Contribution of Rhizobium–Legume Symbiosis in Salt Stress Tolerance in Medicago truncatula Evaluated through Photosynthesis, Antioxidant Enzymes, and Compatible Solutes Accumulation

Zinc oxide nanoparticles influence on plant tolerance to salinity stress: insights into physiological, biochemical, and molecular responses

A slight variation in ecological milieu of plants, like drought, heavy metal toxicity, abrupt changes in temperature, flood, and salt stress disturbs the usual homeostasis or metabolism in plants. Among these stresses, salinity stress is particularly detrimental to the plants, leading to toxic effects and reduce crop productivity. In a saline environment, the accumulation of sodium and chloride ions up to toxic levels significantly correlates with intracellular osmotic pressure, and can result in morphological, physiological, and molecular alterations in plants. Increased soil salinity triggers salt stress signals that activate various cellular-subcellular mechanisms in plants to enable their survival in saline conditions. Plants can adapt saline conditions by maintaining ion homeostasis, activating osmotic stress pathways, modulating phytohormone signaling, regulating cytoskeleton dynamics, and maintaining cell wall integrity. To address ionic toxicity, researchers from diverse disciplines have explored novel approaches to support plant growth and enhance their resilience. One such approach is the application of nanoparticles as a foliar spray or seed priming agents positively improve the crop quality and yield by activating germination enzymes, maintaining reactive oxygen species homeostasis, promoting synthesis of compatible solutes, stimulating antioxidant defense mechanisms, and facilitating the formation of aquaporins in seeds and root cells for efficient water absorption under various abiotic stresses. Thus, the assessment mainly targets to provide an outline of the impact of salinity stress on plant metabolism and the resistance strategies employed by plants. Additionally, the review also summarized recent research efforts exploring the innovative applications of zinc oxide nanoparticles for reducing salt stress at biochemical, physiological, and molecular levels.

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.

Bioprospecting for Rhizobacteria with the Ability to Enhance Drought Tolerance in Lessertia frutescens

Lessertia frutescens is a multipurpose medicinal plant indigenous to South Africa that is used for the management of cancer, stomach ulcers, wounds, etc. The use and demand for the raw materials from this plant have been increasing steadily over the years, putting strain on the dwindling wild populations. Although cultivation may provide relief to the strained supply, the persistent drought climate poses a threat to the plant's growth and productivity. This study explored three plant-growth-promoting rhizobacteria isolates, TUTLFNC33, TUTLFNC37 and TUTLFWC74, obtained from the root nodules of Lessertia frutescens as potential bioinoculants that can improve yield, biological activities and the production of secondary metabolites in the host plant. Isolate TUTLFNC37 was identified as the most promising isolate for inoculation of Lessertia frutescens under drought conditions as it induced drought tolerance through enhanced root proliferation, osmolyte proline accumulation and stomatal closure. Superior biomass yield, phenolics, triterpenes and antioxidant activity were evident in the extracts of Lessertia frutescens inoculated with TUTLFNC37 and under different levels of drought. Furthermore, the metabolomics of the plant extracts demonstrated the ability of the isolate to withstand drastic changes in the composition of unique metabolites, sutherlandiosides A-D and sutherlandins A-D. Molecular families which were never reported in the plant (peptides and glycerolipids) were detected and annotated in the molecular networks. Although drought had deleterious effects on Lessertia frutescens, isolate TUTLFNC37 alleviated the impact of the stress. Isolate TUTLFNC37 is therefore the most promising, environmentally friendly alternative to harmful chemicals such as nitrate-based fertilizers. The isolate should be studied to establish its field performance, cross infectivity with other medicinal plants and competition with inherent soil microbes.

