The in vitro culture of plant cells or tissues in artificial medium is said to be plant tissue culture. It has many applications in crop improvement, preservation, breeding and in industries.

Micropropagation

Micropropagation is the rapid production of a large number of identical clones in vitro within a short duration. It restores the original genetic make up of parent plant in the clone. So it is used to breed crop plants in large number. Micropropagation is practiced in-

• Plants in which seed production is difficult or impossible.
• Plants which fail to produce seeds with desired traits.
• Plants in which frequent changes take place due to cross pollination.

Elimination of Pathogens

The plants free from pathogens can be raised from the existing plants through meristem culture. A small slice of meristematic tissue is cultured in a medium. During this culture, the culture flask is subjected to heat shock treatment to destroy the pathogens. The resulting callus is regenerated into plantlets. They are free from bacteria, viruses and mycoplasmas.

Germplasm Storage

Germplasm storage refers to the collection and storage of different plant species in a limited area. Most of the plants are stored in the form of seeds or as growing plants. In vitro storage is practiced in recent years.

The plant material stored in a minimal medium with low light intensity at a low temperature. Such a medium reduces the growth rate of plant tissue, while the totipotency of tissue is maintained in it. Thus a large number of individuals are stored in a small area. Sub- cuturing may be done at a regular intervals of one year.

Sometimes the actively growing meristematic tissues are stored in liquid nitrogen at -196ÂșC . This method of preservation of plant tissue is named cryopreservation of cold storage.

Somaclonal Variation

Generally clones released through tissue culture show uniformity in their characters. However, a few clones show variations among the clones. They are called somatic variants. They have one or two characters not present in their parents. The formation of variant clones from the cultured callus tissues is called somaclonal variation. It may be caused by mutation in some cells of callus tissue.

Somaclonal variants are obtained from the culture of explants like immature embryos, tip of inflorescences, stems, microspores, ovaries, etc. in the presence of mutagens.
The regenerated variant clones are tested for their resistance to herbicide, heavy metals, temperature, drought, etc. The resistance clones are again tested for their productivity. The selected clones are multiplied by using micropropagation.

Uses: Fiji and powdery mildew resistant sugarcane varieties.
• Potato resistant to early and late blights.
• Potato with altered chromosome number.
• Tobacco with a different grain colour, grain weight.
• Tobacco with a different type of chloroplast
• Maize resistant to Helminthosporium.

Embryo rescue

In some plants, normal fertilization takes place, but the ovule to become a seed. In such cases, the immature embryo is taken from the immature fruits and cultured in a tissue culture medium, consequently develop from embryo. As this method overcomes embryo immaturity, it is often known as embryo rescue.

The Production of Haploids

The haploid plants contain only a single set of chromosomes in their cells. They are produced from anther cultures, pollen cultures and ovule cultures. The haploids are used in plant breeding in order to improve the field and agricultural crops.
Nearly 250 haploid species are established through anther culture method. Example: rice, rye, wheat, potato, tomato, tobacco, etc.

Uses: Haploids plants are used to-
• Identify recessive traits and recessive mutants of crops.
• Breed diploid fertile plants by chromosomal doubling.
• Breed plants by mutation.
• Study the metabolic pathways of cells.

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