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

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Abstract
Members of the Leguminosae form the largest plant family on Earth, with around 18,000 species. The success of legumes can largely be attributed to their ability to form a nitrogen-fixing symbiosis with specific bacteria known as rhizobia, manifested by the development of nodules on the plant roots in which the bacteria fix atmospheric nitrogen, a major contributor to the global nitrogen cycle. Rhizobia described so far belong exclusively to the α-subclass of Proteobacteria, where they are distributed in four distinct phylogenetic branches1,2. Although nitrogen-fixing bacteria exist in other proteobacterial subclasses, for example Herbaspirillum and Azoarcus from the phylogenetically distant β-subclass, none has been found to harbour the nod genes essential for establishing rhizobial symbiosis3,4. Here we report the identification of proteobacteria from the β-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.