Mechanisms of high salinity tolerance in plants

Can abiotic stresses in plants be alleviated by manganese nanoparticles or compounds?

Abiotic stress has become one of the most challenging problems for agriculture as the world population keeps increasing dramatically. Crop stress management using manganese (Mn) compounds has been recently employed to reduce the negative effects caused by drought, harsh temperature, and salinity. In response to abiotic stress, an adequate supply of Mn has shown to remediate plant manganese deficiency, induce Mn superoxide dismutase at the transcriptional level to face reactive oxygen species production, and stimulate manganese-dependent proteins to maintain cell integrity. Lately, nanoparticles (NPs) have been explored in agriculture applications. Recent studies have implied that Mn NPs may help plants to overcome abiotic stresses at higher efficiency and lower toxicity, compared to their bulk or ionic counterparts. Although studies have shown that Mn compounds promote crop growth and alleviate abiotic stress, many questions related to Mn-plant networking, their mode of signaling, and the Mn-dependent regulation processes need to be answered.

Metabarcoding reveals differences in fungal communities between unflooded versus tidal flat soil in coastal saline ecosystem

In the saline-affected ecosystem, fungi have huge potential to promote growth, induce disease resistance and enhance tolerance against salt-stress of host plants. Since areas of plowland are gradually decreasing, the reclamation of coastal saline lands could play a crucial role in maintaining agricultural productivity and crop security globally. Therefore, it is of great significance to explore the fungal diversity in the coastal saline ecosystem. Here, we collected saline soil samples from unflooded areas and tidal flat areas, the two typical distinct landforms in coastal saline ecosystems, and used ITS metabarcoding to depict the diversity of fungal communities. We found that fungal species evenness had a remarkably higher variation from the tidal flat compared to unflooded soil samples. Furthermore, we also confirmed that the fungal niches differentiation reports in the coastal saline ecosystem. Our ITS based DNA sequencing revealed that both unflooded and tidal flat soil were mainly composed of amplicon sequence variants (ASVs) belonging to Ascomycota (93.43% and 86.91% respectively). Based on our findings, understanding the associations and distinctions of fungal microbiome between unflooded soil and tidal flat could provide the basis for the development of reclamation in coastal saline lands.

Phenomic and Physiological Analysis of Salinity Effects on Lettuce

Salinity is a rising concern in many lettuce-growing regions. Lettuce (Lactuca sativa L.) is sensitive to salinity, which reduces plant biomass, and causes leaf burn and early senescence. We sought to identify physiological traits important in salt tolerance that allows lettuce adaptation to high salinity while maintaining its productivity. Based on previous salinity tolerance studies, one sensitive and one tolerant genotype each was selected from crisphead, butterhead, and romaine, as well as leaf types of cultivated lettuce and its wild relative, L. serriola L. Physiological parameters were measured four weeks after transplanting two-day old seedlings into 350 mL volume pots filled with sand, hydrated with Hoagland nutrient solution and grown in a growth chamber. Salinity treatment consisted of gradually increasing concentrations of NaCl and CaCl2 from 0 mM/0 mM at the time of transplanting, to 30 mM/15 mM at the beginning of week three, and maintaining it until harvest. Across the 10 genotypes, leaf area and fresh weight decreased 0-64% and 16-67%, respectively, under salinity compared to the control. Salinity stress increased the chlorophyll index by 4-26% in the cultivated genotypes, while decreasing it by 5-14% in the two wild accessions. Tolerant lines less affected by elevated salinity were characterized by high values of the chlorophyll fluorescence parameters Fv/Fm and instantaneous photosystem II quantum yield (QY), and lower leaf transpiration.

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

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

Endophytic Bacteria Potentially Promote Plant Growth by Synthesizing Different Metabolites and their Phenotypic/Physiological Profiles in the Biolog GEN III MicroPlateTM Test

Endophytic bacteria, as the most promising components of effective, biofertilizers biostimulating and biocontrol preparations, should be very intensively obtained from various plants and studied in terms of the conditions determining the potential ability to promote plant growth. For this reason, endophytic bacteria have been isolated from both stems and roots of up to six systematically distant species of vascular plants: one species belonging to the seedless vascular plants (Monilophyta), and five seed plants (Spermatophyta). The 23 isolated strains represented nine genera: Delftia, Stenotrophomonas, Rhizobium, Brevundimonas, Variovorax, Achromobacter, Novosphingobium, Comamonas and Collimonas, notably which were closely related—belonging to the phylum Proteobacteria. Stenotrophomonas sp. strains showed the greatest ability to synthesize indole-3-acetic acid (IAA)-like compounds, while Achromobacter sp. strains produced the highest levels of siderophores. The presence of the nifH gene and nitrogen binding activity was demonstrated for 95% of the strains tested. Stenotrophomonas maltophila (ES2 strain) showed the highest metabolic activity based on Biolog GEN III test. The ability to solubilize phosphate was determined only for three tested strains from genus: Delftia, Rhizobium and Novosphingobium. The presented work demonstrated that the metabolic and phenotypic properties of plant growth-promoting endophytes are correlated with the genus of bacteria and are not correlated with the host plant species or part of plant (stem, root).

Comparative Transcriptome and Metabolic Profiling Analysis of Buckwheat (Fagopyrum Tataricum (L.) Gaertn.) under Salinity Stress

Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) is a nutritional crop, which has high flavonoid content. However, buckwheat is a salt sensitive glycophyte cereal crop and the growth and grain yield of buckwheat are significantly affected by soil salinity. In this study, we performed a comprehensive analysis of the transcriptome and metabolome of salt treated-buckwheat to understand the effects of salinity on buckwheat. A total of 50,681,938 clean reads were acquired from all samples. We acquired 94,950 unigenes with a mean length of 1133 bp and N50 length of 1900 bp assembly. Of these, 63,305 unigenes (66.7%) were matched in public databases. Comparison of the transcriptome expression patterns between control and salt treated groups showed that 4098 unigenes were up-regulated and 3292 unigenes were down-regulated significantly. Further, we found that genes involved with amino acid, lipid and nucleotide metabolism were most responsive to salt stress. Additionally, many genes involved in secondary metabolite biosynthesis changed significantly following treatment. Those affected included phenylpropanoid biosynthesis and flavonoid biosynthesis. Chromatographic analysis was used to examine the differences in concentration of flavonoids, carotenoids, amino acids and organic acids in the samples following treatment. There was a significant increase in rutin (12.115 mg/g dry weight), following salt stress; whereas, six carotenoids (lutein, zeaxanthin, 13Z-β-carotene, α-carotene, E-β-carotene and 9Z-β-carotene) did not significantly respond to salt stress. Ultimately, our data acts as a valuable resource for future research on buckwheat and can be used as the basis for future analysis focused on gene-to-metabolite networks in buckwheat.

Proteomic and physiological responses in mangrove Kandelia candel roots under short-term high-salinity stress

Kandelia candel is one of the mangrove species that are most resistant to environmental stress. As a typical nonsalt-secreting mangrove plant, K. candel is an ideal biological material to analyze the molecular mechanism of salt tolerance in woody plants. In this study, changes in protein abundance and expression profile in K. candel roots under high-salinity stress of 600 mmol L^-1 NaCl were analyzed using isobaric tags for relative and absolute quantification (iTRAQ) assay. Moreover, the physiological parameters associated with metabolic pathways in which the differentially abundant proteins (DAPs) are involved were determined. A total of 5577 proteins were identified by iTRAQ analysis of the K. candel root proteins, of which 227 were DAPs with a fold change ratio >1.2 or a fold change ratio <0.83 and a P-value <0.05. A total of 227 DAPs consisting of 110 up-regulated and 117 down-regulated proteins were identified. Our Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that the DAPs were primarily involved in biological processes including carbohydrate and energy metabolisms, stress response and defense, cell wall structure, and secondary metabolism. The results of the physiological parameters showed that their profile changes were consistent with those of the proteome analysis. The results of the proteome and physiological parameters showed that K. candel roots could resist high-salinity stress by maintaining a normal Embden-Meyerhof-Parnas and tricarboxylic acid (EMP-TCA) pathway, increasing the activities of various antioxidant enzymes and antioxidant contents, stabilizing the cell wall structure, and accumulating secondary metabolites such as triterpenoids.

