Molecular Biology of Agrobacterium Infection - The molecular mechanisms involved in Agrobacterium infection of plant cells became known only during 1980’s. The process of infection by A. tumefaciens culminates in the transfer of a small part of pTi into the plant cell genome; this DNA sequence is celled T-DNA (transferred DNA).

The infection process is governed by both chromosomal and plasmid-borne genes of A. tumefaciens. Infection begins when Agrobacterium cells become attached to plant cells; this step determines the host range of bacterium, is a function of host-parasite interaction, and is governed by bacterial chromosomal genes, generally the chv (chromosomal virulence) genes.

chromosomal genes are expressed constitutively, i.e., expressed in all bacterial cells at all the times.