Localization and Pattern of Graviresponse across the Pulvinus of Barley Hordeum vulgare

Pulvini of excised stem segments from barley (Hordeum vulgare cv `Larker') were pretreated with 1 millimolar coumarin before gravistimulation to reduce longitudinal cell expansion and exaggerate radial cell enlargement. The cellular localization and pattern of graviresponse across individual pulvini were then evaluated by cutting the organ in cross-section, photographing the cross-section, and then measuring pulvinus thickness and the radial width of cortical and epidermal cells in enlargements of the photomicrographs. With respect to orientation during gravistimulation, we designated the uppermost point of the cross-section 0° and the lowermost point 180°. A gravity-induced increase in pulvinus thickness was observable within 40° of the vertical in coumarin-treated pulvini. In upper halves of coumarin-treated gravistimulated pulvini, cells in the inner cortex and inner epidermis had increased radial widths, relative to untreated gravistimulated pulvini. In lower halves of coumarin-treated pulvini, cells in the central and outer cortex and in the outer epidermis showed the greatest increase in radial width. Cells comprising the vascular bundles also increased in radial width, with this pattern following that of the central cortex. These results indicate (a) that all cell types are capable of showing a graviresponse, (b) that the graviresponse occurs in both the top and the bottom of the responding organ, and (c) that the magnitude of the response increases approximately linearly from the uppermost point to the lowermost. These results are also consistent with models of gravitropism that link the pattern and magnitude of the graviresponse to graviperception via statolith sedimentation.     

Cellular Dimorphism in the Maize Root Cortex: Involvement of Microtubules,Ethylene and Gibberellin in the Differentiation of Cellular Behaviour in Postmitotic Growth Zones

Detailed morphometric analysis of cell shapes and an immunofluorescent study of microtubules were carried out on primary roots of Zea mays L. Two types of cells were found to be formed within the postmitotic isodiametric growth (PIG) region of the root cortex that were differentially responsive to low level of exogenous ethylene. The innermost and central cell rows of the cortex were sensitive to ethylene treatment and showed a disturbed distribution of cortical microtubules (CMTs) as well as changed polarity of cell growth, whereas the 2–3 outermost cell rows were less sensitive in this respect. This suggests that post-mitotic cells of the inner cortex are specific targets for ethylene action. These properties of the inner cortex are compatible with its cells being involved in the formation of aerenchyma; they may also favour root growth in compacted soil. By contrast, the specific properties of the outer cortex indicate that this tissue domain is necessary for the gaseous impermeability and the mechanical strengthening of subjacent aerenchymatous cortex, especially in the mature region of the root. Ethylene affected neither the pattern of cortical cell expansion in the meristem nor the position of the PIG region with respect to the root tip. This contrasts with gibberellin-deficiency which affected these parameters in both parts of the cortex. These observations indicate a fundamental difference between the role of these two phytohormones in the morphogenesis and development of maize roots.     

A role for gibberellic acid in orienting microtubules and regulating cell growth polarity in the maize root cortex

The role of gibberellins and cortical microtubules in determining the polarity of cell growth in the root cortex of maize (Zea mays L.) was examined. Inhibition of gibberellin biosynthesis, either naturally through mutation (d5 mutant) or by means of chemicals such as 2S,3S paclobutrazol, caused thickening of root apices and increased their starch content. Immunofluorescence microscopy of cortical microtubules, coupled with a comparison of cell widhts, lengths and shapes, indicated that the meristem and immediate post-mitotic zone were the targets of gibberellin deficiency. Cortical cells in these regions were impaired in their ability to develop highly ordered transversal arrays of cortical microtubules. Consequently, the cells became wider and shorter. Application of gibberellic acid re-established the arrangements of cortical microtubules and the polarity of cell growth characteristic for roots having normal levels of gibberellins, it also decreased the starch content. These results indicate that gibberellins are morphogenetically active substances, not only in shoots but also in roots of maize.Abbreviations CMT cortical microtubule - GA gibberellin - GA₃ gibberellic acid - MT microtubule - PIG postmitotic isodiametric growth The authors acknowledge the support to F.B. from the Royal Society (London UK). We also thank Dr. J. Lenton (University of Bristol, Long Ashton Research Station) who kindly supplied us with 2S,3S paclobutrazol and grains of the GA-deficient d5 mutant of maize.     

