The Leguminosae are recognised as the largest plant family with approximately 18,000 documented species. Their success is in no small part due to their ability to form nitrogen-fixing symbioses with rhizobial bacteria. All rhizobia so far identified belong to the a-subclass of the Proteobacteria. In the June 21 Nature, Lionel Moulin and colleagues at l'Institut de Recherché pour le Dévelopment, Paris, report the discovery of legume nodulation by members of the ß-subclass of Proteobacteria.
Moulin et al. characterized rhizobia from a number of tropical legumes and found that those from the nodules of Aspalatus and Machaerium were phylogenetically distant from known rhizobia (Nature 2001, 411:948-949). Analysis of strain STM678 from the South African legume A. carnosa showed it to be most closely related to the Burkholderia bacteria (~97% identity), a genus within the ß-subclass of Proteobacteria. A subsequent PCR screen of this strain revealed the presence of nodA, nodB and nodC genes encoding the Nod (nodulation) factors that act as signalling molecules in the initiation of the nodules characteristic of legumes.
The discovery that Burkholderia bacteria are also involved in rhizobium formation could have important implications environmentally. Some of the genus are capable of breaking down certain organic compounds and could provide a cheap and effective technique for cleaning up contaminated land.
So far the rhizobia of less than 10% of the Leguminosae have been characterized. The new results suggest that the symbiotic nitrogen-fixing bacteria are a considerably more diverse group than was originally assumed.