A positively gravitropic mutant mirrors the wild-type protonemal response in the moss Ceratodon purpureus

Wild-type Ceratodon purpureus (Hedw.) Brid. protonemata grow up in the dark by negative gravitropism. When upright wild-type protonemata are reoriented 90°, they temporarily grow down soon after reorientation (“initial reversal”) and also prior to cytokinesis (“mitotic reversal”). A positively gravitropic mutant designated wrong-way response (wwr-1) has been isolated by screening ultraviolet light-mutagenized Ceratodon protonemata. Protonemata of wwr-1 reoriented from the vertical to the horizontal grow down with kinetics comparable to those of the wild-type. Protonemata of wwr-1 also show initial and mitotic reversals where they temporarily grow up. Thus, the direction of gravitropism, initial reversal, and mitotic reversal are coordinated though each are opposite in wwr-1 compared to the wild-type. Normal plastid zonation is still maintained in dark-grown wwr-1 apical cells, but the plastids are more numerous and plastid sedimentation is more pronounced. In addition, wwr-1 apical cells are wider and the tips greener than in the wild-type. These data suggest that a functional WWR gene product is not necessary for the establishment of some gravitropic polarity, for gravitropism, or for the coordination of the reversals. Thus, the WWR protein may normally transduce information about cell orientation. Received: 4 November 1996 / Accepted: 26 November 1996     

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.     

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 ptr116 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. Received: 11 July 1997 / Accepted: 30 January 1998     

Hypergravity can reduce but not enhance the gravitropic response ofChara globularis protonemata

Summary The relationship between the position of the statoliths and the direction and rate of tip growth in negatively gravitropic protonemata ofChara globularis was studied with a centrifuge video microscope. Cells placed perpendicularly to the acceleration vector (stimulation angle 90 °) showed a gradual reduction of the gravitropic curvature with increasing accelerations from 1g to 8g 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 6g and a stimulation angle of 170 ° the gravitropic curvature after 1 h was identical to that determined for the same cells at 1g 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 1g. When the statoliths were moved along the apical cell wall at 8g and the stimulation angle was gradually increased from 170 ° to 220 ° 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 Spitzenkörper 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 (6g, stimulation angle 0 °, 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.Dedicated to Prof. Dr. Zygmunt Hejnowicz on the occasion of his 70th birthday     

Curvature Induced by Amyloplast Magnetophoresis in Protonemata of the Moss Ceratodon purpureus

