Winding threads around plant cells: a geometrical model for microfibril deposition

In this paper, a geometrical model is put forward to account for the deposition orientation of plant cell wall microfibrils (CMFs). The model presupposes the insertion in the plasma membrane of CMF initiation complexes, which, once inserted, are moved through the fluid plane of the plasma membrane by the kinetic force of CMF synthesis, leaving CMFs in their wake. Deposition occurs in a limited space and the CMFs are linked to wall matrix molecules. CMF orientation is governed by the laws of geometry and, taking space-limiting conditions into account, therefore depends on (1) cell geometry, (2) the other wall molecules linked to the CMFs, and (3) the number of CMF initiation complexes inserted into the plasma membrane. The model does not exclude the idea that cortical microtubules may determine initial CMF orientation after cell division by determining the cell elongation direction.     

Orientation of cellulose microfibrils in cortical cells of tobacco explants

The deposition of nascent cellulose microfibrils (CMFs) was studied in the walls of cortical cells in explants of Nicotiana tabacum L. flower stalks. In freshly cut explants the CMFs were deposited in two distinct and alternating orientations — all given with respect to the longitudinal axis of the cell —, at 75° and 115°, in a left-handed (S-helix) and right-handed (Z-helix) form, respectively. The CMFs deposited in these orientations did not form uninterrupted layers, but sheets in which both orientations were present. After explantation, the synthesis of CMFs and their deposition in bundles continued. New orientations occurred within 6 h. After 6 h a new sheet was deposited, with orientations of 15° (S-helix) and 165° (Z-helix). The changes could be seen as sudden bends in individual CMFs or in small bundles of CMFs. In the next stage, more CMFs were deposited with these new orientations and the bundles became larger. New orientations arose by a shift towards more longitudinal directions, starting from either the S-helix or the Z-helix form. It was only after an almost longitudinal orientation was reached that the CMFs were deposited in two opposing directions again and a new sheet was formed. Neither colchicine nor cremart influenced the changes in CMF deposition. It is concluded that microtubules do not control CMF deposition in cortical cells of tobacco explants; control of CMF deposition and microtubule orientation occurs by factors related to cell polarity.Abbreviations CMF cellulose microfibril - MT microtubule We thank Professor M.M.A. Sassen and Dr. G.W.M. Barendse (Department of Experimental Botany, University of Nijmegen, Nijmegen, The Netherlands) for helpful discussions and Mrs. A. Kemp for her assistance in the ethylene experiments.     

A dynamical model for plant cell wall architecture formation

We discuss a dynamical mathematical model to explain cell wall architecture in plant cells. The highly regular textures observed in cell walls reflect the spatial organisation of the cellulose microfibrils (CMFs), the most important structural component of cell walls. Based on a geometrical theory proposed earlier [A. M. C. Emons, Plant, Cell and Environment 17, 3–14 (1994)], the present model describes the space-time evolution of the density of the so-called rosettes, the CMF synthesizing complexes. The motion of these rosettes in the plasma membrane is assumed to be governed by an optimal packing constraint on the CMFs plus adherent matrix material, that couples the direction of motion, and hence the orientation of the CMF being deposited, to the local density of rosettes. The rosettes are created inside the cell in the endoplasmatic reticulum and reach the cell-membrane via vesicles derived from Golgi-bodies. After being inserted into the plasma membrane they are assumed to be operative for a fixed, finite lifetime. The plasma membrane domains within which rosettes are activated are themselves also supposed to be mobile. We propose a feedback mechanism that precludes the density of rosettes to rise beyond a maximum dictated by the geometry of the cell. The above ingredients lead to a quasi-linear first order PDE for the rosette-density. Using the method of characteristics this equation can be cast into a set of first order ODEs, one of which is retarded. We discuss the analytic solutions of the model that give rise to helicoidal, crossed polylamellate, helical, axial and random textures, since all cell walls are composed of (or combinations of) these textures. Received: 10 July 1999 / Revised version: 7 June 2000 / Published online: 16 February 2001     

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

The Geometrical Model for Microfibril Deposition and the Influence of the Cell Wall Matrix

Abstract: A theory for cell wall deposition has been formulated consistent with present day experimental data on cell walls and cellular processes. This theory has a generic origin, geometrical constraints, as the underlying cause for the cell wall architecture. The theory has been worked out as a fully mathematical model, allowing for specific predictions of a qualitative and quantitative nature. The key point of the geometrical theory is the coupling of the trajectory of the cellulose microfibril synthases, i.e., rosettes, to their density. This coupling provides the cell with a mechanism for manipulating the cell wall texture by creating controlled local variations in the number of active synthases. In the present paper we show that the geometrical model can explain the helicoidal, crossed polylamellate, helical and axial wall textures, which are the basic textures found in plant cell walls. In addition, we discuss the role of cortical microtubules in the wall deposition process and how the cell wall matrix contributes to cell wall texture determination.     

