Thigmomorphogenesis: a complex plant response to mechano-stimulation

Expression responses of XTH genes in tomato and potato to environmental mechanical forces: focus on behavior in response to rainfall, wind and touch

Rainfall, wind and touch, as mechanical forces, were mimicked on 6-week-old soil-grown tomato and potato under controlled conditions. Expression level changes of xyloglucan endotransglucosylase/hydrolase genes (XTHs) of tomato (Solanum lycopersicum L. cv. Micro Tom; SlXTHs) and potato (Solanum tuberosum L. cv. Desirée; StXTHs) were analyzed in response to these mechanical forces. Transcription intensity of every SlXTHs of tomato was altered in response to rainfall, while the expression intensity of 72% and 64% of SlXTHs was modified by wind and touch, respectively. Ninety-one percent of StXTHs (32 out of 35) in potato responded to the rainfall, while 49% and 66% of the StXTHs were responsive to the wind and touch treatments, respectively. As previously demonstrated, all StXTHs were responsive to ultrasound treatment, and all were sensitive to one or more of the environmental mechanical factors examined in the current study. To our best knowledge, this is the first study to demonstrate that these ubiquitous mechanical environmental cues, such as rainfall, wind and touch, influence the transcription of most XTHs examined in both species.

Mechanically induced localisation of SECONDARY WALL INTERACTING bZIP is associated with thigmomorphogenic and secondary cell wall gene expression

Plant growth requires the integration of internal and external cues, perceived and transduced into a developmental programme of cell division, elongation and wall thickening. Mechanical forces contribute to this regulation, and thigmomorphogenesis typically includes reducing stem height, increasing stem diameter, and a canonical transcriptomic response. We present data on a bZIP transcription factor involved in this process in grasses. Brachypodium distachyon SECONDARY WALL INTERACTING bZIP (SWIZ) protein translocated into the nucleus following mechanostimulation. Classical touch-responsive genes were upregulated in B. distachyon roots following touch, including significant induction of the glycoside hydrolase 17 family, which may be unique to grass thigmomorphogenesis. SWIZ protein binding to an E-box variant in exons and introns was associated with immediate activation followed by repression of gene expression. SWIZ overexpression resulted in plants with reduced stem and root elongation. These data further define plant touch-responsive transcriptomics and physiology, offering insights into grass mechanotranduction dynamics.

Ethylene and jasmonate signaling converge on gibberellin catabolism during thigmomorphogenesis in Arabidopsis

Touch induces marked morphological changes in plants, including reduced rosette diameters and delayed flowering, a process called thigmomorphogenesis. Previous studies have revealed that thigmomorphogenesis in Arabidopsis (Arabidopsis thaliana) results from touch-induced accumulation of jasmonic acid (JA) and GIBBERELLIN 2-OXIDASE7 (GA2ox7) transcripts, which encode a gibberellin (GA) catabolism enzyme, leading to reduced levels of active GAs. However, the mechanisms underlying thigmomorphogenesis remain uncharacterized. Here, we showed that touch induces ethylene (ET) production in Arabidopsis. After touch treatment, ET biosynthesis and signaling mutants exhibited even greater thigmomorphogenic changes and more decreased GA4 contents than did wild-type (WT) plants. Biochemical analysis indicated that the transcription factor ETHYLENE INSENSITIVE3 (EIN3) of the ET pathway binds to the promoter of GA2ox8 (encoding another GA 2-oxidase performing the same GA modification as GA2ox7) and represses GA2ox8 transcription. Moreover, MYC2, the master regulator of JA signaling, directly promoted GA2ox7 expression by binding the G-box motif on GA2ox7 promoter. Further genetic analysis suggested that the ET and JA pathways independently control the expression of GA2ox8 and GA2ox7, respectively. This study reveals that the ET pathway is a novel repressor of touch-induced thigmomorphogenesis and highlights that the ET and JA pathways converge on GA catabolism but play opposite roles to fine-tune GA4 content during thigmomorphogenesis.

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.

Flexure wood formation via growth reprogramming in hybrid aspen involves jasmonates and polyamines and transcriptional changes resembling tension wood development

Stem bending in trees induces flexure wood but its properties and development are poorly understood. Here, we investigated the effects of low-intensity multidirectional stem flexing on growth and wood properties of hybrid aspen, and on its transcriptomic and hormonal responses. Glasshouse-grown trees were either kept stationary or subjected to several daily shakes for 5 wk, after which the transcriptomes and hormones were analyzed in the cambial region and developing wood tissues, and the wood properties were analyzed by physical, chemical and microscopy techniques. Shaking increased primary and secondary growth and altered wood differentiation by stimulating gelatinous-fiber formation, reducing secondary wall thickness, changing matrix polysaccharides and increasing cellulose, G- and H-lignin contents, cell wall porosity and saccharification yields. Wood-forming tissues exhibited elevated jasmonate, polyamine, ethylene and brassinosteroids and reduced abscisic acid and gibberellin signaling. Transcriptional responses resembled those during tension wood formation but not opposite wood formation and revealed several thigmomorphogenesis-related genes as well as novel gene networks including FLA and XTH genes encoding plasma membrane-bound proteins. Low-intensity stem flexing stimulates growth and induces wood having improved biorefinery properties through molecular and hormonal pathways similar to thigmomorphogenesis in herbaceous plants and largely overlapping with the tension wood program of hardwoods.

