The Ti plasmid of Agrobacterium tumefaciens, is of great importance. ^[4]

A. tumefaciens is a soil microorganism that causes crown gall disease in many species of dicotyledonous plants. ^[4] The ability to cause crown gall disease is associated with the presence of the Ti (tumor inducing) plasmid within the bacterial cell. This is a large (greater than 200 kb) plasmid that carries numerous genes involved in the infective process. A remarkable feature of the Ti plasmid is that, after infection, part of the molecule is integrated into the plant chromosomal DNA. This segment, called the T-DNA, is between 15 and 30 kb in size, depending on the strain. It is maintained in a stable form in the plant cell and is passed on to daughter cells as an integral part of the chromosomes. ^[4]

But the most remarkable feature of the Ti plasmid is that the T-DNA contains eight or so genes that are expressed in the plant cell and are responsible for the cancerous properties of the transformed cells. ^[4] These genes also direct synthesis of unusual compounds, called opines, that the bacteria use as nutrients. A. tumefaciens genetically engineers the plant cell for its own purposes. ^[4]

Image source: http://upload.wikimedia.org/wikipedia/commons/d/d1/Ti_plasmid.svg 

Genes in the virulence region are grouped into the operons virABCDEFG, which code for the enzymes responsible for mediating transduction of T-DNA to plant cells.^[22]

·         virA codes for a receptor which reacts to the presence of phenolic which leak out of damaged plant tissues.^[24]

·         virB encodes proteins which produce a pore/pilus-like structure.^[23]

·         virC binds the overdrive sequence.^[23]

·         virD1 and virD2 produce endonucleases which target the direct repeat borders of the T-DNA segment,

·         vir E Binds to T-strand protecting it from nuclease attack, and intercalates with lipids to form channels in the plant membranes through which the T-complex passes, ^[23] beginning with the right border.^[24]

·         virG (TRANSCRIPTIONAL FACTOR) activates vir-gene expression after binding to a consensus sequence,^[23] once it has been phosphorylated by virA.^[24]

Image source: http://www.discoverbiotech.com/image/image_gallery?uuid=c2aa87a0-9c4c-4783-a69c-1f396e3b7aa5&groupId=11406&t=1332308806765

Click on the link to watch the animation for better understanding:

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120078/bio40.swf::The%20Ti%20Plasmid 

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