Probing the potential of salinity-tolerant endophytic bacteria to improve the growth of mungbean [Vigna radiata (L.) Wilczek]

Soil salinity is one of the major limiting factors in plant growth regulation. Salinity-tolerant endophytic bacteria (STEB) can be used to alleviate the negative effects of salinity and promote plant growth. In this study, thirteen endophytic bacteria were isolated from mungbean roots and tested for NaCl salt-tolerance up to 4%. Six bacterial isolates, TMB2, TMB3, TMB5, TMB6, TMB7 and TMB9, demonstrated the ability to tolerate salt. Plant growth-promoting properties such as phosphate solubilization, indole-3-acetic acid (IAA) production, nitrogen fixation, zinc solubilization, biofilm formation and hydrolytic enzyme production were tested in vitro under saline conditions. Eight bacterial isolates indicated phosphate solubilization potential ranging from 5.8-17.7 μg mL-1, wherein TMB6 was found most efficient. Ten bacterial isolates exhibited IAA production ranging from 0.3-2.1 μg mL-1, where TMB7 indicated the highest potential. All the bacterial isolates except TMB13 exhibited nitrogenase activity. Three isolates, TMB6, TMB7 and TMB9, were able to solubilize zinc on tris-minimal media. All isolates were capable of forming biofilm except TMB12 and TMB13. Only TMB2, TMB6 and TMB7 exhibited cellulase activity, while TMB2 and TMB7 exhibited pectinase production. Based on in vitro testing, six efficient STEB were selected and subjected to the further studies. 16S rRNA gene sequencing of efficient STEB revealed the maximum similarity between TMB2 and Rhizobium pusense, TMB3 and Agrobacterium leguminum, TMB5 and Achromobacter denitrificans, TMB6 and Pseudomonas extremorientalis, TMB7 and Bradyrhizobium japonicum and TMB9 and Serratia quinivorans. This is the first international report on the existence of A. leguminum, A. denitrificans, P. extremorientalis and S. quinivorans inside the roots of mungbean. Under controlled-conditions, inoculation of P. extremorientalis TMB6, B. japonicum TMB7 and S. quinivorans TMB9 exhibited maximum potential to increase plant growth parameters; specifically plant dry weight was increased by up to 52%, 61% and 45%, respectively. Inoculation of B. japonicum TMB7 displayed the highest potential to increase plant proline, glycine betaine and total soluble proteins contents by 77%, 78% and 64%, respectively, compared to control under saline conditions. It is suggested that the efficient STEB could be used as biofertilizers for mungbean crop productivity under saline conditions after field-testing.

Serendipita indica mitigates drought-triggered oxidative burst in trifoliate orange by stimulating antioxidant defense systems

Soil drought is detrimental to plant growth worldwide, particularly by triggering reactive oxygen species (ROS) burst. Serendipita indica (Si), a culturable root-associated endophytic fungus, can assist host plants in dealing with abiotic stresses; however, it is unknown whether and how Si impacts the drought tolerance of citrus plants. To unravel the effects and roles of Si on drought-stressed plants, trifoliate orange (Poncirus trifoliata L. Raf.; a citrus rootstock) seedlings were inoculated with Si and exposed to soil drought, and growth, gas exchange, ROS levels, antioxidant defense systems, and expression of genes encoding antioxidant enzymes and fatty acid desaturases in leaves were measured. Soil drought suppressed plant biomass, whereas Si inoculation significantly increased plant biomass (10.29%-22.47%) and shoot/root ratio (21.78%-24.68%) under ample water and drought conditions, accompanied by improved net photosynthetic rate (105.71%), water use efficiency (115.29%), chlorophyll index (55.34%), and nitrogen balance index (63.84%) by Si inoculation under soil drought. Soil drought triggered an increase in leaf hydrogen peroxide and superoxide anion levels, while Si inoculation significantly reduced these ROS levels under soil drought, resulting in lower membrane lipid peroxidation with respect to malondialdehyde changes. Furthermore, Si-inoculated seedlings under soil drought had distinctly higher levels of ascorbate and glutathione, as well as catalase, peroxidase, and glutathione peroxidase activities, compared with no-Si-inoculated seedlings. Si inoculation increased the expression of leaf PtFAD2, PtFAD6, PtΔ9, PtΔ15, PtFe-SOD, PtCu/Zn-SOD, PtPOD, and PtCAT1 genes under both ample water and soil drought conditions. Overall, Si-inoculated trifoliate orange plants maintained a low oxidative burst in leaves under drought, which was associated with stimulation of antioxidant defense systems. Therefore, Si has great potential as a biostimulant in enhancing drought tolerance in plants, particularly citrus.

