Recurrent symmetrical bendings cause dwarfing in Hydrangea through spatial molecular regulation of xylem cell walls

Environmental prejudices progressively lead to the ban of dwarfing molecules in agriculture, and alternatives are urgently required. Mechanical stimulation (MS) is a promising, eco-friendly, and economical technique, but some responses to mechanical stimulation vary from one plant species to another. Additionally, as more frequent and violent wind episodes are forecasted under global climate change, knowledge of plant responses to stimuli mimicking wind sways is decisive for agriculture. However, little is known about plant mechanosensitive responses after long-term, recurrent MS. Here, the effects of 3-week, recurrent, symmetrical bendings (1 or 12 per day) in Hydrangea macrophylla stems are examined. Bendings repressed internode elongation and leaf area development, whereas the diametrical growth of the basal internode is increased. Responses were dose-dependent, and no desensitization was observed during the 3 weeks of treatment. MS was almost as efficient as daminozide for plant dwarfing, and it improved stem robustness. Histological and molecular responses to MS were spatially monitored and were concordant with ongoing primary or secondary growth in the internodes. Our molecular data provide the first knowledge on the molecular paths controlled by mechanical loads in Hydrangea and revealed for the first time the involvement of XYP1 in thigmomorphogenetic responses. MS still had a transcriptional impact 48 h after the last bending session, promoting the expression of XYP1, FLA11, and CAD1 while repressing the expression of EXP3 and XTH33 homologs in accordance with xylogenesis, cell wall thickening, and lignin deposition in the xylem of basal internodes. In upper elongating internodes, repression of XYP1, CAD1, SAMS1, and CDC23 homologs is correlated with ongoing primary, even though stunted, growth. For producers, our findings highlight the potential of MS as a sustainable and economical option for controlling plant compactness in Hydrangea and show valuable reinforcement of stem strength.

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.

The Course of Mechanical Stress: Types, Perception, and Plant Response

Mechanical stimuli, together with the corresponding plant perception mechanisms and the finely tuned thigmomorphogenetic response, has been of scientific and practical interest since the mid-17th century. As an emerging field, there are many challenges in the research of mechanical stress. Indeed, studies on different plant species (annual/perennial) and plant organs (stem/root) using different approaches (field, wet lab, and in silico/computational) have delivered insufficient findings that frequently impede the practical application of the acquired knowledge. Accordingly, the current work distils existing mechanical stress knowledge by bringing in side-by-side the research conducted on both stem and roots. First, the various types of mechanical stress encountered by plants are defined. Second, plant perception mechanisms are outlined. Finally, the different strategies employed by the plant stem and roots to counteract the perceived mechanical stresses are summarized, depicting the corresponding morphological, phytohormonal, and molecular characteristics. The comprehensive literature on both perennial (woody) and annual plants was reviewed, considering the potential benefits and drawbacks of the two plant types, which allowed us to highlight current gaps in knowledge as areas of interest for future research.

Stronger wind, smaller tree: Testing tree growth plasticity through a modeling approach

Plants exhibit plasticity in response to various external conditions, characterized by changes in physiological and morphological features. Although being non-negligible, compared to the other environmental factors, the effect of wind on plant growth is less extensively studied, either experimentally or computationally. This study aims to propose a modeling approach that can simulate the impact of wind on plant growth, which brings a biomechanical feedback to growth and biomass distribution into a functional-structural plant model (FSPM). Tree reaction to the wind is simulated based on the hypothesis that plants tend to fit in the environment best. This is interpreted as an optimization problem of finding the best growth-regulation sink parameter giving the maximal plant fitness (usually seed weight, but expressed as plant biomass and size). To test this hypothesis in silico, a functional-structural plant model, which simulates both the primary and secondary growth of stems, is coupled with a biomechanical model which computes forces, moments of forces, and breakage location in stems caused by both wind and self-weight increment during plant growth. The Non-dominated Sorting Genetic Algorithm II (NSGA-II) is adopted to maximize the multi-objective function (stem biomass and tree height) by determining the key parameter value controlling the biomass allocation to the secondary growth. The digital trees show considerable phenotypic plasticity under different wind speeds, whose behavior, as an emergent property, is in accordance with experimental results from works of literature: the height and leaf area of individual trees decreased with wind speed, and the diameter at the breast height (DBH) increased at low-speed wind but declined at higher-speed wind. Stronger wind results in a smaller tree. Such response of trees to the wind is realistically simulated, giving a deeper understanding of tree behavior. The result shows that the challenging task of modeling plant plasticity may be solved by optimizing the plant fitness function. Adding a biomechanical model enriches FSPMs and opens a wider application of plant models.

