Differential efficiency of nitrate respiration between Bradyrhizbium diazoefficiens and Bradyrhizobium japonicum
1Touhoku University – Graduate School of Life Sciences
Denitrification is an alternative respiratory mechanism that occurs in limitation of
oxygen, characterized by using nitrate or other nitrogen oxide as electron acceptor,
reducing it sequentially in NO3 → NO2 → NO → N2O → N2 by nitrate reductase, nitrite
reductase, nitric oxide reductase and nitrous oxide reductase (encoded by nosZ gene),
respectively. This process is found in Bradyrhizobium diazoefficiens and B. japonicum
that nodulate soybean plants for symbiotic nitrogen fixation. B. diazoefficiens strains
contain nos gene cluster (nosZ+) and are able of complete denitrification, while B.
japonicum strains lack nos gene cluster (nosZ-). Shiina et al. (2014) observed B.
diazoefficiens (Bd) dominance in alluvial soils, characterized as anaerobic
environments. Moreover B. japonicum (Bj) strains were dominant in Andosol soils,
characterized as aerobic environment, suggesting that different ability of nitrate
respiration might drive the distribution of Bj and Bd strains in soybean fields. Growth
experiments performed under free-living conditions showed that Bj strain USDA 6
(natural nosZ-) and Bd strain USDA 110 (natural nosZ+) have the same growth in HMM
medium under aerobiosis, however when in anaerobic nitrate-respiring condition (10mM
KNO3), USDA 6 grew less than USDA 110. The lack of nosZ gene seems not to be the
cause of this difference since nosZ mutant of USDA 110 had the same growth as the
wild-type. To further investigate this and extrapolate as global phenomena, the
aerobic, microaerobic (2% O2) and anaerobic growth of randomly selected Bd and Bj
strains was tested. In total, 27 strains were selected, 12 of Bj and 15 of Bd. Anaerobic
growth curves in presence of nitrate were statistically different between Bd and Bj
strains, however no difference was observed in aerobic or microaerobic growth curves.
Our results strongly suggest a differential response of nitrate reduction to N2O between
Bd and Bj strains.