Estimation of Growth Fractions in Meristems of Zea mays L.

Three ways of estimating the fraction of cycling cells in ameristematic population have been examined region by regionthroughout the root meristem of Zea mays. Only in the quiescentcentre and in the nearby proliferating regions did the threemethods produce similar results. The estimate based on the ratioof the average cycle duration of cycling cells and the cell-doublingtime is the most reliable. The two methods that use labellingindex either immediately after a short pulse of ³H-thymidineor after a long labelling period exaggerate the growth fractionby counting endoreduplicating cells which become prominent evenin the proximal half of the meristem and in the second and subsequenttiers of the cap meristem. The nature of the non-cycling cellsis discussed.     

A Morphometric Analysis of the Ultrastructure of Columella Statocytes in Primary Roots of Zea mays L.

A morphometric analysis of the ultrastructure of columella statocytesin primary roots of Zea mays was performed to determine theprecise location of cellular organelles in graviperceptive cells.Vacuoles occupy the largest volume in the cell (11.4 per centof the protoplasm). The nucleus (9.51 per cent), amyloplasts(7.57 per cent), mitochondria (3.42 per cent), spherosomes (2.13per cent) and dictyosomes (0.55 per cent) occupy progressivelysmaller volumes of the statocytes. All organelles are distributedasymmetrically within the cell. Amyloplasts, spherosomes anddictyosomes are found in greatest numbers (and relative volumes)in the lower (i.e. ‘bottom’) third of the cell.The largest numbers and relative volumes of mitochondria arein the lower and middle thirds of the cell. Nuclei tend to befound in the middle third of the statocytes. Only the hyaloplasmis concentrated in the upper (i.e. ‘top’) thirdof Z. mays statocytes. When the sedimentation of amyloplasts(and the resulting exclusion of other organelles from the lowerthird of the cell) is corrected for, all cellular constituentsremain asymmetrically distributed within the cell. Therefore,the sedimentation of amyloplasts alone is not responsible forthe differential distribution of other cellular organelles inZ. mays statocytes. The quantitative ultrastructure of Z. maysstatocytes is discussed relative to the graviperceptive functionof these cells. Zea mays, corn, maize, root cap, stereology, columella, statocytes, graviperception, ultrastructure     

Nuclear Chromatin Structure in Relation to Cell Differentiation and Cell Activation in the Cap and Quiescent Centre of Zea mays L.

Barlow, P. W. 1985. Nuclear chromatin structure in relationto cell differentiation and cell activation in the cap and quiescentcentre of Zea mays L —J. exp. Bot. 36: 1492–1503.Nuclear chromatin structure has been analysed by electron microscopyof thin sections of cells in four zones of the root cap—meristem,central, slime-secreting and outermost cells—and alsoin the quiescent centre of the root before and after decapping.The chromatin pattern has been related to the DNA and RNA syntheticactivity of the nuclei. During cap cell maturation there wasa progressive condensation of the chromatin and this was accompaniedby some reduction of RNA synthesis. The degree of condensationwas estimated from the area and number of pieces of electrondense chromatin which increased and decreased, respectively,during cap maturation. The volume fraction of condensed chromatinwas also estimated but, in the cap, was not found to be a goodindicator of nuclear activity. The outermost cells of the capshowed the greatest degree of chromatin condensation but werestill active in RNA synthesis. Microdensitometry of their nuclearDNA contents gave an indication of loss of DNA in some of thenuclei. Decapping activated DNA and RNA synthesis in the quiescentcentre and also stimulated a decondensation of chromatin: thenumber of condensed pieces of chromatin increased, and theirsize and volume fraction both decreased 4 h after decapping.The number of pores per unit length of nuclear envelope profilewas also estimated. In the cap this number increased duringcap maturation; in the activated quiescent centre the numberremained constant except for a small rise 4 h after decapping Key words: Zea mays, chromatin, root cap, quiescent centre     

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     

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     

Deterministic Patterns of Cellular Growth and Division Within a Meristem

The primary root meristem of maize (Zea mays L.) is composedof longitudinal files of cells arranged in groups of familialdescent (sisters, cousins, etc.). In the proximal portion ofthe meristem, the cells in these groups, or packets, show orderedsequences of division that are transverse with respect to theapico-basal axis of the root. The division sequences fall intoa relatively small number of pathways which can be describedusing deterministic 'bootstrap' L-systems. Although these systemscan operate through the assignment of determinate lifespansto sister cells which thus specify their subsequent interdivisionalperiod, because of their exponential growth kinetics the systemscan also operate with determinate units of cell extension. Thisdeterministic type of system allows simulation not only of thedivision sequences, but also of the lengths of the cells thatare present within the packets which participate in the differentdivision pathways. The types of L-systems used to describe thesepathways also predict the distributions and ranges of cell andpacket lengths found after varying numbers of cell generations.These distributions compare favourably with those actually foundin the maize root meristem. Theoretical aspects of bootstrapL-systems, essential for their application to the one-dimensionalcellular arrays of the meristematic cell-files of the maizeroot apex, are also presented.Copyright 1994, 1999 AcademicPress Cell division, cell elongation, cell polarity, L-system, root meristem, Zea mays     

Lateral Root Anlage Development in Excised Roots of Vicia faba L., Pisum sativum L., Zea mays L. and Phaseolus vulgaris L.

