A unique inventory of ion transporters poises the Venus flytrap to fast-propagating action potentials and calcium waves

The "plant neurobiology" revolution

The 21st-century "plant neurobiology" movement is an amalgam of scholars interested in how "neural processes", broadly defined, lead to changes in plant behavior. Integral to the movement (now called plant behavioral biology) is a triad of historically marginalized subdisciplines, namely plant ethology, whole plant electrophysiology and plant comparative psychology, that set plant neurobiology apart from the mainstream. A central tenet held by these "triad disciplines" is that plants are exquisitely sensitive to environmental perturbations and that destructive experimental manipulations rapidly and profoundly affect plant function. Since destructive measurements have been the norm in plant physiology, much of our "textbook knowledge" concerning plant physiology is unrelated to normal plant function. As such, scientists in the triad disciplines favor a more natural and holistic approach toward understanding plant function. By examining the history, philosophy, sociology and psychology of the triad disciplines, this paper refutes in eight ways the criticism that plant neurobiology presents nothing new, and that the topics of plant neurobiology fall squarely under the purview of mainstream plant physiology. It is argued that although the triad disciplines and mainstream plant physiology share the common goal of understanding plant function, they are distinct in having their own intellectual histories and epistemologies.

Revisiting plant electric signaling: Challenging an old phenomenon with novel discoveries

Higher plants efficiently orchestrate rapid systemic responses to diverse environmental stimuli through electric signaling. This review explores the mechanisms underlying two main types of electric signals in plants, action potentials (APs) and slow wave potentials (SWPs), and how new discoveries challenge conventional neurophysiological paradigms traditionally forming their theoretical foundations. Animal APs are biophysically well-defined, whereas plant APs are often classified based on their shape, lacking thorough characterization. SWPs are depolarizing electric signals deviating from this shape, leading to an oversimplified classification of plant electric signals. Indeed, investigating the generation and propagation of plant APs and SWPs showcases a complex interplay of mechanisms that sustain self-propagating signals and internally propagating stimuli, resulting in membrane depolarization, cytosolic calcium increase, and alterations in reactive oxygen species and pH. A holistic understanding of plant electric signaling will rely on unraveling the network of ion-conducting proteins, signaling molecules, and mechanisms for signal generation and propagation.

Light-gated channelrhodopsin sparks proton-induced calcium release in guard cells

Although there has been long-standing recognition that stimuli-induced cytosolic pH alterations coincide with changes in calcium ion (Ca2+) levels, the interdependence between protons (H+) and Ca2+ remains poorly understood. We addressed this topic using the light-gated channelrhodopsin HcKCR2 from the pseudofungus Hyphochytrium catenoides, which operates as a H+ conductive, Ca2+ impermeable ion channel on the plasma membrane of plant cells. Light activation of HcKCR2 in Arabidopsis guard cells evokes a transient cytoplasmic acidification that sparks Ca2+ release from the endoplasmic reticulum. A H+-induced cytosolic Ca2+ signal results in membrane depolarization through the activation of Ca2+-dependent SLAC1/SLAH3 anion channels, which enabled us to remotely control stomatal movement. Our study suggests a H+-induced Ca2+ release mechanism in plant cells and establishes HcKCR2 as a tool to dissect the molecular basis of plant intracellular pH and Ca2+ signaling.

Ion Channels in Electrical Signaling in Higher Plants

Electrical signals (ESs) appearing in plants under the action of various external factors play an important role in adaptation to changing environmental conditions. Generation of ES in higher plant cells is associated with activation of Ca2+, K+, and anion fluxes, as well as with changes in the activity of plasma membrane H+-ATPase. In the present review, molecular nature of the ion channels contributing to ESs transmission in higher plants is analyzed based on comparison of the data from molecular-genetic and electrophysiological studies. Based on such characteristics of ion channels as selectivity, activation mechanism, and intracellular and tissue localization, those ion channels that meet the requirements for potential participation in ES generation were selected from a wide variety of ion channels in higher plants. Analysis of the data of experimental studies performed on mutants with suppressed or enhanced expression of a certain channel gene revealed those channels whose activation contributes to ESs formation. The channels responsible for Ca2+ flux during generation of ESs include channels of the GLR family, for K+ flux - GORK, for anions - MSL. Consideration of the prospects of further studies suggests the need to combine electrophysiological and genetic approaches along with analysis of ion concentrations in intact plants within a single study.