Which cells respond to gravitropic stimulus in the lentil root?

When roots of lentil ( Lens culinaris L., cv. Large blonde) were placed in horizontal position for 2 h, their upper side elongated faster than their lower side, and also faster than vertical controls. The length of the cortical cells was greater in the upper half than in the lower half of roots which had been horizontally stimulated for 2 h. The zone of curvature extended from the distal part of the meristem to the proximal part of the cell elongation zone. The curvature in the meristem was due to early differentiation of the cells of its upper part. In the proximal part of the cell elongation zone, bending took place due to inhibition of cell growth in the lower half of the root. The results obtained are in agreement with the hypothesis of lateral transport of an inhibitor in gravistimulated roots. This inhibitor should be present in greater amounts in the lower side of the stimulated root and in lower amounts in its upper side than in the vertical controls.     

ROOT CONTRACTION IN HYACINTH. I. EFFECTS OF IAA ON DIFFERENTIAL CELL EXPANSION

Contractile roots of Hyacinthus orientalis L. cv ‘Pink Pearl’ shorten as a result of growth of inner cortical cells which expand radially and contract longitudinally. Brief treatment with IAA (indole-3-acetic acid—0.5 and 1.0 mg/1) induces subapical swelling, root cap proliferation and decreased rates of elongation in potentially contractile roots. Growth resumes with removal of IAA from the culture medium and contraction subsequently occurs. The pattern of subsequent contraction is affected by prior IAA treatment; contraction occurs in the normal manner both acropetal and basipetal to the points of IAA-induced swelling, but does not occur in the swollen region itself. Microscopic examination of the swollen region reveals that cells of the middle and outer cortex are radially expanded and longitudinally shortened relative to middle and outer cortical cells of contracted and uncontracted portions of the same root and control roots. In contrast, inner cortical cells in swollen regions of IAA-treated roots show approximately 50% less radial expansion than inner cortical cells of control contracted roots. Middle and outer cortical cells in the swollen region of IAA-treated roots undergo radial expansion, while middle and outer cortical cells in adjacent contracting zones are compressed by radially expanding inner cortical cells. Average volumes of cortical cells in the IAA-induced swollen region increased approximately two-fold when contraction occurred in adjacent regions. These results suggest that in hyacinth roots, under certain circumstances, inner and outer cortical cells alike possess the ability for growth reorientation and expansion. However, during the usual course of contractile root development, cells of the outer cortex are restricted in this ability, through an as yet unknown mechanism, and are passively compressed by the radially expanding inner cortical cells.     

Graviresponse and the localization of its initiating cells in roots of Phleum pratense L.

Roots of Phleum pratense L. were photographed during both vertical growth and gravitropic bending, and positions of anticlinal rhizodermal cell walls were digitized on the physically upper and lower flanks of the root in the curvature plane. By using B-splines, arc lengths of these positions, i.e. distances along the root surface, values of curvature, and relative elemental rates of elongation were estimated. The whole graviresponse can be divided into phases according to growth-rate values: (i) an increase of rates on the upper side of the root and a decrease on the lower side during the first 1–11/2h after the root has been moved from the vertical to a horizontal position, (ii) a transient equality of the rates on both sides, (iii) 2–3 h after the beginning of graviresponse, the growth gradient is inverted, and (iv) finally, after about 4 h, the growth rates of both flanks are approximately equal again. Curvature begins 15–20 min after horizontal placement of the root. During the first 2 h of graviresponse, plots of curvature versus arc length show one maximum value. After 2–21/2 h, two maximum values can be observed, the apical one near the root tip always keeping the same distance from the tip, the other one drifting basipetally relative to the growing tip. By evaluating photographs of high magnification, a group of six rhizodermal cells on each side of the root was identified which are the first cells showing gravitropic bending. These cells are located at the beginning of the elongation zone, enclosing the region 480–680 m from the root tip. These cells might be target cells for a signal which the statenchyma, the site of graviperception, sends to the reacting zone of gravicurvature.Abbreviations curvature - RELEL relative elemental rate of elongation A preliminary report was presented at the Meeting of the Deutsche Botanische Gesellschaft, Regensburg, 30 Sept–5 Oct 1990This work was supported by Deutsche Forschungsgemeinschaft. We thank Dr. Brigitte Buchen and Professor Zygmunt Hejnowicz (Botanisches Institut, Universität Bonn, Bonn, FRG) for critical reading of the mansucript.     