The development of lateral root primordia has been investigatedin excised roots of Vicia faba, Pisum sativum, Zea mays andPhaseolus vulgaris cultured in White's medium supplemented with2 per cent sucrose and compared with previously published dataon such development in primaries of the corresponding intactplants (control roots). Primordia were produced in each batchof excised roots over the 6 day culture period but at a lowerrate (number day^–1) than in the controls. Such primordia in cultured roots of Zea and Phaseolus completedtheir development and grew out as lateral roots over a periodsimilar in length to that found in the controls, but with acell number of only about 33 per cent of that attained at thetime of secondary emergence in the primaries of the latter roots.These lower cell numbers were at least partly a reflection ofincreases in mean cell doubling time over the period of anlagedevelopment investigated in the excised roots relative to thecorresponding values found in the controls. Primordia initiated in excised roots of Pisum and Vicia didnot complete their development in culture, i.e. no lateral rootsemerged and arrest took place with cell numbers of only 37 (Pisum)and 17 (Vicia) per cent of the numbers determined at the timeof secondary root emergence in the controls. Such arrested primordiahad few nuclei in S and none in mitosis. Moreover, at leastin Pisum, the frequency distribution of the relative DNA contentof the nuclei in the latter primordia approximated that foundin the apical meristem of primary roots following the establishmentof the stationary phase under conditions of carbohydrate starvation. It has also been demonstrated in the course of these investigationsthat lateral root primordium development in all four speciesis at least biphasic and possibly triphasic. Vicia faba L., broad bean, Pisum sativum L., garden pea, Zea mays L., maize, Phaseolus vulgaris L., dwarf bean, root primordia, anlage, cell doubling time, lateral root emergence     

Flow Cytometric Determination of Relative Nuclear DNA Contents in Bicellulate and Tricellulate Pollen

Nuclear DNA content in mature pollen was measured with a flowcytometer Pollen of Lilium longiflorum, Dendranthema grandiflora(syn Chrysanthemum monfolium) and Zea mays was chopped and stainedwith the DNA fluorochrome DAPI DNA levels, expressed as arbitraryC values, were compared with those of nuclei isolated from leafor root material of the same plants In mature tricellulate pollen the generative cell is dividedafter second pollen mitosis into two sperm cells Tricellulatepollen from maize and chrysanthemum gave rise to one large 1Cpeak and, only in the case of chrysanthemum, a much smallerone at the 2C level These results suggest that the haploid nucleiof the vegetative as well as both sperm cells in tricellulatepollen are arrested in the G₁ stage of nuclear division Thesmall 2C peak in the case of chrysanthemum probably arose froma fraction of pollen with the sporophytic chromosome number(2n pollen) In contrast to this, mature bicellulate lily pollengave rise to two identical peaks at the 1C and the 2C levelFrom this result it was concluded that in bicellulate pollen,the 1C peak is caused by the signal of the haploid vegetativenucleus arrested in the G₁ stage of nuclear division, whereasthe 2C peak originates from the haploid generative nucleus whichhas already undergone DNA synthesis and is arrested in G₂ Lilium longiflorumThunb, lily, Dendranthema grandiflora Tzelev (syn Chrysanthemum morifolium Ramat ), chrysanthemum, Zea maysL, maize, male gametophytic cells, vegetative cells, generative cells, sperm cells, unreduced pollen, sporophytic cells, relative nuclear DNA contents, replication stage     

Use of Lanthanum to Trace Apoplastic Solute Transport in Intact Plants

This electron microscopic study revealed that solutes enterthe apoplasm of root mcristems and move from there to the steleof the root and to the shoots of intact plants. Lanthanum wasused as a plasma membrane-impermeable electron-dense markerof apoplastic solute flux in Hordeum vulgare L., Saiicorniavirgimca L., Spartina alternflora Loisel. and Zea mays L. Thepresence of lanthanum in EM specimens was confirmed by X-raymicroanalysis. Lanthanum that entered the root apoplasm wasalso localized in membrane-bound compartments within cells ofeach plant Lanthanum was localized in vesicles, ER, and vacuolesof root and leaf cells. Following root application, lanthanumwas evident in the leaves of the three grass species studied.Lanthanum was rarely observed in S. virginica leaves. Only plantsexposed to 23 mol m^–3 lanthanum for 24 h or more showedlanthanum in root cell cytosol and this was concluded to bea toxic response. Key words: Apoplast, halophyte, lanthanum, root meristem, transport     

The Nuclear Endoreduplication Cycle in Metaxylem Cells of Primary Roots of Zea mays L.

