Progress in Studying Salt Secretion from the Salt Glands in Recretohalophytes: How Do Plants Secrete Salt?

Comparative Transcriptome Analysis Reveals New lncRNAs Responding to Salt Stress in Sweet Sorghum

Long non-coding RNAs (lncRNAs) can enhance plant stress resistance by regulating the expression of functional genes. Sweet sorghum is a salt-tolerant energy crop. However, little is known about how lncRNAs in sweet sorghum respond to salt stress. In this study, we identified 126 and 133 differentially expressed lncRNAs in the salt-tolerant M-81E and the salt-sensitive Roma strains, respectively. Salt stress induced three new lncRNAs in M-81E and inhibited two new lncRNAs in Roma. These lncRNAs included lncRNA13472, lncRNA11310, lncRNA2846, lncRNA26929, and lncRNA14798, which potentially function as competitive endogenous RNAs (ceRNAs) that influence plant responses to salt stress by regulating the expression of target genes related to ion transport, protein modification, transcriptional regulation, and material synthesis and transport. Additionally, M-81E had a more complex ceRNA network than Roma. This study provides new information regarding lncRNAs and the complex regulatory network underlying salt-stress responses in sweet sorghum.

Comparative Transcriptome Analysis of Halophyte Zoysia macrostachya in Response to Salinity Stress

As one of the most severe environmental stresses, salt stress can cause a series of changes in plants. In salt tolerant plant Zoysia macrostachya, germination, physiology, and genetic variation under salinity have been studied previously, and the morphology and distribution of salt glands have been clarified. However, no study has investigated the transcriptome of such species under salt stress. In the present study, we compared transcriptome of Z. macrostachya under normal conditions and salt stress (300 mmol/L NaCl, 24 h) aimed to identify transcriptome responses and molecular mechanisms under salt stress in Z. macrostachya. A total of 8703 differently expressed genes (DEGs) were identified, including 4903 up-regulated and 3800 down-regulated ones. Moreover, a series of molecular processes were identified by Gene Ontology (GO) analysis, and these processes were suggested to be closely related to salt tolerance in Z. macrostachya. The identified DEGs concentrated on regulating plant growth via plant hormone signal transduction, maintaining ion homeostasis via salt secretion and osmoregulatory substance accumulation and preventing oxidative damage via increasing the activity of ROS (reactive oxygen species) scavenging system. These changes may be the most important responses of Z. macrostachya under salt stress. Some key genes related to salt stress were identified meanwhile. Collectively, our findings provided valuable insights into the molecular mechanisms and genetic underpinnings of salt tolerance in Z. macrostachya.

Transcriptomic analysis identifies novel genes and pathways for salt stress responses in Suaeda salsa leaves

Salinity is a critical abiotic stress, which significantly impacts the agricultural yield worldwide. Identification of the molecular mechanisms underlying the salt tolerance in euhalophyte Suaeda salsa is conducive to the development of salt-resistant crops. In the present study, high-throughput RNA sequencing was performed after S. salsa leaves were exposed to 300 mM NaCl for 7 days, and 7,753 unigenes were identified as differently expressed genes (DEGs) in S. salsa, including 3,638 increased and 4,115 decreased unigenes. Moreover, hundreds of pathways were predicted to participate in salt stress response in S. salsa by Gene Ontology (GO), MapMan and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses, including ion transport and sequestration as well as photoprotection of photosystem (PS) II. The GO enrichment analysis indicated that genes related to ion transport, reactive oxygen species (ROS) scavenging and transcriptional factors were highly expressed upon NaCl treatment. The excessive Na+ and Cl- ions were supposed to be absorbed into the vacuole for ion sequestration and balance adjustment by potassium transporters (such as KEA3) with high expressions. Moreover, we predicted that mutiple candidate genes associated with photosynthesis (such as PSB33 and ABA4), ROS (such as TAU9 and PHI8) and transcriptional regulation (HB-7 and MYB78) pathways could mitigate salt stress-caused damage in S. salsa.

Advances in salt tolerance molecular mechanism in tobacco plants

Tobacco, an economic crop and important model plant, has received more progress in salt tolerance with the aid of transgenic technique. Salt stress has become a key research field in abiotic stress. The study of tobacco promotes the understanding about the important adjustment for survival in high salinity environments, including cellular ion transport, osmotic regulation, antioxidation, signal transduction and expression regulation, and protection of cells from stress damage. Genes, which response to salt, have been studied using targeted transgenic technologies in tobacco plants to investigate the molecular mechanisms. The transgenic tobacco plants exhibited higher seed germination and survival rates, better root and shoot growth under salt stress treatments. Transgenic approach could be the promising option for enhancing tobacco production under saline condition. This review highlighted the salt tolerance molecular mechanisms of tobacco.

Cell Cycle Regulation in the Plant Response to Stress

As sessile organisms, plants face a variety of environmental challenges. Their reproduction and survival depend on their ability to adapt to these stressors, which include water, heat stress, high salinity, and pathogen infection. Failure to adapt to these stressors results in programmed cell death and decreased viability, as well as reduced productivity in the case of crop plants. The growth and development of plants are maintained by meiosis and mitosis as well as endoreduplication, during which DNA replicates without cytokinesis, leading to polyploidy. As in other eukaryotes, the cell cycle in plants consists of four stages (G1, S, G2, and M) with two major check points, namely, the G1/S check point and G2/M check point, that ensure normal cell division. Progression through these checkpoints involves the activity of cyclin-dependent kinases and their regulatory subunits known as cyclins. In order for plants to survive, cell cycle control must be balanced with adaption to dynamic environmental conditions. In this review, we summarize recent advances in our understanding of cell cycle regulation in plants, with a focus on the molecular interactions of cell cycle machinery in the context of stress tolerance.

