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.

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

A unified hypothesis for the role of membrane bound enzyme complexes and microtubules in plant cell wall synthesis

The plant cell wall plays a central role in morphogenesis as well as responsiveness to environmental signals Xyloglucans are the principal component of the plant cell wall matrix and serve as cross-links between cellulose microfibrils to form the cellulose-xyloglucan framework Endoxyloglucan transferase (EXGT), which was isolated and characterized in 1992, is an enzyme that mediates molecular grafting reaction between xyloglucan molecules Structural studies on cDNAs encoding EXGT and its related proteins have disclosed the ubiquitous presence in the plant kingdom of a large multigene family of xyloglucan-related proteins (XRPs) Each XRP functions as either hydrolase or transferase acting on xyloglucans and is considered to be responsible for rearrangement of the cellulose-xyloglucan framework, the processes essential for the construction, modification, and degradation of plant cell walls Different XRP genes exhibit potentially different expression profiles with respect to tissue specificity and responsiveness to hormonal and mechanical signals The molecular approach to individual XRP genes will open a new path for exploring the controlling mechanisms by which the plant cell wall is constructed and reformed during plant growth and development

The Role of Endoxyloglucan Transferase in the Organization of Plant Cell Walls

Biosynthesis of glucans occurred in cell-free fractions isolated from onion stem (Allium cepa L.) enriched in either dictyosomes or plasma membranes. beta-1,3- and beta-1, 4-Glucans were synthesized in differing proportions and at different rates as the concentration of uridine diphosphoglucose or the proportion of dictyosomes or plasma membrane varied. At low (1.5 mum) UDP-glucose concentrations synthesis of alkali-insoluble glucan was correlated with abundance of dicytosomes; most of the substrate utilized by plasma membrane was for glycolipid synthesis. At high (1 mm) UDP-glucose concentration, the synthesis of alkali-insoluble glucans correlated with the abundance of plasma membrane. Substrate enhancement of beta-1, 4-glucan synthesis in dictyosome fractions was less than proportional to increases in substrate concentration. In contrast, beta-1, 4-glucan synthesis by plasma membrane was more than proportionately increased. At high substrate concentrations the synthesis of beta-1, 3-glucans predominated in both dictyosome and plasma membrane fractions. The results show that the capacity to synthesize glucans resides in both Golgi apparatus and plasma membranes of onion stem, but that the plasma membrane has the greatest capacity for synthesis of alkali-insoluble glucans at high UDP-glucose concentrations.

β-Glucan Synthetases of Plasma Membrane and Golgi Apparatus from Onion Stem

SUMMARY A brief history of the literature dealing with cellulose microfibril assembly is presented, and a current summary of cellulose microfibril synthesizing complexes among eukaryotic cells is given. Terminal complexes not described before include the following: linear terminal complexes (TCs) with three rows in Eremosphaera , Microdictyon and Chaetomorpha ; globular terminal complexes in Ophioglossum , Psilotum , Equisetum and Gingko . Cellulose microfibril assembly in Acetobacter xylinum is described very briefly and compared with the process among eukaryotic cells. Particular emphasis on structures that may be involved in the spatial control of cellulose synthesis is given: Among these are cytoplasmic structures such as microtubules and microfilaments. Microfilament structures are shown to clearly surround individual microtubules that lie adjacent to the plasma membrane. Using freeze-fracture techniques, these labile associations have been shown for the first time. Microfibril orientation may be mediated through an interaction of cortical microtubules in association with microfilaments. A review of Mueller and Brown’s membrane flow model for microfibril orientation is presented. Cellulose terminal complex clustering and its role in gravitropic response is covered. Definitive membrane changes with TC clustering/disaggregation and intramembranous particle frequencies, occur within 12 min following gravistimulation. These differences are pronounced in the cells from upper and lower hemicylinders of rapidly frozen tissue, which was studied by the freeze-fracture method. A hypothesis for cellulose microfibril interaction in controlling the constraint of the growth axis is presented, and the supporting data for terminal complex clustering/disaggregation as well as fluorescent brightener inhibition of the gravitropic response support this hypothesis. The onset and regulation of cellulose microfibril assembly is presented for synchronized protoplasts generated by Boergesenia , using inhibitors of transcription and translation. These results suggest dynamic turnover of terminal complex subunits during the assembly of the cellulose microfibril. This study is concluded with a brief discussion of possible phylogenetic trends in the evolution of cellulose synthesis. A principal underlying theme is that the specific arrangement and consolidation of the terminal complex subunits determine to a large degree the size and shape of the microfibril, its crystallinity, as well as intramicrofibrillar associations. Three basic types of TCs appear among all eukaryotic cells studied so far: namely, the rosette, the globular and the linear complex.

