Summary auto-generated
This study examined 16S rRNA gene sequences from seven Agrobacterium strains, eight Rhizobium strains, and related species to clarify the taxonomy of the genus Agrobacterium. Phylogenetic analysis revealed that Agrobacterium and Rhizobium species are phylogenetically intertwined and cannot be clearly separated as distinct genera. Within Agrobacterium, the three biovars and Agrobacterium rubi showed distinct groupings. The researchers propose significant taxonomic revisions: Agrobacterium tumefaciens should be rejected as a species name because its type strain belongs to biovar 1. They propose that biovar 1 strains be classified as Agrobacterium radiobacter and biovar 2 strains as Agrobacterium rhizogenes. The genus description is revised with Agrobacterium radiobacter as the type species. These changes reflect the current understanding that pathogenic traits such as tumor and hairy-root formation depend on plasmids rather than chromosomal DNA, and that chromosome-based classification better reflects true evolutionary relationships among these bacteria.
Key findings
- Agrobacterium and Rhizobium species are phylogenetically entwined based on 16S rRNA analysis and cannot be separated as distinct genera
- Agrobacterium tumefaciens should be rejected as a valid species name because its type strain is classified as Agrobacterium radiobacter (biovar 1)
- Biovar 1 and biovar 2 strains represent two distinct species: Agrobacterium radiobacter and Agrobacterium rhizogenes, respectively, based on chromosomal DNA analysis
- The traditional separation of Agrobacterium species based on phytopathogenic characteristics (plasmid-encoded traits) does not reflect true chromosomal DNA relationships
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Abstract
The 16S rRNA sequences of seven representative Agrobacterium strains, eight representative Rhizobium strains, and the type strains of Azorhizobium caulinodans and Bradyrhizobium japonicum were determined. These strains included the type strains of Agrobacterium tumefaciens, Agrobacterium rhizogenes, Agrobacterium radiobacter, Agrobacterium vitis, Agrobacterium rubi, Rhizobium fredii, Rhizobium galegae, Rhizobium huakuii, Rhizobium leguminosarum, Rhizobium loti, Rhizobium meliloti, and Rhizobium tropici. A phylogenetic analysis showed that the 15 strains of Agrobacterium and Rhizobium species formed a compact phylogenetic cluster clearly separated from the other members of the alpha subclass of the Proteobacteria. However, Agrobacterium species and Rhizobium species are phylogenetically entwined with one another, and the two genera cannot be separated. In the Agrobacterium species, the strains of biovar 1, biovar 2, Agrobacterium rubi, and Agrobacterium vitis were clearly separated. The two biovars exhibited homogeneity in their phenotypic, chemotaxonomic, and phylogenetic characteristics, and two species should be established for the two biovars. We considered the nomenclature of the two biovars, and revised descriptions of Agrobacterium radiobacter(for the biovar 1 strains) and Agrobacterium rhizogenes(for the biovar 2 strains) are proposed. The name Agrobacterium tumefaciens is rejected because the type strain of this species was assigned to Agrobacterium radiobacter, and consequently the description of the genus Agrobacterium is revised.