Identification of Genes Regulated by the Antitermination Factor NasT during Denitrification in Bradyrhizobium diazoefficiens

Diversity of Bradyrhizobium in Non-Leguminous Sorghum Plants: B. ottawaense Isolates Unique in Genes for N2O Reductase and Lack of the Type VI Secretion System

Diverse members of Bradyrhizobium diazoefficiens, B. japonicum, and B. ottawaense were isolated from the roots of field-grown sorghum plants in Fukushima, and classified into “Rhizobia” with nodulated soybeans, “Free-living diazotrophs”, and “Non-diazotrophs” by nitrogen fixation and nodulation assays. Genome analyses revealed that B. ottawaense members possessed genes for N₂O reduction, but lacked those for the Type VI secretion system (T6SS). T6SS is a new bacterial weapon against microbial competitors. Since T6SS-possessing B. diazoefficiens and B. japonicum have mainly been isolated from soybean nodules in Japan, T6SS-lacking B. ottawaense members may be a cryptic lineage of soybean bradyrhizobia in Japan.

Nitrogen Cycling in Soybean Rhizosphere: Sources and Sinks of Nitrous Oxide (N2O)

Nitrous oxide (N₂O) is the third most important greenhouse gas after carbon dioxide and methane, and a prominent ozone-depleting substance. Agricultural soils are the primary anthropogenic source of N₂O because of the constant increase in the use of industrial nitrogen (N) fertilizers. The soybean crop is grown on 6% of the world's arable land, and its production is expected to increase rapidly in the future. In this review, we summarize the current knowledge on N-cycle in the rhizosphere of soybean plants, particularly sources and sinks of N₂O. Soybean root nodules are the host of dinitrogen (N₂)-fixing bacteria from the genus Bradyrhizobium. Nodule decomposition is the main source of N₂O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N₂O. This N₂O is either emitted into the atmosphere or further reduced to N₂ by the bradyrhizobial N₂O reductase (N₂OR), encoded by the nos gene cluster. The dominance of nos ^- indigenous populations of soybean bradyrhizobia results in the emission of N₂O into the atmosphere. Hence, inoculation with nos ⁺ or nos ⁺⁺ (mutants with enhanced N₂OR activity) bradyrhizobia has proved to be promising strategies to reduce N₂O emission in the field. We discussed these strategies, the molecular mechanisms underlying them, and the future perspectives to develop better options for global mitigation of N₂O emission from soils.