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

Potassium ameliorates cotton (Gossypium hirsutum L.) fiber length by regulating osmotic and K+ /Na+ homeostasis under salt stress

Potassium (K) application can alleviate cotton salt stress, but the regulatory mechanisms affecting cotton fiber elongation and ion homeostasis are still unclear. A two-year field experiment was conducted to explore the effects of K on the osmolyte contents (soluble sugar, K⁺ content, and malate) and related enzyme activities during the fiber elongation of two cotton cultivars with contrasting salt sensitivity (CCRI-79; salt tolerant cultivar, and Simian 3; salt-sensitive cultivar). Three K application treatments (0, 150, and 300 kg K₂ O ha^-1 ) were applied at three soil salinity levels (low salinity, EC = 1.73 ± 0.05 dS m^-1 ; medium salinity, EC = 6.32 ± 0.10 dS m^-1 ; high salinity, EC = 10.84 ± 0.24 dS m^-1 ). K application improved fiber length and alleviated salt stress by increasing the maximum velocity of fiber elongation (V_max ). The increase rate of K on fiber length decreased with elevating salt stress, and the increase rate of K on V_max of CCRI-79 was greater than that of Simian3. K application can increase the enzyme activities (phosphoenolpyruvate carboxylase, PEPC, E.C. 4.1.1.31; pyrophosphatase, PPase, E.C. 3.6.1.1; and plasma membrane H⁺ -ATPase, PM H⁺ -ATPase, E.C. 3.6.3.14) as well as the content of osmolytes associated with the enzymes mentioned above. K increased the osmolyte contents under salt stress, and the increase in the K⁺ content of the fibers was much higher than that of soluble sugar and malate. The results of this study indicated K fertilizer application rates regulate the metabolism of osmolytes in cotton fiber development under salt stress, K⁺ is more critical to fiber elongation.

Glycine max (L.) Merr. (Soybean) metabolome responses to potassium availability

Potassium (K⁺) has vital physiological and metabolic functions in plants and its availability can impact tolerance to biotic and abiotic stress conditions. Limited studies have investigated the effect of K⁺ fertilization on soybean metabolism. Using integrated omics, ionomics and metabolomics, we investigated the field-grown Glycine max (soybean) response, after four K⁺ soil fertilization rates. Soybean leaf and pod tissue (valves and immature seeds) extracts were analysed by ultra-performance liquid chromatography coupled to high-resolution mass spectrometry (UPLC-HRMS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Multivariate analyses (PCA-X&Y e O2PLS-DA) showed that 51 compounds of 19 metabolic pathways were regulated in response to K⁺ availability. Under very low potassium availability, soybean plants accumulated of Ca²⁺, Mg²⁺, Fe²⁺, Cu²⁺, and B in young and old leaves. Potassium fertilization upregulated carbohydrate, galactolipid, and flavonol glycoside biosynthesis in leaves and pod valves, while K⁺ deficient pod tissues showed increasing amino acids, oligosaccharides, benzoic acid derivatives, and isoflavones contents. Severely K⁺ deficient soils elicited isoflavones, coumestans, pterocarpans, and soyasaponins in trifoliate leaves, likely associated to oxidative and photodynamic stress status. Additionally, results demonstrate that L-asparagine content is higher in potassium deficient tissues, suggesting this compound as a biomarker of K⁺ deficiency in soybean plants. These results demonstrate that potassium soil fertilization did not linearly contribute to changes in specialised constitutive metabolites of soybean. Altogether, this work provides a reference for improving the understanding of soybean metabolism as dependent on K⁺ availability.

Advances and future research in ecological stoichiometry under saline-alkali stress

Saline-alkali stress is a serious abiotic factor which negatively impacts agricultural production and the ecological environment. Thus, improving the development of saline-alkali soil and reducing the effects of saline-alkali stress is a key issue for sustainable agricultural development and environmental protection. As such, it is unsurprising that researchers have lately focused on how to improve saline-alkali soil, increase the agricultural yield of saline-alkali land, and promote the adaptive growth of plants in saline-alkali soil. This paper reviews the latest research concerning nutrient content changes in saline-alkali soil, along with the associated changes in key nutrients in plants, to summarize which methods are most effective for improving the plant growth under saline-alkali stress. Finally, the prospects for alleviating saline-alkali stress and improving saline-alkali soil are put forward as a theoretical foundation for the stabilization of plant growth in saline-alkali soil, expansion of arable land area, crop yield improvement, and effective environmental protection.

Efficacy of endophytic SB10 and glycine betaine duo in alleviating phytotoxic impact of combined heat and salinity in Glycine max L. via regulation of redox homeostasis and physiological and molecular responses

Adverse environmental stresses occurring simultaneously exhibit a lethal effect on crop productivity at the global level. Here, we investigated the individual and synergistic effects of endophytic T. virens SB10 and glycine betaine (GB) on the physiological and biochemical responses of Glycine max L. to alleviate the devastating effects of combined heat and salinity (HS) stress. Screening against HS stress tolerance showed that SB10 has significant tolerance against heat stress and produces hormones such as gibberellins and indole-3-acetic acid upon GB amendment of the growth medium under HS stress. Moreover, the current findings illustrated that the synergistic application of SB10 and GB was effective in alleviating the negative effects of HS stress on plant growth and physiology. The findings revealed that SB10 + GB led to a reduction in proline accumulation and Na⁺ uptake. It also maintained a high K⁺/Na ⁺ ratio by regulating GmHKT1 and GmSOS1 expression and enhanced macronutrient uptake (N, Ca, K) in plants. In turn, plants exhibited a higher growth rate and gaseous exchange attributes coupled with the upregulation of APX, SOD, POD, and GSH antioxidant activities and transcript accumulation of GmSOD1 and GmAPX1 to overcome HS-induced oxidative damage. Furthermore, SB10 + GB downregulated DREB2, DREB1B, and GmNCED3 expression and resulted in the reduced accumulation of endogenous ABA while enhancing endogenous SA accumulation via upregulation of PAL genes. In addition, enhanced accumulation of bioactive gibberellins (GA1, GA3, GA4, and GA7) was detected under HS stress in the SB10 + GB treatment group. Moreover, SB10 + GB also significantly regulated GmHsp90A2 and GmHsfA2 expression in tolerance against HS stress. The combination of SB10 and GB was shown to be an effective and alternative approach for growing G. max at high temperature coupled with saline conditions for sustainable agriculture.

An assessment of the physicochemical characteristics and essential oil composition of Mentha longifolia (L.) Huds. exposed to different salt stress conditions

Salt stress adversely influences growth, development, and productivity in plants, resulting in a limitation on agriculture production worldwide. Therefore, this study aimed to investigate the effect of four different salts, i.e., NaCl, KCl, MgSO₄, and CaCl₂, applied at various concentrations of 0, 12.5, 25, 50, and 100 mM on the physico-chemical properties and essential oil composition of M. longifolia. After 45 days of transplantation, the plants were irrigated at different salinities at 4-day intervals for 60 days. The resulting data revealed a significant reduction in plant height, number of branches, biomass, chlorophyll content, and relative water content with rising concentrations of NaCl, KCl, and CaCl₂. However, MgSO₄ poses fewer toxic effects than other salts. Proline concentration, electrolyte leakage, and DPPH inhibition (%) increase with increasing salt concentrations. At lower-level salt conditions, we had a higher essential oil yield, and GC-MS analysis reported 36 compounds in which (-)-carvone and D-limonene covered the most area by 22%-50% and 45%-74%, respectively. The expression analyzed by qRT-PCR of synthetic Limonene (LS) and Carvone (ISPD) synthetic genes has synergistic and antagonistic relationships in response to salt treatments. To conclude, it can be said that lower levels of salt enhanced the production of essential oil in M. longifolia, which may provide future benefits commercially and medicinally. In addition to this, salt stress also resulted in the emergence of novel compounds in essential oils, for which future strategies are needed to identify the importance of these compounds in M. longifolia.

HvNCX, a prime candidate gene for the novel qualitative locus qS7.1 associated with salinity tolerance in barley

A major QTL (qS7.1) for salinity damage score and Na+ exclusion was identified on chromosome 7H from a barley population derived from a cross between a cultivated variety and a wild accession. qS7.1 was fine-mapped to a 2.46 Mb physical interval and HvNCX encoding a sodium/calcium exchanger is most likely the candidate gene. Soil salinity is one of the major abiotic stresses affecting crop yield. Developing salinity-tolerant varieties is critical for minimizing economic penalties caused by salinity and providing solutions for global food security. Many genes/QTL for salt tolerance have been reported in barley, but only a few of them have been cloned. In this study, a total of 163 doubled haploid lines from a cross between a cultivated barley variety Franklin and a wild barley accession TAM407227 were used to map QTL for salinity tolerance. Four significant QTL were identified for salinity damage scores. One (qS2.1) was located on 2H, determining 7.5% of the phenotypic variation. Two (qS5.1 and qS5.2) were located on 5H, determining 5.3-11.7% of the phenotypic variation. The most significant QTL was found on 7H, explaining 27.8% of the phenotypic variation. Two QTL for Na⁺ content in leaves under salinity stress were detected on chromosomes 1H (qNa1.1) and 7H(qNa7.1). qS7.1 was fine-mapped to a 2.46 Mb physical interval using F₄ recombinant inbred lines. This region contains 23 high-confidence genes, with HvNCX which encodes a sodium/calcium exchanger being most likely the candidate gene. HvNCX was highly induced by salinity stress and showed a greater expression level in the sensitive parent. Multiple nucleotide substitutions and deletions/insertions in the promoter sequence of HvNCX were found between the two parents. cDNA sequencing of the HvNCX revealed that the difference between the two parents is conferred by a single Ala77/Pro77 amino acid substitution, which is located on the transmembrane domain. These findings open new prospects for improving salinity tolerance in barley by targeting a previously unexplored trait.

