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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

Redistribution of annexin in gravistimulated pea plumules

We used immunocytochemistry to investigate the effects of gravistimulation on annexin localization in etiolated pea plumule shoots. In longitudinal sections, an asymmetric annexin immunostaining pattern was observed in a defined group of cells located just basipetal to apical meristems at the main shoot apex and at all of the axillary buds, an area classically referred to as the leaf gap. The pattern was observed using both protein-A-purified anti-annexin and affinity-purified anti-annexin antibodies for the immunostaining. A subset of the cells with the annexin staining also showed an unusually high level of periodic acid Schiff (PAS) staining in their cell walls. Prior to gravistimulation, the highest concentration of annexin was oriented toward the direction of gravity along the apical end of these immunostained cells. In contrast, both at 15 and 30 min after gravistimulation, the annexin immunostain became more evenly distributed all around the cell and more distinctly cell peripheral. The asymmetry along the lower wall of these cells was no longer evident. In accord with current models of annexin action, we interpret the results to indicate that annexin-mediated secretion in the leaf gap area is preferentially toward the apical meristem prior to gravistimulation, and that gravistimulation results in a redirection of this secretion. These data are to our knowledge the first to show a correlation between the vector of gravity and the distribution of annexins in the cells of flowering plants. c 2000 Editions scientifiques et medicales Elsevier SAS.

Non-NASA Center↗
NASA NTRS2000

Two distinct regions of response drive differential growth in Vigna root electrotropism

Although exogenous electric fields have been reported to influence the orientation of plant root growth, reports of the ultimate direction of differential growth have been contradictory. Using a high-resolution image analysis approach, the kinetics of electrotropic curvature in Vigna mungo L. roots were investigated. It was found that curvature occurred in the same root toward both the anode and cathode. However, these two responses occurred in two different regions of the root, the central elongation zone (CEZ) and distal elongation zone (DEZ), respectively. These oppositely directed responses could be reproduced individually by a localized electric field application to the region of response. This indicates that both are true responses to the electric field, rather than one being a secondary response to an induced gravitropic stimulation. The individual responses differed in the type of differential growth giving rise to curvature. In the CEZ, curvature was driven by inhibition of elongation, whereas curvature in the DEZ was primarily due to stimulation of elongation. This stimulation of elongation is consistent with the growth response of the DEZ to other environmental stimuli.

Non-NASA Center↗
NASA NTRS2001

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

In maize (Zea mays) and other grasses, changes in orientation of stems are perceived by pulvinal tissue, which responds to the stimulus by differential growth resulting in upward bending of the stem. The amyloplast-containing bundle sheath cells are the sites of gravity perception, although the initial steps of gravity perception and transmission remain unclear. In columella cells of Arabidopsis roots, we previously found that cytoplasmic pH (pH(c)) is a mediator in early gravitropic signaling (A.C. Scott, N.S. Allen [1999] Plant Physiol 121: 1291-1298). The question arises whether pH(c) has a more general role in signaling gravity vector changes. Using confocal ratiometric imaging and the fluorescent pH indicator carboxy seminaphtorhodafluor acetoxymethyl ester acetate, we measured pH(c) in the cells composing the maize pulvinus. When stem slices were gravistimulated and imaged on a horizontally mounted confocal microscope, pH(c) changes were only apparent within the bundle sheath cells, and not in the parenchyma cells. After turning, cytoplasmic acidification was observed at the sides of the cells, whereas the cytoplasm at the base of the cells where plastids slowly accumulated became more basic. These changes were most apparent in cells exhibiting net amyloplast sedimentation. Parenchyma cells and isolated bundle sheath cells did not show any gravity-induced pH(c) changes although all cell types responded to external stimuli in the predicted way: Propionic acid and auxin treatments induced acidification, whereas raising the external pH caused alkalinization. The results suggest that pH(c) has an important role in the early signaling pathways of maize stem gravitropism.

