This first method relies on the ability of the soil bacterium Agrobacteri-um tumefaciens to transfer some of its plasmid DNA to plant cells. Agrobacterium is a plant pathogen that, after infecting its host, causes the disease called crown gall. This disease is characterized by the for mation of tumors at the point of infection. In the infection process, Agrobacterium transfers its plasmid DNA to the plant cell, and this DNA is subsequently integrated within the DNA double strand of the infected plant cells. Within this plasmid are tumor-causing genes, so the plasmid is called the Ti (for tumor inducing) plasmid. The important point here is that Agrobacterium is a natural genetic engineer, since it can naturally transfer its DNA to a plant host. In order to insert foreign genes into plants, scientists remove the tumor-causing genes from the Ti plasmid and insert a foreign gene in their place. The Ti plasmid is thus a natural plasmid vector useful for plant transformation.
A typical plant transformation experiment consists in harvesting plant tissues (for example, leaf pieces) and incubating them with Agrobacterium cells containing the gene of interest in its Ti plasmid. After just a few hours, Agrobacterium has transferred its Ti plasmid with the foreign genes to the plant cells. Whole plants are then regenerated from these treated plant pieces. This is accomplished by cultivating the plant pieces in a petri dish with a nutrient medium containing plant hormones that promote both root and stem growth. Thus whole fertile plants can be regenerated from the treated tissues. These plants can then be propagated by the usual means, seeds. The complete genetic modification process is shown in figure 6.1.
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