Plant acoustics: in the search of a sound mechanism for sound signaling in plants

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.

Cracking the plant VOC sensing code and its practical applications

Volatile organic compounds (VOCs) are essential airborne mediators of interactions between plants. These plant-plant interactions require sophisticated VOC-sensing mechanisms that enable plants to regulate their defenses against pests. However, these interactions are not limited to specific plants or even conspecifics, and can function in very flexible interactions between plants. Sensing and responding to VOCs in plants is finely controlled by their uptake and transport systems as well as by cellular signaling via, for example, chromatin remodeling system-based transcriptional regulation for defense gene activation. Based on the accumulated knowledge about the interactions between plants and their major VOCs, companion plants and biostimulants are being developed for practical applications in agricultural and horticultural pest control, providing a sustainable alternative to harmful chemicals.

Transcriptomic and Physiological Analysis Reveals the Possible Mechanism of Inhibiting Strawberry Aroma Changes by Ultrasound after Harvest

The volatile compounds in strawberries play a significant role in the formation of strawberry aroma. However, these compounds undergo continual changes during storage, resulting in a decline in quality. In this study, a total of 67 volatile organic compounds (VOCs) were identified in strawberries through quantitative analysis. At the end of the storage period, the VOC content in the ultrasonic group was 119.02 µg/kg higher than that in the control group. The results demonstrated that the ultrasonic treatment increased the contents of terpenes and esters at the end of storage. Among these, linalool increased from 67.09 to 91.41 µg/kg, while ethyl cinnamate increased from 92.22 to 106.79 µg/kg. Additionally, the expression of the key metabolic genes closely related to these substances was significantly up-regulated. The expression of the FaNES gene, related to terpene metabolism, was up-regulated by 2.8 times in the second day, while the expression of the FaAAT gene, related to ester metabolism, was up-regulated by 1.5 times. In summary, this study provides a theoretical basis for exploring the mechanism of ultrasonic effect on strawberry flavor and quality after harvest.

How (not) to Talk to a Plant: An Application of Automata Theory to Plant Communication

Plants are capable of a range of complex interactions with the environment. Over the last decade, some authors have used this as evidence to argue that plants are cognitive agents. While there is no consensus on this view, it is certainly interesting to approach the debate from a comparative perspective, trying to understand whether different lineages of plants show different degrees of responsiveness to environmental cues, and how their responses compare with those of animals or humans. In this paper, I suggest that a potentially fruitful approach to these comparative studies is provided by automata theory. Accordingly, I shall present a possible application of this theory to plant communication. Two tentative results will emerge. First, that different lineages may exhibit different levels of complexity in response to similar stimuli. Second, that current evidence does not allow to infer great cognitive sophistication in plants.

Sound Matrix Shaping of Living Matter: From Macrosystems to Cell Microenvironment, Where Mitochondria Act as Energy Portals in Detecting and Processing Sound Vibrations

Vibration and sound are the shaping matrix of the entire universe. Everything in nature is shaped by energy vibrating and communicating through its own sound trail. Every cell within our body vibrates at defined frequencies, generating its peculiar "sound signature". Mitochondria are dynamic, energy-transforming, biosynthetic, and signaling organelles that actively transduce biological information. Novel research has shown that the mitochondrial function of mammalian cells can be modulated by various energetic stimuli, including sound vibrations. Regarding acoustic vibrations, definite types of music have been reported to produce beneficial impacts on human health. In very recent studies, the effects of different sound stimuli and musical styles on cellular function and mitochondrial activity were evaluated and compared in human cells cultured in vitro, investigating the underlying responsible molecular mechanisms. This narrative review will take a multilevel trip from macro to intracellular microenvironment, discussing the intimate vibrational sound activities shaping living matter, delving deeper into the molecular mechanisms underlying the sound modulation of biological systems, and mainly focusing our discussion on novel evidence showing the competence of mitochondria in acting as energy portals capable of sensing and transducing the subtle informational biofields of sound vibration.

Symphonies of Growth: Unveiling the Impact of Sound Waves on Plant Physiology and Productivity

The application of sound wave technology to different plant species has revealed that variations in the Hz, sound pressure intensity, treatment duration, and type of setup of the sound source significantly impact the plant performance. A study conducted on cotton plants treated with Plant Acoustic Frequency Technology (PAFT) highlighted improvements across various growth metrics. In particular, the treated samples showed increases in the height, size of the fourth expanded leaf from the final one, count of branches carrying bolls, quantity of bolls, and weight of individual bolls. Another study showed how the impact of a 4 kHz sound stimulus positively promoted plant drought tolerance. In other cases, such as in transgenic rice plants, GUS expression was upregulated at 250 Hz but downregulated at 50 Hz. In the same way, sound frequencies have been found to enhance the osmotic potential, with the highest observed in samples treated with frequencies of 0.5 and 0.8 kHz compared to the control. Furthermore, a sound treatment with a frequency of 0.4 kHz and a sound pressure level (SPL) of 106 dB significantly increased the paddy rice germination index, as evidenced by an increase in the stem height and relative fresh weight. This paper presents a complete, rationalized and updated review of the literature on the effects of sound waves on the physiology and growth parameters of sound-treated plants.

