Plant neurobiology

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Plant neurobiology sees itself as a new, interdisciplinary research area which, in connection with the question of how plants perceive and react to their environment, investigates plant communication and signal processing . This takes place on both an electrical and a molecular level. It is assumed that the anatomical structures and physiological processes involved correspond to those of animals in many ways. The terms used, as well as some of the conclusions drawn by the representatives of plant neurobiology, which extend to the postulation of “plant synapses ” and “plant intelligence”, are controversial. They are criticized or rejected by a majority of plant physiologists .

Initiators and goal

In order to be able to react to changes in environmental conditions, be it fluctuations in the supply of light, water or nutrients or threats from pests, the mostly multicellular plant body needs sensors and systems for information absorption and coordinated signal transmission from cell to cell. Since, in the opinion of some scientists, correspondences between animals and plants have not been adequately taken into account in this regard, they used the term "plant neurobiology" at the beginning of the 21st century - also with reference to studies on electrophysiological signal processing in plants that had been carried out decades ago. ) into the scientific discourse.

The initiators of this research direction and at the same time the first advocates of the term include working groups and institutes of the University of Bonn (Institute for Cellular & Molecular Botany, Working Group Cytoskeleton-Membrane Interactions) and the University of Florence (International Laboratory of Plant Neurobiology). The first international symposium on the subject of plant neurobiology in Florence in 2005 was followed by others over the next few years (the sixth in Japan in 2011). The "Society for Plant Neurobiology" was also founded in 2005 (renamed "Society of Plant Signaling and Behavior" in 2009), and in 2006 this society launched the journal "Plant Signaling & Behavior" as a specialist organ.

Plant neurobiology sees itself as an interdisciplinary research approach in which results from fields such as electrophysiology, cell biology , molecular biology and ecology converge. It deals with the question of how plants perceive their environment and respond to it holistically. The aim is also a better understanding of how the linking and processing of information regulate the metabolism and growth of plants.

Basics

Reaction of a mimosa to a mechanical stimulus
Catching sheet of the Venus flytrap (with feeler bristles)

Plant neurobiology often invokes the “root-brain” hypothesis of Charles and Francis Darwin , which was established at the end of the 19th century , according to which the root tips of plants act like the brains of lower animals. Therefore, special attention is paid to the plant root tips, which, among other things, use mechanisms for gravity perception. With emphasis on analogies between animals and plants, plant neurobiology takes the view that the root, equipped with sensory organs and the ability to eat, forms the front end of the plant, while the shoot forms the rear end. According to this, plants are, so to speak, “head” stuck in the ground.

The finding that electrical signals in the form of action potentials are not only restricted to animal cells, but can also occur in plants, also goes back to the 19th century. The English physiologist John Scott Burdon-Sanderson was the first to describe electrical signals in plants in 1873 . Initially it was assumed that this was limited to plants with a known rapid reaction to mechanical stimuli, such as the mimosa ( Mimosa pudica ), the Venus flytrap ( Dionaea muscipula ) or tendrils of climbing plants. In the 1930s, however, action potentials could also be measured in giant internodial cells from candelabrum algae . More recently, electrical activities at the cellular level have also been detected in a number of other plant species, such as cucurbits , using the patch-clamp technique , an electrophysiological measurement method.

The search for plant structures that would take on the role of animal nerves in plants also began early on. It concentrated on the vascular bundles of plants , and experimental findings suggested that electrical signals can propagate along them over longer distances. The Austrian botanist Gottlieb Haberlandt (1854–1945) and others saw the phloem as analogies to nerves. Other structures that extend across several cells were later found in plant roots or between the stalked glandular hairs and the sessile, digestive secretion-producing glands in species of the insectivorous genus butterwort ( pinguicula ). Nevertheless, the prevailing view in plant physiology was that plants basically have no nerves and that signals are mainly transmitted chemically over longer distances. This theory was reinforced by the discovery of phytohormones . Plant cells were also regarded as fundamentally unsuitable for the transmission of electrical impulses because of their physical-structural properties such as turgor or thick cell walls .

