Aspects of plant intelligence (ii)
Dictionaries also contain intelligence definitions such as good mental capacity. Such qualities are limited, by communication, to one species, mankind. Because of our inability to communicate meaningfully with virtually all other species on earth, whether the other eight or more living species on earth have good mental capacities remains, unfortunately, entirely speculative.
The AI and computational expert, 55 (p. 1–19) provides a valuable but critical discussion on the early failings of Artificial intelligence (AI) as it relates to the nature of intelligence; criticizing in particular the narrow context in which early problems of intelligence were to be solved. He argues (Beer.10) that intelligence is the much broader ability to cope with the complex, dynamic unpredictable world which all life (including mankind) experiences. “Our higher cognitive functions are our own particular human elaboration of this more basic competence for effectively coping with the world” 55 (p. 11). The more “basic competence” reflects evolutionary strategies and adaptive capabilities 3 (p. 31) of all animals, plants, and microbes in natural environments. His statement mirrors earlier statements by the prominent psychologist, Anastasi, 8 who recognized that intelligence is adaptive insofar as it represents ways of meeting the demands of a changing environment. Such behavior varies with the species and the specific environments in which it lives. Again, this emphasizes the generic nature of intelligence and adaptive behavior.
There is of course, great interest in artificial intelligence, but the cell and the computer are distinctly different entities. Although there is some form of structural arrangement through membranes and organelles, the cytoplasm itself still acts as a semi-fluid structure, and the cell operates as an open system. Protein complements are changed as does their interactions. Computers, no matter how complex, still have structural defined connections and thus cannot directly mimic the nature of cells. Nobel and Noble 56 continue some useful discussion on these issues.
George Romanes was a close colleague of Charles Darwin during the last eight years of Darwin's life. The interaction is described briefly in Trewavas. 57 He wrote the first book on Animal Intelligence (1883). Romanes defined biological intelligence 58 (p. 17) as “intelligence is the faculty which is concerned in the intentional adaptation of means to ends”. The “means” references the behavioral capabilities of the organism and the “ends” refers to evolutionary benefit. I interpret his intentional adaptability as meaning adaptive. His close association with Darwin enabled him to correct Darwin's (naïve) view of intelligence. Darwin thought that if organisms behaved as he would, then that behavior is intelligent. Romanes pointed out to Darwin that there had to be profit (improved survival) in the behavior, or it was not intelligent.
Alfred Binet was a French psychologist who authored a short text on protozoan behavior and concluded that such organisms were intelligent. 59 In the early 20th century, the French government asked Alfred Binet and Theodore Simon in 1905 to construct a series of tests to help identify children with special educational needs. The original test in French quickly attracted the interest of other psychologists because they felt it more accurately assessed the higher and more complex functions of the human brain. These psychological measurements blossomed as other psychologists realized they could investigate various aspects of mental ability and thinking, such as Spearman's “g” (general mental ability) suggested in 1904 as a single underlying ability for all cognitive tasks.
Most plant scientists will be aware of some aspect of those terms identified as IQ, (intelligence quotient) since they have become the public view of intelligence. Anastasi 8 summarized IQ as simply measuring scholastic aptitude. That description, which I consider correct, indicates its limitations.
The leading psychologist, Robert Sternberg, is the most prominent contributor to the enormous psychological literature on human intelligence. He has published over a thousand articles, chapters in books, and authored and edited numerous books on the subject of human intelligence. He developed his own triarchic theory of human intelligence, involving analytical, creative, and practical aspects of human intelligence. His cognitive psychology is in its fourth edition. 60
Despite his long history on intelligence as IQ and g (Spearman's general ability), he now firmly rejects them, arguing that human intelligence is instead adaptive behavior. 7 , 61-64 “Adaptive intelligence-intelligence that is used in order to serve the purpose of biological adaptation in real world contexts”. 61 “Without adaptation, one has little or nothing resembling intelligence”. “Our latest work on adaptive competence is based on my theory of adaptive intelligence, and always has been primarily about adaptation to the environment”. The basic idea of the theory is that we can no longer afford to define intelligence merely as [Spearman's] g or IQ. Doing so has been a disaster-literally, not merely figuratively. At the same time that IQs have “benefited” from the 30-point increase of the 20th century, the world has been coping—or often failing adequately to cope—with disasters that are perhaps unprecedented in human history. These disasters include human-induced global warming, air and water pollution that are killing millions of people, income disparity that has resulted in staggering gains for the wealthiest 1% of the population at the expense of everyone else. “(https://robertjsternberg.com/successful-intelligence)”. “Theories of intelligence have failed to account for why so many people who have relatively high levels of intelligence as an ability, fail to deploy their ability especially towards positive ends.” 65 The positive ends are those that increase our species survival; the biological role of adaptive behavior.
