Thigmomorphogenesis: the relationship of mechanical perturbation to elicitor-like activity and ethylene production

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.

Penicillium chrysogenum polypeptide extract protects tobacco plants from tobacco mosaic virus infection through modulation of ABA biosynthesis and callose priming

The polypeptide extract of the dry mycelium of Penicillium chrysogenum (PDMP) can protect tobacco plants from tobacco mosaic virus (TMV), although the mechanism underlying PDMP-mediated TMV resistance remains unknown. In our study, we analysed a potential mechanism via RNA sequencing (RNA-seq) and found that the abscisic acid (ABA) biosynthetic pathway and β-1,3-glucanase, a callose-degrading enzyme, might play an important role in PDMP-induced priming of resistance to TMV. To test our hypothesis, we successfully generated a Nicotiana benthamiana ABA biosynthesis mutant and evaluated the role of the ABA pathway in PDMP-induced callose deposition during resistance to TMV infection. Our results suggested that PDMP can induce callose priming to defend against TMV movement. PDMP inhibited TMV movement by increasing callose deposition around plasmodesmata, but this phenomenon did not occur in the ABA biosynthesis mutant; moreover, these effects of PDMP on callose deposition could be rescued by treatment with exogenous ABA. Our results suggested that callose deposition around plasmodesmata in wild-type plants is mainly responsible for the restriction of TMV movement during the PDMP-induced defensive response to TMV infection, and that ABA biosynthesis apparently plays a crucial role in PDMP-induced callose priming for enhancing defence against TMV.

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

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

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.

Auxin mediates the touch-induced mechanical stimulation of adventitious root formation under windy conditions in Brachypodium distachyon

BACKGROUND

It is widely perceived that mechanical or thigmomorphogenic stimuli, such as rubbing and bending by passing animals, wind, raindrop, and flooding, broadly influence plant growth and developmental patterning. In particular, wind-driven mechanical stimulation is known to induce the incidence of radial expansion and shorter and stockier statue. Wind stimulation also affects the adaptive propagation of the root system in various plant species. However, it is unknown how plants sense and transmit the wind-derived mechanical signals to launch appropriate responses, leading to the wind-adaptive root growth.

RESULTS

Here, we found that Brachypodium distachyon, a model grass widely used for studies on bioenergy crops and cereals, efficiently adapts to wind-mediated lodging stress by forming adventitious roots (ARs) from nonroot tissues. Experimental dissection of wind stimuli revealed that not bending of the mesocotyls but physical contact of the leaf nodes with soil particles triggers the transcriptional induction of a group of potential auxin-responsive genes encoding WUSCHEL RELATED HOMEOBOX and LATERAL ORGAN BOUNDARIES DOMAIN transcription factors, which are likely to be involved in the induction of AR formation.

CONCLUSIONS

Our findings would contribute to further understanding molecular mechanisms governing the initiation and development of ARs, which will be applicable to crop agriculture in extreme wind climates.

Gene Interactions Regulating Sex Determination in Cucurbits

The family Cucurbitaceae includes many economically important crops, such as cucumber (Cucumis sativus), melon (Cucumis melo), watermelon (Citrullus lanatus), and zucchini (Cucurbita pepo), which share homologous gene pathways that control similar phenotypes. Sex determination is a research hotspot associated with yield and quality, and the genes involved are highly orthologous and conserved in cucurbits. In the field, six normal sex types have been categorized according to the distribution of female, male, or bisexual flowers in a given plant. To date, five orthologous genes involved in sex determination have been cloned, and their various combinations and expression patterns can explain all the identified sex types. In addition to genetic mechanisms, ethylene controls sex expression in this family. Two ethylene signaling components have been identified recently, which will help us to explore the ethylene signaling-mediated interactions among sex-related genes. This review discusses recent advances relating to the mechanism of sex determination in cucurbits and the prospects for research in this area.

Molecular and functional characterization of CpACS27A gene reveals its involvement in monoecy instability and other associated traits in squash (Cucurbita pepo L.)

A number of Cucurbita pepo genotypes showing instable monoecy or partial andromonoecy, i.e. an incomplete conversion of female into bisexual flowers, have been detected. Given that in melon and cucumber andromonoecy is the result of reduction of ethylene production in female floral buds, caused by mutations in the ethylene biosynthesis genes CmACS7 and CsACS2; we have cloned and characterized two related C. pepo genes, CpACS27A and CpACS27B. The molecular structure of CpACS27A and its specific expression in the carpels of female flowers during earlier stages of flower development suggests that this gene is the Cucurbita ortholog of CmACS7 and CsACS2. CpACS27B is likely to be a paralogous pseudogene since it has not been found to be expressed in any of the analyzed tissues. CpACS27A was sequenced in Bolognese (Bog) and Vegetable Spaghetti (Veg), two monoecious inbred lines whose F2 was segregating for partial andromonoecy. The Bog allele of CpACS27A carried a missense mutation that resulted in a substitution of the conserved serine residue in position 176 by an alanine. Segregation analysis indicated that this mutant variant is necessary but not sufficient to confer the andromonoecious phenotype in squash. In concordance with its involvement in stamen arrest, a reduction in CpACS27A expression has been found in bisexual flower buds at earlier stages of development. This reduction in CpACS27A expression was concomitant with a downregulation of other ethylene biosynthesis and signaling genes during earlier and later stages of ovary development. The role of CpACS27A is discussed regarding the regulation of ethylene biosynthesis and signaling genes in the control of andromonoecy-associated traits, such as the delayed maturation of corolla and stigma as well as the parthenocarpic development of the fruit.

