Phytoalexins Are Natural Plant Antibiotics: Phytochemicals Defend Against Bacteria, Fungi and Herbivores

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Phytoalexins Are Natural Plant Antibiotics
Phytochemicals Defend Against Bacteria, Fungi and Herbivores

Art Ayers - Mar 9, 2010

Plants produce a cornucopia of exquisitely toxic chemicals to attack
essential metabolic functions of any microbe or animal that would
attempt to use them as food.

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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