The Truth About genetically modified plants: Simply a matter of infection and injection

THIS WEEK there's been a lot of talk about genetic engineering, especially of plants. Yet it seems to be a technology people take for granted. Few people seem to ask: how exactly do you get a new gene into a plant?

There are two main methods: infection and injection. The first, and most common, is to use the "Ti plasmid". A plasmid is a loop of DNA found in bacterial cells, separate from the main DNA strands and able to replicate on its own. "Ti" (pronounced tee-eye) stands for tumour-inducing. The Ti plasmid is found in the Agrobacterium tumefaciens bacterium, which normally lives in soil. A tumefaciens can infect plants, causing a characteristic tumor.

When these bacteria infect a plant cell, a segment of the plasmid - called T-DNA - separates from the rest of the bacterium and is taken up into the genetic material of the infected cell. The Ti plasmid can thus be used to transfer foreign genes into plant cells. So, to add a gene (say, to instruct the plant how to produce a particular protein) you inactivate the tumour-causing genes and insert the foreign genes in the T-DNA.

The T DNA and the required foreign genes are thus taken up into the genetic material of the growing plant, so the foreign genes will be "expressed" - and their proteins produced.

The difficult thing is getting the foreign genes into the "germ line" - the pollen or stamen cells of the plants. Once that is achieved, the new genes are incorporated into future generations of the plant.

However, not every plant is susceptible to infection by A tumefaciens. That's where the alternative method, of injection, comes in handy. There are three processes available to put new DNA directly into cells: microinjection, electroporation and particle bombardment. Microinjection is the direct injection of material into the plant cell using a micropipette needle. Electroporation uses short pulses of high voltage electricity to induce the formation of pores in the plant cell membrane. These pores last only briefly but allow foreign DNA to enter the cell. Particle bombardment shoots microscopic pellets coated with DNA through the cell wall, targetting the genetic region of the cell.

These injection methods are integral to the commercial application of plant genetic engineering. They can be used on many basic food crops and have become routine in crops such as maize and potatoes; genes conferring resistance to plant disease have been the most popular.