There's no evidence that genetically modified crops are capable of causing any harm, and plenty of indications say they are safe. Nevertheless, the use of these plants is controversial in a number of countries, limiting their adoption.

One of the most frequently mentioned issues with GMO foods is a vague concern about bringing genes from distantly related organisms into plants. But an international team of biologists has now found that this has occurred naturally in a major crop plant: the sweet potato. The strains of this crop that we currently cultivate had a set of genes inserted into their genomes by bacteria—the same bacteria used to create many genetically modified plants.

The bacteria in question are part of the Agrobacterium genus, a group of plant pathogens. Upon infection, the bacteria insert a small piece of DNA (termed T-DNA) into the plant's genome. The DNA carries a number of genes that interfere with a few normal plant hormones. These changes cause plant cells to start proliferating, forming tumor-like growths.

This has turned out to be a useful tool for plant geneticists. By replacing some of the bacterial DNA with a gene (or collection of genes) of their choice, researchers can get it inserted into plants and, with a bit of work, transmitted to the next generation. This makes for one of the easiest means of making transgenic plants—genetically modified organisms—since the bacteria do a lot of the work for you.

The main barrier for this happening normally appears to be the fact that the bacterial DNA doesn't end up in plant tissues that normally contribute to the next generation (even if it did, the uncontrolled growth would probably interfere with that). Nevertheless, Agrobacterium DNA has been found in a number of plants, including tobacco.

But now researchers have found it in the sweet potato, a food crop. This plant was domesticated in South America over 8,000 years ago, and its use spread globally. While studying the RNA made in sweet potato cells, researchers found a collection of bacterial genes. Tracing them back to the DNA, they found the Agrobacterium T-DNA, along with a handful of genes from the bacteria. Further examination revealed a second cluster of genes, indicating that this natural transgenic process has happened at least twice in this lineage.

One of these two clusters of genes was found in a number of cultivated strains, as well as a handful of their wild relatives. The second was absent in the wild relatives but present in every cultivated strain of sweet potato they looked in. Thus, every sweet potato plant contains foreign genes obtained through a process similar to that used to create GMO foods.

The authors, rather optimistically, suggest that "Our finding, that sweet potato is naturally transgenic while being a widely and traditionally consumed food crop, could affect the current consumer distrust of the safety of transgenic food crops." There was plenty of information indicating GMO foods are safe prior to this finding, and it's had little effect on the public debate yet.

PNAS, 2015. DOI: 10.1073/pnas.1419685112 (About DOIs).