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Plants are continuously analyzing their environment through more than 700 sensors identified in the plant world to measure temperature, humidity, light, etc. This wide range of sensory perception is the subject of much work on the part of plant physiologists.
Stefano Mancuso, an eminent specialist in signaling and behavior in plants, defines plants as
“fixed beings, unable to flee, that do not have organs. They have a modular structure, a bit like coral. Thus, if an herbivore comes and eats 80% of the plant, the remaining 20% survives. This is a major difference from animals. They breathe without lungs, detoxify without a liver, digest without intestines... and have intelligence without a brain.”
Daniel Chamovitz, a biologist at Tel-Aviv University and author of the recently published work
“What a Plant Knows: A Field Guide to the Senses” (Buchet-Chastel, 2014), completes the description by informing us that “they do not have eyes and yet they see, they do not have a nose, yet they smell, they do not have ears and yet they react to sound.”
Sense of hearing
Desmodium gyrans (L.) DC., now renamed Codariocalyx motorius (Houtt.) H. Ohashi, is one of the rare plants capable of rapid movements. Each leaf has a hinge at the bottom, which enables the blade to move in order to optimize exposure to sunlight. This mobility could also be a defensive strategy with regard to various insects: the rapid rotation of the foliage can mimic the flight of a butterfly [Lev-Yadun 2013]. But even more surprising, this legume from tropical Asia nicknamed the “dancing plant”, has the ability to hear. If you make a noise or play music, the leaves move in time, try it...
In S. Mancuso’s laboratory, his team noticed that the roots of various plants move towards the noise made by water flowing in an underground pipe. This suggests that these plants can detect the sound of flowing water. Monica Gagliano from Crawley University (Australia) found that corn roots tend to grow towards a sound source with a frequency of around 200 Hz [Gagliano 2012]. The ability to use sounds is not a prerogative of animals because plants can not only perceive sounds, but also emit them. The exploration of acoustic communication in plants is full of promises [Gagliano 2013].
Sense of smell
Dodder (Cuscuta pentagona Engelm.), a parasitic climbing plant lacking in chlorophyll, smells the presence of tomato plants and goes to attach itself to them.
The same phenomenon occurs when the plant is presented with an item infused with the scent of tomatoes. Cuscuta seedlings can distinguish the volatile compounds of wheat from those of tomatoes and prefer to grow towards the latter. The nature of the sensors that enable the sense of smell is unknown but among the volatile mediators of the tomato, β-phellandrene, β-myrcene and α-pinene can be cited [Runyon 2006; Mescher 2006].
Ian Baldwin from the Max Planck Institute for Chemical Ecology in Germany says “At the moment, we do not know what serves as a nose for plants, but we will discover it within the next decade.”
Sense of touch (mechanisms of mechanical perception)
We can add another sense, that of touch, as in the sensitive plant (Mimosa pudica L.) that closes its leaves when they are touched or carnivorous plants capable of capturing prey (mainly insects, mites and other small invertebrates) using active traps, such as those of the Venus flytrap (Dionaea muscipula Solander ex Ellis) or other drosera.
The closure of the lobes of the leaves of the Venus flytrap is triggered by sensitive hairs (usually 6) located on the inner surfaces. As soon as an insect touches one of the hairs, the mechanism is triggered, but the trap does not close completely yet. This requires a second contact within a relatively short period of time, the plant thus protecting itself from unnecessary effort (for example due to the falling of a dead leaf).
The sense of touch is wonderfully at work during the growth of some climbing plants that have tendrils, which search for a support to wrap themselves around. The level of sensitivity can be extreme, for example with the tendrils of the one-seeded bur cucumber (Sicyos angulatus L.), which wrap around a wire weighing 0.25 grams, while a human finger can only detect the same wire if it weighs 2 grams or more [Monshausen 2009; Monshausen 2013].
The root apices possess an extreme sensitivity to environmental stimuli. Indeed, “the root tip acts as the most important sensory organ of the plant; it detects various physical parameters such as gravity, light, moisture, oxygen and essential inorganic nutrients” [Baluska 2013].
Claude Bernard and his work on plant anesthesia
Considered to be the founder of experimental medicine, Claude Bernard (1813-1878) believed that plants were also capable of perceiving changes in the environment. He called this “sensitivity”.
In order to verify this intuition, he carried out anesthesia on plants and showed the effects of volatile anesthetics on various processes such as germination, photosynthesis and the movement of plants (C. Bernard, “Lectures on the phenomena of life common to animals and plants”, 1878). Anesthesia can be defined as a loss of reactivity to environmental stimuli.
Neurons are the living cells most sensitive to anesthetics, which is due to their specialization in the integration of sensory information and heightened perception of the environment. Similarly, plant cells are all excitable and some of them are even specialized in the perception, transmission and integration of sensory information [Grémiaux 2014]. And Claude Bernard concludes: “that which is living must sense and can be anesthetized; the rest is dead.”
Didier Guédon, Expert on the French Pharmacopoeia Committee
Bibliography:
Baluska F, Mancuso S. Root apex zone as oscillatory zone. Front Plant Sci 2013;4:e354.
Gagliano M, Mancuso S, Robert D. Towards understanding plant bioacoustics. Trends Plant Sci 2012;17:323-5.
Gagliano M. Green symphonies: a call for studies on acoustic communication in plants. Behav Ecol 2013;24:789-96.
Grémiaux A, Yokawa K, Mancuso S, Baluška F. Plant anesthesia supports similarities between animals and plants: Claude Bernard's forgotten studies. Plant Signal Behav 2014;9:e27886.
Lev-Yadun S. The enigmatic fast leaflet rotation in Desmodium motorium. Butterfly mimicry for defense? Plant Signal Behav 2013;8:e24473.
Mescher MC, Runyon JB, De Moraes CM. Plant host finding by parasitic plants: a new perspective on plant to plant communication. Plant Signal Behav 2006;1:284-6.
Monshausen GB, Gilroy S. Feeling green: mechanosensoring in plants. Trends Cell Biol 2009;19:228-35.
Monshausen GB, Haswell ES. A force of nature: molecular mechanisms of mechanoperception in plants. J Exp Bot 2013;64:4663-80.
Runyon JB, Mescher MC, De Moraes CM. Volatile chemical cues guide host location and host selection by parasitic plants. Science 2006;313:1964-7.
