Cellulose synthesizing terminal complexes and morphogenesis in tip-growing cells of Syringoderma phinneyi (Phaeophyceae)

The intramembrane particles and cellulose synthesis of the brown alga Syringoderma phinneyi Henry et Müller were examined using replicas of freeze‐fractured apical cells. Like in other brown algae, linear terminal complexes (TCs) were found in the plasmatic fracture face (PF) of the plasmalemma, which are the putative cellulose synthases. Terminal complexes consist of a single row of particles, each particle composed of two sub‐units, and are found in close relationship with cellulose microfibril imprints. Examination of the distribution of TCs revealed a clear apico‐basal gradient, with a higher density of TCs in the apical part. This seems to reflect the tip growth of the apical cells. The rate of cellulose synthesis per TC subunit was calculated based on the dimensions of the TCs and cellulose microfibrils.     

Occurrence of the putative microfibril-synthesizing complexes (linear terminal complexes) in the plasma membrane of the epiphytic marine red algaErythrocladia subintegra Rosenv.

Summary Cells of thalli at different developmental stages of the epiphytic marine red algaErythrocladia subintegra have been studied by freeze-etching. It was found that the plasma membrane exhibits linear microfibril-termnal synthesizing complexes (TCs), randomly distributed consisting of four rows of linearly-arranged particles (average diameter of particles 8.6 nm); each row of TCs consists of 5–33 particles (average 15). The TCs were observed on both fracture faces (PF and EF) but more clearly on the PF face. These structures appear to span both the outer and inner leaflets of the plasma membrane (transmembrane complexes)-The TCs have stable width (35 nm) and vary in length (41–311 nm, average 181 nm). The TCs subunits are highly ordered arrays forming a semicylinder. The average density of TCs on the PF face is 5.5TC/m². The microfibrils are randomly distributed and have a mean width of 39.4 nm (ranging from 16 to 70 nm). Many TCs are associated with the ends of microfibrils and microfibril imprints. The structural characteristics of linear TCs in the red algaErythrocladia are compared with those of the so far investigated Chlorophyta spp. All results favour the suggestion that TCs in the plasma membrane ofErythrocladia cells are involved in the biosynthesis, assembly and orientation of microfibrils.     

Plasma-membrane rosettes in root hairs of Equisetum hyemale

Particle arrangement in the plasma membrane during cell wall formation was investigated by means of the double-replica technique in root hairs of Equisetum hyemale. Particle density in the protoplasmic fracture face of the plasma membrane was higher than in the extraplasmic fracture face. Apart from randomly distributed particles, particle rosettes were visible in the PF face of the plasma membrane. The rosettes consisted of six particles arranged in a circle and had an outer diameter of approx. 26 nm. No gradient in the number of rosettes was found, which agrees with micrifibril deposition taking place over the whole hair. The particle rosettes were found individually, which might indicate that they spin out thin microfibrils as found in higher-plant cell walls. Indeed microfibril width in these walls, measured in shadowed preparations, is 8.5±1.5 nm. It is suggested that the rosettes are involved in microfibril synthesis. Non-turgid cells lacked microfibril imprints in the plasma membrane and no particle rosettes were present on their PF face. Fixation with glutaraldehyde caused, probably as a result of plasmolysis, the microfibril imprints to disappear together with the particle rosettes. The PF face of the plasma membrane of non-turgid hairs sometimes showed domains in which the intramembrane particles were aggregated in a hexagonal pattern. Microfibril orientation during deposition will be discussed.Abbreviations EF extraplasmic fracture face - PF protoplasmic fracture face     

