Comparative physiology of salt and water stress

Exogenous Application of Zinc to Mitigate the Salt Stress in Vigna radiata (L.) Wilczek-Evaluation of Physiological and Biochemical Processes

Salt stress adversely affects the growth and productivity of crops. However, reports suggest that the application of various micronutrients could help the plant to cope with this stress. Hence, the objective of the study was to examine the effect of exogenous application of Zinc (Zn) on salt tolerance in Vigna radiata (L.) Wilczek (mungbean). Mungbean is considered to be an economically important crop and possess a strategic position in Southeast Asian countries for sustainable crop production. It is rich in quality proteins, minerals and vitamins. Three weeks old grown seedlings were subjected to NaCl (150 mM and 200 mM) alone or with Zn (250 µM). After 21 days of treatment, plants were harvested for investigating morphological, physiological and biochemical changes. We found that the Zn application mitigates the negative effect upon plant growth to a variable extent. This may be attributed to the increased shoot and root length, improved chlorophyll and carotenoid contents, enhanced total soluble sugar (TSS), total soluble protein (TSP) and proline accumulation, decreased H2O2 content and increased enzymatic antioxidant activities. Zn's application improved the performance of the enzymes such as phenylalanine ammonia-lyase (PAL) and tyrosine ammonia-lyase (TAL) of the secondary metabolism, which resulted in the improvement of total phenol and flavonoids. The antioxidant activities such as 1,1diphenyl 2-picryl hydrazine (DPPH) and ferrous reducing antioxidant power assay (FRAP) of the plants also showed improved results in their salt only treatments. Furthermore, hydrogen peroxide (H2O2) and superoxide radical (SOD) scavenging activity were also improved upon the application of 250 µM zinc. Thus, Zn application in low doses offers promising potential for recovering plants suffering from salinity stress. In conclusion, we assume that zinc application improved salt tolerance in mungbean through the improvement of various physiological and photochemical processes which could prove to be useful in nutrient mediated management for crop improvement.

Effect of Salt Stress on Growth and Metabolite Profiles of Cape Gooseberry (Physalis peruviana L.) along Three Growth Stages

Colombia is the main producer of cape gooseberry (Physalis peruviana L.), a plant known for its various consumption practices and medicinal properties. This plant is generally grown in eroded soils and is considered moderately tolerant to unfavorable conditions, such as nutrient-poor soils or high salt concentrations. Most studies conducted on this plant focus on fruit production and composition because it is the target product, but a small number of studies have been conducted to describe the effect of abiotic stress, e.g., salt stress, on growth and biochemical responses. In order to better understand the mechanism of inherent tolerance of this plant facing salt stress, the present study was conducted to determine the metabolic and growth differences of P. peruviana plants at three different BBCH-based growth substages, varying salt conditions. Hence, plants were independently treated with two NaCl solutions, and growth parameters and LC-ESI-MS-derived semi-quantitative levels of metabolites were then measured and compared between salt treatments per growth substage. A 90 mM NaCl treatment caused the greatest effect on plants, provoking low growth and particular metabolite variations. The treatment discrimination-driving feature classification suggested that glycosylated flavonols increased under 30 mM NaCl at 209 substages, withanolides decreased under 90 mM NaCl at 603 and 703 substages, and up-regulation of a free flavonol at all selected stages can be considered a salt stress response. Findings locate such response into a metabolic context and afford some insights into the plant response associated with antioxidant compound up-regulation.

Day and Night Fluctuations in GABA Biosynthesis Contribute to Drought Responses in Nicotiana tabacum L

When synchronized with the light/dark cycle the circadian rhythm is termed a diurnal rhythm and this organizes an organism's daily life cycle in relation to the metabolic shifts during the day/night cycles. This is a complex task, particularly under stress conditions. Accurate maintenance of the diurnal rhythm becomes an issue under environmental extremes, such as drought due to the impairment of metabolism, redox balance, and structural integrity. In plants, the non-proteinogenic amino acid GABA accumulates to high levels in response to several stress factors but this is not always dependent on the activation of its biosynthesis. Here we propose a regulatory role to GABA during the diurnal rhythm in plants which is similar to its function in animals where it adjusts the circadian rhythm. Here we investigated whether GABA-biosynthesis was affected by drought stress during the diurnal cycle. For this, we took samples from leaves of N. tabacum plants subjected to PEG-mediated drought stress (-0.73 MPa) during the day and night cycle during a 24 hour period. Glutamate, GABA, and proline contents, along with GDH, GAD enzyme activities and transcript profiles were analyzed. Overall, we conclude that the oscillations in GABA biosynthesis during day and night cycle have an impact on drought stress responses which needs to be elucidated by further analysis.

Endophytic Bacterial Isolates From Halophytes Demonstrate Phytopathogen Biocontrol and Plant Growth Promotion Under High Salinity

Halophytic endophytes potentially contribute to the host's adaptation to adverse environments, improving its tolerance against various biotic and abiotic stresses. Here, we identified the culturable endophytic bacteria of three crop wild relative (CWR) halophytes: Cakile maritima, Matthiola tricuspidata, and Crithmum maritimum. In the present study, the potential of these isolates to improve crop adaptations to various stresses was investigated, using both in vitro and in-planta approaches. Endophytic isolates were identified by their 16S rRNA gene sequence and evaluated for their ability to: grow in vitro in high levels of NaCl; inhibit the growth of the economically important phytopathogens Verticillium dahliae, Ralstonia solanacearum, and Clavibacter michiganensis and the human pathogen Aspergillus fumigatus; provide salt tolerance in-planta; and provide growth promoting effect in-planta. Genomes of selected isolates were sequenced. In total, 115 endophytic isolates were identified. At least 16 isolates demonstrated growth under increased salinity, plant growth promotion and phytopathogen antagonistic activity. Three showed in-planta suppression of Verticillium growth. Furthermore, representatives of three novel species were identified: two Pseudomonas species and one Arthrobacter. This study provides proof-of-concept that the endophytes from CWR halophytes can be used as "bio-inoculants," for the enhancement of growth and stress tolerance in crops, including the high-salinity stress.

Sequential Antioxidants Foliar Application Can Alleviate Negative Consequences of Salinity Stress in Vicia faba L

Salinity is one of the most limiting abiotic stresses in agricultural productivity. Exogenously applied antioxidants successfully enabled salt-stressed plants to cope with stress. Two-season field experiments were conducted consecutively in 2016/17 and 2017/18 to study the effects of foliar applications of singular (ascorbate, AsA; proline, Pro; and glutathione, GSH) or sequential (AsA-Pro-GSH and GSH-Pro-AsA) antioxidants on growth, yield, physio-biochemical attributes, and enzymatic and non-enzymatic antioxidative defense system of Vicia faba L. (CV. Sakha-1) plants grown under saline soil conditions (EC = 4.53 dS m⁻¹). Under soil salinity conditions, AsA, Pro, or GSH-Pro-ASA improved growth and productivity, photosynthesis efficiency, stomatal conductance (gs), plant water status, as well as enzymatic and non-enzymatic antioxidants. However, sequential AsA-Pro-GSH foliar application followed by singular GSH significantly exceeded all other treatments (i.e., AsA, Pro, and GSH-Pro-AsA), improving growth characteristics (shoot length, shoot fresh and dry weights, and leaves area), photosynthesis efficiency, stomatal conductance, plant water status, and yield and its components (green pods weight/plant⁻¹, green pods yield/hectare⁻¹, and seed yield/hectare⁻¹), as well as enzymatic (ascorbate peroxidase, catalase, superoxide dismutase, and glutathione reductase) and non-enzymatic (AsA, GSH, Pro, phenolic aglycone, phenolic glycosides) antioxidants compared to control. Overall, our results clearly demonstrate that sequential AsA-Pro-GSH foliar application has a positive effect on salt-stressed Vicia faba plants.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Different tolerance to salinity of two populations of Oenothera drummondii with contrasted biogeographical origin

Oenothera drummondii is a native species from the coastal dunes of the Gulf of Mexico that has nowadays extended to coastal areas in temperate zones all over the world, its invasion becoming a significant problem locally. The species grows on back beach and incipient dunes, where it can suffer flooding by seawater, and sea spray. We were interested in knowing how salinity affects this species and if invasive populations present morphological or functional traits that would provide greater tolerance to salinity than native ones. To this end, we conducted a greenhouse experiment where plants from one native and from one invading population were irrigated with five salinity treatments. We measured functional traits on photosynthetic, photochemical efficiency, water content, flowering, Na⁺ content, pigment content, and biomass. Although O. drummondii showed high resistance to salinity, the highest levels recorded high mortality, especially in the invasive population. Plants exhibited differences not only in response to time under salinity conditions, but also according to their biogeographic origin, the native population being more resistant to long exposure and high salt concentration than the invasive one. Native and invasive populations showed different response to salt stress in photosynthesis and transpiration rates, stomatal conductance, water use efficiency, carboxylation efficiency, electron transport rate, electron transport efficiency, energy used in photochemistry, among others. The increasing salinity levels resulted in a progressive reduction of photosynthesis rate due to both stomatal and biochemical limitations, and also in a reduction of biomass and number and size of flowers, compromising the reproductive capacity.

