NAME: Agrobacterium IMAGE WIDTH: 12 microns IMAGE CREDITS: Shirley Owens, Microbe Zoo Project, Comm Tech Lab, Michigan State University. Click image to download large 640 x 480 JPEG image (approximate download times: 28.8 k = 40 sec; T1 = 11 sec.)

Mercenary Microbes

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:

• Glow in the dark genes - The luciferase gene from a firefly has been successfully introduced into tobacco plants with glowing results: the plant glows in the dark! Not only do the plants look cool, but this proved to scientists that the technique worked.
  
• Insect killing genes - An insect toxin from the bacterium Bacillus thuriengensis has been inserted into cotton, potato and corn plants, making the plants toxic to insect pests.
  
• Pesticide resistance genes - Scientists have engineered plants that can degrade herbicides such as Roundup™. This means farmers can spray weeds with Roundup™, which contains the active ingredient glyphosate, and it won't kill the crop. Scientists took a glyphosate resistance gene from the bacterium Salmonella and inserted this into the Ti plasmid which was then introduced into the plant.
  
• Tomato anti-softening genes - A gene genetically engineered into Flavr Savr tomatoes slows the ripening of tomatoes, giving them a longer shelf life. These engineered tomatoes lack the ability to degrade pectin, the stuff that your grandmother uses to make jellies gel and that gives fruit their firmness. The genetically engineered Flavr Savr tomato produces an "antisense" gene that encodes a strand of RNA that "zips" onto the complementary strand of RNA from the pectin degrading gene (polygalacturonase). This bound-up RNA molecule is rendered useless, so the pectin degrading enzymes are not produced and hence the tomato stays firm.
  

For an excellent review of Agrobacterium tumefaciens, visit The Microbial World's "Biology and Control of Crown Gall" web page at:

 © 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.