Hormonal sentience, first described by Robert A. Freitas Jr., describes the information processing rate in plants, which are mostly based on hormones instead of neurons like in all major animals (except sponges). Plants can to some degree communicate with each other and there are even examples of one-way-communication with animals.
Acacia trees produce tannin to defend themselves when they are grazed upon by animals. The airborne scent of the tannin is picked up by other acacia trees, which then start to produce tannin themselves as a protection from the nearby animals. When attacked by caterpillars, some plants can release chemical signals to attract parasitic wasps that attack the caterpillars.
A similar phenomenon can be found not only between plants and animals, but also between fungus and animals. There exists some sort of communication between a fungus garden and workers of the leaf-cutting ant Atta sexdens rubropilosa. If the garden is fed with plants that are poisonous for the fungus, it signals this to the ants, which then will avoid fertilizing the fungus garden with any more of the poisonous plant.The Venus flytrap, during a 1- to 20-second sensitivity interval, counts two stimuli before snapping shut on its insect prey, a processing peak of 1 bit/s. Mass is 10-100 grams, so the flytrap’s SQ is about +1. Plants generally take hours to respond to stimuli though, so vegetative SQs (Sentience Quotient) tend to cluster around -2.In theory even an organism with a hormonal system instead of a nervous system could be intelligent in some degree, but it would be an extremely slow brain, to say the least.And yet, at least higher plants are able to produce electrical signals, even if they do not use them in the same way animals do. František Baluška from the University of Bonn in Germany is one of the authorities on plant neurobiology.
Plants do not have a brain or neuronal network, but reactions within signalling pathways may provide a biochemical basis for learning and memory in addition to computation and problem solving.Controversially, the brain is used as a metaphor in plant intelligence to provide an integrated view of signalling.Plants respond to environmental stimuli by movement and changes in morphology. They communicate while actively competing for resources. In addition, plants accurately compute their circumstances, use sophisticated cost–benefit analysis and take tightly controlled actions to mitigate and control diverse environmental stressors. Plants are also capable of discriminating positive and negative experiences and of "learning" (registering memories) from their past experiences. Plants use this information to update their behaviour in order to survive present and future challenges of their environment.Plant physiology studies the role of signalling, communication, and behaviour to integrate data obtained at the genetic, molecular, biochemical, and cellular levels with the physiology, development, and behaviour of individual organisms, plant ecosystems, and evolution. The neurobiological view sees plants as information-processing organisms with rather complex processes of communication occurring throughout the individual plant organism. It studies how environmental information is gathered, processed, integrated and shared (sensory plant biology) to enable these adaptive and coordinated responses (plant behaviour); and how sensory perceptions and behavioural events are ‘remembered’ in order to allow predictions of future activities upon the basis of past experiences. Plants, it is claimed by some plant physiologists, are as sophisticated in behaviour as animals but this sophistication has been masked by the time scales of plants’ response to stimuli, many orders of magnitude slower than animals’.It has been argued that although plants are capable of adaptation, it should not be called intelligence, as plant neurobiologists are relying primarily on metaphors and analogies to argue that complex responses in plants can only be produced by intelligence."A bacterium can monitor its environment and instigate developmental processes appropriate to the prevailing circumstances, but is that intelligence? Such simple adaptation behaviour might be bacterial intelligence but is clearly not animal intelligence." However, plant intelligence fits a definition of intelligence proposed by David Stenhouse in a book about evolution and animal intelligence where he described it as "adaptively variable behaviour during the lifetime of the individual".Critics of the concept have also argued that a plant cannot have goals once it is past the development stage of plantlet because, as a modular organism, each module seeks its own survival goals and the resultant whole organism behavior is not centrally controlled. This view, however, necessarily accommodates the possibility that a tree is a collection of individually intelligent modules cooperating with, competing with and influencing each other, thus determining organism level behavior from the base up. The development into a larger organism whose modules must deal with different environmental conditions and challenges is not universal across plant species either, as smaller organisms might be subject to the same conditions across their bodies, at least, when the below and above ground parts are considered separately. Moreover, the claim that central control of development is completely absent from plants is readily falsified by apical dominance.Charles Darwin studied the movement of plants and in 1880 published a book The Power of Movement in Plants. In the book he concludes:It is hardly an exaggeration to say that the tip of the radicle thus endowed [..] acts like the brain of one of the lower animals; the brain being situated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.In philosophy, there are few studies of the implications of plant perception. Michael Marder put forth a phenomenology of plant life based on the physiology of plant perception.Paco Calvo Garzon offers a philosophical take on plant perception based on the cognitive sciences and the computational modeling of consciousness.Comparison to neurobiology:.A plant’s sensory and response system has been compared to the neurobiological processes of animals. Plant neurobiology, an unfamiliar misnomer, concerns mostly the sensory adaptive behaviour of plants and plant electrophysiology. Indian scientist J. C. Bose is credited as the first person to research and talk about neurobiology of plants. Many plant scientists and neuroscientists, however, view this as inaccurate, because plants do not have neurons.The ideas behind plant neurobiology were criticised in a 2007 article published in Trends in Plant Science by Amedeo Alpi and other scientists, including such eminent plant biologists as Gerd Jürgens, Ben Scheres, and Chris Sommerville. The breadth of fields of plant science represented by these researchers reflects the fact that the vast majority of the plant science research community reject plant neurobiology. Their main arguments are that:"Plant neurobiology does not add to our understanding of plant physiology, plant cell biology or signaling".
