Nodulation of legumes by members of the beta-subclass of Proteobacteria

Members of the Leguminosae form the largest plant family on Earth, with aro und 18,000 species. The success of legumes can largely be attributed to the ir ability to form a nitrogen-fixing symbiosis with specific bacteria known as rhizobia, manifested by the development of nodules on the plant roots i n which the bacteria rx atmospheric nitrogen, a major contributor to the gl obal nitrogen cycle. Rhizobia described so far belong exclusively to the a- subclass of Proteobacteria, where they are distributed in four distinct phy logenetic branches(1,2). Although nitrogen-fixing bacteria exist in other p roteobacterial subclasses, for example Herbaspirillum and Azoarcus from the phylogenetically distant beta -subclass, none has been found to harbour th e nod genes essential for establishing rhizobial symbiosis(3,4). Here we re port the identification of proteobacteria from the b-subclass that nodulate legumes. This finding shows that the ability to establish a symbiosis with legumes is more widespread in bacteria than anticipated to date.