How plants protect themselves

Plant Defenses Against Herbivores

Plants defend against herbivores with mechanical wounding, barriers, secondary metabolites, and attraction of parasitoids.

Learning Objectives

Identify plant defense responses to herbivores

Key Takeaways

Key Points

  • Many plants have impenetrable barriers, such as bark and waxy cuticles, or adaptations, such as thorns and spines, to protect them from herbivores.
  • If herbivores breach a plant’s barriers, the plant can respond with secondary metabolites, which are often toxic compounds, such as glycol cyanide, that may harm the herbivore.
  • When attacked by a predator, damaged plant tissue releases jasmonate hormones that promote the release of volatile compounds, attracting parasitoids, which use, and eventually kill, the predators as host insects.

Defense Responses Against Herbivores

Herbivores, both large and small, use plants as food and actively chew them. Plants have developed a variety of strategies to discourage or kill attackers.

Mechanical Defenses

The first line of defense in plants is an intact and impenetrable barrier composed of bark and a waxy cuticle. Both protect plants against herbivores. Other adaptations against herbivores include hard shells, thorns (modified branches), and spines (modified leaves). They discourage animals by causing physical damage or by inducing rashes and allergic reactions. Some Acacia tree species have developed mutualistic relationships with ant colonies: they offer the ants shelter in their hollow thorns in exchange for the ants’ defense of the tree’s leaves.

Acacia collinsii: The large thorn-like stipules of Acacia collinsii are hollow and offer shelter for ants, which in return protect the plant against herbivores.

Modified leaves on a cactus: The spines on cactus plants are modified leaves that act as a mechanical defense against predators.

Chemical Defenses

A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development.

Many metabolites are toxic and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides. Ingestion can cause nausea, vomiting, hallucinations, convulsions, or death.

Foxgloves: Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides. Ingestion can cause nausea, vomiting, hallucinations, convulsions, or death.

Timing

Mechanical wounding and predator attacks activate defense and protective mechanisms in the damaged tissue and elicit long-distancing signaling or activation of defense and protective mechanisms at sites farther from the injury location. Some defense reactions occur within minutes, while others may take several hours. In addition, long-distance signaling elicits a systemic response aimed at deterring predators. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids: insects that spend their developing stages in or on another insect, eventually killing their host. The plant may activate abscission of injured tissue if it is damaged beyond repair.

When you are stomping through a field, plucking weeds from your backyard, or picking out fresh vegetables from the market, it is unlikely that you think much about the “lives” of plants. Without a central nervous system and an active “brain” like so many mammals, it is easy to disregard plants as life forms. For all the vegetarians and vegans out there, who defend their food choices by their dinner’s lack of sentience, this article might just change their perspective, if not their dietary habits.

While plants cannot walk, run, cry out or feel pain (at least in a way that we are able to quickly detect), they are far from defenseless, senseless life forms. In fact, plants often have more defensive mechanisms that mammals as a result of their inert and silent existence. With that in mind, how exactly do plants defend themselves?

Short answer: In countless ways, from physical and chemical responses to signaling behaviors, camouflage and mimicry.

The Quiet Intelligence of Plants

From grass species and massive trees to flowers and tiny bushes, plants have evolved with a multitude of defensive mechanisms. As with any other evolutionary development in animal species, this has been the result of millions of years of mutation and countless generations that have either survived to reproduce or fallen to external threats.

The primary threats to plants are obviously herbivores (animals that eat only plants, such as insects, birds and various other mammals). While some destruction is simply unavoidable, such as the great grazing herds of the world’s grasslands, these threats can often be countered by volume; there is simply too much grass producing too many seeds to worry about those species being eradicated or prevented from reproducing.

Most plants also have other techniques to ensure their survival, such as their phototropic abilities (physically reorienting to capture the most light) and incredible skills at finding nutrients and water even in barren soil. There is even “communication” between nearby plants, either through a connected root structure that can share/trade resources, or through the detection of sun-soaking competitors, resulting in growth in an opposing direction. These hidden agendas and actions are often missed by those who think of plants as dumb, mindless carbon-based life forms.

When it comes to more specialized plants, perhaps those that aren’t as hardy or tolerant of harsh conditions, more advanced adaptations are necessary for protection and survival. Now, let’s take a closer look at some of the most impressive defenses that plants have developed over the past 700 million years since they first emerged.

