Legume Symbiotic Nitrogen Fixation byβ-Proteobacteria Is Widespread inNature

Abstract

Following the initial discovery of two legume-nodulating Burkholderia strains (L. Moulin, A. Munive, B. Dreyfus, and C. Boivin-Masson, Nature 411:948-950, 2001), we identified as nitrogen-fixing legume symbionts at least 50 different strains of Burkholderia caribensis and Ralstonia taiwanensis, all belonging to the β-subclass of proteobacteria, thus extending the phylogenetic diversity of the rhizobia. R. taiwanensis was found to represent 93% of the Mimosa isolates in Taiwan, indicating thatβ -proteobacteria can be the specific symbionts of a legume. The nod genes of rhizobial β-proteobacteria (β-rhizobia) are very similar to those of rhizobia from theα -subclass (α-rhizobia), strongly supporting the hypothesis of the unique origin of common nod genes. Theβ -rhizobial nod genes are located on a 0.5-Mb plasmid, together with the nifH gene, in R. taiwanensis and Burkholderia phymatum. Phylogenetic analysis of available nodA gene sequences clustered β-rhizobial sequences in two nodA lineages intertwined with α-rhizobial sequences. On the other hand, theβ -rhizobia were grouped with free-living nitrogen-fixingβ -proteobacteria on the basis of the nifH phylogenetic tree. These findings suggest that β-rhizobia evolved from diazotrophs through multiple lateral nod gene transfers.