Ensifer meliloti denitrification is involved in infection effectiveness and N2O emissions from alfalfa root nodules.

Purpose: Alfalfa is one of the most valuable forage crops in temperate climate zones. Ensifer meliloti, the endosymbiont of alfalfa, contains all the denitrification genes but the capacity of alfalfa root nodules to produce nitrous oxide (N₂O) is not known. In this work, N₂O emissions as well as the influence of bacteroidal denitrification on nodulation competitiveness and N₂O release from alfalfa nodules has been investigated. Methods: Medicago sativa cv. Victoria plants were inoculated with E. meliloti 1021, a periplasmic nitrate reductase (Nap) defective mutant, a Nap overexpressing strain and a nitrous oxide reductase defective mutant. Plants were grown in the presence of different nitrate and copper treatments and subjected to flooding during one week before harvesting. N₂O production by the nodules was analysed by using gas chromatography. Methyl viologen-dependent nitrate reductase (MV⁺-NR), nitrite reductase (MV⁺-NIR) and nitrous oxide reductase (N₂OR) enzymatic activities were measured in isolated bacteroids. Results: Alfalfa root nodules produce N₂O in response to nitrate and flooding. Overexpression of Nap improved nodulation competitiveness and induced N₂O emissions from nodules. Copper is required for an effective symbiosis as well as triggered a reduction of N₂O production due to the induction of the N₂OR and a reduction of NIR activities in the bacteroids. Conclusion: Alfalfa root nodules emit N₂O. Nap is involved in nodulation competitiveness and in N₂O emissions by the nodules. Bacteroidal N₂OR and NIR activities are modulated by Cu and may be considered as effective targets for the mitigation strategies of N₂O emissions derived from alfalfa crops. [ABSTRACT FROM AUTHOR]