For more than 30 years, many researchers have investigated plant cell cultures for production of a variety of phytochemicals; however, in spite of their many efforts only two products such as shikonins and ginseng cells are so far being manufactured commercially. The reasons why this technology has scarcely been applied in industry are; low yield of plant metabolites, unstable producing ability of cultured cells and their slow growth rate. Therefore, at present the plant cell culture is not a cost-effective technology. In particular, any process using plant cell cultures is not favorable if the desirable products can be easily manufactured by chemical- or microbial fermentation methods.

However, a variety of scientific strategies, as described in this review, have been investigated for improving the production ability of cultured cells and substantial progress has been made. Undoubtedly, some plant metabolites are likely to be manufactured through plant cell cultures. In addition to the approaches described in this review, several research groups have paid an attention to using recombinant DNA technology as a tool for improvement of cultured cells although it is not an easy task in secondary metabolism.

To overcome barriers hindering industrial application of plant cell cultures, however, it is required not only to follow the approaches reviewed, but also to conduct more fundamental research, including elucidation of biosynthetic pathways of many useful secondary metabolites in plants and of the regulation mechanisms in their biosyntheses. Collaboration with a number of researchers in other scientific fields is also very helpful.