Global plant-responding mechanisms to salt stress: physiological and molecular levels and implications in biotechnology

An ABA Functional Analogue B2 Enhanced Salt Tolerance by Inducing the Root Elongation and Reducing Peroxidation Damage in Maize Seedlings

Salt stress negatively affects maize growth and yield. Application of plant growth regulator is an effective way to improve crop salt tolerance, therefore reducing yield loss by salt stress. Here, we used a novel plant growth regulator B2, which is a functional analogue of ABA. With the aim to determine whether B2 alleviates salt stress on maize, we studied its function under hydroponic conditions. When the second leaf was fully developed, it was pretreated with 100 µM ABA, 0.01 µM B2, 0.1 µM B2, and 1 µM B2, independently. After 5 days treatment, NaCl was added into the nutrient solution for salt stress. Our results showed that B2 could enhance salt tolerance in maize, especially when the concentration was 1.0 µMol·L⁻¹. Exogenous application of B2 significantly enhanced root growth, and the root/shoot ratio increased by 7.6% after 6 days treatment under salt stress. Compared with control, the ABA level also decreased by 31% after 6 days, which might have resulted in the root development. What is more, B2 maintained higher photosynthetic capacity in maize leaves under salt stress conditions and increased the activity of antioxidant enzymes and decreased the generation rate of reactive oxygen species by 16.48%. On the other hand, B2 can enhance its water absorption ability by increasing the expression of aquaporin genes ZmPIP1-1 and ZmPIP1-5. In conclusion, the novel plant growth regulator B2 can effectively improve the salt tolerance in maize.

RsSOS1 Responding to Salt Stress Might Be Involved in Regulating Salt Tolerance by Maintaining Na+ Homeostasis in Radish (Raphanus sativus L.)

Radish is a kind of moderately salt-sensitive vegetable. Salt stress seriously decreases the yield and quality of radish. The plasma membrane Na+/H+ antiporter protein Salt Overly Sensitive 1 (SOS1) plays a crucial role in protecting plant cells against salt stress, but the biological function of the RsSOS1 gene in radish remains to be elucidated. In this study, the RsSOS1 gene was isolated from radish genotype ‘NAU-TR17’, and contains an open reading frame of 3414 bp encoding 1137 amino acids. Phylogenetic analysis showed that RsSOS1 had a high homology with BnSOS1, and clustered together with Arabidopsis plasma membrane Na+/H+ antiporter (AtNHX7). The result of subcellular localization indicated that the RsSOS1 was localized in the plasma membrane. Furthermore, RsSOS1 was strongly induced in roots of radish under 150 mmol/L NaCl treatment, and its expression level in salt-tolerant genotypes was significantly higher than that in salt-sensitive ones. In addition, overexpression of RsSOS1 in Arabidopsis could significantly improve the salt tolerance of transgenic plants. Meanwhile, the transformation of RsSOS1△999 could rescue Na+ efflux function of AXT3 yeast. In summary, the plasma membrane Na+/H+ antiporter RsSOS1 plays a vital role in regulating salt-tolerance of radish by controlling Na+ homeostasis. These results provided useful information for further functional characterization of RsSOS1 and facilitate clarifying the molecular mechanism underlying salt stress response in radish.

Water, Soil, and Plants Interactions in a Threatened Environment

The unprecedented pressure posed by a growing population on water resources generates a significant shortage between the available resources and water demand, which together with water scarcity, floods, and droughts, can affect the world population and various other consumers. On the other hand, soil resources, which represent an essential and complex environmental ecosystem, as a support for the biological cycle, source of nutrients, and water for cultivated and wild plants, forestry, etc., are a provider of raw materials, and are increasingly degrading due to unsustainable use. Since both soil and water are vital resources and support for growth and life of plants, their preservation and sustainable management have become an urgent issue for policy makers, governmental factors, academia, and stakeholders. An important question to be answered is what the disturbing factors of soil–plants–water cycles are and how their negative influence can be reduced, since they affect the quality of life and human health. This work proposes an overview on new research into the links between soil and water, and the interactions among soil, water, and plants in a changing and threatened environment, which can determine human welfare. The analysis addresses the global context of water and soil resources, factors that affect their equilibrium and dynamics, especially toxic pollutants such as heavy metals and others, and their mutual relationship with plant growth.

Calcium amendment for improved germination, plant growth, and leaf photosynthetic electron transport in oat (Avena sativa) under NaCl stress

Calcium (Ca2+) is an essential nutrient element for plants as it stabilizes the membrane system structure and controls enzyme activity. To investigate the effects of Ca2+ on plant growth and leaf photosynthetic electron transport in oat (Avena sativa) under NaCl stress, oat seeds and plants were cultivated in nutrient solutions with single NaCl treatment and NaCl treatment with CaCl2 amendment. By measuring the seed germination rate, plant growth, Na+ and Cl- accumulation in leaves, ion leakage in seedlings and leaves, prompt chlorophyll a fluorescence (PF) transient (OJIP), delayed chlorophyll a fluorescence (DF), and modulated 820 nm reflection (MR) values of the leaves at different growth phases, we observed that Ca2+ alleviated the inhibition of germination and plant growth and decreased Na+ and Cl- accumulation and ion leakage in the leaves under NaCl stress. NaCl stress changed the curves of the OJIP transient, induced PF intensity at P-step (FP) decrease and PF intensity at J-step (FJ) increase, resulted in obvious K and L bands, and altered the performance index of absorption (PIABS), the absorption of antenna chlorophyll (ABS/RC), electron movement efficiency (ETo/TRo), and potential maximum photosynthetic capacity (FV/FM) values. With the time extension of NaCl stress, I1 and I2 in the DF curve showed a decreasing trend, the lowest values of MR/MRO curve increased, and the highest points of the MR/MRO curve decreased. Compared with NaCl treatment, the extent of change induced by NaCl in the values of OJIP, DF and MR was reduced in the NaCl treatment with CaCl2 amendment. These results revealed that Ca2+ might improve the photosynthetic efficiency and the growth of salt-stressed plants by maintaining the integrity of oxygen-evolving complexes and electron transporters on the side of the PSI receptor and enhancing the relationship between the functional units of the photosynthetic electron transport chain. The findings from this study could be used for improving crop productivity in saline alkali lands.

Modulation of salt-induced stress impact in Gladiolus grandiflorus L. by exogenous application of salicylic acid

Salinity is challenging threats to the agricultural system and leading cause of crop loss. Salicylic acid (SA) is an important endogenous signal molecule, which by regulating growth and physiological processes improves the plant ability to tolerate salt stress. Considering the prime importance of Gladiolus grandiflorus (L.) in the world's cut-flower market, the research work was undertaken to elucidate salinity tolerance in G. grandiflorus by exogenous application of SA irrigated with saline water. Results revealed that increasing salinity (EC: 2, 4 and 6 dS m^-1) considerably altered morpho-growth indices (corm morphology and plant biomass) in plants through increasing key antioxidants including proline content and enzymes activity (superoxide dismutase, catalase and peroxidase), while negatively affected the total phenolic along with activity of defense-related enzymes (phenylalanine ammonia lyase, and polyphenol oxidase activity). SA application (50-200 ppm) in non-saline control or saline conditions improved morpho-physiological traits in concentration-dependent manners. In saline conditions, SA minimized salt-stress by enhancing chlorophyll content, accumulating organic osmolytes (glycine betaine and proline content), total phenolic, and boosting activity of antioxidant and defense-related enzymes. Principle component analysis based on all 16 morphological and physiological variables generated useful information regarding the classification of salt tolerant treatment according to their response to SA. These results suggest SA (100 or 150 ppm) could be used as an effective, economic, easily available and safe phenolic agent against salinity stress in G. grandiflorus.

