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GENETIC ENGINEERING (TRANSGENIC PLANTS)

The genetic engineering overcomes the limits of interspecific crosses and permits the introduction of foreign genes from distant species that is not possible following conventional breeding. It is now possible to manipulate the genome of almost any plant species including
agricultural crop plants by introducing advantageous new traits (by incorporating isolated gene from an organism or chemically synthesized gene) that produces a new protein/other product or by preventing the expression of an existing native gene following various techniques.

Agrobacterium tumefaciens has been used extensively to transfer the desired foreign DNA by inserting it into the Ti plasmid and then integrating into the dicotyledonous host plant genome. After screening of the desired recombinant cells, they are regenerated into the complete templets, e.g., Loblolly pine used in forestry. Ti plasmids have also been used to insert ‘antisense genes’ to negate the functions of specific plant genes concerned with an undesirable trait. Such genes are produced by reversing the orientation of a gene in relation to its promoter (in DNA at promoter region RNA polymerase binds to start the transcription giving rise to mRNA) so that a RNA, that is complementary to mRNA, is synthesized and after base pairing with the normal mRNA, double-stranded RNA is produced that is unable to be translated to give respective proteins (Fig. 19.2).

This method of using Agrobacterium tumefaciens to manipulate the genome is unsuitable for monocotyledonous plants, like wheat, rice, maize, etc. In monocotyledonous plants gene transfer is mostly achieved by gene gun by bombarding the foreign DNA-coated gold or tungsten particles (micro projectiles) into the host plant cells that penetrate the cell wall and deliver the DNA into the nucleus. The recombinant cells can be regenerated into whole plants.

The widespread use of insecticides, fungicides, herbicides (to kill the plant weeds) and pesticides has the damaging effect on the environment and human health. Many crop plants nave not only been improved with respect to the pest and disease control but also to increase the yield and quality of the product by genetic engineering technology.

Fig. 19.2 Introduction of antisense gene for polygalacturonase enzyme for the development of Flavr Savr tomato.

(i)Resistance to Insect Pests: Poroteinacious toxin genes (cry genes, where cry means crystalline toxins) from Bacillus thruringiensis (Bt), that is toxic to larvae of certain insects, have been introduced into several crops, so that they become resistant to that particular insects, e.g., cotton, tomato e.g., cotton, tomato etc. In India also, some insect resistant cry genes containing cotton varieties are grown in the fields.

(ii)Resistance to Microbial Diseases: Microbial diseases, particularly fungal and viral, cause huge losses and limit the plant productivity worldwide. Resistance to many viruses by integrating the genes for viral coat proteins have been achieved in several crop plant e.g., rice.

(iii) Resistance to Herbicides: Herbicide-tolerant crop plants have been produced by genetically engineering the crop plant genomes that would reduce the use of herbicides, e.g., potato, rapeseed etc. This method of weed control is more effective, less costly and more environment friendly.

(iv)Delayed Ripening Fruits: Losses during storage and transportation of some crops is very high due to over-ripening and softening of fruits and vegetables. The ripening is due to some endogenous enzyme activities that can be genetically stopped or slowed down e.g., in tomato, the polygalacturonase breaks down the cell wall, leading to softening of fruits during ripening. ‘Flavr Savr tomato’ has been engineered in USA by blocking the expression of polygalactarunase enzyme by introducing the antisense gene for this enzyme.This tomato which has improved flavour and shelf life and is now marketed in USA, Such techniques will now be used in a wide variety of soft fruits. Other studies are considering inhibition of ethylene synthesis for controlling fruit ripening (ethylene is a plant hormone;which induces fruit ripening).

(v)Stress Resistance: Cold resistant tobacco plant has been produced by introducing the gene for glycerol-l-phosphate acyl transferase from the plant Arabidopsis.

Despite their beneficial effects, it has been argued that transgenic plants may be harmful to the environment as.

(i) There may be possibility of transfer of transgene (foreign gene) from transgenic crops to their wild relatives through pollens, making them more persistent and damaging. To prevent this, transgenics should not be grown near the occurance of their wild relatives.

(ii)Transgenic crops may themselves become persistent weeds.

(iii)They may damage the environment in some unknown manner.