A plasmid, or type of circular DNA, found in the bacteria Agrobacterium tumefaciens which infects dicot plants. Part of the plasmid inserts itself into the plant genome and causes tumors to form in the roots or in the stems nearest the roots. The plasmid has been used by geneticists, minus the tumor-causing parts, as a vector towards the genetic engineering of plants.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.