In this article we will discuss about:- 1. Meaning of Transgenic Breeding 2. Achievements of Transgenic Breeding 3. Advantages 4. Disadvantages.
Meaning of Transgenic Breeding:
Genetic improvement of crop plants, domestic animals and useful micro-organisms through biotechnology (tissue culture and genetic engineering) in relation to their economic use for mankind is referred to as transgenic breeding. Transgenic plant breeding refers to genetic improvement of crop plants through the use of transgenes.
Transgenic breeding plays an important role in the genetic improvement of crop plants for various economic characters. However, there are some limitations or drawbacks of this technique of crop improvement.
Now-a-days, transgenic breeding is being used for specific genetic improvement of different field crops. Transgenic breeding is expected to play an important role in the genetic improvement of field crops in the years ahead.
Achievements of Transgenic Breeding:
Transgenic breeding is being carried out in almost all important field crops. In different crops, 1257 transgenic plants have been developed and released the world over till 1992. Now this number would be quite high.
Various plant characters which have been improved or modified through the use of transgenes include:
(1) Insect and disease resistance,
(2) Herbicide resistance,
(3) Drought resistance,
(4) Metal toxicity resistance,
(5) Quality, and
(6) Male sterility (Table 31.4).
Transgenic plants have been recovered in various field crops such as wheat, barley, oat, maize, sugarcane, rapeseed, soybean, peanut, cotton, tobacco tomato, potato, sunflower and others.
The maximum transgenic plants have been released in oil seed rape (290), followed by potato (133), tobacco and tomato (72 each) and maize (65). Other crops in descending order include flax (49), soybean (40), cotton (37), sugar-beet (28), Lucerne (21), etc.
The character wise achievements are as follows:
(1) Insect and Disease Resistance:
Insect and disease resistance have been achieved in many field crops through the use of transgenes. Some examples are given here. In cotton, gene for Helicoverpa resistance has been transferred from soil borne bacterium Bacillus thuringiensis by Monsanto Company in USA.
In corn, gene for resistance to European corn borer has also been transferred from above bacterium. In tobacco, insect resistance has been achieved by transferring trypsin inhibitor gene from cowpea. Transgenic virus resistant genotypes have been developed in crops like tobacco, tomato, potato and cucumbers.
(2) Cold Resistance:
Cold resistance has been achieved in some crop plants through transgenic breeding. In tomato and tobacco, anti-freezing gene has been transferred from fish (winter flounder). In tobacco, cold resistance has been achieved by transferring a gene from Arabidopsis thaliana.
(3) Improvement in Quality:
Improvement in quality has been achieved in some crop plants through transgenic breeding. Some examples are given here. In alfalfa, protein quality has been improved by transferring ovalbumin gene from chicken. In potato, protein quality has been improved by transferring serum albumin gene from human. In tobacco, protein quality has been enhanced by transferring glutenin gene of wheat.
(4) Induction of Male Sterility:
Male sterility is gaining increasing importance these days due to increasing demand of hybrid varieties in various crops. Transgenic breeding is an effective means of inducing male sterility in crop plants. In rapeseed, male sterility has been induced by transferring a gene from Bacillus amyloliquefaciens.
(5) Herbicide Resistance:
In almost all crop plants, weeds pose a serious problem. The manual cleaning of weeds is a very expensive method. The cheapest and the most practical method of weed control, in the mechanized agriculture, is the use of herbicides. In other words, herbicides are the cheapest and the best means of weed control.
However, herbicides have adverse effect on many crop plants. Hence there is need to develop herbicide resistant genotypes in various crop plants. Biotechnology has helped in developing transgenic plants resistant to herbicides in many crop plants.
Herbicide resistant transgenic plants have been developed in cotton, tobacco, wheat, maize, potato, tomato, soybean, flax, rapeseed, sugar-beet, alfalfa, cabbage and other crops. In majority of cases, herbicide resistant genes have been transferred from microorganisms.
These are only few examples of the use of transgenes in crop improvement. In other words, this is just beginning of the application of biotechnology in crop improvement. Transgenic breeding is expected to play a key role in the genetic improvement of crop plants in the years ahead.
Advantages of Transgenic Breeding:
The main advantages of transgenic crop breeding are briefly discussed below:
1. Rapid Method of Crop Improvement:
Transgenic breeding is a rapid method of crop improvement. By this technique, stable transgenic plants can be developed in 3-4 years, whereas it takes 12-15 years to develop a new variety through conventional methods of breeding such as pedigree, bulk and backcross methods.
