Screening of rhizosphere and soil bacteria for transformability

Area of Science:

• Environmental microbiology
• Bacterial genetics

Background:

• Horizontal DNA transfer is crucial for bacterial evolution.
• Natural transformation is a key mechanism for DNA uptake by bacteria.
• Understanding natural transformation in soil environments is vital for assessing risks associated with transgenic plant DNA.

Purpose of the Study:

• To determine the frequency of naturally transformable bacteria in bulk and rhizosphere soil.
• To investigate the potential for horizontal DNA transfer from transgenic plants to soil bacteria.
• To assess the efficiency of different transformation strategies using plasmid and genomic DNA.

Main Methods:

• Transformation assays using IncQ plasmids (pSM1890, pSM1885) and mini-Tn5 tagged genomic DNA.
• Testing 262 bacterial isolates and complex soil communities in microtiter plates and on filters.
• Inoculating tobacco plants with bacterial isolates and applying genomic DNA in rhizosphere studies.
• Utilizing Acinetobacter baylyi BD413 as a positive control.

Main Results:

• Only one putative transformant (Brevundimonas vesicularis) was obtained, but results were not reproducible.
• No transformants were detected in bulk soil isolates or rhizosphere communities.
• Positive control (Acinetobacter baylyi BD413) showed reduced transformation frequencies when mixed with rhizosphere samples.
• Sequencing confirmed transformants belonged to Acinetobacter sp., suggesting genomic rearrangements or stimulated competence.

Conclusions:

• The proportion of naturally transformable bacteria in bulk and rhizosphere soil is negligibly low.
• Horizontal gene transfer from transgenic plants to soil bacteria via natural transformation appears to be a rare event.
• Further research may be needed to understand factors influencing bacterial competence in soil environments.