Bradyrhizobium ottawaense has prospects as an environmentally friendly inoculant for soybean farming because of its higher N2O reductase (N2OR) activity than that of B. diazoefficiens. To examine high N2O-reducing B. ottawaense, we performed a PCR analysis of nosZ genes in 8,640 soybean nodules from 68 fields in Japan. Of 384 PCR-positive nodules, we obtained 90 isolates of bradyrhizobia with B. ottawaense-type nosZ, derived exclusively from 18 fields in Gunma and Osaka prefectures. Of 77 monophyletic isolates, 73 had significantly higher N2OR activity than B. diazoefficiens USDA110. Another 13 isolates from Osaka were phylogenetically placed outside of the B. ottawaense clade with B. liaoningense or B. betae, 8 of which also exhibited significantly higher N2OR activity than B. diazoefficiens USDA110. An analysis of nopP gene sequences revealed amino acid sequence variations in the NopP effector protein among these high N2O-reducing isolates, with the NopPUSDA122 type being one of the variations identified. The NopP-mediated symbiotic incompatibility of soybean host plants may eliminate nodulation by indigenous bradyrhizobia and facilitate inoculant nodulation to reduce N2O emissions. Therefore, 90 isolates and their observed NopP types are potentially important resources for N2O mitigation. Furthermore, the dense geographical map of Bradyrhizobium species based on Internal Transcribed Spacer-Restriction Fragment Length Polymorphisms (ITS-RFLP) of the 16S-23S rRNA gene from 8,640 nodules revealed the recent northward expansion of B. elkanii to central Japan potentially due to global warming. This change in indigenous soybean bradyrhizobia is important for application strategies of bradyrhizobial inoculants under field conditions.
Itakura et al. (Thu,) studied this question.