Photosynthetic Regulation Under Salt Stress and Salt-Tolerance Mechanism of Sweet Sorghum

Sweet sorghum is a C4 crop with the characteristic of fast-growth and high-yields. It is a good source for food, feed, fiber, and fuel. On saline land, sweet sorghum can not only survive, but increase its sugar content. Therefore, it is regarded as a potential source for identifying salt-related genes. Here, we review the physiological and biochemical responses of sweet sorghum to salt stress, such as photosynthesis, sucrose synthesis, hormonal regulation, and ion homeostasis, as well as their potential salt-resistance mechanisms. The major advantages of salt-tolerant sweet sorghum include: 1) improving the Na+ exclusion ability to maintain ion homeostasis in roots under salt-stress conditions, which ensures a relatively low Na+ concentration in shoots; 2) maintaining a high sugar content in shoots under salt-stress conditions, by protecting the structures of photosystems, enhancing photosynthetic performance and sucrose synthetase activity, as well as inhibiting sucrose degradation. To study the regulatory mechanism of such genes will provide opportunities for increasing the salt tolerance of sweet sorghum by breeding and genetic engineering.

Genetic mechanisms of salt stress responses in halophytes

Abiotic stress is a major threat to plant growth and development, resulting in extensive crop loss worldwide. Plants react to abiotic stresses through physiological, biochemical, molecular, and genetic adaptations that promote survival. Exploring the molecular mechanisms involved in abiotic stress responses across various plant species is essential for improving crop yields in unfavorable environments. Halophytes are characterized as plants that survive to reproduce in soils containing high salt concentrations, and thus act as an ideal model to comprehend complicated genetic and physiological mechanisms of salinity stress tolerance. Plant ecologists classify halophytes into three main groups: euhalophytes, recretohalophytes, and pseudo-halophytes. Recent genetic and molecular research has showed complicated regulatory networks by which halophytes coordinate stress adaptation and tolerance. Furthermore, investigation of natural variations in these stress responses has supplied new perspectives on the evolution of mechanisms that regulate tolerance and adaptation. This review discusses the current understanding of the genetic mechanisms that contribute to salt-stress tolerance among different classes of halophytes.

Identification of Superior Alleles for Seedling Stage Salt Tolerance in the USDA Rice Mini-Core Collection

Salt stress is a major constraint to rice acreage and production worldwide. The purpose of this study was to evaluate the natural genetic variation available in the United States Department of Agriculture (USDA) rice mini-core collection (URMC) for early vigor traits under salt stress and identify quantitative trait loci (QTLs) for seedling-stage salt tolerance via a genome-wide association study (GWAS). Using a hydroponic system, the seedlings of 162 accessions were subjected to electrical conductivity (EC) 6.0 dS m⁻¹ salt stress at the three-to-four leaf stage. After completion of the study, 59.4% of the accessions were identified as sensitive, 23.9% were identified as moderately tolerant, and 16.7% were identified as highly tolerant. Pokkali was the most tolerant variety, while Nerica-6 was the most sensitive. Adapting standard International Rice Research Institute (IRRI) protocols, eight variables associated with salt tolerance were determined. The GWAS of the URMC, using over three million single-nucleotide polymorphisms (SNPs), identified nine genomic regions associated with salt tolerance that were mapped to five different chromosomes. Of these, none were in the known Saltol QTL region, suggesting different probable genes and mechanisms responsible for salt tolerance in the URMC. The study uncovered genetic loci that explained a large portion of the variation in salt tolerance at the seedling stage. Fourteen highly salt-tolerant accessions, six novel loci, and 16 candidate genes in their vicinity were identified that may be useful in breeding for salt stress tolerance. Identified QTLs can be targeted for fine mapping, candidate gene verification, and marker-assisted breeding in future studies.

The relationship between cuticular lipids and associated gene expression in above ground organs of Thellungiella salsugineum (Pall.) Al-Shehbaz & Warwick

The cuticle plays a critical role as barrier between plant and environment. Here, cuticular wax morphology, cuticular wax and cutin monomer composition, and expression of associated genes in five above ground organs were examined in model extremophyte Thellungiella salsugineum. Alkanes, ketones, and 2-alcohols were the predominant wax constitutes in rosette leaves, inflorescence stem leaves, stems, and siliques, whereas alkanes and acids were the predominant cuticular lipids in whole flowers. Unsubstituted acids were the most abundant cutin monomers in vegetative organs, especially C18:2 dioic acids, which reached the highest levels in stems. Hydroxy fatty acids were the predominant cutin monomers in flowers, especially 16-OH C16:0 and diOH C16:0. High-throughput RNA-Seq analysis using the Hiseq4000 platform was performed on these five above organs of T. salsugineum, and the differentially expressed lipid-associated genes and their associated metabolic pathways were identified. Expression of genes associated in previous reports to cuticle production, including those having roles in cuticle lipid biosynthesis, transport, and regulation were examined. The association of cuticle lipid composition and gene expression within different organs of T. salsugineum, and potential relationships between T. salsugineum's extreme cuticle and its adaptation to extreme environments is discussed.

Haloconduction as a remediation strategy: Capture and quantification of salts excreted by recretohalophytes

Recretohalophytes employ specialized glands to excrete salt ions onto their tissue surfaces, which then have the potential to be transported away from the plant via wind in a process referred to as 'haloconduction'. Spartina pectinata and Distichlis spicata were selected to investigate the potential to remediate a cement kiln dust landfill in Bath, ON via salt excretion and haloconduction. Under ideal conditions in the laboratory, measurements of salt excreted by large (>15 shoots and > 50 cm height) plants of each species were 280 ± 164 g/m² and 164 ± 75 g/m², respectively, resulting in potential remediation timeframes of 1.4 ± 0.9 and 2.4 ± 1.1 years. Three salt collection methods were developed and installed in the field to test their efficacy for capturing and measuring windborne salt mobilized from plant surfaces. All three methods (two ground-level and one at 260 cm height) were successful in capturing and quantifying airborne salts up to 15 m from the plots. This study is the first to collect and quantify dispersed salt from recretohalophytes and hence confirm the theory of haloconduction, a promising new remediation technology for salt-impacted soils.

Mangroves in the Leaves: Anatomy, Physiology, and Immunity of Epithemal Hydathodes

Hydathodes are organs found on aerial parts of a wide range of plant species that provide almost direct access for several pathogenic microbes to the plant vascular system. Hydathodes are better known as the site of guttation, which is the release of droplets of plant apoplastic fluid to the outer leaf surface. Because these organs are only described through sporadic allusions in the literature, this review aims to provide a comprehensive view of hydathode development, physiology, and immunity by compiling a historic and contemporary bibliography. In particular, we refine the definition of hydathodes.We illustrate their important roles in the maintenance of plant osmotic balance, nutrient retrieval, and exclusion of deleterious chemicals from the xylem sap. Finally, we present our current understanding of the infection of hydathodes by adapted vascular pathogens and the associated plant immune responses.

