Root Cap Structure in Isoetes macrospora Dur

Root meristem cells of Isoetes macrosporausually have one plastidwhich is associated with the prominent nucleus, numerous ribosomesand mitochondria, and small vacuoles. During mitosis each plastidappears to replicate so that each daughter cell contains oneplastid. The cell walls of the meristem cells are traversedby numerous plasmodesmata. Central cells of the root cap lackdistally displaced plastids but have one or more amyloplastsassociated with the nucleus. These cells also contain largeprotein deposits. Peripheral root cap cells are characterizedby being vacuolated, and by possessing a few dictyosomes andprotein deposits. They appear to be sloughed infrequently. Isoetes macrospora Dur, root cap, protein bodies, 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     

Cap Formation during the Elongation of Lateral Roots of Vicia faba L.

Cell proliferation was examined in the cap initials of newly-emerged0·2 and 4·0 cm long lateral roots of Vicia fabafrom an investigation of the passage of labelled cells throughmitosis following a 1-h pulse with tritiated thymidine. Celldoubling time was found to increase in this group of initialcells as the secondary roots elongated, this increase beinga result of a gradual lengthening in the duration of the mitoticcycle of both the fast and slow cycling meristematic cells andof a decrease in the size of both the growth fraction and thoseproliferating cells with a short cycle time. The rate of cellproduction by the cap initials was maximal in the newly emergedsecondary roots and showed a gradual decline with subsequentlateral elongation. This change in the rate of cell proliferationin the cap initials has been shown to be related to the initiationof the quiescent centre following lateral root emergence fromthe tissues of the primary. The number of cells making up the cap of 0·2–4·0cm long secondary roots is known to be constant and thus therate at which cells are sloughed off of the cap into the soilmust be the same as the rate of cap cell formation in theseroots, all of the cap cells being replaced every 6–9 days.The rate of cap cell formation in 0·2–4·0cm long secondary roots was used to calculate that one 11-dayold Vicia plant will have contributed between 56 000 and 85000 cap cells to the rhizosphere from its root system sincegermination.     

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     

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.     

Cellular Differentiation and Tissue Partitioning in Caps of Primary and Lateral Roots of Helianthus annuus

Cellular and tissue volumes in caps of primary and lateral rootsof Helianthus annuus have been measured in order to determinequantitatively how tissues and their functions are partitionedin root caps. Patterns of change in cellular dimensions andvolumes are similar in caps of primary and lateral roots. Significantincreases in cellular dimensions and volume occur during thedifferentiation of columella cells and the innermost peripheralcells. There are no significant changes in cellular dimensionsas either (i) the production and secretion of mucilage begins,or (ii) cells are sloughed from the cap. Tissues are partitionedsimilarly in caps of primary and lateral roots. indeed, rootcaps allocate 7–8 per cent of their volume for regeneration(i.e. calyptrogen tissue), 16–19 per cent of their volumefor graviperception (i.e. columella tissue), and approx. 38per cent of their volume for the production and secretion ofmucilage. These results are discussed relative to patterns ofcellular differentiation and tissue function in root caps. Helianthus annuus, root caps, primary root, lateral root, calyptrogen, columella, peripheral cells, tissue partitioning     

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 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.     

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     

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.     

Duration of the Mitotic Cycle in a Meristem

The length of the mitotic cycle and the time spent in mitosisare calculated for six regions of the apical meristem of rootsof Zea. The two methods used are the measurement of the ratesof labelling nuclei with radioactive thymidine and the ratesof accumulation of metaphases in roots grown in colchicine.The mitotic cycle lasts from 12 hours in the cap initials toabout 200 hours in the quiescent centre of the same roots. Somepoints about the organization of apices are discussed in thelight of these estimates.     

Meristem Initial Cells in Irradiated Roots of Vicia faba: With one Figure in the Text

Beans (Vicia faba) were germinated for 24 hours and irradiatedwith X-rays. Primary roots were fixed after 9, 11, and 21 days.Aberrant chromosome complements, the result of chromosome changesinduced by irradiation, were present. They were used as cellmarkers to estimate the number of cell types present in themeristem initial cells of the growing root and the number ofprimordium initial cells from which root regeneration occurred.Up to 9 cell types occur as meristem initials. From the relativefrequencies of the different cell types, it is estimated thatthere are at least 32 actual meristem initial cells in regeneratingirradiated primary roots. This result is compatible with observationsmade on normal roots. The chimaerical nature of the regeneratingroot apparently does not interfere with the normal organizationof the meristem, but it serves to reveal what part of the organizationis.     

