Mode of action and toxicology of plant toxins and poisonous plants

Plants have evolved the strategy to produce bioactive natural products as a means of defence against herbivores and microbes. Some plants produce toxins that can severely damage or kill a herbivore. The molecular mode of action of neurotoxins, cytotoxins, metabolic poisons, mutagens and toxins that affect skin and mucosal tissues are summarised and discussed. Important poisonous plants of Europe, their toxins and toxicology are tabulated, as this group of plants can provide lead compounds for the development of natural pesticides against insects, slugs or rodents. 1. Why do plants need toxins? It is a trivial observation that most animals can run away when attacked by a predator. When challenged by bacteria, fungi, viruses or parasites, the immune system takes care of the problem. Some marine animals, but also most toads and frogs, are not mobile enough to escape a predator. As a common theme, sessile or slow-moving animals have evolved a battery of poisons that make them unpalatable and toxic. Chemically, the poisons are peptides but also low molecular weight compounds, such as alkaloids, terpenoids, saponins or other secondary metabolites (as these natural products are usually called) (Wink 1999a,b; Wink and van Wyk, 2008). Considering the situation of plants, we recognise a similarity to sessile marine organisms. Plants can neither evade herbivores by flight nor do they have an immune system to fight invading microorganisms. From the early days of the evolution of land plants in the Devonian, herbivores and microbes were present and challenging plants (Wink 2003, 2008). The evolutionary solution of plants was the production of a wide variety of secondary metabolites (Fig. 1), which can interfere with the biochemistry and physiology of herbivores on one hand and some with bacteria, fungi, viruses and even competing plants on the other hand (Fig. 2) (Wink 1988, 2007b). Today, plants usually produce and accumulate not single entities but mixtures of secondary metabolites that mostly belong to several classes. For example, even plants that produce lethal alkaloids, also sequester terpenoids or polyphenols. The components in a mixture may be additive or even synergistic in their overall properties and activities (Wink 2008c). Number of natural products With nitrogen •Alkaloids (1) 20,000 •Non-protein amino acids (2) 700 •Amines (3) 100 •Cyanogenic glucosides (4) 60 •Glucosinolates (5) 100 •Alkamides 150 Without nitrogen •Monoterpenes (incl. Iridoids) (6) 2,500 •Sesquiterpenes (7) 5,000 •Diterpenes (8) 2,500 •Triterpenes, Saponins, steroids (9) 5,000 •Tetraterpenes 500 •Phenylpropanoids, coumarins, lignans 2,000 •Flavonoids (10), tannins 4,000 •Polyacetylenes, fatty acids, waxes (11) 1,500 •Polyketides (12) 750 •Carbohydrates, acids >200 N N