THE SITES OF CELLULOSE SYNTHESIS IN ALGAE: DIVERSITY AND EVOLUTION OF CELLULOSE-SYNTHESIZING ENZYME COMPLEXES

Information on the sites of cellulose synthesis and the diversity and evolution of cellulose-synthesizing enzyme complexes (terminal complexes) in algae is reviewed. There is now ample evidence that cellulose synthesis occurs at the plasma membrane-bound cellulose synthase, with the exception of some algae that produce cellulosic scales in the Golgi apparatus. Freeze-fracture studies of the supramolecular organization of the plasma membrane support the view that the rosettes (a six-subunit complex) in higher plants and both the rosettes and the linear terminal complexes (TCs) in algae are the structures that synthesize cellulose and secrete cellulose microfibrils. In the Zygnemataceae, each single rosette forms a 5-nm or 3-nm single “elementary” microfibril (primary wall), whereas rosettes arranged in rows of hexagonal arrays synthesize criss-crossed bands of parallel cellulose microfibrils (secondary wall). In Spirogyra, it is proposed that each of the six subunits of a rosette might synthesize six β-1,4-glucan chains that cocrystallize into a 36-glucan chain “elementary” microfibril, as is the case in higher plants. One typical feature of the linear terminal complexes in red algae is the periodic arrangement of the particle rows transverse to the longitudinal axis of the TCs. In bangiophyte red algae and in Vaucheria hamata, cellulose microfibrils are thin, ribbon-shaped structures, 1–1.5 nm thick and 5–70 nm wide; details of their synthesis are reviewed. Terminal complexes appear to be made in the endoplasmic reticulum and are transferred to Golgi cisternae, where the cellulose synthases are activated and may be transported to the plasma membrane. In algae with linear TCs, deposition follows a precise pattern directed by the movement and the orientation of the TCs (membrane flow). A principal underlying theme is that the architecture of cellulose microfibrils (size, shape, crystallinity, and intramicrofibrillar associations) is directly related to the geometry of TCs. The effects of inhibitors on the structure of cellulose-synthetizing complexes and the relationship between the deposition of the cellulose microfibrils with cortical microtubules and with the membrane-embedded TCs is reviewed In Porphyra yezoensis, the frequency and distribution of TCs reflect polar tip growth in the apical shoot cell.The evolution of TCs in algae is reviewed. The evidence gathered to date illustrates the utility of terminal complex organization in addressing plant phylogenetic relationships.     

The plasma membrane of growing root hairs is composed of zones of local differentiation

Growing root hairs of cress (Lepidium sativum L.) were investigated using freeze-fracture and electron-microscopic techniques. Three zones of differentiation could be detected: the tip zone, the zone of vacuolation and the foot zone. Corresponding to these zones, the plasmatic fracture face of the plasma membrane showed areas of pronounced differentiation with respect to the distribution and frequency of intramembranous particles (IMPs). The tip zone was characterized by an irregular fracture plane caused by a large number of blisters which were more or less free of IMPs. These blisters coincided in size and shape with Golgi vesicles accumulated in the ground cytoplasm near the very tip. Outside these blisters, IMPs were randomly distributed. The surrounding cell wall was very thin and mainly composed of amorphous material. The plasma membrane of the vacuolation zone often revealed areas of hexagonally ordered particles (HOPS). Such patterns of particles were observed in chemically fixed and unfixed root hairs with a maximum surface density of 1200 HOPS per area. Mostly, however, 15–50 HOPS per area were found. The number of such areas increased with increasing distance from the tip up to five areas per m². Additionally, imprints of large cellulose microfibrils could be detected in unfixed material; they were mainly parallel to the root-hair axis and sometimes ended in areas of HOPS. However, HOPS were observed only in approximately 60% of the root hairs. Otherwise, large areas free of IMPs were interspersed between areas of randomly distributed IMPs. The particle frequency was relatively low and varied greatly in the tip as well as in the vacuolation zone, that is, from 1200 to 2000 IMPs m^-2. Finally, the plasma membrane of the foot zone showed a very constant number of approx. 2000 IMPs m^-2. These particles were mainly distinct and randomly distributed. In this zone, HOPS were never observed in spite of the fact that the cell wall was composed of numerous parallel-running cellulose microfibrils. Since membrane material is mainly incorporated in the tip zone where IMPs are statistically distributed, the results indicate that the plasma membrane of the outgrowing part of the root-hair cells is characterized by a high lateral mobility of its components. Furthermore, they indicate that specifically arranged particles are involved in the synthesis of cellulose microfibrils. These areas of HOPS seem to be locally restricted and — or limited with respect to their lifetime.Abbreviations cmf(s) cellulose microfibril(s) - EF extraplasmatic fracture face - HOPS hexagonally ordered particles - IMP intramembranous particle - PF plasmatic fracture face - pm plasma membrane Dedicated to Professor Dr. Kurt Mühlethaler, Zürich, on the occasion of his 65th birthday     

