Microfibrillar Structure, Cortical Microtubule Arrangement and the Effect of Amiprophos-methyl on Microfibril Orientation in the Thallus Cells of the Filamentous Green Alga, Chamaedoris orientalis

Microfibrillar structure, cortical microtubule orientation andthe effect of amiprophos-methyl (APM) on the arrangement ofthe most recently deposited cellulose microfibrils were investigatedin the marine filamentous green alga, Chamaedoris orientalis.The thallus cells of Chamaedoris showed typical tip growth.The orientation of microfibrils in the thick cell wall showedorderly change in longitudinal, transverse and oblique directionsin a polar dependent manner. Microtubules run parallel to thelongitudinally arranged microfibrils in the innermost layerof the wall but they are never parallel to either transverseor obliquely arranged microfibrils. The ordered change in microfibrilorientation is altered by the disruption of the microtubuleswith APM. The walls, deposited in the absence of the microtubules,showed typical helicoidal pattern. However, the original crossedpolylamellate pattern was restored by the removal of APM. Thissuggests that cortical microtubules in this alga do not controlthe direction of microfibril orientation but control the orderedchange of microfibril orientation. Amiprophos-methyl, Chamaedoris orientalis, coenocytic green alga, cortical microtubule, microfibrillar structure, tip growth     

Cell wall structure in the xylem parenchyma ofCryptomeria

Summary Cell wall structure in ray and axial parenchyma cells in the wood ofCryptomeria was shown to be typically crossed polylamellate and dissimilar to the characteristically layered wall of fibers and tracheids. Ray cells differed from axial cells in terms of form and also in the relative inclination of crossed microfibrillar helices in the cell wall. This feature was reflected by positive birefringence in ray cells and negative birefringence in axial cells. Localized wall thickenings,viz. transverse bars in ray cells and longitudinal ribs in axial cells, also displayed crossed polylamellate structure. This observation contrasts with the exclusively longitudinal microfibrillar orientation previously reported for longitudinal ribs in elongated parenchyma cells of primary tissue. On the basis of similar microfibrillar orientations between outer and inner wall lamellae, the cell walls ofCryptomeria parenchyma were judged to be predominantly secondary.Lignin was heterogeneously distributed in lamellate fashion and a high concentration characterized the thin middle lamella. Both types of parenchyma suggested a higher lignin content than adjacent longitudinal tracheids.     

Orientation of microfibrils and microtubules in cortical parenchyma cells of poplar during elongation growth

Three types of microfibrillar orientation, namely parallel, perpendicular and oblique to the main cell axis were found not only in the innermost surface of but also throughout the developing wall. Furthermore, three types of microtubule orientation, namely parallel, perpendicular and oblique to the main cell axis, were found, coinciding with those of microfibrils. As a whole, the wall was shown to be a crossed polylamellate structure. These observations suggest that the orientation of microfibrils is determined at the time of wall formation, and not influenced by the extension of the wall.     

Observations on the ultrastructure of the thickened sieve cell wall inPinus strobus L.

Summary The thickened sieve cell wall of white pine is shown to comprise a crossed-helical polylamellate structure in which the predominant microfibrillar orientation is greater than 45° with respect to the cell axis. The previously reported observation that microfibrils may be oriented other than parallel to the plane of the cell wall is disputed and it is demonstrated that such an appearance may derive from appropriately oblique sectioning of the wall.     

