Genome sequence of Bradyrhizobium embrapense strain CNPSo 2833T, isolated from a root nodule of Desmodium heterocarpon

Abstract Bradyrhizobium embrapense CNPSo 2833T is a nitrogen-fixing symbiont of the legume pasture Desmodium. Its draft genome contains 8,267,832 bp and 7876 CDSs. The symbiotic island includes nodulation and nitrogen fixation genes resembling the operon organization of B. japonicum. Several CDSs related to secretion proteins and stress tolerance were also identified.

Interrogantes en la tecnología de la inoculación de semillas de soja con Bradyrhizobium spp / Queries related to the technology of soybean seed inoculation with Bradyrhizobium spp

With the aim of exploiting symbiotic nitrogen fixation, soybean crops are inoculated with selected strains of Bradyrhizobium japonicum, Bradyrhizobium diazoefficiens or Bradyrhizobium elkanii (collectively referred to as Bradyrhizobium spp.). The most common method of inoculation used is seed inoculation, whether performed immediately before sowing or using preinoculated seeds or pretreated seeds by the professional seed treatment. The methodology of inoculation should not only cover the seeds with living rhizobia, but must also optimize the chances of these rhizobia to infect the roots and nodulate. To this end, inoculated rhizobia must be in such an amount and condition that would allow them to overcome the competition exerted by the rhizobia of the allochthonous population of the soil, which are usually less effective for nitrogen fixation and thus dilute the effect of inoculation on yield. This optimization requires solving some queries related to the current knowledge of seed inoculation, which are addressed in this article. I conclude that the aspects that require further research are the adhesion and survival of rhizobia on seeds, the release of rhizobia once the seeds are deposited in the soil, and the movement of rhizobia from the vicinity of the seeds to the infection sites in the roots

Con el fin de aprovechar la fijación simbiótica de nitrógeno, el cultivo de soja se inocula con cepas seleccionadas de Bradyrhizobium japonicum, Bradyrhizobium diazoefficiens o Bradyrhizobium elkanii (conjuntamente referidas como Bradyrhizobium spp.). El método más común de hacerlo es la inoculación en semillas, ya sea que esta se realice en el momento previo a la siembra o que se utilicen semillas preinoculadas o pretratadas mediante el tratamiento profesional de semillas. La metodología de inoculación no debe limitarse a recubrir las semillas con rizobios vivos, sino que también debe optimizar las chances de esos rizobios para infectar las raíces y nodular. Para ello los rizobios inoculados deben estar en una cantidad y un estado tales que les permitan superar la competición ejercida por los rizobios de la población alóctona del suelo, los cuales usualmente son menos eficaces para la fijación de nitrógeno y así diluyen el efecto de la inoculación sobre el rendimiento. Esta optimización requiere resolver algunos interrogantes, que son abordados en el presente artículo. Concluyo que los aspectos que requieren más investigación son la adhesión y supervivencia de los rizobios en las semillas, la liberación de los rizobios una vez que las semillas se depositan en el suelo y el movimiento de los rizobios desde las inmediaciones de las semillas hasta los sitios de infección en las raíces

Nitrogen fixation in lichens is important for improved rock weathering.

It is known that cyanobacteria in cyanolichens fix nitrogen for their nutrition.However, specific uses of the fixed nitrogen have not been examined. The present study shows experimentally that a mutualistic interaction between a heterotrophic N2 fixer and lichen fungi in the presence of a carbon source can contribute to enhanced release of organic acids, leading to improved solubilization of the mineral substrate. Three lichen fungi were isolated from Xanthoparmelia mexicana, a foliose lichen, and they were cultured separately or with a heterotrophic N2 fixer in nutrient broth media in the presence of a mineral substrate. Cells of the N2-fixing bacteria attached to the mycelial mats of all fungi, forming biofilms. All biofilms showed higher solubilizations of the substrate than cultures of their fungi alone. This finding has bearing on the significance of the origin and existence of N2-fixing activity in the evolution of lichen symbiosis. Further, our results may explain why there are N2-fixing photobionts even in the presence of non- fixing photobionts (green algae) in some remarkable lichens such as Placopsis gelida. Our study sheds doubt on the idea that the establishment of terrestrial eukaryotes was possible only through the association between a fungus and a phototroph.

Can mushrooms fix atmospheric nitrogen?

It is generally reported that fungi like Pleurotus spp. can fix nitrogen (N2). The way they do it is still not clear. The present study hypothesized that only associations of fungi and diazotrophs can fix N2. This was tested in vitro. Pleurotus ostreatus was inoculated with a bradyrhizobial strain nodulating soybean and P. ostreatus with no inoculation was maintained as a control. At maximum mycelial colonization by the bradyrhizobial strain and biofilm formation, the cultures were subjected to acetylene reduction assay (ARA). Another set of the cultures was evaluated for growth and nitrogen accumulation. Nitrogenase activity was present in the biofilm, but not when the fungus or the bradyrhizobial strain was alone. A significant reduction in mycelial dry weight and a significant increase in nitrogen concentration were observed in the inoculated cultures compared to the controls. The mycelial weight reduction could be attributed to C transfer from the fungus to the bradyrhizobial strain, because of high C cost of biological N2 fixation. This needs further investigations using 14C isotopic tracers. It is clear from the present study that mushrooms alone cannot fix atmospheric N2. But when they are in association with diazotrophs, nitrogenase activity is detected because of the diazotrophic N2 fixation. It is not the fungus that fixes N2 as reported earlier. Effective N2 fixing systems, such as the present one, may be used to increase protein content of mushrooms. Our study has implications for future identification of as yet unidentified N2 systems occurring in the environment.