Salicylic acid improves the salinity tolerance of Medicago sativa in symbiosis with Sinorhizobium meliloti by preventing nitrogen fixation inhibition

Thriving in a salty future: morpho-anatomical, physiological and molecular adaptations to salt stress in alfalfa (Medicago sativa L.) and other crops

BACKGROUND

With soil salinity levels rising at an alarming rate, accelerated by climate change and human interventions, there is a growing need for crop varieties that can grow on saline soils. Alfalfa (Medicago sativa) is a cool-season perennial leguminous crop, commonly grown as forage, biofuel feedstock and soil conditioner. It demonstrates significant potential for agricultural circularity and sustainability, for example by fixing nitrogen, sequestering carbon and improving soil structures. Although alfalfa is traditionally regarded as a moderately salt-tolerant species, modern alfalfa varieties display specific salt-tolerance mechanisms, which could be used to pave its role as a leading crop able to grow on saline soils.

SCOPE

Alfalfa's salt tolerance underlies a large variety of cascading biochemical and physiological mechanisms. These are partly enabled by its complex genome structure and out-crossing nature, but which entail impediments for molecular and genetic studies. This review first summarizes the general effects of salinity on plants and the broad-ranging mechanisms for dealing with salt-induced osmotic stress, ion toxicity and secondary stress. Second, we address the defensive and adaptive strategies that have been described for alfalfa, such as the plasticity of alfalfa's root system, hormonal crosstalk for maintaining ion homeostasis, spatiotemporal specialized metabolite profiles and the protection of alfalfa-rhizobia associations. Finally, bottlenecks for research of the physiological and molecular salt-stress responses as well as biotechnology-driven improvements of salt tolerance are identified and discussed.

CONCLUSION

Understanding morpho-anatomical, physiological and molecular responses to salinity is essential for the improvement of alfalfa and other crops in saline land reclamation. This review identifies potential breeding targets for enhancing the stability of alfalfa performance and general crop robustness for rising salt levels as well as to promote alfalfa applications in saline land management.

Decoding early stress signaling waves in living plants using nanosensor multiplexing

Increased exposure to environmental stresses due to climate change have adversely affected plant growth and productivity. Upon stress, plants activate a signaling cascade, involving multiple molecules like H2O2, and plant hormones such as salicylic acid (SA) leading to resistance or stress adaptation. However, the temporal ordering and composition of the resulting cascade remains largely unknown. In this study we developed a nanosensor for SA and multiplexed it with H2O2 nanosensor for simultaneous monitoring of stress-induced H2O2 and SA signals when Brassica rapa subsp. Chinensis (Pak choi) plants were subjected to distinct stress treatments, namely light, heat, pathogen stress and mechanical wounding. Nanosensors reported distinct dynamics and temporal wave characteristics of H2O2 and SA generation for each stress. Based on these temporal insights, we have formulated a biochemical kinetic model that suggests the early H2O2 waveform encodes information specific to each stress type. These results demonstrate that sensor multiplexing can reveal stress signaling mechanisms in plants, aiding in developing climate-resilient crops and pre-symptomatic stress diagnoses.

Continuous In Vivo Monitoring of Indole-3-Acetic Acid and Salicylic Acid in Tomato Leaf Veins Based on an Electrochemical Microsensor

Indole-3-acetic acid (IAA) and salicylic acid (SA), as critical plant hormones, are involved in multiple physiological regulatory processes of plants. Simultaneous and continuous in vivo detection of IAA and SA will help clarify the mechanisms of their regulation and crosstalk. First, this study reports the development and application of an electrochemical microsensor for simultaneous and continuous in vivo detection of IAA and SA. This electrochemical microsensor system consisted of a tip (length, 2 mm) of platinum wire (diameter, 0.1 mm) modified with carbon cement and multi-walled carbon nanotubes, an untreated tip (length, 2 mm) of platinum wire (diameter, 0.1 mm), as well as a tip (length, 2 mm) of Ag/AgCl wire (diameter, 0.1 mm). It was capable of detecting IAA in the level ranging from 0.1 to 30 µM and SA ranging from 0.1 to 50 µM based on the differential pulse voltammetry or amperometric i-t., respectively. The dynamics of IAA and SA levels in tomato leaf veins under high salinity stress were continuously detected in vivo, and very little damage occurred. Compared to conventional detection methods, the constructed microsensor is not only suitable for continuously detecting IAA and SA in microscopic plant tissue in vivo, it also reduces the damage done to plants during the detection. More importantly, the continuous and dynamic changes in IAA and SA data obtained in stiu through this system not only can help clarify the interaction mechanisms of IAA and SA in plants, it also helps to evaluate the health status of plants, which will promote the development of basic research in botany and precision agriculture.

Drought priming modulates ABF, GRFs, related microRNAs and induce metabolic adjustment during heat stress in chickpea

Drought and high temperature stress may occur concomitantly or individually in succession causing cellular dysfunctions. Abscisic acid (ABA) is a key stress regulator, and its responsive genes are controlled by ABRE (Abscisic acid Responsive Element)-binding factors (ABFs)and G-Box Regulatory factors (GRFs). Here, we identify ABFs, GRFs and targeting miRNAs in desi and kabuli chickpea. To validate their role after drought priming and subsequent high temperature stress, two contrasting chickpea varieties (PBG1 and PBG5) were primed and exposed to 32 °C, 35 °C and 38 °C for 12, 6 and 2 h respectively and analyzed for Physio-biochemical, expression of ABFs, GRFs and MiRNAs, and GC-MS based metabolite analysis. To ascertain the ABF-GRF protein-protein interactions, docking studies were carried out between the ABF3 and GRF14. Genome-wide analysis identified total 9 & 11 ABFs, and 11 GRFsin desi and kabuli respectively. Their gene structure, and motif composition were conserved in all subfamilies and only 10 and 12 genes have undergone duplication in both desi and kabuli chickpea respectively. These genes were differentially expressed in-silico. MiR172 and miR396 were identified to target ABFs and GRFs respectively. Protein-protein interaction (ABF3 and GRF14) might be successful only when the ABF3 was phosphorylated. Drought priming downregulated miR172 and miR396 and eventually upregulated targeting ABFs, and GRFs. Metabolite profiling (GC-MS) revealed the accumulation of 87 metabolites in Primed (P) and Non-Primed (NP) Chickpea plants. Tolerant cultivar (PBG5) responded better in all respects however both severity of stress and exposure are important factors and can produce broadly similar cellular response.

Halomonas ventosae JPT10 promotes salt tolerance in foxtail millet (Setaria italica) by affecting the levels of multiple antioxidants and phytohormones

Plant growth-promoting bacterias (PGPBs) can increase crop output under normal and abiotic conditions. However, the mechanisms underlying the plant salt tolerance-promoting role of PGPBs still remain largely unknown. In this study, we demonstrated that Halomonas ventosae JPT10 promoted the salt tolerance of both dicots and monocots. Physiological analysis revealed that JPT10 reduced reactive oxygen species accumulation by improving the antioxidant capability of foxtail millet seedlings. The metabolomic analysis of JPT10-inoculated foxtail millet seedlings led to the identification of 438 diversely accumulated metabolites, including flavonoids, phenolic acids, lignans, coumarins, sugar, alkaloids, organic acids, and lipids, under salt stress. Exogenous apigenin and chlorogenic acid increased the salt tolerance of foxtail millet seedlings. Simultaneously, JPT10 led to greater amounts of abscisic acid (ABA), indole-3-acetic acid (IAA), salicylic acid (SA), and their derivatives but lower levels of 12-oxo-phytodienoic acid (OPDA), jasmonate (JA), and JA-isoleucine (JA-Ile) under salt stress. Exogenous JA, methyl-JA, and OPDA intensified, whereas ibuprofen or phenitone, two inhibitors of JA and OPDA biosynthesis, partially reversed, the growth inhibition of foxtail millet seedlings caused by salt stress. Our results shed light on the response of foxtail millet seedlings to H. ventosae under salt stress and provide potential compounds to increase salt tolerance in foxtail millet and other crops.

