Abstract

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 recog-nized, which belongs to a subgroup of proteobacteria covering the genera, Rhizobium, Sinorhizobium (renamed as Ensifer), Mesorhizobium, Bradyrhizobium andAzorhizobium. Recently, several non-rhizobial species, belonging to a and b subgroup of Proteobacteria such as Methylobacterium, Blastobacter, Devosia, Phyllobac-terium, Ochro-bactrum, Agrobacterium, Cupriavidus, Herbaspirillum, Burkholde-ria and some g-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.

Key words: Legumes, Nodulation, Proteobacteria, Rhizobium