Nutrient Solution Flowing Environment Affects Metabolite Synthesis Inducing Root Thigmomorphogenesis of Lettuce (Lactuca sativa L.) in Hydroponics

The principal difference between hydroponics and other substrate cultivation methods is the flowing liquid hydroponic cultivation substrate. Our previous studies have revealed that a suitable flowing environment of nutrient solution promoted root development and plant growth, while an excess flow environment was unfavorable for plants. To explain the thigmomorphogenetic response of excess flow-induced metabolic changes, six groups of lettuce (Lactuca sativa L.), including two flow conditions and three time periods, were grown. Compared with the plants without flow, the plants with flow showed decreased root fresh weight, total root length, root surface area, and root volume but increased average root diameter and root density. The roots with flow had more upregulated metabolites than those without flow, suggesting that the flow may trigger metabolic synthesis and activity. Seventy-nine common differential metabolites among six groups were screened, and enrichment analysis showed the most significant enrichment in the arginine biosynthesis pathway. Arginine was present in all the groups and exhibited greater concentrations in roots with flow than without flow. It can be speculated from the results that a high-flowing environment of nutrient solution promotes arginine synthesis, resulting in changes in root morphology. The findings provide insights on root thigmomorphogenesis affected by its growing conditions and help understand how plants respond to environmental mechanical forces.

Deciphering the role of mechanosensitive channels in plant root biology: perception, signaling, and adaptive responses

MAIN CONCLUSION

Mechanosensitive channels are integral membrane proteins that rapidly translate extrinsic or intrinsic mechanical tensions into biological responses. They can serve as potential candidates for developing smart-resilient crops with efficient root systems. Mechanosensitive (MS) calcium channels are molecular switches for mechanoperception and signal transduction in all living organisms. Although tremendous progress has been made in understanding mechanoperception and signal transduction in bacteria and animals, this remains largely unknown in plants. However, identification and validation of MS channels such as Mid1-complementing activity channels (MCAs), mechanosensitive-like channels (MSLs), and Piezo channels (PIEZO) has been the most significant discovery in plant mechanobiology, providing novel insights into plant mechanoperception. This review summarizes recent advances in root mechanobiology, focusing on MS channels and their related signaling players, such as calcium ions (Ca2+), reactive oxygen species (ROS), and phytohormones. Despite significant advances in understanding the role of Ca2+ signaling in root biology, little is known about the involvement of MS channel-driven Ca2+ and ROS signaling. Additionally, the hotspots connecting the upstream and downstream signaling of MS channels remain unclear. In light of this, we discuss the present knowledge of MS channels in root biology and their role in root developmental and adaptive traits. We also provide a model highlighting upstream (cell wall sensors) and downstream signaling players, viz., Ca2+, ROS, and hormones, connected with MS channels. Furthermore, we highlighted the importance of emerging signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), and neurotransmitters (NTs), and their association with root mechanoperception. Finally, we conclude with future directions and knowledge gaps that warrant further research to decipher the complexity of root mechanosensing.

The dynamics of touch-responsive gene expression in cereals

Wind, rain, herbivores, obstacles, neighbouring plants, etc. provide important mechanical cues to steer plant growth and survival. Mechanostimulation to stimulate yield and stress resistance of crops is of significant research interest, yet a molecular understanding of transcriptional responses to touch is largely absent in cereals. To address this, we performed whole-genome transcriptomics following mechanostimulation of wheat, barley, and the recent genome-sequenced oat. The largest transcriptome changes occurred ±25 min after touching, with most of the genes being upregulated. While most genes returned to basal expression level by 1-2 h in oat, many genes retained high expression even 4 h post-treatment in barley and wheat. Functional categories such as transcription factors, kinases, phytohormones, and Ca2+ regulation were affected. In addition, cell wall-related genes involved in (hemi)cellulose, lignin, suberin, and callose biosynthesis were touch-responsive, providing molecular insight into mechanically induced changes in cell wall composition. Furthermore, several cereal-specific transcriptomic footprints were identified that were not observed in Arabidopsis. In oat and barley, we found evidence for systemic spreading of touch-induced signalling. Finally, we provide evidence that both the jasmonic acid-dependent and the jasmonic acid-independent pathways underlie touch-signalling in cereals, providing a detailed framework and marker genes for further study of (a)biotic stress responses in cereals.

Mechanodetection of neighbor plants elicits adaptive leaf movements through calcium dynamics

Plants detect their neighbors via various cues, including reflected light and touching of leaf tips, which elicit upward leaf movement (hyponasty). It is currently unknown how touch is sensed and how the signal is transferred from the leaf tip to the petiole base that drives hyponasty. Here, we show that touch-induced hyponasty involves a signal transduction pathway that is distinct from light-mediated hyponasty. We found that mechanostimulation of the leaf tip upon touching causes cytosolic calcium ([Ca2+]cyt induction in leaf tip trichomes that spreads towards the petiole. Both perturbation of the calcium response and the absence of trichomes reduce touch-induced hyponasty. Finally, using plant competition assays, we show that touch-induced hyponasty is adaptive in dense stands of Arabidopsis. We thus establish a novel, adaptive mechanism regulating hyponastic leaf movement in response to mechanostimulation by neighbors in dense vegetation.

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.

