Arabidopsis Leaf Trichomes as Acoustic Antennae

Is plant acoustic communication fact or fiction?

In recent years, the idea has flourished that plants emit and perceive sound and could even be capable of exchanging information through the acoustic channel. While research into plant bioacoustics is still in its infancy, with potentially fascinating discoveries awaiting ahead, here we show that the current knowledge is not conclusive. While plants do emit sounds under biotic and abiotic stresses such as drought, these sounds are high-pitched, of low intensity, and propagate only to a short distance. Most studies suggesting plant sensitivity to airborne sound actually concern the perception of substrate vibrations from the soil or plant part. In short, while low-frequency, high-intensity sounds emitted by a loudspeaker close to the plant seem to have tangible effects on various plant processes such as growth - a finding with possible applications in agriculture - it is unlikely that plants can perceive the sounds they produce, at least over long distances. So far, there is no evidence of plants communicating with each other via the acoustic channel.

The plant is neither dumb nor deaf; it talks and hears

Animals and insects communicate using vibrations that are frequently too low or too high for human ears to detect. Plants and trees can communicate and sense sound. Khait et al. used a dependable recording system to capture airborne sounds produced by stressed plants. In addition to allowing plants to communicate their stress, sound aids in plant defense, development, and resilience. It also serves as a warning that danger is approaching. Demey et al. and others discussed the audit examinations that were conducted to investigate sound discernment in plants at the atomic and biological levels. The biological significance of sound in plants, the morphophysiological response of plants to sound, and the airborne noises that plants make and can hear from a few meters away were all discussed.

Forces on and in the cell walls of living plants

Environmental influences and differential growth subject plants to mechanical forces. Forces on the whole plant resolve into tensile forces on its primary cell walls and both tensile and compression forces on the secondary cell wall layers of woody tissues. Forces on cell walls are further resolved into forces on cellulose microfibrils and the noncellulosic polymers between them. Many external forces on plants oscillate, with time constants that vary from seconds to milliseconds. Sound waves are a high-frequency example. Forces on the cell wall lead to responses that direct the oriented deposition of cellulose microfibrils and the patterned expansion of the cell wall, leading to complex cell and tissue morphology. Recent experiments have established many of the details of which cell wall polymers associate with one another in both primary and secondary cell walls, but questions remain about which of the interconnections are load bearing, especially in primary cell walls. Direct cellulose-cellulose interactions appear to have a more important mechanical role than was previously thought, and some of the noncellulosic polymers may have a role in keeping microfibrils apart rather than cross-linking them as formerly envisaged.

Understanding the mechanobiology of phytoacoustics through molecular Lens: Mechanisms and future perspectives

BACKGROUND

How plants emit, perceive, and respond to sound vibrations (SVs) is a long-standing question in the field of plant sensory biology. In recent years, there have been numerous studies on how SVs affect plant morphological, physiological, and biochemical traits related to growth and adaptive responses. For instance, under drought SVs navigate plant roots towards water, activate their defence responses against stressors, and increase nectar sugar in response to pollinator SVs. Also, plants emit SVs during stresses which are informative in terms of ecological and adaptive perspective. However, the molecular mechanisms underlying the SV perception and emission in plants remain largely unknown. Therefore, deciphering the complexity of plant-SV interactions and identifying bonafide receptors and signaling players will be game changers overcoming the roadblocks in phytoacoustics.

AIM OF REVIEW

The aim of this review is to provide an overview of recent developments in phytoacoustics. We primarily focuss on SV signal perception and transduction with current challenges and future perspectives.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Timeline breakthroughs in phytoacoustics have constantly shaped our understanding and belief that plants may emit and respond to SVs like other species. However, unlike other plant mechanostimuli, little is known about SV perception and signal transduction. Here, we provide an update on phytoacoustics and its ecological importance. Next, we discuss the role of cell wall receptor-like kinases, mechanosensitive channels, intracellular organelle signaling, and other key players involved in plant-SV receptive pathways that connect them. We also highlight the role of calcium (Ca2+), reactive oxygen species (ROS), hormones, and other emerging signaling molecules in SV signal transduction. Further, we discuss the importance of molecular, biophysical, computational, and live cell imaging tools for decoding the molecular complexity of acoustic signaling in plants. Finally, we summarised the role of SV priming in plants and discuss how SVs could modulate plant defense and growth trade-offs during other stresses.

