Thigmotropic Responses Research Articles

Arabidopsis thaliana RPL13aC affects root system architecture through shoot potassium accumulation.

Plant root system architecture shows complex patterns adapting to different nutritional conditions. In Arabidopsis thaliana, root slanting is a behaviour that is observed when plants are grown on a solid agar plate vertically. However, the regulatory mechanisms of root slanting in response to nutrient conditions are not fully understood. In this study, we found that mutants of A. thaliana ribosome protein RPL13aC, which is expressed in root tips and leaves, exhibit a decreased root-slanting phenotype. Ionomic analysis revealed that rpl13ac mutants have a reduced K content in shoots but not in roots. Because K+ availability has been suggested to affect root coiling, we hypothesized that the decreased root slanting of rpl13ac mutants is caused by the decrease in K content in their shoots. Decapitating shoots or limiting K supply dramatically decreased root slanting in wild-type (WT) plants. We found that the expression of HIGH-AFFINITY K+ TRANSPORTER 5 (HAK5) significantly decreased in the roots of rpl13ac mutants. Mutants of hak5 showed decreased shoot K contents and decreased root slanting, supporting that the decreased shoot K+ accumulation results in less root slanting. K+ replenishment to the shoots of rpl13ac, hak5 mutants and K-starved WT plants recovered their root slanting significantly. These results indicate that plants adjust root slanting in response to K+ accumulation in shoots. Further analysis showed that rpl13ac mutants have abnormal thigmotropic responses, which may be responsible for their defects in root slanting. Altogether, these results revealed K+ -dependent mechanisms that affect root system architecture.

Conditions for the emergence of circumnutations in plant roots.

The plant root system shows remarkably complex behaviors driven by environmental cues and internal dynamics, whose interplay remains largely unknown. A notable example is circumnutation growth movements, which are growth oscillations from side to side of the root apex. Here we describe a model capable of replicating root growth behaviors, which we used to analyze the role of circumnuntations, revealing their emergence I) under gravitropic stress, as a combination of signal propagation and sensitivity to the signal carriers; II) as a result of the interplay between gravitropic and thigmotropic responses; and III) as a behavioral strategy to detect and react to resource gradients. The latter function requires the presence of a hypothetical internal oscillator whose parameters are regulated by the perception of environmental resources.

Thigmotropic responses of Oryza sativa L. to external rubbing stimulation

Our aim was to study the morphological and physiological responses of rice to rubbing stimulation. Rice was subjected to rubbing 30 times/day (R30), 60 times/day (R60), 90 times/day (R90) and 0 times/day (control) for 35 days. The height, elongation rates and second internode length were significantly decreased by the three treatments, whereas stem width increased significantly. The tiller number and chlorophyll contents of the top third and top fifth of leaves increased significantly after R30 and R60. In R90, the aboveground biomass was significantly decreased and dead leaf biomass was increased. In R30 and R60, the transpiration rates were 16% and 13% higher than in the control, whereas photosynthetic rates increased 25% and 23%, respectively. Root biomass was significantly increased in R30, and root/aboveground was enhanced in R90. Stomatal conductance and root triphenyltetrazolium chloride-deoxidizing ability was significantly increased by the three treatments. The SOD activities in all treatments and the control were similar after stimulation. POD and CAT activities increased significantly in R30 and R60, and malondialdehyde increased by 42% in R90. Membrane permeability in R30 and R60 decreased 26% and 15%, respectively. The calcium content and soluble protein content increased in R30, whereas the magnesium content decreased. The nitrogen content increased significantly in R30 and R60. The silicon content in the whole plant and the size of stomata increased significantly in the three treatments. Thus, rubbing stimulation had complex effects on rice growth.

Regulation of vectorial supply of vesicles to the hyphal tip determines thigmotropism in Neurospora crassa

Thigmotropism is the ability of an organism to respond to a topographical stimulus by altering its axis of growth. The thigmotropic response of the model fungus Neurospora crassa was quantified using microfabricated glass slides with ridges of defined height. We show that the polarity machinery at the hyphal tip plays a role in the thigmotropic response of N. crassa. Deletion of N. crassa genes encoding the formin, BNI-1, and the Rho-GTPase, CDC-42, an activator of BNI-1 in yeast, CDC-24, its guanine nucleotide exchange factor (GEF), and BEM-1, a scaffold protein in the same pathway, were all shown to significantly decrease the thigmotropic response. In contrast, deletion of genes encoding the cell end-marker protein, TEA-1, and KIP-1, the kinesin responsible for the localisation of TEA-1, significantly increased the thigmotropic response. These results suggest a mechanism of thigmotropism involving vesicle delivery to the hyphal tip via the actin cytoskeleton and microtubules. Neurospora crassa thigmotropic response differed subtly from that of Candida albicans where the stretch-activated calcium channel, Mid1, has been linked with thigmotropic behaviour. The MID-1 deficient mutant of N. crassa (Δmid-1) and the effects of calcium depletion were examined here but no change in the thigmotropic response was observed. However, SPRAY, a putative calcium channel protein, was shown to be required for N. crassa thigmotropism. We propose that the thigmotropic response is a result of changes in the polarity machinery at the hyphal tip which are thought to be downstream effects of calcium signalling pathways triggered by mechanical stress at the tip.

