Inhibition of the Uptake and Long-Distance Transport of Calcium by Aluminium and Other Polyvalent Cations

Aluminium, scandium, and iron inhibit the uptake of calciumby week-old barley plants from acid culture solutions (pH 4.0–4.2).The inhibition by scandium can be detected when its ratio tocalcium is 1:1000. The onset of the inhibition may be quit rapidand will persist for at least. 24 h in the absence of the polyvalentcation. The inhibition caused by 25 and 50 µM aiuminiumsulphate may be overcome if the calicum chloride concentrationin the medium is raised to 15mM, but in this situation aluminiumstill inhibits root growth by more than 50 per cent. Elutionexperiments show that polyvalent cations reduce the amount ofcalcium held in the water free space (WFS) and the Donnan freespace (DFS) but increase both the exchangeable and absorbedchloide content of the root. Aluminium-treated roots transportedmuch less calcium to the shoot system than untreated plants.Autoradiographs showed that this difference was reflected ina greatly reduced labelled-calcium concentration over the tissuesof the stele. By contrast the non-exchangeable fraction of labelledcalicum in the cortex was similar in both treatments. Autoradiographsof ⁴⁶Sc showed that it was restricted to the epidermis and outerrank of cortical cells from whence it controls calicum movementthroughout the root. A theory to account for this control isoutlined.     

Analysis of Cell Division and Elongation Underlying the Developmental Acceleration of Root Growth in Arabidopsis thaliana

To investigate the relation between cell division and expansion in the regulation of organ growth rate, we used Arabidopsis thaliana primary roots grown vertically at 20°C with an elongation rate that increased steadily during the first 14 d after germination. We measured spatial profiles of longitudinal velocity and cell length and calculated parameters of cell expansion and division, including rates of local cell production (cells mm⁻¹ h⁻¹) and cell division (cells cell⁻¹ h⁻¹). Data were obtained for the root cortex and also for the two types of epidermal cell, trichoblasts and atrichoblasts. Accelerating root elongation was caused by an increasingly longer growth zone, while maximal strain rates remained unchanged. The enlargement of the growth zone and, hence, the accelerating root elongation rate, were accompanied by a nearly proportionally increased cell production. This increased production was caused by increasingly numerous dividing cells, whereas their rates of division remained approximately constant. Additionally, the spatial profile of cell division rate was essentially constant. The meristem was longer than generally assumed, extending well into the region where cells elongated rapidly. In the two epidermal cell types, meristem length and cell division rate were both very similar to that of cortical cells, and differences in cell length between the two epidermal cell types originated at the apex of the meristem. These results highlight the importance of controlling the number of dividing cells, both to generate tissues with different cell lengths and to regulate the rate of organ enlargement.     

DISTRIBUTION AND RELATIONSHIP OF CELL DIVISION AND MATURATION EVENTS IN PISUM SATIVUM (FABACEAE) SEEDLING ROOTS

Pea roots have open apical organization, where discrete initial cells do not exist. Differentiation of all tissues occurs in cylinders and vascular sectors that blend gradually with each other. This study reports the distribution of dividing cells and their relationship to maturation events in the 2 mm root tip, and in the 8–10 and 18–20 mm segments. Up to 200 μm from the root body/cap junction, cell division is uniformly distributed throughout all meristem regions. By 350 to 500 μ, xylem tracheary elements and cells of the pith parenchyma and middle cortex have stopped dividing. At this level cell division is almost entirely restricted to two cylinders, one composed of the inner root cap, the epidermis, and the outer cortex (outer cortex cylinder) and another composed of cells of the inner cortex, the pericycle and vascular tissue (inner cortex cylinder). When the protophloem matures, all cells in the phloem sector of the inner cortex cylinder, including the 1 layered pericycle, the endodermis and the phloem parenchyma, stop dividing. The 3–4 layered pericycle in the xylem sectors continues dividing until about 10 mm from the body/cap junction following the maturation of the protoxylem tracheary elements.     

