Finding a cure for potato blight

USA Today (Society for the Advancement of Education), June, 1995

A team of Purdue University researchers successfully has engineered a potato that is resistant to late blight disease, the infestation that caused the catastrophic Irish potato famine of 1845. It may seem like the breakthrough is coming 150 years too late, but the disease is still the numberone pest of potato plants worldwide, and new varieties of the fungus are resistant to current pesticides.

In addition, the research should result in plants that naturally can fight off a number of fungal infestations, greatly reducing the amount of pesticides needed to grow these crops. The researchers are working to transfer this natural resistance to crops such as corn, soybeans, peanuts, and any other plants susceptible to fungal diseases.

Late blight disease, caused by Phytophthora infestans, may be remembered forever for bringing about the Irish potato famine in 1845-46 in which 1,000,000 people died and another 1,000,000 emigrated to avoid starvation.

"It's a pretty nasty disease that's been controlled in the United States through the use of pesticides, hygiene, and quarantine," points out Ray Bressan, professor of horticulture. "The fungus has to have the precise weather conditions to develop, but when those are present, a farmer can go into the field later and the plants are all down on the ground, wiped out completely."

Although the fungus has been controlled in the U.S. for the past 140 years, a new variant of the fungus, known as A2, came into the country in 1991 from Mexico or Central America, presumably on tomatoes. "Now there's a lot more concern about this disease, especially since the two types can mate and recombine."

To develop the naturally resistant variety of potato, the researchers first identified a gene in tobacco that codes for a protein called osmotin. This protein, which is produced by many plants under stress, was known to kill the fungus in a petri dish. "We've been working on this gene for 10 years," Bressan explains. "Its role as an anti-fungal gene was proven in 1991. We'd had suspicions of its anti-fungal characteristics because the DNA sequence of that gene is similar to that of other anti-fungal genes, but other groups developed the tests and showed it did kill the fungus." The gene codes for the osmotin protein, which has two parts: a receptor that recognizes specific fungi, and an active component that kills the fungi by chemically drilling holes in their cell membranes.

The team removed the gene from tobacco, combined it with a promoter gene sequence that would increase the amount of protein produced, and moved the genetic material into potato plants. The additional osmotin in the genetically engineered potato plants enables them to fight off the late blight fungal infestations for one or two days. "Weather conditions have to be just right for the onset of the disease, and a delay of a couple of days could have a large effect on decreasing the disease."

The osmotin protein has no known human health effects. "It's very, very unlikely that it has any toxicity because it occurs naturally in all plants." The researchers' next step is to try to create varieties that produce the fungi-fighting proteins in specific parts of the plants where diseases strike. "We're trying to get it secreted outside of the plant cells on the surface of the plant, which we think will improve the plant's resistance," Bressan indicates.