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Legume-rhizobia symbiotic associations have beneficial effects on food security and nutrition, health and climate change. Hypoxia induced by flooding produces nitric oxide (NO) in nodules from soybean plants cultivated in nitrate-containing soils. As NO is a strong inhibitor of nitrogenase expression and activity, this negatively impacts symbiotic nitrogen fixation in soybean and limits crop production. In  Bradyrhizobium diazoefficiens , denitrification is the main process involved in NO formation by soybean flooded nodules. In addition to denitrification, nitrate assimilation is another source of NO in free-living  B. diazoefficiens  cells and a single domain hemoglobin (Bjgb) has been shown to have a role in NO detoxification during nitrate-dependent growth. However, the involvement of Bjgb in protecting nitrogenase against NO in soybean nodules remains unclear. In this work, we have investigated the effect of inoculation of soybean plants with a  bjgb  mutant on biological nitrogen fixation. By analyzing the proportion of N in shoots derived from N 2 -fixation using the  15 N isotope dilution technique, we found that plants inoculated with the  bjgb  mutant strain had higher tolerance to flooding than those inoculated with the parental strain. Similarly, reduction of nitrogenase activity and  nifH  expression by flooding was less pronounced in  bjgb  than in WT nodules. These beneficial effects are probably due to the reduction of NO accumulation in  bjgb  flooded nodules compared to the wild-type nodules. This decrease is caused by an induction of expression and activity of the denitrifying NO reductase enzyme in  bjgb  bacteroids. As  bjgb  deficiency promotes NO-tolerance, the negative effect of NO on nitrogenase is partially prevented and thus demonstrates that inoculation of soybean plants with the  B. diazoefficiens bjgb  mutant confers protection of symbiotic nitrogen fixation during flooding.

The Hemoglobin Bjgb From Bradyrhizobium diazoefficiens Controls NO Homeostasis in Soybean Nodules to Protect Symbiotic Nitrogen Fixation