INCREASED PRODUCTION OF SEROTONIN BY SUSPENSION AND ROOT CULTURES OF PEGANUM-HARMALA TRANSFORMED WITH A TRYPTOPHAN DECARBOXYLASE CDNA CLONE FROM CATHARANTHUS-ROSEUS

Cell suspension and root cultures of Peganum harmala were established expressing a tryptophan decarboxylase cDNA clone from Carharanthus roseus under the control of the cauliflower mosaic virus (CaMV) 35S promoter and terminator sequences. The tryptophan decarboxylase activity of some of the transgenic lines was greatly enhanced (25-40 pkat/mg protein) as compared to control cultures (1-5 pkat per mg protein) and remained high during the growth cycle. While the levels of tryptamine, the product of the reaction catalysed by tryptophan decarboxylase, were unchanged in the transgenic lines, their serotonin contents were enhanced up to 10-fold, reaching levels of 1.5 to 2% dry mass. Thus, tryptamine produced by the engineered reaction was apparently immediately used for enhanced serotonin biosynthesis. The yields of serotonin in transgenic lines overexpressing tryptophan decarboxylase activity were further enhanced to 3-5% dry mass by feeding L-tryptophan, while no or only minor effects were seen when control cultures were fed. These data demonstrate that the production of a plant secondary metabolite can be enhanced greatly via genetic manipulation of the level of activity of the rate-limiting enzyme. The amounts of beta-carboline alkaloids, the other tryptamine-derived metabolites of P. harmala, in contrast, were not affected by the overproduction of tryptamine. The information needed for successfully predicting manipulations that enhance production of a secondary metabolite is discussed.