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     

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     

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.     

Root Cap Inhibitor Formation in Isolated Root Caps of Zea mays

Root caps were isolated and cultured aseptically on variousdefined media. Under appropriate culture conditions plus suitableillumination a substance able to produce a positive geotropicresponse (i.e. a downward bending of roots) was formed in isolatedroot caps. The presence of this substance, or root cap inhibitor,was assessed by substituting cultured caps in place of capsof dark-grown roots. Normally these roots if kept in continuousdarkness will not respond to gravity (i.e. no bending). Optimalroot cap inhibitor production occurs on a relatively simplemedium, lacking sucrose, but supplemented with 10^–9 MIAA. Protein synthesis is necessary for inhibitor productionand/or expression, whereas DNA synthesis is not.     

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     

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     

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.     

The Proportion of Cells that Divide in Root Meristems of Zea mays L.

The proportion of cells that divide in four regions of the rootmeristem of Zea mays has been determined by an analysis of itscells pulse-labelled with tritiated thymidine. In the quiescent centre less than half of the cells divide andthe fastest of these (less than half of them) have a mitoticcycle duration of about 40 h at 23 °C compared with a cell-doublingtime of 230 h for the region. In the cap initials 80–90per cent of the cells divide and about 80 per cent of thesedivide once in 10 h. In the stele about 80 per cent of cellsdivide near the quiescent centre and all divide at 200 µmfrom the quesecent centre. The fast cells divide every 14 hin both regions, but the cell-doubling time increases from 18to 25 h near the quiescent centre. The root cap is completely replaced by its initials every dayand 10 000 cells are sloughed off. The rest of the meristemadds about 170 000 cells to the root every day. These figures are discussed in relation to the role of the quiescentcentre and the control of cell division.     

Characterizing Pathways by Which Gravitropic Effectors Could Move from the Root Cap to the Root of Primary Roots of Zea mays

Plasmodesmata linking the root cap and root in primary rootsZea mays are restricted to approx. 400 protodermal cells borderingapprox. 110000 µm² of the calyptrogen of the root cap.This area is less than 10% of the cross-sectional area of theroot-tip at the cap junction. Therefore, gravitropic effectorsmoving from the root cap to the root can move symplasticallyonly through a relatively small area in the centre of the root.Decapped roots are non-responsive to gravity. However, decappedroots whose caps are replaced immediately after decapping arestrongly graviresponsive. Thus, gravicurvature occurs only whenthe root cap contacts the root, and symplastic continuity betweenthe cap and root is not required for gravicurvature. Completelyremoving mucilage from the root tip renders the root non-responsiveto gravity. Taken together, these data suggest that gravitropiceffectors move apoplastically through mucilage from the capto the root. Calyptrogen, open meristem, protoderm, root cap, root gravitropism, Zea mays     

Osmotic Stress Decreases the Activity of ATPase Associated with the Root Cap Plasmodesmata in Zea mays

With light and electron microscopy the substructural change and the ATPase activity of corn (Zea mays L. ) root cap cells after short-term osmotic stress were studied. Some spoke-like fine strands originating from the departed periplasm and stretching towards cell wall could be observed even after plasmolysis. By observing the precipitation of ATPase activity product (lead phosphate) at plasma membrane and plasmodesmata, it was found that the fine strands were plasma membrane-lined channels surrounding the cytoplasm and that they still firmly connected to the plasmodesmata during plasmolysis. Compared with the control (unstressed), a sharp decrease of ATPase activity in the plasmodesmata of the stressed cells was observed. Inhibition of energy metabolism in these limited locales would affect the physiological activity, maybe including the regulation of permeability and the change of size exclusion limit (SEL) of plasmodesmata.     

Influence of Temperature on Proton Secretion and Hexacyanoferrate (III) Reduction of Zea mays L. Roots

Responses of potassium hexacyanoferrate (III) [HCF(III)] reduction and net proton secretion by Zea mays L. cv Goldprinz roots to changes in ambient temperature were investigated. Arrhenius plots of proton secretion and redox activity showed a constant slope between 5 and 20[deg]C, indicating that reaction kinetics do not change. Proton secretion without HCF(III) was strongly temperature dependent. This dependence was not altered when H+ efflux was stimulated by fusicoccin or by increased K+ concentration. The temperature coefficient for HCF(III) reduction was low, indicating that the velocity of this reaction was limited by apoplastic diffusion of the ferric complex. In the presence of HCF(III) but not hexacyanoferrate (II), temperature dependence of proton efflux markedly declined, indicating fundamental changes in the process(es) contributing to net proton secretion. It is concluded that HCF(III) establishes a proton extrusion path that is directly linked with the reduction reaction.     

