Early ontogeny of vascular cambium

Observations were made on structural changes of procambium and the subsequent appearance of cambium in the developing shoot. The procambium in early stages shows radial seriation of cells as a result of repeated tangential divisions. In tangential view the procambium has initially a rather homogeneous structure and later becomes organized into two distinct systems, one composed of long cells and the other of short cells. The latter cells are arranged tangentially in axial files and transversely in radial files. They show repeated transverse divisions. Some of the short cells elongate to intrude between neighboring cells. Therefore, long cells may be derived both from cells of homogeneous structure in the first stage and from elongation of some of the short cells in axial files. Long cells have mostly tapering end walls and elongate actively. In the subsequent stages, the frequency of transverse divisions of short cells in axial files decreases and these cells expand radially. Short cells in axial files are separated from one another vertically by the elongation of neighboring long cells which break up the axial files as seen in tangential view. The vascular meristem in this stage is believed to initiate the cambium. Eventually, short cells remain mostly single, or form files of two or three cells in height in tangential view. The observations are discussed in relation to the structure of the vascular meristem in other plants.     

The ontogeny of the vascular cambium inGinkgo biloba roots

Observation was made on early ontogeny of vascular cambium in the developing root ofGinkgo biloba L. After completion of root elongation, the vascular meristem gradually acquires cambial characteristics. Strips of the periclinal division of cells in transverse section are observed on the inner side of phloem when the primary xylem and phloem in the stele have been established. The strips are united into a continuous layer between phloem and xylem. In tangenital section, the procambium shows a homogeneous structure, which is initially composed of short cells with transverse end walls and subsequently, of long cells with tapering ends. Then, the procambium is organized into two systems of cells; axial strands of short cells with transverse end walls resulting from the sporadic transverse divisions of long cells, and long cells with tapering ends. Still later, the short cells are divided frequently in a trasverse plane exhibiting one or a few cells in width and several decades of cells in height, while the long cells are elongated. The frequency of transverse divisions of the short cells decreases in subsequent stages. Eventually, the short cells in axial strands are vertically separated from one another by the elongation of neighboring long cells and by the decrease in the frequency of transverse divisions of short cells themselves. Cambial initials occur in two forms; ray initials a few cells in height and one cell in width derived from the short cells, and fusiform initials with tapering ends derived from the long cells.     

Occurrence of crystals in vascular cambium

Summary Crystals are of common occurrence in plant tissues and considered to be waste products resulting from active metabolism of plant cells. They mostly occur in ray and vertical parenchyma of wood and bark of angiosperm trees and often develop in tissues which soon cease to be functional. While studying the annual rhythm of cambial activity in two species of theVerbenaceae, Tectona grandis (Rao andDave 1981) andGmelina arborea (Dave andRao 1982), crystals of calcium oxalate have been observed in ray initials (Figs. 1–10). The crystals were identified as calcium oxalate after a convincing chemical behaviour towards dilute (10%) acetic, hydrochloric, and sulphuric acids. They appear colorless when viewed with brightfield illumination (Fig. 3), and show birefringence in polarized light (Figs. 1 and 6). Radial, tangential, and transverse sections of 10–15 m thick were used to study the crystals in polarized light with a Carl Zeiss Axiomat microscope.     

Early ontogeny of vascular cambium III

Observations were made on structural changes from the procambium to cambium in the developing shoots ofRobinia pseudo-acacia andSyringa oblata, both of which are characterized by relatively short fusiform initials. In both species, the procambium in transverse view shows radial seriations of cells as a result of repeated tangential divisions, and there is an almost continuous procambial cylinder in the young stem in the earlier stage. The procambium in tangential view has initially a homogeneous structure and later develops into two systems, one made of long cells, the other of short cells. Some of the short cells elongate to intrude among neighbouring cells and some long cells divide radially as well as tangentially. InRobinia, long cells have transverse or tapering end walls at a relatively earlier stage and mainly tapering end walls in the subsequent stages. Although some of the short cells develop into long cells, the height of axial files of the short cells decreases only a little, because of subsequent transverse divisions and growth of cells. InSyringa, long cells have mainly transverse end walls at a relatively earlier stage and usually tapering end walls in the subsequent stages. Short cells in axial files have predominantly transverse end walls. A number of additional long cells are derived from elongating short cells in the later stages. Therefore, the height of axial files becomes apparently lower than that of earlier stages. Radial divisions in short cells occur to some extent. Results are discussed in relation to the structure of the vascular meristem inGinkgo, Aucuba, Weigela, and others.     

Structure and functions of the vascular cambium.

