Search Results (72)

The benefits of legume-nitrogen-fixing bacteria symbioses are vital in agricultural systems globally. Cross-infectivity studies are important for identifying rhizobial strains with potential for use as inoculants. The native rhizobial isolates inoculated on different legume species are the first step to determining host range and ecological adaptive traits. This study reports on the water-use efficiency and mineral nutrition of diverse legume species cross-inoculated by native rhizobial isolates from Eswatini, Ghana and South Africa under glasshouse conditions. A portable infrared red gas analyzer was used for water use efficiency. Data from a gas exchange study shows that rhizobial strains can significantly influence the photosynthetic functioning of their host plants. As a result, photosynthetic rates differed depending on bacterial compatibility with the host plant, as well as its symbiotic efficacy. Isolate TUTGmGH2 induced greater accumulation of P, K, Mg, Zn, Cu and Mn in soybean and Winged bean, clearly suggesting that rhizobia do have an influence on the mineral nutrition of their host plants. Therefore, these findings further show that native rhizobial isolates can be manipulated to enhance mineral nutrient uptake, promote growth and development and also produce nutrient-dense food with a low environmental impact globally since rhizobia do have an influence on the mineral nutrition of their host plants. Full article

Mediterranean-type ecosystems (METs) occur on five continents and represent some of the most climatically constrained yet biologically rich regions on Earth. In these environments, legumes and their nitrogen-fixing rhizobial symbionts—including widely distributed genera such as Rhizobium, Bradyrhizobium, and Ensifer—play a pivotal role in sustaining plant productivity, nutrient cycling, and ecosystem resilience. This review synthesizes current knowledge on the environmental regulation of legume–Rhizobium symbiosis specifically within Mediterranean-type ecosystems, focusing on how nitrogen (N) and phosphorus (P) availability, light conditions, and carbon allocation trade-offs shape symbiotic performance across the five Mediterranean-type regions of the world (California, central Chile, the Cape Region of South Africa, southwestern Australia, and the Mediterranean Basin). By integrating physiological, ecological, and biogeochemical perspectives, we highlight how the shared features of these regions—strong seasonal drought, chronic nutrient limitation (particularly P in southwestern Australia and the Cape Region), recurrent fires, and exceptionally high plant diversity—constrain and, at the same time, favor the ecological success of symbiotic legumes. Throughout the review, we use case studies from key legume genera such as Lupinus in Chile and southwestern Australia, Virgilia and other Cape legumes in South Africa, Acacia in Australian kwongan and woodlands, and Medicago and Cytisus in the Mediterranean Basin and California to illustrate how general principles of legume–Rhizobium ecology manifest under Mediterranean-type climatic and edaphic constraints. Beyond summarizing established mechanisms, we critically examine the limitations of current metagenomic approaches, which often provide descriptive inventories of soil microbial communities without linking microbial composition to functional outcomes. We argue that advancing the field requires integrated, hypothesis-driven research that combines multi-omic tools with plant eco-physiology, soil nutrient dynamics, and temporal replication. Finally, we outline key priorities for future research, including the integration of functional ‘omics’, the study of microbiome interactions beyond rhizobia, the development of predictive models for Mediterranean-type ecosystems under climate change, and the application of symbiotic principles to restoration and agroecological management. By bridging molecular, physiological, and ecosystem perspectives, this review provides a conceptual framework for understanding and enhancing legume–Rhizobium symbiosis across five continents in a rapidly changing world. Full article

