Aluminium distribution in soybean root tip for a short time Al treatment

Using the lumogallion staining method which we developed (Kataoka et al. 1997a), Al movement in soybean (Glycine max. (L.)Merr. cv. Tsurunoko) root tips treated for a short time was studied. We have indicated that the majority of Al accumulated in the root was found between 0 and 2 mm from the root apex within 2 h (Kataoka et al. 1997a, b). In the study presented here two-day seedlings of the soybean were treated with 50 μmol/L AlCl₃ (pH 4.4), including 0.2 mmol/L CaCl₂, for 1 h, and Al accumulation in the root sections at both 1 and 2 mm apart from root apex was analyzed by a confocal laser microscopy. Although the early indicators, callose induction and the decrease of growth recovery, were not observed in the root when treated for 15 min, a trace amount of Al was already incorporated into the nucleus of cells and the middle tissue of the root. The non-toxic level of Al was more rapidly absorbed than previously thought. The initial increase of callose accumulation and the reduction of the growth recovery were found after 30 min. Therefore, the difference between Al accumulation profiles of 15 and 30 min was analyzed to find out what triggered a toxic Al effect. Increase of Al accumulation in whole root tissue was observed in the root sections, at both 1 and 2 mm from the root apex, and the greatest amount of Al was found in the cytoplasm of the outer cortex, 1 mm away from the root apex. These results are consistent with the fact that Al exclusion from root tip cells is an important mechanism of Al tolerance.     

Aluminum accumulation and associated effects on 15NO3 − influx in roots of two soybean genotypes differing in Al tolerance

A study was conducted to examine aluminum (Al) exclusion by roots of two differentially tolerant soybean (Glycine max L. Merr.) lines, Pl-416937 (Al-tolerant) and Essex (Al-sensitive). Following exposure to 80μM Al for up to 2 h, roots were rinsed with a 10 mM potassium citrate solution and rapidly dissected to allow estimation of intracellular Al accumulation in morphologically distinct root regions. Using 10 min exposures to 300μM ¹⁵NO₃ ⁻ and dissection, accompanying effects on NO₃ ⁻ uptake were measured. With Al exposures of 20 min or 2 h, there was greater Al accumulation in all root regions of Essex than in those of Pl-416937. The genotypic difference in Al accumulation was particularly apparent at the root apex, both in the tip and in the adjacent root cap and mucilage. Exposure of roots to Al inhibited the uptake of ¹⁵NO₃ ⁻ to a similar extent in all root regions. The results are consistent with Al exclusion from cells in the root apical region being an important mechanism of Al tolerance.     

Redistribution of actin, profilin and phosphatidylinositol-4,5-bisphosphate in growing and maturing root hairs

The continuously changing polar cytoplasmic organization during initiation and tip growth of root hairs is reflected by a dynamic redistribution of cytoskeletal elements. The small G-actin binding protein, profilin, which is known to be a widely expressed, potent regulator of actin dynamics, was specifically localized at the tip of root hairs and co-distributed with a diffusely fluorescing apical cap of actin, but not with subapical actin microfilament (MF) bundles. Profilin and actin caps were present exclusively in the bulge of outgrowing root hairs and at the apex of elongating root hairs; both disappeared when tip growth terminated, indicating a tip-growth mechanism that involves profilin-actin interactions for the delivery and localized exocytosis of secretory vesicles. Phosphatidylinositol-4,5-bisphosphate (PIP₂), a ligand of profilin, was localized almost exclusively in the bulge and, subsequently, formed a weak tip-to-base gradient in the elongating root hairs. When tip growth was eliminated by the MF-disrupting inhibitor cytochalasin D, the apical profilin and the actin fluorescence were lost. Mastoparan, which is known to affect the PIP₂ cycle, probably by stimulating phospholipases, caused the formation of a meshwork of distinct actin MFs replacing the diffuse apical actin cap and, concomittantly, tip growth stopped. This suggests that mastoparan interferes with the PIP₂-regulated profilin-actin interactions and hence disturbs conditions indispensable for the maintenance of tip growth in root hairs. Received: 11 March 1999 / Accepted: 27 May 1999     