Enhanced tolerance to salinity stress in grapevine plants through application of carbon quantum dots functionalized by proline

Salinity has destructive impacts in plant production; therefore, application of new approaches such as nanotechnology and plant priming is attracting increasing attention as an innovative means to ameliorate salt stress effects. Considering the unique properties and recorded beneficial influence of carbon quantum dots (CQDs) and proline in plant growth and physiological parameters when applied individually, their conjugation in the form of carbon quantum dot nanoparticles functionalized by proline (Pro-CQDs NPs) could lead to synergistic effects. Accordingly, an experiment was conducted to evaluate the impact of this advanced nanomaterial (Pro-CQDs NPs) as a chemical priming agent, in grapevine plants cv. 'Rasha'. For this purpose, proline, CQDs, and Pro-CQDs NPs at three concentrations (0, 50, and 100 mg L^-1) were applied exogenously 48 h prior to salinity stress (0 and 100 mM NaCl) that was imposed for a month. Three days after imposing salt stress, an array of biochemical measurements was recorded, while agronomic and some physiological parameters were noted at the end of the stress period. Results revealed that proline treatment at both concentrations, as well as CQDs and Pro-CQDs NPs at low concentration, positively affected grapevine plants under both non-stress and stress conditions. Specifically, the application of proline at 100 mg L^-1 and Pro-CQDs NPs at 50 mg L^-1 resulted in optimal performance identifying 50 mg L^-1 Pro-CQDs NPs as the optimal treatment. Proline treatment at 100 mg L^-1 increased leaf fresh weight (FW) and dry weight (DW); chl a, b, and proline content; SOD activity under both non-stress and stress conditions; Y (II) under salinity and carotenoid content; and CAT activity under control conditions. Pro-CQDs NP treatment at 50 mg L^-1 enhanced total phenol, anthocyanin, and ^Fv/_Fo, as well as APX and GP activities under both conditions, while increasing carotenoid, Y (II), ^Fv/_Fo, and CAT activity under salinity. Furthermore, it decreased MDA and H₂O₂ contents at both conditions and EL and Y (NO) under salt stress. Overall, conjugation of CQDs with proline at 50 mg L^-1 resulted in further improving the protective effect of proline application at 100 mg L^-1. Therefore, functionalization of NPs with chemical priming agents appears to be an effective means of optimizing plant-priming approaches towards efficient amelioration of abiotic stress-related damage in plants.

The Effects of Plant-Associated Bacterial Exopolysaccharides on Plant Abiotic Stress Tolerance

Plant growth-promoting rhizobacteria (PGPR) are beneficial soil microorganisms that can stimulate plant growth and increase tolerance to biotic and abiotic stresses. Some PGPR are capable of secreting exopolysaccharides (EPS) to protect themselves and, consequently, their plant hosts against environmental fluctuations and other abiotic stresses such as drought, salinity, or heavy metal pollution. This review focuses on the enhancement of plant abiotic stress tolerance by bacterial EPS. We provide a comprehensive summary of the mechanisms through EPS to alleviate plant abiotic stress tolerance, including salinity, drought, temperature, and heavy metal toxicity. Finally, we discuss how these abiotic stresses may affect bacterial EPS production and its role during plant-microbe interactions.

Overexpression of isochorismate synthase enhances salt tolerance in barley

Isochorismate synthase (ICS) is a key enzyme for the synthesis of salicylic acid (SA) in plants. SA plays an important role in the response of plants to abiotic stress. In this study, transgenic barley was constructed to evaluate the function of ICS under salt stress. ICS_OE lines showed obvious salt stress tolerance, this results from the increased outward Na⁺ flux and inward K⁺ flux in roots, thereby maintaining a lower cytosolic Na⁺/K⁺ ratio under salt stress. Overexprssion of ICS also improved Na⁺ sequestration in shoots under salt stress. In addition, ICS_OE lines displayed less accumulation of reactive oxygen species and oxidative damage, accompanied by higher activity of antioxidant enzymes. The improved Na⁺/K⁺ ratio, Na⁺ sequestration, and antioxidative competence play an important role in the enhanced salt tolerance of ICS_OE lines. These findings help to elucidate the abiotic stress resistance of the ICS pathway in barley.

Genotypic differences relative photochemical activity, inorganic and organic solutes and yield performance in clones of the forage cactus under semi-arid environment

Plants with the crassulacean acid metabolism commonly present good adaptation to arid and semi-arid environments, but it highly depends on the type of species. In this study, chlorophyll fluorescence, the concentration of inorganic and organic solutes and the productive performance were evaluated along with their relationships in different clones of the genera Opuntia and Nopalea. The experiment was conducted from 2016 to 2018. Four clones of genus Opuntia were evaluated: 'Orelha de Elefante Mexicana' (OEM), 'Orelha de Elefante Africana' (OEA), V19 and F8; and two clones of genus Nopalea: 'IPA Sertânia' and 'Miúda'. The experiment was arranged in a randomised block design, with six treatments and three replications. The following parameters were measured when harvesting: initial, maximum and variable fluorescence; the quantum yield of PSII (Fv/Fm); light-induction curves of the photochemical parameters (ΔF/Fm', qP, NPQ and ETR); the chlorophyll and carotenoid content; carbohydrates; the sodium (Na⁺) and potassium (K⁺) content; morphometry; and dry matter accumulation. The values for the effective quantum yield of PSII (ΔF/Fm') and the alterations in photochemical quenching were higher in the OEM clone (P < 0.05). There was a difference between clones for non-photochemical quenching, with the F8 clone having the highest values. The Fv/Fm was 0.87 for the OEM. 'IPA Sertânia' obtained the greatest Chl a/b, and the highest values for carbohydrate concentration were found in the OEA clone. The OEM clone showed the greatest accumulation of K⁺, in addition to a higher cladode area index and greater dry matter accumulation. The results of this study show the high physiological tolerance of the forage cactus to a semi-arid environment, which varies according to the clone.

Understanding the Integrated Pathways and Mechanisms of Transporters, Protein Kinases, and Transcription Factors in Plants under Salt Stress

Abiotic stress is the major threat confronted by modern-day agriculture. Salinity is one of the major abiotic stresses that influence geographical distribution, survival, and productivity of various crops across the globe. Plants perceive salt stress cues and communicate specific signals, which lead to the initiation of defence response against it. Stress signalling involves the transporters, which are critical for water transport and ion homeostasis. Various cytoplasmic components like calcium and kinases are critical for any type of signalling within the cell which elicits molecular responses. Stress signalling instils regulatory proteins and transcription factors (TFs), which induce stress-responsive genes. In this review, we discuss the role of ion transporters, protein kinases, and TFs in plants to overcome the salt stress. Understanding stress responses by components collectively will enhance our ability in understanding the underlying mechanism, which could be utilized for crop improvement strategies for achieving food security.

ZxNHX1 indirectly participates in controlling K+ homeostasis in the xerophyte Zygophyllum xanthoxylum

The succulent xerophyte Zygophyllum xanthoxylum (Bunge) Engl. can absorb Na+ from the soil as an osmoticum in order to resist osmotic stress. The tonoplast Na+/H+ antiporter ZxNHX1 is essential for maintaining the salt-accumulation characteristics of Z. xanthoxylum by compartmentalizing Na+ into vacuoles. Previous results revealed that the silencing of ZxNHX1 greatly decreased Na+ accumulation in Z. xanthoxylum under 50 mM NaCl due to the weakened compartmentalisation; in addition, K+ concentration also significantly reduced in ZxNHX1-RNAi lines. Yet, whether the reduction of K+ concentration was directly triggered by the silencing of ZxNHX1 remains unclear. In this study, the growth parameters and expression levels of ZxSOS1, ZxHKT1;1, ZxAKT1 and ZxSKOR were measured in wild-type and ZxNHX1-RNAi lines under control or -0.5 MPa osmotic stress. The results showed that the silencing of ZxNHX1 inhibited the plant growth, decreased Na+ concentration in leaves, reduced the transcript abundance of ZxSOS1 and dramatically increased that of ZxHKT1;1 in roots of Z. xanthoxylum under osmotic stress; whereas tissue K+ concentrations and the expression level of ZxSKOR displayed no significant variations, and the expression of ZxAKT1 were significantly reduced in ZxNHX1-RNAi lines under osmotic stress, compared with the wild type. These results suggest that in Z. xanthoxylum, ZxNHX1 can maintain the normal growth by compartmentalizing Na+ into vacuoles, and regulate the spatial distribution of Na+ indirectly by affecting the expressions of ZxSOS1 and ZxHKT1;1. Moreover, the silencing of ZxNHX1 is not the main reason that led to the reduction of K+ concentration in ZxNHX1-RNAi lines under 50 mM NaCl, and ZxNHX1 might be indirectly involved in regulating K+ homeostasis.