Evaluation of Salicylic Acid Effects on Growth, Biochemical, Yield, and Anatomical Characteristics of Eggplant (Solanum melongena L.) Plants under Salt Stress Conditions

Salt stress is a major issue in agriculture and crop production that influences global food security. Mitigation options to address salt stress through agronomic practices can help manage this issue. Experiments were performed in two summer seasons in an experimental farm to test the impact of three salinity levels (S): 300 (control), 1000, 2000, and 3000 ppm, and two salicylic acid (SA) levels, including 1.0 and 1.50 mM, and their interaction on growth and yield of eggplant (Solanum melongena L.) hybrid Suma. The results showed that increasing S levels up to 3000 ppm reduced plant and fruit physical characteristics, as well as leaf and fruit chemical characteristics, especially leaf total chlorophyll, carotenoids, relative water, fruit nitrogen, phosphorus, and potassium contents, which led to a reduction in total yield per plant. However, an insignificant effect was observed in the control level and 1000 ppm saline water in leaf area, fruit length, leaf total chlorophyll content, fruit phosphorus content, and total yield per plant. In contrast, leaf sugars, proline contents, electrolyte leakage, fruit TSS (total soluble solids), and ascorbic acid contents were improved with S levels up to the concentration of 3000 ppm compared to the control. However, tested parameters were significantly higher due to the SA foliar spray of 1.0 mM besides photosynthetic pigments of leaves enhanced by using 1.0 and 1.50 mM. Using 1.0 mM SA concentration alleviated the adverse impact of S on eggplant plants until 1000 ppm saline water, reflecting an increase in eggplant yield. The anatomical structure of eggplant leaves revealed positive variations in mature leaf blades in both the stressed and SA-treated plants. Based on these results, the use of SA at a concentration of 1.0 mM may lessen the negative impacts of salt on the growth of eggplant, which increases the overall yield.

Salicylic acid-mitigates abiotic stress tolerance via altering defense mechanisms in Brassica napus (L.)

Under the changing climate due to global warming, various abiotic stresses including drought (D) and salinity (S) are expected to further trigger their devastating effects on the already vulnerable crop production systems. This experiment was designed to unravel and quantify the potential role of exogenous application of salicylic acid (SA) in mitigating both D and S stresses and their combination (D+S), with three replications using CRD (Completely Randomized Design). The obtained results of the current study demonstrated significant effects of all three types of stresses (D, S, and D+S) on various parameters in Brassica napus plants. Quantifying these parameters provides a more informative and precise understanding of the findings. Current results revealed that all three stress types (D, S, and D+S) resulted in a reduction in leaf area (13.65 to 21.87%), chlorophyll levels (30 to 50%), gaseous exchange rate (30 to 54%) and the concentration of mineral ions compared to non-stressed plants. However, application of SA helped in mitigating these stresses by ameliorating the negative effects of these stresses. Moreover, Malondialdehyde (MDA) contents, an indicator of lipid per-oxidation and oxidative stress, the levels of antioxidants, proline content, an osmolyte associated with stress tolerance, and sugar content in the leaves were elevated in response to all stress conditions. In addition, the ultra-structures within the leaves were negatively affected by the stresses, while an application of SA considerably minimized the deterioration of these structures thus providing protection to the brassica plants against the stresses. In a nutshell, the findings of this study suggest that SA application in S, D and S+ D stresses provides evasion to the plants by improving different physiological and growth indices. The application of Salicylic Acid (SA) mitigated the negative effects of the stresses on all the above parameters, reducing MDA contents (47%), antioxidants (11 to 20%), proline (28%), sugar contents (20.50%), and minimizing the deterioration of ultra-structures. The findings emphasize the potential mitigatory role of SA in mitigating D and S stresses and highlight the need for further research to understand the underlying mechanisms in detail and explore its practical application in farming practices.

Spatiotemporal, physiological and transcriptomic dynamics of wild jujube seedlings under saline conditions

Soil salinity is a major constraint limiting jujube production in China. Wild jujube (Ziziphus jujuba var. spinosa (Bunge) Hu ex H. F. Chow) is widely used as the rootstock of jujube (Z. jujuba) to overcome the saline conditions. To understand the adaptive mechanism in wild jujube under saline conditions, we combined spatiotemporal and physiological assessments with transcriptomic analysis on wild jujube seedlings undergoing various salt treatments. These salt treatments showed dose and duration effects on biomass, photosynthesis, (K+) and (Na+) accumulation. Salt treatments induced higher levels of salicylic acid in roots and leaves, whereas foliar abscisic acid was also elevated after 8 days. The number of differential expression genes increased with higher doses and also longer exposure of NaCl treatments, with concomitant changes in the enriched Gene Ontology terms that were indicative of altered physiological activities. Gene co-expression network analysis identified the core gene sets associated with salt-induced changes in leaves, stems and roots, respectively. The nitrogen transporters, potassium transporters and a few transcription factors belonging to WRKY/MYB/bHLH families were clustered as the hub genes responding to salt treatments, which were related to elevated nitrogen and K+/Na+. Ectopic overexpression of two WRKY transcription factor genes (ZjWRKY6 and ZjWRKY65) conferred stronger salt-tolerance in Arabidopsis thaliana transformants by enhancing the activities of antioxidant enzymes, decreasing malondialdehyde accumulation and maintaining K+/Na+ homeostasis. This study provided evidence about the spatiotemporal, physiological and transcriptomic dynamics of wild jujube during salt stress and identified potential genes for further research to improve salt tolerance in jujube.

Functional characterization of 5'-regulatory region of flavonoid 3',5'-hydroxylase-1 gene of banana plants

Generation of crops with broad-spectrum tolerance to biotic and abiotic stress conditions depends upon availability of genetic elements suitable for varied situations and diverse genotypes. Here, we characterize the 5'-upstream regulatory region of flavonoid 3'5'-hydroxylase-1 (F3'5'H-1) gene from banana and analyzed its tissue-specific and stress-mediated activation in genetic background of tobacco plants. MusaF3'5'H-1 is a stress-responsive gene as its expression is induced in banana after application of salicylic acid and methyl jasmonate while its transcript levels were drastically reduced in response to drought, high salinity and abscisic acid. P_MusaF3'5'H-1 harbours cis-elements associated with stress conditions and those responsible for tissue-specific expression. Transgenic lines harbouring P_MusaF3'5'H-1-GUS displays strong GUS expression in guard cells of stomata indicating guard cell preferred activity of P_MusaF3'5'H-1 while its activity was undetectable in roots. Drought and high salinity induce strong expression of GUS in transgenic tobacco lines and exposure to abscisic acid, salicylic acid and methyl jasmonate revealed distinct profiles of GUS expression in transgenic lines confirming involvement of F3'5'H-1 in plant stress responses. Fluorescent β-galactosidase assay revealed induction profiles of P_MusaF3'5'H-1 at different time points in transgenic lines exposed to salicylic acid and abscisic acid while strong suppression in GUS expression was observed after application of methyl jasmonate. The guard cell preferred activity of P_MusaF3'5'H-1 and stress-mediated expression profiles of MusaF3'5'H-1 indicated the suitability of P_MusaF3'5'H-1 for generating stress-enduring crops and analyzing guard cell functions.

