When a caterpillar, or some other pesky herbivore, nibbles on a leaf, the plant starts sending danger signals to its furthest fronds. These signals can activate the plant’s defense mechanisms, such as producing noxious compounds to chase off the pest, or turning on pathways to heal damaged tissues.
Plant researchers have thought that this signaling process involves calcium ions and protein receptors that bind the molecule glutamate. Now, using fluorescence imaging and genetic engineering, a research team led by Masatsugu Toyota at Saitama University has confirmed this mechanism (Science 2018, DOI: 10.1126/science.aat7744).
Glutamate is a fast-acting neurotransmitter in mammalian nervous systems. So the findings suggest plants can send internal signals through a nervelike process.
In the new study, the team either enticed caterpillars to munch on, or cut Arabidopsis plants engineered to express a protein sensor that fluoresces when it binds calcium ions. The plant’s defense mechanisms are triggered in part by increases in the concentration of calcium ions.
Using sensitive fluorescent microscopy, the team detected spikes in calcium ion levels immediately at the sites of damage and two minutes later in distant leaves. The timing was too fast to be explained by the ions diffusing through the plant. Instead, the scientists hypothesized that the damage triggers the release of glutamate that then activates glutamate-like receptors. These proteins are ion channels that open up when turned on, triggering the influx of calcium ions to the furthest reaches of the plant. Glutamate receptors similar to these proteins are involved in brain processes such as learning and memory in humans.
To show that glutamate was responsible, the team also engineered the plants to express fluorescent glutamate sensors. The scientists observed that glutamate levels increased at the site of the wound on the plant. Toyota says that they think this glutamate comes from veins in the leaf.
“Our take-home message is that plants, which do not have a central nervous system, produce nervous-like functions,” Toyota says. But he’s not certain if scientists can now definitively say that plants have nervous systems.
František Baluška of the University of Bonn, who was not involved in the work, suggested in 2006 that plants might have neuronal-like structures (Trends Plant Sci. 2006, DOI: 10.1016/j.tplants.2006.06.009). He thinks the new work provides the “final evidence” that glutamate is acting as a classical neurotransmitter in plants.