signaling on salt perception, signaling, gene expression, ion homeostasis and adaptive responses.

Assessing trait contribution and mapping novel QTL for salinity tolerance using the Bangladeshi rice landrace Capsule

BACKGROUND

Salinity is one of the most widespread abiotic stresses affecting rice productivity worldwide. The purpose of this study was to establish the relative importance of different traits associated with salinity tolerance in rice and to identify new quantitative trait loci (QTL) conferring tolerance to salinity at seedling stage. A total of 231 F_2:3 plants derived from a cross between a sensitive variety BRRI dhan29 (BR29 hereafter) and Capsule, a salt tolerant Bangladeshi indica landrace, were evaluated under salt stress in a phytotron.

RESULTS

Out of the 231 F₂ plants, 47 highly tolerant and 47 most sensitive lines were selected, representing the two extreme tails of the phenotypic distribution. These 94 plants were genotyped for 105 simple sequence repeat (SSR) and insertion/deletion (InDel) markers. A genetic linkage map spanning approximately 1442.9 cM of the 12 linkage groups with an average marker distance of 13.7 cM was constructed. QTL were identified on the long arm of chromosome 1 for Na⁺ concentration, K⁺ concentration, Na⁺-K⁺ ratio and survival; chromosome 3 for Na⁺ concentration, survival and overall phenotypic evaluation using the Standard Evaluation system (SES); and chromosome 5 for SES. A total of 6 pairwise epistatic interactions were also detected between QTL-linked and QTL-unlinked regions. Graphical genotyping indicated an association between the phenotypes of the extreme families and their QTL genotypes. Path coefficient analysis revealed that Na⁺ concentration, survival, Na⁺-K⁺ ratio and the overall phenotypic performance (SES score) are the major traits associated with salinity tolerance of Capsule.

CONCLUSIONS

Capsule provides an alternative source of salinity tolerance aside from Pokkali and Nona Bokra, the two Indian salt tolerant landraces traditionally used for breeding salt tolerant rice varieties. Pyramiding the new QTL identified in this study with previously discovered loci, such as Saltol, will facilitate breeding varieties that are highly tolerant of salt stress.

The elemental composition of halophytes correlates with key morphological adaptations and taxonomic groups

Halophytes are crucial in the light of increasing soil salinization, yet our understanding of their chemical composition and its relationship to key morphological traits such as succulence or salt excretion is limited. This study targets this issue by exploring the relationship between the elemental composition of 108 plant species from saline environments in Iran and their eco-morphological traits and taxonomy. Leaves and/or photosynthetic shoots of individual species and soils were sampled and analyzed for 20 elements in plant samples and 5 major elements plus % gypsum content, pH, and EC in soil samples. Eu-halophytes and leaf- and stem-succulent and salt-recreting plants showed high concentrations of Na, S, and Mg and low concentrations of Ca and K. In contrast, pseudo-halophytes, facultative-halophytes and eury-hygro-halophytes, which often lack succulent shoots, showed low Na, S, and Mg and high Ca and K concentrations in their leaves. Clear patterns were identified among taxonomic families, with Chenopodiaceae and Plumbaginaceae having high Na and Mg and low Ca and K concentrations, Caryophyllaceae having high K, Poaceae having low Na, and Asteraceae, Boraginaceae, and Brassicaceae showing high foliar Ca concentrations. We conclude that the elemental composition of halophytes and pseudo-halophytes is related to salt-tolerance categories, eco-morphological types and respective taxonomic groups.

Transcriptomic profiling and analysis of differentially expressed genes in asparagus bean (Vigna unguiculata ssp. sesquipedalis) under salt stress

Asparagus bean (Vigna unguiculata ssp. sesquipedalis) is a warm season legume which is widely distributed over subtropical regions and semiarid areas. It is mainly grown as a significant protein source in developing countries. Salinity, as one of the main abiotic stress factors, constrains the normal growth and yield of asparagus bean. This study used two cultivars (a salt-sensitive genotype and a salt-tolerant genotype) under salt stress vs. control to identify salt-stress-induced genes in asparagus bean using RNA sequencing. A total of 692,086,838 high-quality clean reads, assigned to 121,138 unigenes, were obtained from control and salt-treated libraries. Then, 216 root-derived DEGs (differentially expressed genes) and 127 leaf-derived DEGs were identified under salt stress between the two cultivars. Of these DEGs, thirteen were assigned to six transcription factors (TFs), including AP2/EREBP, CCHC(Zn), C2H2, WRKY, WD40-like and LIM. GO analysis indicated four DEGs might take effects on the "oxidation reduction", "transport" and "signal transduction" process. Moreover, expression of nine randomly-chosen DEGs was verified by quantitative real-time-PCR (qRT-PCR) analysis. Predicted function of the nine tested DEGs was mainly involved in the KEGG pathway of cation transport, response to osmotic stress, and phosphorelay signal transduction system. A salt-stress-related pathway of "SNARE interactions in vesicular transport" was concerned. As byproducts, 15, 321 microsatellite markers were found in all the unigenes, and 17 SNP linked to six salt-stress induced DEGs were revealed. These candidate genes provide novel insights for understanding the salt tolerance mechanism of asparagus bean in the future.

Contrasting microbial community responses to salinization and straw amendment in a semiarid bare soil and its wheat rhizosphere

Soil salinization is a major constraint of agriculture in semiarid ecosystems. In this study soil microcosms were applied to evaluate the impact of a lower- and higher-level salinization treatment of a pristine scrubland soil on the abundance of Bacteria, Archaea, and Fungi, and on prokaryotic diversity in bare soil and the rhizosphere of wheat assessed by qPCR and high-throughput sequencing of 16S rRNA gene amplicons. Furthermore, the impact of soil straw amendment as a salt-stress alleviation strategy was studied. While the low-level salinity stimulated plant growth, the seedlings did not survive under the higher-level salinity unless the soil was amended with straw. Without the straw amendment, salinization had only minor effects on the microbial community in bare soil. On the other hand, it decreased prokaryotic diversity in the rhizosphere of wheat, but the straw amendment was effective in mitigating this effect. The straw however, was not a significant nutrient source for the rhizosphere microbiota but more likely acted indirectly by ameliorating the salinity stress on the plant. Members of Proteobacteria, Actinobacteria, and Firmicutes were abundant among the bacteria that reacted to soil salinization and the straw amendment but showed inconsistent responses indicating the large physiological diversity within these phyla.