Effect of strong wind on laminas and petioles of Farfugium japonicum (L.) Kitam. var. japonicum (Asteraceae)

Farfugium japonicum (L.) Kitam. var. japonicum grows mainly in the coastal areas of Japan. Meteorological recording data from natural habitats were used to investigate the factors associated with the laminas and petioles of radical leaves of F. japonicum var. japonicum to avoid or resist higher wind stress. Our morphological and mechanical results indicated that petiole length and petiole cross-sectional area had a weak correlation with wind speed and breaking strength, and the petiole second area moment of inertia did not differ significantly among populations. However, both lamina area and petiole length per petiole cross-sectional area decreased with increasing wind speed, indicating that F. japonicum var. japonicum resisted or avoided an increase in wind speed outdoors by reducing the lamina area and petiole length per petiole cross-sectional area without qualitative changes in their petioles. The results of this study indicated that densely distributed recording stations of the Automated Meteorological Data Acquisition System (AMeDAS) by the Japan Meteorological Agency can be used for environmental adaptation studies of plants in the field using nearby plant populations.

Pavement cells distinguish touch from letting go

A micro-cantilever technique applied to individual leaf epidermis cells of intact Arabidopsis thaliana and Nicotiana tabacum synthesizing genetically encoded calcium indicators (R-GECO1 and GCaMP3) revealed that compressive forces induced local calcium peaks that preceded delayed, slowly moving calcium waves. Releasing the force evoked significantly faster calcium waves. Slow waves were also triggered by increased turgor and fast waves by turgor drops in pressure probe tests. The distinct characteristics of the wave types suggest different underlying mechanisms and an ability of plants to distinguish touch from letting go.

Mechanical Stimulation Decreases Auxin and Gibberellic Acid Synthesis but Does Not Affect Auxin Transport in Axillary Buds; It Also Stimulates Peroxidase Activity in Petunia × atkinsiana

Thigmomorphogenesis (or mechanical stimulation-MS) is a term created by Jaffe and means plant response to natural stimuli such as the blow of the wind, strong rain, or touch, resulting in a decrease in length and an increase of branching as well as an increase in the activity of axillary buds. MS is very well known in plant morphology, but physiological processes controlling plant growth are not well discovered yet. In the current study, we tried to find an answer to the question if MS truly may affect auxin synthesis or transport in the early stage of plant growth, and which physiological factors may be responsible for growth arrest in petunia. According to the results of current research, we noticed that MS affects plant growth but does not block auxin transport from the apical bud. MS arrests IAA and GA3 synthesis in MS-treated plants over the longer term. The main factor responsible for the thickening of cell walls and the same strengthening of vascular tissues and growth arrestment, in this case, is peroxidase (POX) activity, but special attention should be also paid to AGPs as signaling molecules which also are directly involved in growth regulation as well as in cell wall modifications.

Mechanical stimulation reprograms the sorghum internode transcriptome and broadly alters hormone homeostasis

Stem structural failure, or lodging, affects many crops including sorghum, and can cause large yield losses. Lodging is typically caused by mechanical forces associated with severe weather like high winds, but exposure to sub-catastrophic forces may strengthen stems and improve lodging resistance. The responses of sorghum internodes at different developmental stages were examined at 2 and 26 h after initiating moderate mechanical stimulation with an automated apparatus. Transcriptome profiling revealed that mechanical stimulation altered the expression of over 900 genes, including transcription factors, cell wall-related and hormone signaling-related genes. IAA, GA₁ and ABA abundances generally declined following mechanical stimulation, while JA increased. Weighted Gene Co-expression Network Analysis (WGCNA) identified three modules significantly enriched in GO terms associated with cell wall biology, hormone signaling and general stress responses, which were highly correlated with mechanical stimulation and with biomechanical and geometrical traits documented in a separate study. Additionally, mechanical stimulation-triggered responses were dependent on the developmental stage of the internode and the duration of stimulation. This study provides insights into the underlying mechanisms of plant hormone-regulated thigmomorphogenesis in sorghum stems. The critical biological processes and hub genes described here may offer opportunities to improve lodging resistance in sorghum and other crops.

Mechanosensing, from forces to structures

Sessile plants evolve diverse structures in response to complex environmental cues. These factors, in essence, involve mechanical stimuli, which must be sensed and coordinated properly by the plants to ensure effective growth and development. While we have accumulated substantial knowledge on plant mechanobiology, how plants translate mechanical information into three-dimensional structures is still an open question. In this review, we summarize our current understanding of plant mechanosensing at different levels, particularly using Arabidopsis as a model plant system. We also attempt to abstract the mechanosensing process and link the gaps from mechanical cues to the generation of complex plant structures. Here we review the recent advancements on mechanical response and transduction in plant morphogenesis, and we also raise several questions that interest us in different sections.

Cajanus cajan shows multiple novel adaptations in response to regular mechanical stress

Cajanus cajan is one of the least studied crop plants regarding its responses to stress conditions. Regular mechanical stress suppresses plant physiology and growth at the cellular and systemic levels. In the current study, we have explored morphological, physiological, and anatomical adaptations of C. cajan seedlings to regular mechanical stress. Young seedlings of C. cajan were given mechanical stress in the form of touch for fifteen days and observed for various changes. Touch stimuli caused an immediate release of oxidative burst, suppressed plant growth, increased compactness of the stem tissue, and altered the chlorophyll a/b ratio. We have also identified two novel phenotypes; regular touch stimuli affected the nyctinasty movements of the leaves and also affected the root nodule development. We have identified and studied the expression of four putative touch responsive calcium binding genes, TCH gene homologs, in C. cajan using Arabidopsis TCH gene sequences. At an early time point, the expression of two TCH gene homologs (CcTCH1-1 and CcTCH2-2) were found to be upregulated. This study unravels the novel adaptation displayed by C. cajan in response to mechanical stress that can be used as a phenotypic marker for future studies in this plant.

