Colorado State University scientists have made an important breakthrough in studies of bacteria that cause disease in crops worldwide but whose basic functions have eluded researchers for years.
The breakthrough proves that plants use an arsenal of gates and passwords to defend themselves against a group of gram-positive bacteria–known as club-shaped bacteria–the same way they defend themselves against other types of bacteria. The discovery could ultimately lead to the development of genetically-engineered crops that resist a wide range of diseases.
The study centered on bacteria that causes ring rot in potatoes, a disease that spawns millions of dollars in prevention and treatment costs on American farms each year. A single plant with symptoms of ring rot infection in a field of potatoes can cost a farmer as much as $80,000 in lost revenue. Because ring rot spreads so quickly and potato plants have zero tolerance for the disease, potatoes grown in the United States must be inspected and certified disease-free before being shipped out of state or overseas.
When this bacteria encounters its potato plant host, a series of complex interactions take place that signal the bacteria to invade and infect the plant. In their study, Colorado State researchers found that plants that are not specific hosts for these gram-positive bacteria display a much different reaction known as a suicide response. Like an internal quarantine, the plant protects itself from bacterial invasion by killing its own cells and surrounding the bacteria, thus cutting off the potential for disease. The Colorado State project documented this response in tobacco plants exposed to the potato ring rot bacteria.
While the suicide response has been documented in plants exposed to another group of well-known bacteria called gram-negative bacteria, Colorado State researchers were the first to document this reaction in plants exposed to gram-positive bacteria. Plant breeders have successfully engineered plants resistant to diseases caused by gram-negative bacteria, which has been a focus of research for a number of years. But gram-positive bacteria have remained a mystery, in part because they are difficult to grow in a laboratory setting and because they grow very slowly, which hinders research.
Yet gram-positive bacteria are a particularly troublesome class of bacteria that cause a wide range of diseases in a number of crops, including wheat, potatoes, alfalfa and apples. Once a crop is infected with a disease caused by a gram-positive bacteria, it is almost impossible to combat or is too costly to treat with chemicals.
"This is an exciting find because the suicide response has been used as a tool for cloning of disease resistance genes from tomato, lettuce and rice," said Carol Ishimaru, a lead researcher on the project and associate professor of bioagricultural sciences and pest management at Colorado State. "Now that we know this suicide response occurs in plants exposed to this specific form of bacteria, we can work on finding ways to develop potato plants that give this suicide response even when they are invaded by bacteria for which they are hosts."
Ishimaru said the suicide response is the result of a sophisticated interplay between plant and bacteria that works like this: When a disease-causing bacteria invades a plant, the plant will only display a suicide response if it has the resistance genes for that bacteria. The plant host manufactures products encoded by resistance genes that recognize specific antivirulence genes, or genes that fight infection, made by the bacteria. In plants that aren’t hosts for bacteria–like the tobacco plants used in the Colorado State study–the same defense reaction is triggered but it is not clear what activates the response.
The next phase of research at Colorado State will investigate the cues gram-positive bacteria give off that activates the suicide response and why plants do not express the suicide response when they are exposed to bacteria for which they are hosts.
"This has led to the identification of genes in gram-negative bacteria that are required for the bacteria to cause disease," Ishimaru said. "We hope there will be features in the ring rot bacteria that are common to gram-negative bacteria as well. If we find common traits, we may be able to develop ways to prevent disease in crops affected by forms of gram-positive bacteria."
Ishimaru has collaborated on the research with a number of other Colorado State faculty, including Dennis Knudson, Susan Knudson, and Penelope Bauer, all of the department of bioagricultural sciences and pest management; and Nora Lapitan, professor of soil and crop sciences.