A LIGHT- AND ELECTRON-MICROSCOPIC SURVEY OF ALGAL CELL WALLS. I. PHAEOPHYTA AND RHODOPHYTA

Dawes , Clinton J., Flora M. Scott , and E. Bowler . (U. California, Los Angeles.) A light- and electron-microscopic survey of algal cell walls. I. Phaeophyta and Rhodophyta. Amer. Jour. Bot. 48(10): 925–934. Illus. 1961.—An introductory survey of the structure of the cell walls of brown, red, and green algae, as seen under light and electron microscopes, has been completed. In the present paper (Part I) the structure of the thalli of the Phaeophyta and Rhodophyta is compared, and the occurrence of intercellular spaces, pitting, and microfibrillar patterns is discussed. A detailed comparison of the cell-wall structure and growth of a brown alga, Dictyota flabellata, and of a red alga, Helminthocladia californica, is also presented. In Dictyota, typical of the brown algae, the microfibrillar pattern in the apical cells and in the adjacent cells of the thallus tip is reticulate. In mature cells, the microfibrils are dominantly parallel in orientation. Pits, which are fields of closely set pores, are distinctive. The microfibrils in the pit areas are masked by non-fibrillar material. Helminthocladia, with a cell wall characteristic of the red algae, differs from Dictyota in that the microfibrillar pattern is reticulate at all ages of the cell and throughout the thallus. In the pit areas, the microfibrils are not masked by amorphous material. Pit connections, characteristic of the Florideae, can be divided into 2 major groups. Either the pit connection is an open channel between 2 adjacent cells, or it is composed of numerous plasmodesmata traversing a continuous, loose, microfibrillar wall. The techniques of the survey are emphasized in that fragmented cell walls were studied, and, also, chemically cleared material was constantly compared with fresh material under light and electron microscopes. It is concluded that the cell wall, as a taxonomic character, is of value only in delimiting the Phaeophyta and Rhodophyta.     

CERCIDIUM FLORIDUM SEED COAT,LIGHT AND ELECTRON MICROSCOPIC STUDY

Scott , Flora Murray , B. G. Bystrom , and E. Bowler . (U. California, Los Angeles.) Cercidium floridum seed coat, light and electron microscopic study. Amer. Jour. Bot. 49(8): 821–833. Illus. 1962.—The structure of the seed coat of the desert tree Cercidium is typical of the family Leguminosae, but the inner integument is mucilaginous. The characteristic palisade cells of the epidermis and the many-armed cells of the mucilaginous zone are discussed in detail. The minute wax rodlets on the surface of the young seed fuse later into a film of wax. The epidermal palisade cells are polygonal in transection and the walls are strengthened by cellulose flanges. The palisade cells are enclosed in a thin suberin-like sheath which connects with the film which lines all air-filled intercellular spaces in the outer integument. Plasmodesmata extend from the protoplast through the subcuticular zone to the cuticle and also interconnect with adjacent cells through numerous pits in the interflange areas of the main wall. The cells of the inner integument are many-armed. Intercellular spaces at all stages of growth observed are occluded with mucilage; nevertheless, they are lined with a suberin-like film similar in reaction to that of the air-filled intercellular spaces of the outer integument. The distribution of wall materials and ergastic substances was determined by microchemical tests. For electron microscopic study, the epidermis and the mucilaginous zone were fragmented ultrasonically, after chemical treatment when necessary. Cuticular fragments, frequently polygonal in outline, are dense to the electron beam at photographic illumination. When exposed to the electron beam, full-strength, volatile components are driven off and condense on other wall fragments or on substrate. Unique structures occur in the subcuticular zone, termed here, collar, cone and paddle structure. The basic material of the cell wall as a whole is cellulose impregnated with matrix materials, mainly non-cellulosic polysaccharides. In addition, lipid-like substances are conspicuous particularly in the flanges of the palisade cells. Under the electron beam, full-strength, these substances volatilize and leave the flanges with a blistered outline except in the region of the light line. The volatile substances may be absent from this zone or the microfibrillar structure may prevent distortion. The microfibrillar pattern throughout the length of the flanges is dominantly parallel, but a change to reticulate arrangement occurs in the light-line region. Numerous pits mark the paths of the plasmodesmata in the subcuticular zone and also through flange and interflange areas of the wall. The dominant microfibrillar pattern in the mucilage cells is reticulate in young and old cells, except for helical orientation in extending arms. Pits occur on arm ends but are infrequent elsewhere. The loose microfibrillar structure presumably allows continuous secretion of mucilage.     