ANATOMY OF MACROZAMIA COMMUNIS LATERAL ROOTS AND ROOT NODULES FORMED IN VITRO STUDIED WITH LIGHT AND SCANNING ELECTRON MICROSCOPY

The anatomy of Macrozamia communis L. Johnson lateral roots and nodules was studied following axenic culture in light and darkness. Pointed lateral roots from dark cultures had an open apical organization similar to that of other cycads and gymnosperms. A distinct protoderm-derived epidermis was not observed. At the apex, the dermis was formed by the outer root capcortical cell layer. Subapically, the outer cortex formed the dermis. No evidence of an algal zone was observed in these roots. The stele was bounded by a distinct endodermis and contained an exarch, diarch xylem. Apogeotropic nodules which developed at the root-shoot junction in darkness, branched dichotomously and had rounded tips covered by tangentially-enlarged root cap cells. The root cap was reduced to a few cell layers and was confined to the extreme nodule apex. The central region of the apical meristem was enlarged, and meristematic cells contained differentiated amyloplasts. A presumptive algal zone was present in some but not all nodules and divided the cortex into inner and outer regions. Stelar anatomy was similar to that observed in pointed, dark-grown lateral roots, except that there was greater xylem differentiation. Nodules which developed in the light were similar to dark-formed nodules, except that root cap cells were radially enlarged and extended over the flanks of the nodule forming a persistent root cap. The heteromorphic lateral roots of M. communis formed a developmental continuum not a heterorhizic root system.     

Regions of differential cell elongation and mitosis, and root meristem morphology in different tissues of geotropically stimulated maize root apices

We examined cell length, mitosis, and root meristem “cuticle” in different tissues of geostimulated, red light-exposed primary roots of corn (Zea Mays, Wisconsin hybrid 64A × 22R). The examination was done at 15-minute intervals for a period of 240 minutes. Differences in cell elongation between the upper and lower sides were most prominent between 1.5 and 2.5 mm from the root meristem; the outer cortex had the greatest elongation growth, and the upper cells showed a significant increase in length compared to the lower. A differential mitosis was also found, with the lower tissue being greater. We infer that the mitotic activity is indicative of cell division, and this division occurs strictly in the first 1.5 mm of the root meristem. The combined effect of differential cell elongation and cell division results in the localization of the geotropic curvature in the 1.5- to 2.5-mm region from the root meristem. Mitosis that occurs primarily in the cortex and stele were asynchronous; the peak of cortical division preceded that of the stele. Both peaks occurred before the peak of geotropism. A densely stained layer separates the cap from the root meristem. This layer is thinner at the apex of the root meristem. The area of the thin region increased with time and peaked at 180 minutes after geostimulation, which was coincidental with the peak of the geotropic response.     

Aluminum and calcium distribution patterns in aluminum-intoxicated roots of Allium cepa do not support the calcium-displacement hypothesis and indicate signal-mediated inhibition of root growth

The aluminum and calcium distributions in the root tips of aluminum-intoxicated onions, Allium cepa L., were mapped using PIXE (particle-induced X-ray emission) microanalysis. Not enough aluminum was present to have replaced, atom-for-atom, more than a minor fraction of the calcium. Furthermore, no inverse relationship between variations in aluminum and calcium concentrations was observed for pairs of adjacent 30-μm-diameter regions. Our observations, therefore, do not support the hypothesis that aluminum substantially reduces the quantity of bound calcium by competing with calcium for binding sites. Instead we suggest that reductions in calcium content are a non-local and indirect consequence of aluminum-intoxication. We found that aluminum accumulates almost exclusively in a surface layer. Observations of wounded roots indicated that exposed internal tissue binds aluminum avidly, so we contend that the surface accumulation pattern indicates that little aluminum penetrates into the interior of the root. We argue that aluminum does not directly inhibit growth in the interior of the apical root meristem because root growth rate was unaffected by root cap removal which should greatly increase the aluminum concentration in the exposed interior region. We hypothesize that growth inhibition in the interior of the meristem is mediated by a signal initiated or disrupted by excess aluminum in the periphery of the meristem. Received: 29 January 1997 / Accepted: 10 June 1997     

