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New strains of symbiotic nitrogen fixing Proteobacteria
June 19, 2001Nitrogen is essential for plant growth. Legumes, representing the Earth's largest family of plants, can fix atmospheric nitrogen necessary for their growth thanks to an association with bacteria, the rhizobia. This ability proves to be particularly valuable when soils are poor in nitrogen. The nitrogen fixing symbiosis occurs in nodules, outgrowths induced by bacteria, usually on roots but also, more rarely, on stems.
Taxonomically the rhizobia were up to now classified in three distinct branches of the alpha-Proteobacteria subclass (1). Recently, IRD scientists from the research unit UMR "Symbioses tropicales et méditerranéennes" described rhizobia belonging to a fourth branch of this subclass. They subsequently began to exlpore the genetic diversity of symbiotic nitrogen-fixing bacteria and analysed a collection of rhizobia isolated from roots of tropical legumes. Among these, two bacterial strains appeared to be genetically different from all the rhizobia hitherto identified. Ribosomal DNA sequencing of these two strains has shown that they belong to the Burkholderia genus, a member of the beta-Proteobacteria subclass. They are therefore phylogenetically distant from rhizobia described to date. The nodule-forming ability of the two strains was confirmed by root inoculation of a tropical legume. The two strains, one of which was isolated from South Africa, the other from French Guiana, would very likely be different species. Investigations under way should determine this.
Three genes (nodABC) involved in triggering nodule induction are essential for establishing the symbiosis. Polymer chain reaction (PCR) amplification analysis of the genome of the one bacterial strain has shown that it also contain these genes. It appears thus that both the alpha- and beta-Proteobacteria to install symbioses with leguminous plants.
The rhizobia so far characterized in detail have been isolated from only less than 10 % of the 18 000 species of Leguminosae recorded on Earth. These recent results of this latest research show that the symbiotic nitrogen-fixing bacteria are much more diverse than previous studies suggested. They open the way for discoveries of new rhizobia among the beta-Proteobacteria or even in other taxonomic groups. They should contribute to better understanding of the origin and evolution of legume-rhizobia symbioses. Interesting prospects could also emerge for applications. For instance, bacteria of the Burkholderia genus are known for their ability to degrade certain organic compounds. The legumes that form an association with the Burkholderia could serve as a reservoir of in situ pollution-removing bacteria and therefore be used to regenerate vegetation on contaminated land.
(1) The Proteobacteria are one of the 25 phyla of bacteria; undoubtedly one of the largest, this phylum comprises 387 genera which are grouped into 5 phylogenetic branches.
Institut de Recherche pour le Développement, Paris (IRD)
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