In situ synthesis of beta-glucan microfibrils on tobacco plasma membrane sheets

Microfluidic preparation of anchored cell membrane sheets for in vitro analyses and manipulation of the cytoplasmic face

Molecular networks on the cytoplasmic faces of cellular plasma membranes are critical research topics in biological sciences and medicinal chemistry. However, the selective permeability of the cell membrane restricts the researchers from accessing to the intact intracellular factors on the membrane from the outside. Here, a microfluidic method to prepare cell membrane sheets was developed as a promising tool for direct examination of the cytoplasmic faces of cell membranes. Mammalian cells immobilized on a poly(ethylene glycol)-lipid coated substrate were rapidly and efficiently fractured, with the sheer stress of laminar flow in microchannels, resulting in isolation of the bottom cell membrane sheets with exposed intact cytoplasmic faces. On these faces of the cell membrane sheets, both ligand-induced phosphorylation of receptor tyrosine kinases and selective enzymatic modification of a G-protein coupling receptor were directly observed. Thus, the present cell membrane sheet should serve as a unique platform for studies providing new insights into juxta-membrane molecular networks and drug discovery.

Arabidopsis Regenerating Protoplast: A Powerful Model System for Combining the Proteomics of Cell Wall Proteins and the Visualization of Cell Wall Dynamics

The development of a range of sub-proteomic approaches to the plant cell wall has identified many of the cell wall proteins. However, it remains difficult to elucidate the precise biological role of each protein and the cell wall dynamics driven by their actions. The plant protoplast provides an excellent means not only for characterizing cell wall proteins, but also for visualizing the dynamics of cell wall regeneration, during which cell wall proteins are secreted. It therefore offers a unique opportunity to investigate the de novo construction process of the cell wall. This review deals with sub-proteomic approaches to the plant cell wall through the use of protoplasts, a methodology that will provide the basis for further exploration of cell wall proteins and cell wall dynamics.

Microarray analysis of Etrog citron (Citrus medica L.) reveals changes in chloroplast, cell wall, peroxidase and symporter activities in response to viroid infection

Viroids are small (246-401 nucleotides), single-stranded, circular RNA molecules that infect several crop plants and can cause diseases of economic importance. Citrus are the hosts in which the largest number of viroids have been identified. Citrus exocortis viroid (CEVd), the causal agent of citrus exocortis disease, induces considerable losses in citrus crops. Changes in the gene expression profile during the early (pre-symptomatic) and late (post-symptomatic) stages of Etrog citron infected with CEVd were investigated using a citrus cDNA microarray. MaSigPro analysis was performed and, on the basis of gene expression profiles as a function of the time after infection, the differentially expressed genes were classified into five clusters. FatiScan analysis revealed significant enrichment of functional categories for each cluster, indicating that viroid infection triggers important changes in chloroplast, cell wall, peroxidase and symporter activities.

A new callose function: involvement in differentiation and function of fern stomatal complexes

Callose in polypodiaceous ferns performs multiple roles during stomatal development and function. This highly dynamic (1→3)-β-D-glucan, in cooperation with the cytoskeleton, is involved in: (a) stomatal pore formation, (b) deposition of local GC wall thickenings, and (c) the mechanism of stomatal pore opening and closure. This behavior of callose, among others, probably relies on the particular mechanical properties as well as on the ability to form and degrade rapidly, to create a scaffold or to serve as a matrix for deposition of other cell wall materials, and to produce fibrillar deposits in the periclinal GC walls, radially arranged around the stomatal pore. The local callose deposition in closing stomata is an immediate response of the external periclinal GC walls experiencing strong mechanical forces induced by the neighboring cells. The radial callose fibrils transiently co-exist with radial cellulose microfibrils and, like the latter, seem to be oriented via cortical MTs.

Biosynthesis of callose and cellulose by detergent extracts of tobacco cell membranes and quantification of the polymers synthesized in vitro

The conditions that favor the in vitro synthesis of cellulose from tobacco BY-2 cell extracts were determined. The procedure leading to the highest yield of cellulose consisted of incubating digitonin extracts of membranes from 11-day-old tobacco BY-2 cells in the presence of 1 mM UDP-glucose, 8 mM Ca(2+) and 8 mM Mg(2+). Under these conditions, up to nearly 40% of the polysaccharides synthesized in vitro corresponded to cellulose, the other polymer synthesized being callose. Transmission electron microscopy analysis revealed the occurrence of two types of structures in the synthetic reactions. The first type consisted of small aggregates with a diameter between 3 and 5 nm that associated to form fibrillar strings of a maximum length of 400 nm. These structures were sensitive to the acetic/nitric acid treatment of Updegraff and corresponded to callose. The second type of structures was resistant to the Updegraff reagent and corresponded to straight cellulose microfibrils of 2-3 nm in diameter and 200 nm to up to 5 microm in length. In vitro reactions performed on electron microscopy grids indicated that the minimal rate of microfibril elongation in vitro is 120 nm/min. Measurements of retardance by liquid crystal polarization microscopy as a function of time showed that small groups of microfibrils increased in retardance by up to 0.047 nm/min per pixel, confirming the formation of organized structures.