Following a general outline on the functioning of the cambium, the authors review the data acquired over the last 20 years. The interest is focused on: i) the ultrastructural characteristics of the two kinds of initials; ii) the early structural, metabolic and molecular modifications occurring during the first stages of daughter cell differentiation into either xylem or phloem; iii) the complex rhythmic changes of structure, metabolism and activity undergone by cambial cells during the seasonal cycle; iv) the characteristics and control of the cessation of cambial activity in autumn and of its reactivation in spring; v) the main research approaches in cell and molecular biology presently open to the students of the cambial meristem.     

The extracellular matrix of Volvox carteri: molecular structure of the cellular compartment

The extracellular matrix (ECM) of Volvox contains insoluble fibrous layers that surround individual cells at a distance to form contiguous cellular compartments. Using immunological techniques, we identified a sulfated surface glycoprotein (SSG 185) as the monomeric precursor of this substructure within the ECM. The primary structure of the SSG 185 poly-peptide chain has been derived from cDNA and genomic DNA. A central domain of the protein, 80 amino acid residues long, consists almost exclusively of hydroxyproline residues. The chemical structure of the highly sulfated polysaccharide covalently attached to SSG 185 has been determined by permethylation analysis. As revealed by EM, SSG 185 is a rod-shaped molecule with a 21-nm-long polysaccharide strand protruding from its central region. The chemical nature of the cross- links between SSG 185 monomers is discussed.     

MicroRNA-directed cleavage of Nicotiana sylvestris PHAVOLUTA mRNA regulates the vascular cambium and structure of apical meristems

Leaf initiation in the peripheral zone of the shoot apical meristem involves a transition to determinate cell fate, but indeterminacy is maintained in the vascular cambium, a tissue critical to the continuous growth of vascular tissue in leaves and stems. We show that the orientation of cambial growth is regulated by microRNA (miRNA)-directed cleavage of mRNA from the Nicotiana sylvestris ortholog of PHAVOLUTA (NsPHAV). Loss of miRNA regulation in semidominant phv1 mutants misdirects lateral growth of leaf midveins and stem vasculature away from the shoot, disrupting vascular connections in stem nodes. The phv1 mutation also expands the central zone in vegetative and inflorescence meristems, implicating miRNA and NsPHAV in regulation of meristem structure. In flowers, phv1 causes reiteration of carpel initiation, a phenocopy for loss of CARPEL FACTORY/DICER LIKE1, indicating that miRNA is critical to the termination of indeterminacy in floral meristems. Results point to a common role for miRNA in spatial and temporal restriction of HD-ZIPIII mediated indeterminacy in apical and vascular meristems.     

The origin and development of vascular cambium in girdled stems ofEucommia ulmoides Oliv.

We conducted anatomical studies of girdled stems ofEucommia ulmoides at various developmental stages to elucidate the origin and development of callus and the vascular cambium. In the transverse view, ray initial cells in the cambial zone began to divide both periclinally and anticlinally 2 d after girdling. Fusiform initial cells started to enlarge at 3 d, then gradually proliferated via periclinal divisions. Thus, the callus was derived from the ray initial cells of the cambial zone as well as from fusiform initial cells. In the tangential view, callus cells derived from ray initial cells were short while those from fusiform initial cells were long, thereby producing a heterogeneous structure. However, the fusiform initial cells underwent transverse divisions 10 d after girdling, which resulted in shorter cells and a homogeneous callus structure. Afterward, some short cells divided transversely while others elongated, so that a heterogeneous form was regained. Finally, the vascular meristem that was girdled early in its development redifferentiated from short and long cells in the callus. The long cells developed into fusiform initials, with the short ones becoming ray initials.     

The missing link: implementation of morphogenetic growth control on the cellular and molecular level

In the wing imaginal disc of Drosophila melanogaster, the morphogen Dpp controls growth, probably in an instructive manner. Many models for growth control by Dpp have been proposed and have been extensively discussed elsewhere. In this review, we speculate on how instructive growth control could provide a link between Dpp signaling and cell growth and/or cell cycle progression and so implement morphogenetic growth control on the cellular and molecular levels.     

Early ontogeny of vascular cambium II.Aucuba japonica andWeigela coraeensis

InAucuba andWeigela the six vascular bundles distributed as a hexagon become connected tangentially by meristematic cells into a procambial cylinder in the early stage. In the tangential view, the procambial cylinder shows a rather homogeneous structure. InAucuba, some cells of the procambium elongate in a relatively earlier stage and the rest also elongate during subsequent stages. All of these cells have tapering end walls. Then some long cells divide transversely and form two systems in the vascular meristem, one made up of long cells and the other of short ones. The long cells become the fusiform initials and the short cells, the ray initials. InWeigela, the homogeneous procambium is organized in the later stages into two systems, one of long cells and the other of short cells in axial files. Most of the long cells have tapering end walls and the short cells transverse end walls. Some of the short cells elongate to intrude between adjacent cells and become long cells. The long cells become the fusiform initials. Radial divisions in some short cells occur occasionally. Some of these cells elongate and the rest remain in the axial files. Some short cells in the axial files are vertically separated from each other by the elongation of adjacent long cells. however, this occurs infrequently and the height of axial files is still several decades of cells. Short cells in axial files eventually become ray initials.     

