Cytoplasmic pH dynamics in maize pulvinal cells induced by gravity vector changes

Strategies of seedlings to overcome their sessile nature: auxin in mobility control

Plants are sessile organisms that are permanently restricted to their site of germination. To compensate for their lack of mobility, plants evolved unique mechanisms enabling them to rapidly react to ever changing environmental conditions and flexibly adapt their postembryonic developmental program. A prominent demonstration of this developmental plasticity is their ability to bend organs in order to reach the position most optimal for growth and utilization of light, nutrients, and other resources. Shortly after germination, dicotyledonous seedlings form a bended structure, the so-called apical hook, to protect the delicate shoot meristem and cotyledons from damage when penetrating through the soil. Upon perception of a light stimulus, the apical hook rapidly opens and the photomorphogenic developmental program is activated. After germination, plant organs are able to align their growth with the light source and adopt the most favorable orientation through bending, in a process named phototropism. On the other hand, when roots and shoots are diverted from their upright orientation, they immediately detect a change in the gravity vector and bend to maintain a vertical growth direction. Noteworthy, despite the diversity of external stimuli perceived by different plant organs, all plant tropic movements share a common mechanistic basis: differential cell growth. In our review, we will discuss the molecular principles underlying various tropic responses with the focus on mechanisms mediating the perception of external signals, transduction cascades and downstream responses that regulate differential cell growth and consequently, organ bending. In particular, we highlight common and specific features of regulatory pathways in control of the bending of organs and a role for the plant hormone auxin as a key regulatory component.

Self-incompatibility-induced programmed cell death in field poppy pollen involves dramatic acidification of the incompatible pollen tube cytosol

Self-incompatibility (SI) is an important genetically controlled mechanism to prevent inbreeding in higher plants. SI involves highly specific interactions during pollination, resulting in the rejection of incompatible (self) pollen. Programmed cell death (PCD) is an important mechanism for destroying cells in a precisely regulated manner. SI in field poppy (Papaver rhoeas) triggers PCD in incompatible pollen. During SI-induced PCD, we previously observed a major acidification of the pollen cytosol. Here, we present measurements of temporal alterations in cytosolic pH ([pH]cyt); they were surprisingly rapid, reaching pH 6.4 within 10 min of SI induction and stabilizing by 60 min at pH 5.5. By manipulating the [pH]cyt of the pollen tubes in vivo, we show that [pH]cyt acidification is an integral and essential event for SI-induced PCD. Here, we provide evidence showing the physiological relevance of the cytosolic acidification and identify key targets of this major physiological alteration. A small drop in [pH]cyt inhibits the activity of a soluble inorganic pyrophosphatase required for pollen tube growth. We also show that [pH]cyt acidification is necessary and sufficient for triggering several key hallmark features of the SI PCD signaling pathway, notably activation of a DEVDase/caspase-3-like activity and formation of SI-induced punctate actin foci. Importantly, the actin binding proteins Cyclase-Associated Protein and Actin-Depolymerizing Factor are identified as key downstream targets. Thus, we have shown the biological relevance of an extreme but physiologically relevant alteration in [pH]cyt and its effect on several components in the context of SI-induced events and PCD.

A type III ACC synthase, ACS7, is involved in root gravitropism in Arabidopsis thaliana

Ethylene is an important plant hormone that regulates developmental processes in plants. The ethylene biosynthesis pathway is a highly regulated process at both the transcriptional and post-translational level. The transcriptional regulation of these ethylene biosynthesis genes is well known. However, post-translational modifications of the key ethylene biosynthesis enzyme 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) are little understood. In vitro kinase assays were conducted on the type III ACS, AtACS7, fusion protein and peptides to determine whether the AtACS7 protein can be phosphorylated by calcium-dependent protein kinase (CDPK). AtACS7 was phosphorylated at Ser216, Thr296, and Ser299 by AtCDPK16 in vitro. To investigate further the function of the ACS7 gene in Arabidopsis, an acs7-1 loss-of-function mutant was isolated. The acs7-1 mutant exhibited less sensitivity to the inhibition of root gravitropism by treatment with the calcium chelator ethylene glycol tetraacetic acid (EGTA). Seedlings were treated with gradient concentrations of ACC. The results showed that a certain concentration of ethylene enhanced the gravity response. Moreover, the acs7-1 mutant was less sensitive to inhibition of the gravity response by treatment with the auxin polar transport inhibitor 1-naphthylphthalamic acid, but exogenous ACC application recovered root gravitropism. Altogether, the results indicate that AtACS7 is involved in root gravitropism in a calcium-dependent manner in Arabidopsis.

Perturbation of auxin homeostasis by overexpression of wild-type IAA15 results in impaired stem cell differentiation and gravitropism in roots

Aux/IAAs interact with auxin response factors (ARFs) to repress their transcriptional activity in the auxin signaling pathway. Previous studies have focused on gain-of-function mutations of domain II and little is known about whether the expression level of wild-type Aux/IAAs can modulate auxin homeostasis. Here we examined the perturbation of auxin homeostasis by ectopic expression of wild-type IAA15. Root gravitropism and stem cell differentiation were also analyzed. The transgenic lines were less sensitive to exogenous auxin and exhibited low-auxin phenotypes including failures in gravity response and defects in stem cell differentiation. Overexpression lines also showed an increase in auxin concentration and reduced polar auxin transport. These results demonstrate that an alteration in the expression of wild-type IAA15 can disrupt auxin homeostasis.