Intelligence is an aspect of adaptive behavior, even in humans 66 (p. 1). Artificial intelligence, human intelligence, animal and plant intelligence all identify the critical adaptive capability that over aeons of time has enabled life to survive. Adaptive behavior is an irreducible property of life itself.
In the context of signal transduction, the word “cognitive” equates to “any relation established by interpretation.” 51 What it concerns is the unique, subjective, sensory worlds inhabited by different organisms and, in this context, plant environments in wilderness. “Cell signaling as a cognitive process” is the title of a review by Koseska and Bastiaens. 67 These authors consider that the function of a tissue emerges through intracellular networks that are shared between tissue cells. In this context, Koseska and Bastiaens are using the word cognitive to refer to the way in which knowledge of the environment is acquired. “On this basis, cell identity is not intrinsically predetermined by the genetic code but dynamically maintained in a cognitive manner”. For a non-neural version of cognitive signaling, see Stewart. 68 In my view, this concept is an important advance in understanding the processes of signal transduction and communication. An associated topic is phytosemiotics. That is how plants interpret environmental information and use that information to understand its meaning. Previously in this article, it was indicated that plants interpret lack of water by specific proliferation off the root system and shade conditions by increasing the size of leaves.
It is not uncommon to find diagrams of plant signal transduction events in which the known constituents are placed in a box to indicate a cell. Arrows are then drawn between the transduction constituents indicating potential connections. This diagram is a useful summary, but it can also suggest that signal transduction is a simple process. The primary problem with the diagram is that multi-omics investigations have identified the quantitative variations in transcriptomes and proteomes between individual tissue cells. This is commonly referred to as cellular noise. 69-71 The processes of signal transduction leading from [Ca2+]I or cyclic nucleotides involve at least several hundred proteins.
Cellular noise indicates that there are likely to be substantial molecular variations in some or many of these proteins between individual cells. On a cellular basis, individual protein species in a tissue can be summarized as a Gaussian distribution. 71 Transduction involves interactions between the constituent proteins. With variation in protein number, the simple rate kinetics of these interactions will also exhibit cell variation when transduction is initiated. This problem of evident variation is well exposed by. 72 Provision of supposedly exact growth room conditions and laboratory operations, identical seed origin and soil used, still failed to get reproducible results on cell numbers and growth characteristics between ten different laboratories. Transfer that information to wilderness conditions and the real problems are laid bare. Physiology tries to obviate the problem of variation by the use of statistics, sometimes implying that the mean value is the only one worth considering. Variation is the reality of life; anything that fails to emphasize that fact entirely misleads.
Explanations involve networks of varying complexity and using feedback mechanisms which help but do not eliminate the variation. 70 Noise applies to all cellular processes; not just signal transduction. The common result is individuality. 37 Individuality is the substance of selection. Plants in wilderness are metapopulations of leaves, stems and roots whose number varies widely between individual plants. As indicated by Herrera, 73 such variation enables plants to much more finely exploit the local resources than the animal with fixed numbers of limbs. Chapter 1 in Williams 38 is a valuable introduction to the problems of variation and its consequences, particularly if students are told to find equivalent examples from the plant and fungal kingdoms.
Crawford and Zambryski 74 bombarded leaves with microprojectiles containing transgenic GFP. They were thus able to assess functioning plasmodesmatal connections from the observed movement of GFP into adjacent cells. They reported considerable aperture variation. Some plasmodesmata were fully open, some fully closed, and a range of aperture openings in between these two extremes. Age and physiological conditions were two features that additionally modified the active aperture numbers. Plasmodesmata exist in varying states and cell-to-cell connectivity is heterogenous. 75 Molecular noise may be responsible and has important consequences for understanding tissue functioning.