THE ASSOCIATION OF ROSETTE AND GLOBULE TERMINAL COMPLEXES WITH CELLULOSE MICROFIBRIL ASSEMBLY IN NITELLA TRANSLUCENS VAR. AXILLARIS (CHAROPHYCEAE)1,2

A freeze-fracture investigation of the putative cellulose synthesizing complex (terminal complex) morphology in Nitella translucens var. axillaris (A. Br.) R.D.W. internodal cells revealed single solitary EF globules and PF rosettes on the plasma membrane. The average density of rosettes in elongating internodal cells was 5.6 μm^?2 with slight spatial variation observed. In only three other algal genera (all zygnematalean) have rosette / globule terminal complexes been observed, while this characteristic is common to all vascular plants and one moss thus far investigated. This evidence strongly suggests that the rosette type of terminal complex morphology is an additional characteristic of charophycean algae and lends further support to the hypothesis that this group of algae represents the evolutionary line that gave rise to vascular plants. Observations were also made from the freeze-fracture of Nitella internodal cells concerning the orientation of cell wall microfibrils and cytoskeletal elements near the plasma membrane. The pattern of microfibril orientation in growing internodal cells is initially transverse to the cell long axis, becoming progressively axial presumably due to the strain of elongation. In mature internodal cells, the pattern of microfibril orientation is helicoidal. Microtubules appressed to the inner surface of the plasma membrane are oriented parallel to the most recently formed microfibrils in elongating and mature internodal cells.     

Tunicamycin Prevents Cellulose Microfibril Formation in Oocystis solitaria

The effect of tunicamycin (TM) on the development of the cell wall in Oocystis solitaria has been investigated. It was found that 10 micromolar TM completely stops the assembly of new microfibrils as observed at the ultrastructural level. During cell wall formation, freeze fracture replicas of the E-face of the plasma membrane reveal two major substructures: the terminal complexes (TC), paired and unpaired, and the microfibril imprints extending from unpaired TCs. In cells treated for 3 hours or longer with TM, the TCs are no longer visible, whereas microfibril imprints are still present. Because of the reported highly selective mode of action of TM, our results implicate a role for lipid-intermediates in cellulose synthesis in O. solitaria. It is assumed that TM prevents the formation of a glycoprotein which probably is a fundamental part of the TCs and may act as a primer for the assembly of the microfibrils.     

The assembly of cellulose microfibrils in Valonia macrophysa Kütz

The assembly of cellulose microfibrils was investigated in artificially induced protoplasts of the alga, Valonia macrophysa (Siphonocladales). Primary-wall microfibrills, formed within 72 h of protoplast induction, are randomly oriented. Secondary-wall lamellae, which are produced within 96 h after protoplast induction, have more than three orientations of highly ordered microfibrils. The innermost, recently deposited micofibrils are not parallel with the cortical microtubules, thus indicating a more indirect role of microtubules in the orientation of microfibrils. Fine filamentous structures with a periodicity of 5.0–5.5 nm and the dimensions of actin were observed adjacent to the plasma membrane. Linear cellulose-terminal synthesizing complexes (TCs) consisting of three rows, each with 30–40 particles, were observed not only on the E fracture (EF) but also on P fracture (PF) faces of the plasma membrane. The TC appears to span both faces of the bimolecular leaflet. The average length of the TC is 350 nm, and the number of TCs per unit area during primary-wall synthesis is 1 per m². Neither paired TCs nor granule bands characteristic of Oocystis were observed. Changes in TC structure and distribution during the conversion from primary- to secondary-wall formation have been described. Cellulose microfibril assembly in Valonia is discussed in relation to the process among other eukaryotic systems.Abbreviations TC terminal complex - EF E (outer leaflet) fracture face of the plasma membrane - PF P (inner leaflet) fracture face of the plasma membrane - MT microtubule - PS protoplasmic surface of the membrane     

