Induction and ionic basis of slow wave potentials in seedlings of Pisum sativum L

Electrical Signaling Beyond Neurons

Neural action potentials (APs) are difficult to interpret as signal encoders and/or computational primitives. Their relationships with stimuli and behaviors are obscured by the staggering complexity of nervous systems themselves. We can reduce this complexity by observing that "simpler" neuron-less organisms also transduce stimuli into transient electrical pulses that affect their behaviors. Without a complicated nervous system, APs are often easier to understand as signal/response mechanisms. We review examples of nonneural stimulus transductions in domains of life largely neglected by theoretical neuroscience: bacteria, protozoans, plants, fungi, and neuron-less animals. We report properties of those electrical signals-for example, amplitudes, durations, ionic bases, refractory periods, and particularly their ecological purposes. We compare those properties with those of neurons to infer the tasks and selection pressures that neurons satisfy. Throughout the tree of life, nonneural stimulus transductions time behavioral responses to environmental changes. Nonneural organisms represent the presence or absence of a stimulus with the presence or absence of an electrical signal. Their transductions usually exhibit high sensitivity and specificity to a stimulus, but are often slow compared to neurons. Neurons appear to be sacrificing the specificity of their stimulus transductions for sensitivity and speed. We interpret cellular stimulus transductions as a cell's assertion that it detected something important at that moment in time. In particular, we consider neural APs as fast but noisy detection assertions. We infer that a principal goal of nervous systems is to detect extremely weak signals from noisy sensory spikes under enormous time pressure. We discuss neural computation proposals that address this goal by casting neurons as devices that implement online, analog, probabilistic computations with their membrane potentials. Those proposals imply a measurable relationship between afferent neural spiking statistics and efferent neural membrane electrophysiology.

pH change accompanying long-distance electrical signal controls systemic jasmonate biosynthesis

Local damaging stimuli cause a rapid increase in the content of the defense phytohormone jasmonic acid (JA) and its biologically active derivative jasmonoyl-L-isoleucine (JA-Ile) in undamaged distal tissues. The increase in JA and JA-Ile levels was coincident with a rapid decrease in the levels of the precursor 12-oxo-phytodienoic acid (OPDA). The propagation of a stimulus-induced long-distance electrical signal, variation potential (VP), which is accompanied by intracellular changes in pH and Ca2+ levels, preceded systemic changes in jasmonate content. The decrease in pH during VP, mediated by transient inactivation of the plasma membrane H+-ATPase, induced the conversion of OPDA to JA, probably by regulating the availability of the OPDA substrate to JA biosynthetic enzymes. The regulation of systemic synthesis of JA and JA-Ile by the Ca2+ wave accompanying VP most likely occurs by the same mechanism of pH-induced conversion of OPDA to JA due to Ca2+-mediated decrease in pH as a result of H+-ATPase inactivation. Thus, the transient increase in intracellular Ca2+ levels and the transient decrease in intracellular pH are most likely the key mechanisms of VP-mediated regulation of jasmonate production in systemic tissues upon local stimulation.

Local Action of Moderate Heating and Illumination Induces Electrical Signals, Suppresses Photosynthetic Light Reactions, and Increases Drought Tolerance in Wheat Plants

Local actions of stressors induce electrical signals (ESs), influencing photosynthetic processes and probably increasing tolerance to adverse factors in higher plants. However, the participation of well-known depolarization ESs (action potentials and variation potentials) in these responses seems to be rare under natural conditions, particularly in the case of variation potentials, which are induced by extreme stressors (e.g., burning). Earlier, we showed that the local action of moderate heating and illumination can induce low-amplitude hyperpolarization ESs influencing photosynthetic light reactions in wheat plants cultivated in a vegetation room. In the current work, we analyzed ESs and changes in photosynthetic light reactions and drought tolerance that were induced by a combination of moderate heating and illumination in wheat plants cultivated under open-ground conditions. It was shown that the local heating and illumination induced low-amplitude ESs, and the type of signal (depolarization or hyperpolarization) was dependent on distance from the irritated zone and wheat age. Induction of depolarization ESs was not accompanied by photosynthetic changes in plants under favorable conditions or under weak drought. In contrast, the changes were observed after induction of these signals under moderate drought. Increasing drought tolerance was also observed in the last case. Thus, low-amplitude ESs can participate in photosynthetic regulation and increase tolerance to drought in plants cultivated under open-ground conditions.

Changes in Activity of the Plasma Membrane H+-ATPase as a Link Between Formation of Electrical Signals and Induction of Photosynthetic Responses in Higher Plants

Action of numerous adverse environmental factors on higher plants is spatially-heterogenous; it means that induction of a systemic adaptive response requires generation and transmission of the stress signals. Electrical signals (ESs) induced by local action of stressors include action potential, variation potential, and system potential and they participate in formation of fast physiological changes at the level of a whole plant, including photosynthetic responses. Generation of these ESs is accompanied by the changes in activity of H+-ATPase, which is the main system of electrogenic proton transport across the plasma membrane. Literature data show that the changes in H+-ATPase activity and related changes in intra- and extracellular pH play a key role in the ES-induced inactivation of photosynthesis in non-irritated parts of plants. This inactivation is caused by both suppression of CO2 influx into mesophyll cells in leaves, which can be induced by the apoplast alkalization and, probably, cytoplasm acidification, and direct influence of acidification of stroma and lumen of chloroplasts on light and, probably, dark photosynthetic reactions. The ES-induced inactivation of photosynthesis results in the increasing tolerance of photosynthetic machinery to the action of adverse factors and probability of the plant survival.