"There is no evidence for structures such as neurons, synapses or a brain in plants".The common occurrence of plasmodesmata in plants which "poses a problem for signaling from an electrophysiological point of view" since extensive electrical coupling would preclude the need for any cell-to-cell transport of a ‘neurotransmitter-like’ compounds.The authors call for an end to "superficial analogies and questionable extrapolations" if the concept of "plant neurobiology" is to benefit the research community.There were several responses to the criticism clarifying that the term "plant neurobiology" is a metaphor and metaphors have proved useful on several previous occasions. Plant ecophysiology describes this phenomenon.Parallels in other taxa. As described above in the case of a plant, similar mechanisms exist in a bacterial cell, a choanoflagellate, a fungal hypha, or a sponge, among the many other examples. All of these individual organisms of the respective taxa, despite being devoid of a brain or nervous system, are capable of sensing their immediate and momentary environment and responding accordingly. In the case of single-celled life, the sensory pathways are even more primitive in the sense that they take place on the surface of a single cell, as opposed to a network of many cells.
Recent surprising similarities between plant cells and neuronsPlant cells and neurons share several similarities, including non-centrosomal microtubules, motile post-Golgi organelles, separated both spatially/structurally and functionally from the Golgi apparatus and involved in vesicular endocytic recycling, as well as cell-cell adhesion domains based on the actin/myosin cytoskeleton which serve for cell-cell communication. Tip-growing plant cells such as root hairs and pollen tubes also resemble neurons extending their axons. Recently, surprising discoveries have been made with respect to the molecular basis of neurodegenerative disorders known as Hereditary Spastic Paraplegias and tip-growth of root hairs. All these advances are briefly discussed in the context of other similarities between plant cells and neurons.There are very prominent similarities between tip-growing plant cells and the extending axons of neurons. However, recent advances reveal that these visible similarities stretch beyond the tip-growing plant cells and include plant tissue cells generating action potentials3 and accomplishing vesicle trafficking and recycling, typically at actin/myosin enriched cell-cell adhesion domains resembling neuronal synapses. Moreover, plant cells and neurons are similar from the cellular perspective, when most of their microtubules and Golgi apparatus organelles are not associated with the perinuclear centrosomes.In plant cells, Golgi stacks and Trans-Golgi Networks (TGNs) are motile organelles extending through the whole plant cells. Similarly in neurons centrosome-independent cortical microtubules are abundant in axons. They transport, among other cargo, so-called Golgi Outposts—which correspond to the TGNs of plant cells toward neuronal synapses. In both plant cells and neurons, TGNs act as independent organelles separated both spatially/structurally and functionally from the Golgi apparatus.Intriguingly, similarly as in neurons, also the TGN of plant cells is the inherent part of the endosomal/vesicular recycling pathways, supporting the dynamic and communicative nature of plant synapses.Plant action potentials (electric spikes) run in an axial direction, along the longitudinal axis of any plant organ, and the highest spike activity was scored in the transition zone of the root apex in maize.Hereditary spastic paraplegia (HSP) represents a heterogeneous group of genetic neurodegenerative disorders affecting the longest neurons of the human body, extending from the brain along the spinal cord /down to the legs.21 In the HSP disorders, axons of these long neurons degenerate causing problems in controlling leg muscles. One of the major genes in which mutation results in the HSP is Atlastin. Recent study has reported that Atlastin is homologous to the RHD3 protein of Arabidopsis.RHD3 protein is essential for proper growth and development of root hairs in Arabidopsis.Moreover, RHD3 is also important for the proper arrangement of root cell files which underlies the direction of root growth.In order to maintain their ordered cell files, root apex cross-walls (plant root synapses) perform active vesicle recycling. Both Arabidopsis RHD3 and Drosophila Atlastin are important for shaping tubular ER networks.RHD3 is also known to be required for the proper arrangement of the actin cytoskeleton and cell wall maintenance via vesicle trafficking.Moreover, similarly as Atlastin in neurons,RHD3 is important for the GA morphogenesis in plant cells too Importantly, both RHD3 and