“Roger that!” Do plants have sound perception skills?

A wide range of abilities have long been attributed to the animals (within pluricellular organisms of the living world) to animals only: the ability to move, to stimulate an organ repetitively and very quickly, to communicate with one another or to act in a network. In practice, these skills are easily noticeable in animals by humans, as the nature of the signals or the timescale in which they occur are generally well adapted to the direct identification by own senses. Furthermore, we used to attribute these capacities only to organisms endowed with what humans call “Intelligence”, so primitive it may be, accompanied at the physiological level by the presence of a more or less complex nervous system. Indeed it seemed that these faculties were so complex in their implementation that we could not imagine organisms deprived of this so-called “Intelligence” being able to carry out such various achievements. And yet, Stefano Mancuso, a researcher in botany working in the International Laboratory of Vegetable Neurobiology (Firenze, Italy), prompts us in a video to change our look on plants, these creatures with no brain which may not be completely deprived of intelligence: Stefano Mancuso, The roots of the vegetable intelligence (on TED).

Communication amongst the plant kingdom is a fairly new topic and has long been overlooked for reasons mentioned above. However, this look is currently changing and studies have shown recently an unsuspected richness. It appears that communication between plants is highly diverse and may involve a large variety of strategies acquired during evolution. It can be thus achieved by systems involving mechano-receptors which can detect stimuli related to movements, or again via highly sophisticated light-sensitive captors which may detect wavelengths reflected by neighboring plants… Many other examples include ways of chemical communication pathway implying volatile organic compounds. For instance, a wild species of tobacco (Nicotiana attenuata, Utah desert) attacked by herbivores (caterpillar, bug or beetle) synthesizes a variety of volatile molecules (including jasmonic acid) which attract the predators of the latter, thus reducing subsequent herbivore attacks up to 90%(1) & (2). Another interesting case concerns acacia (Acacia erioloba, South Africa), which can synthesize ethylene, a highly volatile molecule, in case of an attack by herbivores (giraffe, antelope) to prevent neighbours from possible grazing. Afterwards, these nearby trees will synthesize tannins within their leaves, making them bitter and indigestible to repulse other herbivores(3).

Stefano Mancuso and his team have published a study in 2012 in which they tried to identify other pathways of communication between plants from two different species: the Florence fennel plant (Foeniculum vulgare) and the Chilli (Capsicum anuum). Considering that the chemical, mechanical or luminous means of communication were already well known, they aimed at exploring if other mechanisms might be involved. For this, they decided to block experimentally all the previously cited means of communication in order to assess if an interaction could still be maintained between plants. Two hypotheses were specifically tested: first, does the neighborhood of a plant influence the rate of seed germination when the chemical and/or light communications are impossible? And second, does the rate of germination or sprouting seeds depend on the identity of the neighbouring plant?

To test these hypotheses, the authors developed experiments with different types of interactions between a pot of fennel in the center and several chilli or fennel plants placed around it (seeds in Petri dishes or potted seedlings). Fennel has been of interest here for its well-known ability to secrete volatile compounds that inhibit the growth of neighbouring plants in order to preserve natural resources. Using a transparent or opaque cylinder placed (or not) around the fennel plant, the researchers decided to test the chemical and light communication pathways with plants arranged around (chilli or fennel). The experimental unit was then located under vacuum to isolate each experiment unit from the others. Different treatments were tested: first, the cylinder around the fennel was removed, to allow either the chemical or light communication pathways to occur, then the cylinder was replaced to block volatile compounds while allowing the possibility of light communication, and finally the cylinder was covered in black plastic to mask the fennel to other plants. Controls corresponded to no plant in the central cylindrical box (transparent or covered in black plastic).

And the results were rather intriguing! The authors showed that the chilli seeds germinated faster when one fennel plant was in the cylindrical box in comparison with the control (no plant in the cylinder). More strikingly, the germination of chilli seeds was even faster when the fennel plant was in the cylinder rather than when the cylinder was removed allowing all sources of communication signals. It can be concluded that when the fennel is present, the germination of seeds is accelerated which corresponds to a typical behavior of competition. A model with two opposing signals can be then proposed: a negative effect from light or chemical signal on germination when the fennel is separated from the chilli seeds by a cylinder, and a still unknown signal which has a positive effect on germination rate.

What is the nature of this unknown signal? The scientists suggest two possibilities. The first one would involve weak interferences between magnetic fields induced by the plants themselves. Several studies(5) have shown that plants could indeed feel the Earth’s geomagnetic field. Therefore, why could they not feel the fields that they generate locally around them? The second possibility would consist in a “sound-based” communication. Mechanisms emitting and receiving vibrations are still unknown but this study highlights a new putative way of communication. This would allow the seeds of chilli “to feel” the presence of the competitive fennel, and as a consequence increase their rate of germination and growth.

We have told you to whisper pretty love words and serenade to your pot plants to make them grow faster!

References
(1) Cortereso A-M. & Thibout E. 2004. ‘Des insectes gardiens de plantes‘. La Recherche n°380 : 54.
(2) Beck C. 2001. ‘Chemical signal mobilises reserve units‘. Max Planck Research 4: 62-63.
(3) Attenborough D. 1995. The Private Life of Plants: A Natural History of Plant Behaviour. London, BBC Books. 320 pp.
(4) Gagliano M., Renton M., Duvdevani N., Timmins M. and Mancuso S. 2012. ‘Out of sight but not out of mind: alternative means of communication in plants’. PLoS ONE 7(5): e37382. doi:10.1371/journal.pone.0037382
(5) Ahmad M., Galland P., Ritz T., Wiltschko R. and Wiltschko W. 2007. ‘Magnetic intensity affects cryptochrome–dependent responses in Arabidopsis thaliana‘. Planta 225: 614-624. PMID:16955271