Plant Cell Walls

Abstract

Cell walls are important features of plant cells that perform a number of essential functions, including providing shape to the many different cell types needed to form the tissues and organs of a plant. Forming the interface between adjacent cells, plant cell walls often play important roles in intercellular communication. Because of their surface location, plant cell walls play an important role in plant-microbe interactions, including defense responses against potential pathogens. The desire to understand these and other plant functions help explain the strong interest in wall structure and biosynthesis. Plant cell walls are usually divided in textbooks into two categories: primary walls that surround growing cells or cells capable of growth and secondary walls that are thickened structures containing lignin and surrounding specialized cells such as vessel elements or fiber cells. In reality, all differentiated cells contain walls with distinct compositions, resulting in a spectrum of specialized cell walls with primary and secondary walls as two extremes. This brief prospective overview focuses mainly on issues that must be resolved if we are to understand the role of cell walls in plant physiology. Many outstanding reviews cover recent progress, including a series of excellent updates in a recent special issue of Plant Physiology focused on this topic (see McCann and Rose, 2010). In addition, the lignin component of secondary cell walls is covered elsewhere in this issue (Li and Chapple, 2010) as is the uses of cell walls as a source of energy (Somerville et al., 2010). The author apologizes to the many colleagues whose work could not be cited because of space limitations. The polysaccharide and glycoprotein components found in plant cell walls have been well characterized structurally. We need now to understand how these components are organized into the three-dimensional matrix needed for plant cell walls to perform their functions. The most characteristic component found in all plant cell walls is cellulose. It consists of a collection of β-1,4-linked glucan chains that interact with each other via hydrogen bonds to form a crystalline microfibril (Somerville, 2006). In addition to cellulose, plant cell walls contain several matrix polysaccharides that are grouped into two general categories: (1) the pectic polysaccharides include homogalacturonan, and rhamnogalacturonan I and II (Harholt et al., 2010) and (2) the hemicellulosic polysaccharides include xyloglucans, glucomannans, xylans, and mixed-linkage glucans (Scheller and Ulvskov, 2010). Plant cell walls also contain many proteins and glycoproteins, including various enzymes and structural proteins (Rose and Lee, 2010). For example, arabinogalactan proteins are structurally complex molecules found on the plasma membrane and in the cell wall; they are thought to play important roles in recognition and signaling events at the cell surface (Ellis et al., 2010). Despite tantalizing evidence for their involvement in many important recognition and signaling events, few details are available regarding how they are recognized or how recognition leads to signal transmission. How are wall components organized into a functional matrix? Over the years, several models have been proposed to explain the organization of wall components (Keegstra et al., 1973; Carpita and Gibeaut, 1993; Somerville et al., 2004). Most of the models have focused on understanding the organization of components in primary cell walls that would allow regulated reorganization of wall components during cell growth and differentiation. Hemicellulosic polysaccharides are known to bind tightly to cellulose microfibrils via hydrogen bonds and most wall models have incorporated this interaction as one important feature of cell wall architecture. Less is known about how the pectic polysaccharides interact with other components in plant cell walls, but there is increasing awareness of their importance in primary cell walls, where they are most abundant. The dynamic nature of plant cell walls is an important feature that is lacking from most models. As cells grow and differentiate, new wall material is laid down near the plasma membrane and older wall material is pushed outward. This process has the potential to create a wall where the composition and architecture are not uniform across the wall. For example, pectic polysaccharides are thought to be deposited early after cell division, leading to a middle lamella that is rich in pectins; other components are deposited later. This differentiation of the wall may be especially important for protein and glycoprotein components, such as AGPs that may change as cells mature and differentiate. Information about such heterogeneity is lost when tissues are ground and subjected to biochemical analysis. Thus, to fully understand the dynamic nature of plant cell walls at the molecular level, new visualization techniques are needed that reveal the three-di