Seasonal changes in the ultrastructure of the vascular cambium of Robinia pseudoacacia

 The ultrastructure of the vascular cambium of Robinia pseudoacacia L. was examined in trunk tissues collected over a 2 ¹/₂ year period. During dormancy, fusiform cells are densely cytoplasmic with many small vacuoles and centrally located nuclei. Mitochondria are round to oval in sectional view. The plastids are variable in shape, have few internal membranes, and generally lack starch grains. The plasmalemma is smooth in outline. Proteinaceous material occurs in the vacuoles and many lipid droplets are scattered throughout the ground substance. Smooth tubular ER, often highly dilated, predominates, but short segments of rough ER are also present. Abundant free ribosomes are evenly distributed throughout the ground substance and the dictyosomes are inactive. Microtubules are parietal and have various orientations. During reactivation, the plasmalemma becomes irregular in outline and begins to form invaginations. Concurrently, the proteinaceous material disappears, the vacuoles begin to fuse, polysomes appear, and the dictyosomes begin to produce vesicles. During the period of cambial activity, fusiform cells are highly vacuolate, and the nuclei are centrally located. The mitochondria are round, oval, or elongate. Now the plastids contain phytoferritin, starch grains, or both. Many large invaginations of the plasmalemma intrude into the vacuole, pushing the tonoplast inward and pinching off into the vacuole, which lacks proteinaceous material. Lipid droplets are scarce. Most ER is rough, and ribosomes are generally aggregated as polysomes. Dictyosomes are actively producing vesicles. During the transition to dormancy, the fusiform cells gradually assume the appearance typical of the dormant cambium.     

A mathematical model of pattern formation in the vascular cambium of trees

The beautiful patterns apparent in wood grain have their origin in the alignment of fusiform initial cells in the vascular cambium of trees. We develop a mathematical model to describe the orientation of fusiform initial cells, and their interaction with the plant hormone indole-3-acetic acid (auxin). The model incorporates the following four assumptions: (1) auxin is actively transported parallel to the long axis of the initials, (2) auxin diffuses perpendicular to the long axis of the initials, (3) the initials tend to orient parallel to the flux of auxin through the cambium, and (4) adjacent initials tend to orient parallel to one another. Each assumption is justified on the basis of available evidence and cast in mathematical form. Our main result is a pair of nonlinear differential equations that describe the coupling between the distribution of auxin in the cambium and the orientation of fusiform initials. Numerical solutions to the equations show qualitative resemblance to the wood grain patterns observed at branch junctions, wounds and knots, and topological defects.     

Single molecule studies of molecular diffusion in cellular membranes: determining membrane structure

Since the advent of single particle/molecule microscopies, researchers have applied these techniques to understanding the fluid membranes of cells. By observing diffusion of membrane proteins and lipids in live cell membranes of eukaryotic cells, it has been found that membranes contain a mosaic of fluid compartments. Such structure may be instrumental in understanding key characteristics of the membrane. Recent single molecule observations on prokaryotic cell membranes will also be discussed.     

Integrative visualization of the molecular structure of a cellular microdomain

An integrative approach to visualization is used to create a visual snapshot of the structural biology of the polar microdomain of Caulobacter crescentus. The visualization is based on the current state of molecular and cellular knowledge of the microdomain and its cellular context. The collaborative process of researching and executing the visualization has identified aspects that are well determined and areas that require further study. The visualization is useful for dissemination, education, and outreach, and the study lays the groundwork for future 3D modeling and simulation of this well‐studied example of a cellular condensate.     

Organisation of vascular cambium during different seasons in some tropical timber trees

The vascular cambium of Albizzia, Tectona, Terminalia, Calophyllum, Mangifera and Morinda was non-storied while that of Dalbergia was semi-storied. The ray initials were uniseriate in Terminalia and Calophyllum , both uniseriate and biseriate in Albizzia, Dalbergia, Mangifera and Morinda and were also multiseriate in Tectona . They were homogeneous in Albizzia and heterogeneous in the other species. The radial walls of cambial cells were always beaded, although beads were more prominent and closer to one another during periods of cambial dormancy than during activity. The fusiform initials were comparatively less vacuolated during dormancy/ least activity. When active, the fusiform and ray initials of all species, except Calophyllum , also showed multinucleate condition (2–10 depending on species). The proportion of ray initials to fusiform initials was almost constant throughout the year in all species.     

Wood structure and function change with maturity: Age of the vascular cambium is associated with xylem changes in current‐year growth

Xylem vessel structure changes as trees grow and mature. Age‐ and development‐related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water‐stress‐induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current‐year growth of young (1–4 years), intermediate (2–7 years), and older (3–10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple‐aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.