MADISON – In a video, you can see a hungry caterpillar, working first around the edges of the leaf, approaching the base of the leaf and, with one last bite, cutting it off from the rest of the plant. Within seconds, a glow of fluorescent light spreads over the other leaves, a sign that they should be prepared for future attacks by the caterpillar or its relatives.

This fluorescent light follows the calcium through the plant tissues, providing an electrical and chemical threat signal. In more than a dozen videos like this, Simon Gilroy, professor of botany at the University of Wisconsin-Madison, and his lab reveal that glutamate – an abundant neurotransmitter in animals – activates this wave of calcium when the plant is injured. Videos offer the best aspect of communication systems within normally hidden factories.

The research is published on September 14 in the journal Science. Masatsugu Toyota led the work as a postdoctoral researcher in Gilroy's laboratory. Gilroy and Toyota, now at Saitama University in Japan, have collaborated with researchers from the Japan Agency of Science and Technology, Michigan State University and the University of Missouri.

"We know there is a systemic signaling system, and if you hurt in the same place, the rest of the plant triggers its defense reactions," says Gilroy. "But we did not know what was behind this system."

"We know that if you hurt a leaf, you get an electric charge and you get a spread that moves through the plant," adds Gilroy. What triggered this electrical charge and its displacement in the plant was unknown.

But calcium was a candidate. Ubiquitous in cells, calcium often acts as a signal on a changing environment. And because calcium carries a charge, it can also produce an electrical signal. But calcium is ephemeral, pungent and plunging quickly into concentration. Researchers needed a way to see calcium in real time.

So, Toyota has developed plants that have shown calcium in a whole new light. Plants produce a protein that only moves around the calcium, allowing researchers to track its presence and concentration. Then there were caterpillar bites, scissors cuts and crushing wounds.

In response to each type of injury, the videos show plants that light up when calcium flows from the site of damage to other leaves. The signal is moved quickly, about one millimeter per second. This is only a fraction of the speed of nerve impulses of animals, but it is very fast in the plant world – fast enough to spread to other leaves in a matter of minutes . It only took a few minutes for the defense-related hormone levels to increase in the distant leaves. These defense hormones help prepare the plant for future threats, for example by increasing the levels of harmful chemicals to prevent predators.

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