Genetically Modified Foods

The first step in developing a transgenic plant is to identify a trait in one type of organism that would make a useful characteristic if transferred to the experimental plant. The components of an experiment to create a transgenic plant are the gene of interest, a piece of "vector" DNA that delivers the gene of interest, and a recipient plant cell. Donor genes are often derived from bacteria, and are chosen because they are expected to confer a useful characteristic, such as resistance to a pest or pesticide.

To begin, the donor DNA and vector DNA are cut with the same restriction enzyme. This creates hanging ends that are the same sequence on both of the DNA molecules. Some of the pieces of donor DNA are then joined with vector DNA, forming a recombinant DNA molecule. The vector then introduces the donor DNA into the recipient plant cell, and a new plant is grown.

For plants that have two seed leaves (dicots), a naturally occurring ring of DNA called a Ti plasmid is a commonly used vector. Dicots include sunflowers, tomatoes, cucumbers, squash, beans, tomatoes, potatoes, beets, and soybeans. For monocots, which have one seed leaf, Ti plasmids do not work as gene vectors. Instead, donor DNA is usually delivered as part of a disabled virus, or sent in with a jolt of electricity (electroporation) or with a "gene gun" (particle bombardment). The monocots include the major cereals (corn, wheat, rice, oats, millet, barley, and sorghum).

Transgenesis in plants is technically challenging because the transgene must penetrate the tough cell walls, which are not present in animal cells. Instead of modifying plant genes in the nucleus, a method called transplastomics alters genes in the chloroplast, which is a type of organelle called a plastid. Chloroplasts house the biochemical reactions of photosynthesis. Transplastomics can give high yields of protein products, because cells have many chloroplasts, compared to one nucleus. Another advantage is that altered chloroplast genes are not released in pollen, and therefore cannot fertilize unaltered plants. However, it is difficult to deliver genes into chloroplasts, and expression of the trait is usually limited to leaves. This is obviously not very helpful in a plant whose fruits or tubers are eaten. The technique may be more valuable for introducing resistances than enhancing food qualities. Someday, transplastomics may be used to create "medicinal fruits" or edible vaccines.