Thigmomorphogenesis: on the mechanical properties of mechanically perturbed bean plants

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.

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.

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.

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.

Genome-Wide Analysis and Expression Profiles of Ethylene Signal Genes and Apetala2/Ethylene-Responsive Factors in Peanut (Arachis hypogaea L.)

Peanut is an important oil and economic crop widely cultivated in the world. It has special characteristics such as blooming on the ground but bearing fruits underground. During the peg penetrating into the ground, it is subjected to mechanical stress from the soil at the same time. It has been proved that mechanical stress affects plant growth and development by regulating the ethylene signaling-related genes. In this study, we identified some genes related to ethylene signal of peanut, including 10 ethylene sensors, two constitutive triple responses (CTRs), four ethylene insensitive 2 (EIN2s), four ethylene insensitive 3 (EIN3s), six EIN3-binding F-box proteins (EBFs), and 188 Apetala2/ethylene-responsive factors (AP2/ERFs). One hundred and eighty-eight AP2/ERFs were further divided into four subfamilies, 123 ERFs, 56 AP2s, 6 Related to ABI3/VP1 (RAVs), and three Soloists, of them one hundred and seventy AP2/ERF gene pairs were clustered into segmental duplication events in genome of Arachis hypogaea. A total of 134, 138, 97, and 150 AhAP2/ERF genes formed 210, 195, 166, and 525 orthologous gene pairs with Arachis duranensis, Arachis ipaensis, Arabidopsis thaliana, and Glycine max, respectively. Our transcriptome results showed that two EIN3s (Arahy.J729H0 and Arahy.S7XF8N) and one EBFs (Arahy.G4JMEM) were highly expressed when mechanical stress increased. Among the 188 AhAP2/ERF genes, there were 31 genes with the fragments per kilobase of exon model per million mapped fragments (FPKM) ≥ 100 at least one of the 15 samples of Tifrunner. Among them, three AhAP2/ERFs (Arahy.15RATX, Arahy.FAI7YU, and Arahy.452FBF) were specifically expressed in seeds and five AhAP2/ERFs (Arahy.HGAZ7D, Arahy.ZW7540, Arahy.4XS3FZ, Arahy.QGFJ76, and Arahy.AS0C7C) were highly expressed in the tissues, which responded mechanical stress, suggesting that they might sense mechanical stress. Mechanical stress simulation experiment showed that three AhAP2/ERFs (Arahy.QGFJ76, Arahy.AS0C7C, and Arahy.HGAZ7D) were sensitive to mechanical stress changes and they all had the conservative repressor motif (DLNXXP) in the C-terminus, indicated that they might transmit mechanical stress signals through transcriptional inhibition. This study reveals the regulatory landscape of ethylene signal-related genes in peanut, providing valuable information for the mining of target genes for further study.

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.

Wound- and mechanostimulated electrical signals control hormone responses

Plants in nature are constantly exposed to organisms that touch them and wound them. A highly conserved response to these stimuli is a rapid collapse of membrane potential (i.e. a decrease of electrical field strength across membranes). This can be coupled to the production and/or action of jasmonate or ethylene. Here, the various types of electrical signals in plants are discussed in the context of hormone responses. Genetic approaches are revealing genes involved in wound-induced electrical signalling. These include clade 3 GLUTAMATE RECEPTOR-LIKE (GLR) genes, Arabidopsis H⁺ -ATPases (AHAs), RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), and genes that determine cell wall properties. We briefly review touch- and wound-induced increases in cytosolic Ca²⁺ concentrations and their temporal relationship to electrical activities. We then look at the questions that need addressing to link mechanostimulation and wound-induced electrical activity to hormone responses. Utilizing recently published results, we also present a hypothesis for wound-response leaf-to-leaf electrical signalling. This model is based on rapid electro-osmotic coupling between the phloem and xylem. The model suggests that the depolarization of membranes within the vascular matrix triggered by physical stimuli and/or chemical elicitors is linked to changes in phloem turgor and that this plays vital roles in leaf-to-leaf electrical signal propagation.