Leveraging physical intelligence for the self-design of high performance engineering structures

The design of complex engineering structures largely relies on computational intelligence in the form of science-based predictive models to support design decisions. This approach requires modeling and manufacturing uncertainties to be accounted for explicitly and leads to an inescapable trade-off of performance for robustness. To remedy this situation, a novel self-design paradigm is proposed that closes the loop between the design and manufacturing processes by leveraging physical intelligence in the form of real-time experimental observations. This allows the real-time product behavior to participate in its own design. The main benefit of the proposed paradigm is that both manufacturing variability and difficult-to-model physics are accounted for implicitly via in situ measurements thus circumventing the performance-robustness trade-off and guaranteeing enhanced performance with respect to standardized designs. This paradigm shift leads to tailored design realizations which could benefit a wide range of high performance engineering applications. The proposed paradigm is applied to the design of a simply-supported plate with a beam-like absorber introduced to reduce vibrations based on an equal peaks performance criteria. The experimental setup includes a low-cost 3D printer driven by a simple decision algorithm and equipped with an online vibration testing system. The performances of a small population of self-designed plates are compared to their standardized counterparts in order to highlight the advantages and limitations of the new self-design manufacturing paradigm.

Thermo-Priming Mediated Cellular Networks for Abiotic Stress Management in Plants

Plants can adapt to different environmental conditions and can survive even under very harsh conditions. They have developed elaborate networks of receptors and signaling components, which modulate their biochemistry and physiology by regulating the genetic information. Plants also have the abilities to transmit information between their different parts to ensure a holistic response to any adverse environmental challenge. One such phenomenon that has received greater attention in recent years is called stress priming. Any milder exposure to stress is used by plants to prime themselves by modifying various cellular and molecular parameters. These changes seem to stay as memory and prepare the plants to better tolerate subsequent exposure to severe stress. In this review, we have discussed the various ways in which plants can be primed and illustrate the biochemical and molecular changes, including chromatin modification leading to stress memory, with major focus on thermo-priming. Alteration in various hormones and their subsequent role during and after priming under various stress conditions imposed by changing climate conditions are also discussed.

Unidirectional versus bidirectional brushing: Simulating wind influence on Arabidopsis thaliana

Plants acclimate to various types of mechanical stresses through thigmomorphogenesis and alterations in their mechanical properties. Although resemblance between wind- and touch-induced responses provides the foundation for studies where wind influence was mimicked by mechanical perturbations, factorial experiments revealed that it is not always straightforward to extrapolate results induced by one type of perturbation to the other. To investigate whether wind-induced changes in morphological and biomechanical traits can be reproduced, we subjected Arabidopsis thaliana to two vectorial brushing treatments. Both treatments significantly affected the length, mechanical properties and anatomical tissue composition of the primary inflorescence stem. While some of the morphological changes were found to be in line with those induced by wind, changes in the mechanical properties exhibited opposite trends irrespective of the brushing direction. Overall, a careful design of the brushing treatment gives the possibility to obtain a closer match to wind-induced changes, including a positive tropic response.

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.

The shaping of plant axes and crowns through tropisms and elasticity: an example of morphogenetic plasticity beyond the shoot apical meristem

Shoot morphogenetic plasticity is crucial to the adaptation of plants to their fluctuating environments. Major insights into shoot morphogenesis have been compiled studying meristems, especially the shoot apical meristem (SAM), through a methodological effort in multiscale systems biology and biophysics. However, morphogenesis at the SAM is robust to environmental changes. Plasticity emerges later on during post-SAM development. The purpose of this review is to show that multiscale systems biology and biophysics is insightful for the shaping of the whole plant as well. More specifically, we review the shaping of axes and crowns through tropisms and elasticity, combining the recent advances in morphogenetic control using physical cues and by genes. We focus mostly on land angiosperms, but with growth habits ranging from small herbs to big trees. We show that generic (universal) morphogenetic processes have been identified, revealing feedforward and feedback effects of global shape on the local morphogenetic process. In parallel, major advances have been made in the analysis of the major genes involved in shaping axes and crowns, revealing conserved genic networks among angiosperms. Then, we show that these two approaches are now starting to converge, revealing exciting perspectives.

Biomechanics of tendrils and adhesive pads of the climbing passion flower Passiflora discophora

The climbing passion flower Passiflora discophora features branched tendrils with multiple adhesive pads at their tips allowing it to attach to large-diameter supports and to flat surfaces. We conducted tensile tests to quantify the performance of this attachment system. We found that the force at failure varies with substrate, ontogenetic state (turgescent or senescent), and tendril size (i.e. tendril cross-sectional area and pad area). The tendrils proved to be well balanced in size and to attach firmly to a variety of substrates (force at failure up to 2N). Pull-off tests performed with tendrils grown on either epoxy, plywood, or beech bark revealed that senescent tendrils could still bear 24, 64, or 100% of the force measured for turgescent tendrils, respectively, thus providing long-lasting attachment at minimal physiological costs. The tendril main axis was typically the weakest part of the adhesive system, whereas the pad-substrate interface never failed. This suggests that the plants use the slight oversizing of adhesive pads as a strategy to cope with 'unpredictable' substrates. The pads, together with the spring-like main axis, which can, as shown, dissipate a large amount of energy when straightened, thus constitute a fail-safe attachment system.

