Aspects of plant intelligence (ii)
This general definition is designed to highlight the fundamental requirements of any organism that lives and learns in variable and uncertain circumstances that both provide its sources of energy and contain threats that can eliminate the individual. That fundamental process is the acquisition and processing of environmental information that, when interpreted (to update the organism)can enable avoidance of threat, construction of resistance, or perception of mates. Learning is fundamental for all organisms like plants that live in the wilderness. Life is about survival, as indicated earlier in this article. Nor does the definition specify the precise mechanisms used by different members of the 10 million or so species on this planet because that would defeat its intention of indicating what exactly is fundamental about the learning process.
Lipid rafts offer some very specific possibilities for the first structural steps of learning through [Ca2+]I. 162 Virtually all plant environmental signals initiate [Ca2+]i transients. It is highly likely that there is a complex structural organization on this lipid raft platform that transfers information through the first few steps of transduction and learning. The following are potential constituents of this transduction structure.
A tissue- or cell-specific calcium channel. 163 Plasma membrane-located calmodulin. 164 A specific CDPK(s), Asi et al. 165 Phospholipase C. 166 A receptor-like protein kinase (RLPK) that acts upstream of MAP kinase cascades 167 and phosphorylates changes in MAP kinase cascades that lead to ERK 2. 168 (ERK2 enters the nucleus initiating changes in gene expression, a common response to calcium signals). A plasma membrane-bound Ca2+ATPase, activated by calmodulin. 169 PIP2 is highly negatively charged and located on the inner plasma membrane leaflet. It is electrostatically attracted to membrane regions where Ca2+ (positive charge) enters. PIP2 accumulates briefly, attracting proteins that act as initiation sites for actin polymerization (the actin refit) before being hydrolyzed by phospholipase C to IP3 that mobilizes Ca2+ from the endoplasmic reticulum.
Calcium waves 85-88 are common results of cellular activation of [Ca2+]I. They were discussed briefly in “Cell signaling as a cognitive process” and represent the continuation of information transfer leading to tissue learning.
MEMORY. “Memory can be defined as the recording of experiences that can modify behavior. This general definition encompasses numerous phenomena from the process by which bacteria sense chemical gradients to cognitive learning in humans.” 170 Another definition of memory, equally general, is the persistence of cellular behaviors after disappearance of their initial cause. 171 Learning and memory are also discussed in 12 (p. 211–216).
The following indicate five, possibly six different categories of plant memory with illustrated examples only. A complete compilation would necessitate a book. Some, or even all, maybe tissue specific.
Hebbian-style constructed memory. Hebb 172 suggested memories in nervous systems could be formed by increasing the strength of synaptic interactions between neurons that fire together. Such an integration would greatly increase the rate of information flow through this part of the nervous system. Retention of this neural pathway acts as a memory or engram too.
In molecular terms, an equivalent suggestion is to increase, by synthesis, the numbers of rate- constraining proteins and enzymes in a transduction pathway. The consequence would be either a more rapid or a greater transfer of information to the response concerning stressful conditions. 16 The length of memory in this case would be determined by the stability of the synthesized proteins. The expression of six proteins (CaM, PLC, Ca2+-ATPase, CPK, CaN, RLPK) were increased by an estimated order of magnitude from signals such, as touch, osmotic stress, salt stress, low temperature. 17 More recently, a number of members of the MAPK pathway can be added to the list; receptor protein kinase, MKK1, MEKK1, MKK2, MPK4, MPK6PK19, and two CDPK's and PK19. 173-178
Whether these proteins are expressed in the same tissues has not been determined, but the data are highly suggestive of a Hebbian-type learning situation. Out of 1000 protein kinases, about 100 are found in the plant MAPK complex. MAPK cascades, crucial to plant memory and resulting in chromatin modification, are reviewed in Sun and Zhang. 179 A diversity in relaxation time scales of MAPKs can create an emergent transduction memory as indicated above. 180 Persistent adaptive dynamics in cascades can arise from the diversity of turnover timescales.