Calcium signaling and salt tolerance are diversely entwined in plants

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

Effects of cerium oxide on rice seedlings as affected by co-exposure of cadmium and salt

Effects of CeO₂ NPs (200 mg.L^-1) on rice (Oryza sativa L.) alone or co-exposure with cadmium (Cd) and salt (sodium chloride, NaCl) were investigated in hydroponic systems for two weeks. Physiological results show that rice biomass was significantly inhibited when NaCl or CdCl₂ added alone or in co-exposure treatment. CeO₂ NPs significantly relieve the chlorophyll damage under CdCl₂ environmental stress. The presence of CeO₂ NPs alleviated both stressors induced damages to rice as indicated by the reduced proline level. Additionally, CeO₂ NPs triggered the antioxidant defense systems to counteract the oxidative stress caused by NaCl and CdCl₂. The level of 8-OHdG, one of the most important indicators for genotoxicity, in rice suggest that the presence of CeO₂ NPs reduced the DNA damage in NaCl treated rice. Elemental analysis indicated that co-exposure to NaCl and CdCl₂ slightly decreased the Cd content as compared to the one in the CdCl₂ alone treatment, and this co-exposure also significantly reduced the Na content when comparing with the NaCl alone treatment. Taken together, our findings suggest that CeO₂ NPs could alleviate the CdCl₂ and NaCl stresses, but could not completely change the phenotype of both contaminants treated rice.

Chitosan-induced enhanced expression and activation of alternative oxidase confer tolerance to salt stress in maize seedlings

This study aimed to investigate the possible alleviating effect of chitosan on salt-induced growth retardation and oxidative stress and to elucidate whether this effect is linked to activation of mitochondrial respiration on the basis of alternative respiration in maize seedlings. Salt stress significantly reduced root length and plant height in comparison to the control, whereas foliar application of chitosan ameliorated the adverse effect of salinity to a certain degree. Moreover, chitosan resulted in plant growth promotion as compared to unstressed seedlings. The separate applications of chitosan and salt had a stimulatory effect on the activities of antioxidant enzymes; however, combined application of chitosan and salt were more effective than that of chitosan or salt alone. Similarly, mitochondrial total respiration rate (V_t) and alternative respiration capacity (V_alt) were increased by separate applications of chitosan and salt; however, the combination of chitosan and salt gave the highest values for these parameters. The highest values of V_alt/V_t was recorded at seedlings treated with salt plus chitosan. Similarly, cytochrome respiration capacity was also increased by chitosan in both stress-free and stressed conditions. In addition, AOX1, encoding alternative oxidase, was significantly upregulated by chitosan and/or salt. The maximum transcript level was recorded at seedlings treated with salt plus chitosan. Chitosan also significantly decreased superoxide anion and hydrogen peroxide contents and lipid peroxidation level under normal and the stressed conditions. These results suggest that the mitigating effect of chitosan on salt stress is linked to activation of alternative respiration at biochemical and molecular level.

Overexpression of PbrNHX2 gene, a Na+/H+ antiporter gene isolated from Pyrus betulaefolia, confers enhanced tolerance to salt stress via modulating ROS levels

Intracellular Na+/H+ antiporters (NHXs) play important roles in plant tolerance to salt stress. However, plant NHXs functioning in salt tolerance and the underlying physiological mechanisms remain poorly understood. In this report, we report the identification and functional characterization of PbrNHX2 isolated from Pyrus betulaefolia. PbrNHX2 expression levels were induced by salt, and dehydration, but was unaffected by cold. PbrNHX2 was localized in the tonoplast. Overexpression of PbrNHX2 in tobacco and Pyrus ussuriensis conferred enhanced tolerance to salt tolerance, whereas down-regulation of PbrNHX2 in Pyrus betulaefolia by virus-induced gene silencing (VIGS) resulted in elevated salt sensitivity. The transgenic lines contained lower levels of Na+, higher levels of K+, and higher K/Na ratio, whereas they were changed in an opposite way when PbrNHX2 was silenced. In addition, the transgenic plants accumulated lower levels of reactive oxygen species compared with wild type, accompanied by higher activities of three antioxidant enzymes. Taken together, the data demonstrate that PbrNHX2 plays a positive role in salt tolerance and that it holds a great potential for engineering salt tolerance in crops.

Antioxidative capacity is highly associated with the storage property of tuberous roots in different sweetpotato cultivars

The activities and gene expression of antioxidative enzymes and the ROS content were analyzed in two typical storage-tolerant cultivars (Xushu 32 and Shangshu 19) and another two storage-sensitive cultivars (Yanshu 25 and Sushu 16) to explore the association between the storage capacity of sweetpotato (Ipomoea batatas (L.) Lam) and ROS scavenging capability. The storage roots of the storage-tolerant cultivars maintained higher activities and expression levels of antioxidative enzymes, including ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD); lower activity and expression of lipoxygenase (LOX); and lower accumulation of ROS metabolites compared with the storage-sensitive cultivars. The antioxidative capability and ROS parameters of leaves were positively correlated with those of storage roots. Our results provide valuable insight for evaluating the storability of sweetpotato cultivars by analyzing the capabilities of the antioxidative system and the contents of ROS metabolites.

Effect of soil aeration on root morphology and photosynthetic characteristics of potted tomato plants (Solanum lycopersicum) at different NaCl salinity levels

BACKGROUND

Salt stress is one of the environmental factors that greatly limits crop production worldwide because high salt concentrations in the soil affect morphological responses and physiological and metabolic processes, including root morphology and photosynthetic characteristics. Soil aeration has been reported to accelerate the growth of plants and increase crop yield. The objective of this study was to examine the effects of 3 NaCl salinity levels (28, 74 and 120 mM) and 3 aeration volume levels (2.3, 4.6 and 7.0 L/pot) versus non-aeration and salinity treatments on the root morphology, photosynthetic characteristics and chlorophyll content of potted tomato plants.

RESULTS

The results showed that both aeration volume and salinity level affected the root parameters, photosynthetic characteristics and chlorophyll content of potted tomato plants. The total length, surface area and volume of roots increased with the increase in aeration volume under each NaCl stress level. The effect was more marked in the fine roots (especially in ≤1 mm diameter roots). Under each NaCl stress level, the photosynthetic rate and chlorophyll content of tomato significantly increased in response to the aeration treatments. The net photosynthetic rate and chlorophyll a and t content increased by 39.6, 26.9, and 17.9%, respectively, at 7.0 L/pot aeration volume compared with no aeration in the 28 mM NaCl treatment. We also found that aeration could reduce the death rate of potted tomato plants under high salinity stress conditions (120 mM NaCl).

CONCLUSIONS

The results suggest that the negative effect of NaCl stress can be offset by soil aeration. Soil aeration can promote root growth and increase the photosynthetic rate and chlorophyll content, thus promoting plant growth and reducing the plant death rate under NaCl stress conditions.

Transcriptome profiling of Puccinellia tenuiflora during seed germination under a long-term saline-alkali stress

BACKGROUND

Puccinellia tenuiflora is the most saline-alkali tolerant plant in the Songnen Plain, one of the three largest soda saline-alkali lands worldwide. Here, we investigated the physicochemical properties of saline-alkali soils from the Songnen Plain and sequenced the transcriptomes of germinated P. tenuiflora seedlings under long-term treatment (from seed soaking) with saline-alkali soil extracts.

