Plant cells do not contain natural plastids that can be utilized as cloning vectors. However, genes may be introduced into plant cells on a plasmid carried by Agrobacterium tumefaciens. The bacterium contains a 200kb Ti(Tumour inducing) plasmid that has the capability of integrating into the host plant's chromosomes. It primarily infects dicotyledonous plants, but has also been shown recently to infect the monocot, rice. Agrobacterium tumefaciens causes crown gall disease of dicotyledonous plants. The bacterium prevalent in the soil infects a wound site on the stem at the crown and the plant cells begin to proliferate and form galls.
The ability of Agrobacterium tumefaciens to induce tumours in plants is controlled by genes carried on a large plasmid called the Ti-plasmid for its tumour inducing capacity. On infection, part of the Ti plasmid, the T-DNA is excised and integrated into the plant cells directed by genes in the T-DNA, and the development of the crown gall. The virulence region of the Ti plasmid contains the genes required for the T-DNA transfer process. These genes encode the DNA processing enzymes required for excision, transfer, and integration of the T-DNA segment during the transformation process.
Foreign genes could be inserted into the T-DNA, and then transferred to the plant. However, the transformed plant cells carrying wild type T-DNAs lose their normal control of cell division and form tumours. This feature of T-DNA renders wild type Ti plasmids incompatible with the goals of most gene transfer experiments. Deleting the genes for crown gall formation from the T-DNA is an improvement but it may be difficult to identify plant cells that have received the disarmed T-DNA. If the host cells are growing in culture, complete recombinant plants can be reconstituted from the transformed cells.