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.

The Chloroplastic Small Heat Shock Protein Gene KvHSP26 Is Induced by Various Abiotic Stresses in Kosteletzkya virginica

Small heat shock proteins (sHSPs) are a group of chaperone proteins existed in all organisms. The functions of sHSPs in heat and abiotic stress responses in many glycophyte plants have been studied. However, their possible roles in halophyte plants are still largely known. In this work, a putative sHSP gene KvHSP26 was cloned from K. virginica. Bioinformatics analyses revealed that KvHSP26 encoded a chloroplastic protein with the typical features of sHSPs. Amino acid sequence alignment and phylogenetic analysis demonstrated that KvHSP26 shared 30%-77% homology with other sHSPs from Arabidopsis, cotton, durian, salvia, and soybean. Quantitative real-time PCR (qPCR) assays exhibited that KvHSP26 was constitutively expressed in different tissues such as leaves, stems, and roots, with a relatively higher expression in leaves. Furthermore, expression of KvHSP26 was strongly induced by salt, heat, osmotic stress, and ABA in K. virginica. All these results suggest that KvHSP26 encodes a new sHSP, which is involved in multiple abiotic stress responses in K. virginica, and it has a great potential to be used as a candidate gene for the breeding of plants with improved tolerances to various abiotic stresses.

Identification and Characterization of Wheat Germplasm for Salt Tolerance

Salinity is one of the limiting factors of wheat production worldwide. A total of 334 internationally derived wheat genotypes were employed to identify new germplasm resources for salt tolerance breeding. Salt stress caused 39, 49, 58, 55, 21 and 39% reductions in shoot dry weight (SDW), root dry weight (RDW), shoot fresh weight (SFW), root fresh weight (RFW), shoot height (SH) and root length (RL) of wheat, respectively, compared with the control condition at the seedling stage. The wheat genotypes showed a wide genetic and tissue diversity for the determined characteristics in response to salt stress. Finally, 12 wheat genotypes were identified as salt-tolerant through a combination of one-factor (more emphasis on the biomass yield) and multifactor analysis. In general, greater accumulation of osmotic substances, efficient use of soluble sugars, lower Na+/K+ and a higher-efficiency antioxidative system contribute to better growth in the tolerant genotypes under salt stress. In other words, the tolerant genotypes are capable of maintaining stable osmotic potential and ion and redox homeostasis and providing more energy and materials for root growth. The identified genotypes with higher salt tolerance could be useful for developing new salt-tolerant wheat cultivars as well as in further studies to underline the genetic mechanisms of salt tolerance in wheat.

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.

Integration of Gas Exchange With Metabolomics: High-Throughput Phenotyping Methods for Screening Biostimulant-Elicited Beneficial Responses to Short-Term Water Deficit

Biostimulants are emerging as a feasible tool for counteracting reduction in climate change-related yield and quality under water scarcity. As they are gaining attention, the necessity for accurately assessing phenotypic variables in their evaluation is emerging as a critical issue. In light of this, high-throughput phenotyping techniques have been more widely adopted. The main bottleneck of these techniques is represented by data management, which needs to be tailored to the complex, often multifactorial, data. This calls for the adoption of non-linear regression models capable of capturing dynamic data and also the interaction and effects between multiple factors. In this framework, a commercial glycinebetaine- (GB-) based biostimulant (Vegetal B60, ED&F Man) was tested and distributed at a rate of 6 kg/ha. Exogenous application of GB, a widely accumulated and documented stress adaptor molecule in plants, has been demonstrated to enhance the plant abiotic stress tolerance, including drought. Trials were conducted on tomato plants during the flowering stage in a greenhouse. The experiment was designed as a factorial combination of irrigation (water-stressed and well-watered) and biostimulant treatment (treated and control) and adopted a mixed phenotyping-omics approach. The efficacy of a continuous whole-canopy multichamber system coupled with generalized additive mixed modeling (GAMM) was evaluated to discriminate between water-stressed plants under the biostimulant treatment. Photosynthetic performance was evaluated by using GAMM, and was then correlated to metabolic profile. The results confirmed a higher photosynthetic efficiency of the treated plants, which is correlated to biostimulant-mediated drought tolerance. Furthermore, metabolomic analyses demonstrated the priming effect of the biostimulant for stress tolerance and detoxification and stabilization of photosynthetic machinery. In support of this, the overaccumulation of carotenoids was particularly relevant, given their photoprotective role in preventing the overexcitation of photosystem II. Metabolic profile and photosynthetic performance findings suggest an increased effective use of water (EUW) through the overaccumulation of lipids and leaf thickening. The positive effect of GB on water stress resistance could be attributed to both the delayed onset of stress and the elicitation of stress priming through the induction of H₂O₂-mediated antioxidant mechanisms. Overall, the mixed approach supported by a GAMM analysis could prove a valuable contribution to high-throughput biostimulant testing.

Comparative transcriptome analysis reveals the regulatory effects of acetylcholine on salt tolerance of Nicotiana benthamiana

Salinity is a major cause of crop losses worldwide. Acetylcholine (ACh) can ameliorate the adverse effects of abiotic stresses on plant growth, including salinity stress; however, the underlying molecular mechanisms of this process are unclear. Here, seedlings of Nicotiana benthamiana grown under normal conditions or exposed to 150 mmol L-1 NaCl salinity stress were then treated with a root application of 10 μM ACh. Exogenous ACh application resulted in the downregulation of the activity of the antioxidant enzymes, ascorbate peroxidase, and catalase. ACh-treated plants had lower levels of reactive oxygen species, including the superoxide anion radical and hydrogen peroxide. Transcriptome analysis indicated that ACh treatment under salt stress promoted the differential expression of 658 genes in leaves of N. benthamiana (527 were upregulated and 131 were downregulated). Gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that exogenous ACh application was associated with a substantial increase in the transcripts of genes related to cell wall peroxidases, xyloglucan endotransglucosylases or hydrolases, and expansins, indicating that ACh activates cell wall biosynthesis in salt-stressed plants. ACh also enhanced the expression of genes associated with the auxin, gibberellin, brassinosteroid, and salicylic acid signalling pathways, indicating that ACh induces the activation of these pathways under salt stress. Collectively, these findings indicate that ACh-induced salt tolerance in N. benthamiana seedlings is mediated by the inhibition of antioxidant enzymes, activation of cell wall biosynthesis, and hormone signalling pathways. Stress-induced genes involved in osmotic regulation and oxidation resistance were induced by ACh under salt stress. The genes whose transcript levels were elevated by ACh treatment in salt-stressed N. benthamiana could be used as molecular markers of the physiological status of plants under salt stress.

Temporal salt stress-induced transcriptome alterations and regulatory mechanisms revealed by PacBio long-reads RNA sequencing in Gossypium hirsutum

Background

Cotton (Gossypium hirsutum) is considered a fairly salt tolerant crop however, salinity can still cause significant economic losses by affecting the yield and deteriorating the fiber quality. We studied a salt-tolerant upland cotton cultivar under temporal salt stress to unfold the salt tolerance molecular mechanisms. Biochemical response to salt stress (400 mM) was measured at 0 h, 3 h, 12 h, 24 h and 48 h post stress intervals and single-molecule long-read sequencing technology from Pacific Biosciences (PacBio) combined with the unique molecular identifiers approach was used to identify differentially expressed genes (DEG).

