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The following points highlight the top six uses of genetic engineering in different fields. The uses are: 1. Protection of Crop Plants Against Insects by BT-Toxins 2. Protection of Plants Against Viruses 3. Herbicide-Resistant Plants 4. Improvement of Yield and Quality of Crops 5. Protection against Environmental Stress 6. Increased N2 Utilization.
Genetic Engineering: Use # 1.
Protection of Crop Plants Against Insects by BT-Toxins:
A major portion of the crop yield is lost from insect attacks. About one-sixth of the global food-production are lost by insect pests. There is no other alternative by farmers for the use of chemical pesticides that cause damage to the environment. Preparations from Bacillus thuringensis are now being used as alternative biological insecticides.
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These bacteria produce toxic peptides called BT-toxins, which impair the absorption of food in the intestine of some insects causing starvation and death. These BT-toxins are not harmful to humans.
For many years bacterial suspensions containing BT-toxins have been sprayed to protect crops from insects. But this is disadvantageous as these preparations are expensive and are easily washed off from the leaf surface by rain. Again it cannot reach the larvae inside plant shoots.
Nowadays, the BT-toxin genes have been cloned and used to transform a number of crop plants. The toxin is decomposed in the soil and digested in human body as all other proteins. Insect-resistant transgenic lines of potato, maize and cotton have now been licensed for agricultural use in the USA. Scientists are still trying to generate transgenic insect-resistant plants for other crops.
Genetic Engineering: Use # 2.
Protection of Plants Against Viruses:
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Viruses are very harmful for certain crops. Cucumber mosaic virus can totally destroy the pumpkin, cucumber, melon and zucchini crops. Viral diseases cannot be controlled by the use of chemicals. The means of control is the destruction of virus-transferring insects with the pesticides. Crop rotation may also be done to check the propagation of viruses.
The introduction of coat protein gene of the tobacco mosaic virus into the genome of tobacco plants makes them resistant to the virus. Sufficient expression of the gene in a plant is the cause of protection. Although the mechanism is not yet understood, still the principle has already been used with success in many cases.
Genetic Engineering: Use # 3.
Herbicide-Resistant Plants:
Non-selective herbicides can be used as selective herbicides by transformed herbicide-resistant plants. Glyphosate, a structural analogue of PEP, is a non-selective herbicide, which inhibits the synthesis of aromatic amino acids via the sikimate pathway at the EPSP (5′-enolpyruvyl shikimate 3-phosphate) synthase step.
Animals are not affected by glyphosate as they do not possess the shikimate pathway. Glyphosate is widely used as a pre-emergence herbicide to kill the weeds before the plantation of crops.
Otherwise, as it is a non-selective herbicide, it will kill the crop plants also. In order to use this powerful herbicide as a selective post-emergence herbicide, glyphosate resistant transformed plants have been generated for a number of crop plants through genetic engineering.
EPSP synthase genes have been isolated from bacteria, cloned and used to transform the crop plants. Glyphosate-resistant cotton, rapeseed and soybean have already been produced. Similarly glufosinate resistant maize and rapeseed plants have also been licensed for cultivation.
Genetic Engineering: Use # 4.
Improvement of Yield and Quality of Crops:
Yield and quality of crops can be improved through the generation of hybrids from genetically engineered male sterile plants. Genetic engineering is being used in many ways to modify the quality of crops.
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It has been possible to increase the starch content and decrease the water content in potato tubers by transferring the ADP-glucose pyrophosphorylase gene from E. coli. By antisense RNA inhibition of the same gene decrease of starch content (3-5%) and increase of sucrose content have been achieved.
The conversion of sucrose to polyglucans, which is used as industrial raw material, has been done through introduction of further genes. Amylose synthesis in potato tubers can also be reduced through genetic engineering resulting in the uniform storage of amylopectin, which is used as an industrial raw material.
Transgenic tomatoes have been raised with altered ripening process and improved quality and storage stability.
Ethylene synthesis in tomato fruits is suppressed through genetic engineering either:
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(1) by decreasing the activity of ACC (aminocyclopropane carboxylate) synthase and ACC oxidase by antisense inhibition technique
(2) by introducing a bacterial gene, the enzyme product of which degrades the ACC the immediate precursor of ethylene.
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When ethylene biosynthesis is decreased in this way, the ripening process continues in the unpicked fruits, but in harvested fruits ripening is delayed and can be transported for several days without decay.
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About 10 per cent of the world production of plant fats is utilized as a raw material for industrial purposes. Fatty acid methyl esters synthesized from rapeseed oil are used in some countries as automobile fuel. Transgenic plants produce fats with short chain fatty acids, which is used as a raw material for the detergent industry.
Erucic acid is used in plastic industry, its content can be increased through transgenesis in certain plants. Through genetic engineering amino acid composition of storage proteins can be altered to increase their nutrition value, attempts are being made to produce transgenic Yew tree to increase the cancer therapeutic agent, taxol. It is obtained from the tree bark in very low amount.
Genetic Engineering: Use # 5.
Protection Against Environmental Stress:
Plants protect themselves against various environmental stresses like temperature, drought, salinity, xenobiotic (toxic substances taken up by the plants) and heavy metal pollution, oxygen radical production, etc.
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Transgenic plants are being produced to increase the stress tolerance of cultivated plants through alteration or over-expression of gene involved in stress resistance. The cold tolerance of tobacco has enhanced by increasing the amount of unsaturated fatty acids in the membrane lipid by genetic engineering.
Strategies have been proposed for making salt tolerant plants by increasing the synthesis of osmotica like mannitol, trehalose, proline, betaine, fructan, etc. In transgenic plants transferred gene for mannitol dehydrogenase could accumulate mannitol to show altered stress tolerance.
A vast portion of land remains uncultivated due to its high salt and low water content. If gene technology were to succeed Ln raising salt-tolerant crop plants, this would be a very important achievement towards the world food supply.
Genetic Engineering: Use # 6.
Increased N2 Utilization:
Higher plants cannot utilize atmospheric nitrogen for their own growth and development. For that reason, billions of rupees are being spent each year on chemical nitrogen fertilizers for the optimum cereal crop yields. Biological nitrogen fixation is the alternative to the use of chemical nitrogen fertilizer.
Certain bacteria and BGA have the ability to fix atmospheric nitrogen. The nitrogen-fixing metabolic machinery is very complex in these organisms.
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In Klebsiella, a free-living N2– fixing bacteria, there are 20 nif (nitrogen fixation) genes under 7 operons. It is very difficult to engineer 20 different chimeric genes to transfer to the same recipient plant and to make them function in a coordinated way. At present, it is largely a fantasy to produce transgenic N2-fixing plants.
Nodule formation in symbiotic associations between Rhizobium and legumes is dependent on genetic information of both host and the bacterium. It might be possible to use genetic engineering to modify non-legume plants, particularly the cereal crops, such that they will become susceptible to Rhizobium infection and nodule formation.
This is also a difficult task as the genetic control of nodule formation is complex. However, experiments are in progress with the goal of modifying bacteria with enhanced N2-fixing capacity and wide host range.
Although genetic engineering has exaggerated lot of expectations, still it has some negative consequences. Always it is to be kept in mind whether,(the transgenic plant represents a hazard to the environment or not. It has to be examined whether crossing between the released trans-formants and wild plants is possible. For example, herbicide-resistant weeds may develop from herbicide-resistant cultivars.