black nightshadeJ. Fujishima/B.W. Halstead, World Life Research Institute
Everyone knows that some plants are poisonous. But what constitutes a “poison” to one organism may very well be an inert compound to another. Birds, for example, are unfazed by urushiol, the toxic oil produced by poison ivy, and in fact seem to love the berries produced by the plants. Monarch butterfly caterpillars chomp on milkweed and sequester the glycosides produced by the plant in their own tissues, making them poisonous to predators. Of course, humans have turned all manner of plant poisons to their own devious ends, from the chrysanthemum-derived pyrethrins used as insecticides to the castor bean-derived ricin, which Walter White of Breaking Bad attempted to use to eliminate his enemies on several occasions (and which was successfully used in the 1978 assassination of a Bulgarian writer).
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Vegetative Vengeance: 8 Ruthless Plant Defense Mechanisms 
Most people think of plants are passive, harmless, healthy, and happy to sacrifice themselves for the caloric benefits of herbivores like cows, and people pretending to be herbivores. But plants do much more than “veg-out”. Evolution is a survival game that has equipped plants with an extensive arsenal of plant defense mechanisms.
Don’t let plants’ lack of a central nervous system fool you. The human ability to run, hit, and scream are crude defenses when compared to a plant’s arsenal of chemicals, stabbers, and adaptive defenses.
Table of Contents
- 1 Poison
- 2 Idioblast
- 2 Chemical Signaling
- 3 Mutualism
- 4 Crypsis
- 5 Trichome
- 6 Spines
- 7 Thorns
- 8 Prickles
- Plant Defense Mechanisms: The Takeaway
1 Poison
Most people have had some experience with plant poisons in poison ivy, oak, and sumac. The rashes and even death caused by these plants is due to the toxic oil urushiol [1][2]. Interestingly, with the rising CO2 in the atmosphere, the biomass and toxicity of poison ivy are on the rise [3].
Urishiol is so toxic that ancient Egyptians used poison sumac to booby trap the inner chambers of the great pyramids. Thousands of years later, tomb raiders were afflicted by terrible rashes that they though was a curse for disturbing the tombs.
Some animals, including humans, have adapted plant poisons to their own uses. Monarch butterfly caterpillars consume milkweed, which is high in glycosides. These poisons accumulate in the caterpillar’s own tissue, making them poisonous to predators.
For eons, humans have used plant poisons to help stun and bring down larger prey. In the early 1930s, adventurer Richard Gill got curious about the plant chemicals that Amazonian tribes brewed up to coat their arrows. His research of Amazonian arrow poisons led to the discovery of compounds that made modern anesthesia possible.
The ricin derived from castor bean oil was popularized when Walter White of Breaking Bad tried to use it to poison his enemies.
Plant Poisons in Food
Though the above examples of plant poisons have acute and dramatic effects, eating a lot of plants chronically exposes your body to a wide array of subtler plant toxins. Over time these poisons can kindle severe health issues.
Phytoalexins
The phytoalexins found in potatoes (especially when old and damaged), green beans, garden peas, and carrots can cause symptoms after even mild exposure. These include headache, vomiting, diarrhea, apathy, and mental confusion. Severe exposure to glycoalkaloids can lead to hallucinations, trembling, stupor, and even death [4].
Saponins
Eating plants subjects your body to between 5,000 and 10,000 different natural pesticides every day, and saponins are among the most common [5].
Plants produce saponins to kill insects. A common way they work is by penetrating the outer surface of larvae, binding, and destroying the intestines [6].
In humans, saponins can bind to cholesterol and interfere with the mucosa lining of the intestines [7]. Though it’s likely that the effects of saponins alone are moderate, when combined with other plant toxins like lectins, gluten, and gliadin, there can be a cumulative risk of intestinal permeability, or ‘leaky gut’.
Permeability of the intestinal membrane allows bacteria, toxins, and undigested food particles to enter into the bloodstream and flow freely through the body to places they’re not supposed to go. This weakens the immune system and can cause chronic inflammation and autoimmune diseases [8].
