Non-rhizobial nodulation in legumes

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 α subgroup of proteobacteria covering the genera, Rhizobium, Sinorhizobium (renamed as Ensifer), Mesorhizobium, Bradyrhizobium and Azorhizobium. Recently, several non-rhizobial species, belonging to α and β subgroup of Proteobacteria such as Methylobacterium, Blastobacter, Devosia, Phyllobacterium, Ochrobactrum, Agrobacterium, Cupriavidus, Herbaspirillum, Burkholderia and some γ-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.