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     

Cell genealogies in a plant meristem deduced with the aid of a 'bootstrap' L-system

Abstract. The primary root meristem of maize ( Zea mays L.) contains longitudinal files of cells arranged in groups of familial descent (sisters, cousins, etc.). These groups, or packets, show ordered sequences of cell division, which are transverse with respect to the apico-basal axis of the root. The sequences have been analysed in three zones of the meristem during the course of the first four cell generations following germination. In this period, the number of cells in the packets increases from one to 16. Theoretically, there are 48 possible division pathways that lead to the eight-cell stage, and nearly 2- × 10⁶ that lead to the 16-cell stage. However, analysis shows that only a few of all the possible pathways are used in any particular zone of the root. This restriction of pathways results from inherited sequences of asymmetric cell divisions, which lead to sister cells of unequal length. All possible division pathways can be generated by deterministic 'bootstrap' L-systems which assign different life spans to sister cells of successive generations and hence specify their subsequent sequence of divisions. These systems simulate propagating patterns of cell divisions, which agree with those actually found within the growing packets that comprise the root meristem. The patterns of division are specific to cells originating in various regions of the meristem of the germinating root. The importance of such systems is that they simulate patterns of cellular proliferation where there is ancestral dependency. They can therefore be applied in other growing and proliferating systems where this is suspected.     

Experimental Modification of Cell Division Patterns in the Root Meristem of Zea mays

In the meristem of the young primary root of maize seedlingsthe first transverse division in the cortex 250 µm fromthe root apex results in two daughter cells of distinctly unequalsize. This division could be rendered equal by raising the seedlingsin up to 7.5% methanol. The pattern of the subsequent two orthree transverse divisions in the cortex, as revealed by thearrangement of the newly divided cells in the resultant cellularpackets, was acropetal in the methanol-treated roots but basipetalin the control roots. The sequence of division within a cellularpacket tended to follow the distribution of cell sizes - largercells divided earlier than smaller cells. A temporary arrestof cell division by exposing roots to cold (5 °C) conditionshad no effect on the sequence of divisions that followed whenthe roots were allowed to recover at 20 °C. The resultssuggest that the normally asymmetric position of the cell wallformed at cytokinesis is subject to active regulation and thatmethanol interferes with this process. The cytoplasm of certaincells in the root meristem was also found to be unequally distributed,as judged by Azure B staining, between the two ends of the cell.Cytoplasmic asymmetry was not directly correlated with inequalityof division, although it too was affected by methanol. Cell polarity, root meristem, unequal division, Zea mays     

The relation between cell size,chromosome length and the orientation of chromosomes in dividing root cortex cells

Cells in the root meristem are organised in longitudinal files. Repeated transverse cell divisions in these files are the prime cause of root growth. Because of the orientation of the cell divisions, we expected to find mitoses with an spindle axis parallel to the file axis. However, we observed in the root cortex ofVicia faba large number of oblique chromosome orientations. From metaphase to telophase there was a dramatic increase of the rotation of the spindle axis. Measurements of both the size of the cortex cells and the chromosome configurations indicated that most cells were too small for an orientation of the spindle parallel to the file axis. Space limitation force the spindle into an oblique position. Despite this spindle axis rotation, most daughter cells remained within the original cell file. Only in extremely flat cells did the position of the daughter nuclei forced the cell to set a plane of division parallel to the file axis, which result in side-by-side orientation of the daughter cells. Telophase spindle axis rotations are also observed inCrepis capillaris andPetunia hybrida.. These species have respectively medium and small sized chromosomes compared toVicia. Since space limitation, which causes the rotation, depends both on cell and chromosome size, the frequency and extent of the phenomenon in former two species is comparatively low.     

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.     

