Development of Golgi Apparatus in the Root Cap Cells of Maize (Zea mays L.) as Affected by Compacted Soil

Effects of soil mechanical impedance on the development of Golgiapparatus in the root cap cells of maize were studied undercontrolled soil-water conditions Heavily compacted soil (bulkdensity = 1.50 g cm^–2) had 3.3 to 3.4 times greater mechanicalimpedance than control soil (bulk density = 1.33 g cm^–3),but their oxygen diffusion rates were not significantly differentThe number of dictyosomes and the number and area of secretoryvesicles per unit area of tangentially sub-peripheral root capcells in the heavily compacted soil increased compared to thosein the control These results suggest that secretory activityof the root cap cells is promoted by soil mechanical impedance Dictyosome, Golgi apparatus, maize, mucilage, root cap, secretory activity, secretory vesicles, soil mechanical impedance, Zea mays L     

A Gradient of Endogenous Calcium Forms in Mucilage of Graviresponding Roots of Zea mays

Agar blocks that contacted the upper sides of tips of horizontally-orientedroots of Zea mays contain significantly less calcium (Ca) thanblocks that contacted the lower sides of such roots. This gravity-inducedgradient of Ca forms prior to the onset of gravicurvature, anddoes not form across tips of vertically-oriented roots or rootsof agravitropic mutants. These results indicate that (1) Cacan be collected from mucilage of graviresponding roots, (2)gravity induces a downward movement of endogenous Ca in mucilageoverlying the root tip, (3) this gravity-induced gradient ofCa does not form across tips of agravitropic roots, and (4)formation of a Ca gradient is not a consequence of gravicurvature.These results are consistent with gravity-induced movement ofCa being a trigger for subsequent redistribution of growth effectors(e.g. auxin) that induce differential growth and gravicurvature. Atomic absorption, calcium, corn, gravitropism (root), Zea mays     

Signal transmission during gravitropic curvature of primary roots of Zea mays

Primary roots of Zea mays cv. Ageotropic are nonresponsive to gravity and elongate approximately 0.80 mm h^?1. Applying mucilage-like material (K-Y Jelly) to the terminal 1.5 cm of these roots induces graviresponsiveness and slow elongation 28% (i.e. from 0.80 to 0.58mm h^?1). Applying mucilage-like material to one side of the terminal 1.5 cm of the root induces curvature toward the mucilage, irrespective of the root's orientation to gravity. Applying a 2-mm-wideband of mucilage-like material to a root's circumference 8 to 10 mm behind the root cap neither induces gravicurvature nor affects elongation significantly. Similarly, applying mucilage-like material to only the root cap does not significantly affect elongation or graviresponsiveness. Gravicurvature of mutant roots occurs only when mucilage-like material is applied to the root/root-cap junction. Reversing the caps of wild-type and mutant roots produces gravitropic responses characteristic of the root cap rather than the host root. These results are consistent with the suggestion that gravitropic effectors are growth inhibitors that move apoplastically through mucilage between the root cap and root.     

The Involvement of Glucose-6-Phosphatase in Mucilage Secretion by Root Cap Cells of Zea mays

In order to determine the involvement of glucose-6-phosphatasein mucilage secretion by root cap cells, we have cytochemicallylocalized the enzyme in columella and peripheral cells of rootcaps of Zea mays. Glucose-6-phosphatase is associated with theplasmalemma and cell wall of columella cells. As columella cellsdifferentiate into peripheral cells and begin to produce andsecrete mucilage, glucose-6-phosphatase staining intensifiesand becomes associated with the mucilage and, to a lesser extent,the cell wall. Cells being sloughed from the cap are characterizedby glucose-6-phosphatase staining being associated with thevacuole and plasmalemma. These changes in enzyme localizationduring cellular differentiation in root caps suggest that glucose-6-phosphataseis involved in the production and/or secretion of mucilage byperipheral cells of Z. mays. Zea mays, corn, glucose-6-phosphatase, columella cell, peripheral cell, mucilage, secretion, cytochemistry     

Defective Secretion of Mucilage is the Cellular Basis for Agravitropism in Primary Roots of Zea mays cv. Ageotropic