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.

Analysis of secondary growth in the Arabidopsis shoot reveals a positive role of jasmonate signalling in cambium formation

After primary growth, most dicotyledonous plants undergo secondary growth. Secondary growth involves an increase in the diameter of shoots and roots through formation of secondary vascular tissue. A hallmark of secondary growth initiation in shoots of dicotyledonous plants is the initiation of meristematic activity between primary vascular bundles, i.e. in the interfascicular regions. This results in establishment of a cylindrical meristem, namely the vascular cambium. Surprisingly, despite its major implications for plant growth and the accumulation of biomass, the molecular regulation of secondary growth is only poorly understood. Here, we combine histological, molecular and genetic approaches to characterize interfascicular cambium initiation in the Arabidopsis thaliana inflorescence shoot. Using genome-wide transcriptional profiling, we show that stress-related and touch-inducible genes are up-regulated in stem regions where secondary growth takes place. Furthermore, we show that the products of COI1, MYC2, JAZ7 and the touch-inducible gene JAZ10, which are components of the JA signalling pathway, are cambium regulators. The positive effect of JA application on cambium activity confirmed a stimulatory role of JA in secondary growth, and suggests that JA signalling triggers cell divisions in this particular context.

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.

RGD-dependent mechanotransduction of suspension cultured Taxus cell in response to shear stress

Plant cells cultured in bioreactors are strongly influenced by mechanical forces. However, the molecular mechanism of plant cell mechanoreception has maintained unclear. In animal cells, the Arg-Gly-Asp (RGD) motif can be found in proteins of the extracellular matrix. Integrins link the intracellular cytoskeleton of cells with the extracellular matrix by recognizing this RGD motif. Integrin has been demonstrated to function as an apparatus not only for adhesion but also for mechanotransduction. In plant cells, the molecules that mediate the structural continuity between wall and membrane are unknown. Here, we found that synthetic RGD peptide could dramatically reduce the level of phosphorylation of MAPK-like cascades that are activated by shear stress and reduce the alkalinization response, production of reactive oxygen species (ROS) and accumulation of phenolics by Taxus cuspidata cells during shear stress. These results implicate that a RGD recognition system may exist in Taxus cells and play an important role in signal transduction of shear stress. Although the Arabidopsis genome database shows that the plant seems to lack a homologue of animal integrin, plant cells may use other RGD-binding proteins to recognize the RGD motif. The correlative mechanism is discussed.

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.

Aluminum-induced gene expression and protein localization of a cell wall-associated receptor kinase in Arabidopsis

Here, we report the aluminum (Al)-induced organ-specific expression of a WAK1 (cell wall-associated receptor kinase 1) gene and cell type-specific localization of WAK proteins in Arabidopsis. WAK1-specific reverse transcriptase-polymerase chain reaction analysis revealed an Al-induced WAK1 gene expression in roots. Short- and long-term analysis of gene expression in root fractions showed a typical "on" and "off" pattern with a first peak at 3 h of Al exposure followed by a sharp decline at 6 h and a complete disappearance after 9 h of Al exposure, suggesting the WAK1 is a further representative of Al-induced early genes. In shoots, upon root Al exposure, an increased but stable WAK1 expression was observed. Using confocal microscopy, we visualized Al-induced closure of leaf stomata, consistent with previous suggestions that the Al stress primarily experienced in roots associated with the transfer of root-shoot signals. Elevated levels of WAK protein in root cells were observed through western blots after 6 h of Al exposure, indicating a lag time between the Al-induced WAK transcription and translation. WAK proteins are localized abundantly to peripheries of cortex cells within the elongation zone of the root apex. In these root cells, disintegration of cortical microtubules was observed after Al treatment but not after the Al analog lanthanum treatments. Tip-growing control root hairs, stem stomata, and leaf stomatal pores are characterized with high amounts of WAKs, suggesting WAKs are accumulating at plasma membrane domains, which suffer from mechanical stress and lack dense arrays of supporting cortical microtubules. Further, transgenic plants overexpressing WAK1 showed an enhanced Al tolerance in terms of root growth when compared with the wild-type plants, making the WAK1 one of the important candidates for plant defense against Al toxicity.

Effects of mechanical vibration on seed germination of Arabidopsis thaliana (L.) Heynh

The effects of sinusoidal vibration (40-120 Hz, amplitude equal to or smaller than 0.42 mm) on seed germination of Arabidopsis thaliana were examined. When the amplitude of vibration was fixed at 0.42 mm, vibration with frequencies higher than 70 Hz increased the rate of seed germination. When the frequency of vibration was fixed at 100 Hz, vibration with amplitudes larger than 0.33 mm also increased the rate of germination. The increase in the rate of germination appeared dependent on acceleration calculated from the frequency and amplitude of vibration. Vibration with a maximum acceleration of 70 m s(-2) increased the rate of germination, but the promotive effects leveled off at higher accelerations. Vibration had little effect on seed germination in a starch-deficient mutant, pgm. Thus, the amyloplasts appeared to act as a susceptor that senses mechanical vibrations. No vibration-induced promotion of germination was seen in an ethylene-insensitive mutant, etr1, or in the wild type in the presence of aminoethoxyvinylglycine, an inhibitor of ethylene synthesis, suggesting that vibration increased the rate of seed germination through the action of ethylene.

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.