The Ultrastructure of the Regenerating Root Cap of Zea mays L

Removal of the cap from the primary root of Zea mays activatescell division in the quiescent centre. It is the descendentsof these cells that eventually regenerate a new cap—aprocess that is complete in about 4 days at 23 °C. The ultrastructureof the cells of the regenerating cap was examined at daily intervals.During the first day after decapping the dictyosomes in theexposed outer layer of cells change from a relatively quiescentstate to one where they are secreting a polysaccharide slimewhich accumulates between the plasmalemma and the outer cellwall. Amyloplasts grow in size and appear to divide, and theendoplasmic reticulum proliferates. Many different cytoplasmicfeatures that are normally characteristic of cells in distinctlocations within the undisturbed cap occur, at first, all togetherwithin the few cells that are the source of the new regeneratingtissue. Regeneration of a normal structure in the new cap isachieved by progressive changes in the structures of the cellorganelles, apparently in response to the position that thecells containing them occupy within the growing cellular ensembleat the root apex. Zea mays, regeneration, root cap, ultrastructure     

Oxygen Distribution in Wetland Plant Roots and Permeability Barriers to Gas-exchange with the Rhizosphere: a Microelectrode and Modelling Study with Phragmites australis

Adventitious roots of intact Phragmites plantlets were securedhorizontally 2–3 mm below the surface of an oxygen-depletedfluid agar across which oxygen-free nitrogen was gently streamedto create a constant oxygen sink; the leafy shoot was fullyexposed to air. Radial oxygen profiles through rhizosphere androot at different distances from the apex were obtained polarographicallyusing Clark-type bevelled microelectrodes servo-driven in stepsof 10 µm (root) or 10–50 µm (rhizosphere).The pattern of radial oxygen loss (ROL) typical of wetland plants,viz. high at the apex and declining sharply sub-apically, wasrelated to synergism between ROL, and oxygen consumption andincreasing impedance to diffusion within the epidermal/hypodermalcylinder rather than to a surface resistance. The smallest oxygendeficit (2 kPa) to develop across the 80 µm thick epidermal/hypodermalcylinder was within the apical 10 mm and was consistent withtissue oxygen diffusivities similar to water. At 100 mm fromthe apex, consumption and impedance had increased the deficitto about 15 kPa and reduced ROL almost to zero. The developingimpedance within the epidermal/hypodermal cylinder was leastin cell layers immediately adjoining the cortex and increasedmost in the hypodermal cell layer abutting the epidermis. Thesub-apical decline in ROL appeared to coincide with the appearanceof aerenchyma in the cortex but thin walled ‘passage areas’(windows) in the hypodermal/epidermal cylinder persisted locallyand remained leaky to oxygen to some degree. It is through thesewindows that lateral roots emerge and the cortex in line withthe windows remains non-aerenchymatous. The radial and longitudinaloxygen profiles were consistent with modelling predictions.The shapes of the stelar oxygen profiles were consistent witha higher oxygen demand in the outer region (viz. pericycle,phloem, protoxylem and early metaxylem cylinder) than in theinner core (late metaxylem cylinder and medulla), but the deficitswere relatively small (     

Effect of copper excess in environment on soybean root viability and morphology

The effects of increase copper concentrations in medium (10–150 μM CuSO₄) on growth and viability of the roots of two-week-old soybean seedlings (Glycine max L., cv. Dorintsa) were studied. Copper excess suppressed biomass accumulation and linear plant growth; copper affected root growth much stronger than shoot growth. The presence of 10 μM CuSO₄ in medium suppressed accumulation of plant biomass by 40% and the root length by 70%; in the presence of 25 μM CuSO₄, these indices were equal to 80 and 90%, respectively. In the presence of 50 μM CuSO₄, roots ceased to grow but biomass and shoot length still increased slightly. 150 μM CuSO₄ was lethal for plants. The earliest sign of excessive copper toxicity was the accumulation of MDA, indicating activation of membrane lipid peroxidation. A significant increase in MDA content was observed at plant incubation in medium with 10 μM CuSO₄ for 1 h; in this case, the content of copper in the roots increased from 36 ±1.8 (in control) to 48 ± 2.4 μg/g dry wt. The number of dead cells (permeable for the dye Evans Blue) was doubled in the presence of 200 μg/g dry wt within the root; this occurred in 72 h of growth in medium with 10 μM CuSO₄, in 6 h at 25 μM CuSO₄, in 3 h at 50 μM CuSO₄, and 1 h at 150 μM CuSO₄. Toxicity of copper excess was manifested stronger in dividing and elongation cells of the root apex (root meristem and the zone of elongation) than in more basal root regions. Copper excess resulted in the formation of breaks in the surface cell layers of the root tips and affect root morphology. When plant grew in medium with 10 μM CuSO₄, a distance of lateral root formation zone from the root tip decreased markedly, and spherical swellings were formed on the tips of lateral roots. The higher copper concentrations (50 and 150 μM) suppressed completely the development of lateral roots.     

