Tryptophan decarboxylase is encoded by two autonomously regulated genes in Camptotheca acuminata which are differentially expressed during development and stress

Abstract

Camptothecin (CPT) is a valuable anti-cancer monoterpene alkaloid produced by the Chinese tree Camptotheca acuminata. Tryptophan decarboxylase (TDC) supplies tryptamine for the indole moiety of CPT and its derivatives, and is considered a key step in monoterpene indole alkaloid biosynthesis as it links primary and secondary metabolism. This report describes the isolation and characterization of tdc1 and tdc2, two autonomously regulated TDC genes from Camptotheca. When expressed in Escherichia coli, the products of each gene could decarboxylate tryptophan, but were inactive against tyrosine, phenylalanine and 3,4-dihydroxyphenylalanine (dopa), tdc1 was developmentally regulated, having its highest expression level in the apex, young stem and bark, tissues which also contain the highest levels of CPT. Expression of tdc1 also increased during seedling development and was correlated with alkaloid accumulation during germination. tdc2 expression was induced in Camptotheca leaf discs and cell suspension cultures treated with fungal elicitor or methyl jasmonate, treatments which did not affect tdc1 expression. Unlike tdc1, tdc2 expression was not detected in any unstressed Camptotheca tissues nor in developing seedlings. These data suggest that tdc1 may be part of a developmentally regulated chemical defense system in Camptotheca, while tdc2 serves as part of a defense system induced during pathogen challenge.