Thigmomorphogenesis and biomechanical responses of shade-grown Serianthes nelsonii plants to stem flexure

The influences of stem flexure on shade-grown Serianthes nelsonii Merr. stem growth and strength were determined in a container nursery setting. Treated stems were bent 90° two times daily for a 14 wk nursery production period. Plant height, internode length, and slenderness were decreased by stem flexure when compared with control plants that received no flexure. Two force-displacement tests revealed stem strength was increased by the flexure treatment. Control plants exhibited undesirable lean of the main stem, and 1 hr of wind stress further increased the angle of lean. Treated plants were close to orthotropic and the wind stress did not change the stem lean. Results indicate stem flexure is a reliable method for increasing the quality of shade-grown S. nelsonii plants and some form of mechanical stimulation should be added to nursery production protocols for the species.

Mitochondrial function modulates touch signalling in Arabidopsis thaliana

Plants respond to short- and long-term mechanical stimuli, via altered transcript abundance and growth respectively. Jasmonate, gibberellic acid and calcium have been implicated in mediating responses to mechanical stimuli. Previously it has been shown that the transcript abundance for the outer mitochondrial membrane protein of 66 kDa (OM66), is induced several fold after 30 min in response to touch. Therefore, the effect of mitochondrial function on the response to mechanical stimulation by touch at 30 min was investigated. Twenty-five mutants targeting mitochondrial function or regulators revealed that all affected the touch transcriptome. Double and triple mutants revealed synergistic or antagonistic effects following the observed responses in the single mutants. Changes in the touch-responsive transcriptome were localised, recurring with repeated rounds of stimulus. The gene expression kinetics after repeated touch were complex, displaying five distinct patterns. These transcriptomic responses were altered by some regulators of mitochondrial retrograde signalling, such as cyclic dependent protein kinase E1, a kinase protein in the mediator complex, and KIN10 (SnRK1 - sucrose non-fermenting related protein kinase 1), revealing an overlap between the touch response and mitochondrial stress signalling and alternative mitochondrial metabolic pathways. Regulatory network analyses revealed touch-induced stress responses and suppressed growth and biosynthetic processes. Interaction with the phytohormone signalling pathways indicated that ethylene and gibberellic acid had the greatest effect. Hormone measurements revealed that mutations of genes that encoded mitochondrial proteins altered hormone concentrations. Mitochondrial function modulates touch-induced changes in gene expression directly through altered regulatory networks, and indirectly via altering hormonal levels.

Omeprazole Enhances Mechanical Stress-Induced Root Growth Reduction in Arabidopsis thaliana

Mechanical sensing is one of the most fundamental processes for sessile plants to survive and grow. The response is known to involve calcium elevation in the cell. Arabidopsis seedlings grown horizontally on agar plates covered with a dialysis membrane show a 2-fold reduction in root growth compared with those grown vertically, a response to mechanical stress generated due to gravitropism of the root. To understand the molecular mechanism of how plant roots sense and respond to mechanical stimuli, we screened chemical libraries for compounds that affect the horizontal root growth in this experimental system and found that, while having no effect on root gravitropism, omeprazole known as a proton pump inhibitor significantly enhanced the mechanical stress-induced root growth reduction especially in lower pH media. In contrast, omeprazole reversed neither the alleviation of the mechanical stress-induced growth reduction caused by calcium depletion nor the insensitivity to the mechanical stress in the ethylene signaling mutant ein2. Together with the finding that omeprazole increased expression of touch-induced genes and ETHYLENE RESPONSE FACTOR1, our results suggest that the target of omeprazole mediates ethylene signaling in the root growth response to mechanical stress.

Arabidopsis touch-induced morphogenesis is jasmonate mediated and protects against pests

Plants cannot change location to escape stressful environments. Therefore, plants evolved to respond and acclimate to diverse stimuli, including the seemingly innocuous touch stimulus [1-4]. Although some species, such as Venus flytrap, have fast touch responses, most plants display more gradual touch-induced morphological alterations, called thigmomorphogenesis [2, 3, 5, 6]. Thigmomorphogenesis may be adaptive; trees subjected to winds develop less elongated and thicker trunks and thus are less likely damaged by powerful wind gusts [7]. Despite the widespread relevance of thigmomorphogenesis, the regulation that underlies plant mechanostimulus-induced morphological responses remains largely unknown. Furthermore, whether thigmomorphogenesis confers additional advantage is not fully understood. Although aspects of thigmomorphogenesis resemble ethylene effects [8], and touch can induce ethylene synthesis [9, 10], Arabidopsis ethylene response mutants show touch-induced thigmomorphogenesis [11]; thus, ethylene response is nonessential for thigmomorphogenesis. Here we show that jasmonate (JA) phytohormone both is required for and promotes the salient characteristics of thigmomorphogenesis in Arabidopsis, including a touch-induced delay in flowering and rosette diameter reduction. Furthermore, we find that repetitive mechanostimulation enhances Arabidopsis pest resistance in a JA-dependent manner. These results highlight an important role for JA in mediating mechanostimulus-induced plant developmental responses and resultant cross-protection against biotic stress.

