Current status of the taxonomy of bacteria able to establish nitrogen-fixing legume symbiosis

Abstract

Bacteria forming nitrogen-fixing symbiosis with legumes, classically named rhizobia, currently include more than 100 species distributed in the old genera Allorhizobium, Azorhizobium, Bradyrhizobium, Ensifer (formerly Sinorhizobium), Mesorhizobium and Rhizobium and in the new genera Neorhizobium and Pararhizobium. In addition, several new rhizobia have been described in the twenty-first century belonging, as the classical rhizobia, to the alpha Proteobacteria genera Aminobacter, Devosia, Methylobacterium, Microvirga, Ochrobactrum, Phyllobacterium and Shinella and to the beta Proteobacteria Burkholderia, Paraburkholderia (formerly Burkholderia) and Cupriavidus. These species carry symbiotic genes encoding for nodulation and nitrogen fixation which are located on plasmids or symbiotic islands. These genes determine the host range and confer rhizobia the ability to fix nitrogen in the legume nodules. Depending on the harboured nodulation genes, several symbiovars have recently been described in the classical rhizobia genera. In this chapter, we review the different groups of bacteria able of forming symbiosis with legumes and their classification based on core genes (genera and species) as well as on auxiliary ones (symbiovars).