The apple palmitoyltransferase MdPAT16 influences sugar content and salt tolerance via an MdCBL1-MdCIPK13-MdSUT2.2 pathway

Protein S-acyltransferases (PATs) are a category of eukaryotic transmembrane proteins that mediate the S-acylation of their target proteins. S-acylation, commonly known as palmitoylation, is a reversible protein modification that regulates the membrane association and function of target proteins. However, the functions and mechanisms of PATs in apple (Malus domestica) remain poorly understood. In this study, an MdPAT family member, MdPAT16, was identified and shown to have palmitoyltransferase activity. We demonstrated that this gene responds to salt stress and that its expression improves plant salt stress resistance. In addition, its overexpression significantly promotes the accumulation of soluble sugars. The same phenotypes were observed in transgenic tissue culture seedlings, transgenic roots, and Arabidopsis thaliana that ectopically expressed MdPAT16. MdPAT16 was shown to interact with MdCBL1 and stabilize MdCBL1 protein levels through palmitoylation. The N-terminal sequence of MdCBL1 contains a palmitoylation site, and its N-terminal deletion led to changes in MdCBL1 protein stability and subcellular localization. The phenotypes of MdCBL1 transgenic roots and transiently injected apple fruits were fully consistent with the sugar accumulation phenotype of MdPAT16. Mutation of the palmitoylation site interfered with this phenotype. These findings suggest that MdPAT16 palmitoylates its downstream target proteins, improving their stability. This may be a missing link in the plant salt stress response pathway and have an important impact on fruit quality.

Silicon biology in crops under abiotic stress: A paradigm shift and cross-talk between genomics and proteomics

Silicon is a beneficial element to improve the biological process, growth, development, and crop productivity. The review mainly focuses on the advantage of crops supplemented with silicon, how Si alleviate abiotic stress as well as regulate the genes and proteins involved in metabolic and biological functions in plants. Abiotic stress causes damage to the proteins, nucleic acids, affect transpiration rate, stomatal conductance, alter the nutrient balance, and cell desiccation which could reduce the growth and development of the plants. To overcome from this problem researchers, focus on beneficial element like silicon to protect the plants against various abiotic stresses. The previous review reports are based on the application of silicon on salinity and drought stress, plant defense mechanism, the elevation of plant metabolism, enhancement of the biochemical and physiological properties, regulation of secondary metabolites and plant hormone. Here, we discuss about the silicon uptake and accumulation in plants, and silicon regulates the reactive oxygen species under abiotic stress, further we mainly focus on the genes and proteins which play a vital role in plants with silicon supplementation. The study can help the researchers to focus further on plants to improve the advancement in them under abiotic stress.

Exponential decay: an approach to model nutrient uptake rates of macrophytes

One of the challenges of integrating phytoremediation into a waste treatment system is the sensitivity of plant species to fluctuations in environmental conditions and the difficulty in estimating subsequent changes to their rates of uptake. In this study, we examine a method using the exponential decay equation to approximate the median uptake rate (MUR) of nutrients for three aquatic macrophyte species, Salvinia molesta, Spirodela polyrhiza, and Lemna minor. These MUR values were then used to directly evaluate the phytoremediation performance between species and at varying levels of salinity stress. The results of this study indicate that an exponential decay relationship produced the most accurate models of the nutrient uptake profile for each species, with highest correlation values in 74.1% of tests for the three species at increasing salinity over a period of 14 d. S. polyrhiza and L. minor began to show significant reductions in nutrient uptake and growth at salinity concentration above 10 g/L. Using MUR, direct comparisons can be made between species in a time and mass-independent manner, allowing for the rapid assessment of phytoremediation performance under conditions of increasing salinity stress. Novelty statementIn this study, we propose the use of an exponential decay model and the use of median uptake rate (MUR) obtained from the model coefficients as a method for directly comparing species performance under different conditions. Subsequently, we show how the use of MUR values obtained from three species of aquatic macrophytes allows for the direct comparison of species performance under increasing salinity stress. The method proposed in this study would improve the ability for easy comparison between species performance under varying environmental conditions. Future works could further build on the parameters proposed in this study and optimize the performance of phytoremediation systems developed for nutrient-affected wastewater management. This study is especially beneficial to phytoremediation researchers and environmental engineers who are implementing or designing macrophyte phytoremediation systems.

Wuxal amino (Bio stimulant) improved growth and physiological performance of tomato plants under salinity stress through adaptive mechanisms and antioxidant potential

In the present study, ameliorative capabilities of wuxal amino (bio stimulant) under salt stress has been investigated through adaptive mechanisms and antioxidant potential in tomato plants. In the experiment, two different concentrations (2 cm L^-1 and 3 cm L^-1) of wuxal amino through foliar application and soil irrigation were applied to the salt (150 mM) treated tomato plants and then morphological traits, photosynthetic pigments, osmolytes, secondary metabolites, oxidative stress and antioxidant enzymes activity were assessed at 60 days after planting. The results revealed that salt stress decreased the growth parameters, photosynthetic pigments, soluble sugars and soluble protein whereas, content of proline, ascorbic acid, total phenols, malondialdehyde, hydrogen peroxide and the activity of antioxidant enzymes activity increased under salt stress. Moreover, Wuxal amino application through foliar or soil to salt stressed plants improved morphological traits, photosynthetic pigments, osmolytes, total phenol and antioxidant enzymes activity. Interestingly, the deleterious impact of salinity on tomato plants were significantly reduced and it can be evident from reduced MDA and H₂O₂ levels. These responses varied with the mode (foliar or soil) of application of Wuxal amino under different concentrations (2 cm L^-1 and 3 cm L^-1). It was concluded that application of Wuxal amino (2 cm L^-1, foliar) and (3 cm L^-1; soil) proved best and could be commercially used as eco-friendly tool for the protection of tomato plants grown under salinity stress.

Transcriptome analysis of upland cotton revealed novel pathways to scavenge reactive oxygen species (ROS) responding to Na2SO4 tolerance

Salinity is an extensive and adverse environmental stress to crop plants across the globe, and a major abiotic constraint responsible for limited crop production threatening the crop security. Soil salinization is a widespread problem across the globe, threatening the crop production and food security. Salinity impairs plant growth and development via reduction in osmotic potential, cytotoxicity due to excessive uptake of ions such as sodium (Na⁺) and chloride (Cl^-), and nutritional imbalance. Cotton, being the most cultivated crop on saline-alkaline soils, it is of great importance to elucidate the mechanisms involved in Na₂SO₄ tolerance which is still lacking in upland cotton. Zhong 9835, a Na₂SO₄ resistant cultivar was screened for transcriptomic studies through various levels of Na₂SO₄ treatments, which results into identification of 3329 differentially expressed genes (DEGs) in roots, stems and leave at 300 mM Na₂SO₄ stress till 12 h in compared to control. According to gene functional annotation analysis, genes involved in reactive oxygen species (ROS) scavenging system including osmotic stress and ion toxicity were significantly up-regulated, especially GST (glutathione transferase). In addition, analysis for sulfur metabolism, results in to identification of two rate limiting enzymes [APR (Gh_D05G1637) and OASTL (Gh_A13G0863)] during synthesis of GSH from SO₄^2-. Furthermore, we also observed a crosstalk of the hormones and TFs (transcription factors) enriched in hormone signal transduction pathway. Genes related to IAA exceeds the rest of hormones followed by ubiquitin related genes which are greater than TFs. The analysis of the expression profiles of diverse tissues under Na₂SO₄ stress and identification of relevant key hub genes in a network crosstalk will provide a strong foundation and valuable clues for genetic improvements of cotton in response to various salt stresses.

