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Background and aims: Drought is one of the main causes of global crop decline. Plant growth-promoting rhizobacteria enhance plant tolerance to adverse environmental conditions. This study aimed to determine whether the rhizobacteria Microvirga vignae (BR 3296 and BR 3299) and Bradyrhizobium sp. (BR 3301) can maintain cowpea growth under drought stress. Methods: We analyzed biomass, nodulation, nitrogen accumulation, and physiological traits of the inoculated plants. Rhizobacterial strains were assessed for exopolysaccharide (EPS) and indole acetic acid (IAA) production, growth, and biofilm formation in a water-stress medium induced by polyethylene glycol (PEG)-6000. The expression of genes associated with abscisic acid (ABA) biosynthesis in root nodules was also investigated. Results: All evaluated strains were grown in a culture medium supplemented with PEG. M. vignae strains exhibited increased biofilm formation and EPS production, while Bradyrhizobium showed high IAA production. Cowpea plants inoculated with Bradyrhizobium exhibit higher levels of nodulation, biomass, and nitrogen accumulation. Conversely, M. vignae strains were more efficient at alleviating drought stress and maintaining nodulation, biomass, nitrogen accumulation, and stomatal conductance similar to well-watered plants. Drought-inducible genes were more strongly upregulated in the nodules of plants inoculated with Bradyrhizobium than in those inoculated with M. vignae. Conclusion: Our results suggest that M. vignae strains, isolated from a semi-arid region, help plants withstand water-stress, whereas the strain of Bradyrhizobium sp. isolated from a wet region did not effectively alleviate drought stress. However, Bradyrhizobium sp. conferred growth and nitrogen accumulation to cowpea superior to M. vignae and like plants supplied with nitrogen fertilizer. [ABSTRACT FROM AUTHOR]

Aims: Our objective was to examine short-term responses of A.sinensis agronomic, soil productivity to agricultural management (i.e. amino acid (AAs) and nitrogen-fixing bacteria (NFB) as alone or co-applicationed exogenous nitrogen (N) after cover planting). Methods: A field experiment was established with four fertilization treatments, including NFB (Y1), NFB and AAs (Y2), AAs (Y3), and sterile water (CK). The plant yield, biomass, N and C content, photosynthesis and the soil chemical properties, microbial community and metabolite spectrum were evaluated. Results: Y2 improved the crop biomass by 32–66% than CK, Y1 and Y3. It also decreased soil C/N by 24.29% than CK. 16S rRNA and ITS genes revealed that Y2 markedly enhanced the growth of copiotrophic taxa (Alloprevotella, Bactriodies and Alistipes) while inhibiting the growth of oligotrophic taxa (Sphingomonas and Ellin6055), among the dominant bacteria. Y2 reduced the growth of pathogenic taxa (Thanatephorus and Fusarium) that are encouraged by AAs. Further, AAs and NFB addition directly affected the diversity of bacteria and fungi and indirectly affected fungal diversity by altering soil total organic carbon (TOC), total nitrogen (TN), C/N. Additionally, soil metabolomics analysis revealed that Y3 elevated the concentration of chemical compounds that impede microbial growth, while decreasing fructose, N-Acetylglutamic acid, and arginine involved in C and N metabolism. Y2 augmented the content of disease-resistant compounds and lysine in N metabolism. Conclusion: Inoculating AAs with NFB (Bacillus amyloliquefaciae and Pseudomonas fluorescence) is a strategy with immense benefits as a biofertilizer for sustainable A.sinensis production, which also renders soil ecological services. [ABSTRACT FROM AUTHOR]

Aims: To understand the species composition, abundance and diversity of nitrogen-fixing bacteria in the Cenchrus fungigraminus rhizosphere and to screen nitrogen-fixing bacteria to study their potential role in plant growth promotion. Methods: Soil were collected from 4 depths (G1, G2, G3 and G4) of the C. fungigraminus rhizosphere, and their physical and chemical properties were determined. The diversity and abundance of nitrogen-fixing bacteria and the nifH gene was analysed. Nitrogen-fixing bacteria were screened and selected to promote growth of C. fungigraminus seedlings. Results: The highest diversity and abundance of nitrogen-fixing bacteria was observed in the G2 samples collected from the C. fungigraminus rhizosphere. These bacteria mainly included Proteobacteria (93.91%), Actinobacteria (0.42%), and Firmicutes (0.18%) and were significantly affected by total nitrogen, available nitrogen and soil depth. The nifH gene copy number was highest (1.56 ± 0.17 × 107copies/g) in G2. Rhizobium pusense No. 8 and No. 28 were isolated from G1 and G2, with nitrogenase activities of 145.06 ± 4.10 and 199.78 ± 7.50 U/L. The promotion experiment revealed that the plant height, root length, and leaf length of C. fungigraminus seedlings treated with both strains significantly increased by 56.79%, 76.99% and 55.71%, and the soil moisture and total nitrogen were significantly increased compared to the control (P < 0.05). The available nitrogen, organic matter and organic carbon in soil with strain No. 28 significantly increased compared to CK. Conclusion: Rhizobacteria in the C. fungigraminus rhizosphere play an important role as plant growth promoting rhizobacteria (PGPR). Two strains showed potential for the development of biological fertilizers. [ABSTRACT FROM AUTHOR]

