Rhizobia are the traditional soil bacteria capable of forming root or stem nodules on various leguminous plants, where they undertake symbiotic fixation of atmospheric nitrogen. Currently, they are divided into six genera with approximately 30 species, including Allorhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium and Sinorhizobium. Phylogenetically, all these bacteria belong to the tx-subclass of proteobacteriat. Till recently, none of the bacteria belonging to the #beta#-subclass of proteobacteria were known to form root or stem nodules on legumes. Moulin et al. reported the ability of Burkholderia species, a member of the #beta#-subclass of proteobacteria, to nodulate the African legumes Aspalathus and Machaerium. Later, Chen et al. described Ralstonia taiwanensis as the first member of the #beta#-subclass of proteobacteria capable of nodulating two species of Mimosa, i.e. M. pudica and M. diplotricha. Careful reading of the two papers indicates that authors of these two papers were in touch with each other and yet, each group claims to be the first to report the #alpha#-subclass of proteobacteria as being involved in nodulation in legumes.
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