Effects, tolerance mechanisms and management of salt stress in grain legumes

Exogenous application of 5-azacitidin, royal jelly and folic acid regulate plant redox state, expression level of DNA methyltransferases and alleviate adverse effects of salinity stress on Vicia faba L. plants

DNA methylation is one of induced changes under salinity stress causing reduction in the expression of several crucial genes required for normal plant's operation. Potential use of royal jelly (RJ), folic acid (FA) and 5-azacitidine (5-AZA) on two Egyptian faba bean varieties (Sakha-3 and Giza-716) grown under saline conditions was investigated. Salinity stress affects negatively on seeds germination (G %), mitotic index, membrane stability and induced a significant increase in chromosomal abnormalities (CAs). DNA methyltransferases genes (MT1 and MT2) were highly up-regulated (∼23 and 8 folds for MT1 and MT2 in shoots of Giza-716 stressed plants). On the other hand, down regulation of other studied stress related genes: superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), heat shock protein (HSP-17.9) and proline-rich protein (GPRP) were detected in stressed plants of both studied varieties. Treating plants with RJ and FA increase G%, chlorophyll content, improves membrane properties and reduces CAs compared to non-treated stressed plants. Exogenous application of 5-AZA, RJ and FA on salinity stressed plants was associated with a significant reduction in the transcription of MT1 and MT2 which was associated with significant up regulation in the expression of Cu/Zn-SOD, CAT, GR, GPRP and HSP-17.9 encoding genes. The Lowest expression of MT1 and MT2 were induced with 5-AZA treatment in both studied varieties. Exogenous application of the FA, RJ and 5-AZA modified the methylation state of stressed plants by regulation the expression of DNA methyltransferases, subsequently, modulated the expression of studied genes and could be proposed as a promising treatment to ameliorate hazardous effects of salt stress on different plants.

Improving crop salt tolerance through soil legacy effects

Soil salinization poses a critical problem, adversely affecting plant development and sustainable agriculture. Plants can produce soil legacy effects through interactions with the soil environments. Salt tolerance of plants in saline soils is not only determined by their own stress tolerance but is also closely related to soil legacy effects. Creating positive soil legacy effects for crops, thereby alleviating crop salt stress, presents a new perspective for improving soil conditions and increasing productivity in saline farmlands. Firstly, the formation and role of soil legacy effects in natural ecosystems are summarized. Then, the processes by which plants and soil microbial assistance respond to salt stress are outlined, as well as the potential soil legacy effects they may produce. Using this as a foundation, proposed the application of salt tolerance mechanisms related to soil legacy effects in natural ecosystems to saline farmlands production. One aspect involves leveraging the soil legacy effects created by plants to cope with salt stress, including the direct use of halophytes and salt-tolerant crops and the design of cropping patterns with the specific crop functional groups. Another aspect focuses on the utilization of soil legacy effects created synergistically by soil microorganisms. This includes the inoculation of specific strains, functional microbiota, entire soil which legacy with beneficial microorganisms and tolerant substances, as well as the application of novel technologies such as direct use of rhizosphere secretions or microbial transmission mechanisms. These approaches capitalize on the characteristics of beneficial microorganisms to help crops against salinity. Consequently, we concluded that by the screening suitable salt-tolerant crops, the development rational cropping patterns, and the inoculation of safe functional soils, positive soil legacy effects could be created to enhance crop salt tolerance. It could also improve the practical significance of soil legacy effects in the application of saline farmlands.

Residual efficiency of iron-nanoparticles and different iron sources on growth, and antioxidants in maize plants under salts stress: life cycle study

Exogenous application of iron (Fe) may alleviate salinity stress in plants growing in saline soils. This comparative study evaluated the comparative residual effects of iron nanoparticles (FNp) with two other Fe sources including iron-sulphate (FS) and iron-chelate (FC) on maize (Zea mays L.) crop grown under salt stress. All three Fe sources were applied at the rate of 15 and 25 mg/kg of soil before the sowing of wheat (an earlier crop; following the sequence of crop rotation) and no further Fe amendments were added later for the maize crop. Results revealed that FNp application at 25 mg/kg (FNp-2) substantially increased maize height, root length, root dry weight, shoot dry weight, and grain weightby 80.7%, 111.1%, 45.7%, 59.5%, and 77.2% respectively, as compared to the normal controls; and 62.6%, 81.3%, 65.1%, 78%, and 61.2% as compared to salt-stressed controls, respectively. The FNp-2 treatment gave higher activities of antioxidant enzymes, such as superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase compared to salt stressed control (50.6%, 51%, 48.5%, and 49.2%, respectively). The FNp-2 treatment also produced more photosynthetic pigments and better physiological markers: higher chlorophyll a contents by 49.9%, chlorophyll b contents by 67.2%, carotenoids by 62.5%, total chlorophyll contents by 50.3%, membrane stability index by 59.1%, leaf water relative contents by 60.3% as compared to salt stressed control. The highest Fe and Zn concentrations in maize roots, shoots, and grains were observed in FNp treatment as compared to salts stressed control. Higher application rates of Fe from all the sources also delivered better outcomes in alleviating salinity stress in maize compared to their respective low application rates. The study demonstrated that FNp application alleviated salinity stress, increased nutrient uptake and enhanced the yield of maize grown on saline soils.

Exogenous ascorbic acid as a potent regulator of antioxidants, osmo-protectants, and lipid peroxidation in pea under salt stress

Pea (Pisum sativum L.), a globally cultivated leguminous crop valued for its nutritional and economic significance, faces a critical challenge of soil salinity, which significantly hampers crop growth and production worldwide. A pot experiment was carried out in the Botanical Garden, The Islamia University of Bahawalpur to alleviate the negative impacts of sodium chloride (NaCl) on pea through foliar application of ascorbic acid (AsA). Two pea varieties Meteor (V1) and Sarsabz (V2) were tested against salinity, i.e. 0 mM NaCl (Control) and 100 mM NaCl. Three levels of ascorbic acid 0 (Control), 5 and 10 mM were applied through foliar spray. The experimental design was completely randomized (CRD) with three replicates. Salt stress resulted in the suppression of growth, photosynthetic activity, and yield attributes in pea plants. However, the application of AsA treatments effectively alleviated these inhibitory effects. Under stress conditions, the application of AsA treatment led to a substantial increase in chlorophyll a (41.1%), chl. b (56.1%), total chl. contents (44.6%) and carotenoids (58.4%). Under salt stress, there was an increase in Na+ accumulation, lipid peroxidation, and the generation of reactive oxygen species (ROS). However, the application of AsA increased the contents of proline (26.9%), endogenous AsA (23.1%), total soluble sugars (17.1%), total phenolics (29.7%), and enzymatic antioxidants i.e. SOD (22.3%), POD (34.1%) and CAT (39%) in both varieties under stress. Salinity reduced the yield attributes while foliarly applied AsA increased the pod length (38.7%), number of pods per plant (40%) and 100 seed weight (45.2%). To sum up, the application of AsA alleviated salt-induced damage in pea plants by enhancing photosynthetic pigments, both enzymatic and non-enzymatic activities, maintaining ion homeostasis, and reducing excessive ROS accumulation through the limitation of lipid peroxidation. Overall, V2 (Sarsabz) performed better as compared to the V1 (Meteor).

Impact of two Erwinia sp. on the response of diverse Pisum sativum genotypes under salt stress

Currently, salinization is impacting more than 50% of arable land, posing a significant challenge to agriculture globally. Salt causes osmotic and ionic stress, determining cell dehydration, ion homeostasis, and metabolic process alteration, thus negatively influencing plant development. A promising sustainable approach to improve plant tolerance to salinity is the use of plant growth-promoting bacteria (PGPB). This work aimed to characterize two bacterial strains, that have been isolated from pea root nodules, initially called PG1 and PG2, and assess their impact on growth, physiological, biochemical, and molecular parameters in three pea genotypes (Merveille de Kelvedon, Lincoln, Meraviglia d'Italia) under salinity. Bacterial strains were molecularly identified, and characterized by in vitro assays to evaluate the plant growth promoting abilities. Both strains were identified as Erwinia sp., demonstrating in vitro biosynthesis of IAA, ACC deaminase activity, as well as the capacity to grow in presence of NaCl and PEG. Considering the inoculation of plants, pea biometric parameters were unaffected by the presence of the bacteria, independently by the considered genotype. Conversely, the three pea genotypes differed in the regulation of antioxidant genes coding for catalase (PsCAT) and superoxide dismutase (PsSOD). The highest proline levels (212.88 μmol g-1) were detected in salt-stressed Lincoln plants inoculated with PG1, along with the up-regulation of PsSOD and PsCAT. Conversely, PG2 inoculation resulted in the lowest proline levels that were observed in Lincoln and Meraviglia d'Italia (35.39 and 23.67 μmol g-1, respectively). Overall, this study highlights the potential of these two strains as beneficial plant growth-promoting bacteria in saline environments, showing that their inoculation modulates responses in pea plants, affecting antioxidant gene expression and proline accumulation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01419-8.