Root caps of primary, secondary, and seminal roots of Z. mayscv. Kys secrete large amounts of mucilage and are in close contactwith the root all along the root apex. These roots are stronglygraviresponsive. Secondary and seminal roots of Z. mays cv.Ageotropic are also strongly graviresponsive. Similarly, theircaps secrete mucilage and closely appress the root all alongthe root apex. However, primary roots of Z. mays cv. Ageotropicare non-responsive to gravity. Their caps secrete negligibleamounts of mucilage and contact the root only at the extremeapex of the root along the calyptrogen. These roots become graviresponsivewhen their tips are coated with mucilage or mucilage-like materials.Peripheral cells of root caps of roots of Z. mays cv. Kys containmany dictyosomes associated with vesicles that migrate to andfuse with the plasmalemma. Root-cap cells of secondary and seminal(i.e. graviresponsive) roots of Z. mays cv. Ageotropic are similarto those of primary roots of Z. mays cv. Kys. However, root-capcells of primary (i.e. non-graviresponsive) roots of Z. mayscv. Ageotropic have distended dictyosomal cisternae filled withan electron-dense, granular material. Large vesicles full ofthis material populate the cells and apparently do not fusewith the plasmalemma. Taken together, these results suggestthat non-graviresponsiveness of primary roots of Z. mays cv.Ageotropic results from the lack of apoplastic continuity betweenthe root and the periphery of the root cap. This is a resultof negligible secretion of mucilage by cells along the edgeof the root cap which, in turn, appears to be due to the malfunctioningof dictyosomes in these cells. Corn, dictyosomes, mucilage, root gravitropism, Zea mays cv. Ageotropic, Zea mays cv. Kys     

The Locations and Amounts of Endogenous lons and Elements in the Cap and Elongating Zone of Horizontally Oriented Roots of Zea mays L: An Electron-probe EDS Study

We used quantitative electron-probe energy-dispersive x-raymicroanalysis to localize endogenous Na, Cl, K, P, S, Mg andCa in cryofixed and freeze-dried cryosections of the cap (i.e.the putative site of graviperception) and elongating zone (i.e.site of gravicurvature) of horizontally oriented roots of Zeamays. Ca, Na, Cl, K and Mg accumulate along the lower side ofcaps of horizontally oriented roots. The most dramatic asymmetriesof these ions occur in the apoplast, especially the mucilage.We could not detect any significant differences in the concentrationsof these ions in the central cytoplasm of columella cells alongthe upper and lower sides of caps of horizontally-oriented roots.However, the increased amounts of Na, Cl, K and Mg in the longitudinalwalls of columella cells along the lower side of the cap suggestthat these ions may move down through the columella tissue ofhorizontally-oriented roots. Ca also accumulates (largely inthe mucilage) along the lower side of the elongating zone ofhorizontally-oriented roots, while Na, P, Cl and K tend to accumulatealong the upper side of the elongating zone. Of these ions,only K increases in concentration in the cytoplasm and longitudinalwalls of cortical cells in the upper vs lower sides of the elongatingzone. These results indicate that (1) gravity-induced asymmetriesof ions differ significantly in the cap and elongating zoneof graviresponding roots, (2) Ca accumulates along the lowerside of the cap and elongating zone of graviresponding roots,(3) increased growth of the upper side of the elongating zoneof horizontally-oriented roots correlates positively with increasedamounts of K in the cytoplasm and longitudinal walls of corticalcells, and (4) the apoplast (especially the mucilage) may bean important component of the pathway via which ions move ingraviresponding rots of Zea mays. These results are discussedrelative to mechanisms for graviperception and gravicurvatureof roots. Corn, gravitropism (root), ions, x-ray microanalysis, Zea mays     

Inducing Gravitropic Curvature of Primary Roots of Zea mays cv Ageotropic

Primary roots of the mutant `Ageotropic' cultivar of Zea mays are nonresponsive to gravity. Their root caps secrete little or no mucilage and touch the root only at the extreme apex. A gap separates the cap and root at the periphery of the cap. Applying mucilage from normal roots or substances with a consistency similar to that of mucilage to tips of mutant roots causes these roots to become strongly graviresponsive. Gravicurvature stops when these substances are removed. Caps of some mutants secrete small amounts of mucilage and are graviresponsive. These results indicate that (a) the lack of graviresponsiveness in the mutant results from disrupting the transport pathway between the cap and root, (b) movement of the growth-modifying signal from the cap to the root occurs via an apoplastic pathway, and (c) mucilage is necessary for normal communication between the root cap and root in Zea mays cv Ageotropic.     