Physical Defenses

Given the incredible physical diversity of plants in the world, it comes as no surprise that their physical defense mechanisms are equally impressive and varied. Thorns, prickles and spines are the three most common and recognizable forms of physical defenses, and while many people might mistake them, they are distinctly unique. Thorns are essentially sharpened branches or twigs, whereas prickles are actual growths from the epidermis of the plant, intentionally designed as smaller defensive weapons than thorns. Finally, there are spines, which most people would recognize from cacti. These spines differ from the first two examples, as they tend to be even sharper, often thinning to a point that is microscopically fine, and can be broader or larger than thorns or prickles, since they also provide shade for the body of the plant.

All of these physical adaptations are specifically tailored for a number of things, including preventing common larger predators from alighting or consuming the fruits/flowers, while also welcoming those essential seed and pollen dispersers in safely. The size and function differences may seem minute, but it took millions of years for them to evolve those separate features, so everything is purposeful (albeit accidental, from a mutational perspective).

Some plant physical defenses are harder to see, both for predators and humans, such as trichomes, which are essentially a sharp fur that will leave a painful sting where it brushes on the exposed skin. The reason for this discomfort, despite the gentle contact, is that peripheral glands inject trace amounts of toxin and poison into the same wound that the bristling fur inflicted; the entire process can take mere milliseconds.

Some other plants, particularly some species of ferns, are able to close up at the slightest physical contact, clenching up and protecting their leaves, and hanging low, as though it were dying or sick. This makes the plant look less appealing, often helping it avoid any further predation until the threat has passed, when it will reopen its leaves and flourish in the sunlight once again.

However, these topical defenses are nothing compared to the physically mobile nature of certain carnivorous plants, such as the Pitcher plant, Drosera, or Venus Flytrap, but going into those fascinating and aggressive plants is beyond the scope of this article. The last thing I’ll say is that those plants are anything but helpless, and have some of the most fascinating adaptations in the natural world.

Chemical Defenses

Not all plants choose to show their defenses “on their sleeve”, per se, but instead rely on complex chemical reactions and lightning-fast delivery systems to ward off would-be diners. The leaves, plants, flowers and stems of certain plants are delicious food sources for animals, but insects masticate more plant matter than mammalian herbivores – by a LONG shot. It is firmly believed that insects represent more biomass than any other mammals, outnumbering humans alone by 200,000,000:1. Therefore, since insects can easily crawl around thorns, prickles, and spines, defenses on a cellular level are required.

Many plants will allow an insect to take an initial bite or two, but it might just be their last meal. For instance, specialized cells on the surface of many plants will instantaneously release unpleasant chemicals when they are breached or consumed, making the taste unpalatable or even poisonous to the insect. Other plants release sticky sap or liquid that traps the insect in place, where they will eventually die or be eaten by a larger predator looking for an easy, incapacitated snack.

Idioblasts are some of the coolest types of these specialized cells. When the cells are punctured or broken, they release barbed crystals into the mouths of the hungry insects, as well as venom that can paralyze or kill a hungry insect. These idioblasts cover much of the surface of the plants, acting as “land mines” for any tiny herbivores that can avoid spines and prickles.

Social Defenses

Plants may not be able to speak, but there are other ways to communicate and collaborate with the rest of the world. For example, when certain plants are under attack, the stress of that munching insect stimulates the release of powerful airborne chemicals that attract a variety of larger predators, such as wasps, dragonflies or even small mammals and lizards. These animals will be drawn to the plant that is being eaten, where it can make quick work of the irritating insects doing the damage. This is a type of chemical signaling, but the social aspect – essentially direct communication with other species – makes this defensive mechanism a thing of wonder.

Finally, many plant species have long-standing arrangements with some insects, in a relationship called commensalism. This is particularly prevalent in large trees, namely in South America, where species of vicious ants will take up residence on a tree, which houses and feeds them, and has no negative effects on these “invaders”. In exchange for room and board, the ants will defend the tree to the death, against other insects, birds, larger mammals and even plants that attempt to steal the tree’s sunlight.

As you can see, plants are far from the vulnerable salad fodder that we so often think of them as. In fact, most plants are better equipped than humans to fend off the environmental threats they face. I suppose it’s a good thing that humans have houses, air conditioning, insect spray and fences!

  1. University of Nebraska–Lincoln
  2. Wikipedia
  3. National Center for Biotechnology Information

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Plant Resistance against Herbivory

In comparison to qualitative defenses, quantitative defenses are generally effective against all herbivores but require larger doses. As a result, these compounds are typically mass produced and are rarely recycled. Condensed tannins are common quantitative defenses that bind to proteins, interfering with digestion and potentially leading to malnutrition (Ayres et al. 1997). Other quantitative defenses include chemicals that cause pain, inflammation, or swelling in the skin or mouth when touched as in the case of stinging nettle (see ‘Structural defenses’ above) or poison ivy (Toxicodendron radicans).