Salicylic acid interacts with other plant growth regulators and signal molecules in response to stressful environments in plants

Salicylic acid (SA) is one of the potential plant growth regulators (PGRs) that regulate plant growth and development by triggering many physiological and metabolic processes. It is also known to be a crucial component of plant defense mechanisms against environmental stimuli. In stressed plants, it is documented that it can effectively modulate a myriad of metabolic processes including strengthening of oxidative defense system by directly or indirectly limiting the buildup of reactive nitrogen and oxygen radicals. Although it is well recognized that it performs a crucial role in plant tolerance to various stresses, it is not fully elucidated that whether low or high concentrations of this PGR is effective to achieve optimal growth of plants under stressful environments. It is also not fully understood that to what extent and in what manner it cross-talks with other potential growth regulators and signalling molecules within the plant body. Thus, this critical review discusses how far SA mediates crosstalk with other key PGRs and molecular components of signalling pathways mechanisms, particularly in plants exposed to environmental cues. Moreover, the function of SA exogenously applied in regulation of growth and development as well as reinforcement of oxidative defense system of plants under abiotic stresses is explicitly elucidated.

Emerging Roles of Salicylic Acid in Plant Saline Stress Tolerance

One of the most important phytohormones is salicylic acid (SA), which is essential for the regulation of plant growth, development, ripening, and defense responses. The role of SA in plant-pathogen interactions has attracted a lot of attention. Aside from defense responses, SA is also important in responding to abiotic stimuli. It has been proposed to have great potential for improving the stress resistance of major agricultural crops. On the other hand, SA utilization is dependent on the dosage of the applied SA, the technique of application, and the status of the plants (e.g., developmental stage and acclimation). Here, we reviewed the impact of SA on saline stress responses and the associated molecular pathways, as well as recent studies toward understanding the hubs and crosstalk between SA-induced tolerances to biotic and saline stress. We propose that elucidating the mechanism of the SA-specific response to various stresses, as well as SA-induced rhizosphere-specific microbiome modeling, may provide more insights and support in coping with plant saline stress.

MusaNAC29-like transcription factor improves stress tolerance through modulation of phytohormone content and expression of stress responsive genes

Understanding the molecular mechanisms governed by genes and cross-talks among stress signaling pathways is vital for generating a broad view on stress responses in plants. Here, we analysed the effects of MusaNAC29-like transcription factor of banana on stress responses and report the quantitative modulation of phytohormone and flavonoid content and analysed the growth parameters and yield trait in transgenic banana plants. Expression of MusaNAC29-like transcription factor was strongly altered in responses to stress conditions and application of signaling molecules. Under control conditions, P_{MusaNAC29-like}-GUS is activated in cells bordering xylem vessel elements and is strongly triggered in other cells types after influence of salicylic acid and abscisic acid. Transgenic banana plants of cultivar Rasthali and Grand Naine overexpressing MusaNAC29-like transcription factor displayed superior tolerance towards drought and salinity stress. LC-MS analysis indicated elevated levels of jasmonic acid and salicylic acid while content of zeatin was significantly reduced in leaves of transgenic banana lines. Transgenic banana lines displayed increased levels of gallic acid, coumaric acid, naringenin, chlorogenic acid while levels of vanillic acid and piperine were significantly reduced. Expression of stress related genes coding for antioxidants, thiol peptidase proteins, cold-regulated proteins, late embryogenesis abundant proteins, ethylene-responsive transcription factors, bHLH proteins, jasmonate-zim-domain proteins and WRKY transcription factors were significantly induced in transgenic banana lines. Though MusaNAC29-like transcription factor improved stress tolerance, its overexpression resulted in retarded growth of transgenic lines resulting in reduced yield of banana fruits.

Seed nanopriming: How do nanomaterials improve seed tolerance to salinity and drought?

Salt and drought stress are major environmental issues world-widely. These stresses can result in failures of seed germination, limiting agricultural production. New approaches are needed to increase crop production, ensuring food safety, quality, and agriculture sustainability. Nanopriming (priming seeds with nanomaterials) is an emerging seed technology improving crop production under the drastic climate change associated with stress factors. The present review not only provided an overview of nanopriming achieved salt and drought tolerance but also tried to discuss the behind mechanisms. We argued that the physico-chemical properties of the nanomaterials are key factors affecting their negative or positive effects on seed germination in terms of seed nanopriming. Furthermore, we highlighted the possible critical role of seed coat anatomy in effective nanopriming, in terms of saving costs and reducing biosafety issues. This review aims to help researchers to better understand and follow this fast-developing, cost-effective, and environmentally friendly research area.

Encapsulation Reduces the Deleterious Effects of Salicylic Acid Treatments on Root Growth and Gravitropic Response

The role of salicylic acid (SA) on plant responses to biotic and abiotic stresses is well documented. However, the mechanism by which exogenous SA protects plants and its interactions with other phytohormones remains elusive. SA effect, both free and encapsulated (using silica and chitosan capsules), on Arabidopsis thaliana development was studied. The effect of SA on roots and rosettes was analysed, determining plant morphological characteristics and hormone endogenous levels. Free SA treatment affected length, growth rate, gravitropic response of roots and rosette size in a dose-dependent manner. This damage was due to the increase of root endogenous SA concentration that led to a reduction in auxin levels. The encapsulation process reduced the deleterious effects of free SA on root and rosette growth and in the gravitropic response. Encapsulation allowed for a controlled release of the SA, reducing the amount of hormone available and the uptake by the plant, mitigating the deleterious effects of the free SA treatment. Although both capsules are suitable as SA carrier matrices, slightly better results were found with chitosan. Encapsulation appears as an attractive technology to deliver phytohormones when crops are cultivated under adverse conditions. Moreover, it can be a good tool to perform basic experiments on phytohormone interactions.

Transcriptome analysis of perennial ryegrass reveals the regulatory role of Aspergillus aculeatus under salt stress

Perennial ryegrass (Lolium perenne) is an important turf grass and forage grass with moderately tolerant to salinity stress. Aspergillus aculeatus has been documented to involved in salt stress response of perennial ryegrass, while the A. aculeatus-mediated molecular mechanisms are unclear. Therefore, to investigate the molecular mechanisms underlying A. aculeatus-mediated salt tolerance, the comprehensive transcriptome analysis of the perennial ryegrass roots was performed. Twelve cDNA libraries from roots were constructed after 12 h of plant-fungus cocultivation under 300 mM salt stress concentrations. A total of 21,915 differentially expressed genes (DEGs) were identified through pairwise comparisons. Enrichment analysis revealed that potentially important A. aculeatus-induced salt responsive genes belonging to specific categories, such as hormonal metabolism (auxin and salicylic acid metabolism related genes), secondary metabolism (flavonoid's metabolism related genes) and transcription factors (MYB, HSF and AP2/EREBP family). In addition, weighted gene co-expression network analysis (WGCNA) showed that blue and black modules were significantly positively correlated with the peroxidase activity and proline content, then the hub genes within these two modules were further identified. Taken together, we found the categories of A. aculeatus-induced salt responsive genes, revealing underlying fungus-induced molecular mechanisms of salt stress response in perennial ryegrass roots. Besides, fungus-induced salt-tolerant hub genes represent a foundation for further exploring the molecular mechanisms.

