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At least 19 recordsLinked to original sources
PubMed1994

Negative gravitropism in Chara protonemata: a model integrating the opposite gravitropic responses of protonemata and rhizoids.

The unicellular protonema of Chara fragilis Desv. was investigated in order to establish a reaction chain for negative gravitropism in tip-growing cells. The time course of gravitropic bending after stimulation at angles of 45 degrees or 90 degrees showed three distinct phases of graviresponse. During the first hour after onset of stimulation a strong upward shift of the tip took place. This initial response was followed by an interval of almost straight growth. Complete reorientation was achieved in a third phase with very low bending rates. Gravitropic reorientation could be completely abolished by basipetal centrifugation of the cells, which lastingly removed conspicuous dark organelles from the protonema tip, thus identifying them as statoliths. Within minutes after onset of gravistimulation most or all statoliths were transported acropetally from their resting position 20-100 micrometers from the cell apex to the lower side of the apical dome. This transport is actin-dependent since it could be inhibited with cytochalasin B. Within minutes after arrival of the statoliths, the apical dome flattened on its lower side and bulged on the upper one. After this massive initial response the statoliths remained firmly sedimented, but the distance between this sedimented complex and the cell vertex increased from 7 micrometers to 22 micrometers during the first hour of stimulation and bending rates sharply declined. From this it is concluded that only statoliths inside the apical dome convey information about the spatial orientation of the cell in the gravitropic reaction chain. After inversion of the protonema the statoliths transiently arranged into a disk-shaped complex about 8 micrometers above the vertex. When this statolith complex tilted towards one side of the apical dome, growth was shifted in the opposite direction and bending started. It is argued that the statoliths intruding into the apical dome may displace a growth-organizing structure from its symmetrical position in the apex and may thus cause bending by bulging. In the positively gravitropic Chara rhizoids only a more stable anchorage of the growth-organizing structure is required. As a consequence, sedimented statoliths cannot dislocate this structure from the vertex. Instead they obstruct a symmetrical distribution of cell-wall-forming vesicles around the structure and thus cause bending by bowing.

Actin Cytoskeleton↗
PubMed1995

Gravitropic response in Eichhornia cressipes (Water Hyacinth) 1. Process of gravitropic bending in the peduncle.

In the last half of the anthokinetic cycle in Eichhornia cressipes the peduncle showed a two-step bending response which started after the completion of the flowering: the primary bending in the upper region and the second one in the base of the peduncle. In the present study, only the primary bending response was examined and the following results were obtained. (1) Under the condition in the Western Japan, the anthokinetic cycle completed in 36-40 hr. The inflorescence started to grow during night, reaching the full flowering stage next morning, and the bending was initiated in the evening and completed next morning. (2) The bending was a positive gravitropism since the peduncle did not show bending when it was placed on a horizontal clinostat rotating at 2-3 rpm, or when the plant was placed up-side-down. (3) Before the end of flowering phase, the peduncle showed a normal negative gravitropic response but afterward it acquired the property to show a positive gravitropic response. (4) The bending was due to the extension of convex side of the peduncle, accompanied by a shrinkage of the concave side. The extension of the upper side was caused by cell extension. At the turning point from negative to positive gravitropic response, the extension of the peduncle ceased for several hours. From the above results it is concluded that the primary bending response of the peduncle in water hyacinth was a positive gravitropism.

Biomechanical Phenomena↗
PubMed2003

Touch and gravitropic set-point angle interact to modulate gravitropic growth in roots.

Plant roots must sense and respond to a variety of environmental stimuli as they grow through the soil. Touch and gravity represent two of the mechanical signals that roots must integrate to elicit the appropriate root growth patterns and root system architecture. Obstacles such as rocks will impede the general downwardly directed gravitropic growth of the root system and so these soil features must be sensed and this information processed for an appropriate alteration in gravitropic growth to allow the root to avoid the obstruction. We show that primary and lateral roots of Arabidopsis do appear to sense and respond to mechanical barriers placed in their path of growth in a qualitatively similar fashion. Both types of roots exhibited a differential growth response upon contacting the obstacle that directed the main axis of elongation parallel to the barrier. This growth habit was maintained until the obstacle was circumvented, at which point normal gravitropic growth was resumed. Thus, the gravitational set-point angle of the primary and lateral roots prior to encountering the barrier were 95 degrees and 136 degrees respectively and after growing off the end of the obstacle identical set-point angles were reinstated. However, whilst tracking across the barrier, quantitative differences in response were observed between these two classes of roots. The root tip of the primary root maintained an angle of 136 degrees to the horizontal as it traversed the barrier whereas the lateral roots adopted an angle of 154 degrees. Thus, this root tip angle appeared dependent on the gravitropic set-point angle of the root type with the difference in tracking angle quantitatively reflecting differences in initial set-point angle. Concave and convex barriers were also used to analyze the response of the root to tracking along a continuously varying surface. The roots maintained the a fairly fixed angle to gravity on the curved surface implying a constant resetting of this tip angle/tracking response as the curve of the surface changed. We propose that the interaction of touch and gravity sensing/response systems combine to strictly control the tropic growth of the root. Such signal integration is likely a critical part of growth control in the stimulus-rich environment of the soil.