Trigger hair thermoreceptors provide for heat-induced calcium-electrical excitability in Venus flytrap

Most plants suffer greatly from heat in general and fire in particular, but some can profit from what is called fire ecology.1Dionaea muscipula, the Venus flytrap, is one such plant. In its natural habitat in the Green Swamps, Dionaea often faces challenges from excessive growth of grass and evergreen shrubs that overshadow the plant.2 Without natural fire, the Dionaea populations would decline.3 How does Dionaea survive and even thrive after swamp fires? Here, we ask whether flytraps recognize heat waves at the forefront of swamp fires and demonstrate that a heat-sensor-based alarm may provide a fire survival strategy for them. In this study, we show that flytraps become electrically excited and close in response to a heat wave. Over a critical temperature of 38°C, traps fire action potentials (APs), which are interconnected with cytosolic Ca2+ transients. The heat-induced Ca2+-AP has a 3-min refractory period, yet traps still respond to cold, voltage, and glutamate. The heat responses were trap specific, emerging only when the trap became excitable. Upon heat stimulation, the Ca2+ wave originates in the trigger hair podium, indicating that the mechanosensory zone serves as a heat receptor organ. In contrast to the human heat receptor, the flytrap sensor detects temperature change rather than the absolute body temperature. We propose that by sensing the temperature differential, flytraps can recognize the heat of an approaching fire, thus closing before the trigger hairs are burned, while they can continue to catch prey throughout hot summers.

Mechanosensation in leaf veins

Whether the plant vasculature has the capacity to sense touch is unknown. We developed a quantitative assay to investigate touch-response electrical signals in the leaves and veins of Arabidopsis thaliana. Mechanostimulated electrical signaling in leaves displayed strong diel regulation. Signals of full amplitude could be generated by repeated stimulation at the same site after approximately 90 minutes. However, the signals showed intermediate amplitudes when repeatedly stimulated in shorter timeframes. Using intracellular electrodes, we detected touch-response membrane depolarizations in the phloem. On the basis of this, we mutated multiple Arabidopsis H+-ATPase (AHA) genes expressed in companion cells. We found that aha1 aha3 double mutants attenuated touch-responses, and this was coupled to growth rate reduction. Moreover, propagating membrane depolarizations could be triggered by mechanostimulating the exposed primary vasculature of wild-type plants but not of aha1 aha3 mutants. Primary veins have autonomous mechanosensory properties which depend on P-type proton pumps.

Demystifying the Venus flytrap action potential

All plants are electrically excitable, but only few are known to fire a well-defined, all-or-nothing action potential (AP). The Venus flytrap Dionaea muscipula displays APs with an extraordinarily high firing frequency and speed, enabling the capture organ of this carnivorous plant to catch small animals as fast as flies. The number of APs triggered by the prey is counted and serves as the basis for decisions within the flytrap's hunting cycle. The archetypical Dionaea AP lasts 1 s and consists of five phases: Starting from the resting state, an initial cytosolic Ca2+ transient is followed by depolarization, repolarization and a transient hyperpolarization (overshoot) before the original membrane potential is finally recovered. When the flytrap matures and becomes excitable, a distinct set of ion channels, pumps and carriers is expressed, each mastering a distinct AP phase.

Long-Distance Electrical and Calcium Signals Evoked by Hydrogen Peroxide in Physcomitrella

