Bacteria Involved in Nitrogen-Fixing Legume Symbiosis: Current Taxonomic Perspective

Abstract

Bacteria forming nitrogen-fixing symbiosis with legumes were classically named ``rhizobia{''} and currently include more than 50 species distributed in genera Rhizobium, Ensifer, Mesorhizobium, Azorhizobium, and Bradyrhizobium. These species carry symbiotic genes codifying for nodulation and nitrogen fixation that are located on plasmids or symbiotic islands determining the host range and the ability to fix nitrogen in the nodules. These genes can be transferred from rhizobia to other alpha or beta-Proteobacteria conferring them the ability to nodulate legumes and fix atmospheric nitrogen. In this chapter, an overview of the main different groups of bacteria known up to date to be able of forming symbiosis with legumes is made. The symbiotic features of such bacteria scrutinizing the frontier of legume and bacteria promiscuity are also discussed.