Structure and immunocytochemical localization of photosynthetic enzymes in the lamina joint and sheath pulvinus of the C4 grass Arundinella hirta

The C(4) grass Arundinella hirta exhibits a unique C(4) anatomy, with isolated Kranz cells (distinctive cells) and C(4)-type expression of photosynthetic enzymes in the leaf sheath and stem as well as in the leaf blade. The border zones between these organs are pale green. Those between the leaf blade and sheath and between the sheath and stem are called the lamina joint and sheath pulvinus, respectively, and are involved in gravity sensing. We investigated the structure and localization of C(3) and C(4) photosynthetic enzymes in these tissues. In both zones the epidermis lacked stomata. The inner tissue was composed of parenchyma cells and vascular bundles. The parenchyma cells were densely packed with small intercellular spaces and contained granal chloroplasts with large starch grains. No C(4)-type cellular differentiation was recognized. Western blot analysis showed that the lamina joint and pulvinus accumulated substantial amounts of phosphoenolpyruvate carboxylase (PEPC), pyruvate,Pi dikinase (PPDK), and ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco). Immunogold electron microscopy revealed PEPC in the cytosol and both PPDK and rubisco in the chloroplasts of parenchyma cells, suggesting the occurrence of C(3) and C(4) enzymes within a single type of chlorenchyma cell. These data indicate that the lamina joint and pulvinus have unique expression patterns of C(3) and C(4) enzymes, unlike those in C(4)-type anatomy.

Cereal grass pulvini: agronomically significant models for studying gravitropism signaling and tissue polarity

Cereal grass pulvini have emerged as model systems that are not only valuable for the study of gravitropism, but are also of agricultural and economic significance. The pulvini are regions of tissue that are apical to each node and collectively return a reoriented stem to a more vertical position. They have proven to be useful for the study of gravisensing and response and are also providing clues about the establishment of polarity across tissues. This review will first highlight the agronomic significance of these stem regions and their benefits for use as model systems and provide a brief historical overview. A detailed discussion of the literature focusing on cell signaling and early changes in gene expression will follow, culminating in a temporal framework outlining events in the signaling and early growth phases of gravitropism in this tissue. Changes in cell wall composition and gene expression that occur well into the growth phase will be touched upon briefly. Finally, some ongoing research involving both maize and wheat pulvini will be introduced along with prospects for future investigations.

Gravitropism and mechanical signaling in plants

Mechanical stress is a critical signal affecting morphogenesis and growth and is caused by a large variety of environmental stimuli such as touch, wind, and gravity in addition to endogenous forces generated by growth. On the basis of studies dating from the early 19th century, the plant mechanical sensors and response components related to gravity can be divided into two types in terms of their temporal character: sensors of the transient stress of reorientation (phasic signaling) and sensors capable of monitoring and responding to the extended, continuous gravitropic signal for the duration of the tropic growth response (tonic signaling). In the case of transient stress, changes in the concentrations of ions in the cytoplasm play a central role in mechanosensing and are likely a key component of initial gravisensing. Potential candidates for mechanosensitive channels have been identified in Arabidopsis thaliana and may provide clues to these rapid, ionic gravisensing mechanisms. Continuous mechanical stress, on the other hand, may be sensed by other mechanisms in addition to the rapidly adapting mechnaosensitive channels of the phasic system. Sustaining such long-term responses may be through a network of biochemical signaling cascades that would therefore need to be maintained for the many hours of the growth response once they are triggered. However, classical physiological analyses and recent simulation studies also suggest involvement of the cytoskeleton in sensing/responding to long-term mechanoresponse independently of the biochemical signaling cascades triggered by initial graviperception events.

A role for lipid-mediated signaling in plant gravitropism

Gravitropism is a universal plant response. It is initiated by the sensing of the primary signal (mass or pressure), which is then converted into chemical signals that are transduced and propagated in a precise spatial and temporal fashion, resulting in a differential growth response. Our thesis is that membrane lipids and lipid-mediated signaling pathways play critical roles in the initial signaling and in the establishment of polarity. In this review, we highlight results from recent literature and discuss the major questions that remain unanswered.

Gravity sensing and signal transduction in vascular plant primary roots

During gravitropism, the potential energy of gravity is converted into a biochemical signal. How this transfer occurs remains one of the most exciting mysteries in plant cell biology. New experiments are filling in pieces of the puzzle. In this review, we introduce gravitropism and give an overview of what we know about gravity sensing in roots of vascular plants, with special highlight on recent papers. When plant roots are reoriented sideways, amyloplast resedimentation in the columella cells is a key initial step in gravity sensing. This process somehow leads to cytoplasmic alkalinization of these cells followed by relocalization of auxin efflux carriers (PINs). This changes auxin flow throughout the root, generating a lateral gradient of auxin across the cap that upon transmission to the elongation zone leads to differential cell elongation and gravibending. We will present the evidence for and against the following players having a role in transferring the signal from the amyloplast sedimentation into the auxin signaling cascade: mechanosensitive ion channels, actin, calcium ions, inositol trisphosphate, receptors/ligands, ARG1/ARL2, spermine, and the TOC complex. We also outline auxin transport and signaling during gravitropism.

Molecular mechanisms of root gravity sensing and signal transduction

Plants use gravity as a guide to direct their roots down into the soil to anchor themselves and to find resources needed for growth and development. In higher plants, the columella cells of the root tip form the primary site of gravity sensing, and in these cells the sedimentation of dense, starch-filled plastids (amyloplasts) triggers gravity signal transduction. This generates an auxin gradient across the root cap that is transmitted to the elongation zone where it promotes differential cell elongation, allowing the root to direct itself downward. It is still not well understood how amyloplast sedimentation leads to auxin redistribution. Models have been proposed to explain how mechanosensitive ion channels or ligand-receptor interactions could connect these events. Although their roles are still unclear, possible second messengers in this process include protons, Ca(2+), and inositol 1,4,5-triphosphate. Upon gravistimulation, the auxin efflux facilitators PIN3 and PIN7 relocalize to the lower side of the columella cells and mediate auxin redistribution. However, evidence for an auxin-independent secondary mechanism of gravity sensing and signal transduction suggests that this physiological process is quite complex. Furthermore, plants must integrate a variety of environmental cues, resulting in multifaceted relationships between gravitropism and other directional growth responses such as hydro-, photo-, and thigmotropism.

