Ti plasmid of Agrobacterium tumefaciens: Agrobacterium  tumefaciens, a pathogen of several dicot plants is able to deliver a piece of DNA known as ‘T-DNA’ to transform normal plant cells into a tumor and direct these tumor cells to produce the chemicals required by the pathogen. This bacterium invades plants at the site of wound, transforming them and nearby cells to form a tumor called crown gall.

When the bacterium contacts a damaged plant cell, it delivers a T-DNA (transferred DNA) fragment from plasmid into host cell that integrates at a random position in plant cells chromosome. The tumor inducing (Ti) plasmid of A. tumefaciens has now been modified into a cloning vector which is no more pathogenic to the plants but is still able to use the mechanism to deliver genes of interest.