Ion Transporters and Abiotic Stress Tolerance in Plants

Weighted gene co-expression network analysis reveals hub genes regulating response to salt stress in peanut

Peanut (Arachis hypogaea L.) is an important oilseed crop worldwide. However, soil salinization becomes one of the main limiting factors of peanut production. Therefore, developing salt-tolerant varieties and understanding the molecular mechanisms of salt tolerance is important to protect peanut yield in saline areas. In this study, we selected four peanut varieties with contrasting response to salt challenges with T1 and T2 being tolerance and S1 and S2 being susceptible. High-throughput RNA sequencing resulted in more than 314.63 Gb of clean data from 48 samples. We identified 12,057 new genes, 7,971of which have functional annotations. KEGG pathway enrichment analysis of uniquely expressed genes in salt-tolerant peanut revealed that upregulated genes in the root are involved in the MAPK signaling pathway, fatty acid degradation, glycolysis/gluconeogenesis, and upregulated genes in the shoot were involved in plant hormone signal transduction and the MAPK signaling pathway. Na+ content, K+ content, K+/ Na+, and dry mass were measured in root and shoot tissues, and two gene co-expression networks were constructed based on weighted gene co-expression network analysis (WGCNA) in root and shoot. In this study, four key modules that are highly related to peanut salt tolerance in root and shoot were identified, plant hormone signal transduction, phenylpropanoid biosynthesis, starch and sucrose metabolism, flavonoid biosynthesis, carbon metabolism were identified as the key biological processes and metabolic pathways for improving peanut salt tolerance. The hub genes include genes encoding ion transport (such as HAK8, CNGCs, NHX, NCL1) protein, aquaporin protein, CIPK11 (CBL-interacting serine/threonine-protein kinase 11), LEA5 (late embryogenesis abundant protein), POD3 (peroxidase 3), transcription factor, and MAPKKK3. There were some new salt-tolerant genes identified in peanut, including cytochrome P450, vinorine synthase, sugar transport protein 13, NPF 4.5, IAA14, zinc finger CCCH domain-containing protein 62, beta-amylase, fatty acyl-CoA reductase 3, MLO-like protein 6, G-type lectin S-receptor-like serine/threonine-protein kinase, and kinesin-like protein KIN-7B. The identification of key modules, biological pathways, and hub genes in this study enhances our understanding of the molecular mechanisms underlying salt tolerance in peanuts. This knowledge lays a theoretical foundation for improving and innovating salt-tolerant peanut germplasm.

The Spartina alterniflora genome sequence provides insights into the salt-tolerance mechanisms of exo-recretohalophytes

Spartina alterniflora is an exo-recretohalophyte Poaceae species that is able to grow well in seashore, but the genomic basis underlying its adaptation to salt tolerance remains unknown. Here, we report a high-quality, chromosome-level genome assembly of S. alterniflora constructed through PacBio HiFi sequencing, combined with high-throughput chromosome conformation capture (Hi-C) technology and Illumina-based transcriptomic analyses. The final 1.58 Gb genome assembly has a contig N50 size of 46.74 Mb. Phylogenetic analysis suggests that S. alterniflora diverged from Zoysia japonica approximately 21.72 million years ago (MYA). Moreover, whole-genome duplication (WGD) events in S. alterniflora appear to have expanded gene families and transcription factors relevant to salt tolerance and adaptation to saline environments. Comparative genomics analyses identified numerous species-specific genes, significantly expanded genes and positively selected genes that are enriched for 'ion transport' and 'response to salt stress'. RNA-seq analysis identified several ion transporter genes including the high-affinity K+ transporters (HKTs), SaHKT1;2, SaHKT1;3 and SaHKT1;8, and high copy number of Salt Overly Sensitive (SOS) up-regulated under high salt conditions, and the overexpression of SaHKT2;4 in Arabidopsis thaliana conferred salt tolerance to the plant, suggesting specialized roles for S. alterniflora to adapt to saline environments. Integrated metabolomics and transcriptomics analyses revealed that salt stress activate glutathione metabolism, with differential expressions of several genes such as γ-ECS, GSH-S, GPX, GST and PCS in the glutathione metabolism. This study suggests several adaptive mechanisms that could contribute our understanding of evolutional basis of the halophyte.

GhCLCc-1, a Chloride Channel Gene from Upland Cotton, Positively Regulates Salt Tolerance by Modulating the Accumulation of Chloride Ions

The ionic toxicity induced by salinization has adverse effects on the growth and development of crops. However, researches on ionic toxicity and salt tolerance in plants have focused primarily on cations such as sodium ions (Na+), with very limited studies on chloride ions (Cl-). Here, we cloned the homologous genes of Arabidopsis thaliana AtCLCc, GhCLCc-1A/D, from upland cotton (Gossypium hirsutum), which were significantly induced by NaCl or KCl treatments. Subcellular localization showed that GhCLCc-1A/D were both localized to the tonoplast. Complementation of Arabidopsis atclcc mutant with GhCLCc-1 rescued its salt-sensitive phenotype. In addition, the silencing of the GhCLCc-1 gene led to an increased accumulation of Cl- in the roots, stems, and leaves of cotton seedlings under salt treatments, resulting in compromised salt tolerance. And ectopic expression of the GhCLCc-1 gene in Arabidopsis reduced the accumulation of Cl- in transgenic lines under salt treatments, thereby enhancing salt tolerance. These findings elucidate that GhCLCc-1 positively regulates salt tolerance by modulating Cl- accumulation and could be a potential target gene for improving salt tolerance in plants.

Designing salt stress-resilient crops: Current progress and future challenges

Excess soil salinity affects large regions of land and is a major hindrance to crop production worldwide. Therefore, understanding the molecular mechanisms of plant salt tolerance has scientific importance and practical significance. In recent decades, studies have characterized hundreds of genes associated with plant responses to salt stress in different plant species. These studies have substantially advanced our molecular and genetic understanding of salt tolerance in plants and have introduced an era of molecular design breeding of salt-tolerant crops. This review summarizes our current knowledge of plant salt tolerance, emphasizing advances in elucidating the molecular mechanisms of osmotic stress tolerance, salt-ion transport and compartmentalization, oxidative stress tolerance, alkaline stress tolerance, and the trade-off between growth and salt tolerance. We also examine recent advances in understanding natural variation in the salt tolerance of crops and discuss possible strategies and challenges for designing salt stress-resilient crops. We focus on the model plant Arabidopsis (Arabidopsis thaliana) and the four most-studied crops: rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), and soybean (Glycine max).

γ-Aminobutyric acid enhances salt tolerance by sustaining ion homeostasis in apples

Soil salinization had become a global ecological problem, which restricts the plant growth, and the quantity and quality of fruits. As a signaling molecule, γ-Aminobutyric acid (GABA) mediates a series of physiological processes and stress responses. Our previous research showed that GABA could alleviate drought, low phosphorus, cadmium stresses in apples, but the further research about its physiological mechanisms under salt stress was even more needed. The present study showed that the inhibition of salt stress on plant growth might be effectively alleviated by the treatment of 0.5 mM GABA, and the osmotic balance and photosynthetic capacity of plants could be maintained. Exogenous GABA could effectively inhibit the enrichment of reactive oxygen species and the uptake of Na+, while maintaining ion homeostasis. The experiment results indicated GABA could markedly promote the expression amount of Na+ and K+ transport-related genes (e.g., HKT1, AKT1, NHX1, SOS1, SOS2, and SOS3) in apples under salt stress. Overexpression and interference (RNAi) of MdGAD1 in apple roots, which is a crucial enzyme in the GABA biosynthesis, affected the salt tolerance of plants. Transgenic apple plants with roots of overexpression MdGAD1 showed less relative electrolyte leakage and more expression level of related ion transport genes than CK group, but RNAi MdGAD1 led to the opposite results. These results indicated that GABA accumulation could effectively strengthen the resistance of apple plants to salt stress and alleviate the injury of apple seedlings resulted from salinity.