The Anatomical Basis of Heavy Metal Responses in Legumes and Their Impact on Plant-Rhizosphere Interactions

Rapid industrialization, urbanization, and mine tailings runoff are the main sources of heavy metal contamination of agricultural land, which has become one of the major constraints to crop growth and productivity. Finding appropriate solutions to protect plants and agricultural land from heavy metal pollution/harmful effects is important for sustainable development. Phytoremediation and plant growth-promoting rhizobacteria (PGPR) are promising methods for this purpose, which both heavily rely on an appropriate understanding of the anatomical structure of plants. Specialized anatomical features, such as those of epidermis and endodermis and changes in the root vascular tissue, are often associated with heavy metal tolerance in legumes. This review emphasizes the uptake and transport of heavy metals by legume plants that can be used to enhance soil detoxification by phytoremediation processes. Moreover, the review also focuses on the role of rhizospheric organisms in the facilitation of heavy metal uptake, the various mechanisms of enhancing the availability of heavy metals in the rhizosphere, the genetic diversity, and the microbial genera involved in these processes. The information presented here can be exploited for improving the growth and productivity of legume plants in metal-prone soils.

Salt stress resilience in plants mediated through osmolyte accumulation and its crosstalk mechanism with phytohormones

Salinity stress is one of the significant abiotic stresses that influence critical metabolic processes in the plant. Salinity stress limits plant growth and development by adversely affecting various physiological and biochemical processes. Enhanced generation of reactive oxygen species (ROS) induced via salinity stress subsequently alters macromolecules such as lipids, proteins, and nucleic acids, and thus constrains crop productivity. Due to which, a decreasing trend in cultivable land and a rising world population raises a question of global food security. In response to salt stress signals, plants adapt defensive mechanisms by orchestrating the synthesis, signaling, and regulation of various osmolytes and phytohormones. Under salinity stress, osmolytes have been investigated to stabilize the osmotic differences between the surrounding of cells and cytosol. They also help in the regulation of protein folding to facilitate protein functioning and stress signaling. Phytohormones play critical roles in eliciting a salinity stress adaptation response in plants. These responses enable the plants to acclimatize to adverse soil conditions. Phytohormones and osmolytes are helpful in minimizing salinity stress-related detrimental effects on plants. These phytohormones modulate the level of osmolytes through alteration in the gene expression pattern of key biosynthetic enzymes and antioxidative enzymes along with their role as signaling molecules. Thus, it becomes vital to understand the roles of these phytohormones on osmolyte accumulation and regulation to conclude the adaptive roles played by plants to avoid salinity stress.

Biochemical and molecular responses of Rosa damascena mill. cv. Kashan to salicylic acid under salinity stress

BACKGROUND

Today, salinity stress is one of the most important abiotic stresses in the world, because it causes damage to many agricultural products and reduces their yields. Oxidative stress causes tissue damages in plants, which occurs with the production of reactive oxygen species (ROS) when plants are exposed to environmental stresses such as salinity. Today, it is recommended to use compounds that increase the resistance of plants to environmental stresses and improve plant metabolic activities. Salicylic acid (SA), as an intracellular and extracellular regulator of the plant response, is known as one of these effective compounds. Damask rose (Rosa damascena Mill.) is a medicinal plant from the Rosaceae, and its essential oils and aromatic compounds are used widely in the cosmetic and food industries in the world. Therefore, considering the importance of this plant from both medicinal and ornamental aspects, for the first time, we investigated one of the native cultivars of Iran (Kashan). Since one of the most important problems in Damask rose cultivation is the occurrence of salinity stress, for the first time, we investigated the interaction of several levels of NaCl salinity (0, 4, 8, and 12 ds m^- 1) with SA (0, 0.5, 1, and 2 mM) as a stress reducer.

RESULTS

Since salinity stress reduces plant growth and yield, in this experiment, the results showed that the increase in NaCl concentration caused a gradual decrease in photosynthetic and morphological parameters and an increase in ion leakage. Also, increasing the level of salinity stress up to 12 ds m^- 1 affected the amount of chlorophyll, root length and leaf total area, all of which reduced significantly compared to plants under no stress. However, many studies have highlighted the application of compounds that reduce the negative effects of stress and increase plant resistance and tolerance against stresses. In this study, the application of SA even at low concentration (0.5 mM) could neutralize the negative effects of salinity stress in the Rosa damascena. In this regard, the results showed that salinity increases the activity of antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) and the concentration of proline, protein and glycine betaine (GB). Overexpression of antioxidant genes (Ascorbate Peroxidase (APX), CAT, Peroxidase (POD), Fe-SOD and Cu-SOD) showed an important role in salt tolerance in Damascus rose. In addition, 0.5 mm SA increased the activity of enzymatic and non-enzymatic systems and increased salinity tolerance.

CONCLUSIONS

The change in weather conditions due to global warming and increased dryness contributes to the salinization of the earth's surface soils. Therefore, it is of particular importance to measure the threshold of tolerance of roses to salinity stress and the effect of stress-reducing substances in plants. In this context, SA has various roles such as increasing the content of pigments, preventing ethylene biosynthesis, increasing growth, and activating genes involved in stress, which modifies the negative effects of salinity stress. Also, according to the results of this research, even in the concentration of low values, positive results can be obtained from SA, so it can be recommended as a relatively cheap and available material to improve production in saline lands.

Pivotal Role of Phytohormones and Their Responsive Genes in Plant Growth and Their Signaling and Transduction Pathway under Salt Stress in Cotton

The presence of phyto-hormones in plants at relatively low concentrations plays an indispensable role in regulating crop growth and yield. Salt stress is one of the major abiotic stresses limiting cotton production. It has been reported that exogenous phyto-hormones are involved in various plant defense systems against salt stress. Recently, different studies revealed the pivotal performance of hormones in regulating cotton growth and yield. However, a comprehensive understanding of these exogenous hormones, which regulate cotton growth and yield under salt stress, is lacking. In this review, we focused on new advances in elucidating the roles of exogenous hormones (gibberellin (GA) and salicylic acid (SA)) and their signaling and transduction pathways and the cross-talk between GA and SA in regulating crop growth and development under salt stress. In this review, we not only focused on the role of phyto-hormones but also identified the roles of GA and SA responsive genes to salt stress. Our aim is to provide a comprehensive review of the performance of GA and SA and their responsive genes under salt stress, assisting in the further elucidation of the mechanism that plant hormones use to regulate growth and yield under salt stress.

Mitigative effect of biosynthesized SeNPs on cyanobacteria under paraquat toxicity

Increasing population has resulted in increased food demand. Pesticides like paraquat (PQ) have been used indiscriminately to increase the growth and yield of crops. However, this has adversely affected a wide spectrum of non-target organisms like cyanobacteria that are used as a bio-fertilizer in the rice field. In the present study, biogenic- Gloeocaspa gelatinosa NCCU -430 mediated selenium nanoparticles (SeNPs) were synthesized and characterized using different techniques including UV-Visible spectroscopy, XRD, FTIR, TEM and SEM-EDX for their use as PQ toxicity mitigator in cyanobacterial biofertilizer (Anabaena variabilis NCCU-442). Therefore, a comparative study was performed among control, PQ, SeNPs and SeNPs+PQ to check the efficacy of SeNPs in mitigation of PQ induced toxicity. Supplementation of SeNPs in PQ treated culture enhanced antioxidant enzymes activity i.e., SOD (7.55%), CAT (57.94%), APX (17.45%) and GR (14.72%) as compared to only PQ treated culture. The outcomes of the present study suggested that SeNPs can ameliorate the PQ induced stress that may be used in sustainable rice cultivation needed for filing the gap between requirement and supply.

