Genetically modified plants
Transgenic plants are plants that have been genetically modified by inserting genes directly into a single plant cell. Transgenic crop plants modified for improved flavor, pest resistance, or some other useful property are being used increasingly.
Transgenic plants are unique in that they develop from only one plant cell. In normal sexual reproduction, plant offspring are created when a pollen cell and an ovule fuse. In a similar laboratory procedure, two plant cells that have had their cell walls removed can be fused to create an offspring.
Genetic Engineering Techniques
There are three general approaches that can be used to insert the DNA into a plant cell: vector -mediated transformation, particle-mediated transformation, and direct DNA absorption. With vector-mediated transformation, a plant cell is infected with a virus or bacterium that, as part of the infection process, inserts the DNA. The most commonly used vector is the crown-gall bacterium, Agrobacterium tumefaciens. With particle-mediated transformation (particle bombardment), using a tool referred to as a "gene gun," the DNA is carried into the cell by metal particles that have been accelerated, or "shot," into the cell. The particles are usually very fine gold pellets onto which the DNA has been stuck. With direct DNA absorption, a cell is bathed in the DNA, and an electric shock usually is applied ("electroporation") to the cell to stimulate DNA uptake.
No matter what gene insertion method is used, a series of events must occur to allow a whole genetically modified plant to be recovered from the genetically modified cell: The cell must incorporate the new DNA into its own chromosomes, the transformed cell must initiate division, the new cells need to organize themselves into all the tissues and organs of a normal plant ("regeneration"), and finally, the inserted gene must continue to work properly ("gene expression ") in the regenerated plant.
To help ensure all this occurs, a "cassette" of genes is inserted during the initial transformation. In addition to the gene coding for the desired trait, other genes are added. Some of these genes promote the growth of only those plant cells that have successfully incorporated the inserted DNA. It might do this by providing the transformed cells with resistance to a normally toxic antibiotic that is added to the growth medium, for example. Other genes ("promoters ") may be added to control the functioning of the trait gene by directing when and where in the transformed plant it will operate.
The genes put into plants using genetic engineering can come from any organism. Most genes used in the genetic engineering of plants have come from bacteria. However, as scientists learn more about the genetic makeup of plants ("plant genomics"), more plant-derived genes will be used.
Inserted genes can be classified into three groups based on their use: those that protect a crop, those that improve the quality of a harvested product, and those that let the plant perform some new function.
Genes That Protect a Crop.
The major use of plant genetic engineering has been to make crops easier to grow by decreasing the impact of pests. Insect resistance has been achieved by transforming a crop using a Bt gene. Bt genes were isolated from Bacillus thuringiensis, a common soil bacterium. They code for proteins that severely disrupt the digestive system of insects. Thus an insect eating the leaf of a plant expressing a Bt gene stops eating and dies of starvation. There are many Bt genes, each of which targets a particular group of insects. Some Bt genes, for example, target caterpillars. Others target beetles.
Genetic engineering also has been used in the battle against weeds. Bacterial genes allow crops to either degrade herbicides or be resistant to them. The herbicides that are used are generally very effective, killing most plants. They are considered environmentally benign, degrading rapidly in the soil and having little impact on humans or other organisms. Thus a whole field of transgenic crops can be sprayed with broad-spectrum herbicides, killing all plants except the crops. Corn, soybeans, canola, and cotton that have been engineered to withstand either insects or herbicides, or both, are widely planted in some countries, including the United States. In addition, other crops, including potatoes, tomatoes, tropical fruits, and melons, have been engineered for resistance to viral diseases.
Genes That Improve Crop Quality.
An emerging major use of genetic engineering for crops is to alter the quality of the crop. Fresh fruits and vegetables begin to deteriorate immediately after being harvested. Delaying or preventing this deterioration not only preserves a produce's flavor, and appearance, but maintains the nutritional value of the produce. Genes that change the hormonal status of the harvested crops are the major targets for genetic engineering toward longer shelf-life.
For example, the plant hormone ethylene is associated with accelerated ripening, as well as leaf and flower deterioration, in fruits that are injured or harvested. Scientists insert genes that interfere with a plant's ability to synthesize or respond to ethylene, thereby extending postharvest quality for many fresh products, including tomatoes, lettuce, and cut flowers. Scientists are also using gene insertion to improve a plant's nutritional value and color.
Genes That Introduce New Traits.
One approach to improving the economic value of crops is to give them traits that are completely new for that plant. Some crops, including potatoes, tomatoes, and bananas, have been engineered with genes from pathogenic organisms. This is done to make animals, including humans, that eat the crops immune to the diseases caused by the pathogens. The genes code for proteins that act as antigens to induce immunity. Thus edible parts of plants are engineered to act as oral vaccines. This approach may be particularly effective for pathogens, such as those causing diarrhea and other gastrointestinal disorders, that enter the body through mucous membranes . This is because the "medicine" in the food comes into direct contact with these membranes and does not have to be absorbed into the blood stream. Genes have also been engineered into crop plants to direct the plants to produce industrial enzymes used in the manufacture of paper. Other genes direct crops to produce small polymers useful in the manufacture of plastics. This general approach is being termed "plant molecular farming."
Rice is another plant that has been engineered for a new trait. During commercial processing, a substantial part of the white rice grains are removed, leaving very little vitamin A. Vitamin A deficiency is a significant health problem in regions dependent on rice as a dietary staple. Scientists engineered a certain form of rice, known as "golden rice" because it has a yellow tinge, to express three introduced genes. These genes let the plant produce the precursor of vitamin A in the portion of the grain that remains after processing, thereby providing a dietary source of the vitamin.
see also Agricultural Biotechnology; Biopesticides; Biotechnology; Cloning Genes; Cloning Organisms; Gene Targeting; Genetically Modified Foods; Plant Genetic Engineer; Transformation.
Transgenic plants are produced when a gene from one plant is inserted into the genome of another. For the gene to work properly in the transgenic plant, it must be engineered to have a promoter before its start codon (to turn it on) and a terminator after its end (to turn it off). Generally the gene also needs some deoxyribonucleic acid (DNA) at either end that will allow it to be inserted into the host genome. The transfer DNA (tDNA) from the bacterial microbe Agrobacterium is used routinely for this purpose. Also required is an antibiotic resistance gene that will allow the transgenic plant cells to be recovered from a mixture with untransformed cells. In a typical project, the desired gene is engineered with the correct elements, placed in a plasmid vector , and then inserted into Agrobacterium cells. Agrobacterium is then used to infect the plant cells with the plasmid vector. The tDNA allows the desired gene and the antibiotic resistance gene to be incorporated into the plant cell's genome. The transgenic cells are selected using the antibiotic in a tissue culture medium; only cells having antibiotic resistance as a result of transformation will survive on this medium. Finally, the transgenic cells are manipulated with the hormones in the tissue culture medium to regenerate whole transgenic plants. To be sure that the gene is inserted and is functioning, botanists will check for the presence of inserted DNA using a Southern blot and will confirm the presence of modified ribonucleic acid (RNA) with a Northern blot . Development of an expected phenotype in mature plants can also confirm the insertion.
see also Breeding; Genetic Engineer; Genetic Engineering; Molecular Plant Genetics; Tissue Culture.
Hopkins, W. G. Introduction for Plant Physiology, 2nd ed. New York: John Wiley & Sons, 1999.
Raven, Peter. H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Co., 1999.
Watson, J. D., M. Gilman, J. Witkowski, M. Zoller. Recombinant DNA, 2nd ed. New York: W. H. Freeman and Company, 1992.