Search Results (27)

Developing effective rhizobial inoculants for high-latitude soybean production requires strains that are both well-adapted to local soil conditions and capable of sustaining symbiotic nitrogen fixation under abiotic stress. In this study, 66 indigenous rhizobial strains isolated from four ecological regions of Heilongjiang Province, China, i.e., the Songnen Plain, Sanjiang Plain, Northwest Arid Sandy Region, and Daxing’anling–Xiaoxing’anling mountainous area, were characterized for phenotypic diversity, abiotic stress tolerance, symbiotic nitrogen fixation capacity, and plant growth-promoting (PGPR) traits. Carbon and nitrogen source utilization, physiological and biochemical properties, and tolerance to salinity, pH extremes, temperature, and drought were assessed and analyzed by hierarchical cluster analysis. The strains showed habitat-associated phenotypic differentiation, with stress-tolerance traits accounting for the greatest proportion of variation. Isolates from the Northwest Arid Sandy Region displayed the broadest stress tolerance, while Sanjiang Plain isolates showed more conservative phenotypic profiles. Eight representative strains were subsequently evaluated for symbiotic performance and PGPR activity. Nodule dry weight, nitrogenase activity, and total plant nitrogen content were strongly intercorrelated and appeared more reliable indicators of nitrogen-fixation efficiency than nodule number alone. Songnen Plain strains SN1 and SN8 showed the highest symbiotic performance, while Northwest Arid Sandy Region strain FS1 exhibited the greatest ACC deaminase activity. A two-dimensional ranking framework integrating symbiotic and non-symbiotic traits ordered the strains as SN1 > SN8 > SJ1 > SJ11 > DX5 > DX1 > FS7 > FS1, identifying SN1 and SN8 as the most promising candidates requiring field validation for inoculant applications. These findings suggest that multi-trait evaluation frameworks may offer a practical and reproducible approach to screening indigenous rhizobia for region-specific biofertilizer applications in cold-region soybean agriculture. Full article

Rhizobial inoculation is an important strategy for improving nitrogen supply in soybean systems, but its field performance is often limited by soil constraints and interactions with indigenous microbiomes. Multifunctional microbial consortia may complement rhizobia by modifying rhizosphere soil conditions and nutrient availability. Here, we combined pot and one-season field experiments in a meadow albic black soil of the Sanjiang Plain, China, to evaluate the effects of rhizobia, a multifunctional bacterial consortium, and their co-inoculation (CRF) on soybean performance, soil properties, enzyme activities, and bacterial communities. CRF produced the highest soybean yield, reaching 2230 kg ha⁻¹, which was 6.74%, 1.54%, and 1.50% higher than conventional fertilization (F), single consortium inoculation (CF), and single rhizobial inoculation (RF), respectively. Compared with F, CRF showed higher values of selected short-term soil indicators at specific growth stages, including pH, soil organic matter, alkali-hydrolyzable nitrogen (+7.51–21.14%), and available phosphorus (+19.29–42.68%). At maturity, sucrase, urease, and acid phosphatase activities under CRF were 18.6%, 15.3%, and 22.4% higher than those under F, respectively. Bacterial OTU richness increased by 11.70% under CRF, whereas evenness-related indices decreased, suggesting treatment-associated bacterial community reassembly. Soil pH explained 11.63% and 13.24% of bacterial community variation at the phylum and genus levels, respectively. These findings suggest that rhizobia–consortium co-inoculation was associated with improved soybean yield, altered rhizosphere soil indicators, and bacterial community shifts during one soybean growing season. Longer-term field trials are needed to confirm the persistence and mechanisms of these effects. 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

