Although animal neuroscience is an established and accepted fact, the neurobiology of plants remains controversial despite the fact that electrical signaling in plants was described by M.L. Berthelon in De l’Electricité des Végétaux (Aylon, Paris) 1783, eight years before the first reference of animal electrical signaling by L. Galvani in 1791. This is likely because plant responses to environmental stimuli are significantly (1000 to 100,000 times based on measured refractory periods for action potentials (APs) in Lupinus shoots by Adam Paszewski and Tadeusz Zawadzki, Action Potentials in Lupinus angustifolius L. Shoots (Maria Curie-Sklodowska University, Lublin, Poland 1976)) slower than those in animals (with the exception of a few – the touch-sensitive mimosa (Mimosa pudica) and Venus flytrap (Dionaea muscipula) that require speed to close their leaves and shut their traps since in general, plants do not require the speed of animals to escape predators or capture prey) and because of flawed views that persisted until recently that plants are helpless, passive organisms at the mercy of their environment with little need for rapid signaling.
In reality, plants possess neurobiology analogous to cnidarian nerve nets, in which the existence of a brain or central nervous system is not a prerequisite. This should not be surprising when considering the identical nature between plants and animals as pointed out by Frantisek Baluska, Dieter Volkmann, Andrej Hlavacka, Stefano Mancuso and Peter W. Barlow in Neurobiological View of Plants and Their Body Plan (Communication in Plants, Springer-Verlag Berlin Heidelberg 2006) in that both rely on identical sexual processes utilizing fusion between sperm cells and oocytes (female egg cells), both develop immunity when attacked by pathogens, and both use the same methods and means to drive their circadian rhythms (patterns of biological activity synchronized to day-night cycles). In addition, plants and animals transmit electrical signals over both short and long distances and rely on the same pathways and molecules to control their physiological responses (e.g. movement in animals, growth in plants).
Cnidarians and Plants: Convergent Neurobiology
Plants and cnidarians (e.g. anemones, hydra, jellyfish) have analogous nervous systems, in which stimuli is communicated via a nerve network or web of interconnecting neurons. Neither have a brain (though some theories postulate that root apices may serve as a brain in plants) or central nervous system in the context of advanced animal life. Consistent with plant neurobiology, in which a network of electrical and chemical signaling is used to detect and respond to environmental stimuli (biotic and abiotic), cnidarians do not feel pain per se; they merely react to stimuli.
Cnidaria (a phylum of over 9000 simple aquatic animals) rely on decentralized nerve nets consisting of sensory neurons that generate signals in response to stimuli, motor neurons that instruct muscles to contract and “cobwebs” of intermediate neurons. Hydras rely on a structurally simple nerve net to bridge sensory photoreceptors and touch-sensitive nerve cells located on their body wall and tentacles. Jellyfish also depend on a loose network of nerves located within their epidermal and gastrodermal tissue (outer and inner body walls, respectively) to detect touch and a circular ring throughout the rhopalial lappet located at the rim of their body. Intercellular communication occurs in cnidaria through electronic signaling via synapses or small gaps across which electro-chemicals (called neurotransmitters) flow.
Cnidarian nerves (unlike those in advanced species) rely on neurotransmitters on both sides of their synapses enabling bi-directional action potential (AP) transmission. Cnidarian neurons communicate with all other neurons wherever they cross with such communication utilizing at least three specific pathways without preference. Basically, in cnidaria, stimuli at any point results in an impulse that radiates away in every direction providing optimal intercellular communication throughout the organism.
In both plants and cnidaria, electrical signals are transmitted through non-nerve tissues, from cell to cell through utilization of gap junctions. These gap junctions in a plant’s cell wall are called plasmodesmata.
Consistent with cnidaria, plants rely on action potentials (AP) and synaptic intercellular communication utilizing auxin as their primary neurotransmitter with vascular strands representing nerves. Like cnidarians, plants rely on electrical signaling and developed pathways (phloem and sieve tubes in vascular plants; non-phloem tissue in non-vascular plants such as algae and liverworts) analogous to a nerve net “to respond rapidly to environmental stress factors (e.g. insect herbivory, pathogens, mechanical damage, etc.)” and environmental conditions (e.g. changes in temperature, light intensity, water availability, osmotic pressure, and the presence of chemical compounds). Through electrical signaling, plants “are able to rapidly transmit information over long distances… at the tissue and whole plant levels from leaves to roots and shoots and vice versa through utilization of ion channels.”
