The role of actin filaments in the gravitropic response of snapdragon flowering shoots

The involvement of the actin and the microtubule cytoskeleton networks in the gravitropic response of snapdragon ( Antirrhinum majus L.) flowering shoots was studied using various specific cytoskeleton modulators. The microtubule-depolymerizing drugs tested had no effect on gravitropic bending. In contrast, the actin-modulating drugs, cytochalasin D (CD), cytochalasin B (CB) and latrunculin B (Lat B) significantly inhibited the gravitropic response. CB completely inhibited shoot bending via inhibiting general growth, whereas CD completely inhibited bending via specific inhibition of the differential flank growth in the shoot bending zone. Surprisingly, Lat B had only a partial inhibitory effect on shoot bending as compared to CD. This probably resulted from the different effects of these two drugs on the actin cytoskeleton, as was seen in cortical cells. CD caused fragmentation of the actin cytoskeleton and delayed amyloplast displacement following gravistimulation. In contrast, Lat B caused a complete depolymerization of the actin filaments in the shoot bending zone, but only slightly reduced the amyloplast sedimentation rate following gravistimulation. Taken together, our results suggest that the actin cytoskeleton is involved in the gravitropic response of snapdragon shoots. The actin cytoskeleton within the shoot cells is necessary for normal amyloplast displacement upon gravistimulation, which leads to the gravitropic bending.     

Characterization of the asymmetric growth of gravistimulated snapdragon spikes by stem and cell dimension analyses

Growth patterns of detached spikes of gravistimulated snapdragon (Antirrhinum majus L.) were analyzed in detail. The length increment of 5-mm marked subsections in the upper and lower flanks of the stem-bending zone was measured during gravistimulation using time-lapse photographs. At the onset of bending, a negative relative growth rate of the upper flank was detected, followed by increased relative growth rate in both lower and upper flanks. Consequently, a differential stem growth pattern was obtained during gravistimulation, which was significantly and specifically abolished by calcium antagonists reported previously to inhibit stem curvature of snapdragon. The differential growth patterns resulted from dynamic modifications of the cell dimensions in the epidermal and cortical stem layers. Bending started with both shrinking and widening of the epidermal cells and a parallel decrease in length and height of cortical cells at the upper stem flank. These changes were accompanied with a concomitant increase in length and height of the cortical cells on the lower stem flank, followed by a growth increase of epidermal cells. Our results suggest that both the epidermal and cortical cells play an important role in gravitropic shoot bending of snapdragon.     

Actomyosin mediates gravisensing and early transduction events in reoriented cut snapdragon spikes

We investigated the involvement of the actomyosin network in the early events of the gravitropic response of cut snapdragon (Antirrhinum majus L.) spikes. The effects of the actin-modulating drug, cytochalasin D (CD) and/or the myosin inhibitor, 2,3-butanedione-2-monoxime (BDM) on amyloplast displacement, lateral auxin transport and consequently on stem bending were examined. The inhibitory effect on cytoskeleton integrity was studied by using indirect immunofluorescence double-labeling of actin and myosin. Our results demonstrate that no organizational changes in actin filaments occurred in cortical and endodermal cells of the stem bending zone during reorientation. These results suggest that actin depolymerization is not required for amyloplast sedimentation. Unlike the chloroplasts in the cortex, the amyloplasts in the endodermis were surrounded by actin and myosin, indicating that amyloplasts may be attached to the actin filaments via the motor protein, myosin. This suggests the involvement of myosin as part of the actomyosin complex in amyloplast movement in vertical as well as in reoriented stems. This suggestion was supported by the findings showing that: (a) BDM or CD disrupted the normal organization of actin either by altering characteristic distribution patterns of myosin-like protein in the cortex (BDM), or by causing actin fragmentation (CD); (b) both compounds inhibited the gravity-induced amyloplast displacement in the endodermis. Additionally, these compounds also inhibited lateral auxin transport across the stem and stem gravitropic bending. Our study suggests that during stem reorientation amyloplasts possibly remain attached to the actin filaments, using myosin as a motor protein. Thus, gravisensing and early transduction events in the gravitropic response of snapdragon spikes, manifested by amyloplast displacement and lateral auxin transport, are mediated by the actomyosin complex.     

