What is tissue culture?
Tissue culture is a technique used for the rapid multiplication of plants using a few living cells or tissues. As this process uses tissues to produce new plants asexually, it is also called micropropagation.
Principle of tissue culture
Tissue culture is based on a property of living plant cells called cellular totipotency. Cellular totipotency refers to the ability of a nucleated, mature, living somatic cell to produce the complete organism.
The new individuals formed by tissue culture, just like those formed by any other means of asexual reproduction, are genetically (and morphologically) identical to one another as well as to the parent, and are called clones.
Steps in micropropagation
- Selection of explants
An explant is the part of the plant containing living cells that is used for tissue culture. - Sterilization of explants
The collected explant is surface-sterilized using disinfectants to avoid contamination. - Preparation of a suitable nutrient medium
- Transfer of the selected explants to the nutrient medium
- Initiation of the culture / callus formation
Initiation is done using two plant hormones, auxin and cytokinin, in an equal ratio. This promotes dedifferentiation of the permanent, non-dividing cells of the explant, resulting in the formation of a mass of undifferentiated cells called the callus. The cells of the callus are actively dividing and totipotent, i.e. each cell is capable of producing a new plant body. - Shoot formation
The dedifferentiated cells of the callus are used to promote shoot development by providing both auxin and cytokinin, but with cytokinin in excess. - Root formation
The young shoots are induced to develop roots by again using both auxin and cytokinin, but with auxin in excess. The young plants thus formed, having both shoots and roots, are called plantlets. - Hardening
The young plantlets developed in the laboratory are exposed to conditions similar to the external environment for their acclimatisation. - Transplantation
After hardening, the plantlets are transferred to greenhouses or crop fields.
Remember the hormone ratios: high cytokinin to auxin gives shoots, high auxin to cytokinin gives roots, and roughly equal amounts give callus.
Types of tissue culture
On the basis of the type of explant selected, tissue culture can be divided into the following types:
Single cell culture
Single cells are isolated from explants by mechanical or enzymatic methods.
Somatic embryo culture
In this method of tissue culture, somatic cells (e.g. leaf cells) are used to produce an embryo by using a high concentration of an auxin such as 2,4-dichlorophenoxyacetic acid (2,4-D). The mature somatic embryos are called embryoids, which germinate to produce complete plantlets.
Meristem culture
The cells of the shoot apical meristem or axillary meristem are used to perform tissue culture. Meristem cells are always dividing, so they can easily be used to produce new plants. Additionally, meristem cells are usually free of viruses, so healthy plants can be obtained from an infected plant.
Meristem culture gives virus-free plants because the apical and axillary meristems are usually free of virus even when the plant is infected. This is a frequent NEET question.
Haploid/ anther/ pollen grain culture
Haploid cells of the plant (e.g. pollen grains) are used to obtain haploid plants. This type of tissue culture is highly useful for the improvement of crop plants, as it helps in studying the expression of mutations and in forming pure lines.
Ovary culture/ ovule culture
Ovaries or ovules are cultured to raise embryos from crosses that would otherwise fail to develop.
Advantages of tissue culture
- A large number of copies of the same plant can be obtained in a short time.
- Plants can be prepared inside laboratories throughout the year under controlled conditions.
- It prevents the loss of hybrid characters in the next generations of genetically engineered plants.
- Rare and endangered plant species can be multiplied by this method to save them from extinction.
- Genetically similar plants (somaclones) are formed by this method.
- The effects of induced mutations can be easily studied.