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.

Acclimation to wind loads and/or contact stimuli? A biomechanical study of peltate leaves of Pilea peperomioides

Plants are exposed to various environmental stresses. Leaves immediately respond to mechano-stimulation, such as wind and touch, by bending and twisting or acclimate over a longer time period by thigmomorphogenetic changes of mechanical and geometrical properties. We selected the peltate leaves of Pilea peperomioides for a comparative analysis of mechano-induced effects on morphology, anatomy, and biomechanics of petiole and transition zone. The plants were cultivated for 6 weeks in a phytochamber divided into four treatment groups: control (no stimulus), touch stimulus (brushing every 30 s), wind stimulus (constant air flow of 4.6 m s-1), and a combination of touch and wind stimuli. Comparing the four treatment groups, neither the petiole nor the transition zone showed significant thigmomorphogenetic acclimations. However, comparing the petiole and the transition zone, the elastic modulus (E), the torsional modulus (G), the E/G ratio, and the axial rigidity (EA) differed significantly, whereas no significant difference was found for the torsional rigidity (GK). The twist-to-bend ratios (EI/GK) of all petioles ranged between 4.33 and 5.99, and of all transition zones between 0.67 and 0.78. Based on the twist-to-bend ratios, we hypothesize that bending loads are accommodated by the petiole, while torsional loads are shared between the transition zone and petiole.

Environmental-biomechanical reciprocity and the evolution of plant material properties

Abiotic-biotic interactions have shaped organic evolution since life first began. Abiotic factors influence growth, survival, and reproductive success, whereas biotic responses to abiotic factors have changed the physical environment (and indeed created new environments). This reciprocity is well illustrated by land plants who begin and end their existence in the same location while growing in size over the course of years or even millennia, during which environment factors change over many orders of magnitude. A biomechanical, ecological, and evolutionary perspective reveals that plants are (i) composed of materials (cells and tissues) that function as cellular solids (i.e. materials composed of one or more solid and fluid phases); (ii) that have evolved greater rigidity (as a consequence of chemical and structural changes in their solid phases); (iii) allowing for increases in body size and (iv) permitting acclimation to more physiologically and ecologically diverse and challenging habitats; which (v) have profoundly altered biotic as well as abiotic environmental factors (e.g. the creation of soils, carbon sequestration, and water cycles). A critical component of this evolutionary innovation is the extent to which mechanical perturbations have shaped plant form and function and how form and function have shaped ecological dynamics over the course of evolution.

Between Stress and Response: Function and Localization of Mechanosensitive Ca2+ Channels in Herbaceous and Perennial Plants

Over the past three decades, how plants sense and respond to mechanical stress has become a flourishing field of research. The pivotal role of mechanosensing in organogenesis and acclimation was demonstrated in various plants, and links are emerging between gene regulatory networks and physical forces exerted on tissues. However, how plant cells convert physical signals into chemical signals remains unclear. Numerous studies have focused on the role played by mechanosensitive (MS) calcium ion channels MCA, Piezo and OSCA. To complement these data, we combined data mining and visualization approaches to compare the tissue-specific expression of these genes, taking advantage of recent single-cell RNA-sequencing data obtained in the root apex and the stem of Arabidopsis and the Populus stem. These analyses raise questions about the relationships between the localization of MS channels and the localization of stress and responses. Such tissue-specific expression studies could help to elucidate the functions of MS channels. Finally, we stress the need for a better understanding of such mechanisms in trees, which are facing mechanical challenges of much higher magnitudes and over much longer time scales than herbaceous plants, and we mention practical applications of plant responsiveness to mechanical stress in agriculture and forestry.

Mechanical stimulation in wheat triggers age- and dose-dependent alterations in growth, development and grain characteristics

BACKGROUND AND AIMS

Wheat crops are exposed to a range of mechanical stimulations in their natural environment, yet we know very little about their response to such conditions. The aim of this study was to better understand the effect of mechanical stimulation on wheat growth and development, stem mechanical properties and grain measures. We focused on the following questions: (1) Does plant age affect the response to mechanical stimulation? (2) Is there a minimum threshold for the perception of mechanical stimuli? (3) Is the effect of manual brushing different to natural wind stimulation?