An insight into the different responses to salt stress in growth characteristics of two legume species during seedling growth

Legumes play a crucial role in the restoration and utilization of salinized grassland. To explore the physiological response mechanism of Astragalus membranaceus and Medicago sativa seedlings to salt stress, salt stress culture experiments with five NaCl concentration treatments (0 mmol/L, 50 mmol/L, 100 mmol/L, 200 mmol/L, and 300 mmol/L) were conducted on these two legume seedlings. Morphological characteristics, physiological features, biomass, and the protective enzyme system were measured for both seedlings. Correlation analysis, principal component analysis (PCA), and membership function analysis (MFA) were conducted for each index. Structural equation modeling (SEM) was employed to analyze the salt stress pathways of plants. The results indicated that number of primary branches (PBN), ascorbate peroxidase (APX) activity in stems and leaves, catalase (CAT) activity in roots, etc. were identified as the primary indicators for evaluating the salt tolerance of A. membranaceus during its seedling growth period. And CAT and peroxidase (POD) activity in roots, POD and superoxide dismutase (SOD) activity in stems and leaves, etc. were identified as the primary indicators for evaluating the salt tolerance of M. sativa during its growth period. Plant morphological characteristics, physiological indexes, and underground biomass (UGB) were directly affected by salinity, while physiological indexes indirectly affected the degree of leaf succulence (LSD). Regarding the response of the protective enzyme system to salt stress, the activity of POD and APX increased in A. membranaceus, while the activity of CAT increased in M. sativa. Our findings suggest that salt stress directly affects the growth strategies of legumes. Furthermore, the response of the protective enzyme system and potential cell membrane damage to salinity were very different in the two legumes.

Changes in epigenetic features in legumes under abiotic stresses

Legume crops are rich in nutritional value for human and livestock consumption. With global climate change, developing stress-resilient crops is crucial for ensuring global food security. Because of their nitrogen-fixing ability, legumes are also important for sustainable agriculture. Various abiotic stresses, such as salt, drought, and elevated temperatures, are known to adversely affect legume production. The responses of plants to abiotic stresses involve complicated cellular processes including stress hormone signaling, metabolic adjustments, and transcriptional regulations. Epigenetic mechanisms play a key role in regulating gene expressions at both transcriptional and posttranscriptional levels. Increasing evidence suggests the importance of epigenetic regulations of abiotic stress responses in legumes, and recent investigations have extended the scope to the epigenomic level using next-generation sequencing technologies. In this review, the current knowledge on the involvement of epigenetic features, including DNA methylation, histone modification, and noncoding RNAs, in abiotic stress responses in legumes is summarized and discussed. Since most of the available information focuses on a single aspect of these epigenetic features, integrative analyses involving omics data in multiple layers are needed for a better understanding of the dynamic chromatin statuses and their roles in transcriptional regulation. The inheritability of epigenetic modifications should also be assessed in future studies for their applications in improving stress tolerance in legumes through the stable epigenetic optimization of gene expressions.

Silicic and Humic Acid Priming Improves Micro- and Macronutrient Uptake, Salinity Stress Tolerance, Seed Quality, and Physio-Biochemical Parameters in Lentil (Lens culinaris spp. culinaris)

Lentil is an important grain legume crop which is mostly grown on marginal soils that hamper its productivity. Improvement of salt tolerance in lentils is considered to be a useful strategy of utilizing salt-affected lands in an economic manner. This study was conducted to evaluate the effectiveness of seed priming using silicic acid and humic acid both seperately and in combination to improve salt stress tolerance among three different lentil varieties: IPL-316 (tolerant), PSL-9, and PDL-1 (susceptible). The concentrations and durations of treatments were standardized under the normal condition and the salinity stress condition. Salt stress hindered seedling emergence and biomass production and accelerated Na+ toxicity and oxidative damage at the seedling stage in untreated seeds. Nevertheless, chemical priming improved early seedling emergence, increased root length, shoot length, and seed vigor index I and II, and reduced the mean germination time. A significant quantitative change in biochemical parameters under normal and salinity stress conditions was observed in IPL-316,viz. Specifically, for IPL-316, the following parameters were observed (values under the normal condition and values under salt stress conditions, respectively): chlorophyll-a (16 and 13 mg/g Fw), chlorophyll-b (25 and 16 mg/g FW), total chlorophyll content (42 and 30 mg/g FW), relative leaf water content (92% and 82%), total soluble sugars (26 and 33 ug/g FW), free amino acid (10 and 7 mg/g FW), total phenol (26 and 24 mg of GAE/g FW), total protein (35 and 29 mg/g FW), carbohydrate (208 and 173 mg/g FW), superoxide dismutase (SOD) (29 and 35 unit/min./g FW), proline (0.28 and 0.32 u mol/g FW), catalase (CAT) (84 and 196 unit/mL/g FW), and peroxidase (POX) (217 and 738 unit/mL/g FW). Furthermore, histochemical analysis of H2O2 and O2-, micronutrients, and macronutrients also increased, while malondialdehyde (MDA) (0.31 and 0.47 nmol/mL FW) content decreased using silicic and humic acid priming under salt stress conditions. The combination of silicic and humic acids improved seedling growth and reduced oxidative damage in lentil plants under salt stress conditions. The combination of silicic and humic acid priming hastened seedling emergence, seed quality parameters, and biochemical parameters under salt stress over respective control. To the best of our knowledge, this is the first report of integrated chemical priming in lentils for salinity stress. In conclusion, chemical priming using a combination of silicic and humic acid performed better in terms of seed quality due to enhanced antioxidant machinery, better membrane stability and osmolyte protection, and enhanced nutrient uptake under salt stress conditions.

Cell Mutagenic Autopolyploidy Enhances Salinity Stress Tolerance in Leguminous Crops

Salinity stress affects plant growth and development by causing osmotic stress and nutrient imbalances through excess Na+, K+, and Cl- ion accumulations that induce toxic effects during germination, seedling development, vegetative growth, flowering, and fruit set. However, the effects of salt stress on growth and development processes, especially in polyploidized leguminous plants, remain unexplored and scantly reported compared to their diploid counterparts. This paper discusses the physiological and molecular response of legumes towards salinity stress-based osmotic and ionic imbalances in plant cells. A multigenic response involving various compatible solutes, osmolytes, ROS, polyamines, and antioxidant activity, together with genes encoding proteins involved in the signal transduction, regulation, and response mechanisms to this stress, were identified and discussed. This discussion reaffirms polyploidization as the driving force in plant evolution and adaptation to environmental stress constraints such as drought, feverish temperatures, and, in particular, salt stress. As a result, thorough physiological and molecular elucidation of the role of gene duplication through induced autopolyploidization and possible mechanisms regulating salinity stress tolerance in grain legumes must be further studied.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Haplotype-Resolution Transcriptome Analysis Reveals Important Responsive Gene Modules and Allele-Specific Expression Contributions under Continuous Salt and Drought in Camellia sinensis

The tea plant, Camellia sinensis (L.) O. Kuntze, is one of the most important beverage crops with significant economic and cultural value. Global climate change and population growth have led to increased salt and drought stress, negatively affecting tea yield and quality. The response mechanism of tea plants to these stresses remains poorly understood due to the lack of reference genome-based transcriptional descriptions. This study presents a high-quality genome-based transcriptome dynamic analysis of C. sinensis' response to salt and drought stress. A total of 2244 upregulated and 2164 downregulated genes were identified under salt and drought stress compared to the control sample. Most of the differentially expression genes (DEGs) were found to involve divergent regulation processes at different time points under stress. Some shared up- and downregulated DEGs related to secondary metabolic and photosynthetic processes, respectively. Weighted gene co-expression network analysis (WGCNA) revealed six co-expression modules significantly positively correlated with C. sinensis' response to salt or drought stress. The MEpurple module indicated crosstalk between the two stresses related to ubiquitination and the phenylpropanoid metabolic regulation process. We identified 1969 salt-responsive and 1887 drought-responsive allele-specific expression (ASE) genes in C. sinensis. Further comparison between these ASE genes and tea plant heterosis-related genes suggests that heterosis likely contributes to the adversity and stress resistance of C. sinensis. This work offers new insight into the underlying mechanisms of C. sinensis' response to salt and drought stress and supports the improved breeding of tea plants with enhanced salt and drought tolerance.