Local Action of Increased Pressure Induces Hyperpolarization Electrical Signals and Influences Photosynthetic Light Reactions in Wheat Plants

Long-distance electrical signals caused by the local action of stressors influence numerous physiological processes in plants including photosynthesis and increase their tolerance to the action of adverse factors. Depolarization electrical signals were mainly investigated; however, we earlier showed that hyperpolarization electrical signals (HESs) can be caused by moderate stressors (e.g., local moderate heating) and induce photosynthetic inactivation. We hypothesized that HESs are related to stressor-induced increases in the hydrostatic pressure in the zone of action of the stressor and following the propagation of a hydraulic wave. In the current work, we tested this hypothesis through the direct investigation of electrical signals induced by the local action of artificially increased pressure and an analysis of the subsequent photosynthetic changes in the nonirritated parts of plants. The electrical signals and parameters of photosynthetic light reactions were investigated in wheat plants. The local action of the increased pressure was induced by the action of weights on the wheat leaf. Extracellular electrodes were used for electrical signal measurements. Pulse-amplitude-modulation fluorescent imaging was used for measurements of the quantum yield of photosystem II and nonphotochemical quenching of chlorophyll fluorescence in wheat leaves. It was shown that the local action of pressure on wheat leaf induced electrical signals near the irritated zone: HESs were caused by low pressure (10 kPa) and depolarization signals were induced by high pressure (100 kPa). The local action of moderate pressure (50 kPa) induced weak electrical signals near the irritated zone; however, HESs were observed with increasing distance from this zone. It was also shown that the local action of this moderate pressure induced the photosynthetic inactivation (decreasing the quantum yield of photosystem II and increasing the nonphotochemical quenching of chlorophyll fluorescence) in the nonirritated parts of the wheat leaves. Thus, our results show that the local action of the increased pressure and, probably, subsequent propagation of the hydraulic wave induce electrical signals (including HESs) and photosynthetic inactivation in nonirritated parts of plants that are similar to ones caused by the local action of moderate stressors (e.g., moderate heating). This means that both HESs and depolarization electrical signals can have a hydraulic mechanism of propagation.

Local action of moderate heating and illumination induces propagation of hyperpolarization electrical signals in wheat plants

Electrical signals (ESs), which are generated in irritated zones of plants and propagate into their non-irritated parts, are hypothesized to be an important mechanism of a plant systemic response on the local action of adverse factors. This hypothesis is supported by influence of ESs on numerous physiological processes including expression of defense genes, production of stress phytohormones, changes in photosynthetic processes and transpiration, stimulation of respiration and others. However, there are several questions, which require solution to support the hypothesis. Particularly, the non-physiological stimuli (e.g., strong heating or burning) are often used for induction of ESs; in contrast, the ES induction under action of physiological stressors with moderate intensities requires additional investigations. Influence of long-term environmental factors on generation and propagation of ESs is also weakly investigated. In the current work, we investigated ESs induced by local action of the moderate heating and illumination in wheat plants under irrigated and drought conditions. It was shown that combination of the moderate heating (40°C) and illumination (blue light, 540 μmol m−2s−1) induced electrical signals which were mainly depolarization electrical signals near the irritation zone and hyperpolarization electrical signals (HESs) on the distance from this zone. The moderate soil drought did not influence HESs; in contrast, the strong soil drought significantly decreased amplitude of HESs. Finally, it was shown that the moderate heating could induce HESs without additional action of illumination. It was hypothesized that both hyperpolarization and depolarization ESs could be caused by the hydraulic wave.

Electrical Signaling and Its Functions Under Conditions of Abiotic Stress: A Review of Methodological Approaches and Physiological Implications

In contrast to chemical messengers, electrical signals such as action potentials and variation potentials can transmit information much faster over long distances. Electrical signals can be triggered by various abiotic stress factors and are propagated via plasmodesmata over short distances and within the phloem over long distances. Thus, in addition to assimilate transport from sources to sinks, the phloem serves as a communication highway for various types of information. Key factors for systemic signaling in the phloem are peptides, RNAs, hormones, and electrical signals. In recent years, there has been increasing evidence that rapid communication by means of electrical signals is essential for various plant physiological processes. Thus, this chapter focuses on electrical signaling and various associated physiological effects, such as regulation of leaf movements, assimilate transport, photosynthesis, and gas exchange, as well as plant water status.

Summation of activation at the branch-stem transition of Mimosa pudica; a comparison with summation in cardiac tissue

In Mimosa pudica plants, local and global responses to environmental stimuli are associated with different types of electrical activity. Non-damaging stimuli (e.g. cooling) generate action potentials (APs), whereas damaging stimuli (e.g. heating) are associated with variation potentials (VPs). Local cooling of Mimosa branches resulted in APs that propagated up to the branch-stem interface and caused drooping of the branch (local response). This electrical activation did not pass the interface. If the branch was triggered by heat, however, a VP was transferred to the stem and caused activation of the entire plant (global response). VPs caused by heat were always preceded by APs and summation of the two types of activation appeared to be necessary for the activation to pass the branch-stem interface. Mechanical cutting of leaves also resulted in VPs preceded by APs, but in those cases a time delay was present between the two activations, which prevented adequate summation and transmission of activation. Simultaneous cold-induced activation of a branch and the stem below the interface occasionally resulted in summation sufficient to activate the stem beyond the interface. To investigate the effect of activation delay on summation, a similar structure of excitable converging pathways, consisting of a star-shaped pattern of neonatal rat heart cells, was used. In this model, summation of activation was not hindered by a small degree of asynchrony. The observations indicate that summation occurs in excitable branching structures and suggest that summation of activation plays a role in the propagation of nocuous stimuli in Mimosa.