Electrical and calcium signals in plants are some of the basic carriers of information that are transmitted over a long distance. Together with reactive oxygen species (ROS) waves, electrical and calcium signals can participate in cell-to-cell signaling, conveying information about different stimuli, e.g. abiotic stress, pathogen infection or mechanical injury. There is no information on the ability of ROS to evoke systemic electrical or calcium signals in the model moss Physcomitrella nor on the relationships between these responses. Here, we show that the external application of hydrogen peroxide (H2O2) evokes electrical signals in the form of long-distance changes in the membrane potential, which transmit through the plant instantly after stimulation. The responses were calcium-dependent since their generation was inhibited by lanthanum, a calcium channel inhibitor (2 mM), and EDTA, a calcium chelator (0.5 mM). The electrical signals were partially dependent on glutamate receptor (GLR) ion channels since knocking-out the GLR genes only slightly reduced the amplitude of the responses. The basal part of the gametophyte, which is rich in protonema cells, was the most sensitive to H2O2. The measurements carried out on the protonema expressing fluorescent calcium biosensor GCaMP3 proved that calcium signals propagated slowly (>5 µm/s) and showed a decrement. We also demonstrate upregulation of a stress-related gene that appears in a distant section of the moss 8 min after the H2O2 treatment. The results help understand the importance of both types of signals in the transmission of information about the appearance of ROS in the plant cell apoplast.

Plant electrophysiology with conformable organic electronics: Deciphering the propagation of Venus flytrap action potentials

Electrical signals in plants are mediators of long-distance signaling and correlate with plant movements and responses to stress. These signals are studied with single surface electrodes that cannot resolve signal propagation and integration, thus impeding their decoding and link to function. Here, we developed a conformable multielectrode array based on organic electronics for large-scale and high-resolution plant electrophysiology. We performed precise spatiotemporal mapping of the action potential (AP) in Venus flytrap and found that the AP actively propagates through the tissue with constant speed and without strong directionality. We also found that spontaneously generated APs can originate from unstimulated hairs and that they correlate with trap movement. Last, we demonstrate that the Venus flytrap circuitry can be activated by cells other than the sensory hairs. Our work reveals key properties of the AP and establishes the capacity of organic bioelectronics for resolving electrical signaling in plants contributing to the mechanistic understanding of long-distance responses in plants.

TPC1 vacuole SV channel gains further shape - voltage priming of calcium-dependent gating

Across phyla, voltage-gated ion channels (VGICs) allow excitability. The vacuolar two-pore channel AtTPC1 from the tiny mustard plant Arabidopsis thaliana has emerged as a paradigm for deciphering the role of voltage and calcium signals in membrane excitation. Among the numerous experimentally determined structures of VGICs, AtTPC1 was the first to be revealed in a closed and resting state, fueling speculation about structural rearrangements during channel activation. Two independent reports on the structure of a partially opened AtTPC1 channel protein have led to working models that offer promising insights into the molecular switches associated with the gating process. We review new structure-function models and also discuss the evolutionary impact of two-pore channels (TPCs) on K⁺ homeostasis and vacuolar excitability.

A touchy subject: Ca2+ signaling during leaf movements in Mimosa

Mimosa pudica, the sensitive plant, responds to stimuli such as touch and wounding with leaf movements that propagate throughout the plant. The motion is driven by changes in the turgor of specialized cells in a set of motor organs called pulvinae. By imaging cellular Ca²⁺ levels as the wave of movement propagates through the leaf, Hagihara and colleagues now show that Ca²⁺ signals precede and predict the pulvinar movements. These results provide compelling support for a model where Mimosa uses a Ca²⁺-related response system to trigger its leaf movements. These researchers then used CRISPR to delete a critical genetic regulator of pulvinar development, producing plants with immobile leaves. These plants experienced more herbivory than wild type, suggesting that the Ca²⁺-triggered leaf movements are an adaptation to deter herbivory.

Cracking the code of plant herbivore defense

In 1916, Ricca hypothesized that plant defense mediators are transported by xylem vessels. While it was discovered that electrical waves generated at plant wounds also transmit information over great distances, the molecular nature of the so-called Ricca factor remained unclear. In this issue of Cell, Gao et al. identify thioglucoside glucohydrolases as a Ricca factor in Arabidopsis.