Activation of beta-glucan synthases by wall-bound purple acid phosphatase in tobacco cells

Wall-bound purple acid phosphatases have been shown to be potentially involved in the regulation of plant cell growth. The aim of this work was to further investigate the function of one of these phosphatases in tobacco (Nicotiana tabacum), NtPAP12, using transgenic cells overexpressing the enzyme. The transgenic cells exhibited a higher level of phosphatase activity in their walls. The corresponding protoplasts regenerating a cell wall exhibited a higher rate of beta-glucan synthesis and cellulose deposition was increased in the walls of the transgenic cells. A higher level of plasma membrane glucan synthase activities was also measured in detergent extracts of membrane fractions from the transgenic line, while no activation of Golgi-bound glycan synthases was detected. Enzymatic hydrolysis and methylation analysis were performed on the products synthesized in vitro by the plasma membrane enzymes from the wild-type and transgenic lines extracted with digitonin and incubated with radioactive UDP-glucose. The data showed that the glucans consisted of callose and cellulose and that the amount of each glucan synthesized by the enzyme preparation from the transgenic cells was significantly higher than in the case of the wild-type cells. The demonstration that callose and cellulose synthases are activated in cells overexpressing the wall-bound phosphatase NtPAP12 suggests a regulation of these carbohydrate synthases by a phosphorylation/dephosphorylation process, as well as a role of wall-bound phosphatases in the regulation of cell wall biosynthesis.

Microtubule involvement in the deposition of radial fibrillar callose arrays in stomata of the fern Asplenium nidus L

Aniline blue staining and callose immunolabeling revealed the deposition of significant callose quantities, in the form of fibrils, in the periclinal walls of guard cells (GCs) of stomata of the fern Asplenium nidus. The stomata that were at an early stage of differentiation displayed short callose fibrils at the junctions of the periclinal walls with the dorsal ones, which converged on the site of the future stomatal pore. In stomata being at an advanced stage of differentiation, callose fibrils were radially arranged around the stomatal pore, while in mature closed ones they were focused on the margins of the wall thickenings lining the stomatal pore. The pattern of the callose fibril organization resembled that of cellulose microfibrils in the same walls. Like the cellulose microfibrils, callose fibrils appeared coaligned with the underlying radial arrays of cortical microtubules (MTs). Moreover, the stomata treated with cellulose synthesis inhibitors (coumarin or dichlobenil) and those recovering from treatments with callose synthesis inhibitors (2-deoxy-D-glucose or tunicamycin) exhibited distinct radial callose fibril arrays. Cytochalasin B did not affect the organization of the radial callose fibril arrays. In contrast, oryzalin completely disturbed the pattern of callose deposition in the affected GCs. Therefore, the fibrillar callose orientation in the periclinal GC walls is probably controlled by MTs but not by actin filaments. The MTs seem to orient callose synthases in the plasmalemma, thus determining the fibrillar nature of callose deposits and their radial mode of arrangement. The cellulose microfibrils are not involved in the callose fibril alignment.

Regulation of callose synthase activity in situ in alamethicin-permeabilized Arabidopsis and tobacco suspension cells

BACKGROUND

The cell wall component callose is mainly synthesized at certain developmental stages and after wounding or pathogen attack. Callose synthases are membrane-bound enzymes that have been relatively well characterized in vitro using isolated membrane fractions or purified enzyme. However, little is known about their functional properties in situ, under conditions when the cell wall is intact. To allow in situ investigations of the regulation of callose synthesis, cell suspensions of Arabidopsis thaliana (Col-0), and tobacco (BY-2), were permeabilized with the channel-forming peptide alamethicin.