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.

H2 S works synergistically with rhizobia to modify photosynthetic carbon assimilation and metabolism in nitrogen-deficient soybeans

Hydrogen sulfide (H₂ S) performs a crucial role in plant development and abiotic stress responses by interacting with other signalling molecules. However, the synergistic involvement of H₂ S and rhizobia in photosynthetic carbon (C) metabolism in soybean (Glycine max) under nitrogen (N) deficiency has been largely overlooked. Therefore, we scrutinised how H₂ S drives photosynthetic C fixation, utilisation, and accumulation in soybean-rhizobia symbiotic systems. When soybeans encountered N deficiency, organ growth, grain output, and nodule N-fixation performance were considerably improved owing to H₂ S and rhizobia. Furthermore, H₂ S collaborated with rhizobia to actively govern assimilation product generation and transport, modulating C allocation, utilisation, and accumulation. Additionally, H₂ S and rhizobia profoundly affected critical enzyme activities and coding gene expressions implicated in C fixation, transport, and metabolism. Furthermore, we observed substantial effects of H₂ S and rhizobia on primary metabolism and C-N coupled metabolic networks in essential organs via C metabolic regulation. Consequently, H₂ S synergy with rhizobia inspired complex primary metabolism and C-N coupled metabolic pathways by directing the expression of key enzymes and related coding genes involved in C metabolism, stimulating effective C fixation, transport, and distribution, and ultimately improving N fixation, growth, and grain yield in soybeans.

Water Content of Plant Tissues: So Simple That Almost Forgotten?

The aim of the present review was to reconsider basic information about various functional aspects related to plant water content and provide evidence that the usefulness of measuring absolute water content in plant sciences is undervalued. First, general questions about water status in plants as well as methods for determining water content and their associated problems were discussed. After a brief overview of the structural organization of water in plant tissues, attention was paid to the water content of different parts of plants. Looking at the influence of environmental factors on plant water status, the differences caused by air humidity, mineral supply, biotic effects, salinity, and specific life forms (clonal and succulent plants) were analyzed. Finally, it was concluded that the expression of absolute water content on a dry biomass basis makes easily noticeable functional sense, but the physiological meaning and ecological significance of the drastic differences in plant water content need to be further elucidated.

Insights on strain 115 plant growth-promoting bacteria traits and its contribution in lead stress alleviation in pea (Pisum sativum L.) plants

The present study aims to characterize the plant growth-promoting bacterial traits of Bacillus simplex (strain 115). This bacterium was inoculated in hydroponically conditions to improve pea (Pisum sativum L.) growth submitted to lead (Pb) toxicity. Root nodulation system was developed enough in 23-day-old plants attesting the interaction between the two organisms. In addition to its phosphate solubilization and siderophore production traits that reached 303.8 μg P mL-1 and 49.6 psu respectively, the Bacillus strain 115 exhibited Pb bio-sorption ability. Inoculation of Pb-stressed pea with strain 115 showed roots and shoots biomass recovery (+ 70% and + 61%, respectively). Similarly, water and protein contents were increased in Pb-treated plants after bacterial inoculation. In the presence of strain 115, Pb relative toxicity level decreased (- 39.3% compared to Pb stress only). Moreover, catalase and superoxide dismutase activities were upregulated in Pb-exposed plants (+ 56% and + 51%, respectively). After inoculation with strain 115, catalase and superoxide dismutase activities were restored by - 38% and - 44% respectively. Simultaneously, oxidant stress indicator (H2O2 and 4-hydroxynonenal) and osmo-regulators (proline and glycine-betaine) contents as well as lipoxygenase activity decreased significantly in Pb-treated plants after Bacillus strain's inoculation. Taken together, the results give some evidences for the plant growth-promoting capacity of strain 115 in helping alleviation of Pb stress.