METHODS

For age- and dose-response experiments, wheat plants were grown under controlled glasshouse conditions with brushing treatments applied using a purpose-built rig. The results of the controlled experiments are compared with those from an outside experiment where wheat plants were exposed to natural wind, with or without additional brushing. Detailed phenotypic measurements were conducted and treatment effects on grain characteristics were determined using micro-computed tomography imaging.

KEY RESULTS

Two-week-old wheat plants were particularly sensitive to mechanical stimulation by controlled brushing treatments. Amongst others, plants exhibited a large reduction in height and grain yield, and an increase in tillers, above-ground biomass and stiffness of stem segments. Plants responded significantly to doses as small as one daily brushstroke. Outdoor experiments by and large confirmed results from controlled environment experiments.

CONCLUSIONS

The morphological and developmental response to mechanical brushing treatment, in relation to vegetative above-ground biomass and grain yield, is dependent on plant age as well as the dose of the treatments. This study shows that mechanical stimulation of wheat impacts on a multitude of agriculturally relevant traits and provides a much needed advancement of our understanding of wheat thigmomorphogenesis and the potential applications of mechanical conditioning to control relevant traits.

Effect of Nutrient Solution Flow Rate on Hydroponic Plant Growth and Root Morphology

Crop production under hydroponic environments has many advantages, yet the effects of solution flow rate on plant growth remain unclear. We conducted a hydroponic cultivation study using different flow rates under light-emitting diode lighting to investigate plant growth, nutrient uptake, and root morphology under different flow rates. Swiss chard plants were grown hydroponically under four nutrient solution flow rates (2 L/min, 4 L/min, 6 L/min, and 8 L/min). After 21 days, harvested plants were analyzed for root and shoot fresh weight, root and shoot dry weight, root morphology, and root cellulose and hemicellulose content. We found that suitable flow rates, acting as a eustress, gave the roots appropriate mechanical stimulation to promote root growth, absorb more nutrients, and increase overall plant growth. Conversely, excess flow rates acted as a distress that caused the roots to become compact and inhibited root surface area and root growth. Excess flow rate thereby resulted in a lower root surface area that translated to reduced nutrient ion absorption and poorer plant growth compared with plans cultured under a suitable flow rate. Our results indicate that regulating flow rate can regulate plant thigmomorphogenesis and nutrient uptake, ultimately affecting hydroponic crop 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.

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.

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.

Mechanical stimulation in Brachypodium distachyon: Implications for fitness, productivity, and cell wall properties

Mechanical stimulation, including exposure to wind, is a common environmental variable for plants. However, knowledge about the morphogenetic response of the grasses (Poaceae) to mechanical stimulation and impact on relevant agronomic traits is very limited. Two natural accessions of Brachypodium distachyon were exposed to wind and mechanical treatments. We surveyed a wide range of stem-related traits to determine the effect of the two treatments on plant growth, development, and stem biomass properties. Both treatments induced significant quantitative changes across multiple scales, from the whole plant down to cellular level. The two treatments resulted in shorter stems, reduced biomass, increased tissue rigidity, delayed flowering, and reduced seed yield in both accessions. Among changes in cell wall-related features, a substantial increase in lignin content and pectin methylesterase activity was most notable. Mechanical stimulation also reduced the enzymatic sugar release from the cell wall, thus increasing biomass recalcitrance. Notably, treatments had a distinct and opposite effect on vascular bundle area in the two accessions, suggesting genetic variation in modulating these responses to mechanical stimulation. Our findings highlight that exposure of grasses to mechanical stimulation is a relevant environmental factor affecting multiple traits important for their utilization in food, feed, and bioenergy applications.

Sorghum tiller bud growth is repressed by contact with the overlying leaf

Basal branching in grasses, or tillering, is an important trait determining both form and function of crops. Although similarities exist between eudicot and grass branching programs, one notable difference is that the tiller buds of grasses are covered by the subtending leaf, whereas eudicot buds are typically unconstrained. The current study shows that contact with the leaf sheath represses sorghum bud growth by providing a mechanical signal that cues the bud to refrain from rapid growth. Leaf removal resulted in massive reprogramming of the bud transcriptome that included signatures of epigenetic modifications and also implicated several hormones in the response. Bud abscisic acid transiently increased, then decreased following leaf removal relative to controls, and abscisic acid was necessary to repress bud growth in the presence of the leaf. Jasmonic acid (JA) levels and signalling increased in buds following leaf removal. Remarkably, application of JA to buds in situ promoted growth. The repression of bud growth by leaf contact shares characteristics of thigmomorphogenic responses in other systems, including the involvement of JA, though the JA effect is opposite. The repression of bud growth by leaf contact may represent a mechanism to time tillering to an appropriate developmental stage of the plant.

