Researchers have discovered a gene that could protect potatoes against late blight, a disease that turns them into black goo. Transgenic potatoes outfitted with the gene, described in the 14 July online edition of the Proceedings of the National Academy of Sciences, could save farmers billions of dollars in fungicides.
Phytophthora infestans, a plant pathogen related to fungi, caused the Irish potato famine in the 1840s. More than 1 million of Ireland's 8 million inhabitants starved to death, and 1.5 million emigrated. Today, no commercial potato variety can fully resist the pest, which is one of the most prevalent agricultural diseases worldwide.
A wild potato, Solanum bulbocastanum, in Mexico, however, has been known for 50 years to be resistant to potato blight. But it would be troublesome to breed commercial potatoes with this trait, says Rebecca Nelson of Cornell University in Ithaca, New York. Because potatoes are generally hard to breed for just one trait, “it takes a long time, and it's hard to recover a potato that is very similar to the type demanded in the marketplace.” The goal is to develop resistant commercial varieties that are in high demand, such as the Russet Burbank potato served at McDonald's.
That's why plant pathologist John Helgeson and plant geneticist Jiming Jiang of the University of Wisconsin, Madison, and colleagues decided to try genetic engineering instead. They cloned the resistance gene, called RB, and then inserted it into a disease-susceptible potato variety called Katahdin. They inoculated transgenic Katahdin plants and normal varieties with six isolates of P. infestans in the greenhouse. In nonengineered plants, dark lesions spread from the lower to upper leaves and stalks, and the plants died within a month. Symptoms developed much more slowly in transgenic Katahdins, with dark lesions restricted to lower leaves, and the plants survived the infection.
William Fry of Cornell believes engineering potatoes to carry the gene “is an exciting possibility.” But he points out that the effectiveness of the RB gene against late blight has yet to be tested in the field. Anything can happen, he says, including “rare, unintended side effects like heightened susceptibility to other pathogens.” Helgeson says that field tests on potatoes engineered to carry the RB gene are now being conducted in Minnesota and Mexico.
Jiming Jiang's Web site 
More about late blight from the International Potato Center