Morphological, Physiological, and Genetic Responses to Salt Stress in Alfalfa: A Review

Effects of SNP marker density and training population size on prediction accuracy in alfalfa (Medicago sativa L.) genomic selection

Effects of individual single-nucleotide polymorphism (SNP) markers and the size of "training" and "test" populations affect prediction accuracy in genomic selection (GS). This study evaluated 11 subsets of 4932 SNPs using six genetic additive methods to understand marker density in GS prediction in alfalfa (Medicago sativa L.). In the GS methods, the effect of "training" to "test" population size was also evaluated. Fourteen alfalfa populations sampled from long-term grazing sites were genotyped using genotyping by sequencing for the identification of SNPs. These populations were also phenotyped for six agromorphological and three nutritive traits from 2018 to 2020. The accuracy of GS prediction improved across six GS methods when the ratio of "training" to "test" population size increased. However, the prediction accuracy of the six GS methods reduced to a range of -0.27 to 0.11 when random, uninformative SNPs were used. In this study, five Bayesian methods and ridge-regression best linear unbiased prediction (rrBLUP) method had similar GS accuracies for "training" sets, but rrBLUP tended to outperform Bayesian methods in independent "test" sets when SNP subsets with high mean-squared-estimated-marker effect were used. These findings can enhance the application of GS in alfalfa genetic improvement.

Transcriptome analysis and physiological changes in the leaves of two Bromus inermis L. genotypes in response to salt stress

Soil salinity is a major factor threatening the production of crops around the world. Smooth bromegrass (Bromus inermis L.) is a high-quality grass in northern and northwestern China. Currently, selecting and utilizing salt-tolerant genotypes is an important way to mitigate the detrimental effects of salinity on crop productivity. In our research, salt-tolerant and salt-sensitive varieties were selected from 57 accessions based on a comprehensive evaluation of 22 relevant indexes, and their salt-tolerance physiological and molecular mechanisms were further analyzed. Results showed significant differences in salt tolerance between 57 genotypes, with Q25 and Q46 considered to be the most salt-tolerant and salt-sensitive accessions, respectively, compared to other varieties. Under saline conditions, the salt-tolerant genotype Q25 not only maintained significantly higher photosynthetic performance, leaf relative water content (RWC), and proline content but also exhibited obviously lower relative conductivity and malondialdehyde (MDA) content than the salt-sensitive Q46 (p < 0.05). The transcriptome sequencing indicated 15,128 differentially expressed genes (DEGs) in Q46, of which 7,885 were upregulated and 7,243 downregulated, and 12,658 DEGs in Q25, of which 6,059 were upregulated and 6,599 downregulated. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the salt response differences between Q25 and Q46 were attributed to the variable expression of genes associated with plant hormone signal transduction and MAPK signaling pathways. Furthermore, a large number of candidate genes, related to salt tolerance, were detected, which involved transcription factors (zinc finger proteins) and accumulation of compatible osmolytes (glutathione S-transferases and pyrroline-5-carboxylate reductases), etc. This study offers an important view of the physiological and molecular regulatory mechanisms of salt tolerance in two smooth bromegrass genotypes and lays the foundation for further identification of key genes linked to salt tolerance.

Graphene nanoparticles improve alfalfa (Medicago sativa L.) growth through multiple metabolic pathways under salinity-stressed environment

