Careful, they might hear you

On my first visit to China, ten years ago, to the Xishuangbanna Tropical Botanical Garden, I was intrigued by this plant Codariocalyx motorius. According to my Chinese colleagues it responded to singing by wiggling its stipules, small flaps at the base of its leaves.

As I mentioned in a post a few years ago, the plant failed to perform. It may have been our fault. Rather than sing, a mobile phone ring tone was used to entice the Dancing Plant, as it was called, to dance. I assumed it was probably the vibrations caused by particular noises that caused the response, when and if it did respond.

Now I can report that plants can 'hear' insects munching on their leaves. This discovery by scientists at Missouri University comes close on the heals of a revival in interest in plants communicating with one another (extending the sharing of genetic material). The distinction between plants and animals becomes increasingly blurry.

The plant with ears is the popular experimental mustard plant or cress, Arabidopsis, and the noisy eater the caterpillar of the Cabbage Butterfly. Vibrations from the caterpillar's eating travel through the leaf and stems, triggering chemical defenses by the plant. Wind and other vibrations applied to the plant don't cause any reaction - it's all to do the frequency of the caterpillar's crunch.

Whether you could really describe this as the plant hearing noise is of course a moot point. Although sounds are usually defined as vibrations traveling through air and received (heard) by something else. That fits pretty well what is going on in the mustard-caterpillar interaction.

As you'd expect, these sounds are very quiet. Microphones can't be used due to the small size of the leaves and the caterpillar's mouth, but a piece of reflective tape nearby (see attached to the leaf next to the white caterpillar above) reflects a laser beam which gets disturbed by the (minutely) noisy eating. This response can be amplified to a speaker so that we can hear the caterpillar's crunching.

In response to this quiet racket, the plant produces a number of defensive chemicals, one of which is called glucosinolate. Glucosinolate makes mustards spicy, may cure cancer in humans, gives red wine its colour and making chocolate healthy. For a caterpillar, however, it is repellent.

Playing the chewing noise back to plants caused higher levels of glucosinolate to be produced. Other sounds and frequencies had no effect so it isn't just the leaf shaking about that causes this response. 

Returning to my Dancing Plant, the researchers say that “The field is somewhat haunted by its history of playing music to plants . That sort of stimulus is so divorced from the natural ecology of plants that it’s very difficult to interpret any plant responses.” 

I suspect they are referring to people suggesting that playing music to plants makes them 'happy' and grow better, as featured in the book, The Secret Life of Plants. When such responses are detected (not often in experimental situations) it is most usually attributed to increased carbon dioxide from humans exhaling nearby or perhaps air movement encouraging greater gas exchange into and out of the leaf.

For a more learned summary of research into what are more properly called 'acoustic emissions' from plants, see Monica Gagliano's (from the University of Western Australia) 2012 summary in Behavioral Ecology. Gagliano concludes that more and more we find that plants 'highly sensitive organisms that perceive, assess, interact, and even facilitate each other by actively acquiring information from their environment'. She says they effectively 'cry for help' when they respond like the cress does to the caterpillar and that there is likely to be more to discover in this controversial but fascinating area of plant biology.

Next up for the Missouri team is to find out whether other plants respond in similar ways and what it is precisely in the vibration, or sound, that triggers the plant response. It's nice to think that plants might do more than dance in response to niggling noises.

Notes: the image of the caterpillar on the Arabidopsis comes from the Decoding Science website, and the idea for the story from colleague Peter Symes.