Mechanisms of cerium-induced stress in plants: A meta-analysis

A comprehensive evaluation of the effects of cerium on plants is lacking even though cerium is extensively applied to the environment. Here, the effects of cerium on plants were meta-analyzed using a newly developed database consisting of approximately 8500 entries of published data. Cerium affects plants by acting as oxidative stressor causing hormesis, with positive effects at low concentrations and adverse effects at high doses. Production of reactive oxygen species and its linked induction of antioxidant enzymes (e.g. catalase and superoxide dismutase) and non-enzymatic antioxidants (e.g. glutathione) are major mechanisms driving plant response mechanisms. Cerium also affects redox signaling, as indicated by altered GSH/GSSG redox pair, and electrolyte leakage, Ca²⁺, K⁺, and K⁺/Na⁺, indicating an important role of K⁺ and Na⁺ homeostasis in cerium-induced stress and altered mineral (ion) balance. The responses of the plants to cerium are further extended to photosynthesis rate (A), stomatal conductance (g_s), photosynthetic efficiency of PSII, electron transport rate, and quantum yield of PSII. However, photosynthesis response is regulated not only by physiological controls (e.g. g_s), but also by biochemical controls, such as via changed Hill reaction and RuBisCO carboxylation. Cerium concentrations <0.1-25 mg L^-1 commonly enhance chlorophyll a and b, g_s, A, and plant biomass, whereas concentrations >50 mg L^-1 suppress such fitness-critical traits at trait-specific concentrations. There was no evidence that cerium enhances yields. Observations were lacking for yield response to low concentrations of cerium, whereas concentrations >50 mg Kg^-1 suppress yields, in line with the response of chlorophyll a and b. Cerium affects the uptake and tissue concentrations of several micro- and macro-nutrients, including heavy metals. This study enlightens the understanding of some mechanisms underlying plant responses to cerium and provides critical information that can pave the way to reducing the cerium load in the environment and its associated ecological and human health risks.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Identification of Halophilic and Halotolerant Bacteria from the Root Soil of the Halophyte Sesuvium verrucosum Raf

Soil salinity is a condition that limits crop growth and productivity, and soil-dwelling bacteria from halophytic plant roots may be a viable strategy to cope with low productivity due to salt stress. Halophilic and halotolerant bacteria of the root soil of Sesuvium verrucosum were analyzed in this study as there is little evidence regarding its associated microbiology. Soil was sampled from the roots of Sesuvium verrucosum to obtain the cultivable bacteria. Their morphological characteristics were identified and they were molecularly identified by the 16S sequence. The growth capacity of the bacteria was determined at different levels of pH and salinity, and several growth promotion characteristics were identified, such as phosphorus solubilization, indole acetic acid production by the tryptophan-dependent (AIAt) and tryptophan-independent (IAA) pathways, ammonium production from organic sources, solubilization of carbonates, and zinc and sodium capture capacity. In addition, the bacteria that presented the best characteristics for germination variables of Solanum lycopersicum were evaluated. A total of 20 bacteria from root soil of Sesuvium verrucosum Raf. belonging to the phyla Proteobacteria (50%), Firmicutes (45%) and Actinobacteria (5%) were identified, with each one having different morphological characteristics. Among the bacterial isolates, 45% had the ability to resist different levels of salinity and pH, ranging from 0 to 20% of NaCl, and pH between 5 and 11. Moreover, these bacteria had the capacity to solubilize carbonates, phosphorus and zinc, capture sodium, produce ammonium from organic substrates and IAA (indole acetic acid), and promote enzymatic activity of amylases, proteases, lipases and cellulases. The bacteria evaluated on the germination of Solanum lycopersicum had an influence on germination at different salinity levels, with greater influence at 100 mM NaCl. This demonstrated that halophilic bacteria belonging to the rhizosphere of Sesuvium verrucosum have the ability to promote growth in extreme salinity conditions, making them candidates for the recovery of productivity in saline soils.

Mining for salt-tolerant genes from halophyte Zoysia matrella using FOX system and functional analysis of ZmGnTL

Zoysia matrella is a salt-tolerant turfgrass grown in areas with high soil salinity irrigated with effluent water. Previous studies focused on explaining the regulatory mechanism of Z. matrella salt-tolerance at phenotypic and physiological levels. However, the molecular mechanism associated with salt tolerance of Z. matrella remained unclear. In this study, a high-efficient method named FOX (full-length cDNA overexpression) hunting system was used to search for salt-tolerant genes in Z. matrella. Eleven candidate genes, including several known or novel salt-tolerant genes involved in different metabolism pathways, were identified. These genes exhibited inducible expression under salt stress condition. Furthermore, a novel salt-inducible candidate gene ZmGnTL was transformed into Arabidopsis for functional analysis. ZmGnTL improved salt-tolerance through regulating ion homeostasis, reactive oxygen species scavenging, and osmotic adjustment. In summary, we demonstrated that FOX is a reliable system for discovering novel genes relevant to salt tolerance and several candidate genes were identified from Z. matrella that can assist molecular breeding for plant salt-tolerance improvement.

Regulation of Na+/H+ exchangers, Na+/K+ transporters, and lignin biosynthesis genes, along with lignin accumulation, sodium extrusion, and antioxidant defense, confers salt tolerance in alfalfa

Accumulation of high sodium (Na⁺) leads to disruption of metabolic processes and decline in plant growth and productivity. Therefore, this study was undertaken to clarify how Na⁺/H⁺ exchangers and Na⁺/K⁺ transporter genes contribute to Na⁺ homeostasis and the substantial involvement of lignin biosynthesis genes in salt tolerance in alfalfa (Medicago sativa L.), which is poorly understood. In this study, high Na⁺ exhibited a substantial reduction of morphophysiological indices and induced oxidative stress indicators in Xingjiang Daye (XJD; sensitive genotype), while Zhongmu (ZM; tolerant genotype) remained unaffected. The higher accumulation of Na⁺ and the lower accumulation of K⁺ and K⁺/(Na⁺ + K⁺) ratio were found in roots and shoots of XJD compared with ZM under salt stress. The ZM genotype showed a high expression of SOS1 (salt overly sensitive 1), NHX1 (sodium/hydrogen exchanger 1), and HKT1 (high-affinity potassium transporter 1), which were involved in K⁺ accumulation and excess Na⁺ extrusion from the cells compared with XJD. The lignin accumulation was higher in the salt-adapted ZM genotype than the sensitive XJD genotype. Consequently, several lignin biosynthesis-related genes including 4CL2, CCoAOMT, COMT, CCR, C4H, PAL1, and PRX1 exhibited higher mRNA expression in salt-tolerant ZM compared with XJD. Moreover, antioxidant enzyme (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) activity was higher in ZM relative to XJD. This result suggests that high antioxidant provided the defense against oxidative damages in ZM, whereas low enzyme activity with high Na⁺ triggered the oxidative damage in XJD. These findings together illustrate the ion exchanger, antiporter, and lignin biosysthetic genes involving mechanistic insights into differential salt tolerance in alfalfa.

Pretreatment with LEDs regulates antioxidant capacity and polyphenolic profile in two genotypes of basil under salinity stress

In the present study, we evaluated a pretreatment with four LED light sources (red, blue, red + blue, and white) in two genotypes (green and purple) of basil on the growth parameters, stress oxidative markers, non-enzymatic antioxidants, osmoprotectant compounds, ion content, and polyphenolic profile under both control and salinity stress conditions. The results indicated that 150 mM of NaCl decreased biomass, RWC, and K⁺/Na⁺ ratio but increased the content of proline and antioxidant capacity in the leaves of both genotypes of basil grown under GH (greenhouse) conditions. The results suggested that RB LED-exposed plants in the green genotype and R LED-exposed plants in the purple genotype improved accumulation of shoot biomass, K⁺/Na⁺ ratio, proline and soluble sugars, glutathione and ascorbate, polyphenolic profile, and thioredoxin reductase activity in the leaves of basil under both control and salinity stress conditions. NaCl stress (150 mM) increased oxidative markers, which are responsible for disturbance of routine functions of various plant cellular modules. LED light pretreatments diminished these markers under both control and salinity stress conditions. It could be concluded that intensification of non-enzymatic antioxidant systems during light-mediated priming can diminish the deleterious effects of ROS induced by NaCl stress (150 mM) through preventing the lipid peroxidation, scavenging cytotoxic H₂O₂, and enhancement of antioxidant potentials. Therefore, usage of LED lighting systems as a pretreatment or to supplement natural photoperiods under both control and salinity stress conditions may be advantageous for increasing biomass and phytochemical accumulation in basil.