Nanoparticle cellular internalization is not required for RNA delivery to mature plant leaves

Rapidly growing interest in the nanoparticle-mediated delivery of DNA and RNA to plants requires a better understanding of how nanoparticles and their cargoes translocate in plant tissues and into plant cells. However, little is known about how the size and shape of nanoparticles influence transport in plants and the delivery efficiency of their cargoes, limiting the development of nanotechnology in plant systems. In this study we employed non-biolistically delivered DNA-modified gold nanoparticles (AuNPs) of various sizes (5-20 nm) and shapes (spheres and rods) to systematically investigate their transport following infiltration into Nicotiana benthamiana leaves. Generally, smaller AuNPs demonstrated more rapid, higher and longer-lasting levels of association with plant cell walls compared with larger AuNPs. We observed internalization of rod-shaped but not spherical AuNPs into plant cells, yet, surprisingly, 10 nm spherical AuNPs functionalized with small-interfering RNA (siRNA) were the most efficient at siRNA delivery and inducing gene silencing in mature plant leaves. These results indicate the importance of nanoparticle size in efficient biomolecule delivery and, counterintuitively, demonstrate that efficient cargo delivery is possible and potentially optimal in the absence of nanoparticle cellular internalization. Overall, our results highlight nanoparticle features of importance for transport within plant tissues, providing a mechanistic overview of how nanoparticles can be designed to achieve efficacious biocargo delivery for future developments in plant nanobiotechnology.

Nanoparticle cellular internalization is not required for RNA delivery to mature plant leaves

Rapidly growing interest in the nanoparticle-mediated delivery of DNA and RNA to plants requires a better understanding of how nanoparticles and their cargoes translocate in plant tissues and into plant cells. However, little is known about how the size and shape of nanoparticles influence transport in plants and the delivery efficiency of their cargoes, limiting the development of nanotechnology in plant systems. In this study we employed non-biolistically delivered DNA-modified gold nanoparticles (AuNPs) of various sizes (5-20 nm) and shapes (spheres and rods) to systematically investigate their transport following infiltration into Nicotiana benthamiana leaves. Generally, smaller AuNPs demonstrated more rapid, higher and longer-lasting levels of association with plant cell walls compared with larger AuNPs. We observed internalization of rod-shaped but not spherical AuNPs into plant cells, yet, surprisingly, 10 nm spherical AuNPs functionalized with small-interfering RNA (siRNA) were the most efficient at siRNA delivery and inducing gene silencing in mature plant leaves. These results indicate the importance of nanoparticle size in efficient biomolecule delivery and, counterintuitively, demonstrate that efficient cargo delivery is possible and potentially optimal in the absence of nanoparticle cellular internalization. Overall, our results highlight nanoparticle features of importance for transport within plant tissues, providing a mechanistic overview of how nanoparticles can be designed to achieve efficacious biocargo delivery for future developments in plant nanobiotechnology.

Evaluating Root Mechanosensing Response in Rice

Unable to move, plants are physically restrained to the place where they grow. Remarkably, plants have developed a myriad of mechanisms to perceive the surrounding environment in order to maximize growth and survival. One of those mechanisms is the ability to perceive mechanical stimulus such as touch (thigmomorphogenesis), in order to adjust growth patterns (in different organs) to either attach to or surround an object. Roots are able to perceive several mechanical forces (e.g., gravity, touch). However, being the "hidden part" of a plant, it is difficult to assess their response to mechanical stimulation. In this chapter, our team presents a simple method to evaluate rice (Oryza sativa L.) root mechanosensing response that can be used to test different conditions (e.g., hormones) affecting rice root response to touch stimulus. This method is affordable to any lab and can be upgraded with a fully automated image recording system. We provide a detailed protocol with several notes for a more comprehensive application.

Methods for a Quantitative Comparison of Gravitropism and Posture Control Over a Wide Range of Herbaceous and Woody Species

Quantitative measurements of plant gravitropic response are challenging. Differences in growth rates between species and environmental conditions make it difficult to compare the intrinsic gravitropic responses of different plants. In addition, the bending movement associated with gravitropism is competing with the tendency of plants to grow straight, through a mechanism called proprioception (ability to sense its own shape). Disentangling these two tendencies is not trivial. Here, we use a combination of modeling, experiment and image analysis to estimate the intrinsic gravitropic and proprioceptive sensitivities of stems, using Arabidopsis as an example.

Mechano-transduction via the pectin-FERONIA complex activates ROP6 GTPase signaling in Arabidopsis pavement cell morphogenesis

During growth and morphogenesis, plant cells respond to mechanical stresses resulting from spatiotemporal changes in the cell wall that bear high internal turgor pressure. Microtubule (MT) arrays are reorganized to align in the direction of maximal tensile stress, presumably reinforcing the local cell wall by guiding the synthesis of cellulose. However, how mechanical forces regulate MT reorganization remains largely unknown. Here, we demonstrate that mechanical signaling that is based on the Catharanthus roseus RLK1-like kinase (CrRLK1L) subfamily receptor kinase FERONIA (FER) regulates the reorganization of cortical MT in cotyledon epidermal pavement cells (PCs) in Arabidopsis. Recessive mutations in FER compromised MT responses to mechanical perturbations, such as single-cell ablation, compression, and isoxaben treatment, in these PCs. These perturbations promoted the activation of ROP6 guanosine triphosphatase (GTPase) that acts directly downstream of FER. Furthermore, defects in the ROP6 signaling pathway negated the reorganization of cortical MTs induced by these stresses. Finally, reduction in highly demethylesterified pectin, which binds the extracellular malectin domains of FER and is required for FER-mediated ROP6 activation, also impacted mechanical induction of cortical MT reorganization. Taken together, our results suggest that the FER-pectin complex senses and/or transduces mechanical forces to regulate MT organization through activating the ROP6 signaling pathway in Arabidopsis.

