Search Results (59)

Azospirillum is a well-studied genus of plant growth-promoting rhizobacteria (PGPR) and one of the most extensively researched diazotrophs. This genus can colonize rhizosphere soil and enhance plant growth and productivity by supplying essential nutrients to the host. Azospirillum–plant interactions involve multiple mechanisms, including nitrogen fixation, the production of phytohormones (auxins, cytokinins, indole acetic acid (IAA), and gibberellins), plant growth regulators, siderophore production, phosphate solubilization, and the synthesis of various bioactive molecules, such as flavonoids, hydrogen cyanide (HCN), and catalase. Thus, Azospirillum is involved in plant growth and development. The genus Azospirillum also enhances membrane activity by modifying the composition of membrane phospholipids and fatty acids, thereby ensuring membrane fluidity under water deficiency. It promotes the development of adventitious root systems, increases mineral and water uptake, mitigates environmental stressors (both biotic and abiotic), and exhibits antipathogenic activity. Biological nitrogen fixation (BNF) is the primary mechanism of Azospirillum, which is governed by structural nif genes present in all diazotrophic species. Globally, Azospirillum spp. are widely used as inoculants for commercial crop production. It is considered a non-pathogenic bacterium that can be utilized as a biofertilizer for a variety of crops, particularly cereals and grasses such as rice and wheat, which are economically significant for agriculture. Furthermore, Azospirillum spp. influence gene expression pathways in plants, enhancing their resistance to biotic and abiotic stressors. Advances in genomics and transcriptomics have provided new insights into plant-microbe interactions. This review explored the molecular mechanisms underlying the role of Azospirillum spp. in plant growth. Additionally, BNF phytohormone synthesis, root architecture modification for nutrient uptake and stress tolerance, and immobilization for enhanced crop production are also important. A deeper understanding of the molecular basis of Azospirillum in biofertilizer and biostimulant development, as well as genetically engineered and immobilized strains for improved phosphate solubilization and nitrogen fixation, will contribute to sustainable agricultural practices and help to meet global food security demands. Full article

Sustainable agriculture and food security are challenged by the indiscriminate use of synthetic nitrogen (N₂) fertilizers, inefficient water management, and land degradation. Hydroponic cultivation uses nutrient-rich aqueous media and is a climate-resilient and resource-efficient alternative to traditional farming methods, whose dependence on synthetic N₂ fertilizers reduces their long-term sustainability. Biological nitrogen fixation (BNF), which is mediated by diazotrophs that reduce atmospheric N₂ to plant-available ammonium, has emerged as a sustainable alternative to synthetic N₂ input in hydroponic systems. This review discusses the integration of BNF into hydroponic systems by exploring the functional diversity of diazotrophs, root–microbe interactions, and environmental constraints. It further highlights recent advances in strain improvement, microbial consortia development, nitrogenase protection, and genome editing tools, novel bioformulation strategies to enhance microbial compatibility with hydroponic nutrient regimes, and omics-based tools for the real-time assessment of N₂ fixation and microbial functionality. Key challenges, such as microbial leaching, nitrate-induced inhibition of nitrogenase activity, and the absence of standardized biostimulant protocols, are discussed. Case studies on staple crops have demonstrated enhanced NUE and yield productivity following diazotroph applications. This review concludes with future perspectives on synthetic biology, regulatory policies, and omics-based tools for the real-time assessment of N₂ fixation and microbial functionality. Full article