Regulation of pollen tube growth by cellular pH and ions

Tip growth of the pollen tube is a model system for the study of cell polarity establishment in flowering plants. The tip growth of the pollen tube displays an oscillating pattern corresponding to cellular ion and pH dynamics. Therefore, cellular pH and ions play an important role in pollen growth and development. In this review, we summarized the current advances in understanding the function of cellular pH and ions in regulating pollen tube growth. We analyzed the physiological roles and underlying mechanisms of cellular pH and ions, including Ca²⁺, K⁺, and Cl^-, in regulating pollen tube growth. We further examined the function of Ca²⁺ in regulating cytoskeletons, small G proteins, and cell wall development in relation to pollen tube growth. We also examined the regulatory roles of cellular pH in pollen tube growth as well as pH regulation of ion flow, cell wall development, auxin signaling, and cytoskeleton function in pollen. In addition, we assessed the regulation of pollen tube growth by proton pumps and the maintenance of pH homeostasis in the trans-Golgi network by ion transporters. The interplay of ion homeostasis and pH dynamics was also assessed. We discussed the unanswered questions regarding pollen tube growth that need to be addressed in the future.

Silicon Supplementation Alleviates the Salinity Stress in Wheat Plants by Enhancing the Plant Water Status, Photosynthetic Pigments, Proline Content and Antioxidant Enzyme Activities

Silicon (Si) is the most abundant element on earth after oxygen and is very important for plant growth under stress conditions. In the present study, we inspected the role of Si in the mitigation of the negative effect of salt stress at three concentrations (40 mM, 80 mM, and 120 mM NaCl) in two wheat varieties (KRL-210 and WH-1105) with or without Si (0 mM and 2 mM) treatment. Our results showed that photosynthetic pigments, chlorophyll stability index, relative water content, protein content, and carbohydrate content were reduced at all three salt stress concentrations in both wheat varieties. Moreover, lipid peroxidation, proline content, phenol content, and electrolyte leakage significantly increased under salinity stress. The antioxidant enzyme activities, like catalase and peroxidase, were significantly enhanced under salinity in both leaves and roots; however, SOD activity was drastically decreased under salt stress in both leaves and roots. These negative effects of salinity were more pronounced in WH-1105, as KRL-210 is a salt-tolerant wheat variety. On the other hand, supplementation of Si improved the photosynthetic pigments, relative water, protein, and carbohydrate contents in both varieties. In addition, proline content, MDA content, and electrolyte leakage were shown to decline following Si application under salt stress. It was found that applying Si enhanced the antioxidant enzyme activities under stress conditions. Si showed better results in WH-1105 than in KRL-210. Furthermore, Si was found to be more effective at a salt concentration of 120 mM compared to low salt concentrations (40 mM, 80 mM), indicating that it significantly improved plant growth under stressed conditions. Our experimental findings will open a new area of research in Si application for the identification and implication of novel genes involved in enhancing salinity tolerance.

Sodium Accumulation in Infected Cells and Ion Transporters Mistargeting in Nodules of Medicago truncatula: Two Ugly Items That Hinder Coping with Salt Stress Effects

The maintenance of intracellular nitrogen-fixing bacteria causes changes in proteins’ location and in gene expression that may be detrimental to the host cell fitness. We hypothesized that the nodule’s high vulnerability toward salt stress might be due to alterations in mechanisms involved in the exclusion of Na+ from the host cytoplasm. Confocal and electron microscopy immunolocalization analyses of Na+/K+ exchangers in the root nodule showed the plasma membrane (MtNHX7) and endosome/tonoplast (MtNHX6) signal in non-infected cells; however, in mature infected cells the proteins were depleted from their target membranes and expelled to vacuoles. This mistargeting suggests partial loss of the exchanger’s functionality in these cells. In the mature part of the nodule 7 of the 20 genes encoding ion transporters, channels, and Na+/K+ exchangers were either not expressed or substantially downregulated. In nodules from plants subjected to salt treatments, low temperature-scanning electron microscopy and X-ray microanalysis revealed the accumulation of 5–6 times more Na+ per infected cell versus non-infected one. Hence, the infected cells’ inability to withstand the salt may be the integral result of preexisting defects in the localization of proteins involved in Na+ exclusion and the reduced expression of key genes of ion homeostasis, resulting in premature senescence and termination of symbiosis.

Combined transcriptomic and metabolomic analysis reveals a role for adenosine triphosphate-binding cassette transporters and cell wall remodeling in response to salt stress in strawberry

Strawberry (Fragaria × ananassa Duch) are sensitive to salt stress, and breeding salt-tolerant strawberry cultivars is the primary method to develop resistance to increased soil salinization. However, the underlying molecular mechanisms mediating the response of strawberry to salinity stress remain largely unknown. This study evaluated the salinity tolerance of 24 strawberry varieties, and transcriptomic and metabolomic analysis were performed of 'Sweet Charlie' (salt-tolerant) and 'Benihoppe' (salt-sensitive) to explore salt tolerance mechanisms in strawberry. Compared with the control, we identified 3412 differentially expressed genes (DEGs) and 209 differentially accumulated metabolites (DAMs) in 'Benihoppe,' and 5102 DEGs and 230 DAMs in 'Sweet Charlie.' DEGs Gene Ontology (GO) enrichment analyses indicated that the DEGs in 'Benihoppe' were enriched for ion homeostasis related terms, while in 'Sweet Charlie,' terms related to cell wall remodeling were over-represented. DEGs related to ion homeostasis and cell wall remodeling exhibited differential expression patterns in 'Benihoppe' and 'Sweet Charlie.' In 'Benihoppe,' 21 ion homeostasis-related DEGs and 32 cell wall remodeling-related DEGs were upregulated, while 23 ion homeostasis-related DEGs and 138 cell wall remodeling-related DEGs were downregulated. In 'Sweet Charlie,' 72 ion homeostasis-related DEGs and 275 cell wall remodeling-related DEGs were upregulated, while 11 ion homeostasis-related DEGs and 20 cell wall remodeling-related DEGs were downregulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses showed only four KEGG enriched pathways were shared between 'Benihoppe' and 'Sweet Charlie,' including flavonoid biosynthesis, phenylalanine metabolism, phenylpropanoid biosynthesis and ubiquinone, and other terpenoid-quinone biosynthesis. Integrating the results of transcriptomic and metabolomics analyses showed that adenosine triphosphate-binding cassette (ABC) transporters and flavonoid pathway genes might play important roles in the salt stress response in strawberry, and DAMs and DEGs related to ABC transporter and flavonoid pathways were differentially expressed or accumulated. The results of this study reveal that cell wall remodeling and ABC transporters contribute to the response to salt stress in strawberry, and that related genes showed differential expression patterns in varieties with different salt tolerances. These findings provide new insights into the underlying molecular mechanism of strawberry response to salt stress and suggest potential targets for the breeding of salt-tolerant strawberry varieties.

Potassium in plant physiological adaptation to abiotic stresses

Potassium (K) is an integral part of plant nutrition, playing essential roles in plant growth and development. Despite its abundance in soils, the limitedly available form of K ion (K⁺) for plant uptake is a critical factor for agricultural production. Plants have evolved complex transport systems to maintain appropriate K⁺ levels in tissues under changing environmental conditions. Adequate stimulation and coordinated actions of multiple K⁺-channels and K⁺-transporters are required for nutrient homeostasis, reproductive growth, cellular signaling and stress adaptation responses in plants. Various contemporary studies revealed that K⁺-homeostasis plays a substantial role in plant responses and tolerance to abiotic stresses. The beneficial effects of K⁺ in plant responses to abiotic stresses include its roles in physiological and biochemical mechanisms involved in photosynthesis, osmoprotection, stomatal regulation, water-nutrient absorption, nutrient translocation and enzyme activation. Over the last decade, we have seen considerable breakthroughs in K research, owing to the advances in omics technologies. In this aspect, omics investigations (e.g., transcriptomics, metabolomics, and proteomics) in systems biology manner have broadened our understanding of how K⁺ signals are perceived, conveyed, and integrated for improving plant physiological resilience to abiotic stresses. Here, we update on how K⁺-uptake and K⁺-distribution are regulated under various types of abiotic stress. We discuss the effects of K⁺ on several physiological functions and the interaction of K⁺ with other nutrients to improve plant potential against abiotic stress-induced adverse consequences. Understanding of how K⁺ orchestrates physiological mechanisms and contributes to abiotic stress tolerance in plants is essential for practicing sustainable agriculture amidst the climate crisis in global agriculture.

Identification of Alkaline Salt Tolerance Genes in Brassica napus L. by Transcriptome Analysis

Soil salt alkalization is one major abiotic factor reducing the productivity of crops, including rapeseed, an indispensable oil crop and vegetable. The mechanism studies of alkali salt tolerance can help breed highly resistant varieties. In the current study, rapeseed (B. napus) line 2205 exhibited more tolerance to alkaline salt than line 1423 did. In line 2205, the lesser plasma membrane damage index, the accumulated osmotic solute, and higher antioxidant enzyme activities contributed to alkaline tolerance. A more integrated mesophyll-cell structure was revealed under alkali salt stress by ultrastructure observation in line 2205, which also implied a lesser injury. Transcriptome analysis showed that more genes responded to alkaline salt in line 2205. The expression of specific-response genes in line 1423 was lower than in line 2205. However, most of the specific-response genes in line 2205 had higher expression, which was mainly enriched in carbohydrate metabolism, photosynthetic processes, ROS regulating, and response to salt stress. It can be seen that the tolerance to alkaline salt is attributed to the high expression of some genes in these pathways. Based on these, twelve cross-differentially expressed genes were proposed as candidates. They provide clues for further analysis of the resistance mechanism of rapeseed.