DURATION OF CELL CYCLES IN CULTURED ROOTS OF CONVOLVULUS

The durations of the cell cycle in physiologically different regions of the meristem of cultured roots of Convolvulus arvensis were determined by the metaphase-accumulation technique involving colchicine. The cell cycle in the root cap increases from 13 hr in the actively dividing initials of the first tier to 155 hr in the slowly dividing initials of tiers 2–4 to an indeterminate value for derivatives of the initials in the root cap columella. The cycle times for the cells of the central cylinder and cortex are 21 and 27 hr, respectively. The cells of the quiescent center have a cycle of an estimated 420 hr. The duration of the cell cycle in these different regions is discussed in relation to the increased duration of G₁ in slowly or non-dividing cells. The possible regulation of cell division by the synthesis of a cell-division factor in the quiescent center is also discussed.     

Macrocyclic complexes of scandium radionuclides as precursors for diagnostic and therapeutic radiopharmaceuticals

The aim of this study was to evaluate new ligands which can be applied for labeling biomolecules with scandium radionuclides. Two radionuclides of scandium, ⁴⁷Sc and ⁴⁴Sc, are perspective radioisotopes for radiotherapy and diagnostic imaging. ⁴⁷Sc decays with a half-life of 3.35 days and a maximum β⁻ energy of 600 keV and could be an alternative to carrier added ¹⁷⁷Lu radionuclide for targeted radionuclide therapy. Another scandium radionuclide ⁴⁴Sc (t_1/2 = 3.92 h) is an ideal β⁺ emitter for PET diagnosis. It can be obtained as a daughter of the long-lived ⁴⁴Ti (t_1/2 = 60.4 y) from ⁴⁴Ti/⁴⁴Sc generator. For complexation of scandium radionuclides macrocyclic ligands having a cavity size similar to Sc³⁺ ionic radius were selected: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), 1,4,7-triazacyclononane-1,4,7 triacetic acid (NOTA), 1,4,7-triazacyclodecane-1,4,7 triacetic acid and 1,4,7-triazacycloundecane triacetic acid, and analogs of NOTA with 10, 11 and 12 atoms of the carbon in the ring. Our results have shown that from the studied macrocyclic ligands studied DOTA is most efficient for binding scandium radionuclides ⁴⁴Sc and ⁴⁷Sc to biomolecules. The determined stability constant of Sc-DOTA complex logK = 27.0 is comparable with stability constants for Y³⁺ and heaviest lanthanides but is higher than those for In³⁺ and Ga³⁺. Also ⁴⁶Sc-DOTATATE conjugate exhibits high stability in-vitro studies. The ¹³C NMR studies have shown that Sc-DOTA like Lu-DOTA forms in solution complexes with eight-coordination geometry. The lipophilicity of Sc-DOTATATE is nearly identical to that of Lu-DOTATATE, which suggests similar receptor affinity of both radioconjugates.     

Effects of heavy metals and strontium on division of root cap cells and meristem structural organization

In order to define relations between the behavior of quiescent center cells and the condition of root cap cells, effects of various metal salts on the root meristem structure, root growth, and division of root cap cells were investigated. Two-day-old maize (Zea mays L., cv. Diamant) seedlings were incubated on solutions containing 35 μM Ni(NO₃)₂), 10 μM Pb(NO₃)₂, or 3 mM Sr(NO₃)₂ in the absence or in the presence of 3 mM Ca(NO₃)₂. Toxic effects of metals were assessed from inhibition of the primary root length increment following 24-h and 48-h incubations as compared to the roots grown on water or on 3 mM Ca(NO₃)₂ solution. Metal localization in the root apex tissues following 24-h and 48-h incubations was determined using histochemical techniques. Cell lengths in three upper layers of root cap columella were determined, and the mitotic index in these cells was calculated. In the absence of Ca(NO₃)₂, the metals were found both in the meristem and in the root cap. Pb and Sr were revealed primarily in the cell walls, and Ni, in the cell protoplasts. In the presence of Ca(NO₃)₂, metal content in all root tissues was decreased, and their toxic effect on root growth was ameliorated. Pb and Ni inhibited cell division in the root cap. Pb caused an increase in the root cap cell length as early as following 24-h incubation, and Ni, only following 48-h incubation. Pb activated division of quiescent center cells in the direction of root cap. These effects, as well as possible involvement of dermatogen and cortex cells, resulted in a regrowth of a new root cap already after a 24-h incubation period. In this case, the meristem was transformed from a closed structure into the open one. Following 48-h incubation, Ni brought about only few divisions of quiescent center cells in the direction of root cap. It was suggested that inhibition of divisions of the root cap upper layer cells and a decrease in the sloughing off its cells can stimulate the quiescent center cell divisions. A similarity of the quiescent center and animal stem cells is discussed.     