Cellulose-synthesizing terminal complexes and microfibril structure in the brown alga Sphacelaria rigidula (Sphacelariales,Phaeophyceae)*

The brown alga Sphacelaria rigidula Kützing synthesizes cellulose microfibrils as determined by CBH I-gold labeling. The cellulose microfibrils are thin, ribbon-like structures with a uniform thickness of about 2.6 nm and a variable width in the range of 2.6-30 nm. Some striations appear along the longitudinal axis of the microfibrils. The developed cell wall in Sphacelaria is composed of three to four layers, and cellulose micro-fibrils are deposited in the third layer from the outside of the wall. A freeze fracture investigation of this alga revealed cellulose-synthesizing terminal complexes (TCs), which are associated with the tip of microfibril impressions in the plasmatic fracture face of the plasma membrane. The TCs consist of subunits arranged in a single linear row. The average diameter of the sub-units is about 6 nm, and the intervals between the neighboring subunits, about 9 nm, are relatively constant. The number of subunits constituting the TC varies between 10 and 100, so that the length of the whole TC varies widely. A model that has been proposed for the assembly of thin, ribbon-like microfibrils was applied to microfibril assembly in Sphacelaria.     

Arrays of plasma-membrane “rosettes” involved in cellulose microfibril formation of Spirogyra

The cell-wall structure and plasma-membrane particle arrangement during cell wall formation of the filamentous chlorophycean alga Spirogyra sp. was investigated with the freeze-fracture technique. The cell wall consists of a thick outer slime layer and a multilayered inner wall with ribbon-like microfibrils. This inner wall shows three differing orientations of microfibrils: random orientation on its outside, followed by axial bundles of parallel microfibrils, and several internal layers of bands of mostly five to six parallel associated microfibrils with transverse to oblique orientation. The extraplasmatic fracture face of the plasma membrane shows microfibril imprints, relatively few particles, and “terminal complexes” arranged in a hexagonal package at the end of the imprint of a microfibril band. The plasmatic fracture face of the plasma membrane is rich in particles. In places, it reveals hexagonal arrays of “rosettes”. These rosettes are best demonstrable with the double-replica technique. These findings on rosette arrays of the zygnematacean alga Spirogyra are compared in detail with the published data on the desmidiacean algae Micrasterias and Closterium.     

Interference of cell wall regeneration ofBoergesenia forbesii protoplasts by Tinopal LPW,a fluorescent brightening agent

Summary Wounding cells ofBoergesenia forbesii (Harvey) Feldmann induces the synchronous formation of numerous protoplasts which synthesize large cellulose microfibrils within 2–3 hours after wounding. The microfibrils appear to be assembled by linear terminal synthesizing complexes (TCs). TC subunits appear on both E- and P-faces of the plasma membrane, thus suggesting the occurrence of a transmembrane complex. The direction of microfibril synthesis is random during primary wall assembly and becomes ordered during secondary wall assembly. The average density of TCs during secondary wall deposition is 1.7/m², and the average length of the TC is 510 nm. TC organization is similar to that ofValonia macrophysa; however, the larger TCs ofBoergesenia (510 nm vs. 350 nm) produce correspondingly larger microfibrils (30 nm vs. 20 nm).The effects of a fluorescent brightening agent (FBA), Tinopal LPW, on cell wall regeneration ofBoergesenia protoplasts was investigated. The threshold level of Tinopal LPW for interfering with microfibril assembly is 1.5 M. At 95 M Tinopal (for short periods up to 15 minutes), microfibril impressions have atypical spherical impressions at their termini. At longer incubations (24 hours), TCs and microfibril impressions are absent. When washed free of Tinopal, the protoplasts eventually resume normal wall assembly; however, TCs do not reappear until at least 30 minutes after the removal of Tinopal. In consideration of the presence of ordered TCs before FBA treatment, their random distribution upon recovery implies an intermediate stage of assembly or possiblyde novo synthesis.     