Background and Aims: Mutualistic root symbioses, particularly those involving mycorrhizal fungi and nitrogen-fixing bacteria, are pivotal to ecosystem productivity and stability. Plant-soil feedbacks (PSFs) and climate serve as primary regulators of these symbiotic interactions, determining their establishment, maintenance, and diversity. PSFs, encompassing the complex interactions between plants and soil biota, modulate nutrient uptake and ultimately influence plant growth and development. Climate not only shapes the abundance, composition and performance of soil biota, but also directly impacts the distribution and response of plant symbioses to environmental shifts including rising temperatures and modified precipitation patterns.This review compiles recent advancements in the ecology and diversity of mycorrhizal and nitrogen-fixing associations, emphasizing the interaction between soil biota and climate, and their implications for ecosystem functions in the context of climate change. It also identifies key gaps in our understanding, such as the molecular mechanisms at play, the genetic variability involved, and the impact of global environmental changes on symbiotic networks.Addressing these questions is essential for a more profound comprehension of the complex plant-soil dynamics that sculpt terrestrial ecosystems.Results: Mutualistic root symbioses, particularly those involving mycorrhizal fungi and nitrogen-fixing bacteria, are pivotal to ecosystem productivity and stability. Plant-soil feedbacks (PSFs) and climate serve as primary regulators of these symbiotic interactions, determining their establishment, maintenance, and diversity. PSFs, encompassing the complex interactions between plants and soil biota, modulate nutrient uptake and ultimately influence plant growth and development. Climate not only shapes the abundance, composition and performance of soil biota, but also directly impacts the distribution and response of plant symbioses to environmental shifts including rising temperatures and modified precipitation patterns.This review compiles recent advancements in the ecology and diversity of mycorrhizal and nitrogen-fixing associations, emphasizing the interaction between soil biota and climate, and their implications for ecosystem functions in the context of climate change. It also identifies key gaps in our understanding, such as the molecular mechanisms at play, the genetic variability involved, and the impact of global environmental changes on symbiotic networks.Addressing these questions is essential for a more profound comprehension of the complex plant-soil dynamics that sculpt terrestrial ecosystems.Conclusion: Mutualistic root symbioses, particularly those involving mycorrhizal fungi and nitrogen-fixing bacteria, are pivotal to ecosystem productivity and stability. Plant-soil feedbacks (PSFs) and climate serve as primary regulators of these symbiotic interactions, determining their establishment, maintenance, and diversity. PSFs, encompassing the complex interactions between plants and soil biota, modulate nutrient uptake and ultimately influence plant growth and development. Climate not only shapes the abundance, composition and performance of soil biota, but also directly impacts the distribution and response of plant symbioses to environmental shifts including rising temperatures and modified precipitation patterns.This review compiles recent advancements in the ecology and diversity of mycorrhizal and nitrogen-fixing associations, emphasizing the interaction between soil biota and climate, and their implications for ecosystem functions in the context of climate change. It also identifies key gaps in our understanding, such as the molecular mechanisms at play, the genetic variability involved, and the impact of global environmental changes on symbiotic networks.Addressing these questions is essential for a more profound comprehension of the complex plant-soil dynamics that sculpt terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: Straw amendment can increase nitrogen fixation in paddy field, however, the efficiency of carbon sources with different biochemical properties to enhance N2 fixation and nitrogen fixation activity is still unclear. Methods: A 15N2-labelling system was used in the field environment to determine biological nitrogen fixation (BNF) under the addition of three kinds of straw. The nifH gene (DNA) and nifH RNA gene (cDNA) of soil were amplified by real-time fluorescent quantitative PCR. The diversity and community composition of nitrogen fixing microorganisms were studied by high-throughput sequencing. The study is expected to reveal how different carbon sources impact biological nitrogen fixation and its mechanism in the paddy field system. Results: Results showed that the absolute abundance diazotrophs in the treatment of mature stage wheat straw (MWS) was 4.88 times as high as that in the CK treatment, but jointing stage wheat straw (JWS) and poplar branch (PB) did not induce any significant changes. Straw amendment had no impact on cyanobacteria abundance. The proportion of N2 fixation increased by MWS was 2.07 times, but which was much lower than the increase proportion of the heterotrophic diazotrophs, leading to a decrease of diazotrophic nitrogen fixation activity. Conclusions: Mature wheat straw addition increased biologically fixed nitrogen in paddy field by increasing the number of heterotrophic nitrogen fixing bacteria. The results indicated that to increase biological N2 fixation in paddy system, straws with low nitrogen content and high C/N ratio were recommended. [ABSTRACT FROM AUTHOR]

Background: Straw return may affect the soil nitrogen (N) balance by changing soil carbon (C) and N components and contents. To understand how straw return affects the soil N fixation capacity, the relationship between biological N fixation and changes in soil C and N contents at different depths need to be examined. Methods: Soil samples were collected at 20-cm intervals from 0 to 100 cm after five years of a rice-rape rotation system with straw return (ST) and without (CK). Organic C and inorganic N contents, potential N fixation rate, and abundance and population composition of N-fixing bacteria were analyzed at different soil depths. Results: Compared with CK, ST increased soil organic C (SOC), particulate organic C (POC), mineral-associated organic C (MOC), and total N (TN) but decreased nitrate–N (NO3−-N). Straw return also increased the abundance of N-fixing bacteria from 2.9- to 18-fold at depths from 0 to 40 cm and increased the potential N fixation rate at 0–20 cm by 25%. The annual biological N fixation rates at 0–60 cm by soil N-fixing bacteria were estimated to be approximately 0.13–2.70 and 0.12–2.16 kg·ha−1 in ST and CK, respectively. Long-term ST significantly affected the composition of N-fixing bacteria at depths from 0 to 40 cm. Conclusions: Long-term straw return can increase the abundance of the N-fixing bacteria and effectively alter the structure of the N-fixing bacterial community. SOC, POC, TN, and NO3−-N were the main environmental factors driving changes in abundance, activity, and population composition of soil N-fixing bacteria under ST. [ABSTRACT FROM AUTHOR]

