Agricultural biology is based on the technique of manufacturing transgenic plants. The ability to insert different genes to be expressed in plants was first discovered with Agrobacterium tumifaciens. The agrobacterium infects plants, forms tumors, and transfers a piece of its own DNA into the plant's genome. As a result, the plant can perform the functions specified by the bacterial genes. The agrobacterium causes crown gall disease by transferring into a plant cell its plasmid (Ti plasmid) - this results in the formation of growths at the base of the plant. When the tumors are placed in lab cultures, they will grow and assume cancerous properties of growth and division. Scientists discovered the tumors could be grown in the absence of the bacteria as well. The Ti (tumor inducing) plasmid was responsible for this behavior. On the plasmid there is a region of tDNA (transfer DNA) which gets transferred to the plant cell, a region of virulence (vir) genes, and another set of genes involved in the catabolism of unusual amino acids.
Eugene Nestor and Mary Dell Chilton studied tDNA. They found that
there was a repeated sequence at each end, like a border; genes
involved in hormone production (the expression of these genes
are responsible for the tumors), and genes for producing unusual
amino acids. Nestor and Chilton realized this plasmid could transfer
genetic information between Kingdoms (bacteria <--> plants). All that was necessary for transfer was the border sequence,
and any kind of gene could be placed between them. This method
of genetic transfer is similar to conjugation in bacteria and
forms the basis for cloning new genetic material into plants.
The first gene was cloned in 1973. In 1983, genes from one plant species could be expressed in another species. The first transgenic plant was a herbicide resistant plant. Roundup, a non-selective herbicide also known as glyphosate, will kill weeds but might also kill crops. Roundup works by inhibiting amino acid biosynthesis and prevents plants from making aromatic amino acids (EPSP synthase is the enzyme that is inhibited). Scientists wanted to engineer a plant resistant to glyphosate. They isolated a bacteria that were resistant and cloned the resistance gene which turned out to be an altered EPSP synthase gene (called AroA). They took the AroA gene from the resistant bacteria and inserted it into plant, thus the glyphosate resistant gene was produced in the plant. This method was used to transform tobacco plants. The fields planted with the glyphosate resistant plants could be sprayed with glyphosate to kill all the weeds without killing the tobacco plants themselves.
BT toxin, made by bacillus, can kill an insect if ingested. If this toxin could be expressed in plants, it could kill insects that tried to eat the plants while not having any effect on harmless insects. This could be dangerous if the toxins affected humans and other herbivores and also because the toxin is not completely selective. Additionally, this could create a population of insects that were resistant to the pesticide as a result of their constant exposure to the toxin. As a matter of fact, insects are already showing resistance to BT toxin.
There is now a "Pure Foods" campaign that some restaurants agree to whereby they agree not to serve transgenic foods. The first transgenic food product was a tomato, called the Flavr Savr tomato, which was released two years ago. The tomato has now been pulled from the shelves because there was a problem with the quality of its taste - not with a genetic engineering issue. An enzyme called polygalacturonase is made in mature fruits and is responsible for ripening and aging of the fruits and the breakdown of the structural components of the fruit. Producers wanted to inhibit this process to increase the shelf life of the tomato. They used anti-sense technology to do so. Anti-sense technology involves running transcription in the opposite direction along a coding DNA strand so that the transcription does not give rise to a functional mRNA (it cannot be translated). The normal (sense) mRNA in the cell can then basepair with the anti-sense strand, which ties up the genetic message and keeps it from being sent to the ribosomes to be translated. This inhibits production of the enzyme without introducing any new gene products into the tomatoes.
To market these transgenic tomatoes, a company called MacGregor began to sell normal, high quality tomatoes in order to develop a customer loyalty. When the transgenic tomatoes are approved, MacGregor can then simply place a small sticker on the tomato or box saying that the tomato was genetically engineered or something subtle, and people will continue to purchase them - based on their high quality. The added shelf life means that the producers can pick the fruit when it is ripe and be confident that it will last longer on the shelf.
Some concerns with transgenic foods are as follows:
Producers of transgenic foods must constantly check for unexpected events. The FDA has decided that the transgenic food industry will regulate itself, so no guidelines have been set by the government. The food producers continually check all the nutrients in the tomatoes as well as the toxin levels.
We have already mentioned the Pure Food campaign. Some companies like Heinz and Campbell's soup have agreed not to buy transgenic tomatoes. Along with the desire of some to eat only "pure foods", another concern is religious dietary laws. For example, if a religion restricts the consumption of pork, yet a pig gene is engineered in a tomato, can a person eat the tomato?
Commercial uses for transgenic products are also being studied. For example, Agracetus Co. has found a way to insert a gene into a cotton plant so that the plant expresses a polyester polymer inside the cotton fiber, thus automatically producing a cotton-polyester blend. Because this eliminates the manufacture of plastics it reduces environmental pollution.
Another emphasis is on the production of oils. Most palm and coconut oils are grown in foreign countries. People want to decrease our dependence on other countries for these products. Enzymes can be inserted into sunflowers so that they can convert their own sunflower oil into palm or coconut oils.
Chris Summerville has expressed a gene in potatoes that makes a plastic polymer which is biodegradable. This fiber can be used to manufacture items like grocery bags that will be totally recycled after use because of their biodegradability.
If a vaccine can be expressed in a plant, the plant can then be eaten for the vaccination, and a person wouldn't have to go to a doctor to have a shot administered. This could benefit third world countries which lack the infrastructure and resources to provide access to doctors. The first test of edible vaccines was performed by expressing a surface protein of Hepatitis B in potatoes which were then fed to mice. The mice developed antibodies to the Hepatitis surface protein, and developed a mucosal immunity to infection by the virus. It is important to note that the antibodies are secreted by the mucosal membranes (lining of nose, mouth, digestive track) which is the site that the virus is likely to invade the body.. There are many problems related to this, though. For one, any particular protein might not be immunogenic. Also, ingesting too much protein could create a tolerance instead of an immune response. If vaccinations are put into food that is cooked, because many proteins aren't heat stable, cooking them would cause them to be denatured. Charlie Atsen decided to use bananas, which have been called the fourth most important food in the world. Bananas are eaten without cooking and are available throughout the world. He has produced the first transgenic banana, and believes that eventually it could be used in baby food to immunize babies without trips to the doctor's office.