Cellulose microfibril assembly inErythrocladia subintegra Rosenv.: An ideal system for understanding the relationship between synthesizing complexes (TCs) and microfibril crystallization

Summary The marine red algaErythrocladia subintegra synthesizes cellulose microfibrils as determined by CBH I-gold labelling, X-ray and electron diffraction analyses. The cellulose microfibrils are quite thin, ribbon-like structures, 1–1.5 nm in thickness (constant), and 10–33 nm in width (variable). Several laterally associated minicrystal components contribute to the variation in microfibrillar width. Electron diffraction analysis suggested a uniplanar orientation of the microfibrils with their (101) lattice planes parallel to the plasma membrane surface of the cell. The linear particle arrays bound in the plasma membrane and associated with microfibril impressions recently demonstrated inErythrocladia have been shown in this study to be the cellulose-synthesizing terminal complexes (TCs). The TCs appear to be organized by a repetition of transverse rows consisting of four TC subunits, rather than by four rows of longitudinallyarranged TC subunits. The number of transverse rows varied between 8–26, corresponding with variation in the length of the TCs and the width of the microfibrils. The spacings between the neighboring transverse rows are almost constant being 10.5–11.5 nm. Based on the knowledge thatAcetobacter, Vaucheria, andErythrocladia synthesize similar thin, ribbon-like cellulose microfibrils, the structural characteristics common to the organization of distinctive TCs occurring in these three organisms has been discussed, so that the mode of cellulose microfibril assembly patterns may be deciphered.     

Freeze-fracture studies in the brown algaAsteronema rhodochortonoides

Summary The gross structure of the cell wall and the organization of the plasmalemma of the filamentous brown algaAsteronema rhodochortonoides were examined in replicas of freeze-fractured cells. The protoplasmic fracture face (PF) of the plasmalemma, apart from the single particles, exhibits two particular particle complexes, i.e., single linear arrays of closely packed particles, and well defined particle pentads. The former display a consistent relationship with the ends of microfibril imprints and therefore are considered as terminal complexes (TCs). They seem to be composed of subunits, each one consisting of two particles. The average diameter of the particles is 7 nm. The number of the subunits forming the TCs varies between 2 and 40. Short TCs, consisting of 3–5 subunits were also found on the PF of dictyosome vesicles, a fact suggesting the involvement of the Golgi apparatus in exocytosis of preformed TC portions. The occurrence, distribution and size of the TCs appear to be related to the developmental stage of the cell. A large number of TCs occur in actively growing cells, while a few or no TCs are found in differentiated cells. The pentads are rectangular structures consisting of five particles, four in the corners and one in the centre. Their dimensions are very constant, but their occurrence and distribution varies. They occur in young developing cells where TCs are few or absent, but were also observed in areas showing many TCs. In differentiated cells no pentads were found. Pentad-like structures were rarely observed on the PF of dictyosome vesicles or cisternae. The observations support the hypothesis that pentads are involved in the synthesis of matrix polysaccharides, which are the major components of brown algal cell wall and their synthesis begins before that of cellulose.Dedicated to Prof. Dr. Dr. h.c. Eberhard Schnepf on the occasion of his retirement     

A new putative cellulose-synthesizing complex ofColeochaete scutata

Summary Cells of the charophycean alga,Coleochaete scutata active in cell wall formation were freeze fractured in the search for cellulose synthesizing complexes (TCs) since this alga is considered to be among the most advanced and a progenitor to land plant evolution. We have found a new TC which consists of two geometrically distinctive particle complexes complementary to one another in the plasma membrane and occasionally associated with microfibril impressions. In the E-fracture face is found a cluster of 8–50 closely packed particles, each with a diameter of 5–17 nm. Most of these particles are confined within an 80 nm circle. In the P-fracture face is found an 8-fold symmetrical arrangement of 10 nm particles circumferentially arranged around a 28 nm central particle. The TCs ofC. scutata are quite distinctive from the rosette/globule TCs of land plants. The 5.5×3.1 nm microfibril inC. scutata is also distinctive from the 3.5×3.5 nm microfibril typical of land plants. The phylogenetic implications of this unique TC in land plant evolution are discussed.     