After gravistimulation of Ceratodon purpureus (Hedw.) Brid. protonemata in the dark, amyloplast sedimentation was followed by upward curvature in the wild-type (WT) and downward curvature in the wwr mutant (wrong way response). We used ponderomotive forces induced by high-gradient magnetic fields (HGMF) to simulate the effect of gravity and displace the presumptive statoliths. The field was applied by placing protonemata either between two permanent magnets at the edge of the gap, close to the edge of a magnetized ferromagnetic wedge, or close to a small (<1 mm) permanent magnet. Continuous application of an HGMF in all three configurations resulted in plastid displacement and induced curvature in tip cells of WT and wwr protonemata. WT cells curved toward the HGMF, and wwr cells curved away from the HGMF, comparable to gravitropism. Plastids isolated from protonemal cultures had densities ranging from 1.24 to 1.38 g cm⁻³. Plastid density was similar for both genotypes, but the mutant contained larger plastids than the WT. The size difference might explain the stronger response of the wwr protonemata to the HGMF. Our data support the plastid-based theory of gravitropic sensing and suggest that HGMF-induced ponderomotive forces can substitute for gravity.The force exerted by gravity is proportional to an object''s volume and density. Therefore, objects denser than the surrounding medium fall or sediment. Much evidence suggests that gravity sensing in higher plants depends on the sedimentation of dense, starch-filled amyloplasts inside specialized cells, so-called statocytes (Sack, 1991, 1997; Kuznetsov and Hasenstein, 1996, 1997b; Balu$\breve{s}$ka and Hasenstein, 1997).Dark-grown protonemata of the moss Ceratodon purpureus are tip-growing cells that are negatively gravitropic, i.e. they grow upward (Fig. ​(Fig.1).1). The wwr mutant (wrong way response) of C. purpureus is positively gravitropic, with reaction kinetics similar to the WT (Wagner et al., 1997). In horizontal WT (Fig. ​(Fig.1)1) and wwr protonemata, amyloplasts sediment in a specific zone located behind the apical dome. Plastid sedimentation is probably responsible for gravity sensing in both genotypes because it precedes curvature and because the recovery of gravitropism after basipetal centrifugation correlates with the return and sedimentation of amyloplasts (Walker and Sack, 1990, 1991; Wagner et al., 1997; Sack et al., 1998). Figure 1Gravitropic curvature and amyloplast sedimentation (arrowheads) in WT protonemata of C. purpureus that were rotated from the vertical to the horizontal 4 to 5 h before fixation.To study further the possible role of amyloplast sedimentation in gravity sensing, it is helpful to displace amyloplasts without reorienting the cell in the gravitational field. This can be achieved by exposing cells to an HGMF, thereby inducing the intracellular magnetophoretic displacement of starch-containing plastids (Kuznetsov and Hasenstein, 1996, 1997a, 1997b).Dense plastids such as amyloplasts and the cytoplasm differ in their chemical composition and physical properties, including their magnetic characteristics. When subjected to a nonuniform magnetic field, magnetically heterogeneous systems experience ponderomotive forces that depend on their relative magnetic susceptibilities (Kuznetsov and Hasenstein, 1996). Therefore, a magnetic field of sufficient intensity and gradient should be able to displace plastids inside the cell and provide an excellent test for plastid-based gravity sensing.If gravity sensing is plastid dependent, negatively gravitropic WT protonemata should curve toward stronger field intensities. In contrast, wwr cells should curve toward lower field intensities or in a positive gravitropic sense, similar to previous experiments with positively gravitropic roots (Audus, 1960; Kuznetsov and Kuznetsov, 1989; Kuznetsov and Hasenstein, 1996) and negatively gravitropic shoots (Schwarzacher and Audus, 1973; Kuznetsov and Hasenstein, 1997b). These experiments suggest that intracellular magnetophoresis is equivalent to plastid-based gravity sensing. However, these experiments were performed with higher plant organs, where the sites for perception and response are different, rather than with single cells that are capable of both sensing and responding to gravity. Moreover, the small size of moss protonemata and the availability of genotypes with opposite gravitropic responses warrants the use of HGMFs to study the possible involvement of plastid-based sensing in C. purpureus. If gravitropic sensing depends on the amyloplast sedimentation, then exposure to a magnetic field should induce both amyloplast displacement and the curvature of the tip cells in directions that are genotype dependent.This hypothesis was tested using several configurations to produce magnetic fields of different intensities and geometries. We report here that exposure to HGMF caused magnetophoretic displacement of amyloplasts and induced curvature in both WT and wwr protonemata in the predicted directions.     

Microtubule distribution in gravitropic protonemata of the mossCeratodon

Summary Tip cells of dark-grown protonemata of the mossCeratodon purpureus are negatively gravitropic (grow upward). They possess a unique longitudinal zonation: (1) a tip group of amylochloroplasts in the apical dome, (2) a plastid-free zone, (3) a zone of significant plastid sedimentation, and (4) a zone of mostly non-sedimenting plastids. Immunofluorescence of vertical cells showed microtubules distributed throughout the cytoplasm in a mostly axial orientation extending through all zones. Optical sectioning revealed a close spatial association between microtubules and plastids. A majority (two thirds) of protonemata gravistimulated for >20 min had a higher density of microtubules near the lower flank compared to the upper flank in the plastid-free zone. This apparent enrichment of microtubules occurred just proximal to sedimented plastids and near the part of the tip that presumably elongates more to produce curvature. Fewer than 5% of gravistimulated protonemata had an enrichment in microtubules near the upper flank, whereas 14% of vertical protonemata were enriched near one of the side walls. Oryzalin and amiprophos-methyl (APM) disrupted microtubules, gravitropism, and normal tip growth and zonation, but did not prevent plastid sedimentation. We hypothesize that a microtubule redistribution plays a role in gravitropism in this protonema. This appears to be the first report of an effect of gravity on microtubule distribution in plants.Abbreviations APM amiprophos-methyl - DIC differential interference contrast - DMSO dimethyl sulfoxide - EGTA ethylene glycolbis-(-amino-ethylether) N,N,N',N'-tetraacetic acid - FITC fluorescein isothiocyanate - GS gravitropic stimulus - MT microtubule - PIPES piperazine-N,N'-bis-2-ethanesulfonic acid     