Cellulose microfibril assembly and orientation: recent developments.

The plasmalemma of Oocystis apiculata, W. West when freezeetched has been shown to bear granules of several sizes. At the earliest stage of development the outer face of the plasmalemma of the naked autospore has small (8.5 nm diameter) granules aligned in rows, in pairs. These rows are stacked together forming extensive “granule-bands” over the plasmalemma surface. The orientation of these “granule-bands” corresponds exactly to one of the major microfibril directions. Occasionally, the bands are reduced to patches, some of which are at right angles to each other. Banding of granules on the inner plasmalemma face of naked autospores is also seen. During development the plasmalemma is seen to change so that in the final stages it bears reticulate invaginations, the granule bands occurring within them. The significance of the “granulebands” in terms of cellulose microfibril biosynthesis is discussed.

Plasmalemma structure in relation to microfibril biosynthesis in Oocystis.

Studies on the Fine Structure of Glaucocystis nostochinearum Itzigs. I. WALL STRUCTURE

The fine structure of the alga Glaucocystis nostochinearum Itzigs. has been studied by thinsectioning and freeze-etching. Many of the features already established by sectioning have been verified but different conclusions are drawn, particularly with regard to the photosynthetic organs. The action of glycerol solutions on the morphology of the plasmalemma as revealed by freeze-etching is shown to be of general importance for the freeze-etching of plant cells and of particular importance in elucidating the true form of the plasmalemma in Glaucocystis. The literature regarding the taxonomic position of Glaucocystis has been reviewed, and, along with a completely new demonstration of plasmalemma structure, an attempt has been made to reclassify this organism.

https://www.tandfonline.com/doi/pdf/10.1080/00071617100650141

Studies on the fine structure of Glaucocystis nostochinearum Itzigs. II. Membrane morphology and taxonomy

A study of the fine structure of Oocystis apiculata W. West has been made. The internal cytology has been described and, where appropriate, comparisons have been made with the externally similar alga, Glaucocystis. Particular attention has been paid to the structure and composition of the wall; the microfibrils immediately external to the plasmalemma are shown to be highly oriented; they are also shown by freeze-etching to close in to a ‘pole’. A brief discussion on the size difference in microfibrils as determined by shadowing and staining is also made.

https://www.tandfonline.com/doi/pdf/10.1080/00071617200650131

The fine structure of Oocystis apiculata W. West with particular reference to the wall

Naked swarmers of Cladophora have been collected and wall synthesis and development have been followed using the techniques of freeze-etching and sectioning. Swarmers frozen after 9 hours liberation have lost their flagella, developed the characteristic fibrous layer and show the initial stages of wall production. Both the first formed (randomly oriented) and the later (more ordered) microfibrils appear to have a distinct granular texture. Occasionally linear arrays of granules up to 4 μm long may be seen. After 5 days settling a thick wall composed of almost transversely oriented microfibrils is present and a rhizoid is pushed out. Also characteristic of this stage is the central localisation of cell components and peripheral vacuolar distribution. Longitudinally oriented microtubules also reappear at this stage having been absent during carlier wall formation.

David G. Robinson

Papers

Plasmalemma structure in relation to microfibril biosynthesis in Oocystis.

Studies on the Fine Structure of Glaucocystis nostochinearum Itzigs. I. WALL STRUCTURE

Studies on the fine structure of Glaucocystis nostochinearum Itzigs. II. Membrane morphology and taxonomy

The fine structure of Oocystis apiculata W. West with particular reference to the wall

Fine structure of swarmers of Cladophora and Chaetomorpha : III. Wall synthesis and development.