The first genetically modified foods were not human creations. Scientists have now found that sweet potatoes all over the world naturally contain bacterial genes that the microbes introduced. Such transgenes may have provided attractive traits for domestication, researchers added. The scientists detailed their findings online April 20 in PNAS.
The sweet potato (Ipomoea batatas) is an ancient domesticated crop. It was independently domesticated twice—in Mexico and also in western South America, the latter by about 5000 years ago. Sweet potatoes remain a key food crop, especially in sub-Saharan Africa, parts of Asia and the Pacific Islands, where they spread even before Europeans brought back foods native to the New World. Despite its name, the sweet potato is only distantly related to the potato (Solanum tuberosum); and it’s distinct from the yam.
Study co-author Jan Kreuze, a plant virologist at the International Potato Center in Lima, Peru, and his colleagues were analyzing sweet potatoes for viral material when they discovered genetic sequences from Agrobacterium, a genus of bacteria that can transfer DNA into the genomes of plants they infect. Plant biotechnologists have adapted this capability for use in creating genetically modified crops.
The researchers weren’t convinced that the bacterial DNA they found was merely bacterial contamination of the plant samples. So Kreuze enlisted the aid of study co-author Lieve Gheysen, a plant molecular biologist at Ghent University in Belgium, and her colleagues to study these transfer DNA (T-DNA) sequences in more detail. The researchers analyzed 291 samples of cultivated I. batatas plants from the Americas, Africa, Asia, and Oceania, nine samples of wild I. batatas plants from the Americas, and four samples from three related Ipomoea species.
The scientists discovered the presence of two Agrobacterium T-DNA regions in the genomes of the cultivated sweet potatoes. One of these bacterial regions was found in all the cultivated plants, but not in any of the wild plants. The other DNA region was 42 of the cultivated sweet potato genomes they examined, two of the wild specimens, and one of the other Ipomoea species.
The group identified at least 10 genes in the T-DNAs they analyzed. The genes that are best known either produce plant hormones known as auxins or change sensitivity of plant cells to auxins.
Auxins are involved in a lot of developmental pathways in plants; one of their functions is to stimulate root growth. “Our hypothesis is that maybe these genes made these sweet potatoes different by facilitating root growth,” Gheysen says. Sweet potato farmers usually cultivate the plant through vegetative propagation — growers take stem cuttings from sweet potato vines, which then grow roots and form new plants. The T-DNA may have provided key traits for cultivation during the domestication process.
The presence of this bacterial DNA in the plant genomes means that sweet potatoes can be seen as naturally genetically modified (GM) crops, Gheysen says.
“I think it’s important to recognize that genomes are very dynamic, and what the researchers find here with the sweet potato is one example of how genetic changes can occur,” says plant geneticist Dan Voytas, director of the University of Minnesota’s Center for Genome Engineering, who did not take part in this research. “Whether this changes public opinion about the dynamic nature of genomes and whether GM crops should be part of the food supply remains to be seen.” The recent and ongoing sequencing of thousands of plant genomes should provide some indication as to how often such DNA transfer events occur, Voytas adds.
Confirming the role of these bacterial genes will be difficult. There are at least four copies of each T-DNA in cultivated sweet potatoes, and all would have to be removed. Gheysen and his group may instead try adding a copy of these T-DNAs to wild relatives to see what happens.