Omeprazole Promotes Chloride Exclusion and Induces Salt Tolerance in Greenhouse Basil

The role of small bioactive molecules (<500 Da) in mechanisms improving resource use efficiency in plants under stress conditions draws increasing interest. One such molecule is omeprazole (OMP), a benzimidazole derivative and inhibitor of animal proton pumps shown to improve nitrate uptake and exclusion of toxic ions, especially of chloride from the cytosol of salt-stressed leaves. Currently, OMP was applied as substrate drench at two rates (0 or 10 μM) on hydroponic basil (Ocimum basilicum L. cv. Genovese) grown under decreasing NO3−:Cl− ratio (80:20, 60:40, 40:60, or 20:80). Chloride concentration and stomatal resistance increased while transpiration, net CO2 assimilation rate and beneficial ions (NO3−, PO43−, and SO42−) decreased with reduced NO3−:Cl− ratio under the 0 μM OMP treatment. The negative effects of chloride were not only mitigated by the 10 μM OMP application in all treatments, with the exception of 20:80 NO3−:Cl−, but plant growth at 80:20, 60:40, and 40:60 NO3−:Cl− ratios receiving OMP application showed maximum fresh yield (+13%, 24%, and 22%, respectively), shoot (+10%, 25%, and 21%, respectively) and root (+32%, 76%, and 75%, respectively) biomass compared to the corresponding untreated treatments. OMP was not directly involved in ion homeostasis and compartmentalization of vacuolar or apoplastic chloride. However, it was active in limiting chloride loading into the shoot, as manifested by the lower chloride concentration in the 80:20, 60:40, and 40:60 NO3−:Cl− treatments compared to the respective controls (−41%, −37%, and −24%), favoring instead that of nitrate and potassium while also boosting photosynthetic activity. Despite its unequivocally beneficial effect on plants, the large-scale application of OMP is currently limited by the molecule’s high cost. However, further studies are warranted to unravel the molecular mechanisms of OMP-induced reduction of chloride loading to shoot and improved salt tolerance.

Integration of mRNA and miRNA Analysis Reveals the Molecular Mechanism Underlying Salt and Alkali Stress Tolerance in Tobacco

Salinity is one of the most severe forms of abiotic stress and affects crop yields worldwide. Plants respond to salinity stress via a sophisticated mechanism at the physiological, transcriptional and metabolic levels. However, the molecular regulatory networks involved in salt and alkali tolerance have not yet been elucidated. We developed an RNA-seq technique to perform mRNA and small RNA (sRNA) sequencing of plants under salt (NaCl) and alkali (NaHCO3) stress in tobacco. Overall, 8064 differentially expressed genes (DEGs) and 33 differentially expressed microRNAs (DE miRNAs) were identified in response to salt and alkali stress. A total of 1578 overlapping DEGs, which exhibit the same expression patterns and are involved in ion channel, aquaporin (AQP) and antioxidant activities, were identified. Furthermore, genes involved in several biological processes, such as "photosynthesis" and "starch and sucrose metabolism," were specifically enriched under NaHCO3 treatment. We also identified 15 and 22 miRNAs that were differentially expressed in response to NaCl and NaHCO3, respectively. Analysis of inverse correlations between miRNAs and target mRNAs revealed 26 mRNA-miRNA interactions under NaCl treatment and 139 mRNA-miRNA interactions under NaHCO3 treatment. This study provides new insights into the molecular mechanisms underlying the response of tobacco to salinity stress.

Genome-wide identification and characterization of the metal tolerance protein (MTP) family in grape (Vitis vinifera L.)

Metal tolerance proteins (MTPs) play an important role in the transport of metals at the cellular, tissue and whole plant levels. In the present study, 11 MTP genes were identified and these clustered in three major sub-families Fe/Zn-MTP, Zn-MTP, and Mn-MTP, and seven groups, which are similar to the grouping of MTP genes in both Arabidopsis and rice. Vitis vinifera metal tolerance proteins (VvMTP) ranged from 366 to 1092 amino acids, were predicted to be located in the cell vacuole, and had four to six putative TMDs, except for VvtMTP12 and VvMTP1. The VvMTPs had putative cation diffusion facilitator (CDF) domains and the putative Mn-MTPs also had zinc transporter dimerization domains (ZD-domains). V. vinifera Mn-MTPs had gene structures and motif distributions similar to those of the Fe/Zn-MTP and Zn-MTP sub-families. The upstream regions of VvMTP genes had variable frequencies of cis-regulatory elements that could indicate regulation at different developmental stages and/or differential regulation in response to stress. Comparison of the VvMTP coding sequences with known miRNAs found in various plant species indicated the presence of 13 putative miRNAs, with 7 of these associated with VvMTPs. Temporal and spatial expression profiling indicates a potential role for VvMTP genes during growth and development in grape plants, as well as the involvement of these genes in plant responses to environmental stress, especially osmotic stress. The data generated from this study provides a basis for further investigation of the roles of MTP genes in grapes.

Trichoderma harzianum mitigates salt stress in cucumber via multiple responses

Trichoderma harzianum T-soybean plays an important role in controlling soybean root rot disease. However, the mechanism by which it improves plant tolerance to salt stress is not clear. In this study, we investigated the possible mechanism of T-soybean in mitigating the damage caused by salt stress in Cucumis sativus L plants. Our results suggest that T-soybean improved salt tolerance of cucumber seedlings by affecting the antioxidant enzymes including peroxidase (POD) (EC 1.11.1.6), polyphenol oxidase (PPO) (EC 1.14.18.1), phenylalanine ammonia-lyase (PAL) (EC 4.3.1.5), catalase (CAT) (EC 1.11.1.6), superoxide dismutase (SOD) (EC 1.15.1.1), ascorbate peroxidase (APX) (EC 1.11.1.11), and glutathione reductase (GR) (EC 1.6.4.2), by increasing the levels of proline, soluble sugars, soluble protein, ascorbic acid (AsA) and chlorophyll as well as improving root activity. Treatment with T-soybean improved the ratio of glutathione (GSH)/oxidized glutathione (GSSG) and AsA/dehydroascorbate (DHA), and up-regulated the expression of CsAPX and CsGR genes involved in the AsA-GSH cycle. In addition, treatment with T-soybean increased the K⁺ content and K⁺/Na⁺ ratio while decreased the Na⁺ concentration and ethylene level. In summary, the improved salt tolerance of cucumber plants may be due to multiple mechanisms of T-soybean, such as the increase in reactive oxygen species (ROS) scavenging, as well as maintaining osmotic balance and metabolic homeostasis under salt stress.

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

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

Transcriptome response of roots to salt stress in a salinity-tolerant bread wheat cultivar

Salt stress is one of the major adverse environmental factors limiting crop productivity. Considering Iran as one of the bread wheat origins, we sequenced root transcriptome of an Iranian salt tolerant cultivar, Arg, under salt stress to extend our knowledge of the molecular basis of salinity tolerance in Triticum aestivum. RNA sequencing resulted in more than 113 million reads and about 104013 genes were obtained, among which 26171 novel transcripts were identified. A comparison of abundances showed that 5128 genes were differentially expressed due to salt stress. The differentially expressed genes (DEGs) were annotated with Gene Ontology terms, and the key pathways were identified using Kyoto Encyclopedia of Gene and Genomes (KEGG) pathway mapping. The DEGs could be classified into 227 KEGG pathways among which transporters, phenylpropanoid biosynthesis, transcription factors, glycosyltransferases, glutathione metabolism and plant hormone signal transduction represented the most significant pathways. Furthermore, the expression pattern of nine genes involved in salt stress response was compared between the salt tolerant (Arg) and susceptible (Moghan3) cultivars. A panel of novel genes and transcripts is found in this research to be differentially expressed under salinity in Arg cultivar and a model is proposed for salt stress response in this salt tolerant cultivar of wheat employing the DEGs. The achieved results can be beneficial for better understanding and improvement of salt tolerance in wheat.