Rhythmic plant morphogenesis: Recurrent patterns of idioblast cell production

Most plants are constructed from repeating modular units such as phytomers, merophytes, and cell packets. Even an organism as simple as the filamentous cyanobacterium Anabaena shows recurrent patterns of differentiated cellular structures, notably with respect to its heterocysts. These examples reflect the inherent rhythms established within developmental processes of living organisms. In the present article, attention is paid to repetitious production of idioblasts—isolated cells, or clusters of cells, with an identity different to that of neighbouring cells from which they are derived. In higher plant root tissues, idioblasts are contained within cell packets that grow up from mother cells during the course of a number of cycles of cell production. The heterocysts of Anabaena are also discussed; they, too, are a type of idioblast. The idioblasts of root tissues originate as small cells which result from unequal cell divisions. Such divisions are usually the final ones within a cell packet which has already undergone a number of division cycles and are characteristically located at one or both ends of a packet. The packet end walls are suggested to have a role in regulating division asymmetry. Idioblastic systems discussed are root cortical trichosclereids and diaphragm cells; in their earliest stage, the cells from which lateral root primordia arise are also considered as clusters of idioblasts because they, too, are the products of asymmetric divisions of pericyclic mother cells. The division patterns of all these idioblastic systems were modelled in a consistent way using L-systems, with the assumption that the age of a cell-packet end wall plays a special role in cell determination. This article is dedicated to Vsevelod Ya. Brodsky, doyen of Russian studies of rhythms in cell division and development, who celebrates his 80th birthday on August 4, 2008 This article was presented in original.     

Apical organization and maturation of the cortex and vascular cylinder inArabidopsis thaliana (Brassicaceae) roots

Developmental and physiological studies of roots are frequently limited to a post-germination stage. In Arabidopsis, a developmental change in the root meristem architecture during plant ontogenesis has not previously been studied and is addressed presently. Arabidopsis thaliana have closed root apical organization, in which all cell file lineages connect directly to one of three distinct initial tiers. The root meristem organization is dynamic and changes as the root ages from 1 to 4 wk post-germination. During the ontogeny of the root, the number of cells within the root apical meristem (RAM) increases and then decreases due to changes in the number of cortical layers and number of cell files within a central cylinder. The architecture of the initial tiers also changes as the root meristem ages. Included in the RAM's ontogeny is a pattern associated with the periclinal divisions that give rise to the middle cortex and endodermis; the three-dimensional arrangement of periclinally dividing derivative cells resembles one gyre of a helix. Four- or 5-wk-old roots exhibit a disorganized array of vacuolated initial cells that are a manifestation of the determinate nature of the meristem. Vascular cambium is formed via coordinated divisions of vascular parenchyma and pericycle cells. The phellogen is the last meristem to complete its development, and it is derived from pericycle cells that delineate the outer boundary of the root.     

CORTICAL ONTOGENY IN ROOTS. I. ZEA MAYS

Serial growth stages of young Zea mays primary roots were analyzed for patterns of ground meristem ontogeny. The number of cell layers in the cortex decreases from approximately 15 to 11 during early root growth. The cortex arises mostly by periclinal divisions in the outer portions of the ground meristem at levels 50–150 μm from the meristem tip, although some layers of outer cortex arise beyond 150 μm. The proendodermis contributes 3–5 cell layers to the cortex, but this contribution diminishes during early seedling growth as anticlinal divisions occur in the proendodermis. The relationship between the ground meristem and protoderm changes at the tip of the meristem during root elongation.     

Clonal analysis of the Arabidopsis root confirms that position, not lineage, determines cell fate

 The cellular organization of the Arabidopsis thaliana (L.) Heynh. root meristem suggests that a regular pattern of cell divisions occurs in the root tip. Deviations from this pattern of division might be expected to disrupt the organization of cells and tissues in the root. A clonal analysis of the 3-d-old primary root meristem was carried out to determine if there is variability in division patterns, and if so to discover their effect on cellular organization in the root. Clones induced in the seedling meristem largely confirmed the predicted pattern of cell divisions. However, the cellular initials that normally give rise to the different cell files in the root were shown to exhibit some instability. For example, it was calculated that a lateral root cap/epidermal initial is displaced every 13 d. Furthermore, the existence of large marked clones that included more than two adjacent cell layers suggests that intrusive growth followed by cell division may occur at low frequency, perhaps in response to local cell deaths in the meristem. These findings support the view that even in plant organs with stereotypical cell division patterns, positional information is still the key determinant of cell fate. Received: 27 August 1999 / Accepted: 4 December 1999     

