Features Production Research
Modified crops reveal hidden cost of resistance

November 9, 2009 —
Genetically modified squash plants that are resistant to a debilitating viral
disease become more vulnerable to a fatal bacterial infection, according to
biologists.

November 9, 2009 —
Genetically modified squash plants that are resistant to a debilitating viral
disease become more vulnerable to a fatal bacterial infection, according to
biologists.

“Cultivated squash is
susceptible to a variety of viral diseases and that is a major problem for
farmers,” said Andrew Stephenson, Penn State professor of biology. “Infected
plants grow more slowly and their fruit becomes misshapen.”

In the mid-1990s, the U.S.
Department of Agriculture approved genetically modified squash, which are
resistant to three of the most important viral diseases in cultivated squash.
However, while disease-resistant crops have been a boon to commercial farmers,
ecologists worry there might be certain hidden costs associated with the
modified crops.

“There is concern in the
ecological community that, when the transgenes that confer resistance to these
viral diseases escape into wild populations, they will (change) those plants,”
said Stephenson, whose team’s findings appear on October 26 in the Proceedings
of the National Academy of Sciences. “That could impact the biodiversity of
plant communities where wild squash are native.”

Stephenson and his
colleagues James A. Winsor, professor of biology; Matthew J. Ferrari, research
associate; and Miruna A. Sasu, doctoral student, all at Penn State; and Daolin
Du, visiting professor, Jiangsu University, China, crossed the genetically
modified squash into wild squash native to the southwestern United States and examined
the resulting flower and fruit production.

Unlike a lab experiment,
the researchers tried to mimic a real world setting during their three-year
study.

The researchers then
looked at the effects of the virus-resistant transgenes on prevalence of the
three viral diseases, herbivory by cucumber beetles, as well as the occurrence
of bacterial wilt disease that is spread by the cucumber beetles.

"”When the cucumber
beetles start to feed on infected plants they pick up the bacteria through
their digestive system,” explained Sasu. “This feeding creates open wounds on
the leaves and when the bugs’ feces falls on these open wounds, the bacteria
find their way into the plumbing of the plant.”

The researchers discovered
that as the viral infection swept the fields containing both genetically
modified and wild crops, the damage from cucumber beetles is greater on the
genetically modified plants. The modified plants are therefore more susceptible
to the fatal bacterial wilt disease.

“Plants that do not have the
virus-resistant transgene get the viral disease,” explained Stephenson, whose
team’s work is funded by the National Science Foundation. “However, since
cucumber beetles prefer to feed on healthy plants rather than viral infected
plants, the beetles become increasingly concentrated on the healthy – mostly
transgenic – plants."

During a viral epidemic,
the transgene provides modified plants with a fitness advantage over the wild
plants. But when both the bacterial and viral pathogens are present, the beetles
tend to avoid the smaller viral infected plants and concentrate on the healthy
transgenic plants. This exposes those plants to the bacterial wilt disease
against which they have no defense.

“Wild and transgenic
plants had the same amount of damage from beetles before viral diseases were
prevalent in our fields,” said Stephenson. “Once the virus infected the wild
plants, the transgenic plants had significantly greater damage from the
beetles.”

Results from the study
show that over the course of three years, the prevalence of bacterial wilt
disease was significantly greater on transgenic plants than on non-transgenic
plants.

According to the
researchers, their findings suggest that the fitness advantage enjoyed by
virus-resistant plants comes at a price. Once the virus infects susceptible
plants, cucumber beetles find the genetically modified plants a better source
for food and mating.

“Our study has sought to
uncover the ecological cost that might be associated with modified plants
growing in the full community of organisms, including other insects and other
diseases,” said Ferrari. “We have shown that while genetic engineering has
provided a solution to the problem of viral diseases, there are also these
unintended consequences in terms of additional susceptibility to other
diseases.”