Estimating indigenous rhizobial populations is crucial for understanding soil rhizobia abundance, determining the potential need for inoculation, and evaluating the performance of introduced inoculant strains. However, in southern Ethiopia, information on the population abundance of soybean-nodulating rhizobia is limited. To address this gap, the present study was conducted to evaluate the population abundance of indigenous soybean-nodulating rhizobia and to assess the influence of cropping history and soil properties on rhizobial abundance. The study was conducted across five sites suitable for soybean cultivation in southern Ethiopia: Arsi-Negelle, Boricha, Dore, Hawassa, and Wondo Genet. The study sites represented a range of cropping systems, including sole maize, sole tobacco, sole haricot bean, maize–potato intercropping, and crop rotation. Composite soil samples were collected from a depth of 0–20 cm, and rhizobial abundance was determined using the most probable number (MPN) technique. Indigenous rhizobial populations ranged from 0 to 1.7 × 10¹ cells g⁻¹ of dry soil. Overall, the population levels were low, suggesting that inoculation with effective rhizobial strains would likely improve nodulation and biological nitrogen fixation. Relatively higher rhizobial population densities were observed at Arsi-Negelle under haricot bean cropping history. Statistically significant positive correlations were found between rhizobial abundance and cation exchange capacity, organic carbon, and organic matter. In general, native rhizobial populations across all study locations were below levels considered sufficient to support effective soybean nodulation and nitrogen fixation, indicating the need for inoculation to enhance soybean productivity in the study areas. Full article

Soil fertility constraints, particularly N and P deficiencies, limit the productivity of common bean (Phaseolus vulgaris L.) in Ethiopia. This study is the first to systematically evaluate the combined effects of phosphorus (P) fertilization and Rhizobium inoculation on biological nitrogen fixation (BNF) and yield across soils with varying levels of indigenous rhizobia populations in southern Ethiopia. The aim of the study was to evaluate the effects of P fertilization and Rhizobium inoculation on nodulation, growth, yield, nutrient concentration, and BNF in soils characterized by high, moderate, and low indigenous rhizobia populations. Field experiments were conducted over two years using four P rates (0, 10, 20, and 30 kg P ha⁻¹) and five Rhizobium strains (uninoculated, 102CB, 106CB, 44CB, and HB-429). P application significantly improved nodulation, growth, yield, nutrient concentration, and N₂ fixation, with 20 kg P ha⁻¹ consistently resulting in superior performance. At this rate, grain yields reached 2.51, 2.25, and 2.31 t ha⁻¹ in soils with high, moderate, and low indigenous rhizobia populations, respectively. Inoculation responses depended strongly on indigenous rhizobia abundance: in soils with high indigenous populations, inoculation did not significantly improve growth and yield, whereas in low-population soils, inoculation with strain 102CB produced the highest yield (2.06 t ha⁻¹). For BNF, 20 kg P ha⁻¹ resulted in the highest fixation (44.61 and 36.82 kg N ha⁻¹) in soils with high and moderate indigenous rhizobia populations. Inoculation with 102CB further enhanced N fixation to 44.77 and 36.13 kg N ha⁻¹ in these soils. In low-population soils, the combined application of 102CB and 20 kg P ha⁻¹ significantly increased BNF to 55.34 kg N ha⁻¹. Overall, these findings demonstrate that P fertilization provides universal benefits, while inoculation effectiveness is site-specific. Integrating P fertilization with effective Rhizobium strains offers a practical and sustainable strategy to improve common bean productivity and sustainability of common bean–based farming systems in Ethiopia. Full article

Our previous studies identified a new efficient and broad-spectrum rhizobium strain Bradyrhizobium elkanii Y63-1. This study evaluated the symbiotic effects of Y63-1 inoculation on Zhongdou 63 (ZD63) in native environments and under different nitrogen levels. The evaluation of symbiotic effects in native environments was conducted through pot experiments and field trials. Pot experiments were performed in greenhouse using three soil types. Field trials were conducted in three regions with different soil nitrogen levels. The symbiotic effect of soybean ZD63 inoculated with Y63-1 under different nitrogen levels was investigated through pot experiments in greenhouse. The results showed that Y63-1 is more competitive than the indigenous rhizobia of the three soil types in the nodulation of soybean ZD63. The nodulation ability and yield-related traits of soybean ZD63 were improved after inoculation with Y63-1 in the three regions, especially in Hanchuan, where the soil nitrogen level is relatively rich. The symbiotic effect of soybean ZD63 inoculated with Y63-1 in a pot experiment with four levels of N from 0 to 3.75 mmol/L was superior at N 2.81 mmol/L. Our findings provided technical support for the application of Y63-1 in China, and a theoretical basis for increasing the yield potential of soybean through inoculation with highly efficient rhizobia in agricultural production. Full article