Relocalization of the calcium gradient and a dihydropyridine receptor is involved in upward bending by bulging of Chara protonemata, but not in downward bending by bowing of Chara rhizoids

The localization of cytoplasmic free calcium and a dihydropyridine (DHP) receptor, a putative calcium channel, was recorded during the opposite graviresponses of tip-growing Chara rhizoids and Chara protonemata by using the calcium indicator Calcium Crimson and a fluorescently labeled dihydropyridine (FL-DHP). In upward (negatively gravitropically) growing protonemata and downward (positively gravitropically) growing rhizoids, a steep Ca²⁺ gradient and DHP receptors were found to be symmetrically localized in the tip. However, the localization of the Ca²⁺ gradient and DHP receptors differed considerably during the gravitropic responses upon horizontal positioning of the two cell types. During the graviresponse of rhizoids, a continuous bowing downward by differential flank growth, the Ca²⁺ gradient and DHP receptors remained symmetrically localized in the tip at the centre of growth. However, after tilting protonemata into a horizontal position, there was a drastic displacement of the Ca²⁺ gradient and FL-DHP to the upper flank of the apical dome. This displacement occurred after the apical intrusion and sedimentation of the statoliths but clearly before the change in the growth direction became evident. In protonemata, the reorientation of the growth direction started with the appearence of a bulge on that site of the upper flank which was predicted by the asymmetrically displaced Ca²⁺ gradient. With the upward shift of the cell tip, which is suggested to result from a statolith-induced displacement of the growth centre, the Ca²⁺ gradient and DHP receptors became symmetrically relocalized in the apical dome. No major asymmetrical rearrangement was observed during the following phase of gravitropic curvature which is characterized by slower rates of bending. Labeling with FL-DHP was completely inhibited by a non-fluorescently labeled dihydropyridine. From these results it is suggested that FL-DHP labels calcium channels in rhizoids and protonemata. In rhizoids, positive gravitropic curvature is caused by differential growth limited to the opposite subapical flanks of the apical dome, a process which does not involve displacement of the growth centre, the calcium gradient or calcium channels. In protonemata, however, it is proposed that a statolith-induced asymmetrical relocalization of calcium channels and the Ca²⁺ gradient precedes, and might mediate, the rearrangement of the centre of growth, most likely by the displacement of the Spitzenk?rper, to the upper flank, which results in the negative gravitropic reorientation of the growth direction. Received: 13 February 1999 / Accepted: 25 June 1999     

Tip-growing cells of the moss Ceratodon purpureus Are gravitropic in high-density media

Gravity sensing in plants and algae is hypothesized to rely upon either the mass of the entire cell or that of sedimenting organelles (statoliths). Protonemata of the moss Ceratodon purpureus show upward gravitropism and contain amyloplasts that sediment. If moss sensing were whole-cell based, then media denser than the cell should prevent gravitropism or reverse its direction. Cells that were inverted or reoriented to the horizontal displayed distinct negative gravitropism in solutions of iodixanol with densities of 1.052 to 1.320 as well as in bovine serum albumin solutions with densities of 1.037 to 1.184 g cm(-3). Studies using tagged molecules of different sizes and calculations of diffusion times suggest that both types of media penetrate through the apical cell wall. Estimates of the density of the apical cell range from 1.004 to 1.085. Because protonemata grow upward when the cells have a density that is lower than the surrounding medium, gravitropic sensing probably utilizes an intracellular mass in moss protonemata. These data provide additional support for the idea that sedimenting amyloplasts function as statoliths in gravitropism.     