Role of Acetic Acid and Nitric Oxide against Salinity and Lithium Stress in Canola (Brassica napus L.)

In this study, canola (Brassica napus L.) seedlings were treated with individual and combined salinity and lithium (Li) stress, with and without acetic acid (AA) or nitric acid (NO), to investigate their possible roles against these stresses. Salinity intensified Li-induced damage, and the principal component analysis revealed that this was primarily driven by increased oxidative stress, deregulation of sodium and potassium accumulation, and an imbalance in tissue water content. However, pretreatment with AA and NO prompted growth, re-established sodium and potassium homeostasis, and enhanced the defense system against oxidative and nitrosative damage by triggering the antioxidant capacity. Combined stress negatively impacted phenylalanine ammonia lyase activity, affecting flavonoids, carotenoids, and anthocyanin levels, which were then restored in canola plants primed with AA and NO. Additionally, AA and NO helped to maintain osmotic balance by increasing trehalose and proline levels and upregulating signaling molecules such as hydrogen sulfide, γ-aminobutyric acid, and salicylic acid. Both AA and NO improved Li detoxification by increasing phytochelatins and metallothioneins, and reducing glutathione contents. Comparatively, AA exerted more effective protection against the detrimental effects of combined stress than NO. Our findings offer novel perspectives on the impacts of combining salt and Li stress.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

The Euphrates Poplar Responses to Abiotic Stress and Its Unique Traits in Dry Regions of China (Xinjiang and Inner Mongolia): What Should We Know?

At the moment, drought, salinity, and low-temperature stress are ubiquitous environmental issues. In arid regions including Xinjiang and Inner Mongolia and other areas worldwide, the area of tree plantations appears to be rising, triggering tree growth. Water is a vital resource in the agricultural systems of countries impacted by aridity and salinity. Worldwide efforts to reduce quantitative yield losses on Populus euphratica by adapting tree plant production to unfavorable environmental conditions have been made in response to the responsiveness of the increasing control of water stress. Although there has been much advancement in identifying the genes that resist abiotic stresses, little is known about how plants such as P. euphratica deal with numerous abiotic stresses. P. euphratica is a varied riparian plant that can tolerate drought, salinity, low temperatures, and climate change, and has a variety of water stress adaptability abilities. To conduct this review, we gathered all available information throughout the Web of Science, the Chinese National Knowledge Infrastructure, and the National Center for Biotechnology Information on the impact of abiotic stress on the molecular mechanism and evolution of gene families at the transcription level. The data demonstrated that P. euphratica might gradually adapt its stomatal aperture, photosynthesis, antioxidant activities, xylem architecture, and hydraulic conductivity to endure extreme drought and salt stress. Our analyses will give readers an understanding of how to manage a gene family in desert trees and the influence of abiotic stresses on the productivity of tree plants. They will also give readers the knowledge necessary to improve biotechnology-based tree plant stress tolerance for sustaining yield and quality trees in China's arid regions.

How do plants maintain pH and ion homeostasis under saline-alkali stress?

Salt and alkaline stresses often occur together, severely threatening plant growth and crop yields. Salt stress induces osmotic stress, ionic stress, and secondary stresses, such as oxidative stress. Plants under saline-alkali stress must develop suitable mechanisms for adapting to the combined stress. Sustained plant growth requires maintenance of ion and pH homeostasis. In this review, we focus on the mechanisms of ion and pH homeostasis in plant cells under saline-alkali stress, including regulation of ion sensing, ion uptake, ion exclusion, ion sequestration, and ion redistribution among organs by long-distance transport. We also discuss outstanding questions in this field.

Overexpression of JsFLS5 in calli improves salinity tolerance by maintaining active oxygen balance and reducing Na+ toxicity in Juglans sigillata

The escalating global climate change significantly threatens plant growth, development, and production through salinity stress. Flavonoids, a crucial category of secondary metabolites, have been extensively studied for their role in modulating plant growth and development mechanisms in the face of biological and abiotic stress. The flavonol synthetase (FLS) gene plays a key role in the biosynthesis and accumulation of flavonoids. To investigate the correlation between salt tolerance and flavonol synthesis, JsFLS5 was overexpressed in the callus of Juglans sigillata cv. "Qianhe-7." This study shows that the upregulation of JsFLS5 significantly elevates the overall flavonoid content by modulating the expression of genes associated with flavonoid synthesis under salinity stress conditions. Additionally, the overexpressing callus exhibited enhanced resistance to salt stress compared to the wild-type callus, as evidenced by reduced levels of reactive oxygen species accumulation, electrolyte leakage, and malondialdehyde content in the overexpressing callus relative to the wild type (WT). Moreover, the overexpressing callus showed higher antioxidant enzyme activity and a more efficient ascorbic acid-glutathione cycle. Furthermore, the concentration of Na+ in the overexpressing callus was lower than WT, resulting in a decreased Na+ /K+ ratio. These findings suggest that JsFLS5 overexpression in calli effectively mitigates the oxidative damage induced by osmotic stress and reduces Na+ toxicity by enhancing flavonoid synthesis under salt stress conditions. Consequently, this study offers a novel perspective for comprehending the role of JsFLS5 in the response to abiotic stress in J. sigillata.

Salt-Induced Modulation of Ion Transport and PSII Photoprotection Determine the Salinity Tolerance of Amphidiploid Brassicas

Brassica species show varying levels of resistance to salt stress. To understand the genetics underlying these differential stress tolerance patterns in Brassicas, we exposed two widely cultivated amphidiploid Brassica species having different genomes, Brassica juncea (AABB, n = 18) and Brassica napus (AACC, n = 19), to elevated levels of NaCl concentration (300 mM, half the salinity of seawater). B. juncea produced more biomass, an increased chlorophyll content, and fewer accumulated sodium (Na+) and chloride (Cl-) ions in its photosynthesizing tissues. Chlorophyll fluorescence assays revealed that the reaction centers of PSII of B. juncea were more photoprotected and hence more active than those of B. napus under NaCl stress, which, in turn, resulted in a better PSII quantum efficiency, better utilization of photochemical energy with significantly reduced energy loss, and higher electron transport rates, even under stressful conditions. The expression of key genes responsible for salt tolerance (NHX1 and AVP1, which are nuclear-encoded) and photosynthesis (psbA, psaA, petB, and rbcL, which are chloroplast-encoded) were monitored for their genetic differences underlying stress tolerance. Under NaCl stress, the expression of NHX1, D1, and Rubisco increased several folds in B. juncea plants compared to B. napus, highlighting differences in genetics between these two Brassicas. The higher photosynthetic potential under stress suggests that B. juncea is a promising candidate for genetic modifications and its cultivation on marginal lands.