Arabidopsis↗
PubMed1995

Gravitropic responses of the Avena coleoptile in space and on clinostats. I. Gravitropic response thresholds.

We conducted a series of gravitropic experiments on Avena coleoptiles in the weightlessness environment of Spacelab. The purpose was to test the threshold stimulus, reciprocity rule and autotropic reactions to a range of g-force stimulations of different intensities and durations The tests avoided the potentially complicating effects of earth's gravity and the interference from clinostat ambiguities. Using slow-speed centrifuges, coleoptiles received transversal accelerations in the hypogravity range between 0.l and 1.0 g over periods that ranged from 2 to 130 min. All responses that occurred in weightlessness were compared to clinostat experiments on earth using the same apparatus. Characteristic gravitropistic response patterns of Atuena were not substantially different from those observed in ground-based experiments. Gravitropic presentation times were extrapolated. The threshold at 1.0 g was less than 1 min (shortest stimulation time 2 min), in agreement with values obtained on the ground. The least stimulus tested, 0.1 g for 130 min, produced a significant response. Therefore the absolute threshold for a gravitropic response is less than 0.1 g.

Acceleration↗
PubMed2004

Control of gravitropic orientation. II. Dual receptor model for gravitropism.

Gravitropism of vascular plants has been assumed to require a single gravity receptor mechanism. However, based on the evidence in Part I of this study, we propose that maize roots require two. The first mechanism is without a directional effect and, by itself, cannot give rise to tropism. Its role is quantitative facilitation of the second mechanism, which is directional like the gravitational force itself and provides the impetus for tropic curvature. How closely coupled the two mechanisms may be is, as yet, unclear. The evidence for dual receptors supports a general model for roots. When readiness for gravifacilitation, or gravifacilitation itself, is constitutive, orthogravitropic curvature can go to completion. If not constitutively enabled, gravifacilitation can be weak in the absence of light and water deficit or strong in the presence of light and water deficit. In either case, it can decay and permit roots to assume reproducible non-vertical orientations (plagiogravitropic or plagiotropic orientations) without using non-vertical setpoints. In this way roots are deployed in a large volume of soil. Gravitropic behaviours in shoots are more diverse than in roots, utilising oblique and horizontal as well as vertical setpoints. As a guide to future experiments, we assess how constitutive v. non-constitutive modes of gravifacilitation might contribute to behaviours based on each kind of setpoint.

Abscisic Acid↗
PubMed1984

Gravitropism in Higher Plant Shoots : III. Cell Dimensions during Gravitropic Bending; Perception of Gravity.

Cross and longitudinal sections were prepared for light microscopy from vertical control plants (Xanthium strumarium L. Chicago strain), free-bending horizontal stems, plants restrained 48 hours in a horizontal position, and plants restrained 48 hours and then released, bending immediately about 130 degrees . Top cells of free-bending stems shrink or elongate little; bottom cells continue to elongate. In restrained stems, bottom cells elongate some and increase in diameter; top cells elongate about as much but decrease in diameter. Upon release, bottom cells elongate more and decrease in diameter, while top cells shorten and increase in diameter, accounting for the bend. During restraint, bottom cells take up water while tissue pressures increase; top cells fail to take up water although tissue pressures are decreasing.Settling of amyloplasts was observed in cells of the starch sheath.Removal of different amounts of stem (Xanthium; Lycopersicon esculentum Miller, cv Bonny Best; Ricinus communis L. cv Yolo Wonder) showed that perception of gravity occurs in the bending (elongation) zone, although bending of fourth and fifth internodes from the top was less than in uncut controls. Uniform application of 1% indoleacetic acid in lanolin to cut stem surfaces partially restored bending. Reversing the gradient in tension/compression in horizontal stems (top under compression, bottom under tension) did not affect gravitropic bending.