Progress of Research on the Physiology and Molecular Regulation of Sorghum Growth under Salt Stress by Gibberellin

Plant growth often encounters diverse abiotic stresses. As a global resource-based ecological problem, salinity is widely distributed and one of the major abiotic stresses affecting crop yields worldwide. Sorghum, a cereal crop with medium salt tolerance and great value for the development and utilization of salted soils, is an important source of food, brewing, energy, and forage production. However, in soils with high salt concentrations, sorghum experiences low emergence and suppressed metabolism. It has been demonstrated that the effects of salt stress on germination and seedling growth can be effectively mitigated to a certain extent by the exogenous amendment of hormonal gibberellin (GA). At present, most of the studies on sorghum salt tolerance at home and abroad focus on morphological and physiological levels, including the transcriptome analysis of the exogenous hormone on sorghum salt stress tolerance, the salt tolerance metabolism pathway, and the mining of key salt tolerance regulation genes. The high-throughput sequencing technology is increasingly widely used in the study of crop resistance, which is of great significance to the study of plant resistance gene excavation and mechanism. In this study, we aimed to review the effects of the exogenous hormone GA on leaf morphological traits of sorghum seedlings and further analyze the physiological response of sorghum seedling leaves and the regulation of sorghum growth and development. This review not only focuses on the role of GA but also explores the signal transduction pathways of GA and the performance of their responsive genes under salt stress, thus helping to further clarify the mechanism of regulating growth and production under salt stress. This will serve as a reference for the molecular discovery of key genes related to salt stress and the development of new sorghum varieties.

Effects of salt stress on interspecific competition between an invasive alien plant Oenothera biennis and three native species

Biological invasions and soil salinization have become increasingly severe environmental problems under global change due to sea-level rise and poor soil management. Invasive species can often outcompete native species, but few studies focus on whether invasive alien species are always superior competitors under increasing stressors. We grew an invasive grass species, Oenothera biennis L., and three native grass species (Artemisia argyi Lévl. et Vant., Chenopodium album L., and Inula japonica Thunb.) as a monoculture (two seedlings of each species) or mixture (one seedling of O. biennis and one native species seedling) under three levels of salt treatments (0, 1, and 2 g/kg NaCl) in a greenhouse. We found that invasive O. biennis exhibited greater performance over native C. album and I. japonica, but lower performance compared to A. argyi, regardless of the soil salinity. However, salinity did not significantly affect the relative dominance of O. biennis. Interspecific competition enhanced the growth of O. biennis and inhibited the growth of I. japonica. Although O. biennis seedlings always had growth dominance over C. album seedlings, C. album was not affected by O. biennis at any salt level. At high salt levels, O. biennis inhibited the growth of A. argyi, while A. argyi did not affect the growth of O. biennis. Salt alleviated the competitive effect of O. biennis on I. japonica but did not mitigate the competition between O. biennis and the other two native species. Therefore, our study provides evidence for a better understanding of the invasive mechanisms of alien species under various salinity conditions.

Transcriptome Analysis of Differentially Expressed Genes Associated with Salt Stress in Cowpea (Vigna unguiculata L.) during the Early Vegetative Stage

Cowpea (Vigna unguiculata (L.), 2n = 22) is a tropical crop grown in arid and semiarid regions that is tolerant to abiotic stresses such as heat and drought. However, in these regions, salt in the soil is generally not eluted by rainwater, leading to salt stress for a variety of plant species. This study was conducted to identify genes related to salt stress using the comparative transcriptome analysis of cowpea germplasms with contrasting salt tolerance. Using the Illumina Novaseq 6000 platform, 1.1 billion high-quality short reads, with a total length of over 98.6 billion bp, were obtained from four cowpea germplasms. Of the differentially expressed genes identified for each salt tolerance type following RNA sequencing, 27 were shown to exhibit significant expression levels. These candidate genes were subsequently narrowed down using reference-sequencing analysis, and two salt stress-related genes (Vigun_02G076100 and Vigun_08G125100) with single-nucleotide polymorphism (SNP) variation were selected. Of the five SNPs identified in Vigun_02G076100, one that caused significant amino acid variation was identified, while all nucleotide variations in Vigun_08G125100 was classified as missing in the salt-resistant germplasms. The candidate genes and their variation, identified in this study provide, useful information for the development of molecular markers for cowpea breeding programs.

Alfalfa growth and nitrogen fixation constraints in salt-affected soils are in part offset by increased nitrogen supply

INTRODUCTION

In China, alfalfa (Medicago sativa L.) is often grown on marginal land with poor soil fertility and suboptimal climate conditions. Soil salt stress is one of the most limiting factors for alfalfa yield and quality, through its inhibition of nitrogen (N) uptake and N fixation.

METHODS

To understand if N supply could improve alfalfa yield and quality through increasing N uptake in salt-affected soils, a hydroponic experiment and a soil experiment were conducted. Alfalfa growth and N fixation were evaluated in response to different salt levels and N supply levels.

RESULTS AND DISCUSSION

The results showed that salt stress not only significantly decreased alfalfa biomass, by 43%-86%, and N content, by 58%-91%, but also reduced N fixation ability and N derived from the atmosphere (%Ndfa) through the inhibition of nodule formation and N fixation efficiency when the salt level was above 100 mmol Na₂SO₄ L^-1. Salt stress also decreased alfalfa crude protein by 31%-37%. However, N supply significantly improved shoot dry weight by 40%-45%, root dry weight by 23%-29%, and shoot N content by 10%-28% for alfalfa grown in salt-affected soil. The N supply was also beneficial for the %Ndfa and N fixation for alfalfa with salt stress, and the increase reached 47% and 60%, respectively. Nitrogen supply offset the negative effects on alfalfa growth and N fixation caused by salt stress, in part through improving plant N nutrition status. Our results suggest that optimal N fertilizer application is essential to alleviate the loss of growth and N fixation in alfalfa in salt-affected soils.

Exogenous Proline Enhances Systemic Defense against Salt Stress in Celery by Regulating Photosystem, Phenolic Compounds, and Antioxidant System

This study aimed to explore how exogenous proline induces salinity tolerance in celery. We analyzed the effects of foliar spraying with 0.3 mM proline on celery growth, photosystem, phenolic compounds, and antioxidant system under salt stress (100 mM NaCl), using no salt stress and no proline spraying as control. The results showed that proline-treated plants exhibited a significant increase in plant biomass due to improved growth physiology, supported by gas exchange parameters, chlorophyll fluorescence, and Calvin cycle enzyme activity (Ketosasaccharide-1,5-diphosphate carboxylase and Fructose-1,6-diphosphate aldolase) results. Also, proline spraying significantly suppressed the increase in relative conductivity and malondialdehyde content caused by salt stress, suggesting a reduction in biological membrane damage. Moreover, salt stress resulted in hydrogen peroxide, superoxide anions and 4-coumaric acid accumulation in celery, and their contents were reduced after foliar spraying of proline. Furthermore, proline increased the activity of antioxidant enzymes (superoxide dismutase, peroxidase, and catalase) and the content of non-enzymatic antioxidants (reduced ascorbic acid, glutathione, caffeic acid, chlorogenic acid, total phenolic acids, and total flavonoids). Additionally, proline increased the activity of key enzymes (ascorbate oxidase, ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase) in the ascorbic acid-glutathione cycle, activating it to counteract salt stress. In summary, exogenous proline promoted celery growth under salt stress, enhanced photosynthesis, increased total phenolic acid and flavonoid contents, and improved antioxidant capacity, thereby improving salt tolerance in celery.

Morphological and antioxidant responses of Cicer arietinum L. genotypes exposed to combination stress of anthracene and sodium chloride

Various mutagenic, carcinogenic pollutants such as Polycyclic Aromatic Hydrocarbons (PAHs) are released into the environment posing a negative effect on plant metabolism. All the pollutants that are emitted into the atmosphere, ultimately find their way into the plant. Soil salinity stress is one of the major determinants of crop productivity. Different plants respond differently to different abiotic stress present alone or in combination. One such combination of abiotic stress is PAHs and salinity stress. The present research aims to study the effect of the application of NaCl and Anthracene alone and in various combinations on two chickpea genotypes GPF2 and PDG4. A 21 days laboratory experiment was conducted in petriplates and growth pouches. Different concentrations of NaCl and Anthracene were given to two chickpea genotypes viz. GPF2 and PDG4, alone as well as in combinations to study morphological, physiological and antioxidant responses. Results obtained were further analyzed by using various statistical measures such as Principle Component Analysis and Two-way ANOVA. Results indicated that under the dual presence of NaCl and Anthracene, GPF2 exhibited higher activities of antioxidant enzymes and was shown to have a negative correlation with plant height and chlorophyll content. Based on the results of the present investigation, it was concluded that GPF2 was a better performing chickpea genotype towards the combined presence of Anthracene and NaCl as compared to PDG4.