Early Growth Stage Characterization and the Biochemical Responses for Salinity Stress in Tomato

Salinity is one of the most significant environmental stresses for sustainable crop production in major arable lands of the globe. Thus, we conducted experiments with 27 tomato genotypes to screen for salinity tolerance at seedling stage, which were treated with non-salinized (S1) control (18.2 mM NaCl) and salinized (S2) (200 mM NaCl) irrigation water. In all genotypes, the elevated salinity treatment contributed to a major depression in morphological and physiological characteristics; however, a smaller decrease was found in certain tolerant genotypes. Principal component analyses (PCA) and clustering with percentage reduction in growth parameters and different salt tolerance indices classified the tomato accessions into five key clusters. In particular, the tolerant genotypes were assembled into one cluster. The growth and tolerance indices PCA also showed the order of salt-tolerance of the studied genotypes, where Saniora was the most tolerant genotype and P.Guyu was the most susceptible genotype. To investigate the possible biochemical basis for salt stress tolerance, we further characterized six tomato genotypes with varying levels of salinity tolerance. A higher increase in proline content, and antioxidants activities were observed for the salt-tolerant genotypes in comparison to the susceptible genotypes. Salt-tolerant genotypes identified in this work herald a promising source in the tomato improvement program or for grafting as scions with improved salinity tolerance in tomato.

Assessment of Morpho-Physiological, Biochemical and Antioxidant Responses of Tomato Landraces to Salinity Stress

Understanding salt tolerance in tomato (Solanum lycopersicum L.) landraces will facilitate their use in genetic improvement. The study assessed the morpho-physiological variability of Hail tomato landraces in response to different salinity levels at seedling stages and recommended a tomato salt-tolerant landrace for future breeding programs. Three tomato landraces, Hail 548, Hail 747, and Hail 1072 were tested under three salinity levels: 75, 150, and 300 mM NaCl. Salinity stress reduced shoots' fresh and dry weight by 71% and 72%, and roots were 86.5% and 78.6%, respectively. There was 22% reduced chlorophyll content, carotene content by 18.6%, and anthocyanin by 41.1%. Proline content increased for stressed treatments. The 300 mM NaCl treatment recorded the most proline content increases (67.37 mg/g fresh weight), with a percent increase in proline reaching 61.67% in Hail 747. Superoxide dismutase (SOD) activity decreased by 65% in Hail 548, while it relatively increased in Hail 747 and Hail 1072 treated with 300 mM NaCl. Catalase (CAT) activity was enhanced by salt stress in Hail 548 and recorded 7.6%, increasing at 75 and 5.1% at 300 mM NaCl. It revealed a reduction in malondialdehyde (MDA) at the 300 mM NaCl concentration in both Hail 548 and Hail 1072 landraces. Increasing salt concentrations showed a reduction in transpiration rate of 70.55%, 7.13% in stomatal conductance, and 72.34% in photosynthetic rate. K+/Na+ ratios decreased from 56% for 75 mM NaCl to 85% for 300 mM NaCl treatments in all genotypes. The response to salt stress in landraces involved some modifications in morphology, physiology, and metabolism. The landrace Hail 548 may have better protection against salt stress and observed protection against reactive oxygen species (ROS) by increasing enzymatic "antioxidants" activity under salt stress.

Multifaceted intervention of Bacillus spp. against salinity stress and Fusarium wilt in tomato

AIM

This study aimed to screen halotolerant Bacillus strains able to promote growth and protect tomato plants against salt stress and Fusarium wilt (Fusarium oxysporum f. sp. lycopersici).

METHODS AND RESULTS

We evaluated some halotolerant strains of Bacillus spp. (Bacillus velezensis (AP-3) and Bacillus spp. (AP-6, AP-85 and AP-100)) to promote growth of tomato plants grown under salinity stress conditions and to protect them against Fusarium wilt disease. Such strains had been previously selected among 154 bacterial strains through biochemical tests (siderophores and indoleacetic acid productions, cellulase and catalase activity, nitrogen fixation and phosphate solubilization) in the presence of 100-mmol l^-1 NaCl. Besides the above-mentioned strains, B. subtilis QST-713 (Serenade^TM ) was also evaluated. Compared to control plants, aboveground dry weight increased in plants inoculated with AP-6, AP-85, AP-3, AP-100 and QST-713 strains developed in the absence of salt stress. The same tendency occurred for root dry weight; however, AP-3 strain was more effective, promoting an increase of 163%, when compared to control. Chlorophyll index and height increased >40 and 53%, respectively, for all Bacillus strains. Saline stress reduced plant growth regardless of the presence of Bacillus. Height, stem diameter, and aboveground and root dry weights increased in plants treated with Bacillus strains grown under saline conditions when compared to control. Bacillus velezensis AP-3 reduced the severity of Fusarium wilt in tomato by 50% when compared to control.

CONCLUSION

Halotolerant Bacillus strains controlled tomato Fusarium wilt, increased growth as well as tolerance to salt stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

We demonstrated the efficacy of halotolerant Bacillus strains to control Fusarium wilt and improve tomato growth. We also demonstrated that these Bacillus strains protect tomato plants against salt stress. Bacillus can be used in an eco-friendly way because they are considered Generally Recognized As Safe.

Application of Upstream Open Reading Frames (uORFs) Editing for the Development of Stress-Tolerant Crops

Global population growth and climate change are posing increasing challenges to the production of a stable crop supply using current agricultural practices. The generation of genetically modified (GM) crops has contributed to improving crop stress tolerance and productivity; however, many regulations are still in place that limit their commercialization. Recently, alternative biotechnology-based strategies, such as gene-edited (GE) crops, have been in the spotlight. Gene-editing technology, based on the clustered regularly interspaced short palindromic repeats (CRISPR) platform, has emerged as a revolutionary tool for targeted gene mutation, and has received attention as a game changer in the global biotechnology market. Here, we briefly introduce the concept of upstream open reading frames (uORFs) editing, which allows for control of the translation of downstream ORFs, and outline the potential for enhancing target gene expression by mutating uORFs. We discuss the current status of developing stress-tolerant crops, and discuss uORF targets associated with salt stress-responsive genes in rice that have already been verified by transgenic research. Finally, we overview the strategy for developing GE crops using uORF editing via the CRISPR-Cas9 system. A case is therefore made that the mutation of uORFs represents an efficient method for developing GE crops and an expansion of the scope of application of genome editing technology.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Differential responses of dwarf cashew clones to salinity are associated to osmotic adjustment mechanisms and enzymatic antioxidative defense

This study evaluate growth, gas exchange, solute accumulation and activity of antioxidant enzymes in dwarf cashew clones subjected to salinity. Shoot dry mass reduced 26.8% (CCP06) and 41.2% (BRS189) at 16 dS m-1, concerning control. For net photosynthesis, CCP06 and BRS189 presented 69.8% and 34.7% of reduction, respectively. Na+ and Cl- contents increased in leaves and roots, in both clones, although CCP06 leaves presented Na+ concentrations lower than those of BRS189, the first one was the clone that the most accumulated such toxic ion, whereas K+ content remained almost unchanged for both clones. Soluble N-amino was the organic solute that more varied with salinity in cashew seedlings. Salt stress increased the activity of superoxide dismutase in both clones, mainly 16 dS m-1 treatment. Additionally, salinity promoted increases in ascorbate and guaiacol peroxidase activities, and the last enzyme was the main involved in H2O2 removal. Despite the reductions in growth and gas exchange, dwarf cashew seedlings of both clones presented an osmotic adjustment mechanism, and an efficient enzymatic antioxidant system that were able to attenuate the salt and oxidative stress, respectively. Our research suggested that BRS189 clone is more tolerant to salt stress than CCP06.