Integrated mRNA and miRNA transcriptome analysis of grape in responses to salt stress

Salt stress is an important factor which may negatively affect plant growth and development. High concentrations of Na⁺ ions can destroy the ion balance in plant somatic cells, as well as destroying cell membranes and forming a large number of reactive oxygen species (ROS) and other damage mechanisms. However, plants have evolved numerous defense mechanisms in response to the damages caused by salt stress conditions. Grape (Vitis vinifera L.), a type of economic crop, is widely planted throughout the world. It has been found that salt stress is an important factor affecting the quality and growth of grape crops. In this study, a high-throughput sequencing method was used to identify the differentially expressed miRNAs and mRNAs in grapes as responses to salt stress. A total of 7,856 differentially expressed genes under the salt stress conditions were successfully identified, of which 3,504 genes were observed to have up-regulated expressions and 4,352 genes had down-regulated expressions. In addition, this study also identified 3,027 miRNAs from the sequencing data using bowtie and mireap software. Among those, 174 were found to be highly conserved, and the remaining miRNAs were less conserved. In order to analyze the expression levels of those miRNAs under salt stress conditions, a TPM algorithm and DESeq software were utilized to screen the differentially expressed miRNAs among different treatments. Subsequently, a total of thirty-nine differentially expressed miRNAs were identified, of which fourteen were observed to be up-regulated miRNAs and twenty-five were down-regulated under the salt stress conditions. A regulatory network was built in order to examine the responses of grape plants to salt stress, with the goal of laying a solid foundation for revealing the molecular mechanism of grape in responses to salt stress.

Quantitative elemental imaging in eukaryotic algae

All organisms, fundamentally, are made from the same raw material, namely the elements of the periodic table. Biochemical diversity is achieved by how these elements are utilized, for what purpose, and in which physical location. Determining elemental distributions, especially those of trace elements that facilitate metabolism as cofactors in the active centers of essential enzymes, can determine the state of metabolism, the nutritional status, or the developmental stage of an organism. Photosynthetic eukaryotes, especially algae, are excellent subjects for quantitative analysis of elemental distribution. These microbes utilize unique metabolic pathways that require various trace nutrients at their core to enable their operation. Photosynthetic microbes also have important environmental roles as primary producers in habitats with limited nutrient supplies or toxin contaminations. Accordingly, photosynthetic eukaryotes are of great interest for biotechnological exploitation, carbon sequestration, and bioremediation, with many of the applications involving various trace elements and consequently affecting their quota and intracellular distribution. A number of diverse applications were developed for elemental imaging, allowing subcellular resolution, with X-ray fluorescence microscopy (XFM, XRF) being at the forefront, enabling quantitative descriptions of intact cells in a non-destructive method. This Tutorial Review summarizes the workflow of a quantitative, single-cell elemental distribution analysis of a eukaryotic alga using XFM.

Multifaceted regulatory functions of CsBPC2 in cucumber under salt stress conditions

BASIC PENTACYSTEINE (BPC) transcription factors are essential regulators of plant growth and development. However, BPC functions and the related molecular mechanisms during cucumber (Cucumis sativus L.) responses to abiotic stresses, especially salt stress, remain unknown. We previously determined that salt stress induces CsBPC expression in cucumber. In this study, Csbpc2 transgene-free cucumber plants were created using a CRISPR/Cas9-mediated editing system to explore CsBPC functions associated with the salt stress response. The Csbpc2 mutants had a hypersensitive phenotype, with increased leaf chlorosis, decreased biomass, and increased malondialdehyde and electrolytic leakage levels under salt stress conditions. Additionally, a mutated CsBPC2 resulted in decreased proline and soluble sugar contents and antioxidant enzyme activities, which led to the accumulation of hydrogen peroxide and superoxide radicals. Furthermore, the mutation to CsBPC2 inhibited salinity-induced PM-H⁺-ATPase and V-H⁺-ATPase activities, resulting in decreased Na⁺ efflux and increased K⁺ efflux. These findings suggest that CsBPC2 may mediate plant salt stress resistance through its effects on osmoregulation, reactive oxygen species scavenging, and ion homeostasis-related regulatory pathways. However, CsBPC2 also affected ABA signaling. The mutation to CsBPC2 adversely affected salt-induced ABA biosynthesis and the expression of ABA signaling-related genes. Our results indicate that CsBPC2 may enhance the cucumber response to salt stress. It may also function as an important regulator of ABA biosynthesis and signal transduction. These findings will enrich our understanding of the biological functions of BPCs, especially their roles in abiotic stress responses, thereby providing the theoretical basis for improving crop salt tolerance.

Integrative transcriptomic and proteomic analyses reveal a positive role of BES1 in salt tolerance in Arabidopsis

INTRODUCTION

Salt stress is a major environmental factor limiting plant growth and development. Previous studies have indicated that the steroidal hormones-brassinosteroids (BRs) are important regulators of plant responses to salt stress. However, the underlying molecular mechanisms have not been fully understood.

METHODS

(1) Phenotypic analysis of bes1-D, BES1-RNAi and their wild-type (Col-0) under salt treatments with different concentrations of NaCl. (2) Transcriptomic and proteomic profiling of BES1-regulated genes and proteins under salt treatment; (3) qRT-PCR validation of selected BES1-regulated genes under salt stress; (4) Transient transcriptional assay of BES1 regulation on its putative target genes in Arabidopsis protoplasts; (5) Electrophoresis Mobility Shift Assay (EMSA) of BES1 binding with its potential target genes.

RESULTS AND DISCUSSION

Phenotypic analysis indicated that bes1-D, a gain-of-function mutant of the BR-regulated transcription factor BES1 in Arabidopsis showed better salt tolerance than the wild-type plant, while a BES1 RNA interference (BES1-RNAi) line was more sensitive to salt stress. Global gene expression profiling and time series clustering analyses identified a total of 1,170 genes whose expression was boosted in bes1-D under salt stress. Further GO enrichment and gene functional network analyses identified several key modules that are regulated by BES1 and most sensitive to salt stress perturbations, including stress response, response to ABA and ROS, flavonoid biosynthesis and transmembrane transport. A comparative proteomic analysis performed under the same stress conditions supported the results from the transcriptome analysis. In addition, transient gene transcription assays in Arabidopsis protoplasts and in vitro DNA binding assays verified that BES1 regulates the expression of some ion transporter genes directly and indirectly. Taken together, our results support a positive role of BES1 in plant salt tolerance.

Integrated metabolomic and transcriptomic analysis reveals the role of phenylpropanoid biosynthesis pathway in tomato roots during salt stress

As global soil salinization continues to intensify, there is a need to enhance salt tolerance in crops. Understanding the molecular mechanisms of tomato (Solanum lycopersicum) roots' adaptation to salt stress is of great significance to enhance its salt tolerance and promote its planting in saline soils. A combined analysis of the metabolome and transcriptome of S. lycopersicum roots under different periods of salt stress according to changes in phenotypic and root physiological indices revealed that different accumulated metabolites and differentially expressed genes (DEGs) associated with phenylpropanoid biosynthesis were significantly altered. The levels of phenylpropanoids increased and showed a dynamic trend with the duration of salt stress. Ferulic acid (FA) and spermidine (Spd) levels were substantially up-regulated at the initial and mid-late stages of salt stress, respectively, and were significantly correlated with the expression of the corresponding synthetic genes. The results of canonical correlation analysis screening of highly correlated DEGs and construction of regulatory relationship networks with transcription factors (TFs) for FA and Spd, respectively, showed that the obtained target genes were regulated by most of the TFs, and TFs such as MYB, Dof, BPC, GRAS, and AP2/ERF might contribute to the regulation of FA and Spd content levels. Ultimately, FA and Spd attenuated the harm caused by salt stress in S. lycopersicum, and they may be key regulators of its salt tolerance. These findings uncover the dynamics and possible molecular mechanisms of phenylpropanoids during different salt stress periods, providing a basis for future studies and crop improvement.