Climate change threatens crop yields and farming profitability, especially in drought-prone regions, requiring a transition to climate-resilient farming systems. Concurrently, growing demand for health-promoting and bio-based materials is creating new market opportunities for farmers. Sulla (Sulla coronaria Medik; syn. Hedysarum coronarium L.), a Mediterranean forage crop, may represent a strategic resource for sustainable intensification by simultaneously providing high-value commodities and a wide range of ecosystem services. This review explores the multifunctional potential of sulla following a holistic approach and is structured in thematic chapters, exploring: i. agronomy, ii. ecosystem services and agroecological value, iii. plant biochemical profile, iv. emerging applications for the bio-based industry, v. genetic diversity (including rhizobia diversity) and breeding perspectives for target environments and end-use. A SWOT analysis synthesizes strengths, research gaps and bottlenecks hindering large-scale adoption and valorization. The review proposes a strategic framework matching research priority with specific, actionable goals. The review aims to increase awareness of the multifaceted value of sulla as a promising model legume to increase sustainability in agriculture, promote product diversification and farming profitability, while assuring important ecosystem benefits. Full article

Medicago ruthenica L., a superior forage crop within the genus Medicago (Fabaceae), is endowed with remarkable stress tolerance and an abundance of bioactive compounds, conferring significant ecological and forage value. Existing reviews primarily focus on a single research direction, and the most recent findings are dated, failing to cover breakthroughs at the molecular level. This paper systematically synthesizes the latest research progress in five key areas: genetic diversity and genomic studies, biotic stress responses, abiotic stress tolerance mechanisms (drought, salinity, and low temperature, etc.), utilization (including genetic breeding, ecological restoration, and forage development), and future research prospects. This review addresses critical gaps in existing literature, particularly regarding advances in genomic sequencing, biotic stresses, and research on stress-associated microorganisms. Research indicates that M. ruthenica exhibits extensive genetic diversity, and its genome contains numerous positive selection signals associated with stress resistance. It can tolerate multiple abiotic and biotic stresses through morphoplasticity, physiological metabolic regulation, and transcriptional regulation. Furthermore, its symbiosis with microorganisms such as rhizobia significantly enhances its stress tolerance. M. ruthenica demonstrates outstanding application potential in degraded grassland restoration and high-quality forage production. Future research should focus on mining stress-resistant genes, optimizing molecular breeding techniques, and integrating artificial intelligence into breeding practices. That will facilitate its transformation from a regional endemic resource to a commercially viable functional species, thereby providing robust support for ecological security and the sustainable development of grassland-based livestock husbandry in cold and arid regions. Full article

Background: The adaptability of leguminous plant–rhizobia symbionts enables enhanced plant stress tolerance in environmentally stressed areas. However, how rock desertification (RD) severity affects the endophytic and nitrogen-fixing bacterial communities in Pisum sativum root nodules remains unclear. Methods: We systematically surveyed the microbial communities of P. sativum nodules across a gradient of four RD areas. We sequenced 16S rRNA and nifH amplicons, determined soil physicochemical properties, and performed bioinformatic analyses to relate nodule microbiome diversity to soil variables. Results: The dominant endophytic genera across all sites were Allorhizobium–Neorhizobium–Pararhizobium–Rhizobium and Pseudomonas, with Rhizobium identified as the primary nitrogen-fixing taxon. Soil pH and total phosphorus (TP) showed significant correlations with the overall endophytic bacterial community, whereas total nitrogen (TN), TP, and soil water content (SWC) were associated with nitrogen-fixing taxa. Notably, P. sativum nodules from areas of slight rocky desertification (SRD) harbored higher endophytic bacterial diversity and enhanced carbohydrate metabolism compared to those from moderately rocky desertified (MRD) sites. Conclusions: This study sheds light on how bacterial communities within legume root nodules respond to RD stress, deepening our understanding of plant–microbe co-adaptation and informing microbial-assisted restoration strategies in karst desertification areas. Full article

