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Professor C Kameswara Rao
Foundation for Biotechnology Awareness and Education
Bangalore 560 004, India

Tissue Culture and Micropropagation

Cells differentiated through natural life processes in several different specific ways form the tissues and organs in organisms.  The earlier undifferentiated cells can be made to develop along specific directed lines to develop into tissues, organs and even whole organisms in synthetic media in the laboratory, through process a called tissue culture.   While natural cells, tissues and organs can be made to respond in tissue culture, the technique can also be used to modify natural or tissue culture originated cells, tissues and organs in almost any manner.  On the medical side the stem cells are being explored and exploited to develop tissues and organs for transplantation to replace defunct parts. 

The undifferentiated cells in the growing points of plants and animals are totipotent in the sense they can be made to develop into any type of cell of the organism and so the whole organism, through appropriate procedures in the laboratory.  Elegant tissue culture protocols are widely used in a variety of experimental and production phases both for plants and animals.  Since all this happens in the laboratory glassware, it is called in vitro (in glass), in contrast to in vivo that happens in the living systems.

In plants the growing points that contain undifferentiated are called meristems.  The meristematic cells can be tissue cultured to produce tissues, organs and whole plants.  Tissue culture produced tissues, organs and plantlets, that constitute germplasm, can be cryopreserved regenerated at a later date when needed using tissue culture protocols.  Pollen can be cultured to produce haploid plants (containing only one set of chromosomes instead of the usual two sets) and even fertilization and seed development can be achieved in vitro.  This facility provides for the conservation of plant germplasm in germplasm banks, variously called in vitro banks or tissue banks or gene banks. 

When a piece of meristematic plant tissue (explant) of any part of a plant is subjected to culture protocols, it develops into an undifferentiated mass of cells (callus), from which numerous heart shaped structures (embryoids) arise.  Each one of these embryoids can be cultured into a whole plant in the laboratory.  The embryoids encapsulated in calcium alginate or other suitable material, called synthetic or artificial ‘seeds’, can be cryopreserved for long periods till required for use.

As a very large number of embryoids develop from cultured callus, it is possible to produce thousands of plantlets.  The plantlets are transferred to soil to acclimatize (hardened) and taken to the field to raise a crop.  This process is micropropagation, a way of cloning, which is used extensively to produce genetically uniform plants.  These plants are disease free at least till they are taken to the soil.   There are hundreds of success stories of mass cultivation of plants produced through micropropagation with such crops as banana, plantation tree species as eucalyptus and poplars, and ornamental species as orchids.

Cell and tissue culture techniques are also largely required in developing whole plants from transformed cells in genetic engineering.

In vitro Banks require expensive infrastructure and expertise in collecting, processing and culturing plant material and to preserve and regenerate the cultured material into plants.  The viability of the banked germplasm varies with the experimental finesse and the species, and hence it has to be periodically checked.  Germplasm banking is more reassuring than seed banks, where once the seed viability is lost, the collections become unusable.  Like the seeds in seed banks, banked in vitro germplasm can be a source of DNA, for use in genetic engineering even when it is unviable. 

Thousands of institutions have been engaged in plant tissue culture for over three decades and several succeeded in culturing a very large number of economically plant species, but only a few worked on crop plants.  While many institutions claim in vitro banks, only a few such as the Plant DNA Bank in Korea, are credible.   Some like the National Bureau of Plant Genetic Resources, New Delhi, India, bank only explant material.  Seed in the seed banks can also be used for tissue culture purposes.

Material in the in vitro banks suffers from some disadvantages.  Being much more sensitive to storage conditions, particularly temperature and humidity, than seeds and extracted DNA, the material requires very precisely controlled facilities to ensure longer viability periods.  Though plantlets produced through micropropagation can be transported rather easily, other kinds of in vitro banked material require special facilities for transit.  Tissue culturing requires high level of technical expertise and production costs are very high. 

Responsibilities of Germplasm Banks

The in vitro banks have serious responsibilities in order to fulfill their mandates.  The more important of them are:

a) Ensuring the authenticity of the scientific identity, source and geneology of the explant source species;

b) Adopting state of the art procedures of collection, recording, processing, culturing  and preserving the cultured material; 

c)Maintaining viability of the banked material and replenishing material of doubtful quality;    

d)Ensuring responsible use of the material supplied to others, assuring equitable benefit sharing by all parties;

e) Networking internationally, with other in vitro banks, facilitating exchange of knowledge and material and to prevent duplication of efforts; and 

f)Updating websites frequently and fulfilling the promises made. 

November 11, 2008