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.     

Sloughing of Root Cap Cells

Root cap cells of Zen mays sloughed into water have been countedat short intervals and after 24 h. The results have been comparedwith rates of cell production calculated from the use of thestathmokinetic agent, colchicine, on the four active tiers ofthe cap meristem. More cells are produced by the cap meristem and sloughed bythe cap when roots are grown in low densities in water or whenthe water is changed at more frequent intervals. The range atbetween 250 and 50 roots per litre is from 3000 to 7000 cellsper root per day for seedling primary roots grown continuouslyin water at 23 °C for 24 h The results are discussed in relation to previous estimatesof cap cell proliferation and the possibility of periodic sloughingof cells and slime.     

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     

Models of the Use of Root-zone CO2 by Selected North American Isoetids

Sediment CO₂, entering via the roots, contributes a significantportion of the total carbon uptake for isoetids (small, evergreen,submersed, vascular plants). Laboratory studies of inorganiccarbon uptake via the roots and shoots by five isoetids wereused to model the use of root-zone CO₂. Simple first-order linearmodels accounted for at least 75 per cent of the variation inthe data for Gratiola aurea, Isoetes macrospora, Littorellauniflora and Lobelia dortmanna. For Eriocaulon septangulare,which relies almost exclusively on root-zone CO₂, models couldaccount for only about 62 per cent of the variation in root-zoneCO₂ use. For each species, we present the best fitting regressionof root-zone CO₂ use as a function of root- and shoot-zone CO₂concentrations. For the species studied, carbon uptake was not saturated atfield concentrations of root and shoot-zone CO₂. Maximum ratesof carbon uptake were lower for species that naturally occurredat greater depths, compared with species more common in shallowwater. At equal external CO₂ concentrations carbon entry perunit root surface area was several times more rapid than entryper unit shoot surface area for L. dortmanna. The entry ratesper unit root and shoot surface area were about equal for G.aurea and E. septangulare. Shoots were equally or more permeablethan the roots of L. uniflora and I. macrospora, a fact thatmay be related to the functioning of crassulacean acid metabolismin these plants. Carbon, CO₂, photosynthesis, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna     

The Fine Structure of the Quiescent Centre and Neighbouring Tissues3

The quiescent centre of the root meristem of Zea provides asource of undiffer-entiated, non-meristematic cells whose finestructure we have compared with that of the meristematic, non-differentiatedcells of the meristem and that of the differentiated, non-meristematiccells of the cap. We have shown an association between the developmentof the endoplasmic reticulum and the development of cell walls.Differentiation in the cap is accompanied by a decrease in thenumber of mitochondria and plastics per unit volume of cytoplasmand an increase in their number per cell. There is an even greaterincrease in the number of Golgi bodies per cell. The structureof the mitochondria becomes more elaborate in differentiatingcells especially in the cap and the Golgi increase their sizein meristematic and differentiating cells. These differencesare discussed in relation to some of the views about radiosensitivityand organization.     

Image Analysis of Maize Root Caps--Estimating Cell Numbers from 2-D Longitudinal Sections

The cap of the primary root of maize produces several thousandborder cells that are shed from the outside of the cap eachday. Border cell production is important in the penetrationof soil by roots, and in influencing the activity of both beneficialand pathogenic organisms in the rhizosphere. To improve understandingof the dynamics of border cell production, it is desirable toknow the number of cells in different parts of the root cap.An image analysis procedure was used to quantify cell dimensionsand locations in the median longitudinal section of maize (Zeamays L.) root caps. Calculations based on root symmetry werethen used to estimate the number of cells in 3-dimensions. Ourestimation procedure was tested initially using regular arraysof identical square and hexagonal shapes to represent cells.It was then tested using two different tissues showing analogousarrays: a transverse section through the maize root cap junction,and a transverse section through a barley root. Good linearcorrelations were obtained between the number of cells estimatedand the number of cells actually counted in the microscope.The numbers of cells in the whole maize root cap (8870 ±390) were then estimated from longitudinal sections. These numbersof cap cells agreed with values that had been estimated formaize by other methods. In the first tier of the cap meristem,ten-times more meristematic cells were located in the cap flanks(>500 cells) than were present in the columella portion.Similarly, only 7% of cells in the outermost layer of the rootwere associated with the columella region of the cap, a fractionwhich compared well with previous measurements of sloughed cellsextracted from rhizosphere sand. This present technique canbe applied to estimate the numbers of cells in any cylindricallysymmetrical tissue from two-dimensional sections. Copyright2001 Annals of Botany Company Anatomy, border cells, cell production, image analysis, maize, rhizosphere, root cap, sloughing, stereology, Zea mays L     