Although drunken horse grass (Achnatherum inebrians) can be simultaneously infected by the foliar endophyte Epichloë gansuensis and colonized by Bacillus subtilis, it remains unclear whether Epichloë endophyte symbiosis influences B. subtilis colonization, as well as how their interaction affects nitrogen fixation and assimilation. The purpose of the present study was to investigate whether E. gansuensis endophyte infection facilitates the colonization of B. subtilis in the roots of host plants, with a focus on understanding the interaction effects of the E. gansuensis endophyte and B. subtilis on plant growth and nutrient absorption. In this study, we measured the colony growth rate of B. subtilis LZU7 when co-cultured with E. gansuensis strains. In addition to an in vitro test, we investigated the root colonization of Epichloë endophyte-infected plants (E+) and Epichloë endophyte-free plants (E−) with the GFP-tagged B. subtilis LZU7 in an inoculation test. Furthermore, we evaluated the interactions between E. gansuensis endophyte symbiosis and B. subtilis LZU7 colonization on the dry weight, nitrogen fixation, nitrogen converting-enzyme activity, and nutrients for E+ and E− plants by labeling with ¹⁵N₂. The results showed that the growth rates of B. subtilis LZU7 were altered and increased in a co-culture with the E. gansuensis endophyte. A significantly greater colonization of GFP-tagged B. subtilis LZU7 was detected in the roots of E+ plants compared with the roots of E− plants, suggesting that E. gansuensis endophyte symbiosis enhances the colonization of beneficial microorganisms. The combination of E. gansuensis endophyte symbiosis and B. subtilis LZU7 inoculation significantly altered the expression of the nitrogenase (nifH) gene, thereby promoting increased biological nitrogen fixation (BNF). The E. gansuensis endophyte infection and inoculation with B. subtilis LZU7 significantly increased δ15NAir in plants. Co-inoculation with the E. gansuensis endophyte and B. subtilis LZU7 significantly elevated NH₄⁺ accumulation in the roots, depleted the NH₄⁺ availability in the surrounding soil, and showed no measurable impact on the foliar NH₄⁺ content. The observed alterations in the NH₄⁺ content were linked to nitrogen-fixing microorganisms that promoted nitrogen fixation, thereby enhancing nitrogen uptake and contributing to greater biomass production in A. inebrians. Our findings highlighted the fact that a foliar symbiosis with the E. gansuensis endophyte enhances the recruitment of beneficial bacteria, and that the resulting interaction significantly impacts nitrogen fixation, assimilation, and allocation in host plants. Full article

Common vetch (Vicia sativa L.) is a legume widely used both as a grain and as forage due to its high protein content, which provides considerable nutritional enrichment for livestock feed. As a cover crop, it has the potential to fix atmospheric nitrogen through symbiosis with rhizobia, contributing to sustainable agricultural systems by enhancing soil fertility and reducing the dependence on chemical fertilizers. Although much research has been focused on optimizing Rhizobium inoculants to enhance biological nitrogen fixation (BNF) in leguminous crops, the role of host plant genetic diversity in BNF has been underexplored. This study analyses a collection of V. sativa genotypes to evaluate their BNF by assaying their nodulation capacity, nodule nitrogenase activity, nitrogen fixation potential, and impact on biomass development. Our results reveal large variability in these parameters among the different genotypes, emphasizing the relevance of host legume diversity in the Rhizobium symbiosis. These findings show a direct relationship between nodule biomass development, nitrogen fixation capacity, shoot biomass production, and nitrogen content. However, no correlation was observed for other parameters such as the number of nodules, nitrogenase activity, and shoot nitrogen content. Taken together, these results suggest that selecting genotypes with high BNF capacity could be a promising strategy to improve nitrogen fixation in legume-based agricultural systems. Full article

Incorporating temperate legumes is a strategy for increasing nitrogen (N) in tall fescue (Schedonorus arundinaceus (Schreb.) Dumort, nom. Cons) systems. However, when temperatures are elevated, biological N-fixation (BNF) by temperate legumes is limited. Sunn hemp (Crotalaria juncea L.), a warm-season annual legume, may provide greater N input during the warm season. This 2-year study aimed to (1) determine BNF in sunn hemp-tall fescue mixed pastures and (2) determine N transfer from sunn hemp to tall fescue. The experiment included four replicates of two treatments: tall fescue (TF) and tall fescue intercropped with sunn hemp (TF+SH), arranged in a randomized complete block design. Response variables included δ¹⁵N, N derived from the atmosphere (%NDFA), BNF, N concentration, N transferred (%Ntran), N stock, and herbage accumulation (HA). Herbage accumulation was 16% greater in TF+SH compared to TF (p < 0.05). Root mass was 43% greater for TF compared to both species combined in TF+SH (p < 0.05). Herbage N was 40% greater in sunn hemp shoots than tall fescue shoots in TF or TF+SH (p < 0.05). Sunn hemp root N was 34% greater than tall fescue (p < 0.05). NDFA by sunn hemp was 88% and 100% in 2017 and 2018, respectively. BNF by sunn hemp was greater (p < 0.05) in 2018 than in 2017 (53.8 and 44.3 kg ha⁻¹, respectively). The %Ntran from sunn hemp to tall fescue was 13 and 20% in 2017 and 2018, respectively. Interseeding sunn hemp into tall fescue pastures can provide an alternate N source to tall fescue-based forage-livestock systems, increasing herbage accumulation during the summer grazing season. Full article