HKT sodium and potassium transporters in Arabidopsis thaliana and related halophyte species

High salinity induces osmotic stress and often leads to sodium ion-specific toxicity, with inhibitory effects on physiological, biochemical and developmental pathways. To cope with increased Na⁺ in soil water, plants restrict influx, compartmentalize ions into vacuoles, export excess Na⁺ from the cell, and distribute ions between the aerial and root organs. In this review, we discuss our current understanding of how high-affinity K⁺ transporters (HKT) contribute to salinity tolerance, focusing on HKT1-like family members primarily involved in long-distance transport, and in the recent research in the model plant Arabidopsis and its halophytic counterparts of the Eutrema genus. Functional characterization of the salt overly sensitive (SOS) pathway and HKT1-type transporters in these species indicate that they utilize similar approaches to deal with salinity, regardless of their tolerance.

Aeroponic systems: A unique tool for estimating plant water relations and NO3 uptake in response to salinity stress

The study of transpiration, water, and nutrient uptake during abiotic stress in the root zone is hindered because of the hidden nature of the root zone. In this study, a modified aeroponic system was used to evaluate whole plant transpiration, nitrate and water uptake in the growth and development of tomato plants in response to salinity. Tomato seedlings were exposed to three levels of salinity (1.5, 4.5, and 9 dSm-1) and three levels of nitrate (1, 4, and 8 mM NO3) in a separate experiments conducted concurrently. Whole plant transpiration, water and nitrate uptake were estimated. Our study revealed that ~30 to 35 days after treatment (DAT), water uptake rate per plant increased from a common initial rate of about 0.05 to 1.1, 0.6, and 0.4 kg/day at 1.5, 4.5, and 9 dSm-1 respectively. The NO3 uptake rates in tomatoes grown in 1 and 4 mM NO3 were 5.5 and 22% respectively, of the uptake of tomatoes grown in 8 mM NO3. The estimation of nitrate uptake and lower sensitivity to salinity stress in the aeroponic showed the effectiveness and cost efficiency of the system in the cultivation of vegetables during abiotic stresses. The novelty of the system described is the continuous estimation of root and nutrient uptake by the whole plant at any given time.

Reassessing the role of ion homeostasis for improving salinity tolerance in crop plants

Soil salinity is a constraint for major agricultural crops leading to severe yield loss, which may increase with the changing climatic conditions. Disruption in the cellular ionic homeostasis is one of the primary responses induced by elevated sodium ions (Na⁺ ). Therefore, unraveling the mechanism of Na⁺ uptake and transport in plants along with the characterization of the candidate genes facilitating ion homeostasis is obligatory for enhancing salinity tolerance in crops. This review summarizes the current advances in understanding the ion homeostasis mechanism in crop plants, emphasizing the role of transporters involved in the regulation of cytosolic Na⁺ level along with the conservation of K⁺ /Na⁺ ratio. Furthermore, expression profiles of the candidate genes for ion homeostasis were also explored under various developmental stages and tissues of Oryza sativa based on the publicly available microarray data. The review also gives an up-to-date summary on the efforts to increase salinity tolerance in crops by manipulating selected stress-associated genes. Overall, this review gives a combined view on both the ionomic and molecular background of salt stress tolerance in plants.

Evaluating and Screening of Agro-Physiological Indices for Salinity Stress Tolerance in Wheat at the Seedling Stage

Soil salinity is a worldwide issue that affects wheat production. A comprehensive understanding of salt-tolerance mechanisms and the selection of reliable screening indices are crucial for breeding salt-tolerant wheat cultivars. In this study, 30 wheat genotypes (obtained from a rapid selection of 96 original varieties) were chosen to investigate the existing screening methods and clarify the salinity tolerance mechanisms in wheat. Ten-day-old seedlings were treated with 150 mM NaCl. Eighteen agronomic and physiological parameters were measured. The results indicated that the effects of salinity on the agronomic and physiological traits were significant. Salinity stress significantly decreased K⁺ content and K⁺/Na⁺ ratio in the whole plant, while the leaf K⁺/Na⁺ ratio was the strongest determinant of salinity tolerance and had a significantly positive correlation with salt tolerance. In contrast, salinity stress significantly increased Na⁺ concentration and relative gene expression (TaHKT1;5, TaSOS1, and TaAKT1-like). The Na⁺ transporter gene (TaHKT1;5) showed a significantly greater increase in expression than the K⁺ transporter gene (TaAKT1-like). We concluded that Na⁺ exclusion rather than K⁺ retention contributed to an optimal leaf K⁺/Na⁺ ratio. Furthermore, the present exploration revealed that, under salt stress, tolerant accessions had higher shoot water content, shoot dry weight and lower stomatal density, leaf sap osmolality, and a significantly negative correlation was observed between salt tolerance and stomatal density. This indicated that changes in stomata density may represent a fundamental mechanism by which a plant may optimize water productivity and maintain growth under saline conditions. Taken together, the leaf K⁺/Na⁺ ratio and stomatal density can be used as reliable screening indices for salt tolerance in wheat at the seedling stage.

Silicon in the Soil-Plant Continuum: Intricate Feedback Mechanisms within Ecosystems

Plants' ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon's roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

Leaf apoplastic alkalization promotes transcription of the ABA-synthesizing enzyme Vp14 and stomatal closure in Zea mays

The chloride component of NaCl salinity causes the leaf apoplast to transiently alkalinize. This transition in pH reduces stomatal aperture. However, whether this apoplastic pH (pHapo) transient initiates stomatal closure by interacting with other chloride stress-induced responses or whether the pH transient alone initiates stomatal closure is unknown. To clarify the problem, the transient alkalinization of the leaf apoplast was mimicked in intact maize (Zea mays L.) by infiltrating near-neutral pH buffers into the leaf apoplast. Effects of the pHapo transient could thus be investigated independently from other chloride stress-derived effects. Microscopy-based ratiometric live pHapo imaging was used to monitor pHapoin planta. LC-MS/MS and real-time quantitative reverse transcription-PCR leaf analyses showed that the artificially induced pHapo transient led to an increase in the concentrations of the stomata-regulating plant hormone abscisic acid (ABA) and in transcripts of the key ABA-synthesizing gene ZmVp14 in the leaf. Since stomatal aperture and stomatal conductance decreased according to pHapo, we conclude that the pHapo transient alone initiates stomatal closure. Therefore, the functionality does not depend on interactions with other compounds induced by chloride stress. Overall, our data indicate that the pH of the leaf apoplast links chloride salinity with the control of stomatal aperture via effects exerted on the transcription of ABA.

Quantitative Dissection of Salt Tolerance for Sustainable Wheat Production in Sodic Agro-Ecosystems through Farmers’ Participatory Approach: An Indian Experience

To explore the comparative effects of field sodicity (soil pH) and irrigation water residual alkalinity (RSCiw) on physiological and biochemical attributes of salt tolerance, and crop performance of two wheat varieties (KRL 210, HD 2967), a total of 308 on-farm trials were carried out in sodicity affected Ghaghar Basin of Haryana, India. Salt tolerant variety KRL 210 maintained relatively higher leaf relative water content (RWC; 1.9%), photosynthetic rate (Pn; 5.1%), stomatal conductance (gS; 6.6%), and transpiration (E; 4.1%) with lower membrane injury (MII; −8.5%), and better control on accumulation of free proline (P; −18.4%), Na+/K+ in shoot (NaK_S; −23.1%) and root (NaK_R; −18.7%) portion compared to traditional HD 2967. Altered physiological response suppressed important yield-related traits revealing repressive effects of sodicity stress on wheat yields; albeit to a lesser extent in KRL 210 with each gradual increase in soil pH (0.77–1.10 t ha−1) and RSCiw (0.29–0.33 t ha−1). HD 2967 significantly outyielded KRL 210 only at soil pH ≤ 8.2 and RSCiw ≤ 2.5 me L−1. By comparisons, substantial improvements in salt tolerance potential of KRL 210 with increasing sodicity stress compensated in attaining significantly higher yields as and when soil pH becomes >8.7 and RSCiw > 4 me L−1. Designing such variety-oriented threshold limits of sodicity tolerance in wheat will help address the challenge to enhance crop resilience, closing the yield gaps and improve rural livelihood under the existing or predicted levels of salt stress.