Hyperpolarization electrical signals induced by local action of moderate heating influence photosynthetic light reactions in wheat plants

Local action of stressors induces fast changes in physiological processes in intact parts of plants including photosynthetic inactivation. This response is mediated by generation and propagation of depolarization electrical signals (action potentials and variation potentials) and participates in increasing plant tolerance to action of adverse factors. Earlier, we showed that a local action of physiological stimuli (moderate heating and blue light), which can be observed under environmental conditions, induces hyperpolarization electrical signals (system potentials) in wheat plants. It potentially means that these signals can play a key role in induction of fast physiological changes under the local action of environmental stressors. The current work was devoted to investigation of influence of hyperpolarization electrical signals induced by the local action of the moderate heating and blue light on parameters of photosynthetic light reactions. A quantum yield of photosystem II (Ф_PSII) and a non-photochemical quenching of chlorophyll fluorescence (NPQ) in wheat plants were investigated. It was shown that combination of the moderate heating (40°C) and blue light (540 µmol m^-2s^-1) decreased Ф_PSII and increased NPQ; these changes were observed in 3-5 cm from border of the irritated zone and dependent on intensity of actinic light. The moderate soil drought (7 days) increased magnitude of photosynthetic changes and shifted their localization which were observed on 5-7 cm from the irritated zone; in contrast, the strong soil drought (14 days) suppressed these changes. The local moderate heating decreased Ф_PSII and increased NPQ without action of the blue light; in contrast, the local blue light action without heating weakly influenced these parameters. It meant that just local heating was mechanism of induction of the photosynthetic changes. Finally, propagation of hyperpolarization electrical signals (system potentials) was necessary for decreasing Ф_PSII and increasing NPQ. Thus, our results show that hyperpolarization electrical signals induced by the local action of the moderate heating inactivates photosynthetic light reactions; this response is similar with photosynthetic changes induced by depolarization electrical signals. The soil drought and actinic light intensity can influence parameters of these photosynthetic changes.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H⁺-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K⁺ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

Integration of electric, calcium, reactive oxygen species and hydraulic signals during rapid systemic signaling in plants

The sensing of abiotic stress, mechanical injury or pathogen attack by a single plant tissue results in the activation of systemic signals that travel from the affected tissue to the entire plant. This process is essential for plant survival during stress and is termed systemic signaling. Among the different signals triggered during this process are calcium, electric, reactive oxygen species and hydraulic signals. These are thought to propagate at rapid rates through the plant vascular bundles and to regulate many of the systemic processes essential for plant survival. Although the different signals activated during systemic signaling are thought to be interlinked, their coordination and hierarchy still need to be determined. Here, using a combination of advanced whole-plant imaging and hydraulic pressure measurements, we studied the activation of all four systemic signals in wild-type and different Arabidopsis thaliana mutants subjected to a local treatment of high-light (HL) stress or wounding. Our findings reveal that activation of systemic membrane potential, calcium, reactive oxygen species and hydraulic pressure signals, in response to wounding, is dependent on glutamate receptor-like proteins 3.3 and 3.6. In contrast, in response to HL stress, systemic changes in calcium and membrane potential depended on glutamate receptor-like 3.3 and 3.6, while systemic hydraulic signals did not. We further show that plasmodesmata functions are required for systemic changes in membrane potential and calcium during responses to HL stress or wounding. Our findings shed new light on the different mechanisms that integrate different systemic signals in plants during stress.

Breaking Bad News: Dynamic Molecular Mechanisms of Wound Response in Plants

Recognition and repair of damaged tissue are an integral part of life. The failure of cells and tissues to appropriately respond to damage can lead to severe dysfunction and disease. Therefore, it is essential that we understand the molecular pathways of wound recognition and response. In this review, we aim to provide a broad overview of the molecular mechanisms underlying the fate of damaged cells and damage recognition in plants. Damaged cells release the so-called damage associated molecular patterns to warn the surrounding tissue. Local signaling through calcium (Ca2+), reactive oxygen species (ROS), and hormones, such as jasmonic acid, activates defense gene expression and local reinforcement of cell walls to seal off the wound and prevent evaporation and pathogen colonization. Depending on the severity of damage, Ca2+, ROS, and electrical signals can also spread throughout the plant to elicit a systemic defense response. Special emphasis is placed on the spatiotemporal dimension in order to obtain a mechanistic understanding of wound signaling in plants.