Cdc42 GTPase dynamics control directional growth responses.

Polarized cells reorient their direction of growth in response to environmental cues. In the fungus Candida albicans, the Rho-family small GTPase, Cdc42, is essential for polarized hyphal growth and Ca(2+) influx is required for the tropic responses of hyphae to environmental cues, but the regulatory link between these systems is unclear. In this study, the interaction between Ca(2+) influx and Cdc42 polarity-complex dynamics was investigated using hyphal galvanotropic and thigmotropic responses as reporter systems. During polarity establishment in an applied electric field, cathodal emergence of hyphae was lost when either of the two Cdc42 apical recycling pathways was disrupted by deletion of Rdi1, a guanine nucleotide dissociation inhibitor, or Bnr1, a formin, but was completely restored by extracellular Ca(2+). Loss of the Cdc42 GTPase activating proteins, Rga2 and Bem3, also abolished cathodal polarization, but this was not rescued by Ca(2+). Expression of GTP-locked Cdc42 reversed the polarity of hypha emergence from cathodal to anodal, an effect augmented by Ca(2+). The cathodal directional cue therefore requires Cdc42 GTP hydrolysis. Ca(2+) influx amplifies Cdc42-mediated directional growth signals, in part by augmenting Cdc42 apical trafficking. The Ca(2+)-binding EF-hand motif in Cdc24, the Cdc42 activator, was essential for growth in yeast cells but not in established hyphae. The Cdc24 EF-hand motif is therefore essential for polarity establishment but not for polarity maintenance.

Effect of nutrient availability on hyphal maturation and topographical sensing in Aspergillus niger

Topographical sensing (thigmotropism) is an essential component of efficient fungal growth. It is an important element in the complex pathway of sensory and mechanical elements that drive and control the growing hyphal tip, a fuller understanding of which will bring the mycological community a step closer to complete comprehension of the hyphal growth mode. Previous work has led us to hypothesize that the stress induced by nutrient deficiency causes structural changes in the hyphal tip that induces a thigmotropic response in Aspergillus niger, a soil fungus that does not display thigmotropism under normal conditions. In this study, we have sought to identify some of the factors that influence this induction of thigmotropism using a novel combination of microengineered substrates and imaging and analysis techniques to quantify thigmotropic behavior in complex hyphal systems. We have shown that the sensitivity of fungal contour sensing appears to be directly linked to nutrient availability and hypothesize that this may be caused by a stress-induced fiattening of the tip and increased immaturity of the hyphal apex.

Induction of contour sensing in Aspergillus niger by stress and its relevance to fungal growth mechanics and hyphal tip structure

Thigmotropism (contour sensing) has been assigned an important role in both plant and human fungal pathogens. However, outside these systems, our knowledge of the function of thigmotropism in fungal growth control is relatively poor. Furthermore, the effects of environmental stress on thigmotropic responses have received scant attention. To try to elucidate some of the mechanisms behind hyphal contour sensing in response to nutrient-poor environments, we have used micro-engineered substrates and several imaging techniques to investigate the thigmotropic reactions of the ubiquitous fungus Aspergillus niger. This organism not appear to demonstrate thigmotropic growth under normal conditions. Our results show that A. niger undergoes significant morphological changes during growth on solid substrates and demonstrate that the intensity of contour sensing varies depending on the area of the hyphal tip which initiates contact with the substrate. We propose that growth under nutrient-limited conditions triggers several factors that combine to increase thigmotropic sensitivity while conversely creating a 60° arc at the hyphal tip which is blind to topographical variations. This has important consequences for our general understanding of the hyphal mode of growth in fungi as well as more specific aspects of hyphal tip development under stress.