Results

Antioxidant enzymes including, catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) were found significantly induced under temporal salt stress, suggesting that reactive oxygen species scavenging antioxidant machinery is an essential component of salt tolerance mechanism in cotton. We identified a wealth of novel transcripts based on the PacBio long reads sequencing approach. Prolonged salt stress duration induces high number of DEGs. Significant numbers of DEGs were found under key terms related to stress pathways such as “response to oxidative stress”, “response to salt stress”, “response to water deprivation”, “cation transport”, “metal ion transport”, “superoxide dismutase”, and “reductase”. Key DEGs related to hormone (abscisic acid, ethylene and jasmonic acid) biosynthesis, ion homeostasis (CBL-interacting serine/threonine-protein kinase genes, calcium-binding proteins, potassium transporter genes, potassium channel genes, sodium/hydrogen exchanger or antiporter genes), antioxidant activity (POD, SOD, CAT, glutathione reductase), transcription factors (myeloblastosis, WRKY, Apetala 2) and cell wall modification were found highly active in response to salt stress in cotton. Expression fold change of these DEGs showed both positive and negative responses, highlighting the complex nature of salt stress tolerance mechanisms in cotton.

Conclusion

Collectively, this study provides a good insight into the regulatory mechanism under salt stress in cotton and lays the foundation for further improvement of salt stress tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07260-z.

TaMYB86B encodes a R2R3-type MYB transcription factor and enhances salt tolerance in wheat

The MYB transcription factor family is important for plant responses to abiotic stresses. In this study, we identified three wheat TaMYB86 genes encoding R2R3-type MYB transcription factors. Analyses of the phylogenetic relationships and gene structures of TaMYB86A, TaMYB86B, and TaMYB86D revealed considerable similarities in gene structures and the encoded amino acid sequences. Additionally, TaMYB86B was highly expressed in the roots, stems, and leaves, suggesting it is critical for regulating salt stress responses in wheat. Moreover, TaMYB86B expression was induced by NaCl, abscisic acid (ABA), methyl jasmonate (MeJA), gibberellin (GA), auxin and low temperature treatments. The TaMYB86B protein localized in the nucleus and exhibited transcriptional activation activity. Under salt stress, TaMYB86B-overexpressing plants had a higher biomass and potassium ion (K⁺) content, but lower MDA, H₂O₂, O₂^-., and sodium ion (Na⁺) contents, when compared with the wild-type plants. Quantitative real-time PCR results indicated that the overexpression of TaMYB86B improved the expression of many stress-related genes. These findings suggest that TaMYB86B influences the salt tolerance of wheat by regulating the ion homeostasis to maintain an appropriate osmotic balance and decrease ROS levels.

Salinity induced physiological and biochemical changes in plants: An omic approach towards salt stress tolerance

Salinity is one of the major threats to sustainable agriculture that globally decreases plant production by impairing various physiological, biochemical, and molecular function. In particular, salinity hampers germination, growth, photosynthesis, transpiration, and stomatal conductance. Salinity decreases leaf water potential and turgor pressure and generates osmotic stress. Salinity enhances reactive oxygen species (ROS) content in the plant cell as a result of ion toxicity and disturbs ion homeostasis. Thus, it imbalances nutrient uptake, disintegrates membrane, and various ultrastructure. Consequently, salinity leads to osmotic and ionic stress. Plants respond to salinity by modulating various morpho-physiological, anatomical, and biochemical traits by regulating ion homeostasis and compartmentalization, antioxidant machinery, and biosynthesis of osmoprotectants and phytohormones, i. e, auxins, abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellins, salicylic acid, jasmonic acid, and polyamines. Thus, this further modulates plant osmoticum, decreases ion toxicity, and scavenges ROS. Plants upregulate various genes and proteins that participate in salinity tolerance. They also promote the production of various phytohormones and metabolites that mitigate the toxic effect of salinity. Based on recent papers, the deleterious effect of salinity on plant physiology is discussed. Furthermore, it evaluates the physiological and biochemical responses of the plant to salinity along with phytohormone response. This review paper also highlights omics (genomics, transcriptomics, proteomics, and metabolomics) approach to understand salt stress tolerance.

The bZIP Transcription Factor GmbZIP15 Negatively Regulates Salt- and Drought-Stress Responses in Soybean

Soybean (Glycine max), as an important oilseed crop, is constantly threatened by abiotic stress, including that caused by salinity and drought. bZIP transcription factors (TFs) are one of the largest TF families and have been shown to be associated with various environmental-stress tolerances among species; however, their function in abiotic-stress response in soybean remains poorly understood. Here, we characterized the roles of soybean transcription factor GmbZIP15 in response to abiotic stresses. The transcript level of GmbZIP15 was suppressed under salt- and drought-stress conditions. Overexpression of GmbZIP15 in soybean resulted in hypersensitivity to abiotic stress compared with wild-type (WT) plants, which was associated with lower transcript levels of stress-responsive genes involved in both abscisic acid (ABA)-dependent and ABA-independent pathways, defective stomatal aperture regulation, and reduced antioxidant enzyme activities. Furthermore, plants expressing a functional repressor form of GmbZIP15 exhibited drought-stress resistance similar to WT. RNA-seq and qRT-PCR analyses revealed that GmbZIP15 positively regulates GmSAHH1 expression and negatively regulates GmWRKY12 and GmABF1 expression in response to abiotic stress. Overall, these data indicate that GmbZIP15 functions as a negative regulator in response to salt and drought stresses.

Zinc and Paclobutrazol Mediated Regulation of Growth, Upregulating Antioxidant Aptitude and Plant Productivity of Pea Plants under Salinity

Soil salinity is the main obstacle to worldwide sustainable productivity and food security. Zinc sulfate (Zn) and paclobutrazol (PBZ) as a cost-effective agent, has multiple biochemical functions in plant productivity. Meanwhile, their synergistic effects on inducing salt tolerance are indecisive and not often reported. A pot experiment was done for evaluating the defensive function of Zn (100 mg/L) or PBZ (200 mg/L) on salt (0, 50, 100 mM NaCl) affected pea plant growth, photosynthetic pigment, ions, antioxidant capacity, and yield. Salinity stress significantly reduces all growth and yield attributes of pea plants relative to nonsalinized treatment. This reduction was accompanied by a decline in chlorophyll, nitrogen, phosphorus, and potassium (K⁺), the ratio between K⁺ and sodium (Na⁺), as well as reduced glutathione (GSH) and glutathione reductase (GR). Alternatively, salinity increased Na⁺, carotenoid (CAR), proline (PRO), ascorbic acid (AsA), superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) over nonsalinized treatment. Foliar spraying with Zn and PBZ under normal condition increased plant growth, nitrogen, phosphorus, potassium, K⁺/Na⁺ ratio, CAR, PRO, AsA, GSH, APX, GR, and yield and its quality, meanwhile decreased Na⁺ over nonsprayed plants. Application of Zn and PBZ counteracted the harmful effects of salinity on pea plants, by upregulating the antioxidant system, ion homeostasis, and improving chlorophyll biosynthesis that induced plant growth and yield components. In conclusion, Zn plus PBZ application at 30 and 45 days from sowing offset the injuries of salinity on pea plant growth and yield by upregulating the antioxidant capacity and increasing photosynthetic pigments.

The potential mitigation effect of ZnO nanoparticles on [Abelmoschus esculentus L. Moench] metabolism under salt stress conditions

Salt stress is known to be momentous abiotic stress which treats agricultural lands and crop production throughout the world and effects the system of food security. The current study aims to investigate the effect of foliar application of 10 mg/l of zinc oxide (ZnO) as a bulk or as a green synthesized nanoparticle (ZnO-NPs) which were hexagonal and spherical in shape and at size 16–35 nm to alleviate the impact of salinity concentrations (0, 10, 25, 50, 75 and 100% SW) on Okra (Abelmoschus esculentus L. Moench cv. Hasawi) species. The results demonstrated a gradual decrease in the photosynthetic pigments (i.e., chlorophyll a and b with total chlorophylls and carotenoids) with the growth of salinity conc. However, the sea water levels between 0 and 75% will led to increase in proline, total soluble sugar and activity of the antioxidant enzymes i.e., superoxide dismutase (SOD) and catalase (CAT) and then decrease at 100% SW. The addition of bulk ZnO or ZnO-NPs enhances the contents of the photosynthetic pigments, activity of both SOD and CAT and then lowers the accumulation of proline and total soluble sugar when equated with controls. Plants treated with ZnO-NPs showed the greatest results when compared with other treatments. The results of current study showed ZnO-NPs as an appropriate eco-friendly and low-cost application for plant growth under salinity which has an ability to moderate the salt stress effect of plants.

