Abstract

The symbiosis between legumes and nodulating Proteobacteria (so-called rhizobia) contributes greatly to nitrogen fixation in terrestrial ecosystems. Root nodulating Proteobacteria produce nodulation (Nod) factors during the initiation of rhizobial nodule organogenesis on the roots of legumes. Here, we screened the Nod factor production capacity of the previously reported nodule inducing Proteobacteria genera using their genome sequences and assessed the evolutionary history of symbiosis based on phylogenomics. Our analysis revealed 12 genera as potentially Nod factor producing taxa exclusively from alpha- and beta-Proteobacteria. Based on molecular clock analysis, we estimate that rhizobial nitrogen-fixing symbiosis appeared for the first time about 51 Mya (Eocene epoch) in Rhizobiaceae, and it was laterally transferred to multiple symbiotic taxa in alpha- and beta-Proteobacteria. Coevolutionary tests conducted for measuring the phylogenetic congruence between hosts and symbionts revealed only weak topological similarity between legumes and their bacterial symbionts. We conclude that frequent lateral transfer of symbiotic genes, facultative symbiotic nature of rhizobia, differential evolutionary processes of chromosome versus plasmids, and complex multispecies coevolutionary processes have shaped the rhizobia-host associations.

Keywords

Horizontal gene transfer; Molecular clock analysis; Nitrogen fixation; Nod factor; Phylogenomics; Rhizobiaceae

Published in

Microbiological Research
2021, volume: 250, article number: 126788
Publisher: ELSEVIER GMBH

Authors' information