Ricca's factors as mobile proteinaceous effectors of electrical signaling

Leaf-feeding insects trigger high-amplitude, defense-inducing electrical signals called slow wave potentials (SWPs). These signals are thought to be triggered by the long-distance transport of low molecular mass elicitors termed Ricca's factors. We sought mediators of leaf-to-leaf electrical signaling in Arabidopsis thaliana and identified them as β-THIOGLUCOSIDE GLUCOHYDROLASE 1 and 2 (TGG1 and TGG2). SWP propagation from insect feeding sites was strongly attenuated in tgg1 tgg2 mutants and wound-response cytosolic Ca²⁺ increases were reduced in these plants. Recombinant TGG1 fed into the xylem elicited wild-type-like membrane depolarization and Ca²⁺ transients. Moreover, TGGs catalyze the deglucosidation of glucosinolates. Metabolite profiling revealed rapid wound-induced breakdown of aliphatic glucosinolates in primary veins. Using in vivo chemical trapping, we found evidence for roles of short-lived aglycone intermediates generated by glucosinolate hydrolysis in SWP membrane depolarization. Our findings reveal a mechanism whereby organ-to-organ protein transport plays a major role in electrical signaling.

DYSCALCULIA, a Venus flytrap mutant without the ability to count action potentials

The Venus flytrap Dionaea muscipula estimates prey nutrient content by counting trigger hair contacts initiating action potentials (APs) and calcium waves traveling all over the trap.^1,2,3 A first AP is associated with a subcritical rise in cytosolic calcium concentration, but when the second AP arrives in time, calcium levels pass the threshold required for fast trap closure. Consequently, memory function and decision-making are timed via a calcium clock.^3,4 For higher numbers of APs elicited by the struggling prey, the Ca²⁺ clock connects to the networks governed by the touch hormone jasmonic acid (JA), which initiates slow, hermetic trap sealing and mining of the animal food stock.⁵ Two distinct phases of trap closure can be distinguished within Dionaea's hunting cycle: (1) very fast trap snapping requiring two APs and crossing of a critical cytosolic Ca²⁺ level and (2) JA-dependent slow trap sealing and prey processing induced by more than five APs. The Dionaea mutant DYSC is still able to fire touch-induced APs but does not snap close its traps and fails to enter the hunting cycle after prolonged mechanostimulation. Transcriptomic analyses revealed that upon trigger hair touch/AP stimulation, activation of calcium signaling is largely suppressed in DYSC traps. The observation that external JA application restored hunting cycle progression together with the DYSC phenotype and its transcriptional landscape indicates that DYSC cannot properly read, count, and decode touch/AP-induced calcium signals that are key in prey capture and processing.

Stellate Trichomes in Dionaea muscipula Ellis (Venus Flytrap) Traps, Structure and Functions

The digestive organs of carnivorous plants have external (abaxial) glands and trichomes, which perform various functions. Dionaea muscipula Ellis (the Venus flytrap) is a model carnivorous plant species whose traps are covered by external trichomes. The aim of the study was to fill in the gap regarding the structure of the stellate outer trichomes and their immunocytochemistry and to determine whether these data support the suggestions of other authors about the roles of these trichomes. Light and electron microscopy was used to show the trichomes' structure. Fluorescence microscopy was used to locate the carbohydrate epitopes that are associated with the major cell wall polysaccharides and glycoproteins. The endodermal cells and internal head cells of the trichomes were differentiated as transfer cells, and this supports the idea that stellate trichomes transport solutes and are not only tomentose-like trichomes. Trichome cells differ in the composition of their cell walls, e.g., the cell walls of the internal head cells are enriched with arabinogalactan proteins (AGPs). The cell walls of the outer head cells are poor in both low and highly homogalacturonans (HGs), but the immature trichomes are rich in the pectic polysaccharide (1-4)-β-D-galactan. In the immature traps, young stellate trichomes produce mucilage which may protect the trap surface, and in particular, the trap entrance. However, the role of these trichomes is different when the outer head cells collapse. In the internal head cells, a thick secondary wall cell was deposited, which together with the thick cell walls of the outer head cells played the role of a large apoplastic space. This may suggest that mature stellate trichomes might function as hydathodes, but this should be experimentally proven.

Plant physiology: Anatomy of a plant action potential

The Venus flytrap possesses modified leaves that can snap shut fast enough to catch a fly. A new study identifies the major components of the toolkit that allows the flytrap to fire action potentials, illustrating how different ion channels and transporters are recruited to give rise to this unique plant behavioural response.