Legume - Rhizobium associations are undoubtedly form the most important N₂-fixing symbiosis and play a subtle role in contributing nitrogen and maintaining/improving soil fertility. A great diversity in the rhizobial species nodulating legumes has been recognized, which belongs to Alpha subgroup of proteobacteria covering the genera, Rhizobium, Sinorhizobium (renamed as Ensifer), Mesorhizobium, Bradyrhizobium and Azorhizobium. Recently, several non-rhizobial species, belonging to alpha and beta subgroup of Proteobacteria such as Methylobacterium, Blastobacter, Devosia, Phyllobacterium, Ochrobactrum, Agrobacterium, Cupriavidus, Herbaspirillum, Burkholderia and some gamma-Proteobacteria have been reported to form nodules and fix nitrogen in legume roots. The phylogenetic relationship of these non-rhizobial species with the recognized rhizobial species and the diversity of their hosts are discussed in this review.

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