Defining key metabolic roles in osmotic adjustment and ROS homeostasis in the recretohalophyte Karelinia caspia under salt stress

The recretohalophyte Karelinia caspia is of forage and medical value and can remediate saline soils. We here assess the contribution of primary/secondary metabolism to osmotic adjustment and ROS homeostasis in Karelinia caspia under salt stress using multi-omic approaches. Computerized phenomic assessments, tests for cellular osmotic changes and lipid peroxidation indicated that salt treatment had no detectable physical effect on K. caspia. Metabolomic analysis indicated that amino acids, saccharides, organic acids, polyamine, phenolic acids, and vitamins accumulated significantly with salt treatment. Transcriptomic assessment identified differentially expressed genes closely linked to the changes in above primary/secondary metabolites under salt stress. In particular, shifts in carbohydrate metabolism (TCA cycle, starch and sucrose metabolism, glycolysis) as well as arginine and proline metabolism were observed to maintain a low osmotic potential. Chlorogenic acid/vitamin E biosynthesis was also enhanced, which would aid in ROS scavenging in the response of K. caspia to salt. Overall, our findings define key changes in primary/secondary metabolism that are coordinated to modulate the osmotic balance and ROS homeostasis to contribute to the salt tolerance of K. caspia.

Modulation of Cellular Redox Status and Antioxidant Defense System after Synergistic Application of Zinc Oxide Nanoparticles and Salicylic Acid in Rice (Oryza sativa) Plant under Arsenic Stress

The objective of this research was to determine the effect of zinc oxide nanoparticles (ZnONPs) and/or salicylic acid (SA) under arsenic (As) stress on rice (Oryza sativa). ZnONPs are analyzed for various techniques viz., X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). All of these tests established that ZnONPs are pure with no internal defects, and can be potentially used in plant applications. Hence, we further investigated for better understanding of the underlying mechanisms and the extent of ZnONPs and SA induced oxidative stress damages. More restricted plant growth, gas exchange indices, significant reduction in the SPAD index and maximum quantum yield (Fv/Fm) and brutal decline in protein content were noticed in As-applied plants. In contrast, foliar fertigation of ZnONPs and/or SA to As-stressed rice plants lessens the oxidative stress, as exposed by subordinate levels of reactive oxygen species (ROS) synthesis. Improved enzymatic activities of catalase (CAT), peroxidase (POX), and superoxide dismutase (SOD), proline and total soluble protein contents under ZnONPs and SA treatment plays an excellent role in the regulation of various transcriptional pathways participated in oxidative stress tolerance. Higher content of nitrogen (N; 13%), phosphorus (P; 10%), potassium (K; 13%), zinc (Zn; 68%), manganese (Mn; 14%), and iron (Fe; 19) in ZnONPs and SA treated plants under As-stress, thus hampered growth and photosynthetic efficiency of rice plants. Our findings suggest that toxicity of As was conquering by the application of ZnONPs and SA in rice plants.

Physiological Responses of Salinized Fenugreek (Trigonellafoenum-graecum L.) Plants to Foliar Application of Salicylic Acid

Considering the detrimental effects of salt stress on the physiological mechanisms of plants in terms of growth, development and productivity, intensive efforts are underway to improve plant tolerance to salinity. Hence, an experiment was conducted to assess the impact of the foliar application of salicylic acid (SA; 0.5 mM) on the physiological traits of fenugreek (Trigonellafoenum-graecum L.) plants grown under three salt concentrations (0, 75, and 150 mM NaCl). An increase in salt concentration generated a decrease in the chlorophyll content index (CCI); however, the foliar application of SA boosted the CCI. The malondialdehyde content increased in salt-stressed fenugreek plants, while a reduction in content was observed with SA. Likewise, SA application induced an accumulation of proline, total phenolics, and flavonoids. Moreover, further increases in total free amino acids and shikimic acid were observed with the foliar application of SA, in either control or salt-treated plants. Similar results were obtained for ascorbate peroxidase, peroxidase, polyphenol oxidase, and catalase with SA application. Hence, we concluded that the foliar application of SA ameliorates salinity, and it is a growth regulator that improves the tolerance of fenugreek plants under salt stress.

Regulation of superoxide dismutase activity in soybean plants by inoculating seeds with rhizobia containing nanoparticles of metal carboxylates under conditions of different water supply

Soybean is one of the most profitable advanced crops in agricultural production in Ukraine and the world as a whole. Therefore, studies of means of regulation and increase in the adaptive capacity of soybeans in symbiosis with nodule bacteria under the action of unfavourable environmental factors are relevant and should be aimed at the use of complex bacterial compositions involving modern nanotechnological approaches. Nanocarboxylates of ferrum, molybdenum and germanium metals were used as components of rhizobia inoculation suspension for soybean seed treatment to study the effectiveness of their complex effect on the regulation of the activity of the key antioxidant enzyme superoxide dismutase in plants under drought. Various symbiotic systems were used, which included soybean plants and inoculation suspensions based on the active, virulent Tn5-mutant Bradyrhizobium japonicum B1-20 by adding nanoparticles of ferrum, germanium and molybdenum carboxylates to the culture medium in a ratio of 1: 1000. Citric acid was the chelator. A model drought lasting 14 days was created during the period of active fixation of atmospheric molecular nitrogen by root nodules of soybeans in the budding and flowering stages, by means of controlled watering of plants to 30% of the total moisture content. In the stage of bean formation, watering of plants was resumed to the optimal level – 60% of the total moisture content. The control was soybean plants, the seeds of which were inoculated with a suspension of rhizobia without the addition of chelated metals. The following research methods were used in the work – microbiological, physiological and biochemical. According to the results, it was found that when nanoparticles of carboxylates of ferrum, molybdenum and germanium were added to the inoculation suspension of rhizobia, there was an increase in superoxide dismutase activity in root nodules and a decrease in soybean leaves under optimal water supply conditions of plants. This indicates the initial changes in the activity of the antioxidant enzyme in these symbiotic systems, induced by the influence of chelated metals in combination with the rhizobia of the active Tn5-mutant B. japonicum B1-20. Prolonged drought induced an increase in the overall level of superoxide dismutase activity in soybean nodules and leaves, compared to plants grown under optimal watering conditions. The symbiotic system formed by soybeans and B. japonicum with molybdenum carboxylate nanoparticles was the most sensitive to long-term drought exposure, compared to two other soybean-rhizobial symbioses using ferrum and germanium nanocarboxylates. This was manifested in the unstable reaction of the enzyme to the action of drought – suppression or intensification of the level of its activity in the root nodules and leaves of soybeans inoculated with rhizobia containing molybdenum carboxylate nanoparticles. In symbiotic systems with the participation of germanium and ferrum nanocarboxylates, slight changes were revealed in superoxide dismutase activity in root nodules and leaves of plants during drought and restoration of enzyme activity to the level of plants with optimal watering after water stress. It is concluded that the addition to the culture medium of rhizobia Tn5-mutant B1-20 of nanocarboxylates of germanium or ferrum is an effective means of regulating the activity of the antioxidant enzyme superoxide dismutase in soybean root nodules and leaves, which can contribute to an increase in the protective properties and adaptation of plants to the action of dehydration.

Roles of Si and SiNPs in Improving Thermotolerance of Wheat Photosynthetic Machinery via Upregulation of PsbH, PsbB and PsbD Genes Encoding PSII Core Proteins

Photosystem II is extremely susceptible to environmental alterations, particularly high temperatures. The maintenance of an efficient photosynthetic system under stress conditions is one of the main issues for plants to attain their required energy. Nowadays, searching for stress alleviators is the main goal for maintaining photosynthetic system productivity and, thereby, crop yield under global climate change. Potassium silicate (K2SiO3, 1.5 mM) and silicon dioxide nanoparticles (SiO2NPs, 1.66 mM) were used to mitigate the negative impacts of heat stress (45 °C, 5 h) on wheat (Triticum aestivum L.) cv. (Shandawelly) seedlings. The results showed that K2SiO3 and SiO2NPs diminished leaf rolling symptoms and electrolyte leakage (EL) of heat-stressed wheat leaves. Furthermore, the maximum quantum yield of photosystem II (Fv/Fm) and the performance index (PIabs), as well as the photosynthetic pigments and organic solutes including soluble sugars, sucrose, and proline accumulation, were increased in K2SiO3 and SiO2NPs stressed leaves. At the molecular level, RT-PCR analysis showed that K2SiO3 and SiO2NPs treatments stimulated the overexpression of PsbH, PsbB, and PsbD genes. Notably, this investigation indicated that K2SiO3 was more effective in improving wheat thermotolerance compared to SiO2NPs. The application of K2SiO3 and SiO2NPs may be one of the proposed approaches to improve crop growth and productivity to tolerate climatic change.