Controversial publications such as the 1973 book " The Secret Life of Plants " by Peter Tompkins and Christopher Bird , which also dealt with paranormal phenomena and ascribed emotions to plants, caused, from the point of view of plant neurobiology, that research into plant sensory functions in science was associated with a "stigma des Esoteric ”and thereby additionally hindered.

Central themes

Central topics and research focuses of plant neurobiology are in particular the complex processes of plant intercellular signal processing, which are often not understood, but which take place not only on an electrical basis, but also on a molecular level.

Electrical signal transmission

Since the findings of John Scott Burdon-Sanderson, electrical signals have been detected in numerous plant species (see above). Ion channels coupled with them and transport systems in cell membranes are also known. Plant neurobiology sees one of its central research tasks in the transfer of these results to an understanding of the electrical transmission of stimuli over long distances.

Plants apparently use two types of electrical signal transmission. Both are characterized by a temporary depolarization of the membrane potential and have a refractory period (period in which the excited cell cannot react again to a stimulus): The catch leaves of the Venus flytrap ( Dionaea ) or the water trap ( Aldrovanda ) as well as some lower plants produce omnidirectional ( action potentials running in all directions; but more common are directional action potentials (running in one direction) along the vessels in higher plants . In addition, so-called “slow wave potentials” are a second type. While action potentials follow an all-or-nothing principle , slow wave potentials can be of variable size. They follow hydraulic pressure fluctuations of the xylem along the plant axis . Plant action potentials are coupled with calcium, chloride and potassium channels in the cell membranes, whereas other mechanisms seem to be involved in slow wave potentials. When looking for the anatomical structures that take on the function of animal nerves in plants, sieve tubes , escort cells , the forisomes described in butterflies (protein complexes in phloem ) and connecting channels between cells ( plasmodesmata ) must be examined more closely. How the electrical processes observed so far are related to the diverse reactions of plants to their environment is still largely unclear.

Transmitter-like substances

Molecules were found in the plant organism that apparently play similar roles as neuroreceptors and neurotransmitters in the animal nervous system. The neurotransmitters (groups) acetylcholines , catecholamines , histamines , serotonin , dopamine , melatonin , GABA and glutamate known in animals could all be detected in plants. So far it is largely unknown whether these substances only play a role in the metabolism or also in plant stimulus processing. At least for glutamate, the discovery of corresponding receptors seems likely. In the case of GABA and acetylcholine, there is also evidence that they also function as transmitters in plants. Proponents of plant neurobiology also described intercellular fissures in plant root tissue, to which they attribute properties of animal neuronal synapses as “plant synapses”. Since plant roots are highly sensitive to the neurotoxic aluminum , which is also assigned a role in Alzheimer's disease , their neuronal properties could also contribute to a better understanding of this disease.

Meaning of auxin

In plant neurobiology, the phytohormone auxin , which has been known for a long time and which takes on a variety of tasks in plant growth and differentiation, is also a neurotransmitter-like substance. Effective cell-to-cell transport mechanisms are available for auxin that include both the intracellular space ( symplast ) and cell walls and spaces ( apoplast ). Apparently, transport through the cytoplasmic channels of the plasmodesmata is actively avoided, which favors polar transport through the apoplast. However, this transcellular transport is still poorly understood; vesicle-based processes ( endosomes ) and the involvement of special auxin transport molecules are suspected . Overall, from the point of view of plant neurobiology, the similarity between auxin and neurotransmitters in animal nerve cells is supported by the observation that extracellularly applied auxin induces rapid electrical responses in cells within a few seconds. Apparently, different mechanisms act here than in the long-term response in the context of the phytohormonal auxin effects on transcription (an important intermediate step in the “translation” of genes into proteins).