The mechanistic basis of sodium exclusion in Puccinellia tenuiflora under conditions of salinity and potassium deprivation

Soil salinity is a significant threat to global agriculture. Understanding salt exclusion mechanisms in halophyte species may be instrumental in improving salt tolerance in crops. Puccinellia tenuiflora is a typical salt-excluding halophytic grass often found in potassium-deprived saline soils. Our previous work showed that P. tenuiflora possesses stronger selectivity for K⁺ than for Na⁺ ; however, the mechanistic basis of this phenomenon remained elusive. Here, P. tenuiflora PutHKT1;5 was cloned and the functions of PutHKT1;5 and PutSOS1 were characterized using heterologous expression systems. Yeast assays showed that PutHKT1;5 possessed Na⁺ transporting capacity and was highly selective for Na⁺ over K⁺ . PutSOS1 was located at the plasma membrane and operated as a Na⁺ /K⁺ exchanger, with much stronger Na⁺ extrusion capacity than its homolog from Arabidopsis. PutHKT2;1 mediated high-affinity K⁺ and Na⁺ uptake and its expression levels were upregulated by mild salinity and K⁺ deprivation. Salinity-induced changes of root PutHKT1;5 and PutHKT1;4 transcript levels matched the expression pattern of root PutSOS1, which was consistent with root Na⁺ efflux. The transcript levels of root PutHKT2;1 and PutAKT1 were downregulated by salinity. Taken together, these findings demonstrate that the functional activity of PutHKT1;5 and PutSOS1 in P. tenuiflora roots is fine-tuned under saline conditions as well as by operation of other ion transporters/channel (PutHKT1;4, PutHKT2;1, and PutAKT1). This leads to the coordination of radial Na⁺ and K⁺ transport processes, their loading to the xylem, or Na⁺ retrieval and extrusion under conditions of mild salinity and/or K⁺ deprivation.

Transcriptome analysis of halophyte Nitraria tangutorum reveals multiple mechanisms to enhance salt resistance

As a typical halophyte, Nitraria tangutorum Bobr. has attracted the interest of many researchers with the excellent salt tolerance. Elucidation of the mechanism of N. tangutorum salinity tolerance will facilitate the genetic improvement of productive plants faced with salinity. To reveal the molecular response to gradually accumulated salt stress in N. tangutorum, RNA-sequencing and analysis of gradually accumulated NaCl treated samples and control samples were performed, and a total of 1419 differentially expressed genes were identified, including 949 down-regulated genes and 470 up-regulated genes. Detailed analysis uncovered that the catabolism of organic compounds mainly based on oxidative phosphorylation genes was up-regulated. Additionally, various antioxidant genes, especially anthocyanin-related genes, were found to help N. tangutorum remove reactive oxygen species. Moreover, the Mitogen activated protein kinase signaling pathway and other signaling pathways co-regulated various salt tolerance activities. Additionally, intracellular ion homeostasis was maintained via regulation of osmotic regulator-related genes, cutin-related genes, and cell elongation-related genes to retain cellular water and reduce ion concentration. In particularly, simultaneous up-regulation in cytoskeleton-related genes, cell wall-related genes, and auxin-related genes, provided evidence of important role of cell expansion in plant salt tolerance. In conclusion, complex regulatory mechanisms modulated by multiple genes might contribute to the salt tolerance by N. tangutorum.

RNA-Seq Analysis of Magnaporthe grisea Transcriptome Reveals the High Potential of ZnO Nanoparticles as a Nanofungicide

Magnaporthe grisea is one of the most destructive pathogen that encounters a challenge to rice production around the worldwide. The unique properties of ZnO nanoparticles (NPs), have high attractiveness as nanofungicide. In the present study, the response of fungi to ZnO NPs was evaluated using RNA sequencing (RNA-seq). Two different aligners (STAR and Hisat2) were used for aligning the clean reads, and the DEseq2 package was used to identify the differentially expressed genes (DEGs). In total, 1,438 and 761 fungal genes were significantly up- and down-regulated in response to ZnO NPs, respectively. The DEGs were subjected to functional enrichment analysis to identify significantly enriched biological pathways. Functional enrichment analysis revealed that "cell membrane components," "ion (calcium) transmembrane transporter activity," "steroid biosynthesis pathway" and "catalytic activity" were the contributed terms to fungal response mechanisms. The genes involved in aflatoxin efflux pumps and ribosome maturation were among the genes showing significant up- and down-regulation after ZnO NPs application. To confirm the obtained RNA-seq results, the expression of six randomly selected genes were evaluated using q-RT-PCR. Overall, the RNA-seq results suggest that ZnO NPs primarily act on the fungal cell membrane, but accumulation of ROS inside the cell induces oxidative stress, the fungal catalytic system is disrupted, resulting into the inhibition of ROS scavenging and eventually, to the death of fungal cells. Our findings provide novel insights into the effect of ZnO NPs as a promising nanofungicide for effective control of rice blast disease.

Identification of TCP family in moso bamboo (Phyllostachys edulis) and salt tolerance analysis of PheTCP9 in transgenic Arabidopsis

Bioinformatic analysis of moso bamboo TEOSINTE BRANCHED 1, CYCLOIDEA, and PROLIFERATING CELL FACTORS (TCP) transcription factors reveals their conservation and variation as well as the probable biological functions in abiotic stress response. Overexpressing PheTCP9 in Arabidopsis thaliana illustrates it may exhibit a new vision in different aspects of response to salt stress. Plant specific TCPs play important roles in plant growth, development and stress response, but studies of TCP in moso bamboo are limited. Therefore, in this study, a total of 40 TCP genes (PheTCP1 ~ 40) were identified and characterized from moso bamboo genome and divided into three different subfamilies, namely, 7 in TEOSINTE BRANCHED 1 / CYCLOIDEA (TB1/CYC), 14 in CINCINNATA (CIN) and 19 in PROLIFERATING CELL FACTOR (PCF). Subsequently, we analyzed the gene structures and conserved domain of these genes and found that the members from the same subfamilies exhibited similar exon/intron distribution patterns. Selection pressure and gene duplication analysis results indicated that PheTCP genes underwent strong purification selection during evolution. There were many cis-elements related to phytohermone and stress responsive existing in the upstream promoter regions of PheTCP genes, such as ABRE, CGTCA-motif and ARE. Subcellular localization experiments showed that PheTCP9 was a nuclear localized protein. As shown by β-glucuronidase (GUS) activity, the promoter of PheTCP9 was significantly indicated by salt stress. PheTCP9 was significantly induced in the roots, stems and leaves of moso bamboo. It was also significantly induced by NaCl solution. Overexpressing PheTCP9 increased the salt tolerance of transgenic Arabidopsis. Meanwhile, H₂O₂ and malondialdehyde (MDA) contents were significantly lower in PheTCP9 over expression (OE) transgenic Arabidopsis than WT. Catalase (CAT) activity, K⁺/Na⁺ ratio as well as CAT2 expression level was also much improved in transgenic Arabidopsis than WT under salt conditions. In addition, PheTCP9 OE transgenic Arabidopsis held higher survival rates of seedlings than WT under NaCl conditions. These results showed the positive regulation functions of PheTCP9 in plants under salt conditions.