Microhair on the adaxial leaf surface of salt secreting halophytic Oryza coarctata Roxb. show distinct morphotypes: Isolation for molecular and functional analysis

Halophytic Oryza coarctata is a good model system to examine mechanisms of salinity tolerance in rice. O. coarctata leaves show the presence of microhairs in adaxial leaf surface furrows that secrete salt under salinity. However, detailed molecular and physiological studies of O. coarctata microhairs are limited due to their relative inaccessibility. This work presents a detailed characterization of O. coarctata leaf features. O. coarctata has two types of microhairs on the adaxial leaf surface: longer microhairs (three morphotypes) lining epidermal furrow walls and shorter microhairs (reported first time) arising from bulliform cells. Microhair morphotypes include (i) finger-like, tubular structures, (ii) tubular hairs with bilobed and flattened heads and (iii) bi-or trifurcated hairs. The unicellular nature of microhairs was confirmed by propidium iodide (PI) staining. An efficient method for the isolation and enrichment of O. coarctata microhairs is presented (yield averaging ˜2 × 10⁵/g leaf tissue). The robustness of the microhair isolation procedure was confirmed by subsequent viability staining (PI), total RNA isolation and RT-PCR amplification of O. coarctata trichome-specific WUSCHEL-related homeobox 3B (OcWox3B) and transporter gene-specific cDNA sequences. The present microhair isolation work from O. coarctata paves the way for examining genes involved in ion secretion in this halophytic wild rice model.

Roles of malic enzymes in plant development and stress responses

Malic enzyme (ME) comprises a family of proteins with multiple isoforms located in different compartments of eukaryotic cells. It is a key enzyme regulating malic acid metabolism and can catalyze the reversible reaction of oxidative decarboxylation of malic acid. And it is also one of the important enzymes in plant metabolism and is involved in multiple metabolic processes. ME is widely present in plants and mainly discovered in cytoplasmic stroma, mitochondria, chloroplasts. It is involved in plant growth, development, and stress response. Plants are stressed by various environmental factors such as drought, high salt, and high temperature during plant growth, and the mechanisms of plant response to various environmental stresses are synergistic. Numerous studies have shown that ME participates in the process of coping with the above environmental factors by increasing water use efficiency, improving photosynthesis of plants, providing reducing power, and so on. In this review, we discuss the important role of ME in plant development and plant stress response, and prospects for its application. It provides a theoretical basis for the future use of ME gene for molecular resistance breeding.

Characterization of the complete plastome of Atriplex centralasiatica (Chenopodiaceae), an annual halophytic herb

Atriplex centralasiatica, an annual halophytic herb, is one of the most important Chinese herbal medicines, forages and indicator plants for saline-alkali soil. In this study, we report the complete plastome of A. centralasiatica. The plastome was 152,237 bp in length and comprises a large single-copy region (83,721 bp), a small single-copy region (18,096 bp), and a pair of inverted repeats (25,210 bp). It encodes 113 unique genes, including 79 protein-coding genes (PCGs), 30 tRNAs and 4 rRNAs. The overall GC content of this plastome was 37.3%. Phylogenomic analysis based on 21 plastomes revealed that A. centralasiatica was closely related to the genus Chenopodium.

Na+ compartmentation strategy of Chinese cabbage in response to salt stress

Na⁺/H⁺ antiporter (NHX), responsible for counter-transport of Na⁺ and H⁺ across membranes (Na⁺ compartmentalization), plays a central role in plant salt-tolerance. In order to explore the Na⁺ compartmentalization modes and salt tolerance strategy in Chinese cabbage (Brassica rapa L. ssp. pekinensis), the seedlings of a salt-susceptible cabbage cultivar (Kuaicai 38) and a salt-tolerant cabbage cultivar (Qingmaye) were exposed to 100-400 mM NaCl for 30 days. Both of these cultivars showed a gradual decrease in fresh weight and water content and an increase in root-shoot ratio with the increasing NaCl-treatment concentration. The distribution of Na⁺ in these two cultivars was similar, with the green leaves showing the highest Na⁺ content, followed by inflated midribs, stems, and roots. The Na⁺ concentration in the apoplast was higher than that in the protoplast of the leaves. The expression levels of BrNHX1-1 and BrNHX1-2 in the leaves of Qingmaye were the highest among all BrNHX members, and increased after salt treatment. However, only BrNHX1-1 was expressed in Kuaicai 38. These results indicate that Na⁺ compartmentation into vacuoles is the major salt-adaptation strategy in Chinese cabbage. Coordinated overexpression of BrNHX1-1 and BrNHX1-2 may confer greater salt-tolerance for Chinese cabbage.

Biological features and regulatory mechanisms of salt tolerance in plants

Halophytes play a vital role in saline agriculture because these plants are necessary to increase the food supply to meet the demands of the growing world population. In addition, the transfer of salt-resistance genes from halophytes using genetic technologies has the potential to increase the salt tolerance of xerophytes. Characterization of some particularly promising halophyte model organisms has revealed the important new insights into the salt tolerance mechanisms used by plants. Numerous advances using these model systems have improved our understanding of salt tolerance regulation and salt tolerance-associated changes in gene expression, and these mechanisms have important implications for saline agriculture. Recent findings provide a basis for future studies of salt tolerance in plants, as well as the development of improved strategies for saline agriculture to increase yields of food, feed, and fuel crops.

Research advances of MYB transcription factors in plant stress resistance and breeding

Plants face various stresses during the growth and development processes. The specific transcription factors bind to the cis-acting elements upstream of the stress resistance genes, specifically regulating the expression of the gene in plants and increasing the adaptability of plants to environmental stress. The transcription factor-mediated gene expression regulatory networks play an important role in plant stress response pathways. MYB (v-myb avian myeloblastosis viral oncogene homolog) transcription factor is one of the largest members of the transcription factor family in plants. It participates and has a great influence on all aspects of plant growth and development. It plays an important role in plant secondary metabolic regulation, hormone and environmental factor responses, cell differentiation, organ morphogenesis, and cell cycle regulation. This review mainly introduces the characteristics, structure, and classification of MYB transcription factors, as well as the abiotic stress resistance to drought, salt, temperature, and other functions in breeding, and provides a reference for the research and utilization of transcription factors in the future.