Recovery of Geotropism after Removal of the Root Cap

Removal of the cap from the primary roots of Zea mays and Triticumaestivum renders the roots unresponsive to gravity. In bothspecies a geotropic response is recovered before a new cap hasstarted to regenerate. Immediately after decapping amyloplastsstart to develop in cells of the root apex and it is proposedthat as the development of amyloplasts continues so they becomefunctional as gravity sensors. It is also suggested that theamyloplasts may be the source of an inhibitor that has beenpostulated to be the intermediary between the perception ofgravity and the geotropic response.     

Cytochemical Localization of Calcium in Cap Cells of Primary Roots of Zea mays L.

The distribution of calcium (Ca) in caps of vertically- andhorizontally-oriented roots of Zea mays was monitored to determineits possible role in root graviresponsiveness. A modificationof the antimonate precipitation procedure was used to localizeCa in situ. In vertically-oriented roots, the presumed graviperceptive(i.e., columella) cells were characterized by minimal and symmetricstaining of the plasmalemma and mitochondria. No precipitatewas present in plasmodesmata or cell walls. Within 5 min afterhorizontal reorientation, staining was associated with the portionof the cell wall adjacent to the distal end of the cell. Thisasymmetric staining persisted throughout the onset of gravicurvature.No staining of lateral cell walls of columella cells was observedat any stage of gravicurvature, suggesting that a lateral flowof Ca through the columella tissue of horizontally-orientedroots does not occur. The outermost peripheral cells of rootsoriented horizontally and vertically secrete Ca through plasmodesmata-likestructures in their cell walls. These results are discussedrelative to proposed roles of root-cap Ca in root gravicurvature. Key words: Antimonate, calcium, columella cell, peripheral cell, root gravitropism, Zea mays L.     

Root Growth Inhibitors from Root Cap and Root Meristem of Zea mays L.

A micro-assay based on the growth inhibition of root segmentsof the seminal roots of Zea mays has been used to investigatethe root-growth-inhibiting substances in root caps and meristemsrespectively of the roots of Zea mays. This micro-assay is sensitiveto 50 pg of IAA or less. Paper chromatography of the acid fractionof methanolic extracts shows the presence of one main inhibitorin root caps and a different main inhibitor in root meristems.Neither is IAA, whose presence in meristems is sometimes indicatedby small inhibitions (or stimulations) at the characteristicRf of IAA. A Commelina leaf-epidermis assay shows the presenceof one stomata-closing ABA-like substance in root caps and onein meristems, one corresponding in Rf to the main root-growthinhibitor from the root cap. The implications of these findingsfor the geotropic responses of roots is briefly discussed.     

Nuclear and Cell Sizes in Different Regions of Root Meristems of Zea mays L.

Nuclear volumes and cell areas were determined for seven regionsof the meristem of roots of Zea mays. Roots were fixed in 10per cent neutral buffered formalin, in 3 per cent glutaraldehydeor in acetic acid/alcohol; they were prepared as sections oralls were teased apart. Mean volumes of interphase nuclei weresimilar in all regions of the root except the vascular tissueof the stele. Mean nuclear volumes and the overall range ofvolumes were similar in sub-populations of cells with differentproportions of G¹, S and G² cells, e.g. in row I of root capinitials, whose cells lack a G¹ phase, and in quiescent centrecells, which are mainly in G¹. Nuclear volume does not appearto be closely correlated with DNA content. Nuclear volumes covereda 6 to 12-fold range within a meristem and even within specificregions, in which cells are part of the same cell lineages,there was a 4- to 9-fold range. Nuclear volumes were comparedin sister cells in rows I and II of the root cap initials. In10 per cent of the pairs, sister nuclei had identical volumes;the other pain had different volumes and mean difference was68 µm³. Mechanisms by which this variability could begenerated are discussed, particularly asymmetry, at mitoses,of factors that regulate nuclear growth. Zea mays L., nuclear volume, cell size, root mcristem, DNA content, mitosis     

Comparison of [14C]oryzalin Uptake in Root Segments of a Sensitive and a Resistant Species