Transcriptomic Analysis Elaborates the Resistance Mechanism of Grapevine Rootstocks against Salt Stress

Grapes are subject to a wide range of climatic conditions during their life cycle, but the use of rootstocks can effectively ameliorate the effects of abiotic stress. However, the tolerance mechanism of different grape rootstock varieties varies under various stresses, and systematic research on this aspect is limited. On the basis of previous research, transcriptome sequencing was performed on three tolerant grape rootstock varieties (3309C, 520A, 1103P) and three intolerant grape rootstock varieties (5BB, 101-14, Beta). In total, 56,478,468 clean reads were obtained. One hundred and ten genes only existed in all combinations during P1 with a downregulated trend, and 178 genes existed only in P1 of tolerant grape rootstock varieties. Salt treatment firstly affected the photosynthesis of leaves, and tolerant varieties weakened or even eliminated this effect through their own mechanisms in the later stage. Tolerant varieties mobilized a large number of MFs during the P2 stage, such as hydrolase activity, carboxypeptidase activity, and dioxygenase activity. Carbon metabolism was significantly enriched in P1, while circadian rhythm and flavonoid biosynthesis were only enriched in tolerant varieties. In the intolerant varieties, photosynthesis-related pathways were always the most significantly enriched. There were large differences in the gene expression of the main signal pathways related to salt stress in different varieties. Salt stress affected the expression of genes related to plant abiotic stress, biotic stress, transcription factors, hormones, and secondary metabolism. Tolerant varieties mobilized more bHLH, WRKY, and MYB transcription factors to respond to salt stress than intolerant varieties. In the tolerant rootstocks, SOS was co-expressed. Among these, SOS1 and SOS2 were upregulated, and the SOS3 and SOS5 components were downregulated. The genes of heat shock proteins and the phenylalanine pathway were upregulated in the tolerant varieties. These findings outline a tolerance mechanism model for rootstocks for coping with osmotic stress, providing important information for improving the resistance of grapes under global climate change.

Root Na+ Content Negatively Correlated to Salt Tolerance Determines the Salt Tolerance of Brassica napus L. Inbred Seedlings

Soil salinization is a major environmental stressor that reduces the growth and yield of crops. Maintaining the balance of ions under salinity is vital for plant salt tolerance; however, little is known about the correlation between the salt tolerance of crops and the ion contents of their roots and shoots. Here, we investigated the poorly understood salt-tolerance mechanisms, particularly regarding ion contents (particularly Na+), in Brassica napus subsp. napus L., an agriculturally important species. Twenty B. napus inbred lines were randomly chosen from five salt-tolerance categories and treated with increasing concentrations of NaCl (0–200 mmol) for this work. We found that the root Na+ content is the most correlated limiting factor for the salt tolerance of B. napus; the higher the salt tolerance, the lower the root Na+ content. Correspondingly, the Ca2+/Na+ and K+/Na+ ratios of the roots were highly correlated with B. napus salt tolerance, indicating that the selective absorption ability of these ions by the roots and their translocation to the shoots play a pivotal role in this trait. These data provide a foundation for the further study of the molecular mechanisms underlying salt tolerance and for breeding salt-tolerant B. napus cultivars.

Linking genome wide RNA sequencing with physio-biochemical and cytological responses to catalogue key genes and metabolic pathways for alkalinity stress tolerance in lentil (Lens culinaris Medikus)

BACKGROUND

Alkaline soils cause low productivity in crop plants including lentil. Alkalinity adaptation strategies in lentil were revealed when morpho-anatomical and physio-biochemical observations were correlated with transcriptomics analysis in tolerant (PDL-1) and sensitive (L-4076) cultivars at seedling stage.

RESULTS

PDL-1 had lesser salt injury and performed better as compared to L-4076. Latter showed severe wilting symptoms and higher accumulation of Na⁺ and lower K⁺ in roots and shoots. PDL-1 performed better under high alkalinity stress which can be attributed to its higher mitotic index, more accumulation of K⁺ in roots and shoots and less aberrantly dividing cells. Also, antioxidant enzyme activities, osmolytes' accumulation, relative water content, membrane stability index and abscisic acid were higher in this cultivar. Differentially expressed genes (DEGs) related to these parameters were upregulated in tolerant genotypes compared to the sensitive one. Significantly up-regulated DEGs were found to be involved in abscisic acid (ABA) signalling and secondary metabolites synthesis. ABA responsive genes viz. dehydrin 1, 9-cis-epoxycarotenoid dioxygenase, ABA-responsive protein 18 and BEL1-like homeodomain protein 1 had log₂fold change above 4.0. A total of 12,836 simple sequence repeats and 4,438 single nucleotide polymorphisms were identified which can be utilized in molecular studies.

CONCLUSIONS

Phyto-hormones biosynthesis-predominantly through ABA signalling, and secondary metabolism are the most potent pathways for alkalinity stress tolerance in lentil. Cultivar PDL-1 exhibited high tolerance towards alkalinity stress and can be used in breeding programmes for improving lentil production under alkalinity stress conditions.

Deciphering Molecular Mechanisms Involved in Salinity Tolerance in Guar (Cyamopsis tetragonoloba (L.) Taub.) Using Transcriptome Analyses

Guar is a commercially important legume crop known for guar gum. Guar is tolerant to various abiotic stresses, but the mechanisms involved in its salinity tolerance are not well established. This study aimed to understand molecular mechanisms of salinity tolerance in guar. RNA sequencing (RNA-Seq) was employed to study the leaf and root transcriptomes of salt-tolerant (Matador) and salt-sensitive (PI 340261) guar genotypes under control and salinity. Our analyses identified a total of 296,114 unigenes assembled from 527 million clean reads. Transcriptome analysis revealed that the gene expression differences were more pronounced between salinity treatments than between genotypes. Differentially expressed genes associated with stress-signaling pathways, transporters, chromatin remodeling, microRNA biogenesis, and translational machinery play critical roles in guar salinity tolerance. Genes associated with several transporter families that were differentially expressed during salinity included ABC, MFS, GPH, and P-ATPase. Furthermore, genes encoding transcription factors/regulators belonging to several families, including SNF2, C₂H₂, bHLH, C3H, and MYB were differentially expressed in response to salinity. This study revealed the importance of various biological pathways during salinity stress and identified several candidate genes that may be used to develop salt-tolerant guar genotypes that might be suitable for cultivation in marginal soils with moderate to high salinity or using degraded water.