Dynamics of auxin-dependent Ca2+ and pH signaling in root growth revealed by integrating high-resolution imaging with automated computer vision-based analysis

Plants adapt to a changing environment by entraining their growth and development to prevailing conditions. Such 'plastic' development requires a highly dynamic integration of growth phenomena with signal perception and transduction systems, such as occurs during tropic growth. The plant hormone auxin has been shown to play a key role in regulating these directional growth responses of plant organs to environmental cues. However, we are still lacking a cellular and molecular understanding of how auxin-dependent signaling cascades link stimulus perception to the rapid modulation of growth patterns. Here, we report that in root gravitropism of Arabidopsis thaliana, auxin regulates root curvature and associated apoplastic, growth-related pH changes through a Ca2+-dependent signaling pathway. Using an approach that integrates confocal microscopy and automated computer vision-based image analysis, we demonstrate highly dynamic root surface pH patterns during vertical growth and after gravistimulation. These pH dynamics are shown to be dependent on auxin, and specifically on auxin transport mediated by the auxin influx carrier AUX1 in cells of the lateral root cap and root epidermis. Our results further indicate that these pH responses require auxin-dependent changes in cytosolic Ca2+ levels that operate independently of the TIR1 auxin perception system. These results demonstrate a methodology that can be used to visualize vectorial auxin responses in a manner that can be integrated with the rapid plant growth responses to environmental stimuli.

Directional gravity sensing in gravitropism

Plants can reorient their growth direction by sensing organ tilt relative to the direction of gravity. With respect to gravity sensing in gravitropism, the classic starch statolith hypothesis, i.e., that starch-accumulating amyloplast movement along the gravity vector within gravity-sensing cells (statocytes) is the probable trigger of subsequent intracellular signaling, is widely accepted. Several lines of experimental evidence have demonstrated that starch is important but not essential for gravity sensing and have suggested that it is reasonable to regard plastids (containers of starch) as statoliths. Although the word statolith means sedimented stone, actual amyloplasts are not static but instead possess dynamic movement. Recent studies combining genetic and cell biological approaches, using Arabidopsis thaliana, have demonstrated that amyloplast movement is an intricate process involving vacuolar membrane structures and the actin cytoskeleton. This review covers current knowledge regarding gravity sensing, particularly gravity susception, and the factors modulating the function of amyloplasts for sensing the directional change of gravity. Specific emphasis is made on the remarkable differences in the cytological properties, developmental origins, tissue locations, and response of statocytes between root and shoot systems. Such an approach reveals a common theme in directional gravity-sensing mechanisms in these two disparate organs.

Changes in the effective gravitational field strength affect the state of phosphorylation of stress-related proteins in callus cultures of Arabidopsis thaliana

In a recent study it was shown that callus cell cultures of Arabidopsis thaliana respond to changes in gravitational field strengths by changes in protein expression. Using ESI-MS/MS for proteins with differential abundance after separation by 2D-PAGE, 28 spots which changed reproducibly and significantly in amount (P <0.05) after 2 h of hypergravity (18 up-regulated, 10 down-regulated) could be identified. The corresponding proteins were largely involved in stress responses, including the detoxification of reactive oxygen species (ROS). In the present study, these investigations are extended to phosphorylated proteins. For this purpose, callus cell cultures of Arabidopsis thaliana were exposed to hypergravity (8 g) and simulated weightlessness (random positioning; RP) for up to 30 min, a period of time which yielded the most reliable data. The first changes, however, were visible as early as 10 min after the start of treatment. In comparison to 1 g controls, exposure to hypergravity resulted in 18 protein spots, and random positioning in 25, respectively, with increased/decreased signal intensity by at least 2-fold (P <0.05). Only one spot (alanine aminotransferase) responded the same way under both treatments. After 30 min of RP, four spots appeared, which could not be detected in control samples. Among the protein spots altered in phosphorylation, it was possible to identify 24 from those responding to random positioning and 12 which responded to 8 g. These 12 proteins (8 g) are partly (5 out of 12) the same as those changed in expression after exposure to 2 h of hypergravity. The respective proteins are involved in scavenging and detoxification of ROS (32%), primary metabolism (20.5%), general signalling (14.7%), protein translation and proteolysis (14.7%), and ion homeostasis (8.8%). Together with our recent data on protein expression, it is assumed that changes in gravitational fields induce the production of ROS. Our data further indicate that responses toward RP are more by post-translational protein modulation (most changes in the degree of phosphorylation occur under RP-treatment) than by protein expression (hypergravity).

A confocal technique applicable to studies of cellular pH-related signaling in plants

pH may act as a crucial signal in both animal and plant cells. It is very difficult to monitor pH signals and this has largely hindered progress in the investigation of pH signaling, particularly systematic pH signaling. Here, we report the development of a confocal technique to monitor leaf apoplastic pH in intact plants, which is particularly suitable for the studies on root to shoot signaling. A variety of different pH indicators and plant species were tested. It was found that different pH indicators, for example, 2',7'-Bis-(2-carboxyethyl)-5-(and-6)-carboxyfluoresce (BCECF), SNARF-4F 5-(and-6)-carboxylic acid (SNARF) and DM-NERF (NERF), were of different properties, and to successfully monitor pH at a sub-cellular level, the comparability between the pH indicator and plant species must be involved according to their suitable pH range and loading characteristics. The loading characteristics of different pH indicators differ with different plant species, cell types and their developing stages. No matter what methods were adopted, BCECF and SNARF could not be loaded specifically in the leaf apoplast in sunflower, tomato, and Comelina communis L. In contrast, regardless of the methods adopted, NERF could be loaded efficiently and specifically in the leaf apoplast in C. communis, but not in other plants. In C. communis, the determination coefficient for in vitro and in situ calibration of NERF was very high, which was respectively 0.9951 and 0.9916, and therefore, the adoption of NERF together with C. communis could construct an ideal experimental system that is suitable for the investigation of pH systematic signaling. Ratio image analysis demonstrated that the leaf apoplastic pH was about 5.5 in non-stressed conditions, and water deficit could trigger an increase in pH by about half a pH unit, which is the first evidence to directly indicate that pH is able to act as a systematic signal under water deficit conditions.