Auxin, transport and growth in intact roots of Vicia faba

Transport of IAA applied to the intact root of Vicia seedlingswas investigated in relation to root growth. The root was treatedat 3–4, 4–5 or 7–8 mm from the tip with anarrow ring of lanolin paste containing IAA or IAA-2-¹⁴C ingrowth or transport experiments, respectively. The growth processalong the root axis was examined in every 1-mm part from thetip at 30 min, 1 or 4 hr intervals. The elongation zone of thecontrol root was 1–9 mm from the tip. IAA treatment broughtabout no significant change in the growth of the region apicalto the treated site, whereas distinct inhibition of growth occurredin the region basal to the treated site within 1 hr. The transportof radioactivity was observed in both acropetal and basipetaldirections within 1 hr, but the latter predominated for 8 hror more; the nearer to the tip the treatment site, the longerthe predominance lasted. The velocities of acropetal and basipetaltransport were estimated at about 4 and 8 mm/hr, respectively.Autoradiographs of transverse section of roots showed that basipetaltransport occurred mainly through the outer part of the root,whereas acropetal transport occurred mainly through the innerpart, the central cylinder. It may be concluded that the basipetallytransported IAA which passed through the outer part of the rootinhibited the elongation of the intact root. (Received November 25, 1975; )     

Distribution and Toxic Effects of Cadmium and Lead on Maize Roots

Two-day-old seedlings of maize (Zea mays L.) were incubated on Cd and Pb nitrate solutions at the concentrations that inhibited root growth approximately by 50% after two-day-long incubation (LC₅₀; 10^–4 and 10^–3 M, respectively) or completely terminated growth of the primary root after one-day-long incubation (LC; 5 × 10^–4 and 10^–2 M, respectively). Cd and Pb contents were measured using an anodic inversion voltammetric technique in a flow injection system and a histochemical method. At LC₅₀, Cd and Pb were discerned, by histochemical techniques, in all root apical tissues, whereas in the root hair zone, the heavy metals were primarily accumulated in the apoplast of the rhizodermis and cortex and to a lesser extent, in the vascular tissues and parenchyma cells surrounding the metaxylem vessels. Insignificant accumulation of Cd and Pb in the pericycle probably explains why root branching was tolerant to these agents. At LC, Cd and Pb were found in the apoplast of all root tissues, in accordance with the practically complete inhibition of root growth and branching. Irrespectively of Cd and Pb concentrations in the external solution, the metal contents in the root apex exceeded those in the basal region. Procion dyes were used to assess cell death inflicted by Cd and Pb. At LC, the root cap and meristematic cells perished, together with the rhizodermal cells and the outer cortical cells of the root apex, whereas only the rhizodermal cells in the root apical region died at LC₅₀. The evidence that Cd and Pb cross the endodermal barrier at LC presumes that, at lower metal concentrations, the Casparian strip and plasmalemma of the endodermis regulate the transport of these metals into the central cylinder. The authors conclude that the identical barriers control Cd and Pb transport in root tissues.     