Quantitative and functional posttranslational modification proteomics reveals that TREPH1 plays a role in plant touch-delayed bolting

Environmental mechanical forces, such as wind and touch, trigger gene-expression regulation and developmental changes, called "thigmomorphogenesis," in plants, demonstrating the ability of plants to perceive such stimuli. In Arabidopsis, a major thigmomorphogenetic response is delayed bolting, i.e., emergence of the flowering stem. The signaling components responsible for mechanotransduction of the touch response are largely unknown. Here, we performed a high-throughput SILIA (stable isotope labeling in Arabidopsis)-based quantitative phosphoproteomics analysis to profile changes in protein phosphorylation resulting from 40 seconds of force stimulation in Arabidopsis thaliana Of the 24 touch-responsive phosphopeptides identified, many were derived from kinases, phosphatases, cytoskeleton proteins, membrane proteins, and ion transporters. In addition, the previously uncharacterized protein TOUCH-REGULATED PHOSPHOPROTEIN1 (TREPH1) became rapidly phosphorylated in touch-stimulated plants, as confirmed by immunoblots. TREPH1 fractionates as a soluble protein and is shown to be required for the touch-induced delay of bolting and gene-expression changes. Furthermore, a nonphosphorylatable site-specific isoform of TREPH1 (S625A) failed to restore touch-induced flowering delay of treph1-1, indicating the necessity of S625 for TREPH1 function and providing evidence consistent with the possible functional relevance of the touch-regulated TREPH1 phosphorylation. Taken together, these findings identify a phosphoprotein player in Arabidopsis thigmomorphogenesis regulation and provide evidence that TREPH1 and its touch-induced phosphorylation may play a role in touch-induced bolting delay, a major component of thigmomorphogenesis.

Photosynthetic opportunity cost and energetic cost of a rapid leaf closure behavior in Mimosa pudica

PREMISE OF THE STUDY

The rapid leaf movement of Mimosa pudica is expected to be costly because of energetic trade-offs with other processes such as growth and reproduction. Here, we assess the photosynthetic opportunity cost and energetic cost of the unique leaf closing behavior of M. pudica.

METHODS

In the greenhouse, we employed novel touch-stimulation machines to expose plants to one of three treatments: (1) untouched control plants; (2) plants touch-stimulated to close their leaves during the day to incur energetic costs associated with leaf movement and reduced photosynthesis; (3) plants touched at night to assess the effects of touch alone. M. pudica is nyctinastic and closes its leaves at night; thus, touching at night does not impart additional costs. We directly assessed costs by comparing physical traits. Leaf re-opening response was measured to assess the potential for plant behavioral plasticity to impact photosynthetic opportunity costs.

KEY RESULTS

The cost of rapid leaf closure behavior was expressed as a 47% reduction in reproductive biomass accounting for the effect of touch. Touch itself changed physical traits such as biomass, with touched plants being generally bigger. Plants touched at night re-opened their leaflets 26% quicker than plants touched during the day.

CONCLUSIONS

We reason that the reproductive allocation costs incurred by M. pudica can be attributed to a combination of photosynthetic opportunity cost and the energetic cost associated with increased stimulation of leaf movement and that behavioral plasticity has the potential to alter photosynthetic opportunity costs.

Vine tendrils use contact chemoreception to avoid conspecific leaves

Movement and growth habit of climbing plants have attracted attention since the time of Charles Darwin; however, there are no reports on whether plants can choose suitable hosts or avoid unsuitable ones based on chemoreception. Here, I show that the tendrils of Cayratia japonica (Vitaceae) appear to avoid conspecific leaves using contact chemoreception for oxalates, which are highly concentrated in C. japonica leaves. The coiling experiments show that C. japonica has a flexible plastic response to avoid coiling around conspecific leaves. The coiling response is negatively correlated with the oxalate content in the contacted leaves. Experiments using laboratory chemicals indicate that the tendrils avoid oxalate-coated plastic sticks. These results indicate that the tendrils of C. japonica avoid coiling around a conspecific leaf based on contact chemoreception for oxalate compounds. The tendrils of climbing plants may function as a chemoreceptor system to detect the chemical cues of a contacted plant.