Inhibition of the Gravitropic Response of Snapdragon Spikes by the Calcium-Channel Blocker Lanthanum Chloride

The putative Ca²⁺-channel blocker LaCl₃ prevented the gravitropic bending of cut snapdragon (Antirrhinum majus L.) spikes (S. Philosoph-Hadas, S. Meir, I. Rosenberger, A.H. Halevy [1996] Plant Physiol 110: 301–310) and inhibited stem curvature to a greater extent than vertical and horizontal stem elongation at the bending zone. This might indicate that LaCl₃, which modulates cytosolic Ca²⁺, does not influence general stem-growth processes but may specifically affect other gravity-associated processes occurring at the stem-bending zone. Two such specific gravity-dependent events were found to occur in the bending zone of snapdragon spikes: sedimentation of starch-containing chloroplasts at the bottom of stem cortex cells, as seen in cross-sections, and establishment of an ethylene gradient across the stem. Our results show that the lateral sedimentation of chloroplasts associated with gravity sensing was prevented in cross-sections taken from the bending zone of LaCl₃-treated and subsequently gravistimulated spikes and that LaCl₃ completely prevented the gravity-induced, asymmetric ethylene production established across the stem-bending zone. These data indicate that LaCl₃ inhibits stem curvature of snapdragon spikes by preventing several gravity-dependent processes. Therefore, we propose that the gravitropic response of shoots could be mediated through a Ca²⁺-dependent pathway involving modulation of cytosolic Ca²⁺ at various stages.     

The role of the epidermis and cortex in gravitropic curvature of maize roots

In order to determine the role of the epidermis and cortex in gravitropic curvature of seedling roots of maize (Zea mays L. cv. Merit), the cortex on the two opposite flanks was removed from the meristem through the growing zone; gravitropic curvature was measured with the roots oriented horizontally with the cut flanks either on the upper and lower side, or on the lateral sides as a wound control. Curvature was slower in both these treatments (53° in 5 h) than in intact roots (82°), but there was no difference between the two orientations in extent and rate of curvature, nor in the latent time, showing that epidermis and cortex were not the site of action of the growth-regulating signal. The amount of cortex removed made no difference in the extent of curvature. Curvature was eliminated when the endodermis was damaged, raising the possibility that the endodermis or the stele-cortex interface controls gravitropic curvature in roots. The elongation rate of roots from which just the epidermis had been peeled was reduced by 0.01 mM auxin (indole-3-acetic acid) from 0.42 to 0.27 mm h^-1, contradicting the hypothesis that only the epidermis responds to changes in auxin activity during gravistimulation. These observations indicate that gravitropic curvature in maize roots is not driven by differential cortical cell enlargement, and that movement of growth regulator(s) from the tip to the elongating zone is unlikely to occur in the cortex.Abbreviations df degrees of freedom - IAA indole-3-acetic acid     

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     

Changes in starch and inositol 1,4,5-trisphosphate levels and auxin transport are interrelated in graviresponding oat (Avena sativa) shoots

This study was conducted to unravel a mechanism for the gravitropic curvature response in oat (Avena sativa) shoot pulvini. For this purpose, we examined the downward movement of starch-filled chloroplast gravisensors, differential changes in inositol 1,4,5-trisphosphate (IP(3)) levels, transport of indole-3-acetic acid (IAA) and gravitropic curvature. Upon gravistimulation, the ratio for IAA levels in lower halves versus those in upper halves (L/U) increased from 1.0 at 0 h and reached a maximum value of 1.45 at 8 h. When shoots were grown in the dark for 10 d, to deplete starch in the chloroplast, the gravity-induced L/U of IAA was reduced to 1.0. N-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), both auxin transport inhibitors, significantly reduced the amount of gravitropic curvature and gravity-induced lateral IAA transport, but did not reduce the gravity-induced late change in the L/U ratio of IP(3) levels. U73122, a specific phospholipase C (PLC) inhibitor, decreased gravity-induced curvature. Because U73122 reduced the ratio of L/U of IAA imposed by gravistimulation, it is clear that IAA transport is correlated with changes in IP(3) levels upon gravistimulation. These results indicate that gravistimulation-induced differential lateral IAA transport may result from the onset of graviperception in the chloroplast gravisensors coupled with gravity-induced asymmetric changes in IP(3) levels in oat shoot pulvini.     