Links to behavioral science and ecology

In zoology, the concept of neurobiology is closely linked to behavioral research , but also to the behavior of entire animal communities. In a similar way, plant neurobiology also searches for the signals that coordinate and control not only the individual plant, but also entire plant communities or societies, for example with regard to influencing the microbiological communities of their root space ( rhizosphere ). Plant neurobiology regards plants as territorial "behavioral organisms" comparable to animals, which have the ability to absorb, store, divide, process and use information from the biotic (living) and abiotic (inanimate) environment . This has long been overlooked from the point of view of plant neurobiology because of the slower time and reaction processes compared to animals. The question of how plants acquire all this information and integrate it into their response behavior is an essential research focus of plant neurobiology. These also include ecological issues such as the interactions and recognition mechanisms that take place between plants of the same and different species.

Plant intelligence

The plants euros biology states that plants have at least many of the components that are also found in animal nervous systems and engages in this consequence also the question of a "plant intelligence" (plant intelligence ) on. She also refers to the Indian natural scientist Jagadish Chandra Bose (1858–1937), who concluded from observing electrical signals between plant cells as a reaction to environmental influences that they must have a nervous system, a form of intelligence as well as memory and learning . Accordingly, plant neurobiology endeavors to find correspondingly broad definitions that allow plants to be assigned intelligence. Intelligence is defined by detailed sensory perception, information processing, learning, memory, optimized development of (food) resources, self-recognition, foresight and the ability to solve problems in recurring and new situations. All of these properties also apply to plants.

Scientific controversy

In 2007, 36 scientists from 33 different institutions wrote a letter published in the journal “Trends in Plant Science” to the specialist public (with David G. Robinson , Director of the Heidelberg Institute for Plant Science, as the corresponding author). Under the title Plant neurobiology: no brain, no gain? (a play on words with the English "no pain, no gain" = "without diligence, no price") the term "plant neurobiology" was initially criticized, which hardly contributes to a better understanding of plant physiological processes. Furthermore, a number of basic assumptions about this line of research were called into question. Its advocates are particularly accused of the inadmissible transfer of similarities between animal and plant cells at the molecular level (such as the presence of action potentials or neurotransmitter-like substances) to higher functional levels (such as tissue or organs). Because of the cellular connections ( plasmodesmata ) that are common in plants, the transport and functioning of neurotransmitter-like substances do not correspond to the conditions in animal cells, and because of the inevitably close electrical coupling of plant cells, transmitter substances are not required at all. Furthermore, the auxin transport molecules known to date are regarded as sufficient for the transport of this phytohormone, and there is no need for additional vesicular transport mechanisms, which have not yet been documented. If vesicular transport from cell to cell actually takes place in plants, this should by no means be equated with processes in animal nerves and synapses. Under the charge of superficial analogy, it is emphasized that there is no evidence that plants actually have neurons, synapses or even a brain. The breadth of the institutions represented in the letter (from Italy, Germany, Switzerland, Canada, USA, France, Great Britain and the Netherlands) makes it clear that the concept of plant neurobiology is rejected by a majority in specialist circles.

In response to this criticism, the other scientific side emphasized, among other things, that the terms they used were metaphors . These have already proven useful in previous cases and would generally direct research interest to new types of questions. In addition to Darwin's “root-brain” hypothesis (ie the assumption that root tips behave as if they had a brain), reference was also made to Nobel Prize winner Barbara McClintock and her metaphor of “genetic intelligence”. Ultimately, it is less about terms than about phenomena that botany has so far overlooked.

In 2009 the prestigious journal Proceedings of the National Academy of Sciences published an article written by, among others, employees of the International Laboratory of Plant Neurobiology at the University of Florence. According to this, time-synchronized, spontaneous electrical oscillations were measured in the tips of maize roots . According to the authors, this observation is a clear sign that the plant root tip functions as a sensor area for various sensory perceptions from the environment. The plants electrophysiologist Dietrich degree man ( University of Göttingen , emeritus) threw the authors then methodically experimental shortcomings and misinterpretations of artifacts , and the editors of the journal shortcomings in the review process before; the article must be withdrawn.