Non-NASA Center↗
NASA NTRS2000

Columella cells revisited: novel structures, novel properties, and a novel gravisensing model

A hundred years of research has not produced a clear understanding of the mechanism that transduces the energy associated with the sedimentation of starch-filled amyloplast statoliths in root cap columella cells into a growth response. Most models postulate that the statoliths interact with microfilaments (MF) to transmit signals to the plasma membrane (or ER), or that sedimentation onto these organelles produces the signals. However, no direct evidence for statolith-MF links has been reported, and no asymmetric structures of columella cells have been identified that might explain how a root turned by 90 degrees knows which side is up. To address these and other questions, we have (1) quantitatively examined the effects of microgravity on the size, number, and spatial distribution of statoliths; (2) re-evaluated the ultrastructure of columella cells in high-pressure frozen/freeze-substituted roots; and (3) followed the sedimentation dynamics of statolith movements in reoriented root tips. The findings have led to the formulation of a new model for the gravity-sensing apparatus of roots, which envisages the cytoplasm pervaded by an actin-based cytoskeletal network. This network is denser in the ER-devoid central region of the cell than in the ER-rich cell cortex and is coupled to receptors in the plasma membrane. Statolith sedimentation is postulated to disrupt the network and its links to receptors in some regions of the cell cortex, while allowing them to reform in other regions and thereby produce a directional signal.

NASA Discipline Cell Biology↗
NASA NTRS1999

Early root cap development and graviresponse in white clover (Trifolium repens) grown in space and on a two-axis clinostat

White clover (Trifolium repens) was germinated and grown in microgravity aboard the Space Shuttle (STS-60, 1994; STS-63, 1995), on Earth in stationary racks and in a slow-rotating two-axis clinostat. The objective of this study was to determine if normal root cap development and early plant gravity responses were dependent on gravitational cues. Seedlings were germinated in space and chemically fixed in orbit after 21, 40, and 72 h. Seedlings 96 h old were returned viable to earth. Germination and total seedling length were not dependent on gravity treatment. In space-flown seedlings, the number of cell stories in the root cap and the geometry of central columella cells did not differ from those of the Earth-grown seedlings. The root cap structure of clinorotated plants appeared similar to that of seedlings from microgravity, with the exception of three-day rotated plants, which displayed significant cellular damage in the columella region. Nuclear polarity did not depend on gravity; however, the positions of amyloplasts in the central columella cells were dependent on both the gravity treatment and the age of the seedlings. Seedlings from space, returned viable to earth, responded to horizontal stimulation as did 1 g controls, but seedlings rotated on the clinostat for the same duration had a reduced curvature response. This study demonstrates that initial root cap development is insensitive to either chronic clinorotation or microgravity. Soon after differentiation, however, clinorotation leads to loss of normal root cap structure and plant graviresponse while microgravity does not.

manned↗
NASA NTRS1997

Stereological analysis of gravitropism in protonemata of the moss Ceratodon

Apical cells of dark-grown protonemata of the moss Cerotodon purpureus are negatively gravitropic. Previous light microscopy has shown that reorientation to the horizontal induces amyloplast sedimentation and redistribution of microtubules. To determine whether other components become redistributed laterally or axially, the apical 35 micrometers of both vertical and horizontal apical cells were compared stereologically using transmission electron microscopy. Reorientation to the horizontal changed the longitudinal distributions of tubular ER, Golgi stacks, and vesicles but not cisternal ER, mitochondria, and plastids. Only plastids showed a statistically significant lateral redistribution after horizontal placement. Qualitative examination of the sedimentation zone showed plastids sedimented close to peripherally located ER with vacuoles displaced above plastids. These results argue against a model where differential tip growth results from a redistribution of Golgi stacks or exocytic vesicles.

