Symbiotic Bradyrhizobium japonicum Reduces N2O Surrounding the Soybean Root System via Nitrous Oxide Reductase

摘要

N2O reductase activity in soybean nodules formed with Bradyrhizobium japonicum was evaluated from N2O uptake and conversion of ¹⁵N-N2O into ¹⁵N-N2. Free-living cells of USDA110 showed N2O reductase activity, whereas a nosZ mutant did not. Complementation of the nosZ mutant with two cosmids containing the nosRZDFYLX genes of B. japonicum USDA110 restored the N2O reductase activity. When detached soybean nodules formed with USDA110 were fed with ¹⁵N-N2O, they rapidly emitted ¹⁵N-N2 outside the nodules at a ratio of 98.5% of ¹⁵N-N2O uptake, but nodules inoculated with the nosZ mutant did not. Surprisingly, N2O uptake by soybean roots nodulated with USDA110 was observed even in ambient air containing a low concentration of N2O (0.34 ppm). These results indicate that the conversion of N2O to N2 depends exclusively on the respiratory N2O reductase and that soybean roots nodulated with B. japonicum carrying the nos genes are able to remove very low concentrations of N2O.