Transcriptional reprogramming in sound-treated Micro-Tom plants inoculated with Pseudomonas syringae pv. tomato DC3000

Sound vibrations (SV) are known to influence molecular and physiological processes that can improve crop performance and yield. In this study, the effects of three audible frequencies (100, 500 and 1000 Hz) at constant amplitude (90 dB) on tomato Micro-Tom physiological responses were evaluated 1 and 3 days post-treatment. Moreover, the potential use of SV treatment as priming agent for improved Micro-Tom resistance to Pseudomonas syringae pv. tomato DC3000 was tested by microarray. Results showed that the SV-induced physiological changes were frequency- and time-dependent, with the largest changes registered at 1000 Hz at day 3. SV treatments tended to alter the foliar content of photosynthetic pigments, soluble proteins, sugars, phenolic composition, and the enzymatic activity of polyphenol oxidase, peroxidase, superoxide dismutase and catalase. Microarray data revealed that 1000 Hz treatment is effective in eliciting transcriptional reprogramming in tomato plants grown under normal conditions, but particularly after the infection with Pst DC3000. Broadly, in plants challenged with Pst DC3000, the 1000 Hz pretreatment provoked the up-regulation of unique differentially expressed genes (DEGs) involved in cell wall reinforcement, phenylpropanoid pathway and defensive proteins. In addition, in those plants, DEGs associated with enhancing plant basal immunity, such as proteinase inhibitors, pathogenesis-related proteins, and carbonic anhydrase 3, were notably up-regulated in comparison with non-SV pretreated, infected plants. These findings provide new insights into the modulation of Pst DC3000-tomato interaction by sound and open up prospects for further development of strategies for plant disease management through the reinforcement of defense mechanisms in Micro-Tom plants.

Sound waves alter the viability of tobacco cells via changes in cytosolic calcium, membrane integrity, and cell wall composition

The effect of sound waves (SWs) on plant cells can be considered as important as other mechanical stimuli like touch, wind, rain, and gravity, causing certain responses associated with the downstream signaling pathways on the whole plant. The objective of the present study was to elucidate the response of suspension-cultured tobacco cells (Nicotiana tabacum L. cv Burley 21) to SW at different intensities. The sinusoidal SW (1,000 Hz) was produced through a signal generator, amplified, and beamed to the one layer floating tobacco cells inside a soundproof chamber at intensities of 60, 75, and 90 dB at the plate level for 15, 30, 45, and 60 min. Calibration of the applied SW intensities, accuracy, and uniformity of SW was performed by a sound level meter, and the cells were treated. The effect of SW on tobacco cells was monitored by quantitation of cytosolic calcium, redox status, membrane integrity, wall components, and the activity of wall modifying enzymes. Cytosolic calcium ions increased as a function of sound intensity with a maximum level of 90 dB. Exposure to 90 dB was also accompanied by a significant increase of H2O2 and membrane lipid peroxidation rate but the reduction of total antioxidant and radical scavenging capacities. The increase of wall rigidity in these cells was attributed to an increase in wall-bound phenolic acids and lignin and the activities of phenylalanine ammonia-lyase and covalently bound peroxidase. In comparison, in 60- and 75 dB, radical scavenging capacity increased, and the activity of wall stiffening enzymes reduced, but cell viability showed no changes. The outcome of the current study reveals that the impact of SW on plant cells is started by an increase in cytosolic calcium. However, upon calcium signaling, downstream events, including alteration of H2O2 and cell redox status and the activities of wall modifying enzymes, determined the extent of SW effects on tobacco cells.

Developmental programming by prenatal sounds: insights into possible mechanisms

In recent years, the impact of prenatal sound on development, notably for programming individual phenotypes for postnatal conditions, has increasingly been revealed. However, the mechanisms through which sound affects physiology and development remain mostly unexplored. Here, I gather evidence from neurobiology, developmental biology, cellular biology and bioacoustics to identify the most plausible modes of action of sound on developing embryos. First, revealing often-unsuspected plasticity, I discuss how prenatal sound may shape auditory system development and determine individuals' later capacity to receive acoustic information. I also consider the impact of hormones, including thyroid hormones, glucocorticoids and androgen, on auditory plasticity. Second, I review what is known about sound transduction to other - non-auditory - brain regions, and its potential to input on classical developmental programming pathways. Namely, the auditory pathway has direct anatomical and functional connectivity to the hippocampus, amygdala and/or hypothalamus, in mammals, birds and anurans. Sound can thus trigger both immediate and delayed responses in these limbic regions, which are specific to the acoustic stimulus and its biological relevance. Third, beyond the brain, I briefly consider the possibility for sound to directly affect cellular functioning, based on evidence in earless organisms (e.g. plants) and cell cultures. Together, the multi-disciplinary evidence gathered here shows that the brain is wired to allow multiple physiological and developmental effects of sound. Overall, there are many unexplored, but possible, pathways for sound to impact even primitive or immature organisms. Throughout, I identify the most promising research avenues for unravelling the processes of acoustic developmental programming.

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.

The role of sound stimulation in production of plant secondary metabolites

Sound vibration is one of natural stimuli trigging physiological changes in plants. Recent studies showed that sound waves stimulated production of a variety of plant secondary metabolites, including flavonoids, in order to enhance seed germination, flowering, growth or defense. In this review, we examine the potential role of sound stimulation on the biosynthesis of secondary metabolites and the followed cascade of physiological changes in plants, from the perspective of transcriptional regulation and epigenetic regulation for the first time. A systematic summary showed that a wide range of factors may regulate the production of secondary metabolites, including plant species, growth stage, sound types, sound frequency, sound intensity level and exposure time, etc. Biochemical and physiological changes due to sound stimulation were thoroughly summarized as well, for secondary metabolites can also act as a free radical scavenger, or a hormone signaling molecule. We also discussed the limits of previous studies, and the future application of sound waves in biosynthesis of plant secondary metabolites.