Comparative morphoanatomy and transcriptomic analyses reveal key factors controlling floral trichome development in Aristolochia (Aristolochiaceae)

Trichomes are specialized epidermal cells in aerial plant parts. Trichome development proceeds in three stages, determination of cell fate, specification, and morphogenesis. Most genes responsible for these processes have been identified in the unicellular branched leaf trichomes from the model Arabidopsis thaliana. Less is known about the molecular basis of multicellular trichome formation across flowering plants, especially those formed in floral organs of early diverging angiosperms. Here, we aim to identify the genetic regulatory network (GRN) underlying multicellular trichome development in the kettle-shaped trap flowers of Aristolochia (Aristolochiaceae). We selected two taxa for comparison, A. fimbriata, with trichomes inside the perianth, which play critical roles in pollination, and A. macrophylla, lacking specialized trichomes in the perianth. A detailed morphoanatomical characterization of floral epidermis is presented for the two species. We compared transcriptomic profiling at two different developmental stages in the different perianth portions (limb, tube, and utricle) of the two species. Moreover, we present a comprehensive expression map for positive regulators and repressors of trichome development, as well as cell cycle regulators. Our data point to extensive modifications in gene composition, expression, and putative roles in all functional categories when compared with model species. We also record novel differentially expressed genes (DEGs) linked to epidermis patterning and trichome development. We thus propose the first hypothetical genetic regulatory network (GRN) underlying floral multicellular trichome development in Aristolochia, and pinpoint key factors responsible for the presence and specialization of floral trichomes in phylogenetically distant species of the genus.

Sound perception in plants: from ecological significance to molecular understanding

In addition to positive effects on plant growth and resilience, sound alerts plants of potential danger and aids in defense. Sound guides plants towards essential resources, like water, through phonotropic root growth. Sound also facilitates mutualistic interactions such as buzz pollination. Molecularly, sound induces Ca2+ signatures, K+ fluxes, and an increase in reactive oxygen species (ROS) levels in a mechanosensitive ion channel-dependent fashion. We review the two major open questions in the field of plant acoustics: (i) what is the ecological relevance of sound in plant life, and (ii) how is sound sensed and transduced to evoke a morphophysiological response? We highlight the clear need to combine the ecological and molecular perspectives for a more holistic approach to better understand plant behavior.

A discrete shear lag model of the mechanics of hitchhiker plants, and its prospective application to tendon-to-bone repair

Tendon-to-bone repairs often fail when sutures pull through tendon, like a wire through cheese. Repair strength is maximized when loads are balanced equally among all sutures, relative to the pullout resistance of the tendon and the strength of the sutures. This problem of balancing loads across multiple, discrete attachment sites has been solved in nature by hitchhiker plants that proliferate by adhering relatively stiff fruit to relatively soft fur and fabrics through arrays of hooks. We, therefore, studied the fruits of such a plant, Harpagonella palmeri , and developed a discrete shear lag analysis of the force distributions in H. palmeri 's linear arrays of long, slender hooks of varied lengths and spacing. Results suggested that strategies were used by the plant to distribute loads, including variations in the spacing and stiffnesses of hooks that serve to equalize forces over attachment sites. When applying these models to suturing schemes for surgical reattachment of tendon to bone, results suggested that strategies exhibited by H. palmeri show promise for balancing forces over sutures, potentially doubling repair strength relative to what could be achieved with a uniform suture distribution. Results suggest a potential pathway for strengthening surgical repairs, and more broadly for optimizing fasteners for bi-material attachment.

Plants detect and respond to sounds

Specific sound patterns can affect plant development. Plants are responsive to environmental stimuli such as sound. However, little is known about their sensory apparatus, mechanisms, and signaling pathways triggered by these stimuli. Thus, it is important to understand the effect of sounds on plants and their technological potential. This review addresses the effects of sounds on plants, the sensory elements inherent to sound detection by the cell, as well as the triggering of signaling pathways that culminate in plant responses. The importance of sound standardization for the study of phytoacoustics is demonstrated. Studies on the sounds emitted or reflected by plants, acoustic stress in plants, and recognition of some sound patterns by plants are also explored.

