infecting bacteria, which catabolizes the respective opines using genes encoded in the Ti plasmid. Accordingly, Ti plasmids have been classified based on the type of opine they catabolize, namely: nopaline-, octopine- or mannityl-types, which are amino acid derivatives, or agrocinopine-type, which are sugar phosphate derivatives. Historical discovery The identification of A. tumefaciens as the cause of gall tumours in plants paved the way for insights into the molecular basis of crown gall disease. The first indication of a genetic effect on host plant cells came in 1942-1943, where plant cells of secondary tumours were found to lack any bacterial cells within. However, these tumour cells did possess the ability to produce opines metabolized by the infecting bacterial strain. Crucially, the production of the respective opines occurred regardless of the plant species and occasionally only within crown gall tissues, indicating that the bacteria had transferred some genetic material to the host plant cells in order to allow opine synthesis. However, how and to what extend did DNA transfer occur remained an open question. Adding A. tumefaciens DNA alone did not cause tumors in plants, while very little A. tumefaciens DNA was found to be integrated into the host plant cell genome. The addition of deoxyribonucleases (DNases) to degrade DNA also failed to prevent the formation and growth of the plant tumors. These suggested that little, if any, of the A. tumefaciens DNA is transferred to the host plant cell to cause disease and, if DNA is indeed transferred from the bacteria to the plant, it must occur in a protected manner. Subsequently, oncogenic bacterial strains were found to be able to convert non-pathogenic bacteria into pathogens via the process of conjugation, where the genes responsible for virulence were transferred to the non-pathogenic cells. The role of a plasmid in this pathogenic ability was further supported when large plasmids were found only in pathogenic bacteria but not avirulent bacteria. Eventually, the detection of parts of bacterial plasmids in host plant cells was established, confirming that this was the genetic material responsible for the genetic effect of infection. With the identification of the Ti plasmid, many studies were carried out to determine the characteristics of the Ti plasmid and how the genetic material is transferred from the Agrobacterium to the plant host. Some notable early milestones in the studies of Ti plasmids include the mapping of a Ti plasmid in 1978 and the