The nuclear DNA content of metaxylem cells in roots of Zea mayscv. Golden Bantam reaches 16C or 32C by successive rounds ofDNA endoreduplication. Each phase of endoreduplication (endo-S)is separated by a non-DNA synthetic phase (endo-G). These phasesseem to occur in zones at fixed distances from the root tip.The duration of the phases in two of the endoreduplication cycles(4C–8C, 8C–16C) has been estimated in two ways.The first makes use of the rate of movement of cells throughthe positions along the root where the different phases of thecycle are occurring, the second uses labelling with methyl-[³H]thymidineand autoradiography. Both methods indicate that the endo-S phaseswhich cause the nuclear DNA content to rise from 4C to 8C andfrom 8C to 16C last 8–10 h, and that the intervening endo-Gphase lasts 8–12 h. DNA endoreduplication keeps pace withthe increase of nuclear volume; cell volume increases at a morerapid rate, however. Comparison of the endoreduplication cyclein the metaxylem with the mitotic cycle in the adjoining filesof parenchyma cells shows that the mitotic cells complete theircycle more slowly. DNA synthesis, endoreduplication cycle, mitotic cycle, root apex, Zea mays     

The Induced Resistance Response of Carrot Root Slices to Heat-killed Conidia and Cell-free Germination Fluid of Botrytis cinerea Pers. ex Pers.: 2. Nuclear Migration, Nucleolar Volume and Uptake of Tritiated Uracil

Sixty-eight per cent of nuclei in the cells of the upper fourlayers of carrot slices treated with heat-killed conidia ofBotrytis cinerea for 6 h followed by inoculation with live sporesfor 18 h, migrated to the cell face nearest to the treated surface,compared with 46 per cent in cells of control slices showinga wound-healing response only. Nucleolar volumes in the surfacecell layers of control slices increased from a mean of 1.0 µm³to 3.8 µm³ over 24 h, and in ‘induced’ slicesto 7.28 µm³. Using a 40 min pulse of [5–³H]uracil,there was an increase within 15 h of slicing in the number oflabelled nuclei in cells from control slices undergoing healing.Within 8 h after treatment of slice surfaces with heat-killedconidia, there was an accelerated incorporation of label into‘nuclear’ RNA. Slices from roots cold-stored for12 months failed to show an induction response and nucleolarvolumes did not increase more than in control slices. Theseresults are discussed in relation to active defence mechanismsin plant tissue. Botrytis cinerea, carrot, induced resistance, nuclear migration, nucleolar volume, RNA incorporation     

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     

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 Application of 31P Nuclear Magnetic Resonance to Higher Plant Tissue: II. DETECTION OF INTRACELLULAR CHANGES

^3IP nuclear magnetic resonance (NMR) spectra are used to monitorintracellular changes in sections of potato tuber (Solariumtuberosum), maize root tips (Zea mays) and the roots of singlemaize seedlings. Intracellular changes that result from theuptake of inorganic phosphate, D-mannose, the spectroscopicbroadening probe Mn²+ and 2, 4-dinitrophenol are described.It is concluded that NMR provides a powerful, direct methodfor following intracellular changes in plant tissues.     

The Difference Between Open and Closed Meristems

An open and a closed root meristem have been compared by investigatingthe cell kinetics of small regions of the apices of Helianthusand Zea. The cells of the stelar pole are quiescent in both and thereis no exchange of cells between stele and cortex or stele andcap. The immediately distal cells in the closed meristem (Zea)are also quiescent and the few divisions that do occur can betransverse or longitudinal. In the open meristem (Helianthus)these cells are not quiescent, but they go out of cycle transiently,prolonging the potential cell-doubling time. Their divisionsare transverse. It is a consequence of these differences thatclosed meristems form root caps discrete from the cortex whereasopen meristems force instability in the boundary between theperipheral part of the cap and the cortex. Another consequencein roots with open meristems is a succession of columella complexestransversely displaced from each other by the state of fluxin the meristem during the non-cycling phase of the proximaltier of cells, those immediately distal to the stelar pole. The results are discussed in relation to the ontogenetic onsetof quiescence and the evidence for switches between open andclosed operation of meristems. meristem, root apex, Helianthus annuus, Zea mays L.     