Impacts of biochar application on upland agriculture: A review

Soil degradation has become an emerging global problem limiting sustainable upland crop production. Soil erosion, soil acidity, low fertility, inorganic/organic contamination, and salinization challenge food security and lead to severe economic constraints. Therefore, a new research agenda to develop cost-beneficial amendments for improving upland soil quality and productivity is urgently required. Biochar has been used in recent years to mitigate the problems mentioned above. Application of biochar improves the upland soil quality through significant changes in soil physicochemical and biological properties, thereby substantially increasing crop yield. This review article aims to discuss the effects of biochar on upland soil quality and productivity based on biochar-soil interactions. The yield of various upland crops can be enhanced by biochar-induced increases of nutrient availability and topsoil retention/recovery. Furthermore, biochar can assist in controlling unsuitable soil acidity/alkalinity/salinity and remediating a contaminated soil while increasing the retention of soil organic carbon, water content, and thereby high crop yield. Biochar is strongly recommended as one of the best management practices to meet the challenges of upland agriculture. However, the properties of biochar and soil type should be considered carefully prior to application.

Hybrid identification for Glycine max and Glycine soja with SSR markers and analysis of salt tolerance

Glycine max cultivars Lee68, Nannong 1138-2, and Nannong 8831 were used as the female parents, and hybrid lines (F5) 4,111, 4,076 (N23674 × BB52), 3,060 (Lee68 × N23227), and 185 (Jackson × BB52) that selected for salt tolerance generation by generation from the cross combination of G. max and G. soja were used as the male parents, 11 (A-K) backcrosses or three-way crosses were designed and 213 single hybrids were harvested. The optimized soybean simple sequence repeat (SSR)-polymerase chain reaction (PCR) system was used to analyze the SSR polymorphism of above parental lines and get the parental co-dominant SSR markers for hybrid identification, and in which 30 true hybrids were gained. The true hybrids (G1, G3, G9, G12, G13, G16) of G cross combination were chosen as the representative for the salt tolerance test, and the results showed that, as exposed to salt stress, the seedlings of G9 line displayed higher salt tolerant coefficient, relative growth rate, and dry matter accumulation, when compared with their female parent Nannong 1138-2, and even performed equally strong salt tolerance as the male parent 3,060. It provides a feasible method of the combination of molecular SSR markers and simple physiological parameters to identify the true hybrids of G. max and G. soja, and to innovate the salt-tolerant soybean germplasms.

Overexpression of LeNHX2 and SlSOS2 increases salt tolerance and fruit production in double transgenic tomato plants

Transgenic tomato plants (Solanum lycopersicum L. cv. MicroTom) overexpressing both the K⁺,Na⁺/H⁺ antiporter LeNHX2 and the regulatory kinase SlSOS2 were produced by crossing transgenic homozygous lines overexpressing LeNHX2 and SlSOS2. LeNHX2 expression was enhanced in plants overexpressing LeNHX2 but surprisingly even more in plants overexpressing SlSOS2 with and without LeNHX2. All transgenic plants showed better NaCl tolerance than wild type controls and plants overexpressing both LeNHX2 and SlSOS2 grew better under saline conditions than plants overexpressing only one of these genes. Yield related parameters indicated that single and above all double transgenic plants performed significantly better than wild type controls. All transgenic plants produced fruits with a higher K⁺ content than wild-type plants and plants overexpressing SlSOS2 accumulated more Na⁺ in fruits than the rest of the plants when grown with NaCl. Roots, stems and leaves of transgenic plants overexpressing LeNHX2 showed a higher K⁺ content than wild type and single transgenic plants overexpressing SlSOS2. Na⁺ content in stems and leaves of NaCl treated plants was higher in SlSOS2 overexpressing plants than in wild type and LeNHX2 single transgenic plants. All transgenic lines showed a higher leaf relative water content and a higher plant water content and water use efficiency than wild type controls when both were grown in the presence of NaCl. Results in this work indicate that the joint overexpression of LeNHX2 and SlSOS2 improves growth and water status under NaCl stress, affects K⁺ and Na⁺ homeostasis and enhances fruit yield of tomato plants.

Natural variation in cytokinin maintenance improves salt tolerance in apple rootstocks

Plants experiencing salt-induced stress often reduce cytokinin levels during the early phases of stress-response. Interestingly, we found that the cytokinin content in the apple rootstock "robusta" was maintained at a high level under salt stress. Through screening genes involved in cytokinin biosynthesis and catabolism, we found that the high expression levels of IPT5b in robusta roots were involved in maintaining the high cytokinin content. We identified a 42 bp deletion in the promoter region of IPT5b, which elevated IPT5b expression levels, and this deletion was linked to salt tolerance in robusta×M.9 segregating population. The 42 bp deletion resulted in the deletion of a Proline Response Element (ProRE), and our results suggest that ProRE negatively regulates IPT5b expression in response to proline. Under salt stress, the robusta cultivar maintains high cytokinin levels as IPT5b expression cannot be inhibited by proline due to the deletion of ProRE, leading to improve salt tolerance.

Uncovering Differentially Methylated Regions (DMRs) in a Salt-Tolerant Rice Variety under Stress: One Step towards New Regulatory Regions for Enhanced Salt Tolerance

Chromatin structure, DNA methylation, and histone modifications act in a concerted manner to influence gene expression and therefore plant phenotypes. Environmental stresses are often associated with extensive chromatin rearrangements and modifications of epigenetic levels and patterns. Stress-tolerant plants can be a good tool to unveil potential connections between specific epigenetic modifications and stress tolerance capacity. We analyzed genome wide DNA methylation of a salt-tolerant rice variety under salinity and identified a set of differentially methylated regions (DMRs) between control and stress samples using high-throughput sequencing of DNA immunoprecipitated with the 5-methylcytosine antibody (MeDIP-Seq). The examination of DNA methylation pattern at DMRs regions revealed a general tendency for demethylation events in stress samples as compared to control. In addition, DMRs appear to influence the expression of genes located in their vicinity. We hypothesize that short regions as DMRs can shape the chromatin landscape of specific genomic regions and, therefore, may modulate the function of several genes. In this sense, the identification of DMRs represents one step towards to uncover new players in the regulation of stress-responsive genes and new target genes with potential application in enhancement of plant salinity-tolerance.