ROS interplay between plant growth and stress biology: Challenges and future perspectives

In plants, reactive oxygen species (ROS) have emerged as a multifunctional signaling molecules that modulate diverse stress and growth responses. Earlier studies on ROS in plants primarily focused on its toxicity and ROS-scavenging processes, but recent findings are offering new insights on its role in signal perception and transduction. Further, the interaction of cell wall receptors, calcium channels, HATPase, protein kinases, and hormones with NADPH oxidases (respiratory burst oxidase homologues (RBOHs), provides concrete evidence that ROS regulates major signaling cascades in different cellular compartments related to stress and growth responses. However, at the molecular level there are many knowledge gaps regarding how these players influence ROS signaling and how ROS regulate them during growth and stress events. Furthermore, little is known about how plant sensors or receptors detect ROS under various environmental stresses and induce subsequent signaling cascades. In light of this, we provided an update on the role of ROS signaling in plant growth and stress biology. First, we focused on ROS signaling, its production and regulation by cell wall receptor like kinases. Next, we discussed the interplay between ROS, calcium and hormones, which forms a major signaling trio regulatory network of signal perception and transduction. We also provided an overview on ROS and nitric oxide (NO) crosstalk. Furthermore, we emphasized the function of ROS signaling in biotic, abiotic and mechanical stresses, as well as in plant growth and development. Finally, we conclude by highlighting challenges and future perspectives of ROS signaling in plants that warrants future investigation.

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.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

Sounds emitted by plants under stress are airborne and informative

Stressed plants show altered phenotypes, including changes in color, smell, and shape. Yet, airborne sounds emitted by stressed plants have not been investigated before. Here we show that stressed plants emit airborne sounds that can be recorded from a distance and classified. We recorded ultrasonic sounds emitted by tomato and tobacco plants inside an acoustic chamber, and in a greenhouse, while monitoring the plant's physiological parameters. We developed machine learning models that succeeded in identifying the condition of the plants, including dehydration level and injury, based solely on the emitted sounds. These informative sounds may also be detectable by other organisms. This work opens avenues for understanding plants and their interactions with the environment and may have significant impact on agriculture.

Ultrasonic washing as an abiotic elicitor to increase the phenolic content in fruits and vegetables: A review

Ultrasonic washing has been widely applied to the postharvest storage of fruits and vegetables as a residue-free physical washing technology, which plays an important role in improving shelf-life, safety, and nutritional value. Phenolics are a large group of phytochemicals widespread in fruits and vegetables, and they have been considered potential protective factors against some diseases because of potent antioxidative properties. Previous studies have shown that ultrasonic washing can increase the phenolic content of fruits and vegetables immediately or during storage through the induction of plant stress responses, which is of great significance for improving the functional and nutritional value of fruits and vegetables. However, the mechanisms of ultrasound as an elicitor to improve the phenolic content remain controversial. Therefore, this review summarizes the applications of ultrasonic washing to increase the phenolic content in fruits and vegetables. Meanwhile, the corresponding physiological stress response mechanisms of the phenolic accumulation in terms of immediate stress responses (i.e., higher extractability of phenolics) and late stress responses (i.e., metabolism of phenolics) are expounded. Moreover, a hypothetical model is proposed to explain phenolic biosynthesis triggered by signaling molecules produced under ultrasound stress, including primary signal (i.e., extracellular adenosine triphosphate) and secondary signals (e.g., reactive oxygen species, Ca²⁺ , NO, jasmonates, and ethylene). Additionally, the techno-economic feasibility of ultrasonic washing technology is also discussed. Further, challenges and trends for further development of ultrasonic washing as an abiotic elicitor applied to the postharvest storage of fruits and vegetables are presented.

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.

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.

Coupling computational vibrational models and experimental biotremology to develop a green pest control strategy against the greenhouse whitefly Trialeurodes vaporariorum

In applied biotremology, vibrational signals or cues are exploited to manipulate the target species behaviour. To develop an efficient pest control strategy, other than a detailed investigation into the pest biology and behaviour, the role of the substrate used to transmit the signal is an important feature to be considered, since it may affect vibrations spreading and effective signal transmission and perception. Therefore, we used a multi-disciplinary approach to develop a control technique against the greenhouse whitefly, Trialeurodes vaporariorum. First, an ad hoc vibrational disruptive noise has been developed, based on the acquired knowledge about the mating behaviour and vibrational communication of the mated species. Subsequently, we employed finite-element models to investigate a growing tomato plant response to the aforesaid noise. Modelling how vibrations spread along the plant allowed us to set up a greenhouse experiment to assess the efficacy in terms of insect population of the vibrational treatment, which was administrated through vibrational plates. The green methodology applied in this study represents an innovative, environmentally sound alternative to the usage of synthetic pesticides.

Stress-Induced Volatile Emissions and Signalling in Inter-Plant Communication

The sessile plant has developed mechanisms to survive the "rough and tumble" of its natural surroundings, aided by its evolved innate immune system. Precise perception and rapid response to stress stimuli confer a fitness edge to the plant against its competitors, guaranteeing greater chances of survival and productivity. Plants can "eavesdrop" on volatile chemical cues from their stressed neighbours and have adapted to use these airborne signals to prepare for impending danger without having to experience the actual stress themselves. The role of volatile organic compounds (VOCs) in plant-plant communication has gained significant attention over the past decade, particularly with regard to the potential of VOCs to prime non-stressed plants for more robust defence responses to future stress challenges. The ecological relevance of such interactions under various environmental stresses has been much debated, and there is a nascent understanding of the mechanisms involved. This review discusses the significance of VOC-mediated inter-plant interactions under both biotic and abiotic stresses and highlights the potential to manipulate outcomes in agricultural systems for sustainable crop protection via enhanced defence. The need to integrate physiological, biochemical, and molecular approaches in understanding the underlying mechanisms and signalling pathways involved in volatile signalling is emphasised.