Tee and Faulkner 75 provide a table that indicates that proteins about the size of GFP can move with relative ease between plasmodesmata if the aperture is open. Small GTP-binding proteins (GTPase), calmodulin, for example, would then likely equilibrate between adjacent cells with an open aperture. Calmodulin, when combined with its four Ca2+ ions binds to and activates several hundred proteins. 76 sRNAs and peptides should also equilibrate by diffusion between relevant cells. But a critical question surrounds the initiating molecules that are generated as a result of environmental change, [Ca2+]I , cyclic nucleotides, GTP-binding proteins, etc. Would any be shared as required for cognitive signaling?
The apparent consensus is that elevation of [Ca2+]I blocks plasmodesmatal transmission. 75 , 77-80 Concentrations of [Ca2+]I higher than 1 µM can activate gelsolin and villin that lead to fragmentation of actin cables, essential to plasmodesmatal closure control. 81-83 Callose synthase is activated slightly beyond 1 µM Ca2+ and blocks plasmodesmatal aperture. 84 Fortuitious closure can result if [Ca2+]I exceeds this critical concentration. Plasmodesmata contain calcium-binding proteins also thought to be responsible for initiating closure. 80
When single plants containing active aequorin are placed in ice-cold water, they generate immediate [Ca2+]i transients that reach a peak 1–1.2 µM [Ca2+]I from a resting level of 0.05–0.1 µ M in less than 0.1 second. The resting level is recovered in about 15 seconds. 20 We have imaged some of these Ca2+ transients in leaves and roots. 85 , 86 A cold shock stimulus was applied at a discrete but tiny part of the leaf. After a brief lag, a divided but elevated [Ca2+]I signal was recorded that progressively moved away from the inducing source and across the leaf but disappeared within 2–3 minutes. Providing a cold shock to the roots leads after a lag of about three minutes to an elevation of [Ca2+]i in the leaves.
Pavement cells respond to touch of a single cell by inducing the formation and directional movement of calcium waves. 87 The time course of the Ca2+ movement in pavement cells is very similar to Knight et al. 86 taking several minutes to traverse the leaf from a point of impact before fading away. Transmission of insect damage induces a Ca2+ wave dependent on glutamate synthesis and glutamate-dependent channels. 88 It would be useful to examine whether cyclic nucleotides likewise move between cells when synthesized is elevated.
Holdaway-Clarke et al. 77 have clarified some of the uncertainty over elevation of [Ca2+]i and aperture control. If the elevation of [Ca2+]i in the plasmodesmata is in the physiological range (below 1 µM), then plasmodesmatal closure lasts only about 10–15 seconds. By iontophoretically loading Ca2+ into a plasmodesmata, closure is limited to some 10–15 seconds. These authors suggest that the ER in the plasmodesmata may sequester physiological levels rapidly, enabling rapid recovery of opening after transient closure.
What then could interpret the apparent movement of elevated [Ca2+]I? The plasma membrane of cells is contiguous with the connecting plasmodesmatal plasma membranes. In turn, this is contiguous with that of the plasma membranes of the adjacent cell. Initiation of a Ca2+ transient in one cell is then shared through all the connecting plasmodesmatal plasma membranes and thence to adjacent connecting cells.
In animal cells that have gap junctions, elevation of [Ca2+]I in one cell seems to be shared to just a few others in the form of a simple wave. 89 , 90 In plant leaves, anywhere from 5 to 25% is vascular tissue. Bose 91 demonstrated that the phloem transmits electrical signals. These electrical changes due to cold and touch will surely impact the phloem during their progress and initiate electrical information, which can then be transferred towards the apical meristem. Light/dark variation is a primary signal altering the electrical potential characteristics of leaves. 92 Conveyance of that electrical information to the apical meristem could be the signal that results in the shade avoidance-adaptive mechanisms of increased length and diminished thickness of stem tissues. Other recent information has reported that leaves touching each other increase the stress potential of the participants. 93 , 94 Cold signals have obvious preparative effects, including abscission via the pulvinus. These are all cognitive events using signal transduction.
