A different breed
The ability to shift genes from one species to another is revolutionising agriculture and plant breeding. Previously, breeders looking for a gene that confers resistance to a pest, had to restrict their search to wild relatives of the plant. If the relatives didn't have the required gene, the search had to be abandoned. But now, researchers can search the whole world for a wanted gene and simply add it to the crop. Additionally, a host of other unwanted genes that are found in the wild relative can also be eliminated.
Agricultural genetic engineering successes have not been as dramatic as in medicine, but experts predict biotechnology in the food-processing and agricultural sector will range from $10 billion to $100 billion by 2000. So far, agricultural genetic engineering has been driven largely by companies manufacturing pesticides and herbicides and more recently, by seed companies.
With increasing concern about the environmental effects of agrochemicals, the $8-billion herbicide industry is looking at genetically transformed plants that are herbicide tolerant. Another reason for promoting such crops is the difficulty in developing new and effective herbicides. A recent study by the Organisation for Economic Cooperation and Development shows that of all the genetically manipulated crops that were field-tested since 1986, almost 60 per cent were herbicide tolerant ones. Scientists hope to develop crops that are resistant to a broad spectrum of herbicides, which means herbicides can be sprayed with impunity to destroy weeds leaving an unaffected healthy crop behind.
Manipulating tolerance
Manipulating plants to be herbicide tolerant has been relatively easy because this trait is usually imparted by a single gene from herbicide-resistant bacteria. Another approach has been to incorporate genes that increase the number of proteins affected by the herbicide, thereby making more proteins available for the plant to survive. The US-based agrochemical giant Monsanto used this strategy to field-test maize, cotton, soya bean, sugarbeet, tobacco and tomato crops for herbicide resistance.
Gene transfer has also been used extensively to enhance pest and disease resistance in crops. Toxin-producing genes from the bacteria, Bacillus thuringiensis (Bt), are extremely popular. The Bt toxin is extremely poisonous for several insects, like the Colorado potato beetle and the cotton pod borer Heliothis.
Scientists have incorporated the toxins into several crops and insect-resistant cotton, which has been extensively field-tested, will probably be the first insect-resistant crop to be commercialised. Because several types of Bt toxins are produced by the bacteria, experts say insect resistance that develops to one toxin can be overcome simply by using another toxin.
The favourite vector for transferring genes into most plants, except cereals, is a crown-gall disease-causing bacterium called Agrobacterium tumefaciens. This bacterium can transfer a part of its DNA into the plant genome and induce plant cells to reproduce madly, forming a tumour-like gall. By substituting the bacterium's tumour-causing genes with a required gene, the plant genome can be transformed.
Cereals have a problem because they are not natural hosts for Agrobacterium. So, scientists from Cornell University in Ithaca developed an instrument called a "particle gun", which delivers the DNA by "firing" DNA-coated particles through cell walls.
Rice was the first to fall to this technique and transgenic rice was first produced in 1988. Maize followed in 1990 and wheat proved to be recalcitrant till the middle of last year. Though reports from laboratories come regularly, most experts point out the commercial development of cereals will be a while yet.
Among non-cereal, tobacco was the first to be modified. Afterwards, potatoes have been engineered to reduce starch content, corn to make it resistant to certain insects and cotton to be resistant to herbicides. Nitrogen-fixing bacteria have been manipulated to form nodules on leguminous plants that are not their natural hosts. Attempts to produce plants that can survive in adverse environmental conditions like high soil salinity and aridity, are also being made. In more outlandish attempts, plants have been made to produce biodegradable polymers that can be used in plastics and others that glow in the dark.
A report from Japan indicates that scientists are genetically manipulating rice to sell "physiologically functional food". For unexplained reasons, allergy to rice has become common in Japan. The allergy causes unsightly red lesions on the skin and the only cure is to avoid rice and its products, including sake, which is not welcomed by the Japanese. Cashing in on the opportunity, Japan's leading cosmetics manufacturer Shiseido now markets as a health product rice from which the allergy causing protein globulin has been removed.
Transgenic tomatoes with the brand name Flavr Savr, developed by the Californian company Calgene, can stay fresh longer and are likely to be the first genetically altered food product to reach American supermarkets this year.
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