Swapping Genes

Exactly how do you get a vaccine into a tomato? Arizona State University scientists are working on the recipe.

To turn a vegetable into a drug, scientists take a gene from the virus that causes a disease and insert it into a plant cell. The cell has to adopt the gene as part of its own DNA, the master molecule of the genetic code in every living thing. Each cell will pass on that inserted DNA when it divides into two new cells.

The first task is finding the right gene. Scientists use enzymes to chop up the viral DNA into manageable fragments. Each fragment is then spliced into bacterial DNA. The modified bacteria are allowed to multiply into cell colonies. The new “recombinant” DNA sequences are transmitted to their offspring.

Only one bacterial colony will contain the viral DNA fragment the scientists want to use. To find it, the scientists use a gene probe to recognize which bacteria contain the DNA sequence needed to produce a vaccine. Once the colony is identified, scientists allow these bacteria to reproduce even further, creating millions of copies of the DNA fragment.

The scientists now have a plentiful source of the DNA they need. Next, they have to insert it into a target plant cell. There are two primary ways to do this.

The most common method is to use a bacterium that naturally infects the plant. For example, Agrobacterium tumefaciens easily infects many plants. During infection, the bacterium transfers part of its DNA plasmid into the plant cell nucleus. In doing this, the bacterium causes the plant to grow tumors.

Scientists have figured out how to remove the tumor-causing DNA from this bacterium. This makes it an effective vehicle for transporting DNA into plant cells without damaging the plant in any way.

A newer technique for delivering genes is called the “gene gun.” Scientists load genes into microscopic gold particles. They accelerate the particles and shoot them into the plant cell. As the particles pass through the cell, some of the DNA is left behind to mix with the cell’s own DNA.

Once the new DNA is embedded in the cell nucleus, the genetic blueprint for the new vaccine is replicated whenever the cell divides. When the plant reproduces, its offspring will contain the recombinant DNA as well. —Diane Boudreau