Non-NASA Center↗
NASA NTRS1999

Gravitropism of inflorescence stems in starch-deficient mutants of Arabidopsis

Previous studies have assayed the gravitropic response of roots and hypocotyls of wild type Arabidopsis thaliana, two reduced-starch strains, and a starchless strain. Because there have been few reports on inflorescence gravitropism, in this article, we use microscopic analyses and time-course studies of these mutants and their wild type to study gravitropism in these stems. Sedimentation of plastids was observed in endodermal cells of the wild type and reduced-starch mutants but not in the starchless mutant. In all of these strains, the short inflorescence stems (1.0-2.9 cm) were less responsive to the gravistimulus compared with the long stems (3.0-6.0 cm). In both long and short inflorescence stems, the wild type initially had the greatest response; the starchless mutant had the least response; and the reduced starch mutants exhibited an intermediate response. Furthermore, growth rates among all four strains were approximately equal. At about 6 h after reorientation, inflorescences of all strains returned to a position parallel to the gravity vector. Thus, in inflorescence stems, sedimentation of plastids may act as an accelerator but is not required to elicit a gravitropic response. Furthermore, the site of perception appears to be diffuse throughout the inflorescence stem. These results are consistent with both a plastid-based statolith model and the protoplast pressure hypothesis, and it is possible that multiple systems for gravity perception occur in plant cells.

Non-NASA Center↗
NASA NTRS1998

Growth dynamics and cytoskeleton organization during stem maturation and gravity-induced stem bending in Zea mays L

Characterization of gravitropic bending in the maize stem pulvinus, a tissue that functions specifically in gravity responses, demonstrates that the pulvinus is an ideal system for studying gravitropism. Gravistimulation during the second of three developmental phases of the pulvinus induces a gradient of cell elongation across the non-growing cells of the pulvinus, with the most elongation occurring on the lower side. This cell elongation is spatially and temporally separated from normal internodal cell elongation. The three characterized growth phases in the pulvinus correspond closely to a specialized developmental sequence in which structural features typical of cells not fully matured are retained while cell maturation occurs in surrounding internodal and nodal tissue. For example, the lignification of supporting tissue and rearrangement of transverse microtubules to oblique that occur in the internode when cell elongation ceases are delayed for up to 10 d in the adjacent cells of the pulvinus, and only occurs as a pulvinus loses its capacity to respond to gravistimulation. Gravistimulation does not modify this developmental sequence. Neither wall lignification nor rearrangement of transverse microtubules occurs in the rapidly elongating lower side or non-responsive upper side of the pulvinus until the pulvinus loses the capacity to bend further. Gravistimulation does, however, lead to the formation of putative pit fields within the expanding cells of the pulvinus.

Non-NASA Center↗
NASA NTRS1998

The development of spaceflight experiments with Arabidopsis as a model system in gravitropism studies

Experiments with Arabidopsis have been developed for spaceflight studies in the European Space Agency's Biorack module. The Biorack is a multiuser facility that is flown on the United States Space Shuttle and serves as a small laboratory for studying cell and developmental biology in unicells, plants, and small invertebrates. The purpose of our spaceflight research was to investigate the starch-statolith model for gravity perception by studying wild-type (WT) and three starch-deficient mutants of Arabidopsis. Since spaceflight opportunities for biological experimentation are scarce, the extensive ground-based testing described in this paper is needed to ensure the success of a flight project. Therefore, the specific aims of our ground-based research were: (1) to modify the internal configuration of the flight hardware, which originally was designed for large lentil seeds, to accommodate small Arabidopsis seeds; (2) to maximize seed germination in the hardware; and (3) to develop favorable conditions in flight hardware for the growth and gravitropism of seedlings. The hardware has been modified, and growth conditions for Arabidopsis have been optimized. These experiments were successfully flown on two Space Shuttle missions in 1997.