Journal Article↗
PubMed1997

The RHG gene is involved in root and hypocotyl gravitropism in Arabidopsis thaliana.

In higher plants, shoots show negative gravitropism and roots show positive gravitropism. To elucidate the molecular mechanisms of root and hypocotyl gravitropism, we segregated the second mutation from the original phyB-1 mutant line which impaired both root and hypocotyl gravitropism and characterized this novel mutation named rhg (for root and hypocotyl gravitropism). The rhg is a single recessive nuclear mutation and it is mapped on the lower part of the chromosome 1. Analyses on the gravitropic responses of the rhg mutant indicate that root and hypocotyl gravitropism are severely impaired but inflorescence stem gravitropism is not affected by the rhg mutation. In the rhg mutant seedlings, amyloplasts (statoliths for gravity-perception) were present in the presumptive statocytes of roots and hypocotyls. Phototropism by roots and hypocotyls was not impaired in the rhg mutant. These results suggest that the RHG gene product probably acts on the gravity-perception and/or the gravity-signal transduction in root and hypocotyl gravitropism. This is the first report about the genetic locus specifically involved in both root and hypocotyl gravitropism but not inflorescence stem gravitropism, supporting our hypothesis that the mechanisms of gravitropism are genetically different between hypocotyls and inflorescence stems.

Arabidopsis↗
PubMed1982

Physiology of Movements in Stems of Seedling Pisum sativum L. cv. Alaska : I. Experimental Separation of Nutation from Gravitropism.

Gravitropism and nutation in the stems of dark-grown, seedling peas (Pisum sativum L. cv. Alaska) were recorded on time-lapse photographs made with photomorphogenetically inactive light. Although gravitropism and nutation have been connected by several different theories in the past, our experiments indicate that the two processes are in fact dissociable. The evidence is as follows: (a) Nutational patterns are asymmetric. There is much greater amplitude of oscillation in the plane parallel ( parallel) to the plane of the apical hook than in the plane perpendicular ( perpendicular), yet the average gravitropic response is equal in these two planes. (b) Brief red light irradiation given 16 to 24 hours before observation greatly increases the amplitude of nutation in the parallel-plane, but has no influence on the kinetics of gravitropic response. (c) An inhibitor of auxin transport, alpha-naphthylphthalamic acid, strongly inhibits nutation at 5 micromolar but affects gravitropism only at higher concentrations. (d) Nutation is also strongly inhibited by removal of the apical bud, but gravitropism is unaffected. (e) The period of nutation does not exhibit a constant relationship to the response time of gravitropism. The above evidence is inconsistent with theories that gravitropism is an asymmetrically modified nutation or, alternatively, that nutational oscillations result in a simple fashion from gravitropic overshoots. The evidence is consistent with, although not proof of, autonomous factors such as an endogenous rhythm of growth as the cause of nutation in pea stems. However, gravity and nutation do interact. Nutation in a population of seedlings can be synchronized and brought into phase by a single gravitropic induction. Furthermore, the response time and initial rate of gravitropic curvature depend to some extent on the phase of nutational curvature at which gravitropic induction is begun.

Journal Article↗
PubMed1996

SGR1, SGR2, SGR3: novel genetic loci involved in shoot gravitropism in Arabidopsis thaliana.

In higher plants shoots show a negative gravitropic response but little is known about its mechanism. To elucidate this phenomenon, we have isolated a number of mutants with abnormal shoot gravitropic responses in Arabidopsis thaliana. Here we describe mainly three mutants: sgr1-1, sgr2-1, and sgr3-1 (shoot gravitropism). Genetic analysis confirmed that these mutations were recessive and occurred at three independent loci, named SGR1, SGR2, and SGR3, respectively. In wild type, both inflorescence stems and hypocotyls show negative gravitropic responses. The sgr1-1 mutants showed no response to gravity either by inflorescence stems or by hypocotyls. The sgr2-1 mutants also showed no gravitropic response in inflorescence stems but showed a reduced gravitropic response in hypocotyls. In contrast, the sgr3-1 mutant was found to have reduced gravitropic responses in inflorescence stems but normal gravitropic responses in hypocotyls. These results suggest that some genetic components of the regulatory mechanisms for gravitropic responses are common between inflorescence stems and hypocotyls, but others are not. In addition, these sgr mutants were normal with respect to root gravitropism, and their inflorescence stems and hypocotyls could carry out phototropism. We conclude that SGR1, SGR2, and SGR3 are novel genetic loci specifically involved in the regulatory mechanisms of shoot gravitropism in A. thaliana.