Characterization of Differentially Expressed Genes under Salt Stress in Olive

Climate change, currently taking place worldwide and also in the Mediterranean area, is leading to a reduction in water availability and to groundwater salinization. Olive represents one of the most efficient tree crops to face these scenarios, thanks to its natural ability to tolerate moderate salinity and drought. In the present work, four olive cultivars (Koroneiki, Picual, Royal de Cazorla and Fadak86) were exposed to high salt stress conditions (200 mM of NaCl) in greenhouse, in order to evaluate their tolerance level and to identify key genes involved in salt stress response. Molecular and physiological parameters, as well as plant growth and leaves’ ions Na⁺ and K⁺ content were measured. Results of the physiological measurements showed Royal de Cazorla as the most tolerant cultivar, and Fadak86 and Picual as the most susceptible ones. Ten candidate genes were analyzed and their complete genomic, CDS and protein sequences were identified. The expression analysis of their transcripts through reverse transcriptase quantitative PCR (RT-qPCR) demonstrated that only OeNHX7, OeP5CS, OeRD19A and OePetD were upregulated in tolerant cultivars, thus suggesting their key role in the activation of a salt tolerance mechanism.

Integrated Physiological, Transcriptomic, and Metabolomic Analyses Revealed Molecular Mechanism for Salt Resistance in Soybean Roots

Salinity stress is a threat to yield in many crops, including soybean (Glycine max L.). In this study, three soybean cultivars (JD19, LH3, and LD2) with different salt resistance were used to analyze salt tolerance mechanisms using physiology, transcriptomic, metabolomic, and bioinformatic methods. Physiological studies showed that salt-tolerant cultivars JD19 and LH3 had less root growth inhibition, higher antioxidant enzyme activities, lower ROS accumulation, and lower Na+ and Cl- contents than salt-susceptible cultivar LD2 under 100 mM NaCl treatment. Comparative transcriptome analysis showed that compared with LD2, salt stress increased the expression of antioxidant metabolism, stress response metabolism, glycine, serine and threonine metabolism, auxin response protein, transcription, and translation-related genes in JD19 and LH3. The comparison of metabolite profiles indicated that amino acid metabolism and the TCA cycle were important metabolic pathways of soybean in response to salt stress. In the further validation analysis of the above two pathways, it was found that compared with LD2, JD19, and LH3 had higher nitrogen absorption and assimilation rate, more amino acid accumulation, and faster TCA cycle activity under salt stress, which helped them better adapt to salt stress. Taken together, this study provides valuable information for better understanding the molecular mechanism underlying salt tolerance of soybean and also proposes new ideas and methods for cultivating stress-tolerant soybean.

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

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

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

The TCP transcription factor PeTCP10 modulates salt tolerance in transgenic Arabidopsis

PeTCP10 can be induced by salt stresses and play important regulation roles in salt stresses response in transgenic Arabidopsis. Salt stress is one of the major adverse environmental factors that affect normal plant development and growth. PeTCP10, a Class I TCP member, was markedly expressed in moso bamboo mature leaf, root and stem under normal conditions and also induced by salt stress. Overexpressed PeTCP10 was found to enhance salt tolerance of transgenic Arabidopsis at the vegetative growth stage. It was also found capable to increase relative water content, while decreasing relative electrolyte leakage and Na⁺ accumulation of transgenic Arabidopsis versus wild-type (WT) plants at high-salt conditions. In addition, it improved antioxidant capacity of transgenic Arabidopsis plants by promoting catalase activity and enhanced their H₂O₂ tolerance. In contrast to WT plants, transcriptome analysis demonstrated that multiple genes related to abscisic acid, salt and H₂O₂ response were induced after NaCl treatment in transgenic plants. Meanwhile, overexpressed PeTCP10 improved the tolerance of abscisic acid. Moreover, luciferase reporter assay results showed that PeTCP10 is able to directly activate the expression of BT2 in transgenic plants. In contrary, the germination rates of transgenic plants were significantly lower than those of WT plants under high-NaCl conditions. Both primary root length and survival rate at the seedling stage are also found lower in transgenic plants than in WT plants. It is concluded that overexpressed PeTCP10 enhances salt stress tolerance of transgenic plants at the vegetative growth stage, and it also improves salt sensitiveness in both germination and seedling stages. These research results will contribute to further understand the functions of TCPs in abiotic stress response.

The CrRLK1L subfamily: One of the keys to versatility in plants

Catharanthus roseous kinase 1L receptors (CrRLK1Ls) are a subfamily of membrane receptors unique to plant cells that perceive internal and external signals, integrate metabolic, physiological, and molecular processes, and regulate plant development. Recent genomic studies have suggested that this receptor subfamily arose during the emergence of terrestrial plants and has since diversified, preserving its essential functions. Participation of some of these CrRLK1Ls in different processes is presented and discussed herein, as well as the increasing number of interactors necessary for their function. At least five different responses have been detected after activating these receptors, such as physiological changes, formation or disassembly of protein complexes, metabolic responses, modification of gene expression, and modulation of phytohormone activity. To date, a common response mechanism for all processes involving CrRLK1Ls has not been described. In this review, the information available on the different functions of CrRLK1Ls was compiled. Additionally, the physiological and/or molecular mechanisms involved in the signaling processes triggered by these receptors are also discussed. In this review, we propose a possible common signaling mechanism for all processes regulated by CrRLK1Ls and pose questions to be answered in the future.

Nanoparticles enhances the salinity toxicity tolerance in Linum usitatissimum L. by modulating the antioxidative enzymes, photosynthetic efficiency, redox status and cellular damage

The contribution of nanoparticles (NPs) in physiology of the plants became the new area of interest for the physiologists; as it is very much cost effective compared to the phytohormones. Our present investigation was also based on this interest in which the same doses (50 mg/L) of four different NPs were sprayed on stressed and non-stressed foliage. The experiment was conducted to assess the impact of four NPs viz., zinc oxide (ZnO), silicon dioxide (SiO₂), titanium dioxide (TiO₂), and ferric oxide (Fe₂O₃) on the morphology and physiology of linseed in the presence of sodium chloride (NaCl). Plants responded positively to all the treated NPs and improved the growth, carbon and nutrient assimilation, while salt stress increased the content of proline, hydrogen peroxide and superoxide anion. Application of NPs over the stressed plants further increased the antioxidant enzymatic system and other physiochemical reactions. Results indicate that application of NPs increased the growth and physiology of the plant and also increased the salt tolerance capacity of the plant.

Comprehensive transcriptional analysis reveals salt stress-regulated key pathways, hub genes and time-specific responsive gene categories in common bermudagrass (Cynodon dactylon (L.) Pers.) roots

BACKGROUND

Despite its good salt-tolerance level, key genes and pathways involved with temporal salt response of common bermudagrass (Cynodon dactylon (L.) Pers.) have not been explored. Therefore, in this study, to understand the underlying regulatory mechanism following the different period of salt exposure, a comprehensive transcriptome analysis of the bermudagrass roots was conducted.

RESULTS

The transcripts regulated after 1 h, 6 h, or 24 h of hydroponic exposure to 200 mM NaCl in the roots of bermudagrass were investigated. Dataset series analysis revealed 16 distinct temporal salt-responsive expression profiles. Enrichment analysis identified potentially important salt responsive genes belonging to specific categories, such as hormonal metabolism, secondary metabolism, misc., cell wall, transcription factors and genes encoded a series of transporters. Weighted gene co-expression network analysis (WGCNA) revealed that lavenderblush2 and brown4 modules were significantly positively correlated with the proline content and peroxidase activity and hub genes within these two modules were further determined. Besides, after 1 h of salt treatment, genes belonging to categories such as signalling receptor kinase, transcription factors, tetrapyrrole synthesis and lipid metabolism were immediately and exclusively up-enriched compared to the subsequent time points, which indicated fast-acting and immediate physiological responses. Genes involved in secondary metabolite biosynthesis such as simple phenols, glucosinolates, isoflavones and tocopherol biosynthesis were exclusively up-regulated after 24 h of salt treatment, suggesting a slightly slower reaction of metabolic adjustment.