Cell expansion in the epidermis: microtubules, cellulose orientation and wall loosening enzymes

Two models of isolated epidermis were used to demonstrate that the net orientation of cellulose microfibrils in the cell wall is related to mechanical properties of the tissue, and can be used as an indicator for wall anisotropy. In the developing plant epidermis, cells expand in one or two directions in the plane of the plant surface. In epidermis cells actively expanding in one direction (elongation), the orientation of cortical microtubules closely matches the net cellulose orientation. In epidermis cells expanding in two directions, the orientation of the parallel microtubules does not coincide with the net cellulose orientation in the adjacent cell wall. The orientation of cortical microtubules is thus not always a reliable indicator of wall characteristics. In both types of epidermis, a high rate of expansion correlates with a high activity of xyloglucan endotransglycosylase (XET), as determinedin situ. This high activity alone cannot explain unidirectional wall expansion.     

STRUCTURAL SPECIALIZATION OF THE SITE OF RESPONSE TO VECTORIAL PHOTO-EXCITATION IN THE SOLAR-TRACKING LEAF OF LAVATERA CRETICA

Structural features of the pulvinus of the solar-tracking leaf of Lavatera cretica L. that are involved in its capacity for omnidirectional and fully reversible bending in response to vectorial excitation of the lamina were studied by light- and scanning electron microscopy. Pulvini that had bent in the plane at right angles to the midvein were bisected along that plane and the symmetrical tissues of the expanded and contracted flanks were compared. The pulvinus contains a central vascular core and exhibits a transversely furrowed exterior. These specialized features enable the fully mature tissue of this region of the petiole to bend reversibly. The epidermis, chlorenchyma, peripheral collenchyma, and cortical parenchyma in the pulvinus form concentric, radially symmetric sheaths around the vascular core and exhibit structural features in their cell walls that would allow considerable changes in cell volume and consequently enable the omnidirectional bending of this pulvinus. Thickened wall portions of the pulvinar epidermis and peripheral collenchyma exhibit a highly specialized architecture, consisting of alternating thick and thin strips, that enhances their flexibility, while maintaining mechanical support. Cell walls of the chlorenchyma and the cortical parenchyma are thin and capable of reversible infolding. Those of the cortical parenchyma also exhibit numerous prominent transverse pit fields, indicative of anisotropic orientation of their microfibrillar lattice transverse to the pulvinar axis. This orientation is compatible with elastic deformation of the cortical parenchyma cells along the pulvinar axis. Filament-like cytoplasmic strands were observed along the walls of pulvinar motor cells, predominantly transverse to the pulvinar axis, but their function (if any) in volume changes of these cells is unknown.     

Relationship between microtubule orientation and patterns of cell expansion in developing cortical cells of Lemna minor roots

In actively growing cortical cells in the elongation zone of Lemna minor L. roots, both longitudinal (radial and tangential) and transverse walls expand in both length and width. The longitudinal walls of the three types of cortical cells in the root (i.e. outer, middle and inner) showed the largest expansion in the longitudinal axis. In contrast, the inner cortical cells exhibited the least expansion in width, whereas the middle cortical cells displayed the largest expansion in width. Thus, the profiles of the expansion of longitudinal walls were characteristic for the three types of cortical cells. In this study, both the orientation of cortical microtubule (MT) arrays and their dynamic reorientation, and the density of cortical MTs, were documented and correlated to the patterns of cell wall expansion. Significantly, transverse arrays of cortical MTs were most prominent in the radial walls of the inner cortical cells, and least so in those of the middle cortical cells. Toward the base of roots, beyond the elongation zone, the orientation of cortical MTs shifted continuously from transverse to oblique and then to longitudinal. In this case, the rate of shift in the orientation of cortical MTs along the root axis was appreciably faster in the middle cortical cells than in the other two types of cortical cells. Interestingly, the continuous change in cortical MT orientation was not confirmed in the transverse walls which showed much smaller two-dimensional expansion than the radial walls. Additionally, the presence of fragmented or shortened cortical MTs rapidly increased concomitantly with the decrease of transversely oriented cortical MTs. This relationship was especially prominent in the transverse walls of the inner cortical cells, which displayed the least expansion among the three types of cortical cells investigated. In the root elongation zone, the density of cortical MTs in the inner cortical cells was about three times higher than that in the other two cortical cell types. These results indicate that in the early stage of cell expansion, the orientation of cortical MTs determines a preferential direction of cell expansion and both the shifting orientation and density of cortical MTs affect the magnitude of expansion in width of the cell wall.     

