APPLICATIONS OF TRANSGENIC PLANTS
Plant genetic engineering has made possible the transfer of genes across all taxonomic barriers. Genes from bacteria, fungi, insects, viruses and even mammals have been introduced into plants. The different transgenic varieties are developed by recombinant DNA technology such as tomato, cotton, maize, potatoes, soyabean, oilseed rape, tobacco, corn, papaya, melon, cucumber, rice etc.
The production of homologous high quality plant material for pharmaceutical purpose requires the use of genetically defined plant material with adequate phytochemical and agrochemical characters. Medicinal plant breeding aims at the development and supply of such plant material to the growers and the pharmaceutical industry. The transgenic plants are developed to produce high degree of tolerance or resistance to pests and diseases. The different applications of transgenic plants are described as follows:
- Production of pharmaceuticals
The pharmaceutical products can be manufactured by transgenic plants are antigens antibodies, starch, oligopeptides, proteins, cyclodextrins, polymers, alkaloids, vitamins etc
Hepatitis B surface antigen (HBSAG) in transgenic plants was made by Arntzen and associates. Antigens such as CT-B, LT-B, capsid protein, surface antigen, malaria epitope, rabies virus Drg 24 antigen are produced from transgenic plants of potato and tobacco.
Arntzen and coworkers expressed hepatitis B surface antigen in tobacco to produce immunologically active ingredients via genetic engineering of plants. Various proteins such as serum albumin, human a-interferon, human erythropoietin, murine igG and lgA immuno-globulins have been successfully expressed in plants. Antigens and antibodies expressed in plants can be administered orally or by parenteral route. The plants are capable of producing vaccines in large quantities at low cost. Transgenic potato tubers are used for the preparation of cyclodextrins (CDs). Genetic transformation and selection in the kanamycin-containing medium have resulted in transgenic A. belladonna plants which produce alkaloids and phenolics. Vitamin A’ is synthesized from carotenoid which is precursor of vitamin A .Genetically engineered rice is prepared by introducing three genes associated with biosynthesis of carotenoid. The transgenic rice was rich in pro-vitamin A. The seeds of transgenic rice is yellow in colour due to pro-vitamin A, the rice is commonly known as golden rice. Different enzymes produced by transformed plants are shown in Table 18.2.
Transgenic plants are used by biotechnology industries as bioreactor’ for preparation of chemicals and pharmaceutical compounds.
- Agricultural or horticultural uses
The production of new plant strains with improved resistance to infectious diseases is another major goal of plant breeders.
Virus resistant transgenic plants: Viruses are responsible for several diseases to plants causing considerable losses in agricultural products. There are three approaches for developing genetically engineered resistance in plants such as:) expression of the virus-coat protein (CP) gene, (1) expression of satellite RNAS and (m) Use of antisense viral RNA. Coat protein-mediated resistance (CPMR) is the most favoured strategy to make virus-resistant plants. Virus-resistance transgenics have been developed by introducing either CP gene or replicate gene encoding sequences.
Protection against fungal and bacterial pathogens may be obtained by transferring a fungal or bacterial gene for virulence into the target plants. The success depends on the type of strains, plant species, controlled glasshouse conditions and natural field conditions.
- Insect-resistant transgenic plants: Insects are serious pests of agricultural products in the field and during storage. Insects belonging to the orders Coleoptera, Lepidoptera and Diptera are the most senous plant pests. Bacillus thuringiensis (Bt), a Gram-positive soil bacterium has been employed as an insecticide. The insecticidal toxin of B. thuringiensis has been classified into cry I, cry II, cry II and cry IV based on insecticidal activities. The Bt gene is isolated and introduced into Ti-DNA plasmid of Agrobacterium tumifaciens. The genetically modified A. tumifaciens is allowed to infect the desired plant. The first group to report success in producing insect resistant plants by inserting Bt gene was Belgian biotech company. The Bt gene inserted into tobacco plants produced enough of the endotoxin to kill Manduca sexta larva attempting to feed on their leaves. Many transgenic crops having cry gene are developed. e.g. cotton, maize, potato, tobacco, rice, soyabean, brinjal, cabbage, corn etc. Transgenic cotton containing Bt gene is introduced in India by Maharashtra Hybrid Seeds Co. (MAHYCO), Jalna.
- Herbicide-resistant transgenic plants: Herbicides are used in agriculture for killing unwanted plants (weeds). Weeds compete with crop plants for nutrients, moisture and light and cause a considerable decline in the yields. Herbicides such as chlorsulfuron, imazapur, DL- phosphinothricin, bromoxynil are effective for broad-spectrum weed control. They act by inactivating target proteins or enzymes present in plants. They are non-selective and kill the crop plants also. Genes coding for herbicide sensitive proteins have been isolated from plants and used to produce several herbicide resistant transgenic crop plants.
A herbicide-resistant gene for EPSPS (5-enol pyruvate – shikimate – 3- phosphate-synthase) is isolated from plants resistant to glyphosate (Roundup herbicide). The resistant gene for EPSPS is transferred to petunia plants and transgenic petunia is developed which is resistant to glyphosate. Roundup tolerance in transgenic tobacco plants has been introduced by using other EPSPS genes along with chloroplast transit peptide sequence.