PBIO 100 Lecture Notes

I. How a plant compound can affect humans and animals

• A. Compounds that act like hormones, neurotransmitters (e.g. morphine is similar to natural opiates in body; insect hormones which induce molting made by plants and found in paper towels, human estrogen[female sex hormone] mimics found in food plants); plants of the genus Papaver are nonetheless poisonous to humans and to animals.
  B. Compounds which look like a substrate and inhibit specific enzymes (e.g. caffeine: phosphodiesterase)
  C. Compounds which damage cells or cell components (e.g. saponins: in green tomato)

II. Reason plants contain poisonous or potentially poisonous compounds

• A. "Plants can not move- they have to sit there and take it" but doesn't mean they are defenseless - instead, plants are very diverse chemical factories and resort to chemical warfare
  B. There is an evolutionary advantage to a plant that discourages herbivores from eating it by making compounds which kill, sicken or discourage a herbivore
  • 1. Milder form of discouragement: compounds which make bitter and unpalatable
    2. Cattle, horse, sheep deaths common on western rangelands, will avoid larkspur (Delphinium) and locoweed (Astragalus) unless the lack of food forces them to eat it

  C. Timing and place of chemical production well choreographed to manipulate animal behavior -e.g. green tomatoes have tomatine (saponin-intestinal irritation), discourages herbivores while seeds immature, but once ripens, toxicity decreases and attractive flavors develop to attract animals to spread seed.
  D. Compared to animals which have motility and more active defenses, plants generally have more toxic chemicals for defense

  • 1. Many animals which are toxic derive their toxin from plants - e.g Monarch gets cardiac toxin from milkweed- birds find the butterfly bitter taste

  E. Animals are not the only target: some plants also make compounds poisonous to other plants -e.g. walnut tree and many desert shrubs reduces competition from neighboring plants with compound that washes off leaves and onto understory plants and kills them

III. Types of compounds and plants

• A. Poisonous (toxic )plants [See "THE SILENT WEAPON Poisons and Antidotes in the Middle Age" by Mark S. Harris which traces historical aspects of poisonings.]
  • 1. Alkaloids (bitter compounds, contain Nitrogen atoms):
    • a. Yew (Taxus baccata): taxine (sudden death)
      b. Buttercups (Ranunculus bulbosus): Ranunculin (diarrhea, vomiting)
      c. Boxwood: buxene (convulsions and death) -small dose = intestinal disturbance, large dose = convulsions and death)
      d. Poison Hemlock (Conium maculatum): coniine (paralysis)
      • i. Death from asphyxiation due to paralysis of diaphragm
        ii. Socrates was forced to consume hemlock for corruption of young boys minds by his teachings

      e. Strychnine tree (seeds of Strychnos nux-vomica): strychnine (convulsions,death)

      • i. Tree native to India, orange-like fruit contains a few flattish seeds
        ii. Used in rat poison and to kill gophers
        iii. Tried by Cleopatra on servants while deciding on method of suicide - didn't like severe seizures, doubling over, with facial distortion - used asp instead.

    2. Cardiac glycosides

    • a. Oleander (Nerium oleander): nerioside and oleandroside - a single leaf can kill an adult, poisonings from use of twigs as skewers and honey made from pollen
      b. Lily of the Valley (Convallaria majalis): convallarin (irregular heartbeat) - similar to digitalis (Digitalis purpurea) - digitoxin (quickens heartbeat)

    3. Cyanide - History

    • a. Seeds of rose family (apples, apricot); also in peach pits and in leaves (poisonings often occur from fresh "peach leaf tea" but very low in fruit. Wilting leaves of native cherries, especially wild black cherry (Prunus serotina), can cause death of horses and cattle if consumed. Prunasin and amygdalin are the toxic compound.
      b. Cassava (Manihot esculenta) - an important staple throughout the developing world - soaked extensively to remove cyanide in the "bitter" forms of the plant.
      c. Lima beans(Phaseolus lunatus) (American varieties- reduced by breeding to a safe level)

    4. Other compounds

    • a. Castor bean (Rincus communis): ricin (toxic protein one seed can kill child) and is one of the most potent of poisonous plant compounds. Oil was used in small quantities as bowel purgative - did not contain protein. Ricin is a lectin or a toxalbumin which causes blood cells to aggregate.
      b. Wisteria (Wisteria floribunda): wistarine (two seeds can potentially kill a child)
      c. Mistletoe (Phoradendron leucarpum; occasionally called P. serotinum, a later name, or incorrectly known as P. flavescens): tryamine and phoratoxin (mildly poisonous but in large amounts can cause death). The European mistletoe (Viscum album) contains viscotoxin.
      d. Pokeweed (Phytolacca americana): phytolaccigenin and phytolaccatoxin (roots and berries cause respiratory depression, but cooked leaves with water drained are commonly eaten)
      