Genome-Wide Association Study Reveals Marker-Trait Associations for Early Vegetative Stage Salinity Tolerance in Rice

Rice germplasm is a rich resource for discovering genes associated with salt tolerance. In the current study, a set of 96 accessions were evaluated for seedling stage salinity tolerance and its component traits. Significant phenotypic variation was observed among the genotypes for all the measured traits and eleven accessions with high level of salt tolerance at seedling stage were identified. The germplasm set comprised of three sub-populations and genome-wide association study (GWAS) identified a total of 23 marker-trait associations (MTAs) for traits studied. These MTAs were located on rice chromosomes 1, 2, 5, 6, 7, 9, and 12 and explained the trait phenotypic variances ranging from 13.98 to 29.88 %. Twenty-one MTAs identified in this study were located either in or near the previously reported quantitative trait loci (QTLs), while two MTAs namely, qSDW2.1 and qSNC5 were novel. A total of 18 and 13 putative annotated candidate genes were identified in a genomic region spanning ~200 kb around the MTAs qSDW2.1 and qSNC5, respectively. Some of the important genes underlying the novel MTAs were OsFBA1,OsFBL7, and mTERF which are known to be associated with salinity tolerance in crops. These MTAs pave way for combining salinity tolerance with high yield in rice genotypes through molecular breeding.

Salinity Effects on Morpho-Physiological and Yield Traits of Soybean (Glycine max L.) as Mediated by Foliar Spray with Brassinolide

Salinity episodes that are common in arid regions, characterized by dryland, are adversely affecting crop production worldwide. This study evaluated the effectiveness of brassinolide (BL) in ameliorating salinity stress imposed on soybean at four levels (control (1.10), 32.40, 60.60 and 86.30 mM/L NaCl) in factorial combination with six BL application frequency (control (BL₀), application at seedling (BL₁), flowering (BL₂), podding (BL₃), seedling + flowering (BL₄) and seedling + flowering + podding (BL₅)) stages. Plant growth attributes, seed yield, and N, P, K, Ca and Mg partitioning to leaves, stems and roots, as well as protein and seed-N concentrations, were significantly (p ≤ 0.05) reduced by salinity stress. These trends were ascribed to considerable impairments in the photosynthetic pigments, photosynthetically active radiation, leaf stomatal conductance and relative water content in the leaves of seedlings under stress. The activity of peroxidase and superoxidase significantly (p ≤ 0.05) increased with salinity. Foliar spray with BL significantly (p ≤ 0.05) improved the photosynthetic attributes, as well as nutrient partitioning, under stress, and alleviated ion toxicity by maintaining a favourable K⁺/Na⁺ ratio and decreasing oxidative damage. Foliar spray with brassinolide could sustain soybean growth and seed yield at salt concentrations up to 60.60 mM/L NaCl.

Assessment of Acidic Biochar on the Growth, Physiology and Nutrients Uptake of Maize (Zea mays L.) Seedlings under Salinity Stress

The application of an acidic biochar can improve plant growth and soil properties in saline conditions. In this study, we investigated the effect of acidic biochar on plant growth and nutrients contents in saline soil. Seven treatments were arranged in a complete randomized design, including control (CK), 0, 30, and 45 g biochar added to a soil having 1% and 1.5% salts; these treatments were termed as B0S1, B30S1, B45S1 and B0S1.5, B30S1.5, B45S1.5 respectively. Experimental results showed that the plant height, leaves plant−1, leaf area, and shoot fresh and dry biomass, and root fresh and dry biomass were increased for the B45S1.5, respectively. Similarly, the highest total nitrogen (TN), total phosphorus (TP), total potassium (TK), and total sodium (Na) concentration in maize shoot were observed for B30S1, B0S1.5, CK, and B0S1.5, respectively. The highest concentrations of TN, TP, TK, and Na in root were obtained with the treatments B0S1, B0S1, B45S1, and B0S1, respectively. Soil pH, and EC decreased and nutrients concentration improved by the addition of acidic biochar. We conclude that the use of acidic biochar can be a potential source for the improvement of maize plant growth as well as mitigate the adverse effect of salt stress.

Effect of NaCl road salt on the ionic composition of soils and Aesculus hippocastanum L. foliage and leaf damage intensity

We investigated the accumulation of sodium chloride in roadside soils and common horse chestnut Aesculus hippocastanum L. under urban conditions to evaluate changes in soil and leaf ionic content and their relationship with foliar damage, considering the visual assessment of trees of the same health status. A total of 15 field sites were assessed in late June 2016. The analysis included soil granulometric composition, pH, electrical conductivity, and the content of Cl⁻, Na⁺, K⁺, Ca²⁺, and Mg²⁺ ions in soil and foliage samples. The results showed increased salinity and alkalization of roadside soils together with the decreased magnesium content. Foliage samples manifested significantly higher concentrations of Na⁺ and Cl⁻. A wide range of Cl⁻ content was noted in leaves (2.0–11.8% d.w.) regardless of their damage index. On the contrary, leaf damage was strongly correlated with increasing Na⁺ concentrations and decreasing K⁺ and Mg²⁺. A severe imbalance of nutrients, and therefore poor urban tree vitality, can be attributed to the excessive accumulation of de-icing salt. However, further research would be needed to clarify the discrepancy between the extent of leaf damage and chloride content.

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

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

Silicon alleviates salt stress-induced potassium deficiency by promoting potassium uptake and translocation in rice (Oryza sativa L.)

Under salt stress, plants suffer from potassium (K) deficiency caused by excess salts in growth substrate. Silicon (Si) can promote K status in many plant species under salt stress, however, the underlying mechanisms remain unclear. In this study, we assessed the effects of Si on K homeostasis in rice under salt stress and investigated the mechanisms behind using two low-Si rice mutants (lsi1 and lsi2) and their wild types (WTs). After five days' treatment with Si, plant growth was improved and salt stress-induced K deficiency was alleviated in WTs but not in mutants. Simultaneously, Si significantly enhanced K accumulation content, K uptake index and shoot K distribution rate in WTs but not in mutants. Besides, Si enhanced K concentration in xylem sap in WTs but not in mutants. Scanning ion-selected electrode technique (SIET) analysis showed net K influx rate was raised by Si addition under salt stress in WTs but not in mutants. Moreover, Si up-regulated the expression of genes responsible for K uptake (OsAKT1 and OsHAK1) and xylem loading (OsSKOR) in WTs but not in mutants. Overall, our results strongly indicate that Si can improve K uptake and translocation by up-regulating the expression of relevant genes, thereby promoting K status and alleviating salt stress in rice.

Contrasting response of two Lotus corniculatus L. accessions to combined waterlogging-saline stress

Waterlogging and salinity impair crop growth and productivity worldwide, with their combined effects being larger than the additive effects of the two stresses separately. Here, a common forage tetraploid Lotus corniculatus (cv. San Gabriel) and a diploid L. corniculatus accession, collected from a coastal area with high frequency of waterlogging-saline stress events, were evaluated for tolerance to waterlogging, salinity and these two stresses combined. We hypothesize that, due to its environmental niche, the diploid accession would show better adaptation to combined waterlogging-saline stress compared to the tetraploid L. corniculatus. Plants were evaluated under control conditions, waterlogging, salinity and a combined waterlogging-saline treatment for 33 days. Shoot and root growth were assessed, together with chlorophyll fluorescence and gas exchange measurements. Results showed that salinity and waterlogging effects were more severe for the tetraploid accession, with a larger effect being observed under the combined stress condition. Concentrations of Na⁺ , Cl^- and K⁺ were measured in apical and basal leaves, and in roots. A larger accumulation of Na⁺ and Cl^- was observed under both saline and combined stress treatments for the tetraploid L. corniculatus, for which ion toxicity effects were evident. The expression of CLC gene, coding for a Cl^- transporter, was only increased in diploid L. corniculatus plants in response to the combined stress condition, suggesting that ion compartmentalization mechanisms were induced in this accession. Thus, this recently characterized L. corniculatus could be used for the introduction of new tolerance traits in other Lotus species used as forage.

