The Biological Warfare of Plants

January 4, 2016

Compared to the hectic rush of our bipedal world, a plant’s life may appear an oasis of tranquility. But look a little closer. The voracious appetites of pests put plants under constant stress: They have to fight just to stay alive.

And fight they do. Far from being passive victims, plants have evolved potent defenses: chemical compounds that serve as toxins, signal an escalating attack, and solicit help from unlikely allies.

However, all of this security comes at a cost: energy and other resources that plants could otherwise use for growth and repair. So to balance the budget, plants have to be selective about how and when to deploy their chemical arsenal. Here are five tactics they’ve developed to ward off their insect foes without sacrificing their own wellbeing.

Warning Flares

Rather than pump out chemical defenses 24-7 (a waste of resources), plants hold off production until an attack is underway. As soon as an insect bites a leaf, the leaf sounds the alarm by emitting volatiles—chemical flares that tell other parts of the plant, as well as its neighbors, to start manning the barricades.

This early warning system works via a cascade of molecular events. First, it triggers the release of “jasmonate” hormones, which in turn break down proteins known as JAZ. These proteins silence genes that direct the manufacture of various toxic and protective chemicals. By eliminating JAZ, jasmonate hormones free these genes to express themselves, thus powering up a plant’s weapons assembly line.

Plants also make use of underground networks to warn each other of impending danger. Many species have a symbiotic relationship with a soil-borne fungus, which penetrates the outer layers of a plant’s roots, feeding off its carbon stores and helping it take up vital nutrients such as nitrogen and phosphorus in return. The fungus grows by sending out long, threadlike branches called hyphae, which colonize nearby plants, forming vast underground webs.

In experiments with bean plants, when researchers placed aphids on a plant encased in a polythene bag (so it couldn’t broadcast warnings by air), other plants hooked up to the subterranean fungal web began churning out defensive compounds. Their unconnected neighbors, however, did not. The fungus appears to work like a biological Internet, carrying vital intelligence from plant to plant.

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