The RNA Polymerase-Associated Factor 1 Complex Is Required for Plant Touch Responses

Thigmomorphogenesis is a stereotypical developmental alteration in the plant body plan that can be induced by repeatedly touching plant organs. To unravel how plants sense and record multiple touch stimuli we performed a novel forward genetic screen based on the development of a shorter stem in response to repetitive touch. The touch insensitive (ths1) mutant identified in this screen is defective in some aspects of shoot and root thigmomorphogenesis. The ths1 mutant is an intermediate loss-of-function allele of VERNALIZATION INDEPENDENCE 3 (VIP3), a previously characterized gene whose product is part of the RNA polymerase II-associated factor 1 (Paf1) complex. The Paf1 complex is found in yeast, plants and animals, and has been implicated in histone modification and RNA processing. Several components of the Paf1 complex are required for reduced stem height in response to touch and normal root slanting and coiling responses. Global levels of histone H3K36 trimethylation are reduced in VIP3 mutants. In addition, THS1/VIP3 is required for wild type histone H3K36 trimethylation at the TOUCH3 (TCH3) and TOUCH4 (TCH4) loci and for rapid touch-induced upregulation of TCH3 and TCH4 transcripts. Thus, an evolutionarily conserved chromatin-modifying complex is required for both short- and long-term responses to mechanical stimulation, providing insight into how plants record mechanical signals for thigmomorphogenesis.

Forest trees filter chronic wind-signals to acclimate to high winds

Controlled experiments have shown that trees acclimate thigmomorphogenetically to wind-loads by sensing their deformation (strain). However, the strain regime in nature is exposed to a full spectrum of winds. We hypothesized that trees avoid overreacting by responding only to winds which bring information on local climate and/or wind exposure. Additionally, competition for light dependent on tree social status also likely affects thigmomorphogenesis. We monitored and manipulated quantitatively the strain regimes of 15 pairs of beech (Fagus sylvatica) trees of contrasting social status in an acclimated stand, and quantified the effects of these regimes on the radial growth over a vegetative season. Trees exposed to artificial bending, the intensity of which corresponds to the strongest wind-induced strains, enhanced their secondary growth by at least 80%. Surprisingly, this reaction was even greater - relatively - for suppressed trees than for dominant ones. Acclimated trees did not sense the different types of wind events in the same way. Daily wind speed peaks due to thermal winds were filtered out. Thigmomorphogenesis was therefore driven by intense storms. Thigmomorphogenesis is also likely to be involved in determining social status.

Root biomechanics in Rhizophora mangle: anatomy, morphology and ecology of mangrove's flying buttresses

BACKGROUND AND AIMS

Rhizophora species of mangroves have a conspicuous system of stilt-like roots (rhizophores) that grow from the main stem and resemble flying buttresses. As such, the development of rhizophores can be predicted to be important for the effective transmission of dynamic loads from the top of the tree to the ground, especially where the substrate is unstable, as is often the case in the habitats where Rhizophora species typically grow. This study tests the hypothesis that rhizophore architecture in R. mangle co-varies with their proximity to the main stem, and with stem size and crown position.

METHODS

The allometry and wood mechanical properties of R. mangle (red mangrove) trees growing in a mangrove basin forest within a coastal lagoon in Mexico were compared with those of coexisting, non-buttressed mangrove trees of Avicennia germinans. The anatomy of rhizophores was related to mechanical stress due to crown orientation (static load) and to prevailing winds (dynamic load) at the study site.

KEY RESULTS

Rhizophores buttressed between 10 and 33 % of tree height. There were significant and direct scaling relationships between the number, height and length of rhizophores vs. basal area, tree height and crown area. Wood mechanical resistance was significantly higher in the buttressed R. mangle (modulus of elasticity, MOE = 18·1 ± 2 GPa) than in A. germinans (MOE = 12·1 ± 0·5 GPa). Slenderness ratios (total height/stem diameter) were higher in R. mangle, but there were no interspecies differences in critical buckling height. When in proximity to the main stem, rhizophores had a lower length/height ratio, higher eccentricity and higher xylem/bark and pith proportions. However, there were no directional trends with regard to prevailing winds or tree leaning.

CONCLUSIONS

In comparison with A. germinans, a tree species with wide girth and flare at the base, R. mangle supports a thinner stem of higher mechanical resistance that is stabilized by rhizophores resembling flying buttresses. This provides a unique strategy to increase tree slenderness and height in the typically unstable substrate on which the trees grow, at a site that is subject to frequent storms.