RESULTS

We found that the soils from Songnen Plain were reasonably rich in salts and alkali; moreover, the soils were severely deficient in nitrogen [N], phosphorus [P], potassium [K] and several other mineral elements. This finding demonstrated that P. tenuiflora can survive from not only high saline-alkali stress but also a lack of essential mineral elements. To explore the saline-alkali tolerance mechanism, transcriptional analyses of P. tenuiflora plants treated with water extracts from the saline-alkali soils was performed. Interestingly, unigenes involved in the uptake of N, P, K and the micronutrients were found to be significantly upregulated, which indicated the existence of an efficient nutrition-uptake system in P. tenuiflora. Compared with P. tenuiflora, the rice Oryza sativa was hypersensitive to saline-alkali stress. The results obtained using a noninvasive microtest techniques confirmed that the uptake of NO3- and NH4+ and the regulatory flux of Na+ and H+ were significantly higher in the roots of P. tenuiflora than in those of O. sativa. In the corresponding physiological experiments, the application of additional nutrition elements significantly eliminated the sensitive symptoms of rice to saline-alkali soil extracts.

CONCLUSIONS

Our results imply that the survival of P. tenuiflora in saline-alkali soils is due to a combination of at least two regulatory mechanisms and the high nutrient uptake capacity of P. tenuiflora plays a pivotal role in its adaptation to those stress. Taken together, our results highlight the role of nutrition uptake in saline-alkali stress tolerance in plants.

The Roles of Environmental Factors in Regulation of Oxidative Stress in Plant

Exposure to a variety of environmental factors such as salinity, drought, metal toxicity, extreme temperature, air pollutants, ultraviolet-B (UV-B) radiation, pesticides, and pathogen infection leads to subject oxidative stress in plants, which in turn affects multiple biological processes via reactive oxygen species (ROS) generation. ROS include hydroxyl radicals, singlet oxygen, and hydrogen peroxide in the plant cells and activates signaling pathways leading to some changes of physiological, biochemical, and molecular mechanisms in cellular metabolism. Excessive ROS, however, cause oxidative stress, a state of imbalance between the production of ROS and the neutralization of free radicals by antioxidants, resulting in damage of cellular components including lipids, nucleic acids, metabolites, and proteins, which finally leads to the death of cells in plants. Thus, maintaining a physiological level of ROS is crucial for aerobic organisms, which relies on the combined operation of enzymatic and nonenzymatic antioxidants. In order to improve plants' tolerance towards the harsh environment, it is vital to reinforce the comprehension of oxidative stress and antioxidant systems. In this review, recent findings on the metabolism of ROS as well as the antioxidative defense machinery are briefly updated. The latest findings on differential regulation of antioxidants at multiple levels under adverse environment are also discussed here.

Performance of Aeluropus lagopoides (mangrove grass) ecotypes, a potential turfgrass, under high saline conditions

Climate change has become a real threat, and its impacts are being felt throughout the world. Temperature is considered one of the significant elements by the recent consequences of climate change and global warming, specially the salinity which is increased at higher temperature. Turfgrasses are adversely affected due to an increasing trend in salinity. The main aim of this investigation was to find out salt-tolerant ecotypes from native species of UAE to mitigate the salinity problem. Performance of a native grass, Aeluropus lagopoides, was investigated under high saline conditions during the year 2014 under the UAE climatic conditions. The experiment was planned under randomised complete block design (RCBD) with two factors and four replications. During the experiment, 50 ecotypes of Aeluropus lagopoides, alongside Paspalum vaginatum (as control), were tested at different salt levels, i.e. 0, 15, 30, 45, 60 and 75 dSm^-1. Significant differences were found among various ecotypes as well as salinity levels for different agronomic traits including green cover, canopy stiffness, leaf colour and salinity of leaf rinseates. Most of the ecotypes tolerated salinity up to 30 dSm^-1, maintaining the quality, but beyond this level the quality declined. However, some of the ecotypes survived under high salinity, even beyond sea level (75 dSm^-1). All the ecotypes, except RUA2, RUA3 and RUA1, showed better performance than P. vaginatum, the prevailing commercial turfgrass in the UAE. Based on their performance, the ecotypes RUDA7, FA5, RA3, RUDA2 and RA2 could be used for turf purposes under saline conditions.

Advances in understanding salt tolerance in rice

This review presents a comprehensive overview of the recent research on rice salt tolerance in the areas of genomics, proteomics, metabolomics and chemical genomics. Salinity is one of the major constraints in rice cultivation globally. Traditionally, rice is a glycophyte except for a few genotypes that have been widely used in salinity tolerance breeding of rice. Both seedling and reproductive stages of rice are considered to be the salt-susceptible stages; however, research efforts have been biased towards improving the understanding of seedling-stage salt tolerance. An extensive literature survey indicated that there have been very few attempts to develop reproductive stage-specific salt tolerance in rice probably due to the lack of salt-tolerant phenotypes at the reproductive stage. Recently, the role of DNA methylation, genome duplication and codon usage bias in salinity tolerance of rice have been studied. Furthermore, the study of exogenous salt stress alleviants in rice has opened up another potential avenue for understanding and improving its salt tolerance. There is a need to not only generate additional genomic resources in the form of salt-responsive QTLs and molecular markers and to characterize the genes and their upstream regulatory regions, but also to use them to gain deep insights into the mechanisms useful for developing tolerant varieties. We analysed the genomic locations of diverse salt-responsive genomic resources and found that rice chromosomes 1-6 possess the majority of these salinity-responsive genomic resources. The review presents a comprehensive overview of the recent research on rice salt tolerance in the areas of genomics, proteomics, metabolomics and chemical genomics, which should help in understanding the molecular basis of salinity tolerance and its more effective improvement in rice.

Grasping at straws: a re-evaluation of sweepstakes colonisation of islands by mammals

Natural rafting is an easy, non-evidence-based solution often used to explain the presence of a variety of species on isolated islands. The question arises as to whether this solution is based on solid scientific grounds. It is a plausible colonisation route only if intricate networks of variables are considered and many different conditions satisfied. This review provides a descriptive account of some of the most critical issues underlying the theory of natural rafting that should be addressed by its supporters. These include: (i) biological variables; (ii) characteristics of the vessels; and (iii) physical variables. Natural rafting may explain the dispersal of poikilotherms with low metabolic rates and low resource requirements that could withstand trans-oceanic crossings, but explaining the transport of homeothermic terrestrial mammals to oceanic islands is more problematic. Drifting at sea exposes organisms to high concentrations of salt, high temperature and humidity excursions, starvation, and above all to dehydration. A sufficiently large group of healthy reproductive individuals of the two sexes should either be transported together, or be able to reassemble after separate crossings, to prevent inbreeding, genetic drift and ultimately extinction. Any vessels of flotsam occupied must minimally provide the animals they transport with sufficient provisions to survive the journey, offer minimum friction and drag through water, and be transported by appropriately directed, sustained, high-speed currents. Thus, a 'sweepstakes colonisation' event would be the result of a lucky combination of all, or at least the majority, of these factors. Some cases throw doubt on the use of a natural rafting model to explain known animal colonisations, with one of the most striking examples being Madagascar. This island is far from the nearest mainland coasts and the sea currents in the Mozambique Channel are directed towards Africa rather than Madagascar, yet, the island was colonised by terrestrial mammals (e.g. extinct hippopotamuses, lemurs, carnivores, rodents and tenrecs) unable to swim and to survive long journeys at sea. In order to assess the feasibility of the natural rafting model in a case such as Madagascar, tests were performed using three variables for which enough information could be obtained from the literature: length of survival without food, survival without water, and sea current speed. The distributions of these variables appear to be log-normal and multiplicative, or follow a power-law, rather than being Gaussian. The tests suggest that a distributional analysis is a more suitable approach than the use of geometric probability to calculate the probabilities associated with the examined data. Such non-linear and self-organising systems may reach a critical point governed by different competing factors. Mammals with high survival requirements, such as lemurs and hippopotamuses, thus may have a virtually zero probability of reaching distant islands by natural rafting. Our results raise doubts as to the validity of a natural rafting model, and we urge a rethinking of the modes in which numerous islands were colonised by land mammals and a careful revision of past geological and phylogeographic work.