Plant Growth Stimulators Improve Two Wheat Cultivars Salt-Tolerance: Insights into Their Physiological and Nutritional Responses

Spermine (SPM) and salicylic acid (SA), plant growth stimulators, are involved in various biological processes and responses to environmental cues in plants. However, the function of their combined treatment on wheat salt tolerance is unclear. In this study, wheat (Triticum aestivum L. cvs. Shandawel 1 and Sids 14) plants were grown under non-saline and saline (6.0 and 12.0 dS m^-1) conditions and were foliar sprayed with 100 mgL^-1 SA and/or 30 mgL^-1 SPM. Exogenously applied SA and/or SPM relieved the adverse effects caused by salt stress and significantly improved wheat growth and production by inducing higher photosynthetic pigment (chlorophyll a, chlorophyll b, carotenoids) content, nutrient (N, P, K⁺, Ca²⁺, Mg²⁺, Fe, Zn, Cu) acquisition, ionic (K⁺/Na⁺, Ca²⁺/Na⁺, Mg²⁺/Na⁺) homeostatics, osmolyte (soluble sugars, free amino acids, proline, glycinebetaine) accumulation, protein content, along with significantly lower Na⁺ accumulation and chlorophyll a/b ratio. The best response was registered with SA and SPM combined treatment, especially in Shandawel 1. This study highlighted the recovery impact of SA and SPM combined treatment on salinity-damaged wheat plants. The newly discovered data demonstrate that this treatment significantly improved the photosynthetic pigment content, mineral homeostasis, and osmoprotector solutes buildup in salinity-damaged wheat plants. Therefore, it can be a better strategy for ameliorating salt toxicity in sustainable agricultural systems.

Mild and severe salt stress responses are age-dependently regulated by abscisic acid in tomato

Salt stress hampers plant growth and development through both osmotic and ionic imbalances. One of the key players in modulating physiological responses towards salinity is the plant hormone abscisic acid (ABA). How plants cope with salinity largely depends on the magnitude of the soil salt content (stress severity), but also on age-related developmental processes (ontogeny). Here we studied how ABA directs salt stress responses in tomato plants for both mild and severe salt stress in leaves of different ages. We used the ABA-deficient mutant notabilis, which contains a null-mutation in the gene of a rate-limiting ABA biosynthesis enzyme 9-cis-epoxycarotenoid dioxygenase (NCED1), leading to impaired stomatal closure. We showed that both old and young leaves of notabilis plants keep a steady-state transpiration and photosynthesis rate during salt stress, probably due to their dysfunctional stomatal closure. At the whole plant level, transpiration declined similar to the wild-type, impacting final growth. Notabilis leaves were able to produce osmolytes and accumulate ions in a similar way as wild-type plants, but accumulated more proline, indicating that osmotic responses were not impaired by the NCED1 mutation. Besides NCED1, also NCED2 and NCED6 are strongly upregulated under salt stress, which could explain why the notabilis mutant did not show a lower ABA content upon salt stress, except in young leaves. This might be indicative of a salt-mediated feedback mechanism on NCED2/6 in notabilis and might explain why notabilis plants seem to perform better under salt stress compared to wild-type plants with respect to biomass and water content accumulation.

Mitigating abiotic stress: microbiome engineering for improving agricultural production and environmental sustainability

The responses of plants to different abiotic stresses and mechanisms involved in their mitigation are discussed. Production of osmoprotectants, antioxidants, enzymes and other metabolites by beneficial microorganisms and their bioengineering ameliorates environmental stresses to improve food production. Progressive intensification of global agriculture, injudicious use of agrochemicals and change in climate conditions have deteriorated soil health, diminished the microbial biodiversity and resulted in environment pollution along with increase in biotic and abiotic stresses. Extreme weather conditions and erratic rains have further imposed additional stress for the growth and development of plants. Dominant abiotic stresses comprise drought, temperature, increased salinity, acidity, metal toxicity and nutrient starvation in soil, which severely limit crop production. For promoting sustainable crop production in environmentally challenging environments, use of beneficial microbes has emerged as a safer and sustainable means for mitigation of abiotic stresses resulting in improved crop productivity. These stress-tolerant microorganisms play an effective role against abiotic stresses by enhancing the antioxidant potential, improving nutrient acquisition, regulating the production of plant hormones, ACC deaminase, siderophore and exopolysaccharides and accumulating osmoprotectants and, thus, stimulating plant biomass and crop yield. In addition, bioengineering of beneficial microorganisms provides an innovative approach to enhance stress tolerance in plants. The use of genetically engineered stress-tolerant microbes as inoculants of crop plants may facilitate their use for enhanced nutrient cycling along with amelioration of abiotic stresses to improve food production for the ever-increasing population. In this chapter, an overview is provided about the current understanding of plant-bacterial interactions that help in alleviating abiotic stress in different crop systems in the face of climate change. This review largely focuses on the importance and need of sustainable and environmentally friendly approaches using beneficial microbes for ameliorating the environmental stresses in our agricultural systems.

Metabolomics and lipidomics insight into the effect of different polyamines on tomato plants under non-stress and salinity conditions

Polyamines (PAs) are key signaling molecules involved in plant growth and stress acclimation processes. This work investigated the effect of spermidine, spermine, and putrescine (alone and in a mixture) in tomato plants using a combined metabolomics and lipidomics approach. The experiments were carried out under non-stress and 100 mM NaCl salinity conditions. Shoot and root biomass, as well as SPAD values, were increased by the application of exogenous PAs but with differences across treatments. Similarly, root length density (F: 34, p < 0.001), average root diameter (F: 14, p < 0.001), and very fine roots (0.0-0.5 mm) increased in PA-treated plants, compared to control. Metabolomics and lipidomics indicated that, despite being salinity the hierarchically prevalent factor, the different PA treatments imposed distinct remodeling at the molecular level. Plants treated with putrescine showed the broader modulation of metabolite profile, whereas spermidine and spermine induced a comparatively milder effect. The pathway analysis from differential metabolites indicated a broad and multi-level intricate modulation of several signaling molecules together with stress-related compounds like flavonoids and alkaloids. Concerning signaling processes, the complex crosstalk between phytohormones (mainly abscisic acid, cytokinins, the ethylene precursor, and jasmonates), and the membrane lipids signaling cascade (in particular, sphingolipids as well as ceramides and other glycerophospholipids), was involved in such complex response of tomato to PAs. Interestingly, PA-specific processes could be observed, with peculiar responses under either control or salinity conditions.

A Comprehensive Review on Chickpea (Cicer arietinum L.) Breeding for Abiotic Stress Tolerance and Climate Change Resilience

Chickpea is one of the most important pulse crops worldwide, being an excellent source of protein. It is grown under rain-fed conditions averaging yields of 1 t/ha, far from its potential of 6 t/ha under optimum conditions. The combined effects of heat, cold, drought, and salinity affect species productivity. In this regard, several physiological, biochemical, and molecular mechanisms are reviewed to confer tolerance to abiotic stress. A large collection of nearly 100,000 chickpea accessions is the basis of breeding programs, and important advances have been achieved through conventional breeding, such as germplasm introduction, gene/allele introgression, and mutagenesis. In parallel, advances in molecular biology and high-throughput sequencing have allowed the development of specific molecular markers for the genus Cicer, facilitating marker-assisted selection for yield components and abiotic tolerance. Further, transcriptomics, proteomics, and metabolomics have permitted the identification of specific genes, proteins, and metabolites associated with tolerance to abiotic stress of chickpea. Furthermore, some promising results have been obtained in studies with transgenic plants and with the use of gene editing to obtain drought-tolerant chickpea. Finally, we propose some future lines of research that may be useful to obtain chickpea genotypes tolerant to abiotic stress in a scenario of climate change.

Combating Salinity Through Natural Plant Extracts Based Biostimulants: A Review

Enhanced crop growth and yield are the recurring concerns in agricultural field, considering the soaring world population and climate change. Abiotic stresses are one of the major limiting factors for constraining crop production, for several economically important horticultural crops, and contribute to almost 70% of yield gap. Salt stress is one of these unsought abiotic stresses that has become a consistent problem in agriculture over the past few years. Salinity further induces ionic, osmotic, and oxidative stress that result in various metabolic perturbations (including the generation of reactive oxygen, carbonyl, and nitrogen species), reduction in water potential (ψw), distorted membrane potential, membrane injury, altered rates of photosynthesis, leaf senescence, and reduced nitrogen assimilation, among others); thereby provoking a drastic reduction in crop growth and yield. One of the strategies to mitigate salt stress is the use of natural plant extracts (PEs) instead of chemical fertilizers, thus limiting water, soil, and environmental pollution. PEs mainly consist of seeds, roots, shoots, fruits, flowers, and leaves concentrates employed either individually or in mixtures. Since PEs are usually rich in bioactive compounds (e.g., carotenoids, flavonoids, phenolics, etc.), therefore they are effective in regulating redox metabolism, thereby promoting plant growth and yield. However, various factors like plant growth stage, doses applied, application method, soil, and environmental conditions may greatly influence their impact on plants. PEs have been reported to enhance salt tolerance in plants primarily through modulation of signaling signatures and pathways (e.g., Na+, ANNA4, GIPC, SOS3, and SCaBP8 Ca2+ sensors, etc.), and regulation of redox machinery [e.g., superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), non-specific peroxidase (POX), glutathione peroxidase (GPX), peroxiredoxin (Prx), ascorbic acid (AsA), glutathione (GSH), α-tocopherol, etc.]. The current study highlights the role of PEs in terms of their sources, methods of preparation, and mode of action with subsequent physiological changes induced in plants against salinity. However, an explicit mode of action of PEs remains nebulous, which might be explicated utilizing transcriptomics, proteomics, metabolomics, and bioinformatics approaches. Being ecological and economical, PEs might pave the way for ensuring the food security in this challenging era of climate change.