Effect of nutrient availability on hyphal maturation and topographical sensing in <i>Aspergillus niger</i>

Topographical sensing (thigmotropism) is an essential component of efficient fungal growth. It is an important element in the complex pathway of sensory and mechanical elements that drive and control the growing hyphal tip, a fuller understanding of which will bring the mycological community a step closer to complete comprehension of the hyphal growth mode. Previous work has led us to hypothesize that the stress induced by nutrient deficiency causes structural changes in the hyphal tip that induces a thigmotropic response in Aspergillus niger, a soil fungus that does not display thigmotropism under normal conditions. In this study, we have sought to identify some of the factors that influence this induction of thigmotropism using a novel combination of microengineered substrates and imaging and analysis techniques to quantify thigmotropic behavior in complex hyphal systems. We have shown that the sensitivity of fungal contour sensing appears to be directly linked to nutrient availability and hypothesize that this may be caused by a stress-induced fiattening of the tip and increased immaturity of the hyphal apex.

Mode of Action of a Novel Nonchemical Method of Insect Control: Atmospheric Pressure Plasma Discharge

Atmospheric pressure plasma discharge (APPD) has been applied to a number of industrial applications, including the bacterial sterilization of medical equipment of bacteria. APPD may also have applications in insect control. A positive correlation was found between exposure time to APPD and mortality of western flower thrips, Frankliniella occidentalis (Pergande); tobacco thrips, Frankliniella fusca (Hinds); Asian tiger mosquito, Aedes albopictus (Skuse); twospotted spider mite, Tetranychus urticae Koch; and German cockroach, Blattella germanica (L.), with the level of mortality also increasing with time after treatment. Cockroaches exposed to APPD for 60, 90, 120, and 180 s lost on average 7.5 +/- 0.8, 8.1 +/- 0.6, 8.7 +/- 0.4, and 10.1 +/- 1.1 (+/-1 SEM) mg of water weight, respectively, which was an increase over that of the controls. The metabolic rate of cockroaches exposed to plasma for 180 s increased from 0.79 +/- 0.03 to 1.07 +/- 0.04 ml of oxygen consumed mg-cockroach(-1) h(-1) at standard temperature and pressure. The level of cuticular hydrocarbons identified by electron impact gas chromatography-mass spectrometry were not significantly affected by plasma exposure in the green peach aphid, Myzus persicae (Sulzer), German cockroach, and citrus mealybug, Planococcus citri (Risso), except for a reduction in n-tritriacontane in the latter. However, changes in the behavior of cockroaches after plasma exposure, including the loss of photo-, vibro-, and thigmotropic responses, inability to right themselves, and hyperexcitatory symptoms, suggest that the site of action of APPD in insects is the nervous and/or neuromuscular system.

Cytokines and cytokine receptors in advanced heart failure. An analysis of the cytokine database from the vesnarinone trial (VEST)

Thigmotropism is the ability of an organism to respond to a topographical stimulus by altering its axis of growth. The thigmotropic response of the model fungus Neurospora crassa was quantified using microfabricated glass slides with ridges of defined height. We show that the polarity machinery at the hyphal tip plays a role in the thigmotropic response of N. crassa. Deletion of N. crassa genes encoding the formin, BNI-1, and the Rho-GTPase, CDC-42, an activator of BNI-1 in yeast, CDC-24, its guanine nucleotide exchange factor (GEF), and BEM-1, a scaffold protein in the same pathway, were all shown to significantly decrease the thigmotropic response. In contrast, deletion of genes encoding the cell end-marker protein, TEA-1, and KIP-1, the kinesin responsible for the localisation of TEA-1, significantly increased the thigmotropic response. These results suggest a mechanism of thigmotropism involving vesicle delivery to the hyphal tip via the actin cytoskeleton and microtubules. Neurospora crassa thigmotropic response differed subtly from that of Candida albicans where the stretch-activated calcium channel, Mid1, has been linked with thigmotropic behaviour. The MID-1 deficient mutant of N. crassa (Δmid-1) and the effects of calcium depletion were examined here but no change in the thigmotropic response was observed. However, SPRAY, a putative calcium channel protein, was shown to be required for N. crassa thigmotropism. We propose that the thigmotropic response is a result of changes in the polarity machinery at the hyphal tip which are thought to be downstream effects of calcium signalling pathways triggered by mechanical stress at the tip.

