Selective medium for growth of Rhizobium

Nodule-associated diazotrophic community succession is driven by developmental phases combined with microhabitat of Sophora davidii

Nodule-associated nitrogen-fixing microorganisms (diazotrophs) residing in legume root nodules, and they have the potential to enhance legume survival. However, the succession characteristics and mechanisms of leguminous diazotrophic communities remain largely unexplored. We performed a high-throughput nifH amplicon sequencing with samples of root nodules and soil in the three developmental phases (young nodules, active nodules and senescent nodules) of the Sophora davidii (Franch.) Skeels root nodules, aiming to investigate the dynamics of nodule-endophytic diazotrophs during three developmental phases of root nodules. The results demonstrated the presence of diverse diazotrophic bacteria and successional community shifting dominated by Mesorhizobium and Bradyrhizobium inside the nodule according to the nodule development. The relative abundance decreased for Mesorhizobium, while decreased first and then increased for Bradyrhizobium in nodule development from young to active to senescent. Additionally, strains M. amorphae BT-30 and B. diazoefficiens B-26 were isolated and selected to test the interaction between them in co-cultured conditions. Under co-culture conditions: B. diazoefficiens B-26 significantly inhibited the growth of M. amorphae BT-30. Intriguingly, growth of B. diazoefficiens B-26 was significantly promoted by co'culture with M. amorphae BT-30 and could utilize some carbon and nitrogen sources that M. amorphae BT-30 could not. Additionally, the composition of microbial community varied in root nodules, in rhizosphere and in bulk soil. Collectively, our study highlights that developmental phases of nodules and the host microhabitat were the key driving factors for the succession of nodule-associated diazotrophic community.

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

Biological nitrogen fixation by Rhizobium-legume symbioses represents an environmentally friendly and inexpensive alternative to the use of chemical nitrogen fertilizers in legume crops. Rhizobial inoculants, applied frequently as biofertilizers, play an important role in sustainable agriculture. However, inoculants often fail to compete for nodule occupancy against native rhizobia with inferior nitrogen-fixing abilities, resulting in low yields. Strains with excellent performance under controlled conditions are typically selected as inoculants, but the rates of nodule occupancy compared to native strains are rarely investigated. Lack of persistence in the field after agricultural cycles, usually due to the transfer of symbiotic genes from the inoculant strain to naturalized populations, also limits the suitability of commercial inoculants. When rhizobial inoculants are based on native strains with a high nitrogen fixation ability, they often have superior performance in the field due to their genetic adaptations to the local environment. Therefore, knowledge from laboratory studies assessing competition and understanding how diverse strains of rhizobia behave, together with assays done under field conditions, may allow us to exploit the effectiveness of native populations selected as elite strains and to breed specific host cultivar-rhizobial strain combinations. Here, we review current knowledge at the molecular level on competition for nodulation and the advances in molecular tools for assessing competitiveness. We then describe ongoing approaches for inoculant development based on native strains and emphasize future perspectives and applications using a multidisciplinary approach to ensure optimal performance of both symbiotic partners.

Enhanced nodulation and higher germination using sludge ash as a carrier for biofertilizer production

Sludge ash, a byproduct resulting from the combustion of the dewatered sludge generated in the sewage treatment plants, is recognized as a hazardous solid waste throughout the world. This solid waste, which is produced in behemoth volume, has very limited applications and is directly disposed to landfills. The present study aims to explore a novel strategy for utilizing this solid waste for the development of Rhizobium biofertilizer and thus reducing the burden on landfills. The Rhizobium inoculum was prepared and mixed with sludge ash to formulate Rhizobium biofertilizer, and shelf life of this formulation was evaluated by counting colony forming unit. The prepared formulation was applied on lentil seeds, and its effect was analyzed by assessing the plant growth along with other yield characteristics. Enhanced colony forming unit count (6487.78) was observed in sludge ash carrier combination with a better shelf life of five months in comparison to lignite carrier formulation from the beginning of the experiment (9.1%) till 150th day (69.8%). Lentil seeds treated with sludge ash based biofertilizer showed luxuriant growth with 19% enhanced root length, 42% in number of nodules, 10% in plant growth and 53.40% in seed yield as compared to conventional lignite formulations. Hence, biofertilizer produced from waste material sludge ash is not supposed to cause a hazard for the soil and plants when incorporated in defined amount and can prove to be a substitute for conventional lignite carrier in biofertilizer production.