Sodium Selenate Treatment Using a Combination of Seed Priming and Foliar Spray Alleviates Salinity Stress in Rice

Soil salinity is one of the important abiotic stress factors that affect rice productivity and quality. Research with several dicotyledonous plants indicated that the detrimental effects associated with salinity stress can (partly) be overcome by the external application of antioxidative substances. For instance, sodium selenate (Na₂SeO₄) significantly improved the growth and productivity of several crops under various abiotic stress conditions. At present there is no report describing the impact of Na₂SeO₄ on salinity stressed cereals such as rice. Rice cultivation is threatened by increasing salinity stress, and in future this problem will further be aggravated by global warming and sea level rise, impacting coastal areas. The current study reports on the effect of Na₂SeO₄ in alleviating salinity stress in rice plants. The optimal concentration of Na₂SeO₄ and the most efficient mode of selenium application were investigated. Selenium, sodium, and potassium contents in leaves were determined. Antioxidant enzyme activities as well as proline, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) concentrations were analyzed. In addition, the transcript levels for OsNHX1, an important Na⁺/H⁺ antiporter, were quantified. Treatment of 2-week-old rice plants under 150 mM NaCl stress with 6 mg l^-1 Na₂SeO₄ improved the total biomass. A significantly higher biomass was observed for the plants that received Na₂SeO₄ by a combination of seed priming and foliar spray compared to the individual treatments. The Na₂SeO₄ application enhanced the activity of antioxidant enzymes (SOD, APX, CAT, and GSH-Px), increased the proline content, and reduced H₂O₂ and MDA concentrations in plants under NaCl stress. These biochemical changes were accompanied by increased transcript levels for OsNHX1 resulting in a higher K⁺/Na⁺ ratio in the rice plants under NaCl stress. The results suggest that Na₂SeO₄ treatment alleviates the adverse effect of salinity on rice plant growth through enhancing the antioxidant defense system and increase of OsNHX1 transcript levels.

Impact of Arbuscular Mycorrhizal Fungi on Photosynthesis, Water Status, and Gas Exchange of Plants Under Salt Stress-A Meta-Analysis

Soil salinization is one of the most serious abiotic stress factors affecting plant productivity through reduction of soil water potential, decreasing the absorptive capacity of the roots for water and nutrients. A weighted meta-analysis was conducted to study the effects of arbuscular mycorrhizal fungi (AMF) inoculation in alleviating salt stress in C₃ and C₄ plants. We analyzed the salt stress influence on seven independent variables such as chlorophyll, leaf area, photosynthetic rate (Amax), stomatal conductance (Gs), transpiration rate (E), relative water content (RWC), and water use efficiency (WUE) on AMF inoculated plants. Responses were compared between C₃ and C₄ plants, AMF species, plant functional groups, level of salinity, and environmental conditions. Our results showed that AMF inoculated plants had a positive impact on gas exchange and water status under salt stress. The total chlorophyll contents of C₃ plants were higher than C₄ plants. However, C₃ plants responses regarding Gs, Amax, and E were more positive compared to C₄ plants. The increase in G _s mainly maintained E and it explains the increase in Amax and increase in E. When the two major AMF species (Rhizophagus intraradices and Funnelliformis mosseae) were considered, the effect sizes of RWC and WUE in R. intraradices were lower than those in F. mosseae indicating that F. mosseae inoculated plants performed better under salt stress. In terms of C₃ and C₄ plant photosynthetic pathways, the effect size of C₄ was lower than C₃ plants indicating that AMF inoculation more effectively alleviated salt stress in C₃ compared to C₄ plants.

Metabolomic analysis of date palm seedlings exposed to salinity and silicon treatments

Silicon is known to promote plant growth as well as stress tolerance of plants. The current study was undertaken to assess the growth promoting effect of silicon on date palm seedling development as well as its ability to abate some of the negative effects of salinity. In this study, date palm seedlings were treated with silicon and sodium chloride salts, and the effect of these salts on some physiological parameters of the plants was determined. In addition, a global nontargeted metabolomics analysis was performed for the leaf and root tissues using liquid chromatography-mass spectrometry (LC-MS). The results showed that under non-stress conditions, silicon treatment enhanced the growth of the date palm seedlings, however, under salinity, silicon slightly mitigates the negative effects of salt stress on the date palm seedlings although it enhances the potassium accumulation under this condition. The global metabolomics analysis has identified a total of 1,101 significant differentially accumulated (p, q ≤ 0.05) metabolites in leaves and roots under silicon, salinity or their combination. A differential pairwise metabolic profile comparison revealed the accumulation of distinct metabolites in response to silicon and salinity treatments such as antioxidant compounds pyridoxine, cepharanthine, allithiamine, myristic acid and boldine; osmoregulators such as mucic acid; along with the accumulation of detoxification intermediates such as S-D-lactoylglutathione, beta-cyano-L-alanine and gamma-glutamyl-conjugates. In addition, histochemical analyses revealed that application of silicon significantly (p ≤ 0.05) enhanced the formation of the Casparian strip. Identification of the differentially accumulated metabolites could offer an insight into how silicon is able to promote growth and salinity tolerance in date palms.

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

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

Identification and Functional Characterization of a Cold-Related Protein, BcHHP5, in Pak-Choi (Brassica rapa ssp. chinensis)

In plants, heptahelical proteins (HHPs) have been shown to respond to a variety of abiotic stresses, including cold stress. Up to the present, the regulation mechanism of HHP5 under low temperature stress remains unclear. In this study, BcHHP5 was isolated from Pak-choi (Brassica rapa ssp. chinensis cv. Suzhouqing). Sequence analysis and phylogenetic analysis indicated that BcHHP5 in Pak-choi is similar to AtHHP5 in Arabidopsis thaliana. Structure analysis showed that the structure of the BcHHP5 protein is relatively stable and highly conservative. Subcellular localization indicated that BcHHP5 was localized on the cell membrane and nuclear membrane. Furthermore, real-time quantitative polymerase chain reaction (RT-qPCR) analysis showed that BcHHP5 was induced to express by cold and other abiotic stresses. In Pak-choi, BcHHP5-silenced assay, inhibiting the action of endogenous BcHHP5, indicated that BcHHP5-silenced might have a negative effect on cold tolerance, which was further confirmed. All of these results indicate that BcHHP5 might play a role in abiotic response. This work can serve as a reference for the functional analysis of other cold-related proteins from Pak-choi in the future.