An Effective Screening Method and a Reliable Screening Trait for Salt Tolerance of Brassica napus at the Germination Stage

Salinity is a major and complex abiotic stress that inhibits plant growth and reduces crop yield. Given the global increase in soil salinity, there is a need to develop salt-tolerant species. Brassica napus L. is an important oilseed crop with some level of salt tolerance. However, few studies have evaluated its salt tolerance thoroughly or screened for traits that can be reliably evaluated for salt tolerance. Here, we evaluated salt tolerance in 549 B. napus inbred lines with different genetic backgrounds using the membership function value (MFV) of certain traits, including the germination rate, root and shoot length, root and shoot fresh weight, and total fresh weight. According to the evaluation criteria-mean MFV, 50 highly salt-tolerant, 115 salt-tolerant, 71 moderately salt-tolerant, 202 salt-sensitive, and 111 highly salt-sensitive inbred lines were screened at the germination stage. We also developed a mathematical evaluation model and identified that the salt tolerance index of shoot fresh weight is a single trait that reliably represents the salt tolerance of B. napus germplasm at the germination stage. These results are useful for evaluating and breeding salt-tolerant B. napus germplasm.

Electrochemical Biosensors for Detection of Foodborne Pathogens

Foodborne safety has become a global public health problem in both developed and developing countries. The rapid and precise monitoring and detection of foodborne pathogens has generated a strong interest by researchers in order to control and prevent human foodborne infections. Traditional methods for the detection of foodborne pathogens are often time-consuming, laborious, expensive, and unable to satisfy the demands of rapid food testing. Owing to the advantages of simplicity, real-time analysis, high sensitivity, miniaturization, rapid detection time, and low cost, electrochemical biosensing technology is more and more widely used in determination of foodborne pathogens. Here, we summarize recent developments in electrochemical biosensing technologies used to detect common foodborne pathogens. Additionally, we discuss research challenges and future prospects for this field of study.

Potential eolian dust contribution to accumulation of selected heavy metals and rare earth elements in the aboveground biomass of Tamarix spp. from saline soils in Kazakhstan

In arid and semi-arid zones, atmospheric dust of different origins influences soil chemistry and plant biomass composition. Thus, studies on plant accumulation of heavy metals and rare earth elements (RREs) should include some assessments of potential eolian deposition. Here, we proposed the use of fractionation of metals in soils as an indirect method to assess potential atmospheric dust input to metal content in plant biomass. Our research was performed on individuals of Tamarix spp. growing on saline automorphic and hydromorphic soils in Kazakhstan. Studied soils could be, in general, classified as polluted, especially in industrial areas of Karaganda and Chromtau. However, concentrations of heavy metals and RREs in biomass remained low, as most of the studied elements were present in plant-inaccessible forms. Nevertheless, we recorded a high accumulation of Cd in biomass (70% of this element present in soils as plant-inaccessible fractions), which indicates the impact of Cd atmospheric deposition.

Reproductive Physiology of Halophytes: Current Standing

Background: Halophytes possess efficient salt-tolerance mechanisms and can complete their life cycles in naturally saline soils with NaCl contents exceeding 200 mM. While a significant progress have been made in recent decades elucidating underlying salt-tolerance mechanisms, these studies have been mostly confined to the vegetative growth stage. At the same time, the capacity to generate high-quality seeds and to survive early developmental stages under saline conditions, are both critically important for plants. Halophytes perform well in both regards, whereas non-halophytes cannot normally complete their life cycles under saline conditions. Scope: Research into the effects of salinity on plant reproductive biology has gained momentum in recent years. However, it remains unclear whether the reproductive biology of halophytes differs from that of non-halophytes, and whether their reproductive processes benefit, like their vegetative growth, from the presence of salt in the rhizosphere. Here, we summarize current knowledge of the mechanisms underlying the superior reproductive biology of halophytes, focusing on critical aspects including control of flowering time, changes in plant hormonal status and their impact on anther and pollen development and viability, plant carbohydrate status and seed formation, mechanisms behind the early germination of halophyte seeds, and the role of seed polymorphism. Conclusion: Salt has beneficial effects on halophyte reproductive growth that include late flowering, increased flower numbers and pollen vitality, and high seed yield. This improved performance is due to optimal nutrition during vegetative growth, alterations in plant hormonal status, and regulation of flowering genes. In addition, the seeds of halophytes harvested under saline conditions show higher salt tolerance than those obtained under non-saline condition, largely due to increased osmolyte accumulation, more optimal hormonal composition (e.g., high gibberellic acid and low abcisic acid content) and, in some species, seed dimorphism. In the near future, identifying key genes involved in halophyte reproductive physiology and using them to transform crops could be a promising approach to developing saline agriculture.

Beneficial Effects of Salt on Halophyte Growth: Morphology, Cells, and Genes

Halophytes can survive and complete their life cycle in the presence of ≥200 mM NaCl. These remarkable plants have developed various strategies to tolerate salinity and thrive in high-salt environments. At the appropriate levels, salt has a beneficial effect on the vegetative growth of halophytes but inhibits the growth of non-halophytes. In recent years, many studies have focused on elucidating the salt-tolerance mechanisms of halophytes at the molecular, physiological, and individual level. In this review, we focus on the mechanisms, from the macroscopic to the molecular, underlying the successful growth of halophytes in saline environments to explain why salt has beneficial effects on halophytes but harmful effects on non-halophytes. These mechanisms include the specialized organs of halophytes (for example, ion compartmentalization in succulent leaves), their unique structures (salt glands and hydrophobic barriers in roots), and their salt-tolerance genes. We hope to shed light on the use of halophytes for engineering salt-tolerant crops, soil conservation, and the protection of freshwater resources in the near future.

