Multilocus sequence analysis reveals taxonomic differences among Bradyrhizobium sp. symbionts of Lupinus albescens plants growing in arenized and non-arenized areas

Lupinus albescens is a leguminous plant that belongs to “New World” lupine species, which is native to southern Brazil. This Brazilian region is characterized by poor degraded soils with low organic matter and is designated as an arenized area. The symbiosis between Lupinus plants and nitrogen-fixing bacteria belonging to the Bradyrhizobium genus may help the plant establish itself in these areas. To characterize the bradyrhizobial population symbionts of L. albescens plants grown in arenized and non-arenized areas, a multilocus phylogenetic analysis allied to genetic diversity indices were conducted. Seventy-four bradyrhizobial isolates were analyzed, 38 coming from L. albescens plants growing in an arenized area and 36 from a non-arenized area. Isolates were different between arenized and non-arenized areas. Phylogenetic analysis of the 16S rRNA, dnaK, atpD, recA, glnII, rpoB, gyrB, nodA, nodB, and nodZ genes resulted in three supported clades, which were most likely to be three different new Bradyrhizobium species: one species from the arenized area and two from the non-arenized area. Estimates of genetic diversity, which decreased in arenized areas, were positively correlated with habitat variability. These results suggested that a few resistant and efficient Bradyrhizobium sp. strains were capable of forming nodules on L. albescens plants growing in an arenized area. An in vivo inoculation experiment with L. albescens plants showed that Bradyrhizobium ssp. isolated from this extreme environment were more efficient at promoting plant growth than those from the non-arenized area. This result suggested that the environment affected the selection of more efficient plant growth promoters in order to sustain plant growth.