Inhibition of Acid-Enhanced Elongation of Zea mays Root Segments by Galactose

The effect of sugars and metabolic inhibitors on the elongation of Zea mays root segments was analyzed by a rhizometer which records the elongation of each of 32 root segments at the same time. Galactose suppressed the acid-enhanced rapid elongation after a lag period of 1.5 hours, but it did not inhibit the slow elongation at pH 7. Mannose was less inhibitory than galactose. Arabinose, xylose, glucose, sucrose, mannitol, and sorbitol caused no inhibition. When galactose was removed after a 1-hour treatment, the elongation was partially recovered. Cycloheximide and 2-deoxyglucose suppressed acid-enhanced elongation when these were applied at the same time as acid treatments, whereas cordycepin (3′-deoxyadenosine) inhibited elongation only if it was applied prior to acid treatment. Over the 9-hour period of elongation studied, the inhibition by galactose was comparable to that of cycloheximide. Since galactose has been reported to suppress the sugar metabolism necessary for the cell wall synthesis, the later phase of acid-enhanced elongation of root segments may at least partially depend on the synthesis or metabolism of cell wall components. The inhibition of root growth by galactose may be partially ascribed to a direct effect on the elongation process in roots, an effect that is enhanced by the acidification of the cell walls.     

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     

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     

Golgi apparatus mediated polysaccharide secretion by outer root cap cells of Zea mays

Summary In the outer cap cells of roots of Zea mays, secretion is accompanied by hypertrophy of dictyosome cisternae with formation of large secretory vesicles. Vesicle contents are subsequently released from the protoplast by fusion of the vesicle membrane with the plasma membrane. The secreted material, a highly hydrated polysaccharide, was localized intracellularly by the periodic acid-Schiff reaction. Under appropriate conditions, the product moves outward through the cell wall after discharge from the protoplast, and appears as a droplet adhering to the root tip. Under conditions where the secretory product accumulates at the inner wall surfaces, no external droplet is formed.The secretory activity has an active phase that is sensitive to metabolic inhibitors and influenced by temperature (Q₁₀>2), and a passive phase that is independent of temperature, insensitive to metabolic inhibitors but sensitive to osmotic agents. The active phase is characterized by a temperature-independent periodicity (3 hours). Sucrose supplied to the growth medium increases the amount of polysaccharide secreted. Polysaccharide synthesis, segregation into vesicles, and discharge from the protoplast are assumed to require active metabolism; the step involving extrusion of polysaccharide through the cell wall region appears to be a passive process influenced by the degree of hydration of the polysaccharide and by cell turgor.Purdue University Agricultural Experiment Station Journal Paper No. 2967; Charles F. Kettering Research Laboratory Contribution No. 261.     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

Phytotropins V. The Effect of 1-(2'-carboxyphenyl)-3-phenylpropane-1,3-dione on the Root and Root Cap of Zea mays L.

Treatment of the primary root of Zea mays L. with the phytotropin1-(2'-carboxyphenyl)-3-phenylpropane-1, 3-dione (CPD) gave riseto nastic curvature, loss of georesponse, and root growth inhibition.From these results and a histological examination of the rootcap, it is suggested that interference with the perception mechanismmay not be a factor in the mode of action of CPD. The resultsmay be explained in terms of the known hormone transport inhibitingproperties of the phytotropins.     

Localization of Acid Hydrolase Activity in Zea mays L. Root Tips

Naphthol AS-BI phosphatase, esterase, aryl sulphatase, glucuronidase,and ß-glycerophosphatase have been studied in frozensections of maize root tips. In general these enzymes showedhighest activities at the root surface and at particulate sitesin the cytoplasm although the indigogenic method for esteraseshowed no particulate activity and the naphthol AS-D acetatereaction gave no pronounced surface activity. With electronmicroscopy highest activity for ß-glycerophosphatasewas observed in the cell walls and associated with the vacuoles.The significance of these observations are discussed in relationto the function of surface hydrolytic activity and to the presenceof lysosome-like bodies in higher plant cells.