Plant neurobiology studies how plants interpret external stimuli and convert them into internal electro-chemical signals. The term 'plant neurobiology', was introduced recently in the scientific literature. It refers to the fact that similarities exist between the ways internal signals are transmitted in plants and animals. Strictly speaking, plants does't have real neurons, so the use of the word 'neurobiology' in this context can be seen as a metaphore or as an analogie (Barlow, 2008).
For this reason, some authors consider this terminology as illegitimate, and fear that it could only bring confusion and misunderstanding in the field of plant signalling. However, the association of the words 'plant' and 'neurobiology' can also be seen as an invitation to broaden the definition of what a nervous system actually is, and to take distance from the classical view of a plant as an inanimate and passive organism.
It is now largely accepted that electrical signals are transmitted via the plant's phloem and allow the rapid propagation of information between remote parts of the plant, especially in reaction to an external stress. Those electrical signals has to be interpreted by a specialized molecular machinery at the location where it is supposed to produce an effect. The analogy with the animal's nerve is then easy to be made :
« Phloem is an electrical conductor of bioelectrochemical impulses over long distances . . . structures of phloem and axon can be pictured as hollow tubes ﬁlled with electrolyte solutions » (Volkov, 2000)
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Plants use a lot of chemical signals to regulate their growth, metabolisms and to adapt to external constraints. In the last decades, the study of the key molecules such as auxins, growth regulators, morphogens and other plant hormones has shown a lot of similarities with the animal's hormonal system. The existence of electrical signals in plants, which has been suspected since the '70 (see : the Secret Life of Plants), is now being widely recognized by the scientific community.
It has been shown that the vascular tissues of plants (phloem and xylem) plays a central role in the plant's 'neural' structure. They do not only convey nutients, water and hormones but also electrical impulses through the plant's structrure.
There are two types of electrical impulses in the plant :
The action potentials are largely documented in the scientific literature and the most famous manifestation of it is the rapid reaction of 'sensitive' plants such as Mimosa pudica or the insect trap of Dionaea muscipula. Those signals actually consist in a wave of depolarization of the plasma membrane around the sieve tubes (Volkov, 2004). Just as in animal's neurons, this wave of depolarization is followed by a short period of non-excitability, called the refractory period. This mechanism prevent the action potential travelling back the way it just came. The sensory systems of plants also allow the summation of subthreshold stimuli, that finally induce an internal signal when the threshold is reached.
Recently, the Netherlands-based artist Ivan Enriques made use of the action potentials in his 'Action Plant' installation, a collaboration with Bert van Duijn, from the Biology University of Leiden. This creation actually consist in a plant of Mimosa pudica installed on motorized wheels. Action potentials are measured by a set of electrodes placed on the branches of the plant, and are amplified by an electronic circuit that commands the motors of the structure. The response thresholds are set in such a way that the plant-machine hybrid travels in the room when someone touches its leaves (Henriques, 2011).
The protoplasm (the liquid content of each plant's cell) in highly senstitive to chemical changes, and many external stress can induce an electrical response, whether directly whether as a stimulus. Those external factors can be :
Additionnaly to local Those factors induce of course local responses but also responses in distant parts of the plant. Effects of the electrical signals The speed of propagation of the action potentials in plants depends highly on the species that is studied and on the type of stimulus that induced the signal.
Between two living organisms, a similarity can be explained by a process of convergent evolution (in that case we speak about an homoplasy) or by the existence of a common ancestor that already presented that certain feature. A question that arise here is wheter the 'similar' neural system of plants and animals can be explained as pure homoplasies or could be partially explained via their common origin. The latter hypothesis implies that the common ancestor of plants and animals, that was an unicellular organism, should already present some features that announce the apparition of a neural system. Some authors (Meyerowitz, 2002) defend the idea that neuronal rudiments could have already existed in elongated single cells with a great membrane surface.
“Plants may be more sophisticated and share more in common with animals in their non-cognitive behaviours than previously thought” (Gersani et al., 2001)
Some authors (Volkov & Brown, 2004) suggest that the precise measurement and interpretation of electrical signals in plants could allow to use plants as efficient and cheap bio-sensors. Indeed, the plants are able to detect very tiny amounts of certain volatile compounds, or subtle variations in light intensity, moisture, etc. and to convert those informations into electrical signals. By just measuring the electrical signal geneated by the plant, and then converting it back to environmental data, it would be like using the plant as a living measure tool.
note: The International Laboratory of Plant Neurobiology (LINV), in Florence, is the research group of F. Baluska and S. Mancuso, two prominent figures in plant neurobiology. A serie of publications are freely available on the website of LINV: http://www.linv.org/linv_papers.php