Atypical rhizobia trigger nodulation and pathogenesis on the same legume hosts

Area of Science:

• Microbiology
• Plant Pathology
• Symbiotic Interactions

Background:

• Commensalism and mutualism often evolve from parasitic ancestors.
• Rhizobium-legume interactions are typically studied as either symbiotic or pathogenic, not both on the same host.
• Dual pathogenic and nodulating abilities in a single rhizobial strain on one host have not been previously documented.

Purpose of the Study:

• To isolate and characterize a bacterial strain exhibiting both pathogenic and nodulating capabilities on legume hosts.
• To investigate the genomic and phenotypic basis of this dual ability in rhizobia.
• To identify bacterial clades associated with these mixed interaction phenotypes.

Main Methods:

• Isolation of bacterial strains from Medicago nodules.
• Phenotypic characterization of bacterial interactions with various legume genotypes.
• Genomic analysis and phylogenetic clustering of isolated strains.
• Comparative analysis with known rhizobial groups.

Main Results:

• Isolation of the T4 strain from Medicago nodules, demonstrating dual pathogenic and nodulating abilities.
• The T4 strain induced ineffective nodules competitively and acted as a parasite, causing plant death on specific legume genotypes.
• This dual ability was observed across multiple legume species within the Inverted Repeat-Lacking Clade.
• Genomic analysis placed T4 within the nonsymbiotic Ensifer adhaerens group, clustering with another strain (T173) possessing similar dual traits.
• Identification of a distinct bacterial clade encompassing these dual-ability rhizobial strains.

Conclusions:

• A novel bacterial clade of rhizobia has been identified, characterized by the ability to act as both a pathogen and a symbiont on the same legume host.
• The outcome of the interaction is dependent on the developmental stage of the plant.
• This discovery expands the understanding of the evolutionary pathways and phenotypic plasticity within rhizobium-legume interactions.