<i>Rhizobium etli</i> is able to emit nitrous oxide by connecting assimilatory nitrate reduction with nitrite respiration in the bacteroids of common bean nodules

Legumes can contribute to emissions of the potent greenhouse gas nitrous oxide (N2O) directly by some rhizobia species that are able to denitrify under free-living conditions and in symbiotic association with the plant. In this study, the capacity of Phaseolus vulgaris-Rhizobium etli symbiosis to emit N2O in response to nitrate (NO3-) has been demonstrated for the first time. We found that bacteroidal assimilatory nitrate reductase (NarB) as well as nitrite reductase (NirK) and nitric oxide reductase (cNor) denitrifying enzymes contribute to nitric oxide (NO) and N2O formation in nodules. We also show that R. etli NarK is involved in NO2- extrusion and links NO3- reduction by NarB in the cytoplasm with NirK and cNor denitrification activities in the periplasm. The knowledge generated in this work will be instrumental for exploring strategies and sustainable practices in agricultural soil management to increase legume crop yield and mitigate greenhouse gas emissions. Common bean nodules are able to produce N2O in response to nitrate.Nitrate reduction to nitrite in R. etli bacteroids catalyzed by assimilatory nitrate reductase NarB is involved in N2O emissions from common bean nodules.R. etli respiratory nitrite reductase NirK and nitric oxide reductase cNor support N2O production in common bean nodules.R. etli NarB and cNor are involved in NO homeostasis in common bean nodules.R. etli nitrite exporter NarK links nitrate reduction in the cytoplasm with denitrification activities in the periplasm. Common bean nodules are able to produce N2O in response to nitrate. Nitrate reduction to nitrite in R. etli bacteroids catalyzed by assimilatory nitrate reductase NarB is involved in N2O emissions from common bean nodules. R. etli respiratory nitrite reductase NirK and nitric oxide reductase cNor support N2O production in common bean nodules. R. etli NarB and cNor are involved in NO homeostasis in common bean nodules. R. etli nitrite exporter NarK links nitrate reduction in the cytoplasm with denitrification activities in the periplasm.