Cortical Air Spaces (Aerenchyma) in Roots of Corn Subjected to Oxygen Stress: STRUCTURE AND INFLUENCE ON UPTAKE AND TRANSLOCATION OF RUBIDIUM IONS

When the seminal root system of 14-day-old corn (Zea mays cv. Dekalb 202) was subjected to O₂ stress, nodal roots with well developed cortical air spaces (aerenchyma) grew into the deoxygenated solution. Microscopic examination showed that there was extensive breakdown of cells in the midcortex of these roots, while the stele, endodermis, and inner layer of cortical cells remained complete, as did the outer layers of the cortex and the epidermis. Occasional files of intact cells, and the wall residues of collapsed cells, crossed the space between inner and outer cortex. Experiments with short, intact root segments with and without air spaces showed that in the presence of O₂ the ability to absorb and translocate ⁸⁶Rb⁺, per unit volume or length of root, was little affected by cortical degeneration. The distribution across root sections of recently supplied strontium and rubidium, determined by electron microprobe analysis, indicated that in roots with air spaces the strands of wall residues bridging the cortex could be involved in maintaining the conduction of ions from the outer cortex up to the endodermis.     

Root contraction in hyacinth III. Orientation of cortical microtubules visualized by immunofluorescence microscopy

Summary Cortical microtubules (MTs) were visualized in root cortex cells ofHyacinthus orientalis L. using immunofluorescence techniques. Cellular MT orientation was determined adjacent to radial longitudinal and transverse walls of root tip, uncontracted, contracting, and fully contracted regions. As seen in longitudinal views, MTs formed parallel, apparently helical arrays which were oriented transversely, axially or obliquely depending upon the region. Transverse sectional views showed that MTs adjacent to transverse cell walls formed a variety of patterns which varied with developmental stage and cell location. Microtubules were oriented in crisscross or parallel arrays. The parallel arrays were oriented either parallel, perpendicular or oblique to the radius of the root. There was an apparent temporal progression in MT reorientation from outer cortical to inner cortical cell layers. A resultant progression of reoriented cell growth could account for root contraction. These findings corroborate earlier electron microscopic observations of changing MT orientation accompanying root contraction, and provide cytological evidence to test mathematical and biophysical models of the mechanics of cell expansion.Abbreviations MT microtubule - MF microfibril - MTSB microtubule stabilizing buffer - PBS phosphate buffered saline     

Zeta potential of protoplasts from gravireacting maize roots

Protoplasts were isolated from cortical cells of the elongating zone of maize (Zea mays L. cv. LG 11) roots and submitted to microelectrophoresis. Significant and transient differences in zeta potential between protoplasts from upper and lower root sides were compared with the gravireaction and the differential elongation of these roots. The maximum difference in the zeta potential was obtained between protoplasts from the upper and lower cortical cells after 90 min, exactly the time of gravipresentation for which the maximum rate of gravireaction was observed. In addition, this almost corresponded to the time for which the difference between the elongation rates of upper and lower sides of the extending zone began to increase. Consequently, the changes in the charges of the plasmalemma of the cortical cells from the growing part of roots could be more or less directly related to the root graviresponse.     