The Arabidopsis phosphatase PP2C49 negatively regulates salt tolerance through inhibition of AtHKT1;1

Type 2C protein phosphatases (PP2Cs) are the largest protein phosphatase family. PP2Cs dephosphorylate substrates for signaling in Arabidopsis, but the functions of most PP2Cs remain unknown. Here, we characterized PP2C49 (AT3G62260, a Group G PP2C), which regulates Na⁺ distribution under salt stress and is localized to the cytoplasm and nucleus. PP2C49 was highly expressed in root vascular tissues and its disruption enhanced plant tolerance to salt stress. Compared with wild type, the pp2c49 mutant contained more Na⁺ in roots but less Na⁺ in shoots and xylem sap, suggesting that PP2C49 regulates shoot Na⁺ extrusion. Reciprocal grafting revealed a root-based mechanism underlying the salt tolerance of pp2c49. Systemic Na⁺ distribution largely depends on AtHKT1;1 and loss of function of AtHKT1;1 in the pp2c49 background overrode the salt tolerance of pp2c49, resulting in salt sensitivity. Furthermore, compared with plants overexpressing PP2C49 in the wild-type background, plants overexpressing PP2C49 in the athtk1;1 mutant background were sensitive to salt, like the athtk1;1 mutants. Moreover, protein-protein interaction and two-voltage clamping assays demonstrated that PP2C49 physically interacts with AtHKT1;1 and inhibits the Na⁺ permeability of AtHKT1;1. This study reveals that PP2C49 negatively regulates AtHKT1;1 activity and thus determines systemic Na⁺ allocation during salt stress.

The PalWRKY77 transcription factor negatively regulates salt tolerance and abscisic acid signaling in Populus

High salinity, one of the most widespread abiotic stresses, inhibits photosynthesis, reduces vegetation growth, blocks respiration and disrupts metabolism in plants. In order to survive their long-term lifecycle, trees, such as Populus species, recruit the abscisic acid (ABA) signaling pathway to adapt to a saline environment. However, the molecular mechanism behind the ABA-mediated salt stress response in woody plants remains elusive. We have isolated a WRKY transcription factor gene, PalWRKY77, from Populus alba var. pyramidalis (poplar), the expression of which is repressed by salt stress. PalWRKY77 decreases salt tolerance in poplar. Furthermore, PalWRKY77 negatively regulated ABA-responsive genes and relieved ABA-mediated growth inhibition, indicating that PalWRKY77 is a repressor of the ABA response. In vivo and in vitro assays revealed that PalWRKY77 targets the ABA- and salt-induced PalNAC002 and PalRD26 genes by binding to the W-boxes in their promoters. In addition, overexpression of both PalNAC002 and PalRD26 could elevate salt tolerance in transgenic poplars. These findings reveal a novel negative regulation mechanism for the ABA signaling pathway mediated by PalWRKY77 that results in more sensitivity to salt stress in poplar. This deepens our understanding of the complex responses of woody species to salt stress.

Microbial and physicochemical assessment of irrigation water treatment methods

AIMS

The presence of foodborne pathogens in preharvest agricultural water has been identified as a potential contamination source in outbreak investigations, driving markets and auditing bodies to begin requiring water treatment for high-risk produce. Therefore, it is essential that we identify water treatment methods which are effective as well as practical in their application on farm.

METHODS AND RESULTS

In this work, we evaluated two sanitizers which are most prominent in preharvest agricultural water treatment (calcium hypochlorite (free chlorine: 3-5 ppm) and peracetic acid (PAA: 5 ppm)), an EPA registered antimicrobial device (ultraviolet light (UV)), in addition to a combination approach (chlorine + UV, PAA + UV). Treatments were evaluated for their ability to inactivate total coliforms and generic Escherichia coli and consistency in treatment efficacy over 1 h of operation. Physicochemical variables were measured along with microbial populations at 0, 5, 15, 30, 45 and 60 min of operation. Escherichia coli and coliform counts showed a significant (P < 0·05) reduction after treatment, with combination and singular treatments equally effective at inactivating E. coli and coliforms. A significant increase (P < 0·05) in oxidation-reduction potential was seen during water treatment (Chlorine; UV + Chlorine), and a significant reduction (P < 0·05) in pH was seen after PAA and PAA + UV treatments (60 min).

CONCLUSION

Overall, the results indicate that all treatments evaluated are equally efficacious for inactivating E. coli and coliforms present in surface agricultural water.

SIGNIFICANCE AND IMPACT OF THE STUDY

This information when paired with challenge studies targeting foodborne pathogens of interest can be used to support grower decisions when selecting and validating a preharvest agricultural water treatment programme.

The effect of salt stress on the production of apocarotenoids and the expression of genes related to their biosynthesis in saffron

Saffron stigmas are widely used as food additives and as traditional medicine in Iran and many other countries. The unique taste, flavor and pharmaceutical properties of saffron stigmas are due to the presence of three apocarotenoids secondary metabolites crocin, picrocrocin and safranal. There is limited knowledge about the effect of environmental stresses on the metabolism of apocarotenoids in saffron. We analyzed the content of crocin and picrocrocin and the expression of key genes of apocarotenoid biosynthesis pathways (CsCCD2, CsCCD4, CsUGT2, CsCHY-β and CsLCYB) in saffron plants exposed to moderate (90 mM) and high (150 mM) salt (NaCl) concentrations. Measuring ion concentrations in leaves showed an increased accumulation of Na⁺ and decreased uptake of K⁺ in salt treated compared to control plants indicating an effective salt stress. HPLC analysis of apocarotenoids revealed that crocin production was significantly halted (P < 0.05) with increasing salt concentration while picrocrocin level did not change with moderate salt but significantly dropped by high salt concentration. Real-time PCR analysis revealed a progressive decrease in transcript levels of CsUGT2 and CsLCYB genes with increasing salt concentration (P < 0.05). The expression of CsCCD2 and CsCHY-β tolerated moderate salt concentration but significantly downregulated with high salt concentration. CsCCD4 however responded differently to salt concentration being decreased with moderate salt but increased at higher salt concentration. Our result suggested that salt stress had an adverse effect on the production of saffron apocarotenoids and it is likely influencing the quality of saffron stigma produced.

Application of Phenotyping Methods in Detection of Drought and Salinity Stress in Basil (Ocimum basilicum L.)

Basil is one of the most widespread aromatic and medicinal plants, which is often grown in drought- and salinity-prone regions. Often co-occurrence of drought and salinity stresses in agroecosystems and similarities of symptoms which they cause on plants complicates the differentiation among them. Development of automated phenotyping techniques with integrative and simultaneous quantification of multiple morphological and physiological traits enables early detection and quantification of different stresses on a whole plant basis. In this study, we have used different phenotyping techniques including chlorophyll fluorescence imaging, multispectral imaging, and 3D multispectral scanning, aiming to quantify changes in basil phenotypic traits under early and prolonged drought and salinity stress and to determine traits which could differentiate among drought and salinity stressed basil plants. Ocimum basilicum "Genovese" was grown in a growth chamber under well-watered control [45-50% volumetric water content (VWC)], moderate salinity stress (100 mM NaCl), severe salinity stress (200 mM NaCl), moderate drought stress (25-30% VWC), and severe drought stress (15-20% VWC). Phenotypic traits were measured for 3 weeks in 7-day intervals. Automated phenotyping techniques were able to detect basil responses to early and prolonged salinity and drought stress. In addition, several phenotypic traits were able to differentiate among salinity and drought. At early stages, low anthocyanin index (ARI), chlorophyll index (CHI), and hue (HUE2 D ), and higher reflectance in red (R Red ), reflectance in green (R Green ), and leaf inclination (LINC) indicated drought stress. At later stress stages, maximum fluorescence (F m ), HUE2 D , normalized difference vegetation index (NDVI), and LINC contribute the most to the differentiation among drought and non-stressed as well as among drought and salinity stressed plants. ARI and electron transport rate (ETR) were best for differentiation of salinity stressed plants from non-stressed plants both at early and prolonged stress.

