by Katherine Wu

Herbivores, beware: the humble tomato plant has a trick up its sleeve.

When under attack by pests such as caterpillars, plants can goad their predators into selecting another meal: each other.

In a study published in July 2017 in Nature Ecology & Evolution, a team of scientists led by Dr. John Orrock at the University of Wisconsin-Madison demonstrated for the first time that plants can respond to threats by directly causing cannibalistic behavior.

Few insects have qualms about resorting to cannibalism; when times are particularly desperate, hungry caterpillars will eat whatever is available – including each other. But plants prompting caterpillar cannibalism as a means of self-defense is a new phenomenon, shedding light on the powerful tactics plants have evolved to turn the odds in their favor.

When Dining Turns Deadly

While not equipped with teeth or claws or even fast-moving feet, plants are not as defenseless as they may seem. Scientists have known for years that a compound called methyl jasmonate can act as a chemical distress call, eliciting a suite of defense responses in plants under attack. We now know that one of those responses includes sapping the tomato plant’s leaves of nutrition, such that fellow caterpillars look comparatively scrumptious.

Flipping this “defense” switch is not unlike the initiation of the “fight or flight” response in humans when we experience fear, shock, or anger. Simply put, the last thing you want to be spending your energy on is resting and digesting when you’re being chased by a tiger – or, in this case, a ravenous caterpillar.

Plants can’t fly – but they have their own ways to fight. When plants sense danger in their environment, they will release methyl jasmonate into the surrounding air to warn their neighbors like a silent scream. This airborne alarm prompts plants to divert their resources away from food production and growth and into the production of toxins and cell wall, akin to plant body armor.

Armed with tomato plants and beet armyworm caterpillars, Dr. Orrock and his colleagues decided to test the protective power of methyl jasmonate. They sprayed plants with varying amounts of methyl jasmonate or harmless soapy water, then added eight hungry caterpillars. In this vacuum, the caterpillars had only three choices: eat the tomato plant, eat their fellow caterpillars, or starve.

Eight days later, the plants that had been sprayed with soapy water alone had lost over 90% of their leaves, and most of the caterpillar brethren remained intact. With increasing concentrations of methyl jasmonate, however, the caterpillars took their leaves instead out of Hannibal Lecter’s book. The methyl jasmonate had somehow moved the tomato plants down the caterpillars’ hierarchy of food preferences.

From the tomato plants’ perspective, this is a two-for-one deal: not only are the plants left unscathed at the end of the ordeal, but there are also fewer caterpillars left to come back for seconds.

With Friends Like These, Who Needs Energy Bars?

Orrock and his team wanted to be sure that the consumption of methyl jasmonate-treated leaves was directly responsible for cannibalism. They assumed complete control of caterpillar diet, feeding single caterpillars only leaves from sprayed or unsprayed tomato plants. The researchers then placed the live caterpillars in containers with four dead caterpillars and waited to see how long it took the singletons to begin consuming their fallen comrades.

After two days – relatively early in the cannibalism timeline – the caterpillars that had been fed methyl jasmonate-treated leaves were already consuming the bodies. Something in the leaves the caterpillars had eaten was directly leading to cannibalistic behavior.

The simplest explanation was that the methyl jasmonate had made the leaves less nutritious. As it turns out, caterpillars fed only methyl jasmonate-treated leaves grew remarkably poorly compared to caterpillars fed the same number of untreated leaves; it seemed that the treated leaves were less nutritionally dense. Strikingly, caterpillars fed only the dead bodies of other caterpillars grew just as well as their counterparts eating untreated leaves. Cannibalism, it seems, is a very appetizing plan B.

As docile as plants may appear, they’ve had millennia to evolve the craftiest of defenses – and we are just beginning to appreciate the extent of them. In fact, methyl jasmonate is almost ubiquitous among plants – meaning it has the potential to be the Esperanto of botanical distress, communicating between even distantly related species under attack from predators. And although it’s down the road, this research may someday form the basis of a new form of insecticide or pest-management strategy.

In the meantime, don’t worry – there’s no evidence that serving salsa will turn your next evening soirée into the Donner Party. At least, not yet.

Katherine Wu is a fifth-year graduate student at Harvard University and Co-Director Emeritus of Science in the News.

For more information:

  1. The original study is behind an unfortunate pay wall, but was also covered by Nature News.
  2. A few more articles on the unique ways in which plants defend themselves can be found here and here.

Cover image by David Marquina Reyes.

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