Pea (Pisum sativumL.) Tendril‐Surface Changes Are Correlated with Changes in Functional Development

A scanning electron microscope (SEM) developmental study of tendrils of both conventional (Improved Laxton's Progress) and semileafless (Curly) cultivars of Pisum sativum was carried out in order to ascertain if there were changes in surface features that correlated with tendril functional development and to identify any potential thigmotropic receptors. These results were also compared with leaflet, rachis, and petiole surfaces at comparable developmental stages. Tendrils from the two cultivars did not differ, nor were positional differences found among tendrils from the same leaf. A number of dome‐shaped protrusions were present on the dorsal tip cells of tendrils. These protrusions bear a superficial resemblance to the tactile bleps reported for other tendril species. However, they are unlikely to play any role in the thigmotropic response since they are not located at the site of greatest thigmotropic sensitivity and are also present on leaflet tips. A distinct pattern of epidermal ridges was found on the tendrils that changes from interwoven during the young expanding phase, to primarily radial when thigmotropic, to nearly absent at maturity. The composition of these structures was determined, with transmission electron microscopy (TEM), to be wall material and cuticle. Ridges were also variably present on the other cylindrical organs. These ridges may be related to the requirement by tendrils for directional wall extension during coiling and to similar requirements in rachides and petioles for rapid extension during leaflet unfolding and growth.

Thigmotropism and stretch-activated channels in the pathogenic fungus Candida albicans.

The direction of growth of hyphae of the pathogenic fungus Candida albicans responds thigmotropically to surface contours by following scratches, ridges and grooves and by penetrating pores. Here it is shown that the thigmotropic response to ridges is attenuated by GdCl3 and verapamil [blockers of stretch-activated (SA) ion channels and L-type calcium channels, respectively]. At low concentrations, both compounds reduced the percentage of hyphae reorienting on contact with a ridge without markedly affecting hyphal extension rate, suggesting a possible role for SA or other calcium channels in the transduction of the thigmotropic response. In addition, patch-clamp recordings demonstrated SA channel activity in the plasma membrane of both yeast and hyphal cells of C. albicans. Two distinct SA channels with conductances of 54 pS and 20-25 pS in 200 mM KCl were observed in protoplasts from yeast cells and one channel of 51 pS was found in protoplasts from hyphal cells.

The role of mucilage in the attachment of conidia, germ tubes, and appressoria in the saprobic aquatic Hyphomycetes Lemonniera aquatica and Mycocentrospora filiformis

Spore attachment of the saprobic aquatic Hyphomycetes Lemonniera aquatica and Mycocentrospora filiformis was compared on Thermanox cover slips at the scanning and transmission electron microscope levels. The strength of attachment to Perspex discs was measured with the Fowler cell adhesion measurement module. Data are presented on the initial attachment of conidia and subsequent attachment of germ tubes, hyphae, and appressoria. Attachment at all stages was mediated by mucilage. Both fungi exhibited a similar strength of initial conidial attachment, but attachment resulted from different strategies: (i) in L. aquatica, by active release of mucilage as a result of a thigmotropic response to the substratum; (ii) in M. filiformis, by a passive process involving attachment by pre-existing conidial mucilage prior to contact with the substratum. The strength of attachment increased with settlement time. A fast germination rate, concurrent with mucilage production, resulted in the firm attachment of L. aquatica conidia, while initial germination in M. filiformis was slow and this corresponded to a comparatively weaker conidial attachment. Further increase in the strength of attachment was the result of greater hyphal development, and most importantly, by appressorium formation. All these structures were enrobed in a mucilaginous sheath, although variations were observed in the morphology and texture of mucilages. This suggested differences in chemical composition of mucilage (i) between the two fungi, and (ii) among different structures of the same fungus. Keywords: aquatic Hyphomycetes, appressorium, attachment, germination, mucilage, ultrastructure.

Thermomorphogenic Responses in Stem and Leaf Development

Stem elongation in commercial greenhouse production is controlled mainly by using synthetic plant growth retardants. Concern over the impact of these compounds on human health may limit their future availability (Bidinotto, 1990). The growth retardant daminozide (Alar) has been removed for use on food crops in the United States. The only growth retardant registered for controlling height of greenhouse crops in Germany is chlormequat (Ludolph, 1992). Emphasis, therefore, must be placed on developing effective nonchemical methods of controlling height in controlled-environment facilities. One nonchemical plant height control method that has potential is suppressing elongation through touching (Biddington, 1986; Jaffe, 1973). Touching or shaking (Mitchell et al., 1975) a plant causes a thigmotropic response that reduces internode elongation via ethylene synthesis (Biddington, 1986; Jaffe, 1973; Latimer, 1991, 1993; Latimer and Thomas, 1991; Miller, 1991). Drawbacks to this method are the labor requirements and mechanical damage to plants (Latimer, 1991, 1993; Latimer and Thomas, 1991). Another method of controlling stem elongation is by managing day and night temperatures (DT/NT) over a 24-h period (Erwin, 1991; Erwin and Heins, 1990; Erwin et al., 1989b; Heins and Erwin, 1990). Stem length increases as DT increases (Karlsson, 1986; Smith and Langhans, 1961; Tageras, 1979) and NT decreases (Karlsson, 1986; Tageras, 1979; Went, 1944, 1952, 1957). Interestingly, recent experimentation shows that morphological development in many plants is highly