The Position and Characteristics of 'Leaky' Cells in Root Tip Meristems of Helianthus annuus

Sunflower root meristems are composed of two populations ofcells which respond differently to stress. One population becomesarrested in G₁ and G₂, while the second ‘leaky’population (0.25–1.0 per cent) is able to pass throughS even during carbohydrate starvation. Leaky cells enter S ata rate of 0.06 per cent cells h^–1 after 48 h of starvation.The character of leakiness is retained by roots starved fortwo successive 48 h starvation periods separated by an 8 h sucrosepulse. Single and double layer autoradiograph experiments demonstratedthat leaky cell progeny maintain their leaky character throughat least two cell generations. Leaky cells are located at randomin the root cap, promeristem, ground meristem, protoderm, cortex,and pericycle. The presence of leaky cells may indicate a stressresponse mechanism to repopulate the root meristem.     

NaCl Uptake in Roots of Zea mays Seedlings: Comparison of Root Pressure Probe and EDX Data

The extent by which salinity affects plant growth depends partlyon the ability of the plant to exclude NaCl. To study the uptakeof NaCl into excised roots of Zea mays L. cv. ‘Tanker’,two different techniques were applied. A root pressure probewas used to record steady state as well as transient valuesof root (xylem) pressure upon exposure of the root to mediacontaining NaCl and KCl as osmotic solutes. In treatments withNaCl, pressure/time responses of the root indicated a significantuptake of NaCl into the xylem. NaCl induced kinetics were completelyreversible when the NaCl solution was replaced by an isosmoticKCl solution. This indicated a passive movement of Na⁺-saltsacross the root cylinder. Root samples were taken at differenttimes of exposure to NaCl and prepared for X-ray microanalysis(EDX analysis). Radial profiles of ion concentrations (Na⁺,K⁺, Cl^–) were measured in cell vacuoles and xylem vesselsalong the root axis. Na⁺ appeared rapidly in mature xylem (earlymetaxylem) and living xylem (late metaxylem) before it was detectablein vacuoles of the root cortex. EDX results confirmed that thekinetics observed by the pressure probe technique correspondedmainly to an influx of Na⁺-salts into early metaxylem. In latemetaxylem, the uptake of Na⁺ was associated with a decline ofK⁺. The Na⁺/K⁺ exchange indicated a mechanism to reduce sodiumfrom the transpiration stream. Ion localization, ion transport, maize, root pressure, salinity, water relations, X-ray microanalysis, Zea mays     

The Effects of Modifying Sucrose Concentration on the Development of Maize Kernels Grown in vitro

Intact Zea mays L. kernels attached to cob tissue develop tomaturity when grown in vitro. This experiment was designed todetermine if it is possible to prolong kernel growth by refreshingthe culture medium. Blocks of maize kernels were grown in vitroon media containing several concentrations of sucrose. Kernels,at all concentrations of sucrose, developed to maturity at 30–35d post-pollination, indicating that it is not possible to extendthe kernel growth phase by supplying a carbohydrate source.Kernels grown on media containing 80 g 1^–1 or higher sucroseconcentration had a significantly greater percentage of kernelsthat developed to maturity, and had greater weight and starchcontent per seed. Zea mays, kernel culture, seed development, starch     

The Response of the Primary Root Meristem of Zea mays L. to Various Periods of Cold

Primary roots of maize (Zea mays L.) grown in nutrient solutionat 5?C elongate at about 1% of the rate found at 20?C. The apicalmeristem becomes shorter and shows little proliferative activityat 5?C, but following transfer to 20?C mitoses increase in frequencyand the meristem regrows to its original length. Both the amountby which the meristem shortens and the time for its completeregrowth are related to the period spent at 5?C. The shorteningof the meristem suggests that at the lower temperature the balancewhich normally exists between cell production and differentiationis altered, the latter continuing at a relatively faster ratethan the former. A new, steady-state balance between the twoprocesses is re-established during the recovery period. Themeristem recovers as a result not only of its own mitotic activitybut also through stimulation of cell division in the quiescentcentre. The degree to which the quiescent centre is activated,as judged by its mitotic index and the number of nuclei labelledby feeding with tritiated thymidine, increases as the durationof the preceding cold treatment increases. The close relationshipbetween proliferative activity in the quiescent centre and theminimum length of the meristem following the cold treatmentsuggests that there is communication between these two zoneswhich co-ordinates their respective rates of cell productionand helps to maintain a normal meristem structure. The resultsemphasize the importance of the quiescent centre as a reservoirof cells that can re-establish a meristem rendered non-functionalthrough the impact of unfavourable environmental conditions. Key words: Meristem, quiescent centre, root, temperature, Zea mays