Cellulose plasma membrane, formed

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

It seems likely that the enzyme complexes for hemicelluloses, pectins and cellulose are constructed, at least in part, on the endoplasmic reticulum and then transferred to the Golgi apparatus, where they are modified and sorted so that they can be segregated within the compartments of the Golgi cisternae (30,31). The complex for cellulose synthesis is not normally active within the Golgi apparatus and it is transported to active sites at the plasma membrane (1). The hemicelluloses and pectins are formed within vesicles and cisternae of the Golgi apparatus and the vesicles are transported to the plasma membrane, where fusion occurs and the polysaccharides are packed into the wall (1). It is not known whether particular polysaccharides such as the xylans of the hemicellulose and the arabinogalactans of the pectins are transported in separate vesicles or together in one vesicle. Nor is it known if the complex for cellulose synthesis is transported by vesicles which carry hemicellulose and pectin polysaccharides. 

In one proposed model, cellulose synthase spans the plasma membrane and uses cytosolic UDP-glucose as the precursor for extracellular cellulose synthesis. In another, a membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP-glucose from sucrose directly into cell wall synthesis (Fig. 20-32). 

Cellulose synthesis takes place in terminal complexes (rosettes) in the plasma membrane. Each cellulose chain begins as a sitosterol dextrin formed inside the cell. It then flips to the outside, where the oligosaccharide portion is transferred to cellulose synthase in the rosette and is then extended. Each rosette produces 36 separate cellulose chains simultaneously and in parallel. The chains crystallize into one of the microfibrils that form the cell wall. 

Significantly, the ordered pattern of fibril deposition in the secondary wall of Micrasterias367 was shown to be derived from the structure of the complexes located in the plasma membrane. The results indicated that the widest fibrils, those in the center of a band, are formed by the longest rows of rosettes, those in the center of arosette array. The shorter rows of rosettes within an array give rise to narrower fibrils. This proportionality between the width of a secondary cellulose fibril and the number of rosettes involved in its formation provides strong evidence that the rosette structure plays a significant role in the synthesis of cellulose fibrils. 

Chitin. Like cellulose synthase, fungal chitin synthases are present in the plasma membrane and extrude microfibrils of chitin to the outside.147 150 In the fungus Mucor the majority of the chitin synthesized later has its N-acetyl groups removed hydrolytically to form the deacetylated polymer chitosan.151152 Chitin is also a major component of insect exoskeletons. For this reason, chitin synthase is an appropriate target enzyme for design of synthetic insecticides.153... 

Cell-surface BI cellulase is envisaged as the form which is active against cellulose in peas in vivo, with a function that may be constructive in that it can act synergistically with plasma membrane-bound / -glucan synthetase complexes to enhance the rate of cellulose deposition (7,8,9). BS cellulase never appears to reach the wall in vivo in a form recognized by a BS antiserum (II). BS cellulase does not even bind readily to wall material in homogenates (Table III) despite its ability to bind to cellulose (3) and hydrolyze it (Table I). It is possible that BS cellulase functions intracellularly to hydrolyze a noncellulosic organelle-bound polysac-... 

Polymerization of the D-glucan chains occurs by way of a multi-subunit, enzyme complex embedded in the plasma membrane an almost simultaneous association, by means of hydrogen bonds, of the newly formed chains results in formation of partially crystalline microfibrils. This mechanism of polymerization and crystallization results in the creation of microfibrils whose chains are oriented parallel (cellulose I). In A. xylinum, the complex is apparently immobile, but, in cells in which cellulose is deposited as a cell-wall constituent, it seems probable that the force generated by polymerization of the relatively rigid microfibrils propels the complex through the fluid-mosaic membrane. The direction of motion may be guided through the influence of microtubules. 

Streptomyces pathogens are quite rare in plant diseases. Thaxtomin A from Streptomyces scabies causes scab disease of potato, which is characterized by conspicuous corky lesions on tubers. Molecular genetic investigation has revealed that thaxtomins are biosynthesized by nonribosomal peptide synthetases (txtAB) that condense modified L-phenylalanyl and L-4-nitrotryptophanyl units to form a 2,5-dioxopiperazine skeleton.316 Disruption of txtA results in the formation of nonpathogenic strain. This toxin is shown to affect the movement of calcium ions and protons across the plasma membrane and also inhibit cellulose biosynthesis.317... 

We begin our discussion of the newly introduced C02 resistances by evaluating the components of r, the resistance encountered by C02 as it diffuses through the water-filled interstices between the cellulose microfibrils in the cell wall (Fig. 1-13). In this way, C02 moves from the interface with the intercellular air spaces on one side of the cell wall to the plasma membrane on the other side (Fig. 8-11). We will use an appropriate form of Equation 8.22 to describe this resistance ... 

The membrane structure described must be maintained if the metabolism in cells is to function normally, and for this a variety of macro- and microelements is employed The element B forms stable mono- and diesters with cis-diols such as sugars and sugar alcohols. Thereby, unexchangeable complexes originate within cell walls (cellulose), middle lamellae (rhamnogalacturonan II Match and Kobayashi 1998), and as plasma membrane constituents such as glycoproteins or glycolipids (see Scheme 2.5). These complexes stabilize the structure (Cakmak and Roemheld 1997), and this may be especially important at the inter-... 

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