Circumnutation and gravitropism cause root waving inArabidopsis thaliana

Arabidopsis thaliana roots grow in a wavy pattern on inclined agar plates. This waving behaviour has been interpreted as representing a gravitropism-dependent thigmotropic response. We argue instead that this root waving represents primarily a flattened spiral growth pattern resulting from circumnutation and gravitropism

Substrate surface influences upon germination of Colletotrichum truncatum conidia

Substrates influenced germination of conidia of the mycoherbicide, Colletotrichum truncatum. Very few (< 10%) conidia germinated when incubated while suspended in water or when incubated on or in partially liquefied agar. Many (> 70%) were induced to germinate when incubated on firm agar. The percentage of germinated conidia increased as agar firmness increased. Not all solid substrates equally influenced germination. Over 50% of the conidia germinated on chromatography paper, cellulose acetate filter, or on hard (plastic cover slip) substrates. In contrast, germination was relatively poor on cellulose nitrate filter and glass cover slips. Some natural substrates of dissimilar texture (wood, live plant leaf) produced good germination. Lower O2 levels or limited gas exchange for submerged conidia did not prevent germination because many conidia germinated while submerged on or between firm substrates. Subjecting conidia to motion during incubation between glass cover slips favored germ tube production rather than appressoria formation. Evidence suggests a positive germination response of C. truncatum conidia to solid substrates that occurs prior to the well-documented thigmotropic response of germ tubes of germinated conidia. Key words: Colletotrichum truncatum, conidia germination, thigmotropism, substrate.

Hydrophobicity of contact surface induces appressorium formation inMagnaporthe grisea

Infection by Magnaporthe grisea, the causal agent of rice blast, requires the formation of a melanized, dome-shaped infection cell, called an appressorium. Little is known about the signals and mechanisms regulating this important developmental process. We have previously observed a correlation between hydrophobicity of the contact surface and appressorium formation. To evaluate this thigmotropic response more precisely, we measured appressorium formation on the surfaces of silicon wafers modified to create various degrees of hydrophobicity. We also examined the effects of artificial ridges created on polystyrene surfaces. Hydrophobic surfaces induced a high level of appressorium formation, whereas hydrophilic surfaces did not. Tips of germ-tubes did not respond to ridges of any particular height, but formed appressoria in a random manner. These results indicate that hydrophobicity of the substratum is a primary determinant and is sufficient to induce appressorium formation in M. grisea.

Induction of Curvature in Maize Roots by Calcium or by Thigmostimulation

We examined the response of primary roots of maize (Zea mays L. cv Merit) to unilateral application of calcium with particular attention to the site of application, the dependence on growth rate, and possible contributions of thigmotropic stimulation during application. Unilateral application of agar to the root cap induced negative curvature whether or not the agar contained calcium. This apparent thigmotropic response was enhanced by including calcium in the agar. Curvature away from objects applied unilaterally to the extreme root tip occurred both in intact and detipped roots. When agar containing calcium chloride was applied to one side of the postmitotic isodiametric growth zone ( a region between the apical meristem and the elongation zone), the root curved toward the side of application. This response could not be induced by plain agar. We conclude that curvature away from calcium applied to the root tip results from a thigmotropic response to stimulation during application. In contrast, curvature toward the calcium applied to the postmitotic isodiametric growth zone results from direct calcium-induced inhibition of growth.

The production of nitrocellulose membranes as used in the study of thigmotropism

A detailed description is given of a method employed in making nitrocellulose membranes with orientated polymers for studies of the thigmotropic responses of rust fungi.

Auxin-ethylene interactions in the thigmotropic response of Cucumber tendrils

Coiling of intact or excised cucumber (Cucumis sativus) tendrils can be induced by IAA or ethylene. The velocity of coiling in different regions of the tendrils correlates with the capacity for auxin-stimulated ethylene synthesis. Ethylene (Ethephon) induces an increase in membrane permeability of tendrils, and as a result the efflux of substances previously taken up (glucose) is stimulated. It is assumed that this may contribute to the contraction of the ventral side of the tendril. The excretion of glucose after ethylene treatment can be reduced by Ca2+, and calcium also inhibits coiling of tendrils following incubation in ethephon solution. Auxin stimulated ethylene synthesis in the ventral half of the tendril is several times higher than in the dorsal half and it is hypothesized that this may be a cause for the different reactions of the two sides of a tendril following a mechanical stimulus.