In Vitro Transformation of Root meristem to Shoot and Plantlets in Vanilla planifolia

A study is reported of histogenesis and organogenesis duringthe processes leading up to plantlet formation in tip culturesof aerial roots of Vanilla planifolia. Young root tips excisedfrom aerial roots, less than 15 cm long, when cultured in liquidMS medium containing IAA and KN showed gravitropic responseuntil cap lysis began. With the collapse of the distal halfof the cap, the cells of the quiescent centre divided forminga hemispherical mass of cells. Further localized divisions onthe periphery of the hemisphere resulted in a number of meristemoidseach of which differentiated into a shoot meristem with leafprimordia. Procambium differentiated first beneath the apicalmeristem after two to three leaf primordia had formed and thenat the base of the leaves. After a few leaves have been formeda root meristem differentiated in close lateral proximity tothe basal end of the shoot procambium. Formation of a plateof vasculature at the nodal region of the first formed leaf,procambialization of the root and the bridging up of the shootand root vasculature with the nodal plate are described. Vanilla planifolia, root tip, in vitro, quiescent centre, meristemoid, plantlet     

Cytochemical Localization of Phenolic Compounds in Columella Cells of the Root Cap in Seeds of Brassica napus--Changes in the Localization of Phenolic Compounds during Germination

Changes in the localization of phenolic compounds were investigatedin columella cells of embryonic roots in 1-year-old Brassicanapus seeds during 48 h of imbibition and germination. In dry,dormant seeds, phenolic compounds were located in the apoplasticcompartment between the cell wall and plasmalemma in the outermostlayer of the columella. These apoplastic phenolic deposits disappearedduring the activation processes associated with imbibition andgermination, but new deposits appeared successively in the nucleus,ER cisternae, protein bodies and on the outer surface of theroot cap. A large number of phenolic deposits were observedin the outermost part of the columella, becoming less frequenttowards the initial centre. Their appearance coincided withrestoration of the ER and immediately preceded cytological activation.After the primary root had emerged from the seed coat, depositsof phenolic compounds disappeared from the cytoplasm, but simultaneouslyappeared in the vacuoles. Copyright 1999 Annals of Botany Company Brassica napus var. oleifera, root columella, germination, phenolic compounds localization.     

Localization of Nucleic Acid Synthesis in Root Meristems

Adenine-8-C1¹⁴ was supplied to roots of Vicia faba and Alliumascalonicum and its incorporation into DNA was studied fromautoradiographs of hydrolysed sections. These roots have a quiescentcentre to the meristem where the cells do not synthesize DNAand probably, therefore, play no part in the construction ofthe root. The boundary between the quiescent centre and thecentral cap initials is clearly denned and this suggests thatthere is as little cell interchange between the histogens asthere is in roots with visibly discrete histogens.     

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     

Meristem-Specific Suppression of Mitosis and a Global Switch in Gene Expression in the Root Cap of Pea by Endogenous Signals

Two functionally distinct sets of meristematic cells exist within root tips of pea (Pisum sativum): the root apical meristem, which gives rise to the body of the root; and the root cap meristem, which gives rise to cells that differentiate progressively through the cap and separate ultimately from its periphery as border cells. When a specific number of border cells has accumulated on the root cap periphery, mitosis within the root cap meristem, but not the apical meristem, is suppressed. When border cells are removed by immersion of the root tip in water, a transient induction of mitosis in the root cap meristem can be detected starting within 5 min. A corresponding switch in gene expression throughout the root cap occurs in parallel with the increase in mitosis, and new border cells begin to separate from the root cap periphery within 1 h. The induction of renewed border cell production is inhibited by incubating root tips in extracellular material released from border cells. The results are consistent with the hypothesis that operation of the root cap meristem and consequent turnover of the root cap is self-regulated by a signal from border cells.     

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     

The Quiescent Centre and Rates of Mitosis in the Root Meristem of Allium sativum

The root apices of Allium sativum have been examined by continuous-and pulse-labelling with tritiated thymidine and by colchicinetreatment to measure the time parameters of the mitotic cyclein various parts of the meristem. There is a quiescent centre of 30–50 cells whose averagerate of mitosis is low because the G₁ period is extended toabout 140 h compared with about 4 h in the othe regions of themeristem. The stele just above the quiescent centre and at 200microns above it and the cap initials just below the quiescentcentre are very similar in their mitotic cycles, the total lengthsof which are about 30 h of which nearly half is taken up byDNA synthesis. Allium thus differs from Zea in having root capinitials whose mitotic cycle is not telescoped by the eliminationof the G₁ phase. These facts are discussed in relation to theradiosensitivity of the meristem.     