STS-84 Shuttle Project↗
NASA NTRS1998

Gravitropism in caulonemata of the moss Pottia intermedia

The gravitropism of caulonemata of Pottia intermedia is described and compared with that of other mosses. Spore germination produces primary protonemata including caulonemata which give rise to buds that form the leafy moss plant, the gametophore. Primary caulonemata are negatively gravitropic but their growth and the number of filaments are limited in the dark. Axenic culture of gametophores results in the production of secondary caulonemata that usually arise near the leaf base. Secondary protonemata that form in the light are agravitropic. Secondary caulonemata that form when gametophores are placed in the dark for several days show strong negative gravitropism and grow well in the dark. When upright caulonemata are reorientated to the horizontal or are inverted, upward bending can be detected after 1 h and caulonemata reach the vertical within 1-2 d. Clear amyloplast sedimentation occurs 10-15 minutes after horizontal placement and before the start of upward curvature. This sedimentation takes place in a sub-apical zone. Amyloplast sedimentation also takes place along the length of upright and inverted Pottia protonemata. These results support the hypothesis that amyloplast sedimentation functions in gravitropic sensing since sedimentation occurs before gravitropism in Pottia and since the location and presence of a unique sedimentation zone is conserved in all four mosses known to gravitropic protonomata.

Non-NASA Center↗
NASA NTRS1989

The use of bioelectric currents to study gravity perception in roots

Inhibitor-based studies of gravity perception in plants have yielded ambiguous results because gravitropism consists of many steps in series. Inhibiting any step eliminates gravicurvature; it cannot be shown whether gravity perception and transduction are affected by the inhibitory treatment. The use of the vibrating probe has allowed detection of a response that is measurable independent of stimulus transmission and growth, two post-transduction steps of gravitropism. This application of the vibrating probe requires particular attention to artifacts. Calmodulin action and auxin transport are both believed to be components of gravitropism and specific roles have been proposed for them in graviperception. The perception-dependent current measured with the vibrating probe is eliminated by inhibitors of calmodulin, but not by auxin inhibitors. Thus, the vibrating probe has made it possible to offer more direct evidence for a role of calmodulin in transduction, and to reject one for auxin transport in perception and transduction.

Non-NASA Center↗
NASA NTRS1996

Random root movements in weightlessness

The dynamics of root growth was studied in weightlessness. In the absence of the gravitropic reference direction during weightlessness, root movements could be controlled by spontaneous growth processes, without any corrective growth induced by the gravitropic system. If truly random of nature, the bending behavior should follow so-called 'random walk' mathematics during weightlessness. Predictions from this hypothesis were critically tested. In a Spacelab ESA-experiment, denoted RANDOM and carried out during the IML-2 Shuttle flight in July 1994, the growth of garden cress (Lepidium sativum) roots was followed by time lapse photography at 1-h intervals. The growth pattern was recorded for about 20 h. Root growth was significantly smaller in weightlessness as compared to gravity (control) conditions. It was found that the roots performed spontaneous movements in weightlessness. The average direction of deviation of the plants consistently stayed equal to zero, despite these spontaneous movements. The average squared deviation increased linearly with time as predicted theoretically (but only for 8-10 h). Autocorrelation calculations showed that bendings of the roots, as determined from the 1-h photographs, were uncorrelated after about a 2-h interval. It is concluded that random processes play an important role in root growth. Predictions from a random walk hypothesis as to the growth dynamics could explain parts of the growth patterns recorded. This test of the hypothesis required microgravity conditions as provided for in a space experiment.