Rigidity control mechanism by turgor pressure in plants

The bodies of herbaceous plants are slender, thin, and soft. These plants support their bodies through the action of turgor pressure associated with their internal water stores. The purpose of this study was to apply the principles of structural mechanics to clarify the underlying mechanism of rigidity control that is responsible for turgor pressure in plants and the reason behind the self-supporting ability of herbaceous plants. We modeled a plant a horizontally oriented thin-walled cylindrical cantilever with closed ends enclosing a cavity filled with water that is acted on by its own weight and by internal tension generated through turgor pressure. We derived an equation describing the plant's consequent deflection, introducing a dimensionless parameter to express the decrease in deflection associated with the action of turgor pressure. We found that the mechanical and physical characteristics of herbaceous plants that would appear to be counter-productive from a superficial perspective increase the deflection decreasing effect of turgor pressure.

Acoustic radiation force on a long cylinder, and potential sound transduction by tomato trichomes

Acoustic transduction by plants has been proposed as a mechanism to enable just-in-time up-regulation of metabolically expensive defensive compounds. Although the mechanisms by which this "hearing" occurs are unknown, mechanosensation by elongated plant hair cells known as trichomes is suspected. To evaluate this possibility, we developed a theoretical model to evaluate the acoustic radiation force that an elongated cylinder can receive in response to sounds emitted by animals, including insect herbivores, and applied it to the long, cylindrical stem trichomes of the tomato plant Solanum lycopersicum. Based on perturbation theory and validated by finite element simulations, the model quantifies the effects of viscosity and frequency on this acoustic radiation force. Results suggest that acoustic emissions from certain animals, including insect herbivores, may produce acoustic radiation force sufficient to trigger stretch-activated ion channels.

Mechanical stress acclimation in plants: Linking hormones and somatic memory to thigmomorphogenesis

A single event of mechanical stimulation is perceived by mechanoreceptors that transduce rapid transient signalling to regulate gene expression. Prolonged mechanical stress for days to weeks culminates in cellular changes that strengthen the plant architecture leading to thigmomorphogenesis. The convergence of multiple signalling pathways regulates mechanically induced tolerance to numerous biotic and abiotic stresses. Emerging evidence showed prolonged mechanical stimulation can modify the baseline level of gene expression in naive tissues, heighten gene expression, and prime disease resistance upon a subsequent pathogen encounter. The phenotypes of thigmomorphogenesis can persist throughout growth without continued stimulation, revealing somatic-stress memory. Epigenetic processes regulate TOUCH gene expression and could program transcriptional memory in differentiating cells to program thigmomorphogenesis. We discuss the early perception, gene regulatory and phytohormone pathways that facilitate thigmomorphogenesis and mechanical stress acclimation in Arabidopsis and other plant species. We provide insights regarding: (1) the regulatory mechanisms induced by single or prolonged events of mechanical stress, (2) how mechanical stress confers transcriptional memory to induce cross-acclimation to future stress, and (3) why thigmomorphogenesis might resemble an epigenetic phenomenon. Deeper knowledge of how prolonged mechanical stimulation programs somatic memory and primes defence acclimation could transform solutions to improve agricultural sustainability in stressful environments.

Cell twisting during desiccation reveals axial asymmetry in wall organization

Plant cell size and shape are tuned to their function and specified primarily by cellulose microfibril (CMF) patterning of the cell wall. Arabidopsis thaliana leaf trichomes are unicellular structures that act as a physical defense to deter insect feeding. This highly polarized cell type employs a strongly anisotropic cellulose wall to extend and taper, generating sharply pointed branches. During elongation, the mechanisms by which shifts in fiber orientation generate cells with predictable sizes and shapes are unknown. Specifically, the axisymmetric growth of trichome branches is often thought to result from axisymmetric CMF patterning. Here, we analyzed the direction and degree of twist of branches after desiccation to reveal the presence of an asymmetric cell wall organization with a left-hand bias. CMF organization, quantified using computational modeling, suggests a limited reorientation of microfibrils during growth and a maximum branch length limited by the wall axial stiffness. The model provides a mechanism for CMF asymmetry, which occurs after the branch bending stiffness becomes low enough that ambient bending affects the principal stresses. After this stage, the CMF synthesis results in a constant bending stiffness for longer branches. The bending vibration natural frequencies of branches with respect to their length are also discussed.