Aims: Silicon (Si) uptake and accumulation improves plant resilience to environmental stresses, but most studies examining this functional role of Si have focussed on grasses (Poaceae) and neglected other important plant groups, such as legumes (Fabaceae). Legumes have evolved a symbiotic relationship with nitrogen-fixing bacteria (rhizobia) housed in root nodules. Our study determined the impacts of silicon (Si) supplementation on Medicago truncatula inoculated with Ensifer meliloti rhizobial strains that differed in their capacity for nitrogen fixation: Sm1021 ('low-efficiency') or Sm1022 ('high-efficiency'). Methods: We examined how Si and rhizobial efficacy influence nodule and plant functional traits, including their chemical aspects. These combinations were supplied with or without Si in a glasshouse experiment, where we quantified nodule flavonoids and foliar chemistry (free amino acids, soluble protein, elemental C, N and Si). Results: Si supply increased nodule number per plant, specific nodule flavonoid concentrations, contents of foliar nitrogenous compounds and foliar C, but not foliar Si. We also demonstrated that rhizobial efficacy altered the magnitude of Si effects on certain traits. For example, Si significantly promoted concentrations of foliar N and soluble protein in the plants associated with the 'low-efficiency' strain only, and this was not the case with the 'high-efficiency' one. Conclusions: Collectively, our study indicates that Si generates positive effects on M. truncatula, particularly when the association with rhizobia is relatively inefficient, and may play a more prominent role in rhizobial functionality than previously thought. [ABSTRACT FROM AUTHOR]

Purpose: In cold climates, glyphosate residues may linger in soils, with effects on plant–microbe interactions and, consequently, plant performance. Here, we explore the influence of glyphosate residues on the endophytic microbiota (bacteria and fungi) and performance of the perennial nitrogen-fixing weed Lupinus polyphyllus. Methods: In a common garden, we grew plants from six populations of L. polyphyllus in glyphosate-treated or untreated control soils, with or without additional phosphorus. We sampled plant microbiota (leaves, roots, nodules) and assessed plant performance based on six traits: height, retrogression probability (i.e. shrinkage), biomass, root:shoot ratio, nodule number, and nodule viability. Results: The richness of plant endophytic microbial communities was determined by soil phosphorus level rather than by glyphosate treatment. However, for bacteria, the composition of these communities differed between glyphosate-treated and control soils across plant tissue types; no difference was observed for fungi. The plant bacterial communities in both soil types were dominated by potential nitrogen-fixing bacteria belonging to family Bradyrhizobiaceae, and particularly so in glyphosate-treated soils. Overall, though, these changes in plant bacterial communities had a minor effect on plant performance: the only difference we detected was that the probability of retrogression was occasionally higher in glyphosate-treated soils than in control soils. Conclusion: Our findings indicate that glyphosate-based herbicides, when applied at the recommended frequency and concentration, may not have critical effects on the growth of short-lived weeds after the safety period has passed; however, the endophytic microbiota of such weeds may experience longer-lasting shifts in community structure. [ABSTRACT FROM AUTHOR]

