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IMAGE CREDITS: Shirley Owens, Microbe Zoo Project, Comm Tech Lab, Michigan State University.
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The bacterium, Agrobacterium, is like a mercenary commando. It invades a living plant, commandeers it's genetic machinery, throws the plant's cells into anarchy, and forces the plant to produce K rations for the invading bacterium and its legions in the soil.
The anarchy that Agrobacterium causes plants is uncontrolled growth of cells into masses of tissue called tumors. Different species of Agrobacterium form different types of tumors. Agrobacterium tumefaciens causes a tumor called a crown gall. Agrobacterium rhizogenes, like the name implies, causes the sprouting of root tissue from an infection site, a condition known as hairy root. Agrobacterium rubi causes cane gall of raspberries.
The weapon Agrobacterium tumefaciens weilds is a circular bit of DNA called the Ti plasmid. Ti is short for tumor inducing. Part of the DNA from the Ti plasmid (the T-DNA) infiltrates the plant's DNA. Once this bit of DNA from Agrobacterium's Ti plasmid is integrated into the plant's DNA, it makes a growth hormone which causes the cells to grow into tumors. The Agrobacterium's DNA takes control of the plant's genetic machinery, forcing the plant to produce food, called opines, which only Agrobacterium can eat.
Agrobacterium and Genetic Engineering
Sometimes this mercenary microbe works for humans, helping scientists to genetically engineer plants. The ability of Agrobacterium's Ti plasmid to insert its DNA into that of dicotyledenous plants has been the major tool in genetic engineering of plants. In the early 1980's scientists discovered that they could cut out the tumor forming part of the Ti plasmid and insert genes of novel or commercial interest. Over 35 genetically engineered plants created this way are approved by the United States department of agriculture. Many of these plants, including potatoes, cotton, tomatoes and corn are in commercial production.
Some of the genes which have been inserted into plants DNA using Agrobacterium's Ti plasmid include:
For an excellent review of Agrobacterium tumefaciens, visit The Microbial World's "Biology and Control of Crown Gall" web page at:
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© 1999 Comm Tech Lab, Michigan State Univeristy. This work was created with support from the National Science Foundation and the Center for Microbial Ecology at Michigan State University. Current maintenance is supported by the International Society for Microbial Ecology and the Comm Tech Lab.