Debra Mohnen, Maor Bar-Peled and Chris Somerville

5.1 Introduction

One of the many challenges associated with trying to understand the synthesis, structure, and function of higher plant cell walls is the obvious chemical diversity between tissue types within a plant and between plant species. For instance, an analysis of the cell wall sugar composition of different tissues in Arabidopsis revealed that each of the tissue types analyzed had a strikingly different composition (1). In spite of this diversity, there is sub­stantial evidence in support of the hypothesis that most cells of all higher plants have the same six major types of polysaccharides: cellulose, xyloglucan, xylan, homogalacturonan, rhamnogalacturonan I, and rhamnogalacturonan II (2). According to this idea, variation in wall composition arises from variation in the amounts of the various polysaccharides and in variations of the structures of the various polysaccharides. Thus, in order to understand how cell walls are synthesized, much of the task may be condensed into understanding how these classes of polysaccharides are synthesized and deposited in walls. Additionally, several other polysaccharides are found in some species. These include xylogalacturonan, mannan, and mixed-linkage glucans. Lignin associated with secondary cell walls represents the other major component. Although cell wall proteoglycans are a quantitatively minor component, they may be relevant for understanding certain aspects of assembly. Similarly, callose plays an important role in synthesis of de novo cell walls or in specialized cell walls such as pollen tubes but is not usually a quantitatively significant component of most types of cell walls.

Plant walls are divided into two basic types, the primary and secondary wall (Figure 5.1). The primary wall is the first wall laid down in dividing and growing plant cells and it is the terminal wall in many cells in the soft parts of the plant, including the palisade cells in leaves and parenchyma cells present throughout the plant. The primary wall contains 80-90% polysaccharide and 10-20% protein. Cellulose, hemicellulose, and pectin are the main polysaccharide components in the primary wall. The most abundant hemicellulose in most primary walls is xyloglucan. However, in grasses and other commelinoid monocots glucuronoarabinoxylan is the major hemicellulose and during cell expansion in grasses, (3-1,3;p1,4-mixed linkage glucans are prevalent in the primary wall. Secondary walls are produced by specialized cells that serve a structural role such as fibers and xylem cells in vascular bundles. Secondary walls generally have less pectin, contain more cellulose and more of the hemicellulose (3-1,4-xylan (often as glucuronoxylan), and are often rigidified

plasma membrane (cellulose) and in the Golgi (pectin and hemicellulose) by the action of glycosyltransferases that use nucleotide-sugar substrates. (b) Some cells (e. g., xylem and fiber cells) form secondary walls internal to the primary wall. Secondary walls have increased amounts of cel­lulose and hemicellulose, less pectin, and are often rigidified with lignin. Secondary walls in wood tissue consist of three layers (S1, S2, and S3) that differ in cellulose microfibril orientation and chemical compo­sition. (c) Secondary wall synthesis includes formation of cellulose microfibrils at the plasma membrane, hemicellulose within the Golgi followed by deposition in the wall, and lignin polymerization from mono — lignols within the wall matrix. (Figure with permission by Malcolm O’Neill, CCRC, University of Georgia)

by lignification. The synthesis of both the primary and secondary walls utilizes nucleotide — sugar substrates that are synthesized in the cytosol or in the lumen of the endoplasmic reticulum and the Golgi.

In the following overview, we have summarized the current status of knowledge about how the major classes of cell wall polymers are synthesized, modified, secreted, and assembled.

Because of the importance of Arabidopsis as an experimental model, much of the recent progress has been obtained using Arabidopsis. However, information from other species is included where it is available. The following description of cellulose synthesis is an update of a recent review (3).