Insight into salt tolerance mechanisms of the halophyte Achras sapota: an important fruit tree for agriculture in coastal areas

Sapota (Achras sapota), a fruit tree with nutritional and medicinal properties, is known to thrive in salt-affected areas. However, the underlying mechanisms that allow sapota to adapt to saline environment are yet to be explored. Here, we examined various morphological, physiological, and biochemical features of sapota under a gradient of seawater (0, 4, 8, and 12 dS m^-1) to study its adaptive responses against salinity. Our results showed that seawater-induced salinity negatively impacted on growth-related attributes, such as plant height, root length, leaf area, and dry biomass in a dose-dependent manner. This growth reduction was positively correlated with reductions in relative water content, stomatal conductance, xylem exudation rate, and chlorophyll, carbohydrate, and protein contents. However, the salt tolerance index did not decline in proportional to the increasing doses of seawater, indicating a salt tolerance capacity of sapota. Under salt stress, ion analysis revealed that Na⁺ mainly retained in roots, whereas K⁺ and Ca²⁺ were more highly accumulated in leaves than in roots, suggesting a potential mechanism in restricting transport of excessive Na⁺ to leaves to facilitate the uptake of other essential minerals. Sapota plants also maintained an improved leaf succulence with increasing levels of seawater. Furthermore, increased accumulations of proline, total amino acids, soluble sugars, and reducing sugars suggested an enhanced osmoprotective capacity of sapota to overcome salinity-induced osmotic stress. Our results demonstrate that the salt adaptation strategy of sapota is attributed to increased leaf succulence, selective transport of minerals, efficient Na⁺ retention in roots, and accumulation of compatible solutes.

Could vesicular transport of Na+ and Cl- be a feature of salt tolerance in halophytes?

Background

Halophytes tolerate external salt concentrations of 200 mm and more, accumulating salt concentrations of 500 mm and more in their shoots; some, recretohalophytes, excrete salt through glands on their leaves. Ions are accumulated in central vacuoles, but the pathway taken by these ions from the outside of the roots to the vacuoles inside the cells is poorly understood. Do the ions cross membranes through ion channels and transporters or move in vesicles, or both? Vesicular transport from the plasma membrane to the vacuole would explain how halophytes avoid the toxicity of high salt concentrations on metabolism. There is also a role for vesicles in the export of ions via salt glands.

Scope and Methods

We have collected data on the fluxes of sodium and chloride ions in halophytes, based on the weight of the transporting organs and on the membrane area across which the flux occurs; the latter range from 17 nmol m-2 s-1 to 4.2 μmol m-2 s-1 and values up to 1 μmol m-2 s-1 need to be consistent with whatever transport system is in operation. We have summarized the sizes and rates of turnover of vesicles in plants, where clathrin-independent vesicles are 100 nm or more in diameter and can merge with the plasma membrane at rates of 100 s-1. We gathered evidence for vesicular transport of ions in halophytes and evaluated whether vesicular transport could account for the observable fluxes.

Conclusions

There is strong evidence in favour of vesicular transport in plants and circumstantial evidence in favour of the movement of ions in vesicles. Estimated rates of vesicle turnover could account for ion transport at the lower reported fluxes (around 20 nmol m-2 s-1), but the higher fluxes may require vesicles of the order of 1 μm or more in diameter. The very high fluxes reported in some salt glands might be an artefact of the way they were measured.

Abiotic Stresses: General Defenses of Land Plants and Chances for Engineering Multistress Tolerance

Abiotic stresses, such as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalty worldwide. It is getting imperative to equip crops with multistress tolerance to relieve the pressure of environmental changes and to meet the demand of population growth, as different abiotic stresses usually arise together in the field. The feasibility is raised as land plants actually have established more generalized defenses against abiotic stresses, including the cuticle outside plants, together with unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes inside cells. In stress response, they are orchestrated by a complex regulatory network involving upstream signaling molecules including stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors, particularly transcription factors. In this review, we aimed at presenting an overview of these defensive systems and the regulatory network, with an eye to their practical potential via genetic engineering and/or exogenous application.

Therapeutic targeting of cellular stress responses in cancer

Similar to bacteria, yeast, and other organisms that have evolved pathways to respond to environmental stresses, cancer cells develop mechanisms that increase genetic diversity to facilitate adaptation to a variety of stressful conditions, including hypoxia, nutrient deprivation, exposure to DNA-damaging agents, and immune responses. To survive, cancer cells trigger mechanisms that drive genomic instability and mutation, alter gene expression programs, and reprogram the metabolic pathways to evade growth inhibition signaling and immune surveillance. A deeper understanding of the molecular mechanisms that underlie the pathways used by cancer cells to overcome stresses will allow us to develop more efficacious strategies for cancer therapy. Herein, we overview several key stresses imposed on cancer cells, including oxidative, metabolic, mechanical, and genotoxic, and discuss the mechanisms that drive cancer cell responses. The therapeutic implications of these responses are also considered, as these factors pave the way for the targeting of stress adaption pathways in order to slow cancer progression and block resistance to therapy.

Regulation mechanism of microRNA in plant response to abiotic stress and breeding

microRNAs (miRNAs) in plants are a class of small RNAs consisting of approximately 21-24 nucleotides. The mature miRNA binds to the target mRNA through the formation of a miRNA-induced silencing complex (MIRISC), and cleaves or inhibits translation, thereby achieving negative regulation of the target gene. Based on miRNA plays an important role in regulating plant gene expression, studies on the prediction, identification, function and evolution of plant miRNAs have been carried out. In addition, many researches prove that miRNAs are also involved in many kinds of abiotic and biotic stress, under abiotic stress, plants can express some miRNA, and act on stress-related target genes, which can make plants adapt to stress in physiological response. In this review, the synthetic pathway and mechanism of plant miRNA are briefly described, and we discuss the biological functions and regulatory mechanisms of miRNAs responding to abiotic stresses including low temperature, salt, drought stress and breeding to lay the foundation for further exploring the mechanism of action of miRNAs in stress resistance of plant. And analyze its utilization prospects in plant stress resistance research.

Adaptation Mechanism of Salt Excluders under Saline Conditions and Its Applications

Global soil salinization is increasingly a serious threat to agriculture worldwide. Therefore, it is imperative to improve crop salt tolerance as a means of adaptation to saline habitats. Some halophytes and most monocotyledonous crops are salt-excluders. Understanding the regulatory mechanisms of salt exclusion at the molecular level in salt-exclusion plants is critical for improving the salt tolerance of monocotyledonous crops such as maize, wheat, rice, and sorghum. In this review, we summarize recent research into salt-exclusion mechanisms and the genes that underlie them. Findings related to salt exclusion may accelerate the process of breeding tolerant cultivars by using genomic and molecular tools.