Salt-avoidance tropism in Arabidopsis thaliana

The orientation of plant root growth is modulated by developmental as well as environmental cues. Among the environmental factors, gravity has been extensively studied because of its overpowering effects in modulating root growth direction. However, our knowledge of the effects of other abiotic signals that influence root growth direction is largely unknown. Recently, we have shown that high salinity can modify root growth direction by inducing rapid amyloplast degradation in root columella cells of Arabidopsis thaliana. By exploiting salt overly sensitive (sos) mutants and PIN2 expression analyses, we have shown that the altered root growth direction in response to salt is mediated by ion disequilibrium and is correlated with PIN2 mRNA abundance and expression and localization of the protein. Our study demonstrates that the SOS pathway may mediate this process. Here we discuss our data from broader perspectives. We propose that salt-induced modification of root growth direction is a salt-avoidance behavior, which is an active adaptive mechanism for plants grown under saline conditions. Furthermore, high salinity also stimulates alteration of gravitropic growth of shoots. These findings illustrate that plants have a fine and sophisticated sensory and communication system that enable plants to dynamically and efficiently cope with rapidly changing environment.

Role of xyloglucan in gravitropic bending of azuki bean epicotyl

The mechanism of the gravitropic bending was studied in azuki bean epicotyls. The cell wall extensibility of the lower side became higher than that of the upper side in the epicotyl bending upward. The contents of matrix polysaccharides of the cell wall (pectin and xyloglucan in hemicellulose-II) in the lower side became smaller than those in the upper side. The molecular mass of xyloglucans in the lower side decreased. After an epicotyl was fixed to a metal rod to prevent the bending, gravistimulation was applied. Fundamentally the same results were obtained with respect to rheological and chemical characteristics of the cell wall as those of epicotyls showing gravitropic bending. The present results suggested that the initial gravitropic bending was caused by the increase in extensibility of the lower side and the decrease in extensibility of the upper side via the change of the cell wall matrix, especially xyloglucans.

Characterization of a tobacco TPK-type K+ channel as a novel tonoplast K+ channel using yeast tonoplasts

The tonoplast K(+) membrane transport system plays a crucial role in maintaining K(+) homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K(+) channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased approximately 2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K(+) over Na(+). NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca(2+), but a cytosolic concentration of 45 microM Ca(2+) resulted in a 2-fold increase in the amplitude of the K(+) current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K(+) currents. These results show that NtTPK1 is a novel tonoplast K(+) channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.

Increased levels of reactive oxygen species and expression of a cytoplasmic aconitase/iron regulatory protein 1 homolog during the early response of maize pulvini to gravistimulation

The maize (Zea mays L.) stem pulvinus is a disc of tissue located apical to each node that functions to return a tipped stem to a more upright position via increased cell elongation on its lower side. We investigated the possibility that reactive oxygen species (ROS) and hydrogen peroxide (H2O2), in particular, are involved in the gravitropic response of the pulvinus prior to initiation of the growth response by employing the cytochemical stain 3,3'-diaminobenzidine (DAB). DAB polymers were found in the bundle sheath cells of gravistimulated pulvini in association with amyloplasts after 1 min of gravistimulation, and the signal spread throughout the cytosol of these cells by 30 min. Furthermore, treatment of maize stem explants containing pulvini with 1 mm H2O2 on their upper sides caused reversal of bending polarity. Similar, though less dramatic, results were obtained via application of 1 mm ascorbic acid to the lower side of the explants. In addition, we determined that a maize cytoplasmic aconitase/iron regulatory protein 1 (IRP1) homolog is up-regulated in the pulvinus bundle sheath cells after gravistimulation using suppressive subtractive hybridization PCR (SSH PCR), real-time RT-PCR and in situ hybridization. Although we do not yet know the role of the IRP1 homolog in the pulvinus, the protein is known to be a redox sensor in other systems. Collectively, our results point to an increase in ROS quite early in the gravitropic signalling pathway and its possible role in determining the direction of bending of the pulvini. We speculate that an ROS burst may serve to link the physical phenomenon of amyloplast sedimentation to the changes in cellular biochemistry and gene expression that facilitate directional growth.

Salt modulates gravity signaling pathway to regulate growth direction of primary roots in Arabidopsis

Plant root architecture is highly plastic during development and can adapt to many environmental stresses. The proper distribution of roots within the soil under various conditions such as salinity, water deficit, and nutrient deficiency greatly affects plant survival. Salinity profoundly affects the root system architecture of Arabidopsis (Arabidopsis thaliana). However, despite the inhibitory effects of salinity on root length and the number of roots, very little is known concerning influence of salinity on root growth direction and the underlying mechanisms. Here we show that salt modulates root growth direction by reducing the gravity response. Exposure to salt stress causes rapid degradation of amyloplasts in root columella cells of Arabidopsis. The altered root growth direction in response to salt was found to be correlated with PIN-FORMED2 (PIN2) messenger RNA abundance and expression and localization of the protein. Furthermore, responsiveness to gravity of salt overly sensitive (sos) mutants is substantially reduced, indicating that salt-induced altered gravitropism of root growth is mediated by ion disequilibrium. Mutation of SOS genes also leads to reduced amyloplast degradation in root tip columella cells and the defects in PIN2 gene expression in response to salt stress. These results indicate that the SOS pathway may mediate the decrease of PIN2 messenger RNA in salinity-induced modification of gravitropic response in Arabidopsis roots. Our findings provide new insights into the development of a root system necessary for plant adaptation to high salinity and implicate an important role of the SOS signaling pathway in this process.