RESULTS

Nucleic acid-binding dyes and marker enzymes demonstrated alamethicin permeabilization of plasma membrane, mitochondria and plastids, also allowing callose synthase measurements. In the presence of alamethicin, Ca2+ addition was required for callose synthase activity, and the activity was further stimulated by Mg2+ Cells pretreated with oryzalin to destabilize the microtubules prior to alamethicin permeabilization showed significantly lower callose synthase activity as compared to non-treated cells. As judged by aniline blue staining, the callose formed was deposited both at the cell walls joining adjacent cells and at discrete punctate locations earlier described as half plasmodesmata on the outer walls. This pattern was unaffected by oryzalin pretreatment, showing a quantitative rather than a qualitative effect of polymerized tubulin on callose synthase activity. No callose was deposited unless alamethicin, Ca2+ and UDP-glucose were present. Tubulin and callose synthase were furthermore part of the same plasma membrane protein complex, as judged by two-dimensional blue native SDS-PAGE.

CONCLUSION

Alamethicin permeabilization allowed determination of callose synthase regulation and tubulin interaction in the natural crowded cellular environment and under conditions where contacts between the cell wall, the plasma membrane and cytoskeletal macromolecules remained. The results also suggest that alamethicin permeabilization induces a defense response mimicking the natural physical separation of cells (for example when intercellulars are formed), during which plasmodesmata are transiently left open.

The cytoplasmic domain of the cellulose-synthesizing complex in vascular plants

The cytoplasmic domain of the rosette terminal complex has been imaged in situ in patches of plasma membrane isolated from tobacco BY-2 protoplasts. By partially extracting the plasma membrane lipids, cellulose microfibrils were observed through the plasma membrane. Rosette terminal complexes were identified on the basis of their association with the ends of these cellulose microfibrils. The cytoplasmic domain of the rosette terminal complex has been shown to be hexagonal in shape and has been measured to be 45-50 nm in diameter and 30-35 nm tall. These findings demonstrate that the terminal complex does indeed have a substantial cytoplasmic component, and that the hexagonal array observed in the lipid bilayer by freeze fracture is actually only a small part of the overall complex. These findings will allow better modeling of the terminal complex and may facilitate predictions of how many proteins are associated with the rosette terminal complex in vivo.

Spinning of a gigantic bundle of hollow fibrils by a spirally moving higher plant protoplast

A unique fiber spinning was found in protoplasts from white birch (Betula platyphylla) leaves under an acidic medium containing high concentration of Ca(2+). After expanding from 10 to 100 microm in diameter under the culture condition, the protoplast started secreting a gigantic fiber while moving in a spiral way. Real time video analyses elucidated that the orientation, rate and pattern of the motion were directed due to the inverse force of the fiber spinning. Moreover, observation using several microscopic methods accompanied with histochemical staining and nuclear magnetic resonance (NMR) analysis indicated that the fiber was composed of 400-500 nm wide (1-->3)-beta-glucan hollow sub-fibrils. This entire phenomenon may be a response against the stress imposed. The observation presented provides an understanding of the unique relationship between fiber spinning and the bottom-up fiber fabrication from nano to micro scales.

The cellulose synthase complex: a polymerization driven supramolecular motor

We present a biophysical model for the propulsion of the cellulose synthase complex, the motile transmembrane protein complex responsible for the biosynthesis of cellulose microfibrils, the dominant architectural component of the cell walls of higher plants. Our model identifies the polymerization and the crystallization of the cellulose chains as the combined driving forces and elucidates the role of polymer flexibility and membrane elasticity as force transducers. The model is elaborated using both stochastic simulations and a simplified analytical treatment. On the basis of the model and approximate values for the relevant physical constants, we estimate the speed of the cellulose synthase complex to be in the range v(p) approximately 10(-9)-10(-8) m/s, consistent with the recently reported experimental value of 5.8 x 10(-9) m/s.

Cellulose synthesis in higher plants

Cellulose microfibrils play essential roles in the organization of plant cell walls, thereby allowing a growth habit based on turgor. The fibrils are made by 30 nm diameter plasma membrane complexes composed of approximately 36 subunits representing at least three types of related CESA proteins. The complexes assemble in the Golgi, where they are inactive, and move to the plasma membrane, where they become activated. The complexes move through the plasma membrane during cellulose synthesis in directions that coincide with the orientation of microtubules. Recent, simultaneous, live-cell imaging of cellulose synthase and microtubules indicates that the microtubules exert a direct influence on the orientation of cellulose deposition. Genetic studies in Arabidopsis have identified a number of genes that contribute to the overall process of cellulose synthesis, but the role of these proteins is not yet known.