Using the Automated Botanical Contact Device (ABCD) to Deliver Reproducible, Intermittent Touch Stimulation to Plants

Despite mechanical stimulation having profound effects on plant growth and development and modulating responses to many other stimuli, including to gravity, much of the molecular machinery triggering plant mechanical responses remains unknown. This gap in our knowledge arises in part from difficulties in applying reproducible, long-term touch stimulation to plants. We describe the design and implementation of the Automated Botanical Contact Device (ABCD) that applies intermittent, controlled, and highly reproducible mechanical stimulation by drawing a plastic sheet across experimental plants. The device uses a computer numerical control platform and continuously monitors plant growth and development using automated computer vision and image analysis. The system is designed around an open-source architecture to help promote the generation of comparable datasets between laboratories. The ABCD also offers a scalable system that could be deployed in the controlled environment setting, such as a greenhouse, to manipulate plant growth and development through controlled, repetitive mechanostimulation.

Robust, Long-Term, and Exceptionally Sensitive Microneedle-Based Bioimpedance Sensor for Precision Farming

Precision farming has the potential to increase global food production capacity whilst minimizing traditional inputs. However, the adoption and impact of precision farming are contingent on the availability of sensors that can discern the state of crops, while not interfering with their growth. Electrical impedance spectroscopy offers an avenue for nondestructive monitoring of crops. To that end, it is reported on the deployment of impedimetric sensors utilizing microneedles (MNs) that can be used to pierce the waxy exterior of plants to obtain sensitive impedance spectra in open-air settings with an average relative noise value of 3.83%. The sensors are fabricated using a novel micromolding and release method that is compatible with UV photocurable and thermosetting polymers. Assessments of the quality of the MNs under scanning electron microscopy show that the replication process is high in fidelity to the original design of the master mold and that it can be used for upward of 20 replication cycles. The sensor's performance is validated against conventional planar sensors for obtaining the impedance values of Arabidopsis thaliana. As a change is detected in impedance due to lighting and hydration, this raises the possibility for their widespread use in precision farming.

Linking ecomechanical models and functional traits to understand phenotypic diversity

Physical principles and laws determine the set of possible organismal phenotypes. Constraints arising from development, the environment, and evolutionary history then yield workable, integrated phenotypes. We propose a theoretical and practical framework that considers the role of changing environments. This 'ecomechanical approach' integrates functional organismal traits with the ecological variables. This approach informs our ability to predict species shifts in survival and distribution and provides critical insights into phenotypic diversity. We outline how to use the ecomechanical paradigm using drag-induced bending in trees as an example. Our approach can be incorporated into existing research and help build interdisciplinary bridges. Finally, we identify key factors needed for mass data collection, analysis, and the dissemination of models relevant to this framework.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

Application of naturally occurring mechanical forces in in vitro plant tissue culture and biotechnology

Cues and signals of the environment in nature can be either beneficial or detrimental from the growth and developmental perspectives. Plants, despite their limited spatial mobility, have developed advanced strategies to overcome the various and changing environmental impacts including stresses. In vitro plantlets, tissues and cells are constantly exposed to the influence of their environment that is well controlled. Light has a widely known morphogenetic effect on plants; however, other physical cues and signals are at least as important but were often neglected. In this review, I summarize our knowledge about the role of the mechanical stimuli, like sound, ultrasound, touch, or wounding in in vitro plant cultures. I summarize the molecular, biochemical, physiological, growth, and developmental changes they cause and how these processes are controlled; moreover, how their regulating or stimulating roles are applied in various plant biotechnological applications. Recent studies revealed that mechanical forces can be used for affecting the plant development and growth in plant tissue culture efficiently, and for increasing the efficacy of other plant biotechnological methods, like genetic transformation and secondary metabolite production.

Sound Waves Promote Arabidopsis thaliana Root Growth by Regulating Root Phytohormone Content

Sound waves affect plants at the biochemical, physical, and genetic levels. However, the mechanisms by which plants respond to sound waves are largely unknown. Therefore, the aim of this study was to examine the effect of sound waves on Arabidopsis thaliana growth. The results of the study showed that Arabidopsis seeds exposed to sound waves (100 and 100 + 9k Hz) for 15 h per day for 3 day had significantly longer root growth than that in the control group. The root length and cell number in the root apical meristem were significantly affected by sound waves. Furthermore, genes involved in cell division were upregulated in seedlings exposed to sound waves. Root development was affected by the concentration and activity of some phytohormones, including cytokinin and auxin. Analysis of the expression levels of genes regulating cytokinin and auxin biosynthesis and signaling showed that cytokinin and ethylene signaling genes were downregulated, while auxin signaling and biosynthesis genes were upregulated in Arabidopsis exposed to sound waves. Additionally, the cytokinin and auxin concentrations of the roots of Arabidopsis plants increased and decreased, respectively, after exposure to sound waves. Our findings suggest that sound waves are potential agricultural tools for improving crop growth performance.

Internode elongation and strobili production of Humulus lupulus cultivars in response to local strain sensing

Three different cultivars of Humulus lupulus L. were subjected to a regime of internode touch and bending under greenhouse conditions. Experiments were performed to assess intraspecific variability in plant mechanosensing, flower quality, and yield to quantify the thigmomorphogenic impact on plant compactness and flowering performance. Touching and/or touching plus bending the plant shoot internodes located in the apical meristem zone decreased internode elongation and increased width. The growth responses were due partly to touching and/or touching plus bending perturbation, 25.6% and 28% respectively. Growth of new tissue within the local apical portion of the bine continued to remain mechanosensitive. The number of nodes and female flowers produced was unaffected by either type of mechanical stress. The study provides evidence that thigmomorphogenic cues can be used as a hop crop management tool to increase bine compactness and increase node density per unit area. The findings have broad implications for hop production; production can more readily take place in a confined greenhouse space with the aid of mechanical stimulation to control plant growth without sacrificing yield or flower quality.

