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In a video, you can see a hungry caterpillar, working first on the edges of the leaf, approaching the base of the leaf and, with one last bite, separating it 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 how 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 was 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 that there is a systemic signaling system, and if you hurt at one place, the rest of the factory triggers its defense responses," Gilroy said. "But we did not know what was behind this system, we know that if you hurt a leaf, you get an electrical charge and you get a spread across the plant."
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, as 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 the animal nerve impulse, but it is very fast in the plant world – fast enough to spread to other leaves in 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.
Previous research by Swiss scientist Ted Farmer has shown that defense-related electrical signals depend on the receptors for glutamate, an amino acid that is a major neurotransmitter in animals and also common in plants.
Farmer has shown that mutant plants lacking glutamate receptors have also lost their electrical responses to the threats. Thus, Toyota and Gilroy examined the calcium flux during injury in these mutant plants.
"And lo and behold, the mutants that darken the electrical signaling completely dry up calcium signaling," Gilroy said.
Videos showing burning fluorescent plants at the time of the injury, the videos show that the mutant plants barely spray marginal splinters of light. These results suggest that the release of glutamate from wounds triggers the bursting of calcium that is spreading through the plant.
The study links decades of research that has revealed how plants, often considered inert, respond dynamically to threats by preparing distant tissues to cope with future attacks. Glutamate leads to calcium that leads to defense hormones and altered growth and biochemistry, all without a nervous system.
Gilroy said that in addition to helping to assemble all these pieces, the videos help to visualize the wave of activity within the plants that is normally invisible.
"Without the imagery and seeing everything unfold in front of you, it's never really felt at home, man, this thing is fast!" He concluded.
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