Cellulose microfibril alignment recovers from DCB-induced disruption despite microtubule disorganization

Cellulose microfibril deposition patterns define the direction of plant cell expansion. To better understand how microfibril alignment is controlled, we examined microfibril orientation during cortical microtubule disruption using the temperature-sensitive mutant of Arabidopsis thaliana, mor1-1. In a previous study, it was shown that at restrictive temperature for mor1-1, cortical microtubules lose transverse orientation and cells lose growth anisotropy without any change in the parallel arrangement of cellulose microfibrils. In this study, we investigated whether a pre-existing template of well-ordered microfibrils or the presence of well-organized cortical microtubules was essential for the cell to resume deposition of parallel microfibrils. We first transiently disrupted the parallel order of microfibrils in mor1-1 using a brief treatment with the cellulose synthesis inhibitor 2,6-dichlorobenzonitrile (DCB). We then analysed the alignment of recently deposited cellulose microfibrils (by field emission scanning electron microscopy) as cellulose synthesis recovered and microtubules remained disrupted at the mor1-1 mutant's non-permissive culture temperature. Despite the disordered cortical microtubules and an initially randomized wall texture, new cellulose microfibrils were deposited with parallel, transverse orientation. These results show that transverse cellulose microfibril deposition requires neither accurately transverse cortical microtubules nor a pre-existing template of well-ordered microfibrils. We also demonstrated that DCB treatments reduced the ability of cortical microtubules to form transverse arrays, supporting a role for cellulose microfibrils in influencing cortical microtubule organization.     

Sucrose synthase localizes to cellulose synthesis sites in tracheary elements.

The synthesis of crystalline cellulose microfibrils in plants is a highly coordinated process that occurs at the interface of the cortex, plasma membrane, and cell wall. There is evidence that cellulose biogenesis is facilitated by the interaction of several proteins, but the details are just beginning to be understood. In particular, sucrose synthase, microtubules, and actin have been proposed to possibly associate with cellulose synthases (microfibril terminal complexes) in the plasma membrane. Differentiating tracheary elements of Zinnia elegans L. were used as a model system to determine the localization of sucrose synthase and actin in relation to the plasma membrane and its underlying microtubules during the deposition of patterned, cellulose-rich secondary walls. Cortical actin occurs with similar density both between and under secondary wall thickenings. In contrast, sucrose synthase is highly enriched near the plasma membrane and the microtubules under the secondary wall thickenings. Both actin and sucrose synthase lie closer to the plasma membrane than the microtubules. These results show that the preferential localization of sucrose synthase at sites of high-rate cellulose synthesis can be generalized beyond cotton fibers, and they establish a spatial context for further work on a multi-protein complex that may facilitate secondary wall cellulose synthesis.     

On the alignment of cellulose microfibrils by cortical microtubules: A review and a model

Summary The hypothesis that microtubules align microfibrils, termed the alignment hypothesis, states that there is a causal link between the orientation of cortical microtubules and the orientation of nascent microfibrils. I have assessed the generality of this hypothesis by reviewing what is known about the relation between microtubules and microfibrils in a wide group of examples: in algae of the family Characeae,Closterium acerosum, Oocystis solitaria, and certain genera of green coenocytes and in land plant tip-growing cells, xylem, diffusely growing cells, and protoplasts. The salient features about microfibril alignment to emerge are as follows. Cellulose microfibrils can be aligned by cortical microtubules, thus supporting the alignment hypothesis. Alignment of microfibrils can occur independently of microtubules, showing that an alternative to the alignment hypothesis must exist. Microfibril organization is often random, suggesting that self-assembly is insufficient. Microfibril organization differs on different faces of the same cell, suggesting that microfibrils are aligned locally, not with respect to the entire cell. Nascent microfibrils appear to associate tightly with the plasma membrane. To account for these observations, I present a model that posits alignment to be mediated through binding the nascent microfibril. The model, termed templated incorporation, postulates that the nascent microfibril is incorporated into the cell wall by binding to a scaffold that is oriented; further, the scaffold is built and oriented around either already incorporated microfibrils or plasma membrane proteins, or both. The role of cortical microtubules is to bind and orient components of the scaffold at the plasma membrane. In this way, spatial information to align the microfibrils may come from either the cell wall or the cell interior, and microfibril alignment with and without microtubules are subsets of a single mechanism.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Cellulose-microfibril-orienting mechanisms in plant cells walls

