Insights into the history of the legume-betaproteobacterial symbiosis

Abstract

The interaction between legumes and rhizobia has been well studied in the context of a mutualistic, nitrogen-fixing symbiosis. The fitness of legumes, including important agricultural crops, is enhanced by the plants' ability to develop symbiotic associations with certain soil bacteria that fix atmospheric nitrogen into a utilizable form, namely, ammonia, via a chemical reaction that only bacteria and archaea can perform. Of the bacteria, members of the alpha subclass of the protebacteria are the bestknown nitrogen-fixing symbionts of legumes. Recently, members of the beta subclass of the proteobacteria that induce nitrogen-fixing nodules on legume roots in a species-specific manner have been identified. In this issue, Bontemps et al. reveal that not only are these newly identified rhizobia novel in shifting the paradigm of our understanding of legume symbiosis, but also, based on symbiotic gene phylogenies, have a history that is both ancient and stable. Expanding our understanding of novel plant growth promoting rhizobia will be a valuable resource for incorporating alternative strategies of nitrogen fixation for enhancing plant growth.

Root nodule symbiosis in Lotus japonicus drives the establishment of distinctive rhizosphere, root, and nodule bacterial communities

“…For example, Mimosa genera within South America and legumes from the Papilionoideae subfamily in South Africa are known to form an ancient and stable symbiosis with nitrogen-fixing Burkholderia lineages inside nodules (48)(49)(50). Members of the order Burkholderiales, which are dramatically depleted in the Lotus mutants, are also known for potent plant growth promotion activities in nonleguminous plants (51).…”

MOLECULAR AND BIOCHEMICAL CHARACTERIZATION OF RHIZOBIA FROM CHICKPEA (Cicer arietinum)

“…But currently it is discovered that rhizobia is paraphyletic group, and are categorized into proteobacteria classes i.e. beta, alpha and gamma (Benhizia et al, 2004;Angus and Hirsch, 2010). The atmospheric nitrogen is fixed by biological nitrogen fixation process and because of this, rhizobia are known as symbionts of legume crops.…”

The Structure and Evolution of Beta-Rhizobial Symbiotic Genes Deduced from Their Complete Genomes

“…The acds gene is often located on transferable elements such as plasmids in Rhizobium and Sinorhizobium/Ensifer, and has been reported to be prone to HGT, most likely through symbiosis island and plasmid exchange, and is a common and important plant-beneficial property among Fynbos Rhizobia. In the present study, we examined the divergence and mutual characteristics of symbiotic nod and nif gene organization from whole genomes, and we conclude that although the common nod genes nodABC and nodIJ are present in all symbiotic Rhizobia, with the exception of some photosynthetic bradyrhizobia [75], there is significant distance between the two clades (Alpha-and Beta-rhizobia) in their nodA phylogeny, and also that some discrepancies could be detected only through the full genome and partial sequence analyses conducted in the present study. Previously, the nodA gene of B. tuberum sv.…”

“…It is particularly interesting T is closer to free-living strains than to Beta-rhizobia from Mimosa (86.5% and 80.8% similarity, respectively), and that it was quite close to the photosynthetic symbiont Bradyrhizobium sp. BTAi1 (which does not possess nod genes; [75] with 80.6% similarity. The two types of Burkholderia Beta-rhizobia have divergent traits in nifH, as although they are still distant from most Alpha-rhizobia, the Beta-rhizobia from papilionoids were closer to Alpha-rhizobia (80.2% similarity) than they were to their Mimosanodulating cousins (74% similarity).…”