Sweet potato naturally ‘genetically modified’

Researchers from Ghent University and the International Potato Institute have discovered that sweet potatoes from all over the world naturally contain genes from the bacterium Agrobacterium. Because of the presence of this ‘foreign’ DNA, sweet potato can be seen as a ‘natural genetically modified organism (GMO)’.

The researchers discovered the foreign DNA sequences of Agrobacterium while searching the genome – this is the entire DNA-code – of sweet potato for viral diseases. Instead of contributing this peculiar finding to bacterial contamination of the plant samples, the researchers decided to study these sequences in more detail.

The sequences appeared to be present in each of the 291 tested sweet potato cultivars and even in some wild related species. Different research methods confirmed the same conclusion: the specific sequences are not due to contamination, but they are part of the sweet potato genome.

The genes in the foreign DNA sequences were also shown to be active in sweet potato, which can indicate that they provide a positive characteristic which was selected by farmers during domestication.

It is not the first time that researchers have found bacterial, fungal or viral DNA in the genome of plants or animals or “horizontal gene transfers”. In a horizontal gene transfer there is an exchange of genes between different species – in contrast to normal gene transfer from parents to progeny which occurs within one species.

Finding similar sequences is not a full proof guarantee that they are the result of horizontal gene transfer, but in the case of sweet potato there are strong indications that this has happened. Indeed, Agrobacterium is specialised to transfer a part of its own DNA, the so-called T-DNA, to plants. And it is this T-DNA that has been found in sweet potato.

Presense of Agrobacterium T-DNA in sweet potato demonstrates the possibility of DNA exchange across species barriers

The mechanism that Agrobacterium uses to incorporate its own T-DNA in the genetic material of plants forms the basis of the GMO technology. While Agrobacterium traditionally introduces its T-DNA only in a few plant cells, biotechnologists have succeeded to regenerate plants from these cells. The finding of T-DNAs in sweet potato now reveals that this extra step has also occurred in nature thousands of years ago.

Prof. Lieve Gheysen, one of the researchers involved in the study, said: “The natural presence of Agrobacterium T-DNA in sweet potato and its stable inheritance during evolution is a beautiful example of the possibility of DNA exchange across species barriers. It demonstrates that genetic modification also happens in nature. In comparison to “natural” GMOs, that are beyond our control, human-made GMOs have the advantage that we know exactly which characteristic we add to the plant.”

Sweet potato is one of the most important food crops for human consumption, especially in Sub-Sahara Africa, parts of Asia and the Pacific islands. It is one of the earliest domesticated crops with archaeological findings in caves of the Cholca Canyon in Peru that are 8,000 to 10,000 years old.

The research is published in the Proceedings of the National Academy of Sciences (PNAS).