Global climate change predictions point to an increase in the frequency of droughts and floods, which are a huge challenge to food production. During crop evolution, different mechanisms for drought resilience have emerged, and studies suggest that roots can be an important key in understanding these mechanisms. However, knowledge is still scarce, being fundamental to its exploitation. Plant-based protein, especially grain legume crops, will be crucial in meeting the demand for affordable and healthy food due to their high protein content. In addition, grain legumes have the unique ability for biological nitrogen fixation (BNF) through symbiosis with bacteria, which contributes to sustainable agriculture. The exploitation of root phenotyping techniques in grain legumes is an important step toward understanding their drought resilience mechanisms and selecting more resilient genotypes. Different methodologies are available for root phenotyping, including the paper pouch approach, rhizotrons and the semi-hydroponic system. Additionally, different imaging techniques have been employed to assess root traits. This review provides an overview of the root system architecture (RSA) of grain legumes, its role in drought stress resilience and the phenotyping approaches useful for the identification of accessions resilient to water stress. Consequently, this knowledge will be important in mitigating the effects of climate change and improving grain legume production. Full article

Elephant grass has high biomass production potential and can benefit from biological nitrogen fixation (BNF) as its main external nitrogen source. This study evaluated the effect of BNF on biomass productivity and total nitrogen accumulation in different elephant grass genotypes. This experiment was conducted in a 120 m² concrete tank filled with soil labeled with ¹⁵N to estimate the contribution of BNF. The experimental design was randomized blocks with four replications, and the evaluation was over three years of cultivation, with semiannual cuts. The productivity of fresh and dry mass of the shoot, Nitrogen (N) accumulation, and the contribution of BNF by the ¹⁵N natural abundance technique were evaluated. The annual average of BNF was 38%. There was a statistical difference between the treatments, with the genotype P13G13 presenting fresh and dry mass productivity 50% higher than P6G4. The annual average of fresh mass, dry matter, total N, and N derived from BNF in the genotypes was approximately 70, 30, 100 Mg ha⁻¹, and 35 kg ha⁻¹, respectively. The results obtained by the P13G13 genotype allow us to recommend its use for biomass production aimed at bioenergy, favoring sustainability, reducing greenhouse gas emissions, and dependence on synthetic nitrogen fertilizers. Full article

Legumes play a pivotal role in addressing global challenges of food and nutrition security by offering a sustainable source of protein and bioactive compounds. The capacity of legumes to establish symbiotic relationships with rhizobia bacteria enables biological nitrogen fixation (BNF), reducing the dependence on chemical fertilizers while enhancing soil health. However, the efficiency of this symbiosis is significantly influenced by environmental factors, such as soil acidity, salinity, temperature, moisture content, light intensity, and nutrient availability. These factors affect key processes, including rhizobia survival, nodule formation, and nitrogenase activity, ultimately determining the growth and productivity of legumes. This review summarizes current knowledge on legume-rhizobia interactions under varying abiotic conditions. It highlights the impact of salinity and acidity in limiting nodule development, soil temperature in regulating microbial community dynamics, and moisture availability in modulating metabolic and hormonal responses during drought and waterlogging. Moreover, the role of essential nutrients, including nitrogen, phosphorus, potassium, and trace elements such as iron, molybdenum, and boron, in optimizing symbiosis is critically analyzed. Full article

A Gram-positive, rod-shaped, and obligate anaerobic bacterial strain OS1-26 was isolated from apple orchard soil in Iksan, South Korea. Interestingly, strain OS1-26 was observed to possess the functional genes involved in biological nitrogen fixation (BNF), including nifH, which was actively transcribed during the anaerobic cultivation with excessive production of extracellular NH₄⁺ despite of presence of other fixed N nutrients. The BNF of strain OS1-26 was distinguished from the other well-known Clostridium diazotrophs, such as C. pasteurianum and C. acetobutylicum. The altruistic N-fixing ability of the strain may play a pivotal role in providing N nutrients to the microbial community and plants in the soil ecosystem. The microorganism grew at 25–35 °C (optimum 30–35 °C) and pH 5.0–8.0 (optimum 6.0–8.0) but was not able to grow in the presence of >0.5% NaCl. The major cellular fatty acids of strain OS1-26 were C_16:0, C_14:0, and the summed feature consisted of C_16:1 ω7c and C_16:1 ω6c (35.63%, 25.29%, and 18.84%, respectively). The 16S rRNA phylogeny indicated that strain OS1-26 is a member of the genus Clostridium, and the closest species are C. aciditolerans, C. nitrophenolicum, and C. thailandense, with 16S rRNA sequence similarities such as 99.71%, 98.52%, and 98.45%, respectively. In spite of the high 16S rRNA sequence similarity, strain OS1-26 showed overall genomic relatedness, such as the average nucleotide identity (ANI), and phenotypical features distinctly different from Clostridium aciditolerans. Although the species taxonomy of strain OS1-26 is undetermined within the genus Clostridium based on overall genomic and phenotypic properties, further studies on the soil bacterial strain would enhance our understanding of its taxonomic identity, ecological roles for the terrestrial soil N cycle, and the potential to be developed as a biological N fertilizer. Full article