Morphophysiological and molecular evidence supporting the augmentative role of Piriformospora indica in mitigation of salinity in Cucumis melo L

Salinity is one of the major limiting factors in plant growth and productivity. Cucumis melo L. is a widely cultivated plant, but its productivity is significantly influenced by the level of salinity in soil. Symbiotic colonization of plants with Piriformospora indica has shown a promotion in plants growth and tolerance against biotic stress. In this study, physiological markers such as ion analysis, antioxidant determination, proline content, electrolyte leakage and chlorophyll measurement were assessed in melon cultivar under two concentrations (100 and 200 mM) of NaCl with and without P. indica inoculation. Results showed that the endophytic inoculation consistently upregulated the level of antioxidants, enhanced plants to antagonize salinity stress. The expression level of an RNA editing factor (SLO2) which is known to participate in mitochondria electron transport chain was analyzed, and its full mRNA sequence was obtained by rapid amplification of cDNA ends (RACE). Under salinity stress, the expression level of SLO2 was increased, enhancing the plant's capability to adapt to the stress. However, P. indica inoculation further elevated the expression level of SLO2. These findings suggested that the symbiotic association of fungi could help the plants to tolerate the salinity stress.

CYSTM3 negatively regulates salt stress tolerance in Arabidopsis

CYSTM3, a small mitochondrial protein, acts as a negative regulator in salt stress response by preventing Na⁺ efflux and disturbing reactive oxygen species (ROS) homeostasis in Arabidopsis. Cysteine-rich transmembrane module (CYSTM) is a not well characterized small peptide family in plants. In this study, we identified a novel mitochondrion-localized CYSTM member CYSTM3 from Arabidopsis, which was ubiquitously expressed in different tissues and dramatically induced by salt stress. Transgenic plants overexpressing CYSTM3 (OE) displayed hypersensitivity to salt stress compared with wild type (WT) plants, whereas a knockout mutant cystm3 was more tolerant to high salinity than WT. Moreover, OE lines accumulated higher contents of Na⁺ and ROS than WT and cystm3 upon exposure to high salinity. Further analysis revealed that CYSTM3 could deter root Na⁺ efflux and inhibit the activities of a range of ROS scavenging enzymes in Arabidopsis. In addition, the transcripts of nuclear salt stress-responsive genes were over-activated in cystm3 than those in WT and OE lines. Taken together, Arabidopsis CYSTM3 acts as a negative regulator in salt stress tolerance.

Cloning and molecular characterization of TaERF6, a gene encoding a bread wheat ethylene response factor

Ethylene response factor proteins are important for regulating gene expression under different stresses. Different isoforms for ERF have previously isolated from bread wheat (Triticum aestivum L.) and related genera and called from TaERF1 to TaERF5. We isolated, cloned and molecular characterized a novel one based on TdERF1, an isoform in durum wheat (Triticum turgidum L.) and called TaERF6. Its cDNA was synthesized, sequenced and compared with genomic sequence to figure out intron and exon regions and determine coding sequence region. The length of TdERF1 gene was 1939 bp and cDNA was 1065 bp including two exons, the first one 259 bp and the second one 806 bp separated by a 874 bp intron with a 111 bp 5'-UTR (untranslated region) and 401 bp 3'-UTR. TaERF6 encodes a 353 amino acids protein with nearly 99% identity to TdERF1. Hydrophobic cluster analysis revealed an N-terminal hydrophobic domain contains a highly conserved motif with the consensus sequence of M [C/L/Y] [G/R] [G/R/P] [A/G/V/L/R] [I/L/R/S/P/Q] [L/I/R/H] and hydrophobic clusters in AP2/ERF domain of which tends to form -sheet. Three monopartite nuclear localization signals also identified in TaERF6 that play important role in getting back into the nucleus. The results showed several putative phosphorylation sites in TaERF6 that a motif from residues 246 to 266, the CMVII-4 motif, was predicted to phosphorylate by different kinase proteins and play important roles in TaERF6 function. Phylogenetic analysis showed 7 clusters (I to VII) and 10 subclusters according to their relatedness in Poaceae family.

NtMYB4 and NtCHS1 Are Critical Factors in the Regulation of Flavonoid Biosynthesis and Are Involved in Salinity Responsiveness

High levels of salinity induce serious oxidative damage in plants. Flavonoids, as antioxidants, have important roles in reactive oxygen species (ROS) scavenging. In the present study, the tobacco R2R3 MYB type repressor, NtMYB4, was isolated and characterized. The expression of NtMYB4 was suppressed by salinity. Overexpression of NtMYB4 reduced the salt tolerance in transgenic tobacco plants. NtMYB4 repressed the promoter activity of NtCHS1 and negatively regulated its expression. Rutin accumulation was significantly decreased in NtMYB4 overexpressing transgenic plants and NtCHS1 RNAi silenced transgenic plants. Moreover, high H2O2 andO 2 - contents were detected in both types of rutin-reduced transgenic plants under high salt stress. In addition, exogenous rutin supplementation effectively scavenged ROS (H2O2 andO 2 - ) and improved the salt tolerance of the rutin-reduced transgenic plants. In contrast, NtCHS1 overexpressing plants had increased rutin accumulation, lower H2O2 andO 2 - contents, and higher tolerance to salinity. These results suggested that tobacco NtMYB4 acts as a salinity response repressor and negatively regulates NtCHS1 expression, which results in the reduced flavonoid accumulation and weakened ROS-scavenging ability under salt stress.

Populus euphratica JRL Mediates ABA Response, Ionic and ROS Homeostasis in Arabidopsis under Salt Stress

Sodium chloride (NaCl) induced expression of a jacalin-related mannose-binding lectin (JRL) gene in leaves, roots, and callus cultures of Populus euphratica (salt-resistant poplar). To explore the mechanism of the PeJRL in salinity tolerance, the full length of PeJRL was cloned from P. euphratica and was transformed into Arabidopsis. PeJRL was localized to the cytoplasm in mesophyll cells. Overexpression of PeJRL in Arabidopsis significantly improved the salt tolerance of transgenic plants, in terms of seed germination, root growth, and electrolyte leakage during seedling establishment. Under NaCl stress, transgenic plants retained K⁺ and limited the accumulation of Na⁺. PeJRL-transgenic lines increased Na⁺ extrusion, which was associated with the upward regulation of SOS1, AHA1, and AHA2 genes encoding plasma membrane Na⁺/proton (H⁺) antiporter and H⁺-pumps. The activated H⁺-ATPases in PeJRL-overexpressed plants restricted the channel-mediated loss of K⁺ that was activated by NaCl-induced depolarization. Under salt stress, PeJRL–transgenic Arabidopsis maintained reactive oxygen species (ROS) homeostasis by activating the antioxidant enzymes and reducing the production of O₂⁻ through downregulation of NADPH oxidases. Of note, the PeJRL-transgenic Arabidopsis repressed abscisic acid (ABA) biosynthesis, thus reducing the ABA-elicited ROS production and the oxidative damage during the period of salt stress. A schematic model was proposed to show the mediation of PeJRL on ABA response, and ionic and ROS homeostasis under NaCl stress.

Ecophysiological Responses of Carpinus turczaninowii L. to Various Salinity Treatments

Carpinus turczaninowii L., commonly known as hornbeam, has significant economic and ornamental importance and is largely distributed in the northern hemisphere, including parts of China and Korea, with high adaptation to harsh conditions in very unfertile soils. In this study, the ecophysiological responses of C. turczaninowii seedlings to various salinity stress treatments (NaCl: 0, 17, 34, 51, 68, and 85 mM) were studied for 42 days by determining stress-induced changes in growth parameters and biochemical markers. Salinity stress affected the values of all the examined parameters, both morphological and physiological, and caused the inhibition of plant growth, the degradation of photosynthetic capacity and stomatal behavior, a decrease in the photosynthetic pigments contents and relative water content, an increase in the Malondialdehyde (MDA) content and relative electrolytic conductivity, and the accumulation of Na+ and Cl− content. The presence of relatively high concentrations of organic osmolytes, the activation of antioxidant enzymes, and the ionic transport capacity from the root to shoots may represent a constitutive mechanism of defence against stress in C. turczaninowii seedlings. Our results suggest that C. turczaninowii can tolerate salinity at low and moderate concentrations (17–51 mM) under nursery conditions and can be widely used in roadsides, gardens, parks, and other urban areas.