Revealing mechanisms of salinity tissue tolerance in succulent halophytes: A case study for Carpobrotus rossi

Efforts to breed salt tolerant crops could benefit from investigating previously unexplored traits. One of them is a tissue succulency. In this work, we have undertaken an electrophysiological and biochemical comparison of properties of mesophyll and storage parenchyma leaf tissues of a succulent halophyte species Carpobrotus rosii ("pigface"). We show that storage parenchyma cells of C. rossii act as Na⁺ sink and possessed both higher Na⁺ sequestration (298 vs. 215 mM NaCl in mesophyll) and better K⁺ retention ability. The latter traits was determined by the higher rate of H⁺ -ATPase operation and higher nonenzymatic antioxidant activity in this tissue. Na⁺ uptake in both tissues was insensitive to either Gd³⁺ or elevated Ca²⁺ ruling out involvement of nonselective cation channels as a major path for Na⁺ entry. Patch-clamp experiments have revealed that Caprobrotus plants were capable to downregulate activity of fast vacuolar channels when exposed to saline environment; this ability was higher in the storage parenchyma cells compared with mesophyll. Also, storage parenchyma cells have constitutively lower number of open slow vacuolar channels, whereas in mesophyll, this suppression was inducible by salt. Taken together, these results provide a mechanistic basis for efficient Na⁺ sequestration in the succulent leaf tissues.

Water transport properties of root cells contribute to salt tolerance in halophytic grasses Poa juncifolia and Puccinellia nuttalliana

Plant water uptake and aquaporin-mediated root water transport are among the most salt-sensitive processes in most plants, but even relatively high salt concentrations do not appear to impair water transport processes in halophytes. To develop better understanding of these processes in halophytic plants, we compared the responses to NaCl of the two halophytic grasses varying in salt tolerance, Puccinellia nuttalliana and Poa juncifolia, with the glycophytic grass Poa pratensis. The plants were hydroponically grown and subjected to different NaCl concentrations for up to 10 days. At the lower NaCl concentrations, shoot and root dry weights were drastically reduced in Poa pratensis, but increased in Puccinellia nuttalliana and Poa juncifolia. The examined treatment concentrations of up to 300 mM NaCl had either no effect (Puccinellia nuttalliana) or little effect (Poa juncifolia) on the net photosynthesis and transpiration rates in plants, but severely decreased the gas exchange parameters in Poa pratensis. Similarly, to growth and gas exchange, leaf water content in Puccinellia nuttalliana was not affected even by the highest, 300 mM NaCl concentration, while Poa pratensis showed decreased shoot water content in all examined NaCl treatments and Poa juncifolia in 150 and 300 mM NaCl. Cell hydraulic conductivity in roots of Poa pratensis also showed high sensitivity to NaCl and was drastically reduced in all examined NaCl concentrations. Cell hydraulic conductivity in Poa juncifolia roots was less affected by NaCl compared with Poa pratensis and in Puccinellia nuttalliana, cell hydraulic conductivity increased in response to NaCl treatments. Both Puccinellia nuttalliana and Poa juncifolia accumulated less Na in their shoot tissues compared with Poa pratensis. The concentrations of K in the roots of Poa pratensis sharply decreased with increasing NaCl treatment concentrations while in Puccinellia nuttalliana, K root concentrations remained high in all NaCl treatments and in Poa juncifoila, root K decreased only in the 300 mM NaCl treatment. Since K efflux from the cytoplasm can contribute to the acidification of the cytoplasm, this process could potentially lead to the inhibition of aquaporin function and reduction of root hydraulic conductivity. The, significance of stable K root concentrations in the roots of halophytes should be further investigated as a possible salt tolerance mechanism that could contribute to the maintenance of aquaporin function and root water transport under salt stress conditions.

Survival Mechanisms to Selective Pressures and Implications

Organisms have evolved a spectrum of strategies that facilitate survival in the face of adverse environmental conditions. In order to make full use of the unfavorable resources of nature, human beings usually impose selective pressures to breed phenotypic traits that can survive in adverse environments. Animals are frequently under attack by biotic stress, such as bacterial and viral infections, while plants are more often subjected to abiotic stress, including high salinity, drought, and cold. In response to these diverse stresses, animals and plants initiate wide-ranging changes in gene expression by altering regulation of transcriptional and post-transcriptional activities. Recent studies have identified a number of key responsive components that promote survival of animals and plants in response to biotic and abiotic stresses. Importantly, with recent developments in genome-editing technology based on the CRISPR/Cas9 system, manipulation of genetic elements to generate stress-resistant animals and plants has become both feasible and cost-effective. Herein, we review important mechanisms that govern the response of organisms to biotic and abiotic stresses with the aim of applying our understanding to the agriculture and animal husbandry industries.

Identification of Salt Stress Responding Genes Using Transcriptome Analysis in Green Alga Chlamydomonas reinhardtii

Salinity is one of the most important abiotic stresses threatening plant growth and agricultural productivity worldwide. In green alga Chlamydomonas reinhardtii, physiological evidence indicates that saline stress increases intracellular peroxide levels and inhibits photosynthetic-electron flow. However, understanding the genetic underpinnings of salt-responding traits in plantae remains a daunting challenge. In this study, the transcriptome analysis of short-term acclimation to salt stress (200 mM NaCl for 24 h) was performed in C. reinhardtii. A total of 10,635 unigenes were identified as being differently expressed by RNA-seq, including 5920 up- and 4715 down-regulated unigenes. A series of molecular cues were screened for salt stress response, including maintaining the lipid homeostasis by regulating phosphatidic acid, acetate being used as an alternative source of energy for solving impairment of photosynthesis, and enhancement of glycolysis metabolism to decrease the carbohydrate accumulation in cells. Our results may help understand the molecular and genetic underpinnings of salt stress responses in green alga C. reinhardtii.

Seed Heteromorphism: An Important Adaptation of Halophytes for Habitat Heterogeneity

Seed germination is a very critical and important step for seedling establishment under saline environments, as high level of salinity in the soil can prevent seed germination. However halophytes exhibit an interesting mechanism to cope with salt stress. Many halophytes produce heteromorphic seeds, which have different dormancy and germination behavior under saline conditions. This characteristic is related to the structural and physiological differences among heteromorphic seeds. It was unclear that how heteromorphic seeds differently accumulate organic and inorganic substances under saline conditions, and what are the physiological and molecular mechanisms involved in the production of heteromorphic seeds, and in the development of transgenerational plasticity in heteromorphic seeds. In the current brief review, dormancy and germination and the possible role of seed coat and storage compounds in this process of heteromorphic seeds development have been discussed. Moreover, the role of maternal effects on heteromorphic seeds production under saline environments and growth and reproduction capability of the descendants from them have been highlighted.