Transcriptome Sequence Analysis Elaborates a Complex Defensive Mechanism of Grapevine (Vitis vinifera L.) in Response to Salt Stress

Salinity is ubiquitous abiotic stress factor limiting viticulture productivity worldwide. However, the grapevine is vulnerable to salt stress, which severely affects growth and development of the vine. Hence, it is crucial to delve into the salt resistance mechanism and screen out salt-resistance prediction marker genes; we implicated RNA-sequence (RNA-seq) technology to compare the grapevine transcriptome profile to salt stress. Results showed 2472 differentially-expressed genes (DEGs) in total in salt-responsive grapevine leaves, including 1067 up-regulated and 1405 down-regulated DEGs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotations suggested that many DEGs were involved in various defense-related biological pathways, including ROS scavenging, ion transportation, heat shock proteins (HSPs), pathogenesis-related proteins (PRs) and hormone signaling. Furthermore, many DEGs were encoded transcription factors (TFs) and essential regulatory proteins involved in signal transduction by regulating the salt resistance-related genes in grapevine. The antioxidant enzyme analysis showed that salt stress significantly affected the superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and glutathione S-transferase (GST) activities in grapevine leaves. Moreover, the uptake and distribution of sodium (Na⁺), potassium (K⁺) and chlorine (Cl⁻) in source and sink tissues of grapevine was significantly affected by salt stress. Finally, the qRT-PCR analysis of DE validated the data and findings were significantly consistent with RNA-seq data, which further assisted in the selection of salt stress-responsive candidate genes in grapevine. This study contributes in new perspicacity into the underlying molecular mechanism of grapevine salt stress-tolerance at the transcriptome level and explore new approaches to applying the gene information in genetic engineering and breeding purposes.

Endophytic Fungus Aspergillus japonicus Mediates Host Plant Growth under Normal and Heat Stress Conditions

We have isolated an endophytic fungus with heat stress alleviation potential from wild plant Euphorbia indica L. The phylogenetic analysis and 18S rDNA sequence homology revealed that the designated isolate was Aspergillus japonicus EuR-26. Analysis of A. japonicus culture filtrate displayed higher concentrations of salicylic acid (SA), indoleacetic acid (IAA), flavonoids, and phenolics. Furthermore, A. japonicus association with soybean and sunflower had improved plant biomass and other growth features under high temperature stress (40°C) in comparison to endophyte-free plants. In fact, endophytic association mitigated heat stress by negotiating the activities of abscisic acid, catalase, and ascorbic acid oxidase in both soybean and sunflower. The nutritional quality (phenolic, flavonoids, soluble sugars, proteins, and lipids) of the A. japonicus-associated seedlings has also improved under heat stress in comparison to endophyte-free plants. From the results, it is concluded that A. japonicus can modulate host plants growth under heat stress and can be used as thermal stress alleviator in arid and semiarid regions of the globe (where mean summer temperature exceeds 40°C) to sustain agriculture.

Polysaccharides from Grateloupia filicina enhance tolerance of rice seeds (Oryza sativa L.) under salt stress

Rice (Oryza sativa L.) is a salt-sensitive crop which could be suppressed seriously by salt stress at germination stage. Some seaweeds polysaccharides could enhance plants resistance but there is little research about polysaccharides from Grateloupia filicina in agriculture. Therefore, G. filicina polysaccharide (GFP) and low molecular weight (MW) G. filicina polysaccharide (LGFP) were applied to rice seeds under salt stress (GFP: 2093.4 kDa, LGFP-1: 40.8 kDa, LGFP-2: 22.6 kDa, LGFP-3: 5.1 kDa, LGFP-4: 3.0 kDa). Relatively low MW polysaccharides LGFP1-4 showed better effect than GFP, and LGFP-1 showed the best effect on germination potential, germination index, shoot/root length and vigor index than negative control by 26.67, 14.27, 30.50, 202.65 and 162.78%, respectively. Optimum concentration was determined at 0.1 mg/mL, and LGFP-1 increased proline content, superoxide dismutase (SOD) and peroxidase activities (POD) which improved ability of osmotic adjustment and reactive oxygen species (ROS) scavenging. FITC-labeled LGFP-1 (F-LGFP-1) was to investigate the polysaccharide absorption and it was be observed in root and shoot with different distribution. Finally, expression of Na⁺/H⁺ antiporter gene was up regulated which suggested LGFP-1 could protect rice seeds by regulating Na⁺ content. This research showed potential application of polysaccharides from G. filicina for increasing rice seeds salt tolerance.

Effects of non-uniform root zone salinity on growth, ion regulation, and antioxidant defense system in two alfalfa cultivars

A split-root system was established to investigate the effects of uniform (0/0, 50/50, and 200/200 mM salt [NaCl]) and non-uniform (0/200 and 50/200 mM NaCl) salt stress on growth, ion regulation, and the antioxidant defense system of alfalfa (Medicago sativa) by comparing a salt-tolerant (Zhongmu No.1) and salt-sensitive (Algonquin) cultivar. We found that non-uniform salinity was associated with greater plant growth rate and shoot dry weight, lower leaf Na⁺ concentration, higher leaf potassium cation (K⁺) concentration, lower lipid peroxidation, and greater superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), and peroxidase (EC 1.11.1.7) activities, compared to uniform salt stress in both alfalfa cultivars. Under non-uniform salinity, a significant increase in Na⁺ concentration and Na⁺ efflux and a decline in K⁺ efflux in the no-saline or low-saline part of the roots alleviated salt damage. Our results also demonstrated that proline and antioxidant enzymes accumulated in both the no- or low-saline and high-saline roots, revealing that osmotic adjustment and antioxidant defense had systemic rather than localized effects in alfalfa plants, and there was a functional equilibrium within the root system under non-uniform salt stress. The salt-tolerant cultivar Zhongmu No.1 exhibited greater levels of growth compared to Algonquin under both uniform and non-uniform salt stress, with Na⁺ tolerance and efflux abilities more effective and greater antioxidant defense capacity evident for cultivar Zhongmu No.1.

Exogenous Pi supplementation improved the salt tolerance of maize (Zea mays L.) by promoting Na+ exclusion

The mechanism of phosphate (Pi)-mediated salt tolerance in maize is poorly understood. In this study, the effects of Pi (H2PO4-) on the salt tolerance of two contrasting genotypes was investigated in a pot experiment. We discovered that the application of 3 mM Pi could alleviate salt injury caused by 200 mM NaCl. High amounts of compatible solutes and low amounts of reactive oxygen species (ROS) were also observed under Pi application. Consistent with the increased tolerance, the total number of roots and the growth of shoots increased to relieve salt stress. This phenomenon could be associated with the observed increased expression of nitrate transporters. Furthermore, the seedlings presented a negative relationship between sodium (Na+) and Pi (low Na+ content and high Pi content), which is related to the genes ZmNHX1, ZmPHT1;8, and ZmPHT1;9, indicating that the exclusion of Na+ was promoted by high Pi uptake. However, high Na+ and low potassium (K+) efflux were detected in the roots, and these were positively correlated with two K+ transporters. These observations indicate that Na+ exclusion was directly induced by high K+ retention rather than Pi absorption. We conclude that maize salt tolerance increased in response to Pi application by promoting Na+ exclusion.