The controversies in connection with plant neurobiology and the associated terminology were also taken up in works with a philosophical-pragmatic focus. The philosopher Günther Witzany sees the discussions about the term “plant neurobiology” and the question of whether plants can be assigned intelligence as necessary from the perspective of biocommunication . Others are critical of the terminology of plant neurobiology with its “zoomorphic charge”, since it blurs the fundamental difference between plants as autotrophic creatures and heterotrophic animals. This raises the question of the point of making a plant “an animal” in order to give it a special identity.

The theses and results of the representatives of plant neurobiology were also echoed in popular scientific media, although here too they were adopted by no means uncritically.

literature

  • František Baluška (Ed.): Plant-Environment Interactions. Signaling and Communication in Plants. Springer, Berlin 2009, ISBN 978-3-540-89229-8 .
  • František Baluška, Stefano Mancuso (eds.): Signaling in Plants . Springer, Berlin 2009, ISBN 978-3-540-89227-4 .
  • Eric D. Brenner, Rainer Stahlberg and others: Plant neurobiology: an integrated view of plant signaling. In: Trends in Plant Science. 11 (8), 2006, pp. 413-419.
  • Rainer Stahlberg: Historical Overview on Plant Neurobiology . In: Plant Signaling & Behavior. 1 (1), 2006, pp. 6-8.
  • Peter W. Barlow: Reflections on 'plant neurobiology' . In: BioSystems. 99, 2008, pp. 132-147.
  • Daniel Chamovitz: What plants know: How they see, smell and remember. Munich: Hanser, 2013. ISBN 978-3446435018
  • Stefano Mancuso , Alessandra Viola: The intelligence of plants . Munich: Kunstmann, 2015. ISBN 3956140303 .