Halotolerance is enhanced in carrot callus by sensing hypergravity: influence of calcium modulators and cytochalasin D

Carrot callus was centrifuged at 10 g and compared to callus growing at 1 g on agar in the presence of increasing sodium chloride concentrations. Growth after 14 days was enhanced in the centrifuged samples versus samples kept at 1 g. This effect was not found when the samples were grown on potassium chloride. At 50 mM NaCl, the calcium ionophore ionomycin was applied to centrifuged and noncentrifuged callus samples. In both experiments, the growth of callus increased with increasing ionomycin concentrations but under 10 g this increase was more enhanced. As inhibitors of calcium influx, lanthanum and gadolinium chloride were chosen in the presence of 50 mM NaCl. Both inhibitors inhibited growth at 1 g at low concentrations of around 2 microM, whereas the centrifuged samples were not or much less so inhibited. We tested an involvement of actin by application of cytochalasin D to callus grown in the presence of 50 mM NaCl. In both types of samples, growth at 1 g and growth at 10 g, cytochalasin D enhanced growth but the effect was clearly stronger at 10 g than at 1 g. As increased halotolerance was only observed in the presence of increased sodium ions, not potassium ions, and as halotolerance is known to be induced by an influx of calcium, the data suggest that a calcium influx induced by hypergravity and possibly modulated by actin caused the observed increase in halotolerance at 10 g.

Changes in starch and inositol 1,4,5-trisphosphate levels and auxin transport are interrelated in graviresponding oat (Avena sativa) shoots

This study was conducted to unravel a mechanism for the gravitropic curvature response in oat (Avena sativa) shoot pulvini. For this purpose, we examined the downward movement of starch-filled chloroplast gravisensors, differential changes in inositol 1,4,5-trisphosphate (IP(3)) levels, transport of indole-3-acetic acid (IAA) and gravitropic curvature. Upon gravistimulation, the ratio for IAA levels in lower halves versus those in upper halves (L/U) increased from 1.0 at 0 h and reached a maximum value of 1.45 at 8 h. When shoots were grown in the dark for 10 d, to deplete starch in the chloroplast, the gravity-induced L/U of IAA was reduced to 1.0. N-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), both auxin transport inhibitors, significantly reduced the amount of gravitropic curvature and gravity-induced lateral IAA transport, but did not reduce the gravity-induced late change in the L/U ratio of IP(3) levels. U73122, a specific phospholipase C (PLC) inhibitor, decreased gravity-induced curvature. Because U73122 reduced the ratio of L/U of IAA imposed by gravistimulation, it is clear that IAA transport is correlated with changes in IP(3) levels upon gravistimulation. These results indicate that gravistimulation-induced differential lateral IAA transport may result from the onset of graviperception in the chloroplast gravisensors coupled with gravity-induced asymmetric changes in IP(3) levels in oat shoot pulvini.

Intracellular pH signals in the induction of secondary pathways--the case of Eschscholzia californica

Transient peaks of the cytoplasmic pH are essential elements in a number of signal cascades that activate environmental responses or developmental processes in plant cells but little is known about the mechanisms of their generation. In many plant cells, elicitation of the hypersensitive response is preceded by a perturbation of the ionic balance at the plasma membrane including the inhibition of the proton pump and the influx of H+ from the apoplast. A basically different mechanism of cytoplasmic acidification that is fed by vacuolar protons has been discovered in cell suspensions of the California Poppy (Eschscholzia californica). These cells react to a yeast glycoprotein elicitor with the overproduction of benzophenanthridine alkaloids. Low elicitor concentrations trigger the biosynthesis of these phytoalexins without invoking elements of the hypersensitive response. Accumulated data support the existence of a signal path that includes the following steps: Links between the above events that connect them within a distinct signal path are substantiated by the phenotypes of transformed cell lines that either display lowered Galpha levels due to antisense transformation or express Galpha-binding antibodies in the cytoplasm. All of these cell lines lack the elicitor-activation of PLA2 and of vacuolar proton fluxes and show an impaired phytoalexin response to low elicitor concentrations. High elicitor concentrations trigger alkaloid biosynthesis via an increase of jasmonate at a pH-independent signal path.

A universal role for inositol 1,4,5-trisphosphate-mediated signaling in plant gravitropism

Inositol 1,4,5-trisphosphate (InsP3) has been implicated in the early signaling events of plants linking gravity sensing to the initiation of the gravitropic response. However, at present, the contribution of the phosphoinositide signaling pathway in plant gravitropism is not well understood. To delineate the role of InsP3 in plant gravitropism, we generated Arabidopsis (Arabidopsis thaliana) plants constitutively expressing the human type I inositol polyphosphate 5-phosphatase (InsP 5-ptase), an enzyme that specifically hydrolyzes InsP3. The transgenic plants show no significant differences in growth and life cycle compared to wild-type plants, although basal InsP3 levels are reduced by greater than 90% compared to wild-type plants. With gravistimulation, InsP3 levels in inflorescence stems of transgenic plants show no detectable change, whereas in wild-type plant inflorescences, InsP3 levels increase approximately 3-fold within the first 5 to 15 min of gravistimulation, preceding visible bending. Furthermore, gravitropic bending of the roots, hypocotyls, and inflorescence stems of the InsP 5-ptase transgenic plants is reduced by approximately 30% compared with the wild type. Additionally, the cold memory response of the transgenic plants is attenuated, indicating that InsP3 contributes to gravisignaling in the cold. The transgenic roots were shown to have altered calcium sensitivity in controlling gravitropic response, a reduction in basipetal indole-3-acetic acid transport, and a delay in the asymmetric auxin-induced beta-glucuronidase expression with gravistimulation as compared to the controls. The compromised gravitropic response in all the major axes of growth in the transgenic Arabidopsis plants reveals a universal role for InsP3 in the gravity signal transduction cascade of plants.

Disruption of the F-actin cytoskeleton limits statolith movement in Arabidopsis hypocotyls

The F-actin cytoskeleton is hypothesized to play a role in signal transduction mechanisms of gravitropism by interacting with sedimenting amyloplasts as they traverse statocytes of gravistimulated plants. Previous studies have determined that pharmacological disruption of the F-actin cytoskeleton with latrunculin B (Lat-B) causes increased gravitropism in stem-like organs and roots, and results in a more rapid settling of amyloplasts in the columella cells of Arabidopsis roots. These results suggest that the actin cytoskeleton modulates amyloplast movement and also gravitropic signal transduction. To determine the effect of F-actin disruption on amyloplast sedimentation in stem-like organs, Arabidopsis hypocotyls were treated with Lat-B and a detailed analysis of amyloplast sedimentation kinetics was performed by determining amyloplast positions in endodermal cells at various time intervals following reorientation. Confocal microscopy was used to confirm that Lat-B effectively disrupts the actin cytoskeleton in these cells. The results indicate that amyloplasts in hypocotyl endodermal cells settle more quickly compared with amyloplasts in root columella cells. F-actin disruption with Lat-B severely reduces amyloplast mobility within Arabidopsis endodermal statocytes, and these results suggest that amyloplast sedimentation within the hypocotyl endodermal cell is F-actin-dependent. Thus, a model for gravitropism in stem-like organs is proposed in which F-actin modulates the gravity response by actively participating in statolith repositioning within the endodermal statocytes.