Mechanosensitive control of plant growth: bearing the load, sensing, transducing, and responding

As land plants grow and develop, they encounter complex mechanical challenges, especially from winds and turgor pressure. Mechanosensitive control over growth and morphogenesis is an adaptive trait, reducing the risks of breakage or explosion. This control has been mostly studied through experiments with artificial mechanical loads, often focusing on cellular or molecular mechanotransduction pathway. However, some important aspects of mechanosensing are often neglected. (i) What are the mechanical characteristics of different loads and how are loads distributed within different organs? (ii) What is the relevant mechanical stimulus in the cell? Is it stress, strain, or energy? (iii) How do mechanosensing cells signal to meristematic cells? Without answers to these questions we cannot make progress analyzing the mechanobiological effects of plant size, plant shape, tissue distribution and stiffness, or the magnitude of stimuli. This situation is rapidly changing however, as systems mechanobiology is being developed, using specific biomechanical and/or mechanobiological models. These models are instrumental in comparing loads and responses between experiments and make it possible to quantitatively test biological hypotheses describing the mechanotransduction networks. This review is designed for a general plant science audience and aims to help biologists master the models they need for mechanobiological studies. Analysis and modeling is broken down into four steps looking at how the structure bears the load, how the distributed load is sensed, how the mechanical signal is transduced, and then how the plant responds through growth. Throughout, two examples of adaptive responses are used to illustrate this approach: the thigmorphogenetic syndrome of plant shoots bending and the mechanosensitive control of shoot apical meristem (SAM) morphogenesis. Overall this should provide a generic understanding of systems mechanobiology at work.

Acclimation of mechanical and hydraulic functions in trees: impact of the thigmomorphogenetic process

The secondary xylem (wood) of trees mediates several functions including water transport and storage, mechanical support and storage of photosynthates. The optimal structures for each of these functions will most likely differ. The complex structure and function of xylem could lead to trade-offs between conductive efficiency, resistance to embolism, and mechanical strength needed to count for mechanical loading due to gravity and wind. This has been referred to as the trade-off triangle, with the different optimal solutions to the structure/function problems depending on the environmental constraints as well as taxonomic histories. Thus, the optimisation of each function will lead to drastically different anatomical structures. Trees are able to acclimate the internal structure of their trunk and branches according to the stress they experience. These acclimations lead to specific structures that favor the efficiency or the safety of one function but can be antagonistic with other functions. Currently, there are no means to predict the way a tree will acclimate or optimize its internal structure in support of its various functions under differing environmental conditions. In this review, we will focus on the acclimation of xylem anatomy and its resulting mechanical and hydraulic functions to recurrent mechanical strain that usually result from wind-induced thigmomorphogenesis with a special focus on the construction cost and the possible trade-off between wood functions.

The zinc finger protein PtaZFP2 negatively controls stem growth and gene expression responsiveness to external mechanical loads in poplar

Mechanical cues are essential signals regulating plant growth and development. In response to wind, trees develop a thigmomorphogenetic response characterized by a reduction in longitudinal growth, an increase in diameter growth, and changes in mechanical properties. The molecular mechanisms behind these processes are poorly understood. In poplar, PtaZFP2, a C2H2 transcription factor, is rapidly up-regulated after stem bending. To investigate the function of PtaZFP2, we analyzed PtaZFP2-overexpressing poplars (Populus tremula × Populus alba). To unravel the genes downstream PtaZFP2, a transcriptomic analysis was performed. PtaZFP2-overexpressing poplars showed longitudinal and cambial growth reductions together with an increase in the tangent and hardening plastic moduli. The regulation level of mechanoresponsive genes was much weaker after stem bending in PtaZFP2-overexpressing poplars than in wild-type plants, showing that PtaZFP2 negatively modulates plant responsiveness to mechanical stimulation. Microarray analysis revealed a high proportion of down-regulated genes in PtaZFP2-overexpressing poplars. Among these genes, several were also shown to be regulated by mechanical stimulation. Our results confirmed the important role of PtaZFP2 during plant acclimation to mechanical load, in particular through a negative control of plant molecular responsiveness. This desensitization process could modulate the amplitude and duration of the plant response during recurrent stimuli.