Biotechnological strategies for enhancing heavy metal tolerance in neglected and underutilized legume crops: A comprehensive review

Contamination of agricultural land and water by heavy metals due to rapid industrialization and urbanization including various natural processes have become one of the major constraints to crop growth and productivity. Several studies have reported that to counteract heavy metal stress, plants should be able to maneuver various physiological, biochemical and molecular processes to improve their growth and development under heavy metal stress. With the advent of modern biotechnological tools and techniques it is now possible to tailor legume and other plants overexpressing stress-induced genes, transcription factors, proteins, and metabolites that are directly involved in heavy metal stress tolerance. This review provides an in-depth overview of various biotechnological approaches and/or strategies that can be used for enhancing detoxification of the heavy metals by stimulating phytoremediation processes. Synthetic biology tools involved in the engineering of legume and other crop plants against heavy metal stress tolerance are also discussed herewith some pioneering examples where synthetic biology tools that have been used to modify plants for specific traits. Also, CRISPR based genetic engineering of plants, including their role in modulating the expression of several genes/ transcription factors in the improvement of abiotic stress tolerance and phytoremediation ability using knockdown and knockout strategies has also been critically discussed.

Transcriptome skimming of lentil (Lens culinaris Medikus) cultivars with contrast reaction to salt stress

Extensive transcriptomic skimming was conducted to decipher molecular, morphological, physiological, and biochemical responses in salt-tolerant (PDL-1) and salt-sensitive (L-4076) cultivars under control (0 mM NaCl) and salinity stress (120 mM NaCl) conditions at seedling stage. Morphological, physiological, and biochemical studies revealed that PDL-1 exhibited no salt injury and had higher K⁺/Na⁺ ratio, relative water content (RWC), chlorophyll, glycine betaine, and soluble sugars in leaves while lower H₂O₂ induced fluorescence signals in roots as compared to L-4076. Transcriptomic profile revealed a total of 17,433 significant differentially expressed genes (DEGs) under different treatments and cultivar combinations that include 2557 upregulated and 1533 downregulated transcripts between contrasting cultivars under salt stress. Accuracy of transcriptomic analysis was validated through quantification of 10 DEGs via quantitative real-time polymerase chain reaction (qRT-PCR). DEGs were functionally characterized by Gene Ontology (GO) analysis and assigned to various metabolic pathways using MapMan. DEGs were found to be significantly associated with phytohormone-mediated signal transduction, cellular redox homoeostasis, secondary metabolism, nitrogen metabolism, and cellular stress signaling. The present study revealed putative molecular mechanism of salinity tolerance in lentil together with identification of 5643 simple sequence repeats (SSRs) and 176,433 single nucleotide polymorphisms (SNPs) which can be utilized to enhance linkage maps density along with detection of quantitative trait loci (QTLs) associated with traits of interests. Stress-related pathways identified in this study divulged plant functioning that can be targeted to improve salinity stress tolerance in crop species.

How Plant Hormones Mediate Salt Stress Responses

Salt stress is one of the major environmental stresses limiting plant growth and productivity. To adapt to salt stress, plants have developed various strategies to integrate exogenous salinity stress signals with endogenous developmental cues to optimize the balance of growth and stress responses. Accumulating evidence indicates that phytohormones, besides controlling plant growth and development under normal conditions, also mediate various environmental stresses, including salt stress, and thus regulate plant growth adaptation. In this review, we mainly discuss and summarize how plant hormones mediate salinity signals to regulate plant growth adaptation. We also highlight how, in response to salt stress, plants build a defense system by orchestrating the synthesis, signaling, and metabolism of various hormones via multiple crosstalks.

Exogenous salicylic acid ameliorates heat stress-induced damages and improves growth and photosynthetic efficiency in alfalfa (Medicago sativa L.)

Heat stress is found to be a detrimental factor for growth and development of alfalfa (Medicago sativa L.) which is tremendously invaluable forage due to its high feed value and yield potential. Salicylic acid (SA) has been reported to play a pivotal role in the regulation of plants biotic and abiotic stress response. However, the role of exogenous SA in protecting alfalfa from heat-induced damage has rarely been studied. In this study, four-week-old alfalfa seedlings were treated with 0.25 mM or 0.5 mM SA five days prior to high stress treatment (three day), and various growth and physiological traits were measured. The results showed that exogenous SA pretreatment could improve leaf morphology, plant height, biomass, chlorophyll content, and photosynthetic efficiency of alfalfa under heat stress. Meanwhile, SA could alleviate heat-induced membrane damage by reducing electrolyte leakage (EL) and malondialdehyde (MDA) content, and regulate the activities of antioxidant enzymes including catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). The results revealed that exogenous SA application enhanced alfalfa heat tolerance by modulating various morphological and physiological characteristics under heat stress, with more prominent effect at lower concentration (0.25 mM). Overall, this study provides fundamental insights into the SA-mediated physiological adaptation of alfalfa plants to heat stress, which could have useful implication in managing other plants which are suffering global warming.

Towards a Sustainable Agriculture: Strategies Involving Phytoprotectants against Salt Stress

Salinity is one of the main constraints for agriculture productivity worldwide. This important abiotic stress has worsened in the last 20 years due to the increase in water demands in arid and semi-arid areas. In this context, increasing tolerance of crop plants to salt stress is needed to guarantee future food supply to a growing population. This review compiles knowledge on the use of phytoprotectants of microbial origin (arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria), osmoprotectants, melatonin, phytohormones and antioxidant metabolism-related compounds as alleviators of salt stress in numerous plant species. Phytoprotectants are discussed in detail, including their nature, applicability, and role in the plant in terms of physiological and phenotype effects. As a result, increased crop yield and crop quality can be achieved, which in turn positively impact food security. Herein, efforts from academic and industrial sectors should focus on defining the treatment conditions and plant-phytoprotectant associations providing higher benefits.

Polyamines produced by Sinorhizobium meliloti Rm8530 contribute to symbiotically relevant phenotypes ex planta and to nodulation efficiency on alfalfa

In nitrogen-fixing rhizobia, emerging evidence shows significant roles for polyamines in growth and abiotic stress resistance. In this work we show that a polyamine-deficient ornithine decarboxylase null mutant (odc2) derived from Sinorhizobium meliloti Rm8530 had significant phenotypic differences from the wild-type, including greatly reduced production of exopolysaccharides (EPS; ostensibly both succinoglycan and galactoglucan), increased sensitivity to oxidative stress and decreased swimming motility. The introduction of the odc2 gene borne on a plasmid into the odc2 mutant restored wild-type phenotypes for EPS production, growth under oxidative stress and swimming. The production of calcofluor-binding EPS (succinoglycan) by the odc2 mutant was also completely or mostly restored in the presence of exogenous spermidine (Spd), norspermidine (NSpd) or spermine (Spm). The odc2 mutant formed about 25 % more biofilm than the wild-type, and its ability to form biofilm was significantly inhibited by exogenous Spd, NSpd or Spm. The odc2 mutant formed a less efficient symbiosis with alfalfa, resulting in plants with significantly less biomass and height, more nodules but less nodule biomass, and 25 % less nitrogen-fixing activity. Exogenously supplied Put was not able to revert these phenotypes and caused a similar increase in plant height and dry weight in uninoculated plants and in those inoculated with the wild-type or odc2 mutant. We discuss ways in which polyamines might affect the phenotypes of the odc2 mutant.