Growth, physiology, and transcriptional analysis of Two contrasting Carex rigescens genotypes under Salt stress reveals salt-tolerance mechanisms

Salt stress is a major abiotic stress threatening plant growth and development throughout the world. In this study, we investigated the salt stress adaptation mechanism of Carex rigescens (Franch.) V. Krecz, a stress-tolerant turfgrass species with a wide distribution in northern China. Specifically, we analyzed the growth, physiology, and transcript expression patterns of two C. rigescens genotypes (Huanghua and Lvping No.1) exposed to salt stress. Results show that Huanghua demonstrated better growth performance, and higher turf quality (TQ), photochemical efficiency (Fv/Fm), relative water content (RWC), proline content, and lower relative electrolyte leakage (REL) during seven days of salt treatment compared to Lvping No.1, suggesting that Huanghua is more salt tolerant. Significant differences in reactive oxygen species (ROS), Malondialdehyde (MDA), melatonin, non-enzymatic antioxidants, lignin, and flavonoid content, as well as in antioxidant activity between Huanghua and Lvping No.1 after salt stress indicate the diverse regulation involved in salt stress adaptation in C. rigescens. These results, combined with those of the transcript expression pattern of involved genes, suggest that Huanghua is more active and efficient in ROS scavenging, Ca²⁺ binding, and its phytohormone response than Lvping No.1. Meanwhile, Lvping No.1 showed relatively higher phenylpropanoid synthesis, using flavonoid and lignin as supplements for the inadequate ROS-scavenging capacity and the development of vascular tissues, respectively. These performances illustrate the differences between the two genotypes in multifaceted and sophisticated actions contributing to the tolerance mechanism of salt stress in C. rigescens. In addition, the significantly higher content of melatonin and the rapid induction of Caffeic acid O-methyltransferase (COMT) highlight the role of melatonin in the salt stress response in Huanghua. The results of our study expand existing knowledge of the complexity of the salt stress response involving the antioxidant system, Ca²⁺ signaling, phytohormone response signaling, and phenylpropanoid pathways. It also provides a basis for further study of the underlying mechanism of salt tolerance in C. rigescens and other plant species.

Phosphorus sorption capacity of biochars varies with biochar type and salinity level

Biochar is recognized as an effective material for recovering excess nutrients, including phosphorus (P), from aqueous solutions. Practically, that benefits the environment through reducing P losses from biochar-amended soils; however, how salinity influences P sorption by biochar is poorly understood and there has been no direct comparison on P sorption capacity between biochars derived from different feedstock types under non-saline and saline conditions. In this study, biochars derived from wheat straw, hardwood, and willow wood were used to compare P sorption at three levels of electrical conductivity (EC) (0, 4, and 8 dS m^-1) to represent a wide range of salinity conditions. Phosphorus sorption by wheat straw and hardwood biochars increased as aqueous solution P concentration increased, with willow wood biochar exhibiting an opposite trend for P sorption. However, the pattern for P sorption became the same as the other biochars after the willow wood biochar was de-ashed with 1 M HCl and 0.05 M HF. Willow wood biochar had the highest P sorption (1.93 mg g^-1) followed by hardwood (1.20 mg g^-1) and wheat straw biochars (1.06 mg g^-1) in a 25 mg L^-1 P solution. Although the pH in the equilibrium solution was higher with willow wood biochar (~ 9.5) than with the other two biochars (~ 6.5), solution pH had no or minor effects on P sorption by willow wood biochar. The high sorption rate of P by willow wood biochar could be attributed to the higher concentrations of salt and other elements (i.e., Ca and Mg) in the biochar in comparison to that in wheat straw and hardwood biochars; the EC values were 2.27, 0.53, and 0.27 dS m^-1 for willow wood, wheat straw, and hardwood biochars, respectively. A portion of P desorbed from the willow wood biochar; and that desorption increased with the decreasing P concentration in the aqueous solution. Salinity in the aqueous solution influenced P sorption by hardwood and willow wood but not by wheat straw biochar. We conclude that the P sorption capacity of the studied biochars is dependent on the concentration of the soluble element in the biochar, which is dependent on the biochar type, as well as the salinity level in the aqueous solution.

Silicon improves salt tolerance of Glycyrrhiza uralensis Fisch. by ameliorating osmotic and oxidative stresses and improving phytohormonal balance

Si has a beneficial effect on improving plant tolerance to salt stress. Nevertheless, the mechanisms of Si in mediating the stress responses are still poorly understood. Glycyrrhiza uralensis Fisch. (G. uralensis), a well-known medicinal plant, possesses vast therapeutic potentials. In the present study, a pot experiment was conducted to investigate the long-term effects of Si on growth and physiobiochemical characteristics in 2-year-old G. uralensis subjected to different levels of salinity. Si markedly affected G. uralensis growth in a salt concentration-dependent manner and had no effect on G. uralensis growth under 6 g/kg NaCl. However, it partly reversed the reduction effect induced by 9 g/kg NaCl. In addition, Si significantly increased the contents of soluble sugar and protein but deceased proline content and thus increased water relations; Si markedly increased the activities of SOD, peroxidase, and CAT and further resulted in decreased MDA content and membrane permeability. Moreover, Si altered the levels of phytohormones and their balances. With correlation analysis and principal component analysis (PCA), root biomass had a significant negative correlation with MDA and membrane permeability while a positive correlation with indole-3-acetic acid and GA₃. The PCA partitioned the total variance into three PCs contributing maximum (88.234%) to the total diversity among the salt stress with or without Si due to the study of various traits. In conclusion, Si exerts a beneficial property on salt-induced harmful effects in G. uralensis by relieving osmotic stress, improving water relations, and alleviating oxidative stress; thus, altering the levels and balance of phytohormones results in improved growth of salt-stressed G. uralensis.

Physiological and structural modifications in snail medic (Medicago scutellata L.) plants exposed to salinity

Seeds of snail medic (Medicago scutellata L.) were assessed for their response to salt at the germination and seedling stages. NaCl at concentrations 86 and 170 mM decreased the final germination percentage. Embryonic axis length, water content and dry weight of embryonic axis and cotyledons were also reduced by salt treatment. Furthermore, 28-d-old plants were grown hydroponically with different NaCl concentrations (0, 86 and 170 mM). After 7 days of treatment, growth, water content and development of the different organs of M. scutellata plant were affected especially at the highest NaCl concentration (170 mM). However, NaCl did not affect root length and the number of stem shoots but reduced stem length and total leaf area. Salt treatment increased markedly the concentration of Na⁺ in leaf and root tissues while reduced that of K⁺ only in root and stem tissues. Lipid peroxidation revealed the damage of the membranes of roots and leaves. Moreover, showed a more intense suberization and lignification at the cambial zone of roots of M. scutellata, were observed under the effect of NaCl.

