The next time you wander into a patch of stinging nettles, or contract a bad case of poison ivy, try to see things from the plant’s point of view. Our green neighbors get picked, eaten, infected, and uprooted by any number of organisms, including people, insects, fungi, bacteria, viruses, and countless other herbivores and diseases. What’s a plant to do?
Defend itself.
Plants use spines, bristles, toxins, and irritants as green armor. The noxious oil of poison sumac leaves and berries causes skin to itch and blister. Coatings of silica, wax, and resins make leaves and stems distasteful and indigestible. Blackberry thorns send a painful message: “Eat the berries and spread the seeds, but leave the plants alone!”
Insects are especially adept at penetrating botanical foils. For millions of years, the six-leggeds have engaged in a duel of genetic thrusts and parries with plants – an epic battle between Earth’s most numerous class of species and those that comprise the world’s greatest living mass.
Many plants possess potent alkaloids that inhibit insect digestion and metabolism, interrupt nerve impulses, weaken cells, and even cause cell walls to leak. We have adopted many alkaloids for medicine and as mind-altering drugs, including nicotine, caffeine, strychnine, quinine, morphine, and mescaline. Mandrake root contains an alkaloid called scopolamine that poisons some plant eaters but quells motion sickness in humans.
We use natural insecticides – including menthol, camphor, latex, citronella, cannabis, cocaine, opium, benzoin, and gum resins – in ways that are both helpful and harmful. Steroids and vitamin D were originally derived from insect-fighting plant compounds. Pyrethrin, a natural insecticide in chrysanthemums, is concentrated and sold for pest control.
Chemical precursors to salicylic acid, or aspirin, are found in the sap of tobacco and in other familiar plants, such as willow, teaberry, and both black and yellow birch. Plants concentrate salicylic acid when they are attacked, using it as we do – as an anti-inflammatory to fight infections caused by viruses and other microbes. The powerful cancer-fighting compound Taxol is found in the bark of the pacific yew, in common hazelnut, and in fungi associated with hazelnut.
Many defensive plant compounds, however, are toxic to humans. Pokeweed roots, leaf stalks, and berries, with their attractive purple hues, contain lectin – a protein that causes red blood cells to clot. Lectin is also found in the deadly ricin of castor beans. As few as a dozen berries from a nightshade can be fatal to a child. Foxglove, lily of the valley, oleander, and baneberry produce cardiac glycosides that can cause heart failure. Because children think that colorful leaves and berries look good to eat, nearly 60 percent of all individuals poisoned by glycosides are under seven years old.
Even with this impressive arsenal, plants must continuously adapt. Each time a plant develops a new line of defense, its foes probe for a way around it. When the halo-bright bacterium invades a bean plant, a protein alerts the victim and triggers the bean’s immune response. The bacterium then throws a genetic switch that stops production of that protein, and so invades undetected.
But the plants aren’t caught flat footed. When the fungus Phytophthora sojae attacks, parsley activates a toxic chemical defense. Tomato plants under siege produce greater amounts of a chemical that inhibits the digestion of proteins in insect larvae, causing the ubiquitous leaf-munchers to become undernourished. Certain plants even grow thorns in response to grazing.
Plant defenses are ingenious, intricate, and tailor-made to defeat those who would harm them. Mouth fluids from caterpillars that eat corn, tobacco, and cotton cause the entire plant to emit a chemical scent that attracts insect parasites, which, in turn, attack the offending caterpillars. Each different kind of larva causes the victimized plant to emanate the specific scent needed to lure that pest’s parasites.
Some plant roots can discern whether fungi are hurtful or helpful. Detrimental fungi are attacked as soon as they invade root tissues. But when the root senses a mycorrhizal fungus – the beneficial kind that expands the root’s ability to absorb nutrients from the soil – the plant suppresses its counterattack and allows the fungal threads to penetrate.
Like these symbiotic fungi, people often nourish plants that we find useful. Cultivation under optimal conditions and in healthy soil creates vigorous plants that can protect themselves while attracting advantageous insects.
With or without our help, plants have proven resilient over time. Now, however, they must adapt to challenges posed by rapid climate change. Fossil records of trees show that, when the climate warms, the rate at which insects consume deciduous leaves increases, as does the number of different kinds of insects doing the damage. Time will tell how global warming will tip the delicate balance in the struggle between plants and their ravenous foes.