Whether a plant relies more heavily on qualitative or quantitative defenses may be influenced by factors such as growth rate (Coley et al. 1985), nutrient availability (Stamp 2003), or how easily it can be found by herbivores (Feeny 1976). Regardless of their mode of operation, organic chemical defenses reduce the incentive for herbivores to feed on a plant and, consequently, the amount of damage sustained.

Humans use organic chemical defenses as common ingredients in food and medicine. For example, caffeine belongs to a class of qualitative defenses that block brain signals relaying drowsiness (Fisone et al. 2004). Uses for defensive and other secondary compounds are studied in the fields of ethnobotany and pharmacology (Fowler 2006, Soetan 2008).

Inorganic, elemental defenses likely evolved as a way for plants to cope with toxic elements inadvertently absorbed from the soil such as nickel, zinc, cadmium, and lead. Many plants avoid poisoning by storing these elements away from cell machinery in cell walls, vacuoles, or trichomes until they are released when a plant dies or is consumed. As these elements are also poisonous to most herbivores, plants that absorb and store toxic elements, referred to as ‘metal hyperaccumulators’, benefit from reduced herbivory (Poschenrieder et al. 2006, Boyd 2007, Boyd 2009).

Chemical defenses are not always meant to deter all herbivores. Many plants benefit from interactions with mutualistic herbivores such as pollinators or seed dispersers and have evolved defenses that only target harmful herbivores. For example, chili seeds pass safely through the digestive systems of birds and are dispersed in the droppings but are destroyed when eaten by mammals. Not surprisingly, capsaicin, the compound that gives chilies their hot flavor, affects mammals but not birds (Tewksbury & Nabhan 2001).

Indirect defenses

Instead of directly defending against herbivores, indirect defenses reduce herbivory by increasing the likelihood that herbivores (usually insects) are attacked, removed, or harassed by predators like ants, wasps, and mites (Figure 5). Because indirect defenses rely on a third trophic, or feeding, level in the food web, they are sometimes referred to as tritrophic or biotic defenses. Plants increase predation of herbivores by luring and keeping predators on a plant with food rewards, shelters from harsh conditions, or chemicals signaling prey availability.

Figure 5: Indirect defenses. Indirect defenses function by (a) attracting predators (third trophic level) such as ants, wasps, and mites with incentives including food rewards, domatia, or chemical signals advertising the presence of prey. (b) Once present, predators attack and/or remove herbivores (second trophic level) that can damage a plant (first trophic level). (c) By comparison, direct defenses do not require a mediator to negatively affect herbivores. (d) Decreased feeding by herbivores results in less damage to the plant. © 2012 Nature Education All rights reserved.

Many plants produce energy-rich food rewards to attract predators, decreasing production when predators or herbivores are absent or inactive (Heil et al. 1997, Wäckers & Bonifay 2004). Food rewards used in plant defense include nectar, produced by extrafloral nectaries (EFNs), and solid food bodies (Figure 6a). Unlike floral nectaries, the primary function of EFNs is to attract predators rather than pollinators. EFNs promote defensive mutualisms ranging from absolute requirements for survival in myrmecophytes (i.e., ‘ant plants’) to beneficial but nonessential relationships in a number of other plants (Bentley 1977). Solid food bodies range in form from fruit-like appendages to soft layers of nutritious tissue. These structures contain lipids, carbohydrates, and proteins that represent a substantial investment by the plant. Consequently, food bodies have only been observed on myrmecophytes and are associated with other forms of indirect defense such as domatia (Heil 2008).

Domatia are structures that shelter predators from harsh environmental conditions or other predators. Domatia may range in complexity from shallow crevasses covered with trichomes as in some varieties of avocado (Persea americana; Agrawal 1997) to hollow tissues with multiple chambers and elaborate entrances as in many acacia (Acacia spp.; Figure 6b and c). Although domatia and food rewards may not directly attract predators, they can increase the likelihood that visiting predators will remain on a plant and reduce herbivory.

Figure 6: Food rewards and domatia. The bullhorn acacia (Acacia cornigera) uses indirect defenses to encourage predators to remain on the plant and attack visiting herbivores. These indirect defenses include (a) food rewards for foraging ants including EFNs (red arrow) located at the base of the leaves and food bodies (blue arrow) located on the tips of individual leaflets; and (b, c) domatia formed from hollow spines that provide shelter for ants (b: exterior view; c: interior view). © 2012 Nature Education All rights reserved.