Cell surface changes accompanying aging in human diploid fibroblasts : III. Division age and senescence revealed by concanavalin A-mediated red blood cell adsorption

The relationship between cell size, [³H]thymidine incorporation capacity, and cell surface property of human diploid fibroblasts was investigated using the concanavalin A (ConA)-mediated red blood cell (RBC) adsorption assay. Small cells in late passage populations adsorbed RBCs well with the RBC coating method (in which ConA-coated RBCs are adsorbed to fibroblasts) as did large cells of this population, while small cells in early passage populations did not. The RBC adsorption capacity of rapidly dividing cells with this method differed among young, middle-aged and old cell populations. The results suggest that temporal cell size and [³H]thymidine incorporating capacity is not a measure of the division age of human diploid cells at the individual cell level. On the other hand, RBC adsorption with the fibroblast coating method (in which RBCs are adsorbed to ConA-coated fibroblasts) occurred to non-dividing cells of the populations. Thus, the increase in RBC adsorption with this method is considered to be a reflection of the increase in non-dividing cells at phase III. Our results support the hypothesis that RBC adsorption with the RBC and fibroblast-coating methods represents a cell surface marker for division age and senescence of human diploid cells, respectively, at the individual cell level.     

Cloning of genes whose expression is correlated with mitosis and localized in dividing cells in root caps of Pisum sativum L.

Removal of border cells from pea roots synchronizes and induces root cap cell division, wall biogenesis and differentiation. Three messages which are expressed differentially in such induced root caps have been cloned. Sequence analyses showed that the PsHRGP1-encoded protein has high homology with a hydroxyproline-rich glycoprotein. The PsCaP23-encoded protein has high homology with an alfalfa callus protein or translationally controlled human or mouse tumor protein P23. The PsRbL41-encoded protein has high homology with a highly basic 60S ribosomal protein L41. In situ hybridization showed that PsHRGP1, PsCaP23 and PsRbL41 messages are localized within dividing cells of the root cap. PsHRGP1 is highly expressed in uninduced root caps, but its message is repressed by 10–11 times as soon as cell division and differentiation begin. Expression of PsHRGP1 recovers to higher than (180%) its initial level in 30 min. PsHRGP1 is root-specific. PsCaP23 and PsRbL41 messages increase ca. 3-fold within 15 min after root cap induction. All three genes represent small families of 3–5 closely related genes in the pea genome.     