CONCLUSION

Here, we revealed salt-responsive genes belonging to categories that were commonly or differentially expressed in short-term salt stress, suggesting possible adaptive salt response mechanisms in roots. Also, the distinctive salt-response pathways and potential salt-tolerant hub genes investigated can provide useful future references to explore the molecular mechanisms of bermudagrass.

Recent progress in understanding salinity tolerance in plants: Story of Na+/K+ balance and beyond

High salt concentrations in the growing medium can severely affect the growth and development of plants. It is imperative to understand the different components of salt-tolerant network in plants in order to produce the salt-tolerant cultivars. High-affinity potassium transporter- and myelocytomatosis proteins have been shown to play a critical role for salinity tolerance through exclusion of sodium (Na⁺) ions from sensitive shoot tissues in plants. Numerous genes, that limit the uptake of salts from soil and their transport throughout the plant body, adjust the ionic and osmotic balance of cells in roots and shoots. In the present review, we have tried to provide a comprehensive report of major research advances on different mechanisms regulating plant tolerance to salinity stress at proteomics, metabolomics, genomics and transcriptomics levels. Along with the role of ionic homeostasis, a major focus was given on other salinity tolerance mechanisms in plants including osmoregulation and osmo-protection, cell wall remodeling and integrity, and plant antioxidative defense. Major proteins and genes expressed under salt-stressed conditions and their role in enhancing salinity tolerance in plants are discussed as well. Moreover, this manuscript identifies and highlights the key questions on plant salinity tolerance that remain to be discussed in the future.

MdINT1 enhances apple salinity tolerance by regulating the antioxidant system, homeostasis of ions, and osmosis

Myo-inositol is a versatile compound and plays a vital role in plant growth and stress tolerance. Previously, we found that exogenous application of myo-inositol enhanced the salinity tolerance in Malus hupehensis Rehd. by enhancing myo-inositol metabolism. In this study, we found that the tonoplast-localized myo-inositol transporter 1 (MdINT1) was involved in myo-inositol accumulation and conferred salinity tolerance in apple. MdINT1 is characterized by the highest transcripts among the four apple INT-like genes and could be induced by salt stress at the transcriptional level. Also, it was shown that myo-inositol level was slightly decreased in the leaves of transgenic apple lines over-expressing MdINT1, but was significantly increased in the leaves and roots of MdINT1 silencing line. Interestingly, overexpression of MdINT1 enhanced salinity tolerance by promoting Na⁺ and K⁺ balance, antioxidant activity, and accumulation of osmoprotectants in transgenic apple lines. In contrast, under salinity conditions, the MdINT1-mediated protective roles in the antioxidant activity, homeostasis of ions and osmosis were compromised, which in turn increased the risk of salt intolerance in the MdINT1 silencing line.

Molecular genetic analyses of abiotic stress responses during plant reproductive development

Plant responses to abiotic stresses during vegetative growth have been extensively studied for many years. Daily environmental fluctuations can have dramatic effects on plant vegetative growth at multiple levels, resulting in molecular, cellular, physiological, and morphological changes. Plants are even more sensitive to environmental changes during reproductive stages. However, much less is known about how plants respond to abiotic stresses during reproduction. Fortunately, recent advances in this field have begun to provide clues about these important processes, which promise further understanding and a potential contribution to maximize crop yield under adverse environments. Here we summarize information from several plants, focusing on the possible mechanisms that plants use to cope with different types of abiotic stresses during reproductive development, and present a tentative molecular portrait of plant acclimation during reproductive stages. Additionally, we discuss strategies that plants use to balance between survival and productivity, with some comparison among different plants that have adapted to distinct environments.

Quantitative trait loci (QTL) mapping for physiological and biochemical attributes in a Pasban90/Frontana recombinant inbred lines (RILs) population of wheat (Triticum aestivum) under salt stress condition

Salt stress causes nutritional imbalance and ion toxicity which affects wheat growth and production. A population of recombinant inbred lines (RILs) were developed by crossing Pasban90 (salt tolerant) and Frontana (salt suceptible) for identification of quantitative trait loci (QTLs) for physiological traits including relative water content, membrane stability index, water potential, osmotic potential, total chlorophyll content, chlorophyll a, chlorophyll b and biochemical traits including proline contents, superoxide dismutase, sodium content, potassium content, chloride content and sodium/potassium ratio by tagging 202 polymorphic simple sequence repeats (SSR) markers. Linkage map of RILs comprised of 21 linkage group covering A, B and D genome for tagging and maped a total of 60 QTLs with major and minor effect. B genome contributed to the highest number of QTLs under salt stress condition. Xgwm70 and Xbarc361 mapped on chromosome 6B was linked with Total chlorophyll, water potential and sodium content. The increasing allele for all these QTLs were advanced from parent Pasban90. Current study showed that Genome B and D had more potentially active genes conferring plant tolerance against salinity stress which may be exploited for marker assisted selection to breed salinity tolerant high yielding wheat varieties.

GABA operates upstream of H+-ATPase and improves salinity tolerance in Arabidopsis by enabling cytosolic K+ retention and Na+ exclusion

The non-protein amino acid γ-aminobutyric acid (GABA) rapidly accumulates in plant tissues in response to salinity. However, the physiological rationale for this elevation remains elusive. This study compared electrophysiological and whole-plant responses of salt-treated Arabidopsis mutants pop2-5 and gad1,2, which have different abilities to accumulate GABA. The pop2-5 mutant, which was able to overaccumulate GABA in its roots, showed a salt-tolerant phenotype. On the contrary, the gad1,2 mutant, lacking the ability to convert glutamate to GABA, showed oversensitivity to salinity. The greater salinity tolerance of the pop2-5 line was explained by: (i) the role of GABA in stress-induced activation of H+-ATPase, thus leading to better membrane potential maintenance and reduced stress-induced K+ leak from roots; (ii) reduced rates of net Na+ uptake; (iii) higher expression of SOS1 and NHX1 genes in the leaves, which contributed to reducing Na+ concentration in the cytoplasm by excluding Na+ to apoplast and sequestering Na+ in the vacuoles; (iv) a lower rate of H2O2 production and reduced reactive oxygen species-inducible K+ efflux from root epidermis; and (v) better K+ retention in the shoot associated with the lower expression level of GORK channels in plant leaves.

