All cells except sperm and egg cells of a plant contain the full complement of genes, therefore it is often possible to grow individual cells and pieces of tissue into full healthy plants. This process is called tissue culture or micro propagation when whole plants are grown under artificial controlled conditions (in vitro) from sections of plant tissue. The plants that result are clones of the original.
Under tissue culture, a single original shoot tip is heat-treated to destroy diseases and then used through as many as ten cycles of regeneration to produce up to 1,500 daughter plants. Tissue culture is used to create large amounts of plants or growing material in a sterile environment with the required water and nutrients to suit the plant species and a gelling agent such as agar. During the process, plant hormones that regulate growth can also be added to increase multiplication or to enable the development of roots. Because the tissues are isolated and raised in contained and sterile environments, tissue culture is a highly effectively method for producing pest- and disease-free planting material.
Although most commonly used for high value horticultural crops, today tissue culture propagation has also been very successful in producing improved subsistence crops widely used in developing countries. One major breakthrough made possible by tissue culture is the development of disease-free bananas in East Africa. Bananas are a major source of consumption, nutrition and income in Africa; Ugandans for example eat on average 1kg per person per day. Other notable examples include higher-yielding varieties of rice and a frog-skin disease resistant cassava variety.
Demand for rice in West Africa is rising rapidly, failing to be met by local production. The region imports around 6 million tonnes of rice each year (half of the region’s requirements) at the considerable cost of US$1 billion. In order to address this shortfall, Monty Jones, a Sierra Leone scientist working at the Africa Rice Centre (previously WARDA), began a programme utilising tissue culture technologies to develop crosses between the African species of rice (Oryza glaberrima) and the Asian species (Oryza sativa). The former is better adapted to local environments, but typically returns low yields of around 1 ton per hectare whereas the latter yields around 5 tons per hectare. Crossing the 2 species created numerous embryos but these could only be grown to maturity with the use of tissue culture. The resulting ‘new rices for Africa,’ or NERICAs are tolerant to a variety of harsh conditions and produce much higher yields.
Cassava is a major staple crop for millions of people in East and Central Africa, mostly in the rural areas and it is the second most important staple crop in Africa after maize. Although dependable with respect to high temperatures and water-scarce growing environments, cassava is highly susceptible to a number of pests and diseases, especially African cassava mosaic disease which can lead up to 100% yield losses. Frog-skin disease, which affects cassava in South American countries such as Columbia, has been eliminated from 5 different cassava cultivars using a combination of heat treatment and tissue culture. Developing disease-free cassava varieties through the use of tissue culture in Africa may be possible as innovations seek to reduce the cost of application of the technology.Contribution to Sustainable Intensification
Tissue culture has been shown to improve food and nutrition security, increase incomes and enhance biodiversity. In particular, the technology is useful for reducing plant breeding times or creating large amounts of plant material for research or distribution to farmers especially for crops important to food security in developing countries. Most notably, the technique has been applied to breed disease-free, high-yielding planting material for staple crops such as bananas, rice and cassava. As a prerequisite for conserving germplasm in vitro (the process of taking place outside of a living organism), tissue culture enables the preservation of a wide variety of genetic resources for plants, animals and trees. Whilst widely used, the technology is costly which explains the low adoption rates in developing country breeding programmes.
Tissue culture has been used to develop more than 1,000 plant species, many of which are used commercially. The main advantage that tissue culture technology offers is the ability to produce a significant number of high quality and uniform planting material rapidly that can be multiplied year-round under sterile conditions anywhere irrespective of the season or weather. During multiplication, thousands and even millions of seedlings can be produced in a very short time for transfer to the field. These methods are useful in particular for root and tuber crops such as cassava, potato, and sweet potato as well as for fruit tree crops such as banana and oil palm mainly because healthy planting materials can be produced at reasonable cost. For tree crops, especially, tissue culture has sped up the breeding process considerably compared to the length of time normally required for developing perennial planting material, resulting in comparatively cheap and fast method for multiplying the best genetic stock.
Tissue culture may offer a cost-effective way to multiply a significant quantity of planting materials very rapidly, but the technology is nevertheless capital, labour and energy intensive. First, in developing countries, the resources of trained personnel and equipment are often not readily available. The availability of utilities such as electricity and clean water can be erratic and costly which can be problematic for tissue culture plant multiplication. The energy requirements for tissue culture technology depend on day temperature, day-length and relative humidity, but importantly they must all be controlled which can prove challenging if energy supplies are not consistently reliable and affordable. Without quality control, unintended variations and deformities may occur. Low cost options for multiplication through tissue culture in developing countries must be explored.
Tissue culture has been widely used for more than 50 years and is now used to improve many crops important to developing country food security including major staples such as rice, potato and banana. Other crops important to developing countries that are improved and propagated by tissue culture include cassava, sweet potato, and yam; commercial plantation crops such as cocoa, coffee, oil palm, sugarcane and tea; horticultural crops, such as artichoke, cardamom, garlic, ginger, and vanilla; and fruit trees, such as almond, cactus, citrus, coconut, date palm, grape, lemon, mango, olive, pistachio, pineapple, and plantain. Some of the many countries with well-developed tissue culture programmes in Africa include Gabon, Kenya, Nigeria, and Uganda.
Some of the greatest successes with tissue culture has been demonstrated with vegetatively propagated (asexually) root crops. For example, disease-free sweet potatoes have been adopted on 500,000 hectares in Shandong Province in China, increasing yields between 30%–40% and incomes for 7 million sweet potato producers by 3.6%–1.6%. In India, potato breeders used tissue culture to detect viruses at the initial stages of seed production leading to an estimated 2-3 fold increase in seed health whilst generating more than US$4 million in revenues. Farmers in Vietnam participated in the use of tissue culture for high-yielding, late-blight resistant potatoes enabling them to double their yields from 10 to 20 tonnes per hectare. Zimbabwean farmers similarly improved household food security and generated cash surpluses from growing sweet potato plantings that were produced through tissue culture.
In addition to rapidly breeding superior quality plantings for a multitude of crops, tissue culture is also important for germplasm preservation, the conservation of a variety of genetic resources, many of which are important for developing countries. Depending upon the crop species and method of preservation, tissue culture can aid in the preservation of genetic resources from 1 to 15 years. Cryopreservation – the preservation of germplasm in a dormant state at ultra-low temperatures, usually in liquid nitrogen (-196 °C) – is a type of tissue culture which can be used to preserve seeds, sperm, and embryos from crop, livestock, forest or fish populations. For tree crops such as bananas that do not produce seed, or crops whose seeds do not survive under cold storage such as mango, coffee, oak and many tropical tree species, cryopreservation can offer a viable alternative.