STS-65 Shuttle Project↗
NASA NTRS1989

Reduced gravitropic sensitivity in roots of a starch-deficient mutant of Nicotiana sylvestris

Gravitropism was studied in seedlings of Nicotiana sylvestris Speg. et Comes wild-type (WT) and mutant NS 458 which has a defective plastid phosphoglucomutase (EC 2.7.5.1.). Starch was greatly reduced in NS 458 compared to the WT, but small amounts of starch were detected in rootcap columella cells in NS 458 by light and electron microscopy. The roots of WT are more sensitive to gravity than mutant NS 458 roots since: (1) in mutant roots, curvature was reduced and delayed in the time course of curvature; (2) curvature of mutant roots was 24-56% that of WT roots over the range of induction periods tested; (3) in intermittent-stimulation experiments, curvature of mutant roots was 37% or less than that of WT roots in all treatments tested. The perception time, determined by intermittent-stimulation experiments, was < or = 5 s for WT roots and 30-60 s for mutant roots. The growth rates for WT and NS 458 roots were essentially equal. These results and our previous results with WT and starchless mutant Arabidopsis roots (Kiss et al. 1989, Planta 177, 198-206) support the conclusions that a full complement of starch is necessary for full gravitropic sensitivity and that amyloplasts function in gravity perception. Since a presumed relatively small increase in plastid buoyant mass (N. sylvestris mutant versus Arabidopsis mutant) significantly improves the orientation of the N. sylvestris mutant roots, we suggest that plastids are the likeliest candidates to be triggering gravity perception in roots of both mutants.

Non-NASA Center↗
NASA NTRS1987

USU research helps agriculture enter the space age

Research at the Utah State University College of Agriculture that is relevant to the space life sciences is reviewed. Specific programs detailed are gravitropism of dicot stems, maximization of wheat yields for use in space exploration, and plant development processes in wheat in microgravity.

Non-NASA Center↗
NASA NTRS1985

Abscisic acid, xanthoxin and violaxanthin in the caps of gravistimulated maize roots

The occurrence and distribution of abscisic acid (ABA), xanthoxin (Xa) and the carotenoid violaxanthin (Va) were investigated in root tips of maize (Zea mays L. cv. Merit). In roots grown in the dark, Va and ABA were present in relatively high amounts in the root cap and in low amounts in the adjacent terminal 1.5 mm of the root. Xanthoxin was present in equal concentrations in both regions. In roots exposed to light, the ABA distribution was reversed, with relatively low levels in the root cap and high levels in the adjacent 1.5-mm segment. Light also caused a decrease in Va in both regions of the root and an increase in Xa, especially in the cap. In the maize cultivar used for this work, light is necessary for gravitropic curving. This response occurs within the same time frame as the light-induced ABA redistribution as well as the changes in the levels of Va and Xa. These data are consistent with a role for ABA in root gravitropism and support the proposal that Xa may arise from the turnover of Va.

NASA Discipline Plant Biology↗
NASA NTRS1986

A morphometric analysis of the redistribution of organelles in columella cells of horizontally-oriented roots of Zea mays

In order to determine what structural changes in graviperceptive cells are associated with onset of root gravicurvature, the redistribution of organelles in columella cells of horizontally-oriented, graviresponding roots of Zea mays has been quantified. Root gravicurvature began by 15 min after reorientation, and did not involve significant changes in the (i) volume of individual columella cells or amyloplasts, (ii) relative volume of any cellular organelle, (iii) number of amyloplasts per columella cell, or (iv) surface area of cellular location of endoplasmic reticulum. Sedimentation of amyloplasts began within 1 to 2 min after reorientation, and was characterized by an intensely staining area of cytoplasm adjacent to the sedimenting amyloplasts. By 5 min after reorientation, amyloplasts were located in the lower distal corner of columella cells, and, by 15 min after reorientation, overlaid the entire length of the lower cell wall. No consistent contact between amyloplasts and any cellular structure was detected at any stage of gravicurvature. Centrally-located nuclei initially migrated upward in columella cells of horizontally-oriented roots, after which they moved to the proximal ends of the cells by 15 min after reorientation. No significant pattern of redistribution of vacuoles, mitochondria, dictyosomes, or hyaloplasm was detected that correlated with the onset of gravicurvature. These results indicate that amyloplasts and nuclei are the only organelles whose movements correlate positively with the onset of gravicurvature by primary roots of this cultivar of Zea mays.

NASA Discipline Plant Biology↗