Phytohormones
Phytohormones act as signaling compounds in plants, helping plants survive changing conditions in their environments. But they double as chemical weapons that plants use to defend themselves against plant-eating predators, including humans.
Phytohormones can mimic reproductive hormones including estrogen and androgen in ways that reduce the fertility of herbivores thereby driving down populations of predators [9].
Soy is one of the most common foods containing high levels of phytohormones. These compounds have been shown to interfere with animal and human estrogen signaling pathways, affecting fertility and reproduction [9].
The chart below further details plant poison defense mechanisms and how they can affect humans.
2 Idioblast
If plant poisons are like carpet bombing with chemical weapons, idioblasts are chemical landmines that release specialized defensive compounds in specific situations.
Idioblasts come in bundles of cells that contain toxic compounds called raphides. These barbed calcium oxalate crystals penetrate the mouths, throats, and intestines of predators and then release toxin enzymes resembling the venom of reptiles [10][11].
In humans, eating plants with idioblasts can cause loss of speech and paralysis–symptoms that gave the common house plant “dumb cane” its name.
Though research on idioblasts and raphides in food is limited, they have been found in abundance in over 200 plant species, including common foods such as pineapple and taro.
2 Chemical Signaling
When plant defenses are breached by predators, plants can send out chemical messages in the form of volatile organic compounds (VOCs).
Nearby plants can respond to these warnings in various ways. Common responses include increasing concentrations of protective toxic compounds and releasing other compounds that attract the enemies of plant predators.
3 Mutualism
Mutualism goes hand in hand with the plant defense of chemical signaling. Certain plants like South American and African acacia trees produce certain foods including nectar and leaf-tips specifically to feed helpful ants. The ants then use their own poison to defend the trees against other animals, plants, and fungi.
Research has shown that when the ants are removed from the trees the trees will die [12].
Diagram from: Nature
4 Crypsis
Plants are much more than inanimate clumps of stems, leaves, and flowers, and crypsis makes this visually apparent.
Plants like the mimosa pudica “wilt” their leaves by closing and dropping them in response to touch. These plant defense makes them appear dead and therefore unappetizing to predators looking for fresh forage. That’s right, like possums, plants know how to play dead.
5 Trichome
Trichomes are furry bristles that plants use to shield themselves from herbivores. These arrays of tiny bristles can eviscerate insects and can cause irritating stings and rashes on humans.
When they’re connected to poisonous glands that the plant injects into the lacerations caused by the trichomes they can cause serious symptoms including nerve damage and death. If you’ve ever brushed your hand against stinging nettle you know what trichomes feel like.
Researchers in Australia found that the Gympie-Gympie stinging tree uses trichomes to inject neurotoxins into predators. The pain is equivalent to poisonous spider and scorpion bites and can last for days and even weeks [13].
From: University of Queensland
6 Spines
Perhaps the most recognizable plant defense mechanisms are the visibly sharp parts that stab and prick.
Surprisingly to non-botanists, these stabby parts come in three different varieties.
The cactus, for instance, has spines. These needle-like crowths protect the juicy succulent parts from predators, while also providing shade. Cacti that live in less sunny environments have a lighter “coat” of spines than their desert-dwelling cousins.
7 Thorns
Unlike prickles and spines, thorns are sharp branches and stems. Thorns and spines are modifications of existing organs such as stems, leaves or stipules.
8 Prickles
Roses have neither spines nor thorns. They have prickles–sharp protuberances from the skin of the plant. Think of them as sharp moles.
Plant Defense Mechanisms: The Takeaway
Plants don’t just “veg-out”. They’re not particularly friendly, and they’re certainly not harmless. Like most wild creatures that have survived for million of years, plants have evolved sophisticated defense mechanisms.
Most plant defense mechanisms are invisible, hidden within as toxic chemical agents that get released when eaten by predators.
Other plant defenses are all too obvious. The sting of a nettle, prick of a rose, and rash from poison ivy are a few of the less than comforting encounters many humans have with plant defenses.
