Cell suspension cultures of Catharanthus roseus have been subjected to various media condi­tions in order to stimulate the formation of indole alkaloids. High ajmalicine contents (up to 0.5 mg/g cell fresh weight) were achieved by transferring 2-week-old cell suspensions to a 10-fold volume of a 8% sucrose solution. The alkaloid accumulation started two days after the transfer and reached a plateau after ten days. Furthermore an enhanced level of phenolic compounds was found, whereas growth of the culture was low. The accumulation of both, alkaloids and poly­phenols was stimulated by high concentrations of sucrose and low concentrations of nitrogen con­taining salts and phosphate. When these minerals were added to the sucrose solution in con­centrations commonly used for cell culture media, the accumulation of alkaloids and phenolic compounds was largely suppressed.

Recently medium conditions have been developed which stimulate the formation of the indole alkaloid ajmalicine in cell suspension cultures of Catharanthus roseus [6]. When cells were subjected to these conditions the alkaloid accumulation was preceded by a 12-fold increase of the specific activity of tryptophan decarboxylase. The enzyme activity showed a maximum two days after the cell transfer into the induction medium and subsequently declined. In contrast the activity of strictosidine synthase, the enzyme condensing tryptamine and secologanin, was present over the entire measuring period at a constant level. The intracellular content of tryp­tamine and ajmalicine increased during a period of 6 days after cell transfer and reached a plateau after that time. A possible regulatory function of tryptophan decarboxylase in indole alkaloid biosynthesis is discussed.

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In cell suspension cultures of Catharanthus roseus a rapid accumulation of secondary compounds (tryptamine, indole alkaloids, phenolics) was observed after transfer of the cells into special 'induction'-media devoid of phosphate and other essential growth factors [11,14]. The increase of product levels was suppressed in the presence of phosphate which was almost completely taken up from the medium and accumulated by the cells within 48 h after inoculation. The activities of tryptophan decarboxylase (TDC), the first enzyme in indole alkaloid biosynthesis, and of phenylalanine ammonia-lyase (PAL), the key enzyme of phenytpropanoid biosynthesis, were influenced differently by phosphate. Whereas the accumulation of phenolics and PAL activity were similarly inhibited by low concentration of phosphate, the mediuminduced enhanced activity of TDC was not affected although the product pools were considerably reduced. Some consequences for the regulation of secondary metabolism will be discussed.

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