Atlastin are implicated in membrane tubulation and vesiculation whereas rhd3 mutant line emerges to be less active in endocytic internalization of FM endocytic tracer.Drosophila Atlastin regulates the stability of muscle microtubules and is required for both the axonal maintenance and synapse development. All this suggest that Arabidopsis emerges as an attractive and useful model object for investigations of mechanisms underlying HSP disorders in humans.Glutamate is one of the best understood and the most widespread excitatory .neurotransmitter which is perceived via glutamate receptors at brain synapses in animals and humans. These neuronal receptors have, in fact, deep evolutionary origin in prokaryotic bacteria, and are present also in plants., Importantly, the plant glutamate receptors have all the features of neuronal ones, and glutamate induces plant action potentials., All this strongly suggest that glutamate serves in neurotransmitter-like cell-cell communication in plants too. Interestingly in this respect, especially the root apices are target of the neuronal-like activity of glutamate in plants, with effects on cell development, root growth, morphogenesis, and behavior. The transition zone cells, localized between the apical meristem and basal cell elongation zone, respond to glutamate with rapid depolarization of the plasma membrane and this response is blocked by a specific antagonist of ionotropic glutamate receptors, 2-amino-5-phosphonopentanoate.Cells of the transition zone, also known as the distal elongation zone or the basal meristem, are crucial for root primordia priming,and exogenous glutamate is known to decrease primary root growth and increase lateral root proliferation.Beta-N-methylamino-L-alanine (BMAA) is a neurotoxic amino acid, derived from cycads, which is well-known to act as agonists and antagonists of mammalian glutamate receptors. BMAA inhibits root growth, cotyledon opening, and it stimulates elongation of light-grown hypocotyls in Arabidopsis.BMAA affects growth of Arabidopsis organs at very low concentrations, and these BMAA-induced effects are reversed by the addition of glutamate.This is consistent with a scenario wherein BMAA acts to block plant-specific glutamate receptors.Similarly to glutamate, aluminium also induces very rapid plasma membrane depolarization specifically in cells of the root apex transition zone. Moreover, glutamate and aluminium both induce rapid and strong calcium spikes with unique signatures in cells of the transition zone.These root cells represent the primary target for the aluminium toxicity in plants, whereas aluminium is not toxic to root cells which have already entered the rapid elongation region.Similarly, although aluminium is not so toxic in most plant cells, neuronal-like tip-growing root hairs and pollen tubes1,2 are sensitive to aluminium similarly as are the transition zone cells. In these latter cells, aluminium is specifically internalized via endocytosis. Internalized endocytic aluminium interferes with vesicle trafficking/recycling and endocytosis, inhibiting the PIN2-driven basipetal auxin transport in the transition zone of root apices.Aluminium targets specifically the auxinsecreting plant synapses and affects the polar auxin-transport-based root cell patterning. Moreover, aluminium affects also nitric oxide (NO) production which is highest in cells of the the distal portion of the transition zone. Importantly, the rapidly elongating root cells are not sensitive towards aluminium and neither is there internalization of aluminium into rapidly elongating root cells. In support of the endocytosis of aluminium being the primary process linked to the aluminium toxicity in root cells, endocytosis of aluminium and its toxicity is lowered in the Arabidopsis mutant over-expressing the DnaJ domain protein auxillin which regulates the clathrin-based endocytosis.In animals and humans, neuronal cells are extremely sensitive towards aluminium which is internalized via endocytosis specifically in these cells. Aluminium was found to be enriched in lysosomes, similarly like Alzheimer’s amyloid β-peptide plaque depositions. These are also internalized from cell surface and aluminium was reported to inhibit their degradation.In conclusion, in both transition zone root cells and neurons, endocytosis of aluminium emerges as relevant to its high biotoxicity. In plants, the aluminium toxicity is the most important limiting factor for crop production in acid soil environments worldwide. Further studies on these cells might give us crucial clues not only for plant biology and agriculture but also for our still limited understanding of the Alzheimer disease. In line with the original proposal of Charles and Francis Darwin, root apices of plants represent neuronal/anterior pole of plant bodies