4.4.1 Primary cell walls

4.4.1.1 Pectin-rich walls

Primary walls that are rich in pectic polysaccharides occur in the gymnosperms, eudicotyle — dons, non-commelinid monocotyledons, and palms (Arecaceae) (3) (see Table 4.1). The first model for this type of wall was proposed by Keegstra and coworkers (142) based on detailed analyses of the walls of cell suspension cultures of sycamore (Acer pseudoplatanus). This model depicted the pectic polysaccharides as being covalently linked to the xyloglucans and to the glycoprotein extensin. Hydrogen bonding between the xyloglucans and the cellulose microfibrils provided the link between the matrix complex and the cellulose. However, evi­dence for possible covalent linkages between pectic polysaccharides and xyloglucans was not obtained until recently (86-88). Instead, models, often referred to as “tethered or sticky net­work models,” were developed in which xyloglucans were postulated to form non-covalent bridges between cellulose microfibrils in walls through hydrogen bonding (143, 144).

Evidence for these non-covalent bridges was obtained using transmission electron mi­croscopy after preparation of walls of the non-commelinid monocotyledon onion (Allium cepa) by fast freezing, deep etching, and rotary shadowing (145). From these studies, they developed a model in which there are two co-extensive, but independent, polymer networks: a cellulose-xyloglucan network and a pectic polysaccharide network. The first network is thought to be the main load-bearing structure of the wall, and the second is thought to deter­mine wall porosity (Figure 4.9). A third network composed of extensin may also be present (146). Using the same technique, similar bridges between cellulose have since been observed by other researchers in primary walls of this type from a variety of species (147-149).

In these wall models, it is usually assumed that in addition to cross-linking the cellulose microfibrils, xyloglucans completely coat the surfaces of these microfibrils. However, solid — state 13C NMR spectroscopy on isolated cell walls of mung bean (Vigna radiata) indicated a maximum of only 8% of the surface of the cellulose microfibrils had adsorbed xyloglucan (150).

The organization of the pectic polysaccharide network in these wall models is not clear but is probably in the form of a gel. Thus, the anionic HGA and RG-I have strong gel forming capabilities (53). HGA in the solid/gel state has an extended flexible conformation and may adopt a double or triple helical structure depending on the degree of hydration and the nature of the counter ion. Gel formation is due to the coordination of Ca2+ ions by carboxylic acid groups on adjacent chains enabling the formation of junction zones between HGA or RG-I chains. The strength of the HGA gel depends on the extent of methyl esterification of the GalAp residues.

50 nm

Figure 4.9 A simplified schematic representation of the spatial arrangement of polymers in a pectin-rich primary cell wall, e. g. of a parenchyma cell, as occurs in gymnosperms, eudicotyledons, non-commelinid monocotyledons, and palms (Arecaceae). The cellulosic microfibrils are embedded in a network of non — cellulosic polysaccharides [xyloglucans, pectic polysaccharides, and proteins (not shown)]. The xyloglu — cans are associated with the microfibril surfaces and form bridges between them. The immature primary wall may contain ~60% water but during development of the wall in some cell types (e. g., xylem fiber or tracheid), the water is replaced by lignins, which encrust the cellulosic microfibrils and non-cellulosic polysaccharides and may be covalently linked to them. (Reprinted with permission, from McCann, M. C. & Roberts, K. (1991) Architecture of the primary cell wall. In: The Cytoskeletal Basis of Plant Growth and Form (ed. C. W. Lloyd), pp. 109-129, Fig. 9.19, p. 126. Academic Press, London.)

The conformation of RG-I depends on the length of the alternating ь-Rhap insertions into the galacturonan chain (Figure 4.2). A single ь-Rhap causes a kink in the chain but two or three alternating ь-Rhap residues results in a chain having a fully extended conformation comparable with HGA. RG-I side chain galactans, arabino-4-galactans etc. (see Section 4.2.1.2.4) may not seriously restrict the stereochemistry of the backbone permitting junction zone formation (53) HGA and RG-I gels are important structurally in primary cell walls and in interfaces between walls in the middle lamella, the interfacial layer between adjacent cells.

In addition to calcium bridges between HG domains, cross-linking of the pectic polysac­charide network can occur by formation of borate esters between RG-II substituents, and in the walls of “core” families of the Caryophyllales (eudicotyledons) by DDFA cross-linking of RG-I (3, 52,62). Covalent linking of pectic polysaccharides to xyloglucans may also possibly occur (86-88).