A brief review is given of the changing views over the years, as knowledge of wall structure has developed, concerning the mechanism whereby cellulose chains may be oriented. This leads to an examination of current concepts, particularly those concerning microtubules. It is shown that none of the mechanisms suggested whereby microtubules might cause orientation of cellulose microfibrils is consistent with the known range of molecular architectures found in plant cell walls. It is further concluded that any mechanism which necessitates an indissoluble link between the plasmalemma and the cellulose-synthesising complex at the tip of a microfibril is unacceptable. A new proposal is presented in which it is speculated that both microtubules and microfibrils are oriented by a mechanism separate from both. It is shown that if two vectors are contemplated, one parallel to cell length and one at right angles, and a sensor exists on the plasmalemma surface which responds to changes in the vectors, then all known wall structures may be explained. The possible nature of the vectors and the sensor are considered.     

How do expansins control plant growth? A model for cell wall loosening via defect migration in cellulose microfibrils

Expansins are plant cell wall-loosening proteins that promote cell growth and are essential for many critical developmental processes and stress responses. The molecular basis for expansin action is uncertain. Recently, it has been proposed that expansins loosen the wall by means of the generation of mobile conformational defects at the surface of cellulose microfibrils. The present work addresses this hypothesis by elaborating three assumptions: (1) microfibril–matrix interfaces cause steep stress gradients on the microfibril surface, (2) stress gradients drive the motion of conformational defects along the microfibril surface toward the microfibril–matrix interfaces, and (3) the approach of the defects to the microfibril–matrix interfaces facilitates the dissociation of matrix polysaccharides from cellulose microfibrils.     

A kinesin-like protein is essential for oriented deposition of cellulose microfibrils and cell wall strength

Cortical microtubules have long been hypothesized to regulate the oriented deposition of cellulose microfibrils. However, the molecular mechanisms of how microtubules direct the orientation of cellulose microfibril deposition are not known. We have used fibers in the inflorescence stems of Arabidopsis to study secondary wall deposition and cell wall strength and found a fragile fiber (fra1) mutant with a dramatic reduction in the mechanical strength of fibers. The fra1 mutation did not cause any defects in cell wall composition, secondary wall thickening, or cortical microtubule organization in fiber cells. An apparent alteration was found in the orientation of cellulose microfibrils in fra1 fiber walls, indicating that the reduced mechanical strength of fra1 fibers probably was attributable to altered cellulose microfibril deposition. The FRA1 gene was cloned and found to encode a kinesin-like protein with an N-terminal microtubule binding motor domain. The FRA1 protein was shown to be concentrated around the periphery of the cytoplasm but absent in the nucleus. Based on these findings, we propose that the FRA1 kinesin-like protein is involved in the microtubule control of cellulose microfibril order.     

Cell wall biogenesis in Oocystis: experimental alteration of microfibril assembly and orientation.

Cell wall biogenesis in the unicellular green alga Oocystis apiculata has been studied. Under normal growth conditions, a cell wall with ordered microfibrils is synthesized. In each layer there are rows of parallel microfibrils. Layers are nearly perpendicular to each other. Terminal linear synthesizing complexes are located in the plasma membrane, and they are capable of bidirectional synthesis of cellulose microfibrils. Granule bands associated with the inner leaflet of the plasma membrane appear to control the orientation of newly synthesized microfibrils. Subcortical microtubules also are present during wall synthesis. Patterns of cell wall synthesis were studied after treatment with EDTA and EGTA as well as divalent cations (MgSO4, CaSO4, Cacl2). 0.1 M EDTA treatment for 15 min results in the disassociation of the terminal complexes from the ends of microfibrils. EDTA-treated cells followed by 15 min treatment with MgSO4 results in reaggregation of the linear complexes into a paired state, remote from the original ends to which they were associated. After 90 min treatment with MgSO4, normal synthesis resumes. EGTA and calcium salts do not affect the linear complexes or microfibril orientation. Treatments with colchicine and vinblastine sulphate do not depolymerize the microtubles, but the wall microfibril orientation is altered. With colchicine or vinblastine, the change in orientation from layer to layer is inhibited. The process is reversible upon removal of the drugs. Lumicolchicine has no effect upon microfibril orientation, but granule bands are disorganized. Treatment with coumarin, a known inhibitor of cellulose synthesis, causes the loss of visualization of subunits of the terminal complexes. The possibility of the existence of a membrane-associated colchicine-sensitive orientation protein for cellulose microfibrils is discussed. Transmembrane modulation of microfibril synthesis and orientation is presented.     

