How leaves tell each other about a bug attack

New research explores how plant communication systems respond to threats from hungry insects. The study, published September 14, 2018, in the peer-reviewed journal Science, suggests that once wounded, plants use calcium signals to warn distant tissues of future attacks.

The study found that a chemical called glutamate – which is an abundant neurotransmitter in animals, including humans – activates a wave of calcium when the plant is wounded.

A hungry caterpillar, first working around a leaf’s edges, approaches the base of the leaf and, with one last bite, severs it from the rest of the plant. Within seconds, a blaze of fluorescent light washes over the other leaves, a signal that they should prepare for impending attacks by the caterpillar or its kin. That fluorescent light tracks calcium as it zips across the plant’s tissues, providing an electrical and chemical signal of a threat. Image via Toyota/Gilroy.

Gregg Howe is a professor at Michigan State University and a study co-author. Howe said in a statement:

We often think of plants as being passive and at the mercy of their environment. My jaw literally dropped when I first saw these videos … They beautifully illustrate how active and complex plants really are.

When a plant’s leaf is wounded, an electrical charge races across the plant to warn other tissues of possible danger. Howe said:

For decades, it’s been known that leaf damage, inflicted by mechanical wounding or caterpillar munching, rapidly activates defense responses in distant, undamaged leaves of the plant. But what triggers this rapid response has largely remained a mystery.

The researchers thought the trigger might be calcium. That’s because it’s ubiquitous in cells and often acts as a signal of a changing environment. And because calcium carries a charge, it can produce an electrical signal. But it is hard to track because its concentration levels spike and dip quickly.

The researchers created a method to see the calcium in real time. According to a Michigan State University statement:

[The researchers] developed plants that produce a protein that fluoresces around calcium, letting the researchers track its presence and concentration. Then came caterpillar bites, scissor cuts, and crushing wounds.

In response to each kind of damage, the plants light up as calcium flows from the site of damage to other leaves. The signal moved quickly, about one millimeter per second, reaching out to distant leaves in just a couple minutes.

A few minutes later, levels of a defense hormone – called jasmonate – spiked in those distant leaves. They were preparing the plant for future threats by producing noxious chemicals that ward off predators.

The results suggest that glutamate exiting a plant wound leads to rapid propagation of a calcium wave, which in turn leads to production of jasmonate and defense responses.

Bottom line: In a video from a new study, a caterpillar eats a leaf, and the plant sends a warning signal to other leaves.

Read more from Michigan State University

September 19, 2018

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