Regulation of photosynthesis under salt stress and associated tolerance mechanisms

Photosynthesis is crucial for the survival of all living biota, playing a key role in plant productivity by generating the carbon skeleton that is the primary component of all biomolecules. Salinity stress is a major threat to agricultural productivity and sustainability as it can cause irreversible damage to photosynthetic apparatus at any developmental stage. However, the capacity of plants to become photosynthetically active under adverse saline conditions remains largely untapped. This study addresses this discrepancy by exploring the current knowledge on the impact of salinity on chloroplast operation, metabolism, chloroplast ultrastructure, and leaf anatomy, and highlights the dire consequences for photosynthetic machinery and stomatal conductance. We also discuss enhancing photosynthetic capacity by modifying and redistributing electron transport between photosystems and improving photosystem stability using genetic approaches, beneficial microbial inoculations, and root architecture changes to improve salt stress tolerance under field conditions. Understanding chloroplast operations and molecular engineering of photosynthetic genes under salinity stress will pave the way for developing salt-tolerant germplasm to ensure future sustainability by rehabilitating saline areas.

Responses of Physiology, Photosynthesis, and Related Genes to Saline Stress in Cornus hongkongensis subsp. tonkinensis (W. P. Fang) Q. Y. Xiang

Cornus hongkongensis subsp. tonkinensis (W. P. Fang) Q. Y. Xiang is a native evergreen species with high ornamental value for abundant variations in leaf, bract, fruit, and tree gesture. To broaden its cultivation in coastal saline soil, salt damage and survival rate, physiological responses, photosynthetic performance, and related genes were evaluated for annual seedlings exposed to 0.3% salt (ST) concentrations for 60 days. Syndromes of salt damage were aggravated, and the survival rate decreased with prolonged stress duration; all stressed seedlings displayed salt damage, and 58.3% survived. Under short-term saline stress (5 d), marked increases in malondialdehyde (MDA), relative electrical conductivity (REC), and decreases in superoxide dismutase (SOD), photosynthetic rate (P_n), stomatal conductance (gs), and internal carbon dioxide concentration (C_i) were recorded. The stable leaf water use efficiency (WUE) and chlorophyll content were positive physiological responses to ensure photosynthetic performance. Meanwhile, the expression levels of genes related to photosystem II (psbA) and photorespiration (SGAT and GGAT) were upregulated, indicating the role of photorespiration in protecting photosynthesis from photoinhibition. After 30 days of stress (≥30 d), there was a significant increase in MDA, REC, soluble sugar (SS), soluble protein (SP), and C_i, whereas descending patterns in P_n, gs, WUE, the maximal photochemical efficiency of photosystem II (F_v/F_m), and potential activities of PSII (F_v/F₀) occurred in salt-stressed seedlings, compared with CK. Meanwhile, the expression levels of related genes significantly dropped, such as psbA, LFNR, GGAT, GLYK, and PGK, indicating photoinhibition and worse photosynthetic performance. Our results suggest that the moderate salt tolerance of C. hongkongensis subsp. tonkinensis mostly lies in a better photosynthetic system influenced by active photorespiration. Hence, these results provide a framework for better understanding the photosynthetic responses of C. hongkongensis subsp. tonkinensis to salt stress.

Generalist herbivore response to volatile chemical induction varies along a gradient in soil salinization

Elevated soil salinity directly modifies plant physiology and indirectly alters the biotic interactions that shape plant performance. However, it is unclear how soil salinization interacts with plant defenses to alter patterns of leaf consumption or herbivore survival, development, and performance. In this study, we carried out laboratory feeding trials and a common garden experiment to investigate how gradients in soil salinization interact with plant induction status (modified via exogenous application of methyl jasmonate [MeJA]) to influence feeding consumption and performance of the generalist herbivore Spodoptera exigua on tomato (Solanum lycoperscium) plants. Our results showed that S. exigua consumed less leaf tissue from tomatoes treated with ≥ 50 mM NaCl; at these higher salinity treatments, these herbivores were less likely to pupate and died more quickly. Treatment with MeJA only reduced leaf consumption in the 0 mM NaCl treatment. Our common garden study demonstrated that natural populations of leaf chewing herbivores were less likely to damage tomatoes treated with > 50 mM NaCl solutions. Treatment with MeJA in the common garden reduced damage from natural populations of herbivores, but only for salt treatments at the 50 mM NaCl concentration level and we did observe variation in herbivore damage between cohorts in common garden trials. These results suggest that both soil salinization and volatile jasmonate signals may generate complementary shifts in decreased plant quality for herbivores. Overall, our study concludes that soil salinization could be a potential driver in spatial patterns of variation in both herbivory and herbivore demography.

Salt and Drought Effect on Germination and Initial Growth of Lavandula stoechas: A Potential Candidate for Rehabilitation of the Mediterranean Disturbed Coastal Lands

Information relating to germination and seedling emergence of a plant aids in determining the species spatiotemporal distribution and also facilitates in designing appropriate plant management strategies within an ecosystem. Lavandula stoechas L. (Lamiaceae), a naturally occurring shrub, is particularly used in pharmaceutical and cosmetic industries. This species, indeed, has the potential for rehabilitation of degraded costal lands. However, various aspects of its seed biology have not yet been recognised. Here, we aimed to assess the effects of different soluble salts (NaCl, CaCl2, MgCl2 and Na2SO4) and drought (as simulated by polyethylene glycol, [PEG]6000) on seed germination patterns and early seedling growth responses. Seeds treated with five iso-concentration (0–100 mM) salinities and five PEG6000 (0 to −1 MPa) levels were incubated in a controlled germinator set at 20°C. The preliminary results revealed that seeds of L. stoechas lacked primary/innate dormancy and they germinated abundantly (89.2% germination) and fast (7.4% day−1) in the absence of stress. Regardless of the kind of salt applied, the germination percentage (GP) and germination rate index (GRI) fell significantly with increasing salinity, and germination ceased completely at 100 mM Na2SO4. In fact, the salinity tolerance index (STI) showed that, among all salts tested, Na2SO4 appeared to have more inhibitory action on germination. In addition, L. stoechas was found to be tolerant to moderate salty stress (<50 mM) in early growth phase. The salt solution parameters (i.e. concentration, electrical conductivity [EC] and salt content) were best correlated with seed/seedling metrics. pH was not a good predictor for salt effects at the germination/seedling stages. Overall, this species seems to be sensitive to drought at the germination and initial growth phases. The germination recovery potential of L. stoechas in both stresses stipulates that this species can be regarded as a promising candidate in the rehabilitation of Mediterranean disturbed coastal habitats.

Association analysis of GmMAPKs and functional characterization of GmMMK1 to salt stress response in soybean

Salinity is one of the major abiotic constraints affecting the growth and yield of plants including soybean. In this context, the previous studies have documented the role of the mitogen-activated protein kinase (MAPK) cascade in the regulation of salt signaling in model plants. However, there is not a systematic analysis of salt-related MAPKs in soybean. Hence, in this study, we identified a total of 32 GmMAPKs via., genome-wide reanalysis of the MAPK family using the soybean genome v4.0. Based on the transcriptome datasets in the public database, we observed that GmMAPKs are induced by different abiotic stresses, especially salt stress. Furthermore, based on the candidate gene association mapping and haplotype analysis of the GmMAPKs, we identified a salt-related MAPK member, GmMMK1. GmMMK1 possesses significant sequence variations, which affect salt tolerance in soybean at the germination stage. Besides, the overexpression of the GmMMK1 in soybean hairy roots has a significant negative effect on the root growth, leading to increased sensitivity of the GmMMK1-OE plants to salt stress. Moreover, the heterologous expression of the GmMMK1 in Arabidopsis has been also observed to have a negative effect on the germination and root growth under salt stress. The transcriptome analysis and yeast two-hybrid screening showed that hormone signaling and the homeostasis of reactive oxygen species are involved in the GmMMK1 regulation network. In conclusion, the results of this work demonstrated that GmMMK1 is an important negative regulator of the salt stress response, and provides better insights for understanding the role of the MAPKs in soybean salt signaling.

Conventional and Molecular Breeding Tools for Accelerating Genetic Gain in Faba Bean (Vicia Faba L.)

