Differential growth resulting in the specification of different types of cellular architecture in root meristems

Symplastic growth of plant organs may be described by a continuous growth tensor field. In tensorial analysis of meristems, the trajectories of periclinal and anticlinal cell walls represent trajectories of the principal directions of growth (PDGs); this follows from the maintenance of mutual orthogonality between periclinal and anticlinal wall trajectories during growth. Periclinal and anticlinal cell divisions are also oriented in the principal planes of growth. The growth tensor for the root apex is specified in such a way that the principal directions of the tensor fit the pattern of periclinal and anticlinal walls in the apex, and that the grid formed by material particles aligned along PDG trajectories preserve this alignment during growth. Two growth tensors are formulated--one giving a maximum and the other giving a minimum of the volumetric relative elemental growth rate at the region of the initial cell(s). Temporal sequences of deformation of a grid formed by lines coinciding with the principal directions of growth are shown. The formation of cellular patterns in root apices is simulated. Two types of patterns are obtained: one with an apical cell and merophytes, and another with files of cells converging towards a quiescent centre.     

Auxin Relations of the Woody Shoot: The Distribution of Diffusible Auxin in Shoots of Apple and Plum Rootstock Varieties

Surveys have been made of diffusible auxin in the stem tissuesof growing shoots of apple and plum rootstock varieties. Usingagar plates as carriers auxin was collected from the lower surfaceof isolated stem sections and assayed by the Avena curvaturemethod. The stool and layer shoots studied grew for severalmonths producing many leaves and reaching considerable lengths.The data provide information on selected internodes and showthe auxin status of the shoot at various times during growth,and the auxin gradients down the stem at these various times.Free auxin content of the shoot apex was consistently less thanthat of the internodes below. In 1946 auxin content declinedthroughout growth with a steady positioning of the auxin peakin the upper shoot. In 1947, following a period of drought,when growth almost ceased, a secondary auxin peak occurred positionedin lower internodes distant from the apex. This seasonal contrastwas reflected in the auxin relations of the individual internode,and was observed both in apple and plum. The nature of the auxindecline below the peak region, and the total disappearance offree auxin from the shoot as growth subsides, is discussed.The reappearance of free auxin in mature internodes, which hasnot been transmitted from the stem apex, implies either derivationfrom a stored state or the ability of the internode to produceits own auxin.     

INDETERMINATE GROWTH AND RAMIFICATION OF THE CLIMBING LEAVES OF LYGODIUM JAPONICUM (SCHIZAEACEAE)

Morphological and anatomical specializations of the climbing leaves (CL) of Lygodium japonicum were investigated. Examination of growth relationships between the rachis and pinnae of the circumnutating CL revealed a close relationship to the “searcher” morphology of twining shoots. The CL has resting pinna apices (leafbuds) capable of replacing a damaged leaf apex or ramifying the foliar axis. Their structure and growth is similar to the main leaf apex. CL growth is indeterminate and occurs at a steady rate. Crozier uncoiling and rachis elongation occurs by a mechanism of unequal rates of cell division and elongation. The adaptations of the CL are interpreted as specializations within the basic principles of fern leaf morphogenesis.     

The Relationship Between Cell Size and Cell Number and the Distance from the Root Tip to the Region of Phloem Initiation in Groundnuts (Arachis hypogea. L. var. Bukene Red)

The effects of cell size and number on the distance from theroot tip to phloem inception in groundnuts have been investigated.The roots were treated with a number of plant growth regulatorshaving a wide range of effects on the growth rate. Increasing the growth rate of roots increased the distance ofphloem initiation and lignification from the apex. In all treatments,the average distance to phloem inception varied from about 590to 885 µm behind the root apex. The average lengths of the cortical and epidermal cells, whichwere 16 and 20 µm respectively, remained constant regardlessof the distance from the root tip.     

The hyphal tip structure of Basidiobolus sp.: a zygomycete fungus of uncertain phylogeny

To date, among the zygomycete fungi that have been examined, a Spitzenk?rper has not been reported. In this paper, the cytoplasmic order of hyphal tip cells of Basidiobolus sp.,?a zygomycete genus of uncertain phylogeny, has been examined using light microscopy and transmission electron microscopy methods. With phase-contrast light optics, a phase-dark body was observed at the tips of growing hyphae of Basidiobolus sp. The hyphal apex also showed high affinity for FM4-64 labelling resulting in an intense fluorescence signal. The phase-dark inclusion exhibited independent motility within the hyphal apex and its presence and position were correlated to the rate and direction of hyphal growth. The hyphal apex of Basidiobolus sp. did not contain γ-tubulin. Ultrastructural observations revealed a dense cluster of vesicles at the hyphal apex. These results suggest that the growing hypha of Basidiobolus sp. contains a Spitzenk?rper, a character generally attributed to members of the ascomycete and basidiomycete fungi and not to zygomycete fungi.     