Combining Physio-Biochemical Characterization and Transcriptome Analysis Reveal the Responses to Varying Degrees of Drought Stress in Brassica napus L

Brassica napus L. has become one of the most important oil-bearing crops, and drought stress severely influences its yield and quality. By combining physio-biochemical characterization and transcriptome analysis, we studied the response of B. napus plants to different degrees of drought stress. Some physio-biochemical traits, such as fresh weight (FW), dry weight (DW), abscisic acid (ABA) content, net photosynthetic rate (Pn), stomatal conductance (g_s), and transpiration rate (Tr), were measured, and the total content of the epidermal wax/cutin, as well as their compositions, was determined. The results suggest that both stomatal transpiration and cuticular transpiration are affected when B. napus plants are subjected to varying degrees of drought stress. A total of 795 up-regulated genes and 1050 down-regulated genes were identified under severe drought stress by transcriptome analysis. Gene ontology (GO) enrichment analysis of differentially expressed genes (DEGs) revealed that the up-regulated genes were mainly enriched in the stress response processes, such as response to water deprivation and abscisic acid, while the down-regulated genes were mainly enriched in the chloroplast-related parts affecting photosynthesis. Moreover, overexpression of BnaA01.CIPK6, an up-regulated DEG, was found to confer drought tolerance in B. napus. Our study lays a foundation for a better understanding of the molecular mechanisms underlying drought tolerance in B. napus.

Plant Nutrition: An Effective Way to Alleviate Abiotic Stress in Agricultural Crops

By the year 2050, the world’s population is predicted to have grown to around 9–10 billion people. The food demand in many countries continues to increase with population growth. Various abiotic stresses such as temperature, soil salinity and moisture all have an impact on plant growth and development at all levels of plant growth, including the overall plant, tissue cell, and even sub-cellular level. These abiotic stresses directly harm plants by causing protein denaturation and aggregation as well as increased fluidity of membrane lipids. In addition to direct effects, indirect damage also includes protein synthesis inhibition, protein breakdown, and membranous loss in chloroplasts and mitochondria. Abiotic stress during the reproductive stage results in flower drop, pollen sterility, pollen tube deformation, ovule abortion, and reduced yield. Plant nutrition is one of the most effective ways of reducing abiotic stress in agricultural crops. In this paper, we have discussed the effectiveness of different nutrients for alleviating abiotic stress. The roles of primary nutrients (nitrogen, phosphorous and potassium), secondary nutrients (calcium, magnesium and sulphur), micronutrients (zinc, boron, iron and copper), and beneficial nutrients (cobalt, selenium and silicon) in alleviating abiotic stress in crop plants are discussed.

Exogenous melatonin increases salt tolerance in bitter melon by regulating ionic balance, antioxidant system and secondary metabolism-related genes

BACKGROUND

Melatonin is a multi-functional molecule widely employed in order to mitigate abiotic stress factors, in general and salt stress in particular. Even though previous reports revealed that melatonin could exhibit roles in promoting seed germination and protecting plants during various developmental stages of several plant species under salt stress, no reports are available with respect to the regulatory acts of melatonin on the physiological and biochemical status as well as the expression levels of defense- and secondary metabolism-related related transcripts in bitter melon subjected to the salt stress.

RESULTS

Herewith the present study, we performed a comprehensive analysis of the physiological and ion balance, antioxidant system, as well as transcript analysis of defense-related genes (WRKY1, SOS1, PM H⁺-ATPase, SKOR, Mc5PTase7, and SOAR1) and secondary metabolism-related gene expression (MAP30, α-MMC, polypeptide-P, and PAL) in salt-stressed bitter melon (Momordica charantia L.) plants in response to melatonin treatment. In this regard, different levels of melatonin (0, 75 and 150 µM) were applied to mitigate salinity stress (0, 50 and 100 mM NaCl) in bitter melon. Accordingly, present findings revealed that 100 mM salinity stress decreased growth and photosynthesis parameters (SPAD, ^Fv/_Fo, Y(II)), RWC, and some nutrient elements (K⁺, Ca²⁺, and P), while it increased Y(NO), Y(NPQ), proline, Na⁺, Cl^-, H₂O₂, MDA, antioxidant enzyme activity, and lead to the induction of the examined genes. However, prsiming with 150 µM melatonin increased SPAD, ^Fv/_Fo, Y(II)), RWC, and K⁺, Ca²⁺, and P concentration while decreased Y(NO), Y(NPQ), Na⁺, Cl^-, H₂O₂, and MDA under salt stress. In addition, the antioxidant system and gene expression levels were increased by melatonin (150 µM).

CONCLUSIONS

Overall, it can be postulated that the application of melatonin (150 µM) has effective roles in alleviating the adverse impacts of salinity through critical modifications in plant metabolism.

Structural determinants of phytoremediation capacity in saltmarsh halophyte Diplachne fusca (L.) P. Beauv. ex Roem. & Schult. subsp. fusca

Micro and macro-morphological features contribute to plants' tolerance to a variety of environmental pollutants. The contribution of such structural modifications in the phytoremediation potential of Diplachne fusca populations collected from five saline habitats were explored when treated with 100 to 400 mM NaCl for 75 days along with control. Structural modifications in the populations from the highest salinity included development of aerenchyma in stem instead of chlorenchyma, absence of excretory hairs in stem, and exceptionally large trichomes on the leaf surface to help excretion of excess salt. Large parenchyma cells provided more space for water and solute storage, while broad metaxylem vessels were linked to better conduction water and nutrients, which ultimately excreted via glandular hairs, microhairs, and vesicular hairs. Broad metaxylem vessels and exceptionally long hairs observed in the populations collected from 52 dS m^-1. In conclusion, large stem aerenchyma, exceptionally large trichomes on the leaf surface, and tightly packed outer cortical region in roots with intensive sclerification just inside the epidermis accompanied with salt excretion via glandular hairs, microhairs, and vesicular hairs were the main anatomical modifications involved in the phytoremediation potential of D. fusca in hyper-saline environments.

Assessing Silicon-Mediated Growth Performances in Contrasting Rice Cultivars under Salt Stress

Silicon (Si) application has great potential to improve salt tolerance in a variety of crop plants. However, it is unclear how Si influences the responses of contrasting rice cultivars when exposed to excessive salt. Here, we investigated the functions of Si in alleviating the negative effects of salt stress on two contrasting rice cultivars, namely BRRI dhan48 (salt-sensitive) and Binadhan-10 (salt-tolerant). Rice seedlings was pre-treated with three doses of Si (as silicic acid; 0, 1 and 2 mM) for 14 days at one-day interval before being exposed to salt stress (10 dSm⁻¹) in a sustained water bath system. The results demonstrated that the seedlings of BRRI dhan48 and Binadhan-10, respectively exhibited substantial reductions in shoot height (16 and 9%), shoot fresh weight (64 and 43%) and shoot dry weight (50 and 39%) under salinity. Intriguingly, BRRI dhan48 pre-treated with 1 and 2 mM Si, respectively, showed a higher increase in shoot height (SH) (by 25.90 and 26.08%) as compared with Binadhan-10 (by 3 and 8%) under salt stress compared with their respective controls. Data revealed that a comparatively higher improvement in the growth performances of the salt-induced Si pre-treated BRRI dhan48 than that of Binadhan-10. For example, 1 and 2 mM of Si treatments significantly attributed to elevated leaf relative water content (RWC) (13 and 22%), proline (138 and 165%), chlorophyll a (42 and 44%), chlorophyll b (91 and 72%), total chlorophyll (58 and 53%) and carotenoids (33 and 29%), and recovery in the reductions of electrolyte leakage (13 and 21%), malondialdehyde content (23 and 30%) and shoot Na⁺/K⁺ ratio (22 and 52%) in BRRI dhan48 compared with Si-untreated control plants under salt stress. In addition, we found salt-tolerant Binadhan-10 also had enhanced RWC (9 and 19%), proline (12 and 26%) with pre-treatment with 1 and 2 mM of Si, respectively, under salt stress, while no significant differences were noticed in the case of photosynthetic pigments and Na⁺/K⁺ ratio. Our results showed that Si supplementation potentiated higher salt-tolerance ability in the salt-sensitive BRRI dhan48 as compared with salt-tolerant Binadhan-10. Thus, Si application could be highly beneficial in the growth recovery of the salinity-affected salt-sensitive high yielding rice cultivars in the saline-prone areas.