Background and aims: Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB) can form symbiotic relationships with host plants, and this enhances nutrient uptake and plant performance of host plants. However, how AMF and NFB influence the complementarity and facilitation dynamics between grasses and legumes in grasslands is still not well understood.We conducted a greenhouse experiment to assess effects of different grass: legume ratios (GLRs) and microbial symbionts on plant production comprising two native C3 species from a semiarid steppe in northern China (grass (G): Leymus chinensis; legume (L): Medicago ruthenica). Different GLRs (G4L0: 4:0, G3L1: 3:1, G2L2: 2:2, G1L3: 1:3, G0L4: 0:4) were compared with or without AMF or NFB inoculation.Grass–legume mixtures, especially G1L3, produced significantly higher biomass than monocultures via altering the community structure of soil microorganisms. AMF inoculation increased the availability of nutrients (mainly soil available P (SAP) and/or NO3−–N), which increased plant community productivity due to increased biomass in both grass and legume. Co-occurrence networks revealed that keystone genera (e.g., Arenimonas, Dominikia, Claroideoglomus and Scutellospora) were the primary factors influencing plant community productivity among grass–legume mixtures. Structural equation model showed that GLRs and inoculation types consistently produced overyielding directly and indirectly via affecting SAP and NO3−–N concentrations.Our findings highlight that suitable grass: legume mixture ratios (e.g., 1:3) positively affect productivity, and soil microorganisms that improve soil resource acquisition (SAP and NO3−–N), especially AMF, play an important role in grass: legume mixtures in this process.Methods: Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB) can form symbiotic relationships with host plants, and this enhances nutrient uptake and plant performance of host plants. However, how AMF and NFB influence the complementarity and facilitation dynamics between grasses and legumes in grasslands is still not well understood.We conducted a greenhouse experiment to assess effects of different grass: legume ratios (GLRs) and microbial symbionts on plant production comprising two native C3 species from a semiarid steppe in northern China (grass (G): Leymus chinensis; legume (L): Medicago ruthenica). Different GLRs (G4L0: 4:0, G3L1: 3:1, G2L2: 2:2, G1L3: 1:3, G0L4: 0:4) were compared with or without AMF or NFB inoculation.Grass–legume mixtures, especially G1L3, produced significantly higher biomass than monocultures via altering the community structure of soil microorganisms. AMF inoculation increased the availability of nutrients (mainly soil available P (SAP) and/or NO3−–N), which increased plant community productivity due to increased biomass in both grass and legume. Co-occurrence networks revealed that keystone genera (e.g., Arenimonas, Dominikia, Claroideoglomus and Scutellospora) were the primary factors influencing plant community productivity among grass–legume mixtures. Structural equation model showed that GLRs and inoculation types consistently produced overyielding directly and indirectly via affecting SAP and NO3−–N concentrations.Our findings highlight that suitable grass: legume mixture ratios (e.g., 1:3) positively affect productivity, and soil microorganisms that improve soil resource acquisition (SAP and NO3−–N), especially AMF, play an important role in grass: legume mixtures in this process.Results: Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB) can form symbiotic relationships with host plants, and this enhances nutrient uptake and plant performance of host plants. However, how AMF and NFB influence the complementarity and facilitation dynamics between grasses and legumes in grasslands is still not well understood.We conducted a greenhouse experiment to assess effects of different grass: legume ratios (GLRs) and microbial symbionts on plant production comprising two native C3 species from a semiarid steppe in northern China (grass (G): Leymus chinensis; legume (L): Medicago ruthenica). Different GLRs (G4L0: 4:0, G3L1: 3:1, G2L2: 2:2, G1L3: 1:3, G0L4: 0:4) were compared with or without AMF or NFB inoculation.Grass–legume mixtures, especially G1L3, produced significantly higher biomass than monocultures via altering the community structure of soil microorganisms. AMF inoculation increased the availability of nutrients (mainly soil available P (SAP) and/or NO3−–N), which increased plant community productivity due to increased biomass in both grass and legume. Co-occurrence networks revealed that keystone genera (e.g., Arenimonas, Dominikia, Claroideoglomus and Scutellospora) were the primary factors influencing plant community productivity among grass–legume mixtures. Structural equation model showed that GLRs and inoculation types consistently produced overyielding directly and indirectly via affecting SAP and NO3−–N concentrations.Our findings highlight that suitable grass: legume mixture ratios (e.g., 1:3) positively affect productivity, and soil microorganisms that improve soil resource acquisition (SAP and NO3−–N), especially AMF, play an important role in grass: legume mixtures in this process.Conclusion: Arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing bacteria (NFB) can form symbiotic relationships with host plants, and this enhances nutrient uptake and plant performance of host plants. However, how AMF and NFB influence the complementarity and facilitation dynamics between grasses and legumes in grasslands is still not well understood.We conducted a greenhouse experiment to assess effects of different grass: legume ratios (GLRs) and microbial symbionts on plant production comprising two native C3 species from a semiarid steppe in northern China (grass (G): Leymus chinensis; legume (L): Medicago ruthenica). Different GLRs (G4L0: 4:0, G3L1: 3:1, G2L2: 2:2, G1L3: 1:3, G0L4: 0:4) were compared with or without AMF or NFB inoculation.Grass–legume mixtures, especially G1L3, produced significantly higher biomass than monocultures via altering the community structure of soil microorganisms. AMF inoculation increased the availability of nutrients (mainly soil available P (SAP) and/or NO3−–N), which increased plant community productivity due to increased biomass in both grass and legume. Co-occurrence networks revealed that keystone genera (e.g., Arenimonas, Dominikia, Claroideoglomus and Scutellospora) were the primary factors influencing plant community productivity among grass–legume mixtures. Structural equation model showed that GLRs and inoculation types consistently produced overyielding directly and indirectly via affecting SAP and NO3−–N concentrations.Our findings highlight that suitable grass: legume mixture ratios (e.g., 1:3) positively affect productivity, and soil microorganisms that improve soil resource acquisition (SAP and NO3−–N), especially AMF, play an important role in grass: legume mixtures in this process. [ABSTRACT FROM AUTHOR]

Aims: Revealing genetic diversity in a root nodulation symbiosis under field conditions is critical to understand the formation of ecological communities of organisms associated with hosts and the nitrogen cycle in natural ecosystems. However, our knowledge of the genetic diversity of bacterial mutualists on a local scale is still poor because of the assumption that the genetic diversity of mutualistic bacteria is constrained by their hosts. Methods: We thoroughly investigated the genetic diversity of Frankia in a local forest stand. We collected root nodules from 213 Alnus hirsuta seedlings covering the spatial range of the continuous population, which means that Alnus individuals occurred in a relatively homogeneous distribution in a continuous forest. Then, a phylogenetic and diversity analysis was performed for the nifD-K IGS region, including global Frankia sequences from Alnus hosts. Results: The genetic diversity of Frankia detected even on a local scale measured as high as that shown by previous studies conducted on local and regional scales. Moreover, a genetic structure analysis revealed a spatially mosaic-like distribution of genetic variation in Frankia despite the small spatial scale. Conclusions: The genetic diversity and composition of bacterial mutualists are heterogeneous on a local scale. Our findings demonstrate that genetically different bacterial symbionts simultaneously interact with a single host population and interaction partnerships spatially vary. The standing variation could produce dynamic ecological and evolutionary outcomes in a heterogeneous forest ecosystem. [ABSTRACT FROM AUTHOR]