Variations in the Size of Mitotic Cells in the Root Meristem

Variations in the length of mitotic and interphase cells were analyzed in various tissues of wheat roots and in the cortex of maize roots. Reliable differences were shown in the length of mitotic cells in individual file clones of cells of the same tissue. The mean lengths of dividing cells in different roots differed to a lesser extent than those of different files in the same tissue of one root. Within the file, the length of the sister simultaneously dividing cells differed the least, while the difference of lengths of the neighbor simultaneously dividing nonsister cells was bigger. The mean length of interphase cells in any file was always less than that of mitotic cells by a factor of 1.45. This ratio was almost invariable for files and tissues in both the plants we studied and corresponded to that of an exponentially growing cell population. In addition, a very small number of cells were found (less than 1%) in meristems, which are longer than the mitotic cells. The length of these cells exceeded those of mitotic cells by less than twice. The origin of such cells is discussed. The length of mitotic cells near the quiescent center is more variable than in the middle of the meristem in the cortex of both plants. In the meristem basal part, the mitotic cells were no longer than those in the middle of the meristem but there were no small dividing cells. In the wheat epidermis, the cells are differentiated into trichoblasts and atrichoblasts and, therefore, the length of the dividing cells is highly variable. The cell length is essential for their transition to mitosis for all studied proliferating meristem cells.     

Regions of differential cell elongation and mitosis, and root meristem morphology in different tissues of geotropically stimulated maize root apices

We examined cell length, mitosis, and root meristem “cuticle” in different tissues of geostimulated, red light-exposed primary roots of corn (Zea Mays, Wisconsin hybrid 64A × 22R). The examination was done at 15-minute intervals for a period of 240 minutes. Differences in cell elongation between the upper and lower sides were most prominent between 1.5 and 2.5 mm from the root meristem; the outer cortex had the greatest elongation growth, and the upper cells showed a significant increase in length compared to the lower. A differential mitosis was also found, with the lower tissue being greater. We infer that the mitotic activity is indicative of cell division, and this division occurs strictly in the first 1.5 mm of the root meristem. The combined effect of differential cell elongation and cell division results in the localization of the geotropic curvature in the 1.5- to 2.5-mm region from the root meristem. Mitosis that occurs primarily in the cortex and stele were asynchronous; the peak of cortical division preceded that of the stele. Both peaks occurred before the peak of geotropism. A densely stained layer separates the cap from the root meristem. This layer is thinner at the apex of the root meristem. The area of the thin region increased with time and peaked at 180 minutes after geostimulation, which was coincidental with the peak of the geotropic response.     

3D analysis of mitosis distribution highlights the longitudinal zonation and diarch symmetry in proliferation activity of the Arabidopsis thaliana root meristem

To date CYCB1;1 marker and cortex cell lengths have been conventionally used to determine the proliferation activity of the Arabidopsis root meristem. By creating a 3D map of mitosis distribution we showed that these markers overlooked that stele and endodermis save their potency to divide longer than the cortex and epidermis. Cessation of cell divisions is not a random process, so that mitotic activity within the endodermis and stele shows a diarch pattern. Mitotic activity of all root tissues peaked at the same distance from the quiescent center (QC); however, different tissues stopped dividing at different distances, with cells of the protophloem exiting the cell cycle first and the procambial cells being the last. The robust profile of mitotic activity in the root tip defines the longitudinal zonation in the meristem with the proliferation domain, where all cells are able to divide; and the transition domain, where the cell files cease to divide. 3D analysis of cytokinin deficient and cytokinin signaling mutants showed that their proliferation domain is similar to that of the wild type, but the transition domain is much longer. Our data suggest a strong inhibitory effect of cytokinin on anticlinal cell divisions in the stele.     

Variations in the size of mitotic cells in the root meristem

Variations in the length of mitotic and interphase cells were analyzed in various tissues of wheat roots and in the cortex of maize roots. Reliable differences were shown in the length of mitotic cells in individual files-clones of cells of the same tissue. The mean lengths of dividing cells in different roots differed to a lesser extent than those of different files in the same tissue of one root. Within the file, the length of sister simultaneously dividing cells differed the least, while the difference of lengths of neighbor simultaneously dividing nonsister cells was bigger. The mean length of interphase cells in any file was always less than that of mitotic cells by a factor of 1.45. This ratio was almost invariable for files and tissues in both plants we studied and corresponded to that of an exponentially growing cell population. In addition, a very small number of cells were found (less than 1%) in meristems, which are longer than the mitotic cells. The length of these cells exceeded those of mitotic cells by less than twice. The origin of such cells is discussed. The length of mitotic cells near the quiescent center is more variable than in the middle of the meristem in the cortex of both plants. In the meristem basal part, the mitotic cells were no longer than those in the middle of the meristem but there were no small dividing cells. In the wheat epidermis, the cells are differentiated into trichoblasts and atrichoblasts and, therefore, the length of dividing cells is highly variable. The cell length is essential for their transition to mitosis for all studied proliferating meristem cells.     