Do microtubules orient plant cell wall microfibrils?

Cortical microtubules (MTs) allegedly orient nascent cellulose microfibrils (CMFs) in plant cells. The frequently observed parallelism between them, and the effect of MT-depolymerizing agents, are the bases for this hypothesis. Data have, however, accumulated about cells in which MTs and CMFs are not in parallel alignment. These data will be reviewed. MT orientation cannot be the only factor determining CMF orientation, but MTs could overrule other factors in cells where, for instance, they are more tightly attached to the plasma membrane than in other cells. MT and CMF orientations could, however, both be controlled by a third factor, and CMFs may even impose orientation on MTs.     

Cortical microtubules optimize cell-wall crystallinity to drive unidirectional growth in Arabidopsis

The shape of plants depends on cellulose, a biopolymer that self-assembles into crystalline, inextensible microfibrils (CMFs) upon synthesis at the plasma membrane by multi-enzyme cellulose synthase complexes (CSCs). CSCs are displaced in directions predicted by underlying parallel arrays of cortical microtubules, but CMFs remain transverse in cells that have lost the ability to expand unidirectionally as a result of disrupted microtubules. These conflicting findings suggest that microtubules are important for some physico-chemical property of cellulose that maintains wall integrity. Using X-ray diffraction, we demonstrate that abundant microtubules enable a decrease in the degree of wall crystallinity during rapid growth at high temperatures. Reduced microtubule polymer mass in the mor1-1 mutant at high temperatures is associated with failure of crystallinity to decrease and a loss of unidirectional expansion. Promotion of microtubule bundling by over-expressing the RIC1 microtubule-associated protein reduced the degree of crystallinity. Using live-cell imaging, we detected an increase in the proportion of CSCs that track in microtubule-free domains in mor1-1, and an increase in the CSC velocity. These results suggest that microtubule domains affect glucan chain crystallization during unidirectional cell expansion. Microtubule disruption had no obvious effect on the orientation of CMFs in dark-grown hypocotyl cells. CMFs at the outer face of the hypocotyl epidermal cells had highly variable orientation, in contrast to the transverse CMFs on the radial and inner periclinal walls. This suggests that the outer epidermal mechanical properties are relatively isotropic, and that axial expansion is largely dependent on the inner tissue layers.     

Role of cortical microtubules in the orientation of cellulose microfibril deposition in higher-plant cells

Cortical microtubules (MTs) have been implicated in the morphogenesis of plant cells by regulating the orientation of newly deposited cellulose microfibrils (CMFs). However, the role of MTs in oriented CMF deposition is still unclear. We have investigated the mechanism of CMF deposition with cultured tobacco protoplasts derived from taxol-treated BY-2 cells (taxol protoplasts). The BY-2 protoplasts regenerated patches of beta-l,3-glucan (callose) and fibrils of beta-l,4-glucan (cellulose). Taxol protoplasts possessed the same ordered MT arrays as material cells and regenerated CMFs with patterns almost coincidental with MTs. Electron microscopy revealed that, on the surface of cultured taxol protoplasts, each CMF bundle appeared to be deposited on each cortical MT. These results suggest that MTs may attach directly to the cellulose-synthesizing complexes, by some form of linkage, and regulate the movement of these complexes in higher-plant cells.     

Role of cortical microtubules in the orientation of cellulose microfibril deposition in higher-plant cells

Summary Cortical microtubules (MTs) have been implicated in the morphogenesis of plant cells by regulating the orientation of newly deposited cellulose microfibrils (CMFs). However, the role of MTs in oriented CMF deposition is still unclear. We have investigated the mechanism of CMF deposition with cultured tobacco protoplasts derived from taxol-treated BY-2 cells (taxol protoplasts). The BY-2 protoplasts regenerated patches of β-l,3-glucan (callose) and fibrils of β-l,4-glucan (cellulose). Taxol protoplasts possessed the same ordered MT arrays as material cells and regenerated CMFs with patterns almost coincidental with MTs. Electron microscopy revealed that, on the surface of cultured taxol protoplasts, each CMF bundle appeared to be deposited on each cortical MT. These results suggest that MTs may attach directly to the cellulose-synthesizing complexes, by some form of linkage, and regulate the movement of these complexes in higher-plant cells.     

