A team of researchers with the University of Tennessee (UT) has discovered a unique plant gene that may alter research and production of genetically modified crops.

Ayalew examines plant tissue regeneration from leaf discs transformed with the plant marker gene. Photo by R. Maxey, The University of Tennessee Institute of Agriculture.

In a paper to be published in the September issue of Nature Biotechnology, UT plant scientists Mentewab Ayalew and Neal Stewart document the first use of a plant gene as an antibacterial selection marker. Selection markers are invaluable tools for producing genetically modified plants.

“Our discovery of a plant-based selection marker will certainly impact the public debate over the use of genetically modified crops,” Stewart said. “It has the potential to make genetically modified crops more attractive to markets overseas.”

Since the 1980s researchers have been pairing genes for desirable plant traits with bacterial markers and inserting them into target plants. By using bacterial genes resistant to certain antibiotics as markers, researchers can separate genetically modified plants from parent tissue. The genetically modified plants exposed to the antibiotics live while parent tissue is killed.

However, inserting bacterial selection markers into plant genomes has raised questions about the possible transfer of the engineered genes. The theoretical risk is an increase in antibiotic resistance in bacteria that might come in contact with modified plants, which could lead to human health risks.

“Because our marker originates from a plant, it is highly unlikely any horizontal gene transfer would result in antibiotic-resistant bacteria,” Stewart said. Concerns over the biosafety of genetically modified crops are among the chief objections to their production and sale. Both researchers say their discovery will influence scientific debate about genetically modified crops and be a valuable open-source tool for researchers worldwide.

Stewart holds the Racheff Chair of Excellence in Plant Molecular Genetics in the UT Institute of Agriculture and is the author of the book Genetically Modified Planet (Oxford University Press, 2004). Ayalew holds a post-doctoral appointment in the UT Department of Plant Sciences.

Their paper, “Overexpression of an Arabidopsis thaliana ABC transporter confers kanamycin resistance to transgenic plants,” is available in the August 21 advance online version of Nature Biotechnology.

Further research involves monitoring the gene’s expression in canola and other mustard species as well as investigating its activity in bacteria.

Click here for a PDF explaining two theoretical pathways for horizontal gene transfer (89 KB)

Related news release from Nature

Replacement found for bacterial DNA in transgenic crops
Possible spread of antibiotic resistance to gut bacteria squelched by using weed genes.

Roxanne Khamsi, Nature

Scientists may have developed a potentially less controversial way to bioengineer plants, by replacing a marker gene normally borrowed from bacteria with a gene from weeds. The new technique could make genetically modified crops less contentious in places such as Europe, the team says.

Modern technology allows experts to mix and match DNA from different organisms to enhance favourable crop properties; a gene from fish, for example, can make tomato plants frost resistant.

Most transgenic crops also contain a bacterial gene, which helps researchers distinguish between plants that have successfully picked up foreign genes and those that haven't during crop development. The two genes, one for the favourable trait and one for antibiotic resistance, are tacked together and inserted into seeds. When the growing plants are then doused with antibiotic, those that haven't picked up the foreign genes die off.

The marker gene typically comes from the Escherichia coli bacterium. But critics of the technology have pointed out that the code for antibiotic resistance could hop, in a process known as horizontal gene transfer, from the bioengineered food we eat into the bacteria that live in our gut, thereby creating a superbug and a health menace.

Gene for gene

Some companies take an extra step to remove the antibiotic-resistant gene before marketing their seeds. But this doesn't always happen.

Now Neal Stewart and Ayalew Mentewab of the University of Tennessee in Knoxville, Tennessee think they have a more foolproof way to eliminate this threat, which involves scrapping the E. coli gene and using one from a plant instead.

A gene called Atwbc19 in the well-studied weed Arabidopsis thaliana also confers antibiotic resistance; when this gene is expressed at unusually high levels it helps to capture and squelch antibiotic compounds.

Stewart and Mentewab designed a piece of DNA including this gene and another that codes for blueish pigments, making plants that pick it up easily identifiable. The Atwbc19 gene is three times larger than the antibiotic-resistance gene from bacteria. Both the large size of the gene and the fact that it comes from plants makes it less likely to hop into microbes, they say.

Weeding out fears

To test whether the Arabidopsis gene worked, they incorporated the linked genes into tobacco plants; the tobacco seedlings with the Arabidopsis gene continued to grow when blasted with antibiotics. The results appear in the journal Nature Biotechnology.

Microbiologist Michael Syvanen of the University of California, Davis agrees that the study could calm fears about GM crops, particularly since the plant gene simply can't be expressed by bacteria. "It produces a gene which, if displaced by horizontal gene transfer back into bacteria, would never be able to confer resistance to antibiotics," he says.

The technique could be adopted in parts of the world that have remained skeptical about bioengineered foods, suggests Stewart. "There would be some interest, especially in Europe, to move away from a bacterial gene towards a plant gene," he says. But he cautions that further testing is needed. Scientists must demonstrate, for example, that the protein made from the weed gene has no negative effect in humans.