Effects of salinity changes on aquatic organisms in a multiple stressor context

Under global change, the ion concentration of aquatic ecosystems is changing worldwide. Many freshwater ecosystems are being salinized by anthropogenic salt inputs, whereas many naturally saline ones are being diluted by agricultural drainages. This occurs concomitantly with changes in other stressors, which can result in additive, antagonistic or synergistic effects on organisms. We reviewed experimental studies that manipulated salinity and other abiotic stressors, on inland and transitional aquatic habitats, to (i) synthesize their main effects on organisms' performance, (ii) quantify the frequency of joint effect types across studies and (iii) determine the overall individual and joint effects and their variation among salinity-stressor pairs and organism groups using meta-analyses. Additive effects were slightly more frequent (54%) than non-additive ones (46%) across all the studies (n = 105 responses). However, antagonistic effects were dominant for the stressor pair salinity and toxicants (44%, n = 43), transitional habitats (48%, n = 31) and vertebrates (71%, n = 21). Meta-analyses showed detrimental additive joint effects of salinity and other stressors on organism performance and a greater individual impact of salinity than the other stressors. These results were consistent across stressor pairs and organism types. These findings suggest that strategies to mitigate multiple stressor impacts on aquatic ecosystems should prioritize restoring natural salinity concentrations.This article is part of the theme issue 'Salt in freshwaters: causes, ecological consequences and future prospects'.

Dissecting genomic hotspots underlying seed protein, oil, and sucrose content in an interspecific mapping population of soybean using high-density linkage mapping

The cultivated [Glycine max (L) Merr.] and wild [Glycine soja Siebold & Zucc.] soybean species comprise wide variation in seed composition traits. Compared to wild soybean, cultivated soybean contains low protein, high oil, and high sucrose. In this study, an interspecific population was derived from a cross between G. max (Williams 82) and G. soja (PI 483460B). This recombinant inbred line (RIL) population of 188 lines was sequenced at 0.3× depth. Based on 91 342 single nucleotide polymorphisms (SNPs), recombination events in RILs were defined, and a high-resolution bin map was developed (4070 bins). In addition to bin mapping, quantitative trait loci (QTL) analysis for protein, oil, and sucrose was performed using 3343 polymorphic SNPs (3K-SNP), derived from Illumina Infinium BeadChip sequencing platform. The QTL regions from both platforms were compared, and a significant concordance was observed between bin and 3K-SNP markers. Importantly, the bin map derived from next-generation sequencing technology enhanced mapping resolution (from 1325 to 50 Kb). A total of five, nine, and four QTLs were identified for protein, oil, and sucrose content, respectively, and some of the QTLs coincided with soybean domestication-related genomic loci. The major QTL for protein and oil were mapped on Chr. 20 (qPro_20) and suggested negative correlation between oil and protein. In terms of sucrose content, a novel and major QTL were identified on Chr. 8 (qSuc_08) and harbours putative genes involved in sugar transport. In addition, genome-wide association using 91 342 SNPs confirmed the genomic loci derived from QTL mapping. A QTL-based haplotype using whole-genome resequencing of 106 diverse soybean lines identified unique allelic variation in wild soybean that could be utilized to widen the genetic base in cultivated soybean.

Salicylic acid improves arbuscular mycorrhizal symbiosis, and chickpea growth and yield by modulating carbohydrate metabolism under salt stress

Salt stress is a major abiotic stress restricting plant growth and reproductive yield. Salicylic acid (SA) and arbuscular mycorrhizal (AM) symbioses play key roles in eliminating adverse effects of salt stress by modulating ion homeostasis and carbohydrate metabolism in crop plants. Sugars synthesized via carbohydrate metabolism act as osmotic adjustors and signaling molecules in activation of various defense responses against salt stress. The present study investigated the role of SA (0.5 mM) seed priming in establishment of AM symbiosis with Rhizoglomus intraradices and the impact on growth, ion-homeostasis, nutrient uptake, and sugar metabolism in Cicer arietinum L. (chickpea) genotypes under salt stress. Salinity had a negative correlation with plant growth and AM symbiosis in both genotypes with more negative effects in relatively salt-sensitive genotype than tolerant. SA enhanced the percent root colonization by significantly increasing the number of arbuscules and vesicles under salt stress. AM symbiosis was more effective in improving root biomass, root to shoot ratio, and nutrient acquisition than SA, while SA was more effective in maintaining ion equilibrium and modulating carbohydrate metabolism and reproductive yield when compared with AM inoculation. SA priming directed the utilization of total soluble sugars (TSS) towards reproductive attributes more efficiently than did AM inoculation by activating TSS metabolic consumption. In AM plants, TSS concentrations were more directed towards sink demand by the fungus itself rather than developing reproductive structures. SA priming further increased sugar export to roots of AM plants, thus favored AM symbiosis. Hence, SA seed priming-induced improvement in AM symbiosis can be a promising strategy in achieving sustainable production of chickpea genotypes under salt stress.

Comparative Proteomic and Physiological Analyses of Two Divergent Maize Inbred Lines Provide More Insights into Drought-Stress Tolerance Mechanisms

Drought stress is the major abiotic factor threatening maize (Zea mays L.) yield globally. Therefore, revealing the molecular mechanisms fundamental to drought tolerance in maize becomes imperative. Herein, we conducted a comprehensive comparative analysis of two maize inbred lines contrasting in drought stress tolerance based on their physiological and proteomic responses at the seedling stage. Our observations showed that divergent stress tolerance mechanisms exist between the two inbred-lines at physiological and proteomic levels, with YE8112 being comparatively more tolerant than MO17 owing to its maintenance of higher relative leaf water and proline contents, greater increase in peroxidase (POD) activity, along with decreased level of lipid peroxidation under stressed conditions. Using an iTRAQ (isobaric tags for relative and absolute quantification)-based method, we identified a total of 721 differentially abundant proteins (DAPs). Amongst these, we fished out five essential sets of drought responsive DAPs, including 13 DAPs specific to YE8112, 107 specific DAPs shared between drought-sensitive and drought-tolerant lines after drought treatment (SD_TD), three DAPs of YE8112 also regulated in SD_TD, 84 DAPs unique to MO17, and five overlapping DAPs between the two inbred lines. The most significantly enriched DAPs in YE8112 were associated with the photosynthesis antenna proteins pathway, whilst those in MO17 were related to C5-branched dibasic acid metabolism and RNA transport pathways. The changes in protein abundance were consistent with the observed physiological characterizations of the two inbred lines. Further, quantitative real-time polymerase chain reaction (qRT-PCR) analysis results confirmed the iTRAQ sequencing data. The higher drought tolerance of YE8112 was attributed to: activation of photosynthesis proteins involved in balancing light capture and utilization; enhanced lipid-metabolism; development of abiotic and biotic cross-tolerance mechanisms; increased cellular detoxification capacity; activation of chaperones that stabilize other proteins against drought-induced denaturation; and reduced synthesis of redundant proteins to help save energy to battle drought stress. These findings provide further insights into the molecular signatures underpinning maize drought stress tolerance.

