Metabolites that are not required by the producing organism for its support system are called secondary metabolites.
Application of plant tissue culture for production of useful secondary compounds has been recognised since the early 1950s. Plant cells generally produce only small amounts of useful compounds. Techniques are now available to induce and select stable genetic variables arising from the cultured tissue and these can be applied for production of high amounts of vitamins, pigments, alkaloids, food flavours and useful metabolites and plant species. About 25% prescription medicines and various raw materials used in industries are obtained from plants.
The impact of utilizing a specific tissue culture system for producing increased levels of certain secondary metabolites over the intact plant has been observed in cell lines producing high nicotine in Nicotiana tabacum, anthocynin in Euphorbia millii, antheaquinones in Morinda. Citrifolia, Crosus sativa and in about 18 species from genera Asperula, Galium and Sherardia, Shikonin derivatives in Lithispermum erythorhizon, phenolic compounds in Acer pseudoplatanus, 1-hydroxyquiphenylalanine ( L-dopa) in Stizolobium hassjoo, ferruginol in Salvia miltorrhiza, sanguinarine in Papaver Sommiferum, ajmalcine and serpentine in Catheranthus roseus, Ros-marinic acid in Coleus blumei, diosgenin in Dioscoarea deltoids etc.
Various evidence suggest that the regulation of secondary metabolism is linked with induction of morphological differentiation in plants of some species. For example, cardiac glycoside of Digitalis are principally found in leaf cells quinine and quinine in the bark of Cinchona trees and tropane alkaloids in roots of many solanaceous species. This shows that cells not only undergo a morphological specialization during plant growth and maturation, but also differentiate in their capacity to produce specific chemicals. Investigation on cell differentiation versus secondary product synthesis using celery tissue culture revealed that cultures initiated from less differentiated tissues ( globular and heart-shaped embryos demonstrated no flavour compounds, while those initiated from better differentiated tissues torpedo-shaped embryos, petiole tissues) possessed the characteristics celery phthalide flavour compounds. It was further assumed that presence of chlorophyll in differentiated tissues may be someway associated with the synthesis of flavour compounds.