Feb. 6, 2006 A single gene apparently thwarts a disease-causing invader that creates a fuzzy gray coating on flowers, fruits and vegetables. But the same gene provides access to a different type of pathogen.
A Purdue University plant molecular biologist and his collaborators in Austria and North Carolina identified the gene that helps plants recognize pathogens and also triggers a defense against disease. The gene and its defense mechanisms are similar to an immunity pathway found in people and in the laboratory research insect, the fruit fly.
As Botrytis cinerea, a pathogen that makes strawberries gray and fuzzy, tries to invade a plant, the gene BIK1 recognizes the pathogen and sets off a defensive reaction. Botrytis is a type of pathogen that can infect and obtain nutrients from dead cells on a plant and actually secretes toxic substances into plant tissue in order to gain entry. Another type of pathogen, called a biotroph, must feed on live plant cells. As a strategy to contain a pathogen, plants actually kill their own cells at the site where a biotrophic pathogen is attempting to invade.
"This gene, BIK1, makes plants resistant to pathogens such as Botrytis, but it allows biotrophic pathogens to invade," said Tesfaye Mengiste, a Purdue plant molecular biologist and assistant professor of botany and plant pathology. "The mutant plant that doesn't have BIK1 actually shows decreased immunity to two pathogens, including Botrytis. But unexpectedly, it is completely resistant to virulent strains of the biotrophic bacteria."
The study of BIK1's role in plant resistance to these two types of pathogens appeared in the January issue of the journal Plant Cell. The study also shows that the gene impacts plant growth and development as evidenced by abnormally short roots, overabundance of root hairs and wrinkly leaves on plants lacking the gene, according to the scientists.
The gene produces a protein located in the plant cell membranes and shows activity that is characteristic of proteins that act as enzymes. This finding led researchers to believe that these molecules give the early signals needed to set off a relay of biochemical events allowing the plant to fight off the pathogen, Mengiste said.
"Basically the BIK1 protein does this by regulating a plant defense hormone called salicylic acid," he said. "The amount of salicylic acid determines the type and level of a plant's response to the pathogen. This is very important in terms of disease resistance.
"In this paper, we speculate that there is an optimum level of salicylic acid that is required for pathogen defense. When that level is exceeded, in some cases it may promote susceptibility to other pathogens by interfering with other defense strategies of the plants."
The research team first looked at normal plants and then at the BIK1 mutant when they began to study the effect of different hormones on plant growth and pathogen defense, Mengiste said. The scientists were surprised to find that the mutants had reduced primary root growth but increased numbers of root hairs. Along with their other findings, this revelation is leading the scientists to future research.
"It looks like this gene actually links pathogen response to plant growth and development," Mengiste said. "But how a single protein regulates these two processes that are singularly independent, we don't know. That is the main purpose of our future studies.
"We need to figure out the details of how it regulates root growth and the length and amount of root hair. This may have implications in terms of nutrient absorption or total plant biomass."
The answers eventually could lead to increased crop yield and decreased produce loss due to Botrytis and other similar pathogens, he said.
Currently, the gray mold disease caused by Botrytis destroys about 10 percent of the grape crop annually and about 25 percent to 30 percent of tomato and strawberry crops in some seasons. It also infects many other fruits, vegetables, bulbs and a variety of flowers, including petunias, geraniums and chrysanthemums. Cool, humid weather fosters the fungus, which is spread by spores. The mold can appear in fields on growing plants and on strawberries, raspberries and other foods stored in the refrigerator.
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