Thigmomorphogenesis in Solanum lycopersicum: morphological and biochemical responses in stem after mechanical stimulation

The activation of the phenylpropanoid pathway in plants by environmental stimuli is one of the most universal biochemical stress responses known. In tomato plant, rubbing applied to a young internode inhibit elongation of the rubbed internode and his neighboring one. These morphological changes were correlated with an increase in lignification enzyme activities, phenylalanine ammonia-lyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidases (POD), 24 hours after rubbing of the forth internode. Furthermore, a decrease in indole-3-acetic acid (IAA) content was detected in the rubbed internode and the upper one. Taken together, our results suggest that decrease in rubbed internode length is a consequence of IAA oxidation, increases in enzyme activities (PAL, CAD and POD), and cell wall rigidification associated with induction of lignification process.

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.

Characterization and expression analysis under bending and other abiotic factors of PtaZFP2, a poplar gene encoding a Cys2/His2 zinc finger protein

In plants, mechanoperception and transduction of mechanical signals have been studied essentially in Arabidopsis thaliana L. and Lycopersicon esculentum L. plants, i.e., in nonwoody plants. Here, we have described the isolation of both the full-length cDNA and the regulatory region of PtaZFP2, encoding a member of Cys2/His2 zinc finger protein (ZFP) family in Populus tremula L. x Populus alba L. Time course analysis of expression demonstrated that PtaZFP2 mRNA accumulated as early as 5 min in response to a controlled stem bending and is restricted to the organ where the mechanical stimulus is applied. The real-time quantitative Reverse Transcriptase Polymerase Chain Reaction experiments showed that PtaZFP2 was also rapidly up-regulated in poplar stems in response to gravitropism suggesting that PtaZFP2 is induced by different mechanical signals. Abundance of PtaZFP2 transcripts also increased highly in response to wounding and to a weaker extent to salt treatment and cold, which is consistent with the numerous putative cis-elements found in its regulatory region. As in other species, these data suggest that Cys2/His2 ZFPs could function in poplar as key transcriptional regulators in the acclimation response to different environmental factors.

Thigmomorphogenesis: a complex plant response to mechano-stimulation

In nature, plants are challenged with hurricane winds, monsoon rains, and herbivory attacks, in addition to many other harsh mechanical perturbations that can threaten plant survival. As a result, over many years of evolution, plants have developed very sensitive mechanisms through which they can perceive and respond to even subtle stimuli, like touch. Some plants respond behaviourally to the touch stimulus within seconds, while others show morphogenetic alterations over long periods of time, ranging from days to weeks. Various signalling molecules and phytohormones, including intracellular calcium, jasmonates, ethylene, abscisic acid, auxin, brassinosteroids, nitric oxide, and reactive oxygen species, have been implicated in touch responses. Many genes are induced following touch. These genes encode proteins involved in various cellular processes including calcium sensing, cell wall modifications, and defence. Twenty-three per cent of these up-regulated genes contain a recently identified promoter element involved in the rapid induction in transcript levels following mechanical perturbations. The employment of various genetic, biochemical, and molecular tools may enable elucidation of the mechanisms through which plants perceive mechano-stimuli and transduce the signals intracellularly to induce appropriate responses.

Unique ethylene-regulated touch responses of Arabidopsis thaliana roots to physical hardness

Although touch responses of plant roots are an important adaptive behavior, the molecular mechanism remains unclear. We have developed a bioassay for measuring root-bending responses to physical hardness in Arabidopsis thaliana seedlings. Our test requires a two-layer solid medium. Primary roots growing downward through an upper layer of 0.3% phytagel either penetrate the lower layer or bend along an interface between the upper and lower layers with different concentrations (0.2-0.5%, corresponding to 1.57-6.79 gw mm(-2) in hardness). In proportion to increasing hardness of the lower layer, we found that the percentage of bending roots increased and ethylene production decreased, suggesting an inverse relationship between the root-bending response and ethylene production. Studies with ethylene biosynthesis modulators and mutants also suggested that bending and non-bending responses of roots to medium hardness depend, respectively, on decreased and increased ethylene biosynthesis. In addition, the degrees of root-tip softening and differential root-cell growth, both possible factors determining root-bending response, were enhanced and attenuated by decreased and increased amounts of ethylene, respectively--also in bending roots and non-bending roots. Our findings indicate that ethylene regulates root touch responses, probably through a combination of root-tip softening (or hardening) and differential root-cell growth.