Faba bean is a cool-season grain legume crop, which is grown worldwide for food and feed. Despite a decrease in area under faba bean in the past, the interest in growing faba bean is increasing globally due to its high seed protein content and its excellent ecological service. The crop is, however, exposed to diverse biotic and abiotic stresses causing unstable, low grain yield. Although, sources of resistance to main diseases, such as ascochyta blight (Ascochyta fabae Speg.), rust (Uromyces viciae-fabae (Pers.) Schroet.), chocolate spot (Botrytis fabae Sard.) and gall disease (Physioderma viciae), have been identified, their resistance is only partial and cannot prevent grain yield losses without agronomical practices. Tightly associated DNA markers for host plant resistance genes are needed to enhance the level of resistance. Less progress has been made for abiotic stresses. Different breeding methods are proposed, but until now line breeding, based on the pedigree method, is the dominant practice in breeding programs. Nonetheless, the low seed multiplication coefficient and the requirement for growing under insect-proof enclosures to avoid outcrossing hampers breeding, along with the lack of tools such as double haploid system and cytoplasmic male sterility. This reduces breeding population size and speed of breeding hence the chances of capturing rare combinations of favorable alleles. Availability and use of the DNA markers such as vicine-convicine (vc -) and herbicide tolerance in breeding programs have encouraged breeders and given confidence in marker assisted selection. Closely linked QTL for several biotic and abiotic stress tolerance are available and their verification and conversion in breeder friendly platform will enhance the selection process. Recently, genomic selection and speed breeding techniques together with genomics have come within reach to accelerate the genetic gains in faba bean. Advancements in genomic resources with other breeding tools, methods and platforms will enable to accelerate the breeding process for enhancing genetic gain in this species.

De-novo Domestication for Improving Salt Tolerance in Crops

Global agriculture production is under serious threat from rapidly increasing population and adverse climate changes. Food security is currently a huge challenge to feed 10 billion people by 2050. Crop domestication through conventional approaches is not good enough to meet the food demands and unable to fast-track the crop yields. Also, intensive breeding and rigorous selection of superior traits causes genetic erosion and eliminates stress-responsive genes, which makes crops more prone to abiotic stresses. Salt stress is one of the most prevailing abiotic stresses that poses severe damages to crop yield around the globe. Recent innovations in state-of-the-art genomics and transcriptomics technologies have paved the way to develop salinity tolerant crops. De novo domestication is one of the promising strategies to produce superior new crop genotypes through exploiting the genetic diversity of crop wild relatives (CWRs). Next-generation sequencing (NGS) technologies open new avenues to identifying the unique salt-tolerant genes from the CWRs. It has also led to the assembly of highly annotated crop pan-genomes to snapshot the full landscape of genetic diversity and recapture the huge gene repertoire of a species. The identification of novel genes alongside the emergence of cutting-edge genome editing tools for targeted manipulation renders de novo domestication a way forward for developing salt-tolerance crops. However, some risk associated with gene-edited crops causes hurdles for its adoption worldwide. Halophytes-led breeding for salinity tolerance provides an alternative strategy to identify extremely salt tolerant varieties that can be used to develop new crops to mitigate salinity stress.

Investigating the Drought and Salinity Effect on the Redox Components of Sulla Coronaria (L.) Medik

For the Mediterranean region, climate models predict an acceleration of desertification processes, thus threatening agriculture. The present work aimed to investigate the effect of drought and salinity on Sulla coronaria (L.) Medik., a Mediterranean forage legume, for understanding plant defence systems activated by these stressors. In detail, we focused our attention on the variations on the plant redox status. Plants were subjected to suboptimal watering and irrigation with sodium chloride (NaCl) solutions. The same salt treatment was applied for in vitro tests on seedlings. Water content did not change after treatments. Salt negatively influenced seed germination and seedling development, but it did not affect photosynthesis parameters, contrary to what was observed in adult plants. Proline concentration increased in all samples, while abscisic acid level increased exclusively in seedlings. NaCl caused accumulation of superoxide anion in plants and seedlings and hydrogen peroxide only in seedlings; nevertheless, lipid peroxidation was not detected. Total phenolics, glutathione, expression level, and activity of antioxidant enzymes were assayed, revealing a complex antiradical molecular response, depending on the type of stress and development stage. Our results confirm Sulla as a drought- and salt-tolerant species and highlight its ability to counteract oxidative stress. This evidence suggests a key role for the redox components, as signal transduction messengers, in Sulla acclimation to desertification. Finally, plants and seedlings showed different acclimation capacity to salinity, revealing a greater genomic plasticity for seedlings.

Modulation of the Antioxidant Defense System by Exogenous l-Glutamic Acid Application Enhances Salt Tolerance in Lentil (Lens culinaris Medik.)

Salt stress greatly disturbs the growth, morpho-physiological, and biochemical performance of plants. However, different physiological processes and acclimation mechanisms can be induced under stress, while some of them can be modulated by the appropriate chemical stimulus. The objective of this study was to evaluate the impact of exogenous pretreatment with 10 mM l-glutamic acid (l-Glu) on the physiological and biochemical parameters of lentil (Lensculinaris Medik.) under 110 mM NaCl stress. Salt stress inhibited the growth and reduced the photosynthetic pigment (chlorophylls and carotenoids) level, water content, and survival of lentil seedlings during recovery from the stress. Salt stress also induced oxidative damage, as indicated by higher hydrogen peroxide and malonaldehyde contents and electrolyte leakage, by interrupting the antioxidant defense system and promoting the accumulation of toxic levels of Na⁺. However, l-Glu pretreatment mitigated the salt-induced damage in lentil seedlings by reducing the accumulation of Na⁺, maintaining ion homeostasis, and increasing the activities of antioxidant enzymes (catalase and ascorbate peroxidase). As a result, salt-induced oxidative damage was reduced, seedling growth and photosynthetic pigment contents were enhanced, and the survival rate of the lentil seedlings was improved in response to salt stress, indicating an ameliorative role for l-Glu in lentil seedling growth under salt stress.

Hydropriming Applied on Fast Germinating Solanum villosum Miller Seeds: Impact on Pre-germinative Metabolism

Seed priming can circumvent poor germination rate and uniformity, frequently reported in eggplant (Solanum melongena L.) and its crop wild relatives (CWRs). However, there is still a gap of knowledge on how these treatments impact the pre-germinative metabolism in a genotype- and/or species-dependent manner. The CWR Solanum villosum Miller (hairy nightshade) investigated in this study showed a quite unique profile of fast germination. Although this accelerated germination profile would not apparently require further improvement, we wanted to test whether priming would still be able to impact the pre-germinative metabolism, eventually disclosing the predominant contribution of specific antioxidant components. Hydropriming followed by dry-back resulted in synchronized germination, as revealed by the lowest MGR (Mean Germination Rate) and U (Uncertainty) values, compared to unprimed seeds. No significant changes in ROS (reactive oxygen species) were observed throughout the treatment. Increased tocopherols levels were detected at 2 h of hydropriming whereas, overall, a low lipid peroxidation was evidenced by the malondialdehyde (MDA) assay. Hydropriming resulted in enhanced accumulation of the naturally occurring antioxidant phenolic compounds chlorogenic acid and iso-orientin, found in the dry seeds and ex novo accumulation of rutin. The dynamic changes of the pre-germinative metabolism induced by hydropriming are discussed in view of future applications that might boost the use of eggplant CWRs for breeding, upon upgrade mediated by seed technology.

Kinetics of DNA Repair in Vicia faba Meristem Regeneration Following Replication Stress

The astonishing survival abilities of Vicia faba, one the earliest domesticated plants, are associated, among other things, to the highly effective replication stress response system which ensures smooth cell division and proper preservation of genomic information. The most crucial pathway here seems to be the ataxia telangiectasia-mutated kinase (ATM)/ataxia telangiectasia and Rad3-related kinase (ATR)-dependent replication stress response mechanism, also present in humans. In this article, we attempted to take an in-depth look at the dynamics of regeneration from the effects of replication inhibition and cell cycle checkpoint overriding causing premature chromosome condensation (PCC) in terms of DNA damage repair and changes in replication dynamics. We were able to distinguish a unique behavior of replication factors at the very start of the regeneration process in the PCC-induced cells. We extended the experiment and decided to profile the changes in replication on the level of a single replication cluster of heterochromatin (both alone and with regard to its position in the nucleus), including the mathematical profiling of the size, activity and shape. The results obtained during these experiments led us to the conclusion that even “chaotic” events are dealt with in a proper degree of order.

