OVEREXPRESSION OF A TRYPTOPHAN DECARBOXYLASE CDNA IN CATHARANTHUS-ROSEUS CROWN GALL CALLUSES RESULTS IN INCREASED TRYPTAMINE LEVELS BUT NOTIN INCREASED TERPENOID INDOLE ALKALOID PRODUCTION

The enzyme tryptophan decarboxylase (TDC) (EC 4.1.1.28) catalyses a ke y step in the biosynthesis of terpenoid indole alkaloids in C. roseus by converting tryptophan into tryptamine. Hardly any tdc mRNA could be detected in hormone-independent callus and cell suspension cultures t ransformed by the oncogenic T-DNA of Agrobacterium tumefaciens. Supply of tryptamine may therefore represent a limiting factor in the biosyn thesis of alkaloids by such cultures. To investigate this possibility, chimaeric gene constructs, in which a tdc cDNA is linked in the sense or antisense orientation to the cauliflower mosaic virus 35S promoter and terminator, were introduced in C. roseus cells by infecting seedl ings with an oncogenic A. tumefaciens strain. In the resulting crown g all tumour calluses harbouring the tdc sense construct, an increased T DC protein level, TDC activity and tryptamine content but no significa nt increase in terpenoid indole alkaloid production were observed comp ared to empty-vector-transformed tumour calluses. In tumour calluses c ontaining the tdc antisense construct, decreased levels of TDC activit y were measured. Factors which might be responsible for the lack in in creased terpenoid indole alkaloid production in the tdc cDNA overexpre ssing crown gall calluses are discussed.