Nov. 5, 2001 For the first time, a technique known as "gene silencing" has produced resistance to a bacterial disease in crop plants, report researchers at the University of California, Davis.
The technique holds promise for creating genetically engineered trees and vines that can stave off crown gall, a costly disease that affects many perennial fruit and nut crops, including walnuts, apples and grapes.
Professor Abhaya Dandekar and doctoral candidate Matthew Escobar, both of the UC Davis pomology department, discovered that gene silencing can be used to interrupt the process of tumor formation in crown gall disease. Their findings will appear in the Oct. 30 issue of the Proceedings of the National Academy of Sciences.
"Gene silencing is one of the most exciting things happening in the plant sciences," said Dandekar, an authority on genetically engineered tree crops. "There have been clues that this technique might be used to produce disease resistance in plants, but we were surprised to discover just how well it actually works in the case of crown gall disease."
Crown Gall Disease
Crown gall is caused by the common soil bacterium Agrobacterium tumefacians, which has the unique ability to transfer its own DNA into the DNA of the plant it infects in a process known as "horizontal gene transfer."
The bacteria usually invade the plant at a spot where it has been wounded. The wounded plant cells release certain chemicals that make it possible for the bacterial DNA to be incorporated into the plant's DNA. Genes contained in this bacterial DNA then become active in the nuclei of the plant cells.
This molecular takeover of the plant's genetic code causes the plant to produce the proteins that trigger tumor formation. The tumors appear as galls or lumps of tissue near the base of the plant's stem. Galls can damage the plant by blocking transport of nutrients and water up and down the plant stem or tree trunk.
"This is a unique mode of parasitizing the plant," said Escobar. "By inserting a section of its own DNA into the plant's DNA, the bacteria cause the plant to form the gall, which, in turn, provides the food and shelter for production of more bacteria."
In this study, Dandekar and Escobar targeted two bacterial genes that are key to tumor formation. They used the gene-silencing technique to "turn off" those genes and prevent the galls from forming on the plants. The work was done on the tomato plant and on Arabidopsis thaliana, a small flowering plant commonly used in research.
The job of all genes is to produce proteins. Gene silencing works by interrupting or suppressing the activity of a targeted gene, preventing it from coordinating production of specific proteins. In this case, the researchers targeted the bacterial genes that cause overproduction of plant growth hormones. It is this hormonal overproduction that results in uncontrolled cell growth and gall formation.
Thus, by using the gene-silencing technique, the researchers produced genetically engineered plants that could still be infected by Agrobacterium tumefaciens but would not produce the hormones that lead to gall formation.
"Usually when you try to prevent disease, you look at stopping the bacteria or other disease-causing agents at the 'front door' and preventing them from entering the plant," said Dandekar. "Here, we're doing the opposite. We're allowing the bacteria in, then slamming the door on them."
Analytic tests indicated that there is greater than a 90 percent reduction in gall formation among the genetically engineered tomato and Arabidopsis plants. Other than the lack of galls, however, the genetically engineered plants did not look any different than their non-transgenic counterparts.
Applications for gene silencing
The researchers suggest that this gene-silencing technique will be especially useful for crops that use rootstocks -- the trunk and root portions of a plant -- on which the desired variety is grafted. Farmers use grafted rootstocks to grow many tree and vine crops.
Some day, gene silencing may be used to produce disease-resistant rootstocks for growing non-transgenic crops. The rootstocks would carry the disease-resistance traits introduced through gene silencing. The harvested crop, however, would not be genetically altered, and there would be no possibility of unintentionally transferring the foreign genes to other domestic or wild plants through pollen.
Dandekar and Escobar are now using the gene-silencing technique to produce crown-gall resistance in walnut trees.
Their research is funded by the Walnut Marketing Board of California and the University of California BioSTAR Program.
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