Title: Enhanced salt tolerance and photosynthetic performance: Implication of ɤ-amino butyric acid application in salt-exposed lettuce (Lactuca sativa L.) plants

Gamma-Amino Butyric Acid (GABA) is a substantial component of the free amino acid pool with low concentration in plant tissues. Enhanced GABA content occurs during plant growth and developmental processes like seed germination. GABA level, basically, alters in response to many endogenous and exogenous stimuli. In the current study, GABA effects were studied on germination, photosynthetic performance and oxidative damages in salt-exposed lettuce plants. Three NaCl (0, 40 and 80 mM) and two GABA (0 and 25 μM) concentrations were applied on lettuce during two different developmental (seed germination and seedlings growth) stages. Negative effects of salinity on germination and plant growth were removed by GABA application. GABA significantly reduced mean germination time (MGT) in salt-exposed lettuce seeds. Although, salinity caused a significant decline in maximum quantum yield of photosystem II (Fv/Fm) during distinct steps of plant growth, GABA application improved Fv/Fm particularly on high salinity level. GABA decreased specific energy fluxes per reaction center (RC) for energy absorption and dissipation, while enhanced-electron transport flux in photosynthetic apparatus of lettuce plants was observed in GABA-supplemented plants. Moreover, decline in non-photochemical quenching (NPQ) and quenching coefficients (qP, qL, qN) by salt stress were recovered by GABA application. Elevated electrolyte leakage considerably decreased by GABA exposure on salt-treated plants. Although, proline level increased by NaCl treatments in a concentration dependent manner, combined application of salt with GABA caused a significant reduction in proline content. Catalase; EC 1.11.1.6 (CAT), l-ascorbate peroxidase; EC 1.11.1.11 (APX), and superoxide dismutase; EC 1.15.1.1 (SOD) activities were increased by GABA exposure in salt-supplemented plants that resulted in regulated hydrogen peroxide level. In conclusion, a multifaceted role for GABA is suggested for minimizing detrimental effects of salinity on lettuce through improvement of photosynthetic functionality and regulation of oxidative stress.

Cropland Soil Salinization and Associated Hydrology: Trends, Processes and Examples

While global food demand and world population are rapidly growing, land potential for cropping is steadily declining due to various soil degradation processes, a major one of them being soil salinization. Currently, approximately 20% of total cropland and 33% of irrigated agricultural land are salinized as a result of poor agricultural practices and it is expected that by 2050, half of the croplands worldwide will become salinized. Thus, there is a real need to better understand soil salinization processes and to develop agricultural practices that will enable production of the needed amount of food to feed humanity, while minimizing soil salinization and other degradation processes. The major sources of solutes in agricultural environments are: (i) the soil itself, and the parent geological material; (ii) shallow and salt rich groundwater; and (iii) salt rich irrigation water. The salinization of soil is a combination of transport of solutes towards the root zone to replenish evaporation and transpiration and limited washing of the soil by rain or irrigation. Therefore, most salinized soils are present in arid and semi-arid environments where precipitation is low and evaporation is high. In this manuscript, examples of soil salinization processes from croplands around the world will be presented and discussed to bring attention to this important topic, to present the latest scientific insights and to highlight the gaps that should be filled, from both scientific and practical perspectives.

Genome-Wide Analysis of Multidrug and Toxic Compound Extrusion (MATE) Family in Gossypium raimondii and Gossypium arboreum and Its Expression Analysis Under Salt, Cadmium, and Drought Stress

The extrusion of toxins and substances at a cellular level is a vital life process in plants under abiotic stress. The multidrug and toxic compound extrusion (MATE) gene family plays a large role in the exportation of toxins and other substrates. We carried out a genome-wide analysis of MATE gene families in Gossypium raimondii and Gossypium arboreum and assessed their expression levels under salt, cadmium and drought stresses. We identified 70 and 68 MATE genes in G. raimondii and G. arboreum, respectively. The majority of the genes were predicted to be localized within the plasma membrane, with some distributed in other cell parts. Based on phylogenetic analysis, the genes were subdivided into three subfamilies, designated as M1, M2 and M3. Closely related members shared similar gene structures, and thus were highly conserved in nature and have mainly evolved through purifying selection. The genes were distributed in all chromosomes. Twenty-nine gene duplication events were detected, with segmental being the dominant type. GO annotation revealed a link to salt, drought and cadmium stresses. The genes exhibited differential expression, with GrMATE18, GrMATE34, GaMATE41 and GaMATE51 significantly upregulated under drought, salt and cadmium stress, and these could possibly be the candidate genes. Our results provide the first data on the genome-wide and functional characterization of MATE genes in diploid cotton, and are important for breeders of more stress-tolerant cotton genotypes.

Exogenous salicylic acid regulates reactive oxygen species metabolism and ascorbate-glutathione cycle in Nitraria tangutorum Bobr. under salinity stress

The effect of 0.5-1.5 mM salicylic acid (SA) on modulating reactive oxygen species metabolism and ascorbate-glutathione cycle in NaCl-stressed Nitraria tangutorum seedlings was investigated. The individual plant fresh weight (PFW) and plant dry weight (PDW) significantly increased under 100 mM NaCl while remained unchanged or decreased under 200-400 mM NaCl compared to the control. Superoxide anion (O₂^·-), hydrogen peroxide (H₂O₂), thiobarbituric acid reactive substances (TBARS), reduced ascorbate (AsA), dehydroascorbate (DHA), reduced glutathione (GSH) and oxidized glutathione (GSSG) increased whereas the ratios of AsA/DHA and GSH/GSSG decreased under varied NaCl treatments. Ascorbate peroxidase (APX) and glutathione reductase (GR) activities were enhanced while dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) activities remained unvaried under 100-400 mM NaCl stresses. In addition, exogenous SA further increased PFW, PDW and root/shoot ratio. SA effectively diminished O₂^·- accumulation. H₂O₂ and TBARS decreased under 0.5 and 1.0 mM SA treatments compared to those without SA. 0.5 mM of SA increased while 1.0 and 1.5 mM SA decreased APX activities. DHAR activities were elevated by 0.5 and 1.0 mM SA but not by 1.5 mM SA. MDHAR and GR activities kept constant or significantly increased at varying SA concentrations. Under SA treatments, AsA and GSH contents further increased, DHA and GSSG levels remained unaltered, while the decreases in AsA/DHA and GSH/GSSG ratios were inhibited. The above results demonstrated that the enhanced tolerance of N. tangutorum seedlings conferred by SA could be attributed mainly to the elevated GR and DHAR activities as well as the increased AsA/DHA and GSH/GSSG ratios.