New techniques enable comparative analysis of microtubule orientation, wall texture, and growth rate in intact roots of Arabidopsis

This article explores root epidermal cell elongation and its dependence on two structural elements of cells, cortical microtubules and cellulose microfibrils. The recent identification of Arabidopsis morphology mutants with putative cell wall or cytoskeletal defects demands a procedure for examining and comparing wall architecture and microtubule organization patterns in this species. We developed methods to examine cellulose microfibrils by field emission scanning electron microscopy and microtubules by immunofluorescence in essentially intact roots. We were able to compare cellulose microfibril and microtubule alignment patterns at equivalent stages of cell expansion. Field emission scanning electron microscopy revealed that Arabidopsis root epidermal cells have typical dicot primary cell wall structure with prominent transverse cellulose microfibrils embedded in pectic substances. Our analysis showed that microtubules and microfibrils have similar orientation only during the initial phase of elongation growth. Microtubule patterns deviate from a predominantly transverse orientation while cells are still expanding, whereas cellulose microfibrils remain transverse until well after expansion finishes. We also observed microtubule-microfibril alignment discord before cells enter their elongation phase. This study and the new technology it presents provide a starting point for further investigations on the physical properties of cell walls and their mechanisms of assembly.     

Alteration of oriented deposition of cellulose microfibrils by mutation of a katanin-like microtubule-severing protein

It has long been hypothesized that cortical microtubules (MTs) control the orientation of cellulose microfibril deposition, but no mutants with alterations of MT orientation have been shown to affect this process. We have shown previously that in Arabidopsis, the fra2 mutation causes aberrant cortical MT orientation and reduced cell elongation, and the gene responsible for the fra2 mutation encodes a katanin-like protein. In this study, using field emission scanning electron microscopy, we found that the fra2 mutation altered the normal orientation of cellulose microfibrils in walls of expanding cells. Although cellulose microfibrils in walls of wild-type cells were oriented transversely along the elongation axis, cellulose microfibrils in walls of fra2 cells often formed bands and ran in different directions. The fra2 mutation also caused aberrant deposition of cellulose microfibrils in secondary walls of fiber cells. The aberrant orientation of cellulose microfibrils was shown to be correlated with disorganized cortical MTs in several cell types examined. In addition, the thickness of both primary and secondary cell walls was reduced significantly in the fra2 mutant. These results indicate that the katanin-like protein is essential for oriented cellulose microfibril deposition and normal cell wall biosynthesis. We further demonstrated that the Arabidopsis katanin-like protein possessed MT-severing activity in vitro; thus, it is an ortholog of animal katanin. We propose that the aberrant MT orientation caused by the mutation of katanin results in the distorted deposition of cellulose microfibrils, which in turn leads to a defect in cell elongation. These findings strongly support the hypothesis that cortical MTs regulate the oriented deposition of cellulose microfibrils that determines the direction of cell elongation.     