Exogenous melatonin improves salt stress adaptation of cotton seedlings by regulating active oxygen metabolism

Melatonin is a small-molecule indole hormone that plays an important role in participating in biotic and abiotic stress resistance. Melatonin has been confirmed to promote the normal development of plants under adversity stress by mediating physiological regulation mechanisms. However, the mechanisms by which exogenous melatonin mediates salt tolerance via regulation of antioxidant activity and osmosis in cotton seedlings remain largely unknown. In this study, the regulatory effects of melatonin on reactive oxygen species (ROS), the antioxidant system, and osmotic modulators of cotton seedlings were determined under 0-500 µM melatonin treatments with salt stress induced by 150 mM NaCl treatment. Cotton seedlings under salt stress exhibited an inhibition of growth, excessive hydrogen peroxide (H₂O₂), superoxide anion (O₂ ^-), and malondialdehyde (MDA) accumulations in leaves, increased activity levels of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX), and elevated ascorbic acid (AsA) and glutathione (GSH) content in leaves. However, the content of osmotic regulators (i.e., soluble sugars and proteins) in leaves was reduced under salt stress. This indicates high levels of ROS were produced, and the cell membrane was damaged. Additionally, osmotic regulatory substance content was reduced, resulting in osmotic stress, which seriously affected cotton seedling growth under salt stress. However, exogenous melatonin at different concentrations reduced the contents of H₂O₂, O₂ ^-, and MDA in cotton leaves, increased the activity of antioxidant enzymes and the content of reductive substances (i.e., AsA and GSH), and promoted the accumulation of osmotic regulatory substances in leaves under salt stress. These results suggest that melatonin can inhibit ROS production in cotton seedlings, improve the activity of the antioxidant enzyme system, raise the content of osmotic regulation substances, reduce the level of membrane lipid peroxidation, and protect the integrity of the lipid membrane under salt stress, which reduces damage caused by salt stress to seedlings and effectively enhances inhibition of salt stress on cotton seedling growth. These results indicate that 200 µM melatonin treatment has the best effect on the growth and salt tolerance of cotton seedlings.

Evaluation of indigenous Omani alfalfa landraces for morphology and forage yield under different levels of salt stress

Alfalfa is the major fodder crop of Sultanate Oman, but salinity is a major problem in its cultivation. Therefore, thirty-four alfalfa (Medicago sativa L.) landraces of Oman were evaluated for morphology and forage yield response to different salinity levels viz. 1 (control), 3, 6, 9, and 12 dS m^-1 under greenhouse conditions. The experiment was conducted under a completely randomized design. Different alfalfa landraces responded differently to the five salinity levels for plant height, number of branches, number of leaves, leaflet width, leaflet length, forage fresh weight, and forage dry matter yield. Salt stress caused a reduction in growth and dry matter yield of alfalfa landraces with exception of some, which responded positively to the salinity levels of 3 and 6 dS m^-1 compared to control for the number of leaves per plant. Moreover, some landraces had better forage fresh weight and dry matter yield at 6 dS m^-1 than 3 dS m^-1. Alfalfa landraces OMA 257, OMA, 245, OMA 270, OMA 315, OMA 211, OMA 117, OMA 56, OMA 239, OMA 148, OMA 131, OMA 95, OMA 263, OMA 262, OMA 289 and OMA 220 were designated as salt tolerant based on their overall performance across salinity levels of 6, 9 and 12 dS m^-1. However, the landraces OMA 305, OMA 100, OMA 211, OMA 148, OMA 60, OMA 248, OMA 9, OMA 88, and OMA 302 collected were sensitive to 6, 9 and 12 dS m^-1 salinity stress. The study showed the variation of alfalfa landraces potential for salinity tolerance, and their potential for cultivation in saline areas and/or use in breeding programs aimed to develop salt tolerant alfalfa genotypes.

Proteomic responses to progressive dehydration stress in leaves of chickpea seedlings

Background

Chickpea is an important food legume crop with high protein levels that is widely grown in rainfed areas prone to drought stress. Using an integrated approach, we describe the relative changes in some physiological parameters and the proteome of a drought-tolerant (MCC537, T) and drought-sensitive (MCC806, S) chickpea genotype.

Results

Under progressive dehydration stress, the T genotype relied on a higher relative leaf water content after 3 and 5 d (69.7 and 49.3%) than the S genotype (59.7 and 40.3%) to maintain photosynthetic activities and improve endurance under stress. This may have been facilitated by greater proline accumulation in the T genotype than the S genotype (14.3 and 11.1 μmol g^− 1 FW at 5 d, respectively). Moreover, the T genotype had less electrolyte leakage and lower malondialdehyde contents than the S genotype under dehydration stress, indicating greater membrane stability and thus greater dehydration tolerance. The proteomic analysis further confirmed that, in response to dehydration, the T genotype activated more proteins related to photosynthesis, stress response, protein synthesis and degradation, and gene transcription and signaling than the S genotype. Of the time-point dependent proteins, the largest difference in protein abundance occurred at 5 d, with 29 spots increasing in the T genotype and 30 spots decreasing in the S genotype. Some of the identified proteins—including RuBisCo, ATP synthase, carbonic anhydrase, psbP domain-containing protein, L-ascorbate peroxidase, 6-phosphogluconate dehydrogenase, elongation factor Tu, zinc metalloprotease FTSH 2, ribonucleoproteins and auxin-binding protein—may play a functional role in drought tolerance in chickpea.

Conclusions

This study highlights the significance of genotype- and time-specific proteins associated with dehydration stress and identifies potential resources for molecular drought tolerance improvement in chickpea.

Transcriptome profile analysis of two Vicia faba cultivars with contrasting salinity tolerance during seed germination

Faba bean (Vicia faba L.) is an important food legume crop. Salinity soils severely constrain the production of faba bean, however, the seed germination of faba bean, which is a vital plant growth stage, is sensitive to salinity. Planting improved varieties of faba bean, which exhibit salt tolerance in seed germination stage, is an optimal strategy for faba bean product. To investigate the genes dynamics during the seed germination stage under salinity, RNA-seq method was used to investigate genome-wide transcription profiles of two faba bean varieties with contrast salt-tolerance during the seed germination. A total of 4,486 differentially expressed genes (DEGs) were identified among the comparison of salt-tolerant variety Y134 and salt-sensitive variety Y078 treated with salinity or not. Of these, 1,410 candidate DEGs were identified as salt-stress response genes. Furthermore, 623 DEGs were identified as variety-specific response gene during seed germination at 16 h or 24 h with salt treatment. Based on the pathway enrichment according to the Kyoto Encyclopedia of Genes and Genomes database (KEGG), these DEGs involving in cell wall loosening (e.g., xyloglucan endotransglucosylase/hydrolase, chitinase, and expansin), hormone metabolism (e.g., LEA genes, genes associated with ABA or ethylene signal pathway), chromatin remodeling (e.g., chromatin structure proteins, LHP1), small interfering RNA pathway, etc., were significantly up-regulated in salt-tolerance variety with salt treatment, indicating that they play critical roles in regulation of seed germination. The results indicated that a clearer mechanism of gene regulation that regulates the seed germination responding to salinity in faba bean. These findings are helpful to increase the understanding of the salt tolerance mechanism of crops during seed germination, and provide valuable genetic resource for the breeding of salt-tolerant faba bean varieties in future.

Arbuscular mycorrhizal fungi (AMF) enhance the tolerance of Euonymus maackii Rupr. at a moderate level of salinity

Salt stress is one of the major environmental constraints for plant growth. Although the ways in which mycorrhizal plants deal with salt stress have been well documented, it still is blank for Euonymus maackii, an important local ecological restoration tree, to arbuscular mycorrhizal fungi (AMF) inoculation and salt stress. In this study, we tested the effect of different salt levels (0, 50, 100,150 and 200 mM) and AMF inoculation on E. maackii growth rate, photosynthesis, antioxidant enzymes, nutrient absorption and salt ion distribution. The results indicated negative effect of salt on height, photosynthesis capacity, nutrition accumulation, while salt stimulated the antioxidant defense system and salt ions accumulation. The toxic symptom by excessive accumulation of salt ions worsen with salt level increased gradually (except for the 50 mM NaCl treatment). AMF inoculation alleviated the toxic symptom under moderate salt levels (100 and 150 mM) by increasing photosynthesis capacity, accelerating nutrient absorption and activating antioxidant enzyme activities under salt stress. Meanwhile, effect of AMF was not detected on seedlings under slight (0 and 50 mM) and high (200 mM) NaCl concentration. Our study indicated AMF had positive impact on E. maackii subjected to salt, which suggested potential application of AMF- E. maackii on restoration of salt ecosystems.