Rhizobia have been used for decades as biopreparations, successfully replacing synthetic nitrogen fertilizers in legume cultivation. They have a beneficial effect on the growth and yield of these plants when cultivated in soils that are deficient in both nitrogen and indigenous rhizobia. However, such preparations, containing strains that are characterized by high effectiveness in reducing atmospheric dinitrogen, are not universal. Their use is ineffective when plants are grown in soils that are already rich in strains with low effectiveness, because such inoculant strains are unable to effectively compete with native soil populations. This review discusses issues related to the rhizobia–legume symbiosis, with particular emphasis on inter-strain competition occurring in the soil and in the colonized plant tissues. The importance of Nod factors (NFs) in symbiosis and their broad impact on plant physiological and developmental processes are also discussed. Research results on the effects of NF-containing biopreparations on legume growth and yield are summarized. Moreover, this review explains how such preparations can support the growth and yield of legumes growing in soils containing numerous populations of low-effectiveness rhizobia. Finally, the potential for the application of this technology to non-legume plants is presented. Full article

The agronomic effectiveness of biofertilizers is influenced by strain origin, genetic identity, crop genotype, soil type, and environmental conditions. For best results, both the plant and rhizobia strain must be adapted to the common harsh soil conditions in the tropics. While plant varieties have changed over the years, complementary research on new strains effectiveness under varying soil fertility conditions has lagged in southern Africa. Seven field experiments were established in the main soybean-producing areas of Zimbabwe in the north, central, and north–east regions to evaluate agronomic benefits of new rhizobia strains against the current exotic commercial strain (MAR1491). One site was irrigated (site 3), and the other six sites were rainfed (sites 1, 2, 4, 5, 6, and 7). While trends in inoculation response varied from site to site due to site conditions, inoculation with the strains NAZ15, NAZ25, and NAK128 consistently yielded high grain yields, which were similar to the current commercial strain MAR1491 and to application of mineral fertilizer (51.75 and 100 kg N ha⁻¹). Grain yield levels were generally below 2 t ha⁻¹ for sites 2, 3, and 5 and above 2 t ha⁻¹ for sites 1, 4, and 6, while for the irrigated site 3, they ranged upwards of 3 t ha⁻¹. When irrigated, all strains except NAK9 performed similarly in terms of grain yields and aboveground N uptake. Further testing on the inclusion of the indigenous strains NAZ15, NAZ25, and NAK128 in multi-strain commercial inoculant production targeting application in regions and soils where they excel beyond the current exotic strain MAR1491 is recommended. 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

Increasing soil salinity is threatening agricultural productivity which implies the development of new sustainable strategies to deal with this challenge. The main objective here is to assess the potential for improving the tolerance of alfalfa to salinity by combining inoculations with rhizobia and AMF. However, the distinguishing feature of this study is the comparison of two alfalfa varieties’ microbial response to salinity. The greenhouse trial was conducted on an Australian variety Siriver and an indigenous Demnate population, which were inoculated with Rhizoglomus irregulare and/or native AMF, and/or a RhOL1 rhizobial strain. The RhOL1 strain was selected from nine rhizobia tested for their plant growth promoting rhizobacteria (PGPR) activities. In addition to its ability to tolerate high salinity levels (769 mM) and solubilize insoluble phosphate as well as potassium, it can also synthesize auxins such as IAA. The application of these biofertilizers was carried out in the absence and the presence of the saline stress (0 and 120 mM NaCl). The double inoculations of native AMF and RhOL1 significantly improve the shoot and root dry biomass, plant elongation, number of formed leaves, and mineral nutrition, as well as the number of nodules and the rate of mycorrhizal root colonization. The synergistic effects between the native AMF and RhOL1 strain have been demonstrated in this study. However, the behavior of alfalfa genotypes towards microbial inoculation was significantly different. The ability to react to the double indigenous RhOL1 + AMF inoculation is more important in the Siriver than in the Demnate population. Thus, the possibility of formulating biofertilizers is based on the AMF–rhizobia–hote tripartite combination for alfalfa production in saline areas. Full article

