Plants Send Distress Signals When Eaten
Scientists have known for several years now that while plants may not have brains, they do know when their livelihood is being threatened. Previous research has shown that plants can tell when they are being eaten, and in response to this, they send out defense mechanisms to try to stop the transgressor from continuing to chomp away. 
For example, thale cress secretes mustard oils that are slightly toxic when eaten and sends these oils throughout its leaves in an attempt to discourage the aggressor from further munching. Researchers have also shown that plants will secrete these oils even if they “hear” sounds that resemble eating, indicating that plants can “sense” danger.
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But, until recently, scientists weren’t quite sure how the plant relayed these signals throughout its body.
Appearing in the journal Science, researchers observed that when a leaf of a plant gets eaten, it transmits signals to other leaves using glutamate -- an excitatory neurotransmitter. 
When a nerve cell is excited, glutamate is released, setting off a wave of calcium ions in nearby cells, warning them that danger is nearby. The calcium wave travels to the adjacent nerve cell, which subsequently relays a signal to the next cell in line. This information propagation continues down the line resulting in a sort of long-distance game of telephone in the plant.
A “beneficial” side effect of this is that the nutrition of the plant may be increased due to the overall increase in calcium content of the plant.
Perhaps most interestingly, now that scientists understand how plants communicate within their leaf network, they are beginning to investigate how they may be able to manipulate the plant’s internal communication network.
Can any good come from this?
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1) Appel, H. M., & Cocroft, R. B. (2014). Plants respond to leaf vibrations caused by insect herbivore chewing. Oecologia, 175(4), 1257–1266. https://doi.org/10.1007/s00442-014-2995-6
2) Toyota, M., Spencer, D., Sawai-Toyota, S., Jiaqi, W., Zhang, T., Koo, A. J., … Gilroy, S. (2018). Glutamate triggers long-distance, calcium-based plant defense signaling. Science, 361(6407), 1112 LP-1115. https://doi.org/10.1126/science.aat7744