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     

An Auxin Transport Inhibitor Interferes With Unicellular Gravitropism in Protonemata of the Moss Ceratodon purpureus

Abstract: Gravitropism of the protonemata of the moss Ceratodon purpureus (Hedw.) Brid. was studied after treatment with auxin transport inhibitors and auxin-related substances. The phytotropins NPA (naphthylphthalamic acid) and PBA (pyrenoylbenzoic acid), known to block auxin efflux in higher plants, strongly inhibited gravitropic curvature of the apical protonemal cell. At 3 μM NPA or PBA, approximately 60 % inhibition of curvature was observed; growth rates were less affected. Tyrphostin A47, a known antagonist of NPA effects in higher plants, released the inhibition of moss protonemal gravitropism and restored the full curvature response. Exogenous IAA, even at high concentrations (40 μM), did not interfere with protonemal gravitropism. To account for the results, modified hypotheses for auxin transport and action are discussed.     

Ethylene Interacts with Auxin in Regulating Developmental Attenuation of Gravitropism in Flax Root

Previous research shows that gravity-sensing in flax (Linum usitatissimum) root is initiated during seed imbibition and precedes root emergence. In this study we investigated the developmental attenuation of flax root gravitropism post-germination and the involvement of ethylene. Gravity response deteriorated significantly from 3 to 11?h after root emergence, which occurred at around 19?h after imbibition (that is, from “age” 22 to 30?h). Although the root elongation rate increased from 22 to 30?h, the gravitropic curving rate declined steadily. Older roots were able to tolerate higher levels of exogenous IAA before inhibition of elongation and gravitropism occurred. The age-dependent effect of IAA on root growth and gravitropism suggests that young roots are more sensitive to auxin and respond to a smaller vertical auxin gradient than older roots upon horizontal gravistimulation. The ethylene synthesis inhibitor AVG (2-aminoethoxyvinyl glycine, 10?μM) or ethylene action inhibitor Ag⁺ (10?μM) stimulated gravitropic curvature of 30?h roots by 24 and 32%, respectively, but had no effect on 22?h roots, suggesting that as roots age, ethylene begins to play a role in root gravitropism. The auxin transport inhibitor NPA (N-naphthylphthalamic acid, 50?μM) reduced gravitropic curvature of 30?h roots by 24% but had no effect on 22?h roots. On the other hand, treating roots simultaneously with the auxin transport inhibitor and ethylene synthesis or action inhibitor stimulated gravitropic curvature of 30?h roots but not 22?h roots. Taken together, these data indicate that as roots develop, their weakened gravity response is due to decreased auxin sensitivity and possibly auxin transport regulated by ethylene.     

Conservation of the Plastid Sedimentation Zone in All Moss Genera with Known Gravitropic Protonemata

Moss protonemata from several species are known to be gravitropic. The characterization of additional gravitropic species would be valuable to identify conserved traits that may relate to the mechanism of gravitropism. In this study, four new species were found to have gravitropic protonemata, Fissidens adianthoides, Fissidens cristatus, Physcomitrium pyriforme, and Barbula unguiculata. Comparison of upright and inverted apical cells of P. pyriforme and Fissidens species showed clear axial sedimentation. This sedimentation is highly regulated and not solely dependent on amyloplast size. Additionally, the protonemal tip cells of these species contained broad subapical zones that displayed lateral amyloplast sedimentation. The conservation of a zone of lateral sedimentation in a total of nine gravitropic moss species from five different orders supports the idea that this sedimentation serves a specialized and conserved function in gravitropism, probably in gravity sensing.     

Irradiance-dependent regulation of gravitropism by red light in protonemata of the moss Ceratodon purpureus

Apical cells of protonemata of the moss Ceratodon purpureus (Hedw.) Brid. are negatively gravitropic in the dark and positively phototropic in red light. Various fluence rates of unilateral red light were tested to determine whether both tropisms operate simultaneously. At irradiances ≥140 nmol m⁻² s⁻¹ no gravitropism could be detected and phototropism predominated, despite the presence of amyloplast sedimentation. Gravitropism occurred at irradiances lower than 140 nmol m⁻² s⁻¹ with most cells oriented above the horizontal but not upright. At these low fluence rates, phototropism was indistinct at 1 g but apparent in microgravity, indicating that gravitropism and phototropism compete at 1 g. The frequency of protonemata that were negatively phototropic varied with the fluence rate and the duration of illumination, as well as with the position of the apical cell before illumination. These data show that the fluence rate of red light regulates whether gravitropism is allowed or completely repressed, and that it influences the polarity of phototropism and the extent to which apical cells are aligned in the light path. Received: 19 January 1999 / Accepted: 19 March 1999     