Rhizobium inoculation and exogenous melatonin synergistically increased thermotolerance by improving antioxidant defense, photosynthetic efficiency, and nitro-oxidative homeostasis in Medicago truncatula

Global warming negatively affects plant growth due to the detrimental effects of high temperature-induced heat stress. Rhizobium inoculation (RI) and exogenous melatonin (MT) have shown a positive role in resisting abiotic stress. However, their synergistic effect on avoiding heat-induced damages in Medicago truncatula has not been studied yet. Hence, the objective of the present study was to evaluate the impact of these amendments (RI and MT) to ameliorate the heat damages in Medicago truncatula. The study was comprised of two factors: (1) heat-induced stress: (i) optimum temperature (26 ± 1°C): (23 ± 1°C) (day: night), (ii) moderate heat (35 ± 1°C): (28 ± 1°C), and (iii) severe heat (41 ± 1°C): (35 ± 1°C) for 72 h, and (2) amendments: (i) no RI + no MT (NRI + NMT), (ii) Rhizobium inoculation (RI), (iii) 60 μM melatonin (MT), and (iii) RI + MT. Results showed that the combined application of RI and MT was better than their individual applications, as it prevented heat-induced membrane damages by declining the hydrogen peroxide (34.22% and 29.78%), superoxide anion radical (29.49% and 26.71%), malondialdehyde contents (26.43% and 21.96%), and lipoxygenase activity (44.75% and 25.51%) at both heat stress levels as compared to NRI + NMT. Moreover, RI + MT treated plants showed higher antioxidative and methylglyoxal detoxification enzymes (Gly I and Gly II) activities under heat stress. While, NRI + NMT treated plants showed a higher level of methylglyoxal contents (47.99% and 46.71%) under both levels of heat stress. Relative to NRI + NMT plants, RI + MT pretreated plants exhibited improved heat tolerance as indicated by higher chlorophyll (37.42% and 43.52%), carotenoid contents (32.41% and 47.08%), and photosynthetic rate (42.62% and 64.63%), under moderate and severe heat stress, respectively. Furthermore, RI + MT pretreated plants had considerably higher indole-3 acetic acid and abscisic acid concentrations under moderate (54.02% and 53.92%) and severe (68.36% and 64.61%) heat stress conditions. Similarly, plant dry biomass, NPK uptake, nitric oxide, and nitrate reductase activity were high in RI + MT treated plants, under both levels of stress. Therefore, this study advocates the positive synergistic effect of RI and MT pretreatment against moderate and severe heat-induced stress and for possible maintenance of plant growth under changing scenarios of global warming.

Zn alleviated salt toxicity in Solanum lycopersicum L. seedlings by reducing Na+ transfer, improving gas exchange, defense system and Zn contents

Soil secondary salinization is a serious menace that has significant influence on the sustainability of agriculture and threatens food security around the world. Zinc (Zn) as an essential plant nutrient associated with many physio-biochemical processes in plants and improve resistance against various abiotic stresses. The role of Zn in acclimation of Solanum lycopersicum L. challenged with salt stress is miserly understood. A hydroponic study was performed with two tomato varieties (Riogrande and Sungold) exposed to the salinity stress (0 mM and 160 mM NaCl) under two Zn concentrations (15 μM and 30 μM ZnSO₄). The results revealed that salt stress exerted strongly negative impacts on root and shoot length, fresh and dry biomass, plant water relations, membrane stability, chlorophyll contents, Na⁺/K⁺ ratio along with inferior gas exchange attributes and activities of antioxidant enzymes. Moreover, Riogrande was found to be more resistant to salinity stress than Sungold. However, Zn supply significantly alleviated the hazardous effects of salinity by altering compatible solutes accumulation, photosynthetic activity, water relation, soluble sugar contents and providing antioxidant defense against salt stress. The salinity + Zn2 treatment more obviously enhanced RWC (19.0%), MSI (30.8%), SPAD value (17.8%), and activities of SOD (31.7%), POD (28.5%), APX (64.5%) and CAT (23.3%) in Riogrande than Sungold, compared with the corresponding salinity treatment alone. In addition, salinity + Zn2 treatment significantly (P > 0.05) ameliorated salinity stress due to the depreciation in Na⁺/K⁺ ratio by 63.3% and 40.8%, Na⁺ ion relocation from root to shoot by 10.4% and 6.4%, and thereby significantly reduced Na⁺ ion accumulation by 47.4% and 16.3% in the leaves of Riogrande and Sungold respectively, compared to the salinity treatment alone. Therefore, it was obvious that 30 μM Zn concentration was more effective to induce resistance against salinity stress than 15 μM Zn concentration. Conclusively, it can be reported that exogenous Zn application helps tomato plant to combat adverse saline conditions by modulating photosynthetic and antioxidant capacity along with reduced Na⁺ uptake at the root surface of tomato plant.