Oscillatory chloride efflux at the pollen tube apex has a role in growth and cell volume regulation and is targeted by inositol 3,4,5,6-tetrakisphosphate

Oscillatory growth of pollen tubes has been correlated with oscillatory influxes of the cations Ca(2+), H(+), and K(+). Using an ion-specific vibrating probe, a new circuit was identified that involves oscillatory efflux of the anion Cl(-) at the apex and steady influx along the tube starting at 12 microm distal to the tip. This spatial coupling of influx and efflux sites predicts that a vectorial flux of Cl(-) ion traverses the apical region. The Cl(-) channel blockers 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and 5-nitro-2-(3-phenylpropylamino)benzoic acid completely inhibited tobacco pollen tube growth at 80 and 20 microM, respectively. Cl(-) channel blockers also induced increases in apical cell volume. The apical 50 micro m of untreated pollen tubes had a mean cell volume of 3905 +/- 75 microm(3). DIDS at 80 microM caused a rapid and lethal cell volume increase to 6206 +/- 171 microm(3), which is at the point of cell bursting at the apex. DIDS was further demonstrated to disrupt Cl(-) efflux from the apex, indicating that Cl(-) flux correlates with pollen tube growth and cell volume status. The signal encoded by inositol 3,4,5,6-tetrakisphosphate [Ins(3,4,5,6)P(4)] antagonized pollen tube growth, induced cell volume increases, and disrupted Cl(-) efflux. Ins(3,4,5,6)P(4) decreased the mean growth rate by 85%, increased the cell volume to 5997 +/- 148 microm(3), and disrupted normal Cl(-) efflux oscillations. These effects were specific for Ins(3,4,5,6)P(4) and were not mimicked by either Ins(1,3,4,5)P(4) or Ins(1,3,4,5,6)P(5). Growth correlation analysis demonstrated that cycles of Cl(-) efflux were coupled to and temporally in phase with cycles of growth. A role for Cl(-) flux in the dynamic cellular events during growth is assessed. Differential interference contrast microscopy and kymographic analysis of individual growth cycles revealed that vesicles can advance transiently to within 2 to 4 microm of the apex during the phase of maximally increasing Cl(-) efflux, which temporally overlaps the phase of cell elongation during the growth cycle. In summary, these investigations indicate that Cl(-) ion dynamics are an important component in the network of events that regulate pollen tube homeostasis and growth.     

The role of actin, fimbrin and endocytosis in growth of hyphae in Aspergillus nidulans

Filamentous fungi are ideal systems to study the process of polarized growth, as their life cycle is dominated by hyphal growth exclusively at the cell apex. The actin cytoskeleton plays an important role in this growth. Until now, there have been no tools to visualize actin or the actin-binding protein fimbrin in live cells of a filamentous fungus. We investigated the roles of actin (ActA) and fimbrin (FimA) in hyphal growth in Aspergillus nidulans . We examined the localization of ActA::GFP and FimA::GFP in live cells, and each displayed a similar localization pattern. In actively growing hyphae, cortical ActA::GFP and FimA::GFP patches were highly mobile throughout the hypha and were concentrated near hyphal apices. A patch-depleted zone occupied the apical 0.5 μm of growing hypha. Both FimA::GFP and Act::GFP also localize transiently to septa. Movement and later localization of both was compromised after cytochalasin treatment. Disruption of fimA resulted in delayed polarity establishment during conidium germination, abnormal hyphal growth and endocytosis defects in apolar cells. Endocytosis was severely impaired in apolar fimA disruption cells. Our data support a novel apical recycling model which indicates a critical role for actin patch-mediated endocytosis to maintain polarized growth at the apex.     

THE CUTICULAR ANATOMY OF FRENELOPSIS VARIANS FROM THE LOWER CRETACEOUS OF CENTRAL TEXAS

Stem fragments identified as Frenelopsis varions Fontaine have been found in the Lower Cretaceous (Albian) of central Texas. The cuticle is extremely thick and characterized by 5–6 subsidiary cells with papillae overarching the stomatal chamber. Guard cells are deeply sunken below the epidermis. Stomatal complexes are arranged in axial rows extending from the base of an internode to its apex. The rows of stomata continue into the sheathing leaf where the rows curve towards the leaf apex. The epidermis of F. varions was apparently long persistent and underwent prolonged growth. Axial rows of stomata are frequently disrupted resulting in a random pattern of stomata. A single, highly reduced, sheathing leaf is present at each node. The margin of the leaf has numerous unicellular trichomes and extends to form a slightly triangular blade.     