Alterations in the Cytoskeleton Accompany Aluminum-Induced Growth Inhibition and Morphological Changes in Primary Roots of Maize

Although Al is one of the major factors limiting crop production, the mechanisms of toxicity remain unknown. The growth inhibition and swelling of roots associated with Al exposure suggest that the cytoskeleton may be a target of Al toxicity. Using indirect immunofluorescence microscopy, microtubules and microfilaments in maize (Zea mays L.) roots were visualized and changes in their organization and stability correlated with the symptoms of Al toxicity. Growth studies showed that the site of Al toxicity was associated with the elongation zone. Within this region, Al resulted in a reorganization of microtubules in the inner cortex. However, the orientation of microtubules in the outer cortex and epidermis remained unchanged even after chronic symptoms of toxicity were manifest. Auxin-induced reorientation and cold-induced depolymerization of microtubules in the outer cortex were blocked by Al pretreatment. These results suggest that Al increased the stability of microtubules in these cells. The stabilizing effect of Al in the outer cortex coincided with growth inhibition. Reoriented microfilaments were also observed in Al-treated roots, and Al pretreatment minimized cytochalasin B-induced microfilament fragmentation. These data show that reorganization and stabilization of the cytoskeleton are closely associated with Al toxicity in maize roots.     

Organization of cortical microtubules in graviresponding maize roots

Immunofluorescence labeling of cortical microtubules (MTs) was used to investigate the relationship between MT arrangement and changes in growth rate of the upper and lower sides of horizontally placed roots of maize (Zea mays L. cv. Merit). Cap cells and cells of the elongation zone of roots grown vertically in light or darkness showed MT arrangements that were transverse (perpendicular) to the growth direction. Microtubules of cells basal to the elongation zone typically showed oblique orientation. Two hours after horizontal reorientation, cap cells of gravicompetent, light-grown and curving roots contained MTs parallel to the gravity vector. The MT arrangement on the upper side of the elongation zone remained transverse but the MTs of the outer four to five layers of cortical cells along the lower side of the elongation zone showed reorientation parallel to the axis of the root. The MTs of the lower epidermis retained their transverse orientation. Dark-grown roots did not curve and did not show reorientation of MTs in cells of the root cap or elongation zone. The data indicate that MT depolymerization and reorientation is correlated with reduction in growth rate, and that MT reorientation is one of the steps of growth control of graviresponding roots.Abbreviations MT microtubule - QC quiescent center This work was supported by National Science Foundation grant IBN-9118094.     

THE ROOT APEX OF MALVA SYLVESTRIS. I. STRUCTURAL DEVELOPMENT

The apical organization of the primary root of Malva sylvestris was analyzed at several growth stages, beginning with the embryo, to determine the structural changes that occurred during growth. Seeds were germinated, and plants were grown under controlled conditions. There were three discrete groups of initials in the embryonic root: those of the central cylinder, cortex, and secondary columella. The secondary columella initials consisted of a plate of cells flanked by a ring of cortical initials. The lateral portion of the rootcap shared a common origin with the epidermis. During growth both the initials of the secondary columella and outer cortex produced rootcap cells. The first indication of the outer cortical initials participating in rootcap formation was observed in roots 3 cm long. In 6-, 9-, and 16-cm roots the cellular continuity between the outer cortex and rootcap was marked, but in 23- and 33-cm roots the histogenic continuity between the outer cortex and rootcap was not evident. In all growth stages the initials of the central cylinder and inner cortex retained their histogenic integrity.     

Root contraction in hyacinth. II. Changes in tubulin levels,microtubule number and orientation associated with differential cell expansion

Root contraction in hyacinth (Hyacinthus orientalis L.) is marked by reoriented cell growth in the cortex of the contractile region. Cellular volume of the inner cortex enlarges fourfold during root contraction. This is associated with large increases in the radial and tangential dimensions and decreases in the longitudinal dimension of the cells. In order to determine the possible role of microtubules (MTs) in these changes we compared tubulin levels and MT numbers and orientation in contracted and non-contracted regions of hyacinth roots. Tubulin content was analysed by a radioimmunoassay; MT numbers and orientation were analyzed by counting profiles in sectioned material using transmission electron microscopy. Contracted tissue was found to have significantly higher levels of tubulin on a per-cell basis than non-contracted tissue, and also increased tubulin levels relative to total protein. The spatial MT frequencies were the same in contracted and non-contracted tissues, indicating a proportional increase in MT numbers in the expanded cells. Although the absolute spatial frequency of MTs was constant, the orientation, as determined by morphometric analysis of MT profiles, was not. While in the longitudinal section plane 42% of the MTs in the non-contracted cells were oblique, in the contracted cells the percentage of MTs presenting oblique profiles increased to 87%. Additionally, a qualitative difference in MTs was observed in contracted cells; electron-opaque material was seen peripherally associated with the MTs of the inner cortex. The changes in tubulin levels and in MT numbers as well as the qualitative differences in the MTs of contracted and non-contracted root regions indicate that, in hyacinth, reoriented cellular enlargement associated with root contraction cannot be explained simply by shifts in the arrangement of preexisting cortical MT arrays, but involves more complex changes in the cytoskeleton.Abbreviations MT(s) microtubule(s) - TEM transmission electron microscopy - RIA radioimmunoassay - M_r apparent molecular mass I=Jernstedt (1984b)     