Graphene, one of the emerging carbon nanomaterials, has many advantages and applications. Salinity stress seriously affects ecology and agroforestry worldwide. The effects of graphene on alfalfa under salinity stress were investigated. The results indicated that graphene promoted alfalfa growth under non-salinity stress but caused some degree of damage to root cells and leaf parameters. Graphene used in salinity stress had a positive effect on growth parameters, chlorophyll, photosynthetic gas parameters, stomatal opening, ion balance, osmotic homeostasis, cell membrane integrity and antioxidant system, while it decreased Na+, lipid peroxidation and reactive oxygen species levels. Correlation analysis revealed that most of the parameters were significantly correlated; and principal component analysis indicated that the first two dimensions (78.1% and 4.1%) explained 82.2% of the total variability, and the majority of them exceeded the average contribution. Additionally, Gene Ontology functional enrichment analysis and Kyoto Encyclopedia of Genes and Genomes signaling pathway enrichment analysis showed that there were numerous differentially expressed genes and pathways to regulate alfalfa responding to salinity stress. Taken together, the findings reveal that graphene does not enter the plant, but improves the properties and adsorption of soil to enhance salt tolerance and seedling growth of alfalfa through morphological, physiological, biochemical, and transcriptomic aspects. Furthermore, this study provides a reference for the application of graphene to improve soil environment and agricultural production.

Silicon Induces Heat and Salinity Tolerance in Wheat by Increasing Antioxidant Activities, Photosynthetic Activity, Nutrient Homeostasis, and Osmo-Protectant Synthesis

Modern agriculture is facing the challenges of salinity and heat stresses, which pose a serious threat to crop productivity and global food security. Thus, it is necessary to develop the appropriate measures to minimize the impacts of these serious stresses on field crops. Silicon (Si) is the second most abundant element on earth and has been recognized as an important substance to mitigate the adverse effects of abiotic stresses. Thus, the present study determined the role of Si in mitigating adverse impacts of salinity stress (SS) and heat stress (HS) on wheat crop. This study examined response of different wheat genotypes, namely Akbar-2019, Subhani-2021, and Faisalabad-2008, under different treatments: control, SS (8 dSm^-1), HS, SS + HS, control + Si, SS + Si, HS+ Si, and SS + HS+ Si. This study's findings reveal that HS and SS caused a significant decrease in the growth and yield of wheat by increasing electrolyte leakage (EL), malondialdehyde (MDA), and hydrogen peroxide (H₂O₂) production; sodium (Na⁺) and chloride (Cl^-) accumulation; and decreasing relative water content (RWC), chlorophyll and carotenoid content, total soluble proteins (TSP), and free amino acids (FAA), as well as nutrient uptake (potassium, K; calcium, Ca; and magnesium, Mg). However, Si application offsets the negative effects of both salinity and HS and improved the growth and yield of wheat by increasing chlorophyll and carotenoid contents, RWC, antioxidant activity, TSP, FAA accumulation, and nutrient uptake (Ca, K, and Mg); decreasing EL, electrolyte leakage, MDA, and H₂O₂; and restricting the uptake of Na⁺ and Cl^-. Thus, the application of Si could be an important approach to improve wheat growth and yield under normal and combined saline and HS conditions by improving plant physiological functioning, antioxidant activities, nutrient homeostasis, and osmolyte accumulation.

Co-application of copper oxide nanoparticles and Trichoderma harzianum with physiological, enzymatic and ultrastructural responses for the mitigation of salt stress

Chemical contamination or nutrient pollution is concerning for health, environmental, and economic reasons. Ecofriendly surface modification of nanoparticles is a consistent challenge for agricultural purposes. In response to this environmental concern, CuO-NPs synthesized through biological method using green source and characterized for morphological and structural features through SEM (scanning electron microscope) and TEM (transmission electron microscope) spectroscopy. Our research findings illustrate that the presence of salt stress induces a notable decline in both physiological and biochemical parameters within plants. Nevertheless, the utilization of T. harzianum and CuO-NPs exhibited a mitigating effect on the detrimental consequences induced by salt stress in plants. The application of T. harzianum and the simultaneous co-inoculation with CuO-NPs notably enhanced fresh biomass and facilitated vegetative growth in comparison to the control group. Furthermore, the exposure of both T. harzianum inoculum and Copper oxide nanoparticles resulted in a significant reduction of oxidative stresses, including reactive oxygen species (ROS) levels, H₂O₂, and lipid peroxidation (MDA) levels in the above-ground parts of the plant, while also minimizing electrolyte leakage (EL) by reducing root growth. Additionally, the co-inoculation of the endophyte and CuO-NPs led to a significant enhancement in antioxidant enzymatic activities, such as superoxide dismutase (SOD) and chitinase (CAT) activity in the above-ground parts, under salt stress conditions. The inoculum, along with its combination with CuO-NPs, decreased electrolyte conductivity and improved total chlorophyll contents as compared to the control. The combined application of T. harzianum and CuO-NPs improved salt tolerance in A. thaliana plants by triggering salt-associated gene expression. These findings suggest that the application of T. harzianum and CuO-NPs can considerably promote leaf anatomical changes in A. thaliana and have ability to enhance salt tolerance, particularly in saline areas.