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     

A SEASONAL STUDY OF THE VEGETATIVE SHOOT APEX AND THE PATTERN OF PITH DEVELOPMENT IN HIBISCUS SYRIACUS

Tolbert , Robert J. (West Virginia U., Morgantown.) A seasonal study of the vegetative shoot apex and the pattern of pith development in Hibiscus syriacus. Amer. Jour. Bot. 48(3): 249–255. Illus. 1961.—The shoot apex of Hibiscus syriacus L. is described as having a cytohistological zonation superimposed on a tunica-corpus configuration. The apex is flat-topped or may have a saddle-back or concave appearance as seen in median longitudinal section. The metrameristem, consisting of the tunica and corpus initials, is comprised of large, light-staining, vacuolate cells that have thick cell walls and exhibit much dark-staining intercellular substance. Surrounding the metrameristem is the flanking meristem, which is responsible for the outer layers of the shoot, and from which the leaf primordia arise. The pith rib meristem lies below the metrameristem and consists of files of cells that are responsible for the pith. There are no major seasonal changes in the structure of the apex during the yearly cycle. The pith displays a long-shoot type of development with the cells remaining in distinct files during the first flush of growth in the spring. As growth slows and internode elongation is gradually reduced, the pith displays the characteristic short-shoot type of development, consisting of a spongy tissue of rounded cells with many intercellular spaces and no distinct files of cells. A crown is differentiated across the top of the pith at the end of the growth period. This consists of a band of cells with thick, dark-staining cell walls, which separates by the apex from the last year's growth. In contrast to many gymnosperms, this crown is dispersed by renewed cell activity the following spring.     

Localization of starch in shoot apices of vegetative and photoperiodically induced plants ofChenopodium rubrutn

Starch was determined by means of IKI reaction in shoot apices ofChenopodium rubrum plants induced to flowering by two short days and in non-induced plants. Small starch grains were already observed in the meristematic cells at an age of four days after sowing. Larger grains were found in the subapical region of the apex. Heterogeneity increases during further growth of the plants in induced, as well as in non-induced vegetative plants. Starch disappears from the cells potentially giving rise to axillary buds, while the number and size of starch grains increase in cells from which leaf primordia will be formed. This metabolic specifity of leaf and bud primordia is preserved during morphological differentiation and applies to vegetative, as well as to prefloral apices of photoperiodically induced plants. The amount of starch in the different regions of the apex is linked rather with organogenesis than with the quantitative growth in the apex.     

Growth of the Surface and Inner Parts of the Pea Shoot Apical Meristem during Leaf Initiation

The rate of cell division and the rate of increase in cell numberwere compared in the epidermis and in the underlying cells ofthe apical dome, the incipient primordium, and the axis of thepea shoot apex. These rates did not coincide in any part ofthe apex, but in the primordium and the apical dome there wasa closer correspondence in the epidermis than in the underlyingcells. This is interpreted as showing that the changing shapeof the apex, during growth of the primordium and the apicaldome, is associated with a tendency to local changes in therate of growth in the epidermis but to a tendency to changesin the direction of growth in the underlying cells.     

MODELING OF MERISTEMATIC GROWTH OF ROOT APICES IN A NATURAL COORDINATE SYSTEM

A curvilinear, orthogonal coordinate system, which resembles the pattern of periclines and anticlines in the cellular network of root apices, is presented. The system makes possible an analysis of the dynamics of apical growth: the relationship between growth rates and tensile stresses in cell walls. In this paper the coordinate system is used in modeling the growth and cell partitioning in the apical domes. The symplastic growth is described by means of the growth tensor which is assumed to have diagonal form in the system, so its coordinate lines represent the principal directions of growth rate. The coordinate system and the growth tensor in diagonal form assure temporal stability of form and cellular structure of the modeled apex including transition from the dome to the cylindrical part of the apex. The spatial and temporal aspects of the dome part of two types of root apices—one with maximum of volumetric relative growth rate at the center region of the apex, and another with minimum of the rate (quiescent center)—are described. The maximum of the rate at the center results in the cellular pattern with an apical cell and merophytes, the minimum results in ribs of cells (inside the root proper) converging toward the quiescent center.     

