of the Ti plasmid. This nick creates a region of single stranded DNA from the left border of the T-DNA gene over to the right border which was cut. Then, single stranded binding proteins attach to the single stranded DNA. DNA synthesis displaces the single stranded region and then a second nick at the left border region releases the single stranded T-DNA fragment. Further this fragment can be incorporated into a host genome. Agrobacterium has been known to evolve a control system that uses plant host factors and cellular processes for several pathways of host-plant defense response to invade the host cell nucleus. For the integration of T-DNA into the target host genome, Agrobacterium carries out multiple interactions with host-plant factors. To interact with host plant proteins many Agrobacterium virulence proteins encoded by vir genes. Agrobacterium vir gene expression occurs via the VirA-VirG sensor that results in generation of a mobile single-stranded T-DNA copy (T-strand). A processed form of VirB2 is the major component of the T-complex that is required for transformation. VirD2 is the protein that caps the 5′ end of the transferred T-strand by covalent attachment and is transported to the host cell cytoplasm. VirE2 is the single-stranded DNA binding protein that presumably coats the T- strand in the host cytoplasm by cooperative binding. It is then directed into the nucleus via interactions with the host cell proteins such as importin a, bacterial VirE3, and dynein-like proteins. Several other bacterial virulence effectors like VirB5, VirB7 (the minor components of the T-complex), VirD5, VirE2, VirE3, and VirF that may also interact with proteins of host plant cells. Uses in biotechnology Agrobacterium-mediated T-DNA transfer is widely used as a tool in biotechnology. For more than two decades, Agrobacterium tumefaciens has been exploited for introducing genes into plants for basic research as well as for commercial production of transgenic crops. In genetic engineering, the tumor-promoting and opine-synthesis genes are removed from the T-DNA and replaced with a gene of interest and/or a selection marker, which is required to establish which plants have been successfully transformed. Examples of selection markers include neomycin phosphotransferase, hygromycin B phosphotransferase (which both phosphorylate antibiotics) and phosphinothricin acetyltransferase (which acetylates and deactivates phosphinothricin, a potent inhibitor of glutamine synthetase) or a herbicide formulations such as Basta or Bialophos. Another selection system that can be employed is usage of metabolic markers such as phospho-mannose isomerase. Agrobacterium is then