A Novel Transcriptional Regulator HbERF6 Regulates the HbCIPK2-Coordinated Pathway Conferring Salt Tolerance in Halophytic Hordeum brevisubulatum

Halophytic Hordeum brevisubulatum is a perennial grass which has evolved many distinctive salt-adaptive mechanisms. Our previous studies indicated it could thrive under salt stress through maintaining better K⁺ and Na⁺ homeostasis. Stress-responsive HbCIPK2 can phosphorylate K⁺ channel HbVGKC1 and Na⁺ transporter HbSOS1L to prevent Na⁺ accumulation and K⁺ reduction, hence pathway was not detected in glycophytic plants. In this study, we cloned the inducible promoter of HbCIPK2 by genome-walking, and identified a novel transcriptional regulator HbERF6 through yeast one-hybrid screening. HbERF6 functioned as a transcription factor which can bind to the GCC-box of the HbCIPK2 promoter to activate its expression. HbERF6 transgenic lines in Arabidopsis improved salt tolerance compared with wild type, and especially induced AtCIPK24 (SOS2) expression, resulting in K⁺/Na⁺ homeostasis to enhance salt tolerance. All the results confirmed the inducible function of HbERF6 for CIPK genes during salt tolerance. This regulatory network that integrates transcriptional regulation and post-translation modification will unravel a novel salt stress-responsive mechanism, highlighting the value and utilization of the halophytic resource.

Comparative transcriptome analysis of synthetic and common wheat in response to salt stress

Salt stress reduces wheat yield. Therefore, improvement for enhanced salt stress tolerance is necessary for stable production. To understand the molecular mechanism of salt tolerance in common wheat and synthetic hexaploid (SH) wheat, RNA sequencing was performed on the roots of three wheat lines salt-tolerant SH wheat, salt-tolerant common wheat, and salt-sensitive common wheat. Differentially expressed genes (DEGs) in response to salt stress were characterized using gene ontology enrichment analysis. Salt tolerance in common wheat has been suggested to be mainly regulated by the activation of transporters. In contrast, salt tolerance in SH wheat is enhanced through up-regulation of the reactive oxygen species signaling pathway, other unknown pathways, and different ERF transcription factors. These results indicate that salt tolerance is differentially controlled between common wheat and SH wheat. Furthermore, QTL analysis was performed using the F₂ population derived from SH and salt-sensitive wheat. No statistically significant QTL was detected, suggesting that numerous QTLs with negligible contributions are involved in salt tolerance in SH wheat. We also identified DEGs specific to each line near one probable QTL. These findings show that SH wheat possesses salt tolerance mechanisms lacking in common wheat and may be potential breeding material for salt tolerance.

Iron-Doped Biochar Regulated Soil Nickel Adsorption, Wheat Growth, Its Physiology and Elemental Concentration under Contrasting Abiotic Stresses

The prevalence of abiotic stresses hampers soil health and plant growth in most ecosystems. In this study, rice husk iron-enriched biochar (BC) was prepared and its superiority in terms of nutrients enrichment, porosity and different acidic functional group (O-H, C=O) relative to simple biochar was confirmed through scanning electron microscopic, X-ray fluorescence and Fourier transform infrared analysis. To further evaluate its nickel (Ni), salt (NaCl) and carbonate (CaCO3) stress mitigating impact on wheat physiology and biochemical attributes, a pot experiment was conducted using; BC (1%), Ni (0.5 mM NiNO3), Na (100 mM NaCl) and CO3 (100 mM CaCO3) and with twelve treatments; T1; Control, T2; NiNO3, T3; CaCO3, T4; NaCl, T5; BC, T6; Ni + BC, T7; CaCO3 + BC, T8; NaCl + BC, T9; Ni + CaCO3 + BC, T10; Ni + NaCl + BC, T11; CaCO3 + NaCl + BC, T12; Ni + NaCl + CaCO3 + BC. The Langmuir isotherm model revealed the maximum Ni adsorption capacity (2433 mg g−1) in treatments where Ni was applied with BC soil. Maximum soil DTPA-extractable Ni was found in the T9 treatment; however, Ni concentration was not reported in wheat roots while only trace amounts of Ni were found in wheat shoots with the T9 treatment. It was suggested that BC has the capacity to induce the immunization effect in plant roots by providing additional Fe so their ionic homeostasis and redox metabolism worked properly. This argument was further paved by the enhanced adsorption of these toxic ions in the presence of BC-favored wheat growth as indicated by maximum increases in shoot iron and potassium concentrations under Ni + CaCO3 + BC, relative to control. Furthermore, the decrease in shoot hydrogen peroxide (H2O2) (20%) and malondialdehyde (32%) concentrations and increase in shoot ascorbate peroxidase (81%) and catalase (three-fold) activities under Ni + BC relative to Ni + NaCl + CaCO3 + BC controlled the cell membrane damage. In conclusion, BC proved to be an excellent amendment to reduce the toxic effects of Ni, NaCl and CaCO3 stresses and enhance wheat growth and nutrition.

Salt Priming as a Smart Approach to Mitigate Salt Stress in Faba Bean (Vicia faba L.)

The present investigation aims to highlight the role of salt priming in mitigating salt stress on faba bean. In the absence of priming, the results reflected an increase in H2O2 generation and lipid peroxidation in plants subjected to 200 mM salt shock for one week, accompanied by a decline in growth, photosynthetic pigments, and yield. As a defense, the shocked plants showed enhancements in ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), peroxidase (POX), and superoxide dismutase (SOD) activities. Additionally, the salt shock plants revealed a significant increase in phenolics and proline content, as well as an increase in the expression levels of glutathione (GSH) metabolism-related genes (the L-ascorbate peroxidase (L-APX) gene, the spermidine synthase (SPS) gene, the leucyl aminopeptidase (LAP) gene, the aminopeptidase N (AP-N) gene, and the ribonucleo-side-diphosphate reductase subunit M1 (RDS-M) gene). On the other hand, priming with increasing concentrations of NaCl (50–150 mM) exhibited little significant reduction in some growth- and yield-related traits. However, it maintained a permanent alert of plant defense that enhanced the expression of GSH-related genes, proline accumulation, and antioxidant enzymes, establishing a solid defensive front line ameliorating osmotic and oxidative consequences of salt shock and its injurious effect on growth and yield.

A New Discovery of Argon Functioning in Plants: Regulation of Salinity Tolerance

Argon, a non-polar molecule, easily diffuses into deeper tissue and interacts with larger proteins, protein cavities, or even receptors. Some of the biological effects of argon, notably its activity as an antioxidant, have been revealed in animals. However, whether and how argon influences plant physiology remains elusive. Here, we provide the first report that argon can enable plants to cope with salinity toxicity. Considering the convenience of the application, argon gas was dissolved into water (argon-rich water (ARW)) to investigate the argon’s functioning in phenotypes of alfalfa seed germination and seedling growth upon salinity stress. The biochemical evidence showed that NaCl-decreased α/β-amylase activities were abolished by the application of ARW. The qPCR experiments confirmed that ARW increased NHX1 (Na⁺/H⁺ antiporter) transcript and decreased SKOR (responsible for root-to-shoot translocation of K⁺) mRNA abundance, the latter of which could be used to explain the lower net K⁺ efflux and higher K accumulation. Subsequent results using non-invasive micro-test technology showed that the argon-intensified net Na⁺ efflux and its reduced Na accumulation resulted in a lower Na⁺/K⁺ ratio. NaCl-triggered redox imbalance and oxidative stress were impaired by ARW, as confirmed by histochemical and confocal analyses, and increased antioxidant defense was also detected. Combined with the pot experiments in a greenhouse, the above results clearly demonstrated that argon can enable plants to cope with salinity toxicity via reestablishing ion and redox homeostasis. To our knowledge, this is the first report to address the function of argon in plant physiology, and together these findings might open a new window for the study of argon biology in plant kingdoms.

Exogenously Supplemented Proline and Phenylalanine Improve Growth, Productivity, and Oil Composition of Salted Moringa by Up-Regulating Osmoprotectants and Stimulating Antioxidant Machinery

Salinity is linked to poor plant growth and a reduction in global food output. Therefore, there is an essential need for plant adaptation and mitigation of salinity stress conditions. Plants combat salinity stress influences by promoting a set of physiological, biochemical, and molecular actions. Tremendous mechanisms are being applied to induce plant stress tolerance, involving amino acid application. For evaluating the growth and productivity of Moringa oleifera trees grown under salt stress conditions, moringa has been cultivated under different levels of salinity and subjected to a foliar spray of proline (Pro) and phenylalanine (Phe) amino acids. Moringa plants positively responded to the lowest level of salinity as the leaves, inflorescences, seeds, and oil yields have been increased, but the growth and productivity slightly declined with increasing salinity levels after that. However, Pro and Phe applications significantly ameliorate these effects, particularly, Pro-treatments which decelerated chlorophyll and protein degradation and enhanced vitamin C, polyphenols, and antioxidant activity. A slight reduction in mineral content was observed under the high levels of salinity. Higher osmoprotectants (proline, protein, and total soluble sugars) content was given following Pro treatment in salted and unsalted plants. A significant reduction in oil yield was obtained as affected by salinity stress. Additionally, salinity exhibited a reduction in oleic acid (C18:1), linoleic (C18:2), and linolenic (C18:3) acids, and an increase in stearic (C18:0), palmitic (C16:0), eicosenoic (C20:2), and behenic (C22:0) acids. Generally, Pro and Phe treatments overcome the harmful effects of salinity in moringa trees by stimulating the osmoprotectants, polyphenols, and antioxidant activity, causing higher dry matter accumulation and better defense against salinity stress.