Reduced Gravitropism in Hypocotyls of Starch-Deficient Mutants of Arabidopsis

Gravitropism was examined in dark- and light-grown hypocotylsof wild-type (WT), two reduced starch mutants (ACG 20 and ACG27), and a starchless mutant (ACG 21) of Arabidopsis. In addition,the starch content of these four strains was studied with lightand electron microscopy. Based on time course of curvature andorientation studies, the graviresponse in hypocotyls is proportionalto the amount of starch in a genotype. Furthermore, starch mutationsseem to primarily affect gravitropism rather than differentialgrowth since both phototropic curvature and growth rates amongthe four genotypes are approximately equal. Our results suggestthat gravity perception may require a greater plastid mass inhypocotyls compared to roots. The kinetics of gravitropic curvaturealso was compared following reorientation at 45°, 90°,and 135°. As has been reported for other plant species,the optimal angle of reorientation is 135° for WT Arabidopsisand the two reduced starch mutants, but the magnitude of curvatureof the starchless mutant appears to be independent of the initialangle of displacement. Taken together, the results of the presentstudy and our previous experiments with roots of the same fourgenotypes [Kiss et al. (1996) Physiol. Plant. 97: 237] supporta plastid-based hypothesis for gravity perception in plants. (Received December 16, 1996; Accepted February 7, 1997)     

Negative gravitropic response of roots directs auxin flow to control root gravitropism

Root tip is capable of sensing and adjusting its growth direction in response to gravity, a phenomenon known as root gravitropism. Previously, we have shown that negative gravitropic response of roots (NGR) is essential for the positive gravitropic response of roots. Here, we show that NGR, a plasma membrane protein specifically expressed in root columella and lateral root cap cells, controls the positive root gravitropic response by regulating auxin efflux carrier localization in columella cells and the direction of lateral auxin flow in response to gravity. Pharmacological and genetic studies show that the negative root gravitropic response of the ngr mutants depends on polar auxin transport in the root elongation zone. Cell biology studies further demonstrate that polar localization of the auxin efflux carrier PIN3 in root columella cells and asymmetric lateral auxin flow in the root tip in response to gravistimulation is reversed in the atngr1;2;3 triple mutant. Furthermore, simultaneous mutations of three PIN genes expressed in root columella cells impaired the negative root gravitropic response of the atngr1;2;3 triple mutant. Our work revealed a critical role of NGR in root gravitropic response and provided an insight of the early events and molecular basis of the positive root gravitropism.     

Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope

The starch‐statolith hypothesis proposes that starch‐filled amyloplasts act as statoliths in plant gravisensing, moving in response to the gravity vector and signaling its direction. However, recent studies suggest that amyloplasts show continuous, complex movements in Arabidopsis shoots, contradicting the idea of a so‐called ‘static’ or ‘settled’ statolith. Here, we show that amyloplast movement underlies shoot gravisensing by using a custom‐designed centrifuge microscope in combination with analysis of gravitropic mutants. The centrifuge microscope revealed that sedimentary movements of amyloplasts under hypergravity conditions are linearly correlated with gravitropic curvature in wild‐type stems. We next analyzed the hypergravity response in the shoot gravitropism 2 (sgr2) mutant, which exhibits neither a shoot gravitropic response nor amyloplast sedimentation at 1  g . sgr2 mutants were able to sense and respond to gravity under 30  g conditions, during which the amyloplasts sedimented. These findings are consistent with amyloplast redistribution resulting from gravity‐driven movements triggering shoot gravisensing. To further support this idea, we examined two additional gravitropic mutants, phosphoglucomutase (pgm) and sgr9, which show abnormal amyloplast distribution and reduced gravitropism at 1  g . We found that the correlation between hypergravity‐induced amyloplast sedimentation and gravitropic curvature of these mutants was identical to that of wild‐type plants. These observations suggest that Arabidopsis shoots have a gravisensing mechanism that linearly converts the number of amyloplasts that settle to the ‘bottom’ of the cell into gravitropic signals. Further, the restoration of the gravitropic response by hypergravity in the gravitropic mutants that we tested indicates that these lines probably have a functional gravisensing mechanism that is not triggered at 1  g .     