Silicon has long been recognized as a beneficial element in plant biology. Recent advances in nanosilicon technology have revealed its transformative potential in legume-rhizobia symbiosis. This review synthesizes current knowledge on how silicon and SiO₂ nanoparticles (Si-NPs) influence nodulation, microbial metabolism, and soil–plant interactions. We highlight emerging evidence that Si-NPs enhance symbiotic signaling, strengthen infection pathways, and mitigate oxidative stress, thereby supporting nitrogen fixation efficiency. Beyond the rhizosphere, nanosilicon improves soil structure, microbial diversity, and plant resilience under abiotic stress, offering a multifaceted approach to sustainable agriculture. The novelty of this review lies in its integrative perspective, connecting molecular mechanisms with ecological impacts and climate-smart applications. By examining Si-NPs across three domains—soils, rhizosphere metabolites, and plants—we provide a framework for understanding their role in enhancing productivity while reducing environmental costs. Importantly, we identify critical research gaps, including the need for standardized application protocols, large-scale field validation, sustainable nanosilicon production, and robust regulatory frameworks. These insights position nanosilicon as a promising tool for advancing legume productivity, reducing reliance on synthetic fertilizers, and contributing to global food security. This review underscores silicon’s potential not only as a plant nutrient but also as a strategic agent in climate-resilient agriculture. Full article

Rhizobia are a polyphyletic group of Proteobacteria comprising approximately 700 different species. Despite significant advancements in their taxonomy, evolutionary history, and ecological importance, substantial knowledge gaps remain regarding a detailed understanding of rhizobial biodiversity in a geographical context and the interest in studying and valorizing native rhizobial strains. This bibliometric study used data from the last four decades (1985–2025) to review the taxonomic and functional diversity of the documented novel taxa of rhizobia described from African ecosystems, as well as their valorization status as biofertilizers. It aims to discuss the interest in knowing, preserving, and valorizing native rhizobial resources in the global context of climate change and biodiversity erosion. The study revealed that the first African indigenous novel species of rhizobia was published in 1988, although research on rhizobia dates back to the 1950s in Africa. To date, ~63 species (approximately 9% of the total in the world) and two genera of rhizobia have been described using native isolates from 11 African countries, with substantial discoveries from the Succulent Karoo hotspot of biodiversity in South Africa. Approximately 51% of species are affiliated with Bradyrhizobium and Mesorhizobium, with Vachellia karroo and Senegalia spp. (formerly Acacia spp.) as their primary hosts. Most species-type strains (~89%) were found to be infective on legumes and are good candidates for biofertilizer development. However, there is a limited level of commercial valorization of indigenous isolates as inoculants, mainly because the production of biological intrants is still at the experimental stage in Africa. Interestingly, important breaking point discoveries have been made using native rhizobial strains from Africa, including the pioneering demonstration in 2001 that Burkholderia (beta-rhizobia) is a symbiotic genus with legumes. It also includes the discovery of stem-nodulating rhizobia and Nod factor-independent symbiotic processes in some rhizobia. Together, this review highlights the importance of native African rhizobial strains. This underscores the need to accelerate their agronomic valorization to better support the transition to more resilient and sustainable legume-based farming systems in African countries. Full article

Microbiomes play a pivotal role in sustaining plant function and broader ecosystem processes. Leguminous plants host vast populations of intracellular bacteria within specialized root organs known as nodules. The intricate mutualism between legumes and rhizobia ensures a stable supply of biologically fixed nitrogen (N) essential for plant growth. While rhizobia remain the central actors in this symbiosis, recent discoveries reveal the presence of non-rhizobial endophytes within nodules, suggesting a complex interplay shaped by host selection and compatibility with rhizobial partners. Understanding the structure and dynamics of crop nodule-associated microbial communities is thus critical for optimizing host responses to rhizobia and for leveraging beneficial plant–microbe interactions. This review explores the dualistic nature—both facilitative and inhibitory—of the nodule microbiome in relation to nodulation. We examine the diversity of soil bacteria that stimulate nodulation and those that ultimately colonize nodule tissues, questioning whether these functional groups overlap. Furthermore, we discuss the molecular dialogs and counter-signaling mechanisms that regulate endophyte ingress into nodules, and evaluate how nodule endophytes contribute to plant performance and soil fertility. Full article

