Symbiotic efficiency and genetic diversity of soybean bradyrhizobia in Brazilian soils

The symbiotic N2-fixing genus Bradyrhizobium includes 29 species distributed throughout different geographic regions. Only five species have recently been described based on isolates from tropical soils, three species from Brazil (B. manausense, B. ingae, B. neotropicale) and two species from Peru (B. paxllaere e B. icense), although tropical region is considered to be the origin of legume rhizobia symbiosis. Besides, some authors suggested that Bradyrhizobium was introduced in Brazil with first soybeans inoculants from USA. In this work, 46 Bradyrhizobium strains were isolated from soils collected in different regions of Brazil (Midwest, Northeast, Southeast, and South), using soybean as a trap plant. These strains were characterized genetically by analyzing the 16S rRNA gene and five housekeeping genes (atpD, gyrB, dnaK, recA, and rpoB). They were also characterized in terms of their symbiotic efficiency with soybean plants grown under axenic conditions in Leonard jars. The phylogenetic analysis of housekeeping genes revealed the possible presence of novel species in the Northeast and Southeast soils, some of which exhibited high symbiotic efficiency with soybean plants. These results emphasize the great diversity among native strains belonging to Bradyrhizobium genus in Brazilian soils as well as potential ones to be used as inoculants. They also indicate that symbiotically efficient native bradyrhizobia occur in Brazilian soils and are independent of strains introduced as soybean inoculant.