Bacteria spread genes to fungi on plants

27 October 2010 by Tom Marshall

A bacterium that's used to modify plants' genes can also change the DNA of completely different lifeforms in the wild, new research shows.

Genes being transferred from bacteria to fungi on plants in the lab

If the bacteria come into contact with particular fungi at a wound in a plant's outer skin, the fungi can come away with new genes from the bacteria. If these help it survive, they could become a permanent part of its genetic makeup.

This is a way genes could potentially escape genetically-modified organisms (GMOs) and move into other living things. It underscores the need to make sure these microbes are removed completely from genetically-modified plants before they leave the lab. It also shows that genes can move between organisms in more ways than has previously been assumed.

"This study suggests that the encounter between this bacterium and a fungus on the plant surface may lead to gene flow in a previously overlooked way, potentially leaking GM genes into the natural world," says Professor Gary Foster of the University of Bristol, one of the study's authors.

Agrobacterium tumefaciens is a bacterium that in the wild infects plants through wounds in their outer skin and transfers its own genetic material into them, making them form what are called 'crown galls'.

Scientists already know which bit of the bacterium's DNA gets transferred to the new host when it's infected. Removing this DNA and adding new genetic material from another source is the main tool they've used to genetically modify plants like soya beans or oilseed rape to give them desired characteristics like resistance to pesticides or higher vitamin content.

"Agrobacterium is the industry standard for getting new DNA into plants," says Dr Andy Bailey, a plant pathologist at the University of Bristol who took part in the research. "It infects many different plants in the wild and effectively carries out natural genetic engineering. But our work raises the question of whether its host range is wider than we had thought - maybe it's not confined only to plants after all."

To transfer DNA to a plant, Agrobacterium needs a wound in the plant's skin, as well as a hormone called acetosyringone, which plants make when they're injured.

Researchers already knew the bacterium can transform the DNA of living things other than plants, including fungi - whether inserting its own genes, or any others it's provided with. But until now they've had to supply the acetosyringone themselves to make this happen.

In this new research, published in the open-access journal PLoS ONE, Claire Knight, the University of Bristol PhD student on the project, used lab-grown plant samples to show that there is enough acetosyringone around a naturally-occurring wound on a plant to let the bacterium transfer genes to the common fungal pest Verticillium albo-atrum.

In most cases this won't be a problem, as the new genes won't do anything to help the fungus survive. But in particular cases, it's potentially harmful, transferring genes that confer pesticide resistance or some other valuable trait to a harmful pest species.

Companies that produce commercial GM crops already use antibiotics to remove Agrobacterium after it's done its work. But Bailey says this research suggests they may need to do more to ensure this is working. "If a plant is still carrying Agrobacterium and is planted out, not only other plants but also fungi could receive the new DNA," Bailey says. "So it's important to be absolutely certain all the Agrobacterium has been removed."

'Investigating Agrobacterium-Mediated Transformation of Verticillium albo-atrum on Plant Surfaces' - Knight, Bailey & Foster, published in PLoS ONE.