Agrobacterium tumefaciens
"Agrobacterium tumefaciens" causes crown-gall disease in plants. The disease is characterised by a tumour-like growth or gall on the infected plant, often at the junction between the root and the shoot. Tumors are incited by the conjugative transfer of a DNA segment (T-DNA) from the bacterial tumour-inducing (Ti) plasmid. The closely related species, "Agrobacterium rhizogenes", induces root tumors, and carries the distinct Ri (root-inducing) plasmid. Although the taxonomy of "Agrobacterium" is currently under revision it can be generalised that 3 biovars exist within the genus, "Agrobacterium tumefaciens", "Agrobacterium rhizogenes", and "Agrobacterium vitis". Strains within "Agrobacterium tumefaciens" and "Agrobacterium rhizogenes" are known to be able to harbour either a Ti or Ri-plasmid, whilst strains of "Agrobacterium vitis", generally restricted to grapevines, can harbour a Ti-plasmid. Non-"Agrobacterium" strains have been isolated from environmental samples which harbour a Ri-plasmid whilst laboratory studies have shown that non-"Agrobacterium" strains can also harbour a Ti-plasmid. Some environmental strains of "Agrobacterium" possess neither a Ti nor Ri-plasmid. These strains are avirulent.