Wound- and mechanostimulated electrical signals control hormone responses

Plants in nature are constantly exposed to organisms that touch them and wound them. A highly conserved response to these stimuli is a rapid collapse of membrane potential (i.e. a decrease of electrical field strength across membranes). This can be coupled to the production and/or action of jasmonate or ethylene. Here, the various types of electrical signals in plants are discussed in the context of hormone responses. Genetic approaches are revealing genes involved in wound-induced electrical signalling. These include clade 3 GLUTAMATE RECEPTOR-LIKE (GLR) genes, Arabidopsis H⁺ -ATPases (AHAs), RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), and genes that determine cell wall properties. We briefly review touch- and wound-induced increases in cytosolic Ca²⁺ concentrations and their temporal relationship to electrical activities. We then look at the questions that need addressing to link mechanostimulation and wound-induced electrical activity to hormone responses. Utilizing recently published results, we also present a hypothesis for wound-response leaf-to-leaf electrical signalling. This model is based on rapid electro-osmotic coupling between the phloem and xylem. The model suggests that the depolarization of membranes within the vascular matrix triggered by physical stimuli and/or chemical elicitors is linked to changes in phloem turgor and that this plays vital roles in leaf-to-leaf electrical signal propagation.

Two glutamate- and pH-regulated Ca2+ channels are required for systemic wound signaling in Arabidopsis

Plants defend against herbivores and nematodes by rapidly sending signals from the wounded sites to the whole plant. We investigated how plants generate and transduce these rapidly moving, long-distance signals referred to as systemic wound signals. We developed a system for measuring systemic responses to root wounding in Arabidopsis thaliana We found that root wounding or the application of glutamate to wounded roots was sufficient to trigger root-to-shoot Ca²⁺ waves and slow wave potentials (SWPs). Both of these systemic signals were inhibited by either disruption of both GLR3.3 and GLR3.6, which encode glutamate receptor-like proteins (GLRs), or constitutive activation of the P-type H⁺-ATPase AHA1. We further showed that GLR3.3 and GLR3.6 displayed Ca²⁺-permeable channel activities gated by both glutamate and extracellular pH. Together, these results support the hypothesis that wounding inhibits P-type H⁺-ATPase activity, leading to apoplastic alkalization. This, together with glutamate released from damaged phloem, activates GLRs, resulting in depolarization of membranes in the form of SWPs and the generation of cytosolic Ca²⁺ increases to propagate systemic wound signaling.

Measurement of Electropotential Waves in Intact Phloem Sieve Elements Using Microelectrodes

Electropotential waves (EPW) are involved in plant responses to both abiotic and biotic stresses. Three different types of EPWs have been identified: action potential, variation potential, and system potential, all of which have been indicated to participate in phloem-based communication between plant organs. In this chapter we describe in detail how to measure EPWs in plants, including how to access the phloem, and how to insert microelectrodes. Such experiments can be used, for example, to study the local and systemic signaling in response to diverse stimuli like microbial threat or herbivore attack.

Insect-damaged Arabidopsis moves like wounded Mimosa pudica

Slow wave potentials (SWPs) are damage-induced electrical signals which, based on experiments in which organs are burned, have been linked to rapid increases in leaf or stem thickness. The possibility that pressure surges in injured xylem underlie these events has been evoked frequently. We sought evidence for insect feeding-induced positive pressure changes in the petioles of Arabidopsis thaliana Instead, we found that petiole surfaces of leaves distal to insect-feeding sites subsided. We also found that insect damage induced longer-duration downward leaf movements in undamaged leaves. The transient petiole deformations were contemporary with and dependent on the SWP. We then investigated if mutants that affect the xylem, which has been implicated in SWP transmission, might modify SWP architecture. irregular xylem mutants strongly affected SWP velocity and kinetics and, in parallel, restructured insect damage-induced petiole deformations. Together, with force change measurements on the primary vein, the results suggest that extravascular water fluxes accompany the SWP. Moreover, petiole deformations in Arabidopsis mimic parts of the spectacular distal leaf collapse phase seen in wounded Mimosa pudica We genetically link electrical signals to organ movement and deformation and suggest an evolutionary origin of the large leaf movements seen in wounded Mimosa.

Signal coordination before, during and after stomatal closure in response to drought stress

Signal coordination in response to changes in water availability remains unclear, as does the role of embolism events in signaling drought stress. Sunflowers were exposed to two drought treatments of varying intensity while simultaneously monitoring changes in stomatal conductance, acoustic emissions (AE), turgor pressure, surface-level electrical potential, organ-level water potential and leaf abscisic acid (ABA) concentration. Leaf, stem and root xylem vulnerability to embolism were measured with the single vessel injection technique. In both drought treatments, it was found that AE events and turgor changes preceded the onset of stomatal closure, whereas electrical surface potentials shifted concurrently with stomatal closure. Leaf-level ABA concentration did not change until after stomata were closed. Roots and petioles were equally vulnerable to drought stress based on the single vessel injection technique. However, anatomical analysis of the xylem indicated that the increased AE events were not a result of xylem embolism formation. Additionally, roots and stems never reached a xylem pressure threshold that would initiate runaway embolism throughout the entire experiment. It is concluded that stomatal closure was not embolism-driven, but, rather, that onset of stomatal closure was most closely correlated with the hydraulic signal from changes in leaf turgor.