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     

Effects of Zinc on Meristem Size and Proximity of Root Hairs and Xylem Elements to the Root Tip in a Zinc-tolerant and a Non-tolerant Cultivar of Festuca rubra L.

A 4 d exposure to zinc (0.1 and 02 µg Zn cm^–3) reducedthe length of the root apical meristem in a Zn-sensitive cultivar(S59) of Festuca rubra L. to a much greater extent than in aZn-tolerant cultivar (Merlin). In S59, Zn treatment also inducedroot hair and xylem formation much closer to the root cap boundarythan in control roots, whereas Merlin was only marginally affectedby Zn treatment. The data are discussed in relation to previouslyestablished effects of Zn on the cell cycle and other cellularcharacters of the two cultivars. zinc, meristem size, root hair, xylem, Festuca rubra     

ROOT APEX STRUCTURE IN EPHEDRA MONOSPERMA AND EPHEDRA CHILENSIS (EPHEDRACEAE)

The root meristem of E. monosperma and E. chilensis possesses a central group of distinctive, large cells. These cells have large nuclei with scattered heterochromatin, proplastids with no starch, small vacuoles, mitochondria, few dictyosomes and endoplasmic reticulum cisternae, and lipid deposits. Over a 24 hr labelling period, the large cells fail to incorporate ³H-thymidine, whereas cells both distal and proximal to this region do. A quiescent center which includes these large cells is present therefore. Both species have an extensive root cap, the length being contributed by mitoses in many tiers of cells distal to the quiescent center. The root cap consists of a columella and peripheral regions. Distinctive amyloplasts, an increase in the number of endoplasmic reticulum cisternae and dictyosomes, large vacuoles, and lipid deposits are characteristic of differentiated columella cells. Peripheral cells elongate, lose most of their starch, and are eventually sloughed from the root.     

Root apical organization inArabidopsis thaliana 1. Root cap and protoderm

Summary We investigated the development of the root cap and protoderm inArabidopsis thaliana root tips.A. Thaliana roots have closed apical organization with the peripheral root cap, columella root cap and protoderm developing from the dermatogen/calyptrogen histogen. The columella root cap arises from columella initials. The initials for the peripheral root cap and protoderm are arranged in a collar and the initiation event for these cells occurs in a sequential pattern that is coordinated with the columella initials. The resulting root cap appears as a series of interconnected spiraling cones. The protoderm, in three-dimensions, is a cylinder composed of cell files made up of packets of cells. The number of cell files within the protoderm cylinder increases as the root ages from one to two weeks. The coordinated division sequence of the dermatogen/calyptrogen and the increase in the number of protoderm cell files are both features of post-embryonic development within the primary root meristem.Abbreviations RCP root cap/protoderm - CI columella initial - PI protoderm initial     

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.     

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     

Tissue-6

In order to study a possible involvement of cdc-like proteinkinases in cell development and tissue differentiation, a polyclonalantibody raised against the evolutionary conserved PSTAIR-regionof p34^cdc2-homologue protein kinases (PSTAIR-proteins) was appliedto sections of the maize root apices. PSTAIR-proteins were localizedin the nuclei and the cytoplasm of cells in the root meristem,including the quiescent centre (QC), and of all dividing cellsthat form the lateral root primordia. In most root tissues,the amount of cytoplasmic PSTAIR-proteins progressively declinedwith increasing distance from the root cap junction, becomingrestricted to the nucleus after the cessation of cell divisions.This occurred much nearer to the root cap junction in cellsof the stele, especially in metaxylem cells, than in cells ofthe root cortex. Interesting exceptions were cells of the pericycle,endodermis and the outermost cell rows of stelar parenchyma,which exhibited relatively high levels of the cytoplasmic PSTAIR-proteinsthroughout all developmental zones. After root wounding, rapid cytoplasmic accumulation of PSTAIR-proteinsin cells adjacent to the wound was observed in all tissues ofthe meristem and of the elongation zone. This wound response,which was usually followed by newly-induced cell divisions,was delayed with increasing distance from the root cap junctionin a tissue-specific manner. Since PSTAIR-proteins were foundin the cell nuclei throughout all developmental zones, theyseem to have some nuclear functions which continue even aftercell division has stopped. Key words: Cell cycle, maize roots, cyclin-dependent protein kinases, wounding