慈姑匍匐茎的起源和球茎的膨大研究

匍匐茎的发生一般见于主茎倒二或倒三叶原基的叶腋部位。在匍匐茎发生区域的主茎一侧,匍匐茎原始细胞的基部形成壳状区;壳状区的形成对匍匐茎原基的外凸起一定作用。匍匐茎无居间分生组织;它的伸长依靠顶端分生组织细胞的横向分裂,使轴向细胞数目增多,并使细胞的轴向延伸。球茎的膨大是通过匍匐茎第8—10节基本分生组织的细胞有丝分裂,增加细胞数目,然后细胞体积的扩大来实现的。球茎中的淀粉一般为单粒淀粉;匍匐茎中的淀粉由单粒和复合两种淀粉粒组成。     

The simulation model of growth and cell divisions for the root apex with an apical cell in application to Azolla pinnata

In contrast to seed plants, the roots of most ferns have a single apical cell which is the ultimate source of all cells in the root. The apical cell has a tetrahedral shape and divides asymmetrically. The root cap derives from the distal division face, while merophytes derived from three proximal division faces contribute to the root proper. The merophytes are produced sequentially forming three sectors along a helix around the root axis. During development, they divide and differentiate in a predictable pattern. Such growth causes cell pattern of the root apex to be remarkably regular and self-perpetuating. The nature of this regularity remains unknown. This paper shows the 2D simulation model for growth of the root apex with the apical cell in application to Azolla pinnata. The field of growth rates of the organ, prescribed by the model, is of a tensor type (symplastic growth) and cells divide taking principal growth directions into account. The simulations show how the cell pattern in a longitudinal section of the apex develops in time. The virtual root apex grows realistically and its cell pattern is similar to that observed in anatomical sections. The simulations indicate that the cell pattern regularity results from cell divisions which are oriented with respect to principal growth directions. Such divisions are essential for maintenance of peri-anticlinal arrangement of cell walls and coordinated growth of merophytes during the development. The highly specific division program that takes place in merophytes prior to differentiation seems to be regulated at the cellular level.     

Kinematics of plant growth.

Many of the concepts and equations which have been used in the study of compressible fluids can be applied to problems of plant development. Growth field variables, i.e. functions of position in the plant and of time, can be specified in either Eulerian (spatial) or Lagrangian (material) terms. The two specifications coincide only when the spatial distribution of the variable is steady, and steady patterns are most likely to emerge when an apex is chosen as origin of the co-ordinate system. The growth field itself can be described locally by the magnitude and orientation of the principal axes of the rate of strain tensor and by the vorticity tensor. Material derivatives can be calculated if the temporal and spatial variation in both growth velocity, u (rate of displacement from a material origin), and the variable of interest are known. The equation of continuity shows the importance of including both growth velocity, u, and growth rate, ▽ ·u in estimates of local biosynthesis and transport rates in expanding tissue, although the classical continuity equation must be modified to accommodate the compartmentalized distributions characteristic of plant tissue. Relatively little information on spatial variation in plant organs can be found in the botanical literature, but the current availability of interactive computer graphics equipment suggests that analysis of the spatial distribution of growth rates at least is no longer difficult.     

Signaling mechanisms in the establishment of plant and fucoid algal polarity

The establishment of polarity is a fundamental property of most cells. In tip‐growing plant and in fucoid algal cells, polarization specifies a growth pole, the center of localized secretion of new plasma membrane and cell wall material, generating a protrusion with a dome‐shaped apex. Although much progress has been made concerning the cellular machinery required to execute tip growth, less is known regarding the signaling mechanisms involved in selecting the growth site and regulating vectorial cell division and expansion. Fucoid algal zygotes use extrinsic cues to orient their growth axes and are thus well‐suited for studies of de novo selection of an axis. This process has been investigated largely by both pharmacological and immuno‐localization studies. In tip growing plant cells, polarity is often predetermined, as in the formation of root hairs or moss protonema branches. More focus has been on genomic and genetic studies to reveal the molecules involved in expressing a growth axis. Here we review the common roles of the cytoskeleton and signal transduction pathways in the formation of a developmental axis in fucoid algal cells and the control of tip growth in higher plant cells. Mol. Reprod. Dev. 77: 751–758, 2010. © 2010 Wiley‐Liss, Inc.     

A Catastrophe Model for the Switch from Vegetative to Reproductive Growth in the Shoot Apex

A mathematical model is constructed to describe the morphopneticswitch that occurs when a vegetative plant apex becomes reproductive.The cusp equation from catastrophe theory is modified, and isused to relate primordial size at initiation to apex size. Theresulting equation may be viewed as an equation of state definingthe allowed organizational modes of the shoot apex. The modelsimulates the growth of the apex from the vetative stage toearly reproductive growth, and makes reasonable predictionsabout apex size and growth rate, primordial sizes, and the lengthsof the plastochron. flowering, mathematical model, catastrophe theory, shoot apex