Arabidopsis↗
PubMed1998

Blue light- and genetically-reversed gravitropic response in protonemata of the moss Ceratodon purpureus.

In darkness, protonemal filaments of Ceratodon purpureus (Brid.) grow negatively gravitropically (upwards). Red light induces a positive phototropic response mediated by the photoreceptor phytochrome. A red light treatment also has an inhibitory effect on the gravitropic response, an effect also mediated by phytochrome. In this study the effects of blue light on phototropism and on gravitropism were analysed. Unilateral blue light resulted in only a weak phototropic response, but markedly randomised growth direction. Blue light given together with a gravitropic stimulus reversed the gravitropism, changing it from negative to positive (filaments grow downward). The effect of blue light was also analysed with the mutant ptr116, which is defective in the biosynthesis of the phytochrome chromophore, and in a newly isolated mutant wwr2, which is positively gravitropic in darkness. Blue light induced the same reversal of gravitropism in ptrll6 as in the wild type, indicating that phytochrome is not involved in this process. In wwr2 the direction of gravitropism was unaltered by the blue light treatment. Light also affects chlorophyll content and the size of plastids, potential statoliths for gravitropism. Red light induced an increase in plastid size and chlorophyll content in the wild type but not in ptr116. Blue light induced a similar change in wild type plastids. It seems as though light-induced alterations of gravitropism are not simply mediated by alterations in plastid properties, and that red light and blue light evoke fundamentally different responses.

Bryopsida↗
PubMed2006

Circumnutation of rice coleoptiles: its relationships with gravitropism and absence in lazy mutants.

Although circumnutation occurs widely in higher plants, its mechanism is little understood. The idea that circumnutation is based on gravitropism has long been investigated, but the reported results have been controversial. We used dark-grown coleoptiles of rice (Oryza sativa L.) to re-investigate this issue. The following results supported the existence of a close relationship between gravitropism and circumnutation: (1) circumnutation disappears on a horizontal clinostat; (2) circumnutation is interrupted by a gravitropic response and re-initiated at a definable phase after gravitropic curvature; (3) circumnutation can be re-established by submergence and a brief gravitropic stimulation in the coleoptiles that have stopped nutating in response to red light; and (4) lazy mutants show no circumnutation. In spite of these results, however, there were cases in which gravitropism and circumnutation could be separated. Firstly, the non-circumnutating lazy coleoptile showed nearly a wild-type level of gravitropic responsiveness in its upper half, although this part was an active site of both gravitropism and circumnutation in wild-type coleoptiles. Secondly, coleoptiles could nutate without overshooting the vertical when developing phototropic curvature. It is concluded that gravitropism influences, but it is not directly involved in the process of circumnutation. It is further suggested that a gravity signal, shared with gravitropism, contributes to the maintenance of circumnutation.

Genes, Plant↗
PubMed1997

Molecular genetic analysis of plant gravitropism.

The analysis of mutants is a powerful approach for elucidating the components of complex biological processes. A growing number of mutants have been isolated which affect plant gravitropism and the classes of mutants found thus far provide important information about the gravity response mechanism. The wide variety of mutants isolated, especially in Arabidopsis, indicates that gravitropism is a complex, multi-step process. The existence of mutants altered in either root gravitropism alone, shoot gravitropism alone, or both indicates that the root and shoot gravitropic mechanisms have both separate and common steps. Reduced starch mutants have confirmed the role of amyloplasts in sensing the gravity signal. The hormone auxin is thought to act as the transducing signal between the sites of gravity perception (the starch parenchyma cells surrounding the vascular tissue in shoots and the columella cells of root caps) and asymmetric growth (the epidermal cells of the elongation zone(s) of each organ). To date, all mutants that are resistant to high concentrations of auxin have also been found to exhibit a reduced gravitropic response, thus supporting the role of auxin. Not all gravitropic mutants are auxin-resistant, however, indicating that there are additional steps which do not involve auxin. Studies with mutants of tomato which exhibit either reduced or reversed gravitropic responses further support the role of auxin redistribution in gravitropism and suggest that both red light and cytokinin interact with gravitropism through controlling lateral auxin transport. Plant responses to gravity thus likely involve changes in both auxin transport and sensitivity.

