Agrobacterium tumefaciens is a plant pathogen and a natural genetic engineer. It integrates its plasmid genes into the plant genome and transforms it. When the bacteria infect a plant, a part of Ti plasmid called T-DNA is transferred to the plant chromosome. The tumour inducing gene (T-DNA) is replaced with a foreign DNA and this modified or disarmed plasmid is then used as a vector to infect and transfer our gene of interest into the plant cells.