Chemical genetic screening identifies a novel inhibitor of parallel alignment of cortical microtubules and cellulose microfibrils

It is a well-known hypothesis that cortical microtubules control the direction of cellulose microfibril deposition, and that the parallel cellulose microfibrils determine anisotropic cell expansion and plant cell morphogenesis. However, the molecular mechanism by which cortical microtubules regulate the orientation of cellulose microfibrils is still unclear. To investigate this mechanism, chemical genetic screening was performed. From this screening, 'SS compounds' were identified that induced a spherical swelling phenotype in tobacco BY-2 cells. The SS compounds could be categorized into three classes: those that disrupted the cortical microtubules; those that reduced cellulose microfibril content; and thirdly those that had neither of these effects. In the last class, a chemical designated 'cobtorin' was found to induce the spherical swelling phenotype at the lowest concentration, suggesting strong binding activity to the putative target. Examining cellulose microfibril regeneration using taxol-treated protoplasts revealed that the cobtorin compound perturbed the parallel alignment of pre-existing cortical microtubules and nascent cellulose microfibrils. Thus, cobtorin could be a novel inhibitor and an attractive tool for further investigation of the mechanism that enables cortical microtubules to guide the parallel deposition of cellulose microfibrils.     

Cobtorin target analysis reveals that pectin functions in the deposition of cellulose microfibrils in parallel with cortical microtubules

Cellulose and pectin are major components of primary cell walls in plants, and it is believed that their mechanical properties are important for cell morphogenesis. It has been hypothesized that cortical microtubules guide the movement of cellulose microfibril synthase in a direction parallel with the microtubules, but the mechanism by which this alignment occurs remains unclear. We have previously identified cobtorin as an inhibitor that perturbs the parallel relationship between cortical microtubules and nascent cellulose microfibrils. In this study, we searched for the protein target of cobtorin, and we found that overexpression of pectin methylesterase and polygalacturonase suppressed the cobtorin-induced cell-swelling phenotype. Furthermore, treatment with polygalacturonase restored the deposition of cellulose microfibrils in the direction parallel with cortical microtubules, and cobtorin perturbed the distribution of methylated pectin. These results suggest that control over the properties of pectin is important for the deposition of cellulose microfibrils and/or the maintenance of their orientation parallel with the cortical microtubules.     

Chitinase-like1/pom-pom1 and its homolog CTL2 are glucan-interacting proteins important for cellulose biosynthesis in Arabidopsis

Plant cells are encased by a cellulose-containing wall that is essential for plant morphogenesis. Cellulose consists of β-1,4-linked glucan chains assembled into paracrystalline microfibrils that are synthesized by plasma membrane-located cellulose synthase (CESA) complexes. Associations with hemicelluloses are important for microfibril spacing and for maintaining cell wall tensile strength. Several components associated with cellulose synthesis have been identified; however, the biological functions for many of them remain elusive. We show that the chitinase-like (CTL) proteins, CTL1/POM1 and CTL2, are functionally equivalent, affect cellulose biosynthesis, and are likely to play a key role in establishing interactions between cellulose microfibrils and hemicelluloses. CTL1/POM1 coincided with CESAs in the endomembrane system and was secreted to the apoplast. The movement of CESAs was compromised in ctl1/pom1 mutant seedlings, and the cellulose content and xyloglucan structures were altered. X-ray analysis revealed reduced crystalline cellulose content in ctl1 ctl2 double mutants, suggesting that the CTLs cooperatively affect assembly of the glucan chains, which may affect interactions between hemicelluloses and cellulose. Consistent with this hypothesis, both CTLs bound glucan-based polymers in vitro. We propose that the apoplastic CTLs regulate cellulose assembly and interaction with hemicelluloses via binding to emerging cellulose microfibrils.     

Mutation or drug-dependent microtubule disruption causes radial swelling without altering parallel cellulose microfibril deposition in Arabidopsis root cells

As critical determinants of growth anisotropy in plants, cortical microtubules are thought to constrain the movement of cellulose synthase complexes and thus align newly deposited cellulose microfibrils. We tested this cellulose synthase constraint model using the temperature-sensitive mor1-1 mutant of Arabidopsis. Contrary to predictions, the disruption of cortical microtubules in mor1-1 root epidermal cells led to left-handed root twisting and radial swelling but did not alter the transverse orientation of cellulose microfibrils. We also found that drug-dependent disassembly or hyperstabilization of cortical microtubules did not alter the parallel order of cellulose microfibrils. By measuring cellulose content in mor1-1 seedlings, we verified that cellulose synthesis is not reduced at the restrictive temperature. The independence of cortical microtubule organization and cellulose microfibril alignment was supported by the observation that double mutants of mor1-1 and rsw1-1, the cellulose-deficient mutant with misaligned microfibrils, had additive phenotypes. Our results suggest that cortical microtubules regulate growth anisotropy by some mechanism other than cellulose microfibril alignment or synthesis.     