  B. Milder discouragement- poisonous but generally not toxic

  • 1. Oxalic acid (crystals)
    • a. Rhubarb (Rheum rhaponticum), Dumbcane (Dieffenbachia seguine), Caladium (Caladium bicolor): oxalate (crystals cause damage, paralyze vocal cords, swelling of mouth and throat)
      • i. During World War I, British government mistakenly advised people to eat rhubarb leaves which resulted in many poisonings

    2. Saponins (e.g. tomatine in tomato, solanine in potato)

    • a. Natural soaps (as in century plants (Agave))- develop bubbles but poke holes in membranes (by aggregating cholesterol)
      b. Causes nausea, vomiting, diarrhea
      c. Solanine present in potato (Solanum tuberosum) - most of plant except tubers is poisonous. Tuber contains solanine too, but toxic dose is about 100 potatoes. Green potatoes (which turn green after exposure to sunlight) can cause fatal poisonings. Green and damaged potatoes are even more toxic.

    3. Tannins in potato leaves, stems, roots-(e.g. tea)

    • a. Make unpalatable- bitter or astringent
      b. Astringent due to cross-linking of components (glycosides) in saliva- precipitates mucus

    4. Allergenic components: poison ivy (Toxicodendron radicans or Rhus radicans), western poison oak (T. diversiloba or R. diversiloba): urushiol oil (allergenic compounds- "harmless" compounds that your body encounters and, instead of ignoring, treats like an invading bacteria or virus. Urushiol: oil from poison ivy triggers bodies defenses producing a reaction like an infection - reddening, warming, bubbling of skin.
    5. Alkaloids: Death camas (Zigadenus venosus: steriodal alkaloids (frequent poison for livestock (sheep and cattle particularly) in the West; can kill persons eating bulbs - especially children - and even pollinators who gather the pollen).
    6. Food allergies: compounds in certain foods that certain people's immune systems do not ignore, producing an immune reaction (swelling, redness) on skin (rash, hives), in gastrointestinal tract (pain, diarrhea), and in the respiratory tract (asthma). Examples include peanuts, soybeans, wheat, tree nuts, strawberries, citrus.
    7. See these sites for more information on allergies caused by plant pollens . The term "hayfever" is frequently applied to pollen-caused allergies. More than 35 million Americans suffer from hayfever; antihistamines are used to treat the symptoms.

  C. Human uses: not just for diabolical plots in Agatha Christie novels

  • 1. Crop plants need defenses against insects and diseases, otherwise would have to always spray huge amounts of insecticide and fungicide
    2. Selective poisons of pests: rat poison with strychnine. Rotenone from the bean family (Fabaceae) and pyrethrum are from sunflower family (Asteraceae); these kill fish and insects, mammals can eat the killed animals with no effect. Numerous other plants are used to kill fish.
    3. Medicines in smaller doses [REQUIRED READING]
    4. Poison arrows and darts - common in many societies (Greece, China, Africa, South America) with different plants. In the Old World (e.g. oubane) compounds which stimulate heart were used while in the New world (e.g. curare) most compounds relaxed muscles and heart functions. Also used frogs.
    5. Jewelry: Rosary beads (Abrus precatorius): abrin, a ribosome inactivating protein (one of the most deadly plant toxins known; decorative jewelry occasionally imported into the United States; plant native in warm areas where kids use the seeds for "pea shooters").

Take home lessons:

1. Plants have effective chemical defenses mechanisms specifically designed to make you leave them alone.
   
2. Many household plants are very poisonous
   
3. Poisons are present in food plants
   
4. 99% of carcinogens in food are in there naturally, not what we spray on them
   
5. Just a matter of location (what part of plant we eat) and certain times (ripe vs. unripe) that makes them edible
   
6. By avoiding a diet focusing on one food will reduce the level of any one toxin

Other sites of interest:
Major summary
Review: See section on plant defenses
Important WWW Plant Poison sites
Poisonous Plants Database at the University of Maryland
Canadian Poisonous Plant Information System
Guide to poisonous Plants
Illustrations of Poisons Plants from Cornell University
Allergies Drug Information
AllerDays Allergy Resource Center

Last revised: October 19, 1998, Straney