Amyloplasts are necessary for full gravitropic sensitivity in roots of Arabidopsis thaliana

The observation that a starchless mutant (TC7) of Arabidopsis thaliana (L.) Heynh. is gravitropic (T. Caspar and B.G. Pickard, 1989, Planta 177, 185–197) raises questions about the hypothesis that starch and amyloplasts play a role in gravity perception. We compared the kinetics of gravitropism in this starchless mutant and the wild-type (WT). Wild-type roots are more responsive to gravity than TC7 roots as judged by several parameters: (1) Vertically grown TC7 roots were not as oriented with respect to the gravity vector as WT roots. (2) In the time course of curvature after gravistimulation, curvature in TC7 roots was delayed and reduced compared to WT roots. (3) TC7 roots curved less than WT roots following a single, short (induction) period of gravistimulation, and WT, but not TC7, roots curved in response to a 1-min period of horizontal exposure. (4) Wild-type roots curved much more than TC7 roots in response to intermittent stimulation (repeated short periods of horizontal exposure); WT roots curved in response to 10 s of stimulation or less, but TC7 roots required 2 min of stimulation to produce a curvature. The growth rates were equal for both genotypes. We conclude that WT roots are more sensitive to gravity than TC7 roots. Starch is not required for gravity perception in TC7 roots, but is necessary for full sensitivity; thus it is likely that amyloplasts function as statoliths in WT Arabidopsis roots. Furthermore, since centrifugation studies using low gravitational forces indicated that starchless plastids are relatively dense and are the most movable component in TC7 columella cells, the starchless plastids may also function as statoliths.Abbreviations S2 story two - S3 story three - WT wild-type     

Amyloplasts as possible statoliths in gravitropic protonemata of the moss Ceratodon purpureus

The kinetics of gravitropism and of amyloplast sedimentation were studied in dark-grown protonemata of the moss Ceratodon purpureus (Hedw.) Brid. The protonemata grew straight up at a rate of 20–25 m·h^– in nutrient-supplemented agar. After they were oriented to the horizontal, upward curvature was first detected after 1–1.5 h and reached 84° by 24 h. The tip cells exhibited an amyloplast zonation, with a tip cluster of nonsedimenting amyloplasts, an amyloplast-free zone, and a zone with pronounced amyloplast sedimentation. This latter zone appears specialized more for lateral than for axial sedimentation since amyloplasts sediment to the lower wall in horizontal protonemata but do not fall to the basal wall in vertical protonemata. Amyloplast sedimentation started within 15 min of gravistimulation; this is within the 12–17-min presentation time. The data support the hypothesis that some amyloplasts function as statoliths in these cells.This work was supported by the National Aeronautics and Space Administration grant NAGW-780. We thank Professor E. Hartmann and J. Schwuchow for providing Ceratodon cultures, Dr. John Z. Kiss and Jeff Young for valuable discussions, and Professor Rainer Hertel (University of Freiburg, FRG) for bringing this material to our attention.     

Blood vessel adaptation to gravity in a semi-arboreal snake

The effects of vasoactive agonists on systemic blood vessels were examined with respect to anatomical location and gravity acclimation in the semi-arboreal snake, Elaphe Obsoleta. Major blood vessels were reactive to putative neurotransmitters, hormones or local factors in vessel specific patterns. Catecholamines, adenosine triphosphate, histamine and high potassium (80 mM) stimulated significantly greater tension per unit vessel mass in posterior than anterior arteries. Anterior vessels were significantly more sensitive to catecholamines than midbody and posterior vessels. Angiotensin II stimulated significantly greater tension in carotid artery than in midbody and posterior dorsal aorta. Arginine vasotocin strongly contracted the left and right aortic arches and anterior dorsal aorta. Veins were strongly contracted by catecholamines, high potassium and angiotensin II, but less so by adenosine triphosphate, arginine vasotocin and histamine. Precontracted vessels were relaxed by acetylcholine and sodium nitroprusside, but not by atrial natriuretic peptide or bradykinin. Chronic exposure of snakes to intermittent hypergravity stress (+1.5 G_z at tail) did not affect the majority of vessel responses. These data demonstrate that in vitro tension correlates with known patterns of sympathetic innervation and suggest that catecholamines, as well as other agonists, are important in mediating vascular responses to gravitational stresses in snakes.Abbreviations ACH acetylcholine - ADA anterior dorsal aorta - ANG II salmon asn¹-val⁵-angiotensin II - ANP rat ile²⁶-atrial natriuretic peptide - ATP adenosine triphosphate - AVT arginine vasotocin - BK human bradykinin - BL total body length - CA carotid artery - CONT control - EC ₅₀ effective concentration producing 50% maximal response - EPI epinephrine - + G _z earth's gravity force - HI-G high gravity acclimation - HI K ⁺ 80 mM high potassium - JV jugular vein - LAA left aortic arch - MDA midbody dorsal aorta - MPV midbody portal vein - MS Mackenzie's solution - NEPI norepinephrine - pD ₂ log EC₅₀ - PDA posterior dorsal aorta - PPV posterior portal vein - RAA right aortic arch - SNP sodium nitroprusside     

Hydrostatic pressure mimics gravitational pressure in characean cells

Summary Hydrostatic pressure applied to one end of a horizontalChara cell induces a polarity of cytoplasmic streaming, thus mimicking the effect of gravity. A positive hydrostatic pressure induces a more rapid streaming away from the applied pressure and a slower streaming toward the applied pressure. In contrast, a negative pressure induces a more rapid streaming toward and a slower streaming away from the applied pressure. Both the hydrostatic pressure-induced and gravity-induced polarity of cytoplasmic streaming respond identically to cell ligation, UV microbeam irradiation, external Ca²⁺ concentrations, osmotic pressure, neutral red, TEA Cl^–, and the Ca²⁺ channel blockers nifedipine and LaCl₃. In addition, hydrostatic pressure applied to the bottom of a vertically-oriented cell can abolish and even reverse the gravity-induced polarity of cytoplasmic streaming. These data indicate that both gravity and hydrostatic pressure act at the same point of the signal transduction chain leading to the induction of a polarity of cytoplasmic streaming and support the hypothesis that characean cells respond to gravity by sensing a gravity-induced pressure differential between the cell ends.     

Mapping of neurons in the gravity receptor system of the octopus statocyst by iontophoretic cobalt staining

Summary The presence of uni-, bi- and multipolar neurons beneath the hair cell epithelium of the Octopus gravity receptor system has been demonstrated by iontophoretic cobalt staining. Counts give an average number of 1,940 neurons per macula. Whether the hair cells are primary of secondary sensory cells is discussed.This work was supported by grant Wo 160/3 of the Deutsche Forschungsgemeinschaft (DFG) to H.G.W. Thanks are due to the Director and staff of the Zoological Station in Naples for their hospitality and help     

Long-distance navigation in the wandering desert spider<Emphasis Type="Italic"> Leucorchestris arenicola</Emphasis>: can the slope of the dune surface provide a compass cue?

Males of the nocturnal spider Leucorchestris arenicola (Araneae: Sparassidae) wander long distances over seemingly featureless dune surfaces in the Namib Desert searching for females. The spiders live in burrows to which they return after nearly every such excursion. While the outward path of an excursion may be a meandering search, the return path is often a nearly straight line leading towards the burrow. This navigational behaviour resembles that of path integration known from other arthropods, though on a much larger scale (over tens to hundreds of meters). Theoretically, precise navigation by path integration over long distances requires an external compass in order to adjust for inevitable accumulation of navigational errors. As a first step towards identifying any nocturnal compass cues used by the male spiders, a method for detailed 3-D recordings of the spiders paths was developed. The 3-D reconstructions of the paths revealed details about the processes involved in the spiders nocturnal way of navigation. Analyses of the reconstructed paths suggest that gravity (slope of the dune surface) is an unlikely parameter used in path integration by the L. arenicola spiders.     

Mechanotransduction molecules in the plant gravisensory response: Amyloplast/statolith membranes contain a β1 integrin-like protein

Summary It has been hypothesized that the sedimentation of amyloplasts within root cap cells is the primary event in the plant gravisensory-signal transduction cascade. Statolith sedimentation, with its ability to generate weighty mechanical signals, is a legitimate means for organisms to discriminate the direction of the gravity vector. However, it has been demonstrated that starchless mutants with reduced statolith densities maintain some ability to sense gravity, calling into question the statolith sedimentation hypothesis. Here we report on the presence of a ₁ integrin-like protein localized inside amyloplasts of tobacco NT-1 suspension culture, callus cells, and whole-root caps. Two different antibodies to the ₁ integrin, one to the cytoplasmic domain and one to the extracellular domain, localize in the vicinity of the starch grains within amyloplasts of NT-1. Biochemical data reveals a 110-kDa protein immunoprecipitated from membrane fractions of NT-1 suspension culture indicating size homology to known ₁ integrin in animals. This study provides the first direct evidence for the possibility of integrin-mediated signal transduction in the perception of gravity by higher plants. An integrin-mediated pathway, initiated by starch grain sedimentation within the amyloplast, may provide the signal amplification necessary to explain the gravitropic response in starch-depleted cultivars.Abbreviations BA 6-benzylaminopurine - ETOH ethyl alcohol - LP liquid propane - LR London Resin - PBST phosphate-buffered saline with Tween - TEM transmission electron microscopy - OSM optical-sectioning microscopy     

Microgravity dependence of excitable biological and physicochemical media

Neuronal tissue and especially the central nervous system (CNS) is an excitable medium. Self-organisation, pattern formation, and propagating excitation waves as typical characteristics in excitable media consequently have been found in neuronal tissue. The properties of such phenomena in excitable media do critically depend on the parameters (i.e., electromagnetic fields, temperature, chemical drugs) of the system and on small external forces to which gravity belongs. The spreading depression, a propagating excitation depression wave of neuronal activity, is one of the best described of the those wave phenomena in the CNS. Especially in the retina as a true part of the CNS it can be easily observed with optical techniques due to the high intrinsic optical signal of this tissue. Another of such waves in neuronal tissue is the propagating action potential in nerve fibres. In this paper, data from our laboratories concerning the influence of gravity on the velocity of propagating waves in excitable media are summarized mainly in terms of the retinal spreading depression and propagating action potentials. Additionally, we have used waves in gels of the Belousov-Zhabotinsky reaction as the physicochemical model system of biological activity as the properties of these waves follow the same theories as the spreading depression and action potentials and they have some striking similarities in wave behavior. Thus propagating Belousov-Zhabotinsky waves are described by their gravity dependence.     

Gravitropism of hypocotyls of wild-type and starch-deficient Arabidopsis seedlings in spaceflight studies

The major purpose of this spaceflight project was to investigate the starch-statolith hypothesis for gravity perception, and a secondary goal was to study plant growth and development under spaceflight conditions. This research was based on our ground studies of gravity perception in the wild type and three starch-deficient (one starchless and two reduced starch) mutants of Arabidopsis thaliana (L.) Heynh. Dark-grown seedlings that developed in microgravity were given one of several (30 min, 60 min, or 90 min) 1-g stimuli by an on-board centrifuge, and additional controls for seedling development also were performed. These latter control experiments included a morphological study of plants that developed in space in microgravity (F μg), in space on a centrifuge (F 1g), on the ground (G 1g), and on a rotating clinostat on the ground. Since elevated levels of ethylene were reported in the spacecraft atmosphere, additional controls for morphology and gravitropism with added ethylene also were performed. While exogenous ethylene reduced the absolute magnitude of the response in all four strains of Arabidopsis, this gas did not appear to change the relative graviresponsiveness among the strains. The relative response of hypocotyls of microgravity-grown seedlings to the stimuli provided by the in-flight centrifuge was: wild type > starch-deficient mutants. Although the protoplast pressure model for gravity perception cannot be excluded, these results are consistent with a statolith-based model for perception in plants. Received: 12 February 1999 / Accepted: 9 March 1999     

Using geotagged photographs and GIS analysis to estimate visitor flows in natural areas

Protected area management requires detailed information about visitors’ travel patterns. Unfortunately, methods commonly applied to obtain this information (e.g. monitoring, simulation) are expensive and time consuming. As a result, for many protected areas worldwide there are no reliable data upon which management actions (e.g. trail maintenance, introduction of use limits) can be based. Today georeferenced material shared on the Internet (e.g. geotagged photographs, GPS tracks) provides a new valuable source of information about people's movements, though its low density (e.g. few geotagged photographs per unit area) in natural areas has discouraged applications in such contexts so far. We propose a new approach that uses geotagged photographs to identify popular locations, and a gravity model to estimate the volume of visitor flows from access points (e.g. parking lots, bus stops) to those locations. The model, which assumes volumes to be proportional to the popularity of access points and destinations, and inversely proportional to the travel time for going from one to the other, is set up in a GIS environment and calibrated by means of actual visitor counts over a set of locations in the study area.Results of a first application to the Dolomites UNESCO World Heritage Site (Italy) are encouraging as the model provided good estimates and a consistent map of visitor flows. The method enables park managers to roughly estimate visitor flows over large trail networks with only limited field work required. Future applications may include the analysis of off-trail movements to predict the intensity of trampling.