208
BIOLOGY
(ii) organic agriculture; and
(iii) genetically engineered crop-based agriculture.
The Green Revolution succeeded in tripling the food supply but yet
it was not enough to feed the growing human population. Increased yields
have partly been due to the use of improved crop varieties, but mainly
due to the use of better management practices and use of agrochemicals
(fertilisers and pesticides). However, for farmers in the developing world,
agrochemicals are often too expensive, and further increases in yield with
existing varieties are not possible using conventional breeding. Is there
any alternative path that our understanding of genetics can show so that
farmers may obtain maximum yield from their fields? Is there a way to
minimise the use of fertilisers and chemicals so that their harmful effects
on the environment are reduced? Use of genetically modified crops is a
possible solution.
Plants, bacteria, fungi and animals whose genes have been altered by
manipulation are called Genetically Modified Organisms (GMO). GM
plants have been useful in many ways. Genetic modification has:
(i) made crops more tolerant to abiotic stresses (cold, drought, salt, heat).
(ii) reduced reliance on chemical pesticides (pest-resistant crops).
(iii) helped to reduce post harvest losses.
(iv) increased efficiency of mineral usage by plants (this prevents early
exhaustion of fertility of soil).
(v) enhanced nutritional value of food, e.g., golden rice, i.e., Vitamin ‘A’
enriched rice.
In addition to these uses, GM has been used to create tailor-made
plants to supply alternative resources to industries, in the form of starches,
fuels and pharmaceuticals.
Some of the applications of biotechnology in agriculture that you will
study in detail are the production of pest resistant plants, which could
decrease the amount of pesticide used. Bt toxin is produced by a
bacterium called Bacillus thuringiensis (Bt for short). Bt toxin gene has
been cloned from the bacteria and been expressed in plants to provide
resistance to insects without the need for insecticides; in effect created a
bio-pesticide. Examples are Bt cotton, Bt corn, rice, tomato, potato and
soyabean etc.
Bt Cotton: Some strains of Bacillus thuringiensis
produce proteins that
kill certain insects such as lepidopterans (tobacco budworm, armyworm),
coleopterans (beetles) and dipterans (flies, mosquitoes). B. thuringiensis
forms protein crystals during a particular phase of their growth. These
crystals contain a toxic insecticidal protein. Why does this toxin not kill
the Bacillus? Actually, the Bt toxin protein exist as inactive protoxins but
once an insect ingest the inactive toxin, it is converted into an active form
of toxin due to the alkaline pH of the gut which solubilise the crystals.
The activated toxin binds to the surface of midgut epithelial cells and