Evidence of Plant Neurobiological Processes:
1. Voltage levels change when mimosa leaves are touched (causing them to close) and the hairs of a Venus flytrap are triggered (causing the trap to snap shut). Scientists have measured action potentials (APs) in both plants dating back to the 1870s. Furthermore based on studies involving electrical stimuli applied between the midrib and lobe of a Venus flytrap’s trap (which results in closing without the need to stimulate the trigger hairs), Dionaea muscipula have demonstrated three stages of electrical responses – stimulus perception, electrical signal transmission, and induction of response – and the existence of a short-term electrical memory (in that the trap does not close if repeated sub-threshold charges are applied).
2. Wounding in one part of a plant initiates a response elsewhere, which is not confined to the injury site nor duration of the initial wounding.
3. Electrical activity consistent with that in cnidaria occurs in the shoots of tomato plants when their leaves are crushed. Such signals are strongest in the phloem, which carries nutrients from the leaves to the roots and triggers the release of proteinase, which serves as a defense mechanism against insect herbivory.
4. Plants generate electrical signals in response to water and light/darkness conditions based on the results of studies involving fruit trees (namely, avocado, blueberry, lemon, and olive) in which electrical potential (EP) fluctuates between light and dark periods, decreases as water stress increases and increases as the transpiration rate rises per Luis A. Gurovich and Paulo Hermosilla, Electric signaling in fruit trees in response to water applications and light-darkness conditions (Journal of Plant Physiology 166, Elsevier, 2009). In addition, when a young leaf is exposed to light, such light elicits a release of action potentials (APs) that unleash a sequence of metabolic events that lead to growth.
5. Per Carol Kaesuk Yoon, Plants Found to Send Nerve-Like Messages (The New York Times, November 17, 1992) when electrical signals were permitted to flow freely from a caterpillar damaged tomato plant leaf, unaffected leaves initiated chemical defense mechanisms; when such electrical signals were blocked, no such defense response was initiated; and when movement of hormones was blocked, unaffected leaves still initiated defense mechanisms proving that electrical rather than chemical signals activate a plant’s defense mechanisms.
6. Plants produce glutamate, GABA dopamine, and serotonin – substances associated with animal neurotransmitters even though not much is known about their function in plants and auxin appears to be their primary neurotransmitter.
Types of Electrical Signaling in Plants:
Plants rely on three known types of electrical signals – Action Potentials (AP), Variation Potentials (VP), and System Potentials (SP) to carry out critical life functions (e.g. respiration, photosynthesis, water uptake, etc.) and to respond to environmental stimuli and stresses. All three types of electrical signals travel intracellularly (within a cell) and extra/inter-cellularly or apoplastically (outside or between cells) and per Matthias R. Zimmermann et al System Potentials, a Novel Electrical Long-Distance Apoplastic Signal in Plants, Induced by Wounding, “may act as forerunners of slower traveling chemical signals” while serving as a back up to each other and overlapping in some instances.
APs are rapid “all or nothing” self-propagating electrical signals that occur in both plants and animals characterized (based on Eric Davies, Electrical Signals in Plants: Facts and Hypotheses (Plant Electrophysiology – Theory & Methods, Springer-Verlag Berlin Heidelberg, 2006)) by “a sharp rise, brief peak, and sharp return to near baseline.” They travel at a constant speed and amplitude regardless of distance when stimuli reach a certain threshold (consistent with AP activity in cnidaria) that triggers membrane depolarization. Based on Electrical signals and their physiological significance in plants (Plant, Cell and Environment, Blackwell Publishing Ltd. 2007) by Jörg Fromm and Silke Lautner, “an AP can propagate over short distances through plasmodesmata, and after it has reached the sieve element/companion cell complex, it can travel over long distances along the sieve element plasma membrane (in the phloem, which extends throughout a vascular plant) in both directions.” APs travel at a speed of between 20-400 cm per minute. The duration of a typical AP is less than 20 seconds. Once an AP passes, a period of delay called a refractory period follows in which an additional AP cannot be generated.