Materials science and mechanosensitivity of living matter

Living systems are composed of molecules that are synthesized by cells that use energy sources within their surroundings to create fascinating materials that have mechanical properties optimized for their biological function. Their functionality is a ubiquitous aspect of our lives. We use wood to construct furniture, bacterial colonies to modify the texture of dairy products and other foods, intestines as violin strings, bladders in bagpipes, and so on. The mechanical properties of these biological materials differ from those of other simpler synthetic elastomers, glasses, and crystals. Reproducing their mechanical properties synthetically or from first principles is still often unattainable. The challenge is that biomaterials often exist far from equilibrium, either in a kinetically arrested state or in an energy consuming active state that is not yet possible to reproduce de novo. Also, the design principles that form biological materials often result in nonlinear responses of stress to strain, or force to displacement, and theoretical models to explain these nonlinear effects are in relatively early stages of development compared to the predictive models for rubberlike elastomers or metals. In this Review, we summarize some of the most common and striking mechanical features of biological materials and make comparisons among animal, plant, fungal, and bacterial systems. We also summarize some of the mechanisms by which living systems develop forces that shape biological matter and examine newly discovered mechanisms by which cells sense and respond to the forces they generate themselves, which are resisted by their environment, or that are exerted upon them by their environment. Within this framework, we discuss examples of how physical methods are being applied to cell biology and bioengineering.

Sculpting the surface: Structural patterning of plant epidermis

Plant epidermis are multifunctional surfaces that directly affect how plants interact with animals or microorganisms and influence their ability to harvest or protect from abiotic factors. To do this, plants rely on minuscule structures that confer remarkable properties to their outer layer. These microscopic features emerge from the hierarchical organization of epidermal cells with various shapes and dimensions combined with different elaborations of the cuticle, a protective film that covers plant surfaces. Understanding the properties and functions of those tridimensional elements as well as disentangling the mechanisms that control their formation and spatial distribution warrant a multidisciplinary approach. Here we show how interdisciplinary efforts of coupling modern tools of experimental biology, physics, and chemistry with advanced computational modeling and state-of-the art microscopy are yielding broad new insights into the seemingly arcane patterning processes that sculpt the outer layer of plants.

Cellular autofluorescence and browning in trichomes of Chinese cabbage (Brassica campestris) in response to mechanical stimulation and senescence

There is limited information concerning the formation of dot-like browning appearing at the base of trichomes on mature leaves on the Chinese cabbage (Brassica campestris L. ssp. pekinensis). This study confirmed for the first time that enhanced autofluorescence can be induced in the base of trichomes when pressure stimuli is applied to trichomes; the enhanced autofluorescence gradually moves to the top of trichomes and the neighbouring mesophyll tissue within 15min. The excitation of autofluorescence in trichomes was found to be more effective in mature leaves compared to newly emergent leaves. Increased polyphenol oxidase (PPO) activities and reactive oxygen species (ROS) accumulation were also detected in the basal region of trichomes that were subjected to mechanical stimuli. Enhanced fluorescence was observed at the top of the trichomes in senescencing leaves. A browning in the base of the trichomes during leaf senescence was observed. In contrast, no browning occurred at the base of the trichomes in leaves that were subject to pressure stimuli. The blue fluorescence in the trichomes in senescent leaves arises mainly from the condensed cytoplasm. No direct evidence was able to prove that the enhanced autofluorescent substances in the trichomes during leaf senescence are the cause of the browning at the early growth stages.

Functions of plant importin β proteins beyond nucleocytoplasmic transport

In eukaryotic cells, nuclear activities are isolated from other cellular functions by the nuclear envelope. Because the nuclear envelope provides a diffusion barrier for macromolecules, a complex nuclear transport machinery has evolved that is highly conserved from yeast to plants and mammals. Among those components, the importin β family is the most important one. In this review, we summarize recent findings on the biological function of importin β family members, including development, reproduction, abiotic stress responses, and plant immunity. In addition to the traditional nuclear transport function, we highlight the new molecular functions of importin β, including protein turnover, miRNA regulation, and signaling. Taken together, our review will provide a systematic view of this versatile protein family in plants.

