TRANSGENIC PLANTS

SAIMA IBRAHIM
(feedback@pgeconomist.com)

Nov 1 - 7, 2010

A transgenic crop plant contains a gene or genes, which have been artificially inserted instead of the plant acquiring them through pollination. The inserted gene sequence (transgene) may come from another unrelated plant, or from a completely different species. For example, transgenic Bt corn, which produces its own insecticide, contains a gene from a bacterium. Plants containing transgenes are often called genetically modified or GM crops, although in reality all crops have been genetically modified from their original wild state by domestication, selection, and controlled breeding over long periods of time.

WHY MAKE TRANSGENIC CROP PLANTS?

A plant breeder tries to assemble a combination of genes in a crop plant, which will make it as useful and productive as possible. Transgenic plants are generally produced for the purpose of higher yield or improved quality, pest or disease resistance, or tolerance to heat, cold, and drought. Combining the best genes in one plant is a long and difficult process, especially as traditional plant breeding has been limited to artificially crossing plants within the same species or with closely related species to bring different genes together. For example, a gene for protein in soybean could not be transferred to a completely different crop such as corn using traditional techniques. Transgenic technology enables plant breeders to bring together in one plant useful genes from a wide range of living sources, not just from within the crop species or from closely related plants. This technology provides the means for identifying and isolating genes controlling specific characteristics in one kind of organism, and for moving copies of those genes into another quite different organism, which will then also have those characteristics. This powerful tool enables plant breeders to do what they have always done - generate more useful and productive crop varieties containing new combinations of genes - but it expands the possibilities beyond the limitations imposed by traditional cross-pollination and selection techniques.

There are several methods for introducing genes into plants, i.e.

* Infecting plant cells with plasmids as vectors carrying the desired gene

* Shooting microscopic pellets containing the gene directly into the cell.

MORE DISTANT NATURAL GENE TRANSFERS

The best-documented route for natural formation of transgenic plants is gene transfer between a plant epiphyte (such as mosses), or a parasitic plant (like dodder) and the host plant it colonises. Another mechanism for horizontal gene transfer is Agrobacterium tumefaciens and similar bacteria that inject DNA into plant cells. Biotechnology laboratories exploit Agrobacterium tumefaciens bacteria to make artificial transgenic plants with small segments of added transgenic DNA inserted in the host cell chromosomes. Others mechanisms may include plant sucking insects, mites, and possibly viruses. Recent comparative studies of gene content of different genomes provides strong circumstantial evidence that natural horizontal gene transfer does occur in plants at a frequency that is significant over evolutionary time scales. Over the evolutionary time-scales, plant mitochondria are a stopping point for genes that may enter the nuclear genome from other species, and can in some cases be very active in inter-species gene-traffic.

SOME ACHIEVEMENTS

1. Improved Nutritional Quality: Milled rice is the staple food for a large fraction of the world's human population. Milling rice removes the husk and any beta-carotene it contained. Beta-carotene is a precursor to vitamin A. The synthesis of beta-carotene requires a number of enzyme-catalyzed steps. In January 2000, a group of European researchers reported that they had succeeded in incorporating three transgenes into rice that enabled the plants to manufacture beta-carotene in their endosperm.

2. Insect Resistance: Bacillus thuringiensis is a bacterium that is pathogenic for a number of insect pests. Its lethal effect is mediated by a protein toxin it produces. Through recombinant DNA methods, the toxin gene can be introduced directly into the genome of the plant where it is expressed and provides protection against insect pests of the plant.

3. Disease Resistance: Genes that provide resistance against plant viruses have been successfully introduced into such crop plants as tobacco, tomatoes, and potatoes.

4. Herbicide Resistance: Alternatives are available, but they may damage the crop as well as the weeds growing in it. However, genes for resistance to some of the newer herbicides have been introduced into some crop plants and enable them to thrive even when exposed to the weed killer.

5. Salt Tolerance: A large fraction of the world's irrigated crop land is so laden with salt that it cannot be used to grow most important crops. However, researchers at the University of California Davis campus have created transgenic tomatoes that grew well in saline soils.

6. Terminator Genes: This term is used for transgenes introduced into crop plants to make them produce sterile seeds and thus force the farmer to buy fresh seeds for the following season rather than saving seeds from the current crop.

The process involves introducing three transgenes into the plant:

* A gene encoding a toxin, which is lethal to developing seeds but not to mature seeds or the plant. This gene is normally inactive because of a stretch of DNA inserted between it and its promoter.

* A gene encoding a recombinase, an enzyme that can remove the spacer in the toxin gene thus allowing to be expressed.

* A repressor gene whose protein product binds to the promoter of the recombinase thus keeping it inactive.

How they work: When the seeds are soaked (before their sale) in a solution of tetracycline

* synthesis of the repressor is blocked

* the recombinase gene becomes active

* the spacer is removed from the toxin gene and it can now be turned on.

7. Biopharmaceuticals: The genes for proteins to be used in human (and animal) medicine can be inserted into plants and expressed by them.

ADVANTAGES:

* Glycoproteins can be made.

* Virtually unlimited amounts can be grown in the field rather than in expensive fermentation tanks.

* There is no danger from using mammalian cells and tissue culture medium that might be contaminated with infectious agents.

* Purification is often easier.

Corn is the most popular plant for these purposes, but tobacco, tomatoes, potatoes, and rice are also being used.

Some of the proteins that are being produced by transgenic crop plants:

* Human growth hormone with the gene inserted into the chloroplast DNA of tobacco plants.

* Humanised antibodies against such infectious agents as

* HIV

* Respiratory syncytial virus (RSV)

* Sperm (a possible contraceptive)

* Herpes simplex virus, HSV, the cause of "cold sores"

* Protein antigens to be used in vaccines

* An example: patient-specific antilymphoma (a cancer) vaccines. B-cell lymphomas are clones of malignant B cells expressing on their surface a unique antibody molecule. Making tobacco plants transgenic for the RNA of the variable (unique) regions of this antibody enables them to produce the corresponding protein. This can then be incorporated into a vaccine in the hopes (early trials look promising) of boosting the patient's immune system ˇ especially the cell-mediated branch ˇ to combat the cancer.

* ther useful proteins like lysozyme and trypsin

NEED OF GM TECHNOLOGY IN AGRICULTURE

Today, there are some 800 million people, who do not have access to sufficient food to meet their needs primarily because of poverty and unemployment. Malnutrition plays a significant role in half of the nearly 12 million deaths each year of children under five in developing countries (Unicef 1998). In addition to lack of food, deficiencies in micronutrients (especially vitamin A, iodine and iron) are widespread. Furthermore, changes in the patterns of global climate and alterations in use of land will exacerbate the problems of regional production and demands for food. Dramatic advances are required in food production, distribution, and access if we are going to address these needs. Some of these advances will occur from non-GM technologies, but others will come from the advantages offered by GM technologies.

Achieving the minimum necessary growth in total production of global staple cropsŚmaize, rice, wheat, cassava, yams, sorghum, potatoes, and sweet potatoesŚwithout further increasing land under cultivation will require substantial increases in yields per acre. Increases in production are also needed for other crops, such as legumes, millet, cotton, rape, bananas, and plantains. It is important to increase yield on land that is already intensively cultivated.