The only indirect defenses that actively attract predators are volatile organic chemicals (VOCs). These gaseous signals are often released from damaged plant tissues, advertising the presence of potential prey. VOCs can vary with time of attack (e.g., night vs. day) or herbivore identity to attract predators best adapted for a particular herbivore. For example, broad bean plants (Vicia faba) attacked by different species of aphid (Acyrthosiphon pisum and A. fabae) release different VOCs that attract different predators (Powell et al. 1998). Many VOCs also repel herbivores, including adult hawkmoths (Manduca quinquemaculata) that avoid laying eggs on tobacco plants (Nicotiana attenuata) emitting predator-attracting VOCs (Kessler & Baldwin 2001). VOCs, therefore, serve dual defensive purposes by indirectly reducing herbivory via predator attraction and directly deterring herbivores.

Resistance costs

The resistance traits discussed above can be costly in terms of energy, resource, and opportunity costs (Strauss et al. 2002). As plants have limited resources, an increase in either resistance or growth must be balanced by a decrease in the other (Coley et al. 1985), a relationship referred to as a tradeoff. Consequently, fast-growing plant species are typically less resistant to herbivory than slower growing species (Endara & Coley 2011). Tradeoffs may also exist among resistance traits so that a plant cannot maximize all forms of resistance against all possible herbivores. As a result, plants may express different resistance traits that minimize consumption by different herbivores in different places (e.g., Berenbaum & Zangerl 2006) or at different times (e.g., Wäckers et al. 2004).

To avoid negative effects of tradeoffs, many plants maintain low baseline, or constitutive, defensive levels until stimulated, or induced, by herbivore damage, VOCs, light availability, or day length (Zangerl 2003, Wäckers & Bonifay 2004, Conrath et al. 2006, Radhika et al. 2008). In fact, many direct (Chen 2008) and indirect (Heil 2008) resistance traits are only expressed following induction by some stimulus. In this way plants are able to avoid investing resource in unneeded resistance traits, thus allowing more resources for growth and reproduction.

Conclusion

The goal of this article is to briefly introduce various forms of plant resistance against herbivores; however, resistance traits against pathogens such as fungi, viruses, and nematodes also exist. Despite some overlap between herbivore and pathogen resistance, a number of uniquely anti-pathogenic defenses exist (Lambers et al. 1998, Anderson et al. 2010).

Natural selection from herbivory has prompted plants to evolve a wide array of resistance traits to reduce losses from herbivory. Plants avoid herbivory by hiding, building structural barriers, producing and acquiring chemical toxins, and recruiting predatory ‘bodyguards’. Thus, plants are not the helpless victims of herbivory but defend themselves against the loss of resources and energy, allowing for greater investment in reproduction and survival.

17 Totally Genius Ways Plants Protect Themselves When Under Attack

Animals have incredible innate strategies for staying safe when there’s imminent danger. For example, skunks spray a foul-smelling odor, porcupines put up their quills, and bees sting. But what about plants? Just like mammals and amphibians, they’re living things that also come under attack. But without arms or legs, plants have to get crafty when it comes to self-defense. We’ve rounded up some of the strangest and most genius tactics that plants use protect themselves.

1 They play dead.

Mimosa pudica, better known as the sensitive plant, is quite cunning and creative when it comes to protecting itself from predators. When the plant is moved in any way, it will fold its leaves inward and droop down in order to appear dead and therefore unpalatable.

2 They sting.

Urtica dioica, or common nettle, is a species of flowering plant defined by its trichomes, AKA stinging hairs. These hollow hairs on the plant’s leaves and stems act like needles when something comes too close.

Upon contact, the stinging hairs inject histamine and other chemicals to induce a searing stinging sensation.

3 They release venom.

You might not be able to see the defense mechanisms of dieffenbachia, or the dumb cane, but they’re there. Inside the plant’s leaves are calcium oxalate crystals. When released, the crystals deliver a venomous enzyme called raphides, which, when ingested, can cause everything from paralysis to speech impairment.

These symptoms are where the houseplant gets its common name from. It’s also why the dieffenbachia is hilariously referred to as mother-in-law’s tongue.

4 They form a partnership with ants.

Vachellia cornigera, or Bullhorn Acacia trees, get aggressive ants to do their dirty work for them. In this relationship—a prime example of what’s known in nature as commensalism—both parties win. The ants protect the trees against anything that poses a threat, and the ants get both a place to live and food to eat in return.