Gravistimulation forces the cortical microtubules to reorientate from transverse to random in the lower flank of actively-growing primary roots of French beans

The gravistimulation of primary roots of seedlings of Phaseolus vulgaris for 0.5 h did not cause any conspicuous difference in the microtubule arrays between the upper and the lower flanks. Exposure to gravistimulation of 1 h resulted in the transversely-orientated microtubules being reduced significantly in the lower flank of the actively elongating region, especially in the epidermis and the outer first and second layers of the cortex. This reduction was compensated for by an increase of randomly-orientated microtubules. In the region where the elongation rate of cells began to decrease, the microtubule arrays did not show a distinct difference between the two opposite flanks. When gravistimulation was prolonged to 2 h, the microtubule arrangement was about the same as in the control. The present results are not compatible with the concept that longitudinal microtubule arrays are induced in the lower flank under gravistimulation and thereby hinder the longitudinal expansion of cells in the lower flanks of roots.     

Inhibition of the gravitropic bending response of flowering shoots by salicylic acid.

The upward gravitropic bending of cut snapdragon, lupinus and anemone flowering shoots was inhibited by salicylic acid (SA) applied at 0.5 mM and above. This effect was probably not due to acidification of the cytoplasm, since other weak acids did not inhibit bending of snapdragon shoots. In order to study its mode of inhibitory action, we have examined in cut snapdragon shoots the effect of SA on three processes of the gravity-signaling pathway, including: amyloplast sedimentation, formation of ethylene gradient across the stem, and differential growth response. The results show that 1 mM SA inhibited differential ethylene production rates across the horizontal stem and the gravity-induced growth, without significantly inhibiting vertical growth or amyloplast sedimentation following horizontal placement. However, 5 mM SA inhibited all three gravity-induced processes, as well as the growth of vertical shoots, while increasing flower wilting. It may, therefore, be concluded that SA inhibits bending of various cut flowering shoots in a concentration-dependent manner. Thus, at a low concentration SA exerts its effect in snapdragon shoots by inhibiting processes operating downstream to stimulus sensing exerted by amyloplast sedimentation. At a higher concentration SA inhibits bending probably by exerting general negative effects on various cellular processes.     

Regulation of the gravitropic response and ethylene biosynthesis in gravistimulated snapdragon spikes by calcium chelators and ethylene inhibitors

The possible involvement of Ca2+ as a second messenger in snapdragon (Antirrhinum majus L.) shoot gravitropism, as well as the role of ethylene in this bending response, were analyzed in terms of stem curvature and gravity-induced asymmetric ethylene production rates, ethylene-related metabolites, and invertase activity across the stem. Application of Ca2+ chelators (ethylenediaminetetraacetic acid, trans-1,2-cyclohexane dinitro-N,N,N',N'-tetraacetic acid, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N',-tetraacetic acid) or a Ca2+ antagonist (LaCl3) to the spikes caused a significant loss of their gravitropic response following horizontal placement. Conversely, the Ca2+ ionophore A23187 or the agonist Bay K-8644 increased gravibending. Longitudinally halved stem sections had significantly higher amounts of ethylene, 1-aminocyclopropane-1-carboxylic acid, and 1-(malonylamino) cyclopropane-1-carboxylic acid compared with vertical controls, with the extra production arising exclusively from the lower half of the stem. trans-1,2-cyclohexane dinitro-N,N,N',N'-tetraacetic acid pretreatment completely abolished the gravity-induced ethylene gradient across the stem, thereby leading to a significant reduction of the curvature. Similarly, reduction of the ethylene produced in the gravistimulated with CoCl2 or inhibition of its action by silver thiosulfate or 2,5-norbornadiene significantly inhibited the subsequent gravibending. Silver thiosulfate and CoCl2 also abolished the gravity-induced gradient of invertase activity across the stem, which is associated with the asymmetric stem elongation. These results suggest that cytosolic Ca2+ may regulate auxin action in snapdragon spikes, manifested as increased ethylene production, which is, in turn, intimately correlated with stem bending. Therefore, both hormones seem to play significant roles in induction and progress of the gravibending of snapdragon spikes.     