Uptake of the dinitroaniline herbicide oryzalin (3,5-dinitro-N⁴,N⁴-dipropylsulfamlamide) and its effect on root growth werestudied using 5 mm corn (Zea mays L.) and pea (Pisum sativum)root apices. Pea root growth was much less susceptible to oryzalinthan corn root growth. Uptake studies showed that pea root apicesalso accumulated much less [¹⁴C]oryzalin and had a lower bindingaffinity for this herbicide. [¹⁴C]oryzalin was not metabolizedin root apices from either species. Thus, the differential susceptibilityto oryzalin in the case of corn versus pea can be explained,at least in part, by differences in oryzalin uptake and accumulationby roots. Oryzalin, dinitroaniline herbicides, Zea mays, Pisum sativum     

Cell Displacement Through the Columella of the Root Cap of Zea mays L

Exposing roots of Zea mays to a solution of caffeine for 1 hinduces a small population of binucleate cells in the meristem.The progress of the binucleate cell population was then followed,in time, as it was displaced along the length of the cap columella.Since this method of marking cells seems to have no effect onthe subsequent pattern of cell proliferation in the cap meristem,the movement of the binucleate cells through the cap is inferredto be similar to the movement of cells in an undisturbed cap.The binculeate cells that persist in the cap are believed tobe cells that were engaged in their final mitosis at the timeof the caffeine treatment, so the time that it takes for themto appear at the edge of the cap is a measure of the periodfor which a cell is contained in the non–dividing portionof the tissue before being lost from the cap surface. In rootsof Zea grown at 22 °C cells take about 7 days to reach thetip of the cap columella and about 2 to 3 days to reach theflanks of the cap following their displacement from the capmeristem. Zea mays, root cap, cell displacement, binucleate cells     

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     

Movement of Endogenous Calcium in the Elongating Zone of Graviresponding Roots of Zea mays

Endogenous calcium (Ca) accumulates along the lower side ofthe elongating zone of horizontally oriented roots of Zea mayscv. Yellow Dent. This accumulation of Ca correlates positivelywith the onset of gravicurvature, and occurs in the cytoplasm,cell walls and mucilage of epidermal cells. Corresponding changesin endogenous Ca do not occur in cortical cells of the elongatingzone of intact roots. These results indicate that the calciumasymmetries associated with root gravicurvature occur in theoutermost layers of the root Calcium, corn, gravitropism (root), Zea mays     

A Morphometric Analysis of the Redistribution of Organelles in Columella Cells of Horizontally-oriented Roots of Zea mays

In order to determine what structural changes in graviperceptivecells are associated with the onset of root gravicurvature,the redistribution of organelles in columella cells of horizontally-oriented,graviresponding roots of Zea mays has been quantified. Rootgravicurvature began by 15 min after reorientation, and didnot involve significant changes in the (i) volume of individualcolumella cells or amyloplasts, (ii) relative volume of anycellular organelle, (iii) number of amyloplasts per columellacell, or (iv) surface area or cellular location of endoplasmicreticulum. Sedimentation of amyloplasts began within 1 to 2min after reorientation, and was characterized by an intenselystaining area of cytoplasm adjacent to the sedimenting amyloplasts.By 5 min after reorientation, amyloplasts were located in thelower distal corner of columella cells, and, by 15 min afterreorientation, overlaid the entire length of the lower cellwall. No consistent contact between amyloplasts and any cellularstructure was detected at any stage of gravicurvature. Centrally-locatednuclei initially migrated upward in columella cells of horizontally-orientedroots, after which they moved to the proximal ends of the cellsby 15 min after reorientation. No significant pattern of redistributionof vacuoles, mitochondra, dictyosomes, or hyaloplasm was detectedthat correlated with the onset of gravicurvature. These resultsindicate that amyloplasts and nuclei are the only organelieswhose movements correlate positively with the onset of gravicurvatureby primary roots of this cultivar of Zea mays. Zea mays, root gravitropism, ultrastructure, morphometry, graviperception     

The Possible Involvement of Root-cap Mucilage in Gravitropism and Calcium Movement Across Root Tips of Allium cepa L