Applications of SERS in the Detection of Stress-Related Substances

A wide variety of biotic and abiotic stresses continually attack plants and animals, which adversely affect their growth, development, reproduction, and yield realization. To survive under stress conditions, highly sophisticated and efficient tolerance mechanisms have been evolved to adapt to stresses, which consist of the variation of effector molecules playing vital roles in physiological regulation. The development of a sensitive, facile, and rapid analytical methods for stress factors and effector molecules detection is significant for gaining deeper insight into the tolerance mechanisms. As a nondestructive analysis technique, surface-enhanced Raman spectroscopy (SERS) has unique advantages regarding its biosensing applications. It not only provides specific fingerprint spectra of the target molecules, conformation, and structure, but also has universal capacity for simultaneous detection and imaging of targets owing to the narrow width of the Raman vibrational bands. Herein, recent progress on biotic and abiotic stresses, tolerance mechanisms and effector molecules is summarized. Moreover, the development and promising future trends of SERS detection for stress-related substances combined with nanomaterials as substrates and SERS tags are discussed. This comprehensive and critical review might shed light on a new perspective for SERS applications.

Transcriptional regulation of bHLH during plant response to stress

Basic helix-loop-helix protein (bHLH) is the most extensive class of transcription factors in eukaryotes, which can regulate gene expression through interaction with specific motif in target genes. bHLH transcription factor is not only universally involved in plant growth and metabolism, including photomorphogenesis, light signal transduction and secondary metabolism, but also plays an important role in plant response to stress. In this review, we discuss the role of bHLH in plants in response to stresses such as drought, salt and cold stress. To provide a strong evidence for the important role of bHLH in plant stress response, in order to provide new ideas and targets for the prevention and treatment of plant stress resistance.

Effect of combined waterlogging and salinity stresses on euhalophyte Suaeda glauca

Salinity and waterlogging are abiotic stresses that have a significant impact on agricultural production and ecosystem conservation. The response of euhalophyte Suaeda glauca to waterlogging and salinity stresses was investigated. The results revealed that waterlogging markedly decreased seedling emergence. Compared to drained conditions, waterlogging inhibited the shoot dry weight, net photosynthetic rate, stomatal conductance, maximal efficiency of PSII photochemistry and chlorophyll content at salinity. Salinity decreased these values in both waterlogged and drained conditions, while the opposite trend was found in intercellular CO₂ concentrations. Waterlogging increased Na⁺, Cl^-, O₂^-, H₂O₂ and MDA content in the leaves compared with those in drained conditions, but this was not the case for K⁺ content and SOD and APX activity. Salinity increased these values except that salinity decreased K⁺ content in both waterlogged and drained conditions. In conclusion, S. glauca is not tolerant to combined waterlogging and salinity stresses during both seedling emergence and seedling growth stages, and this trait may limit the distribution of the species in lowland saline areas.

Mechanisms of Salt Tolerance in Halophytes: Current Understanding and Recent Advances

Halophytes are plants that exhibit high salt tolerance, allowing them to survive and thrive under extremely saline conditions. The study of halophytes advances our understanding about the important adaptations that are required for survival in high salinity conditions, including secretion of salt through the salt glands, regulation of cellular ion homeostasis and osmotic pressure, detoxification of reactive oxygen species, and alterations in membrane composition. To explore the mechanisms that contribute to tolerance to salt stress, salt-responsive genes have been isolated from halophytes and expressed in non-salt tolerant plants using targeted transgenic technologies. In this review, we discuss the mechanisms that underpin salt tolerance in different halophytes.

Ecophysiological response of native and exotic salt marsh vegetation to waterlogging and salinity: Implications for the effects of sea-level rise

The ecophysiological characteristics of native Phragmites australis and exotic Spartina alterniflora grown under waterlogging and salinity were investigated to explore their adaptation potential to sea level rise. The seasonal course of phenotypic traits, photosynthetic activity and chlorophyll fluorescence parameters of P. australis did not change remarkably under shallow flooding, whereas these variables were sensitive to increasing salinity. Waterlogging exacerbated the negative effects of salinity on shoot growth and photosynthetic activity of P. australis, and the combined stresses led to an absence of tassel and reproductive organs. By contrast, S. alterniflora performed well under both stresses and showed an obvious adaptation of salt secretion with increasing salinity. Light salinity was the optimal condition for S. alterniflora, and the tassel growth, chlorophyll content and fluorescence characters under moderate stresses did not differ notably. The Na+ and Cl- concentrations in leaves of both species increased, and the K+ content decreased in response to salinity. Under moderate and high saline levels, the ion concentrations in S. alterniflora were maintained at relatively consistent levels with increased salt secretion. We expect the degradation of P. australis and further colonization of S. alterniflora under prolonged flooding and saltwater intrusion from sea level rise on the coastline of China.

Molecular mechanisms underlying stress response and adaptation

Environmental stresses are ubiquitous and unavoidable to all living things. Organisms respond and adapt to stresses through defined regulatory mechanisms that drive changes in gene expression, organismal morphology, or physiology. Immune responses illustrate adaptation to bacterial and viral biotic stresses in animals. Dysregulation of the genotoxic stress response system is frequently associated with various types of human cancer. With respect to plants, especially halophytes, complicated systems have been developed to allow for plant growth in high salt environments. In addition, drought, waterlogging, and low temperatures represent other common plant stresses. In this review, we summarize representative examples of organismal response and adaptation to various stresses. We also discuss the molecular mechanisms underlying the above phenomena with a focus on the improvement of organismal tolerance to unfavorable environments.