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.     

THE ROOT APICAL MERISTEM OF ASPLENIUM BULBIFERUM: STRUCTURE AND DEVELOPMENT

The root apical meristem of Asplenium bulbiferum Forst. f. has a prominent four-sided pyramidal cell with its base in contact with the rootcap. Derivatives (merophytes) that contribute to the main body of the root are produced from the three proximal faces of the apical cell. The rootcap has its origin from the fourth (distal) face of the apical cell. The first division in a proximal merophyte is periclinal to the root surface, separating an outer cell and an inner cell. The outer cell is the origin of the outer part of the cortex and the epidermis; the larger inner cell is the origin of the inner cortex, endodermis, pericycle, and vascular tissue. After the establishment of the basic number of cells in a unilayered merophyte, the cells undergo transverse divisions forming longitudinal files of cells. The mitotic index of the apical cell indicates that it is not a quiescent cell. Also, the first plane of division in a newly formed merophyte dictates that the apical cell is the originator of merophytes.     

Diversity of cell lengths in terminal portions of roots: location of the proliferative cell population

Terminal meristems are responsible for all primary growth of roots. It has been asserted that all cells of root meristems are actively dividing and that the stem cell (proliferative) population expands exponentially. Lengths of cells in roots just proximal to the root cap/root initial boundary were used to determine the numbers of cortex and stele cells in the meristem. Meristem cells were defined as cells that did not have significantly different cell lengths from initial cells at the boundary. Data show that, for five of the six species (Allium cepa, Pisum sativum, Pyrus communis, Triticum aestivum, Vicia faba, and Zea mays) tested, only the first 15 stele and the first 10-35 cortex cells in median longitudinal sections would be in the meristem. For T. aestivum, no discrete meristem was found because all cells proximal to initial cells were longer than initial cells. In addition to this subject area, distributions of lengths of cells in the root meristem using this definition, for the six species were compared with a theoretical cell-age distribution for exponentially dividing cells, to determine if distributions of cell lengths were similar to a theoretical distribution of exponentially dividing cells. For all species tested, distributions of cell lengths were not similar to a theoretical cell-age distribution. From the data of this study with six plant species, we conclude that either contiguous proliferative cell populations of root meristems are very small or the proliferative cell population is not continuous. In addition, such populations do not resemble a theoretical exponential cell-age distribution. Moreover, it seems that the proliferative capacities of cells within terminal root segments differ markedly among species and are not easily characterized.     

Cortical development in roots of the aquatic plant Pontederia cordata (Pontederiaceae)

Adventitious roots of marsh-grown Pontederia cordata were examined to determine cortical development and structure. The innermost layer of the ground meristem forms the endodermis and aerenchymatous cortex. The outermost layer of the early ground meristem undergoes a precise pattern of oblique and periclinal cell divisions to produce a single or double layer of prohypodermis with an anchor cell for each radial file of aerenchyma cells. At maturity, endodermal cell walls are modified only by narrow Casparian bands. The central regions of the ground meristem become proaerenchyma and exhibit asymmetric cell division and expansion. They produce an aerenchymatous zone with barrel-shaped large cells and irregularly shaped small cells traversing the aerenchyma horizontally along radii; some crystalliferous cells with raphides are present in the aerenchyma. The walls of the hypodermis are modified early by polyphenols. The outermost layer of the hypodermis later matures into an exodermis with Casparian bands that are impermeable to berberine, an apoplastic tracer dye. The nonexodermal layer(s) of the hypodermis has suberin-modified walls. Radial files of aerenchyma are usually connected by narrow protuberances near their midpoints, the aerenchyma lacunae having been produced by expansion of cells along walls lining intercellular spaces. We are terming this type of aerenchyma development, which is neither schizogenous nor lysigenous, "differential expansion."