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Plant Tissue Culture

In 1965, French botanist George Morel was attempting to obtain a virus-free orchid plant when he discovered that a millimetre-long shoot could be developed into complete plantlets by micropropagation. This was the beginning of tissue culture. Thereafter, in the 1970s developed countries began commercial exploitation of this technology. It entered the developing world in the 1980s. It was earlier used to develop ornamental plants and flowering plants for export. With tree species, the technique of tissue culture remained confined for many years to the laboratory stage and had generally invited only academic interest. But in most developing countries, the shortage of biomass and the ever-increasing energy requirements created the need to explore possibilities of mass propagation of trees by tissue culture.

Tissue culture or mass cloning methods of elite tree species is done for increasing land productivity. They are being modified or adapted for large-scale modification.

Species are selected for tissue culture on the following basis.

bullet.gif (62 bytes) Species that have regeneration problems, specially because of poor seed set or germination (as in Anogeissus and bamboo). In these cases, seeds collected from superior trees are used for initiating cultures.
bullet.gif (62 bytes) Species that vary markedly in their desirable traits, i.e. Eucalyptus. The selected trees are marked from the variant population for the desirable trait such as disease resistance, straight bole, higher productivity, etc. in consultation with officials from state forest department or growers.
bullet.gif (62 bytes) Species where plants of any one particular sex is of commercial importance, for example female plants of papaya and male plants of asparagus

In tissue culture cells, tissues, and organs of a plant are separated. These separated cells are grown especially in containers with a nutrient media under controlled conditions of temperature and light. The cultured plant requires a source of energy from sugar, salts, a few vitamins, amino acids, etc. that are provided in the nutrient media. From these cultured parts, an embryo or a shoot bud may develop, which then grows into a whole new plantlet. Similarly, portions of organs or tissues can be cultured in a culture media. Generally, these give rise to an unorganized mass of cells called callus (soft tissue that forms over a cut surface).

Tissue culture plantlets have poor photosynthesis efficiency and lack the proper mechanism to control water loss. They need to be hardened gradually by moving them along a humidity gradient in the greenhouse. Once these plants are in the research fields, they are evaluated under field conditions and the data is collected every 6 months. A large number of tissue culture plants that have grown into trees are remarkably uniform and show an increase in biomass production over the conventionally raised plants.

Tissue culture

                                            Figure Tissue culture and totipotency

Application of tissue culture

bullet.gif (62 bytes)Micropropagation
Rapid vegetative multiplication of valuable plant material for agriculture, horticulture, and forestry.

bullet.gif (62 bytes)Production of disease-free plants
When the apex of shoot is used for multiplication by tissue culture, we get disease free plants because the shoot apical meristem, a group of dividing cells at the tip of a stem or root, is free from pathogens.

bullet.gif (62 bytes)Plant breeding
Tissue culture has also been successfully used in plant breeding programmes.

bullet.gif (62 bytes)Production of disease- and pest-resistant plants
Plants grown from tissue culture usually pass trough callus phase and show many variations. These show some agronomic characteristics like tolerance to pests, diseases, etc.

Cloning

Genetically identical plants derived from an individual are called clones. Processes that produce clones can be put under the term ‘cloning’. This includes all the methods of vegetative propagation such as cutting, layering, and grafting. Propagation by tissue culture also helps in producing clones. Using the shoot tip, it is possible to obtain a large number of plantlets. This technique is used extensively in the commercial field for micropropagation of ornamental plants like chrysanthemum, gladiolus, etc. and also crops such as sugar cane, tapioca, and potato. Thus an unlimited number of plants that are genetically similar or are clones can be produced in a short span of time by tissue culture.

Large-scale propagation

To bridge the gap between research and application, the Department of Biotechnology, Government of India sponsored the setting-up of two pilot-scale facilities for large-scale propagation of elite planting material of forest trees through tissue culture. One of these facilities has been established at TERI’s 36-hectare-campus in Gual Pahari, Haryana with an annual capacity of a million plantlets. Research at these facilities focuses exclusively on developing new protocols for mass cloning of elite planting material, mainly of trees.

Till date, over 4 million plants have been dispatched for field plantation from these facilities. The tissue culture raised plants are presently being evaluated under field conditions. This is being done in tandem with the forest departments of Haryana, Uttar Pradesh, Madhya Pradesh, Bihar, Jammu and Kashmir, and Orissa. For initial screening for phenotypically superior trees only a few hundred plantlets of the same are raised and tested under various agroclimatic zones. The best clones are then mass multiplied and monitored regularly for their performance. Field data suggest a survival percentage of more than 90% even in the harsh conditions of Aravalis without the life-saving irrigation. At half the rotation age some of the selected clones of Eucalyptus are showing a significant increase in productivity as compared to the conventional seed raised progenies.

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