Remodeling of chloroplast proteome under salinity affects salt tolerance of Festuca arundinacea

Acclimation of photosynthetic apparatus to variable environmental conditions is an important component of tolerance to dehydration stresses, including salinity. The present study deals with the research on alterations in chloroplast proteome of the forage grasses. Based on chlorophyll fluorescence parameters, two genotypes of a model grass species-Festuca arundinacea with distinct levels of salinity tolerance: low salt tolerant (LST) and high salt tolerant (HST), were selected. Next, two-dimensional electrophoresis and mass spectrometry were applied under both control and salt stress conditions to identify proteins accumulated differentially between these two genotypes. The physiological analysis revealed that under NaCl treatment the studied plants differed in photosystem II activity, water content, and ion accumulation. The differentially accumulated proteins included ATPase B, ATP synthase, ribulose-1,5-bisphosphate carboxylase large and small subunits, cytochrome b6-f complex iron-sulfur subunit, oxygen-evolving enhancer proteins (OEE), OEE1 and OEE2, plastidic fructose-bisphosphate aldolase (pFBA), and lipocalin. A higher level of lipocalin, potentially involved in prevention of lipid peroxidation under stress, was also observed in the HST genotype. Our physiological and proteomic results performed for the first time on the species of forage grasses clearly showed that chloroplast metabolism adjustment could be a crucial factor in developing salinity tolerance.

Patellin1 Negatively Modulates Salt Tolerance by Regulating PM Na+/H+ Antiport Activity and Cellular Redox Homeostasis in Arabidopsis

Soil salinity significantly represses plant development and growth. Mechanisms involved sodium (Na+) extrusion and compartmentation, intracellular membrane trafficking as well as redox homeostasis regulation play important roles in plant salt tolerance. In this study, we report that Patellin1 (PATL1), a membrane trafficking-related protein, modulates salt tolerance in Arabidopsis. The T-DNA insertion mutant of PATL1 (patl1) with an elevated PATL1 transcription level displays a salt-sensitive phenotype. PATL1 partially associates with the plasma membrane (PM) and endosomal system, and might participate in regulating membrane trafficking. Interestingly, PATL1 interacts with SOS1, a PM Na+/H+ antiporter in the Salt-Overly-Sensitive (SOS) pathway, and the PM Na+/H+ antiport activity is lower in patl1 than in Col-0. Furthermore, the reactive oxygen species (ROS) content is higher in patl1 and the redox signaling of antioxidants is partially disrupted in patl1 under salt stress conditions. Artificial elimination of ROS could partially rescue the salt-sensitive phenotype of patl1. Taken together, our results indicate that PATL1 participates in plant salt tolerance by regulating Na+ transport at least in part via SOS1, and by modulating cellular redox homeostasis during salt stress.

Chrysanthemum DgWRKY2 Gene Enhances Tolerance to Salt Stress in Transgenic Chrysanthemum

WRKY transcription factors (TFs) play a vital part in coping with different stresses. In this study, DgWRKY2 was isolated from Dendranthema grandiflorum. The gene encodes a 325 amino acid protein, belonging to the group II WRKY family, and contains one typical WRKY domain (WRKYGQK) and a zinc finger motif (C-X4-5-C-X22-23-H-X1-H). Overexpression of DgWRKY2 in chrysanthemum enhanced tolerance to high-salt stress compared to the wild type (WT). In addition, the activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT)), proline content, soluble sugar content, soluble protein content, and chlorophyll content of transgenic chrysanthemum, as well as the survival rate of the transgenic lines, were on average higher than that of the WT. On the contrary, hydrogen peroxide (H₂O₂), superoxide anion (O₂-), and malondialdehyde (MDA) accumulation decreased compared to WT. Expression of the stress-related genes DgCAT, DgAPX, DgZnSOD, DgP5CS, DgDREB1A, and DgDREB2A was increased in the DgWRKY2 transgenic chrysanthemum compared with their expression in the WT. In conclusion, our results indicate that DgWRKY2 confers salt tolerance to transgenic chrysanthemum by enhancing antioxidant and osmotic adjustment. Therefore, this study suggests that DgWRKY2 could be used as a reserve gene for salt-tolerant plant breeding.

Calcium Signaling during Salt Stress and in the Regulation of Ion Homeostasis

Soil composition largely defines the living conditions of plants and represents one of their most relevant, dynamic and complex environmental cues. The effective concentrations of many either tolerated or essential ions and compounds in the soil usually differ from the optimum that would be most suitable for plants. In this regard, salinity - caused by excess of NaCl - represents a widespread adverse growth condition but also shortage of ions like K+, NO3- and Fe2+ restrains plant growth. During the past years many components and mechanisms that function in the sensing and establishment of ion homeostasis have been identified and characterized. Here, we reflect on recent insights that extended our understanding of components and mechanisms, which govern and fine-tune plant salt stress tolerance and ion homeostasis. We put special emphasis on mechanisms that allow for interconnection of the salt overly sensitivity pathway with plant development and discuss newly emerging functions of Ca2+ signaling in salinity tolerance. Moreover, we review and discuss accumulating evidence for a central and unifying role of Ca2+ signaling and Ca2+ dependent protein phosphorylation in regulating sensing, uptake, transport and storage processes of various ions. Finally, based on this cross-field inventory, we deduce emerging concepts and arising questions for future research.

Molecular response of canola to salt stress: insights on tolerance mechanisms

Canola (Brassica napus L.) is widely cultivated around the world for the production of edible oils and biodiesel fuel. Despite many canola varieties being described as ‘salt-tolerant’, plant yield and growth decline drastically with increasing salinity. Although many studies have resulted in better understanding of the many important salt-response mechanisms that control salt signaling in plants, detoxification of ions, and synthesis of protective metabolites, the engineering of salt-tolerant crops has only progressed slowly. Genetic engineering has been considered as an efficient method for improving the salt tolerance of canola but there are many unknown or little-known aspects regarding canola response to salinity stress at the cellular and molecular level. In order to develop highly salt-tolerant canola, it is essential to improve knowledge of the salt-tolerance mechanisms, especially the key components of the plant salt-response network. In this review, we focus on studies of the molecular response of canola to salinity to unravel the different pieces of the salt response puzzle. The paper includes a comprehensive review of the latest studies, particularly of proteomic and transcriptomic analysis, including the most recently identified canola tolerance components under salt stress, and suggests what researchers should focus on in future studies.

Proteomic and physiological analyses reveal the role of exogenous spermidine on cucumber roots in response to Ca(NO3)2 stress

The mechanism of exogenous Spd-induced Ca(NO₃)₂ stress tolerance in cucumber was studied by proteomics and physiological analyses. Protein-protein interaction network revealed 13 key proteins involved in Spd-induced Ca(NO₃)₂ stress resistance. Ca(NO₃)₂ stress is one of the major reasons for secondary salinization that limits cucumber plant development in greenhouse. The conferred protective role of exogenous Spd on cucumber in response to Ca(NO₃)₂ stress cues involves changes at the cellular and physiological levels. To investigate the molecular foundation of exogenous Spd in Ca(NO₃)₂ stress tolerance, a proteomic approach was performed in our work. After a 9 days period of Ca(NO₃)₂ stress and/or exogenous Spd, 71 differential protein spots were confidently identified. The resulting proteins were enriched in seven different categories of biological processes, including protein metabolism, carbohydrate and energy metabolism, ROS homeostasis and stress defense, cell wall related, transcription, others and unknown. Protein metabolism (31.2%), carbohydrate and energy metabolism (15.6%), ROS homeostasis and stress defense (32.5%) were the three largest functional categories in cucumber root and most of them were significantly increased by exogenous Spd. The Spd-responsive protein interaction network revealed 13 key proteins, whose accumulation changes could be critical for Spd-induced resistance; all 13 proteins were upregulated by Spd at transcriptional and protein levels in response to Ca(NO₃)₂ stress. Furthermore, accumulation of antioxidant enzymes, non-enzymatic antioxidant and polyamines, along with reduction of H₂O₂ and MDA, were detected after exogenous Spd application during Ca(NO₃)₂ stress. The results of these proteomic and physiological analyses in cucumber root may facilitate a better understanding of the underlying mechanism of Ca(NO₃)₂ stress tolerance mediated by exogenous Spd.