Negative phototropism of rice root and its influencing factors

Some characteristics of the rice (Oryza sativa L.) root were found in the experiment of unilaterally irradiating the roots which were planted in water: (i) All the seminal roots, adventitious roots and their branched roots bent away from light, and their curvatures ranged from 25° to 60°. The curvature of adventitious root of the higher node was often larger than that of the lower node, and even larger than that of the seminal root, (ii) The negative phototropic bending of the rice root was mainly due to the larger growth increment of root-tip cells of the irradiated side compared with that of the shaded side, (iii) Root cap was the site of light perception. If root cap was shaded while the root was irradiated the root showed no negative phototropism, and the root lost the characteristic of negative phototropism when root cap was divested. Rice root could resume the characteristic of negative phototropism when the new root cap grew up, if the original cells of root cap were well protected while root cap was divested, (iv) The growth increment and curvature of rice root were both influenced by light intensity. Within the range of 0–100 μmol · m² -s⁻¹, the increasing of light intensity resulted in the decreasing of the growth increment and the increasing of the curvature of rice root, (v) The growth increment and the curvature reached the maximum at 30°C with the temperature treatment of 10–40°C. (vi) Blue-violet light could prominently induce the negative phototropism of rice root, while red light had no such effect. (vii) The auxin (IAA) in the solution, as a very prominent influencing factor, inhibited the growth, the negative phototropism and the gravitropism of rice root when the concentration of IAA increased. The response of negative phototropism of rice root disappeared when the concentration of IAA was above 10 mg · L⁻¹     

Gas chromatography-mass spectrometric determinations of abscisic acid levels in the cap and the apex of maize roots

Quantitative analyses of abscisic acid (ABA) in different parts of maize root tips (Zea mays L. cv. Kelvedon 33) were performed by mass fragmentography using the hexadeuterated analog of ABA as internal standard. It was found that the cap and the apex contained 36.1 g and 66.5 g ABA kg^–1 fresh weight, respectively. The possibility that the growth regulator formed in the cap and inhibiting the elongation of the extending zone of the root is ABA is discussed.Abbreviations ABA abscisic acid - ABA-D₆ hexadeuterated ABA - ABA-Me and ABA-D₆-Me methyl esters of ABA and ABA-D₆, respectively - GC-MS gas chromatograph(y)-mass spectrometry/spectrometer - IAA indol-3-yl-acetic acid - MF mass fragmentography - TMS trimethylsilyl     

Development of Vacuolar Volume in the Root Tips of Pea

Cell and vacuole areas were measured by light microscopy inlongitudinal and transverse sections cut at 0.4-mm intervalsalong the apical 7.2 mm of the primary root of pea. The vacuolararea (or volume) fraction — that is, vacuole area (orvolume) divided by cell area (or volume) — increased fromabout 15 % in cells 0.4 mm from the distal boundary of the apicalmeristem (the cap /root junction), to about 85% in cells situated6.8–7.2 mm from that boundary. At each distance, vacuoledevelopment tended to be greater in the cortex than in the stele.Vacuoles occupied about 22% of the tissue volume in the first1 mm length of root (measured from the cap/root junction), about31 % of the tissue volume in the first 2 mm, and about 45% whensummed over the apical 5-mm length of root. Phosphorus supplyor deprivation produced only minor and non-significant changesin vacuole development. The results have implications affectingprevious estimates of cytoplasmic and vacuolar phosphate concentrationsin pea root tips. Pisum sativum L., pea, root, vacuole, volume     

Effect of zinc and cadmium on physiological and production characteristics in <Emphasis Type="Italic">Matricaria recutita</Emphasis>

Effects of zinc (12–180 μM) alone and in mixtures with 12 μM Cd on metal accumulation, dry masses of roots and shoots, root respiration rate, variable to maximum fluorescence ratio (F_V/F_M), and content of photosynthetic pigments were studied in hydroponically cultivated chamomile (Matricaria recutita) plants. The content of Zn in roots and shoots increased with the increasing external Zn concentration and its accumulation in the roots was higher than that in the shoots. While at lower Zn concentrations (12 and 60 μM) the presence of 12 μM Cd decreased Zn accumulation in the roots, treatment with 120 and 180 μM Zn together with 12 μM Cd caused enhancement of Zn content in the root. Presence of Zn (12–120 μM) decreased Cd accumulation in roots. On the other hand, Cd content in the shoots of plants treated with Zn + Cd exceeded that in the plants treated only with 12 μM Cd. Only higher Zn concentrations (120 and 180 μM) and Zn + Cd mixtures negatively influenced dry mass, chlorophyll (Chl) and carotenoid content, F_V/F_M and root respiration rate. Chl b was reduced to a higher extent than Chl a.     

