Above: Sweet potatoes (image © istockphoto.com/ppkontokuri)
Did you know? When Agrobacterium bacteria insert their genes into plant DNA, the bacterial DNA can land smack in the middle of a plant gene and cause it to stop working properly.The words “genetically modified organism” (GMO) normally describe living things that have had their genetic information altered by humans. This can happen through selective breeding, which has been going on for thousands of years. But it can also happen through genetic engineering (GE), which is done in a lab and only recently became possible.
Because they are made by scientists in a lab, genetically engineered organisms are often seen as unnatural and raise a variety of safety concerns. However, research on the sweet potato suggests that there may also be a natural form of genetic engineering. Scientists have discovered that the genome of this popular veggie contains bacterial genes. But its DNA has never been genetically engineered by humans! Has nature really been monkeying with DNA the same way that scientists do in the lab?
Genetic engineering techniques are actually based on a natural process called transformation. Bacteria use transformation to share genes with one another and sometimes with other living things. A key tool for transforming plants and fungi is Agrobacterium tumefaciens. This soil bacterium naturally attacks the roots of plants, causing them to grow tumours on their roots.
Did you know? When genes are passed down from parents to offspring, it is called vertical gene transfer. Gene transfer across organisms—such as from a bacterium to a plant, or between two bacteria—is called horizontal gene transfer. When Agrobacterium tumefaciens infects a plant, it injects a small piece of circular DNA called a plasmid. This ring of DNA carries multiple genes in between two marker sequences. These sequences are sort of like the “cut here!” markings on a package. Once inside the plant, the genes are snipped out of the plasmid and randomly inserted into the plant’s DNA. This causes the plant to express bacterial genes and grow tumours. These tumours also provide a cozy environment where the bacteria can grow and multiply.
In the late 1970s, scientists began to realize they could use Agrobacterium’s natural invasion system to their advantage. By inserting different genes between the marker sequences in the plasmid, they could control what genes were integrated into a plant’s genome. This technique can be used to give a plant new abilities, just like when you install a new app on your smartphone. For example, a gene from the bacterium Bacillus thuringiensis (Bt) has been inserted into corn and other plants to create Bt crops that are resistant to insects.
But don’t forget that this process of bacteria inserting their genes into plants has been naturally going on for millions of years without any help from humans. And regardless of how a gene gets into a plant’s DNA, once it’s there it will get reproduced every time an infected cell divides.
Did you know? Genetic researchers uncover the functions of different genes by transferring them from one organism to another, by intentionally causing mutations, or by removing the gene entirely.And when these bacterial genes end up in the germ cells that plants use to reproduce, they can be passed along to the next generation. So as long as the bacterial genes don’t threaten a plant’s ability to survive or reproduce, they can end up in a lot of the plants in a particular species. And because this process has been happening for millions of years, it makes sense that there would be plant species that survived a bacterial attack thousands of years ago and still carries the genetic scars.
Scientists went looking for evidence of ancient bacterial invasions and found bacterial genes in the genomes of some plants, such as the wildflower Linaria vulgaris and Nicotiana glauca, a relative of tobacco. But unlike these two plants, the sweet potato is eaten by millions of people around the world. In fact, it is the only known cultivated edible plant that naturally contains bacterial genes.
In this case, the ancient genetic engineer was probably Agrobacterium rhizogenes, a close cousin of Agrobacterium tumefaciens that causes hairy root disease. Plants infected with its DNA grow big hairy branches instead of tumours. In fact, the hairy roots on infected sweet potatoes may have inspired ancient humans to cultivate them instead of non-infected varieties!
Did you know? The genomes of many plants and animals, including humans, include gene sequences from bacteria, viruses, and other microbes!
The genes inserted into the genomes of the sweet potato and other plants that have been “naturally genetically engineered” are all from Agrobacterium. By contrast, genetically engineered plants created in a lab can contain a wide range of genes from different sources. These genes can also serve a number of different purposes. Their safety and possible impact on human health and the environment always need to be carefully assessed. But the discovery of bacterial genes in the DNA of a popular food crop like sweet potatoes shows that the method used to create genetically engineered crops is not unnatural or harmful on its own. Sweet!
General information on genetically modified organisms and their safety:
Scientific articles on the natural transfer of genes between different species:
The genome of cultivated sweet potato contains Agrobacterium T-DNAs with expressed genes: An example of a naturally transgenic food crop (2015)
T. Kyndt et al., PNAS 112
Expression of multiple horizontally acquired genes is a hallmark of both vertebrate and invertebrate genomes (2015)
A. Crisp, C. Boschetti, M. Perry, A. Tunnacliffe & G. Micklem, Genome Biology 16
Domestication: Sweet! A naturally transgenic crop (2015)
Jonathan Jones, Nature Plants 1
Link to abstract. Registration or subscription required to view full text.
Horizontal gene transfer from genus Agrobacterium to the plant linaria in nature (2012)
T. V. Matveeva, D. I. Bogomaz, O. A. Pavlova, E. W. Nester & L. A. Lutova, Molecular Plant-Microbe Interactions 25
Agrobacterium-mediated plant transformation: the biology behind the “gene-jockeying” tool (2003)
S. B. Gelvin, Microbiology and Molecular Biology Reviews 67
News articles on the presence of bacterial and viral DNA in human and plant genomes: