Hardware Development for Plant Cultivation Allowing In Situ Fluorescence Analysis of Calcium Fluxes in Plant Roots Under Microgravity and Ground-Control Conditions

Maintaining an optimal leaf and stem orientation to yield a maximum photosynthetic output is accomplished by terrestrial plants using sophisticated mechanisms to balance their orientation relative to the Earth's gravity vector and the direction of sunlight. Knowledge of the signal transduction chains of both gravity and light perception and how they influence each other is essential for understanding plant development on Earth and plant cultivation in space environments. However, in situ analyses of cellular signal transduction processes in weightlessness, such as live cell imaging of signaling molecules using confocal fluorescence microscopy, require an adapted experimental setup that meets the special requirements of a microgravity environment. In addition, investigations under prolonged microgravity conditions require extensive resources, are rarely accessible, and do not allow for immediate sample preparation for the actual microscopic analysis. Therefore, supply concepts are needed that ensure both the viability of the contained plants over a longer period of time and an unhindered microscopic analysis in microgravity. Here, we present a customized supply unit specifically designed to study gravity-induced Ca2+ mobilization in roots of Arabidopsis thaliana. The unit can be employed for ground-based experiments, in parabolic flights, on sounding rockets, and probably also aboard the International Space Station.

Simulating Space Conditions Evokes Different DNA Damage Responses in Immature and Mature Cells of the Human Hematopoietic System

The impact of space radiation and microgravity on DNA damage responses has been discussed controversially, largely due to the variety of model systems engaged. Here, we performed side-by-side analyses of human hematopoietic stem/progenitor cells (HSPC) and peripheral blood lymphocytes (PBL) cultivated in a 2D clinostat to simulate microgravity before, during and after photon and particle irradiation. We demonstrate that simulated microgravity (SMG) accelerates the early phase of non-homologous end joining (NHEJ)-mediated repair of simple, X-ray-induced DNA double-strand breaks (DSBs) in PBL, while repair kinetics in HSPC remained unaltered. Repair acceleration was lost with increasing LET of ion exposures, which increases the complexity of DSBs, precluding NHEJ and requiring end resection for successful repair. Such cell-type specific effect of SMG on DSB repair was dependent on the NF-кB pathway pre-activated in PBL but not HSPC. Already under unperturbed growth conditions HSPC and PBL suffered from SMG-induced replication stress associated with accumulation of single-stranded DNA and DSBs, respectively. We conclude that in PBL, SMG-induced DSBs promote repair of radiation-induced damage in an adaptive-like response. HSPC feature SMG-induced single-stranded DNA and FANCD2 foci, i.e., markers of persistent replication stress and senescence that may contribute to a premature decline of the immune system in space.

Human neural network activity reacts to gravity changes in vitro

During spaceflight, humans experience a variety of physiological changes due to deviations from familiar earth conditions. Specifically, the lack of gravity is responsible for many effects observed in returning astronauts. These impairments can include structural as well as functional changes of the brain and a decline in cognitive performance. However, the underlying physiological mechanisms remain elusive. Alterations in neuronal activity play a central role in mental disorders and altered neuronal transmission may also lead to diminished human performance in space. Thus, understanding the influence of altered gravity at the cellular and network level is of high importance. Previous electrophysiological experiments using patch clamp techniques and calcium indicators have shown that neuronal activity is influenced by altered gravity. By using multi-electrode array (MEA) technology, we advanced the electrophysiological investigation covering single-cell to network level responses during exposure to decreased (micro-) or increased (hyper-) gravity conditions. We continuously recorded in real-time the spontaneous activity of human induced pluripotent stem cell (hiPSC)-derived neural networks in vitro. The MEA device was integrated into a custom-built environmental chamber to expose the system with neuronal cultures to up to 6 g of hypergravity on the Short-Arm Human Centrifuge at the DLR Cologne, Germany. The flexibility of the experimental hardware set-up facilitated additional MEA electrophysiology experiments under 4.7 s of high-quality microgravity (10–6 to 10–5 g) in the Bremen drop tower, Germany. Hypergravity led to significant changes in activity. During the microgravity phase, the mean action potential frequency across the neural networks was significantly enhanced, whereas different subgroups of neurons showed distinct behaviors, such as increased or decreased firing activity. Our data clearly demonstrate that gravity as an environmental stimulus triggers changes in neuronal activity. Neuronal networks especially reacted to acute changes in mechanical loading (hypergravity) or de-loading (microgravity). The current study clearly shows the gravity-dependent response of neuronal networks endorsing the importance of further investigations of neuronal activity and its adaptive responses to micro- and hypergravity. Our approach provided the basis for the identification of responsible mechanisms and the development of countermeasures with potential implications on manned space missions.

The enigmatic Placozoa part 2: Exploring evolutionary controversies and promising questions on earth and in space

The placozoan Trichoplax adhaerens has been bridging gaps between research disciplines like no other animal. As outlined in part 1, placozoans have been subject of hot evolutionary debates and placozoans have challenged some fundamental evolutionary concepts. Here in part 2 we discuss the exceptional genetics of the phylum Placozoa and point out some challenging model system applications for the best known species, Trichoplax adhaerens.

The enigmatic Placozoa part 1: Exploring evolutionary controversies and poor ecological knowledge

The placozoan Trichoplax adhaerens is a tiny hairy plate and more simply organized than any other living metazoan. After its original description by F.E. Schulze in 1883, it attracted attention as a potential model for the ancestral state of metazoan organization, the "Urmetazoon". Trichoplax lacks any kind of symmetry, organs, nerve cells, muscle cells, basal lamina, and extracellular matrix. Furthermore, the placozoan genome is the smallest (not secondarily reduced) genome of all metazoan genomes. It harbors a remarkably rich diversity of genes and has been considered the best living surrogate for a metazoan ancestor genome. The phylum Placozoa presently harbors three formally described species, while several dozen "cryptic" species are yet awaiting their description. The phylogenetic position of placozoans has recently become a contested arena for modern phylogenetic analyses and view-driven claims. Trichoplax offers unique prospects for understanding the minimal requirements of metazoan animal organization and their corresponding malfunctions.

ARABIDOMICS-A new experimental platform for molecular analyses of plants in drop towers, on parabolic flights, and sounding rockets

Plants represent an essential part of future life support systems that will enable human space travel to distant planets and their colonization. Therefore, insights into changes and adaptations of plants in microgravity are of great importance. Despite considerable efforts, we still know very little about how plants respond to microgravity environments on the molecular level, partly due to a lack of sufficient hardware and flight opportunities. The plant Arabidopsis thaliana, the subject of this study, represents a well-studied model organism in gravitational biology, particularly for the analysis of transcriptional and metabolic changes. To overcome the limitations of previous plant hardware that often led to secondary effects and to allow for the extraction not only of RNA but also of phytohormones and proteins, we developed a new experimental platform, called ARABIDOMICS, for exposure and fixation under altered gravity conditions. Arabidopsis seedlings were exposed to hypergravity during launch and microgravity during the free-fall period of the MAPHEUS 5 sounding rocket. Seedlings were chemically fixed inflight at defined time points, and RNA and phytohormones were subsequently analyzed in the laboratory. RNA and phytohormones extracted from the fixed biological samples were of excellent quality. Changes in the phytohormone content of jasmonate, auxin, and several cytokinins were observed in response to hypergravity and microgravity.

The MAPHEUS module CellFix for studying the influence of altered gravity on the physiology of single cells

Gravity is the only constant stimulus during the evolution of life. To investigate the impact of the absence of gravity on living systems, their molecular and morphological status has to be studied under microgravity conditions. The experiment unit CellFix was developed in order to provide the possibility of exposure and chemical fixation of small biological systems, such as neurons, stem cells, small animals, yeast cultures, plants, etc., at dedicated time points during a sounding rocket flight. The current version of CellFix consists of two culture bags containing cell cultures in a temperature-controlled pressure vessel. The biosystems in the culture bags can be fixed by pumping the fixative [e.g., paraformaldehyde (PFA), methanol, RNAlater, or others] from a connected bag into the cell suspension. The mechatronic basis of the experiment unit is constructed from compartments of the shelf parts. Open source microcontroller systems (Arduino) or gear pumps, accumulators, etc., from the model making sector are affordable and reliable components to build up an experiment on an unmanned space mission such as a sounding rocket flight. Also, new technologies such as fused deposition modeling were used to construct structures and brackets, which were tested successfully in environmental tests and real space flights (MAPHEUS 7 and 8 sounding rocket missions). In combination with the possibility to handle the experiment as a late access insert in a standardized rocket compartment, CellFix provides a multiusable experiment unit for performing life science experiments in space.

The German Aerospace Center M-42 radiation detector-A new development for applications in mixed radiation fields

In the last few years, the Biophysics Working Group of the Institute of Aerospace Medicine of the German Aerospace Center (DLR) started the development of a small low power consumption radiation detector system for the measurement of the absorbed dose to be applied in various environments, such as onboard aircraft, in space, and also as a demonstration tool for students. These so called DLR M-42 detectors are based on an electronics design, which can easily be adjusted to the user- and mission-requirements. M-42 systems were already applied for measurements in airplanes, during two MAPHEUS (Materialphysikalische Experimente unter Schwerelosigkeit) rocket missions, and are currently prepared for long term balloon experiments. In addition, they will be part of the dosimetry suite of the upcoming Matroshka AstroRad Radiation Experiment on the NASA Artemis I mission. This paper gives an overview of the design and the testing of the DLR M-42 systems and provides highlighted results from the MAPHEUS campaigns where the detectors were tested for the first time under space flight conditions. Results clearly show that the system design enables independent measurements starting upon rocket launch due to the built-in accelerometer sensors and provides data for the relevant 6 min of μ-gravity as given for the MAPHEUS missions. These 6 min of the μ-gravity environment at altitudes between 100 and 240 km lead to a total absorbed dose of 1.21 ± 0.15 µGy being equivalent to half a day of radiation background measured with the M-42 in the laboratory at DLR, Cologne, Germany.

The gravity dependence of pharmacodynamics: the integration of lidocaine into membranes in microgravity

To realize long-term manned space missions, e.g. to Mars, some important questions about pharmacology under conditions of different gravity will have to be answered to ensure safe usage of pharmaceuticals. Experiments on the International Space Station showed that the pharmacokinetics of drugs are changed in microgravity. On Earth, it is well known that the incorporation of substances into cellular membranes depends on membrane fluidity, therefore the finding that membrane fluidity is gravity dependent possibly has effects on pharmacodynamics of hydrophobic and amphiphilic substances in microgravity. To validate a possible effect of gravity on pharmacodynamics, experiments have been carried out to investigate the incorporation of lidocaine into plain lipid membranes under microgravity conditions. In microgravity, the induced increase in membrane fluidity associated with lidocaine incorporation is smaller compared to 1g controls. This experiment concerning the gravity dependence of pharmacodynamics in real microgravity clearly shows that the incorporation of amphipathic drugs into membranes is changed in microgravity. This might have significant impact on the pharmacology of drugs during long-term space missions and has to be investigated in more detail to be able to assess possible risks.

The influence of nitrogen concentration and precipitation on fertilizer production from urine using a trickling filter

Planetary habitation requires technology to maintain natural microbial processes, which make nutrients from biowaste available for plant cultivation. This study describes a 646 day experiment, in which trickling filters were monitored for their ability to mineralize nitrogen when loaded with artificial urine solutions of different concentrations (40, 60, 80 and 100% v/v). Former studies have indicated that increasing urine concentrations slow nitrogen conversion rates and induce growing instability. In the current experiment, nitrogen conversion rates, measured as nitrate production/day, did not differ between concentration levels and increasing instability was not observed. Instead, the buffering capacity of the mussel shells added as buffer system (∼75% calcium carbonate) increased with increasing concentrations of synthetic urine possibly due to the higher phosphate content. The intensified precipitation of calcium phosphates seems to promote carbonate dissolution leading to improved buffering. For space applications, the precipitation of calcium phosphates is not desirable as for the phosphate to be available to the plants the precipitate must be treated with hazardous substances. With regard to terrestrial agriculture the process-integrated phosphate precipitation is a possibility to separate the macronutrients nitrogen and phosphate without addition of other chemicals. Thus, the described process offers a simple and cost-effective approach to fertilizer production from biogenic residues like slurry.

Long term stability of Oligo (dT) 25 magnetic beads for the expression analysis of Euglena gracilis for long term space projects

The unicellular freshwater flagellate Euglena gracilis has a highly developed sensory system. The cells use different stimuli such as light and gravity to orient themselves in the surrounding medium to find areas for optimal growth. Due to the ability to produce oxygen and consume carbon dioxide, Euglena is a suitable candidate for life support systems. Participation in a long-term space experiment would allow for the analysis of changes and adaptations to the new environment, and this could bring new insights into the mechanism of perception of gravity and the associated signal transduction chain. For a molecular analysis of transcription patterns, an automated system is necessary, capable of performing all steps from taking a sample, processing it and generating data. One of the developmental steps is to find long-term stable reagents and materials and test them for stability at higher-than-recommended temperature conditions during extended storage time. We investigated the usability of magnetic beads in an Euglena specific lysis buffer after addition of the RNA stabilizer Dithiothreitol over 360 days and the lysis buffer with the stabilizer alone over 455 days at the expected storage temperature of 19 °C. We can claim that the stability is not impaired at all after an incubation period of over one year. This might be an interesting result for researchers who have to work under non-standard lab conditions, as in biological or medicinal fieldwork.

Pyrocystis noctiluca represents an excellent bioassay for shear forces induced in ground-based microgravity simulators (clinostat and random positioning machine)

Ground-based facilities, such as clinostats and random positioning machines aiming at simulating microgravity conditions, are tools to prepare space experiments and identify gravity-related signaling pathways. A prerequisite is that the facilities are operated in an appropriate manner and potentially induced non-gravitational effects, such as shearing forces, have to be taken into account. Dinoflagellates, here P. noctiluca, as fast and sensitive reporter system for shear stress and hydrodynamic gradients, were exposed on a clinostat (constant rotation around one axis, 60 rpm) or in a random positioning machine, that means rotating around two axes, whose velocity and direction were chosen at random. Deformation of the cell membrane of P. noctiluca due to shear stress results in a detectable bioluminescence emission. Our results show that the amount of mechanical stress is higher on an random positioning machine than during constant clinorotation, as revealed by the differences in photon counts. We conclude that one axis clinorotation induced negligible non-gravitational effects in the form of shear forces in contrast to random operation modes tested. For the first time, we clearly visualized the device-dependent occurrence of shear forces by means of a bioassay, which have to be considered during the definition of an appropriate simulation approach and to avoid misinterpretation of results.

Natural microbial populations in a water-based biowaste management system for space life support

The reutilization of wastewater is a key issue with regard to long-term space missions and planetary habitation. This study reports the design, test runs and microbiological analyses of a fixed bed biofiltration system which applies pumice grain (16-25 mm grain size, 90 m(2)/m(3) active surface) as matrix and calcium carbonate as buffer. For activation, the pumice was inoculated with garden soil known to contain a diverse community of microorganisms, thus enabling the filtration system to potentially degrade all kinds of organic matter. Current experiments over 194 days with diluted synthetic urine (7% and 20%) showed that the 7% filter units produced nitrate slowly but steadily (max. 2191 mg NO3-N/day). In the 20% units nitrate production was slower and less stable (max. 1411 mg NO3-N/day). 84% and 76% of the contained nitrogen was converted into nitrate. The low conversion rate is assumed to be due to the high flow rate, which keeps the biofilm on the pumice thin. At the same time the thin biofilm seems to prevent the activity of denitrifiers implicating the existence of a trade off between rate and the amount of nitrogen loss. Microbiological analyses identified a comparatively low number of species (26 in the filter material, 12 in the filtrate) indicating that urine serves as a strongly selective medium and filter units for the degradation of mixed feedstock have to be pre-conditioned on the intended substrates from the beginning.

Impact of a high magnetic field on the orientation of gravitactic unicellular organisms--a critical consideration about the application of magnetic fields to mimic functional weightlessness

The gravity-dependent behavior of Paramecium biaurelia and Euglena gracilis have previously been studied on ground and in real microgravity. To validate whether high magnetic field exposure indeed provides a ground-based facility to mimic functional weightlessness, as has been suggested earlier, both cell types were observed during exposure in a strong homogeneous magnetic field (up to 30 T) and a strong magnetic field gradient. While swimming, Paramecium cells were aligned along the magnetic field lines; orientation of Euglena was perpendicular, demonstrating that the magnetic field determines the orientation and thus prevents the organisms from the random swimming known to occur in real microgravity. Exposing Astasia longa, a flagellate that is closely related to Euglena but lacks chloroplasts and the photoreceptor, as well as the chloroplast-free mutant E. gracilis 1F, to a high magnetic field revealed no reorientation to the perpendicular direction as in the case of wild-type E. gracilis, indicating the existence of an anisotropic structure (chloroplasts) that determines the direction of passive orientation. Immobilized Euglena and Paramecium cells could not be levitated even in the highest available magnetic field gradient as sedimentation persisted with little impact of the field on the sedimentation velocities. We conclude that magnetic fields are not suited as a microgravity simulation for gravitactic unicellular organisms due to the strong effect of the magnetic field itself, which masks the effects known from experiments in real microgravity.

Analysis of gene expression during parabolic flights reveals distinct early gravity responses in Arabidopsis roots

Plant roots are among most intensively studied biological systems in gravity research. Altered gravity induces asymmetric cell growth leading to root bending. Differential distribution of the phytohormone auxin underlies root responses to gravity, being coordinated by auxin efflux transporters from the PIN family. The objective of this study was to compare early transcriptomic changes in roots of Arabidopsis thaliana wild type, and pin2 and pin3 mutants under parabolic flight conditions and to correlate these changes to auxin distribution. Parabolic flights allow comparison of transient 1-g, hypergravity and microgravity effects in living organisms in parallel. We found common and mutation-related genes differentially expressed in response to transient microgravity phases. Gene ontology analysis of common genes revealed lipid metabolism, response to stress factors and light categories as primarily involved in response to transient microgravity phases, suggesting that fundamental reorganisation of metabolic pathways functions upstream of a further signal mediating hormonal network. Gene expression changes in roots lacking the columella-located PIN3 were stronger than in those deprived of the epidermis and cortex cell-specific PIN2. Moreover, repetitive exposure to microgravity/hypergravity and gravity/hypergravity flight phases induced an up-regulation of auxin responsive genes in wild type and pin2 roots, but not in pin3 roots, suggesting a critical function of PIN3 in mediating auxin fluxes in response to transient microgravity phases. Our study provides important insights towards understanding signal transduction processes in transient microgravity conditions by combining for the first time the parabolic flight platform with the transcriptome analysis of different genetic mutants in the model plant, Arabidopsis.

Changes in morphology, gene expression and protein content in chondrocytes cultured on a random positioning machine

Tissue engineering of chondrocytes on a Random Positioning Machine (RPM) is a new strategy for cartilage regeneration. Using a three-dimensional RPM, a device designed to simulate microgravity on Earth, we investigated the early effects of RPM exposure on human chondrocytes of six different donors after 30 min, 2 h, 4 h, 16 h, and 24 h and compared the results with the corresponding static controls cultured under normal gravity conditions. As little as 30 min of RPM exposure resulted in increased expression of several genes responsible for cell motility, structure and integrity (beta-actin); control of cell growth, cell proliferation, cell differentiation and apoptosis (TGF-β₁, osteopontin); and cytoskeletal components such as microtubules (beta-tubulin) and intermediate filaments (vimentin). After 4 hours of RPM exposure disruptions in the vimentin network were detected. These changes were less dramatic after 16 hours on the RPM, when human chondrocytes appeared to reorganize their cytoskeleton. However, the gene expression and protein content of TGF-β₁ was enhanced during RPM culture for 24 h. Taking these results together, we suggest that chondrocytes exposed to the RPM seem to change their extracellular matrix production behaviour while they rearrange their cytoskeletal proteins prior to forming three-dimensional aggregates.

Ground-based facilities for simulation of microgravity: organism-specific recommendations for their use, and recommended terminology

Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.

Short-term weightlessness produced by parabolic flight maneuvers altered gene expression patterns in human endothelial cells

This study focused on the effects of short-term microgravity (22 s) on the gene expression and morphology of endothelial cells (ECs) and evaluated gravisensitive signaling elements. ECs were investigated during four German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt) parabolic flight campaigns. Hoechst 33342 and acridine orange/ethidium bromide staining showed no signs of cell death in ECs after 31 parabolas (P31). Gene array analysis revealed 320 significantly regulated genes after the first parabola (P1) and P31. COL4A5, COL8A1, ITGA6, ITGA10, and ITGB3 mRNAs were down-regulated after P1. EDN1 and TNFRSF12A mRNAs were up-regulated. ADAM19, CARD8, CD40, GSN, PRKCA (all down-regulated after P1), and PRKAA1 (AMPKα1) mRNAs (up-regulated) provide a very early protective mechanism of cell survival induced by 22 s microgravity. The ABL2 gene was significantly up-regulated after P1 and P31, TUBB was slightly induced, but ACTA2 and VIM mRNAs were not changed. β-Tubulin immunofluorescence revealed a cytoplasmic rearrangement. Vibration had no effect. Hypergravity reduced CARD8, NOS3, VASH1, SERPINH1 (all P1), CAV2, ADAM19, TNFRSF12A, CD40, and ITGA6 (P31) mRNAs. These data suggest that microgravity alters the gene expression patterns and the cytoskeleton of ECs very early. Several gravisensitive signaling elements, such as AMPKα1 and integrins, are involved in the reaction of ECs to altered gravity.

Differential gene regulation under altered gravity conditions in follicular thyroid cancer cells: relationship between the extracellular matrix and the cytoskeleton

Extracellular matrix proteins, adhesion molecules, and cytoskeletal proteins form a dynamic network interacting with signalling molecules as an adaptive response to altered gravity. An important issue is the exact differentiation between real microgravity responses of the cells or cellular reactions to hypergravity and/or vibrations. To determine the effects of real microgravity on human cells, we used four DLR parabolic flight campaigns and focused on the effects of short-term microgravity (22 s), hypergravity (1.8 g), and vibrations on ML-1 thyroid cancer cells. No signs of apoptosis or necrosis were detectable. Gene array analysis revealed 2,430 significantly changed transcripts. After 22 s microgravity, the F-actin and cytokeratin cytoskeleton was altered, and ACTB and KRT80 mRNAs were significantly upregulated after the first and thirty-first parabolas. The COL4A5 mRNA was downregulated under microgravity, whereas OPN and FN were significantly upregulated. Hypergravity and vibrations did not change ACTB, KRT-80 or COL4A5 mRNA. MTSS1 and LIMA1 mRNAs were downregulated/slightly upregulated under microgravity, upregulated in hypergravity and unchanged by vibrations. These data indicate that the graviresponse of ML-1 cells occurred very early, within the first few seconds. Downregulated MTSS1 and upregulated LIMA1 may be an adaptive mechanism of human cells for stabilizing the cytoskeleton under microgravity conditions.

How to activate a plant gravireceptor. Early mechanisms of gravity sensing studied in characean rhizoids during parabolic flights

Early processes underlying plant gravity sensing were investigated in rhizoids of Chara globularis under microgravity conditions provided by parabolic flights of the A300-Zero-G aircraft and of sounding rockets. By applying centrifugal forces during the microgravity phases of sounding rocket flights, lateral accelerations of 0.14 g, but not of 0.05 g, resulted in a displacement of statoliths. Settling of statoliths onto the subapical plasma membrane initiated the gravitropic response. Since actin controls the positioning of statoliths and restricts sedimentation of statoliths in these cells, it can be calculated that lateral actomyosin forces in a range of 2 x 10(-14) n act on statoliths to keep them in place. These forces represent the threshold value that has to be exceeded by any lateral acceleration stimulus for statolith sedimentation and gravisensing to occur. When rhizoids were gravistimulated during parabolic plane flights, the curvature angles of the flight samples, whose sedimented statoliths became weightless for 22 s during the 31 microgravity phases, were not different from those of in-flight 1g controls. However, in ground control experiments, curvature responses were drastically reduced when the contact of statoliths with the plasma membrane was intermittently interrupted by inverting gravistimulated cells for less than 10 s. Increasing the weight of sedimented statoliths by lateral centrifugation did not enhance the gravitropic response. These results provide evidence that graviperception in characean rhizoids requires contact of statoliths with membrane-bound receptor molecules rather than pressure or tension exerted by the weight of statoliths.

Tip-localized actin polymerization and remodeling, reflected by the localization of ADF, profilin and villin, are fundamental for gravity-sensing and polar growth in characean rhizoids

Polar organization and gravity-oriented, polarized growth of characean rhizoids are dependent on the actin cytoskeleton. In this report, we demonstrate that the prominent center of the Spitzenkörper serves as the apical actin polymerization site in the extending tip. After cytochalasin D-induced disruption of the actin cytoskeleton, the regeneration of actin microfilaments (MFs) starts with the reappearance of a flat, brightly fluorescing actin array in the outermost tip. The actin array rounds up, produces actin MFs that radiate in all directions and is then relocated into its original central position in the center of the Spitzenkörper. The emerging actin MFs rearrange and cross-link to form the delicate, subapical meshwork, which then controls the statolith positioning, re-establishes the tip-high calcium gradient and mediates the reorganization of the Spitzenkörper with its central ER aggregate and the accumulation of secretory vesicles. Tip growth and gravitropic sensing, which includes control of statolith positioning and gravity-induced sedimentation, are not resumed until the original polar actin organization is completely restored. Immunolocalization of the actin-binding proteins, actin-depolymerizing factor (ADF) and profilin, which both accumulate in the center of the Spitzenkörper, indicates high actin turnover and gives additional support for the actin-polymerizing function of this central, apical area. Association of villin immunofluorescence with two populations of thick undulating actin cables with uniform polarity underlying rotational cytoplasmic streaming in the basal region suggests that villin is the major actin-bundling protein in rhizoids. Our results provide evidence that the precise coordination of apical actin polymerization and dynamic remodeling of actin MFs by actin-binding proteins play a fundamental role in cell polarization, gravity sensing and gravity-oriented polarized growth of characean rhizoids.