Morphological Responses of Viola Accessions to Nutrient Solution Application and Electrical Conductivity

Growing of ornamental flowering plants represents an important sector of horticulture. Viola × wittrockiana (garden pansy) is used in garden beds and borders due to their colorful blooming, which occurs in early spring and late autumn. Nowadays, breeders focus on applying different nutrient solutions to improve the quality, flowering, and ornamental properties of plants, yet electrical conductivity (EC) level is an important fact to know. It is known that higher EC levels can inhibit plants’ growth. In the present study, pansy seedlings were subjected to different EC nutrient solutions 1 (control), 2, 3.5, 5, and 6.5 mS cm⁻¹ EC to assess the positive or negative effects regarding the plant’s growth and development. The results indicated that an appropriate EC level of nutrient solution can have a positive effect on growth parameters, as well as on the flowering of plants. According to the hierarchical clustering, the used EC nutrient solutions significantly influenced the growth, number of shoots and leaves and the inflorescences number. From the present study results, it can be concluded that even though all EC levels increased growth parameters compared with control, the greatest results were obtained in plants under the effect of the 5 mS cm⁻¹ of EC.

Effects of Salt Stress on Transcriptional and Physiological Responses in Barley Leaves with Contrasting Salt Tolerance

Salt stress negatively impacts crop production worldwide. Genetic diversity among barley (Hordeum vulgare) landraces adapted to adverse conditions should provide a valuable reservoir of tolerance genes for breeding programs. To identify molecular and biochemical differences between barley genotypes, transcriptomic and antioxidant enzyme profiles along with several morpho-physiological features were compared between salt-tolerant (Boulifa) and salt-sensitive (Testour) genotypes subjected to salt stress. Decreases in biomass, photosynthetic parameters, and relative water content were low in Boulifa compared to Testour. Boulifa had better antioxidant protection against salt stress than Testour, with greater antioxidant enzymes activities including catalase, superoxide dismutase, and guaiacol peroxidase. Transcriptome assembly for both genotypes revealed greater accumulation of differentially expressed transcripts in Testour compared to Boulifa, emphasizing the elevated transcriptional response in Testour following salt exposure. Various salt-responsive genes, including the antioxidant catalase 3, the osmoprotectant betaine aldehyde dehydrogenase 2, and the transcription factors MYB20 and MYB41, were induced only in Boulifa. By contrast, several genes associated with photosystems I and II, and light receptor chlorophylls A and B, were more repressed in Testour. Co-expression network analysis identified specific gene modules correlating with differences in genotypes and morpho-physiological traits. Overall, salinity-induced differential transcript accumulation underlies the differential morpho-physiological response in both genotypes and could be important for breeding salt tolerance in barley.

Ion homeostasis in differently adapted populations of Suaeda vera Forssk. ex J.F. Gmel. for phytoremediation of hypersaline soils

Salt-accumulator species are of great interest for the phytoremediation of salt-affected soils to reclaim soil salinization, a major constraints causing germination retardation and growth restriction of plants as well as habitat degradation. Higher biomass production at ECe 23-36 dS m^-1 indicated that this species grows better in high to moderate salinity that was linked to osmotic adjustment through higher ion accumulation (Na⁺, Cl^‒, and Ca²⁺) and organic osmolytes (free amino acids and proline). Plants from highly and moderately saline habitats exhibited broader metaxylem vessels, which was associated with eased conduction of solutes leading to better growth. Leaf anatomical characteristics generally increased with increasing salinity except at the highest ECe 55 dS m^-1. The increased leaf lamina thickness contributed to succulence because of increased storage parenchymatous spongy tissues (that can store high amounts of water), water contents and it is a reflection of maintaining ion homeostasis and colonizing hyper-saline soil. Reduced stomatal density and area under high salinity are critical to cope with environmental hazards. Under high salinity, compartmentalization of excessive Na⁺ and Cl^- ions and accumulation of compatible osmolytes are directly related to high degree of salinity tolerance, and hence are useful for phyto-amelioration of salinity-impacted lands.

The Effective Role of Nano-Silicon Application in Improving the Productivity and Quality of Grafted Tomato Grown under Salinity Stress

This study aims to determine the influence of grafting and nano-silicon fertilizer on the growth and production of tomatoes (Solanumlycopersicum L.) under salinity conditions. A commercial tomato hybrid (cv. Strain B) was used as a scion and two tomato phenotypes were used as rootstocks: S. pimpinellifolium and Edkawy. The rootstock effect was evaluated by growing plants at two NaCl concentrations plus the control (0, 4000, and 8000 ppm NaCl). Nano-silicon foliar application (0.5 ppm) after 20, 28, and 36 days from transplanting was also used to mitigate salinity stress. Antioxidants, hormones, and proline were evaluated for a better understanding of the physiological changes induced by salinity and grafting. The results showed that grafting either on S. pimpinellifolium or Edkawy combined with nano-silicon application enhanced shoot and root growth, fruit yield, and fruit quality. The Edkawy rootstock was more effective than the S. pimpinellifolium rootstock in terms of counteracting the negative effect of salinity. Higher levels of mineral contents, GA3, ABA, and proline were detected in shoots that were subjected to grafting and nano-silicon application compared to the control treatment. This study indicates that grafting and nano-silicon application hold potential as alternative techniques to mitigate salt stress in commercial tomato cultivars.

Metabolites Reprogramming and Na+/K+ Transportation Associated With Putrescine-Regulated White Clover Seed Germination and Seedling Tolerance to Salt Toxicity

Soil salinization is a serious challenge to many countries worldwide. Putrescine (Put) is related to the improvement of seed germination under salt stress, but molecular and metabolic mechanisms are still not fully understood. The objectives of this study were to determine the effect of seed soaking with Put on germination characteristics under salt stress induced by 100 mm sodium chloride (NaCl) and to further analyze subsequent stress tolerance associated with amylolysis, oxidative damage, sodium (Na+)/ potassium (K+) accumulation and transportation, and metabolic homeostasis in white clover (Trifolium repens cv. Haifa) seedlings. The results showed that seed soaking with Put significantly alleviated salt-induced decreases in the endogenous Put content, germination rate, germination vigor, germination index, Rl/SL, and fresh/dry weight of seedlings. Put application also significantly promoted starch metabolism through activating α-amylase and β-amylase activities under salt stress. The metabolomic analysis showed that seed soaking with Put significantly increased the accumulation of polyamines (Put and spermidine), amino acids (γ-aminobutyric acid, glutamate, alanine, proline, citrulline, etc.), organic acids (ketopentanic acid, malonic acid, malic acid, ketopentanic acid, cis-sinapinic acid, etc.), lipids and fatty acids (glycerol, stearic acid, linoleic acid, palmitic acid, etc.), sugars (levoglucosan, fucose, and anhydro-D-galactose), alcohols (myo-inositol, allo-inositol, hexadecanol, and threitol), and other metabolites (thymine, xanthine, adenine, guanine, and glycerol 1-phosphate, etc.) associated with enhanced tricarboxylic acid (TCA) cycle and γ-aminobutyric acid (GABA) shunt contributing to better osmotic adjustment, cell membrane stability, energy supply, and metabolic homeostasis when seeds germinated under salt stress. In addition, Put significantly up-regulated the AsSOS1, NHX6, SKOR, HKT1, and HKT8 expression levels which played critical roles in Na+ rejection and K+ retention resulting in higher K+/Na+ ratio during seed germination under salt stress. The Put-induced up-regulation of HAL2 transcription level could reduce the toxicity of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in cells. Current findings will provide an integrative understanding of Put-induced salt tolerance associated with amylolysis, metabolic regulation, and ionic homeostasis during seed germination.

Protocol for mapping the variability in cell wall mechanical bending behavior in living leaf pavement cells

Mechanical properties, size and geometry of cells, and internal turgor pressure greatly influence cell morphogenesis. Computational models of cell growth require values for wall elastic modulus and turgor pressure, but very few experiments have been designed to validate the results using measurements that deform the entire thickness of the cell wall. New wall material is synthesized at the inner surface of the cell such that full-thickness deformations are needed to quantify relevant changes associated with cell development. Here, we present an integrated, experimental-computational approach to analyze quantitatively the variation of elastic bending behavior in the primary cell wall of living Arabidopsis (Arabidopsis thaliana) pavement cells and to measure turgor pressure within cells under different osmotic conditions. This approach used laser scanning confocal microscopy to measure the 3D geometry of single pavement cells and indentation experiments to probe the local mechanical responses across the periclinal wall. The experimental results were matched iteratively using a finite element model of the experiment to determine the local mechanical properties and turgor pressure. The resulting modulus distribution along the periclinal wall was nonuniform across the leaf cells studied. These results were consistent with the characteristics of plant cell walls which have a heterogeneous organization. The results and model allowed the magnitude and orientation of cell wall stress to be predicted quantitatively. The methods also serve as a reference for future work to analyze the morphogenetic behaviors of plant cells in terms of the heterogeneity and anisotropy of cell walls.

