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close this bookBiotechnology and the Future of World Agriculture (GRAIN, 1991)
close this folderThe tools
View the document(introduction...)
View the documentCutting and pasting
View the documentAbout language and limitations
View the documentCulturing cells and tissues
View the documentControlling the process
View the documentA new threshold?

Culturing cells and tissues

In crop biotechnology, tissue or cell culture techniques, also referred to as clonal propagation, offer the capacity to isolate tissues and individual cells, and grow them out to whole plants. A tissue culture of no more than one cubic centimetre in size may contain a million virtually identical cells, each carrying the potential to become an entire new plant. The technique involves the exposure of the tissue or cell to a cocktail of nutrients and hormones that encourage the development of undifferentiated plant tissue. Once formed, other hormones are added to encourage leaves and roots to be formed, after which the plantlet can be potted in soil.

Tissue culture gives the plant breeder a very powerful tool for speeding up breeding work. Using traditional techniques of crossing and backcrossing different varieties, it can take a breeder from seven to 15 years to produce a new variety. In the case of slow-maturing crops, such as trees, the time scale is even longer. Although scientists still face serious problems with regeneration, tissue culture has already reduced the time necessary to develop oil-palm varieties by a factor of 30! (6) The same technique also enables the evaluation of germplasm for some characteristics to be performed on a growing mass of cells in a Petri dish rather than having to wait until the actual plant has grown out. Also, it can be used to create new variability since spontaneous mutation commonly occurs during regeneration. Breeders can screen this 'somaclonal variation' for useful traits, giving enormous possibilities for the selection and isolation of new strains with valuable characteristics.

Apart from speeding up and expanding the art of plant breeding, tissue culture also offers the possibility of producing plantlets for direct use in the farmers' fields. Though not yet technically feasible for many crops, commercial success has been achieved with some. This offers new possibilities for mass production, particularly of crops which are normally difficult to multiply. Also, virus free plantlets can be produced and distributed for crops which suffer from diseases in the planting material. Finally, tissue culture can provide an effective new tool for the conservation of germplasm, especially for those plants that propagate vegetatively or for crops that produce seeds that cannot be stored in conventional gene banks. The International Centre for Tropical Agriculture (ClAT) in Colombia holds about 3,000 samples of tissue-cultured cassava varieties in vitro. The International Potato Centre in Peru is doing the same with the potato. (7)

While offering exciting possibilities for crop agriculture, the massive use of tissue culture also has its darker sides. One is that widespread use of tissue-cultured crops will result in a tremendous increase of genetic uniformity, as all offspring are genetically identical. This spells high vulnerability, and a resulting increase in the use of farm chemicals. Also, tissuecultured plantlets tend to be more expensive for the farmer. In 1983 Unilever's cloned oil-palm plantlets were 18 times more expensive than the normal ones. (8) Finally, the use of tissue culture in germplasm conservation has serious limitations and even dangers, as pointed out by FAO. The same 'somaclonal variation', that provides breeders with new genetic combinations to select from, is a nuisance in conservation as in the end something quite different from the original parent might be conserved. (9)

The regeneration of tissues and individual cells into entire plants is a formidable tool for the plant breeder, but the culturing of plant cells to directly obtain useful products offers even greater possibilities for food and pharmaceutical industries. Many plants are not grown for direct consumption, but for the useful 'secondary metabolites' which they contain. Medicinal plants are one example, but so are the many shrubs and flowers that produce flavours, fragrances and dyestuffs. What would be nicer than having individual plant cells of such crops produce those valuable substances directly, rather than depending on agriculture to grow out the entire plant? People have used cell cultures for many years with yeast when making wine, beer or bread, but those practices are based on naturally occurring microorganisms. Now biotechnology helps to make it feasible with individual cells. While some commercial success has already been achieved for specific high value plant compounds, this is a technology which is just starting to take off. Scientists still face many technical problems and questions of economic efficiency. But the market stakes are enormous and many firms are now focusing their research in this field. As explained later, cells cultured for these purposes can have tremendous economic benefits for the industries involved, while at the same time spelling disaster for the farmers and countries that now grow the crops which may become obsolete.

Closely linked to plant cell-culture techniques is the work on artificial seeds. Several companies and research institutions are working to perfect techniques whereby plant embryos are mass-produced in fermentation tanks, and later encapsulated in a hard gel to mimic the form and functions of a normal seed. A natural seed is the offspring of two parents, the product of fertilization. An artificial seed is the identical copy of one individual, derived from somatic cells. To some extent, artificial seed technology is a sophisticated method of tissue culture. The end product, in this case, is not a cloned plantlet but an encapsulated somatic embryo. This, again, is a technology that is just starting to take off. Currently, it is only worth pursuing for plants with high value seeds, mainly vegetables. It might, however, have enormous implications for agriculture in the long run.