Characteristics of the Deposition of Microfibrils during Formation of the Polylamellate Walls in the Coenocytic Green Alga Chamaedoris orientalis

Characteristics of the deposition of cellulose microfibrilsduring formation of polylamellate walls and the arrangementof cortical microtubules in the tip-growing bipolar cells ofChamaedoris orientalis were examined by replica preparationmethods and indirect immunofluorescence microscopy. The polylamellatewall is made up of two or three kinds of wall lamella whichdiffer in terms of the orientation of microfibrils. Individuallamellae were periodically initiated one after another fromthe pole that was situated exactly at each growing apex of thecell and they were deposited basipetally. The orientation ofmicrofibrils in each lamella was constant during deposition.Microfibrils in different lamellae were deposited at the sametime through the cell wall but the timing of the depositionwas staggered between neighboring lamellae so that the microfibrilswould not be interwoven. By contrast, cortical microtubuleswere persistently arranged longitudinally all over the celland no focal points to which they converged helically were visible,even around the cell apices. The mechanisms that regulate theformation of the polylamellate wall are discussed and a modelfor interpreting the involvement of the cortical microtubulesin such mechanisms is proposed. (Received July 31, 1989; Accepted January 27, 1990)     

THE MICROFIBRILLAR STRUCTURE OF THE CELL WALLS OF THE FILAMENTOUS FUNGUS, Allomyces

Cell walls of the fungus, Allomyces, were isolated by chemical procedures, using either potassium permanganate oxidation or glacial acetic acid-hydrogen peroxide treatment followed by dilute mineral acid. The structure of the treated walls was investigated by means of electron microscopy and electron diffraction analysis which showed that rhizoidal walls were especially suitable for observation. Chitin microfibrils exist in the extreme tips of rhizoidal walls, and tend to lie in a preferred longitudinal orientation. Older rhizoidal wall segments show a crossed fibrillar structure under a thin layer of short randomly arranged microfibrils. In the possession of systems of crossed fibrils these walls are like the cell walls of certain green algae. Walls of branch rhizoidal filaments were observed in the early stages of development, in which case the observed microfibrillar orientations are such that it is possible to envisage their origin from pre-existing fibrils that have passively reoriented. With respect to the continued growth of the filaments, however, it is difficult to explain the observed microfibrillar arrangements in terms of the "multi-net" theory. Hyphal walls usually show two layers, the outer consisting of microfibrils arranged randomly, and the inner consisting of well oriented microfibrils running parallel with the longitudinal axis of the hypha. The oriented inner layer appears to be similar in structure to the secondary wall of the Phycomyces sporangiophore.     

Observations on the Fine Structure of Plant Cell Walls III. The Sieve Tube Wall in Cucurbita

The sieve tube wall in Cucurbita was examined in ultra-thinsections of petioles treated in different ways for the removalof non-cellulosic wall components. The sections were stainedwith permanganate. The microfibrillar components of the wallare arranged in concentric lamellae. The earliest (outermost)part of the wall is similar to that of ordinary parenchyma inhaving its lamellae composed of thinly-distributed microfibrilsreadily separated from one another by certain treatments suchas pectinase extraction. In the characteristically-thickenedinner (nacreous) layer the microfibrils are very densely packedand the lamellae do not separate readily. The microfibrils inthis layer of the wall are very close to transverse and the‘crossed fibrillar’ orientation is not easily discernible.     