Amyloplasts and vacuolar membrane dynamics in the living graviperceptive cell of the Arabidopsis inflorescence stem

We developed an adequate method for the in vivo analysis of organelle dynamics in the gravity-perceptive cell (endodermis) of the Arabidopsis thaliana inflorescence stem, revealing behavior of amyloplasts and vacuolar membranes in those cells. Amyloplasts in the endodermis showed saltatory movements even before gravistimulation by reorientation, and these movements were confirmed as microfilament dependent. From our quantitative analysis in the wild type, the gravity-oriented movement of amyloplasts mainly occurred during 0 to 3 min after gravistimulation by reorientation, supporting findings from our previous physiological study. Even after microfilament disruption, the gravity-oriented movement of amyloplasts remained. By contrast, in zig/sgr4 mutants, where a SNARE molecule functioning in vacuole biogenesis has been disrupted, the movement of amyloplasts in the endodermis is severely restricted both before and after gravistimulation by reorientation. Here, we describe vacuolar membrane behavior in these cells in the wild-type, actin filament-disrupted, and zig/sgr4 mutants and discuss its putatively important features for the perception of gravity. We also discuss the data on the two kinds of movements of amyloplasts that may play an important role in gravitropism: (1) the leading edge amyloplasts and (2) the en mass movement of amyloplasts.

The plant clock shows its metal: circadian regulation of cytosolic free Ca(2+)

Signal transduction events that lead to circadian control of physiology are poorly understood. Signalling elements that could transmit time information include transcription factors, reversible phosphorylation, and changes in the concentration of cytosolic free calcium ([Ca(2+)](cyt)). [Ca(2+)](cyt) oscillates with a circadian rhythm in Arabidopsis and Nicotiana, but does not have a defined role in circadian signalling. [Ca(2+)](cyt) oscillations with shorter periods encode specific signals in several cell types, therefore circadian [Ca(2+)](cyt) oscillations provide a potential mechanism for signalling time information. Cell types such as stomatal guard cells and legume pulvini represent attractive model systems for dissecting circadian Ca(2+) signalling.

Gravity sensing and signaling

Gravitropism has attracted much attention from plant biologists. Recent studies have provided molecular evidence supporting two long-surviving hypotheses about the mechanism of gravitropism: the starch-statolith hypothesis and the Cholodney-Went hypothesis. Amyloplast movement along the gravity vector within gravity-sensing cells in the root and shoot is the most likely trigger of subsequent intracellular signaling. Several possible events leading from this signaling to differential auxin distribution within the sensing cells have been suggested recently.

The promotion of gravitropism in Arabidopsis roots upon actin disruption is coupled with the extended alkalinization of the columella cytoplasm and a persistent lateral auxin gradient

The actin cytoskeleton has been implicated in regulating plant gravitropism. However, its precise role in this process remains uncertain. We have shown previously that disruption of the actin cytoskeleton with Latrunculin B (Lat B) strongly promoted gravitropism in maize roots. These effects were most evident on a clinostat as curvature that would exceed 90 degrees despite short periods of horizontal stimulation. To probe further the cellular mechanisms underlying these enhanced gravity responses, we extended our studies to roots of Arabidopsis. Similar to our observations in other plant species, Lat B enhanced the response of Arabidopsis roots to gravity. Lat B (100 nm) and a stimulation time of 5-10 min were sufficient to induce enhanced bending responses during clinorotation. Lat B (100 nm) disrupted the fine actin filament network in different regions of the root and altered the dynamics of amyloplasts in the columella but did not inhibit the gravity-induced alkalinization of the columella cytoplasm. However, the duration of the alkalinization response during continuous gravistimulation was extended in Lat B-treated roots. Indirect visualization of auxin redistribution using the DR5:beta-glucuronidase (DR5:GUS) auxin-responsive reporter showed that the enhanced curvature of Lat B-treated roots during clinorotation was accompanied by a persistent lateral auxin gradient. Blocking the gravity-induced alkalinization of the columella cytoplasm with caged protons reduced Lat B-induced curvature and the development of the lateral auxin gradient. Our data indicate that the actin cytoskeleton is unnecessary for the initial perception of gravity but likely acts to downregulate gravitropism by continuously resetting the gravitropic-signaling system.

[Mechanism of gravi-sensing and -transduction in gravitropism of higher plants]

In higher plants, some organs such as roots, hypocotyls, and stems, can sense the direction of gravity to regulate their orientation. Gravitropic response is composed of four steps; 1. gravity sensing and conversion of physical stimuli to biochemical signals, 2. intracellular signal transduction in gravity sensing cells, 3. signal transmitting to responding tissues, 4. differential growth of organs. Here we focus on the former two steps. Recent studies using modern technique have gradually unveiled early events and mechanism of gravitropic response. Genetic approach provided evidences that strongly support the classical theory for gravity sensing (step 1). Computational analysis suggested the existence of another gravity sensing mechanism in roots. Spatial and temporal ion imaging in living organs in real time provided information on step 2. In addition, reverse genetic approach suggested asymmetrical intracellular distribution of auxin transporter [correction of transpoter] is a possible link between step 2 and 3. However, molecular basis of the signaling mechanism remains unknown. We believe extensive molecular genetic approach combined with recent techniques cited here shed the light to this ambiguous area of research.