Drought is the primary limiting factor affecting soybean productivity, and is exacerbated by climate change. In legumes like soybeans, biological nitrogen fixation (BNF) is the main form of nitrogen acquisition, with nitrogen being converted into ureides. A greenhouse experiment was conducted using the soybean cultivar BMX Zeus IPRO, with two water treatments applied during the vegetative phase: control (C) and water deficit (D). The relative water content and number of nodules were reduced in the D plants. Ureide concentrations (allantoin and allantoic acid) were higher in nodules under D conditions. However, no differences were observed in allantoin, total ureide, and soluble sugar concentrations in leaves. Our results suggest that reducing the number of nodules may be a key strategy for maintaining BNF under drought conditions and that ureide accumulation could be the primary metabolic response in this soybean cultivar. These findings indicate that the effects of water restriction on BNF are likely associated with local metabolic responses rather than a systemic ureide feedback mechanism inhibiting BNF. Full article

The use of cover crops (CCs) based on tropical legumes, including Crotalaria ochroleuca, Crotalaria juncea, Crotalaria spectabilis, and Cajanus cajan, represents a pivotal aspect of agricultural rotations. These crops facilitate the incorporation of nitrogen through biological nitrogen fixation (BNF), thereby reducing the necessity for synthetic nitrogen fertilizers. Nevertheless, the capacity for the BNF of these species in Uruguay is relatively modest. To address this limitation, an approach is proposed that involves the immobilization of nitrogen in the soil using a highly energetic material, such as sucrose. The objective of this study was to examine the impact of incorporating sucrose into typical Uruguayan soil on aboveground dry matter production, nitrogen accumulation, and nitrogen fixation by legumes utilized as CCs. The experiments involved the planting of C. ochroleuca, C. juncea, C. spectabilis, and C. cajan in pots containing either soil alone or soil mixed with sucrose and the subsequent maintenance of these in a plant growth chamber for a period of 90 days. The addition of sucrose had a positive impact, with nearly double the aboveground dry matter production and nitrogen content observed. The percentage of nitrogen derived from the atmosphere (%Ndfa) increased significantly in all species, rising from an average of 83% to 96% in the sucrose-amended soil compared to the control. In the case of C. juncea, there was a notable threefold increase in aboveground dry matter and nitrogen accumulation across different treatments, accompanied by a 26% rise in %Ndfa and a fourfold increase in nitrogen fixation amounts. These findings indicate that C. juncea has the potential to significantly enhance performance and ecosystem services in typical Uruguayan soil. Full article

Biological nitrogen fixation (BNF) by symbiotic bacteria plays a vital role in sustainable agriculture. However, current quantification methods are often expensive and impractical. This study explores the potential of Raman spectroscopy, a non-invasive technique, for rapid assessment of BNF activity in soybeans. Raman spectra were obtained from soybean plants grown with and without rhizobia bacteria to identify spectral signatures associated with BNF. δN¹⁵ isotope ratio mass spectrometry (IRMS) was used to determine actual BNF percentages. Partial least squares regression (PLSR) was employed to develop a model for BNF quantification based on Raman spectra. The model explained 80% of the variation in BNF activity. To enhance the model’s specificity for BNF detection regardless of nitrogen availability, a subsequent elastic net (Enet) regularisation strategy was implemented. This approach provided insights into key wavenumbers and biochemicals associated with BNF in soybeans. Full article