Cell growth pattern and wall microfibrillar arrangement: experiments with nitella

In cylindrical cells growing throughout their length, over-all transverse reinforcement of the wall by microfibrils is believed to be required for cell elongation. The multinet theory states that in such cells microfibrils are deposited at the inner surface of the wall with transverse orientation and are then passively reoriented toward the longitudinal direction by the predominant longitudinal strain (surface expension). In the present study young Nitella cells were physically forced to grow in highly abnormal patterns: in length only, in girth only, or with localized suppression of growth. Subsequent gradients of microfibrillar arrangement within the wall cross-section were measured with polarized light and interference microscopes. The novel wall structures produced were in all cases explainable by passive reorientation, i.e. by the multinet theory. The study also showed that orientation of synthesis remains insensitive to several of the physical manipulations that strongly influence the passive behavior of wall microfibrils. Only the localized complete suppression of surface growth led to the deposition of nontransverse cellulose. These results suggest that the presence of strain is needed for continued oriented synthesis, but that the directional aspect of strain is not an “instructional” agent continuously guiding the orientation of synthesis, once this orientation has been established.     

Differential Responsiveness of Cortical Microtubule Orientation to Suppression of Cell Expansion among the Developmental Zones of Arabidopsis thaliana Root Apex

Τhe bidirectional relationship between cortical microtubule orientation and cell wall structure has been extensively studied in elongating cells. Nevertheless, the possible interplay between microtubules and cell wall elements in meristematic cells still remains elusive. Herein, the impact of cellulose synthesis inhibition and suppressed cell elongation on cortical microtubule orientation was assessed throughout the developmental zones of Arabidopsis thaliana root apex by whole-mount tubulin immunolabeling and confocal microscopy. Apart from the wild-type, thanatos and pom2-4 mutants of Cellulose SynthaseA3 and Cellulose Synthase Interacting1, respectively, were studied. Pharmacological and mechanical approaches inhibiting cell expansion were also applied. Cortical microtubules of untreated wild-type roots were predominantly transverse in the meristematic, transition and elongation root zones. Cellulose-deficient mutants, chemical inhibition of cell expansion, or growth in soil resulted in microtubule reorientation in the elongation zone, wherein cell length was significantly decreased. Combinatorial genetic and chemical suppression of cell expansion extended microtubule reorientation to the transition zone. According to the results, transverse cortical microtubule orientation is established in the meristematic root zone, persisting upon inhibition of cell expansion. Microtubule reorientation in the elongation zone could be attributed to conditional suppression of cell elongation. The differential responsiveness of microtubule orientation to genetic and environmental cues is most likely associated with distinct biophysical traits of the cells among each developmental root zone.     

Cell expansion patterns and directionality of wall mechanical properties in nitella

As a means of assessing the extent to which deformation of isolated walls relates to in vivo cell expansion, the directionality of wall mechanical properties was examined in Nitella. Measurements were made of plastic and elastic deformation and creep under both uniaxial and multiaxial stress conditions. Walls of different structural characteristics were obtained from control, isopropyl N-phenylcarbamate (IPC)-treated and IPC recovery cells. Although microfibrils in the inner portion of the wall were transverse for control and recovery cells but random for IPC cells, all walls had similar over-all microfibrillar orientations. Consequently, differences in wall mechanical properties should reflect structural differences in the inner wall. It is the action of the prevailing stress pattern on the inner, not overall, wall microfibrillar organization which dictates the directionality of growth in Nitella. The results indicate that the directional character of expansion is preserved to a large extent in the mechanical properties of isolated walls, and that most, but not all, of the deformation is determined by the inner wall. In addition, directional differences in the threshold for acid-induced extension varied in accord with the pattern of inner wall microfibrils.     

AN ULTRASTRUCTURAL STUDY OF THE RELATIONSHIP BETWEEN MICROTUBULES AND MICROFIBRILS IN COTTON (GOSSYPIUM HIRSUTUM L.) CELL WALL REVERSALS

The arrangement of cellulosic fibrils in the cell walls of cotton fibers is very unusual; rather than exhibiting a continuous spiraling in one direction, they intermittantly reverse their gyre. Microtubules that line the periphery of the protoplasm, subjacent to the plasmalemma, tend to parallel the deployment of the cell wall microfibrils. It was not known whether this parallelism persisted through the reversal. By studying tangential sections of the cell wall/protoplasmic interfaces at the reversals, we show that congruity continues even through the reversals. Colchicine treatment did not appear to inhibit cellulose synthesis but it did abolish microtubules in the cotton fiber cells and deranged normal cell wall microfibrillar orientation. Previously, cotton fibers have been shown to possess most of the familiar organelles, but we found two new features not reported heretofore. They are microfilaments and peculiar “polygonal structures” that appear to be associated with the plasma membrane.     