Inhibition of the gravitropic bending response of flowering shoots by salicylic acid

The upward gravitropic bending of cut snapdragon, lupinus and anemone flowering shoots was inhibited by salicylic acid (SA) applied at 0.5 mM and above. This effect was probably not due to acidification of the cytoplasm, since other weak acids did not inhibit bending of snapdragon shoots. In order to study its mode of inhibitory action, we have examined in cut snapdragon shoots the effect of SA on three processes of the gravity-signaling pathway, including: amyloplast sedimentation, formation of ethylene gradient across the stem, and differential growth response. The results show that 1 mM SA inhibited differential ethylene production rates across the horizontal stem and the gravity-induced growth, without significantly inhibiting vertical growth or amyloplast sedimentation following horizontal placement. However, 5 mM SA inhibited all three gravity-induced processes, as well as the growth of vertical shoots, while increasing flower wilting. It may, therefore, be concluded that SA inhibits bending of various cut flowering shoots in a concentration-dependent manner. Thus, at a low concentration SA exerts its effect in snapdragon shoots by inhibiting processes operating downstream to stimulus sensing exerted by amyloplast sedimentation. At a higher concentration SA inhibits bending probably by exerting general negative effects on various cellular processes.

Mechanotransduction in gravisensing cells

Plant organs can re-orientate themselves with respect to gravity. Gravisensing cells (statocytes) contain movable amyloplasts whose potential energy is apparently used to activate calcium channels by exerting tension on the actin network and/or pressure on the cytoskeleton elements lining the plasma membrane. The chain of events that follows remains to be further analysed but includes transient pH changes in the cytosol and sustained pH changes in the cell wall. Transduction ends with relocation of the auxin efflux carriers responsible for the lateral transport of auxin, which reorients the root tip in the direction of gravity. Many questions remain to be solved but recent studies now herald a better understanding of the molecular events involved in gravisensing.

Enhanced gravitropism of roots with a disrupted cap actin cytoskeleton

The actin cytoskeleton has been proposed to be a major player in plant gravitropism. However, understanding the role of actin in this process is far from complete. To address this problem, we conducted an analysis of the effect of Latrunculin B (Lat B), a potent actin-disrupting drug, on root gravitropism using various parameters that included detailed curvature kinetics, estimation of gravitropic sensitivity, and monitoring of curvature development after extended clinorotation. Lat B treatment resulted in a promotion of root curvature after a 90 degrees reorientation in three plant species tested. More significantly, the sensitivity of maize (Zea mays) roots to gravity was enhanced after actin disruption, as determined from a comparison of presentation time of Lat B-treated versus untreated roots. A short 10-min gravistimulus followed by extended rotation on a 1-rpm clinostat resulted in extensive gravitropic responses, manifested as curvature that often exceeded 90 degrees. Application of Lat B to the cap or elongation zone of maize roots resulted in the disruption of the actin cytoskeleton, which was confined to the area of localized Lat B application. Only roots with Lat B applied to the cap displayed the strong curvature responses after extended clinorotation. Our study demonstrates that disrupting the actin cytoskeleton in the cap leads to the persistence of a signal established by a previous gravistimulus. Therefore, actin could function in root gravitropism by providing a mechanism to regulate the proliferation of a gravitropic signal originating from the cap to allow the root to attain its correct orientation or set point angle.

Enhanced gravitropism of roots with a disrupted cap actin cytoskeleton

The actin cytoskeleton has been proposed to be a major player in plant gravitropism. However, understanding the role of actin in this process is far from complete. To address this problem, we conducted an analysis of the effect of Latrunculin B (Lat B), a potent actin-disrupting drug, on root gravitropism using various parameters that included detailed curvature kinetics, estimation of gravitropic sensitivity, and monitoring of curvature development after extended clinorotation. Lat B treatment resulted in a promotion of root curvature after a 90 degrees reorientation in three plant species tested. More significantly, the sensitivity of maize (Zea mays) roots to gravity was enhanced after actin disruption, as determined from a comparison of presentation time of Lat B-treated versus untreated roots. A short 10-min gravistimulus followed by extended rotation on a 1-rpm clinostat resulted in extensive gravitropic responses, manifested as curvature that often exceeded 90 degrees. Application of Lat B to the cap or elongation zone of maize roots resulted in the disruption of the actin cytoskeleton, which was confined to the area of localized Lat B application. Only roots with Lat B applied to the cap displayed the strong curvature responses after extended clinorotation. Our study demonstrates that disrupting the actin cytoskeleton in the cap leads to the persistence of a signal established by a previous gravistimulus. Therefore, actin could function in root gravitropism by providing a mechanism to regulate the proliferation of a gravitropic signal originating from the cap to allow the root to attain its correct orientation or set point angle.

Rapid alkalinization factors in poplar cell cultures. Peptide isolation, cDNA cloning, and differential expression in leaves and methyl jasmonate-treated cells

A family of peptides inducing rapid pH alkalinization in hybrid poplar (Populus trichocarpa x Populus deltoides) cell culture medium was isolated from hybrid poplar leaves. Five related approximately 5-kD peptides were purified by high-performance liquid chromatography and analyzed by matrix-assisted laser desorption ionization-mass spectrometry. The N-terminal sequence of one of the isolated peptides was very similar to a previously characterized peptide from tobacco (Nicotiana tabacum), rapid alkalinization factor (RALF), which causes a rapid increase in culture medium pH when added to tobacco cell cultures (G. Pearce, D.S. Moura, J. Stratmann, C.A. Ryan [2001] Proc Natl Acad Sci USA 98: 12843-12847). Two unique poplar RALF cDNAs (PtdRALF1 and PtdRALF2) were isolated from a poplar cDNA library and used to study RALF expression in poplar saplings and cultured poplar cells. Both genes were found to be expressed constitutively in poplar saplings and cultured cells. However, PtdRALF2 was expressed in leaves at very low levels, and its expression in suspension culture cells was transiently suppressed by methyl jasmonate (MeJa). Although the function of these novel peptides remains enigmatic, our experiments suggest their role may be developmental rather than stress related. Overall, our study confirms the presence of active RALF peptides in other plants, and provides new data on the complexity of the RALF gene family in poplar.

