Plant Cell Biotechnology for the Production of Alkaloids: Present Status and Prospects

Abstract

hsmcr.-The culture ofplant cells on a large scale in bioreactors has been shown to be feasible. The price of a plant cell biotechnological product is mainly governed by the slow growth of plant cell cultures, making the depreciation costs of the bioreactor the major costdetermining factor. A review of the production of the economically important alkaloids in plant cell cultures shows that presently only berberine and sanguinarine are being produced. Important factors to be considered in connection with accumulation of alkaloids are biosynthetic rate, accumulation site, and catabolism. Recent studies in the field of the regulation of the biosynthesis of terpenoid indole alkaloids on the level of genes and enzymes are reviewed, showing that it is feasible to clone genes from secondary metabolism and express these in various other plants. Concerning storage, it seems that compartmentation also plays a role in the regulation of alkaloid biosynthesis. Furthermore, catabolism of terpenoid indole alkaloids in cell cultures is an important factor, at some point even equalling the rate of de novo biosynthesis. The ongoing studies on regulation of alkaloid biosynthesis might eventually lead to transgenic plants or plant cell cultures with an improved productivity of the desired compounds. This knowledge is also of interest in connection with studies on the role of secondary metabolism for plants, and may contribute to a better understanding of resistance of plants to diseases and various herbivores. Of all known natural products, about 20% (i.e., about 16,000) are classified as alkaloids. For many of these alkaloids biological activities have been reported, but presently only about 30 are commercialized (1). Most of these are medicines, but some are used as flavoring, poison, and model compounds for pharmacological studies. These alkaloids can be qualified as specialty chemicals, as their worldwide production volume is limited; alkaloids such as quinine and quinidine have a yearly production of 300-500 metric tons, ajmalicine about 3600 kg, and compounds like vinblastine and vincristine in the kilogram range only (2,3). The amounts of plant material needed for the extraction of these compounds are, compared to agricultural crops, very small. For the examples mentioned this is in the order of 5000-10,000 metric tons ofCinchonu bark for the extraction of quinine and quinidine and 200-300 tons of Catharanthus roseus roots for the production of ajmalicine. The value ofeach of the markets of the major alkaloids can be estimated to be in the range of several hundred million dollars (4). These specialty chemicals are now produced by extraction from plant material that is cultivated or sometimes even still collected in the wild. There are several problems connected with this production method. Variable quantities and qualities of the plant material, plants that need to grow several years before they are ready for harvesting (e.g., Cinchona bark), and over-collecting of endangered species (e.g., Taxus hev$olia)