(1) Background: Rhizobia can promote plant growth by providing essential nutrients such as NH₄⁺ and PO₄³⁻; thus, rhizobia that can tolerate the stress of heavy metals will be conducive to the phytoremediation of heavy-metal-contaminated soils. Therefore, understanding the dominant heavy-metal-tolerant rhizobia that can be cultured is important for the establishment of an indigenous legume–rhizobia symbiotic remediation system; (2) Methods: Here, we investigated nifH gene diversity in culturable rhizobia from black locust (Robinia pseudoacacia L.) grown in cadmium (Cd)-contaminated soils using high-throughput sequencing.; (3) Results: A total of 16 genera and 26 species were identified from the cultures of root nodules of black locust exposed to five Cd levels. Cadmium did not show a significant effect on the abundance, diversity, and evenness of the culturable rhizobia community. However, Cd significantly affected the community structure of culturable rhizobia containing nifH. Mesorhizobium, Sinorhizobium, and Rhizobium were the absolute dominant genera present in the cultures under five Cd treatments. Additionally, Cd significantly affected the relative abundance of Azohydromonas, Xanthobacter, Skermanella, Bradyrhizobium, Paenibacillus, and Pseudacidovorax in the cultures. Soil pH, total Cd, DTPA-Cd, and C/H ratio were the significant factors on culturable rhizobia community.; (4) Conclusions: Cd showed a negative effect on nifH gene community of culturable rhizobia from black locust, which will provide insight into the selection of excellent strains that can promote phytoremediation of heavy-metal-contaminated soils. Full article

The interplay between soil rhizobial bacteria and leguminous plants, particularly in Africa, has a profound impact on photosynthetic efficiency and overall crop productivity. This review explores the critical role of rhizobia in enhancing photosynthesis through nitrogen fixation, a process crucial for sustainable agriculture. Rhizobial bacteria residing in root nodules provide legumes with symbiotic nitrogen that significantly boosts plant growth and photosynthetic capacity. Recent advances in molecular genomics have elucidated the genetic frameworks underlying this symbiosis, identifying key genes involved in root nodule formation and nitrogen fixation. Comparative genomics of Bradyrhizobium species have revealed seven distinct lineages, with diverse traits linked to nodulation, nitrogen fixation, and photosynthesis. Field studies across Africa demonstrate that rhizobial inoculation can markedly increase nodulation, nitrogen fixation, and grain yields, though outcomes vary depending on local soil conditions and legume species. Notable findings include enhanced nutrient uptake and photosynthetic rates in inoculated legumes compared with nitrate-fed plants. This review highlights the potential of utilizing indigenous rhizobia to improve photosynthesis and crop resilience. Future prospects involve leveraging genomic insights to optimize rhizobial inoculants and enhance legume productivity in water-limited environments. As climate change intensifies, integrating these advancements into agricultural practices could play a crucial role in improving food security and sustainable soil health in Africa. Full article