Spermine reduces the harmful effects of salt stress in Tropaeolum majus

Flowers, leaves, fruits and buds of Tropaeolum majus are used for ornamental, medicinal and food purposes. However, salt stress limits the development and productivity of T. majus due to biochemical, physiological and anatomical disturbances. Polyamine application is an alternative for mitigating the harmful effects of salt stress. Thus, the objective of this work was to evaluate the effects of spermine application in T. majus grown under salt stress. The experiment was carried out in a completely randomized design, in a 3 × 2 factorial scheme, with 0, 40 mM (moderate salt stress) and 80 mM (severe salt stress) NaCl, and 0 and 1 mM spermine, and with five replicates. Growth (plant height, stem diameter, number of leaves, number of flowers, number of buds, leaf dry mass, stem dry mass and flower dry mass), gas exchange (gs, A, E, Ci and WUE), relative water content, contents of free amino acids, phenolic compounds, reducing and non-reducing sugars, lipid peroxidation and enzymatic activities (CAT, POD and APX) were evaluated. Spermine application decreased the harmful effects of salt stress on the growth and gas exchange and increased flowering in T. majus. Furthermore, the relative water content of T. majus increased under severe salt stress conditions. Spermine application reduced the contents of total phenolic compounds, free amino acids, reducing sugars and non-reducing sugars on leaves of T. majus. Spermine application increased CAT and POD activities in plants under severe salt stress and POD and APX in plants under moderate salt stress.

Interactive effect of different salinity sources and their formulations on plant growth, ionic homeostasis and seed quality of maize

Soil salinity is one of the most pernicious environmental hazards affecting crop growth and productivity in arid and semi-arid climates. In saline soils, the crop plants encounter nutrients deficient conditions mainly due to antagonistic affinity of sodium (Na⁺) and chloride (Cl^-). The accumulation of salts in the rhizosphere restricts plant growth, the severity of which depends on the source and concentration of the salt. Therefore, we hypothesized that sodium containing salts could have toxic effects on maize plants either in a single or in combined form. To evaluate the interactive effect of sodium salts on plant growth, ionic homeostasis, and seed quality attributes, a pot study was performed using maize as a test plant at the research area of the College of Agriculture, University of Sargodha. Selected salts including, NaCl, Na₂SO₄ and their combination (NaCl + Na₂SO₄), were applied in equal ratio for different salinity levels (7, 10, 13 and 16 dS m^-1) and the untreated control. The results show that all the measured growth, yield, biochemical and quality attributes of maize were negatively affected with increasing concentration of all the salt sources; however, severity of these effects were more intense when NaCl was applied at all salinity levels. It is concluded that all salts (NaCl, Na₂SO₄ and NaCl + Na₂SO₄) had negative effects on biochemical, qualitative, growth and yield characteristics of maize plant. Most importantly, NaCl was found to be more harmful compared to Na₂SO₄ and mixtures of both salts due to the dominance of Na+ and Cl^-ions. Among all salinity levels, the more detrimental effects of NaCl occurred at salinity level of 16 dS m^-1.

The Halophyte Species Solanum chilense Dun. Maintains Its Reproduction despite Sodium Accumulation in Its Floral Organs

Salinity is a growing global concern that affects the yield of crop species, including tomato (Solanum lycopersicum). Its wild relative Solanum chilense was reported to have halophyte properties. We compared salt resistance of both species during the reproductive phase, with a special focus on sodium localization in the flowers. Plants were exposed to NaCl from the seedling stage. Salinity decreased the number of inflorescences in both species but the number of flowers per inflorescence and sepal length only in S. lycopersicum. External salt supply decreased the stamen length in S. chilense, and it was associated with a decrease in pollen production and an increase in pollen viability. Although the fruit set was not affected by salinity, fruit weight and size decreased in S. lycopersicum. Concentrations and localization of Na, K, Mg, and Ca differed in reproductive structures of both species. Inflorescences and fruits of S. chilense accumulated more Na than S. lycopersicum. Sodium was mainly located in male floral organs of S. chilense but in non-reproductive floral organs in S. lycopersicum. The expression of Na transporter genes differed in flowers of both species. Overall, our results indicated that S. chilense was more salt-resistant than S. lycopersicum during the reproductive phase and that differences could be partly related to dissimilarities in element distribution and transport in flowers.

Physiological response and proteomics analysis of Reaumuria soongorica under salt stress

Soil salinity can severely restrict plant growth. Yet Reaumuria soongorica can tolerate salinity well. However, large-scale proteomic studies of this plant’s response to salinity have yet to reported. Here, R. soongorica seedlings (4 months old) were used in an experiment where NaCl solutions simulated levels of soil salinity stress. The fresh weight, root/shoot ratio, leaf relative conductivity, proline content, and total leaf area of R. soongorica under CK (0 mM NaCl), low (200 mM NaCl), and high (500 mM NaCl) salt stress were determined. The results showed that the proline content of leaves was positively correlated with salt concentration. With greater salinity, the plant fresh weight, root/shoot ratio, and total leaf area increased initially but then decreased, and vice-versa for the relative electrical conductivity of leaves. Using iTRAQ proteomic sequencing, 47 177 136 differentially expressed proteins (DEPs) were identified in low-salt versus CK, high-salt versus control, and high-salt versus low-salt comparisons, respectively. A total of 72 DEPs were further screened from the comparison groupings, of which 34 DEPs increased and 38 DEPs decreased in abundance. These DEPs are mainly involved in translation, ribosomal structure, and biogenesis. Finally, 21 key DEPs (SCORE value ≥ 60 points) were identified as potential targets for salt tolerance of R. soongolica. By comparing the protein structure of treated versus CK leaves under salt stress, we revealed the key candidate genes underpinning R. soongolica’s salt tolerance ability. This works provides fresh insight into its physiological adaptation strategy and molecular regulatory network, and a molecular basis for enhancing its breeding, under salt stress conditions.

Impact of Single and Combined Salinity and High-Temperature Stresses on Agro-Physiological, Biochemical, and Transcriptional Responses in Rice and Stress-Release

Here, for the first time, we aimed to identify in rice the key mechanisms and processes underlying tolerance to high-temperature (HT) or salt stress (SS) alone, the co-occurrence of both stresses, and recovery using physiological and biochemical measurements and gene expression analysis. We also investigated whether recovery from the two stressors depended on the relative intensities/relief of each stressor. Wild type ('Yukinkomai') rice plants were found to be more susceptible to salinity or heat applied individually. SS leads to a depletion of cellular water content, higher accumulation of Na⁺, and alterations in photosynthetic pigments. The stress-tolerant cultivar 'YNU31-2-4' (YNU) displayed a lower Na⁺/K⁺ ratio, higher water content in cells and improved photosynthetic traits, antioxidant system, and expression of defence genes. Strikingly, the SS + HT combination provided a significant level of protection to rice plants from the effects of SS alone. The expression pattern of a selected set of genes showed a specific response and dedicated pathways in plants subjected to each of the different stresses, while other genes were explicitly activated when the stresses were combined. Aquaporin genes were activated by SS, while stress-related (P5CS, MSD1, HSPs, and ions transporters) genes were shaped by HT. Hierarchical clustering and principal component analyses showed that several traits exhibited a gradually aggravating effect as plants were exposed to the combined stresses and identified heat as a mitigating factor, clearly separating heat + salt-stressed from salt-non-heat-stressed plants. Furthermore, seedling recovery was far more dependent on the relative intensities of stressors and cultivars, demonstrating the influence of one stressor over another upon stress-release. Taken together, our data show the uniqueness and complexity of the physiological and molecular network modules used by rice plants to respond to single and combined stresses and recovery.

Molecular Insights into Salinity Responsiveness in Contrasting Genotypes of Rice at the Seedling Stage

Salinity is one of the most common unfavorable environmental conditions that limits plant growth and development, ultimately reducing crop productivity. To investigate the underlying molecular mechanism involved in the salinity response in rice, we initially screened 238 rice cultivars after salt treatment at the seedling stage and identified two highly salt-tolerant cultivars determined by the relative damage rate parameter. The majority of cultivars (94.1%) were ranked as salt-sensitive and highly salt-sensitive. Transcriptome profiling was completed in highly salt-tolerant, moderately salt-tolerant, and salt-sensitive under water and salinity treatments at the seedling stage. Principal component analysis displayed a clear distinction among the three cultivars under control and salinity stress conditions. Several starch and sucrose metabolism-related genes were induced after salt treatment in all genotypes at the seedling stage. The results from the present study enable the identification of the ascorbate glutathione pathway, potentially participating in the process of plant response to salinity in the early growth stage. Our findings also highlight the significance of high-affinity K⁺ uptake transporters (HAKs) and high-affinity K⁺ transporters (HKTs) during salt stress responses in rice seedlings. Collectively, the cultivar-specific stress-responsive genes and pathways identified in the present study act as a useful resource for researchers interested in plant responses to salinity at the seedling stage.