Arabidopsis H+-ATPase AHA1 controls slow wave potential duration and wound-response jasmonate pathway activation

Electrogenic proton pumps have been implicated in the generation of slow wave potentials (SWPs), damage-induced membrane depolarizations that activate the jasmonate (JA) defense pathway in leaves distal to wounds. However, no defined H⁺-ATPases have been shown to modulate these electrical signals. Pilot experiments revealed that the proton pump activator fusicoccin attenuated SWP duration in Arabidopsis Using mutant analyses, we identified Arabidopsis H⁺-ATPase 1 (AHA1) as a SWP regulator. The duration of the repolarization phase was strongly extended in reduced function aha1 mutants. Moreover, the duration of SWP repolarization was shortened in the presence of a gain-of-function AHA1 allele. We employed aphid electrodes to probe the effects of the aha1 mutation on wound-stimulated electrical activity in the phloem. Relative to the wild type, the aha1-7 mutant increased the duration and reduced the amplitudes of electrical signals in sieve tube cells. In addition to affecting electrical signaling, expression of the JA pathway marker gene JAZ10 in leaves distal to wounds was enhanced in aha1-7 Consistent with this, levels of wound-response jasmonoyl-isoleucine were enhanced in the mutant, as was defense against a lepidopteran herbivore. The work identifies a discrete member of the P-type ATPase superfamily with a role in leaf-to-leaf electrical signaling and plant defense.

Long-distance electrical signals as a link between the local action of stressors and the systemic physiological responses in higher plants

Our review is devoted to the analysis of the role of long-distance electrical signals in the development of the fast systemic physiological responses in higher plants. The characteristics and mechanisms of basic electrical signals (variation potential, action potential and system potential) are analyzed, and a potential schema of the generation and propagation of the system potential is proposed. The review summarizes the physiological changes induced by the variation potential, action potential and system potential in higher plants, including changes in gene expressions, the production of phytohormones, photosynthesis, phloem mass-flow, respiration, ATP content, transpiration and plant growth. Potential mechanisms of the changes are analyzed. Finally, a hypothetical schema, which describes a hierarchy of the variation potential, action potential and system potential, in the development of the fast systemic non-specific adaptation of plants to stressors, is proposed.

Parameters of electrical signals and photosynthetic responses induced by them in pea seedlings depend on the nature of stimulus

Local damage induces generation and propagation of variation potentials (VPs) that affect physiological processes in plants. The aims of the work presented here were to investigate parameters of VP induced by burning, heating and mechanical injury in pea seedlings, and to undertake a theoretical analysis of the mechanisms underlying the differences in VP parameters and a study of the photosynthetic responses caused by VPs induced by the damaging factors. The velocity of propagation of burn-induced VP decreased with distance from the damaged area whereas the velocities of heating- and injury-induced VPs were constant. The amplitudes of burn- and heating-induced VPs did not depend on distance whereas the amplitude of VP induced by mechanical injury decreased. VP propagation has been simulated on the basis of wound substance spread. The simulation revealed two possible ways of wound substance propagation: turbulent diffusion from the damaged area and secondary active production in intact cells. The photosynthetic response (decrease in the quantum yield of PSII and raising the level of non-photochemical fluorescence quenching (NPQ)) developed in case of VP entering the intact leaf under heating and burn but was not registered after mechanical injury. An increase in NPQ level was biphasic under burn in comparison with a single-phase one under heating, and the NPQ amplitude was slightly higher under burn. We suggest that differences in photosynthetic responses may be determined by the parameters of VPs induced by stimuli of different nature.

Variation potential-induced photosynthetic and respiratory changes increase ATP content in pea leaves

Local damage induces a physiological response in higher plants by means of generation and propagation of variation potential (VP). The response includes changes in photosynthesis and respiration. The aim of the present study was to investigate the effect of these changes on adenosine triphosphate (ATP) content in pea leaves. VP was induced by local heating of the first mature leaf and registered using extracellular and intracellular electrodes. Photosynthesis and respiration were measured using Dual-PAM-100 and GFS-3000. ATP content was determined using a bioluminescence-based ATP determination kit. Two non-stimulated leaves (second and fourth) were investigated. We showed that heating induced VP that propagated into the second mature leaf, but only a slight electrical reaction was registered in the fourth mature leaf. VP-induced inactivation of photosynthesis developed in the second leaf and included two stages: short- and long-term inactivation. Local heating also caused a two-stage increase in ATP content in the second leaf, which was connected with the photosynthetic responses. Changes in photosynthesis and ATP content were not observed in the fourth leaf. The effect of VP on respiration was investigated under dark conditions. We found that variation potential induced short-term activation of respiration in the second leaf. Local heating induced ATP content increase which included only one stage under dark conditions. Changes in ATP and respiration were absent in the fourth leaf under dark conditions. Thus, VP-induced photosynthetic and respiratory changes are likely to increase ATP content in pea leaves.

Changes in H(+)-ATP Synthase Activity, Proton Electrochemical Gradient, and pH in Pea Chloroplast Can Be Connected with Variation Potential

Local stimulation induces generation and propagation of electrical signals, including the variation potential (VP) and action potential, in plants. Burning-induced VP changes the physiological state of plants; specifically, it inactivates photosynthesis. However, the mechanisms that decrease photosynthesis are poorly understood. We investigated these mechanisms by measuring VP-connected systemic changes in CO2 assimilation, parameters of light reactions of photosynthesis, electrochromic pigment absorbance shifts, and light scattering. We reveal that inactivation of photosynthesis in the pea, including inactivation of dark and light reactions, was connected with the VP. Inactivation of dark reactions decreased the rate constant of the fast relaxation of the electrochromic pigment absorbance shift, which reflected a decrease in the H(+)-ATP synthase activity. This decrease likely contributed to the acidification of the chloroplast lumen, which developed after VP induction. However, VP-connected decrease of the proton motive force across the thylakoid membrane, possibly, reflected a decreased pH in the stroma. This decrease may be another mechanism of chloroplast lumen acidification. Overall, stroma acidification can decrease electron flow through photosystem I, and lumen acidification induces growth of fluorescence non-photochemical quenching and decreases electron flow through photosystem II, i.e., pH decreases in the stroma and lumen, possibly, contribute to the VP-induced inactivation of light reactions of photosynthesis.

