For a very long time, the scientific community considered that plants, while they were certainly capable of adapting to changes in their environment through the mechanisms of natural selection, lacked a nervous system that would enable them to learn - as is the case with animals and human beings. But recent exciting discoveries have challenged this dogma, opening up a huge field of research.
Po develop and survive, plants make remarkable use of their roots, whose sometimes extremely extensive network enables them to reach far and wide for the water and nutrients they need. Working on the plant Arabidopsis thaliana, Researchers from the Montpellier Laboratory of Plant Biochemistry and Molecular Physiology and the Institut Jean-Pierre Bourgin (Inra/AgroParisTech/CNRS) identified, at the end of 2016, a gene, called HCR1, which controls the water permeability of roots, taking into account the potassium and oxygen content of the soil.
This remarkably well-functioning gene allows plants to reduce the entry of water into the roots when the concentration of oxygen becomes too low, but only when the soil is rich in potassium, a mineral salt essential for plant growth. The discovery of this balancing mechanism between oxygen level, mineral content and root permeability is considered major and should have far-reaching agronomic consequences by allowing the selection of species better adapted to their environment.
This discovery also provides genetic support for the hypothesis of Barbara Hohn of the Friedrich-Miescher Institute for Biomedical Research (Basel, Switzerland), who showed in 2006 that the same plant (Arabidopsis thaliana) "remembers" the threats it suddenly faces in the wild. Even more astonishingly, these same researchers have shown that this plant transmits to its descendants its capacity to adapt to the new conditions of its environment.
To highlight this remarkable property of "genetically registered" plants of a sudden change in their living conditions, these scientists have exposed crops ofArabidopsis thaliana to different types of stress (sudden increase in ultraviolet radiation and bacterial attack in particular). They were then surprised to find that the plants very quickly triggered a particularly effective process of recombination of their genes, enabling them to increase their genetic diversity and significantly improve their chances of survival in this new aggressive environment. This rearrangement of their genes continued over the next four generations, even without exposure of the offspring to these stressors...
At the end of 2016, Australian researchers confirmed this astonishing capacity of plants to learn (See study in the magazine Nature), by carrying out a series of very ingenious experiments. They placed pea seedlings in a maze divided into two compartments. In the first compartment, the researchers placed a lamp and a fan on the same side. The scientists' idea was to associate the wind with light and to make the wind be considered by these plants as a predictive event announcing light.
In the other compartment, the light has been placed opposite the fan. In this situation, the wind was therefore used in a neutral way because it is not useful to the plant, even though the plant is quite capable of detecting it. Once this device was installed, the researchers wanted to know whether the plants in these two compartments could see their behaviour conditioned by these combinations of stimuli. They found that by removing the light source, 62 % peas in the first compartment grew on the fan side, even when the light was removed. In the second compartment, the researchers observed that 69 % of the plants grew on the side where the light had previously been.
The scientists who carried out this very rigorous experiment are convinced that these observations show that the plants are well capable of escaping a simple reaction by conditioning and have learned to associate the two elements, light and wind, to choose the place that seems most favourable to their development .
This experimental protocol also shows that the plants know how to anticipate the arrival of light by growing on the side where they had received light exposure in the previous days. As the conclusions of this exciting study point out, "Although the possibility that plants can learn by association has been considered in previous studies, our work provides the first unequivocal proof of this.
Another very interesting observation: for this learning mechanism to work, the experiments had to respect the circadian rhythms of these plants. This learning ability would therefore be subject, as in animals and humans, to precise metabolic constraints.
At the end of this study, which has caused a lot of noise in the scientific community, it must be admitted that the plants studied do not seem to respond only to light stimuli in order to survive. Indeed, their behaviour suggests that these plants are capable of making real choices and predicting when and where light will appear. This quite astonishing capacity for learning and anticipation naturally gives the plants that possess them a decisive competitive advantage that could explain the extraordinary evolutionary "explosion" that characterized plants at the beginning of the Cambrian.
It therefore seems, in the light of these experiments and discoveries, that plants, although they do not have a nervous system, are indeed capable of storing certain important information and experiences in biochemical form. They "remember" important events in their existence.
Recently, researchers at the Centre de recherches sur la cognition animale in Toulouse have shown that the "...". Physarum polycephalum A "yellow undergrowth fungus", a yellow undergrowth fungus that has the particularity of moving, can learn to ignore a caffeine obstacle in its path, even though it is unicellular and devoid of a nervous system. (See PRS).
Appeared on earth more than 500 million years ago, this fungus, several centimetres wide and yet composed of a single cell with thousands of nuclei, never ceases to amaze researchers with its development cycle and its ability to adapt to its environment in a truly extraordinary way. This team from Toulouse wanted to observe how this fungus, which moves very slowly (5 cm/h on average), would react if it was confronted with certain obstacles to access its food.
The researchers divided the mushrooms into two groups: the first group had direct access to their basic food. The second, on the other hand, had to pass through a trail impregnated with chemicals that the fungus did not know about in nature and that could be potentially harmful, such as quinine caffeine. As a result, the mushrooms in the second group were initially much slower and more cautious in their progress than those in the first group. But after six days, having found that the substances to which they were exposed were harmless, they went as fast as their fellow mushrooms in the first group to find their food...
As in the previous experiments that we have mentioned, these researchers are convinced that there was indeed learning and that it was not a simple conditioned reaction, because the response of the mushrooms remained specifically related to the substance used: mushrooms that had become accustomed to caffeine remained resistant to quinine, and vice versa. Previous studies had already shown that the fungus Physarum polycephalum, if forced to go through a maze to find its food, would eventually find the shortest route!
In the face of the quite astonishing results of these latest experiments, the researchers point out that learning, as we understand it - defined as a change in behaviour brought about by the experiment - has so far been studied almost exclusively in multicellular organisms with a nervous system. It was commonly accepted that plants are not capable of learning in the noble sense of the term and that they can only adapt, by expressing their genetic variability, to certain changes in their environment.
However, we must face the facts: plants and trees not only have much more elaborate and subtle adaptive capacities than previously thought, but also seem to be capable of learning from some of their experiences, of storing them in networks whose nature remains to be elucidated, and finally of transmitting this new information to their congeners, insofar as it may prove useful for the survival of the species.
Learning is therefore a fundamental mechanism at the heart of all living things, from bacteria to humans, including the most rudimentary animals and all plants. This extraordinary discovery should have considerable scientific, ethical and philosophical consequences. We now know, even if the uniqueness of the human species within evolution remains undeniable, that mechanisms and phenomena as complex as learning, memory and perhaps consciousness are not peculiar to human beings and are shared by very many living beings, all things being equal, both in the animal and plant kingdoms. (1).
How not to be amazed by this nature whose complexity seems inexhaustible and which reveals, behind an extraordinary diversity, a deep and troubling unity. Let us hope that tomorrow we will be able, while there is still time, to do our utmost to preserve this teeming beauty of life so that our descendants too can continue to admire and study this great mystery of life.
René TRÉGOUËT, Honorary Senator - Founder of the Senate Foresight Group