Consciousness and cognition in plants

Unlike animal behavior, behavior in plants is traditionally assumed to be completely determined either genetically or environmentally. Under this assumption, plants are usually considered to be noncognitive organisms. This view nonetheless clashes with a growing body of empirical research that shows that many sophisticated cognitive capabilities traditionally assumed to be exclusive to animals are exhibited by plants too. Yet, if plants can be considered cognitive, even in a minimal sense, can they also be considered conscious? Some authors defend that the quest for plant consciousness is worth pursuing, under the premise that sentience can play a role in facilitating plant's sophisticated behavior. The goal of this article is not to provide a positive argument for plant cognition and consciousness, but to invite a constructive, empirically informed debate about it. After reviewing the empirical literature concerning plant cognition, we introduce the reader to the emerging field of plant neurobiology. Research on plant electrical and chemical signaling can help shed light into the biological bases for plant sentience. To conclude, we shall present a series of approaches to scientifically investigate plant consciousness. In sum, we invite the reader to consider the idea that if consciousness boils down to some form of biological adaptation, we should not exclude a priori the possibility that plants have evolved their own phenomenal experience of the world. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Philosophy > Consciousness Neuroscience > Cognition.

Effect of hydric stress-related acoustic emission on transcriptional and biochemical changes associated with a water deficit in Capsicum annuum L

At specific vibration frequencies like ones generated by insects such as caterpillar chewing and bee's buzz-pollination turn on the plants secondary metabolism and their respective pathways gets activated. Thus, studies report that vibrations and sound waves applied to plants improves their fitness performance. Commonly, acoustic treatments for plants have used arbitrarily random frequencies. In this work, a group of signals obtained from hydric-stressed plants was recorded as vibrational patterns using a laser vibrometer. These vibration-signals were classified as representative of each condition and then externally applied as Acoustic Emission Patterns (AEP). The present research hypothesized that specific vibration frequencies could "emulate" a plant signal through mechanical energy based on tplant's ability to recognize vibration pattern similarity to a hydric status. This investigation aimed to apply the AEP's as characteristic vibrations classified as Low hydric stress (LHS), medium hydric stress (MHS), and high hydric stress (HHS) to evaluate their effect on healthy-well watered plants at two developmental stages. In the vegetative stage, the gene expression related to antioxidant and hydric stress responses was assessed. The LHS, MHS, and HHS acoustic treatments up-regulated the peroxidase (Pod) (~2.8, 1.9, and 3.6-fold change, respectively). The superoxide dismutase (Mn-sod) and phenylalanine ammonia-lyase (Pal) genes were up-regulated by HHS (~0.23 and ~0.55-fold change, respectively) and, the chalcone synthase (Chs) gene was induced by MHS (~0.63-fold-change). At the fructification stage, the MHS treatment induced a significant increase in Capsaicin content (5.88-fold change), probably through the at3and kas gene activation. Findings are correlated for a better understanding of plant responses to different multi frequency-signals tones from vibrations with potential for agricultural applications.

Barbara G. Pickard - Queen of Plant Electrophysiology

Barbara Gillespie Pickard (1936-2019) studied plant electrophysiology and mechanosensory biology for more than 50 y. Her first papers on the roles of auxin in plant tropisms were coauthored with Kenneth V. Thimann. Later, she studied plant electrophysiology. She made it clear that plant action potentials are not a peculiar feature of so-called sensitive plants, but that all plants exhibit these fast electric signals. Barbara Gillespie Pickard proposed a neuronal model for the spreading of electric signals induced by mechanical stimuli across plant tissues. In later years, she studied the stretch-activated plasma membrane channels of plants and formulated the plasma-membrane control center model. Barbara Pickard summarized all her findings in a new model of phyllotaxis involving waves of auxin fluxes and mechano-sensory signaling.