The plasma membrane of the Funaria caulonema tip cell: morphology and distribution of particle rosettes,and the kinetics of cellulose synthesis

Freeze-fracturing of Funaria hygrometrica caulonema cells leads to a cleavage within the plasma membrane. The extraplasmatic and the plasmatic fracture faces differ in their particle density. The plasmatic fracture face in caulonema tip cells or in tip cells of side branches, but never in other caulonema cells, is further characterized by the occurrence of particle rosettes. The highest density of rosettes is found at the cell apex but decreases steeply toward the cell base. The shape of the rosettes varies remarkably; 20% of them are found in an incomplete, presumably disintegrating or aggregating state. The complete rosette has a diameter of about 25 nm and consists of five to six particles. The size of the single particles varies between 4 nm to 10 nm. The rosettes are thought to posses cellulose-synthase activity. It is assumed that one rosette produces one elementary fibril; rough calculations, considering the number of rosettes and the estimated amount of cellulose produced in the tip region, indicate that an elementary fibrillar length of 900 nm is formed in 1 min by one rosette. The consequence of the kinetics on the life-time of the rosettes and the cellulose-synthase activity are discussed.Abbreviations EF extraplasmatic fracture face - PF plasmatic fracture face     

Assembly of Cellulose Synthesizing Complexes on the Plasma Membrane of Boodlea coacta

The assembly of cellulose synthesizing complexes (terminal complexes,TCs) on the plasma membrane of Boodlea coacta was investigatedduring the formation of both the matrix-rich layer (MRL) andfibril-rich layers (FRLs) of cell walls. The TCs appeared tobe located mostly within the outer leaflet of the plasma membrane,and were observed as elliptical protrusions consisting of manyparticles of about 9 nm in diameter. Their length varied from100 to 500 nm (average, 220 nm) during MRL formation and from100 to 860 nm (average, 360 nm) during FRL formation. A correlationwas found between the length of TCs and the microfibril widthin both MRL and FRL. On the E-face of the plasma membrane, numerous round protrusions(30–130 nm in diameter), consisting of many particles,8–10 nm in diameter, were also present. Their densitywas greater during FRL formation than during MRL formation.Some of these structures larger than 100 nm were associatedwith microfibril impressions and some appeared to be bound tothe TCs. These protrusions increased in number with Calcofluortreatment but decreased in number when the dye was removed fromthe culture medium. Thus, the TCs may be assembled from massesof particles aggregated on the outer surface of the plasma membrane,and may grow longer by incorporation of these masses. The appearanceof the longer TCs during FRL formation is probably due to thegreater density of these masses. (Received May 1, 1985; Accepted August 16, 1985)     

The control of cellulose microfibril deposition in the cell wall of higher plants

In maize (Zea mays L.) and pine (Pinus taeda L.) seedlings, cellulose microfibril impressions are present on freeze-fractured plasma membranes. It has been proposed that impressions of newly synthesized microfibrils are a record of the movement of terminal synthesizing complexes through the plasma membrane (Mueller and Brown, 1980, J. Cell Biol. 84, 315–326). The association of terminal complexes with the ends of microfibril impressions or with the ends of microfibrils torn through the membrane indicates the orientation of microfibril tips. Unidirectionally-oriented microfibril tips (all pointing in the same direction) are associated with the organized deposition of parallel arrays of microfibrils. Multidirectionally-oriented microfibril tips were observed in a cell in which microfibril deposition was unusually disorganized. Microfibril patterns around pit fields are asymmetric and resemble flow patterns. Unidirectionally-oriented tears are associated with these microfibrils. Although microfibril orientations are deflected around pit fields, the main axis of microfibril orientation is maintained across the surface of the cell. The hypothesis is proposed that the interaction of a flowing plasma membrane with microfibril synthesizing complexes in the plane of the membrane may result in unidirectional deposition and asymmetric microfibril impressions around pit fields.Some of this work has been published in preliminary form (Brown 1979)     