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     

DISTRIBUTION AND RELATIONSHIP OF CELL DIVISION AND MATURATION EVENTS IN PISUM SATIVUM (FABACEAE) SEEDLING ROOTS

Pea roots have open apical organization, where discrete initial cells do not exist. Differentiation of all tissues occurs in cylinders and vascular sectors that blend gradually with each other. This study reports the distribution of dividing cells and their relationship to maturation events in the 2 mm root tip, and in the 8–10 and 18–20 mm segments. Up to 200 μm from the root body/cap junction, cell division is uniformly distributed throughout all meristem regions. By 350 to 500 μ, xylem tracheary elements and cells of the pith parenchyma and middle cortex have stopped dividing. At this level cell division is almost entirely restricted to two cylinders, one composed of the inner root cap, the epidermis, and the outer cortex (outer cortex cylinder) and another composed of cells of the inner cortex, the pericycle and vascular tissue (inner cortex cylinder). When the protophloem matures, all cells in the phloem sector of the inner cortex cylinder, including the 1 layered pericycle, the endodermis and the phloem parenchyma, stop dividing. The 3–4 layered pericycle in the xylem sectors continues dividing until about 10 mm from the body/cap junction following the maturation of the protoxylem tracheary elements.     

Effects of Cations on Hormone Transport in Primary Roots of Zea mays

We examined the influence of aluminum and calcium (and certain other cations) on hormone transport in corn roots. When aluminum was applied unilaterally to the caps of 15 mm apical root sections the roots curved strongly away from the aluminum. When aluminum was applied unilaterally to the cap and ³H-indole-3-acetic acid was applied to the basal cut surface twice as much radioactivity (assumed to be IAA) accumulated on the concave side of the curved root as on the convex side. Auxin transport in the apical region of intact roots was preferentially basipetal, with a polarity (basipetal transport divided by acropetal transport) of 6.3. In decapped 5 mm apical root segments, auxin transport was acropetally polar (polarity = 0.63). Application of aluminum to the root cap strongly promoted acropetal transport of auxin reducing polarity from 6.3 to 2.1. Application of calcium to the root cap enhanced basipetal movement of auxin, increasing polarity from 6.3 to 7.6. Application of the calcium chelator, ethylene-glycol-bis-(β-aminoethylether)-N,N,N′, N′-tetraacetic acid, greatly decreased basipetal auxin movement, reducing polarity from 6.3 to 3.7. Transport of label after application of tritiated abscisic acid showed no polarity and was not affected by calcium or aluminum. The results indicate that the root cap is particularly important in maintaining basipetal polarity of auxin transport in primary roots of corn. The induction of root curvature by unilateral application of aluminum or calcium to root caps is likely to result from localized effects of these ions on auxin transport. The findings are discussed relative to the possible role of calcium redistribution in the gravitropic curvature of roots and the possibility of calmodulin involvement in the action of calcium and aluminum on auxin transport.     

Spatial and Directional Variation of Growth Rates in Arabidopsis Root Apex: A Modelling Study

Growth and cellular organization of the Arabidopsis root apex are investigated in various aspects, but still little is known about spatial and directional variation of growth rates in very apical part of the apex, especially in 3D. The present paper aims to fill this gap with the aid of a computer modelling based on the growth tensor method. The root apex with a typical shape and cellular pattern is considered. Previously, on the basis of two types of empirical data: the published velocity profile along the root axis and dimensions of cell packets formed in the lateral part of the root cap, the displacement velocity field for the root apex was determined. Here this field is adopted to calculate the linear growth rate in different points and directions. The results are interpreted taking principal growth directions into account. The root apex manifests a significant anisotropy of the linear growth rate. The directional preferences depend on a position within the root apex. In the root proper the rate in the periclinal direction predominates everywhere, while in the root cap the predominating direction varies with distance from the quiescent centre. The rhizodermis is distinguished from the neighbouring tissues (cortex, root cap) by relatively high contribution of the growth rate in the anticlinal direction. The degree of growth anisotropy calculated for planes defined by principal growth directions and exemplary cell walls may be as high as 25. The changes in the growth rate variation are modelled.