Overexpression of the arginine decarboxylase gene promotes the symbiotic interaction Medicago truncatula-Sinorhizobium meliloti and induces the accumulation of proline and spermine in nodules under salt stress conditions

Legumes have the capacity to fix nitrogen in symbiosis with soil bacteria known as rhizobia by the formation of root nodules. However, nitrogen fixation is highly sensitive to soil salinity with a concomitant reduction of the plant yield and soil fertilization. Polycationic aliphatic amines known as polyamines (PAs) have been shown to be involved in the response to a variety of stresses in plants including soil salinity. Therefore, the generation of transgenic plants overexpressing genes involved in PA biosynthesis have been proposed as a promising tool to improve salt stress tolerance in plants. In this work we tested whether the modulation of PAs in transgenic Medicago truncatula plants was advantageous for the symbiotic interaction with Sinorhizobium meliloti under salt stress conditions, when compared to wild type plants. Consequently, we characterized the symbiotic response to salt stress of the homozygous M. truncatula plant line L-108, constitutively expressing the oat adc gene, coding for the PA biosynthetic enzyme arginine decarboxylase, involved in PAs biosynthesis. In a nodulation kinetic assay, nodule number incremented in L-108 plants under salt stress. In addition, these plants at vegetative stage showed higher nitrogenase and nodule biomass and, under salt stress, accumulated proline (Pro) and spermine (Spm) in nodules, while in wt plants, the accumulation of glutamic acid (Glu), γ-amino butyric acid (GABA) and 1-aminocyclopropane carboxylic acid (ACC) (the ethylene (ET) precursor) were the metabolites involved in the salt stress response. Therefore, overexpression of oat adc gene favours the symbiotic interaction between plants of M. truncatula L-108 and S. meliloti under salt stress and the accumulation of Pro and Spm, seems to be the molecules involved in salt stress tolerance.

Polyamines contribute to salinity tolerance in the symbiosis Medicago truncatula-Sinorhizobium meliloti by preventing oxidative damage

Polyamines (PAs) such as spermidine (Spd) and spermine (Spm) are small ubiquitous polycationic compounds that contribute to plant adaptation to salt stress. The positive effect of PAs has been associated to a cross-talk with other anti-stress hormones such as brassinosteroids (BRs). In this work we have studied the effects of exogenous Spd and Spm pre-treatments in the response to salt stress of the symbiotic interaction between Medicago truncatula and Sinorhizobium meliloti by analyzing parameters related to nitrogen fixation, oxidative damage and cross-talk with BRs in the response to salinity. Exogenous PAs treatments incremented the foliar and nodular Spd and Spm content which correlated with an increment of the nodule biomass and nitrogenase activity. Exogenous Spm treatment partially prevented proline accumulation which suggests that this polyamine could replace the role of this amino acid in the salt stress response. Additionally, Spd and Spm pre-treatments reduced the levels of H₂O₂ and lipid peroxidation under salt stress. PAs induced the expression of genes involved in BRs biosynthesis which support a cross-talk between PAs and BRs in the salt stress response of M. truncatula-S. meliloti symbiosis. In conclusion, exogenous PAs improved the response to salinity of the M. truncatula-S. meliloti symbiosis by reducing the oxidative damage induced under salt stress conditions. In addition, in this work we provide evidences of the cross-talk between PAs and BRs in the adaptive responses to salinity.

Salicylic acid differently impacts ethylene and polyamine synthesis in the glycophyte Solanum lycopersicum and the wild-related halophyte Solanum chilense exposed to mild salt stress

This study aimed to determine the effects of exogenous application of salicylic acid (SA) on the toxic effects of salt in relation to ethylene and polyamine synthesis, and to correlate these traits with the expression of genes involved in ethylene and polyamine metabolism in two tomato species differing in their sensitivity to salt stress, Solanum lycopersicum cv Ailsa Craig and its wild salt-resistant relative Solanum chilense. In S. chilense, treatment with 125 mM NaCl improved plant growth, increased production of ethylene, endogenous salicylic acid and spermine. The production was related to a modification of expression of genes involved in ethylene and polyamine metabolism. In contrast, salinity decreased plant growth in S. lycopersicum without affecting endogenous ethylene, salicylic or polyamine concentrations. Exogenous application of salicylic acid at 0.01 mM enhanced shoot growth in both species and affected ethylene and polyamine production in S. chilense. Concomitant application of NaCl and salicylic acid improved osmotic adjustment, thus suggesting that salt and SA may act in synergy on osmolyte synthesis. However, the beneficial impact of exogenous application of salicylic acid was mitigated by salt stress since NaCl impaired endogenous SA accumulation in the shoot and salicylic acid did not improve plant growth in salt-treated plants. Our results thus revealed that both species respond differently to salinity and that salicylic acid, ethylene and polyamine metabolisms are involved in salt resistance in S. chilense.

Kresoxim-methyl primes Medicago truncatula plants against abiotic stress factors via altered reactive oxygen and nitrogen species signalling leading to downstream transcriptional and metabolic readjustment

Biotic and abiotic stresses, such as fungal infection and drought, cause major yield losses in modern agriculture. Kresoxim-methyl (KM) belongs to the strobilurins, one of the most important classes of agricultural fungicides displaying a direct effect on several plant physiological and developmental processes. However, the impact of KM treatment on salt and drought stress tolerance is unknown. In this study we demonstrate that KM pre-treatment of Medicago truncatula plants results in increased protection to drought and salt stress. Foliar application with KM prior to stress imposition resulted in improvement of physiological parameters compared with stressed-only plants. This protective effect was further supported by increased proline biosynthesis, modified reactive oxygen and nitrogen species signalling, and attenuation of cellular damage. In addition, comprehensive transcriptome analysis identified a number of transcripts that are differentially accumulating in drought- and salinity-stressed plants (646 and 57, respectively) after KM pre-treatment compared with stressed plants with no KM pre-treatment. Metabolomic analysis suggests that the priming role of KM in drought- and to a lesser extent in salinity-stressed plants can be attributed to the regulation of key metabolites (including sugars and amino acids) resulting in protection against abiotic stress factors. Overall, the present study highlights the potential use of this commonly used fungicide as a priming agent against key abiotic stress conditions.

Ethylene, a Hormone at the Center-Stage of Nodulation

Nodulation is the result of a beneficial interaction between legumes and rhizobia. It is a sophisticated process leading to nutrient exchange between the two types of symbionts. In this association, within a nodule, the rhizobia, using energy provided as photosynthates, fix atmospheric nitrogen and convert it to ammonium which is available to the plant. Nodulation is recognized as an essential process in nitrogen cycling and legume crops are known to enrich agricultural soils in nitrogenous compounds. Furthermore, as they are rich in nitrogen, legumes are considered important as staple foods for humans and fodder for animals. To tightly control this association and keep it mutualistic, the plant uses several means, including hormones. The hormone ethylene has been known as a negative regulator of nodulation for almost four decades. Since then, much progress has been made in the understanding of both the ethylene signaling pathway and the nodulation process. Here I have taken a large view, using recently obtained knowledge, to describe in some detail the major stages of the process. I have not only reviewed the steps most commonly covered (the common signaling transduction pathway, and the epidermal and cortical programs), but I have also looked into steps less understood (the pre-infection step with the plant defense response, the bacterial release and the formation of the symbiosome, and nodule functioning and senescence). After a succinct review of the ethylene signaling pathway, I have used the knowledge obtained from nodulation- and ethylene-related mutants to paint a more complete picture of the role played by the hormone in nodule organogenesis, functioning, and senescence. It transpires that ethylene is at the center of this effective symbiosis. It has not only been involved in most of the steps leading to a mature nodule, but it has also been implicated in host immunity and nodule senescence. It is likely responsible for the activation of other hormonal signaling pathways. I have completed the review by citing three studies which makes one wonder whether knowledge gained on nodulation in the last decades is ready to be transferred to agricultural fields.

