Plant Cell Wall Basics

What is a primary cell wall

Growing plant cells are surrounded by a polysaccharide-rich primary wall. This wall is part of the apoplast which itself is largely self-contiguous and contains everything that is located between the plasma membrane and the cuticle. The primary wall and middle lamella account for most of the apoplast in growing tissue. The symplast is another unique feature of plant tissues. This self-contiguous phase exists because tube-like structrues known as plasmodesmata connect the cytoplasm of different cells.

Plants differ in shape and size. These differences result from the different morphologies of the various cells that make up the vegetative and reproductive organs of the plant body. Changes in tissue and organ morphology that occur during plant growth and development result from controlled cell division and growth together with modification and structural reorganiztion of the wall, and the synthesis and insertion of new material into the existing wall.

Some of the functions of the primary wall:

Structural and mechanical support.
maintain and determine cell shape.
resist internal turgor pressure of cell.
control rate and direction of growth.
ultimately responsible for plant architecture and form.
regulate diffusion of material through the apoplast.
carbohydrate storage - walls of seeds may be metabolized.
protect against pathogens, dehydration, and other environmental factors.
source of biologically active signalling molecules.
cell-cell interactions.

Primary walls are the major textural component of plant-derived foods. The ripening of fruits and vegetables is associated with changes in wall structrue and composition. Plant-derived beverages often contain significant amounts of wall polysaccharides. Some wall polysaccharides bind heavy metals, stimulate the immune system or regulate serum cholesterol. Wall polysaccharides are used commercially as gums. gels, and stabilizers. Thus, cell wall structure and organization is of interest to the plant scientist, the food processing industry and the nutritionist.

Primary wall composition and architecture

Primary walls isolated form higher plant tissues and cells are composed predominantly of polysaccharides together with lesser amounts of structural glycoproteins (hydroxyproline-rich extensins) , phenolic esters (ferulic and coumaric acids), ionically and covalently bound minerals (e.g. calcium and boron), and enzymes. In addition walls contain proteins (expansins) that are believed to have a role in regulating wall expansion. Lignin, a macromolecule composed of highly cross-linked phenolic molecules, is a major component of secondary walls.

The major polysaccharides in the primary wall are:

Cellulose - a polysaccharide composed of 1,4-linked β-D-glucose residues

Hemicellulose - branched polysaccharides that are structurally homolgous to cellulose because they have a backbone composed of 1,4-linked β-D-hexosyl residues. The predominant hemicellulose in many primary walls is xyloglucan. Other hemicelluloses found in primary and secondary walls include glucuronoxylan, arabinoxylan, glucomannan, and galactomannan.

Pectin - a family of complex polysaccharides that all contain 1,4-linked α-D-galacturonic acid. To date three classes of pectic polysaccharides have been characterized: Homogalacturonans, rhamnogalacturonans, and substituted galacturonans.

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.

Techniques used at the CCRC to study walls

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	Cultured Cells
	Arabidopsis	Arabidopsis
	Tomato	Tomato
	Tobacco	Sycamore
	Lemna	Rice
	Ferns	Soybean
	Horsetails	Grapefruit
	Lycopods	Physcomitrella (moss)
	Mosses	Anthoceros (hornwort)
	Liverworts	Chara (green algae)
	Hornworts	Coleochaete (green algae)

The conformation and interaction of wall polysaccharides

These studies involve extensive use of 2D NMR spectroscopy and computer-aided molecular modeling techniques. We are also growing plants in a ¹³C-enriched environment to generate isotopically labeled polysaccharides that can be characterized using ¹³C NMR spectroscopy. Further information is available by clicking on Xyloglucan.

The biosynthesis of pectic polysaccharides

We have developed methods for the purification and assay of enzymes involved in pectin biosynthesis, and for enzymes involved in the formation of various nucleotide sugars. In addition we are using molecular techniques to clone and express numerous enzymes involved in cell wall biosynthesis. Further information can be obtained by clicking on Biosynthesis

Selected references

Keegstra et al (1973) The structure of plant cell walls. III. A model of the walls of suspension-cultured sycamore cells based on the interconnections of the macromolecular components. Plant Physiol., 51, 188-196.

Albersheim (1976) The primary cell wall. In Plant Biochemistry (Bonner and Varner eds) 3rd Edition, Academic Press, New York, pp 225-274.

Selvendran and ONeill (1985) Isolation and analysis of cell walls from plant material. In Methods of Biochemical Analysis (Glick ed) Vol 32, John Wiley pp 25-133.

Carpita and Gibeaut (1993) Structural models of primary-cell walls in flowering plants - consistency of molecular structures with the physical properties of the wall during growth. Plant J., 3, 1-30.

Cosgrove (2001) Wall structure and wall loosening. A look backwards and forwards. Plant Physiol.,125, 131-134.

Ebringerova (2006) Structural diversity and application potential of hemicelluloses. Macromol Symp 232, 1-12.

Ragauskus et al (2006) The path forward for biofuels and biomaterials. Science 311, 484-489.

The tether model	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.