Micromachined Tools Using Acoustic Wave Triggering for the Interaction with the Growth of Plant Biological Systems

A plant biological system is exposed to external influences. In general, each plant has its characteristics and needs with specific interaction mechanisms adapted to its survival. Interactions between systems can be examined and modeled as energy exchanges of mechanical, chemical or electrical variables. Thus, each specific interaction can be examined by triggering the system via a specific stimulus. The objective of this work was to study a specific stimulus (mechanical stimulation) as a driver of plants and their interaction with the environment. In particular, the experimental design concerns the setting up and testing of an automatic source of mechanical stimuli at different wavelengths, generated by an electromechanical transducer, to induce a micro-interaction in plants (or in parts of them) that produces a specific behavior (hypothesis) of plants. Four different experimental setups were developed for this work, each pursuing the same objective: the analysis of the germination process induced by stimulation by sound waves in the audible range. It can be said that the introduction of sound waves as a stimulant or a brake for the growth of plants can offer significant advantages when used on a large scale in the primary sector, since these effects can be used instead of polluting chemical solutions.

Is There a Role for Sound in Plants?

Plants have long been considered passive, static, and unchanging organisms, but this view is finally changing. More and more knowledge is showing that plants are aware of their surroundings, and they respond to a surprising variety of stimuli by modifying their growth and development. Plants extensively communicate with the world around them, above and below ground. Although communication through mycorrhizal networks and Volatile Organic Compounds has been known for a long time, acoustic perception and communication are somehow a final frontier of research. Perhaps surprisingly, plants not only respond to sound, they actually seem to emit sound as well. Roots emit audible clicks during growth, and sounds are emitted from xylem vessels, although the nature of these acoustic emissions still needs to be clarified. Even more interesting, there is the possibility that these sounds carry information with ecological implications, such as alerting insects of the hydration state of a possible host plant, and technological implications as well. Monitoring sound emissions could possibly allow careful monitoring of the hydration state of crops, which could mean significantly less water used during irrigation. This review summarizes the current knowledge on sound perception communication in plants and illustrates possible implications and technological applications.

Prenatal acoustic programming of mitochondrial function for high temperatures in an arid-adapted bird

Sound is an essential source of information in many taxa and can notably be used by embryos to programme their phenotypes for postnatal environments. While underlying mechanisms are mostly unknown, there is growing evidence for the involvement of mitochondria-main source of cellular energy (i.e. ATP)-in developmental programming processes. Here, we tested whether prenatal sound programmes mitochondrial metabolism. In the arid-adapted zebra finch, prenatal exposure to 'heat-calls'-produced by parents incubating at high temperatures-adaptively alters nestling growth in the heat. We measured red blood cell mitochondrial function, in nestlings exposed prenatally to heat- or control-calls, and reared in contrasting thermal environments. Exposure to high temperatures always reduced mitochondrial ATP production efficiency. However, as expected to reduce heat production, prenatal exposure to heat-calls improved mitochondrial efficiency under mild heat conditions. In addition, when exposed to an acute heat-challenge, LEAK respiration was higher in heat-call nestlings, and mitochondrial efficiency low across temperatures. Consistent with its role in reducing oxidative damage, LEAK under extreme heat was also higher in fast growing nestlings. Our study therefore provides the first demonstration of mitochondrial acoustic sensitivity, and brings us closer to understanding the underpinning of acoustic developmental programming and avian strategies for heat adaptation.

Plant Communication

Communication occurs when a sender emits a cue perceived by a receiver that changes the receiver's behavior. Plants perceive information regarding light, water, other nutrients, touch, herbivores, pathogens, mycorrhizae, and nitrogen-fixing bacteria. Plants also emit cues perceived by other plants, beneficial microbes, herbivores, enemies of herbivores, pollinators, and seed dispersers. Individuals responding to light cues experienced increased fitness. Evidence for benefits of responding to cues involving herbivores and pathogens is more limited. The benefits of emitting cues are also less clear, particularly for plant–plant communication. Reliance on multiple or dosage-dependent cues can reduce inappropriate responses, and plants often remember past cues. Plants have multiple needs and prioritize conflicting cues such that the risk of abiotic stress is treated as greater than that of shading, which is in turn treated as greater than that of consumption. Plants can distinguish self from nonself and kin from strangers. They can identify the species of competitor or consumer and respond appropriately. Cues involving mutualists often contain highly specific information.

Quantification of crocin, picrocrocin and safranal in saffron stigmas obtained from sounded corms with acoustic waves

INTRODUCTION

Plant acoustic frequency technology (PAFT) is the effect or treatment of a plant with a specific frequency sound wave.

OBJECTIVE

The sound waves with different frequencies and a sound pressure level 77 dB were emitted on the saffron corms in a controlled environment using aeroponic cultivation and the contents of crocin, picrocrocin and safranal in their produced stigmas were analysed by high-performance liquid chromatography. For this purpose, the corms were divided into two groups. In group 1, sound waves with the frequencies of 0.5, 1 and 2 kHz were emitted on saffron corms in different stages of sprouting, flowering and the whole stage of sprouting and flowering. In group 2, sonication was performed on the corms during the flowering stage at 4, 8, 12 and 16 kHz frequencies.

RESULTS

The changes in the contents of crocin, picrocrocin and safranal were not significantly compared to the control at 0.5, 1 and 2 kHz frequencies in the stages of sprouting and flowering of corms. While the higher frequencies (4, 8, 12 and 16 kHz) in flowering stage were affected significantly, the crocin and picrocrocin content increased 8.5% and 30%, applying the frequency of 12 and 8 kHz, respectively. Also, the effect of sound exposure time per day with the frequency of 16 kHz at 15, 30 and 60 min were investigated.

CONCLUSION

The findings showed that the corms could be affected by sounding in the different stages of growth of the corm and also in the content of secondary metabolites.

