There were two related stories that popped up in my twitter feed today about GMO potatoes; the stories intrigued me primarily because they once again call into question what it means to be a genetically modified organism (GMO).
The first story I saw was about using (now-outdated) technology to delete a gene in potato that, when deleted, increases cold-storage longevity and reduces production of the toxin acrylamide when fried.
The second was the publication of a surprising result that sweet potatoes are naturally GMOs - sometime in early stages of cultivation, sweet potatoes were infected with a bacterium that inserted its DNA into the genome of the sweet potato. It appears that this event was then actually selected for during later cultivation in early human history. This means that sweet potatoes are an unregulated GMO.
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There is certainly no question that genetic modification of organisms, and food crops in particular, is a controversial topic. New methods in biotechnology (which I diaried about here) are changing how an organism's genome can be modified, and calling into question how GMOs should be defined and regulated.
GMOs are defined according to this wikipedia article into three categories.
Traditional GMOs, defined in the wiki link as transgenic, include crops resistant to RoundUp or those that produce Bt toxin. These were made by inserting the genes of a different organism (bacterial genes in both of these cases) into plants to provide a specific benefit.
Cisgenic plants are those in which genes from the same or a very similar plant are inserted into a crop. In the recently de-regulated Arctic Apple, which doesn't brown, researchers added extra copies of some apple genes. This effectively tricked the apple into thinking these genes were from viruses, and it turned off both the normal copies of the genes as well as the ones that were inserted. Another use would be to insert a gene for pathogen resistance from corn's ancestor Teosinte back into corn.
Subgenic plants are those in which no genes are added, but genes are removed from a plant. This type of GMO has been made possible through the development of new biotechnological methods that allow researchers to cut DNA at precise positions within the genome. It is this class of mutation that showed up in the first article I saw A Potato Made with Gene Editing. Researchers at a company named Cellectis cut out a single gene in potato that is involved in producing sugar when potatoes are stored in the cold. This allows for the potato to be stored longer, and also results in a decrease in the production of the neurotoxin acrylamide when the potato is fried. Importantly, the biotechnological process used to create this potato means that it is not considered a regulated plant according to the USDA; although the genome has been modified, it is not considered a GMO.
The second article I wanted to discuss is about a GMO that wasn't made by humans, but by nature. In a surprising article published yesterday in PNAS, The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop researchers determined that sweet potato is a naturally occuring GMO.
Most GMO plants are made with the help of Agrobacterium. This naturally occuring plant pathogen transmits a portion of its DNA (called T-DNA for transfer DNA) into the plant it infects. These genes then cause the plants to grow uncontrollably and produce sugar which the bacteria use an energy source. This infection produces a growth on the plant called a crown-gall tumor (google image search results).
According to the authors:
We communicate the rather remarkable observation that among 291 tested accessions of cultivated sweet potato, all contain one or more transfer DNA (T-DNA) sequences. These sequences, which are shown to be expressed in a cultivated sweet potato clone (“Huachano”) that was analyzed in detail, suggest that an Agrobacterium infection occurred in evolutionary times. One of the T-DNAs is apparently present in all cultivated sweet potato clones, but not in the crop’s closely related wild relatives, suggesting the T-DNA provided a trait or traits that were selected for during domestication. This finding draws attention to the importance of plant–microbe interactions, and given that this crop has been eaten for millennia, it may change the paradigm governing the “unnatural” status of transgenic crops.
These two T-DNA insertions into the sweet potato genome resulted in the insertion and expression of up to nine different bacterial genes. The fact that one of the T-DNAs is present in all the cultivated, but not ancestral, sweet potatoes tested strongly suggests that this DNA insertion provided a trait that early farmers then selected for in their breeding programs, although which trait this is is unclear.
This means then that the sweet potato, the 7th most produced staple crop in the world, is a transgenic GMO.
This brings me back to the beginning, with the point that our definition of what constitutes a GMO is important. Should all sweet potatoes, now that we have determined they are a GMO, be subject to GMO regulations? Should Arctic Apples or the Cellectis potato be regulated because they were made using biotechnology that modified the genome? I'd be interested in your thoughts, and can try and answer any questions about the biology/biotechnology you have in the comments.