Nov 3, 2003

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Horizontal gene transfer

“Selection pressure is critical”

Can micro-organisms absorb genetic material from plants and integrate it into their own genome? And could transgenes spread in the environment in this way? Safety research has been considering these questions since the first deliberate release of genetically modified plants in Germany in the early nineties. gmosafety spoke to Kornelia Smalla from the Federal Biological Research Centre for Agriculture and Forestry (BBA) in Braunschweig.

Dr. Kornelia Smalla is a scientist at the Institute for Plant Virology, Microbiology and Biological Safety at the Federal Biological Research Centre for Agriculture and Forestry (BBA)

GMO Safety: Both you and other working groups have been able to demonstrate that horizontal gene transfer from plant DNA to micro-organisms is possible in optimised laboratory conditions. What happens under natural conditions?

Kornelia Smalla: Most studies of bacterial transformation with transgenic plant DNA used the Acinetobacter sp. BD413 strain of bacteria, which has an exceptional ability for absorbing foreign DNA. The studies showed that the absorbed plant DNA is only integrated stably in the bacterial genome, if the recipient has homologous DNA sequences.

All the working groups also used the same principle for detecting horizontal gene transfer: the repair of a damaged antibiotic-resistance gene. To repair this gene, the recipient bacterium needs DNA sections from the transgenic plant. The expression of antibiotic resistance indicates successful transfer.

It is very difficult to detect the transfer of plant DNA in bacteria under natural conditions because of naturally occurring antibiotic-resistant bacteria. It’s a bit like looking for a needle in a haystack, as the saying goes. However, more recent studies by Dr. de Vries and Prof. Wackernagel have shown that transgenic DNA which is released into the soil via the roots or from pollen in field conditions, can be taken up by Acinetobacter sp. BD413 even after several years and used to repair a damaged kanamycin gene.

As part of an EU research project, we are currently investigating which soil and rhizosphere bacteria are actually naturally transformable. These studies will also help us to better estimate the likelihood of bacterial transformation by transgenic plant DNA.

GMO Safety: How would you assess the risk of such a transfer, particularly with regard to the use of antibiotic-resistance genes as marker genes? What are the consequences?

Kornelia Smalla: On the basis of existing experimental data, we assume that the rate of transformation of bacteria with transgenic plant DNA is very low. Since the antibiotic-resistance marker genes used in transgenic plants are frequently found on mobile genetic elements such as plasmids, transposons or gene cassettes in environmental bacteria, the contribution made by the cultivation and consumption of transgenic plants to the spread of antibiotic-resistance genes, even in the light of more recent findings, is considered to be very low. Selection pressure, however, is critical for the establishment of even very rare transfer events. The problem of multi-resistant bacteria stems from the adaptation of bacteria to selection pressure extending over several years, generated by the use of antibiotics in human medicine and agriculture.

GMO Safety: Various new methods of genetic transformation, which aim amongst other things to dispense with the need for antibiotic-resistance genes, are currently under investigation. How safe are such alternative systems? Are new, different problems emerging in the light of the safety assessment of these systems?

Kornelia Smalla: In recent years a wide range of methods of genetic transformation of plants has been developed, which could provide alternatives to the use of antibiotic-resistance genes as transformation markers in the future. It is not possible to make a general statement about the safety of these systems, since each individual system needs to be assessed on a case-by-case basis. Questions also arise about the safety of alternative systems, which have to be addressed before they can be used on a commercial basis. The GMO Panel of the EFSA (European Food Safety Authority) is currently looking at the safety assessment of antibiotic-resistance genes. Its report is expected to be published before the end of the year. The GMO Panel is currently not in a position to undertake a comprehensive evaluation of the alternatives.