Oct 24, 2008

Research Gene Transfer

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Agrobacteria: Natural interspecies gene exchange

“An important driving force for evolution”

The soil bacterium Agrobacterium tumefaciens can transfer segments of its plasmid DNA (called transfer DNA or T-DNA) to the genome of plants. Researchers at the Max Planck Institute for Plant Breeding Research (MPIZ) in Cologne and the University of Bielefeld have discovered during their studies of the model plant Arabidopsis thaliana (thale cress) that in one in 250 plants it is possible to transfer other DNA segments from Agrobacteria (chromosomal DNA) to the plant genome in addition to T-DNA. GMO Safety spoke to the head of the research group, Prof. Bernd Weisshaar, about the results of his work and about whether they raise new questions about the safety of transgenic plants.

Researchers have been following nature’s example and using Agrobacteria to produce genetically modified plants for around 25 years. Agrobacteria are soil-dwelling micro-organisms that are able to modify a plant’s metabolism according to their requirements. To do this, the bacteria transfer genes in the form of transfer DNA(T DNA) to the nuclear genome of plants. This T-DNA is a part of the Ti-plasmid (ti=tumour-inducing), a ring-shaped DNA molecule in the genome of Agrobacteria. T-DNA has signals at both ends which are vital for gene transfer to plants. If the bacteria find an injury site on a plant, they inject their Ti-plasmid into the plant cells. The plant cells’ metabolism is reprogrammed and crown galls are formed. The Agrobacteria are able to transfer T-DNA to plants even if foreign genes, e.g. genes that confer resistance to harmful insects, are inserted between the border signals. This mechanism is exploited when producing genetically modified organisms. In the process, scientists are of course anxious to transfer to the plant only the previously constructed T-DNA and no other DNA segments from the bacterium.

GMO Safety: In a recent research study you report on a transfer of chromosomal Agrobacterium DNA to the plant genome of Arabidopsis thaliana. What exactly are the results of this research?

Bernd Weisshaar: In a nutshell, we found that in less than one per cent of cases it is possible to insert DNA segments from the Agrobacterium chromosomes into the plant genome along with the T-DNA.

GMO Safety: You have demonstrated the transfer of chromosomal bacterium DNA to a plant genome under laboratory conditions. Does this process take place in nature as well?

Bernd Weisshaar: Probably, even though there is no evidence as yet of such a transfer in nature. Our work has shown that Agrobacteria can do more than just transfer T-DNA. Agrobacteria occur in nature and cause crown galls to form on many dicotyledonous plants. I suspect that DNA is also transferred to plants during this natural occurrence and that it has been taking place repeatedly for many millions of years.

GMO Safety: What changes might the chromosomal bacterium genes bring about in the plants?

Bernd Weisshaar: Probably very few in most cases. The likelihood of the chromosomal bacterium genes having an influence on the resulting transgenic plant is very small. For instance, the plants we examined did not have the DNA segments necessary for genes to be expressed in plants.

Nevertheless, the process is still significant. We now know from genome research on various plants that DNA exchange between organisms has taken place frequently over the course of evolution. This exchange was probably an important driving force for development during evolution. Our observations with the model plant thale cress indicate one way in which a recombination of plant DNA with foreign DNA could have taken place in nature.

GMO Safety: Have your research findings uncovered a safety hole for transgenic plants?

Bernd Weisshaar: No. The individual events used in transgenic crops already have to be characterised in detail. Researchers select the cases in which the T-DNA is transferred exactly as expected, i.e. in line with the desired objective. During the subsequent breeding process, the plants are crossed until any undesirable modifications in other parts of the genome have been eliminated.

GMO Safety: Thank you for talking to us.