Prions. Prion proteins provide an unusual method of constructing memory. 181 A conformational change in the prion mother protein enables it to initiate similar changes in other daughter prion proteins that control processes elsewhere in the cell. The presence of prions can have substantial phenotypic consequences after they are transferred to other cells via mitoses. The changes may be beneficial, benign, or detrimental and could contribute to initiating evolutionary changes. Garai et al. 182 uncovered a network of prion-like proteins in rice that were both differentially synthesized and regulated in both stress and developmental changes. This mechanism might be responsible for linking stress and memory pathways constructed by genome changes. Interactions with the chaperone Hsp 90 could increase the range of variation. 183 Prions in animal brains do switch on long term memory by initiating the synthesis of specific proteins (CPEB 3) and this suggests an analogous potential for prion-driven memory in plants.
Changes in chromatin topology and architecture construct long term stress resistance memories initiated via MAPK and cytosolic Ca2+.
Signal transduction systems in both plant and animal cells share the fundamental mechanisms. In both organism groups, these create networks that are highly cross-linked networks of interacting protein species based around calcium, cyclic nucleotides, MAPK pathways, and other interacting protein kinases. Emergent properties will surely be a consequence and different between animals and plants. Virtually every stress that affects plants generates transients of cellular calcium. 184 Slight inhibition of the Ca2+transient can abolish transcription of mRNAs for proteins specific to the stress. 19 , 20 Increasing [Ca2+]I activates the cross-linked MAPK pathway too (Cargnello and Roux), 185 possibly via CDPKs causing Raf kinase dimerization. 186 MAPKK acts as a cytoplasmic anchor of MAPK, but phosphorylation of MAPK by MAPKK activates ERK2 (extracellular signal-regulated kinase) that moves into the nucleus (Lavoie et al). 168 Chromatin architecture is remodeled from changes in PRC2 (polycomb repressor complex), phosphorylation of RNA polymerase, and modification of histone H3. 187
Memory of stressful conditions in plants via chromatin architectural change has now been frequently reported. 21 , 188 , 189 Although originally identified as priming, the mechanism used is similar to those in animal cells, including nerve cells (e.g. 190 , 191 ). Touch and wind are mechanical signals which elevate cytosolic and nuclear calcium in plant cells. 20 Recent data show that the consequent mechanical deformation of the nucleus modifies both nuclear and cytoplasmic behavior. 148
There are a number of interesting examples that suggest electrical memory may be the basis of the observations. Viner et al. 192 investigated leaf unrolling in dark-grown barley leaves. Initiation of leaf unrolling is normally instigated by the generation of 5 minutes of Pfr. EGTA inhibits unrolling, but addition of Ca2+ can initiate unrolling only if given within one hour of the generation of Pfr. Another red-light signal, together with added Ca, given four hours after the initial red light, immediately initiates unrolling. Pfr normally institutes changes in membrane potential and ion channel activity.
For tendril coiling in Pisum, two signals are required: touch and blue light. If the tendril is touched in darkness, the blue light irradiation can be delayed for up to two hours. 193 Touch normally induces changes in membrane potential and entry of Ca2+ with potential secondary influences. Which blue light receptors are needed is not known, but much early work identified changes in membrane potential and redox activity implying electron flow.
A hypo-osmotic shock to cultured cells induces a calcium transient. If extracellular calcium is absent, the transient is not generated but can appear if extracellular calcium is added within 20 minutes of the initial shock. 194 All these events need further research to better understand them and see if they are electrically based.
Volkovet al. 195 indicate electrical memory in the Venus flytrap, a trap which is sprung by electrical and [Ca2=]i changes and has an accumulated memory requiring five touches of the trap hair to initiate digestion and only two to close the trap. 196
In the introduction to this article, it was indicated that Bhalla and Iyengar 22 reported that memory was one emergent property of signal transduction. Another article has identified an emergent memory in cell signaling but more clearly specified Mitra et al. 180 These authors concentrated on the MAP kinase pathway and reported an abundant repertoire of transient adaptive responses with long-lived modulations in frequency. On withdrawing the stimulus to the pathway, the pathway decays only over long-time scales. The MAP kinase cascade retains a memory as well as identifying its central role in information processing by animal and plant cells.
Very brief exposure of dark-grown coleoptiles to a directional blue light stimulus can be overridden by a blue light stimulus from the opposite side, provided it is given in less than an hour after the first. A memory is instituted by the first signal in the form of a labile precursor. A memory is instituted by the first signal in the form of a labile precursor. This memory is truncated by the formation of microtubules oriented in a longitudinal direction after 60 minutes in cells of the outer epidermis. 197 , 198 But if the second irradiation is delayed until 90 minutes or later, the initial irradiation is no longer overridden. Microtubule formation is likely initiated and controlled by spatially-localized generation of cyclic nucleotides (along with an equally located phosphodiesterase and gamma tubulin acting as a microtubule nucleation site Kiwiatowski et al. 199 ).