5 They warn one another when danger is nearby.

Plants can communicate without verbal cues. Instead of using sound, they emit volatile organic compounds, or VOCs, into the air to warn neighboring plants that a threat is nearby.

6 They signal to birds to eat threatening insects.

There are some types of plants that will enlist the help of birds when pests are feeding on them.

In these scenarios, the plants will give off VOCs, signaling that they’re under attack. In response, the birds will come and consume the pests. Another win-win!

7 They choke their predators.

Thousands of plants—including common foods like apples, spinach, and lima beans—are poisonous to other species besides humans.

That’s because these plants produce hydrogen cyanide compounds, which attach to either sugar or fat molecules via a process called cyanogenesis. They remain stored in the plant until they are needed, i.e. when insects try to eat them. At that point, the plants release the hydrogen cyanide, which makes the insects choke until they eventually stop breathing. Nature is brutal.

8 They induce a heart attack.

Digitalis purpurea, or the foxglove, is just as dangerous as it is beautiful. The vibrant vegetation contains a potent toxin known as digitoxin. For humans and insects alike, consuming any part of this plant can potentially lead to heart failure.

9 They enlist the help of wasps.

When corn plants are under attack, they “release volatile chemicals from all their leaves” that “serve as a kind of distress call… to attract wasps,” according to research from the U.S. Department of Agriculture.

Once the wasps receive the call, so to speak, they flock to the corn plant and eliminate the threat by eating it. You might not love them, but wasps do your corn plants good.

10 They poison nearby plants.

There are certain situations in which plants must defend themselves against other plants in order to survive.

When the Black Walnut tree, for example, senses that another plant is starting to grow nearby, it takes action so that the newbie doesn’t steal its resources. As a result, the Black Walnut tree’s roots will emit a toxin called juglone to kill that intruder.

11 They make themselves taste bad.

In an effort to repel pests, certain plants will emit a substance that makes them taste unappetizing.

While the approach is subtle, it leads to some savage results: Researchers have found that when this happens, the bugs will just resort to cannibalism.

12 They pretend to be rocks.

Lithops, or pebble plants, take advantage of their surroundings in order to stay safe. Because these succulents look like rocks, they are able to blend in with actual stones and avoid beating eaten. Genius!

13 They attract predators with nectar.

Think of nectar like an incentive. Basically, plants use this sweet substance to lure over animals like bees and moths who can ward off herbivores.

In exchange, the pollinating animals get nutrients. Just another example of a mutually beneficial plant-pollinator situation.

14 They camouflage themselves.

Image via Yang Niu

Just like animals, some plants have figured out how to camouflage themselves.

Take the corydalis hemidicentra, for instance. Per one study published in the journal Trends in Ecology and Evolution, this plant is able to make itself look like the unappealing elements in its surroundings in order to avoid its predators.

“Different populations of this species look different in different places,” says Dr. Yang Niu of the Kunming Institute of Botany and Exeter. How cool is that?

15 They grow waxy coatings that make them hard to eat.

That waxy covering that you feel on desert plants doesn’t just hold moisture in. This layer also is difficult for insects to eat, thus protecting the plants from being destroyed.

16 They have impenetrable leaves.

Imagine biting through the shell of a walnut. Sounds painful, right? Well, that’s basically what insects experience when they try to eat the leaves on an Inga edulis tree.

These leaves are prone to growing fungus, which attracts certain insects, like Atta cephalotes (fungus-growing ants). But the bugs know better than to try their luck chomping on leaves coated in a hard shell.

17 They trap their predators in goo.

Inside the vascular tissues of certain plants (like milkweeds) is an intricate network of channels with latex sap. When the channels are broken—like, for instance, when an insect eats through the leaves—the sap is released in order to trap whatever is trying to chow down.

Essentially, this defense mechanism is like a spider web, except it’s made out of goo rather than silk.

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How plants defend themselves

“The immune system of plants is more sophisticated than we thought,” says Dr. Stefanie Ranf from the Chair of Phytopathology of the TU Munich. Together with an international research team, the biochemist has discovered substances that activate plant defense.

Until now, scientists have thought that plant cells — similar to those of humans and animals — recognize bacteria through complex molecular compounds, for example from the bacterial cell wall. In particular, certain molecules composed of a fat-like part and sugar molecules, lipopolysaccharides or LPS for short, were suspected of triggering an immune response.

