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

Reiko Sameshima-Saito, Kaori Chiba, Junta Hirayama, Manabu Itakura, Hisayuki Mitsui, Shima Eda, and Kiwamu Minamisawa^*

Graduate School of Life Sciences, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan

Received 23 October 2005/ Accepted 25 January 2006

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.

Present address: Environmental Science for Human Life, Faculty of Agriculture, Shizuoka University, Ohya 836, Shizuoka 422-8529, Japan.