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     

Neuronal and glial enzyme studies in cell culture

Summary Newborn BALB/c mouse brain was cultured as disaggregated cells after serial trypsin dissociations. The ontogeny of the cultures was followed by assays of cell number, deoxyribonucleic acid, and protein content and by the activities of three enzymes considered to be markers of neuronal differentiation. Aliquots of the freshly dissociated cells were assayed for choline acetylase, acetylcholinesterase, and glutamic acid decarboxylase activities and compared with intact brain. The percentages of recovery of activities, expressed as¹⁴C product formed per mg of protein per 10 min, at pH 6.8 and 37°C, were 37% for choline acetylase, 54% for acetylcholinesterase, and 24% for glutamic acid decarboxylase. The remainder of the freshly dissociated cells were placed into culture; enzyme assays were performed as the cells multiplied and then when the cultures became static. Choline acetylase activity increased as the cells rapidly divided, and glutamic acid decarboxylase activity increased only after the cultures became confluent. Under the culture conditions, acetylcholinesterase was not induced, despite active synthesis of acetylcholine. Neuroblastoma clone N18, C1300 cell line, was grown in cell culture, and the activity of acetylcholinesterase was measured as the cells multiplied and came to confluency. The specific activity of mouse neuroblastoma acetylcholinesterase increased 25-fold when the rate of cell division was restricted. The rate of cell division could be regulated by adjusting the serum concentration. By removing fetal calf serum during the growth period, cell division ceased, and acetylcholinesterase activity was significantly and rapidly induced. Choline-O-acetyltransferase specific activity was measured in rapidly dividing and in static cultures. Its specific activity was highest in nondividing cultures, compared to cultures containing actively dividing cells (6-fold), and the specific activity of thymidylate synthetase was increased 2.5-fold in actively dividing cultures, compared to static cultures. Glioblastoma cells obtained from the rat astrocytoma, clone C6, were grown in culture, and glucose metabolism was measured in control cultures, and in cultures containing norepinephrine (0.017 mg per ml). Norepinephrine produced a 50% inhibition in the incorporation ofd-[¹⁴C]glucose. Cells incubated for 2 hr in the presence ofd-[¹⁴C]glucose, washed and then incubated in control medium or in medium containing norepinephrine, resulted in the release of greater than 50% of radioactive metabolites in the norepinephrine treated plates. Norepinephrine caused a 50% increase in¹⁴CO₂ production in glioblastoma cells incubated withd-[1-¹⁴C]glucose. Norepinephrine, under similar conditions, did not affect the metabolism of glucose in clone C46, C1300 mouse neuroblastoma cells. Portions of this work were supported by a research grant (6-444946-58605) from the American Cancer Society.     

MITOTIC INDUCTION AND SYNCHRONIZED CELL DIVISION IN OEDOGONIUM CARDIACUM (HASS.) WITTR.

Waves of mitosis are induced in Oedogonium cardiacum grown under a 15 hr light/9 hr dark cycle. Mitosis starts 4 to 5 hr after the start of the dark period. Each mitotic stage has a high initial rate which plateaus at a lower rate for several additional hours. Partial synchronization of mitotic stages results from this induction of cell division. Mitotic divisions last 9 to 10 hr after induction. During the remainder of the 24-hr light/dark cycle, cells are in interphase. Along a filament, several dividing cells tend to be adjacent, with the most advanced stage in the cap cell. Progressively earlier mitotic stages are basal to the dividing cap cell. This pattern of mitotic division differs from the state in nature where only the cap cell usually divides. Chromosomes probably maintain a telophase arrangement during interphase. The suitability and advantages of Oedogonium, a haploid alga with sexual reproduction, as an experimental plant for cytological, developmental, biochemical, and genetic studies is pointed out.     

Sizes of G1 and G2 populations in starved Tetrahymena

Feulgen cytophotometry and autoradiography were used to study DNA content and DNA synthesis in starved and starved-refed Tetrahymena pyriformis GL-C. It was found that (1) the cell population shows a limited increase in cell number during starvation and this increase is restricted to the first 7 h of starvation; (2) at the end of starvation, there is a portion of the cell population whose DNA content is similar to that for standard G2 cells; (3) a significant portion of the dividing cells at the first division following refeeding in the presence of [³H]TdR are unlabeled; (4) these unlabeled cells are among the first to divide and, upon division, generally enter into a cell cycle either lacking a G1 phase or with a shortened G1 phase.     

