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Chapter 22 : Agrobacterium tumefaciens: Nature's Plant Genetic Engineer
Agrobacterium tumefaciens infects certain types of plants (most dicots [plants with two seed leaves], but not mono-cots) at wound sites. Once in the wound, the bacterium injects a segment of its plasmid, called tumor inducing (Ti), into the adjacent living plant cells. This piece of DNA, called transferred DNA (T-DNA), is only one region of the plasmid. The tumor cells synthesize new chemicals that provide nourishment that is critical to the bacterium but useless to the plant. In the late 1970s, plant scientists realized they might be able to take advantage of A. tumefaciens' natural genetic engineering abilities. They developed an important method for plant genetic engineering based on this organism and its Ti plasmid. In this method, the T-DNA genes that induce tumor formation and nourish the bacterium are removed from the Ti plasmid and replaced with any gene of interest. The new plasmid is returned to A. tumefaciens, which is grown in culture so that many bacteria carrying the engineered plasmid are produced. The plants to be engineered are then infected with the bacterium carrying the "designer" Ti plasmid. A. tumefaciens injects the engineered T-DNA into the plant. Instead of receiving genes for tumor formation, the plant gets the genes inserted into the Ti plasmid by the scientist.