A Comprehensive Evaluation of Salt Tolerance in Tomato (Var. Ailsa Craig): Responses of Physiological and Transcriptional Changes in RBOH's and ABA Biosynthesis and Signalling Genes

Salinity is a ubiquitous stressor, depleting osmotic potential and affecting the tomato seedlings’ development and productivity. Considering this critical concern, we explored the salinity response in tomato seedlings by evaluating them under progressive salt stress duration (0, 3, 6, and 12 days). Intriguingly, besides the adverse effect of salt stress on tomato growth the findings exhibited a significant role of tomato antioxidative system, RBOH genes, ABA biosynthesis, and signaling transcription factor for establishing tolerance to salinity stress. For instance, the activities of enzymatic and non-enzymatic antioxidants continued to incline positively with the increased levels of reactive oxygen species (O₂^•−, H₂O₂), MDA, and cellular damage, suggesting the scavenging capacity of tomato seedlings against salt stress. Notably, the RBOH transcription factors activated the hydrogen peroxide-mediated signalling pathway that induced the detoxification mechanisms in tomato seedlings. Consequently, the increased gene expression of antioxidant enzymes and the corresponding ratio of non-enzymatic antioxidants AsA-GSH suggested the modulation of antioxidants to survive the salt-induced oxidative stress. In addition, the endogenous ABA level was enhanced under salinity stress, indicating higher ABA biosynthesis and signalling gene expression. Subsequently, the upregulated transcript abundance of ABA biosynthesis and signalling-related genes suggested the ABA-mediated capacity of tomato seedlings to regulate homeostasis under salt stress. The current findings have revealed fascinating responses of the tomato to survive the salt stress periods, in order to improve the abiotic stress tolerance in tomato.

Effects of Phytohormone-Producing Rhizobacteria on Casparian Band Formation, Ion Homeostasis and Salt Tolerance of Durum Wheat

Inoculation with plant growth-promoting rhizobacteria can increase plant salt resistance. We aimed to reveal bacterial effects on the formation of apoplastic barriers and hormone concentration in relation to maintaining ion homeostasis and growth of salt-stressed plants. The rhizosphere of a durum wheat variety was inoculated with cytokinin-producing Bacillus subtilis and auxin-producing Pseudomonas mandelii strains. Plant growth, deposition of lignin and suberin and concentrations of sodium, potassium, phosphorus and hormones were studied in the plants exposed to salinity. Accumulation of sodium inhibited plant growth accompanied by a decline in potassium in roots and phosphorus in shoots of the salt-stressed plants. Inoculation with both bacterial strains resulted in faster appearance of Casparian bands in root endodermis and an increased growth of salt-stressed plants. B. subtilis prevented the decline in both potassium and phosphorus concentrations and increased concentration of cytokinins in salt-stressed plants. P. mandelii decreased the level of sodium accumulation and increased the concentration of auxin. Growth promotion was greater in plants inoculated with B. subtilis. Increased ion homeostasis may be related to the capacity of bacteria to accelerate the formation of Casparian bands preventing uncontrolled diffusion of solutes through the apoplast. We discuss the relative impacts of the decline in Na accumulation and maintenance of K and P content for growth improvement of salt-stressed plants and their possible relation to the changes in hormone concentration in plants.

Mitigation of Salinity Stress Effects on Broad Bean Productivity Using Calcium Phosphate Nanoparticles Application

Water salinity is one of the major abiotic stresses, and the use of saline water for the agricultural sector will incur greater demand in the coming decades. Recently, nanoparticles (NPs) have been used for developing numerous plant fertilizers as a smart and powerful form of material with dual action that can alleviate the adverse effects of salinity and provide the plant with more efficient nutrient forms. This study evaluated the influence of calcium phosphate NPs (CaP-NPs) as a soil fertilizer application on the production and bioactive compounds of broad bean plants under salinity stress. Results showed that salinity had deleterious effects on plant yield with 55.9% reduction compared to control. On the other hand, CaP-NPs dramatically improved plant yield by 30% compared to conventional fertilizer under salinity stress. This improvement could be attributed to significantly higher enhancement in total soluble sugars, antioxidant enzymes, proline content, and total phenolics recorded use of nano-fertilizer compared to conventional use under salt stress. Additionally, nano-fertilizer reflected better mitigatory effects on plant growth parameters, photosynthetic pigments, and oxidative stress indicators (MDA and H2O2). Therefore, our results support the replacement of traditional fertilizers comprising Ca2+ or P with CaP-nano-fertilizers for higher plant productivity and sustainability under salt stress.

Integrated Analysis of miRNAs Associated With Sugarcane Responses to Low-Potassium Stress

Sugarcane is among the most important global crops and a key bioenergy source. Sugarcane production is restricted by limited levels of available soil potassium (K⁺). The ability of plants to respond to stressors can be regulated by a range of microRNAs (miRNAs). However, there have been few studies regarding the roles of miRNAs in the regulation of sugarcane responses to K⁺-deficiency. To understand how these non-coding RNAs may influence sugarcane responses to low-K⁺ stress, we conducted expression profiling of miRNAs in sugarcane roots under low-K⁺ conditions via high-throughput sequencing. This approach led to the identification of 324 and 42 known and novel miRNAs, respectively, of which 36 were found to be differentially expressed miRNAs (DEMs) under low-K⁺ conditions. These results also suggested that miR156-x/z and miR171-x are involved in these responses as potential regulators of lateral root formation and the ethylene signaling pathway, respectively. A total of 705 putative targets of these DEMs were further identified through bioinformatics predictions and degradome analyses, and GO and KEGG enrichment analyses revealed these target mRNAs to be enriched for catalytic activity, binding functions, metabolic processes, plant hormone signal transduction, and mitogen-activated protein kinase (MAPK) signaling. In summary, these data provide an overview of the roles of miRNAs in the regulation of sugarcane response to low-K⁺ conditions.

Halotolerant Rhizobacteria for Salinity-Stress Mitigation: Diversity, Mechanisms and Molecular Approaches

Agriculture is the best foundation for human livelihoods, and, in this respect, crop production has been forced to adopt sustainable farming practices. However, soil salinity severely affects crop growth, the degradation of soil quality, and fertility in many countries of the world. This results in the loss of profitability, the growth of agricultural yields, and the step-by-step decline of the soil nutrient content. Thus, researchers have focused on searching for halotolerant and plant growth-promoting bacteria (PGPB) to increase soil fertility and productivity. The beneficial bacteria are frequently connected with the plant rhizosphere and can alleviate plant growth under salinity stress through direct or indirect mechanisms. In this context, PGPB have attained a unique position. The responses include an increased rate of photosynthesis, high production of antioxidants, osmolyte accumulation, decreased Na+ ions, maintenance of the water balance, a high germination rate, and well-developed root and shoot elongation under salt-stress conditions. Therefore, the use of PGPB as bioformulations under salinity stress has been an emerging research avenue for the last few years, and applications of biopesticides and biofertilizers are being considered as alternative tools for sustainable agriculture, as they are ecofriendly and minimize all kinds of stresses. Halotolerant PGPB possess greater potential for use in salinity-affected soil as sustainable bioinoculants and for the bioremediation of salt-affected soil.

Plant salt response: Perception, signaling, and tolerance

Salt stress is one of the significant environmental stressors that severely affects plant growth and development. Plant responses to salt stress involve a series of biological mechanisms, including osmoregulation, redox and ionic homeostasis regulation, as well as hormone or light signaling-mediated growth adjustment, which are regulated by different functional components. Unraveling these adaptive mechanisms and identifying the critical genes involved in salt response and adaption are crucial for developing salt-tolerant cultivars. This review summarizes the current research progress in the regulatory networks for plant salt tolerance, highlighting the mechanisms of salt stress perception, signaling, and tolerance response. Finally, we also discuss the possible contribution of microbiota and nanobiotechnology to plant salt tolerance.

Genome-wide analysis of potassium transport genes in Gossypium raimondii suggest a role of GrHAK/KUP/KT8, GrAKT2.1 and GrAKT1.1 in response to abiotic stress

Potassium (K⁺) is an important macro-nutrient for plants, which comprises almost 10% of plant's dry mass. It plays a crucial role in the growth of plants as well as other important processes related to metabolism and stress tolerance. Plants have a complex and well-organized potassium distribution system (channels and transporters). Cotton is the most important economic crop, which is the primary source of natural fiber. Soil deficiency in K⁺ can negatively affect yield and fiber quality of cotton. However, potassium transport system in cotton is poorly studied. Current study identified 43 Potassium Transport System (PTS) genes in Gossypium raimondii genome. Based on conserved domains, transmembrane domains, and motif structures, these genes were classified as K⁺ transporters (2 HKTs, 7 KEAs, and 16 KUP/HAK/KTs) and K⁺ channels (11 Shakers and 7 TPKs/KCO). The phylogenetic comparison of GrPTS genes from Arabidopsis thaliana, Glycine max, Oryza sativa, Medicago truncatula and Cicer arietinum revealed variations in PTS gene conservation. Evolutionary analysis predicted that most GrPTS genes were segmentally duplicated. Gene structure analysis showed that the intron/exon organization of these genes was conserved in specific-family. Chromosomal localization demonstrated a random distribution of PTS genes across all the thirteen chromosomes except chromosome six. Many stress responsive cis-regulatory elements were predicted in promoter regions of GrPTS genes. The RNA-seq data analysis followed by qRT-PCR validation demonstrated that PTS genes potentially work in groups against environmental factors. Moreover, a transporter gene (GrHAK/KUP/KT8) and two channel genes (GrAKT2.1 and GrAKT1.1) are important candidate genes for plant stress response. These results provide useful information for further functional characterization of PTS genes with the breeding aim of stress-resistant cultivars.