Arabidopsis↗
PubMed1997

Solving the puzzle of gravitropism--has a lost piece been found?

Attempts to devise models to account for the gravitropic behaviour of plant organs have been limited conceptually by the predominance of studies on the gravitropic behaviour of organs of young seedlings. The dramatic growth responses induced by gravitropic stimulation of young shoots or roots, which rapidly restore the elongating axes to vertical, are experimentally convenient but theoretically limiting because gravitropism needs not simply restore an organ to vertical. Evidence is reviewed that suggests that plant organs must possess a mechanism which will allow them to attain a stable gravitropic position at any angle and that each organ has a characteristic gravitropic set-point angle (GSA). The GSA can be changed developmentally and is also regulated in a reversible manner by environmental parameters such as light. It is speculated that gravity may itself influence the GSA of an organ. The recognition that plant organs can grow at angles other than vertically up or down is not new, but previously it has been accepted that angles other than vertical were the consequence of the vectorial resultant effect of two different, opposing mechanisms. The new GSA concept proposes that a single mechanism might be sufficient to account for all forms of gravitropism in roots and shoots. This unifying concept proposes that the ability to change the angle of an organ with respect to the vertical is part of the basic gravitropic mechanism and that models of gravitropism must be able to account for this important feature.

Gravitropism↗
PubMed2001

Gravitropic microtubule reorientation can be uncoupled from growth.

The causal relationship between gravitropic growth responses and microtubule reorientation has been studied. Growth and microtubule reorientation have been uncoupled during the gravitropic response of maize (Zea mays L.) coleoptiles. Microtubule orientation and growth were measured under three different conditions: (i) a gravitropic stimulation where the growth response was allowed to be expressed (intact seedlings were displaced from the vertical position by 90 degrees), (ii) a gravitropic stimulation where the growth response was suppressed (coleoptiles were attached to microscope slides and kept in a horizontal position), (iii) suppression of growth in the absence of gravitropic stimulation (coleoptiles were attached to microscope slides and kept in a vertical position). It was found that (i) gravitropic stimulation can induce a microtubular reorientation from transverse to longitudinal in the upper (slower growing) flank of the coleoptile, and an inhibition of growth; (ii) the reorientation of microtubules precedes the inhibition of growth; (iii) the gravitropic response of microtubules is weaker, not elevated, when the inhibition of growth is artificially enhanced by attaching the coleoptiles to a slide; and (iv) artificial inhibition of growth in the absence of gravitropic stimulation cannot induce a microtubular response. Thus, the extent of microtubule reorientation is not correlated with the extent of growth inhibition. Moreover, these findings demonstrate that microtubules do not reorient passively after growth changes, but actively in response to gravitropic stimulation.

Gravitropism↗
PubMed1986

Gravitropic responses of wild-type and mutant strains of the moss Physcomitrella patens.

The gravitropic responses of dark-grown caulonemata and gametophores of wild-type and mutant strains of the moss Physcomitrella patens have been investigated. In the wild-type both caulonemata and gametophores show negative orthogravitropism. No gravitropic response is observed when plants are rotated slowly on a clinostat and the inductive effect of gravity can be replaced by centrifugal force. The gravitropic response of caulonemanta is biphasic, consisting of an initial phase producing a bend of about 20 degrees within 12 h of 90 degrees reorientation and a subsequent slower phase leading to completion of the 90 degrees curvature. No obvious sedimentation of statoliths accompanies this response. Several mutants have been isolated that are either partially or completely impaired in caulonemal gravitropism and one mutant shows a positive gravitropic response. Complementation analysis using somatic hybrids obtained following protoplast fusion indicates that at least three genes can mutate to give an altered gravitropic phenotype. None of these mutants is altered in gametophore gravitropism, suggesting that the gravitropic response of caulonemal filaments may require at least some gene products that are not required for the response of the multicellular gametophores. One class of mutant with impaired caulonemal gravitropism shows a pleiotropic alteration in leaf shape.