Background: Replacing synthetic fertilizers by biological nitrogen fixation (BNF) is regarded as an environmentally sound practice, but some diazotrophic bacteria are capable of emitting N2O by denitrification. The ability to use nitrate represents an ecological advantage for the survival of some microorganisms under O2-limiting conditions, but may contribute to increased N2O emissions. Scope: The importance of denitrification performed by N2-fixing bacteria used as inoculants in South America is discussed, especially the possibility of these bacteria act as N2O source or sink. Conclusions: There is no doubt of the importance of BNF as a sustainable N source for plants. Through genome investigation, we demonstrated that some strains widely used as inoculants for BNF harbor the entire denitrification pathway to reduce nitrate to N2. Others contain none, or only some of the denitrification genes, resulting in complete absence of denitrification or production of intermediates such as NO2−, NO or N2O. Evidence of differential effects of bacterial strains on soil N2O were reported, but more studies are still needed to affirm crop inoculation can be a driver for source or sink of this gas. Finally, considerations were made about BNF as an indispensable resource to indirectly mitigate greenhouse gas emissions in agroecosystems. [ABSTRACT FROM AUTHOR]

Aims: Several Sarracenia species are endangered due to habitat destruction. Knowledge of their microbiomes may better inform reintroduction efforts. We characterized bacterial communities within rhizomes of three species with an emphasis on diazotrophs. Methods: Microbiomes of wild and greenhouse-grown Sarracenia were characterized using cultivation-independent and cultivation-dependent methods, including Illumina MiSeq, 16S rRNA and nifH qPCR, colony identification via 16S rRNA sequencing, and nifH colony PCR. Rhizomes were tested for biological nitrogen fixation using acetylene reduction assays. Results: Diazotrophs make up a notable proportion of the S. oreophila microbiome according to qPCR. Microbiomes of greenhouse S. oreophila varied from wild ones by having greater proportions of diazotrophs as indicated by qPCR. Wild and greenhouse plant microbiomes also differed with respect to several phylogenetic groups. Acetylene reduction assays indicated that nitrogen fixation occurred in some rhizomes. Multiple colonies from S. leucophylla and S. purpurea were nifH+, and 16S rRNA sequencing likewise revealed dissimilarities between wild and greenhouse-grown plant microbiomes. Conclusion: Our study is the first to characterize bacterial communities within Sarracenia rhizomes and demonstrates that substantial differences exist between wild and greenhouse plants. We provide evidence that Sarracenia can contain large populations of diazotrophs that may help explain how these plants survive in oligotrophic environments. [ABSTRACT FROM AUTHOR]

Background and aims: The adaptation of plants to land ecosystems involves complex rhizosphere interactions between organic matter and microbial communities. Border cells (BC) constitute the first living boundary in plant-soil ecosystems and play an important role in environmental sensing and signaling in response to different biotic and abiotic conditions. In this study, we evaluate the effect of humic acid on the release of BCs and its impact on the colonization of Herbaspirillum seropedicae at maize root tips. Methods: Maize seedlings (1.0 ± 0.05 cm root length) were immersed for 48 h in solutions with different concentrations of humic acid (0, 12, 42, 143 and 500 mg L). Light and scanning electron microscopy were used to evaluate the structural interaction between border cells and H. seropedicae at the root tips. Results: The release of BCs from root tips was significantly increased by humic acid (HA) application and exhibited a bell-shaped dose-response curve; the highest release of BCs occurred at 143 mg HA L and was confirmed by microscopy. The colonization of roots by H. seropedicae strain RAM10 (tagged with green fluorescent protein, GFP) was monitored by epifluorescence microscopy with and without exogenous humic acid (143 mg L). Increased BC release resulted in a high density of diazotrophic bacteria at root tips, and bacteria sometimes aggregated with mucilage and humic acid particles, thus enhancing their viability. Increased BC numbers in response to humic acid might explain previous studies showing a concomitant increase in H. seropedicae populations in the rhizosphere, rhizoplane, and endosphere of grasses. Conclusions: The population of H. seropedicae strain RAM10 colonizing root caps and BCs increased in response to exogenous humic acids. [ABSTRACT FROM AUTHOR]

Background and aims: A major problem in all restoration and rehabilitation projects is to restore the function of the ecosystem. Nitrogen, next to water, is the most limiting factor for productivity of arid terrestrial ecosystems. Methods: We used a successful restored area, completed 10 years earlier, in comparison with an undisturbed scrubland area and a remaining, disturbed area in the southern Sonoran Desert in Baja California, Mexico. We compared the abundance of the nifH gene, estimated by qPCR, potential N fixation activity by acetylene reduction assay, and diversity of diazotrophs by denaturing gradient gel electrophoresis in the rhizosphere of the most representative plant species, the cardon cactus and the mesquite used for restoration. Results: The abundance of N-fixing bacteria in the rhizosphere of cardon growing with mesquite had significantly higher abundance of nifH gene than the rhizosphere of cardon that grew separately. Across all samples, the potential N fixation was significantly higher in soil samples from the restored site than samples from the undisturbed and disturbed sites. Conclusions: Successful long-term restoration improved the potential N fixation to a level similar to undisturbed lands. Beneficial interactions between cardon and mesquite are a promising venue for desert reforestation by their contribution to improve N fixing potential in degraded arid lands and increasing the population of diazotrophs. [ABSTRACT FROM AUTHOR]