“The Directiveness of Organic Activities” (1945) is the title of a small book by the far-sighted biologist, E.S. Russell 14 . He used the term “directiveness” in preference to purpose. In 1945, the ‘Modern Synthesis of Evolution’ dominated evolutionary thinking rather dogmatically claimed that life was purposeless; evolutionary variation was random. Any claimed evolutionary direction was a misinterpretation; it was all down to chance. And like all such embracing ideas, it rapidly became dogmatic. For example, “The universe we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil, no good, nothing but blind pitiless indifference” 95 (p. 133). Russell's groundbreaking book has been consequently ignored.
Biological purpose, that is, goal directiveness, is easily demonstrated in plants. A flowering individual can be generated from a single somatic cell; a single protoplast; regeneration from callus; regeneration via stem, root and leaf cuttings; regeneration from damaged embryos and from individual plants damaged to varying extents by disease; via stiff competition, herbivory or from mixtures of these and of course finally from the growing seed. Some plants like Kalanchoe simply construct plantlets that drop from leaves and become individuals in their own right. 96 There are also some 50 environmental variations known to affect plants and species and from which they survive and then flower 12 (p. 70).
While the routes are multiple, the single goal, the flowering, individual, can be reached from any of the above starting points. Phenomena like these are the essence of biological purpose. Purposeful behavior has the characteristics of feedback control, but it is unlike the simple feedback controls that are familiar, such as end product inhibition of the first step of a metabolic pathway. Development itself can be accessed from many different starting positions!.
The word teleonomy has been coined to indicate biological goal-directed behavior and replaces teleology. The Nobel prize winner 97 (p. 20) makes it clear. “One of the most fundamental characteristics common to all living organisms without exception; that of being objects endowed with a purpose or project. Rather than reject this idea as certain biologists have tried to do, it must be recognized as essential to the very definition of living beings. We shall maintain that the latter are distinct from all other structures or systems present in the universe by this characteristic which we shall call teleonomy”. Goal-directed behavior is a basic characteristic of life itself. There are some interesting consequences that follow. Like survival, reproduction of some kind has always been essential to the continuation of life itself. Survival and reproduction are thus irreducible properties of life. Biological purpose is quite simply goal-directed behavior. There is no implication of conscious control in biological purpose (Vane-Wright and Corning 98 ).
A simple change in language indicates purposeful behavior. It is correct to say that a seed germinates and makes a new plant. To say instead that a seed germinates to make a new plant identifies the purposeful nature of germination. A flower opens to attract pollinators, a fruit ripens to attract animal consumers that distribute its seed; leaves open to increase light absorption; stomata close to reduce water loss; a root branches to increase access to minerals and water for uptake, etc.
Plants exhibit agency in wild circumstances; a capability exemplified by the ability of a single plant to control its own responses to environmental challenge in a purposeful, goal-directed manner. Plants are not passive organisms; they assess their present state and merge this information together with the perceived environment, their previous and present history and make judgements over the present and potentially future threats in order to accomplish survival and eventual reproduction. Agency upsets a common view of plants as passive organisms at the mercy of any environmental shift. 99
In the 19th century, Julius von Sachs was well aware that plant behavior was purposeful. “All those adaptations in the organism are purposeful which contribute to its maintenance and to its existence 100 (p. 601). “Concerning one point I should wish to anticipate: viz, the use of the word ‘Purpose’, a word which many fanatics of the theory of descent would if possible banish entirely from the language” 100 (p. 10). “By the expression this-or- that mechanism has a purpose in an organism one understands nothing more than this contributes to the ability of the organism to exist” 100 (p. 10).
The Modern Synthesis of evolution 101 assumed randomness in mutation followed by selection. Dawkin's 102 “selfish gene” concept best illustrates this issue. Purpose was rejected on the basis that it implied potential evolutionary direction. Lamarckian inheritance was rejected because it implied that the environment could directly modify the genome. Finally, the Modern Synthesis was claimed to be Darwin's view. Ideas on evolutionary mechanisms are changing. What Lamarck actually said is listed 12 (p. 211–219). Lamarck raised the issue of domestication and the multiple end points obtained by selected breeding used by Darwin. 36 He indicated how plants vary enormously in structure when grown in different environments.
Evidence summarized below indicates that change is required, particularly in our understanding of the mechanisms that result in individual variation and thus eventual selection. They do not negate the structure of evolutionary change as it has happened in the past. Kutschera 103 provides a valuable recent article describing the process of evolutionary change since life started in animals and plants.