Effects of MAPKK inhibitor PD98059 on the gravitropism of primary roots of maize

Gravity plays a fundamental role in plant growth and development, yet the molecular details of gravitropism is not fully understood. Here, we report the effects of PD98059, a specific inhibitor of mitogen-activated protein (MAP) kinase kinase, on the gravitropism of primary roots of maize. Unilateral application of PD98059 to horizontal roots led to different gravitropic growth. Placing PD98059-containing agar on the upper side of the root tips accelerated gravitropic curvature, whereas placing the agar on the lower side inhibited gravitropic curvature. However, no effect was detected when asymmetric application of PD98059 to vertical roots. Global application of maize primary root with PD98059 suppressed root gravitropism. Furthermore, the effects of H₂O₂ on horizontal root gravitropism and vertical root bending were compromised by pretreatment with PD98059. These results suggest an involvement of MAP kinase pathway(s) in gravitropism of maize roots.     

The wavy growth 3 E3 ligase family controls the gravitropic response in Arabidopsis roots

Regulation of the root growth pattern is an important control mechanism during plant growth and propagation. To better understand alterations in root growth direction in response to environmental stimuli, we have characterized an Arabidopsis thaliana mutant, wavy growth 3 (wav3), whose roots show a short‐pitch pattern of wavy growth on inclined agar medium. The wav3 mutant shows a greater curvature of root bending in response to gravity, but a smaller curvature in response to light, suggesting that it is a root gravitropism‐enhancing mutation. This wav3 phenotype also suggests that enhancement of the gravitropic response in roots strengthens root tip impedance after contact with the agar surface and/or causes an increase in subsequent root bending in response to obstacle‐touching stimulus in these mutants. WAV3 encodes a protein with a RING finger domain, and is mainly expressed in root tips. RING‐containing proteins often function as an E3 ubiquitin ligase, and the WAV3 protein shows such activity in vitro. There are three genes homologous to WAV3 in the Arabidopsis genome [EMBRYO SAC DEVELOPMENT ARREST 40 (EDA40), WAVH1 and WAVH2 ], and wav3 wavh1 wavh2 triple mutants show marked root gravitropism abnormalities. This genetic study indicates that WAV3 functions positively rather than negatively in root gravitropism, and that enhancement of the gravitropic response in wav3 roots is dependent upon the function of WAVH2 in the absence of WAV3. Hence, our results demonstrate that the WAV3 family of proteins are E3 ligases that are required for root gravitropism in Arabidopsis.     

The polarity of gravitropism in the moss Physcomitrella patens is reversed during mitosis and after growth on a clinostat

Abstract. We report two situations in which the polarity of gravitropism of single protonemal cells of the moss Physcomitrella patens is reversed. Dark-grown protonemata of wild-type P. patens grow negatively gravitropically. Time-lapse video-microscopy reveals that a temporary reversal of growth polarity occurs during mitotic division which is independent of the cells' growth rate. A transitory reversal of growth direction is also observed when the unidirectional gravitropic stimulus is interrupted by a period of growth on a clinostat. A third situation, in which a mutant class responds by growing positively gravitropically, has been described previously (Jenkins, Courtice & Cove, 1986). These observations are discussed in terms of possible mechanisms for cell morphogenesis and tropic growth.     

Negative phototropism in young gametophytes of the fern Schizaea pusilla

Schizaea pusilla is a rare fern that occurs in acidic bogs and is one of the few fern species that maintains a filamentous gametophyte throughout its development. To expand our knowledge of the physiology of this fern, phototropic responses were examined in young gametophytes. In contrast to germ filaments of other fern species, apical protonemata of young gametophytes are negatively phototropic in continuous white, red and blue light at all fluence rates tested. The expression of phototropic curvature is not limited by time since apical protonemata are also negatively phototropic when they are given brief exposures of light and then placed in the dark. In other lower plant groups such as mosses and some algae, the direction of phototropic curvature can change depending on light quality and intensity, but in young gametophytes of Schizaea, negative phototropic curvature was observed in all conditions studied. Blue light is the most effective in promoting the negative phototropic response in Schizaea.