Mechano-sensitive orientation of cortical microtubules during gravitropism in azuki bean epicotyls

The orientation of cortical microtubules (cMT) during gravitropism was studied in epidermal cells of azuki epicotyls. The relative proportion of cells with longitudinal cMT increased in the upper epidermis, and those with transverse cMT increased in the lower epidermis. When epicotyls were kept straight during gravistimulation, no change in cMT orientation occurred in either the upper and lower epidermis. When epicotyls were forced to bend downward, cells with transverse cMT increased in the upper epidermis, and those with longitudinal cMT increased in the lower epidermis. When epicotyls were loaded with naphthylphthalamic acid, an inhibitor of auxin transport, both gravitropic bending and change in cMT orientation were inhibited. However, when a change in cMT orientation was induced by forced downward bending, cells with longitudinal cMT increased in the compressed (lower) side and those with transverse cMT increased in the extended (upper) side. It was suggested that cMT orientation was controlled by the bending of the epicotyl and not by a gravity signal per se. Loading with Gd³⁺, an inhibitor of the stretch-activated channel, did not inhibit gravitropic bending. However, it inhibited cMT reorientation induced by gravitropic bending and by forced bending. Involvement of the stretch-activated channel in mechano-sensitive orientation of cMT was suggested.     

Interaction of auxin, light, and mechanical stress in orienting microtubules in relation to tropic curvature in the epidermis of maize coleoptiles

Summary Changes in the orientation of cortical microtubules (longitudinal vs. transverse with respect to the long cell axis) at the outer epidermal wall of maize coleoptile segments were induced by auxin, red or blue light, and mechanical stresses (cell extension or compression produced by bending). Immunofluorescent techniques were used for the quantitative determination of frequency distributions of microtubule orientation. Detailed kinetic studies showed that microtubule reorientations are temporally correlated with the simultaneously measured changes in growth rate elicited by auxin, red light, or blue light. Growth inhibition induced by depletion of endogenous auxin produces a longitudinal microtubule pattern that can be changed into a transverse pattern in a dose-dependent manner by applying exogenous auxin. A mid-point pattern with equal frequencies of longitudinal and transverse microtubules was adjusted at 2 mol/1 auxin. Bending stress applied under these conditions adjusts permanent, maximally longitudinal and transverse microtubule orientations at the compressed and extended segment sides, respectively, quantitatively mimicking the responses to differential flank growth during phototropic and gravitropic curvature. During tropic curvature the changes in microtubule pattern reflect the distribution of growth rather than the distribution of auxin. The microtubule pattern responds to auxin-dependent growth changes and mechanical stress in a synergistic manner, confirming the functional equivalence of these factors in affecting microtubule orientation. Similar results were obtained when segment growth was altered by blue or red light instead of auxin in the presence or absence of mechanical stress. It is concluded from these results that growth changes, elicited by auxin, light, etc., and mechanical stress affect microtubule orientation through a common signal perception and transduction chain.Abbreviations IAA indole-3-acetic acid (auxin) - MT cortical microtubule     

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°), (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. Received: 23 November 1999 / Accepted: 10 May 2000     

Changes in starch content in oat (Avena sativa) shoot pulvini during the gravitropic response.

In order to determine if components of the signal transduction pathway are involved in starch metabolism during the gravitropic response, the effects of inhibitors of phosphoprotein phosphatases and protein kinases (OA), and calcium channel blockers (LaCl3), on gravitropic bending and starch levels in gravisensitive node/pulvini of oat shoots were examined. Among the compounds tested, okadaic acid (OA) and lanthanum chloride (LaCl3) showed the strongest inhibitory effects on the negative gravitropic curvature response in oat shoot node/pulvini. At the same time, they caused a rapid loss of starch in graviresponding pulvini based on a quantitative analysis of starch levels in the bending tissues over 48 h periods. These two compounds act initially to block the net increase in starch content that occurs during the early stages (0-9 h) in graviresponding oat shoot pulvini. As a result, starch levels drop precipitously in shoots treated with OA and LaCl3, starting at time zero of gravistimulation by reorientation. These findings suggest that protein dephosphorylation and calcium play a role in starch metabolism in oat shoot pulvini in response to a gravistimulation signal. They also indicate that the amount of starch present in the chloroplast gravisensors in oat shoot pulvini may determine the rate of upward bending in graviresponding pulvini.     

