1. Introduction

Summary

Secondary metabolites (SM) occur in plants in a high structural diversity. A typical feature of SM is their storage in relatively high concentrations, sometimes in organs which do not produce them or as inactive ‘prodrugs’ that are enzymatically activated in case of danger. Biochemical and physiological features of secondary metabolism are strongly correlated with the function of SM: SM are not useless waste products (as assumed earlier) but important tools of plants needed against herbivores, microbes (bacteria, fungi) and viruses. Some of the SM also function as signal molecules to attract pollinating arthropods or seed-dispersing animals. During more than 500 million years of evolution, plants have evolved SM with a wide variety of biochemical and pharmacological properties. Many SM interact with proteins (receptors, ion channels, enzymes, cytoskeleton, transcription factors),DNA/RNA and/or biomembranes. Some of the interactions with molecular targets are highly specific, others have pleiotropic properties. Potential modes of action are summarized. As a consequence of the pharmacological properties of SM, several of them are used in medicine to treat disorders and infections. Others are interesting in biotechnology as rational pesticides. Phytomedicine normally employs complex mixtures, as they are present in the producing plant, which may exert additive or even synergistic properties.