A Gram-negative soil bacterium that infects a wide range of plants and causes tumorous growths (galls), especially at the root/stem junction (crown gall). It is of interest because the bacterial cells contain a plasmid, the Ti plasmid (tumour-inducing plasmid), a segment of which is transferred to cells of the plant host. This T-DNA (transfer DNA) segment, which comprises the genes responsible for the tumorous growth, becomes integrated into the genome of infected plant cells. Possession of the Ti plasmid has made A. tumefaciens an important tool in genetic engineering for the introduction of foreign genes into plant tissue. The tumour-inducing genes are usually replaced with the gene of interest, and a marker gene (e.g. the antibiotic resistance gene) is added to enable selection of transformed cells. See genetically modified organisms.