Mailing List

For all the latest news and features, sign up to receive our FREE updates by email:

Plant Poisons: Their Use By Humans

Posted on 20. May, 2010.

Bookmark and Share

Plants produce poisons as a defence against predators.  Many of these substances are biosynthesised from non-protein amino acids by biosynthetic pathways which have been deduced from the results of isotopic tracer analysis. These secondary metabolites have been used by humans over thousands of years, both as drugs and as agents to kill animals and commit homicide.

The myths surrounding the poisonous effects of plants are many and it is not always easy to separate fact from legend. The study of phytochemistry, however, clarified this ambiguity considerably.

Studies of biosynthetic pathways have shown that the majority of plant poisons are the result of secondary metabolism. Historically, plant metabolism was separated into primary and secondary. Primary metabolism produces the building blocks of life: carbohydrates, proteins and fats, while a large number of secondary metabolites are synthesised to protect the plant from attack by animals, fungi, bacteria and viruses. Although it is now accepted that the processes of primary and secondary metabolism are interrelated and that such a sharp division is no longer valid, it is helpful when considering the biosynthetic pathways producing the wealth of compounds found in plants.

Plant poisons are products of secondary metabolism, generally being biosynthesised from quite simple compounds, such as amino acids or sugars through pathways involving catalysis by enzymes. Their main aim is to protect the plant from animal and insect attack. Compounds, such as the alkaloids and cyanogenic glycosides have a bitter taste, warning the animal of their toxicity. Should the animal continue to eat, however, it is likely that it will be killed and toxic plants are of great economic importance to farmers, especially in the developing countries. Herbaceous plants that grow where animals graze are the most dangerous and many losses occur from the pyrrolizidine alkaloids in ragwort.

Stock losses would be much greater however, if animals were not able to recognise and avoid poisonous plants within an area well known to them. Animals moved to a strange environment will not have this ability and are not always deterred by a bitter taste. Cattle can become addicted to buttercups   and will continue to eat the plants even after recovering from a serious bout of poisoning. The compounds most useful to the plant itself though, the insecticides, are generally non-toxic to animals, at least in the concentrations found in plants. Poisonous plants are also dangerous to man as they can be eaten by children or mistaken for edible plants by adults. Particularly dangerous to children are plants with attractive fruits, such as deadly nightshade, while hemlock is easily mistaken for parsley or other edible umbelliferous herbs.

In the past, human epidemics due to plant toxins have occurred, such as milk sickness resulting from Eupatorium rugosum ingested by cows. For thousands of years plants have been used as medicines and many plant drugs are still prescribed today, especially in alternative medicine. Plant poisons have also been used by man to kill animals, commit murder or in warfare. The use of arrow poisons to kill animals is probably almost as old as the arrow head itself. These poisons were generally prepared from plants containing cardiac glycosides or alkaloids, while alkaloid-containing plants such as deadly nightshade, mandrake  and hemlock have been used for centuries to commit homicide.

Isolation of the chemicals present and the testing of their physiological effects established with certainty whether or not a plant is poisonous and the effects the poison has on animals, including man.

This article features in the next issue of Science Progress.

Find out more about Science Progress

Subscribe to Science Progress