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     

Cell Cycling and the Fate of Potato Buds

The cell cycling characteristics of the regions of the apicalmeristems of underground shoots and buds of Solanum tuberosumL. were investigated by stathmokinesis and labelling. The apicaldomes of orthotropic shoots produce cells at twice the elementalrate of those of stolons, and their youngest leaf primordiaat twelve times the rate. Changing the fate of stolons so thatthey will become orthotropic by decapitating the tuber sproutsthat bear them results within 24 h in a general increase incell production especially in the leaf primordia. Axillary buds on tuber sprouts induced to become precursorsof orthotropic shoots instead of stolons undergo a spectacularincrease in cell division within 24 h in all regions, especiallyin the primordia and bud anlagen where the rate increases 20-foldor more. The summit is slower to react than other regions, but,by 24 h, its rate of cell division increases 11-fold and itis contributing 14 cells per day to the flanks from its 80 cells. In all the axillary buds the rate of mitosis in the summit ishalf that of the flanks of the apical dome, but, in both stolonsand orthotropic underground shoots, the rate is higher in thesummit than in the flanks or rib meristem. The results are discussed in relation to what is known of cellcycling changes after floral evocation. Solanum tuberosum L., potato, fate, cell division, apical meristem, stathmokinesis     

Sloughing of cap cells and carbon exudation from maize seedling roots in compacted sand

Sloughing of root cap cells and exudation of mucilage plays an important role in the penetration of compacted soils by roots. For the first time we have quantified the rate of sloughing of root cap cells in an abrasive growth medium that was compacted to create mechanical impedance to root growth. The number of maize ( Zea mays ) root cap cells sloughed into sand increased as a result of compaction, from 1930 to 3220 d⁻¹ per primary root. This represented a 12-fold increase in the number of cells sloughed per mm root extension (from 60 to >700). We estimated that the whole of the cap surface area was covered with detached cells in compacted sand, compared with c . 7% of the surface area in loose sand. This lubricating layer of sloughed cells and mucilage probably decreases frictional resistance to soil penetration. The total carbon deposited by the root was estimated at c . 110 μg g⁻¹ sand d⁻¹. Sloughed cells accounted for <10% of the total carbon, the vast majority of carbon being contained in mucilage exudates.     

Some Cytological Changes accompanying the Regeneration of Meristematic Activity at the Apex of Decapitated Roots of Vicia faba L.

The changes that took place in mitotic index (MI), labellingindex (LI) and the relative proportions of interphase nucleiwith different amounts of DNA have been investigated duringthe regeneration of meristematic activity at the apex of rootsof Vicia faba over the 144 h period following removal of thecap and apical mm of the meristem. Measurements were also madeof the corresponding changes that took place as cells were displacedbasally along the root from the apex over the experimental period.In both parts of the root, MI and the relative proportions ofnuclei with different DNA contents changed from levels similarto those at the apex of the controls at the start and end ofthe experiment to levels resembling those found in more matureparts of the root at 24 and 48 h. In contrast to these results,LI declined over the experimental period. These cytologicalchanges were aresult of the development of lateral root primordiain both the apical 2 mm of the decapitated roots and as cellswere displaced out of the meristem into more basal parts ofthe root. It was concluded that the events leading to the regenerationof meristematic activity at the apex of roots from which thecap and apical mm of the meristem were removed, are no differentfrom those which result in lateral formation as cells are displacedbasally along the primary root from the apex, and they takeplace over the same time interval in both systems.     

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.     

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     

Deoxyribonucleic Acid synthesis in root cap cells of cultured roots of convolvulus

Isolated cultured roots of Convolvulus arvensis L. were incubated in 0.2 microcurie per milliliter methyl-³H-thymidine for 14 hours, for 64 hours, or for 14 hours followed by transfer to fresh nutrient medium without tritiated thymidine. Autoradiographs of serial, longitudinal sections of roots which were continuously incubated with tritiated thymidine showed that cells of the root cap columella did not undergo DNA synthesis after their formation from the root cap initials. In roots pulse-labeled with tritiated thymidine, the movement of labeled cells through the root cap columella was followed. Labeled cells were displaced at a constant rate of 72 microns per day over a period of 6 to 9 days before they were sloughed off from the root cap. The specialized role of the root cap cells in relation to their distinctive metabolism and longevity is discussed.     

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.