Integration of proteomic and transcriptomic profiles reveals multiple levels of genetic regulation of salt tolerance in cotton

BACKGROUND

Salinity is a major abiotic stress that limits upland cotton growth and reduces fibre production worldwide. To reveal genetic regulation via transcript and protein levels after salt stress, we comprehensively analysed the global changes in mRNA, miRNA, and protein profiles in response to salt stress in two contrasting salt-tolerant cotton genotypes.

RESULTS

In the current study, proteomic and mRNA-seq data were combined to reveal that some genes are differentially expressed at both the proteomic and mRNA levels. However, we observed no significant change in mRNA corresponding to most of the strongly differentially abundant proteins. This finding may have resulted from global changes in alternative splicing events and miRNA levels under salt stress conditions. Evidence was provided indicating that several salt stress-responsive proteins can alter miRNAs and modulate alternative splicing events in upland cotton. The results of the stringent screening of the mRNA-seq and proteomic data between the salt-tolerant and salt-sensitive genotypes identified 63 and 85 candidate genes/proteins related to salt tolerance after 4 and 24 h of salt stress, respectively, between the tolerant and sensitive genotype. Finally, we predicted an interaction network comprising 158 genes/proteins and then discovered that two main clusters in the network were composed of ATP synthase (CotAD_74681) and cytochrome oxidase (CotAD_46197) in mitochondria. The results revealed that mitochondria, as important organelles involved in energy metabolism, play an essential role in the synthesis of resistance proteins during the process of salt exposure.

CONCLUSION

We provided a plausible schematic for the systematic salt tolerance model; this schematic reveals multiple levels of gene regulation in response to salt stress in cotton and provides a list of salt tolerance-related genes/proteins. The information here will facilitate candidate gene discovery and molecular marker development for salt tolerance breeding in cotton.

The genus Portulaca as a suitable model to study the mechanisms of plant tolerance to drought and salinity

Drought and soil salinity are at present the major factors responsible for the global reduction of crop yields, and the problem will become more severe in the coming decades because of climate change effects. The most promising strategy to achieve the increased agricultural production that will be required to meet food demands worldwide will be based on the enhancement of crop stress tolerance, by both, traditional breeding and genetic engineering. This, in turn, requires a deep understanding of the mechanisms of tolerance which, although based on a conserved set of basic responses, vary widely among plant species. Therefore, the use of different plant models to investigate these mechanisms appears to be a sensible approach. The genus Portulaca could be a suitable model to carry out these studies, as some of its taxa have been described as tolerant to drought and/or salinity. Information on relevant mechanisms of tolerance to salt and water stress can be obtained by correlating the activation of specific defence pathways with the relative stress resistance of the investigated species. Also, species of the genus could be economically attractive as ‘new’ crops for ‘saline’ and ‘arid’, sustainable agriculture, as medicinal plants, highly nutritious vegetable crops and ornamentals.

An Overview of the Genetics of Plant Response to Salt Stress: Present Status and the Way Forward

Salinity is one of the major threats faced by the modern agriculture today. It causes multidimensional effects on plants. These effects depend upon the plant growth stage, intensity, and duration of the stress. All these lead to stunted growth and reduced yield, ultimately inducing economic loss to the farming community in particular and to the country in general. The soil conditions of agricultural land are deteriorating at an alarming rate. Plants assess the stress conditions, transmit the specific stress signals, and then initiate the response against that stress. A more complete understanding of plant response mechanisms and their practical incorporation in crop improvement is an essential step towards achieving the goal of sustainable agricultural development. Literature survey shows that investigations of plant stresses response mechanism are the focus area of research for plant scientists. Although these efforts lead to reveal different plant response mechanisms against salt stress, yet many questions still need to be answered to get a clear picture of plant strategy to cope with salt stress. Moreover, these studies have indicated the presence of a complicated network of different integrated pathways. In order to work in a progressive way, a review of current knowledge is critical. Therefore, this review aims to provide an overview of our understanding of plant response to salt stress and to indicate some important yet unexplored dynamics to improve our knowledge that could ultimately lead towards crop improvement.

Exogenous nanosilica improves germination and growth of cucumber by maintaining K+/Na+ ratio under elevated Na+ stress

The current work was aimed to elucidate the role of engineered nanosilica (SiNPs) particles to mitigate the damaging impacts of Na⁺-derived salinity on cucumber (Cucumis sativus) Beit Alpha variety by conducting in vitro experiments applying various Na⁺ concentrations i.e. 0, 1000, 2000, 3000, 4000 and 5000 mg L^-1. By treating seeds and seedlings, respectively, of cucumber with SiNPs (0, 100, 200 and 300 ppm) and subsequent determination some germination and vegetative parameters as well as chemical analysis of seedlings, we verified that SiNPs succeeded to alleviate the detrimental effects of high Na⁺ salinity by increasing germination parameters and vegetative growth of cucumber seedlings. Even as little as 100 ppm of N-Si results in considerable improvement of seed germination and seedlings growth of cucumber compared to the control, while 200 ppm was optimal among the doses tested. At 5000 mg Na⁺ L^-1, applying SiNPs with 200 ppm increased final germination percentage by 101% over control, vigor index by 101%, germination rate index by 116%, germination index by 110%, fresh mass by 13%, K⁺/Na⁺ ratio by 77%, shoot dry mass by 384%, root dry mass by 304% and plant height by 70%. The results mentioned in this paper obviously outline the large practical relevance of SiNPs and imply that applying of SiNPs for cucumber seeds and seedlings under high Na⁺-derived salinity enhances germination and growth as a result for decreasing Na⁺ uptake and sequentially improves high K⁺/Na⁺ ratio.

Kadir N, Mustaqim M, Rahmat Z, Ahmad A, Lam SD, Ismail MR. Susceptibility and tolerance of rice crop to salt threat: Physiological and metabolic inspections

Salinity threat is estimated to reduce global rice production by 50%. Comprehensive analysis of the physiological and metabolite changes in rice plants from salinity stress (i.e. tolerant versus susceptible plants) is important to combat higher salinity conditions. In this study, we screened a total of 92 genotypes and selected the most salinity tolerant line (SS1-14) and most susceptible line (SS2-18) to conduct comparative physiological and metabolome inspections. We demonstrated that the tolerant line managed to maintain their water and chlorophyll content with lower incidence of sodium ion accumulation. We also examined the antioxidant activities of these lines: production of ascorbate peroxidase (APX) and catalase (CAT) were significantly higher in the sensitive line while superoxide dismutase (SOD) was higher in the tolerant line. Partial least squares discriminant analysis (PLS-DA) score plots show significantly different response for both lines after the exposure to salinity stress. In the tolerant line, there was an upregulation of non-polar metabolites and production of sucrose, GABA and acetic acid, suggesting an important role in salinity adaptation. In contrast, glutamine and putrescine were noticeably high in the susceptible rice. Coordination of different strategies in tolerant and susceptible lines show that they responded differently after exposure to salt stress. These findings can assist crop development in terms of developing tolerance mechanisms for rice crops.