A common mechanism for efficient N₂O reduction in diverse isolates of nodule‐forming bradyrhizobia

Abstract

Bradyrhizobia are abundant soil bacteria, which can form nitrogen-fixing symbioses with leguminous plants, including important crops such as soybean, cowpea and peanut. Many bradyrhizobia can denitrify, but studies have hitherto focused on a few model organisms. We screened 39 diverse Bradyrhizobium strains, isolated from legume nodules. Half of them were unable to reduce N 2 O, making them sources of this greenhouse gas. Most others could denitrify NO 3 − to N 2. Timeresolved gas kinetics and transcription analyses during transition to anaerobic respiration revealed a common regulation of nirK, norCB and nosZ (encoding NO 2 − , NO and N 2 O reductases), and differing regulation of napAB (encoding periplasmic NO 3 − reductase). A prominent feature in all N 2-producing strains was a virtually complete hampering of NO 3 − reduction in the presence of N 2 O. In-depth analyses suggest that this was due to a competition between electron transport pathways, strongly favouring N 2 O over NO 3 − reduction. In a natural context, bacteria with this feature would preferentially reduce available N 2 O, produced by themselves or other soil bacteria, making them powerful sinks for this greenhouse gas. One way to augment such populations in agricultural soils is to develop inoculants for legume crops with dual capabilities of efficient N 2-fixation and efficient N 2 O reduction.

Diversity of <i>Bradyrhizobium</i> in Non-Leguminous Sorghum Plants: <i>B. ottawaense</i> Isolates Unique in Genes for N₂O Reductase and Lack of the Type VI Secretion System

“…Thus, group W members may mitigate N 2 O emission from soil as well as B. diazoefficiens. Bradyrhizobial isolates phylogenetically close to B. ottawaense from African woody legumes were recently confirmed to reduce N 2 O to N 2 (Mania et al, 2019).…”

Competition for electrons favors N2O reduction in denitrifyingBradyrhizobium isolates

“…The rhizobial isolates were obtained from various culture collections. Table 1 shows a list of host plant and geographic origin of the strains used in the phylogenetic analysis (all listed isolates; details see [31][32][33][34][35][36][37]) and the strains analyzed for their denitrification capacity (all except the four strains from Ethiopia earlier determined to have a complete denitrification pathway [19]).…”

“…Preparation of Bradyrhizobium cultures and growth conditions followed Mania et al [19]. All cultures were grown at 28 °C in Yeast Mannitol Broth (YMB; [38]) (10 g l -1 D-mannitol, 0.5 g l -1 K2HPO4, 0.2 g l -1 MgSO4•7H2O, 0.1 g l -1 NaCl and 0.5 g l -1 yeast extract; pH 7.0).…”

“…Several taxonomic groups within the genus Bradyrhizobium have been reported to denitrify with many strains having truncated denitrification pathways lacking one or more of the steps [12,18,19]. A complete denitrification pathway in bradyrhizobia comprises the periplasmic reductase Nap for dissimilatory NO3reduction, encoded as a part of the napEDABC operon; the Cucontaining nitrite reductase NirK encoded by nirK only or as a part of the nirKV operon, although cd1-type NirS nitrite reductase has also been reported [20]; the cytochrome c dependent nitric oxide reductase cNor (norCBQD operon); and a clade I N2O reductase Nos (nosRZDYFLX operon) [21][22][23][24][25].…”

“…In a recent study, Mania et al [19] screened strains within the genus Bradyrhizobium for their denitrification phenotype. Most of them could perform two or more of the denitrification steps, but only half of them could reduce N2O.…”