Individual evidence

  1. ↑ Laboratorio Internazionale di Neurobiologia Vegetale - LINV (International Laboratory of Plant Neurobiology), University of Florence ( Memento of the original of August 28, 2011 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2Template: Webachiv / IABot / www.linv.org
  2. ^ Institute for Cellular & Molecular Botany (IZMB), Working Group Cytoskeleton-Membrane Interactions, University of Bonn .
  3. ^ First Symposium on Plant Neurobiology .
  4. ↑ Announcement 6th International Symposium on Plant Neurobiology  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. .@1@ 2Template: Dead Link / my.aspb.org  
  5. ^ Website Society of Plant Signaling and Behavior ( Memento of the original from October 28, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2Template: Webachiv / IABot / www.plantbehavior.org
  6. ^ Website of the Plant Signaling & Behavior magazine .
  7. ↑ František Baluška, Andrej Hlavacka et al. (2006): Neurobiological view of plants and their body plan , p. 28. In: Communication in Plants: Neuronal Aspects of Plant Life , Ed .: František Baluška, Stefano Mancuso et al., Springer, Berlin, 2006, pp. 19-35.
  8. ↑ František Baluška (Ed.): Plant-Environment Interactions. Signaling and Communication in Plants , Springer, Berlin, 2009, ISBN 978-3-540-89229-8 , p. 257.
  9. ^ Charles Darwin: The Power of Movements in Plants , John Murry, London, 1880, p. 573.
  10. ^ Robyn M. Perrin, Li-Sen Young et al: Gravity Signal Transduction in Primary Roots . Annals of Botany, 96 (5), 2005, pp. 737-743.
  11. ↑ František Baluška, Stefano Mancuso and others: The 'root-brain' hypothesis of Charles and Francis Darwin . Plant Signaling & Behavior, 4 (12), 2009, pp. 1121-1127, PMC 2819436 (free full text).
  12. ^ John Scott Burdon-Sanderson: Note on the electrical phenomena which accompany stimulation of the leaf of Dionea muscipula . Proceedings of the Royal Society London, 21, 1873, pp. 495-496
  13. ↑ Randy Wayne: The excitability of plant cells: With a special emphasis on Characeae internode cells . Bot Rev., 60, 1994, pp. 265-367.
  14. ↑ a b c d e f Eric D. Brenner, Rainer Stahlberg et al: Plant neurobiology: an integrated view of plant signaling. Trends in Plant Science, 11 (8), 2006, pp. 413-419; PMID 16843034 ; PDF  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. .@1@ 2Template: Toter Link / senaleselectricas.110mb.com  
  15. ↑ Gottlieb Haberlandt: The stimulatory tissue system of the sensory plant . Engelmann-Verl., 1890.
  16. ^ Rainer Stahlberg: Historical Overview on Plant Neurobiology . Plant Signaling & Behavior, 1 (1), 2006, pp. 6-8, PMC 2633693 (free full text).
  17. ↑ Peter Tompkins, Christopher Bird: The Secret Life of Plants , Harper & Row, 1973; German: "The secret life of plants", Fischer Taschenbuch Verlag, Frankfurt, 1977, ISBN 3-596-21977-9 .
  18. ↑ František Baluška (Ed.): Plant-Environment Interactions. Signaling and Communication in Plants . Springer, Berlin, 2009, ISBN 978-3-540-89229-8 , p. 285.
  19. ↑ František Baluska, Dieter Volkmann, Diedrik Menzel: Plant synapses: actin-based domains for cell-to-cell communication. Trends in Plant Science, 10 (3), 2005, pp. 106–111, online (PDF; 157 kB).
  20. ↑ František Baluska: Recent surprising similarities between plant cells and neurons . Plant Signaling & Behavior, 5 (2), 2010, pp. 87-89, PMC 2884105 (free full text).
  21. ↑ Information on the LINV website ( Memento of the original from November 21, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2Template: Webachiv / IABot / www.linv.org
  22. ^ Anthony Trewavas: Aspects of plant intelligence . Annals of Botany 92, 2003, pp. 1-20.
  23. ^ Jagadish Chandra Bose: Plant Response as a Means of Physiological Investigation , Longman, Green & Co., London a. a., 1906.
  24. ^ Anthony Trewavas: Plant intelligence . Naturwissenschaften, 92 (9), 2005, pp. 401-413.
  25. ↑ Amedeo Alpi, Nikolaus Amrhein and others: Plant neurobiology: no brain, no gain? , Trends in Plant Science, 12 (4), 2007, pp. 135-136, online (PDF; 80 kB).
  26. ^ Anthony Trewavas: Response to Alpi et al.: Plant neurobiology - all metaphors have value . Trends in Plant Science, 12 (6), 2007, pp. 231–233, online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 92 kB).@1@ 2Template: Toter Link / ecobio.univ-rennes1.fr  
  27. ↑ Eric D. Brenner, Rainer Stahlberg et al: Response to Alpi et al.: Plant neurobiology: the gain is more than the name , Trends in Plant Science, 12 (7), pp. 285–286, online (PDF; 83 kB).
  28. ↑ E. Masi, M. Ciszak et al .: Spatiotemporal dynamics of the electrical network activity in the root apex , Proceedings of the National Academy of Sciences, 106 (10), 2009, pp. 4048-4053, online (PDF; 878 kB) .
  29. ^ Hubert Rehm, Dietrich Gradmann: Intelligent Plants or Stupid Investigations? , Laborjournal 2010 / 1–2, pp. 20–23, online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 386 kB).@1@ 2Template: Dead Link / www.plant-biotech.net  
  30. ↑ Günther Witzany: Biocommunication and Natural genome editing . Springer, Dordrecht a. a., 2010, ISBN 978-90-481-3318-5 , pp. 42-44.
  31. ↑ Sabine Odparlik, Peter Kunzmann, Nikolaus Knoepffler (ed.): How dignity flourishes. Plants in bioethics . Herbert Utz Verlag, Munich, 2008, ISBN 978-3-8316-0818-8 , pp. 26-27.
  32. ^ Bernhard Epping: The obscure brain of plants , Bild der Wissenschaft 11/2009, pp. 30–33, online (PDF; 849 kB).
  33. ↑ scinexx: Marcus Anhäuser: The metaphor dispute - How legitimate are terms from the animal kingdom? , Springer, 2008, Heidelberg.
  34. ↑ Peggy Freede: Blattgeflüster , Spektrum direkt / Wissenschaft-online.de, 2010, online .

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