Natural Transgenic Plants: The Sweet Potato story

Genetically Modified Organisms (GMOs) have been the source of much controversy, with GM technology provoking very strong reactions. The plant science community is increasingly engaging with GM critics to explain the science better (see the Sense about Science publication and Mark Lynas’s blog for more), however a recent PNAS paper may provide a significant contribution to this debate, with the finding that sweet potatoes are naturally transgenic plants.

Philippe Sakalis - Agrobacterium
Mechanism of T-DNA transfer between Agrobacterium and the plant cell. Image from http://www.science.leidenuniv.nl/index.php/ibl/philippe_sakalis/research

While the authors were small RNA sequencing the genome of sweet potato (Ipomoea batatas [L.] Lam.), and unexpectedly found sequences that had homology to genes of Agrobacterium, a natural plant pathogen. This pathogen inserts copies of a region of DNA known as the T-DNA into the plant genome, resulting in the development of crown galls on the plant, and the production of opines which are sources of nutrition for the bacterium. This T-DNA mechanism is the basis of many of the controversial GM approaches, with scientists inserting genes of interest into the T-DNA. Finding T-DNA sequences in the sweet potato genome indicates that Agrobacterium has naturally genetically modified this crop plant.

After finding T-DNA sequences in one cultivar of sweet potato, the researchers looked in other cultivars and wild relatives of sweet potato. Interestingly, all of the 291 cultivated varieties contained the T-DNA, but these sequences were not found in the wild relatives, suggesting a T-DNA derived trait was actively selected for in the domestication of the species. The researchers analysed the expression of the T-DNA by qPCR, and found that T-DNA genes are expressed in multiple organs of the plant including the tubers, indicating that Agrobacterium-derived proteins are part of the edible part of the plant.

sweetpotato
Sweet Potato – a naturally occurring GMO

Finding that sweet potato contains these T-DNAs in it’s genome is significant, as it provides additional support for the idea that GM is not a completely artificial process, as horizontal gene transfer is common between a range of different species. The fact that sweet potato is a food crop also provides evidence that there are no long term health consequences to introducing transgenic material into the food chain, which is a common concern amongst the public. The battle to change public perception on GMOs is far from over, but this is a very useful study to be able to bring to the table.

Reference: The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop. Kyndt et al 2015, PNAS DOI: 10.1073/pnas.1419685112

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