ARE TISSUE-CULTURES OF PEGANUM-HARMALA A USEFUL MODEL SYSTEM FOR STUDYING HOW TO MANIPULATE THE FORMATION OF SECONDARY METABOLITES

This article reviews our present knowledge on the formation of tryptop han derived secondary metabolites in tissue cultures of Peganum harmal a. With the presence of beta-carboline alkaloids and serotonin, P. har mala contains two rather simple, interrelated biosynthetic pathways. T he long term disadvantage of low and unstable productivity of P. harma la suspension culture has recently been overcome by establishing highl y productive hairy root cultures. The first beta-carboline alkaloid bi osynthetic enzymes, specific for the O-methylation of harmalol and har mol as well as for the oxidation of harmaline to harmine, have been de tected in these cultures, and they should thus provide a suitable sour ce for studying the yet unknown initial two enzymatic steps of beta-ca rboline alkaloid biosynthesis. Seedlings of P harmala have also been s uccessfully transformed with constructed strains of Agrobacterium, as demonstrated by the overexpression of a tryptophan decarboxylase gene from Catharanthus roseus in cultures of P. harmala. In such transgenic cultures a large overproduction of serotonin was observed. The relati ve simplicity of these pathways and the rather easy handling of the cu ltures could make P. harmala a useful and attractive model system for studying the interaction, regulation and manipulation of secondary pat hways in cultured cells.