Agrobacterium tumefaciens is a Gram-negative bacterium found in soil and is the infective agent for crown gall disease in plants. Crown gall disease causes tumors to form at the crown and at the junction of a plant's root and stem. The tumors are caused by the action of the Ti plasmid in the bacterium; this plasmid is being heavily researched by plant genetic engineers because it is a useful way to introduce new genes into a plant cell.

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