Selecting symbiotic rhizobia for use as inoculants in agriculture is a major challenge, though it is necessary for exploiting biological nitrogen fixation as an eco-friendly source of N in contrast to chemical N fertilizers which can pollute the environment. In addition to high symbiotic efficiency, bacterial strain ability to infect and effectively nodulate a wide range of host plants is also desired. Cross-infectivity studies are therefore important for identifying rhizobial strains that are highly effective with a broad host range. The legume/rhizobia symbiosis has the potential to contribute about 80% or more N to agricultural systems, thus providing a sustainable source of N in cropping systems. This study assessed the cross-nodulation, colony morphology, relative symbiotic effectiveness and N₂ fixation of native rhizobial isolates from Africa that nodulate diverse legume species. The results showed that the rhizobial isolates differed significantly in symbiotic performance and relative symbiotic effectiveness. As a result, they differed markedly in nodulation and shoot DM induced in their host plants. Full article

Effective rhizobium–legume symbiosis depends on multiple molecular signaling pathways, integrating not only classical nodulation factors and surface polysaccharides but also diverse protein secretion systems. Among them, the Type VI Secretion System (T6SS) has emerged as a key player, due to its dual roles in interbacterial competition and interactions with eukaryotic hosts, though its contribution to symbiosis remains unclear. Key regulatory messengers, including the main autoinducer of the quorum sensing (QS) systems, the N-acyl homoserine lactones (AHLs), and the second messenger cyclic di-GMP (c-di-GMP), modulate the transition between motility and biofilm formation, especially in the context of bacteria interacting with eukaryotes, including rhizobia. While c-di-GMP’s impact on exopolysaccharide production in these organisms is well established, its influence on protein secretion systems, particularly in conjunction with QS, is largely unexplored. To contribute to the study of such interplay, we artificially increased intracellular c-di-GMP levels by overexpressing a heterologous diguanylate cyclase in three Sinorhizobium fredii strains of agronomic relevance. This engineering revealed strain-specific outcomes, since elevated c-di-GMP enhanced biofilm development in two strains, but reduced it in another. Furthermore, using β-galactosidase expression assays, we confirmed that both high c-di-GMP and/or AHL concentrations contribute to the transcriptional activation of T6SS. These results demonstrate a direct regulatory link between c-di-GMP, QS signals, and T6SS expression, shedding light on the multilayered control mechanisms that structure beneficial rhizobia–plant interactions. Full article

Drought is a significant abiotic stressor affecting crops globally. Beneficial microorganisms, such as rhizobia, have been shown to enhance crop resilience to such stresses. In this study, we isolated a highly efficient rhizobacterial strain (Rhizobium sp. PV-6) from the root system of Phaseolus vulgaris and systematically investigated the phenotypic and physiological responses of the plants across seven growth stages under four treatments: W-NO (watering without inoculation of rhizobium), W-RHI (watering with inoculation of rhizobium), D-NO (drought without inoculation of rhizobium), and D-RHI (drought with inoculation of rhizobium). We also examined the variation in microbial communities in rhizosphere and root compartments. Physiological analyses revealed that rhizobium inoculation significantly enhanced plant height, fresh weight and dry weight, root length, lateral root number, and nodule number of red kidney beans. Alpha diversity analysis suggested that the microbial communities in the roots and rhizosphere of red kidney beans show different variant distributions. Beta diversity and species difference analysis revealed that drought treatments (D-NO, D-RHI) recruit Shinella, Nocardioides, Agromyces, Pseudomonas, and Ensifer at rhizosphere compartments, while D-RHI enrich Pseudomonas, Sphingobacterium, Paenibacillus, Bacillus, Massilia, and Lysobacter at root compartments in the T5 stage. Further, transcriptomic analysis revealed that PV-6 enhances drought tolerance in red kidney beans by modulating the expression of genes associated with abiotic stress-related genes. Our findings highlight the potential of Rhizobium sp. PV-6 as a bioinoculant for improving drought tolerance in red kidney beans (Phaseolus vulgaris), providing a foundation for designing synthetic microbial communities for crop stress resilience. Full article