Progress in understanding salt stress response in plants using biotechnological tools

Salinization is a worldwide environmental problem, which is negatively impacting crop yield and thus posing a threat to the world's food security. Considering the rising threat of salinity, it is need of time, to understand the salt tolerant mechanism in plants and find avenues for the development of salinity resistant plants. Several plants tolerate salinity in a different manner, thereby halophytes and glycophytes evolved altered mechanisms to counter the stress. Therefore, in this review article, physiological, metabolic, and molecular aspects of the plant adaptation to salt stress have been discussed. The conventional breeding techniques for developing salt tolerant plants has not been much successful, due to its multigenic trait. The inflow of data from plant sequencing projects and annotation of genes led to the identification of many putative genes having a role in salt stress. The bioinformatics tools provided preliminary information and were helpful for making salt stress-specific databases. The microRNA identification and characterization led to unraveling the finer intricacies of the network. The transgenic approach finally paved a way for overexpressing some important genes viz. DREB, MYB, COMT, SOS, PKE, NHX, etc. conferred salt stress tolerance. In this review, we tried to show the effect of salinity on plants, considering ion homeostasis, antioxidant defense response, proteins involved, possible utilization of transgenic plants, and bioinformatics for coping with this stress factor. An overview of previous studies related to salt stress is presented in order to assist researchers in providing a potential solution for this increasing environmental threat.

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

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

The pip1s Quintuple Mutants Demonstrate the Essential Roles of PIP1s in the Plant Growth and Development of Arabidopsis

Plasma membrane intrinsic proteins (PIPs) transport water, CO₂ and small neutral solutes across the plasma membranes. In this study, we used the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 system (CRISPR/Cas9) to mutate PIP1;4 and PIP1;5 in a pip1;1,2,3 triple mutant to generate a pip1;1,2,3,4,5 (pip1s⁻) quintuple mutant. Compared to the wild-type (WT) plant, the pip1s⁻ mutants had smaller sized rosette leaves and flowers, less rosette leaf number, more undeveloped siliques, shorter silique and less seeds. The pollen germination rate of the pip1s⁻ mutant was significantly lower than that of the WT and the outer wall of the pip1s⁻ mutant’s pollen was deformed. The transcriptomic analysis showed significant alterations in the expression of many key genes and transcription factors (TFs) in the pip1s⁻ mutant which involved in the development of leaf, flower and pollen, suggesting that the mutant of PIP1s not only directly affects hydraulics and carbon fixation, but also regulates the expression of related genes to affect plant growth and development.

Effect of saline irrigation on plant water traits, photosynthesis and ionic balance in durum wheat genotypes

In the era of climate change, decreased precipitation and increased evapo-transpiration hampers the yield of several cereal crops along with the soil salinity and poor ground water resource. Wheat being the moderately tolerant crop face many challenges in the arid and semi-arid regions under irrigated agriculture. In view of this, the study was planned to explore the potential of durum wheat genotypes under salinity on the basis of physiological traits. Experiment was designed as RBD in three replications to evaluate 15 wheat genotypes with moderate saline irrigation (EC_iw – 6 dS m⁻¹) and extreme saline irrigation (EC_iw – 10 dS m⁻¹) along with one set of control (Best available water). Different physiological traits such as water potential (ψ_p), osmotic potential (ψ_s), relative water content (RWC), Na⁺ and K⁺ content were recorded in roots as well as shoots at the reproductive stage whereas photosynthetic rate and chlorophyll content were measured in the flag leaves. A significant variability (p < 0.001) was noted among the genotypes under different stress environments and it was observed that durum genotype HI 8728 and HI 8737 showed less reduction in plant water traits (RWC, ψ_p and ψ_s) than the salinity tolerant checks of bread wheat KRL 99 and KRL 3–4. HD 4728 and HI 8708 maintained higher photosynthetic rate as well as higher chlorophyll content under the extreme salinity level of EC_iw – 10 dSm⁻¹. No significant differences were found in root Na⁺ in genotypes KRL 99 (3.17^g), KRL 3–4 (3.34^g) and HI 8737 (3.41^g) while in shoots, lowest accumulation was seen in KRL 99, MACS 3949 and KRL 3–4 at EC_iw – 10 dSm⁻¹. The mean range of K⁺ content was 7.60–9.74% in roots and 4.21–6.61% in shoots under control environment which decreased to 50.77% in roots and 46.05% in shoots under extreme salinity condition of EC_iw – 10 dSm⁻¹. At EC_iw – 10 dSm⁻¹, KRL 99 maintained highest K^+/Na⁺ in both root and shoot followed by KRL 3–4, HI 8737, MACS 3949, HD 4728 in roots and MACS 3949, KRL 3–4, MACS 4020, HD 4758, MACS 3972 and HI 8713 in shoots. The differential response of durum wheat genotypes under salinity particularly for physiological traits, confer their adaptability towards stress environments and exhibit their potential as genetic sources in breeding programs for improving salt stress tolerance.

NMR and LC-MS-Based Metabolomics to Study Osmotic Stress in Lignan-Deficient Flax

Lignans, phenolic plant secondary metabolites, are derived from the phenylpropanoid biosynthetic pathway. Although, being investigated for their health benefits in terms of antioxidant, antitumor, anti-inflammatory and antiviral properties, the role of these molecules in plants remains incompletely elucidated; a potential role in stress response mechanisms has been, however, proposed. In this study, a non-targeted metabolomic analysis of the roots, stems, and leaves of wild-type and PLR1-RNAi transgenic flax, devoid of (+) secoisolariciresinol diglucoside ((+) SDG)-the main flaxseed lignan, was performed using 1H-NMR and LC-MS, in order to obtain further insight into the involvement of lignan in the response of plant to osmotic stress. Results showed that wild-type and lignan-deficient flax plants have different metabolic responses after being exposed to osmotic stress conditions, but they both showed the capacity to induce an adaptive response to osmotic stress. These findings suggest the indirect involvement of lignans in osmotic stress response.

Potential risks of nicotine on the germination, growth, and nutritional properties of broad bean

This study evaluated the allelopathy, uptake and accumulation, and potential agricultural and food safety risks of nicotine in broad bean (Vicia faba L.) during seed germination and seedling growth. Nicotine stress has an allelopathic inhibitory effect on seeds and a hormesis effect on germinated seeds and seedlings, which has an enhancement effect (<50 mg kg^-1) and an inhibition effect (>100 mg kg^-1) on the germinated seeds and an enhancement effect (<100 mg kg^-1) and an inhibition effect (>200 mg kg^-1) on the seedlings. Exogenous nicotine can be absorbed by broad bean roots from nicotine-contaminated soil and accumulated in the main organs of the seedlings, especially the leaves, which exceeded the maximum residue level (0.03 mg kg^-1 DW) at 50 mg kg^-1. Moreover, nicotine resulted in a bitter taste in the edible broad bean leaves, disrupting the balance of basic nutritional properties, decreasing sucrose, and increasing bitter substances such as choline and procyanidin. These results demonstrated that residual nicotine in the soil not only poses potential risks to sustainable agricultural development but also a food safety risk for consumers. The present study provides insight into the potential risks of nicotine in agroecosystems.

Mitigation of NaCl Stress in Wheat by Rhizosphere Engineering Using Salt Habitat Adapted PGPR Halotolerant Bacteria

There is a great interest in mitigating soil salinity that limits plant growth and productivity. In this study, eighty-nine strains were isolated from the rhizosphere and endosphere of two halophyte species (Suaeda mollis and Salsola tetrandra) collected from three chotts in Algeria. They were screened for diverse plant growth-promoting traits, antifungal activity and tolerance to different physico-chemical conditions (pH, PEG, and NaCl) to evaluate their efficiency in mitigating salt stress and enhancing the growth of Arabidopsis thaliana and durum wheat under NaCl–stress conditions. Three bacterial strains BR5, OR15, and RB13 were finally selected and identified as Bacillus atropheus. The Bacterial strains (separately and combined) were then used for inoculating Arabidopsis thaliana and durum wheat during the seed germination stage under NaCl stress conditions. Results indicated that inoculation of both plant spp. with the bacterial strains separately or combined considerably improved the growth parameters. Three soils with different salinity levels (S1 = 0.48, S2 = 3.81, and S3 = 2.80 mS/cm) were used to investigate the effects of selected strains (BR5, OR15, and RB13; separately and combined) on several growth parameters of wheat plants. The inoculation (notably the multi-strain consortium) proved a better approach to increase the chlorophyll and carotenoid contents as compared to control plants. However, proline content, lipid peroxidation, and activities of antioxidant enzymes decreased after inoculation with the plant growth-promoting rhizobacteria (PGPR) that can attenuate the adverse effects of salt stress by reducing the reactive oxygen species (ROS) production. These results indicated that under saline soil conditions, halotolerant PGPR strains are promising candidates as biofertilizers under salt stress conditions.

