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.

中文翻译：

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Ensifer meliloti 反硝化作用与苜蓿根瘤的感染效果和 N2O 排放有关

目的

紫花苜蓿是温带气候区最有价值的饲料作物之一。苜蓿内共生体Ensifer meliloti包含所有反硝化基因，但苜蓿根瘤产生一氧化二氮 (N ₂ O) 的能力尚不清楚。在这项工作中，研究了N ₂ O 排放量以及细菌反硝化作用对结瘤竞争力和紫花苜蓿根瘤 N _{2 O 释放的影响。}

方法

紫花苜蓿简历。维多利亚植物接种苜蓿根瘤菌1021，一种周质硝酸还原酶 (Nap) 缺陷突变体，一种 Nap 过表达菌株和一氧化二氮还原酶缺陷突变体。植物在不同的硝酸盐和铜处理的存在下生长，并在收获前一周进行淹水。使用气相色谱分析结核产生的N _{2 O。}在分离的类菌体中测量甲基紫精依赖性硝酸盐还原酶 (MV ⁺ -NR)、亚硝酸盐还原酶 (MV ⁺ -NIR) 和一氧化二氮还原酶 (N _{2 OR) 的酶活性。}

结果

苜蓿根瘤产生 N ₂ O 以响应硝酸盐和水淹。Nap 的过度表达提高了结瘤竞争力并诱导了结核中的 N ₂ O 排放。铜是有效共生所必需的，并且由于 N ₂ OR 的诱导和拟杆菌中 NIR 活性的降低而触发 N _{2 O 产量的减少。}

结论

苜蓿根瘤排放 N ₂ O。Nap 参与结瘤竞争和根瘤的N _{2 O 排放。}细菌的 N ₂ OR 和 NIR 活动受 Cu 调节，可被视为缓解源自苜蓿作物的 N _{2 O 排放的有效目标。}