Background and aims: Our recent research revealed that several key enzymes involved in flavonoid biosynthesis played vital roles in soybean's tolerance to salinity. Since the flavonoids also act as important signals mediating the establishment of symbiosis between rhizobium and leguminous plants, the Sinorhizobium meliloti 1021 was tested to find out whether it had any impact on soybean's tolerance to salt stress. Methods: The roots of soybean seedling ( Glycine max cultivar Union85140) were inoculated by soak in diluted S. meliloti 1021 suspension and then the seedlings were transferred in sphagnum peat and pearlite soil. Five days after inoculation, the seedlings were treated with salt solutions every four days until harvest. Results: Soybean seedlings inoculated with S. meliloti 1021 demonstrated significantly stronger lodging-resistance and the fresh weight of the whole plants increased by 10.95-30.95 % compared with that of the non-inoculated plants. These changes in soybean seedlings might be resulted from: 1. S. meliloti 1021 playing a role in helping soybean plant to exclude Na while absorb K, reducing ionic stress caused by salts; 2. S. meliloti 1021 enhancing the antioxidant enzymes' activities on reducing oxidative stress caused by salts; 3. S. meliloti 1021 increasing the content of osmotic compounds, thus reducing osmotic stress caused by salts. In addition, S. meliloti 1021 played significant roles in regulating the transcription of several key enzymes related to flavonoids metabolism (cytochrome P450 monooxygenase, chalcone synthase and chalcone isomerase), ROS scavenging (catalases, ascorbate peroxidase, glutathione S-transferase and superoxide dismutase), and other salt-responsive genes (Stress-induced protein SAM22, PR10-like protein and Phosphatidylinositol-specific phospholipase C) in soybean seedlings. Conclusion: These results showed that S. meliloti 1021 significantly enhanced soybean's ability to adapt to saline soil, mostly due to its effect on flavonoids metabolism in plants. [ABSTRACT FROM AUTHOR]

Aims: In the semi-arid region of Bahia State, Brazil, sisal ( Agave sisalana Perrine ex Engelm) has been cultivated for fibre production for several decades without the use of chemical fertilizers. The contribution of biological nitrogen fixation (BNF) to this crop and the occurrence of diazotrophic rhizosphere, epiphytic or endophytic bacteria have not been investigated. The aims of this work were to study the occurrence and diversity of diazotrophic bacteria in rhizosphere soil, root and leaf tissues of sisal plants from this region, as well as to test their potential for plant growth promotion. Methods: Burk's nitrogen free semi-solid medium was used for isolation and the MPN method was used for quantification of diazotrophic bacteria. BOX-A1R PCR and 16S rRNA sequence analyses were performed to study the diversity of bacterial isolates harboring the nifH-gene. Six isolates were selected for growth promotion of cucumber plants. Results: A high diversity of BNF bacterial isolates was observed in sisal plants and rhizosphere soil. Bacterial populations were higher in roots, followed by soil and sisal leaves. Burkholderia, Leifsonia and Paenibacillus were the predominant genera. All isolates tested were able to promote cucumber growth. Conclusions: This study showed that sisal-associated diazotrophic bacteria are diverse and may be further exploited to promote plant growth. [ABSTRACT FROM AUTHOR]

Background and aim: Nitrogen-fixing bacteria or diazotrophs have been isolated for many years using different formulations of N-free semi-solid media. However, the strategies used to isolate them, and the recipes of these media, are scattered through the published literature and in other sources that are more difficult to access and which are not always retrievable. Therefore, the aim of this work was to collate the various methods and recipes, and to provide a comprehensive methodological guide and their use by the scientific community working in the field of biological nitrogen fixation (BNF), particularly with non-leguminous plants. Methods: Procedures used for bacterial counting and identification either from rhizosphere soil or on the surface of, or within, plant tissues (to access 'endophytic' bacteria) are presented in detail, including colony and cell morphologies. More importantly, appropriate recipes available for each N-free semi-solid culture medium that are used to count and isolate various diazotrophs are presented. Results: It is recognized by those working in the field of BNF with non-legumes that the development of the N-free semi-solid medium has allowed a tremendous accumulation of knowledge on the ecology and physiology of their associated diazotrophs. At least 20 nitrogen-fixing species have been isolated and identified based on the enrichment method originally developed by Döbereiner, Day and collaborators in the 70's. In spite of all the advances in molecular techniques used to detect bacteria, in most cases the initial isolation and identification of these diazotrophs still requires semi-solid media. Conclusions: The introduction of the N-free semi-solid medium opened new opportunities for those working in the area of BNF with non-legumes not only for elucidating the important role played by their associated microorganisms, but also because some of these bacteria that were isolated using semi-solid media are now being recommended as plant growth-promoting inoculants for sugarcane ( Saccharum sp.), maize ( Zea mays) and wheat ( Triticum aestivum) in Brazil and other countries. Further progress in the field could be made by using a combination of culture-independent molecular community analyses, in situ activity assessments with probe-directed enrichment, and isolation of target strains using modified or standard semi-solid media. [ABSTRACT FROM AUTHOR]

