Conjugative transfer between Rhizobium etli endosymbionts inside the root nodule

Area of Science:

• Microbiology
• Plant-Bacterial Interactions
• Genetics

Background:

• Legume nodules are primarily known as sites for biological nitrogen fixation by rhizobia.
• These nodules were traditionally considered exclusive niches for nitrogen-fixing and plant growth-promoting bacteria.

Purpose of the Study:

• To investigate a novel function of legume nodules beyond nitrogen fixation.
• To determine if nodules serve as a niche for bacterial plasmid acquisition via conjugative transfer.

Main Methods:

• Utilized Rhizobium etli CFN42, a bacterium that nodulates Phaseolus vulgaris (common bean).
• Employed confocal microscopy and flow cytometry to visualize and quantify plasmid transfer.
• Manipulated Quorum-Sensing (QS) regulators (traI, traM) to control conjugation within and outside nodules.

Main Results:

• Demonstrated conjugative transfer of the plasmid pRet42a on the root surface and, unexpectedly, within nodules.
• Confirmed conjugation inside nodules even when transfer was restricted on the plant surface using gene expression controls.
• Showed that nodule-specific gene expression or loss of unstable vectors inside nodules did not prevent conjugation.

Conclusions:

• Legume nodules function as a niche permissive to bacterial plasmid acquisition through conjugation.
• Nodules provide a protected environment facilitating bacterial genetic diversification.
• This finding expands the known roles of legume nodules in plant-microbe interactions.