When Plants Fight Back

By Andrew Scott, Horticulturist for The Gardens on Spring Creek

This is the second installment of a two-part feature on plant defenses. Read Part I online at North Forty News.

Caffeine, nicotine, morphine, cocaine ... sounds like the ingredients label on an old-timey cold medicine, right? These molecules are actually all alkaloids, some of the most common chemical deterrents deployed by plants. As the adage goes, the dose makes the poison. While people can (and do) ingest these alkaloids for their psychoactive effects, they prove to be much more deadly to a small insect that weighs all but a few tenths of a milligram. 

The sap of opium poppies (Papaver somniferum) contains morphine, a potent sedative (via Daniel Prudek, Hodder Education Magazines) 

Still, alkaloids have a strongly bitter taste, which animals have learned to associate with the toxic effects they can have. Eating green potatoes isn’t likely to kill you, but solanine, the alkaloid that triggers this change in potato color, deters both people and insects from munching on the potato plants’ energy stores should they become exposed to sunlight. Another class of chemicals, called cyanogenic glycosides, are essentially a cyanide molecule attached to a sugar. The molecule is harmless within the cell, but when damaged, enzymes zip that sugar off and convert it into the very toxic hydrogen cyanide that Agatha Christie fans are familiar with. Another class of chemicals called terpenoids (e.g., limonene, menthol, camphor, citronellol, and pinene) are toxic to insects and possess antifungal and antibacterial properties, keeping these pests out of plants like mints, lemongrass, laurels, and conifers.

The roots of cassava (Manihot esculenta) contain toxic cyanogenic glycosides; cooking deactivates the toxin of this staple crop of the tropics (via HTS Farms Limited) 

Some plants have evolved to produce compounds that taste just plain nasty, mimicking legitimately toxic chemicals. If you’ve ever bitten into a piece of underripe fruit, you’ve likely tasted tannins. These astringent compounds give red wine and tea their signature drying mouthfeel while also discouraging snacking by animals before the seeds within the fruit have matured and are ready to be distributed. Furthermore, plant hormones such as salicylic acid and jasmonic acid are vital in responding to pathogens and also happen to have a bitter taste.

Consider, too, the effects of poison ivy or stinging nettles. When an animal bushwacks through the forest and damages the plants, chemicals like urushiol and histamine result in contact dermatitis that can last for days. The more yucky or caustic chemicals a plant develops, the less likely it is to get eaten and damaged, and the more likely it will pass on those genes for yucky and caustic chemical production.

But the plant doesn’t always have to defend itself. Mutualism describes an interaction in which two species benefit from each other. For example, predatory insects like parasitoid wasps and lady beetles can detect chemical signals released by damaged plants—a kind of cry for help—and interpret them as a sign that tasty prey insects are nearby to gobble up.

Plants may also offer habitat for insects, as is the case for acacia trees and ants. The swollen thorns of the acacia offer shelter while extrafloral nectaries (nectar sources that aren't flowers) on the tree's leaf stalks provide food for the ants, which swarm anything that threatens the tree (such as the face of a giraffe!).

But the acacia tree doesn’t solely depend on ants for defense. After all, it has those wicked thorns that the ants live in. 

Observing organisms interacting is one of the most fascinating parts of nature—and nothing illustrates this better than seeing how a targeted species defends itself to survive another day. Whether it’s by formulating noxious chemical compounds, putting up a pokey barricade, calling for backup, or avoiding detection altogether, plants have evolved delightfully sophisticated defense systems that keep them one step ahead of their environment’s predators. And, their predators respond in kind to this evolutionary arms race—running, running, running to keep in the same place.

To learn more about the physical defenses plants arm themselves with, head to North Forty to read on Part I!