Genome resequencing reveals demographic history and genetic architecture of seed salinity tolerance in Populus euphratica

Populus euphratica is a dominant tree species in desert riparian forests and possesses extraordinary adaptation to salinity stress. Exploration of its genomic variation and molecular underpinning of salinity tolerance is important for elucidating population evolution and identifying stress-related genes. Here, we identify approximately 3.15 million single nucleotide polymorphisms using whole-genome resequencing. The natural populations of P. euphratica in northwest China are divided into four distinct clades that exhibit strong geographical distribution patterns. Pleistocene climatic fluctuations and tectonic deformation jointly shaped the extant genetic patterns. A seed germination rate-based salinity tolerance index was used to evaluate seed salinity tolerance of P. euphratica and a genome-wide association study was implemented. A total of 38 single nucleotide polymorphisms were associated with seed salinity tolerance and were located within or near 82 genes. Expression profiles showed that most of these genes were regulated under salt stress, revealing the genetic complexity of seed salinity tolerance. Furthermore, DEAD-box ATP-dependent RNA helicase 57 and one undescribed gene (CCG029559) were demonstrated to improve the seed salinity tolerance in transgenic Arabidopsis. These results provide new insights into the demographic history and genetic architecture of seed salinity tolerance in desert poplar.

Chloride is beneficial for growth of the xerophyte Pugionium cornutum by enhancing osmotic adjustment capacity under salt and drought stresses

Chloride (Cl-) is pervasive in saline soils, and research on its influence on plants has mainly focused on its role as an essential nutrient and its toxicity when excessive accumulation occurs. However, the possible functions of Cl- in plants adapting to abiotic stresses have not been well documented. Previous studies have shown that the salt tolerance of the xerophytic species Pugionium cornutum might be related to high Cl- accumulation. In this study, we investigated the Cl--tolerant characteristics and possible physiological functions of Cl- in the salt tolerance and drought resistance of P. cornutum. We found that P. cornutum can accumulate a large amount of Cl- in its shoots, facilitating osmotic adjustment and turgor generation under saline conditions. Application of DIDS (4,4´-diisothiocyanostilbene-2,2´-disulfonic acid), a blocker of anion channels, significantly inhibited Cl- uptake, and decreased both the Cl- content and its contribution to leaf osmotic adjustment, resulting in the exacerbation of growth inhibition in response to NaCl. Unlike glycophytes, P. cornutum was able to maintain NO3- homeostasis in its shoots when large amounts of Cl- were absorbed and accumulated. The addition of NaCl mitigated the deleterious effects of osmotic stress on P. cornutum because Cl- accumulation elicited a strong osmotic adjustment capacity. These findings suggest that P. cornutum is a Cl--tolerant species that can absorb and accumulate Cl- to improve growth under salt and drought stresses.

The tolerance to saline-alkaline stress was dependent on the roots in wheat

Saline-alkaline stress is one of the most serious global problems affecting agriculture, causing enormous economic and yield losses in agricultural production. Wheat, one of the most important crops worldwide, is often subjected to saline-alkaline stress. In this study, two wheat cultivars with different saline-alkaline tolerance, XC-12 (non-tolerance) and XC-45 (tolerance), were used to investigate the influence of saline-alkaline stress on photosynthesis and nitrogen (N) metabolism through hydroponic experiment with aim of elucidating the mechanism of resistance to salt-alkali. These results showed that saline-alkaline stress significantly reduced biomass accumulation, chlorophyll content, photosynthetic ability and N absorption but increased N utilization efficiency. There was no significant difference in photosynthesis between XC-12 and XC-45 under saline-alkaline stress. In addition, XC-45 had lower ratio of Na+/K+ in leaves and Na+-K+ selection rate and higher N absorption ability than XC-12, thereby improving physiological metabolism. Moreover, the roots exhibited greater growth performance in response to saline-alkaline stress as a result of increasing glutamine synthetase activity in roots, thus promoting N metabolism in roots. By coordinating the synergistic effect of increasing soluble protein in root, XC-45 exhibited greater tolerance to saline-alkaline stress. All data pinpoint that the root physiological function was more responsible for resistance to saline-alkaline stress in wheat.

Sodium chloride facilitates the secretohalophyte Atriplex canescens adaptation to drought stress

Atriplex canescens is a C₄ shrub with excellent adaptation to saline and arid environments. Our previous study showed that the secretion of excessive Na⁺ into leaf salt bladders is a primary strategy in salt tolerance of A. canescens and external 100 mM NaCl can substantially stimulate its growth. To investigate whether NaCl could facilitate Atriplex canescens response to drought stress, five-week-old seedlings were subjected to drought stress (30% of field water capacity) in the presence or absence of additional 100 mM NaCl. The results showed that, under drought stress, the addition of NaCl could substantially improve the growth of A. canescens by increasing leaf relative water content, enhancing photosynthetic activity and inducing a significant declined leaf osmotic potential (Ψs). The addition of NaCl significantly increased Na⁺ concentration in leaf salt bladders and the Na⁺ contribution to leaf Ψs, while had no adverse effects on K⁺ accumulation in leaf laminae. Therefore, the large accumulation of Na⁺ in salt bladders for enhancing osmotic adjustment (OA) ability is a vital strategy in A. canescens responding to drought stress. In addition, the concentration of free proline, bataine and soluble sugars exhibited a significant increase in the presence of NaCl under drought stress, and the betaine contribution to leaf Ψs was significantly increased by additional NaCl compared with that under drought treatment alone, suggesting that compatible solutes are also involved in OA in addition to functioning as protectants to alleviate water deficit injury.

Complementary analyses of the transcriptome and iTRAQ proteome revealed mechanism of ethylene dependent salt response in bread wheat (Triticum aestivum L.)

In order to clarify the ethylene dependent salt response mechanism in wheat, 2-week-old wheat seedlings of cultivar 'Qingmai 6' treated with water, sodium chloride (NaCl), NaCl and ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and NaCl and ethylene signaling inhibitor 1-methylcyclopropene (1-MCP) were collected and analyzed by transcriptional sequencing and isobaric tags for relative and absolute quantitation (iTRAQ) proteomics. At least 1140 proteins and 73,401 genes were identified, and proteins including ribosomal proteins (RPs), nucleoside diphosphate kinases (CDPKs), transaldolases (TALs), beta-glucosidases (BGLUs), phosphoenlpyruvate carboxylases (PEPCs), superoxide dismutases (SODs), and 6-phosphogluconate dehydrogenases (6-PGDHs) were significantly differently expressed. These genes and proteins revealed that ethylene dependent salt response through RPs activation, chaperones synthesis, the reactive oxygen species (ROS) scavenging, and carbohydrate metabolites pathway. Our results provided transcriptomics and proteomics information with respect to the molecular mechanisms of ethylene regualted salt response.

Responses to Water Deficit and Salt Stress in Silver Fir (Abies alba Mill.) Seedlings

Forest ecosystems are frequently exposed to abiotic stress, which adversely affects their growth, resistance and survival. For silver fir (Abies alba), the physiological and biochemical responses to water and salt stress have not been extensively studied. Responses of one-year-old seedlings to a 30-day water stress (withholding irrigation) or salt stress (100, 200 and 300 mM NaCl) treatments were analysed by determining stress-induced changes in growth parameters and different biochemical markers: accumulation of ions, different osmolytes and malondialdehyde (MDA, an oxidative stress biomarker), in the seedlings, and activation of enzymatic and non-enzymatic antioxidant systems. Both salt and water stress caused growth inhibition. The results obtained indicated that the most relevant responses to drought are based on the accumulation of soluble carbohydrates as osmolytes/osmoprotectants. Responses to high salinity, on the other hand, include the active transport of Na+, Cl− and Ca2+ to the needles, the maintenance of relatively high K+/Na+ ratios and the accumulation of proline and soluble sugars for osmotic balance. Interestingly, relatively high Na+ concentrations were measured in the needles of A. alba seedlings at low external salinity, suggesting that Na+ can contribute to osmotic adjustment as a ‘cheap’ osmoticum, and its accumulation may represent a constitutive mechanism of defence against stress. These responses appear to be efficient enough to avoid the generation of high levels of oxidative stress, in agreement with the small increase in MDA contents and the relatively weak activation of the tested antioxidant systems.