Ammonium Inhibits Primary Root Growth by Reducing the Length of Meristem and Elongation Zone and Decreasing Elemental Expansion Rate in the Root Apex in Arabidopsis thaliana

The inhibitory effect of ammonium on primary root growth has been well documented; however the underlying physiological and molecular mechanisms are still controversial. To avoid ammonium toxicity to shoot growth, we used a vertical two-layer split plate system, in which the upper layer contained nitrate and the lower layer contained ammonium. In this way, nitrogen status was maintained and only the apical part of the root system was exposed to ammonium. Using a kinematic approach, we show here that 1 mM ammonium reduces primary root growth, decreasing both elemental expansion and cell production. Ammonium inhibits the length of elongation zone and the maximum elemental expansion rate. Ammonium also decreases the apparent length of the meristem as well as the number of dividing cells without affecting cell division rate. Moreover, ammonium reduces the number of root cap cells but appears to affect neither the status of root stem cell niche nor the distal auxin maximum at the quiescent center. Ammonium also inhibits root gravitropism and concomitantly down-regulates the expression of two pivotal auxin transporters, AUX1 and PIN2. Insofar as ammonium inhibits root growth rate in AUX1 and PIN2 loss-of-function mutants almost as strongly as in wild type, we conclude that ammonium inhibits root growth and gravitropism by largely distinct pathways.     

The extent and differences in mitotic activity of the root tip ofVicia faba L.

Mitotic activity does not stop for different meristematic cells of the root apex at the same distance from the initials. The differences are connected with the functional heterogeneity of the apical meristem of the root. The arrangement of vascular bundles,i.e. the alternation of independent xylem and phloem groups, is of major importance. In broad bean roots, the protophloem sieve elements stop dividing first. The centre of the stelei. e. late metaxylem elements stop dividing next. Division in the stele gradually ceases centrifugally, while it ceases centripetally in the peripheral part of the root. The cylindrical region with prolonged cell division includes internal layers of the cortex including endodermis, pericycle and adjoining cells of the stele. Proximally apical meristem is reduced to isolated strands of cells adjacent to the protoxylem poles. Pericycle cells stop dividing last at a distance of approx. 9–10 mm from the initials. The number of the division cycles is limited and is specific for individual cell types. Epidermal and cortical cells divide in broad bean roots transversely approximately seven times, cells of late metaxylem approximately five times. Root apical meristem is an asynchronous cell population with a different duration of the mitotic cycle. We determined local variations in the duration of the mitotic cycle in the apical meristem of broad bean root by means of colchicine-induced polyploidy. The cells of the quiescent centre had the longest mitotic cycle after colchicine treatment. The region of the proper root adjacent to the quiescent centre was mixoploid (2n and 4n). Isolated cells with a long cycle occurred also in the cortex and in the central cylinder. Cells with a division cycle of 18h were found in the root cap, in the epidermis, in the cortex and in the central cylinder. Relatively numerous cells with the shortest division cycle, approx. 12 h, occurred farther of the quiescent centre in the epidermis, in the cortex, in the pericycle, and in adjacent layers of the stele through-out the entire meristematic region. The results derived from the analysis of the apical meristem are discussed in connection with the ontogenesis of different types of cells taking part in the primary structure of the root.     

On the constancy of cell division rate in the root meristem

This review examines under what circumstances the rate of cell division among cells of the root meristem is known to vary. First, methods are compared that have been used to quantify cell division rate. These can be grouped as being either cytological, in which the rate of accumulation of cells in a particular phase of the cell cycle is determined based on some form of cytological labeling, or kinematic, in which the rate of cell accumulation is determined from the net movement of cells. Then, evidence is reviewed as to whether cell division rates vary between different tissues or cell types, between different positions in the root, or finally between different environments. The evidence is consistent with cells dividing at a constant rate, and well documented examples where cell division rate changes substantially are rare. The constancy of cell division rate contrasts with the number of dividing cells, which varies extensively, and implies that a major point for cell cycle control is governing the exit from the proliferative state at the basal boundary of the meristem.     