Demonstration of laser biospeckle method for speedy in vivo evaluation of plant-sound interactions with arugula

In recent years, it is becoming clearer that plant growth and its yield are affected by sound with certain sounds, such as seedling of corn directing itself toward the sound source and its ability to distinguish stuttering of larvae from other sounds. However, methods investigating the effects of sound on plants either take a long time or are destructive. Here, we propose using laser biospeckle, a non-destructive and non-contact technique, to investigate the activities of an arugula plant for sounds of different frequencies, namely, 0 Hz or control, 100 Hz, 1 kHz, 10 kHz, including rock and classical music. Laser biospeckles are generated when scattered light from biological tissues interfere, and the intensities of such speckles change in time, and these changes reflect changes in the scattering structures within the biological tissue. A leaf was illuminated by light from a laser light of wavelength 635 nm, and the biospeckles were recorded as a movie by a CMOS camera for 20 sec at 15 frames per second (fps). The temporal correlation between the frames was characterized by a parameter called biospeckle activity (BA)under the exposure to different sound stimuli of classical and rock music and single-frequency sound stimuli for 1min. There was a clear difference in BA between the control and other frequencies with BA for 100 Hz being closer to control, while at higher frequencies, BA was much lower, indicating a dependence of the activity on the frequency. As BA is related to changes from both the surface as well as from the internal structures of the leaf, LSM (laser scanning microscope) observations conducted to confirm the change in the internal structure revealed more than 5% transient change in stomatal size following exposure to one minute to high frequency sound of 10kHz that reverted within ten minutes. Our results demonstrate the potential of laser biospeckle to speedily monitor in vivo response of plants to sound stimuli and thus could be a possible screening tool for selecting appropriate frequency sounds to enhance or delay the activity of plants. (337 words).

Ethylene Induced by Sound Stimulation Enhances Anthocyanin Accumulation in Grape Berry Skin through Direct Upregulation of UDP-Glucose: Flavonoid 3-O-Glucosyltransferase

Global warming has resulted in the loss of anthocyanin accumulation in berry skin. Sound stimulation can be used as a potential method for enhancing fruit color development since many plants recognize sound vibration as an external stimulus and alter their physiological status in response to it. Sound stimulation (sine wave sound at 1000 Hz) enhanced anthocyanin accumulation in grape cultured cells and berry skins in field-grown grapevines at the early stage of ripening. The transcription of UFGT and ACO2, which encode the key enzymes in anthocyanin and ethylene biosynthesis, respectively, was upregulated in grape cultured cells exposed to sound stimulation. In contrast, the transcription of MybA1 and NCED1, which encode a transcription factor for UFGT and a key enzyme in abscisic acid biosynthesis, respectively, was not affected by the sound stimulation. A treatment with an ethylene biosynthesis inhibitor, aminoethoxyvinyl glycine hydrochloride, revered the enhancement of anthocyanin accumulation by sound stimulation. As the promoter assay using a GUS reporter gene demonstrated that UFGT promoter was directly activated by the ethylene-releasing compound ethephon, which enhanced anthocyanin accumulation in grape cultured cells, we conclude that sound stimulation enhanced anthocyanin accumulation through the direct upregulation of UFGT by ethylene biosynthesis. Our findings suggest that sound stimulation contributes to alleviating poor coloration in berry skin as a novel and innovative practical technique in viticulture.

Meristematic Connectome: A Cellular Coordinator of Plant Responses to Environmental Signals?

Mechanical stress in tree roots induces the production of reaction wood (RW) and the formation of new branch roots, both functioning to avoid anchorage failure and limb damage. The vascular cambium (VC) is the factor responsible for the onset of these responses as shown by their occurrence when all primary tissues and the root tips are removed. The data presented confirm that the VC is able to evaluate both the direction and magnitude of the mechanical forces experienced before coordinating the most fitting responses along the root axis whenever and wherever these are necessary. The coordination of these responses requires intense crosstalk between meristematic cells of the VC which may be very distant from the place where the mechanical stress is first detected. Signaling could be facilitated through plasmodesmata between meristematic cells. The mechanism of RW production also seems to be well conserved in the stem and this fact suggests that the VC could behave as a single structure spread along the plant body axis as a means to control the relationship between the plant and its environment. The observation that there are numerous morphological and functional similarities between different meristems and that some important regulatory mechanisms of meristem activity, such as homeostasis, are common to several meristems, supports the hypothesis that not only the VC but all apical, primary and secondary meristems present in the plant body behave as a single interconnected structure. We propose to name this structure “meristematic connectome” given the possibility that the sequence of meristems from root apex to shoot apex could represent a pluricellular network that facilitates long-distance signaling in the plant body. The possibility that the “meristematic connectome” could act as a single structure active in adjusting the plant body to its surrounding environment throughout the life of a plant is now proposed.

Uncovering Transcriptional Responses to Fractional Gravity in Arabidopsis Roots

Although many reports characterize the transcriptional response of Arabidopsis seedlings to microgravity, few investigate the effect of partial or fractional gravity on gene expression. Understanding plant responses to fractional gravity is relevant for plant growth on lunar and Martian surfaces. The plant signaling flight experiment utilized the European Modular Cultivation System (EMCS) onboard the International Space Station (ISS). The EMCS consisted of two rotors within a controlled chamber allowing for two experimental conditions, microgravity (stationary rotor) and simulated gravity in space. Seedlings were grown for 5 days under continuous light in seed cassettes. The arrangement of the seed cassettes within each experimental container results in a gradient of fractional g (in the spinning rotor). To investigate whether gene expression patterns are sensitive to fractional g, we carried out transcriptional profiling of root samples exposed to microgravity or partial g (ranging from 0.53 to 0.88 g). Data were analyzed using DESeq2 with fractional g as a continuous variable in the design model in order to query gene expression across the gravity continuum. We identified a subset of genes whose expression correlates with changes in fractional g. Interestingly, the most responsive genes include those encoding transcription factors, defense, and cell wall-related proteins and heat shock proteins.