Effect of Three Nanoparticles (Se, Si and Cu) on the Bioactive Compounds of Bell Pepper Fruits under Saline Stress

The bell pepper is a vegetable with high antioxidant content, and its consumption is important because it can reduce the risk of certain diseases in humans. Plants can be affected by different types of stress, whether biotic or abiotic. Among the abiotic factors, there is saline stress that affects the metabolism and physiology of plants, which causes damage, decreasing productivity and quality of fruits. The objective of this work was to evaluate the application of selenium, silicon and copper nanoparticles and saline stress on the bioactive compounds of bell pepper fruits. The bell pepper plants were exposed to saline stress (25 mM NaCl and 50 mM) in the nutrient solution throughout the crop cycle. The nanoparticles were applied drenching solution of these to substrate (Se NPs 10 and 50 mg L-1, Si NPs 200 and 1000 mg L-1, Cu NPs 100 and 500 mg L-1). The results show that saline stress reduces chlorophylls, lycopene, and β-carotene in leaves; but increased the activity of some enzymes (e.g., glutathione peroxidase and phenylalanine ammonia lyase, and glutathione). In fruits, saline stress decreased flavonoids and glutathione. The nanoparticles increased chlorophylls, lycopene and glutathione peroxidase activity in the leaves; and ascorbate peroxidase, glutathione peroxidase, catalase and phenylalanine ammonia lyase activity, and also phenols, flavonoids, glutathione, β-carotene, yellow carotenoids in fruits. The application of nanoparticles to bell pepper plants under saline stress is efficient to increase the content of bioactive compounds in fruits.

Defining the Cell Wall, Cell Cycle and Chromatin Landmarks in the Responses of Brachypodium distachyon to Salinity

Excess salinity is a major stress that limits crop yields. Here, we used the model grass Brachypodium distachyon (Brachypodium) reference line Bd21 in order to define the key molecular events in the responses to salt during germination. Salt was applied either throughout the germination period ("salt stress") or only after root emergence ("salt shock"). Germination was affected at ≥100 mM and root elongation at ≥75 mM NaCl. The expression of arabinogalactan proteins (AGPs), FLA1, FLA10, FLA11, AGP20 and AGP26, which regulate cell wall expansion (especially FLA11), were mostly induced by the "salt stress" but to a lesser extent by "salt shock". Cytological assessment using two AGP epitopes, JIM8 and JIM13 indicated that "salt stress" increases the fluorescence signals in rhizodermal and exodermal cell wall. Cell division was suppressed at >75 mM NaCl. The cell cycle genes (CDKB1, CDKB2, CYCA3, CYCB1, WEE1) were induced by "salt stress" in a concentration-dependent manner but not CDKA, CYCA and CYCLIN-D4-1-RELATED. Under "salt shock", the cell cycle genes were optimally expressed at 100 mM NaCl. These changes were consistent with the cell cycle arrest, possibly at the G1 phase. The salt-induced genomic damage was linked with the oxidative events via an increased glutathione accumulation. Histone acetylation and methylation and DNA methylation were visualized by immunofluorescence. Histone H4 acetylation at lysine 5 increased strongly whereas DNA methylation decreased with the application of salt. Taken together, we suggest that salt-induced oxidative stress causes genomic damage but that it also has epigenetic effects, which might modulate the cell cycle and AGP expression gene. Based on these landmarks, we aim to encourage functional genomics studies on the responses of Brachypodium to salt.

Eco-physiological evaluation of multipurpose tree species to ameliorate saline soils

Salinity is a widespread soil and underground water contaminant threatening food security and economic stability. Phytoremediation is an efficient and environmental-friendly solution to mitigate salinity impacts. The present study was conducted to evaluate the phytoremediation potential of five multipurpose trees: Vachellia nilotica, Concorpus erectus, Syzygium cumini, Tamarix aphylla and Eucalyptus cammaldulensis under four salinity treatments: Control, 10, 20 and 30 dS m^-1. Salinity negatively impacted all the tested species. However, E. cammaldulensis and T. aphylla exhibited the lowest reduction (28%) and (35%) in plant height respectively along with a minimal reduction in leaf gas exchange while V. nilotica, S. cumini and C. erectus showed severe dieback. Similarly, the antioxidant enzymes increased significantly in E. cammaldulensis and T. aphylla as Superoxide Dismutase (87% and 79%), Catalase (66% and 67%) and Peroxidase (89% and 81%), respectively. Furthermore, both of these species maintained optimum Na/K ratio reducing the highest levels of soil EC_e and SAR, suggesting the best phytoremediation potential. The present study identifies that E. cammaldulensis and T. aphylla showed effective tolerance mechanisms and the highest salt sequestration; therefore, may be used for phyto-amelioration of salinity impacted lands. Novelty statement Although previous studies evaluated the tolerance potential of many tree species, comparative and physiochemical evaluation of multipurpose tree species has been remained unexplored. In this scenario, eco-physiological characterization of multipurpose tree species may inform tree species for phytoremediation of saline soils according to the level of salinity. Optimizing tree species selection also improves the success of wood for energy and revenue generation while restoring degraded soils.

Stem Photosynthesis-A Key Element of Grass Pea (Lathyrus sativus L.) Acclimatisation to Salinity

Grass pea (Lathyrus sativus) is a leguminous plant of outstanding tolerance to abiotic stress. The aim of the presented study was to describe the mechanism of grass pea (Lathyrus sativus L.) photosynthetic apparatus acclimatisation strategies to salinity stress. The seedlings were cultivated in a hydroponic system in media containing various concentrations of NaCl (0, 50, and 100 mM), imitating none, moderate, and severe salinity, respectively, for three weeks. In order to characterise the function and structure of the photosynthetic apparatus, Chl a fluorescence, gas exchange measurements, proteome analysis, and Fourier-transform infrared spectroscopy (FT-IR) analysis were done inter alia. Significant differences in the response of the leaf and stem photosynthetic apparatus to severe salt stress were observed. Leaves became the place of harmful ion (Na+) accumulation, and the efficiency of their carboxylation decreased sharply. In turn, in stems, the reconstruction of the photosynthetic apparatus (antenna and photosystem complexes) activated alternative electron transport pathways, leading to effective ATP synthesis, which is required for the efficient translocation of Na+ to leaves. These changes enabled efficient stem carboxylation and made them the main source of assimilates. The observed changes indicate the high plasticity of grass pea photosynthetic apparatus, providing an effective mechanism of tolerance to salinity stress.

Metabolic engineering of osmoprotectants to elucidate the mechanism(s) of salt stress tolerance in crop plants

Previous studies on engineering osmoprotectant metabolic pathway genes focused on the performance of transgenic plants under salt stress conditions rather than elucidating the underlying mechanism(s), and hence, the mechanism(s) remain(s) unclear. Salt stress negatively impacts agricultural crop yields. Hence, to meet future food demands, it is essential to generate salt stress-resistant varieties. Although traditional breeding has improved salt tolerance in several crops, this approach remains inadequate due to the low genetic diversity of certain important crop cultivars. Genetic engineering is used to introduce preferred gene(s) from any genetic reserve or to modify the expression of the existing gene(s) responsible for salt stress response or tolerance, thereby leading to improved salt tolerance in plants. Although plants naturally produce osmoprotectants as an adaptive mechanism for salt stress tolerance, they offer only partial protection. Recently, progress has been made in the identification and characterization of genes involved in the biosynthetic pathways of osmoprotectants. Exogenous application of these osmoprotectants, and genetic engineering of enzymes in their biosynthetic pathways, have been reported to enhance salt tolerance in different plants. However, no clear mechanistic model exists to explain how osmoprotectant accumulation in transgenic plants confers salt tolerance. This review critically examines the results obtained thus far for elucidating the underlying mechanisms of osmoprotectants for improved salt tolerance, and thus, crop yield stability under salt stress conditions, through the genetic engineering of trehalose, glycinebetaine, and proline metabolic pathway genes.