In 2015, Ranf’s team successfully identified the respective receptor protein: lipo-oligosaccharide-specific reduced elicitation, or LORE for short. All experiments indicated that this LORE protein activates the plant cell’s immune system when it detects LPS molecules from the cell wall of certain bacteria.

A throwback leads to the right track

“The surprise came when we wanted to study this receptor protein more closely,” recalls Ranf. “Our goal was to find out how LORE distinguishes different LPS molecules. For this we needed high-purity LPS. “

The researchers found that only LPS samples with certain short fatty acid constituents triggered plant defense. Surprisingly, they found in all these active LPS samples also extremely strong adhering free fatty acid molecules. Only after months of experimentation was the team able to separate these free fatty acids from the LPS.

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“When we finally succeeded in producing highly purified LPS, it became apparent that the plant cell did not respond to them at all! Thus, it was clear that the immune response is not triggered by LPS, but instead by these short fatty acids” said Ranf.

Targeting bacteria building blocks

The 3-hydroxy fatty acids are very simple chemical building blocks compared to the much larger LPS. They are indispensable for bacteria and are produced in large quantities for incorporation into diverse cellular components.

“The strategy of plant cells to identify bacteria through these basic building blocks is extremely sophisticated; the bacteria require these 3-hydroxy fatty acids and therefore cannot bypass the immune response,” summarizes Ranf.

Fitness program for plants

In the future, these results could help in breeding or genetically engineering plants with an improved immune response. It is also conceivable that plants treated with 3-hydroxy fatty acids would have increased resistance to pathogens.

Remarkable!

Although plants do not have a central nervous system and it is unlikely that they can feel pain the same way animals and humans do, they do not lie idly by and watch their greenery get munched on by hungry critters — they protect themselves. Some species have thorns, while others are loaded with poisons. And some have very interesting abilities such as emitting a horrible stench and folding their leaves.

Here are a few examples of plants with strange, but useful, defense mechanisms:

SEE ALSO: A Painful Mishap Leads to the Discovery of the World’s Only Venomous Frog

Mimosa pudica, also known as the sensitive plant, is a creeping herb of the pea family, Fabaceae, that is often grown out of curiosity — the compound leaves fold inward and droop when they are touched or shaken, a way of defending themselves from harm. This makes them appear dead and thus unappetizing. They re-open a few minutes later.

The types of movement the plant undergoes are termed seismonastic, and the movements occur when specific regions of cells lose turgor pressure, which is the force that is applied onto the cell wall by water within the cell vacuoles. So when the plant is disturbed, the stems release chemicals that force water out of the cell vacuoles, leading to a loss of pressure and cell collapse.

It is not clear why Mimosa pudica evolved this trait, but many scientists think that the plant uses the ability to shrink as a defense mechanism from herbivores and dangerous insects.

Mimosa pudica is also known for emitting a foul smell (similar to a common bodily function) when its roots are disturbed by humans.

Stinging Nettle

Photo credit: brewbooks/Flickr (CC BY-SA 2.0)

Stinging nettles (Urtica dioica) grow a bristling fur called trichomes, which are pointed structures that shield the plant from hungry predators. The trichomes act like hypodermic needles whose tips come off when touched, injecting histamine, acetylcholine, serotonin and other chemicals that produce a stinging sensation when touched by humans, animals or insects.

Some of the plants even inject poison into the trichome-inflicted wounds causing permanent nerve damage or death.

Dumb Cane

Photo credit: Fayes4Art/Flickr (CC BY 2.0)

Not all plants wear their defenses on the surface. “Idioblasts” are cells within the plant that store specialized chemicals and are needed when the first line of defense has been breached.

Dieffenbachia, a common houseplant, contains idioblasts that fire barbed calcium oxalate crystals into the mouths of predators and then release an enzyme similar to reptilian venom known as raphides. This can cause paralysis and a loss of speech, hence the plant’s common name, “dumb cane.”

Bullhorn Acacia Tree

Photo credit: Feroze Omardeen/Flickr (CC BY 2.0)

Some plants hire mercenaries to do their dirty work in a process known as commensalism. Bullhorn acacia trees (Vachellia cornigera) both house and feed aggressive ants. The ants dwell inside the trees stipular spines, or thorns, and feed off food bodies produced especially for them by the plant.

The ants will viciously defend the trees against everything that comes near them, including animals, plants and fungi. They have even been known to snip off the foliage of any other plants that get too close to the tree. In experiments where researchers removed the ant colonies, the trees have died.

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