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.Abbreviations 9-HFCA 9-hydroxyfluorenecarboxylic acid - NPA naphthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid - IAA indole-3-acetic acid     

Auxin redistributes upwards in graviresponding gynophores of the peanut plant

The peanut (Arachis hypogaea L.) produces flowers aerially, but buries the recently fertilized ovules into the soil, where fruit and seed development occur. The young seeds are carried down into the soil at the tip of a specialized organ called the gynophore. Although the gynophore has a typical shoot anatomy, it responds positively to gravity like a root. In this study, we explore the role of the plant growth regulator indole-3-acetic acid (IAA) in the growth and the gravitropic response of the peanut gynophore. With an immunolocalization technique using an IAA monoclonal antibody, we localized IAA within the tissues of vertically oriented and gravistimulated gynophores. We found that in vertically oriented gynophores, IAA labeling occurs in the periphery of the gynophore, in the entire cortex and epidermis. Within 20 min of horizontal reorientation, the IAA signal gradually increases in the upper cortex/epidermis and diminishes in the lower cortex/epidermis. At 1.5 h after gravistimulation, all of the IAA immunolocalization signal is detected in the upper cortex and epidermis – none is detected in the lower side. Growth rate measurements also indicate that after 1–2 h of reorientation, the growth rate maximum on the upper side corresponds temporally and spatially to the growth rate minimum on the lower side. Experiments using radioactively labeled IAA corroborate an upper-side redistribution of this hormone upon horizontal reorientation. These results are analyzed with respect to the current theories of plant gravitropic response, and a model for a possible gravity-induced IAA redistribution from the lower to the upper side of the peanut gynophore is proposed. Received: 25 January 1999 / Accepted: 24 February 1999     

A role for inositol 1,4,5-trisphosphate in gravitropic signaling and the retention of cold-perceived gravistimulation of oat shoot pulvini

Plants sense positional changes relative to the gravity vector. To date, the signaling processes by which the perception of a gravistimulus is linked to the initiation of differential growth are poorly defined. We have investigated the role of inositol 1,4,5-trisphosphate (InsP(3)) in the gravitropic response of oat (Avena sativa) shoot pulvini. Within 15 s of gravistimulation, InsP(3) levels increased 3-fold over vertical controls in upper and lower pulvinus halves and fluctuated in both pulvinus halves over the first minutes. Between 10 and 30 min of gravistimulation, InsP(3) levels in the lower pulvinus half increased 3-fold over the upper. Changes in InsP(3) were confined to the pulvinus and were not detected in internodal tissue, highlighting the importance of the pulvinus for both graviperception and response. Inhibition of phospholipase C blocked the long-term increase in InsP(3), and reduced gravitropic bending by 65%. Short-term changes in InsP(3) were unimpaired by the inhibitor. Gravitropic bending of oat plants is inhibited at 4 degrees C; however, the plants retain the information of a positional change and respond at room temperature. Both short- and long-term changes in InsP(3) were present at 4 degrees C. We propose a role for InsP(3) in the establishment of tissue polarity during the gravitropic response of oat pulvini. InsP(3) may be involved in the retention of cold-perceived gravistimulation by providing positional information in the pulvini prior to the redistribution of auxin.     

Basipetal propagation of gravity-induced surface pH changes along primary roots of <Emphasis Type="Italic">Lepidium sativum</Emphasis> L.

