Just as every person is different and unique, so is each plant. Some have traits like better color, yield, or pest resistance. For years, scientists have looked for methods to allow them to make exact copies of these superior individuals.
Plants usually reproduce by forming seeds through sexual reproduction. That is, egg cells in the flowers are fertilized by pollen from the stamens of the plants. Each of these sexual cells contains genetic material in the form of DNA. During sexual reproduction, DNA from both parents is combined in new and unpredictable ways, creating unique organisms.
This unpredictability is a problem for plant breeders as it can take several years of careful greenhouse work to breed a plant with desirable characteristics. Many of us think that all plants grow from seeds but now, researchers have developed several methods of growing exact copies of plants without seeds.
Tissue culture is the cultivation of plant cells, tissues, or organs on specially formulated nutrient media. Under the right conditions, an entire plant can be regenerated from a single cell. Plant tissue culture is a technique that has been around for more than 30 years. Tissue culture is seen as an important technology for developing countries for the production of disease-free, high quality planting material and the rapid production of many uniform plants. Micropropagation, which is a form of tissue culture, increases the amount of planting material to facilitate distribution and large scale planting. In this way, thousands of copies of a plant can be produced in a short time. Micropropagated plants are observed to establish more quickly, grow more vigorously and taller, have a shorter and more uniform production cycle, and produce higher yields than conventional propagules.
Plant tissue culture is a straightforward technique and many developing countries have already mastered it. Its application only requires a sterile workplace, nursery, and green house, and trained manpower. Unfortunately, tissue culture is labor intensive, time consuming, and can be costly. Plants important to developing countries that have been grown in tissue culture are oil palm, plantain, pine, banana, date, eggplant, jojoba, pineapple, rubber tree, cassava, yam, sweet potato, and tomato. This application is the most commonly applied form of biotechnology in Africa.
Examples of the use of tissue culture in crop improvement in Africa include:
A new rice plant type for West Africa (NERICA – New Rice for Africa) resulting from embryo rescue of wide crosses made between Asian rice (Oryza sativa) and African rice (Oryza glaberrima) followed by anther culture of the hybrids to stabilize breeding lines.
Benefits of TC technology for rice farmers in West Africa (Source: WARDA)
For years, scientists dreamed of combining the ruggedness of the African rice species (Oryza glaberrima) with the productivity of the Asian species (Oryza sativa). But the two are so different, attempts to cross them failed as the resulting offspring were all sterile. In the 1990s, rice breeders from the West Africa Rice Development Association (WARDA) turned to biotechnology in an attempt to overcome the infertility problems. Key to the effort were gene banks that hold seeds of 1500 African rices — which had faced extinction as farmers abandoned them for higher-yielding Asian varieties.
Advances in agricultural research helped scientists cross the two species — a breakthrough that is changing the lives of many rice farmers in West Africa.
After cross-fertilization of the two species, embryos were removed and grown on artificial media in a process known as embryo-rescue.
Because the resultant plants are frequently almost sterile, they are re-crossed with the sativa parent wherever possible (known as back-crossing). Once the fertility of the progeny was improved (often after several cycles of back-crossing), anther culture was used to double the gene complement of male sex cells (anthers) and thus produce true-breeding plants.
The first of the new rices dubbed ‘New Rice for Africa’ (or NERICA) was available for testing in 1994 and since then, the techniques have been refined and streamlined, so that many new lines are generated each year. The dream had come true. The new plants had the best of both worlds – some of them combined yield traits of the sativa parent with local adaptation traits from glaberrima.
The NERICAs inherited wide, droopy leaves from their African parent, which smother weeds in early growth. That reduces labor, and allows farmers to work the same land longer, rather than having constantly to clear new land.
The structure of the panicles, or grain heads, has also been changed. Panicles of the African species produce only 75-100 grains. The new rices inherited, from their Asian parent, longer panicles with ‘forked’ branches, and hold up to 400 grains.
Like their Asian parent, the new rices hold grains tightly, not allowing them to shatter. They produce more tillers than either parent, with strong stems to support the heavy grain heads.
The new varieties outyield others with no inputs—but respond bountifully to even modest fertilization. During rice trials, yields as high as 2.5 tonnes per hectare at low inputs—and 5 tonnes or more with just minimum increase in fertilizer use, have been obtained, approximately 25% to 250% increase in production.
The new rices mature 30 to 50 days earlier than current varieties, allowing farmers to grow extra crops of vegetables or legumes. They are taller than most rices, which makes harvesting easier—especially for women with babies strapped to their backs. They resist pests and tolerate drought better than the Asian rices— vitally important for rainfed-rice farmers. The new rices grow better on infertile, acid soils—which comprise 70% of West Africa’s upland rice area.
They also have about 2% more body-building protein than their African or Asian parents.
Because of their success, NERICAs were quickly adopted by farmers. In 2000, it was estimated that the new rices covered some 8,000 ha in Guinea, of which 5000 ha grown by 20,000 farmers was under the supervision of the national extension agency. In 2002, WARDA projected that 330,000 ha would be planted to NERICAs, sufficient to meet the country’s own seed needs, with surplus for export to neighboring countries.
For more, please read “Farmers embrace African ‘miracle’ rice” (http://www.un.org/ecosocdev/geninfo/afrec/
Bananas propagated from apical meristem in Kenya have been shown to have increased vigour and suffer lower yield loss from weevils, nematodes, and fungal diseases.
Benefits of TC technology for small-scale banana producers in Kenya (Source: ISAAA)
In Kenya, as in many parts of the tropical and subtropical developing world, banana is a highly important food crop. In last 20 years, however, there was a rapid decline in banana production due to widespread soil degradation and the infestation of banana orchards with pests and diseases. These problems were further aggravated by the common practice of propagating new banana plants using infected suckers. The situation was threatening food security, employment and incomes in banana-producing areas.
Tissue culture (tc) technology was considered an appropriate option to provide sufficient quality and quantity of such materials.
With proper management and field hygiene, yield losses caused by pests and diseases at farm level have been reduced substantially. Tissue culture technology has made it possible for farmers to have access to the following:
large quantities of superior clean planting material that are early maturing (12-16 months compared to the conventional banana of 2-3 years)
bigger bunch weights (30-45 kg compared to the 10-15 kg from conventional material
higher annual yield per unit of land (40-60 tons per hectare against 15-20 tons previously realized with conventional material)
Moreover, uniformity in orchard establishment and simultaneous plantation development has made marketing easier to coordinate with the possibility of transforming banana growing from merely subsistence to a commercial enterprise. An encouraging finding from a cost-benefit analysis of the project is that tc banana production is more remunerative as an enterprise than traditional banana production. The project has also benefited mainly women who tend the crop, thus helping to narrow the gender gap.
DANIDA.2002. Assessment of potentials and constraints for development and use of plant biotechnology in relation to plant breeding and crop production in developing countries. Working paper. Ministry of Foreign Affairs, Denmark
DeVries, J. and Toenniessen, G. 2001. Securing the harvest: Biotechnology, breeding and seed systems for African crops. The Rockefeller Foundation, New York. USA
FAO 2002 Crop Biotechnology: A working paper for administrators and policy makers in sub-Saharan Africa. Kitch, L., Koch, M., and Sithole-Nang, I.
Wambugu, F. and Kiome, R. 2001. The benefits of biotechnology for small-scale banana farmers in Kenya. ISAAA Briefs No. 22. ISAAA: Ithaca, NY
West Africa Rice Development Association (WARDA)