Effects of glucanase treatment on the structure and mechanical properties of cell walls in the giant‐celled xanthophycean alga Vaucheria frigida

The cell wall of the tip‐growing cells of the giant‐cellular xanthophycean alga Vaucheria frigida is mainly composed of cellulose microfibrils (CMFs) arranged in random directions and the major matrix component into which the CMFs are embedded throughout the cell. The mechanical properties of a cell‐wall fragment isolated from the tip‐growing region, which was inflated by artificially applied pressure, were measured after enzymatic removal of the matrix component by using a protease; the results showed that the matrix component is involved in the maintenance of cell wall strength. Since glucose and uronic acid are present in the matrix component of Vaucheria cell walls, we measured the mechanical properties of the cell wall after treatment with endo‐1,3‐ß‐glucanase and observed the fine structures of its surfaces by atomic force microscopy. The major matrix component was partially removed from the cell wall by glucanase, and the enzyme treatment significantly weakened the cell wall strength without affecting the pH dependence of cell wall extensibility. The enzymatic removal of the major matrix component by using a protease released polysaccharide containing glucose and glucuronic acid. This suggests that the major matrix component of the algal cell walls contains both proteins (or polypeptides) and polysaccharides consisting of glucose and glucuronic acid as the main constituents.     

Cellulose Content in Fibers of Cottons which Differ in their Lint Lengths and Extent of Fuzz

Three cottons differing in their extent of fuzz fibers (linters) and final length of lint fibers were analyzed for amount of fiber cell walls and fiber cellulose at various times postanthesis. Cellulose determinations were performed directly on whole fibers and on fiber cell wall preparations. The data suggest that the presence of fuzz fibers does not account for a rise, followed by a plateau, followed by a rise, in cellulose content expressed as a percentage of cell wall material. It is concluded that: (1) under our greenhouse conditions, all fuzz fibers are initiated by day eight after anthesis; (2) weight per mm length of all fibers increases up to the point of secondary wall deposition and increases even more rapidly after that; (3) deposition of secondary wall cellulose in fuzz fibers probably does not begin until after similar deposition begins in lint fibers; (4) the actual amount of cellulose in primary walls of all elongating fibers (fuzz and lint) is a constant value, about 1 × 10^?16 mg/mm; and (5) secondary wall cellulose deposition in lint fibers begins very sharply, in advance of cessation of elongation, at a time closely related to final lint fiber length. It is speculated that: (1) cell wall preparation procedures may remove significant amounts of noncellulosic wall material, thus making it difficult to define all functional constituents on the basis of what is left in a cell wall residue; and/or (2) primary walls may lose to the cytoplasm some of their constituents in advance of secondary wall deposition, the extent of loss varying due to developmental age of the elongating fibers.     

Alteration in Secondary Wall Deposition by Overexpression of the Fragile Fiber1 Kinesin-Like Protein in Arabidopsis

Secondary walls in fibers and vessels are typically deposited in three distinct layers, which are formed by the successive re-orientation of cellulose microfibrils. Although cortical microtubules have been implicated in this process, the underlying mechanisms for the formation of three distinct wall layers are not known. The Fragile Fiber1 （FRA1） kinesin-like protein has been previously shown to be involved in the oriented deposition of cellulose microfibrils and important for cell wall strength in Arabidopsis thaliana. In the present report, we investigated the expression pattern of the FRA 1 gene and studied the effects of FRA1 overexpression on secondary wall deposition. The FRAI gene was found to be expressed not only in cells undergoing secondary wall deposition including developing interfascicular fibers and xylem cells, but also in dividing cells and expanding/elongating parenchyma cells. Overexpression of FRA1 caused a severe reduction in the thickness of secondary walls in interfascicular fibers and deformation of vessels, which are accompanied with a marked decrease in stem strength. Close examination of secondary walls revealed that unlike the wild-type walls having three typical layers with the middle layer being the thickest, the secondary walls in FRA1 overexpressors exhibited an increased number of layers, all of which had a similar width. Together, these results provide further evidence implicating an important role of the FRA1 kinesin-like protein in the ordered deposition of secondary walls, which determines the strength of fibers and vessels.     

