Nodulation of legumes by members of the beta-subclass of Proteobacteria (vol 411, pg 948, 2001)

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 fix atmospheric nitrogen, a major contributor to the g lobal nitrogen cycle. Rhizobia described so far belong exclusively to the a lpha -subclass of Proteobacteria, where they are distributed in four distin ct phylogenetic branches(1,2). Although nitrogen-fixing bacteria exist in o ther proteobacterial subclasses, for example Herbaspirillum and Azoarcus fr om the phylogenetically distant beta -subclass, none has been found to harb our the nod genes essential for establishing rhizobial symbiosis(3,4). Here we report the identification of proteobacteria from the beta -subclass tha t nodulate legumes. This finding shows that the ability to establish a symb iosis with legumes is more widespread in bacteria than anticipated to date.