Agrobacteria - Transfer of genetic information to other bacteria?
In order to insert new genes into plant cells, a special soil bacterium is used as a ‘gene ferry’. In the past two decades a large number of genetic modifications have been carried out using Agrobacterium tumefaciens – especially on dicotyledonous plants. Since then biosafety research has been dealing with the question of whether the genetically modified agrobacteria can also be detected in the new transgenic plants, and whether they can also transfer their genetic information to bacteria that occur naturally in the plant.
When plant cells are transformed using A. Tumefaciens , plant sections, e.g. leaf pieces, are infected with the genetically modified agrobacteria. On a special selection medium , successfully transformed plant cells initially produce a mass of cells and then, in some cases, complete new plantlets. However, the agrobacteria used can also persist in the plant tissue. Killing the bacteria with substances that would rule out their survival altogether would kill the plant cells as well, or at least severely damage them. Therefore, genetic engineers attempt to minimise the presence of the agrobacteria in several ‘purification steps’ using antibiotics.
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Later in the breeding process, the regenerated plants are usually encouraged to flower and produce seeds after fertilisation. The surface of these seeds can then be sterilised. This kills the agrobacteria and the new seedling carries the new genes only integrated in its plant genome. However, there is a problem with plants that have to be propagated asexually, e.g. woody plants. Transgenic woody plants, particularly woody forest plants, must not be allowed to flower, since the introduced modified trait could ‘escape’ into the wild population, the forest, via pollen or seeds. Gene transfer riskThe fear that genetically modified agrobacteria that enter the environment along with transgenic plants could infect and transform other plants unchecked, is not very probable. Since the agrobacteria used are special bacteria that can no longer lead to the formation of crown galls on plants (plant tumours), any infection would be limited to individual cells. There could be an increased probability of horizontal gene transfer due to the fact that plants contain other bacteria as well – so-called endophytic bacteria. Several possibilities for an exchange of genetic information between agrobacteria and endophytes are conceivable in theory. For instance, bacteria have a mechanism called conjugation, with which they can exchange plasmids with one another. This means that the probability of horizontal gene transfer between bacteria is higher than the transfer of DNA from GM plants to other organisms. A BMBF-funded research project investigated therefore whether there is a possibility of genetic material being transferred between the various bacteria during the course of the transformation itself or later, through so-called persisting agrobacteria. Horizontal gene transfer was detected only in vitro and even then only when the gene transfer had been triggered specifically by an external plasmid from the bacterial community. The persistance of A. tumefaciens in transgenic plants is always taken into account as a possible risk in the safety assessment before deliberate releases. Naturally occurring agrobacteria have been proved to persist in woody plants – a fact that has been known for a long time in the case of e.g. grapevines and roses. In the application procedure for deliberate release trials, the applicant is therefore required to provide information on the agrobacteria-free state of the trial plants. In general, however, the remaining agrobacteria decline in number after passing into the soil culture. In order to prove the absence of agrobacteria, the applicant has to describe the measures taken to remove them. |
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