Plants have been central to genetics, dating back to their use by Gregor Mendel to establish the laws of inheritance of genetic traits. Since the discovery of deoxyribonucleic acid (DNA) and the abilities to transform species, including plants, with DNA derived elsewhere--recombinant DNA technology--the use of plants as vehicles to express traits encoded by recombinant DNA has exploded.
A plant breeder attempts to assemble a combination of genes in a crop plant which will make it as useful and productive as possible. Desirable traits include higher yield, improved quality, pest or disease resistance, or tolerance to environmental extremes. Obtaining the desired mix of traits can be lengthy and difficult. The ability to introduce foreign DNA into a plant via recombinant DNA technology has provided the means for identifying and isolating genes controlling specific characteristics in one kind of organism, and for moving those genes into another quite different organism, which will then express those characteristics. Modern plant genetics augments natural genetic recombination, but expands the natural limitations.
Plant genetics relies upon the universal presence of DNA in the cells of all living organisms. For most organisms, the sequence of events by which the information encoded in the DNA is expressed as proteins operates in a similar fashion. Thus, even species that are very different share similar genetic mechanisms, allowing DNA from one organism to be processed by a very different organism. Plant genetics relies upon the selective DNA sequence recognition and cutting abilities of a battery of enzymes called restriction endonucleases, and the re-splicing ability of other enzymes called ligases. The action of these enzymes makes the introduction of exogenous DNA possible. Identification of a single gene is usually not sufficient--regulation of the gene and its protein product is a complex process involving genetic and environmental factors. Current researchers expend much effort to rapidly sequence and determine the functions of genes of the most important crop species. Once genes having candidate potential are identified, their regulation must be defined.
Transformation of plants occurs mainly by two methods. The first is called the gene gun method. In this method, DNA attached to gold beads is ballistically introduced into the plant tissue. It has been especially useful in transforming monocot species like corn and rice. The second method relies on a bacterium called Agrobacterium tumefaciens. The bacterial method exploits the ability of A. tumefaciens to infect a plant with a piece of its own DNA, which commands the plant's cellular machinery to produce more bacteria. The bacteria accomplish this infection because it possesses a plasmid called the Ti (tumor-inducing) plasmid. Substances released through a wound in the plant activate genes on the plasmid, directing entry of the plasmid into plant tissues.
Engineered plasmids containing the wound-response genes and the genes of commercial or research interest have proved to be a successful means of transforming plants. Transformed plants can be subsequently detected by their growth in the presence of an inhibitory compound (since the plasmid also contains a gene(s) encoding resistance to the compound). Whole plants can then be obtained, seeds produced, and trials in the lab and the field performed to evaluate the performance of the transformed plant variety.
A popular target for transgenic technology has been the production of genetically engineered crops resistant to a target insect pest or herbicide. In 1999, almost 100 million acres of these crops were planted. In that year, the acreage devoted to transgenic varieties was approximately half of the U.S. soybean crop and about 25 per cent of the U.S. corn crop. Herbicide resistant crops contained a gene encoding a protein that can chemically alter the herbicide so as to nullify its killing effect. Of the insect resistant crops, Bt insect-resistance has proved effective and commercially popular. Bt is short for Bacillus thuringiensis, a soil bacterium whose spores contain a crystalline protein. In the insect gut, the protein breaks down to release a toxin. The toxin binds to and creates pores in the intestinal lining, which is ultimately lethal for the insect. The use of Bt as a pest control is not new. However, its introduction to plants via recombinant DNA technology has been a novel application of pest control.
The use of Bt transgenic crops appears to be reducing the need for chemical pesticides, at least for cotton. The situation for other crops, like corn, is as yet unclear.
The production of drugs in genetically altered plants, biopharming, may represent the next wave in agricultural biotechnology. Until now, efforts have mainly been directed at protecting crops from pests and improving the taste and nutrition of the so-called genetically modified foods. About 20 companies worldwide are working on producing biopharmaceuticals in plants. A few are in human clinical trials, including vaccines for hepatitis B contained in potatoes and an antibody to prevent tooth decay, produced in genetically altered tobacco plants. The vaccines have been engineered from subunits of the disease-causing organism, rather than from the whole organism. Thus, the chances of developing an infection from the administration of the vaccine are thought to be much less frequent than traditional vaccination technologies. The production of compounds of industrial relevance in plants is also being investigated.
This is the complete article, containing 853 words
(approx. 3 pages at 300 words per page).