Phospholipases may play multiple roles in anisotropic plant cell growth

Both the cortical microtubule cytoskeleton and cellulose microfibrils are important for the anisotropic growth of plant cells. Although the two systems interact, the details of this interaction are far from clear. It has been shown the inhibitors of phospholipase D, phospholipase A₂ and phospholipase C all cause disorganisation of the microtubule cytoskeleton. Since the phospholipases act on the plasma membrane, which links cortical microtubules to cellulose microfibrils in the cell wall, they may play a key role in the communication between the two structures. This communication may take various forms. Microtubule-linked phospholipase activity may cause the organisation of underlying cellulose microfibril liquid crystals. Alternatively, phospholipases may co-operate in the regulation of plasma membrane fluidity, affecting the movement of cellulose synthase complexes in the underlying plasma membrane. GPI-anchored proteins in the plasma membrane, which are cleaved by phospholipases, may possibly play a role.     

Colchicine and microfibril orientation

Summary Investigations on the mechanism of orientation of the cellulose microfibrils of the green algaOocystis solitaria have been carried out. This organism demonstrates easily observable and highly ordered microfibrils in its wall, which are arranged parallel to one another and regularly alternate at 90 from layer to layer of which there are approximately 30. During the entire wall development, and always parallel to one of the microfibril directions, are microtubules lying in the cortical cytoplasm. In the presence of 10^–2 M colchicine, microtubules are no longer detected and the typical cell wall pattern is not developed. The possible role of microtubules in the orientation of cellulose microfibrils is briefly discussed.     

Roles of microtubules and cellulose microfibril assembly in the localization of secondary-cell-wall deposition in developing tracheary elements

Summary. The roles of cellulose microfibrils and cortical microtubules in establishing and maintaining the pattern of secondary-cell-wall deposition in tracheary elements were investigated with direct dyes to inhibit cellulose microfibril assembly and amiprophosmethyl to inhibit microtubule polymerization. When direct dyes were added to xylogenic cultures of Zinnia elegans L. mesophyll cells just before the onset of differentiation, the secondary cell wall was initially secreted as bands composed of discrete masses of stained material, consistent with immobilized sites of cellulose synthesis. The masses coalesced, forming truncated, sinuous or smeared thickenings, as secondary cell wall deposition continued. The absence of ordered cellulose microfibrils was confirmed by polarization microscopy and a lack of fluorescence dichroism as determined by laser scanning microscopy. Indirect immunofluorescence showed that cortical microtubules initially subtended the masses of dye-altered secondary cell wall material but soon became disorganized and disappeared. Although most of the secondary cell wall was deposited in the absence of subtending cortical microtubules in dye-treated cells, secretion remained confined to discrete regions of the plasma membrane. Examination of non-dye-treated cultures following application of microtubule inhibitors during various stages of secondary-cell-wall deposition revealed that the pattern became fixed at an early stage such that deposition remained localized in the absence of cortical microtubules. These observations indicate that cortical microtubules are required to establish, but not to maintain, patterned secondary-cell-wall deposition. Furthermore, cellulose microfibrils play a role in maintaining microtubule arrays and the integrity of the secondary-cell-wall bands during deposition.Correspondence and reprints: Department of Biological Sciences, University of Rhode Island, Kingston, RI 02881, U.S.A.Present address: Biology Editors Co., Peacedale, Rhode Island, U.S.A.Present address: Department of Biology and Marine Biology, Roger Williams University, Bristol, Rhode Island, U.S.A.Present address: Department of Crop Science and Department of Botany, North Carolina State University, Raleigh, North Carolina, U.S.A.     