Physiological and Molecular Aspects of Tolerance to Environmental Constraints in Grain and Forage Legumes

Despite the agronomical and environmental advantages of the cultivation of legumes, their production is limited by various environmental constraints such as water or nutrient limitation, frost or heat stress and soil salinity, which may be the result of pedoclimatic conditions, intensive use of agricultural lands, decline in soil fertility and environmental degradation. The development of more sustainable agroecosystems that are resilient to environmental constraints will therefore require better understanding of the key mechanisms underlying plant tolerance to abiotic constraints. This review provides highlights of legume tolerance to abiotic constraints with a focus on soil nutrient deficiencies, drought, and salinity. More specifically, recent advances in the physiological and molecular levels of the adaptation of grain and forage legumes to abiotic constraints are discussed. Such adaptation involves complex multigene controlled-traits which also involve multiple sub-traits that are likely regulated under the control of a number of candidate genes. This multi-genetic control of tolerance traits might also be multifunctional, with extended action in response to a number of abiotic constraints. Thus, concrete efforts are required to breed for multifunctional candidate genes in order to boost plant stability under various abiotic constraints.

Salicylic acid-induced abiotic stress tolerance and underlying mechanisms in plants

Abiotic stresses (such as metals/metalloids, salinity, ozone, UV-B radiation, extreme temperatures, and drought) are among the most challenging threats to agricultural system and economic yield of crop plants. These stresses (in isolation and/or combination) induce numerous adverse effects in plants, impair biochemical/physiological and molecular processes, and eventually cause severe reductions in plant growth, development and overall productivity. Phytohormones have been recognized as a strong tool for sustainably alleviating adverse effects of abiotic stresses in crop plants. In particular, the significance of salicylic acid (SA) has been increasingly recognized in improved plant abiotic stress-tolerance via SA-mediated control of major plant-metabolic processes. However, the basic biochemical/physiological and molecular mechanisms that potentially underpin SA-induced plant-tolerance to major abiotic stresses remain least discussed. Based on recent reports, this paper: (a) overviews historical background and biosynthesis of SA under both optimal and stressful environments in plants; (b) critically appraises the role of SA in plants exposed to major abiotic stresses;

Genome-wide identification of CAMTA gene family members in Medicago truncatula and their expression during root nodule symbiosis and hormone treatments

Calmodulin-binding transcription activators (CAMTAs) are well-characterized calmodulin-binding transcription factors in the plant kingdom. Previous work shows that CAMTAs play important roles in various biological processes including disease resistance, herbivore attack response, and abiotic stress tolerance. However, studies that address the function of CAMTAs during the establishment of symbiosis between legumes and rhizobia are still lacking. This study undertook comprehensive identification and analysis of CAMTA genes using the latest updated M. truncatula genome. All the MtCAMTA genes were expressed in a tissues-specific manner and were responsive to environmental stress-related hormones. The expression profiling of MtCAMTA genes during the early phase of Sinorhizobium meliloti infection was also analyzed. Our data showed that the expression of most MtCAMTA genes was suppressed in roots by S. meliloti infection. The responsiveness of MtCAMTAs to S. meliloti infection indicated that they may function as calcium-regulated transcription factors in the early nodulation signaling pathway. In addition, bioinformatics analysis showed that CAMTA binding sites existed in the promoter regions of various early rhizobial infection response genes, suggesting possible MtCAMTAs-regulated downstream candidate genes during the early phase of S. meliloti infection. Taken together, these results provide basic information about MtCAMTAs in the model legume M. truncatula, and the involvement of MtCAMTAs in nodule organogenesis. This information furthers our understanding of MtCAMTA protein functions in M. truncatula and opens new avenues for continued research.

Alleviation of salt-induced photosynthesis and growth inhibition by salicylic acid involves glycinebetaine and ethylene in mungbean (Vigna radiata L.)

The influence of salicylic acid (SA) in alleviation of salt stress in mungbean (Vigna radiata L.) through modulation of glycinebetaine (GB) and ethylene was studied. SA application at 0.5 mM increased methionine (Met) and GB accumulation in plants concomitant with the suppression of ethylene formation by inhibiting 1-aminocyclopropane carboxylic acid synthase (ACS) activity more conspicuously under salt stress than no stress. The increased GB accumulation together with reduced ethylene under salt stress by SA application was associated with increased glutathione (GSH) content and lower oxidative stress. These positive effects on plant metabolism induced by SA application led to improved photosynthesis and growth under salt stress. These results suggest that SA induces GB accumulation through increased Met and suppresses ethylene formation under salt stress and enhances antioxidant system resulting in alleviation of adverse effects of salt stress on photosynthesis and growth. These effects of SA were substantiated by the findings that application of SA-analogue, 2, 6, dichloro-isonicotinic acid (INA) and ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG) resulted in similar effects on Met, GB, ethylene production, photosynthesis and growth under salt stress. Future studies on the interaction between SA, GB and ethylene could be exploited for adaptive responses of plants under salt stress.

Regulation of water, salinity, and cold stress responses by salicylic acid

Salicylic acid (SA) is a naturally occurring phenolic compound. SA plays an important role in the regulation of plant growth, development, ripening, and defense responses. The role of SA in the plant-pathogen relationship has been extensively investigated. In addition to defense responses, SA plays an important role in the response to abiotic stresses, including drought, low temperature, and salinity stresses. It has been suggested that SA has great agronomic potential to improve the stress tolerance of agriculturally important crops. However, the utility of SA is dependent on the concentration of the applied SA, the mode of application, and the state of the plants (e.g., developmental stage and acclimation). Generally, low concentrations of applied SA alleviate the sensitivity to abiotic stresses, and high concentrations of applied induce high levels of oxidative stress, leading to a decreased tolerance to abiotic stresses. In this article, the effects of SA on the water stress responses and regulation of stomatal closure are reviewed.

Salicylic acid alleviates adverse effects of heat stress on photosynthesis through changes in proline production and ethylene formation

We investigated the potential of salicylic acid (SA) in alleviating the adverse effects of heat stress on photosynthesis in wheat (Triticum aestivum L.) cv WH 711. Activity of ribulose 1,5-bisphosphate carboxylase (Rubisco), photosynthetic-nitrogen use efficiency (NUE), and net photosynthesis decreased in plants subjected to heat stress (40 °C for 6 h), but proline metabolism increased. SA treatment (0.5 mM) alleviated heat stress by increasing proline production through the increase in γ-glutamyl kinase (GK) and decrease in proline oxidase (PROX) activity, resulting in promotion of osmotic potential and water potential necessary for maintaining photosynthetic activity. Together with this, SA treatment restricted the ethylene formation in heat-stressed plants to optimal range by inhibiting activity of 1-aminocyclopropane carboxylic acid (ACC) synthase (ACS). This resulted in improved proline metabolism, N assimilation and photosynthesis. The results suggest that SA interacts with proline metabolism and ethylene formation to alleviate the adverse effects of heat stress on photosynthesis in wheat.