Nanosized zinc oxide (n-ZnO) particles pretreatment to alfalfa seedlings alleviate heat-induced morpho-physiological and ultrastructural damages

Global efforts are in rapid progress to tackle the emerging conundrum of climate change-induced heat stress in grassland ecosystems. Zinc oxide nanoparticles (n-ZnO) are known to play a crucial role in plants' abiotic stress regulation, but its response in alfalfa against heat stress has not been explored. This study aimed at assessing the effects of n-ZnO on alfalfa under heat stress by various morpho-physiological and cellular approaches. Five-week-old alfalfa seedlings were subjected to foliar application of n-ZnO as a pretreatment before the onset of heat stress (BHS) to evaluate its effect on heat tolerance, and as a post-treatment after heat stress (AHS) to evaluate recovery efficiency. In vitro studies on Zn release from n-ZnO by Inductively coupled plasma mass spectroscopy (ICPMS) disclosed that the particle uptake and Zn release were concentration dependent. The uptake and translocation of n-ZnO examined by transmission electron microscope (TEM) reveling showed that n-ZnO was primarily localized in the vacuoles and chloroplasts. TEM images showed that ultrastructural modifications to chloroplast, mitochondria, and cell wall were reversible by highest dose of n-ZnO applied before heat stress, and damages to these organelles were not recoverable when n-ZnO was applied after heat stress. The results further enlightened that 90 mg L^-1 n-ZnO better prevented the heat stress-mediated membrane damage, lipid peroxidation and oxidative stress by stimulating antioxidant systems and enhancing osmolyte contents in both BHS and AHS. Although, application of 90 mg L^-1 n-ZnO in BHS was more effective in averting heat-induced damages and maintaining better plant growth and morpho-physiological attributes compared to AHS. Conclusively, foliar application of n-ZnO can be encouraged as an effective strategy to protect alfalfa from heat stress damages while minimizing the risk of nanoparticle transmission to environmental compartments, which could happen with soil application.

Salt stress mitigation in Lathyrus cicera by combining different microbial inocula

Arid and semi-arid areas are considered vulnerable to various environmental constraints which are further fortified by climate change. Salinity is one of the most serious abiotic factors affecting crop yield and soil fertility. Till now, no information is available on the effect of salinity on development and symbiotic nitrogen (N₂) fixation in the legume species Lathyrus cicera. Here, we evaluated the effect of different microbial inocula including nitrogen-fixing Rhizobium laguerreae, arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, a complex mixed inoculum of AMF isolated from rhizospheric soil in "Al Aitha", and various plant growth-promoting bacteria (PGPB) including Bacillus subtilus, Bacillus simplex and Bacillus megaterium combined with Rhizobium, the AMF consortium, or R. irregularis on alleviating salt stress in this legume. A pot trial was conducted to evaluate the ability of different microbial inocula to mitigate adverse effects of salinity on L. cicera plants. The results showed that salinity (100 mM NaCl) significantly reduced L. cicera plant growth. However, inoculation with different inocula enhanced plant growth and markedly promoted various biochemical traits. Moreover, the combined use of PGPB and AMF was found to be the most effective treatment in mitigating deleterious effects of salinity stress on L. cicera. In addition, this co-inoculation upregulated the expression of two marker genes (LcHKT1 and LcNHX7) related to salinity tolerance. Our findings suggest that the AMF/PGPB formulation has a great potential to be used as a biofertilizer to improve L. cicera plant growth and productivity under saline conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01205-4.