Genome-wide analysis and expression profiling of PP2C clade D under saline and alkali stresses in wild soybean and Arabidopsis

Protein phosphatase 2Cs (PP2Cs) belong to the largest protein phosphatase family in plants. Some members have been described as being negative modulators of plant growth and development, as well as responses to hormones and environmental stimuli. However, little is known about the members of PP2C clade D, which may be involved in the regulation of signaling pathways, especially in response to saline and alkali stresses. Here, we identified 13 PP2C orthologs from the wild soybean (Glycine soja) genome. We examined the sequence characteristics, chromosome locations and duplications, gene structures, and promoter cis-elements of the PP2C clade D genes in Arabidopsis and wild soybean. Our results showed that GsPP2C clade D (GsAPD) genes exhibit more gene duplications than AtPP2C clade D genes. Plant hormone and abiotic stress-responsive elements were identified in the promoter regions of most PP2C genes. Moreover, we investigated their expression patterns in roots, stems, and leaves. Quantitative real-time PCR analyses revealed that the expression levels of representative GsPP2C and AtPP2C clade D genes were significantly influenced by alkali and salt stresses, suggesting that these genes might be associated with or directly involved in the relevant stress signaling pathways. Our results established a foundation for further functional characterization of PP2C clade D genes in the future.

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.

Improving Salt Tolerance of Chickpea Using Modern Genomics Tools and Molecular Breeding

INTRODUCTION

The high protein value, essential minerals, dietary fibre and notable ability to fix atmospheric nitrogen make chickpea a highly remunerative crop, particularly in low-input food production systems. Of the variety of constraints challenging chickpea productivity worldwide, salinity remains of prime concern owing to the intrinsic sensitivity of the crop. In view of the projected expansion of chickpea into arable and salt-stressed land by 2050, increasing attention is being placed on improving the salt tolerance of this crop. Considerable effort is currently underway to address salinity stress and substantial breeding progress is being made despite the seemingly highly-complex and environment-dependent nature of the tolerance trait.

CONCLUSION

This review aims to provide a holistic view of recent advances in breeding chickpea for salt tolerance. Initially, we focus on the identification of novel genetic resources for salt tolerance via extensive germplasm screening. We then expand on the use of genome-wide and cost-effective techniques to gain new insights into the genetic control of salt tolerance, including the responsive genes/QTL(s), gene(s) networks/cross talk and intricate signalling cascades.

Transcriptome dynamics provide insights into long-term salinity stress tolerance in Triticum aestivum cv. Kharchia Local

Kharchia Local, a wheat (Triticum aestivum) cultivar, is native to the saline-sodic soils of Pali district, Rajasthan, India and well known for its salinity stress tolerance. In the present study, we performed transcriptome sequencing to compare genome wide differential expression pattern between flag leaves of salinity stressed (15 EC) and control plants at anthesis stage. The 63.9 million paired end raw reads were assembled into 74,106 unigenes, of which, 3197 unigenes were found to be differentially expressed. Functional annotation analysis revealed the upregulation of genes associated with various biological processes including signal transduction, phytohormones signaling, osmoregulation, flavonoid biosynthesis, ion transport and ROS homeostasis. Expression pattern of fourteen differentially expressed genes was validated using qRT-PCR and was found to be consistent with the results of the transcriptome sequencing. Present study is the primary report on transcriptome profiling of Kharchia Local flag leaf under long-term salinity stress at anthesis stage. In conclusion, the data generated in this study can improve our knowledge in understanding the molecular mechanism of salinity stress tolerance. It will also serve as a valuable genomic resource in wheat breeding programs.

Clusia hilariana and Eugenia uniflora as bioindicators of atmospheric pollutants emitted by an iron pelletizing factory in Brazil

The objectives of this work were to evaluate if the pollution emitted by the pelletizing factory causes visual symptoms and/or anatomical changes in exposed Eugenia uniflora and Clusia hilariana, in active biomonitoring, at different distances from a pelletizing factory. We characterize the symptomatology, anatomical, and histochemistry alterations induced in the two species. There was no difference in the symptomatology in relation to the different distances of the emitting source. The foliar symptoms found in C. hilariana were chlorosis, necrosis, and foliar abscission and, in E. uniflora, were observed necrosis punctuais, purple spots in the leaves, and increase in the emission of new leaves completely purplish. The two species presented formation of a cicatrization tissue. E. uniflora presented reduction in the thickness of leaf. In C. hilariana, it was visualized hyperplasia of the cells and the adaxial epidermis did not appear collapsed due to thick cuticle and cuticular flanges. Leaves of C. hilariana showed positive staining for iron, protein, starch, and phenolic compounds. E. uniflora showed positive staining for total phenolic compounds and starch. Micromorphologically, there was accumulation of particulate matter on the leaf surface, obstruction of the stomata, and scaling of the epicuticular wax in both species. It was concluded that the visual and anatomical symptoms were efficient in the diagnosis of the stress factor. C. hilariana and E. uniflora showed to be good bioindicators of the atmospheric pollutants emitted by the pelletizing factory.

Differential Antioxidative Responses to Environmental Constraints in Shoots and Roots of Wild Legumes

The current study aimed to explore the antioxidant system of five legumes inhabiting regions with different conditions. In these legumes, H2O2 generation and lipid peroxidation enhanced in roots of plants inhabiting the Mediterranean region (MR) and Sinai (S) where high soluble salts and low water content in the soil were estimated. High levels of phenols and ascorbic acid were detected in shoots of these plants compared with those inhabiting the Nile region (NR) or Oases (O), which characterized by low soluble salts and high water content. There were great variations between species in their responses to adverse conditions, and enhanced activities of antioxidant enzymes were recorded in plants inhabiting the more stressful habitats. Roots and shoots of legumes responded differentially to oxidative stresses regarding the induction of enhanced or suppressed activities of a definite antioxidative enzym. While CAT activity increased in shoots, GP activity greatly stimulated in roots of legumes at different habitats. The activity of APX decreased in roots but increased in shoots by the harsh conditions of habitate showing minimum and maximum activities in roots and shoots, respectively, in plants inhabiting S. The activity of CAT and APX increased in shoots by increasing the concentration of H2O2, while the over expression of GP gene in roots enhanced scavenging H2O2 to a level between 6% to 37% of its concentration in shoots. Genes expression of the antioxidant enzymes (CAT, GP and APX) more regulated, especially in shoots, by the environmental constraints than the differences between species. 

The physiological and metabolic changes in sugar beet seedlings under different levels of salt stress

Salinity stress is a major limitation to global crop production. Sugar beet, one of the world's leading sugar crops, has stronger salt tolerant characteristics than other crops. To investigate the response to different levels of salt stress, sugar beet was grown hydroponically under 3 (control), 70, 140, 210 and 280 mM NaCl conditions. We found no differences in dry weight of the aerial part and leaf area between 70 mM NaCl and control conditions, although dry weight of the root and whole plant treated with 70 mM NaCl was lower than control seedlings. As salt concentrations increased, degree of growth arrest became obvious In addition, under salt stress, the highest concentrations of Na⁺ and Cl^- were detected in the tissue of petioles and old leaves. N and K contents in the tissue of leave, petiole and root decreased rapidly with the increase of NaCl concentrations. P content showed an increasing pattern in these tissues. The activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione peroxidase showed increasing patterns with increase in salt concentrations. Moreover, osmoprotectants such as free amino acids and betaine increased in concentration as the external salinity increased. Two organic acids (malate and citrate) involved in tricarboxylic acid (TCA)-cycle exhibited increasing contents under salt stress. Lastly, we found that Rubisco activity was inhibited under salt stress. The activity of NADP-malic enzyme, NADP-malate dehydrogenase and phosphoenolpyruvate carboxylase showed a trend that first increased and then decreased. Their activities were highest with salinity at 140 mM NaCl. Our study has contributed to the understanding of the sugar beet physiological and metabolic response mechanisms under different degrees of salt stress.