A rice really interesting new gene H2-type E3 ligase, OsSIRH2-14, enhances salinity tolerance via ubiquitin/26S proteasome-mediated degradation of salt-related proteins

Salinity is a deleterious abiotic stress factor that affects growth, productivity, and physiology of crop plants. Strategies for improving salinity tolerance in plants are critical for crop breeding programmes. Here, we characterized the rice (Oryza sativa) really interesting new gene (RING) H2-type E3 ligase, OsSIRH2-14 (previously named OsRFPH2-14), which plays a positive role in salinity tolerance by regulating salt-related proteins including an HKT-type Na⁺ transporter (OsHKT2;1). OsSIRH2-14 expression was induced in root and shoot tissues treated with NaCl. The OsSIRH2-14-EYFP fusion protein was predominately expressed in the cytoplasm, Golgi, and plasma membrane of rice protoplasts. In vitro pull-down assays and bimolecular fluorescence complementation assays revealed that OsSIRH2-14 interacts with salt-related proteins, including OsHKT2;1. OsSIRH2-14 E3 ligase regulates OsHKT2;1 via the 26S proteasome system under high NaCl concentrations but not under normal conditions. Compared with wild type plants, OsSIRH2-14-overexpressing rice plants showed significantly enhanced salinity tolerance and reduced Na⁺ accumulation in the aerial shoot and root tissues. These results suggest that the OsSIRH2-14 RING E3 ligase positively regulates the salinity stress response by modulating the stability of salt-related proteins.

Shoot sodium exclusion in salt stressed barley (Hordeum vulgare L.) is determined by allele specific increased expression of HKT1;5

High affinity potassium transporters (HKT) are recognized as important genes for crop salt tolerance improvement. In this study, we investigated HvHKT1;5 as a candidate gene for a previously discovered quantitative trait locus that controls shoot Na⁺ and Na⁺/K⁺ ratio in salt-stressed barley lines on a hydroponic system. Two major haplotype groups could be distinguished for this gene in a barley collection of 95 genotypes based on the presence of three intronic insertions; a designated haplotype group A (HGA, same as reference sequence) and haplotype group B (HGB, with insertions). HGB was associated with a much stronger root expression of HKT1;5 compared to HGA, and consequently higher K⁺ and lower Na⁺ and Cl^- concentrations and a lower Na⁺/K⁺ ratio in the shoots three weeks after exposure to 200 mM NaCl. Our experimental results suggest that allelic variation in the promoter region of the HGB gene is linked to the three insertions may be responsible for the observed increase in expression of HvHKT1;5 alleles after one week of salt stress induction. This study shows that in barley - similar to wheat and rice - HKT1;5 is an important contributor to natural variation in shoot Na⁺ exclusion.

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.

Exogenous myo-inositol alleviates salinity-induced stress in Malus hupehensis Rehd

Myo-inositol mediates various physiological processes and stress responses. Here, we investigated its role in Malus hupehensis Rehd. plants when grown hydroponically under saline conditions. Salt-stressed plants showed reduced growth and marked declines in photosynthetic activity and chlorophyll concentrations. However, pretreatment with 50 μM myo-inositol significantly alleviated those inhibitions and enabled plants to maintain their photosynthetic capacity. In addition to changing stomatal behavior, exogenous myo-inositol inhibited ROS accumulation and Na⁺ uptake. In contrast, activities of antioxidant systems were enhanced, and expression was elevated for genes involved in Na⁺ uptake (e.g., HKT1, NHX1, SOS1, and SOS2). This exogenous application also provoked the accumulation of sugars or sugar alcohols, which partially contributed to the maintenance of osmotic balance, and the scavenging of ROS, either directly or indirectly. In summary, myo-inositol appears to alleviate the salt-induced inhibition of physiological processes for M. hupehensis, not only by supporting the plant's antioxidant defense system but also by mediating Na⁺ and K⁺ homeostasis and the osmotic balance.

Unraveling salt stress signaling in plants

Salt stress is a major environmental factor limiting plant growth and productivity. A better understanding of the mechanisms mediating salt resistance will help researchers design ways to improve crop performance under adverse environmental conditions. Salt stress can lead to ionic stress, osmotic stress and secondary stresses, particularly oxidative stress, in plants. Therefore, to adapt to salt stress, plants rely on signals and pathways that re-establish cellular ionic, osmotic, and reactive oxygen species (ROS) homeostasis. Over the past two decades, genetic and biochemical analyses have revealed several core stress signaling pathways that participate in salt resistance. The Salt Overly Sensitive signaling pathway plays a key role in maintaining ionic homeostasis, via extruding sodium ions into the apoplast. Mitogen-activated protein kinase cascades mediate ionic, osmotic, and ROS homeostasis. SnRK2 (sucrose nonfermenting 1-related protein kinase 2) proteins are involved in maintaining osmotic homeostasis. In this review, we discuss recent progress in identifying the components and pathways involved in the plant's response to salt stress and their regulatory mechanisms. We also review progress in identifying sensors involved in salt-induced stress signaling in plants.

Elucidating the molecular mechanisms mediating plant salt-stress responses

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

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

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

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

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

Genomics Approaches For Improving Salinity Stress Tolerance in Crop Plants

Salinity is one of the major factors which reduces crop production worldwide. Plant responses to salinity are highly complex and involve a plethora of genes. Due to its multigenicity, it has been difficult to attain a complete understanding of how plants respond to salinity. Genomics has progressed tremendously over the past decade and has played a crucial role towards providing necessary knowledge for crop improvement. Through genomics, we have been able to identify and characterize the genes involved in salinity stress response, map out signaling pathways and ultimately utilize this information for improving the salinity tolerance of existing crops. The use of new tools, such as gene pyramiding, in genetic engineering and marker assisted breeding has tremendously enhanced our ability to generate stress tolerant crops. Genome editing technologies such as Zinc finger nucleases, TALENs and CRISPR/Cas9 also provide newer and faster avenues for plant biologists to generate precisely engineered crops.

Transcript dynamics at early stages of molecular interactions of MYMIV with resistant and susceptible genotypes of the leguminous host, Vigna mungo

Initial phases of the MYMIV- Vigna mungo interaction is crucial in determining the infection phenotype upon challenging with the virus. During incompatible interaction, the plant deploys multiple stratagems that include extensive transcriptional alterations defying the virulence factors of the pathogen. Such molecular events are not frequently addressed by genomic tools. In order to obtain a critical insight to unravel how V. mungo respond to Mungbean yellow mosaic India virus (MYMIV), we have employed the PCR based suppression subtractive hybridization technique to identify genes that exhibit altered expressions. Dynamics of 345 candidate genes are illustrated that differentially expressed either in compatible or incompatible reactions and their possible biological and cellular functions are predicted. The MYMIV-induced physiological aspects of the resistant host include reactive oxygen species generation, induction of Ca²⁺ mediated signaling, enhanced expression of transcripts involved in phenylpropanoid and ubiquitin-proteasomal pathways; all these together confer resistance against the invader. Elicitation of genes implicated in salicylic acid (SA) pathway suggests that immune response is under the regulation of SA signaling. A significant fraction of modulated transcripts are of unknown function indicating participation of novel candidate genes in restricting this viral pathogen. Susceptibility on the other hand, as exhibited by V. mungo Cv. T9 is perhaps due to the poor execution of these transcript modulation exhibiting remarkable repression of photosynthesis related genes resulting in chlorosis of leaves followed by penalty in crop yield. Thus, the present findings revealed an insight on the molecular warfare during host-virus interaction suggesting plausible signaling mechanisms and key biochemical pathways overriding MYMIV invasion in resistant genotype of V. mungo. In addition to inflate the existing knowledge base, the genomic resources identified in this orphan crop would be useful for integrating MYMIV-tolerance trait in susceptible cultivars of V. mungo.