蜜环菌索发育的研究

利用光学和电子显徽镜对蜜环菌索的发育及其结构分化进行了较系统研究。菌索的顶端有保持细胞不断分裂的分生组织区。由此衍生的菌丝细胞组成菌索的初生结构,包括分化不明显的表皮、皮层及初生髓;初生髓细胞体积大,核同步分裂产生多核体细胞,以一个或几个核为单位在爵体细胞中分化出细长的菌丝后,可以出芽方式自母体细胞中伸出,并且一开始就有薄壁与厚壁之分,同一母体细胞中可同时产生这两类菌丝。发育后期母体细胞破裂形成菌索的髓,两类疏松菌丝分布在其中。观察了成熟菌索的结构和侧枝的形成过程。菌索侧枝起源于皮层细胞,该细胞横向分裂首先形成分枝原基,之后突破菌索壳而分化出新的菌索顶端。讨论了蜜环菌索在不同寄主中的侵染方式。     

Ultrastructural studies on the cell walls in Fusarium sulphureum.

The cell walls of Fusarium sulphureum have a microfibrillar component that is randomly arranged. X-ray-diffraction diagrams of the microfibrils are consistent with a high degree of crystallinity and show that they are chitin. The chitin microfibrils of the peripheral walls envelop the hyphal apex and extend across the septae. During the first 8h in culture, the conversion of conidial cells to chlamydospores is evidenced by a swelling of the cells and the original microfibrils remain randomly arranged. Within 24h new wall material is deposited as the cells expand and the wall thickens. The new microfibrils are indistinguishable from those of the original conidial cells. After 3 days in culture, the chlamydospores are fully developed and have the characteristic thick wall which is a continuous layer of randomly arranged microfibrils. Chlamydospores maintained in a conversion medium for 8 days have microfibrils identical with those in 3-day-old cultures; thus a further change in the microfibril orientation did not occur during that period. Alkaline hydrolysis of the walls removes most of the electron-dense staining constituents from the inner wall layer and leaves the outer wall layer intact. This treatment also reveals some of the wall microfibrils. An additional treatment of the walls with HAc/H2O2 completely removes the wall components that react positively to heavy metal stains. The results are discussed in relation to the structure of other fungal cell walls.     

Microtubule reorganization,cell wall synthesis and establishment of the axis of elongation in regenerating protoplasts of the algaMougeotia

Summary Microtubule reorganization and cell wall deposition have been monitored during the first 30 hours of regeneration of protoplasts of the filamentous green algaMougeotia, using immunofluorescence microscopy to detect microtubules, and the cell-wall stain Tinopal LPW to detect the orientation of cell wall microfibrils. In the cylindrical cells of the alga, cortical microtubules lie in an ordered array, transverse to the long axis of the cells. In newly formed protoplasts, cortical microtubules exhibit some localized order, but within 1 hour microtubules become disordered. However, within 3 to 4 hours, microtubules are reorganized into a highly ordered, symmetrical array centered on two cortical foci. Cell wall synthesis is first detected during early microtubule reorganization. Oriented cell wall microfibrils, co-aligned with the microtubule array, appear subsequent to microtubule reorganization but before cell elongation begins. Most cells elongate in the period between 20 to 30 hours. Elongation is preceded by the aggregation of microtubules into a band intersecting both foci, and transverse to the incipient axis of elongation. The foci subsequently disappear, the microtubule band widens, and microfibrils are deposited in a band which is co-aligned with the band of microtubules. It is proposed that this band of microfibrils restricts lateral expansion of the cells and promotes elongation. Throughout the entire regeneration process inMougeotia, changes in microtubule organization precede and are paralleled by changes in cell wall organization. Protoplast regeneration inMougeotia is therefore a highly ordered process in which the orientation of the rapidly reorganized array of cortical microtubules establishes the future axis of elongation.