Sound Stimulation Can Affect Saccharomyces cerevisiae Growth and Production of Volatile Metabolites in Liquid Medium

The biological effect of sound on microorganisms has been a field of interest for many years, with studies mostly focusing on ultrasonic and infrasonic vibrations. In the audible range (20 Hz to 20 kHz), sound has been shown to both increase colony formation and disrupt microbial growth, depending upon the organism and frequency of sound used. In the brewer's yeast Saccharomyces cerevisiae, sound has been shown to significantly alter growth, increase alcohol production, and affect the metabolite profile. In this study, S. cerevisiae was exposed to a continuous 90 dB @ 20 μPa tone at different frequencies (0.1 kHz, 10 kHz, and silence). Fermentation characteristics were monitored over a 50-h fermentation in liquid malt extract, with a focus on growth rate and biomass yield. The profile of volatile metabolites at the subsequent stationary phase of the ferment was characterised by headspace gas chromatography-mass spectrometry. Sound treatments resulted in a 23% increase in growth rate compared to that of silence. Subsequent analysis showed significant differences in the volatilomes between all experimental conditions. Specifically, aroma compounds associated with citrus notes were upregulated with the application of sound. Furthermore, there was a pronounced difference in the metabolites produced in high- versus low-frequency sounds. This suggests industrial processes, such as beer brewing, could be modulated by the application of audible sound at specific frequencies during growth.

A tuning point in plant acoustics investigation

In a very recent book called Sensory Biology of Plants, published by renowned publisher Springer Nature, the authors stated that the scientific literature gathered so far regarding knowledge around the field of Plant Acoustics allows us to divert the focus from the question "whether plants perceive sound" toward the questions "how and why they are doing it" Some phenomena are well known: roots perceive the sound of flowing water and display a sound-mediated growth toward the water source, while the buzz pollination process allows plants to minimize the pollen lost and maximize which is collected by true pollinators. But plants are far more perceptive and responsive to their environment than we generally consider them to be, and they are communicating far more information than we realize if we only took all their signals (VOCs, sound, exudates, etc.) into a greater picture. Could Volatile Organic Compounds (VOCs) be involved in mediating more responses than we imagine? VOC synthesis and release is known to be elicited also by electrical signals caused by mechanical stimuli, touching and wounding being among these, serving as info-chemicals in the communication between plants ("eavesdropping"), and within the organs of the same plant, in order for it to get synchronized with its surroundings. This paper is an overview of the discoveries around plant perception with a focus on the link between mechanical stimuli, as sound vibrations are, and changes in plant physiology leading to VOC emission.

Seagrass Posidonia is impaired by human-generated noise

The last hundred years have seen the introduction of many sources of artificial noise in the sea environment which have shown to negatively affect marine organisms. Little attention has been devoted to how much this noise could affect sessile organisms. Here, we report morphological and ultrastructural changes in seagrass, after exposure to sounds in a controlled environment. These results are new to aquatic plants pathology. Low-frequency sounds produced alterations in Posidonia oceanica root and rhizome statocysts, which sense gravity and process sound vibration. Nutritional processes of the plant were affected as well: we observed a decrease in the number of rhizome starch grains, which have a vital role in energy storage, as well as a degradation in the specific fungal symbionts of P. oceanica roots. This sensitivity to artificial sounds revealed how sound can potentially affect the health status of P. oceanica. Moreover, these findings address the question of how much the increase of ocean noise pollution may contribute in the future to the depletion of seagrass populations and to biodiversity loss.

Solé et al. report morphological and ultrastructural changes in seagrass, after exposure to human generated noise. These data suggest that noise pollution can potentially affect the health status of seagrass and thereby contribute to the depletion of seagrass populations.

Acoustic developmental programming: a mechanistic and evolutionary framework

Conditions experienced prenatally, by modulating developmental processes, have lifelong effects on individual phenotypes and fitness, ultimately influencing population dynamics. In addition to maternal biochemical cues, prenatal sound is emerging as a potent alternative source of information to direct embryonic development. Recent evidence suggests that prenatal acoustic signals can program individual phenotypes for predicted postnatal environmental conditions, which improves fitness. Across taxonomic groups, embryos have now been shown to have immediate adaptive responses to external sounds and vibrations, and direct developmental effects of sound and noise are increasingly found. Establishing the full developmental, ecological, and evolutionary impact of early soundscapes will reveal how embryos interact with the external world, and potentially transform our understanding of developmental plasticity and adaptation to changing environments.

Mechanostimulation: a promising alternative for sustainable agriculture practices

Plants memorize events associated with environmental fluctuations. The integration of environmental signals into molecular memory allows plants to cope with future stressors more efficiently-a phenomenon that is known as 'priming'. Primed plants are more resilient to environmental stresses than non-primed plants, as they are capable of triggering more robust and faster defence responses. Interestingly, exposure to various forms of mechanical stimuli (e.g. touch, wind, or sound vibration) enhances plants' basal defence responses and stress tolerance. Thus, mechanostimulation appears to be a potential priming method and a promising alternative to chemical-based priming for sustainable agriculture. According to the currently available method, mechanical treatment needs to be repeated over a month to alter plant growth and defence responses. Such a long treatment protocol restricts its applicability to fast-growing crops. To optimize the protocol for a broad range of crops, we need to understand the molecular mechanisms behind plant mechanoresponses, which are complex and depend on the frequency, intervals, and duration of the mechanical treatment. In this review, we synthesize the molecular underpinnings of plant mechanoperception and signal transduction to gain a mechanistic understanding of the process of mechanostimulated priming.

