New Bradyrhizobium species from soils of different Brazilian regions: taxonomy and symbiotic efficiency

Abstract

The Bradyrhizobium genus is an important group of nitrogen-fixing bacteria that nodulate legumes. In Brazilian ecosystems, this genus stands out because it predominates as efficient microsymbionts of important legumes, including grains, forest and forage species, and because it shows high diversity. The aim of this study was to determine the taxonomic position and evaluate the symbiotic efficiency of Bradyrhizobium strains from soils of different Brazilian regions (Northeast, North and Southeast). The strains used in this study are representatives from different phylogenetic groups of Bradyrhizobium genus, indicated in previous studies, based on sequencing of housekeeping genes. Phenotypic characterization, including tests for temperature, pH, salinity, resistance to antibiotics, assimilation of different carbon and nitrogen sources, and analysis of MALDI-TOF MS (Matrix-assisted laser desorption ionizationtime-of-flight mass spectrometry) profiles allowed differentiating the strains of each group from type strains of Bradyrhizobium species phylogenetically close. The results of DNA-DNA hybridization analysis confirmed the existence of four new species, for which the names Bradyrhizobium brasilense, Bradyrhizobium forestalis, Bradyrhizobium piauiense and Bradyrhizobium neoglycinehave been proposed, with the strains UFLA 03-321 T , INPA 54B T , UFLA 06- 13 T and UFLA 06-10 T designated as type strains, respectively. The evaluation of symbiotic efficiency was performed using lima bean,velvet bean and acacia as host species. Strains INPA 54B and INPA 86A stood out in the production of shoot dry matter (SDM) of lima bean. Strains UFLA 03-144 and INPA 104A were more efficient than the control with high mineral N concentration and the inoculant strain BR 2811 in the production of SDM of velvet bean. Strain UFLA 03-268 was the most efficient in symbiosis with acacia, inclusive it was more efficient than the inoculant strain BR 3617. These strains exhibit potential for use as inoculants in their respective host species in which they have established efficient symbiosis. The results presented in this study emphasize the high phenotypic, genotypic and symbiotic diversity of native Bradyrhizobium strains from Brazilian soils.