Background and aims: Azospirillum brasilense, which has the potential to stimulate plant growth, belongs to the group of plant growth-promoting bacteria. The lectin found on the surface of A. brasilense strain Sp7 has the ability to bind specific carbohydrates and ensures adhesion of the bacteria to the root surface. The aim of this work was to investigate possible inductive effects of the Sp7 lectin on the plant cell signal systems. Methods: Enzyme-linked immunosorbent assay, spectrophotometry, and thin-layer and gas-liquid chromatography were used to determine the content of signal intermediates in the cells of wheat root seedlings. Laser scanning confocal microscopy was used to examine the localization of fluorescently labeled lectin on the plant cell. Results: The Sp7 lectin acted on the signal system components in wheat seedling roots by regulating the contents of cAMP, nitric oxide, diacylglycerol, and salicylic acid, as well as by modifying the activities of superoxide dismutase and lipoxygenase. The revealed cell membrane localization of the lectin is of deciding importance for its signal function. Conclusions: The results of the study suggest that the A. brasilense Sp7 lectin acts as a signal molecule involved in the interaction of growth-promoting rhizobacteria with plant roots. [ABSTRACT FROM AUTHOR]

Background and aims: Polyamines are cationic molecules that play an important role in the plant response to environmental stresses. The aim of this work is to determine the role of these compounds in the response to salinity of Medicago sativa plants in symbiosis with the soil bacteria Sinorhizobium meliloti. Methods: M. sativa plants inoculated with S. meliloti were subjected to 100 and 150 mM NaCl treatments. The concentration of nodular polyamines was determined in relation to the nitrogen fixation parameters, proline accumulation, and oxidative damage. In addition, polyamines concentrations were analyzed in different nodular fractions as well as the effect of exogenous polyamines in the nodulation response. Results: The concentration of nodular polyamines decreased by the salinity in correlation with the nitrogenase activity after 2 and 4 weeks of salt treatment while spermine accumulated after 6 weeks. On the contrary, proline accumulation was induced by the salinity at all time points. The analysis of different nodular fractions showed the highest polyamines concentration in bacteroids being homospermidine the most abundant. Conclusion: Proline accumulation had prevalence over polyamines at the earliest response to salinity probably due to nitrogen limitation under salt stress conditions and the existence of a common precursor for both compounds in the nodule. Nevertheless, after long salt exposure, spermine was also accumulated. The analysis of different nodular fractions indicated the bacteroidal origin of polyamines in nodules being homoespermidine, one of the most abundant. [ABSTRACT FROM AUTHOR]

Background and aims: Legume species in the fynbos vegetation of the Cape Floristic Region, that fix N 2 in soils with low P, may have evolved for enhanced acquisition and efficient use of P. It was hypothesized that N 2-fixing and combined-N supplied (N-supplied) A. linearis, P. calyptrata and C. genistoides are adapted to low P and would be relatively unresponsive to increased P of 100 μM. Methods: 18 legume species were evaluated for their nodulation response to low P availability. The N X P interaction was then examined in A. linearis, P. calyptrata and C. genistoides reliant on either N 2-fixation or 300 μM N (NH 4NO 3), and receiving 0.1, 1.0, 10 and 100 μM P (NaH 2PO 4). Results: In the species selection experiment, A. linearis, P. calyptrata and C. genistoides, with the greatest nodule fresh weight (FW) and nodule FW to root FW ratio, were the most prolific nodulating species. In the N X P experiment, with low P supply, the biomass of N 2-fixing P. calyptrata and C. genistoides was consistently greater than that of N-supplied plants. In contrast, with high P supply of 100 μM P, all N-supplied plants accumulated more biomass than the corresponding N 2-fixing plants. High P-use efficiency, poor down-regulation of P uptake and P storage was evident in A. linearis and P. calyptrata. Conclusion: The growth response to P and the significant N X P interactions indicate that N 2-fixing and N-supplied plants were not adapted to low P, but rather colimited by both N and P. [ABSTRACT FROM AUTHOR]

Aims: Previous studies have shown that elephant grass is colonized by nitrogen-fixing bacterial species; however, these results were based on culture-dependent methods, an approach that introduces bias due to an incomplete assessment of the microbial community. In this study, we used culture-independent methods to survey the diversity of endophytes and plant-associated bacterial communities in five elephant grass genotypes used in bioenergy production. Methods: The plants of five genotypes of elephant grass were harvested from the experimental area of Embrapa Agrobiologia and divided into stem and root tissues. Total DNA and RNA were extracted from plant tissues and the bacterial communities were analyzed by DGGE and clone library of the 16S rRNA and nifH genes at both the cDNA and DNA levels. Results: Overall, the patterns based on DNA- and RNA-derived DGGE-profiles differed, especially within tissue samples. DNA-based DGGE indicated that both total bacterial and diazotrophic communities associated with roots (rhizoplane + endophytes) differed clearly from those obtained from stems (endophytes). These results were confirmed by the phylogenetic analyses of RNA-derived sequences of 16S rRNA (total bacteria; 586 sequences), but not for nifH (186). In fact, rarefaction analyses showed a higher diversity of diazotrophic organisms associated with stems than roots. Based on 16S rRNA sequences, the clone libraries were dominated by sequences affiliated to members of Leptotrix (12.8 %) followed by Burkholderia (9 %) and Bradyrhizobium (6.5 %), while most of the nifH clones were closely related to the genus Bradyrhizobium (26 %). Conclusions: Our results revealed an unexpectedly large diversity of metabolically active bacteria, providing new insights into the bacterial species predominantly found in association with elephant grass. Furthermore, these results can be very useful for the development of new strategies for selection of potential bacteria that effectively contribute to biological nitrogen fixation and enhance the sustainable production of elephant grass as bioenergy crop. [ABSTRACT FROM AUTHOR]