While there is ample evidence for a role of auxin in root gravitropism, the seeming rapidity of gravi-induced changes in electrical parameters has so far been an argument against auxin being a primary signal in gravitropic signal transmission. To address this problem, we re-investigated the effect of gravistimulation on membrane voltages of Lepidium sativum L. and Vigna mungo L. root cells. In our hands, gravistimulation did not induce changes in membrane voltage in cells of the root cap statenchyma, root meristem or apical elongation zone that can be correlated with the orientation of the cells relative to the gravity vector. While these results challenge a model of rapid electrically based signal transmission, there is evidence for a slower signal propagation along gravistimulated L. sativum roots. Using multiple proton-selective microelectrodes to simultaneously measure surface pH on opposite root flanks at different distances from the root tip, we observed gravi-induced asymmetric pH changes at the surface of all investigated root zones. Upon gravistimulation, the surface pH decreased on the physically upper root flank and increased on the lower flank. The pH asymmetry appeared first [2.1+/-0.4 min (mean +/- SD) after tilting] at the root cap and then - with incrementing lag times - at the meristem (after 2.5+/-0.3 min at 300 micro m from root tip; after 3.7+/-0.4 min at 700 micro m) and apical elongation zone (4.8+/-0.5 min at 1,000 micro m), suggesting a basipetal progression of differential surface acidification at a rate of 250-350 micro m min(-1), consistent with reported auxin transport rates.     

Effect of gravity on apical dominance in Pharbitis nil.

When the upper part of main shoot of morning glory (Pharbitis nil) is gently bent down, lateral bud on the bending region is released from apical dominance and starts to elongate. But, clinorotating the bending shoots prevents the release of the lateral bud from apical dominance. These results suggest that gravity affects apical dominance in morning glory. Here we verified the gravity-regulated apical dominance by using a weeping morning glory defective in gravitropic response due to abnormal differentiation of endodermis. That is, bending main shoot of the weeping morning glory hardly caused the lateral bud to elongate. In addition, decapitation of apical bud released the lateral bud from apical dominance, and exogenous auxin applied to the cut surface of the decapitated stem was inhibitory to the outgrowth of the lateral bud in the wild type. However, the effect of auxin was much less in the weeping morning glory. Thus, apical dominance of the weeping morning glory was weaker and less influenced by gravity than that of the wild type, which could occur due to abnormal differentiation of endodermis required for graviperception.     

Computer-based video digitizer analysis of surface extension in maize roots

We used a video digitizer system to measure surface extension and curvature in gravistimulated primary roots of maize (Zea mays L.). Downward curvature began about 25 +/- 7 min after gravistimulation and resulted from a combination of enhanced growth along the upper surface and reduced growth along the lower surface relative to growth in vertically oriented controls. The roots curved at a rate of 1.4 +/- 0.5 degrees min-1 but the pattern of curvature varied somewhat. In about 35% of the samples the roots curved steadily downward and the rate of curvature slowed as the root neared 90 degrees. A final angle of about 90 degrees was reached 110 +/- 35 min after the start of gravistimulation. In about 65% of the samples there was a period of backward curvature (partial reversal of curvature) during the response. In some cases (about 15% of those showing a period of reverse bending) this period of backward curvature occurred before the root reached 90 degrees. Following transient backward curvature, downward curvature resumed and the root approached a final angle of about 90 degrees. In about 65% of the roots showing a period of reverse curvature, the roots curved steadily past the vertical, reaching maximum curvature about 205 +/- 65 min after gravistimulation. The direction of curvature then reversed back toward the vertical. After one or two oscillations about the vertical the roots obtained a vertical orientation and the distribution of growth within the root tip became the same as that prior to gravistimulation. The period of transient backward curvature coincided with and was evidently caused by enhancement of growth along the concave and inhibition of growth along the convex side of the curve, a pattern opposite to that prevailing in the earlier stages of downward curvature. There were periods during the gravitropic response when the normally unimodal growth-rate distribution within the elongation zone became bimodal with two peaks of rapid elongation separated by a region of reduced elongation rate. This occurred at different times on the convex and concave sides of the graviresponding root. During the period of steady downward curvature the elongation zone along the convex side extended farther toward the tip than in the vertical control. During the period of reduced rate of curvature, the zone of elongation extended farther toward the tip along the concave side of the root. The data show that the gravitropic response pattern varies with time and involves changes in localized elongation rates as well as changes in the length and position of the elongation zone. Models of root gravitropic curvature based on simple unimodal inhibition of growth along the lower side cannot account for these complex growth patterns.