Long-distance plant signaling pathways in response to multiple stressors: the gap in knowledge

Plants require the capacity for quick and precise recognition of external stimuli within their environment for survival. Upon exposure to biotic (herbivores and pathogens) or abiotic stressors (environmental conditions), plants can activate hydraulic, chemical, or electrical long-distance signals to initiate systemic stress responses. A plant's stress reactions can be highly precise and orchestrated in response to different stressors or stress combinations. To date, an array of information is available on plant responses to single stressors. However, information on simultaneously occurring stresses that represent either multiple, within, or across abiotic and biotic stress types is nascent. Likewise, the crosstalk between hydraulic, chemical, and electrical signaling pathways and the importance of each individual signaling type requires further investigation in order to be fully understood. The overlapping presence and speed of the signals upon plant exposure to various stressors makes it challenging to identify the signal initiating plant systemic stress/defense responses. Furthermore, it is thought that systemic plant responses are not transmitted by a single pathway, but rather by a combination of signals enabling the transmission of information on the prevailing stressor(s) and its intensity. In this review, we summarize the mode of action of hydraulic, chemical, and electrical long-distance signals, discuss their importance in information transmission to biotic and abiotic stressors, and suggest future research directions.

Variation potential in higher plants: Mechanisms of generation and propagation

Long-distance intercellular electrical signals, including variation potential (VP) in higher plants, are a potential mechanism of coordinate functional responses in different plant cells under action of stressors. VP, which is caused by damaging factors (e.g., heating, crushing), is transient depolarization with an irregular shape. It can include a long-term depolarization and fast impulse depolarization ('AP-like' spikes). Mechanisms of VP generation and propagation are still under investigation. It is probable that VP is a local electrical response induced by propagation of hydraulic wave and (or) chemical agent. Both hypotheses are based on numerous experimental results but they predict VP velocities which are not in a good accordance with speed of variation potential propagation. Thus combination of hydraulic and chemical signals is the probable mechanism of VP propagation. VP generation is traditionally connected with transient H(+)-ATPase inactivation, but AP-like spikes are also connected with passive ions fluxes. Ca(2+) influx is a probable mechanism which triggers H(+)-ATPase inactivation and ions channels activation at VP.

Influence of variation potential on resistance of the photosynthetic machinery to heating in pea

Electrical signals [action potentials (APs) and variation potentials (VPs)] induced by local stimuli are a mechanism that underlies rapid plant response to environmental factors. Such signals induce a number of functional responses, including changes in photosynthesis. Ultimately, these responses are considered to increase plant resistance to stress factors, but this question has been poorly investigated. We studied the influence of VP on photosynthesis and resistance of the photosynthetic machinery to heating in leaves of pea (Pisum sativum). Localized burning induced a VP that decreased photosynthesis parameters [CO(2) assimilation rate and quantum yields of photosystem I (PSI) and photosystem II (PSII)]. The photosynthetic response was initiated by a decrease in photosynthesis dark-stage activity, which in turn increased resistance of PSI to heating. Three results supported this hypothesized mechanism: (1) the magnitude of VP-induced decrease in CO(2) assimilation and enhanced PSI resistance to heating were highly correlated; (2) the VP influence on PSI resistance to heating was suppressed under a low external CO(2) concentration and (3) decreasing external CO(2) concentration imitated the VP-induced photosynthetic response and increased PSI resistance to heating.

Proton cellular influx as a probable mechanism of variation potential influence on photosynthesis in pea

Electrical signals (action potential and variation potential, VP) caused by environmental stimuli are known to induce various physiological responses in plants, including changes in photosynthesis; however, their functional mechanisms remain unclear. In this study, the influence of VP on photosynthesis in pea (Pisum sativum L.) was investigated and the proton participation in this process analysed. VP, induced by local heating, inactivated photosynthesis and activated respiration, with the initiation of the photosynthetic response connected with inactivation of the photosynthetic dark stage; however, direct VP influence on the light stage was also probable. VP generation was accompanied with pH increases in apoplasts (0.17-0.30 pH unit) and decreases in cytoplasm (0.18-0.60 pH unit), which probably reflected H(+) -ATPase inactivation and H(+) influx during this electrical event. Imitation of H(+) influx using the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) induced a photosynthetic response that was similar with a VP-induced response. Experiments on chloroplast suspensions showed that decreased external pH also induced an analogous response and that its magnitude depended on the magnitude of pH change. Thus, the present results showed that proton cellular influx was the probable mechanism of VP's influence on photosynthesis in pea. Potential means of action for this influence are discussed.