Overexpression of annexin gene AnnSp2, enhances drought and salt tolerance through modulation of ABA synthesis and scavenging ROS in tomato

Drought and high salinity are two major abiotic stresses that significantly affect agricultural crop productivity worldwide. Annexins are a multigene family that plays an essential role in plant stress responses and various cellular processes. Here, the AnnSp2 gene was cloned from drought-resistant wild tomato (Solanum pennellii) and functionally characterized in cultivated tomato. AnnSp2 protein was localized in the nucleus and had higher expression in leave, flower and fruit. It was induced by several phytohormones and some abiotic stresses. Tomato plants overexpressing AnnSp2 had increased tolerance to drought and salt stress, as determined by analysis of various physiological parameters. AnnSp2-transgenic plants were less sensitive to ABA during the seed germination and seedling stages. However, under drought stress, the ABA content significantly increased in the AnnSp2-overexpressing plants, inducing stomatal closure and reducing water loss, which underlay the plants’ enhanced stress tolerance. Furthermore, scavenging reactive oxygen species (ROS), higher total chlorophyll content, lower lipid peroxidation levels, increased peroxidase activities (including APX, CAT and SOD) and higher levels of proline were observed in AnnSp2-overexpressing plants. These results indicate that overexpression of AnnSp2 in transgenic tomato improves salt and drought tolerance through ABA synthesis and the elimination of ROS.

Transcriptomics analysis of salt stress tolerance in the roots of the mangrove Avicennia officinalis

Salinity affects growth and development of plants, but mangroves exhibit exceptional salt tolerance. With direct exposure to salinity, mangrove roots possess specific adaptations to tolerate salt stress. Therefore, studying the early effects of salt on mangrove roots can help us better understand the tolerance mechanisms. Using two-month-old greenhouse-grown seedlings of the mangrove tree Avicennia officinalis subjected to NaCl treatment, we profiled gene expression changes in the roots by RNA-sequencing. Of the 6547 genes that were differentially regulated in response to salt treatment, 1404 and 5213 genes were significantly up- and down-regulated, respectively. By comparative genomics, 93 key salt tolerance-related genes were identified of which 47 were up-regulated. Upon placing all the differentially expressed genes (DEG) in known signaling pathways, it was evident that most of the DEGs involved in ethylene and auxin signaling were up-regulated while those involved in ABA signaling were down-regulated. These results imply that ABA-independent signaling pathways also play a major role in salt tolerance of A. officinalis. Further, ethylene response factors (ERFs) were abundantly expressed upon salt treatment and the Arabidopsis mutant aterf115, a homolog of AoERF114 is characterized. Overall, our results would help in understanding the possible molecular mechanism underlying salt tolerance in plants.

Metabolomics analysis of rice responses to salinity stress revealed elevation of serotonin, and gentisic acid levels in leaves of tolerant varieties

A GC-MS based analytical approach was undertaken to understand the metabolomic responses of seedlings of 2 salt sensitive (Sujala and MTU 7029) and 2 tolerant varieties (Bhutnath, and Nonabokra) of indica rice (Oryza sativa L.) to NaCl induced stress. The 4 varieties responded differently to NaCl treatment with respect to the conserved primary metabolites (sugars, polyols, amino acids, organic acids and certain purine derivatives) of the leaf of rice seedlings. However, there were significant differences in salt induced production of chorismic acid derivatives. Serotonin level was increased in both the salt tolerant varieties in response to NaCl induced stress. In both the salt tolerant varieties, increased production of the signaling molecule gentisic acid in response to NaCl treatment was noticed. Salt tolerant varieties also produced increased level of ferulic acid and vanillic acid. In the salt sensitive varieties, cinnamic acid derivatives, 4-hydroxycinnamic acid (in Sujala) and 4-hydroxybenzoic acid (in MTU 7029), were elevated in the leaves. So increased production of the 2 signaling molecules serotonin and gentisic acid may be considered as 2 important biomarker compounds produced in tolerant varieties contributing toward NaCl tolerance.

Populus simonii × Populus nigra WRKY70 is involved in salt stress and leaf blight disease responses

WRKY transcription factors (TFs) are important regulators in the complex stress response signaling networks in plants, but the detailed mechanisms underlying these regulatory networks have not been fully characterized. In the present study, we identified a Group III WRKY gene (PsnWRKY70, Potri.016G137900) from Populussimonii × Populusnigra and explored its function under salt and pathogen stresses. The promoter sequence that is located 2471-bp upstream from the start codon (SC) of PsnWRKY70 contained many stress-responsive cis-elements. Yeast one-hybrid assay suggested the upstream regulators, PsnWRKY70, PsnNAM (Potri.009G141600), PsnMYB (Potri.006G000800) and PsnGT1 (Potri.010G055000), probably modulate the expression of the PsnWRKY70 gene by specifically binding to the W-box or GT1GMSCAM4 (GT1) element. Yeast two-hybrid assay and transcriptome analysis revealed that HP1 (Potri.004G092100), RRM (Potri.008G146700), Ulp1 (Potri.002G105700) and some mitogen-activated protein kinase cascade members probably interact with PsnWRKY70 TF to response to salt stress. Compared with non-transgenic (NT) plants, PsnWRKY70-overexpressing (OEX) plants exhibited improved leaf blight disease resistance, while PsnWRKY70-repressing (REX) plants displayed enhanced salt stress tolerance. PsnWRKY70, PsnNAM, PsnMYB and PsnGT1 exhibited similar expression patterns in NT under salt and leaf blight disease stresses. The differentially expressed genes (DEGs) from NT vs OEX1 and the DEGs from NT vs REX1 exhibited considerable diversification. Most of the DEGs between NT and OEX1 were involved in aromatic amino acid biosynthesis, secondary metabolism, programmed cell death, peroxisomes and disease resistance. Most of the DEGs between NT and REX1 were related to desiccation response, urea transmembrane transport, abscisic acid response, calcium ion transport and hydrogen peroxide transmembrane transport. Our findings not only revealed the salt stress response signal transduction pathway of PsnWRKY70, but also provided direct evidence for the opposite biological functions of PsnWRKY70 TF in response to salt stress and leaf blight disease in P. simonii × P. nigra.

Discerning morpho-anatomical, physiological and molecular multiformity in cultivated and wild genotypes of lentil with reconciliation to salinity stress

One hundred and sixty two genotypes of different Lens species were screened for salinity tolerance in hydroponics at 40, 80 and 120 mM sodium chloride (NaCl) for 30 d. The germination, seedling growth, biomass accumulation, seedling survivability, salinity scores, root and shoot anatomy, sodium ion (Na⁺), chloride ion (Cl^-) and potassium ion (K⁺) concentrations, proline and antioxidant activities were measured to evaluate the performance of all the genotypes. The results were compared in respect of physiological (Na⁺, K⁺ and Cl^-) and seed yield components obtained from field trials for salinity stress conducted during two years. Expression of salt tolerance in hydroponics was found to be reliable indicator for similarity in salt tolerance between genotypes and was evident in saline soil based comparisons. Impressive genotypic variation for salinity tolerance was observed among the genotypes screened under hydroponic and saline field conditions. Plant concentrations of Na⁺ and Cl^- at 120 mM NaCl were found significantly correlated with germination, root and shoot length, fresh and dry weight of roots and shoots, seedling survivability, salinity scores and K⁺ under controlled conditions and ranked the genotypes along with their seed yield in the field. Root and shoot anatomy of tolerant line (PDL-1) and wild accession (ILWL-137) showed restricted uptake of Na⁺ and Cl^- due to thick layer of their epidermis and endodermis as compared to sensitive cultigen (L-4076). All the genotypes were scanned using SSR markers for genetic diversity, which generated high polymorphism. On the basis of cluster analysis and population structure the contrasting genotypes were grouped into different classes. These markers may further be tested to explore their potential in marker-assisted selection.