Plant secondary metabolites are biosynthetically derived from primary metabolites—those having an essential role in basic cell metabolism and are not essential for the survival of the individual plant cell. Though being non nutritive and not directly required for growth, they play ecologically significant role in how plants deal with their environment and are important for the survival of the plant as whole. Their distribution may be restricted to specific taxonomic groups and are present in plants in smaller quantities compared to primary metabolites. They are biosynthesized in specialized cell types at distinct developmental stages. Many of them are phytoalexins, that is, produced in response to external physical, chemical or microbiological stimuli. Thus, they may be rapidly synthesized and rapidly destroyed in response to pathogen invasion. Several secondary metabolites are of great economic importance as pharmaceuticals, flavor or fragrance chemicals, pesticides, and so on.

Advances in plant-tissue culture studies have greatly contributed to our understanding of plant secondary products. They have resulted in the identification of more than 200 specific enzymes and this has been invaluable for the study of biosynthetic pathways. Apart from being a source of known compounds, plant-tissue cultures are also known to accumulate newer compounds not found in plants. It has become possible to use plant-tissue culture for bringing about useful chemical conversions on precursor compounds. There has been an increasing interest toward producing plant secondary metabolites by cell cultures to overcome many problems associated with their industrial production from whole plants.

From the time when James Bonner reported rubber bioproduction by guagule plant-tissue cultures in 1940 and Tulecke and Nickells’ demonstration that plant cell biomass could be grown in multi gallon containers in 1950, we have come a long way. A great deal of work especially in Japan and Germany has gone into attempts to generate pharmaceuticals in culture. At least 50 categories of products from plant-tissue culture have been investigated for the feasibility of their generation in large quantities. At least 70 compounds are known to accumulate in plant-tissue culture in concentrations equal to or exceeding that of the parent plant, for example, Catharanthus alkaloids (>1% dry weight), Nicotine and diosgenin (>3.5%), trigonelline and ginseng saponins (>5%), rosemarinic acid (15%), Morinda anthraquinones (18%), proanthocyanins (up to 40%), and Vomelenine alkaloid (>50%) compared to the plant.

Despite extensive research to make possible commercial production of plant secondary metabolites in culture, so far only shikonin, ginseng, and taxol are being commercially produced by tissue culture. Even in successful cases of plant secondary metabolite production in cultures, overall, the technology of growing large quantities of plant cell biomass for biotechnological production of plant secondary metabolites is not yet cost effective.

While several factors such as slow growth of plant cells, their low/variable secondary metabolite yield, genetic instability are quoted, huge volume of work undertaken on plant cell culture have thrown light on many other aspects involved and the problems it poses.

A brief glimpse at the lessons learnt from studies on plant secondary metabolite production: