Plant Cell Wall Basics
The diffuse layer model
The stratified layer model
Xyloglucan molecules are hydrogen bonded to and cross-link cellulose microfibrils. The cellulose-xyloglucan network is emeshed in a non-covalently cross-linked pectic network.
Xyloglucan molecules are hydrogen bonded to the surface of cellulose microfibrils       but do not directly cross link them. The tightly-bound xyloglucan is surrounded       by a layer of less-tightly bound polysaccharides. The cellulose and xyloglucan       are embedded in a pectic matrix.
Xyloglucan molecules are hydrogen bonded to and cross-link cellulose microfibrils. The cellulose-xyloglucan lamellae are separated by strata of pectic polysaccharides.
Much research is sitll required to provide a complete description of the   primary wall at the molecular level. Moreover, there is increasing evidence   that primary walls are dynamic structures whose composition and architecture   changes during plant growth and development.
 
Secondary cell walls
Plants form two types of cell wall that differ in function and in composition. Primary walls surround growing and dividing plant cells. These walls provide mechanical strength but must also expand to allow the cell to grow and divide. The much thicker and stronger secondary wall (see figure on right), which accounts for most of the carbohydrate in biomass, is deposited once the cell has ceased to grow. The secondary walls of xylem fibers, tracheids, and sclereids are further strengthened  by the incorporation of lignin.
The evolution of conducting tissues with rigid secondary cell walls was a critical adaptive event in the history of land plants, as it facilitated the transport of water and nutrients and allowed extensive upright growth. Secondary walls also have a major impact on human life, as they are a major component of wood and are a source of nutrition for livestock. In addition, secondary walls may help to reduce our dependence on petroleum, as they account for the bulk of renewable biomass that can be converted to fuel. Nevertheless, numerous technical challenges must be overcome to enable the efficient utilization of secondary walls for energy production and for agriculture.
Primary and secondary walls contain cellulose, hemicellulose and pectin, albeit in different proportions. Approximately equal amounts of pectin and hemicellulose are present in dicot primary walls whereas hemicellulose is more abundant in grasses (e.g., switchgrass). The secondary walls of woody tissue and grasses are composed predominantly of cellulose, lignin, and hemicellulose (xylan, glucuronoxylan, arabinoxylan, or glucomannan).  The cellulose fibrils are embedded in a network of hemicellulose and lignin.  Cross-linking of this network is believed to result in the elimination of water from the wall and the formation of a hydrophobic composite that limits accessibility of hydrolytic enzymes and is a major contributor to the structural characterisitics of secondary walls.
Xylan, which accounts for up to 30% of the mass of the secondary walls in wood and grasses contributes to the recalcitrance of these walls to enzymic degradation. A high xylan content in wood pulp increases the economic and environmental costs of bleaching in paper manufacturing. Thus, reducing the xylan content of secondary walls and altering xylan structure, molecular weight, ease of extractability, and susceptibility to enzymic fragmentation are key targets for the genetic improvement of plants. However, progress in these areas is limited by our incomplete understanding of the mechanisms of xylan biosynthesis.
 
Xylans have a backbone of 1,4-linked β-D-xylosyl residues with short [α-D-glucosyluronic acid (GlcA), 4-O-methyl-α-D-glucosyluronic acid (MeGlcA), α-L-arabinosyl, O-acetyl, feruloyl, or coumaroyl] sidechains (see figure).
Xylan synthesis requires the coordinated action of numerous enzymes, including glycosyl transferases (GTs) that elongate the backbone and add side chain residues (see left figure on right). None of these GTs have been purified and biochemically characterized, although several candidate genes have been identified.
Little is known about the factors that regulate secondary wall polysaccharide biosynthesis and the mechanisms that control the assembly of these polysaccharides into a functional wall. Moreover, the mechanisms required to initiate and terminate xylan synthesis and to control the chain length of the xylan are as yet unidentified.
We are interested in several aspects of primary and secondary walls including:
The primary structure of wall polysaccharides
The conformation and interactions of wall polysaccharides
The biosynthesis of wall polysaccharides
The molecular and genetic mechanisms involved in wall formation
Developmentally-associated changes in wall structure
The role of the wall in plant-pathogen interactions
We use the following plants and suspension-cultured plants cells as sources of cell walls:
Plants