Biological nitrogen fixation (BNF) can provide the necessary nitrogen for bean crops; however, for this to occur, important limitations involving the inoculant application technology need to be overcome.The use of co-inoculation is a management technique used to obtain benefits and increase the potential of N₂ fixation from the association between bacteria from the rhizobia group, such as R. tropici, and bacteria that promote plant growth, such as A. brasilense, in association with the addition of nutrients that allow greater efficiency of bacteria fixing atmospheric N₂. This study aimed to evaluate the bean response to the reinoculation of R. tropici in co-inoculation with A. brasilense in a mixture with the micronutrients Co/Mo, in the winter season of 2021, in Anápolis-GO, Brazil. A randomized block design was used, with four replications, and the following treatments (TRs) were studied: TR1—reinoculation with R. tropici; TR2—reinoculation with co-inoculation of R. tropici + A. brasilense; TR3—reinoculation of R. tropici + Mo/Co micronutrients; TR4—reinoculation with co-inoculation R. tropici + A. brasilense + Mo/Co micronutrients; TR5—inoculation via seed, without reinoculation; TR6—mineral N fertilization in the sowing furrow and topdressing; TR7—control, without any N source. At stage R6, nodulation characteristics (number and dry mass of nodules) and the morphophysiological parameters of the plants (main root length, root dry mass, plant height, shoot dry mass, leaf area, and leaf N content in the shoot) were evaluated. At harvest, the final plant stand and components (number of pods per plant, number of grains per pod, and average weight of one hundred grains) were determined, in addition to grain yield. It was concluded that inoculation followed by reinoculation in topdressing with R. tropici in co-inoculation with A. brasilense plus Mo/Co, compared to mineral nitrogen fertilization, improves the efficiency of the nodulation process and the morphophysiological characteristics of the common bean crop. Seed inoculation and topdressing application with R. tropici, associated with co-inoculation with A. brasilense + Mo and Co, have the potential to completely replace mineral nitrogen fertilization in common bean crops. Full article

Legume crops establish symbiosis with nitrogen-fixing rhizobia for biological nitrogen fixation (BNF), a process that provides a prominent natural nitrogen source in agroecosystems; and efficient nodulation and nitrogen fixation processes require a large amount of phosphorus (P). Here, a role of GmPAP4, a nodule-localized purple acid phosphatase, in BNF and seed yield was functionally characterized in whole transgenic soybean (Glycine max) plants under a P-limited condition. GmPAP4 was specifically expressed in the infection zones of soybean nodules and its expression was greatly induced in low P stress. Altered expression of GmPAP4 significantly affected soybean nodulation, BNF, and yield under the P-deficient condition. Nodule number, nodule fresh weight, nodule nitrogenase, APase activities, and nodule total P content were significantly increased in GmPAP4 overexpression (OE) lines. Structural characteristics revealed by toluidine blue staining showed that overexpression of GmPAP4 resulted in a larger infection area than wild-type (WT) control. Moreover, the plant biomass and N and P content of shoot and root in GmPAP4 OE lines were also greatly improved, resulting in increased soybean yield in the P-deficient condition. Taken together, our results demonstrated that GmPAP4, a purple acid phosphatase, increased P utilization efficiency in nodules under a P-deficient condition and, subsequently, enhanced symbiotic BNF and seed yield of soybean. Full article

Biological nitrogen fixation (BNF) can reduce synthetic N fertilizer application and improve N-use efficiency. However, knowledge about the effect of biochar and water management regimes on soil diazotrophic microorganisms in tropical paddy fields remains only rudimentary. A field trial was started in the early rice season in 2019 and ended in the late rice season in 2020. We studied the effects of five treatments comprising different water management and biochar applications on the diazotrophic abundance and community composition: no N fertilizer + conventional water management, conventional fertilization + conventional water management, no N fertilizer + flooding, conventional fertilization + flooding, and application of 40 t ha⁻¹ biochar + conventional fertilization + conventional water management. According to the results, biochar increased soil pH and organic carbon (SOC), whereas flooding decreased the soil available phosphorus (P) content. However, the addition of biochar and flooding as well as N application treatments increased nifH abundance. The nifH abundance negatively correlated with available N and P, whereas it significantly positively correlated with SOC (p < 0.05). The results of redundancy analysis unveiled that biochar stimulated the relative abundance of Pelomonas and changed the diazotrophic microbial community structure by increasing soil pH, while flooding stimulated the relative abundance of Azospirllum. Conclusively, both flooding and biochar affect soil diazotrophic microbial community and abundance in paddy fields. Reducing N and P fertilizer application clubbed with biochar amendment and flooding may be beneficial for soil N-fixing in tropical paddy fields. Full article