Advantages of Propagation by Tissue Culture:

* The elimination of diseases and the production of disease free plantlets
* The rapid production of large numbers of genetically identical plantlets
* Introduction of new varieties and or genotypes
* Preservation of germplasm
* Production of haploid plants which can be used for plant breeding
* Production of plantlets from species in which plant development from seed is difficult

When plant cells and tissues are cultured in vitro they generally exhibit a very high degree of plasticity, which allows one type of tissue or organ to be initiated from another type. All plant cells can, given the correct stimuli, express the total genetic potential of the parent plant. This maintenance of genetic potential is called ‘totipotency’.
 

When cultured in vitro, all the needs, both chemical (see Table 2.1) and physical, of the plant cells have to met by the culture vessel, the growth medium and the external environment (light, temperature, etc.). The growth medium has to supply all the essential mineral ions required for growth and development. Many plant cell cultures, as they are not photosynthetic, also require the addition of a fixed carbon source in the form of a sugar (most often sucrose). Physical factors, such as temperature, pH, the gaseous environment, light (quality and duration) and osmotic pressure, also have to be maintained within acceptable limits.

 Plant cell culture media

Culture media used for the in vitro cultivation of plant cells are composed of three basic components:

(1) essential elements, or mineral ions, supplied as a complex mixture of salts;

(2) an organic supplement supplying vitamins and/or amino acids; and

(3) a source of fixed carbon; usually supplied as the sugar sucrose.

The basic components of any cultural medium are:

* Macronutrients: There are six macronutrients, nitrogen, potassium, phosphorous, calcium, magnesium and sulphur.
* Micronutrients: They include, iron (usually added as sulphate with conjunction with EDTA), manganese, zinc, boron, copper, molybdenum, cobalt and iodine.
* A Carbon source: The carbohydrates usually used are sucrose and glucose, maltose, galactose, sorbitol.
* Vitamins: They are thiamine (B1), myoinositol( considered a B vitamin) are considered essential. Moreover: nicotinic acid, pyridoxine (B6).
* Growth Regulators: Plant growth regulators are the critical media components in determining the developmental pathway of the plant cells. The plant growth regulators used most commonly are plant hormones or their synthetic analogues.
* Solidifying or Gelling Agent: The two commonly used are agar, agarose and phytogel.

There are five main classes of plant growth regulator used in plant cell culture,namely:

(1) auxins;

(2) cytokinins;

(3) gibberellins;

(4) abscisic acid;

(5) ethylene.

Auxins

It is more common, though, to use stable chemical analogues of IAA as a source of auxin in plant cell culture media. 2,4-dichlorophenoxyacetic acid (2,4-D) is the most commonly used. 

Cytokinins 

Naturally occurring cytokinins, two have some use in plant tissue culture media (see Table 2.4). These are zeatin and 2iP (2-isopentyl adenine). Their use is not widespread as they are expensive (particularly zeatin) and relatively unstable. The synthetic analogues, kinetin and BAP (benzylaminopurine), are therefore used more frequently.

Gibberellins 

There are numerous, naturally occurring, structurally related compounds termed ‘gibberellins’. Only a few of the gibberellins are used in plant tissue culture media, GA3 being the most common.

Several media formulations are sold  pre-packaged. Some of these are Murashige and Skoog Based Media, Gamborg’s Based  Media, and Orchid Tissue Culture Media. They may contain all components of the media except the carbon source and the solidifying agent, and some may lack vitamins and or growth regulators.   

Plant growth regulators and tissue culture

Generalizations about plant growth regulators and their use in plant cell culture media have been developed from initial observations made in the 1950s. There is, however, some considerable difficulty in predicting the effects of plant growth regulators: this is because of the great differences in culture response between species, cultivars and even plants of the same cultivar grown under different conditions.

However, some principles do hold true and have become the paradigm on which most plant tissue culture regimes are based. Auxins and cytokinins are the most widely used plant growth regulators in plant tissue culture and are usually used together, the ratio of the auxin to the cytokinin determining the type of culture established or regenerated (see Figure 2.1). A high auxin to cytokinin ratio generally favors root formation, whereas a high cytokinin to auxin ratio favors shoot formation. An intermediate ratio favors callus production.