Indirect or Vector-Mediated Gene Transfer

Indirect gene transfer relies on an actual biological vector, a plasmid, to do the delivery - and in practice this almost always means the Ti (tumour-inducing) plasmid carried by the soil bacterium Agrobacterium tumefaciens. In nature, this bacterium already does something remarkable entirely on its own: it transfers a defined segment of its Ti plasmid, called the T-DNA (transfer DNA), directly into the genome of a plant cell at a wound site, which is what naturally causes the crown gall tumours this bacterium is known for in a wide range of dicot plants. Because Agrobacterium is already, unassisted, capable of inserting foreign DNA into a plant's own genome, it is nicknamed the 'natural genetic engineer of plants' - and genetic engineers exploit exactly that natural talent rather than trying to replace it. In practice, researchers replace the Ti plasmid's own disease-causing DNA with the gene they actually want to introduce (for example, a Bt gene for insect resistance), plus a selectable marker gene - commonly npt II, which confers resistance to the antibiotic kanamycin - cloned into the T-DNA region in place of the unwanted native sequences. When this re-engineered Agrobacterium then in …