The first generation of transgenic plants obtained through regeneration is called T0 progeny. Transgenic plants which are obtained from T0 plants are designated as T1 and those obtained by growing embryos of T1 plants are called T2 plants and so on. In barley and sunflower, transgene exhibited Mendelion pattern of segregation i.e. 3: 1 ratio.
2. Overcome Crossing Barriers:
Transgenic breeding permits gene transfer between unrelated species and even between unrelated organisms. It permits gene transfer even between plants and animals. For example, a freezing resistant gene has been transferred from fish to cultivated tomatoes. Similarly, ovalbumin gene of chicken has been transferred in alfalfa for improving protein quality. There are many other examples of gene transfer between plants and animals.
3. Evolution of New Genotypes:
Sometimes, transgenic breeding may lead to evolution of altogether new plant species, because it permits gene transfer between various plant species. Thus it will affect the process of natural evolution.
4. Application:
Transgenic breeding can be used for the genetic improvement of both autogamous and allogamous crop plants. Both seed propagated and vegetatively propagated species can be improved through the use of transgenes.
5. Effectiveness:
Transgenic breeding has been found effective for the genetic improvement of monogenic characters only. It has not been used, so far, for the genetic improvement of polygenic characters. It has been found very effective in development of plants with resistance to various diseases, insects and herbicides.
Disadvantages of Transgenic Breeding:
A brief description of drawbacks or disadvantages of transgenic breeding in presented below:
i. Instable Performance:
Transgenic plants, sometimes, exhibit instable performance for the character under consideration. For example, in Petunia, the flower colour trangene in-hemizygous state, was stable in the glass house but not in the field. Hence transformants should be adequately evaluated under both glass house and field conditions for stable expression of the transgenic character. Similarly, in cotton transgenics developed for Helicoverpa resistance were found to exhibit instable performance after 2-3 years in USA.
ii. Pleiotropic Effects:
Pleiotropy refers to manifold effects of a single gene. Almost all single genes exhibit pleiotropic effects. The side effects of a gene may be sometimes harmful. This problem can be overcome by crossing the transformant (transganic) with several varieties and selecting for desirable combination of characters including transgene.
A gene for orange colour was transferred from corn into Petunia. The transformant had some undesirable side effects of transgene. The undesirable side effects could be eliminated by crossing transgenic with several genotypes and selecting for traits combining noval flower colour and acceptable field performance.
iii. Position Effect:
The position of integration of foreign gene in the host genome affects the expression of tansgene in the transformant. There is no control over the position of integration as well as copy number of foreign gene in the particle bombardment method of gene transfer. However, Agrobacterium mediated transformation has some control over copy number of transgene.
iv. Unable to Transfer Polygenic Traits:
The techniques of genetic engineering have so far been used for the manipulation of monogenic characters only. Majority of plant characters are governed by oligogenes and polygenes. Hence, there is need to develop and perfect techniques for manipulation of oligogenic and polygenic traits. Without manipulation of oligogenes and polygenes, transgenic breeding has very limited scope for genetic improvement of crop plants.
v. Costly Method:
Transgenic breeding is a very expensive method of crop improvement. This method involves application of tissue culture and genetic engineering in the development of new varieties having transgenes.
This requires well equipped tissue culture and genetic engineering laboratories. Thus several lakh rupees are spent for developing a transgenic plant, because the equipments required in the laboratory of genetic engineering are very costly. The varieties developed by genetic engineering will also be very costly. Seed of such varieties cannot be afforded by ordinary farmers.
vi. High Technical Skill:
Development of transgenic plants requires high technical skill. Such task can be accomplished with the close cooperation of scientists from the disciplines of tissue culture, genetic engineering, plant breeding, genetics, entomology, pathology and plant physiology. Lack of cooperation among the scientists of various disciplines often hinders the progress of such worthwhile programmes.
vii. Low Frequency:
Transgenic cell are recovered at a very low frequency in the cell culture medium. Moreover, regeneration of transformed cells into whole plant is also a difficult and time consuming task. For getting a transformed cell, huge number of, single cells or protoplasts have to be screened in the culture medium.
Thus regeneration of whole plant from a single cell is a prerequisite for transgenic breeding. Lack of regeneration of single cell into whole plant restricts the application of transgenic breeding in many crop species.
viii. Effect on Natural Evolution:
Transgenic breeding overcomes the natural barriers of crossing between species and genera. Any two species or genera can be crossed through genetic engineering. Thus, the transgenic breeding fasters the speed of natural evolution. In other words transgenic breeding acts against the process of natural evolution.
ix. Useless Combinations:
Sometimes, the foreign gene has adverse effects on the genome of recipient parent. In such cases, it may give rise to useless combinations which may become a problem. Thus, there are chances of developing new weed species through transgenic breeding.
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