Elucidation of Physiological, Transcriptomic and Metabolomic Salinity Response Mechanisms in Medicago sativa

Alfalfa (Medicago sativa L.) is a widely grown perennial leguminous forage crop with a number of positive attributes. However, despite its moderate ability to tolerate saline soils, which are increasing in prevalence worldwide, it suffers considerable yield declines under these growth conditions. While a general framework of the cascade of events involved in plant salinity response has been unraveled in recent years, many gaps remain in our understanding of the precise molecular mechanisms involved in this process, particularly in non-model yet economically important species such as alfalfa. Therefore, as a means of further elucidating salinity response mechanisms in this species, we carried out in-depth physiological assessments of M. sativa cv. Beaver, as well as transcriptomic and untargeted metabolomic evaluations of leaf tissues, following extended exposure to salinity (grown for 3-4 weeks under saline treatment) and control conditions. In addition to the substantial growth and photosynthetic reductions observed under salinity treatment, we identified 1233 significant differentially expressed genes between growth conditions, as well as 60 annotated differentially accumulated metabolites. Taken together, our results suggest that changes to cell membranes and walls, cuticular and/or epicuticular waxes, osmoprotectant levels, antioxidant-related metabolic pathways, and the expression of genes encoding ion transporters, protective proteins, and transcription factors are likely involved in alfalfa's salinity response process. Although some of these alterations may contribute to alfalfa's modest salinity resilience, it is feasible that several may be disadvantageous in this context and could therefore provide valuable targets for the further improvement of tolerance to this stress in the future.

Biomass composite with exogenous organic acid addition supports the growth of sweet sorghum (Sorghum bicolor 'Dochna') by reducing salinity and increasing nutrient levels in coastal saline-alkaline soil

INTRODUCTION

In coastal saline lands, organic matter is scarce and saline stress is high. Exploring the promotion effect of intervention with organic acid from biological materials on soil improvement and thus forage output and determining the related mechanism are beneficial to the potential cultivation and resourceful, high-value utilization of coastal mudflats as back-up arable land.

METHOD

Three exogenous organic acids [humic acid (H), fulvic acid (F), and citric acid (C)] were combined with four kinds of biomass materials [cottonseed hull (CH), cow manure (CM), grass charcoal (GC), and pine needle (PN)] and applied to about 0.3% of medium-salt mudflat soil. The salinity and nutrient dynamics of the soil and the growth and physiological differences of sweet sorghum at the seedling, elongation, and heading stages were observed under different treatments to screen for efficient combinations and analyze the intrinsic causes and influencing mechanisms.

RESULTS

The soil salinity, nutrient dynamics, and forage grass biological yield during sweet sorghum cultivation in saline soils differed significantly (p < 0.05) depending on the type of organic acid-biomass composite applied. Citric acid-pine needle composite substantially reduced the soil salinity and increased the soil nutrient content at the seedling stage and improved the root vigor and photosynthesis of sweet sorghum by increasing its stress tolerance, allowing plant morphological restructuring for a high biological yield. The improvement effect of fulvic acid-pine needle or fulvic acid-cow manure composite was manifested at the elongation and heading stages.

DISCUSSION

Citric acid-pine needle composite promoted the growth of saline sweet sorghum seedlings, and the effect of fulvic acid-pine needle composite lasted until the middle and late stages.

