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Horizontal gene transfer – experimental research into gene transfer from plants to soil micro-organisms

(1989 – 1993) Free University Berlin, Institute of Applied Genetics

Topic

The transfer of genetic information between different species is called horizontal gene transfer. So far, we are aware of gene transfer taking place between different species of bacteria.

In connection with the release of transgenic plants, it is now feared that gene transfer could also take place from plants to micro-organisms and that the transgene could therefore spread uncontrollably. This possibility is, however, classed as highly improbable because it assumes that the foreign DNA from the plants will integrate in a stable form into the genome of the micro-organisms and that it will be passed on to the progeny.

The aim of the research project was to gather information regarding the spread of the transgene from genetically modified crop plants to bacteria and fungi in the soil.

Summary

The experimental model system provided indicators for a possible horizontal gene transfer from plant to fungus, although it was unable to demonstrate this unambiguously in further tests.

The experimental research showed that after being cultivated with antibiotic-resistant plants of the genus Brassica, colonies of the soil fungus Aspergillus niger exhibited resistance to antibiotics. In general though, a loss of antibiotic resistance was detected after reproduction of the resistant fungus colonies.

Experiment description

Transgenic oilseed rape varieties like Brassica napus and Brassica nigra, which had previously been genetically modified with an antibiotic resistance gene, were selected as model plants for the investigation of possible horizontal gene transfer from plants to micro-organisms.

Resistance to the antibiotic Hygromycin B was selected for the model system. Then the crops were cultivated together with various bacteria and fungi isolates, which had been obtained from soil samples taken from an oilseed rape location. Following joint cultivation, the microbial DNA was investigated for the presence of the antibiotic resistance gene.

The discovery of resistant populations would indicate gene transfer. In order to check that the micro-organisms used did not already have a natural antibiotic resistance gene, the micro-organisms were tested for this beforehand.

Results

Presence of natural antibiotic resistance in bacteria and fungi

The research results corroborate the known fact that a large number of soil micro-organisms – at least those that could be cultivated – have natural antibiotic resistance. Of ten tested species, three species of bacteria and five species of fungus were resistant to the antibiotic Hygromycin B.

Horizontal gene transfer

The results are not clear-cut. Following joint cultivation of transgenic oilseed rape plants with various types of micro-organism, it was possible to select resistant colonies only in the case of the soil fungus Aspergillus niger. In general, however, a loss of antibiotic resistance was detected following reproduction of the resistant fungus colonies. The reason for this is assumed to be the instability of the absorbed foreign DNA, so that its transmission to progeny was not guaranteed.

The process of assimilation of foreign DNA by the fungus is not clear, and it is suspected that instead of being integrated into the genome, the foreign DNA was present outside the genome (extra-chromosomal).

The authors note that more appropriate experimental conditions need to be created for further research to investigate horizontal gene transfer.