T-DNA Plasmid -Plants do not have plasmids indigenous to them as yeasts do. Hence, bacterial plasmids, which can transfer DNA to plants were developed. The best developed plasmid based vector to transfer DNA into the plant is Ti plasmid. Ti plasmids occur naturally in gram negative bacteria called Agrobacterium tumefaciens.

Ti-plasmid is 2.5 kb in size with unique regions called A,B,C and D. Regions 13 and C are involved in plasmid replication. D region is involved in the transfer of DNA from the plasmid into plant. This region of D is 40 kb in length and is referred to as the virulence region.

Ti-plasmids are remarkable because they stand as examples for the insertion of prokaryotic gene into a eukaryotic genome. The plasmid looks like a natural chimeric as it contains two sets of genes, one active in bacteria and the other in plant.

The genes in the T - DNA segment are associated with transcription control signals that operate in plants while those in the remainder of the plasmid are under the control of bacterial promoters.

T - DNA transfer is remarkable from the point of view of interkingdom gene transfer. Although T - DNA is transferred, it has no role in the transfer mechanism. T - DNA region in both Ti and Ri plasmids are flanked by almost perfect 25 base pair direct repeat sequences.

Especially the right hand 25' bp sequence is compulsorily required for T - DNA transfer as they function in a ds-acting manner. Any DNA sequence could be transferred to plant cells as long as it is flanked by the 25 bp repeat sequence in correct order by vir genes.