Effect of genotype and environment on agronomical characters of alfalfa (Medicago sativa L.) in a typical acidic soil environment in southwest China

Alfalfa (Medicago sativa L.), an important perennial legume forage crop with high nutritional value and forage yield, is widely used in animal husbandry. However, it is very sensitive to aluminum, which severely limits its growth in acidic soils. In this study, we analyzed the genotype variation of each agronomic trait in 44 alfalfa varieties in two acidic soil environments. Then, analysis of variance (ANOVA) of the variance components was performed using the Residual Maximum Likelihood (REML). The best linear unbiased predictor analysis was used to obtain the mean trait of each variety, and the mean values were used to construct the mean matrix of varieties × traits and interaction analysis of varieties × years. The results showed that there was significant (P &lt; 0.05) genotypic variation for each trait of the 44 varieties and the genetic diversity was abundant. The average repeatability (R value) of interannual plant height (PH), stem thickness (ST), number of branches (NS), fresh weight (FW), total fresh weight (TFW), and total dry weight (TDW) was high (0.21–0.34), whereas the genetics were relatively stable. PH, NS, FW, TFW, and dry weight (DW) were positively correlated (P &lt; 0.01) with TDW. Six alfalfa varieties (Algonquin, Xinjiang daye, Trifecta, Vernal, WL354HQ, and Boja) with excellent TDW and TFW were identified in different years, environmental regions, and climatic altitudes. Our research results can provide suggestions and critical information regarding the future improvement and development of new alfalfa strains and varieties that are resistant to acidic soil conditions.

Effect and mechanism of the improvement of coastal silt soil by application of organic fertilizer and gravel combined with Sesbania cannabina cultivation

Jiangsu Province of China has a large area of coastal silt soil (CSS) with poor permeability, high salinity, and poor nutrients, which brings great difficulties to the development and utilization of coastal zones, so that needs to be improved as a matter of urgency. In this study, river-sand, serpentine, and organic fertilizer were used as additives in CSS, and Sesbania cannabina, a salt-tolerant cash crop, was planted in these differently treated soils. Through high-throughput sequencing, analysis of soil physico-chemical properties, and detection of plant growth status, the rhizosphere bacterial diversity of S. cannabina growing in CSS under different treatments and their environmental impact factors were studied, while exploring the effect and mechanism of organic fertilizer combined with gravel as a CSS modifier. The results implied that single application of organic fertilizer could significantly increase the fertility levels of total nitrogen (TN), total organic carbon (TOC) and Avail. P in CSS; then, the application of organic fertilizer with river-sand significantly reduced salt content and alkalinity of soil; meanwhile, in the treatment of single application of organic fertilizer and application of organic fertilizer combined with river-sand, the rhizosphere of S. cannabina enriched the bacterial communities of organic matter degradation and utilization to varying degrees. The soil moisture content and indicators related to saline-alkali soil (including total salt, electrical conductivity (EC), exchangeable sodium percentage (ESP), Avail. Na and Avail. K, etc.) were further reduced significantly by the application of organic fertilizer combined with river-sand and serpentine. The method has greatly improved the growth conditions of S. cannabina and promoted the positive development of its rhizosphere bacterial community. Among them, in the treatment of organic fertilizer combined with river-sand and serpentine, a variety of plant growth-promoting rhizobacteria (PGPR, such as Sphingomonas, Ensifer, and Rhodobacter) and nitrogen-cycle-related bacteria (such as nitrate-reduction-related bacteria, nitrogen-fixing bacteria like Ensifer, and purple non-sulfur photosynthetic bacteria like Rhodobacter) were enriched in the rhizosphere of S. cannabina; moreover, the mutual association and robustness of bacterial co-occurrence networks have been significantly enhanced. The results provide a theoretical basis and reference model for the improvement of coastal saline-alkali silt soil.