Soybean (Glycine max) is a protein-rich legume crop that plays an important role in achieving food security. The aim of this study was to isolate soybean-nodulating rhizobia from Côte d’Ivoire soils and evaluate their potential as efficient strains in order to develop local bioinoculants. For this objective, 38 composite soil samples were collected from Côte d’Ivoire’s five major climatic zones. These soils were used as substrate to trap the nodulating rhizobia using the promiscuous soybean variety R2-231. A total of 110 bacterial strains were isolated and subsequently identified. The analysis of ITS (rDNA16S-23S), glnII and recA sequences revealed a relatively low genetic diversity of these native rhizobia. Moreover, the ITS phylogeny showed that these were scattered into two Bradyrhizobium clades dominated by the B. elkanii supergroup, with ca. 75% of all isolates. Concatenated glnII-recA sequence phylogeny confirmed that the isolates belong in the majority to ‘B. brasilense’, together with B. vignae and some putative genospecies of Bradyrhizobium that needs further elucidation. The core gene phylogeny was found to be incongruent with nodC and nifH phylogenies, probably due to lateral gene transfer influence on the symbiotic genes. The diversity and composition of the Bradyrhizobium species varied significantly among different sampling sites, and the key explanatory variables identified were carbon (C), magnesium (Mg), nitrogen (N), pH, and annual precipitation. Based on both shoot biomass and leaf relative chlorophyll content, three isolates consistently showed a higher symbiotic effectiveness than the exotic inoculant strain Bradyrhizobium IRAT-FA3, demonstrating their potential to serve as indigenous elite strains as bioinoculants. 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

Guar (Cyamopsis tetragonoloba L. Taub.) is a significant summer legume used as food for both humans and livestock. In Saudi Arabia, the root nodule bacteria of guar have not been studied. The present work investigated the phenotypic and genetic diversity of guar microsymbionts. Eighty-eight bacterial strains were isolated from the root nodules of guar grown in different locations of Jazan region of Saudi Arabia. The strains were analyzed based on their phenotypic characteristics and variations in their 16S rRNA gene sequences. A significant proportion of the isolates (90%) were fast-growing rhizobia, with 77% showing tolerance to 3–4% NaCl and 91% capable of thriving at temperatures reaching 40 °C. Several isolates exhibited strong plant growth-promoting traits, particularly in IAA production and phosphate solubilization. Genetic analysis indicated considerable diversity, with isolates classified under the genera Rhizobium, Ensifer, Mesorhizobium, Bradyrhizobium, and Agrobacterium. To the best of our knowledge, this study is the first to report on the phenotypic and genetic diversity of guar microsymbionts in Saudi Arabia. Full article

Peanuts are widely cultivated across the world; however, peanut’s rhizobial community and the determinant factors of their composition are still to be elucidated. This study investigates the biogeography and determinant soil environmental factors for peanut rhizobia. A total of 1001 rhizobial isolates were obtained from the peanut root nodules, mainly belonging to two cultivars (X9 and M6) cultivated in 20 sampling sites across China. According to recA sequence analysis, all the isolates were classified as 84 haplotypes, and a representative strain for each haplotype was randomly selected to perform subsequent analyses. Based on multilocus sequence analysis (MLSA) of housekeeping genes dnaK, glnII, gyrB, recA, and rpoB, all the representative strains were classified as 42 genospecies in the genus Bradyrhizobium, including 12 effectively published and 30 undefined genospecies. Strains belonging to six genospecies were predominant (>5%), including B. ottawaense, B. liaoningense, B. yuanmingense, Bradyrhizobium sp. XXIX, B. guangdongense, and B. nanningense. However, only a single isolate was obtained for 15 genospecies. The diversity indices of peanut rhizobia distributed in South China are obviously higher than those in North China, but no obvious peanut cultivar selection for rhizobial genospecies was found. Correlation analyses indicated that the community composition of peanut rhizobia was mainly affected by MAP, MAT, soil AP, and pH. Nodulation tests indicated that the 79 representative strains belonging to 37 genospecies with both nodC and nifH could perform nitrogen-fixing symbiosis with peanuts. This study revealed the great diversity and varied composition of communities of peanut rhizobia in different geographic regions across China. Full article