The Plant Doctor

Potato research has potential for people, too

Plant medicine isn't so very different from people medicine, when you get right down to it. Sometimes even closely related bacteria are involved.

Carol Ishimaru, associate professor in bioagricultural sciences and pest management, tackles potato diseases using many of the methods and tools that other researchers use to tackle human diseases.

Ishimaru's target is bacterial ring rot of potatoes. "The disease is incurable. If just one diseased plant is found in a field, the whole field can be rejected as seed stock. This can cost a grower as much as $80,000, just for that one field."

Because ring rot spreads so quickly and easily, potatoes must be inspected and certified before they can be used as seed stock. Even table stock potatoes must be certified before they can be exported. U.S. certification standards are based on the presence of diseased plants. Canadian and European standards test for the pathogen itself. This difference in standards limits U.S. access to potential export markets.

Plant resistance allows producers to meet U.S. certification standards, but it does nothing to either help potato exporters or slow the spread of the disease. Resistant plants can harbor the bacteria without showing any symptoms. This can mask the presence of the disease and actually contribute to its spread.

"The only true defense against ring rot is prevention," says Ishimaru. Prevention is best accomplished by identifying the bacterium before it does its damage. But not all strains of Clavibacter michiganensis subs. sepedonicus cause disease; some are benign. So it is important to identify not just the bacterium, but the strain as well.

One way to separate disease-causing strains from their benign counterparts is to look for a contained suicide response. Researchers inject a nonhost plant, such as tobacco, with C. michiganensis subs. sepedonicus bacteria. If those bacteria represent a virulent strain, the tobacco plant kills off the infected parts of its leaves to contain the infection. If the strain is not virulent, there is no reaction.

C. michiganensis subs. sepedonicus is gram-positive. That is, one of the methods for identifying this particular bacterium is that it retains a blue color when treated with a special dye called Gram's stain. When Ishimaru began her investigation, very little study had focused on gram-positive bacteria. Ishimaru's research was further complicated by the fact that gram-positive bacteria are very difficult to grow in laboratory conditions.

Ishimaru teamed up with Penny Bauer, professor of bioagricultural sciences and pest management, and researchers in Finland and Canada. In 1997, they showed that gram-positive bacteria can elicit the same contained suicide response in tobacco as their gram-negative counterparts. This breakthrough paves the way for developing genetically engineered resistance to diseases caused by gram-positive bacteria, as has been done for diseases caused by gram-negative bacteria.

But plant resistance to ring rot isn't what Ishimaru is after. She is now focusing on identifying the disease-causing regions in the DNA of these bacteria. She works closely with Dennis Knudson and Susan Brown and their genome mapping team to isolate and mark these regions.

"Our goal is to come up with a specific, reliable, and sensitive test to identify virulent strains of the bacteria," says Ishimaru.

With such a test, potato growers could tell whether a field harbored a virulent strain of C. michiganensis subs. sepedonicus. If it did, they can use sanitation, crop rotation, and other cultural practices to eliminate the disease. Such a test would also open up export markets for U.S. producers.

While Ishimaru focuses on a single potato disease, her work has implications far beyond that. Gram-positive bacteria are responsible for such human scourges as tuberculosis, diphtheria, and staph. Her work with the lowly potato could open new doors for preventing these and other diseases in people, too.