Inhibition by white light of rb absorption in the root apex of corn

Measurements of cell lengths made at 0.5 millimeter intervals in median longitudinal sections of the primary roots of corn (Zea mays) were used to construct a growth curve. The region 1.5 to 4.0 millimeters from the apex contained the largest number of elongating cells. Absorption of ⁸⁶Rb⁺ was measured using intact, dark-grown corn seedlings. Following uptake and exchange, the terminal 8.0 millimeters of each root was cut into four 2.0 millimeter segments. Maximum ⁸⁶Rb⁺ uptake occurred in the region from 0.0 to 4.0 millimeter from the root tip. Washing the intact primary root in fresh 2.0 millimolar CaSO₄ for 2 hours prior to uptake augmented the rate of ⁸⁶Rb⁺ uptake in all regions. Illumination with white light during washing caused a reduction of ⁸⁶Rb⁺ uptake as compared with controls washed in darkness, and the region of greatest light response was the region of elongation. Removal of the coleoptile prior to washing did not prevent the light inhibition of subsequent ⁸⁶Rb⁺ uptake. Removal of the root cap prior to washing in light partially reversed the light-induced inhibition of the washing response.     

Membrane fluxes and comparative toxicities of aluminium, scandium and gallium

Measurements were made of the membrane fluxes and toxicitiesof three cations with trivalent forms, Al, Ga and Sc, in internodalcells of the giant alga Chara corallina. With this species itwas possible to separate the cell wall from the cell contentsto obtain membrane fluxes which were not complicated by adsorptionof cations to the cell wall. Net uptake of Al was low, approximately1.5 pmol m^–2 s^–1, compared to the influxes of thedivalent cation ⁴⁵Ca of 82 pmol m^–2 s^–1 and themonovalent cation ²²Na of 1100 pmol m^–2 s^–1 at thesame external concentration. Traditional desorption methodsfor removing cell wall cations were found to be relatively ineffectivein the case of trivalent cations and, consequently, influx measuredwithout separating the cell wall component would greatly overestimatethe true membrane flux, possibly by several orders of magnitude.Al, Ga and Sc all inhibited growth at 20 mmol m^–3 at pH4.4. Toxicity decreased in the order Sc>Al>Ga. Sc andAl were also toxic to mature non-growing cells. Influx of ⁴⁶Scincreased with increasing pH, consistent with membrane permeationby hydroxy Sc rather than Sc³⁺. However, Sc was more toxic atlow pH where Sc³⁺ was the dominant species and where influxwas low and binding to cell walls was high. These results argueagainst Sc acting intracellularly and favour a toxicity mechanismwhich is initiated extracellularly. Key words: Aluminium toxicity, trivalent cations, Chara corallina, scandium influx, gallium     

Cell growth in the absence of division in a root meristem

Summary In the presence of 10^-2 M hydroxyurea cell division is prevented but cell growth continues. The rate of cell growth varies within the apex, depending on the location of the cell. The rate of growth is low in the quiescent centre and non-dividing region of the root cap but higher elsewhere.Indolyl acetic acid causes a transient increase in the rate of cell enlargement but after about 12 hours has no further effect.     

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     

Modular construction of the protoderm and peripheral root cap in the “open” root apical meristem ofTrifolium repens cv. Ladino

Summary Roots with open apical organization are defined by not having specific tiers of initial cells in the root apical meristem; those with closed apical organization have specific initial tiers to which all cell files can be traced. An example of the clear organization of closed roots is the development protocol of the root cap and protoderm. The key event in differentiating these tissues is the T-division, a periclinal division of the root cap/protoderm (RCP) initial that establishes a module. Each module comprises two packets, the protoderm and peripheral root cap. Consecutive T-divisions of the same RCP initial produce up to five modules on average in a lineage of cells in white clover (Trifolium repens cv. Ladino), with all lineages around the circumference of the root dividing in waves to form one module prior to the next. On average, clover has approximately 32 axial protoderm and peripheral root cap cells in each module, and 32 RCP lineages. The occurrence of RCP T-divisions in white clover, a root with open apical organization, and the subsequent modular construction of the root cap and protoderm, provides a link between open and closed roots and suggests a common developmental feature that most roots of seed plants may share independent of their root meristem organization type. The open apical organization of the white clover root varies from roots with closed apical organization in that the RCP initials occur in staggered positions instead of connected to discrete tiers, and the peripheral root cap and columella daughter cells form additional layers of cells. White clover also forms root hairs on all protoderm cells irrespective of their position relative to the underlying cortical cells.Abbreviations RAM root apical meristem - RCP root cap protoderm - prc peripheral root cap