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Plants have an active social life

  • Marie Narlock
  • Many adjectives describe plants: beautiful, colorful, fragrant. But what about chatty, strategic and altruistic?

    Recently, researchers have sought answers to vexing horticultural questions: can plants perceive and even respond to their environment? Can they think? Listen? Communicate?

    Studies prove that plants are far from uncommunicative. In fact, it turns out there’s a fair bit of chitchat and eavesdropping going on, both above and below ground. Plants warn neighbors of attacks by insects and animals, give each other a heads up on threatening pathogens and looming droughts, and they recognize and protect family members. Some even have Caller ID, changing their flowers to take a call from a friendly pollinator, such as a hummingbird, instead of a damaging feeder like a moth whose eggs will quickly become leaf-eating caterpillars.

    They may not speak English, but plants enjoy an active social life, mostly by exuding airborne chemicals called volatile organic compounds (VOCs) and underground compounds that are exchanged by roots and fungi.

    To understand this phenomenon, consider the tobacco plant, Nicotiana alata. This tough cookie has a sophisticated perception and response system that results in cunning self-defense. When an insect attack occurs, it emits an invisible, but deadly, poison that kills anything that feeds on it. Stop by for a bite of this plant and you’re a goner.

    That is, unless you’re a hornworm caterpillar. These indomitable critters can mow down an entire tobacco plant in days — laughing at the plant’s defense mechanism all the while. But in a fascinating game of one-upmanship, the plant has a secret weapon: it recognizes the critter that’s chewing on it and emits another chemical that attracts the caterpillar’s enemies. Seriously! Like insect mercenaries, in swoop the caterpillar killers. As if that isn’t deadly enough, the plant also exudes a sweet treat that’s irresistible to the caterpillars, but is a foil: one bite and 20 minutes later the caterpillars take on a scent that attracts predators.

    And here’s the best part: the smug tobacco plant alerts neighboring plants, which respond by altering their chemical stew as well. In essence, the plant sends out vibes that protect itself and its neighbors.

    The story continues underground, where a labyrinth of fungal filaments called common mycelial networks carries messages between plants, intertwining like tangled fiber optic cables. This underground web can grow to massive proportions, carrying signals far further than airborne VOCs. One study identified a fungal network that swapped info through an entire forest, connecting each tree to dozens of others.

    Like a driver fixing a stranded motorist’s flat tire, plants help each other out on these underground superhighways. When one tomato plant activates enzymes to fight a disease, molecules produced by its immune system zip along the fungal freeway and prompt nearby plants to turn on their immune systems, too. No two ways about it: these plants are sending, receiving and responding to signals.

    Plants’ underground compounds typically coexist peacefully, but sometimes disagreements occur. In Montana, spotted knapweed has caused hardship for ranchers, who have been unable to stop its march across 4.5 million acres of valuable ag land. Herbicides and grazing sheep are no match for this weed, whose roots exude chemicals that kill the native grasses. The answer? The attractive — and mighty — lupine. This native beauty secretes another chemical, oxalic acid, as it grabs food from the soil. As luck would have it, oxalic acid also acts as a shield against the knapweed’s chemical — and as it unloads its counterattack it protects neighboring plants as well.

    Chemical warfare, Caller ID, booby traps — who knew plants were so clever? Clearly, they’re not just pretty faces. In addition to providing life on earth, fossil fuel, reduced pollutants and the raw material for our food, medicine and equipment, plants also enjoy a good conversation now and then. Considering their attributes, I think they’ve earned it.