Bryopsida↗
PubMed1995

The gravitropic set-point angle (GSA): the identification of an important developmentally controlled variable governing plant architecture.

The angle at which an organ is maintained by gravitropism is characteristic of the organ, its developmental state and the prevailing environmental conditions. We propose that this angle be called the gravitropic set-point angle (GSA), defined as the angle with respect to the gravity vector (with a vertically downward orientation being 0 degrees) at which an organ is maintained as a consequence of gravitropism. Studies or the gravitropic behaviour of organs from trailing plants show that the GSA is subject to developmental regulation. Depending on the developmental age and prevailing environmental conditions, the GSA of an organ can be set at any value between 0 degrees and 180 degrees. The previously reported reversal of the sign of the gravitropic response in such organs, whether this is brought about developmentally or induced by light, represents the change from one common extreme (GSA = 180 degrees, conventionally referred to as negative orthogravitropism) to another (GSA = 0 degrees, or positive orthogravitropism). The concept of a variable gravitropic set-point offers a more unified view of all forms of gravitropic behaviour than has been advanced previously, and places a new constraint on models of gravitropism. Current models of gravitropism appear to be unable to explain either the ability of organs to change their orientation with respect to gravity as they develop, or the re-orientation that can be observed when some organs are exposed to new environmental conditions.

Darkness↗
PubMed1998

Hypergravity can reduce but not enhance the gravitropic response of Chara globularis protonemata.

The relationship between the position of the statoliths and the direction and rate of tip growth in negatively gravitropic protonemata of Chara globularis was studied with a centrifuge video microscope. Cells placed perpendicularly to the acceleration vector (stimulation angle 90 degrees) showed a gradual reduction of the gravitropic curvature with increasing accelerations from 1 g to 8 g despite complete sedimentation of all statoliths on the centrifugal cell flank. It is argued that the increased weight of the statoliths in hypergravity impairs their acropetal transport which is induced when the cell axis deviates from the normal upright orientation. When the statoliths were centrifuged deep into the apical dome at 6 g and a stimulation angle of 170 degrees the gravitropic curvature after 1 h was identical to that determined for the same cells at 1 g and the same stimulation angle. This indicates that gravitropism in Chara protonemata is either independent of the pressure exerted by the statoliths on an underlying structure or is already saturated at 1 g. When the statoliths were moved along the apical cell wall at 8 g and the stimulation angle was gradually increased from 170 degrees to 220 degrees the gravitropic curvature reverted sharply when the cluster of statoliths passed over the cell pole. This experiment supports the hypothesis that in Chara protonemata asymmetrically distributed statoliths inside the apical dome displace the Spitzenkorper and thus the centre of growth, resulting in gravitropic bending. In contrast to the positively gravitropic Chara rhizoids, no modifications either in the transport of statoliths during basipetal acceleration (6 g, stimulation angle 0 degree, 5 h) or in the subsequent gravitropic response could be detected in the protonemata. The different effects of centrifugation on the positioning of statoliths in Chara protonemata and rhizoids indicate subtle differences in the function of the cytoskeleton in both types of cells.

Centrifugation↗
PubMed2002

The kinetics of root gravitropism: dual motors and sensors.

The Cholodny-Went theory of tropisms has served as a framework for investigation of root gravitropism for nearly three quarters of a century. Recent investigations using modern techniques have generated findings consistent with the classical theory, including confirmation of asymmetrical distribution of polar auxin transport carriers, molecular evidence for auxin asymmetry following gravistimulation, and generation of auxin response mutants with predictable lesions in gravitropism. Other results indicate that the classical model is inadequate to account for key features of root gravitropism. Initiation of curvature, for example, occurs outside the region of most rapid elongation and is driven by differential acceleration rather than differential inhibition of elongation. The evidence indicates that there are two motors driving root gravitropism, one of which appears not to be auxin regulated. We have recently developed technology that is capable of maintaining a constant angle of gravistimulation at any selected target region of a root while continuously monitoring growth and curvature kinetics. This review elaborates on the advantages of this new technology for analyzing gravitropism and describes applications of the technology that reveal (1) the existence of at least two phases to gravitropic motor output, even under conditions of constant stimulus input and (2) the existence of gravity sensing outside of the root cap. We propose a revised model of root gravitropism including dual sensors and dual motors interacting to accomplish root gravitropism, with only one of the systems linked to the classical Cholodny-Went theory.

Centrifugation↗