Plant Health and Sound Vibration: Analyzing Implications of the Microbiome in Grape Wine Leaves

Understanding the plant microbiome is a key for plant health and controlling pathogens. Recent studies have shown that plants are responsive towards natural and synthetic sound vibration (SV) by perception and signal transduction, which resulted in resistance towards plant pathogens. However, whether or not native plant microbiomes respond to SV and the underlying mechanism thereof remains unknown. Within the present study we compared grapevine-associated microbiota that was perpetually exposed to classical music with a non-exposed control group from the same vineyard in Stellenbosch, South Africa. By analyzing the 16S rRNA gene and ITS fragment amplicon libraries we found differences between the core microbiome of SV-exposed leaves and the control group. For several of these different genera, e.g., Bacillus, Kocuria and Sphingomonas, a host-beneficial or pathogen-antagonistic effect has been well studied. Moreover, abundances of taxa identified as potential producers of volatile organic compounds that contribute to sensory characteristics of wines, e.g., Methylobacterium, Sphingomonas, Bacillus and Sporobolomyces roseus, were either increased or even unique within the core music-exposed phyllosphere population. Results show an as yet unexplored avenue for improved plant health and the terroir of wine, which are important for environmentally friendly horticulture and consumer appreciation. Although our findings explain one detail of the long-term positive experience to improve grapevine’s resilience by this unusual but innovative technique, more mechanistic studies are necessary to understand the whole interplay.

Sound perception and its effects in plants and algae

Life evolved in an acoustic world. Sound is perceived in different ways by the species that inhabit the Planet. Among organisms, also some algal species seem to respond to sound stimuli with increased cell growth and productivity. The purpose of this Short Communication is to provide an overview of the current literature about various organisms and sound, with particular attention to algal organisms, which, when subjected to sound applications, can change their metabolism accordingly.

Abiotic stress elements in in vitro potato (Solanum tuberosum L.) exposed to air-based and liquid-based ultrasound: A comparative transcriptomic assessment

Ultrasound (US) can modify the plant growth and development. Previous assessments of the transcriptome of in vitro potato (Solanum tuberosum L.) exposed to US transmitted through air (AB-US) or liquid (PE-US) revealed the up- or down-regulation of several stress-related differentially expressed genes (DEGs) related to abiotic stress. In a bid to better characterize stress-related elements over a four-week period, the transcriptome of AB-US was compared to that of PE-US. When comparing the controls of both treatments, DEGs related to hypoxia were not detected. Nevertheless, hypoxia-related DEGs were detected in the combination of liquid medium and ultrasonication. DEGs coding for chitinase, peroxidase, glutathione-S-transferase, transcription factors of ERF (ethylene responsive factor), DREB (dehydration-responsive element-binding), WRKY and MYB were also significantly highly expressed in PE-US, relative to AB-US. Up- and down-regulation of DEGs related to metabolic processes, and enzymes of the antioxidant system also confirm that PE-US is a more acute abiotic stress than AB-US. KEY MESSAGE: A transcriptomic analysis revealed that liquid-based ultrasonication was a stronger abiotic stressor than air-based ultrasonication. Of particular interest were the heat shock proteins and transcription factors in this comparison. Despite the ultrasound stress, explants survived and plantlets developed.

Diffusions of sound frequencies designed to target dehydrins induce hydric stress tolerance in Pisum sativum seedings

Among plant responses to environmentally induced stress modulating protein expression appears to be a key stage in inducible signaling. Our study was focused on an innovative strategy to stimulate plant stress resistance, namely, the use of targeted sequences of specific sound frequencies. The influence of acoustic stimulation on plant protein synthesis was investigated. In our study green peas, Pisum sativum, were cultured under hydric stress conditions with targeted acoustic stimulation. Acoustic sequences targeting dehydrins (DHN) which accumulate in plants in response to dehydration were studied. We experimented on pea seeding with two different sequences of sounds: the first one corresponded to DHN cognate protein and the second one was aimed at the DHN consensus sequence. Shoot elongation after pea seed germination was estimated by fresh weight gain studied in the presence of various conditions of exposure to both sequences of sounds. DHN expression in peas was quantified via ELISA tests and Western-blotting by using specific antibodies. A significant increase in fresh weight in peas grown under exposure to the DHN cognate sound sequence was observed, whereas the consensus sound sequence had no effect on growth. Moreover, the 37kDa DHN amount was increased in peas treated with the consensus acoustic sequence. These results suggest that the expression of DHN could be specifically modulated by a designed acoustic stimulus.

Underground electrotonic signal transmission between plants

Plants can communicate with other plants using wireless pathways above and underground. Some examples of these underground communication pathways are: (1) mycorrhizal networks in the soil; (2) the plants' rhizosphere; (3) acoustic communication; (4) naturally grafting of roots of the same species; (5) signaling chemicals exchange between roots of plants; and (6) electrical signal transmission between plants through the soil. To avoid the possibility of communication between plants using mechanisms (1)-(5), soils in both pots with plants can be connected by Ag/AgCl or platinum wires. Electrostimulation Aloe vera or cabbage plants induces electrotonic potentials transmission in the electro-stimulated plants as well as in the neighboring plants located in the same or different electrically connected pots regardless if plants are the same or different types. The amplitude and sign of electrotonic potentials in both electrostimulated and neighboring plants depend on the amplitude, rise, and fall of the applied voltage. Electrostimulation serves as an important tool for the evaluation of mechanisms of underground communication in the plant-wide web. The previously developed mathematical model of electrotonic potentials transmission within and between tomato plants, which is supported by the experimental data, is generic enough to be used for simulation study and predicting the intercellular and intracellular communication in the form of electrical signals in the electrical networks within and between a variety of plants.

Sound waves affect the total flavonoid contents in Medicago sativa, Brassica oleracea and Raphanus sativus sprouts

BACKGROUND

Sound waves are emerging as a potential biophysical alternative to traditional methods for enhancing plant growth and phytochemical contents. However, little information is available on the improvement of the concentration of functional metabolites like flavonoids in sprouts using sound waves. In this study, different frequencies of sound waves with short and long exposure times were applied to three important varieties to improve flavonoid content. The aim of this study was to investigate the effect of sound waves on flavonoid content on the basis of biochemical and molecular characteristics.

