Transgenic 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.
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.
Main Advantages of Transgenic Plant
Improvement in Yield
Gene technology plays important role in increasing the productivity of food, fibre and vegetable crops ensuring food security which is essential for international peace and stability. Thus it is an important mean to fight hunger.
The transgenes generally are not yield enhancing genes. The increase in yield or productivity is achieved by controlling losses caused by various insects, diseases and abiotic factors. Gene technology is expected to keep pace in food production with increasing would population.
Improvement in Insect and Disease Resistance
In crop plants heavy yield losses are caused every year due to insect and disease attack. Moreover, insecticides and pesticides which are used to control insects and diseases are expensive and have adverse effects on other beneficial organisms (parasites and predators).
Gene technology has played key role in developing insect resistant cultivars in several crops. For example, in cotton bollworm resistant cultivars have been developed by transferring a gene from soil bacterium Bacillus thuringiensis into cotton plants. This leads to saving substantial amount on insecticidal chemicals. Moreover, the technology is environmental friendly.
Improvement in Quality
The quality is adjudged in three ways, viz., nutritional quality, market (keeping) quality and industrial quality. Gene technology has helped in improving all these three types of quality in different crops. For example, gene technology has made it possible to delay the ripening and softening of tomatoes resulting in safe transport and longer storage.
Herbicide Resistance
In crop plants, weeds cause heavy yield losses and also adversely affect the quality of produce. The genetic resistance is the cheapest and the best way of solving this problem.
Resistance to Abiotic Stresses
The gene technology can also be used for developing crop cultivars tolerant to environmental or abiotic stresses such as drought, soil salinity, soil acidity, cold, frost etc. Efforts are being made to develop varieties resistant to abiotic stresses using gene technology.
Rapid and Accurate Technique
Gene technology is a rapid and highly accurate method of crop improvement. The development of cultivar by this technique takes 4-5 years against 10-2 years taken by conventional (hybridization) method. Moreover, this is a highly reliable technology.
Challenges with transgenic plants
Allergenicity
The possibility that we might see an increase in the number of allergic reactions to food as a result of genetic engineering has a powerful emotional appeal because many of us experienced this problem before the advent of transgenic crops, or know of someone who did.
However, there is no evidence so far that genetically engineered foods are more likely to cause allergic reactions than are conventional foods. Tests of several dozen transgenic foods for allergenicity have uncovered only a soybean that was never marketed and the now-famous StarLink corn. Although the preliminary finding is that StarLink corn is probably not allergenic, the scientific debate continues. Every year some people discover that they have developed an allergy to a common food such as wheat or eggs, and some people may develop allergies to transgenic foods in the future, but there is no evidence that transgenic foods pose more of a risk than conventional foods do. More on allergenicity
Horizontal transfer and antibiotic resistance
The use of antibiotic resistance markers in the development of transgenic crops has raised concerns about whether transgenic foods will play a part in our loss of ability to treat illnesses with antibiotic drugs. At several stages of the laboratory process, developers of transgenic crops use DNA that codes for resistance to certain antibiotics, and this DNA becomes a permanent feature of the final product although it serves no purpose beyond the laboratory stage.
Eating foreign DNA
When scientists make a transgenic plant, they insert pieces of DNA that did not originally occur in that plant. Often these pieces of DNA come from entirely different species, such as viruses and bacteria.
We eat DNA every time we eat a meal. DNA is the blueprint for life and all living things contain DNA in many of their cells. What happens to this DNA? Most of it is broken down into more basic molecules when we digest a meal. A small amount is not broken down and is either absorbed into the blood stream or excreted in the feces. We suspect that the body’s normal defense system eventually destroys this DNA. Further research in this area would help to determine exactly how humans have managed to eat DNA for thousands of years without noticing any effects from the tiny bits that sneak into the bloodstream.
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