Purdue News

May 8, 1989

Molecular Biologists Close In On Potato Disease Control

West Lafayette, Ind. -- Purdue University molecular biologists have made a discovery that may lead to a genetic cure for a disease that destroys millions of dollars' worth of potatoes annually.

The two Purdue researchers discovered that by combining genetic information from a less-virulent strain of the disease-causing bacteria with a more-virulent strain, the latter can be disarmed.

The findings of Avtar Handa, associate professor of horticulture, and Jeff Bennetzen, associate professor of biological sciences, will be presented May 15 in New Orleans at the 89th annual meeting of the America Society for Microbiology.

The disease, potato soft rot, strikes potatoes anywhere from the farmer's field to the grocery store to the home pantry, Handa says. One problem with controlling the disease is that it strikes without notice because the bacteria that cause it are always present, either in soil or in the potato. Symptoms of soft rot include darkened patches that appear on the skin. The rotting penetrates into the potato, making it mushy.

In ongoing studies, Handa, Bennetzen and graduate student Jiliang Chiu hope to map the molecular structure of bacterial genes that inhibit soft rot. The ultimate goal is to find a genetic "switch," common to all forms of soft rot, that can be turned on or off, to stop the bacteria in the potato from reproducing and taking over.

Flipping such a genetic switch would provide an environmentally safe method of disease control by reducing the use of chemicals.

Soft rot strikes all varieties of potatoes and is common in all parts of the world. In nations where conditions of sanitation and handling are poor, the disease is more prevalent.

For example, in the Soviet Union, where rough handling and poor sanitation cause many injuries to potatoes, soft rot has claimed as much as 75 percent of the potato crop in years past.

Handling is an important factor in prevention of soft rot. The bacteria gain access to potatoes through injuries, such as bruises, cuts and tissue weakened by temperature extremes. Because of the ease through which the bacteria invade potatoes, a genetic cure for the disease is that much more important, Handa says.

In each strain of soft-rot bacterium that Handa and his research associates studied, they found that the disease easily took over the cells in the potato. After the disease-carrying bacteria were switched on, it was only a matter of time until they took over and soft rot appeared.

Handa began trying to find ways to make the potato more resistant to soft-rot bacteria, but he took a novel approach. Because genetic coding for disease resistance is infinitely complex, he chose the backdoor approach -- attempting to make the bacteria less effective in overcoming host resistance. He knew that if this could be accomplished, it would be a milestone because there's no way to remove bacteria from the potato and its surroundings. The trick is making the bacteria unable to take over.

The researchers developed four soft-rot bacterial strains that are less effective in invading host cells than are normal strains. These four bacterial strains appeared identical to more-destructive relatives, with one important difference: They didn't overcome host resistance, and so couldn't invade healthy cells.

Handa's team then cloned the genes from the weaker strains and inserted them into strains of soft-rot bacteria known for virulence and ability to overcome the host cells. As Handa had hoped, the more-virulent strain absorbed the genetic information, began replicating it, and lost its ability to successfully attack healthy potato cells.

"We hope to take these findings and find a biological or chemical control of potato soft rot on the genetic level," Handa says.

The research is in early stages, so no delivery system is yet envisioned. Two possible methods of delivery are direct spraying on crops in the field or introduction of potato strains resistant to soft rot bacteria.

Purdue News Service: (765) 494-2096; purduenews@purdue.edu