Summary auto-generated
This study describes the characterization and formal naming of nitrogen-fixing bacteria that form root nodules with sulla (Hedysarum coronarium L.), a Mediterranean legume adapted to harsh environments. Researchers analyzed multiple strains previously designated informally as 'Rhizobium hedysari' using comprehensive molecular techniques including 16S rRNA gene sequencing, restriction analysis of the ribosomal RNA operon (ARDRA), DNA-DNA hybridization, and low-molecular-mass RNA profiling. The bacteria were found to form a genetically and phenotypically cohesive group distinct from previously described rhizobial species. These bacteria are fast-growing Gram-negative rods with typical rhizobial features including polyhydroxybutyrate granules, capsules, and flagella. Phylogenetic analysis revealed closest relationships to Rhizobium mongolense and other Rhizobium species. Based on all molecular evidence demonstrating sufficient divergence from known species, the authors propose Rhizobium sullae as a new species, with strain IS123T as the type strain. The bacteria exhibit remarkable host specificity, effectively nodulating only sulla and sporadically some related legumes.
Key findings
- Bacteria isolated from sulla (Hedysarum coronarium) root nodules constitute a novel species, Rhizobium sullae sp. nov., with IS123T (USDA 4950T) as the type strain
- Comprehensive molecular analysis including 16S rRNA sequencing, ARDRA, DNA-DNA hybridization, and LMM RNA profiling demonstrates R. sullae is phylogenetically and genetically distinct from previously described Rhizobium species
- R. sullae shows highest 16S rRNA sequence similarity (98.8% identity) to Rhizobium sp. USDA 1920 from Medicago ruthenica and is classified within the genus Rhizobium based on LMM RNA profiles
- All R. sullae strains display identical or highly similar molecular profiles, indicating a homogeneous species group isolated from diverse geographical origins (Spain, Italy, Northern Africa, Balearic Islands)
- The bacteria are fast-growing rhizobia with host specificity restricted primarily to sulla, with only sporadic ineffective nodulation observed on related legumes
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Abstract
A. Squartini, P. Struffi, H. Doring, S. Selenska-Pobell, E. Tola, A. Giacomini, E. Vendramin, E. Velazquez, P. F. Mateos, E. Martinez-Molina, F. B. Dazzo, S. Casella and M. P. Nuti
Dipartimento di Biotecnologie Agrarie, Universita di Padova, Agripolis, Strada Romea 16, 35020 Legnaro, Padova, Italy
This work is the completion of a series of reports describing the nitrogen-fixing bacterial symbionts of sulla (Hedysarum coronarium L., Leguminosae) and providing the grounds for their proposal as a new taxon. The introduction summarizes a large amount of previous evidence gathered on the physiology, genetics and ecology of such organisms, which have in the past been referred to provisionally as 'Rhizobium hedysari'. Upon adding 16S RNA sequencing, amplified rDNA restriction analysis of the rrn operon, DNA--DNA hybridization homology and analysis of low-molecular-mass RNA species, it is concluded that the group of strains that specifically nodulate sulla consists of a coherent set of isolates that differ from previously described rhizobia to an extent that warrants the constitution of the species boundary. The name Rhizobium sullae sp. nov. is proposed, with isolate IS123(T) (=USDA 4950(T) =DSM 14623(T)) as the type strain.