Plants are the source of a large variety of biochemicals which are metabolites of both primary and secondary metabolism. But secondary metabolites are of much greater interest since they have impressive biological activities like antimicrobial, antibiotic, insecticidal, molluscidal, hormonal properties, and valuable pharmacological and pharmaceutical activities, and many are used as flavours, fragrances, colours etc. The term secondary metabolite is ill-defined but convenient, it is applied to all those compounds which are not directly involved in the primary metabolic processes, e.g. photosynthesis, respiration, protein and lipid biosynthesis etc. Secondary metabolites include a wide variety of compounds, e.g. alkaloids, terpenoids, phenyl propanoids etc.
Biochemicals from cultured plant cells
Cultured plant cells are known to produce biochemicals of interest since 1950's, but initially the yields were very low. Refined culture systems have improved the biochemical yields considerably, and over half a dozen cell cultures produce 2 g/I or more of the biochemical. Shikonin (a naphthoquinone) is produced on a commercial scale from cell cultures of Lithospermum erythrorhizon, while berberine (an alkaloid) is obtained from Coptis japonica cell cultures. Interestingly, Ginserg tissues produced in vitro (19 g dry cells/I) produced in 2000 I bioreactors) are used as additives in tonic drinks, wines, soups, herbal liquors etc. The biochemicals obtained from plant cell cultures are comparable to those derived from intact plants in their chemical, biochemical and biological properties.
In addition, cultured cells of many plant species produce biochemicals which have so far not been detected in whole plants; the list of such compounds has grown rapidly to 140 in 1992 from merely 4 in 1978. For example, cell suspension cultures of Rauwolfia serpentina have been shown to produce 4 highly polar alkaloids which are novel glucosides of ajmaline and its derivatives.
Enhancing biomass yields
Virtually, all high value biochemicals from cultured plant cells are secondary metabolites, which are usually produced in differentiated cells or organized tissues. Therefore, most such biochemicals are not produced by rapidly growing cell cultures, and the culture conditions favouring growth suppress biochemical production (and vice-versa). Therefore, the production strategy should consist of two distinct phases: (i) growth phase for cell biomass accumulation and, (ii) production phase for biosynthesis and accumulation of the biochemicals. Ordinarily, some growth will take place during the production of phase as well, which should be exploited. But in atleast some cases, culture growth and biochemical production occur together, e.g. berberine production in Thalictrum minus.
Improving biochemical production
Biochemical production by cultured cells can be increased chiefly by the following approaches: (i) devising a suitable culture medium and conditions, (ii) development of high producing cultures, (iii) use of elicitors, and (iv) use of organ cultures.