Titanium dioxide nanoparticles (TiO2 NPs) promote growth and ameliorate salinity stress effects on essential oil profile and biochemical attributes of Dracocephalum moldavica

Considering titanium dioxide nanoparticles (TiO2 NPs) role in plant growth and especially in plant tolerance against abiotic stress, a greenhouse experiment was carried out to evaluate TiO2 NPs effects (0, 50, 100 and 200 mg L-1) on agronomic traits of Moldavian balm (Dracocephalum moldavica L.) plants grown under different salinity levels (0, 50 and 100 mM NaCl). Results demonstrated that all agronomic traits were negatively affected under all salinity levels but application of 100 mg L-1 TiO2 NPs mitigated these negative effects. TiO2 NPs application on Moldavian balm grown under salt stress conditions improved all agronomic traits and increased antioxidant enzyme activity compared with plants grown under salinity without TiO2 NP treatment. The application of TiO2 NPs significantly lowered H2O2 concentration. In addition, highest essential oil content (1.19%) was obtained in 100 mg L-1 TiO2 NP-treated plants under control conditions. Comprehensive GC/MS analysis of essential oils showed that geranial, z-citral, geranyl acetate and geraniol were the dominant essential oil components. The highest amounts for geranial, geraniol and z-citral were obtained in 100 mg L-1 TiO2 NP-treated plants under control conditions. In conclusion, application of 100 mg L-1 TiO2 NPs could significantly ameliorate the salinity effects in Moldavian balm.

Transcriptome analyses revealed molecular responses of Cynanchum auriculatum leaves to saline stress

Cynanchum auriculatum is a traditional herbal medicine in China and can grow in saline soils. However, little is known in relation to the underlying molecular mechanisms. In the present study, C. auriculatum seedlings were exposed to 3.75‰ and 7.5‰ salinity. Next, transcriptome profiles of leaves were compared. Transcriptome sequencing showed 35,593 and 58,046 differentially expressed genes (DEGs) in treatments with 3.75‰ and 7.5‰, compared with the control, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of these DEGs enriched various defense-related biological pathways, including ROS scavenging, ion transportation, lipid metabolism and plant hormone signaling. Further analyses suggested that C. auriculatum up-regulated Na⁺/H⁺ exchanger and V-type proton ATPase to avoid accumulation of Na⁺. The flavonoid and phenylpropanoids biosynthesis pathways were activated, which might increase antioxidant capacity in response to saline stress. The auxin and ethylene signaling pathways were upregulated in response to saline treatments, both of which are important plant hormones. Overall, these results raised new insights to further investigate molecular mechanisms underlying resistance of C. auriculatum to saline stress.

Important roles of glycinebetaine in stabilizing the structure and function of the photosystem II complex under abiotic stresses

The molecular and physiological mechanisms of glycinebetaine stabilizing photosystem II complex under abiotic stresses are discussed, helping to address food shortage problems threatening the survival of growing population. In the backdrop of climate change, the frequency, dimensions and duration of extreme events have increased sharply, which may have unintended consequences for agricultural. The acclimation of plants to a constantly changing environment involves the accumulation of compatible solutes. Various compatible solutes enable plants to tolerate abiotic stresses, and glycinebetaine (GB) is one of the most-studied. The biosynthesis and accumulation of GB appear in numerous plant species, especially under environmental stresses. The exogenous application of GB and GB-accumulating transgenic plants have been proven to further promote plant development under stresses. Early research on GB focused on the maintenance of osmotic potential in plants. Subsequent experimental evidence demonstrated that it also protects proteins including the photosystem II complex (PSII) from denaturation and deactivation. As reviewed here, multiple experimental evidences have indicated considerable progress in the roles of GB in stabilizing PSII under abiotic stresses. Based on these advances, we've concluded two effects of GB on PSII: (1) it stabilizes the structure of PSII by protecting extrinsic proteins from dissociation or by promoting protein synthesize; (2) it enhances the oxygen-evolving activity of PSII or promotes the repair of the photosynthetic damage of PSII.

Sustainable wheat (Triticum aestivum L.) production in saline fields: a review

A large part of global agricultural fields, including the wheat (Triticum aestivum L.) ones, are subjected to various stresses including salinity. Given the increasing world population, finding methods and strategies that can alleviate salinity stress on crop yield production is of outmost importance. The presented review has consulted more than 400 articles related to the clean and sustainable production of wheat in saline fields affected by biological, environmental, economical, and social parameters including the important issue of climate change (global warming). The negative effects of salt stress on plant growth and the techniques, which have been so far detected to alleviate salinity stress on wheat growth have been analyzed and presented. The naturally tolerant species of wheat can use a range of mechanisms to alleviate salinity stress including sodium exclusion, potassium retention, and osmoregulation. However, the following can be considered as the most important techniques to enhance wheat tolerance under stress: (1) the biotechnological (crop breeding), biological (soil microbes), and biochemical (seed priming) methods, (2) the use of naturally tolerant genotypes, and (3) their combined use. The proper handling of irrigation water is also an important subject, which must be considered when planting wheat in saline fields. In conclusion, the sustainable and cleaner production of wheat under salt stress is determined by a combination of different parameters including the biotechnological techniques, which if handled properly, can enhance wheat production in saline fields.

Salt-Induced Damage is Alleviated by Short-Term Pre-Cold Treatment in Bermudagrass (Cynodon dactylon)

Excess salinity is a major environmental stress that limits growth and development of plants. Improving salt stress tolerance of plants is important in order to enhance land utilization and crop yield. Cold priming has been reported to trigger the protective processes in plants that increase their stress tolerance. Bermudagrass (Cynodon dactylon) is one of the most widely used turfgrass species around the world. However, the effect of cold priming on salt tolerance of bermudagrass is largely unknown. In the present study, wild bermudagrass was pre-treated with 4 °C for 6 h before 150 mM NaCl treatment for one week. The results showed that the cell membrane stability, ion homeostasis and photosynthesis process which are usually negatively affected by salt stress in bermudagrass were alleviated by short-term pre-cold treatment. Additionally, the gene expression profile also corresponded to the change of physiological indexes in bermudagrass. The results suggest that cold priming plays a positive role in improving salt stress tolerance of bermudagrass.

Less photoprotection can be good in some genetic and environmental contexts

Antioxidant systems modulate oxidant-based signaling networks and excessive removal of oxidants can prevent beneficial acclimation responses. Evidence from mutant, transgenic, and locally adapted natural plant systems is used to interpret differences in the capacity for antioxidation and formulate hypotheses for future inquiry. We focus on the first line of chloroplast antioxidant defense, pre-emptive thermal dissipation of excess absorbed light (monitored as nonphotochemical fluorescence quenching, NPQ) as well as on tocopherol-based antioxidation. Findings from NPQ-deficient and tocopherol-deficient mutants that exhibited enhanced biomass production and/or enhanced foliar water-transport capacity are reviewed and discussed in the context of the impact of lower levels of antioxidation on plant performance in hot/dry conditions, under cool temperature, and in the presence of biotic stress. The complexity of cellular redox-signaling networks is related to the complexity of environmental and endogenous inputs as well as to the need for intensified training and collaboration in the study of plant-environment interactions across biological sub-disciplines.

Spliceosomal protein U1A is involved in alternative splicing and salt stress tolerance in Arabidopsis thaliana

Soil salinity is a significant threat to sustainable agricultural production worldwide. Plants must adjust their developmental and physiological processes to cope with salt stress. Although the capacity for adaptation ultimately depends on the genome, the exceptional versatility in gene regulation provided by the spliceosome-mediated alternative splicing (AS) is essential in these adaptive processes. However, the functions of the spliceosome in plant stress responses are poorly understood. Here, we report the in-depth characterization of a U1 spliceosomal protein, AtU1A, in controlling AS of pre-mRNAs under salt stress and salt stress tolerance in Arabidopsis thaliana. The atu1a mutant was hypersensitive to salt stress and accumulated more reactive oxygen species (ROS) than the wild-type under salt stress. RNA-seq analysis revealed that AtU1A regulates AS of many genes, presumably through modulating recognition of 5′ splice sites. We showed that AtU1A is associated with the pre-mRNA of the ROS detoxification-related gene ACO1 and is necessary for the regulation of ACO1 AS. ACO1 is important for salt tolerance because ectopic expression of ACO1 in the atu1a mutant can partially rescue its salt hypersensitive phenotype. Our findings highlight the critical role of AtU1A as a regulator of pre-mRNA processing and salt tolerance in plants.