Use of nodulation pattern, stress tolerance, nodC gene amplification, RAPD-PCR and RFLP–16S rDNA analysis to discriminate genotypes of Rhizobium leguminosarum biovar viciae

Twenty-seven new Rhizobium isolates were obtained from root nodules of wild and crop legumes belonging to the genera Vicia, Lathyrus and Pisum from different agroecological areas in central and southern Italy. A polyphasic approach including phenotypic and genotypic techniques was used to study their diversity and their relationships with other biovars and species of rhizobia. Analysis of symbiotic properties and stress tolerance tests revealed that wild isolates showed a wide spectrum of nodulation and a marked variation in stress tolerance compared with reference strains tested in this study. All rhizobial isolates (except for the isolate CG4 from Galega officinalis) were presumptively identified as Rhizobium leguminosarum biovar viciae both by their symbiotic properties and the specific amplification of the nodC gene. In particular, we found that the nodC gene could be used as a diagnostic molecular marker for strains belonging to the bv. viciae. RFLP-PCR 16S rDNA analysis confirms these results, with the exception of two strains that showed different RFLP-genotypes from those of the reference strains of R. leguminosarum bv. viciae. Analysis of intraspecies relationship among strains by using the RAPD-PCR technique showed a high level of genetic polymorphism, grouping our isolates and reference strains into six different major clusters with a similarity level of 20%. Data from seven parameters of phenotypic and genotypic analyses were evaluated by using principal component analysis which indicated the differences among strains and allowed them to be divided into seven different groups.

Phylogenetic analysis of the family Rhizobiaceae and related bacteria by sequencing of 16S rRNA gene using PCR and DNA sequencer

The 16S rRNA gene sequences of 19 strains covering 97% of the molecules were determined for the members of the family Rhizobiaceae and related bacteria by PCR and DNA sequencer. The three biovars of Agrobacterium were located separately, whereas Agrobacterium rubi clustered with A. tumefaciens. Phylogenetic locations for the species of the genera Rhizobium, Sinorhizobium, Agrobacterium, Phylobacterium, Mycoplana (M. dimorpha), Ochrobactrum, Brucella and Rochalimaea (a rickettsia) were intermingled with each other with the similarity values higher than 92%. The family Rhizobiaceae should be redefined including the above-mentioned genera despite the ability for plant association and nitrogen fixation. Bradyrhizobium japonicum and Mycoplana bullata were far remote from the other species and should be excluded from this family.

Some ecological and physiological studies on bacteria isolated from salt-affected soils of Egypt

Members of Bacillaceae, Rhizobiaceae, actinomycetes and others were isolated from cultivated and non-cultivated saline soils. The high population of bacteria and actinomycetes were almost coincided with the relatively high levels of organic matter whatever the degree of soil salinity. Bacillus stearothermophilus and B. subtilis were more frequently isolated than other Bacillus species. Most of Rhizobium isolates were salt tolerant being able to grow in media containing 3% and 6% NaCl. The abilities of different bacterial isolates to attack citrus pectin, soluble and insoluble forms of cellulose were also tested.

Radioactive isotope uptake in a grass-legume association

The radioactive uptake of Medicago sativa and Rye grass in a pasture exposed to the fallout from the Chernobyl reactor accident, was determined in four consecutive harvests covering a period of one year after the accident,In plants of Medicago sativa, inoculated with an effective Rhizobia meliloti strain isolated from Greek soils, a high degree of biological nitrogen fixation was observed at all harvests using N-15 techniques. At the second and third harvests, the percentage nitrogen derived from fixation (%NdfF), the percentage nitrogen derived from soil (%NdfS), as well as the radioactive uptake from the soil remained stable. At the fourth harvest, however, the %NdfF decreased while the %NdfS and the radioactive uptake from soil significantly increased. At the first harvest the radioactivity in both plants, caused mainly by direct fallout contamination, was considerably higher than that observed at the later harvests. Medicago sativa contained significantly less radioactivity than the grass at all harvests, although both plants were grown under the same environmental conditions. Even at the fourth harvest, almost one year after the initial contamination, the radioactivity of grass remained at high levels ( 20 Bq g(-1) of protein) while in Medicago sativa it assumed considerably lower values (3.6 Bq g(-1) of protein). A possible involvement of biological nitrogen fixation in the reduction of radioactive uptake is discussed. Finally, certain practical conclusions are drawn with respect to a safer management of pastures exposed to radioactivity.

Evidence of an active glucose uptake in Rhizobium meliloti

Rhizobium meliloti M5N1 showed immediate uptake of glucose. Glucose accumulation was a saturable function of the glucose concentration, and km and Vmax values for uptake were determined. The glucose uptake system was found to be proteic with a turnover of about 6 h. This system was observed to be an active ATP-dependent process, since it was severely inhibited by uncouplers. Glucose analogues and others hexoses had some inhibitory effects on glucose uptake, but not polyols and lactose.