Background and aims: Acacia cyclops is an invasive species within Mediterranean ecosystems, characteristically low in soil nutrients. Thus associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhiza (AM) may provide an advantage to these legumes. This study investigated the role of AM and NFB in the growth and nutritional physiology of A. cyclops. Methods: Seedlings were inoculated with naturally occurring NFB, Glomus mosseae or both, and grown under glasshouse conditions for 5 months. Plants were cultivated in sand and supplied with a 20 % strength nutrient solution. Xylem sap nutrients, photosynthetic rates, biomass and chemical compositions, were recorded. Results: The dual inoculation decreased the colonization of both symbionts, compared to a single symbiosis with either symbiont. Despite low colonization levels, the dual symbiosis increased host biomass and relative growth rates. This was associated with increased photosynthetic rates and enhanced nutrition. Additionally, dual symbiotic plants had enhanced N and P acquisition and utilization rates. Xylem sap analysis showed higher levels of NH being exported from the roots to the shoots in the dual symbiotic plants compared with other treatments. Conclusions: These findings suggest the dual symbiosis is an important factor in the growth and development of A. cyclops under nutrient limiting conditions. [ABSTRACT FROM AUTHOR]

Soil salinity is the major cause limiting plant productivity worldwide. Nitrogen-fixing bacteria were enriched and characterised from roots of Salicornia brachiata, an extreme halophyte which has substantial economic value as a bioresource of diverse and valuable products. Nitrogen-free semisolid NFb medium with malate as carbon source and up to 4% NaCl were used for enrichment and isolation of diazotrophic bacteria. The isolates were tested for plant growth-promoting traits and 16S rRNA, nifH and acdS genes were analysed. For selected strains, plant growth-promoting activities were tested in axenically grown Salicornia seedlings at different NaCl concentrations (0-0.5M). New halotolerant diazotrophic bacteria were isolated from roots of S. brachiata. The isolates were identified as Brachybacterium saurashtrense sp. nov., Zhihengliuella sp., Brevibacterium casei, Haererehalobacter sp., Halomonas sp., Vibrio sp., Cronobacter sakazakii, Pseudomonas spp., Rhizobium radiobacter, and Mesorhizobium sp. Nitrogen fixation as well as plant growth-promoting traits such as indole acetic acid (IAA) production, phosphate solubilisation, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity were demonstrated. For Brachybacterium saurashtrense and Pseudomonas sp., significant plant growth-promoting activities were observed in Salicornia in salt stress conditions. Salicornia brachiata is a useful source of new halotolerant diazotrophic bacteria with plant growth-promoting potential. [ABSTRACT FROM AUTHOR]

The essential oil produced by vetiver can vary in amount and composition depending on the bacterial community associated with its roots. Some of these bacteria could also promote plant growth by fixing nitrogen. This study aimed to analyze the diversity of diazotrophic bacteria tightly associated with roots of different vetiver genotypes. nifH-based PCR-denaturing gradient gel electrophoresis (DGGE) and clone libraries were used. DGGE profiles obtained from bulk and rhizosphere soils and root DNA amplified with nifH primers showed that samples from rhizosphere soil and root were separated at 68% similarity. Twelve bands were excised from the DGGE and sequenced. High similarity with nifH sequences of Bradyrhizobium sp., Pseudacidovorax sp. and Xanthobacter sp. was observed. Moreover, three nifH clone libraries were generated using polF/polR-primers from root DNA samples obtained from vetiver genotypes UFS-VET001, UFS-VET003 and UFS-VET004. In UFS-VET001, 24.2% of 95 clones were affiliated with sequences of Mesorhizobium loti while in UFS-VET003 41.5% of 89 clones were affiliated with Sphingomonas azotifigens, and in UFS-VET004 36.4% of 85 clones were affiliated with Klebsiella pneumoniae. The data obtained can be used to guide the isolation of diazotrophic bacteria, which may contribute to plant growth promotion and improvement of the production of essential oil in vetiver. [ABSTRACT FROM AUTHOR]

Banana and pineapple fruit crops are widely cultivated in tropical areas where high amounts of fertilizers are applied, principally nitrogen. Over 200 kg N.ha-1.yr-1 is often applied to these crops. Nevertheless, developing countries face the problem ofhigh costs of chemical fertilizers. As already demonstrated for other tropical crops, like sugar cane, the utilization of nitrogen-fixingbacteria may support the growth of these fruit plants. In this work,we demonstrate the association of nitrogen-fixing bacteria with banana and pineapple. Samples from roots, stems, leaves and fruits of different genotypes showed the occurrence of diazotrophic bacteria, whenevaluated in semi-specific semi-solid media. These isolates could beseparated into seven different groups according to their morphological and physiological characteristics. Additional, phylogenetic assignments were performed with group- and species-specific oligonucleotideprobes. Bacteria related to the groups of Azospirillum amazonense, Azospirillum lipoferum, Burkholderia sp. and a group similar to the genus Herbaspirillum could be detected in samples of both crops. However, Azospirillum brasilense and another two groups of Herbaspirillum-like bacteria were detected only in banana plants. Two isolates of thelatter group were identified as Herbaspirillum seropedicae, whereas the other isolates may represent a new Herbaspirillum species. [ABSTRACT FROM AUTHOR]