Cellulose microfibril orientation and cell shaping in developing guard cells of Allium: The role of microtubules and ion accumulation

Summary The role of microtubules and ions in cell shaping was investigated in differentiating guard cells of Allium using light and electron microscopy and cytochemistry. Microtubules appear soon after cytokinesis in a discrete zone close to the plasmalemma adjacent to the common wall between guard cells. The microtubules fan out from this zone, which corresponds to the future pore site, towards the other sides of the cell. Soon new cellulose microfibrils are deposited on the wall adjacent to the microtubules and oriented parallel to them. As the wall thickens, the shape of the cell shifts from cylindrical to kidney-like. Studies with polarized light show that guard cells gradually assume a birefringence pattern during development characteristic of wall microfibrils radiating away from the pore site. Retardation increases from 10 Å when cells just begin to take shape, to 80–100 Å at maturity. Both microfibril and microtubule orientation remain constant during development. Observations on aberrant cells including those produced under the influence of drugs such as colchicine, which leads to loss of microtubules, abnormal wall thickenings and disruption of wall birefringence, further support the role of microtubules in cell shaping through their function in the localization of wall deposition and the orientation of cellulose microfibrils in the new wall layer. Potassium first appears in guard mother cells before division and rapidly accumulates afterwards during cell shaping, as judged by the cobaltinitrite reaction. Some chloride and perhaps organic acid anions also accumulate. Thus, these ions, which are known to play a role in the function of mature guard cells, also seem to be important in the early growth and shaping of these cells.Abbreviations IPC isopropyl-N-phenylcarbamate - CB cytochalasin B - GMC guard mother cell - MTOC microtubule organizing center     

Is the recovery of microtubule orientation in pea roots dependent on the cell wall?

This study tested several aspects of a model proposed by Williamson (1990, 1991) in which stresses in plant cell walls, detected by stress-receptive portions of inelastic cellulose microfibrils, orient microtubules via interactions with cell wall-linked transmembrane proteins. Young expanding cells of pea root tips have highly ordered transverse arrays of microtubules oriented perpendicular to the direction of cell expansion. The recovery of these ordered MT arrays after depolymerisation with oryzalin was assessed. It was shown that treating roots with disruptors of microfibril synthesis (2,6-dichlorobenzonitrile and calcofluor white) or the disruption of Arg-Gly-Asp (RGD)-mediated wall-membrane links did not affect the orientation of recovering microtubule arrays. Furthermore, cell wall stresses themselves appeared unnecessary for regeneration of transverse arrays. The relevance of these findings to Williamson's hypothesis is discussed.     

How do cell walls regulate plant growth?

The cell wall of growing plant tissues has frequently been interpreted in terms of inextensible cellulose microfibrils 'tethered' by hemicellulose polymers attached to the microfibril surface by hydrogen bonds, with growth occurring when tethers are broken or 'peeled' off the microfibril surface by expansins. This has sometimes been described as the 'sticky network' model. In this paper, a number of theoretical difficulties with this model, and discrepancies between predicted behaviour and observations by a number of researchers, are noted. (i) Predictions of cell wall moduli, based upon the sticky network model, suggest that the cell wall should be much weaker than is observed. (ii) The maximum hydrogen bond energy between tethers and microfibrils is less than the work done in expansion and therefore breakage of such hydrogen bonds is unlikely to limit growth. (iii) Composites of bacterial cellulose with xyloglucan are weaker than pellicles of pure cellulose so that it seems unlikely that hemicelluloses bind the microfibrils together. (iv) Calcium chelators promote creep of plant material in a similar way to expansins. (v) Reduced relative 'permittivities' inhibit the contraction of cell wall material when an applied stress is decreased. Revisions of the sticky network model that might address these issues are considered, as are alternatives including a model of cell wall biophysics in which cell wall polymers act as 'scaffolds' to regulate the space available for microfibril movement. Experiments that support the latter hypothesis, by demonstrating that reducing cell wall free volume decreases extensibility, are briefly described.     

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.