Surface architecture of the plant cell: Biogenesis of the cell wall,with special emphasis on the role of the plasma membrane in cellulose biosynthesis

Cell wall structure and biogenesis in the unicellular green alga, Oocystis apiculata, is described. The wall consists of an outer amourphous primary layer and an inner secondary layer of highly organized cellulosic microfibrils. The primary wall is deposited immediately after cytokinesis. Golgi-derived products contribute to this layer. Cortical microtubules underlie the plasma membrane immediately before and during primary wall formation. They function in maintaining the elliptical cell shape. Following primary wall synthesis, Golgi-derived materials accumulate on the cell surface to form the periplasmic layer. This layer functions in the deposition of coating and cross-linking substances which associate with cellulosic microfibrils of the incipient secondary wall. Secondary wall microfibrils are assembled in association with the plasma membrane. Freeze-etch preparations of untreated, living cells reveal linear terminal complexes in association with growing cellulosic microfibrils. These complexes are embedded in the EF fracture face of the plasma membrane. The newly synthesized microfibril lies in a groove of the outer leaflet of the plasma membrane. The groove is decorated on the EF fracture face by perpendicular structures termed “ridges.” The ridges interlink with definitive rows of particles associated with the PF fracture face of the inner leaflet of the plasma membrane. These particles are termed “granule bands,” and they function in the orientation of the newly synthesized microfibrils. Microfibril development in relation to a coordinated multienzyme complex is discussed. The process of cell wall biogenesis in Oocystis is compared to that in higher plants.     

Cell-wall formation in Pelvetia embryos. A freeze-fracture study

The cell-wall formation in the egg of Pelvetia fastigiata (J.G. Agardh) DeToni (Fucaceae) was studied with freeze-fracture. 1. The wall is lamellated with microfibrils approximately parallel in each lamella. The average orientation of microfibrils turns about 35° in each subsequent lamella. This slow turn gives rise to bow-shaped arcs when the wall is obliquely cross fractured. 2. The organization of the fibrils in the innermost lamellae is visualized by their imprints on the plasma membrane. These imprints are the result of both turgor pressure and adhesion of fibrils to the membrane. 3. Strings of membrane particles appear on the plasma membrane shortly after fertilization. They seem to be formed by a fertilization-induced aggregation of isolated membrane particles. Later each string comes to lie under a fibril and along its imprint. Peculiar lateral rips indicate that some strings are tightly bound to a fibril and may be involved in its orientation. 4. Wall formation in Pelvetia is marked by pronounced secretory activities. Following fertilization, the fusion of cortical vesicles and other vesicles make numerous loci in the plasma membrane. In older embryos, fibril-free patches in the plasma membrane mark the position of microfibril elongation centers in the wall matrix. Prior to germination, these elongation centers and their corresponding membrane patches reach a high density at the presumptive rhizoid end.We wish dedicate this paper to R.D. Preston     

Cytoskeletal elements in cotton seed hair developmentin vitro: Their possible regulatory role in cell wall organization

Summary The localization and orientation of cytoskeletal elements in developing cotton fibres were studied by the indirect immunofluorescence and the dry cleaving technique. Microtubules are transversely arranged to the cell axis, most probably in a flat helix, in the cortex of expanding fibres. Since the innermost deposited cellulose microfibrils always show primarily the same orientation it is postulated that the microtubules control the transverse deposition of the cellulose fibrils. Little further cell expansion takes place during secondary wall formation and the microfibril pattern corresponds to that of the cortical microtubules,e.g., in the steepness of their helicoidal turns. Microtubules with a length of 7–20 m were observed, probably they are longer. The importance of microtubule length on microfibril deposition is discussed. The density of microtubule packing is in the range of 8–14 m^-1 as in other comparable cell types. In contrast to the microtubules, actin filaments are most likely longitudinally oriented during different phases of fibre development. The dry cleaving technique reveals numerous coated pits in the plasma membrane which are not crossed by microtubules. They seem to be linked to the latter by filamentous structures.