Interactions xyloglucans

From tamarind seed xyloglucan, carboxymethyl derivatives with different levels of DS were prepared in isopropanol medium [440]. Swelling power, solubihty and tolerance to organic solvents of the derivatives increased with increasing DS. The interaction properties of the unmodified xyloglucan with calcium chloride and sodiiun tetraborate were found to be reversed upon car-boxymethylation. 

This essay was written in an attempt to explain our overview of primary cell walls and to reach consensus on the nomenclature of primary cell wall polysaccharides. We present evidence supporting the hypothesis that cellulose, xyloglucan, arabinoxylan, homogalacturonan, RG-I, and RG-II are the six polysaccharides common to all primary cell walls of higher plants. In many cells, these six polysaccharides account for all or nearly all of the primary wall polysaccharides. Like the physically interacting proteins that constitute the electron transport machinery of mitochondria, the structures of the six patently ubiquitous polysaccharides of primary cell walls have been conserved during evolution. Indeed, we hypothesize that the common set of six structural polysaccharides of primary cell walls have been structurally... 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

In Section III, it was mentioned that cell wall is a complex structure formed by different polysaccharides connected to glycoproteins. Hydroxy-L-proline-rich glycoproteins, such as extensin, have been found in almost all plants surveyed, and in some algae.203,281 A network of protein, pectic polymers, and xyloglucan, serving to cross-link the cellulose fibers of the cell wall, has been proposed.282,283 However, covalent links between the different components have not been demonstrated moreover, some of them can be extracted separately,284 and some associations may be artificial.285 Nevertheless, results are consistent with interactions through dipole-dipole (such as hydrogen bonds) or hydrophobic bonds. 

Kim, B. S., Xakemasa, M., and Nishinari, K. 2006. Synergisfic interaction of xyloglucan and xanthan investigated by rheology, differential scanning calorimetry, and NMR. Biomacromolecules 7 1223-1230. 

All other polymers are synthesised inside the cell and transported in lipid vesicles that fuse with the plasma membrane and release their contents into the extracellular space. It is likely that this process occurs alongside cellulose deposition, and hence wall assembly is likely to be influenced by the interaction between cellulose and non-cellulosic polymers. After initial deposition, cell walls can be further modified both by integration of further secreted polymers and by the action of modifying enzymes such as pectin methyl esterase, xyloglucan endo-transglycosylase and peroxidase. 

The primary cell wall of dicotyledonous plants consists of cellulose microfibrils dispersed within a matrix of predominantly non-cellulosic polysaccharides, including xyloglucans and pectic polysaccharides. The xyloglucans are neutral polysaccharides which bind to the cellulose microfibrils through secondary interactions, and have the ability to crosslink the fibrillar cellulose network. This fibrillar network is then dispersed in a network of the pectic polysaccharides.1 The pectic polysaccharide network also forms the middle lamella in dicotyledons and is responsible for cell-cell adhesion. 

Cross-linking between polysaccharide-polysaccharide units can occur in different forms such as ester-linked hydroxycinnamate dimers (Ralph et al., 2004), ferulic acid and/orp-coumaric acid monomers as cyclic dimers (Grabber et al., 2004), and direct linkage between polysaccharides and xyloglucans (Harris and Stone, 2009). Some important interactions between lignin and polysaccharides in the cell walls are the following ... 

Hayashi, T., Marsden, E., Delmer, P., 1987. Pea xyloglucan and cellulose. V. Xyloglucan cellulose interactions in vitro and in vivo. Plant Physiology 83, 384—389. 

Figure 3. Segment of a plant cell wall (schematic drawing). Bundles of cellulose microfibers in near crystalline packing (thick bundles) are crosslinked by hemicelluloses (drawn as double-lined intercalated network) and pectins (fine lines with egg-box interaction sites). Carbohydrate-carbohydrate interaction occurs between individual cellulose chains, between cellulose and xyloglucan (a hemi-cellulose) and between different hemicelluloses and pectins. Besides different forms of non-covalent interactions (Ca + mediated egg-box structures as well as ion-independent forms of molecular interactions) also covalent ester linkages between carbohydrate chains are found (adapted from [23]).

The interaction between cellulose microfibrils and the hemlceUulose-xyloglucan network is believed to represent the major load-bearing structure in the primary ceU waU. In physical terms, cell shape and size are governed by the mechanics of the ceU waU. CeU expansion occurs via strictly regulated reorientation of the waU components and several enzymes play a rule in this mechanism [121]. 

---