Proteomic Analysis of Alfalfa (Medicago sativa L.) Roots in Response to Rhizobium Nodulation and Salt Stress

(1) Background: Alfalfa is an important legume forage throughout the world. Although alfalfa is considered moderately tolerant to salinity, its production and nitrogen-fixing activity are greatly limited by salt stress. (2) Methods: We examined the physiological changes and proteomic profiles of alfalfa with active nodules (NA) and without nodules (NN) under NaCl treatment. (3) Results: Our data suggested that NA roots showed upregulation of the pathways of abiotic and biotic stress responses (e.g., heat shock proteins and pathogenesis-related proteins), antioxidant enzyme synthesis, protein synthesis and degradation, cell wall degradation and modification, acid phosphatases, and porin transport when compared with NN plants under salt stress conditions. NA roots also upregulated the processes or proteins of lipid metabolism, heat shock proteins, protein degradation and folding, and cell cytoskeleton, downregulated the DNA and protein synthesis process, and vacuolar H⁺-ATPase proteins under salt stress. Besides, NA roots displayed a net H⁺ influx and low level of K⁺ efflux under salt stress, which may enhance the salt tolerance of NA plants. (4) Conclusions: The rhizobium symbiosis conferred the host plant salt tolerance by regulating a series of physiological processes to enhance stress response, improve antioxidant ability and energy use efficiency, and maintain ion homeostasis.

Energy vs. Nutritional Potential of Virginia Mallow (Sida hermaphrodita L.) and Cup Plant (Silphium perfoliatum L.)

The world today faces several pressing challenges: energy from non-renewable sources is becoming increasingly expensive, while at the same time the use of agricultural land for food production is decreasing at the expense of biofuel production. Energy crops offer a potential solution to maximizing the use of land. In order to provide new value to the by-product, it is necessary to investigate its possible nutritional and functional potential. Therefore, the main objective of this study was to determine the energetic, nutritional, and functional potential of the species Sida hermaphrodita L. and Silphium perfoliatum L. in different phenophases. The analyzed energy potential of the mentioned species is not negligible due to the high determined calorific value (17.36 MJ/kg for Virginia mallow and 15.46 MJ/kg for the cup plant), high coke content (15.49% for the cup plant and 10.45% for Virginia mallow), and desirably high carbon content, almost 45%, in both species. The phenophase of the plant had a significant influence on the content of the analyzed specialized metabolites (SM) in the leaves, with a high content of ascorbic acid at the full-flowering stage in Virginia mallow (229.79 mg/100 g fw) and in cup plants at the end of flowering (122.57 mg/100 g fw). In addition, both species have high content of polyphenols: as much as 1079.59 mg GAE/100 g were determined in the leaves of Virginia mallow at the pre-flowering stage and 1115.21 mg GAE/100 g fw in the cup plants at the full-flowering stage. An HPLC analysis showed high levels of ellagic acid and naringin in both species. In addition, both species have high total chlorophyll and carotenoid concentrations. Due to their high content of SM, both species are characterized by a high antioxidant capacity. It can be concluded that, in addition to their energetic importance, these two plants are also an important source of bioactive compounds; thus, their nutritional and functional potential for further use as value-added by-products should not be neglected.

Full-Length Transcriptomics Reveals Complex Molecular Mechanism of Salt Tolerance in Bromus inermis L

Bromus inermis L. (commonly known as smooth bromegrass) is a grass species with high nutritional value, great palatability, cold tolerance, and grazing resistance, which has been widely cultivated for pasture and sand fixation in northern and northwestern China. Salt stress is a main environmental factor limiting growth and production of smooth bromegrass. In this study, we performed PacBio Iso-Seq to construct the first full-length transcriptome database for smooth bromegrass under 300 mM NaCl treatment at different time points. Third-generation full-length transcriptome sequencing yielded 19.67 G polymerase read bases, which were assembled into 355,836 full-length transcripts with an average length of 2,542 bp. A total of 116,578 differentially expressed genes were obtained by comparing the results of third-generation sequencing and second-generation sequencing. GO and KEGG enrichment analyses revealed that multiple pathways were differently activated in leaves and roots. In particular, a number of genes participating in the molecular network of plant signal perception, signal transduction, transcription regulation, antioxidant defense, and ion regulation were affected by NaCl treatment. In particular, the CBL-CIPK, MAPK, ABA signaling network, and SOS core regulatory pathways of Ca²⁺ signal transduction were activated to regulate salt stress response. In addition, the expression patterns of 10 salt-responsive genes were validated by quantitative real-time PCR, which were consistent with those detected by RNA-Seq. Our results reveal the molecular regulation of smooth bromegrass in response to salt stress, which are important for further investigation of critical salt responsive genes and molecular breeding of salt-tolerant smooth bromegrass.