In pea (Pisum sativum L.), alleles of the Sym2 gene determine the specificity of the interaction with nodule bacteria (rhizobia). The Sym2^A allele present in landraces from Afghanistan provides higher selectiveness toward rhizobia than the Sym2^E allele present in European cultivars. Rhizobial strains possessing the nodX gene can interact with both Sym2^A and Sym2^E peas, while strains lacking nodX can interact only with Sym2^E peas. Here, we studied the previously obtained introgression line A33.18 bearing Sym2^A in a homozygous state in the genome of the European pea cultivar ‘Rondo’. A33.18 has proved its high selectiveness in pot experiments. Genome sequencing has shown that A33.18 possesses an 18.2 Mb region inherited from Afghanistan pea with 63 genes, including 5 receptor kinase genes, among which was the Sym2 candidate gene LykX. In a field experiment, under inoculation with the nodX⁺ strain TOM, over 95% of nodules of A33.18 contained TOM, as opposed to less than 8% of nodules containing TOM in the parental European cultivar ‘Rondo’. Thus, introgression of Sym2^A enabled peas to interact specifically with the nodX⁺ strain, favoring the formation of nodules by the strain from the inoculum and protecting peas from the indigenous soil microbiota. Full article

Restoring native plant species on degraded landscapes is challenging. Symbiotic partners in the plant rhizosphere can aid in nutrient acquisition, pathogen protection, stress tolerance, and many other processes. However, these microbes are often absent in altered landscapes and need to be re-integrated to improve restoration efforts. We evaluated, within a laboratory setting, the ability of commercial and indigenous rhizobia strains to form nodules on lupine species used for rangeland seedings in the Great Basin region of the Western United States and ascertained if these strains could be applied through a seed coating. We also evaluated if a compost amendment applied via seed coating could further enhance the performance of the rhizobia strains. Our analysis showed that successful nodulation could occur using commercial and wildland-collected indigenous strains through either a liquid culture applied to seedlings or as a dry seed coating. However, the number of root nodules and the presence of a pink color (indicating nitrogen fixation) were typically higher in the commercial product than in the indigenous strains. Compost did not improve nodulation or the performance of the nodules; however, this treatment alone improved shoot growth. Overall, these results suggest that commercial rhizobium may be more effective in improving plant growth, and future research with native rhizobia may want to consider identifying strains compatible with seed-coating delivery. Longer-term studies are now merited for assessing how the rhizobia strains evaluated in this study influence plant growth, particularly in a field setting. Full article

Chickpea (Cicer arietinum L.), encompassing the desi and kabuli varieties, is a beloved pulse crop globally. Its cultivation spans over fifty countries, from the Indian subcontinent and southern Europe to the Middle East, North Africa, the Americas, Australia, and China. With a rich composition of carbohydrates and protein, constituting 80% of its dry seed mass, chickpea is also touted for its numerous health benefits, earning it the title of a ‘functional food’. In the past two decades, research has extensively explored the rhizobial diversity associated with chickpea and its breeding in various countries across Europe, Asia, and Oceania, aiming to understand its impact on the sustainable yield and quality of chickpea crops. To date, four notable species of Mesorhizobium—M. ciceri, M. mediterraneum, M. muleiense, and M. wenxiniae—have been reported, originally isolated from chickpea root nodules. Other species, such as M. amorphae, M. loti, M. tianshanense, M. oportunistum, M. abyssinicae, and M. shonense, have been identified as potential symbionts of chickpea, possibly acquiring symbiotic genes through lateral gene transfer. While M. ciceri and M. mediterraneum are widely distributed and studied across chickpea-growing regions, they remain absent in China, where M. muleiense and M. wenxiniae are the sole rhizobial species associated with chickpea. The geographic distribution of chickpea rhizobia is believed to be influenced by factors such as genetic characteristics, competitiveness, evolutionary adaptation to local soil conditions, and compatibility with native soil microbes. Inoculating chickpea with suitable rhizobial strains is crucial when introducing the crop to new regions lacking indigenous chickpea rhizobia. The introduction of a novel chickpea variety, coupled with the effective use of rhizobia for inoculation, offers the potential not only to boost the yield and seed quality of chickpeas, but also to enhance crop productivity within rotation and intercropped systems involving chickpea and other crops. Consequently, this advancement holds the promise to drive forward the cause of sustainable agriculture on a global scale. Full article