Ethylene stimulates nutations that are dependent on the ETR1 receptor

Ethylene influences a number of processes in Arabidopsis (Arabidopsis thaliana) through the action of five receptors. In this study, we used high-resolution, time-lapse imaging to examine the long-term effects of ethylene on growing, etiolated Arabidopsis seedlings. These measurements revealed that ethylene stimulates nutations of the hypocotyls with an average delay in onset of over 6 h. The nutation response was constitutive in ctr1-2 mutants maintained in air, whereas ein2-1 mutants failed to nutate when treated with ethylene. Ethylene-stimulated nutations were also eliminated in etr1-7 loss-of-function mutants. Transformation of the etr1-7 mutant with a wild-type genomic ETR1 transgene rescued the nutation phenotype, further supporting a requirement for ETR1. Loss-of-function mutations in the other receptor isoforms had no effect on ethylene-stimulated nutations. However, the double ers1-2 ers2-3 and triple etr2-3 ers2-3 ein4-4 loss-of-function mutants constitutively nutated in air. These results support a model where all the receptors are involved in ethylene-stimulated nutations, but the ETR1 receptor is required and has a contrasting role from the other receptor isoforms in this nutation phenotype. Naphthylphthalamic acid eliminated ethylene-stimulated nutations but had no effect on growth inhibition caused by ethylene, pointing to a role for auxin transport in the nutation phenotype.

In touch: plant responses to mechanical stimuli

Perception and response to mechanical stimuli are likely essential at the cellular and organismal levels. Elaborate and impressive touch responses of plants capture the imagination as such behaviors are unexpected in otherwise often quiescent creatures. Touch responses can turn plants into aggressors against animals, trapping and devouring them, and enable flowers to be active in ensuring crosspollination and shoots to climb to sunlit heights. Morphogenesis is also influenced by mechanical perturbations, including both dynamic environmental stimuli, such as wind, and constant forces, such as gravity. Even individual cells must sense turgor and wall integrity, and subcellular organelles can translocate in response to mechanical perturbations. Signaling molecules and hormones, including intracellular calcium, reactive oxygen species, octadecanoids and ethylene, have been implicated in touch responses. Remarkably, touch-induced gene expression is widespread; more than 2.5% of Arabidopsis genes are rapidly up-regulated in touch-stimulated plants. Many of these genes encode calcium-binding, cell wall modifying, defense, transcription factor and kinase proteins. With these genes as tools, molecular genetic methods may enable elucidation of mechanisms of touch perception, signal transduction and response regulation.

Touch modulates gravity sensing to regulate the growth of primary roots of Arabidopsis thaliana

Plants must sense and respond to diverse stimuli to optimize the architecture of their root system for water and nutrient scavenging and anchorage. We have therefore analyzed how information from two of these stimuli, touch and gravity, are integrated to direct root growth. In Arabidopsis thaliana, touch stimulation provided by a glass barrier placed across the direction of growth caused the root to form a step-like growth habit with bends forming in the central and later the distal elongation zones. This response led to the main root axis growing parallel to, but not touching the obstacle, whilst the root cap maintained contact with the barrier. Removal of the graviperceptive columella cells of the root cap using laser ablation reduced the bending response of the distal elongation zone. Similarly, although the roots of the gravisensing impaired pgm1-1 mutant grew along the barrier at the same average angle as wild-type, this angle became more variable with time. These observations imply a constant gravitropic re-setting of the root tip response to touch stimulation from the barrier. In wild-type plants, transient touch stimulation of root cap cells, but not other regions of the root, inhibited both subsequent gravitropic growth and amyloplast sedimentation in the columella. Taken together, these results suggest that the cells of the root cap sense touch stimuli and their subsequent signaling acts on the columella cells to modulate their graviresponse. This interaction of touch and gravity signaling would then direct root growth to avoid obstacles in the soil while generally maintaining downward growth.