Biocontrol Potential of Bacillus amyloliquefaciens LYZ69 Against Anthracnose of Alfalfa (Medicago sativa)

Anthracnose is a destructive disease of alfalfa (Medicago sativa) that causes severe yield losses. Biological control can be an effective and eco-friendly approach to control this alfalfa disease. In the present study, Bacillus amyloliquefaciens LYZ69, previously isolated from healthy alfalfa roots, showed a strong in vitro antifungal activity against Colletotrichum truncatum, an important causal agent of anthracnose of alfalfa. The strain LYZ69 protected alfalfa plants (biocontrol efficacy of 82.59%) from anthracnose under greenhouse conditions. The cell-free culture (CFC) of LYZ69 (20%, vol/vol) caused 60 and 100% inhibition of mycelial growth and conidial germination, respectively. High-performance liquid chromatography tandem mass spectrometry separated and identified cyclic lipopeptides (LPs) such as bacillomycin D and fengycin in the CFC of LYZ69. Light microscopy and scanning electron microscopy revealed that the mixture of cyclic LPs produced by LYZ69 caused drastic changes in mycelial morphology. Fluorescence microscopy showed that the LPs induced reactive oxygen species accumulation and caused apoptosis-like cell death in C. truncatum hyphae. In summary, our findings provide evidence to support B. amyloliquefaciens LYZ69 as a promising candidate for the biological control of anthracnose in alfalfa.

Amelioration of salt induced toxicity in pearl millet by seed priming with silver nanoparticles (AgNPs): The oxidative damage, antioxidant enzymes and ions uptake are major determinants of salt tolerant capacity

Abiotic stresses in plants reduce crop growth and productivity. Nanoparticles (NPs) are effectively involved in the physiochemical processes of crop plants, especially under the abiotic stresses; whereas, less information is available regarding the role of AgNPs in salt-stressed plants. Therefore, in the current study, we investigated the effects of seed priming with commercially available silver nanoparticles (AgNPs) (size range between 50 and 100 nm) on plant morphology, physiology, and antioxidant defence system of pearl millet (Pennisetum glaucum L.) under different concentrations of salt stress (0, 120 and 150 mM NaCl). The seed priming with AgNPs at different levels (0, 10, 20 and 30 mM) mitigated the adverse impacts of salt stress and improved plant growth and defence system. The results demonstrated that salt-stressed plants had restricted growth and a noticeable decline in fresh and dry weight. Salt stress enhanced the oxidative damage by excessive production of hydrogen peroxide (H₂O₂), malondialdehyde (MDA) contents in pearl millet leaves. However, seed priming with AgNPs significantly improved the plant height growth related attributes, relative water content, proline contents and ultimately fresh and dry weight at 20 mM AgNPs alone or with salt stress. The AgNPs reduced the oxidative damage by improving antioxidant enzyme activities in the pearl millet leaves under salt stress. Furthermore, sodium (Na⁺) and Na⁺/K⁺ ratio was decreased and potassium (K⁺) increased by NPs, and the interactive effects between salt and AgNPs significantly impacted the total phenolic and flavonoid content in pearl millet. It was concluded that seed priming with AgNPs could enhance salinity tolerance in crop plants by enhancing physiological and biochemical responses. This might boost global crop production in salt-degraded lands.