An overview on improvement of crop productivity in saline soils by halotolerant and halophilic PGPRs

Salinity of water and soil are of the most important factors limiting the production of crops. Moreover, with the increasing population of the planet and saline fields worldwide there is no choice but to use saline soil and water in the near future. Therefore, to increase plant growth under saline stress condition, provision of sustainable and environmentally friendly management for the use of saline water and soil resources is necessary. The development of saline resistant plants is a potent approach to solve this problem. Generally, soil salinity negatively affects the plant growth through ion toxicity, oxidative stress, osmotic stress and ethylene generation. In recent years, scientists through genetic engineering techniques, which are based on molecular and physiological characteristics of plants, have made salt tolerance plants. However, the validation of the present technique is restricted to laboratory condition and it is not easily applied in the agronomy research under field environment. Another option would be to isolate and utilize salinity resistant microorganisms from the rhizosphere of halophyte plants, namely plant growth-promoting rhizobacteria (PGPR). The mechanisms of these bacteria includes; ACC-deaminase and exopolysachared production, osmolite accumulation, antioxidant system activation, ion hemostasis and etc. In this review, we will discuss mechanisms of PGPR in producing tolerate plants under salt stress and how to improve the plant-microbe interactions in future for increasing agricultural productivity to feed all of the world's people.

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

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

Effects of leaf colorness, pigment contents and allelochemicals on the orientation of the Asian citrus psyllid among four Rutaceae host plants

BACKGROUND

Asian citrus psyllid (ACP) is the primary vector responsible for the transmission of the phloem-limited bacteria Candidatus Liberibacter spp., associated with huanglongbing (HLB), which causes great loss to the citrus industry. Although the roles of leaf color and volatile compounds in the orientation of ACP have been proven, the quantification of color and allelochemicals in the host plant are kept unclear, especially in wild citrus germplasms.

RESULTS

Chongyi wild mandarin significantly attracted more ACP than wild Hong Kong kumquat, 'Gannan zao' navel orange and orange jasmine did in the four-choice and olfactometer assays. The color parameters of the tender leaves from Chongyi wild mandarin and 'Gannan zao' were similar. The yellow color in both of them was less saturated than that of the other two plants species, but Chongyi wild mandarin had significant lower carotenoid content (P < 0.05). Notably metabolic profiling differences were observed among the healthy tender shoots from the four tested plants via UPLC-QQQ-MS and GC-MS analyses. Comparing with the other three plant species, 66 and 50 metabolites with significantly different contents in Chongyi wild mandarin were selected as UPLC-identified and GC-identified metabolites of interest (P < 0.05), respectively. Flavonoids accounted for a large group of secondary metabolites of interest, which may function as stimulants or repellents of ACP. Higher content of salicylic acid o-hexoside and lower content of (+)-jasmonic acid in Chongyi wild mandarin may lead to higher amount of methyl salicylate (an ACP attractant) and lower amount of trans-ocimene (an attractant to herbivores' natural enemies) as well as the suppression of JA-mediated wounding response. This kind of synergistic or antagonistic effect among the metabolites differentially accumulated in Chongyi wild mandarin made it a more attractive host plant to ACP.

CONCLUSIONS

Less saturated yellow color, high amount of attractants, low amount of repellents and insensitivity of JA-mediated wounding response are the four possible reasons why Chongyi wild mandarin attracted more ACP. This work may shed light on the olfactory and visual response of ACP to wild citrus germplasm hosts, and suggest the feasibility of developing ACP attractants or repellents patterned on potential metabolites.

Identification of candidate genes related to salt tolerance of the secretohalophyte Atriplex canescens by transcriptomic analysis

BACKGROUND

Atriplex canescens is a typical C₄ secretohalophyte with salt bladders on the leaves. Accumulating excessive Na⁺ in tissues and salt bladders, maintaining intracellular K⁺ homeostasis and increasing leaf organic solutes are crucial for A. canescens survival in harsh saline environments, and enhanced photosynthetic activity and water balance promote its adaptation to salt. However, the molecular basis for these physiological mechanisms is poorly understood. Four-week-old A. canescens seedlings were treated with 100 mM NaCl for 6 and 24 h, and differentially expressed genes in leaves and roots were identified, respectively, with Illumina sequencing.

RESULTS

In A. canescens treated with 100 mM NaCl, the transcripts of genes encoding transporters/channels for important nutrient elements, which affect growth under salinity, significantly increased, and genes involved in exclusion, uptake and vacuolar compartmentalization of Na⁺ in leaves might play vital roles in Na⁺ accumulation in salt bladders. Moreover, NaCl treatment upregulated the transcripts of key genes related to leaf organic osmolytes synthesis, which are conducive to osmotic adjustment. Correspondingly, aquaporin-encoding genes in leaves showed increased transcripts under NaCl treatment, which might facilitate water balance maintenance of A. canescens seedlings in a low water potential condition. Additionally, the transcripts of many genes involved in photosynthetic electron transport and the C₄ pathway was rapidly induced, while other genes related to chlorophyll biosynthesis, electron transport and C₃ carbon fixation were later upregulated by 100 mM NaCl.

CONCLUSIONS

We identified many important candidate genes involved in the primary physiological mechanisms of A. canescens salt tolerance. This study provides excellent gene resources for genetic improvement of salt tolerance of important crops and forages.

Physiological mechanisms of exogenous calcium on alleviating salinity-induced stress in rice (Oryza sativa L.)

Being more sensitive to salt stress among the cereals, growth of rice (Oryza sativa L.) has been habitually affected by salinity. Although, several practices have evolved to sustain the growth of rice under salinity, the enormous role of calcium (Ca²⁺) as a signalling molecule in salt stress mitigation is still arcane. Considering this fact, an experiment was performed aiming to explicate the mechanism of salt-induced growth inhibition in rice and its alleviation by exogenous Ca²⁺. At germination stage, 10 mM and 15 mM CaCl₂ primed rice (cv. Binadhan-10 & Binadhan-7) seeds were grown in petri dishes for 9 days under 100 mM NaCl stress. At seedling stage, 9-day-old rice seedlings grown on sand were exposed to 100 mM NaCl alone and combined with 10 mM and 15 mM CaCl₂ for 15 days. This research revealed that salinity radically slowed down growth of rice seedlings and Ca²⁺ treatment noticeably improved growth performances. At germination stage, 10 mM CaCl₂ treatment significantly increased the final germination percentage, germination rate index (in Binadhan-7), shoot, root length (89.20, 67.58% in Bindhan-10 & 84.72, 31.15% in Bindhan-7) and biomass production under salinity. Similarly, at seedling stage, 10 mM CaCl₂ supplementation in salt-stressed plants enhanced shoot length (42.17, 28.76%) and shoot dry weight (339.52, 396.20%) significantly in Binadhan-10 & Binadhan-7, respectively, but enhanced root dry weight (36.76%) only in Binadhan-10. In addition, 10 mM CaCl₂ supplementation on salt-stressed seedlings increased the chlorophyll and proline content, and oppressed the accretion of reactive oxygen species thus protecting from oxidative damage more pronouncedly in Binadhan-10 than Binadhan-7 as reflected by the elevated levels of catalase and ascorbate peroxidase activity. The 15 mM CaCl₂ somehow also enhanced some growth parameters but overall was less effective than 10 mM CaCl₂ to alleviate salt stress, and sometimes showed negative effect. Therefore, supplementary application of calcium-rich fertilizers in saline prone soils can be an effective approach to acclimatize salt stress and cultivate rice successfully.

