Conjugation
In order to be virulent, the bacterium must contain a tumour-inducing plasmid (Ti plasmid or pTi), of 200 kb, which contains the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of A. tumefaciens do not contain a pTi.
Since the Ti plasmid is essential to cause disease, pre-penetration events in the rhizosphere occur to promote bacterial conjugation - exchange of plasmids amongst bacteria. In the presence of opines, A. tumefaciens produces a diffusible conjugation signal called 30C8HSL or the Agrobacterium autoinducer. This activates the transcription factor TraR, positively regulating the transcription of genes required for conjugation.
Read more about this topic: Agrobacterium Tumefaciens