Cell membranes suspended across nanoaperture arrays

We present a method for spreading large (>100 microm(2)) cell membrane fragments across nanoapertures in planar supports. Electron-beam and focused-ion-beam lithography were used to fabricate arrays of 50-600 nm diameter holes in free-standing silicon nitride (SiN) solid films 100-500 nm thick. By pressing adhering live cells onto the nanostructured SiN surface and then removing them, planar cell membrane sheets (CMSs) were transferred in a well-defined orientation onto the SiN support. We demonstrate the accessibility to both extracellular and intracellular surfaces of CMSs by targeting membrane constituents side-specifically with fluorescent markers. Our approach is of interest for studying ligand-receptor interactions using optical, electrical, and scanning probe techniques at the single-molecule level.

Beta-D-glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.

Carbon partitioning to cellulose synthesis

This article discusses the importance and implications of regulating carbon partitioning to cellulose synthesis, the characteristics of cells that serve as major sinks for cellulose deposition, and enzymes that participate in the conversion of supplied carbon to cellulose. Cotton fibers, which deposit almost pure cellulose into their secondary cell walls, are referred to as a primary model system. For sucrose synthase, we discuss its proposed role in channeling UDP-Glc to cellulose synthase during secondary wall deposition, its gene family, its manipulation in transgenic plants, and mechanisms that may regulate its association with sites of polysaccharide synthesis. For cellulose synthase, we discuss the organization of the gene family and how protein diversity could relate to control of carbon partitioning to cellulose synthesis. Other enzymes emphasized include UDP-Glc pyrophosphorylase and sucrose phosphate synthase. New data are included on phosphorylation of cotton fiber sucrose synthase, possible regulation by Ca2+ of sucrose synthase localization, electron microscopic immunolocalization of sucrose synthase in cotton fibers, and phylogenetic relationships between cellulose synthase proteins, including three new ones identified in differentiating tracheary elements of Zinnia elegans. We develop a model for metabolism related to cellulose synthesis that implicates the changing intracellular localization of sucrose synthase as a molecular switch between survival metabolism and growth and/or differentiation processes involving cellulose synthesis.

Carbon partitioning to cellulose synthesis

This article discusses the importance and implications of regulating carbon partitioning to cellulose synthesis, the characteristics of cells that serve as major sinks for cellulose deposition, and enzymes that participate in the conversion of supplied carbon to cellulose. Cotton fibers, which deposit almost pure cellulose into their secondary cell walls, are referred to as a primary model system. For sucrose synthase, we discuss its proposed role in channeling UDP-Glc to cellulose synthase during secondary wall deposition, its gene family, its manipulation in transgenic plants, and mechanisms that may regulate its association with sites of polysaccharide synthesis. For cellulose synthase, we discuss the organization of the gene family and how protein diversity could relate to control of carbon partitioning to cellulose synthesis. Other enzymes emphasized include UDP-Glc pyrophosphorylase and sucrose phosphate synthase. New data are included on phosphorylation of cotton fiber sucrose synthase, possible regulation by Ca2+ of sucrose synthase localization, electron microscopic immunolocalization of sucrose synthase in cotton fibers, and phylogenetic relationships between cellulose synthase proteins, including three new ones identified in differentiating tracheary elements of Zinnia elegans. We develop a model for metabolism related to cellulose synthesis that implicates the changing intracellular localization of sucrose synthase as a molecular switch between survival metabolism and growth and/or differentiation processes involving cellulose synthesis.

Beta-D-glycan synthases and the CesA gene family: lessons to be learned from the mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Cellulose synthase genes (CesAs) encode a broad range of processive glycosyltransferases that synthesize (1-->4)beta-D-glycosyl units. The proteins predicted to be encoded by these genes contain up to eight membrane-spanning domains and four 'U-motifs' with conserved aspartate residues and a QxxRW motif that are essential for substrate binding and catalysis. In higher plants, the domain structure includes two plant-specific regions, one that is relatively conserved and a second, so-called 'hypervariable region' (HVR). Analysis of the phylogenetic relationships among members of the CesA multi-gene families from two grass species, Oryza sativa and Zea mays, with Arabidopsis thaliana and other dicotyledonous species reveals that the CesA genes cluster into several distinct sub-classes. Whereas some sub-classes are populated by CesAs from all species, two sub-classes are populated solely by CesAs from grass species. The sub-class identity is primarily defined by the HVR, and the sequence in this region does not vary substantially among members of the same sub-class. Hence, we suggest that the region is more aptly termed a 'class-specific region' (CSR). Several motifs containing cysteine, basic, acidic and aromatic residues indicate that the CSR may function in substrate binding specificity and catalysis. Similar motifs are conserved in bacterial cellulose synthases, the Dictyostelium discoideum cellulose synthase, and other processive glycosyltransferases involved in the synthesis of non-cellulosic polymers with (1-->4)beta-linked backbones, including chitin, heparan, and hyaluronan. These analyses re-open the question whether all the CesA genes encode cellulose synthases or whether some of the sub-class members may encode other non-cellulosic (1-->4)beta-glycan synthases in plants. For example, the mixed-linkage (1-->3)(1-->4)beta-D-glucan synthase is found specifically in grasses and possesses many features more similar to those of cellulose synthase than to those of other beta-linked cross-linking glycans. In this respect, the enzymatic properties of the mixed-linkage beta-glucan synthases not only provide special insight into the mechanisms of (1-->4)beta-glycan synthesis but may also uncover the genes that encode the synthases themselves.