Gene expression underlying floral epidermal specialization in Aristolochia fimbriata (Aristolochiaceae)

BACKGROUND AND AIMS

The epidermis constitutes the outermost tissue of the plant body. Although it plays major structural, physiological and ecological roles in embryophytes, the molecular mechanisms controlling epidermal cell fate, differentiation and trichome development have been scarcely studied across angiosperms, and remain almost unexplored in floral organs.

METHODS

In this study, we assess the spatio-temporal expression patterns of GL2, GL3, TTG1, TRY, MYB5, MYB6, HDG2, MYB106-like, WIN1 and RAV1-like homologues in the magnoliid Aristolochia fimbriata (Aristolochiaceae) by using comparative RNA-sequencing and in situ hybridization assays.

KEY RESULTS

Genes involved in Aristolochia fimbriata trichome development vary depending on the organ where they are formed. Stem, leaf and pedicel trichomes recruit most of the transcription factors (TFs) described above. Conversely, floral trichomes only use a small subset of genes including AfimGL2, AfimRAV1-like, AfimWIN1, AfimMYB106-like and AfimHDG2. The remaining TFs, AfimTTG1, AfimGL3, AfimTRY, AfimMYB5 and AfimMYB6, are restricted to the abaxial (outer) and the adaxial (inner) pavement epidermal cells.

CONCLUSIONS

We re-evaluate the core genetic network shaping trichome fate in flowers of an early-divergent angiosperm lineage and show a morphologically diverse output with a simpler genetic mechanism in place when compared to the models Arabidopsis thaliana and Cucumis sativus. In turn, our results strongly suggest that the canonical trichome gene expression appears to be more conserved in vegetative than in floral tissues across angiosperms.

Effects of hydric stress on vibrational frequency patterns of Capsicum annuum plants

Plants that experience a lack of sufficient irrigation undergo hydric stress, which causes the modification of their mechanical properties. These changes include a complex network of chemical and physical signals that interact between plant-plant and plant-environment systems in a mechanism that is still not well understood, and that differs among species. This mechanical response implies different levels of vibration when the plant experiences structural modifications from self-hydraulic adjustments of flux exchange at specific frequencies, with these carrying behavioral information. To measure these signals, highly sensitive instrumentation that allows the decoding of displacement velocity and displacement of plants, which is possible through calibrated equipment such as 3D scanning laser vibrometers, is necessary. Laser vibrometry technology allows for noninvasive measurements in real-time. Physiological changes could reasonably affect the biomechanical condition of plants in terms of the frequency (hertz) and intensity of the plant's vibration. In this research, it is proposed that the frequency changes of a plant's vibration are related to the plant's hydric condition and that these frequency vibrations have the ecological potential to communicate water changes and levels of hydric stress. The peak of the velocity of plant displacements was found to vary from 0.079 to 1.74 mm/s, and natural frequencies (hertz) range is between 1.8 and 2.6 Hz for plants with low hydric stress (LHS), between 1.3 and 1.6 Hz for plants with medium hydric stress (MHS), and between 6.7 and 7.8 Hz for plants with high hydric stress. These values could act as preliminary references for water management using noninvasive techniques and, knowledge of the range of natural frequencies of hydric stress risk in chili pepper crops can be applied in precision agriculture practices.

Vibration of a liquid-filled capillary tube

Liquid-filled capillary tubes are common structures in nature and engineering fields, which often function via vibration. Although liquid-solid interfacial tension plays important roles in the vibration behavior of the liquid-filled capillary tube, it remains elusive how the interfacial tension influences the natural frequency of capillary tube vibration. To address this, we developed a theory of beam-string structure to analyze the influence of liquid-solid interfacial tension on the vibration of a liquid-filled capillary cantilever. We used glass capillary tubes as a demo and experimentally validated the theory, where the reduced liquid-solid interfacial tension in a capillary tube decreases the natural frequencies of small-order modes. We then performed theoretical analysis and found that the change of elastocapillarity number, slenderness ratio and inner/outer radius ratio of capillary tubes enables: in higher order modes, a nonmonotonic change of natural frequency due to mode transformation between a beam and string; for lower order modes, decrease in the natural frequency to zero (increase from zero) due to mode disappearance (appearance). The developed theory would provide guidelines for high-accuracy design of capillary sensors.