VPs are slower non-self-propagating electrical signals that are elicited by stimuli that trigger a change in potential (depolarization and subsequent repolarization) at the plasma membrane of parenchyma cells that reside adjacent to xylem vessels due to a rapid loss of turgor. They are characterized (based on Eric Davies, Electrical Signals in Plants: Facts and Hypotheses (Plant Electrophysiology) by “a sharp rise followed by a lingering decline, often with spikes.” Unlike APs, membrane repolarization is delayed and the signals are graded and travel at varying amplitudes (which are reduced as the distance increases) based on intensity of the stimulus. VPs also utilize the sieve element plasma membrane and plasmodesmata. VPs travel at a speed of between Electrical Signals in Plants: Facts and Hypotheses APs are generally caused by non-damaging stimuli such as “electrical stimulation, light/dark transitions, brief cooling, pollination [and sometimes] excision” and VPs are caused by damaging stimuli such as “severe wounding, [organ excision, and flaming].” SPs are generally caused by cutting and cations.
Physiological Effects of Plant Electrical Signaling:
Primarily based on Jörg Fromm and Silke Lautner, Electrical signals and their physiological significance in plants (Plant, Cell and Environment, Blackwell Publishing Ltd. 2007)
1. Mechanical (Stimulus) – AP (Signal) – Venus Fly Trap (Dionaea) – Trap Closure; Release of Digestive Enzymes (Physiological Effects)
2. Mechanical (Stimulus) – AP (Signal) – Sundew (Drosera) – Tentacle movement to trap an insect (Physiological Effect)
3. Cold Shock, Mechanical (Stimuli) – AP (Signal) – The Sensitive Plant (Mimosa) – Leaf Movement (Physiological Effect)
4. Electrical (Stimulus) – AP (Signal) – Muskgrass (Chara) – Cessation of cytoplasmic streaming (Physiological Effect)
5. Electrical (Stimulus) – AP (Signal) – Liverwort (Conocephalum) – Increase in Respiration (Physiological Effect)
6. Pollination (Stimulus) – AP (Signal) – Flowering Shrubs (Incarvilea, Hibiscus) – Increase in Respiration (Physiological Effect)
7. Cold Shock (Stimulus) – AP (Signal) – Maize (Zea) – Reduction in Phloem transport (Physiological Effect)
8. Re-Irrigation (Stimulus) – AP (Signal) – Maize (Zea) – Reduction in Phloem Transport (Physiological Effect)
9. Cooling, Electrical (Stimuli) – AP (Signal) – Sponge Gourd (Luffa) – Decrease of Stem elongation growth (Physiological Effect)
10. Electrical (Stimulus) – AP (Signal) – Tomato (Lycopersicon) – Induction of proteinase inhibitor2 (pin2) gene expression (Physiological Effect)
11. Heating (Stimulus) – VP (Signal) – The Sensitive Plant (Mimosa) and Poplar (Populus) – Transient Reduction in Photosynthesis (Physiological Effect)
12. Heating (Stimulus) – VP (Signal) – Bean plants (Vicia) – Increase in Respiration (Physiological Effect)
13. Heating (Stimulus) – VP (Signal) – Nightshade (Solanum) – Induction of jasmonic acid biosynthesis and pin2 gene expression (Physiological Effects)
14. Wounding (Stimulus) – VP (Signal) – Garden Pea (Pisum) – Inhibition of Protein Synthesis, Formation of Polysomes (Physiological Effects)
15. Insect Herbivory (Stimulus) – SP (Signal) – Barley (Hordeum vulgare), Field Bean (Vicia faba), Maize (Zea), Tobacco (Nicotiana tabacum) – Activation of Chemical Defense Mechanisms (Physiological Effects)
Plants, through the use of neurobiology analogous to that of cnidaria, in which the presence of a brain and central nervous system are not prerequisite for interpretation and response, have proven their ability to respond intelligently and rapidly to complex environmental stimuli. Although plants and cnidaria do not feel pain per se as advanced animal life, both, because of their respective neurobiology are not “unfeeling” and can sense, perceive and respond accordingly. Electrical signals in plants and cnidaria provide an efficient means to rapidly transmit information systemically, detect and respond to danger (whether predators – insect herbivory with regard to plants, or pathogens) and other environmental stresses. In short, with their capacity to sense and perceive, plants are not passive, unfeeling organisms as defined by flawed paradigms.