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.

Stretch-activated ion channels identified in the touch-sensitive structures of carnivorous Droseraceae plants

In response to touch, some carnivorous plants such as the Venus flytrap have evolved spectacular movements to capture animals for nutrient acquisition. However, the molecules that confer this sensitivity remain unknown. We used comparative transcriptomics to show that expression of three genes encoding homologs of the MscS-Like (MSL) and OSCA/TMEM63 family of mechanosensitive ion channels are localized to touch-sensitive trigger hairs of Venus flytrap. We focus here on the candidate with the most enriched expression in trigger hairs, the MSL homolog FLYCATCHER1 (FLYC1). We show that FLYC1 transcripts are localized to mechanosensory cells within the trigger hair, transfecting FLYC1 induces chloride-permeable stretch-activated currents in naïve cells, and transcripts coding for FLYC1 homologs are expressed in touch-sensing cells of Cape sundew, a related carnivorous plant of the Droseraceae family. Our data suggest that the mechanism of prey recognition in carnivorous Droseraceae evolved by co-opting ancestral mechanosensitive ion channels to sense touch.

Wind-evoked anemotropism affects the morphology and mechanical properties of Arabidopsis

Plants are known to exhibit a thigmomorphogenetic response to mechanical stimuli by altering their morphology and mechanical properties. Wind is widely perceived as mechanical stress and in many experiments its influence is simulated by applying mechanical perturbations. However, it is known that wind-induced effects on plants can differ and at times occur even in the opposite direction compared with those induced by mechanical perturbations. In the present study, the long-term response of Arabidopsis thaliana to a constant unidirectional wind was investigated. We found that exposure to wind resulted in a positive anemotropic response and in significant alterations to Arabidopsis morphology, mechanical properties, and anatomical tissue organization that were associated with the plant's strategy of acclimation to a windy environment. Overall, the observed response of Arabidopsis to wind differs significantly from previously reported responses of Arabidopsis to mechanical perturbations. The presented results suggest that the response of Arabidopsis is sensitive to the type of mechanical stimulus applied, and that it is not always straightforward to simulate one type of perturbation by another.

AtPiezo Plays an Important Role in Root Cap Mechanotransduction

Plants encounter a variety of mechanical stimuli during their growth and development. It is currently believed that mechanosensitive ion channels play an essential role in the initial perception of mechanical force in plants. Over the past decade, the study of Piezo, a mechanosensitive ion channel in animals, has made significant progress. It has been proved that the perception of mechanical force in various physiological processes of animals is indispensable. However, little is still known about the function of its homologs in plants. In this study, by investigating the function of the AtPiezo gene in the model plant Arabidopsis thaliana, we found that AtPiezo plays a role in the perception of mechanical force in plant root cap and the flow of Ca2+ is involved in this process. These findings allow us to understand the function of AtPiezo from the perspective of plants and provide new insights into the mechanism of plant root cap in response to mechanical stimuli.

Rapid movements in plants

Plant movements are generally slow, but some plant species have evolved the ability to move very rapidly at speeds comparable to those of animals. Whereas movement in animals relies on the contraction machinery of muscles, many plant movements use turgor pressure as the primary driving force together with secondarily generated elastic forces. The movement of stomata is the best-characterized model system for studying turgor-driven movement, and many gene products responsible for this movement, especially those related to ion transport, have been identified. Similar gene products were recently shown to function in the daily sleep movements of pulvini, the motor organs for macroscopic leaf movements. However, it is difficult to explain the mechanisms behind rapid multicellular movements as a simple extension of the mechanisms used for unicellular or slow movements. For example, water transport through plant tissues imposes a limit on the speed of plant movements, which becomes more severe as the size of the moving part increases. Rapidly moving traps in carnivorous plants overcome this limitation with the aid of the mechanical behaviors of their three-dimensional structures. In addition to a mechanism for rapid deformation, rapid multicellular movements also require a molecular system for rapid cell-cell communication, along with a mechanosensing system that initiates the response. Electrical activities similar to animal action potentials are found in many plant species, representing promising candidates for the rapid cell-cell signaling behind rapid movements, but the molecular entities of these electrical signals remain obscure. Here we review the current understanding of rapid plant movements with the aim of encouraging further biological studies into this fascinating, challenging topic.

Morphological plasticity in the kelp Nereocystis luetkeana (Phaeophyceae) is sensitive to the magnitude, direction, and location of mechanical loading

Nereocystis luetkeana is a canopy-forming kelp that exhibits morphological plasticity across hydrodynamic gradients, producing broad, undulate blades in slow flow and narrow, flattened blades in fast flow, enabling thalli to reduce drag while optimizing photosynthesis. While the functional significance of this phenomenon has been well studied, the developmental and physiological mechanisms that facilitate the plasticity remain poorly understood. In this study, we conducted three experiments to characterize how the (1) magnitude, (2) direction, and (3) location of plasticity-inducing mechanical stimuli affect the morphology of Nereocystis blades. We found that applying a gradient of tensile force caused blades to grow progressively longer, narrower, less ruffled, and heavier in a linear fashion, suggesting that Nereocystis is equally well adapted for all conditions within its hydrodynamic niche. We also found that applying tension transversely across blades caused the growth response to rotate 90°, indicating that there is no substantial separation between the sites of stimulus perception and response and suggesting that a long-distance signaling mechanism, such as a hormone, is unlikely to mediate this phenomenon. Meristoderm cells showed morphological changes that paralleled those of their respective blades in this experiment, implying that tissue-level morphology is influenced by cell growth. Finally, we found that plasticity was only induced when tension was applied directly to the growing tissue, reinforcing that long-distance signaling is probably not involved and possibly indicating that the mechanism on display generally requires an intercalary meristem to facilitate mechanoperception.