Plant Salinity Stress Response and Nano-Enabled Plant Salt Tolerance

The area of salinized land is gradually expanding cross the globe. Salt stress seriously reduces the yield and quality of crops and endangers food supply to meet the demand of the increased population. The mechanisms underlying nano-enabled plant tolerance were discussed, including (1) maintaining ROS homeostasis, (2) improving plant's ability to exclude Na+ and to retain K+, (3) improving the production of nitric oxide, (4) increasing α-amylase activities to increase soluble sugar content, and (5) decreasing lipoxygenase activities to reduce membrane oxidative damage. The possible commonly employed mechanisms such as alleviating oxidative stress damage and maintaining ion homeostasis were highlighted. Further, the possible role of phytohormones and the molecular mechanisms in nano-enabled plant salt tolerance were discussed. Overall, this review paper aims to help the researchers from different field such as plant science and nanoscience to better understand possible new approaches to address salinity issues in agriculture.

Effect of Salinity on Growth, Ion Accumulation and Mineral Nutrition of Different Accessions of a Crop Wild Relative Legume Species, Trifolium fragiferum

Crop wild relatives represent a valuable resource for the breeding of new crop varieties suitable for sustainable productivity in conditions of climate change. The aim of the present study was to assess salt tolerance of several wild accessions of T. fragiferum from habitats with different salinity levels in controlled conditions. Decrease of plant biomass and changes in partitioning between different organs was a characteristic response of plants with increasing substrate salinity, but these responses were genotype-specific. In several accessions, salinity stimulated reproductive development. The major differences in salinity responses between various T. fragiferum genotypes were at the level of dry biomass accumulation as well as water accumulation in plant tissues, resulting in relatively more similar effect on fresh mass. Na⁺ and Cl^- accumulation capacity were organ-specific, with leaf petioles accumulating more, followed by leaf blades and stolons. Responses of mineral nutrition clearly were both genotype- and organ-specific, but several elements showed a relatively general pattern, such as increase in Zn concentration in all plant parts, and decrease in Ca and Mg concentration. Alterations in mineralome possibly reflect a reprogramming of the metabolism to adapt to changes in growth, morphology and ion accumulation resulting from effect of NaCl. High intraspecies morphological and physiological variability in responses of T. fragiferum accessions to salinity allow to describe them as ecotypes.

Impact of Two Strains of Rhizobium leguminosarum on the Adaptation to Terminal Water Deficit of Two Cultivars Vicia faba

Drought stress has become one of the most uncontrolled and unpredictable constraints on crop production. The purpose of this study was to evaluate the impacts of two different Rhizobium leguminosarum strains on terminal drought tolerance induction in two faba bean genotypes cultivated in Algeria, Aquadulce and Maltais. To this end, we measured physiological parameters—osmoprotectants accumulation, oxidative stress markers and enzyme activities—to assess the effect of R. leguminosarum inoculation on V. faba under terminal water deficiency conditions in greenhouse trials. Upregulation of anti-oxidative mechanisms and production of compatible solutes were found differentially activated according to Rhizobium strain. Drought stress resilience of the Maltais variety was improved using the local Rhizobium strain OL13 compared to the common strain 3841. Symbiosis with OL13 strain leads in particular to a much better production of proline and soluble sugar in nodules but also in roots and leaves of Maltais plant. Even if additional work is still necessary to decipher the mechanism by which a Rhizobium strain can affect the accumulation of osmoprotectants or cellular redox status in all the plants, inoculation with selected Rhizobium could be a promising strategy for improving water stress management in the forthcoming era of climate change.