Unravelling cell wall formation in the woody dicot stem

Populus is presented as a model system for the study of wood formation (xylogenesis). The formation of wood (secondary xylem) is an ordered developmental process involving cell division, cell expansion, secondary wall deposition, lignification and programmed cell death. Because wood is formed in a variable environment and subject to developmental control, xylem cells are produced that differ in size, shape, cell wall structure, texture and composition. Hormones mediate some of the variability observed and control the process of xylogenesis. High-resolution analysis of auxin distribution across cambial region tissues, combined with the analysis of transgenic plants with modified auxin distribution, suggests that auxin provides positional information for the exit of cells from the meristem and probably also for the duration of cell expansion. Poplar sequencing projects have provided access to genes involved in cell wall formation. Genes involved in the biosynthesis of the carbohydrate skeleton of the cell wall are briefly reviewed. Most progress has been made in characterizing pectin methyl esterases that modify pectins in the cambial region. Specific expression patterns have also been found for expansins, xyloglucan endotransglycosylases and cellulose synthases, pointing to their role in wood cell wall formation and modification. Finally, by studying transgenic plants modified in various steps of the monolignol biosynthetic pathway and by localizing the expression of various enzymes, new insight into the lignin biosynthesis in planta has been gained.     

Disruption of cellulose synthesis by isoxaben causes tip swelling and disorganizes cortical microtubules in elongating conifer pollen tubes

Summary.  In elongating pollen tubes of the conifer Picea abies (Norway spruce), microtubules form a radial array beneath the plasma membrane only at the elongating tip and an array parallel with elongation throughout the tube. Tips specifically swell following microtubule disruption. Here we test whether these radial microtubules coordinate cell wall deposition and maintain tip integrity as tubes elongate. Control pollen tubes contain cellulose throughout the walls, including the tip. Pollen tubes grown in the presence of isoxaben, which disrupts cellulose synthesis, are significantly shorter with a decrease in cellulose throughout the walls. Isoxaben also significantly increases the frequency of tip swelling, with no effect on tube width outside of the swollen tip. The decrease in cellulose is more pronounced in pollen tubes with swollen tips. The effects of isoxaben are reversible. Following isoxaben treatment, the radial array of microtubules persists beneath the plasma membrane of nonswollen tips, while this array is specifically disrupted in swollen tips. Microtubules instead form a random network throughout the tip. Growth in these pollen tubes is turgor driven, but the morphological changes due to isoxaben are not just the result of weakened cell walls since pollen tubes grown in hypoosmotic media are not significantly shorter but do have swollen tips and tubes are wider along their entire length. We conclude that the radial microtubules in the tip do maintain tip integrity and that the specific inhibition of cellulose microfibril deposition leads to the disorganization of these microtubules. This supports the emerging model that there is bidirectional communication across the plasma membrane between cortical microtubules and cellulose microfibrils. Received January 15, 2002; accepted August 3, 2002; published online March 11, 2003     

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.     

Cell wall hydroxyproline-polysaccharide associations in Lupinus hypocotyls

Extraction of lupin hypocotyl cell walls with guanidine thiocynate, both before and after dilute acid treatment does not dissolve the hydroxyproline indicating that compounds containing this amino acid are probably covalently linked to insoluble wall constituents other than through acid labile arabinofuranose-hydroxyproline links. Dilute alkali does extract all of the wall hydroxyproline largely as non-dialysable material. Sequential extraction of cell walls with alkali at two temperatures (2° and 22–25°) removes most of the hemicellulose at the lower temperature but only dissolves the hydroxyproline at the higher temperature. Other studies show that the hydroxyproline containing polymer is co-precipitated with hemicellulose-B arabino-xylan. When cell walls from elongating and non-elongating hypocotyl sections are compared using this sequential extraction, the hemicellulose-B arabino-xylan containing hydroxyproline from the non-elongating wall has a much higher proportion of arabinoseand galactose than the same polymer from the elongating wall. Much more of the hydroxyproline from the elongating wall is dialysable. These results indicate more bonding of the hydroxyproline-containing glycoprotein within the wall of non-elongating tissue consistent with its suggested role in stopping cell elongation. It is suggested that the glycoprotein is linked to insoluble wall constituents such as cellulose through galactose or by direct protein to cellulose links.     

Changes in pectin structure and localization during the growth of unadapted and NaCl-adapted tobacco cells