Touch-induced seedling morphological changes are determined by ethylene-regulated pectin degradation

How mechanical forces regulate plant growth is a fascinating and long-standing question. After germination underground, buried seedlings have to dynamically adjust their growth to respond to mechanical stimulation from soil barriers. Here, we designed a lid touch assay and used atomic force microscopy to investigate the mechanical responses of seedlings during soil emergence. Touching seedlings induced increases in cell wall stiffness and decreases in cell elongation, which were correlated with pectin degradation. We revealed that PGX3, which encodes a polygalacturonase, mediates touch-imposed alterations in the pectin matrix and the mechanics of morphogenesis. Furthermore, we found that ethylene signaling is activated by touch, and the transcription factor EIN3 directly associates with PGX3 promoter and is required for touch-repressed PGX3 expression. By uncovering the link between mechanical forces and cell wall remodeling established via the EIN3-PGX3 module, this work represents a key step in understanding the molecular framework of touch-induced morphological changes.

Proteomic profiles of young and mature cocoa leaves subjected to mechanical stress caused by wind

Cocoa is a perennial and arboreal species intolerant to strong and frequent winds and, for this reason, is usually grown with windbreaks of trees. The mechanical alterations caused by the wind in the field have a great impact on the growth, development and productivity of cocoa. The present work had a main objective to understand the molecular mechanisms of responses to mechanical stress, caused by the action of constant wind flow in young plants of cocoa through alterations of the proteomic profile in young (YL) and mature leaves (ML). Plants were exposed to constant wind (CW) at a speed of 4.5 m s^-1 for 12 h. There was a reduction in the accumulation of proteins in YL and a significant increase in ML submitted to CW in relation to the control. Differentially accumulated proteins, identified in YL and ML, belong to a broad functional group, related to energy production and carbon metabolism. Besides that, there was a higher efficiency in the protein relative abundance associated to energy production and the assimilation of carbon in the ML exposed to CW, in relation to the control. It was observed the appearance of new isoforms and, or post-transitional changes, which represent an acclimatization and tolerance response of these leaves to the stressor factor. In contrast, in YL, the energy production and the synthesis of gene products essential for their growth and development were affected by the mechanical stress caused by the wind, making them more intolerant.

Wind intensity affects fine root morphological traits with consequences for plant-soil feedback effects

Wind influences the development, architecture and morphology of plant roots and may modify subsequent interactions between plants and soil (plant-soil feedbacks-PSFs). However, information on wind effects on fine root morphology is scarce and the extent to which wind changes plant-soil interactions remains unclear. Therefore, we investigated the effects of two wind intensity levels by manipulating surrounding vegetation height in a grassland PSF field experiment. We grew four common plant species (two grasses and two non-leguminous forbs) with soil biota either previously conditioned by these or other species and tested the effect of wind on root:shoot ratio, fine root morphological traits as well as the outcome for PSFs. Wind intensity did not affect biomass allocation (i.e. root:shoot ratio) in any species. However, fine-root morphology of all species changed under high wind intensity. High wind intensity increased specific root length and surface area and decreased root tissue density, especially in the two grasses. Similarly, the direction of PSFs changed under high wind intensity in all four species, but differences in biomass production on the different soils between high and low wind intensity were marginal and most pronounced when comparing grasses with forbs. Because soils did not differ in plant-available nor total nutrient content, the results suggest that wind-induced changes in root morphology have the potential to influence plant-soil interactions. Linking wind-induced changes in fine-root morphology to effects on PSF improves our understanding of plant-soil interactions under changing environmental conditions.

Design and Application of a Rotatory Device for Detecting Transient Ca2+ Signals in Response to Mechanical Stimulation Using an Aequorin-Based Ca2+ Imaging System

Elevation of the cytosolic free calcium ion (Ca²⁺ ) concentration ([Ca²⁺ ]_cyt ) is one of the earliest responses to biotic and abiotic stress in plant cells. Among the various Ca²⁺ detection systems available, aequorin-based luminescence Ca²⁺ imaging systems provide a relatively amenable and robust method that facilitates large-scale genetic-mutant screening based on [Ca²⁺ ]_cyt responses. Compared to that mediated by chemical elicitors, mechanical stimulation-induced elevation of [Ca²⁺ ]_cyt is considerably more rapid, occurring within 10 s following stimulation. Therefore, its assessment using aequorin-based Ca²⁺ imaging systems represents a notable challenge, given that a time interval of ≥1 min is required to reduce the background light before operating the photon imaging detector. In this context, we designed a device that can rotate automatically within the confines of an enclosed dark box, and using this, we can record [Ca²⁺ ]_cyt dynamics immediately after plants had been rotated to induce mechanical stimulation. This tool can facilitate the study of perception and early signal transduction in response to mechanical stimulation on a large scale based on [Ca²⁺ ]_cyt responses. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Detection of background luminance signals in aequorin-transformed Arabidopsis seedlings using a photon imaging detector Basic Protocol 2: Construction of the rotatory device Basic Protocol 3: Calcium measurement in Arabidopsis seedlings after rotatory stimulation Basic Protocol 4: Data analysis and processing.