A chromatin modifying enzyme, SDG8, is involved in morphological, gene expression, and epigenetic responses to mechanical stimulation

Thigmomorphogenesis is viewed as being a response process of acclimation to short repetitive bursts of mechanical stimulation or touch. The underlying molecular mechanisms that coordinate changes in how touch signals lead to long-term morphological changes are enigmatic. Touch responsive gene expression is rapid and transient, and no transcription factor or DNA regulatory motif has been reported that could confer a genome wide mechanical stimulus. We report here on a chromatin modifying enzyme, SDG8/ASHH2, which can regulate the expression of many touch responsive genes identified in Arabidopsis. SDG8 is required for the permissive expression of touch induced genes; and the loss of function of sdg8 perturbs the maximum levels of induction on selected touch gene targets. SDG8 is required to maintain permissive H3K4 trimethylation marks surrounding the Arabidopsis touch-inducible gene TOUCH 3 (TCH3), which encodes a calmodulin-like protein (CML12). The gene neighboring was also slightly down regulated, revealing a new target for SDG8 mediated chromatin modification. Finally, sdg8 mutants show perturbed morphological response to wind-agitated mechanical stimuli, implicating an epigenetic memory-forming process in the acclimation response of thigmomorphogenesis.

Plant neighbor detection through touching leaf tips precedes phytochrome signals

Plants in dense vegetation compete for resources, including light, and optimize their growth based on neighbor detection cues. The best studied of such behaviors is the shade-avoidance syndrome that positions leaves in optimally lit zones of a vegetation. Although proximate vegetation is known to be sensed through a reduced ratio between red and far-red light, we show here through computational modeling and manipulative experiments that leaves of the rosette species Arabidopsis thaliana first need to move upward to generate sufficient light reflection potential for subsequent occurrence and perception of a reduced red to far-red ratio. This early hyponastic leaf growth response is not induced by known neighbor detection cues under both climate chamber and natural sunlight conditions, and we identify a unique way for plants to detect future competitors through touching of leaf tips. This signal occurs before light signals and appears to be the earliest means of above-ground plant-plant signaling in horizontally growing rosette plants.

Partial mechanical stimulation facilitates the growth of the rhizomatous plant Leymus secalinus: modulation by clonal integration

BACKGROUND AND AIMS

Mechanical stimulation (MS) often induces plants to undergo thigmomorphogenesis and to synthesize an array of signalling substances. In clonal plants, connected ramets often share resources and hormones. However, little is known about whether and how clonal integration influences the ability of clonal plants to withstand MS. We hypothesized that the effects of MS may be modulated by clonal integration.

METHODS

We conducted an experiment in which ramet pairs of Leymus secalinus were subjected to three treatments: (1) connected ramet pairs under a homogeneous condition [i.e. the proximal (relatively old) and distal (relatively young) ramets were not mechanically stressed]; (2) connected ramet pairs under a heterogeneous condition (i.e. the proximal ramet was mechanically stressed but the distal ramet was not); and (3) disconnected ramet pairs under the same condition as in treatment 2. At the end of the experiment, we harvested all plants and determined their biomass and allocation.

KEY RESULTS

Clonal integration had no significant influence on measured traits of distal L. secalinus ramets without MS. However, under MS, plants with distal ramets that were connected to a mother ramet produced more total plant biomass, below-ground biomass, ramets and total rhizome length than those that were not connected. Partial MS exerted local effects on stimulated ramets and remote effects on connected unstimulated ramets. Partial MS increased total biomass, root/shoot ratio, number of ramets and total rhizome length of stimulated proximal ramets, and increased total biomass, root weight ratio, number of ramets and total rhizome length of connected unstimulated ramets due to clonal integration.

CONCLUSIONS

These findings suggest that thigmomorphogenesis may protect plants from the stresses caused by high winds or trampling and that thigmomorphogenesis can be strongly modulated by the degree of clonal integration.

Novel thigmomorphogenetic responses in Carica papaya: touch decreases anthocyanin levels and stimulates petiole cork outgrowths

BACKGROUND AND AIMS

Because of its rapid growth rate, relative ease of transformation, sequenced genome and low gene number relative to Arabidopsis, the tropical fruit tree, Carica papaya, can serve as a complementary genetic model for complex traits. Here, new phenotypes and touch-regulated gene homologues have been identified that can be used to advance the understanding of thigmomorphogenesis, a multigenic response involving mechanoreception and morphological change.