Insight into multi-site mechanisms of glycosyl transfer in (1-->4)beta-D-glycans provided by the cereal mixed-linkage (1-->3),(1-->4)beta-D-glucan synthase

Synthases of cellulose, chitin, hyaluronan, and all other polymers containing (1-->4)beta-linked glucosyl, mannosyl and xylosyl units have overcome a substrate orientation problem in catalysis because the (1-->4)beta-linkage requires that each of these sugar units be inverted nearly 180 degrees with respect to its neighbors. We and others have proposed that this problem is solved by two modes of glycosyl transfer within a single catalytic subunit to generate disaccharide units, which, when linked processively, maintain the proper orientation without rotation or re-orientation of the synthetic machinery in 3-dimensional space. A variant of the strict (1-->4)beta-D-linkage structure is the mixed-linkage (1-->3),(1-->4)beta-D-glucan, a growth-specific cell wall polysaccharide found in grasses and cereals. beta-Glucan is composed primarily of cellotriosyl and cellotetraosyl units linked by single (1-->3)beta-D-linkages. In reactions in vitro at high substrate concentration, a polymer composed of almost entirely cellotriosyl and cellopentosyl units is made. These results support a model in which three modes of glycosyl transfer occur within the synthase complex instead of just two. The generation of odd numbered units demands that they are connected by (1-->3)beta-linkages and not (1-->4)beta-. In this short review of beta-glucan synthesis in maize, we show how such a model not only provides simple mechanisms of synthesis for all (1-->4)beta-D-glycans but also explains how the synthesis of callose, or strictly (1-->3)beta-D-glucans, occurs upon loss of the multiple modes of glycosyl transfer to a single one.

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

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.

Arabinogalactan protein and wall-associated kinase in a plasmalemmal reticulum with specialized vertices

Arabinogalactan protein and wall-associated kinase (WAK) are suspected to be regulatory players at the interface between cytoplasm and cell wall. Both WAK(s) and arabinogalactan shown likely to represent arabinogalactan protein(s) have been visualized there with computational optical-sectioning microscopy. The arabinogalactan occurs in a polyhedral array at the external face of the cell membrane. WAK, and other proteins as yet unidentified, appear to fasten the membrane to the wall at vertices of the array. Evidence is presented that the array bears an important part of the mechanical stress experienced by the membrane, and it is speculated that the architectural organization of arabinogalactan protein, WAK, and other components of the array is critical for coordination of endomembrane activities, growth, and differentiation. The array has been named the plasmalemmal reticulum.

The mechanism of synthesis of a mixed-linkage (1-->3), (1-->4)beta-D-glucan in maize. Evidence for multiple sites of glucosyl transfer in the synthase complex

We examined the mechanism of synthesis in vitro of (1-->3), (1-->4)beta-D-glucan (beta-glucan), a growth-specific cell wall polysaccharide found in grasses and cereals. beta-Glucan is composed primarily of cellotriosyl and cellotetraosyl units linked by single (1-->3)beta-linkages. The ratio of cellotriosyl and cellotetraosyl units in the native polymer is strictly controlled at between 2 and 3 in all grasses, whereas the ratios of these units in beta-glucan formed in vitro vary from 1.5 with 5 &mgr;M UDP-glucose (Glc) to over 11 with 30 mM substrate. These results support a model in which three sites of glycosyl transfer occur within the synthase complex to produce the cellobiosyl-(1-->3)-D-glucosyl units. We propose that failure to fill one of the sites results in the iterative addition of one or more cellobiosyl units to produce the longer cellodextrin units in the polymer. Variations in the UDP-Glc concentration in excised maize (Zea mays) coleoptiles did not result in wide variations in the ratios of cellotriosyl and cellotetraosyl units in beta-glucan synthesized in vivo, indicating that other factors control delivery of UDP-Glc to the synthase. In maize sucrose synthase is enriched in Golgi membranes and plasma membranes and may be involved in the control of substrate delivery to beta-glucan synthase and cellulose synthase.