Wednesday, July 2, 2014

Plants Can Differentiate Between Sounds and Respond Accordingly

Gardeners have long been touting the positive effect of talking to their plants, which increases plant yield.  Exposing plants to music has also been shown to promote plant growth and development, but these types of experiments lack an ecological context.  For example, music contains such a wide range of frequencies, amplitudes, and fine-temporal patterns that are unlikely to be found in combination in nature.  In an open access study published today in Oecologia, a group of researchers examined the effect of more ecologically relevant sounds, like those from herbivores and other insects, on plant development.

A P. rapae caterpillar
munching on a leaf.

Chewing herbivores, like Pieris rapae caterpillars (the caterpillar of the white butterfly) make a distinct, high-amplitude vibration that travels throughout the plant.  Predatory insects are often tuned in to these vibrations to detect their prey from a considerable distance.  The authors of the study hypothesized that plants have evolved a pathway for detecting these vibrations and signaling a defense response.  Using the model system Arabidopsis thaliana, the researchers measured the chemical defense responses after exposure to the same vibrations that herbivores produce.

Vibrations from P. rapae herbivory on the leaf being
chewed (top) and on a leaf 5 cm away (bottom)
Chewing vibration are not only felt at the feeding site, but also throughout the plant (left), meaning that the whole plant can respond to the herbivore by heightening its defenses.  To tease out the potential other effects of herbivory that plants could pick up on, the researchers recorded the sound of the caterpillars eating leaves and exposed plants to the sound vibrations.  In a second experiment, they also added two other vibration controls: plants exposed to wind-produced sound vibrations, and another set exposed to the vibrations from the leafhopper mating song.

Plants that were exposed to the herbivore sounds had significantly higher levels of defense compounds, like glucosinolates, than plants that had not been exposed to the vibrations.  This response was both local and systemic - meaning it occurred at the feeding site and at the leaves opposite the feeding site, though to a slightly lesser extent.  In the second experiment, they measured anthocyanin levels (another defense compound) and found that plants exposed to the herbivore vibrations had higher levels of anthocyanin compared to control plants, and the plants exposed to other vibrations.  That means that the plants can detect and differentiate between different sound waves that they are exposed to!

Ultimately, mechanical vibrations - or the actual vibrations produced by feeding insects - produces much higher defense responses than the feeding sound vibrations do, but the authors posit that these two types of vibrations, along with other cues from herbivores, work concertedly to induce and enhance plant defense mechanisms.  Exposure to sound waves alone resulted in a "priming" effect, which is essentially a plant's battle cry.  It allows the plant to heighten its defenses (hence the increased glucosinolates and anthocyanin levels) even before insects start feeding. 

File:Gatling pea 2.png
Other defense compounds, like methyl jasmonate, are used to warn surrounding plants of the presence of herbivores. 
It is also possible that plants eavesdrop on each other, picking up sound vibrations from other plants in order to prepare themselves for battle.

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