Roots of Allium cepa L. grown in aerated water elongate rapidly,but are not graviresponsive. These roots (1) possess extensivecolumella tissues comprised of cells containing numerous sedimentedamyloplasts, (2) lack mucilage on their tips, and (3) are characterizedby a weakly polar movement of calcium (Ca) across their tips.Placing roots in humid air correlates positively with the (1)onset of gravicurvature, (2) appearance of mucilage on tipsof the roots, and (3) onset of the ability to transport Ca polarlyto the lower side of the root tip. Gravicurvature of roots previouslysubmerged in aerated water is more rapid when roots are orientedvertically for 1–2 h in humid air prior to being orientedhorizontally. The more rapid gravicurvature of these roots correlatespositively with the accumulation of mucilage at the tips ofroots during the time the roots are oriented vertically. Therefore,the onset of gravicurvature and the ability of roots to transportCa to the lower sides of their tips correlate positively withthe presence of mucilage at their tips. These results suggestthat mucilage may be important for the transport of Ca acrossroot caps. Allium cepa, root gravitropism, root mucilage, calcium, onion     

Root Gravitropism in a Cultivar of Zea mays Whose Columella Cells Contain Starch-deficient Amyloplasts

Starch occupies 4.2 per cent of the volume of plastids in calyptrogencells in primary roots of Zea mays L. cv. vp-7 wild type. Plastidsin calyptrogen cells are distributed randomly around large,centrally located nuclei. The differentiation of calyptrogencells into columella cells is characterized by cellular enlargementand the sedimentation of plastids to the bottom of the cells.Although sedimented plastids in columella cells do not containsignificantly more starch than those in calyptrogen cells, primaryroots are graviresponsive. The onset of root gravicurvatureis not associated with a significant change in the distributionof plastids in columella cells. These results indicate thatin this cultivar of Z. mays (1) the sedimentation of plastidsin columella cells is not based upon their increased densityresulting from increased starch content alone, (2) starch-ladenamyloplasts need not be present in columella cells for rootsto be graviresponsive, and (3) the onset of root gravicurvaturedoes not require a major redistribution of plastids in columellacells. Columella cell, gravitropism (root), plastids, root cap, Zea mays     

Growth and Graviresponsiveness of Primary Roots of Zea mays Seedlings Deficient in Abscisic Acid and Gibberellic Acid

Moore, R. and Dickey, K. 1985. Growth and graviresponsivenessof primary roots of Zea mays seedlings deficient in abscisicacid and gibberellic acid.—J. exp. Bot. 36: 1793–1798. The objective of this research was to determine if gibberellicacid (GA) and/or abscisic acid (ABA) are necessary for graviresponsivenessby primary roots of Zea mays. To accomplish this objective wemeasured the growth and graviresponsiveness of primary rootsof seedlings in which the synthesis of ABA and GA was inhibitedcollectively and individually by genetic and chemical means.Roots of seedlings treated with Fluridone (an inhibitor of ABAbiosynthesis) and Ancymidol (an inhibitor of GA biosynthesis)were characterized by slower growth rates but not significantlydifferent gravicurvatures as compared to untreated controls.Gravicurvatures of primary roots of d-5 mutants (having undetectablelevels of GA) and vp-9 mutants (having undetectable levels ofABA) were not significantly different from those of wild-typeseedlings. Roots of seedlings in which the biosynthesis of ABAand GA was collectively inhibited were characterized by gravicurvaturesnot significantly different from those of controls. These results(1) indicate that drastic reductions in the amount of ABA andGA in Z. mays seedlings do not significantly alter root graviresponsiveness,(2) suggest that neither ABA nor GA is necessary for root gravicurvature,and (3) indicate that root gravicurvature is not necessarilyproportional to root elongation. Key words: Abscisic acid, Ancymidol, Fluridone, gibberellic acid, root gravitropism, Zea mays     

Regeneration of the Root Cap of Zea mays L. and Pisum sativum L.: A Study with the Scanning Electron Microscope

Removal of the root cap from a root apex initiates regenerationof a new cap. The process has been followed using scanning electronmicroscopy. Quantitative data have been obtained for the growthin area of the exposed acroscopic surface of the quiescent centre(QC) and the increase in volume of the regenerating cap tissue.In Zea the surface of the QC shows an initial rapid increasein area followed by a slower increase. In Pisum the surfacearea increases uniformly, a rapid initial phase being absent.Together with observations on the behaviour of an incision atthe exposed surface, the results indicate that in Zea the capnormally imposes a constraint upon radial growth at the acroscopicsurface of the QC; in Pisum the QC appears not to be so constrained.The different responses may be related to the different arrangementsof cells at the apex of the meristem of these two species. Zea mays, Pisum sativum, maize, peao, scanning electron microscopy, root apex, regeneration