RESULTS

We examined the effects of various sound wave treatments (250 Hz to 1.5 kHz) on flavonoid production in alfalfa (Medicago sativa), broccoli (Brassica oleracea) and red young radish (Raphanus sativus). The results showed that sound wave treatments differentially altered the total flavonoid contents depending upon the growth stages, species and frequency of and exposure time to sound waves. Sound wave treatments of alfalfa (250 Hz), broccoli sprouts (800 Hz) and red young radish sprouts (1 kHz) increased the total flavonoid content by 200%, 35% and 85%, respectively, in comparison with untreated control. Molecular analysis showed that sound waves induce the expression of genes of the flavonoid biosynthesis pathway, which positively corresponds to the flavonoid content. Moreover, the sound wave treatment significantly improves the antioxidant efficiency of sprouts.

CONCLUSIONS

The significant improvement of flavonoid content in sprouts with sound waves makes their use a potential and promising technology for the production of agriculture-based functional foods. © 2019 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Proteomic Changes in the Sound Vibration-Treated Arabidopsis thaliana Facilitates Defense Response during Botrytis cinerea Infection

Sound vibration (SV) treatment can trigger various molecular and physiological changes in plants. Previously, we showed that pre-exposure of Arabidopsis plants to SV boosts its defense response against Botrytis cinerea fungus. The present study was aimed to investigate the changes in the proteome states in the SV-treated Arabidopsis during disease progression. Proteomics analysis identified several upregulated proteins in the SV-infected plants (i.e., SV-treated plants carrying Botrytis infection). These upregulated proteins are involved in a plethora of biological functions, e.g., primary metabolism (i.e., glycolysis, tricarboxylic acid cycle, ATP synthesis, cysteine metabolism, and photosynthesis), redox homeostasis, and defense response. Additionally, our enzyme assays confirmed the enhanced activity of antioxidant enzymes in the SV-infected plants compared to control plants. Broadly, our results suggest that SV pre-treatment evokes a more efficient defense response in the SV-infected plants by modulating the primary metabolism and reactive oxygen species scavenging activity.

Chemical Responses of Nicotiana tabacum (Solanaceae) Induced by Vibrational Signals of a Generalist Herbivore

Plants are able to sense their environment and respond appropriately to different stimuli. Vibrational signals (VS) are one of the most widespread yet understudied ways of communication between organisms. Recent research into the perception of VS by plants showed that they are ecologically meaningful signals involved in different interactions of plants with biotic and abiotic agents. We studied changes in the concentration of alkaloids in tobacco plants induced by VS produced by Phthorimaea operculella (Lepidoptera: Gelechiidae), a generalist caterpillar that naturally feeds on the plant. We measured the concentration of nicotine, nornicotine, anabasine and anatabine in four treatments applied to 11-weeks old tobacco plant: a) Co = undamaged plants, b) Eq = Playback equipment attached to the plant without VS, c) Ca = Plants attacked by P. operculella herbivory and d) Pl = playback of VS of P. operculella feeding on tobacco. We found that nicotine, the most abundant alkaloid, increased more than 2.6 times in the Ca and Pl treatments as compared with the Co and Eq treatments, which were similar between them. Nornicotine, anabasine and anatabine were mutually correlated and showed similar concentration patterns, being higher in the Eq treatment. Results are discussed in terms of the adaptive significance of plant responses to ecologically important VS stimuli.

Root Tropisms: Investigations on Earth and in Space to Unravel Plant Growth Direction

Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.

Methods for Replicating Leaf Vibrations Induced by Insect Herbivores

Testing plant responses to natural sources of mechanical vibration requires methods that can precisely reproduce complex vibrational stimuli. Here we describe a method for conducting high-fidelity vibrational playbacks using consumer audio equipment and custom-written signal processing software.

Exploring the sound-modulated delay in tomato ripening through expression analysis of coding and non-coding RNAs

Background and Aims

Sound is omnipresent in nature. Recent evidence supports the notion that naturally occurring and artificially generated sound waves induce inter- and intracellular changes in plants. These changes, in turn, lead to diverse physiological changes, such as enhanced biotic and abiotic stress responses, in both crops and model plants.

Methods

We previously observed delayed ripening in tomato fruits exposed to 1 kHz sound vibrations for 6 h. Here, we evaluated the molecular mechanism underlying this delaying fruit ripening by performing RNA-sequencing analysis of tomato fruits at 6 h, 2 d, 5 d and 7 d after 1 kHz sound vibration treatment.

Key Results

Bioinformatic analysis of differentially expressed genes and non-coding small RNAs revealed that some of these genes are involved in plant hormone and cell wall modification processes. Ethylene and cytokinin biosynthesis and signalling-related genes were downregulated by sound vibration treatment, whereas genes involved in flavonoid, phenylpropanoid and glucan biosynthesis were upregulated. Furthermore, we identified two sound-specific microRNAs and validated the expression of the pre-microRNAs and the mRNAs of their target genes.

Conclusions

Our results indicate that sound vibration helps to delay fruit ripening through the sophisticated regulation of coding and non-coding RNAs and transcription factor genes.

Beyond Chemical Triggers: Evidence for Sound-Evoked Physiological Reactions in Plants

Sound is ubiquitous in nature. Recent evidence supports the notion that naturally occurring and artificially generated sound waves contribute to plant robustness. New information is emerging about the responses of plants to sound and the associated downstream signaling pathways. Here, beyond chemical triggers which can improve plant health by enhancing plant growth and resistance, we provide an overview of the latest findings, limitations, and potential applications of sound wave treatment as a physical trigger to modulate physiological traits and to confer an adaptive advantage in plants. We believe that sound wave treatment is a new trigger to help protect plants against unfavorable conditions and to maintain plant fitness.