Teamwork to Survive in Hostile Soils: Use of Plant Growth-Promoting Bacteria to Ameliorate Soil Salinity Stress in Crops

Plants and their microbiomes, including plant growth-promoting bacteria (PGPB), can work as a team to reduce the adverse effects of different types of stress, including drought, heat, cold, and heavy metals stresses, as well as salinity in soils. These abiotic stresses are reviewed here, with an emphasis on salinity and its negative consequences on crops, due to their wide presence in cultivable soils around the world. Likewise, the factors that stimulate the salinity of soils and their impact on microbial diversity and plant physiology were also analyzed. In addition, the saline soils that exist in Mexico were analyzed as a case study. We also made some proposals for a more extensive use of bacterial bioinoculants in agriculture, particularly in developing countries. Finally, PGPB are highly relevant and extremely helpful in counteracting the toxic effects of soil salinity and improving crop growth and production; therefore, their use should be intensively promoted.

Endophytic Fusarium oxysporum GW controlling weed and an effective biostimulant for wheat growth

Wheat crop has to compete with several weeds including Avena fatua, a noxious weed that alone is responsible for 30-70% losses in the yield annually. Because of the environmental concerns associated with conventional methods, researchers are on a continuous hunt to find clean alternatives in order to manage weeds. Fungi have shown promising weedicide potential in lab studies. The current study aimed to isolate endophytic fungi from wheat plants which can promote wheat growth and inhibit the growth of common weed, A. fatua. Of several isolates, GW (grayish white) was selected for its promising features, and the strain was identified as Fusarium oxisporum through ITS sequencing technique. This fungus released a number of compounds including Isovitexin, Calycosin, quercetagetin, and dihydroxy-dimethoxyisoflavone that inhibited the growth of A. fatua but did not influence the growth of wheat seedlings. Biomass of this fungus in the soil also reduced growth parameters of the weed and promoted the growth of wheat. For instance, the vigor index of A. fatua seedlings was reduced to only 6% of the control by this endophyte. In contrast, endophyte-associated wheat seedlings showed a higher vigor index than the control. Behind this differential response of the two plants were their contrasting physiological and biochemical status. Lower growth phenotypes of A. fatua seedlings had reduced levels of IAA, GAs, and SA and higher the levels of JA and ABA. Besides, their ROS scavenging ability was also compromised as evident from relatively lower activities of catalase, peroxidase, and ascorbic acid oxidase, as well as higher accumulation of ROS in their leaves. Wheat seedlings response to GW was opposite to the A. fatua. It may be concluded that F. oxysporum GW has the ability to differentially modulate physiology and biochemistry of the two hosts leading to contrasting phenotypic responses.

Addressing the challenge of cold stress resilience with the synergistic effect of Rhizobium inoculation and exogenous melatonin application in Medicago truncatula

Cold stress is an adverse environmental condition that limits the growth and yield of leguminous plants. Thus, discovering an effective way of ameliorating cold-mediated damage is important for sustainable legume production. In this study, the combined use of Rhizobium inoculation (RI) and melatonin (MT) pretreatment was investigated in Medicago truncatula plants under cold stress. Eight-week-old seedlings were divided into eight groups: (i) CK (no stress, noninoculated, no MT), (ii) RI (Rhizobium inoculated), (iii) MT (75 μM melatonin), (iv) RI+MT, (v) CS (cold stress at 4 °C without Rhizobium inoculation and melatonin), (vi) CS+RI, (vii) CS+MT, and (viii) CS+RI+MT. Plants were exposed to cold stress for 24 hrs. Cold stress decreased photosynthetic pigments and increased oxidative stress. Pretreatment with RI and MT alone or combined significantly improved root activity and plant biomass production under cold stress. Interestingly, chlorophyll contents increased by 242.81% and MDA levels dramatically decreased by 34.22% in the CS+RI+MT treatment compared to the CS treatment. Moreover, RI+MT pretreatment improved the antioxidative ability by increasing the activities of peroxidase (POD; 8.45%), superoxide dismutase (SOD; 50.36%), catalase (CAT; 140.26%), and ascorbate peroxidase (APX; 42.63%) over CS treated plants. Additionally, increased osmolyte accumulation, nutrient uptake, and nitrate reductase activity due to the combined use of RI and MT helped the plants counteract cold-mediated damage by strengthening the nonenzymatic antioxidant system. These data indicate that pretreatment with a combined application of RI and MT can attenuate cold damage by enhancing the antioxidation ability of legumes.