It is amazing that plants can used as food by animals, and it is only the powerful detoxifying capabilities of herbivores that permit them to avoid the toxicity of the naturally occurring chemicals in plants, phytochemicals. The diversity of phytochemicals used to defend plants is so intricate, that each taxonomic grouping of plants can be identified by the unique phytochemicals produced by that group.
Pre-existing Defensive Phytochemicals
Plants attract animals to facilitate the spread of pollen, for fertilization, and also for seed dispersal, but most of the biochemical facility of plants is devoted to protection from being used as food by bacteria, fungi, insects and larger herbivores, such as humans.
Domestication of plants has focused on systematic selection of plant mutants with defective phytochemical defenses. Plants that humans cultivate are not as toxic as the wild equivalents. Humans have also developed cultural practices of plant selection and food preparation that avoid or remove the most toxic of residual phytochemicals. Thus, humans learn to avoid the leaves of rhubarb and potato plants. Many other plants are fermented, heated or soaked in various ways to remove the most toxic compounds and permit safe eating.
Damaged or Infected Plants Are Deadly Because of Phytoalexins
Many plants that humans consume become toxic when attacked by pathogens. The plants produce very dangerous phytochemicals, phytoalexins, in response to being attacked by bacterial or fungal pathogens. Most human cultures have prohibitions against eating vegetables that have gone off. Infected parts of plants are carefully removed and disposed. Many of the phytoalexins in infected vegetables, e.g. potatoes, also cause birth defects in babies when pregnant women eat rotted vegetables.
Humans Detoxify Many Phytochemicals
Humans are protected from the natural toxic phytochemicals common in vegetables, because some of those chemicals are not absorbed, human gut flora chemically detoxify some phytochemicals, or the lining of the intestines and the liver produce enzymes that modify phytochemicals to render them harmless or enhance secretion in urine. Thus, humans can eat many vegetables that would kill dogs or cats, because those animals have different defenses against phytochemicals.
The bitter taste of many pre-existing phytochemicals, e.g. alkaloids, triggers vomiting to avoid ingestion. Pregnant women during their first trimester, when the embryo is most sensitive, are nauseated by the smell of cooking vegetables and should be encouraged to avoid eating potentially dangerous plant materials during that time.
Phytochemicals Are Useful to Protect Humans Against Their Pathogens
Although plant phytochemicals are generally toxic, they can be used as antibiotics to reduce populations of human bacterial or fungal pathogens in food. Herbs and spices that have developed for food preparation in various parts of the world have been demonstrated to be very effective in protecting humans against the particular pathogens in foods in each region. Thus, by cultural evolution, humans have learned to use phytochemicals to make their food healthier.
Phytochemicals Are the Foundation of the Pharmaceutical Industry
Phytochemicals have been selected during the evolution of each plant species to provide protection against its pathogens. The resulting phytochemicals are exquisitely adapted to disrupt the activity of essential enzymes of microbial macromolecular synthesis, e.g. protein or DNA synthesis, or insect function. Thus, many antibiotics and common drugs were developed from traditional herbal remedies.
Chemicals Used by Plants for Protection Are Used by People for Entertainment
Phytochemicals protect plants from infecting microbes, but are now used by humans to flavor food, protect against disease and to alter the human brain for entertainment. Phytochemicals are exquisitely adapted to provide the most potent lethal attack on insect nervous systems, e.g. tobacco nicotine, but can be repurposed to be smoked as an addictive stimulant. Fortunately, humans also produce detoxifying enzymes to provide protection against the natural toxicity of plants.
Reference:
Arthur R. Ayers, "Phytoalexins," McGraw-Hill Encyclopedia of Science and Technology, 9th Edition, 2002.
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