Chilton and her coworkers proved beyond doubt that tumor formation is the direct result of the transfer of Ti plasmid DNA from the bacterial cells to the infected plant cells, where the plasmid DNA integrates in the nuclear genome.

A section of Ti plasmid (T -region) was transferred to and maintained as T-DNA in the transformed plant cell by insertion into the plant nuclear genome. Agrobacterium plant cell transformation requires wounding the cell.

The wound is purportedly essential not only for removing a physical barrier, but also for conditioning the plant cell. Indeed, wounding does stimulate the plant cells to switch from a resting stage to one of rapid cell division.

The tumor cells synthesize low molecular weight compounds called opines which are not found in normal plant tissues and which diffuse from the tumor cells. These opines enter the A. tumefaciens cells and lead to the expression of the Ti plasmid opine breakdown genes and certain other genes that promote the transfer of the Ti plasmid to other Agrobacterium strains that lack this plasmid.

The scientists who have contributed to the above understanding of the molecular biology of the Ti plasmid and the design of the vectors include M.D. Chilton and coworkers, Washington State University, Pullman, Washington, USA (1980); R. Fraley and coworkers, (1985) Monsanto Company, St. Louis, Missouri, USA; S.B. Gelvin, and coworkers, (1981) University of Washington, Seattle, Washington, USA;

A. Depicker and coworkers, (1978) State University of Ghent, Belgium; P. Zambryski and coworkers, (1983) Max Planck Institute, Cologne, Germany; R. Schilperoort and G.H. Bomhoff, (1975) University of Leiden, the Netherlands; J. Tempe and coworkers, (1980) National Institute of Agronomy Research, Versailles, France.