Bioprospection of naturalized soybean-nodulating Bradyrhizobium strains in Uruguayan soils: a genetic and symbiotic approach

Soybean (Glycine max (L.) Merr.), an economically relevant crop, establishes a symbiotic association with rhizobia to obtain nitrogen (N2) from the air by biological nitrogen fixation with important benefits. In Uruguay inoculants formulated with two strains, Bradyrhizobium elkanii U1301 and U1302, are recommended since 1984. Besides that, the study of native-naturalized rhizobia populations is relevant because these strains could compete with applied inoculants and may present a better symbiotic efficiency. The aim of this work was to study, genetically and symbiotically, naturalized soybean nodulating rhizobia isolated from Uruguayan soils. A collection of ten naturalized rhizobia was studied and compared with Uruguay's commercial strains and neighbouring countries (B. elkanii U1301 and U1302, Bradyrhizobium japonicum E109 and Bradyrhizobium diazoefficiens SEMIA5080). Using a multilocus sequence analysis (MLSA) (16S rRNA, atpD, gyrB and rpoB genes), five naturalized strains were identified as B. elkanii and four as B. japonicum. The other naturalized strain UYS-CA02 is suggested to belong to Bradyrhizobium ferriligni, considering a second MLSA with 16S rRNA, gyrB, rpoB, dnaK and recA genes. Analysis of symbiotic genes (nodY/K and nifH) indicates that strains U1301 and U1302 may have transferred these genes horizontally to strain UYS-CA02 or its ancestor. Symbiotic efficiency was evaluated in axenic conditions, in which shoot dry weight, total nitrogen in shoots, number of nodules and nodules dry weight, were determined. In that assay, the U1301:U1302 blend outstood in front of other commercial strains. Multivariate analysis of symbiotic efficiency data shows a better performance of B. elkanii-like strains than B. japonicum-like ones.