Cellular basis for the automorphic curvature of rice coleoptiles on a three-dimensional clinostat: possible involvement of reorientation of cortical microtubules

Coleoptiles of rice (Oryza sativa L.) show a spontaneous (automorphic) curvature toward the caryopsis under microgravity conditions. The possible involvement of the reorientation of cortical microtubules in automorphic curvature was studied in rice coleoptiles grown on a three-dimensional clinostat. When rice seedlings that had been grown in the normal gravitational field were transferred to the clinostat in the dark, cortical microtubules of epidermal cells in the dorsal side of the coleoptiles oriented more transversely than the ventral side within 0.5 h. The rotation on the clinostat also increased the cell wall extensibility in the dorsal side and decreased the extensibility in the ventral side, and induced automorphic curvature. The reorientation of cortical microtubules preceded the changes in the cell wall extensibility and the curvature. The irradiation of rice seedlings with white light from above inhibited microtubule reorientation and changes in the cell wall extensibility, as well as curvature of coleoptiles. Also, colchicine, applied to the bending region of coleoptiles, partially inhibited the automorphic curvature. These results suggest that reorientation of cortical microtubules is involved in causing automorphic curvature in rice coleoptiles on the clinostat.     

Aluminum induces changes in the orientation of microtubules and the division plane in root meristem of Zea mays

Aluminum (Al) induces agricultural problems limiting crop productivity in acid soils. Since Al causes morphological changes in roots, and because microtubules (MTs) play important roles in determination of tissue morphology, we investigated whether Al affects the arrangement of MTs in maize root meristem using immunolocalization techniques. When seedling roots were treated with 50 μM Al, the orientations of MTs were dramatically altered in a population of cells located in the protoderm and the two outer layers of cortex: interphase cortical MT arrays lost their normal transverse organization and became random or longitudinal; the preprophase band of MTs, mitotic spindle, and phragmoplast developed at planes 90° rotated compared to their counterparts in controls. These changes in MT orientation resulted in the change of the division plane from transverse to longitudinal, producing daughter cells positioned side by side instead of above and below. The rotation of the otherwise normal MT arrays and the division plane in Al-treated roots indicates that Al interferes with the normal polarity sensing mechanism, which may contribute to the reduced axial growth of the Al-treated roots.     

Microtubules in statocytes from roots of cress (Lepidium sativum L.)

Summary Statocytes in root caps ofLepidium sativum L. were examined by means of ultrathin serial sections to evaluate the amount and distribution of cortical microtubules. The microtubules encircle the cell, oriented normal to the root length axis. In the distal cell edges, microtubules form a network, separating the distal complex of endoplasmic reticulum from the plasmalemma. Preprophase bands in meristem cells are observable rarely, structures which can be regarded as nucleating sites for microtubules are lacking. During ageing of the root cap cells, the number of microtubules increases in combination with a decrease of microtubule length. Development of the roots on a horizontal clinostat preserves a younger developmental stage of the microtubule system regarding amount and length of the individual microtubules. Evidence for an involvement of microtubules in graviperception is low, whereas their role in orienting cellulose microfibrils cannot be ruled out. Compression of the distal network of microtubules after centrifugation of the roots indicates that microtubules in statocytes ofLepidium sativum L. roots might function in stabilizing the distal complex of endoplasmic reticulum.     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

