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 alpha-subclass of Proteobacteria, where they are distributed in four distinct phylogenetic branches. Although nitrogen-fixing bacteria exist in other proteobacterial subclasses, for example Herbaspirillum and Azoarcus from the phylogenetically distant beta-subclass, none has been found to harbour the nod genes essential for establishing rhizobial symbiosis. Here we report the identification of proteobacteria from the beta-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.
SummaryLegumes in the genus Aeschynomene form nitrogen-fixing root nodules in association with Bradyrhizobium strains. Several aquatic and subaquatic species have the additional capacity to form stem nodules, and some of them can symbiotically interact with specific strains that do not produce the common Nod factors synthesized by all other rhizobia. The question of the emergence and evolution of these nodulation characters has been the subject of recent debate.We conducted a molecular phylogenetic analysis of 38 different Aeschynomene species. The phylogeny was reconstructed with both the chloroplast DNA trnL intron and the nuclear ribosomal DNA ITS/5.8S region. We also tested 28 Aeschynomene species for their capacity to form root and stem nodules by inoculating different rhizobial strains, including nodABCcontaining strains (ORS285, USDA110) and a nodABC-lacking strain (ORS278).Maximum likelihood analyses resolved four distinct phylogenetic groups of Aeschynomene. We found that stem nodulation may have evolved several times in the genus, and that all Aeschynomene species using a Nod-independent symbiotic process clustered in the same clade.The phylogenetic approach suggested that Nod-independent nodulation has evolved once in this genus, and should be considered as a derived character, and this result is discussed with regard to previous experimental studies.
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