Plant behaviour under combined stress: tomato responses to combined salinity and pathogen stress

Crop plants are subjected to a variety of stresses during their lifecycle, including abiotic stress factors such as salinity and biotic stress factors such as pathogens. Plants have developed a multitude of defense and adaptation responses to these stress factors. In the field, different stress factors mostly occur concurrently resulting in a new state of stress, the combined stress. There is evidence that plant resistance to pathogens can be attenuated or enhanced by abiotic stress factors. With stress tolerance research being mostly focused on plant responses to individual stresses, the understanding of a plant's ability to adapt to combined stresses is limited. In the last few years, we studied powdery mildew resistance under salt stress conditions in the model crop plant tomato with the aim to understand the requirements to achieve plant resilience to a wider array of combined abiotic and biotic stress combinations. We uncovered specific responses of tomato plants to combined salinity-pathogen stress, which varied with salinity intensity and plant resistance genes. Moreover, hormones, with their complex regulation and cross-talk, were shown to play a key role in the adaptation of tomato plants to the combined stress. In this review, we attempt to understand the complexity of plant responses to abiotic and biotic stress combinations, with a focus on tomato responses (genetic control and cross-talk of signaling pathways) to combined salinity and pathogen stresses. Further, we provide recommendations on how to design novel strategies for breeding crops with a sustained performance under diverse environmental conditions.

Accumulation and speciation of Cd in Avicennia marina tissues

Avicennia marina is a high-Cd-tolerant species in the mangrove wetlands. A hydroponic experiment was carried out to research the accumulation and chemical form distribution of Cd in the tissues of A. marina under different concentrations and durations of Cd stress. It was found that the concentrations of Cd in plant tissues followed the order of root > stem > leaf. The data suggested that root activity decreased, Cd accumulation ability weakened in roots, and the translocation factor increased in stems and leaves with the increase of stress duration. With a proactive defense mechanism, most Cd was bound to pectates, organic acids, and protein, especially in roots and stems with the most proportion of 88.51 and 78.91%, respectively, having lower biological activities. The Cd bounded to water-soluble organic acid and free inorganic aminophenol-Cd showed the lowest concentration. The pectates, organic acids, and protein-integrated Cd seem the most important in affecting Cd detoxification for A. marina; this mechanism of change in Cd biological activities decreases the toxicity of this aggressive pollutant and presents new knowledge about the tolerance of mangrove plants.

Unraveling Salt Tolerance Mechanisms in Halophytes: A Comparative Study on Four Mediterranean Limonium Species with Different Geographic Distribution Patterns

We have performed an extensive study on the responses to salt stress in four related Limonium halophytes with different geographic distribution patterns, during seed germination and early vegetative growth. The aims of the work were twofold: to establish the basis for the different chorology of these species, and to identify relevant mechanisms of salt tolerance dependent on the control of ion transport and osmolyte accumulation. Seeds were germinated in vitro, in the presence of increasing NaCl concentrations, and subjected to "recovery of germination" tests; germination percentages and velocity were determined to establish the relative tolerance and competitiveness of the four Limonium taxa. Salt treatments were also applied to young plants, by 1-month irrigation with NaCl up to 800 mM; then, growth parameters, levels of monovalent and divalent ions (in roots and leaves), and leaf contents of photosynthetic pigments and common osmolytes were determined in control and stressed plants of the four species. Seed germination is the most salt-sensitive developmental phase in Limonium. The different germination behavior of the investigated species appears to be responsible for their geographical range size: L. narbonense and L. virgatum, widespread throughout the Mediterranean, are the most tolerant and the most competitive at higher soil salinities; the endemic L. santapolense and L. girardianum are the most sensitive and more competitive only at lower salinities. During early vegetative growth, all taxa showed a strong tolerance to salt stress, although slightly higher in L. virgatum and L. santapolense. Salt tolerance is based on the efficient transport of Na+ and Cl- to the leaves and on the accumulation of fructose and proline for osmotic adjustment. Despite some species-specific quantitative differences, the accumulation patterns of the different ions were similar in all species, not explaining differences in tolerance, except for the apparent activation of K+ transport to the leaves at high external salinity, observed only in the most tolerant L. narbonense and L. virgatum. This specific response may be therefore relevant for salt tolerance in Limonium. The ecological implications of these results, which can contribute to a more efficient management of salt marshes conservation/regeneration programs, are also discussed.

Phosphorus mediation of cadmium stress in two mangrove seedlings Avicennia marina and Kandelia obovata differing in cadmium accumulation

Mangrove ecosystems are vulnerable to environmental threats. In order to elucidate the effect of phosphorus (P) on cadmium (Cd) tolerance and physiological responses in mangroves under Cd stress, a mangrove specie with salt exclusion Kandelia obovata and a specie with salt secretion Avicennia marina were compared in a hydroponic experiment. The results showed that most Cd was accumulated in mangrove roots and that P addition induced Cd immobilisation in them. Cd stress significantly increased malonaldehyde content, whereas P significantly decreased malonaldehyde in mangroves. Phosphorus positively regulated the photosynthetic pigment, proline content and synthesis of non-protein thiols, glutathione and phytochelatins in the leaves under Cd stress conditions. The results suggest different adaptive strategies adopted by two mangroves in a complex environment and A. marina showed a stronger Cd tolerance than K. obovata. The study provides a theoretical basis for P mediated detoxification of Cd in mangrove plants.

Making Plants Break a Sweat: the Structure, Function, and Evolution of Plant Salt Glands

Salt stress is a complex trait that poses a grand challenge in developing new crops better adapted to saline environments. Some plants, called recretohalophytes, that have naturally evolved to secrete excess salts through salt glands, offer an underexplored genetic resource for examining how plant development, anatomy, and physiology integrate to prevent excess salt from building up to toxic levels in plant tissue. In this review we examine the structure and evolution of salt glands, salt gland-specific gene expression, and the possibility that all salt glands have originated via evolutionary modifications of trichomes. Salt secretion via salt glands is found in more than 50 species in 14 angiosperm families distributed in caryophyllales, asterids, rosids, and grasses. The salt glands of these distantly related clades can be grouped into four structural classes. Although salt glands appear to have originated independently at least 12 times, they share convergently evolved features that facilitate salt compartmentalization and excretion. We review the structural diversity and evolution of salt glands, major transporters and proteins associated with salt transport and secretion in halophytes, salt gland relevant gene expression regulation, and the prospect for using new genomic and transcriptomic tools in combination with information from model organisms to better understand how salt glands contribute to salt tolerance. Finally, we consider the prospects for using this knowledge to engineer salt glands to increase salt tolerance in model species, and ultimately in crops.