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Simulation of variation potential in higher plant cells

Variation potential (VP), a propagating electrical signal unique to plants, induces a number of changes in many physiological processes. However, the mechanisms of its generation and propagation are still under discussion and require experimental and theoretical analysis, including VP simulations. The mathematical model for VP formation in plants has been worked out and is based on our previous description of electrophysiological processes in higher plant cells, including plasma membrane ion transport systems (K(+), Cl(-) and Ca(2+) channels, H(+) and Ca(2+)-ATPase, 2H(+)/Cl(-) symporter and H(+)/K(+) antiporter) and their regulation, ion concentration changes in cells and extracellular spaces and buffers in cytoplasm and apoplast. In addition, the VP model takes into account wound substance diffusion, which is described by a one-dimensional diffusion equation, and ligand-gated Ca(2+) channels, which are activated by this substance. The VP model simulates the experimental dependence of amplitude, velocity and shape of VP on the distance from the wounding site and describes the influence of metabolic inhibitors, divalent cation chelators and anion channel blockers on the generation of this electrical reaction, as shown in experiments. Thus, our model favorably simulates VP in plants and theoretically supports the role of wound substance diffusion and Ca(2+) influx in VP development.

Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.

Accession-dependent action potentials in Arabidopsis

Plant excitability, as measured by the appearance and circulation of action potentials (APs) after biotic and abiotic stress treatments, is a far lesser and more versatile phenomenon than in animals. To examine the genetic basis of plant excitability we used different Arabidopsis thaliana accessions. APs were induced by wounding (W) with a subsequent deposition (D) of 5μL of 1M KCl onto adult leaves. This treatment elicited transient voltage responses (APs) that were detected by 2 extracellular electrodes placed at a distance from the wounding location over an experimental time of 150min. The first electrode (e1) was placed at the end of the petiole and the beginning of the leaf, and the second (e2) electrode was placed on the petiole near the center of the rosette. All accessions (Columbia (Col), Wassilewskija (Ws) and Landsberg erecta (Ler)) responded to the W & D treatment. After W & D treatment was performed on 100 plants for each accession, the number of APs ranged from 0 to 37 (median 8, total 940), 0 to 16 (median 5, total 528) and 0 to 18 (median 2, total 296) in Col, Ws and Ler, respectively. Responding plants (>0 APs) showed significantly different behaviors depending on their accessions of origin (i.e., Col 91, Ws 83 and Ler 76%). Some AP characteristics, such as amplitude and speed of propagation from e1 to e2 (1.28mms(-1)), were the same for all accessions, whereas the average duration of APs was similar in Col and Ws, but different in Ler. Self-sustained oscillations were observed more frequently in Col than Ws and least often in Ler, and the mean oscillation frequency was more rapid in Col, followed by Ws, and was slowest in Ler. In general, Col was the most excitable accession, followed by Ws, and Ler was the least excitable; this corresponded well with voltage elicited action potentials. In conclusion, part of Arabidopsis excitability in AP responses is genetically pre-determined.

Heat-induced electrical signals affect cytoplasmic and apoplastic pH as well as photosynthesis during propagation through the maize leaf

Combining measurements of electric potential and pH with such of chlorophyll fluorescence and leaf gas exchange showed heat stimulation to evoke an electrical signal (propagation speed: 3-5 mm s(-1)) that travelled through the leaf while reducing the net CO(2) uptake rate and the photochemical quantum yield of both photosystems (PS). Two-dimensional imaging analysis of the chlorophyll fluorescence signal of PS II revealed that the yield reduction spread basipetally via the veins through the leaf at a speed of 1.6 +/- 0.3 mm s(-1) while the propagation speed in the intervein region was c. 50 times slower. Propagation of the signal through the veins was confirmed because PS I, which is present in the bundle sheath cells around the leaf vessels, was affected first. Hence, spreading of the signal along the veins represents a path with higher travelling speed than within the intervein region of the leaf lamina. Upon the electrical signal, cytoplasmic pH decreased transiently from 7.0 to 6.4, while apoplastic pH increased transiently from 4.5 to 5.2. Moreover, photochemical quantum yield of isolated chloroplasts was strongly affected by pH changes in the surrounding medium, indicating a putative direct influence of electrical signalling via changes of cytosolic pH on leaf photosynthesis.

Changes in extracellular pH are neither required nor sufficient for activation of mitogen-activated protein kinases (MAPKs) in response to systemin and fusicoccin in tomato

Leaf wounding and the wound signaling peptide systemin induce expression of wound response genes while the fungal toxin fusicoccin (FC) induces expression of pathogenesis-related genes. Consistent with their functional differences, FC and systemin regulate the extracellular pH in opposite ways, with systemin inducing an alkalinization and FC an acidification response. Here we show that systemin, wounding and FC activate the same mitogen-activated protein kinases (MAPKs; MPKs) MPK1 and 2 in tomato (Lycopersicon esculentum) leaves and L. peruvianum suspension-cultured cells. Wounding and FC activated an additional MAPK, MPK3. Pronounced differences were observed with regard to MAPK activation kinetics. FC induced prolonged, and systemin transient activity of the MAPKs. This shows that functionally different elicitors engage the same signaling components, yet induce signal-specific activation dynamics. A comparative analysis of pH effects and MAPK activity in response to specific treatments revealed that the kinetics of pH changes and MAPK activation did not correlate. Simultaneous application of FC and systemin did not lead to immediate pH changes but resulted in rapid increases in MAPK activity. Furthermore, changes in extracellular pH could be induced without concomitant MAPK activation by exchanging conditioned medium with fresh medium. This shows that changes in the extracellular pH are neither required nor sufficient for MAPK activation, suggesting that signaling pathways involving MAPKs and extracellular pH changes operate in parallel and are not part of the same linear pathway.

