The Green Revolution accomplished in multiplying the food supply more than three times but yet it was not sufficient to feed the increasing human population. Improved yields have partially been due to the use of enhanced crop species, but mainly due to the adoption of better management methods and application of agrochemicals (fertilisers and pesticides). However, for farmers, especially in developing countries, agrochemicals are usually too costly, and further improvements in yield with present varieties are not achievable using conventional breeding.
Is there any substitute that genetics can show so that farmers may gain maximum yield from their fields? Is there a way to reduce the use of fertilisers and chemicals so that their ill effects on the environment are subdued?
Use of genetically modified crops is a viable solution.
Plants, fungi, animals and bacteria whose genes have been modified by manipulation are known as Genetically Modified Organisms (GMO). GM plants have been beneficial in many ways.
Genetic modification has:
- made crops more receptive to abiotic pressures (cold, drought, salt, heat).
- decreased dependence on chemical pest-resistant crops (pesticides).
- accommodated to decrease post-harvest damages.
- enhanced performance of mineral usage by plants (this limits early depletion of the fertility of soil).
- the improved nutritional content of food, e.g., golden rice which is rich in Vitamin A.
In addition to these advantages, GM has been utilised to produce tailor-made plants to provide alternative resources to industries, in the form of fuels, pharmaceuticals and starches.
Some of the applicability of biotechnology in agriculture that you will study in detail is the production of pest-resistant plants, which could reduce the amount of pesticide used.
Bt toxin is created by a bacterium known as Bacillus thuringiensis (Bt for short). Bt toxin gene is cloned from the bacteria and is revealed in plants to give immunity against insects without the requirement for insecticides. It works like bio-pesticide without the disadvantages of insecticides. Examples are Bt cotton, Bt rice, tomato, rice, soybean and potato etc.
- Bt Cotton: Some strains of Bacillus thuringiensis create proteins that destroy some insects such as lepidopterans (tobacco, armyworm, budworm), coleopterans (beetles) and dipterans (mosquitoes). B. thuringiensis produces protein crystals during a special phase of their growth. These crystals comprise a toxic insecticidal protein.
- Why does this toxin not destroy the Bacillus?
The Bt toxin protein survives as inactive protoxins, but once an insect ingests the inactive toxin, it is transformed into an active form of toxin due to the alkaline pH of the gut which solubilises the crystals. The activated toxin attaches to the exterior of midgut epithelial cells and forms pores that induce cell swelling and lysis and ultimately affect the death of the insect.
- Specific Bt toxin genes were detached from Bacillus thuringiensis and combined into the various crop plants such as cotton.
- The selection of genes depends upon the targeted pest and the crop, as Bt toxins work on specific insect-group. The gene cryIAc codes the toxin and is called Cry. There are a number of them, for instance, the proteins encoded by the genes cryIIAb and cryIAc control the cotton bollworms, that of cryIAb checks corn borer.
Pest Resistant Plants
Some nematodes parasitise an extensive type of plants and animals including human beings. Meloidegyne incognitia, a nematode, contaminates the roots of tobacco plants and induces a prominent reduction in yield.
- A new approach was selected to check this infestation which was based on the method of RNA interference (RNAi). RNAi occurs in all eukaryotic organisms as a means of cellular defence. This method includes silencing of a specific mRNA due to a complementary dsRNA molecule that attaches to and inhibits translation of the mRNA (silencing).
- The origin of this complementary RNA could be from contamination by viruses having RNA genomes or mobile transposons (genetic elements) that replicate through an RNA intermediate.
- Utilizing Agrobacterium vectors, nematode-specific genes were injected into the host plant. The entrance of DNA was such that it created both anti-sense and sense RNA in the host cells.
- The two RNA’s that are complementary to each other make a dsRNA (double-stranded) that start RNAi and therefore, silenced the specific mRNA of the nematode. The result was that the parasite could not sustain in a transgenic host revealing specific interfering RNA. The transgenic plant hence got itself shielded from the parasite.
Organisms that have had their DNA moulded to hold and express a foreign gene are called transgenic animals. Transgenic rats, pigs, sheep, rabbits, fish and cows have been created, although over 95% of all present transgenic animals are mice.
Why are these animals being produced? How can man benefit from such modifications?
Some of the common reasons:
- Normal physiology and development:
Transgenic animals can be created to study the regulation of genes, and also to study the normal functions of the body and its growth, e.g., the study of complex factors associated with growth such as insulin-like factor. By injecting genes from other species that modify the production of this factor and examining the biological effects that occur, knowledge is obtained about the biological function of the factor in the body.
- Study of disease: Several transgenic animals are produced to improve our knowledge of how genes contribute to the growth of the disease. These are specifically made to assist as prototypes for human diseases so that the investigation of new methods for treatment of diseases are made. Today, transgenic models exist for several human diseases such as cancer, cystic fibrosis, Alzheimer’s and rheumatoid arthritis.
- Biological products: Medicines needed to treat some human diseases can carry biological products, but such products are often costly to make. Transgenic animals that provide useful biological products can be produced by the introduction of the part of genes (or DNA) which codes for a selective product such as human protein, a-1-antitrypsin, is utilised to treat emphysema. Similar efforts are being done for the treatment of cystic fibrosis and phenylketonuria. The first transgenic cow, Rosie, provided human protein-enriched milk (2.4 grams per litre). The milk included the human alpha-lactalbumin and was nutritionally a highly balanced product for human babies than natural cow-milk.
- Vaccine safety: Transgenic mice are being produced for application in examining the safety of vaccines before they are applied to humans. Transgenic mice are being used to examine the safety of the polio vaccine.
If successful and found to be positive, they could follow the use of monkeys to examine the safety of batches of the vaccine.
- Chemical safety testing: This is called toxicity or safety testing. The method is the same as that applied for testing the toxicity of drugs.
Transgenic animals are produced that carry genes which make them more susceptible to toxic elements than non-transgenic animals. They are then exposed to toxic materials, and its effects are then studied. Toxicity experiment in such animals will enable us to get results in less time.