Gravisensing: ionic responses, cytoskeleton and amyloplast behavior

In Zea mays L., changes in orientation of stems are perceived by the pulvinal tissue, which responds to the stimulus by differential growth resulting in upward bending of the stem. Gravity is perceived in the bundle sheath cells, which contain amyloplasts that sediment to the new cell base when a change in the gravity vector occurs. The mechanism by which the mechanical signal is transduced into a physiological response is so far unknown for any gravity perceiving tissue. It is hypothesized that this involves interactions of amyloplasts with the plasma membrane and/or ER via cytoskeletal elements. To gain further insights into this process we monitored amyloplast movements in response to gravistimulation. In a pharmacological approach we investigated how the dynamics of plastid sedimentation are affected by actin and microtubule (MT) disrupting drugs. Dark grown caulonemal filaments of the moss Physcomitrella patens respond to gravity vector changes with a reorientation of tip growth away from the gravity vector. MT distributions in tip cells were monitored over time and MTs were seen to accumulate preferentially on the lower flank of the tip 30 min after a 90 degree turn. Using a self-referencing Ca2+ selective ion probe, we found that growing caulonemal filaments exhibit a Ca2+ influx at the apical dome, similar to that reported previously for other tip growing cells. However, in gravistimulated Physcomitrella filaments the region of Ca2+ influx is not confined to the apex, but extends about 60 micrometers along the upper side of the filament. Our results indicate an asymmetry in the Ca2+ flux pattern between the upper and side of the filament suggesting differential activation of Ca2+ permeable channels at the plasma membrane.

Gravity-stimulated changes in auxin and invertase gene expression in maize pulvinal cells

Maize (Zea mays) stem gravitropism involves differential elongation of cells within a highly specialized region, the stem internodal pulvinus. In the present study, we investigated factors that control gravitropic responses in this system. In the graviresponding pulvinus, hexose sugars (D-Glc and D-Fru) accumulated asymmetrically across the pulvinus. This correlated well with an asymmetric increase in acid invertase activity across the pulvinus. Northern analyses revealed asymmetric induction of one maize acid invertase gene, Ivr2, consistent with transcriptional regulation by gravistimulation. Several lines of evidence indicated that auxin redistribution, as a result of polar auxin transport, is necessary for gravity-stimulated Ivr2 transcript accumulation and differential cell elongation across the maize pulvinus. First, the auxin transport inhibitor, N-1-naphthylphthalamic acid, inhibited gravistimulated curvature and Ivr2 transcript accumulation. Second, a transient gradient of free indole-3-acetic acid (IAA) across the pulvinus was apparent shortly after initiation of gravistimulation. This temporarily free IAA gradient appears to be important for differential cell elongation and Ivr2 transcript accumulation. This is based on the observation that N-1-naphthylphthalamic acid will not inhibit gravitropic responses when applied to pulvinus tissue after the free IAA gradient peak has occurred. Third, IAA alone can stimulate Ivr2 transcript accumulation in non-gravistimulated pulvini. The gravity- and IAA-stimulated increase in Ivr2 transcripts was sensitive to the protein synthesis inhibitor, cycloheximide. Based on these results, a two-phase model describing possible relationships between gravitropic curvature, IAA redistribution, and Ivr2 expression is presented.

Transient dissociation of polyribosomes and concurrent recruitment of calreticulin and calmodulin transcripts in gravistimulated maize pulvini

The dynamics of polyribosome abundance were studied in gravistimulated maize (Zea mays) stem pulvini. During the initial 15 min of gravistimulation, the amount of large polyribosomes transiently decreased. The transient decrease in polyribosome levels was accompanied by a transient decrease in polyribosome-associated mRNA. After 30 min of gravistimulation, the levels of polyribosomes and the amount of polyribosome-associated mRNA gradually increased over 24 h up to 3- to 4-fold of the initial value. Within 15 min of gravistimulation, total levels of transcripts coding for calreticulin and calmodulin were elevated 5-fold in maize pulvinus total RNA. Transcripts coding for calreticulin and calmodulin were recruited into polyribosomes within 15 min of gravistimulation. Over 4 h of gravistimulation, a gradual increase in the association of calreticulin and calmodulin transcripts with polyribosomes was seen predominantly in the lower one-half of the maize pulvinus; the association of transcripts for vacuolar invertase with polyribosomes did not change over this period. Our results suggest that within 15 min of gravistimulation, the translation of the majority of transcripts associated with polyribosomes decreased, resembling a general stress response. Recruitment of calreticulin and calmodulin transcripts into polyribosomes occurred predominantly in the lower pulvinus one-half during the first 4 h when the presentation time for gravistimulation in the maize pulvinus is not yet complete.

Transient dissociation of polyribosomes and concurrent recruitment of calreticulin and calmodulin transcripts in gravistimulated maize pulvini

The dynamics of polyribosome abundance were studied in gravistimulated maize (Zea mays) stem pulvini. During the initial 15 min of gravistimulation, the amount of large polyribosomes transiently decreased. The transient decrease in polyribosome levels was accompanied by a transient decrease in polyribosome-associated mRNA. After 30 min of gravistimulation, the levels of polyribosomes and the amount of polyribosome-associated mRNA gradually increased over 24 h up to 3- to 4-fold of the initial value. Within 15 min of gravistimulation, total levels of transcripts coding for calreticulin and calmodulin were elevated 5-fold in maize pulvinus total RNA. Transcripts coding for calreticulin and calmodulin were recruited into polyribosomes within 15 min of gravistimulation. Over 4 h of gravistimulation, a gradual increase in the association of calreticulin and calmodulin transcripts with polyribosomes was seen predominantly in the lower one-half of the maize pulvinus; the association of transcripts for vacuolar invertase with polyribosomes did not change over this period. Our results suggest that within 15 min of gravistimulation, the translation of the majority of transcripts associated with polyribosomes decreased, resembling a general stress response. Recruitment of calreticulin and calmodulin transcripts into polyribosomes occurred predominantly in the lower pulvinus one-half during the first 4 h when the presentation time for gravistimulation in the maize pulvinus is not yet complete.