Taxonomy of rhizobia and agrobacteria from the Rhizobiaceae family in light of genomics

Abstract

Phylogenomic analyses showed two major superclades within the family Rhizobiaceae that corresponded to the Rhizobium/Agrobacterium and Shinella/Ensifer groups. Within the Rhizobium/Agrobacterium group, four highly supported clades were evident that could correspond to distinct genera. The Shinella/Ensifer group encompassed not only the genera Shinella and Ensifer but also a separate clade containing the type strain of Rhizobium giardinii. Ensifer adhaerens (Casida AT) was an outlier within its group, separated from the rest of the Ensifer strains. The phylogenomic analysis presented provided support for the revival of Allorhizobium as a bona fide genus within the Rhizobiaceae, the distinctiveness of Agrobacterium and the recently proposed Neorhizobium genus, and suggested that R. giardinii may be transferred to a novel genus. Genomics has provided data for defining bacterial-species limits from estimates of average nucleotide identity (ANI) and in silico DNA–DNA hybridization (DDH). ANI reference values are becoming the gold standard in rhizobial taxonomy and are being used to recognize novel rhizobial lineages and species that seem to be biologically coherent, as shown in this study.