Several months ago, a paper was published about sweet potatoes being “natural GMOs”. It got a lot of coverage in the press. I thought that it was high time that I read the original paper to see what it was all about.
The paper is freely available in PNAS (Proceedings of the National Academy of Sciences). The paper starts by defining horizontal gene transfer. This naturally occurring process is when a gene goes from one species to another, and has been studied quite a bit in bacteria. Scientists are starting to identify instances of horizontal gene transfer in non-bacterial organisms: sometimes the gene that gets transferred ends up being non-functional, but sometimes it continues performing its original function. Consequently, horizontal gene transfer can be important in the evolution of species.
When individuals oppose GMOs claiming that these are not natural since scientists are taking a gene from one species and adding it to another, it is often pointed out that horizontal gene transfer happens “naturally” without any human intervention. To understand this point (and the importance of this paper), it is necessary to explain one of the more common methods that scientists use for transgenesis: Agrobacterium-mediated transformation.
The summary below is from this freely available review (any additional references are indicated). The Agrobacterium genus has many different bacteria that cause different plant diseases. For genetic engineering, the species used is Agrobacterium tumefaciens which causes crown gall disease. Crown galls are growths on plants, similar to tumors (when reviewing this piece, my husband informed me that many of the gardening books that he’s read highlight the fact that you’re not supposed to use pruning shears on plants that have galls without cleaning them, so that you don’t transfer the bacteria from one plant to another). Galls develop when a chunk of DNA from the bacteria, known as Ti-DNA (Tumor inducing) gets added to the plant’s own DNA. For this to happen, the DNA needs to get cut out of the bacteria, transported into the plant cell, and integrated into the plant’s genome. This process is carried out by proteins that are made by the bacteria known as vir genes (virulence), and there’s quite a few of them that perform different tasks in the transformation process.
The vir genes get activated by sensing compounds that are released when a plant is injured. Think of the injury as an alarm bell that suddenly alerts the Agrobacterium to the fact that infection is now possible. Once the vir proteins are active, they process the bacterial DNA that will be transported into the plant cell. This DNA is flanked on both sides by a very short segment of DNA that acts as a recognition site for vir proteins which then cut the DNA. Think of the short DNA segments as neon lights flashing “CUT HERE”. Once the vir proteins cut the DNA, it is transported by proteins across a channel in the plant’s cell wall. In this process, different vir proteins transport the DNA, protect the DNA from getting degraded, and also form the channel to get the DNA into the plant cell, so there are many players in this process. Once it’s in the plant cell’s nucleus, the bacterial DNA gets integrated with the plant DNA, and several mechanisms have been proposed as to how this may happen. Once the DNA gets integrated, it can activate gall-causing proteins using the plant’s own cellular machinery. The DNA that gets transferred from the bacterium to the plant is known as T-DNA (Transfer-DNA. Remember this one, because the abbreviation will be used in the paper).
In genetic engineering, the Agrobacterium has been engineered such that the bacteria no longer causes tumors. Additionally, the T-DNA consists of the gene that scientists want to transfer into the plant, such as the gene that confers Round-Up resistance, or a gene that may confer drought resistance.
Many anti-GMO websites will use emotional phrases such as “GMOs use bacteria that cause cancer in plants” (see the image from GMO Inside! that I’ve shared here). Although the statement is correct, it’s a half truth because the bacteria has been engineered to no longer cause tumors in plants. So the intent is to evoke fear by combining scary or emotional phrases and terms.
The Sweet Potato: Nature’s GMO
Why is this Paper Important
- Thousands of years ago, a bacteria closely related to the bacteria used to create GMOs, inserted a bunch of genes into the sweet potato. The GMOs currently on the market add fewer genes than what was naturally introduced into the sweet potato.
- The introduction of these genes into the sweet potato generated an “unintended consequence”: namely, that a sweet potato gene was interrupted.
- The fact that these changes are present in domesticated sweet potatoes and not wild sweet potatoes points to the strong possibility that they were selected by artificial selection. In natural selection, it’s the survival of the fittest where the genes that give reproductive and survival advantages usually win. So a mutant plant that creates a more toxic substance may propagate its genes because fewer predators will eat it. In artificial selection, it’s the genes that are most convenient for humans that win out, and we see it most commonly in agriculture and animal breeding. That means that we might select for genes that create cuddly dogs. Or we might select for genes that give rise to sweeter fruit. But that does not mean that the Chihuahuas that we’ve created and the oranges that we’ve bred are the strongest to survive out in the wild. If the genes examined in this paper did in fact give the sweet potato selective advantage out in the wild, then odds are that the wild sweet potatoes would have the gene, too. So this point, that we humans selected for a mutant that arose through transgenesis, defies the anti-GMO argument that nature has created what is naturally best over the course of evolution. The incredible irony is that what we selected for was transgenic in origin.
- I think the example of
the sweet potato can make the legal definition of the term “GMO” more difficult. If it’s defined as a crop where genes have been added by Agrobacterium, then should the sweet potato be excluded?
- This is a great example for individuals who think that genes from viruses or bacteria in crops are “unnatural” (or what I call “The Ick Factor”).