Fungal endophytes in Astromyelon-type (Sphenophyta,Equisetales, Calamitaceae) roots from the Upper Pennsylvanian of France

A distinctive fungal endophyte, Cashhickia acuminata nov. gen. et sp., is described from permineralized calamite roots from the Upper Pennsylvanian Grand-Croix cherts of France. Heavily infected roots contain numerous intracellular hyphae in the outer cortex that arise from a meshwork-like mycelium extending between cortical cells. All intracellular hyphae are oriented toward the root center; none occur on the inner periclinal host cell walls. Other roots of the same type show localized infection by this fungus in which isolated cortical cells contain or give rise to intracellular fungal growth. Within the cortical cells are host responses in the form of callosities that indicate the roots were alive at the time of infection. Other endophytes are present in the same host tissue but are less frequent. The discovery of this association provides the first detailed account on the morphology of a Carboniferous fungal root endophyte, as well as the spatial distribution within the host, and infection pathways within the cortical tissues.     

Comparison of cytology and distribution of nickel in roots of Ni-hyperaccumulating and non-hyperaccumulating genotypes of Senecio coronatus

Two genotypes of Senecio coronatus (Thunb.) Harv. (Asteraceae) growing on ultramafic outcrops were identified previously: a Ni hyperaccumulator and a non-hyperaccumulator. The aim of the present study was to investigate the cytology of the roots of both genotypes, their Ni content and tissue distribution, and to ascertain whether there was a cytological basis for the differential uptake of Ni. Light and fluorescence microscopy together with histochemical methods were used to study root cytology. X-ray microanalysis by means of a nuclear microprobe—particle-induced X-ray emission (PIXE) and proton backscattering (BS) techniques—was utilized to determine the concentration and distribution of Ni and other elements. Average concentration of Ni and distribution in roots differed significantly between hyperaccumulating and non-hyperaccumulating genotypes. Ni amount in the hyperaccumulating genotype was ca. 60 times higher in the older part of the root (1,760 μg g⁻¹) and ca. 10 times higher (314 μg g⁻¹) in the younger root hair region in comparison with the equivalent parts of the non-accumulating genotype where Ni amounts were 30 μg g⁻¹. Ni distribution pattern was also different in both cases. Cytological differences were observed in the inner cortical region and exodermis of the roots. Distinct groups of specialized cells with an organelle-rich cytoplasm that produced copious numbers of spherical bodies occurred in the inner cortical region of the hyperaccumulator. Such distinct cell groups were absent from the inner cortex of the non-hyperaccumulator. Cortical cells here had a thin parietal cytoplasmic layer and produced fewer spherical bodies. In both genotypes the spherical structures were extruded from the cytoplasm into air spaces between the cells where they coalesced to form amorphous deposits, significantly larger and more abundant in the hyperaccumulator. Histochemical tests identified these deposits as a mixture of lipids, alkaloids and terpenoids. Specialized cells present in the inner cortex of the hyperaccumulating genotype demonstrated significant relative Ni depletion in comparison with the adjacent inner cortex and phloem. Casparian bands were identified in exodermal cell walls of both genotypes but the bands fluoresced more intensely in the non-accumulator suggesting differences in chemical composition and probably also a more efficient apoplastic barrier. This feature was correlated with the observed Ni distribution pattern. The highest Ni enrichment in the hyperaccumulating genotype occurred in the outer cortex; 20 times more than in the adjacent epidermis/exodermis in older portions of roots and 3 times more than in the epidermis/exodermis in younger root hair regions. In contrast, in the non-hyperaccumulating genotype, a higher concentration of nickel was found in the epidermis/exodermis compared to the outer cortex. The Ni ratio between the outer cortex and epidermis/exodermis was about 0.4 in the non-hyperaccumulator. Different cytological features exhibited by the genotypes may represent adaptive responses to the presence of high concentrations of Ni in the soil and subsequent differential uptake of Ni. Basic characteristics and elemental content of soil collected from the close vicinity of roots of two S. coronatus genotypes are reported. Wojciech Przybyłowicz on leave from the Faculty of Physics and Applied Computer Science, AGH University of Science & Technology, Kraków, Poland     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.