Touch and gravitropic set-point angle interact to modulate gravitropic growth in roots

Plant roots must sense and respond to a variety of environmental stimuli as they grow through the soil. Touch and gravity represent two of the mechanical signals that roots must integrate to elicit the appropriate root growth patterns and root system architecture. Obstacles such as rocks will impede the general downwardly directed gravitropic growth of the root system and so these soil features must be sensed and this information processed for an appropriate alteration in gravitropic growth to allow the root to avoid the obstruction. We show that primary and lateral roots of Arabidopsis do appear to sense and respond to mechanical barriers placed in their path of growth in a qualitatively similar fashion. Both types of roots exhibited a differential growth response upon contacting the obstacle that directed the main axis of elongation parallel to the barrier. This growth habit was maintained until the obstacle was circumvented, at which point normal gravitropic growth was resumed. Thus, the gravitational set-point angle of the primary and lateral roots prior to encountering the barrier were 95 degrees and 136 degrees respectively and after growing off the end of the obstacle identical set-point angles were reinstated. However, whilst tracking across the barrier, quantitative differences in response were observed between these two classes of roots. The root tip of the primary root maintained an angle of 136 degrees to the horizontal as it traversed the barrier whereas the lateral roots adopted an angle of 154 degrees. Thus, this root tip angle appeared dependent on the gravitropic set-point angle of the root type with the difference in tracking angle quantitatively reflecting differences in initial set-point angle. Concave and convex barriers were also used to analyze the response of the root to tracking along a continuously varying surface. The roots maintained the a fairly fixed angle to gravity on the curved surface implying a constant resetting of this tip angle/tracking response as the curve of the surface changed. We propose that the interaction of touch and gravity sensing/response systems combine to strictly control the tropic growth of the root. Such signal integration is likely a critical part of growth control in the stimulus-rich environment of the soil.

Mechanically stimulated TCH3 gene expression in Arabidopsis involves protein phosphorylation and EIN6 downstream of calcium

Mechanical signals are important both as environmental and endogenous developmental cues in plants. Among the quickest measurable responses to mechanical stimulation (MS) in plants is the up-regulation of specific genes, including TCH3, in Arabidopsis. Little is known about the signaling events and components that link perception of mechanical signals to gene expression in plants. Calcium has been identified previously as being potentially involved, and a role for ethylene has also been suggested. Using the protein kinase inhibitor staurosporine, we determined that MS up-regulation of TCH3 expression requires protein kinase activity in young Arabidopsis seedlings. Our data from studies on the Arabidopsis ein6 mutant demonstrate that the EIN6 protein is also required, but that its role in mechanically induced TCH3 expression appears to be independent of ethylene. Challenge of seedlings with protein phosphatase inhibitors calyculin A and okadaic acid stimulated TCH3 expression even in the absence of MS, implying protein phosphatase activity acting to negatively regulate TCH3 gene expression. This phosphatase activity acts either downstream or independently of EIN6. EIN6 and protein kinase activity, on the other hand, operate downstream of calcium to mediate mechanically stimulated TCH3 expression.

Arabidopsis thaliana responses to mechanical stimulation do not require ETR1 or EIN2

Plants exposed to repetitive touch or wind are generally shorter and stockier than sheltered plants. These mechanostimulus-induced developmental changes are termed thigmomorphogenesis and may confer resistance to subsequent stresses. An early response of Arabidopsis thaliana to touch or wind is the up-regulation of TCH (touch) gene expression. The signal transduction pathway that leads to mechanostimulus responses is not well defined. A role for ethylene has been proposed based on the observation that mechanostimulation of plants leads to ethylene evolution and exogenous ethylene leads to thigmomorphogenetic-like changes. To determine whether ethylene has a role in plant responses to mechanostimulation, we assessed the ability of two ethylene-insensitive mutants, etr1-3 and ein2-1, to undergo thigmomorphogenesis and TCH gene up-regulation of expression. The ethylene-insensitive mutants responded to wind similarly to the wild type, with a delay in flowering, decrease in inflorescence elongation rate, shorter mature primary inflorescences, more rosette paraclades, and appropriate TCH gene expression changes. Also, wild-type and mutant Arabidopsis responded to vibrational stimulation, with an increase in hypocotyl elongation and up-regulation of TCH gene expression. We conclude that the ETR1 and EIN2 protein functions are not required for the developmental and molecular responses to mechanical stimulation.