Tobacco cells adapted to grow in high concentrations of NaCl exhibit a drastically altered growth physiology that results in cells whose fully expanded volume is only one-fifth to one-eighth those of unadapted cells. Comparison between NaCl-adapted and unadapted tobacco cells provides an opportunity to evaluate current concepts of the structural and mechanical determinants of cell wall expansion. Both biochemical studies of pectic polymers and the ultrastructural localization of pectic epitopes at three specific phases of cell culture, maximal cell division, maximal elongation, and stationary phase are reported here. One-half of the galactosyluronic acid units in wall polymers of NaCl-adapted cells are esterified throughout the culture period, while wall polymers of unadapted cells show a rise in esterified polygalacturonic acid from 50 to 80% during elongation and then a decrease to 70% at stationary phase. Methyl esters account for only a proportion of the total esterified polygalacturonic acid at any stage in both unadapted and NaCl-adapted cell walls. Using monoclonal antibodies, we show differences in the localization of relatively methyl-esterified and unesterified pectic epitopes at different stages of growth and corroborate the chemical determinations. Fourier transform infrared (FTIR) microspectroscopy of representative walls of both NaCl-adapted and unadapted cells confirms, at the single cell wall level, that results obtained from chemical analysis of bulk samples are applicable to the entire cell population. FTIR microspectroscopy also reveals an increase in wall protein in the walls of adapted cells. Images obtained by the fast-freeze, deep-etch, rotary-shadowed replica technique show clearly different cell wall architectures in NaCl-adapted compared with unadapted cells; walls of elongating unadapted cells contain long, thin fibres that show a net orientation with respect to the long axis of the cell, whereas walls of adapted cells have thicker, flatter bundles of fibres with no clear net orientation. Polarized FTIR microspectroscopy indicates that, in unadapted tobacco cells during elongation, pectin molecules may be oriented within the wall in a similar manner to cellulose. Possible ways in which pectin structure and conformation may affect the behaviour of the cellulose-xyloglucan network are discussed.     

Spatial relationship between microtubules and plasma-membrane rosettes during the deposition of primary wall microfibrils in Closterium sp.

The mechanism by which cortical microtubules (MTs) control the orientation of cellulose microfibril deposition in elongating plant cells was investigated in cells of the green alga, Closterium sp., preserved by ultrarapid freezing. Cellulose microfibrils deposited during formation of the primary cell wall are oriented circumferentially, parallel to cortical MTs underlying the plasma membrane. Some of the microfibrils curve away from the prevailing circumferential orientation but then return to it. Freeze-fracture electron microscopy shows short rows of particle rosettes on the P-face of the plasma membrane, also oriented perpendicular to the long axis of the cell. Previous studies of algae and higher plants have provided evidence that such rosettes are involved in the deposition of cellulose microfibrils. The position of the rosettes relative to the underlying MTs was visualized by deep etching, which caused much of the plasma membrane to collapse. Membrane supported by the MTs and small areas around the rosettes resisted collapse. The rosettes were found between, or adjacent to, MTs, not directly on top of them. Rows of rosettes were often at a slight angle to the MTs. Some evidence of a periodic structure connecting the MTs to the plasma membrane was apparent in freeze-etch micrographs. We propose that rosettes are not actively or directly guided by MTs, but instead move within membrane channels delimited by cortical MTs attached to the plasma membrane, propelled by forces derived from the polymerization and crystallization of cellulose microfibrils. More widely spaced MTs presumably allow greater lateral freedom of movement of the rosette complexes and result in a more meandering pattern of deposition of the cellulose fibrils in the cell wall.Abbreviations E-face exoplasmic fracture face - MT microtubule - P-face protoplasmic fracture-face     

THE COMPOSITION AND PHYLOGENETIC SIGNIFICANCE OF THE MOUGEOTIA (CHAROPHYCEAE) CELL WALL1

The two-layered, fibrillar cell wall of Mougeotia C. Agardh sp. consisted of 63.6% non-cellulosic carbohydrates and 13.4% cellulose. The orientation of cellulose microfibrils in the native cell wall agrees with the multinet growth hypothesis, which has been employed to explain the shift in microfibril orientation from transverse (inner wall) toward axial (outer wall). Monosaccharide analysis of isolated cell walls revealed the presence of ten sugars with glucose, xylose and galactose most abundant. Methylation analysis of the acid-modified, 1 N NaOH insoluble residue fraction showed that it was composed almost exclusively of 4-linked glucose, confirming the presence of cellulose. The major hemicellulosic carbohydrate was semi-purified by DEAE Sephacel (Cl^?) anion-exchange chromatography of the hot 1 N NaOH soluble fraction. This hemicellulose was a xylan consisting of a 4-xylosyl backbone and 2,4-xylosyl branch points. The major hot water soluble neutral polysaccharide was identified as a 3-linked galactan. Mougeotia cell wall composition is similar to that of (Charophyceae) and has homologies with vascular plant cell walls. Our observations support transtructural evidence which suggests that members of the Charophyceae represent the phylogenetic line that gave rise to vascular plants. Therefore, the primary cell walls of vascular plants many have evolved directly from structures typical of the filamentous green algal cell walls found in the Charophyceae.     

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.