Regeneration of Plants from Crown Gall Cells A step in the T-DNA mediated genetic engineering of plants

The mechanism whereby the plant pathogenic soil bacterium Ag-robacterium tumefaciens induces Crown Gall plant tumors is the result of a remarkable example of naturally occurring genetic engineering. A fragment of about 20 kb (called the T-region) of a 120 kb plasmid (called Ti plasmid for tumor-inducing plasmid) is transferred to the plant cell where it becomes covalently linked to the nuclear DNA (1–7). This integrated DNA has been named the T-DNA. Crown Gall cells have two distinctive properties: They grow without hormones (8,9) and they produce so-called opines: compounds not found in normal plant cells and used specifically by Agrobacterium tumefaciens as a carbon, nitrogen and energy source with the help of a Ti plasmid-encoded uptake and degradation system (10–13). Opines moreover have the capacity to induce a plasmid transfer system which can spread the opine degradation system through an Agrobacterium population (14,15). For recent reviews see 3, 16–20.