Thigmomorphogenesis: the effect of mechanical perturbation on plants

Thigmomorphogenetic responses occur in many environmental settings. The most pronounced effects are found under conditions of extremely high rates of turbulent wind or water flow. However, it is an ubiquitous phenomenon, since mechanical perturbations are to be encountered under all but the most stringent laboratory conditions. Our present understanding of these phenomena is the result of studies at the ecological, anatomical, physiological, biochemical, biophysical and molecular biological levels.

Regulation of expression of calmodulin and calmodulin-related genes by environmental stimuli in plants

Plants are very sensitive to environmental stimuli and have evolved the ability to adapt to many environmental stresses by altering development. In particular, mechanical stimuli such as touch or wind, result in growth changes that result in plants with greater resistance to such mechanical stimuli. We have initiated a molecular dissection of the pathways that enable perception of and responses to these environmental stimuli in plants. We have discovered five genes--termed the TCH genes--whose expression levels are strongly and rapidly increased in response to stimuli such as touch, wind, rain, wounding and darkness. Three of the TCH genes encode proteins related to calmodulin thereby implicating roles for calcium ions and calmodulin in the transduction of signals from the environment.

Rain-, wind-, and touch-induced expression of calmodulin and calmodulin-related genes in Arabidopsis

In response to water spray, subirrigation, wind, touch, wounding, or darkness, Arabidopsis regulates the expression of at least four touch-induced (TCH) genes. Ten to thirty minutes after stimulation, mRNA levels increase up to 100-fold. Arabidopsis plants stimulated by touch develop shorter petioles and bolts. This developmental response is known as thigmomorphogenesis. TCH 1 cDNA encodes the putative Arabidopsis calmodulin differing in one amino acid from wheat calmodulin. Sequenced regions of TCH 2 and TCH 3 contain 44% and 70% amino acid identities to calmodulin, respectively. The regulation of this calmodulin-related gene family in Arabidopsis suggests that calcium ions and calmodulin are involved in transduction of signals from the environment, enabling plants to sense and respond to environmental changes.

Thigmomorphogenesis: the role of ethylene in the response of Pinus taeda and Abies fraseri to mechanical perturbation

Ethylene production was monitored for 48 h in two half-sibs of Pinus taeda L. grown in the greenhouse and given mechanical perturbation (MP) by flexing; and for 22 h in Abies fraseri (Pursh) Poir. grown in the field and exposed to wind-mediated MP. Both species produced a peak of ethylene 18 h after MP. Seedlings of P. taeda exposed to MP for the duration of the growing season (preconditioned) produced less ethylene compared to non-MP controls, with a peak production at 8 h. One half-sib which responded to MP by an increase in radial growth produced 16 times more ethylene than another half-sib which had no significant change in radial growth. Preconditioned A. fraseri produced no significant quantities of ethylene after MP. The production of wound ethylene appears to be different from MP-induced ethylene. When an ethylene-generating solution was applied to P. taeda seedlings, it mimicked many of the morphological and mechanical characteristics of MP seedlings. The putative role of ethylene in the thigmomorphogenetic response is addressed.

Mechanical perturbation-induced ethylene releases apical dominance in Pharbitis nil by restricting shoot growth

Mechanical perturbation (MP, rubbing) or internodes of Pharbitis nil shoots initiates release of lateral buds (LB) from apical dominance within 48 h. Evidence is presented which suggests that MP promotion of LB outgrowth is mediated by ethylene-induced restriction of main shoot growth. Ethylene production in the internodes is stimulated by MP within 2 h. Effects of MP are mimicked by treatments with 1-aminocyclopropane-1-carboxylic acid (ACC) and are negated by the inhibitors of ethylene production or action, aminoethoxy vinylglycine (AVG) and AgNO3. The fact that effects of MP, ACC, and ethylene inhibitors are observed to occur on main shoot growth at least 24 h before they are observed to occur on LB growth suggests a possible cause and effect relationship. MP also causes an increase in internode diameter. MP stimulation of ethylene production appears to be mediated by ACC synthase. The results of this study and our previous studies suggest that apical dominance may be released by any mechanism which induces ethylene restriction of main shoot growth.