Novel rhizobia exhibit superior nodulation and biological nitrogen fixation even under high nitrate concentrations
No Thumbnail Available
Date
2019-12-20
Journal Title
Journal ISSN
Volume Title
Publisher
FEMS Microbiology Ecology
Abstract
Legume–rhizobium symbiosis leads to the formation of nitrogen-fixing root nodules. However, externally applied chemical
nitrogen fertilizers (nitrate and ammonia) strongly inhibit nodule formation and nitrogen fixation. Here, we isolated several
rhizobial strains exhibiting a superior nodulation and nitrogen fixation with soybean at high nitrate concentrations. The
nodulation of soybean symbiont Bradyrhizobium diazoefficiens USDA110 was significantly inhibited at 12.5 mM nitrate;
however, three isolates (NKS4, NKM2 and NKTG2) were capable of forming nitrogen-fixing nodules, even at 20 mM nitrate.
These isolates exhibited higher nodulation competitiveness and induced larger nodules with higher nitrogen-fixation
activity than USDA110 at 5 mM nitrate. Furthermore, these isolates induced more nodules than USDA110 even in
nitrate-free conditions. These isolates had a distant lineage within the Bradyrhizobium genus; though they were relatively
phylogenetically close to Bradyrhizobium japonicum, their morphological and growth characteristics were significantly
different. Notably, in the presence of nitrate, expression of the soybean symbiosis-related genes (GmENOD40 and GmNIN)
was significantly higher and expression of GmNIC1 that is involved in nitrate-dependent nodulation inhibition was lower in
the roots inoculated with these isolates in contrast with inoculation of USDA110. These novel rhizobia serve as promising
inoculants for soybeans cultivated in diverse agroecosystems, particularly on nitrate-applied soils.
Description
Research Article
Keywords
soybean, symbiosis, rhizobia, nodulation, nitrate inhibition, nitrogen fixation