METHODS

Morphological alterations were quantified, and microscopy of tissue was conducted. Assays for hypocotyl anthocyanins, lignin and chlorophyll were performed, and predicted genes from C. papaya were compared with Arabidopsis touch-inducible (TCH) and Mechanosensitive channel of Small conductance-like genes (MscS-like or MSL). In addition, the expression of two papaya TCH1 homologues was characterized.

KEY RESULTS

On the abaxial side of petioles, treated plants were found to have novel, hypertrophic outgrowths associated with periderm and suberin. Touched plants also had higher lignin, dramatically less hypocotyl anthocyanins and chlorophyll, increased hypocotyl diameter, and decreased leaf width, stem length and root fresh weight. Papaya was found to have fewer MSL genes than Arabidopsis, and four touch-regulated genes in Arabidopsis had no counterparts in papaya. Water-spray treatment was found to enhance the expression of two papaya TCH1 homologues whereas induction following touch was only slightly correlated.

CONCLUSIONS

The novel petiole outgrowths caused by non-wounding, mechanical perturbation may be the result of hardening mechanisms, including added lignin, providing resistance against petiole movement. Inhibition of anthocyanin accumulation following touch, a new phenotypic association, may be caused by diversion of p-coumaroyl CoA away from chalcone synthase for lignin synthesis. The absence of MSL and touch-gene homologues indicates that papaya may have a smaller set of touch-regulated genes. The genes and novel touch-regulated phenotypes identified here will contribute to a more comprehensive view of thigmomorphogenesis in plants.

Clonal plasticity of aquatic plant species submitted to mechanical stress: escape versus resistance strategy

BACKGROUND AND AIMS

The plastic alterations of clonal architecture are likely to have functional consequences, as they affect the spatial distribution of ramets over patchy environments. However, little is known about the effect of mechanical stresses on the clonal growth. The aim of the present study was to investigate the clonal plasticity induced by mechanical stress consisting of continuous water current encountered by aquatic plants. More particularly, the aim was to test the capacity of the plants to escape this stress through clonal plastic responses.

METHODS

The transplantation of ramets of the same clone in two contrasting flow velocity conditions was carried out for two species (Potamogeton coloratus and Mentha aquatica) which have contrasting clonal growth forms. Relative allocation to clonal growth, to creeping stems in the clonal biomass, number and total length of creeping stems, spacer length and main creeping stem direction were measured.

KEY RESULTS

For P. coloratus, plants exposed to water current displayed increased total length of creeping stems, increased relative allocation to creeping stems within the clonal dry mass and increased spacer length. For M. aquatica, plants exposed to current displayed increased number and total length of creeping stems. Exposure to current induced for both species a significant increase of the proportion of creeping stems in the downstream direction to the detriment of creeping stems perpendicular to flow.

CONCLUSIONS

This study demonstrates that mechanical stress from current flow induced plastic variation in clonal traits for both species. The responses of P. coloratus could lead to an escape strategy, with low benefits with respect to sheltering and anchorage. The responses of M. aquatica that may result in a denser canopy and enhancement of anchorage efficiency could lead to a resistance strategy.

The effects of mechanical stress and spectral shading on the growth and allocation of ten genotypes of a stoloniferous plant

BACKGROUND AND AIMS

Because plants protect each other from wind, stand density affects both the light climate and the amount of mechanical stress experienced by plants. But the potential interactive effects of mechanical stress and canopy shading on plant growth have rarely been investigated and never in stoloniferous plants which, due to their creeping growth form, can be expected to respond differently to these factors than erect plants.

METHODS

Plants of ten genotypes of the stoloniferous species Potentilla reptans were subjected to two levels of mechanical stress (0 or 40 daily flexures) and two levels of spectral shading (15 % of daylight with a red:far red ratio of 0.3 vs. 50 % daylight and a red:far red ratio of 1.2).

KEY RESULTS

Mechanically stressed plants produced more leaves with shorter more flexible petioles, more roots, and more but less massive stolons. Responses to spectral shading were mostly in the opposite direction to thigmomorphogenesis, including the production of thinner, taller petioles made of more rigid tissue. The degree of thigmomorphogenesis was either independent of light climate or stimulated by spectral shading. At the genotypic level there were no clear correlations between responses to shade and mechanical stress.

CONCLUSIONS

These results suggest that in stoloniferous plants mechanical stress results in clones with a more compact, shorter shoot structure and more roots. This response does not appear to be suppressed by canopy shading, which suggests that wind shielding (reduced mechanical stress) by neighbours in dense vegetation serves as a cue that induces shade avoidance responses such as increased petiole elongation.