One polarizing signal impacting on the Fucus zygote are the near presence of other seaweeds. Jaffe 200 reported that the seaweed signal is not blocked by an intervening glass slide. He concluded that luminescent light, biophotons, was the polarizing signal. Albrecht-Buehler 201 reported that a culture of cells whose axes were oriented by infrared light when placed behind a glass screen, oriented others whose initial axes were random. Metallizing the glass screen prevented communication. He described it as primitive vision from infrared radiation emitted by the axes-oriented cells.
Biophotons or ultra-weak photon emission (UWPE) are usually characterized as photon fluxes below 104 photons cm−2 .s−1. 202 All living organisms have been found to emit them, they may, in part, result from cell oxidative reactions and increase in amount under stressful conditions. The introduction of photomultipliers has demonstrated the universal nature of UWPE. 203 The critical result of Jaffe's 200 paper is not just the conclusion of biophotons synthesized by other seaweeds but that they initiated a biological response in Fucus development. Does this perception require a specific receptor?
UWPE has a broad spectrum of visible light, but simple calculation indicates its intensity is well below the fluence response of the most highly sensitive blue light-initiated phototropism in bean. 204 But it is approximately within the range of the very low fluence response of phytochrome in lettuce seed. 205 But red light does not polarize the Fucus zygote, Jaffe. 206 Jaffe 200 considered the seaweed biophotons to be concentrated in the red part of the spectrum. A common theme in the literature on biophotons is an association of UWPE with microtubule activities and function, which is a later part of axis formation. 200 , 203 Jaffe exposed his Fucus zygotes continuously to UWPE and given the very low density of UWPE, this suggests the need for a cumulative memory storing information within the developmental window that finally initiates the program leading to the first asymmetric division.
Light is of course electromagnetic, but visible light covers only one narrow range of electromagnetic radiation. In the past, many different electrical wavelengths have been shown to be both generated and sensed by cells and organisms. 203 The Book “Fields of the Cells” edited by Fels, Cifra, and Scholkmann, 207 is free to download on the web and contains 16 chapters that cover many aspects of this large research area.
Investigations of electromagnetic signal synthesis, transduction, memory, and response of radiation beyond the visible are simply absent in plants. But two papers and a book by 208 , 209 indicate the potential in this area. It is commonly supposed that morphogenesis of new developing tissues requires the translocation of specifically synthesized information to the correct place and at the correct time and within a developmental window. As Pietack points out, a dynamic system in a self-developing organ can self-trap electromagnetic energy that acts as a symmetry-breaking mechanism informing on cell position, polarization, orientation, genetic expression, and differentiation state. Pietack 208 , 209 demonstrates this beautifully for the construction of an electromagnetic field that mimics the eventual distribution of cell types and internal structure in the formation of the cucurbit ovary and for vascular tissue distribution in differently-shaped leaves. The 2009 paper is expanded in Chapter 5 of her book Pietack. 210 These patterns develop amongst a mass of undifferentiated dividing cells, but the field pattern is determined by the developing shape of the organ.
I have indicated five examples of memory. Memory can only be acquired if it is learnt in some way. The use of these plant memories by plants needs investigation to understand how widespread their memory basis is. Memory three, chromatin changes, has received some characterization, but the detailed mechanisms that lead to modifications of chromatin structure need proper characterization in plant systems. Plant memories are currently an open field for investigation. They need better characterization, not only for plants in many environmental circumstances but for crop development too. The full panoply of memory and learning should surely be used by plants in wilderness conditions. Investigations of that potential would require taking molecular investigations into wilderness. Adaptive responses acting as intelligence behavior would then receive much greater understanding. While recognizing the difficulties of so doing, the rewards in real understanding would surely be legion. The laboratory might be convenient at present, but concentration on taking molecular studies into natural circumstances is the logical way forward. There will be abundant new and exciting times to come if this particular road is followed.
No grant supported this article. I am grateful to the suggestions made by five referees. They have greatly improved the quality of the manuscript.