Introduction to Plant Secondary Metabolites

INTRODUCTION

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 ‘pro drugs’ 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.

Primary metabolism vs. Secondary Metabolism

Primary metabolism in a plant comprises all metabolic pathways that are essential to the plant's survival. Primary metabolites are compounds that are directly involved in the growth and development of a plant whereas secondary metabolites are compounds produced in other metabolic pathways that, although important, are not essential to the functioning of the plant. However, secondary plant metabolites are useful in the long term, often for defense purposes, and give plants characteristics such as color. Secondary plant metabolites are also used in signalling and regulation of primary metabolic pathways. Plant hormones, which are secondary metabolites, are often used to regulate the metabolic activity within cells and oversee the overall development of the plant. As mentioned above in the History tab, secondary plant metabolites help the plant maintain an intricate balance with the environment, often adapting to match the environmental needs. Plant metabolites that color the plant are a good example of this, as the coloring of a plant can attract pollinators and also defend against attack by animals (https://en.wikipedia.org/wiki/Plant_secondary_metabolism).

Secondary Metabolism from photosynthesis

Secondary metabolism from glucose