Halotolerant Marine Rhizosphere-Competent Actinobacteria Promote Salicornia bigelovii Growth and Seed Production Using Seawater Irrigation

Salicornia bigelovii is a promising halophytic cash crop that grows in seawater of the intertidal zone of the west-north coast of the UAE. This study assess plant growth promoting (PGP) capabilities of halotolerant actinobacteria isolated from rhizosphere of S. bigelovii to be used as biological inoculants on seawater-irrigated S. bigelovii plants. Under laboratory conditions, a total of 39 actinobacterial strains were isolated, of which 22 were tolerant to high salinity (up to 8% w/v NaCl). These strains were further screened for their abilities to colonize S. bigelovii roots in vitro; the most promising ones that produced indole-3-acetic acid, polyamines (PA) or 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase (ACCD) were selected for rhizosphere-competency under naturally competitive environment. Three outstanding rhizosphere-competent isolates, Streptomyces chartreusis (Sc), S. tritolerans (St), and S. rochei (Sr) producing auxins, PA and ACCD, respectively, were investigated individually and as consortium (Sc/St/Sr) to determine their effects on the performance of S. bigelovii in the greenhouse. Individual applications of strains on seawater-irrigated plants significantly enhanced shoot and root dry biomass by 32.3-56.5% and 42.3-71.9%, respectively, in comparison to non-inoculated plants (control). In addition, plants individually treated with Sc, St and Sr resulted in 46.1, 60.0, and 69.1% increase in seed yield, respectively, when compared to control plants. Thus, the synergetic combination of strains had greater effects on S. bigelovii biomass (62.2 and 77.9% increase in shoot and root dry biomass, respectively) and seed yield (79.7% increase), compared to the control treatment. Our results also showed significant (P < 0.05) increases in the levels of photosynthetic pigments, endogenous auxins and PA, but a reduction in the levels of ACC in tissues of plants inoculated with Sc/St/Sr. We conclude that the consortium of isolates was the most effective treatment on S. bigelovii growth; thus confirmed by principal component and correlation analyses. To this best of our knowledge, this is the first report about halotolerant rhizosphere-competent PGP actinobacteria thriving in saline soils that can potentially contribute to promoting growth and increasing yield of S. bigelovii. These halotolerant actinobacterial strains could potentially be exploited as biofertilizers to sustain crop production in arid coastal areas.

Brassinosteroid seed priming with nitrogen supplementation improves salt tolerance in soybean

This study was conducted to evaluate the influence of brassinosteroid (24-epibrassinolide, EBL) seed priming and optimal nitrogen (N) supply in improving salt tolerance in soybean. The experimental treatments were (a) control (nutrient solution without N and without EBL priming), (b) nutrient solution without N and EBL seed priming, (c) N supplemented nutrient solution without EBL priming and (d) EBL seed priming + N supplemented nutrient solution under optimal (0 mM NaCl) and salt stress (0 mM NaCl) conditions. Salt stress caused significant reduction in growth and biomass accumulation of soybean. However, EBL seed priming and application of N improved the soybean performance under optimal and salt stress conditions. In this regard, treatments receiving both EBL and N were more effective. EBL priming and N, alone and in combination, triggered the accumulation of osmolytes including proline, glycine betaine and sugars resulting in better photo-protection through maintenance of tissue water content. Antioxidant activity and osmolyte accumulation significantly increased due to combined treatment of N and EBL under normal as well as salt stress conditions. In conclusion, salt stress caused reduction in growth and biomass soybean due to oxidative damage and osmotic stresses. However, soybean performance was improved by seed priming with EBL. Supplementation of N further improved the effectiveness of EBL treatment in improving salt tolerance in soybean.

Molecular dynamics of pre-germinative metabolism in primed eggplant (Solanum melongena L.) seeds

Seed priming, a pre-sowing technique that enhances the antioxidant/DNA repair activities during the pre-germinative metabolism, still retains empirical features. We explore for the first time the molecular dynamics of pre-germinative metabolism in primed eggplant (Solanum melongena L.) seeds in order to identify hallmarks (expression patterns of antioxidant/DNA repair genes combined with free radical profiles) useful to discriminate between high- and low-quality lots. The hydropriming protocol hereby developed anticipated (or even rescued) germination, when applied to lots with variable quality. ROS (reactive oxygen species) raised during hydropriming and dropped after dry-back. Upregulation of antioxidant/DNA repair genes was observed during hydropriming and the subsequent imbibition. Upregulation of SmOGG1 (8-oxoguanine glycosylase/lyase) gene detected in primed seeds at 2 h of imbibition appeared as a promising hallmark. On the basis of these results, the investigation was restricted within the first 2 h of imbibition, to verify whether the molecular landscape was reproducible in different lots. A complex pattern of antioxidant/DNA repair gene expression emerged, reflecting the preponderance of seed lot-specific profiles. Only the low-quality eggplant seeds subjected to hydropriming showed enhanced ROS levels, both in the dry and imbibed state, and this might be a useful signature to discriminate among lots. The plasticity of eggplant pre-germinative metabolism stimulated by priming imposes a plethora of heterogeneous molecular responses that might delay the search for quality hallmarks. However, the information hereby gained could be translated to eggplant wild relatives to speed-up their use in breeding programs or other agronomical applications.

How Does Proline Treatment Promote Salt Stress Tolerance During Crop Plant Development?

Soil salinity is one of the major abiotic stresses restricting the use of land for agriculture because it limits the growth and development of most crop plants. Improving productivity under these physiologically stressful conditions is a major scientific challenge because salinity has different effects at different developmental stages in different crops. When supplied exogenously, proline has improved salt stress tolerance in various plant species. Under high-salt conditions, proline application enhances plant growth with increases in seed germination, biomass, photosynthesis, gas exchange, and grain yield. These positive effects are mainly driven by better nutrient acquisition, water uptake, and biological nitrogen fixation. Exogenous proline also alleviates salt stress by improving antioxidant activities and reducing Na⁺ and Cl^- uptake and translocation while enhancing K⁺ assimilation by plants. However, which of these mechanisms operate at any one time varies according to the proline concentration, how it is applied, the plant species, and the specific stress conditions as well as the developmental stage. To position salt stress tolerance studies in the context of a crop plant growing in the field, here we discuss the beneficial effects of exogenous proline on plants exposed to salt stress through well-known and more recently described examples in more than twenty crop species in order to appreciate both the diversity and commonality of the responses. Proposed mechanisms by which exogenous proline mitigates the detrimental effects of salt stress during crop plant growth are thus highlighted and critically assessed.

Improvisation of salinity stress response in mung bean through solid matrix priming with normal and nano-sized chitosan

Nano particles open up a new avenue in crop science because of their unique physicochemical properties. Chitosan is preferred as priming agent in nano-form due to its biodegradability and non toxicity. Prepared nano-chitosan was characterized by Dynamic light scattering study, Fourier-transform infrared spectroscopy study and scanning electron microscopy. In present study the morphological, physiological and biochemical responses with an emphasis on several oxidative stress markers like proline, malondialdehyde (MDA), hydrogen peroxide (H₂O₂) contents of mung bean seedlings to 0, 4 and 8 dS/m salt stress conditions after Solid Matrix Priming (SMP) using the elicitors like nano-chitosan, chitosan and water were studied. The activities of different antioxidant enzymes like superoxide dismutase, catalase and oxidative stress markers like proline, H₂O₂ and MDA increased considerably over water control in accordance with salt stress gradient. However, SMP with nano-chitosan showed significant improvement showing reduction in H₂O₂ and MDA contents over control leading to the better growth, increased chlorophyll content and metabolism. Thus, our study shows that SMP with both normal and nano sized chitosan will be able to overcome the adverse effect of salt stress in mung bean seedlings allowing the activation of their defense mechanisms for better protection against salt stress.

Salt-Tolerant Plant Growth Promoting Rhizobacteria for Enhancing Crop Productivity of Saline Soils

Soil salinity has emerged as a serious issue for global food security. It is estimated that currently about 62 million hectares or 20 percent of the world's irrigated land is affected by salinity. The deposition of an excess amount of soluble salt in cultivable land directly affects crop yields. The uptake of high amount of salt inhibits diverse physiological and metabolic processes of plants even impacting their survival. The conventional methods of reclamation of saline soil which involve scraping, flushing, leaching or adding an amendment (e.g., gypsum, CaCl2, etc.) are of limited success and also adversely affect the agro-ecosystems. In this context, developing sustainable methods which increase the productivity of saline soil without harming the environment are necessary. Since long, breeding of salt-tolerant plants and development of salt-resistant crop varieties have also been tried, but these and aforesaid conventional approaches are not able to solve the problem. Salt tolerance and dependence are the characteristics of some microbes. Salt-tolerant microbes can survive in osmotic and ionic stress. Various genera of salt-tolerant plant growth promoting rhizobacteria (ST-PGPR) have been isolated from extreme alkaline, saline, and sodic soils. Many of them are also known to mitigate various biotic and abiotic stresses in plants. In the last few years, potential PGPR enhancing the productivity of plants facing salt-stress have been researched upon suggesting that ST-PGPR can be exploited for the reclamation of saline agro-ecosystems. In this review, ST-PGPR and their potential in enhancing the productivity of saline agro-ecosystems will be discussed. Apart from this, PGPR mediated mechanisms of salt tolerance in different crop plants and future research trends of using ST-PGPR for reclamation of saline soils will also be highlighted.