Shade-Induced Action Potentials in Helianthus annuus L. Originate Primarily from the Epicotyl

Repeated observations that shading (a drastic reduction in illumination rate) increased the generation of spikes (rapidly reversed depolarizations) in leaves and stems of many cucumber and sunflower plants suggests a phenomenon widespread among plant organs and species. Although shaded leaves occasionally generate spikes and have been suggested to trigger systemic action potentials (APs) in sunflower stems, we never found leaf-generated spikes to propagate out of the leaf and into the stem. On the contrary, our data consistently implicate the epicotyl as the location where most spikes and APs (propagating spikes) originate. Microelectrode studies of light and shading responses in mesophyll cells of leaf strips and in epidermis/cortex cells of epicotyl segments confirm this conclusion and show that spike induction is not confined to intact plants. 90% of the epicotyl-generated APs undergo basipetal propagation to the lower epicotyl, hypocotyl and root. They propagate with an average rate of 2 +/- 0.3 mm s(-1) and always undergo a large decrement from the hypocotyl to the root. The few epicotyl-derived APs that can be tracked to leaf blades (< 10%) undergo either a large decrement or fail to be transmitted at all. Occasionally (5% of the observations) spikes were be generated in hypocotyl and lower epicotyl that moved towards the upper epicotyl unaltered, decremented, or amplified. This study confirms that plant APs arise to natural, nontraumatic changes. In simultaneous recordings with epicotyl growth, AP generation was found to parallel the acceleration of stem growth under shade. The possible relatedness of both processes must be further investigated.

Decrement and amplification of slow wave potentials during their propagation in Helianthus annuus L. shoots

Slow wave potentials (SWPs) are transitory depolarizations occurring in response to treatments that result in a pressure increase in the xylem conduits (P(x)). Here SWPs are induced by excision of the root under water in 40- to 50-cm-tall light-grown sunflower plants in order to determine the effective signal range to a naturally sized pressure signal. The induced slow wave depolarization appears to move up the stem while it is progressively decremented (i.e. the amplitude decreases with increasing distance from the point of excision) with a rate that appears to rise acropetally from 2.5 to 5.5% cm(-1). The decline of the SWP signal, in both amplitude and range, could be experimentally increased (i) when root excision was carried out in air and (ii) when the transpiration of the sunflower shoot was minimized by a preceding removal or coating of the leaves. A further decline of the SW signal was expected to occur when leaves were included in the measured path. However, when the most distant apical electrode was attached to an upper leaf, it showed a considerably larger depolarization than a neighboring stem position. This apparent amplification of the SWP signal is not confined to the leaf blade but includes the petiole as well. The amplification disappeared (i) when the illumination level was lowered to room light, (ii) when the blade was excised either completely or along the remaining midvein and (iii) when the intact leaf blade was submersed in water. These treatments reduce the SWP at the petiole to a small fraction of the signal in the opposite control leaf and specify bright illumination and blade-mediated transpiration as prerequisites of a signal increase that is confined to young, expanding leaves.

Electrical perception of "death message" in Chara: involvement of turgor pressure

Plants show various defense responses upon wounding. Surviving cells must perceive a "death message" from killed cells in order to start the signal processing that results in defense responses. The initial step in perception of the death message by a surviving cell was studied by taking advantage of the filamentous morphology of characean algae. A specimen comprising two adjoining internodal cells was prepared. One cell (the victim cell) was killed by cutting and any changes in the membrane potential of the neighboring cell (the receptor cell) were analyzed. Upon cutting the victim cell, at least one of three kinds of response were induced in the receptor cell: (1) slow depolarization lasting more than 10 min, (2) action potentials and (3) small spikes. The first of these response types, slow depolarization, was ubiquitous and is the focus of the present study. Two cell properties were essential for generation of this depolarization. (1) Presence of high cell turgor pressure was necessary. (2) The depolarization was generated only at the nodal end of the receptor cell, not at the flank. I concluded that the death message from the killed cell contains the information that turgor pressure has been lost. The mechanism by which this is translated into the slow depolarization of the receptor cell was discussed.

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

The positive xylem pressure (Px) in cucumber hypocotyls is a direct extension of root pressure and therefore depends on the root environment. Solutions of the electrolyte KCl (0-10 osm) reduced the hypocotyl Px transiently (biphasic response), while the Px reduction by mannitol solutions was sustained. The amplitudes of the induced Px reduction depended directly, and the degree of Px restoration after stress release depended indirectly, on the size of the initial positive Px indicating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pressure reduction revealed two separate growth responses in the affected cucumber hypocotyls. Only steep Px drops (following root excision or root pressure release in mannitol) directly cause a rapid, transient drop in growth rate (GR). Both rapid and slow (after root incubation in KCN or NEM) decreases in root pressure, however, led to a sustained growth inhibition of cucumber hypocotyls after about 30 min. This delay characterizes the growth response as an indirect consequence of the Px change. Pea seedlings, which lacked root pressure and had a negative Px throughout, showed extremely small changes in epicotyl Px and GR after root incubation in mannitol. It is apparent that the higher sensitivity of cucumber growth to mannitol depended on the presence and release of root pressure.