Ameliorating effects of exogenous Ca2+ on foxtail millet seedlings under salt stress

In the present study, we investigated whether Ca2+ application alleviates salinity-induced damage in foxtail millet (Setaria italica L.). We evaluated the stress-related ion balance, physiological activity and gene expression involved in plant defences against salinity exposure. Twenty-one-day-old foxtail millet was maintained in sand culture for 7 days and subjected to one of seven treatments: half-strength modified Hoagland solution (the control), 1.0% NaCl, 1.0% NaCl+2.5mM Ca2+, 1.0% NaCl+5.0mM Ca2+, 1.0% NaCl+7.5mM Ca2+, 1.0% NaCl+10.0mM Ca2+, 1.0% NaCl+12.5mM Ca2+. The addition of Ca2+ significantly increased shoot and root height and weight relative to calcium absent treatment and corrected the ion imbalance by increasing Ca2+, Mg2+ and K+, and decreasing Na+ in the leaves and roots. It increased chlorophyll content and root activity and decreased the relative electrolyte leakage in the roots and leaves. Calcium application significantly upregulated superoxide dismutase and catalase, increased total protein, and decreased malondialdehyde and H2O2. Finally, the addition of Ca2+ upregulated the expression of APX, SOD and CAT. It was found that 10.0mM Ca2+ was the optimal concentration for alleviating salt stress injury in foxtail millet.

Genetic engineering of the biosynthesis of glycinebetaine enhances the fruit development and size of tomato

Glycinebetaine has been widely considered as an effective protectant against abiotic stress in plants, and also found to promote plant growth under normal growing conditions, especially during the reproductive stage. Betaine aldehyde dehydrogenase (BADH) and choline oxidase (COD) are two key enzymes which have been used to confer glycinebetaine synthesis in plant which normally does not synthesis glycinebetaine. In this study, we used the tomato (Solanum lycopersicum, cv 'Moneymaker') plants of wild-type and the transgenic lines codA (L1, L2) and BADH (2, 46), which were transformed with codA and BADH, respectively, to study the impact of glycinebetaine on tomato fruit development. Our results showed that the codA and BADH transgenes induced the formation of enlarged flowers and fruits in transgenic tomato plants. In addition, the transgenic tomato plants had a higher photosynthetic rate, higher assimilates content, and higher leaf chlorophyll content than the wild-type plants. We also found that the enlargement of fruit size was related to the contents of phytohormones, such as auxin, brassinolide, gibberellin, and cytokinin. Additionally, qPCR results indicated that the expressions levels of certain genes related to fruit growth and development were also elevated in transgenic plants. Finally, transcriptome sequencing results revealed that the differences in the levels of gene expression in tomato fruit between the transgenic and wild-type plants were observed in multiple pathways, predominantly those of photosynthesis, DNA replication, plant hormone signal transduction, and biosynthesis. Taken together, our results suggest that glycinebetaine promotes tomato fruit development via multiple pathways. We propose that genetic engineering of glycinebetaine synthesis offers a novel approach to enhance the productivity of tomato and other crop plants.

Transcript profiling reveals an important role of cell wall remodeling and hormone signaling under salt stress in garlic

Salt stress is one of the environmental factors that evidently limit plant growth and yield. Despite the fact that understanding plant response to salt stress is important to agricultural practice, the molecular mechanisms underlying salt tolerance in garlic remain unclear. In this study, garlic seedlings were exposed to 200 mM NaCl stress for 0, 1, 4, and 12 h, respectively. RNA-seq was applied to analyze the transcriptional response under salinity conditions. A total of 13,114 out of 25,530 differentially expressed unigenes were identified to have pathway annotation, which were mainly involved in purine metabolism, starch and sucrose metabolism, plant hormone signal transduction, flavone and flavonol biosynthesis, isoflavonoid biosynthesis, MAPK signaling pathway, and circadian rhythm. In addition, 272 and 295 differentially expressed genes were identified to be cell wall and hormone signaling-related, respectively, and their interactions under salinity stress were extensively discussed. The results from the current work would provide new resources for the breeding aimed at improving salt tolerance in garlic.

The synergistic effects of sodium and potassium on the xerophyte Apocynum venetum in response to drought stress

Apocynum venetum is an eco-economic plant species with high adaptability to saline and arid environments. Our previous work has found that A. venetum could absorb large amount of Na⁺ and maintain high K⁺ level under saline conditions. To investigate whether K⁺ and Na⁺ could simultaneously enhance drought resistance in A. venetum, seedlings were exposed to osmotic stress (-0.2 MPa) in the presence or absence of additional 25 mM NaCl under low (0.01 mM) and normal (2.5 mM) K⁺ supplying conditions, respectively. The results showed that A. venetum should be considered as a typical K⁺-efficient species since its growth was unimpaired and possessed a strong K⁺ uptake and prominent K⁺ utilization efficiency under K⁺ deficiency condition. Leaf K⁺ concentration remained stable or was even significantly increased under osmotic stress in the presence or absence of NaCl, compared with that under control condition, regardless of whether the K⁺ supply was sufficient or not, and the contribution of K⁺ to leaf osmotic potential consistently exceeded 37%, indicating K⁺ is the uppermost contributor to osmotic adjustment of A. venetum. Under osmotic stress, the addition of 25 mM NaCl significantly increase Na⁺ accumulation in leaves and the contribution of Na⁺ to osmotic adjustment, thus improving the relative water content, concomitantly, promoting the photosynthetic activity resulting in an enhancement of overall plant growth. These findings suggested that, K⁺ and Na⁺ simultaneously play crucial roles in the osmotic adjustment and the maintenance of water status and photosynthetic activity, which is beneficial for A. venetum to cope with drought stress.

Analytical Methods for Detection of Plant Metabolomes Changes in Response to Biotic and Abiotic Stresses

Abiotic and biotic stresses are the main reasons of substantial crop yield losses worldwide. Research devoted to reveal mechanisms of plant reactions during their interactions with the environment are conducted on the level of genome, transcriptome, proteome, and metabolome. Data obtained during these studies would permit to define biochemical and physiological mechanisms of plant resistance or susceptibility to affecting factors/stresses. Metabolomics based on mass spectrometric techniques is an important part of research conducted in the direction of breeding new varieties of crop plants tolerant to the affecting stresses and possessing good agronomical features. Studies of this kind are carried out on model, crop and resurrection plants. Metabolites profiling yields large sets of data and due to this fact numerous advanced statistical and bioinformatic methods permitting to obtain qualitative and quantitative evaluation of the results have been developed. Moreover, advanced integration of metabolomics data with these obtained on other omics levels: genome, transcriptome and proteome should be carried out. Such a holistic approach would bring us closer to understanding biochemical and physiological processes of the cell and whole plant interacting with the environment and further apply these observations in successful breeding of stress tolerant or resistant crop plants.

SRAPs and EST-SSRs provide useful molecular diversity for targeting drought and salinity tolerance in Indian mustard

Abiotic stress tolerance is one of the target trait in crop breeding under climate change scenario. Selection of suitable gene pools among available germplasm is first requisite for any crop improvement programme. Drought and salinity traits, being polygenic, are most difficult to target. The present investigation aimed at exploring and assessment of the genetic variability in Indian mustard at molecular level. A total of twenty-five genotypes and five related species were used. Sixty-three molecular markers including sequence related amplified polymorphism (SRAP) markers along with twenty-three expressed sequence tag-simple sequence repeats (EST-SSRs) were used for diversity analysis. Thirty-seven SRAPs and 18 EST-SSRs showed amplification producing a total of 423 alleles of which 422 were polymorphic. These markers gave an overall polymorphism of 99.78%, with 99.67% polymorphism in SRAPs and 100% polymorphism in EST-SSRs. The study revealed the genetic relationships among different genotypes of B. juncea and related species which could be used for Indian mustard improvement for targeting drought and salinity tolerance in future. Four SRAP and two EST-SSRs identified unique bands which may be related to abiotic stress tolerance. EST sequence BRMS-040 (IM7) was similar to Brassica and radish sequences related to PR-5 (pathogenesis-related) protein.