If Homogalacturonan Were a Side Chain of Rhamnogalacturonan I. Implications for Cell Wall Architecture

Abstract

Pectin, an important cell wall component of dicotyledonous plants, is probably the most complex macromolecule in nature. Here, we critically summarize the large amount of data on pectin structure. An alternative model for the macromolecular structure of pectin is put forward, together with ideas on how pectins are integrated into the plant cell wall. Pectin is composed of as many as 17 different monosaccharides (Fig. 1; for review, see Ridley et al., 2001; Voragen et al., 2003). These monosaccharides are organized in a number of distinct polysaccharides, the structures of which are schematically shown in Figure 1. Together, these polymers form the pectin network (Visser and Voragen, 1996; Ridley et al., 2001; Voragen et al., 2003). Schematic representative structures of the constituent polysaccharides of pectin. The symbols for the various monosaccharide building units are explained in the accompanying legend. The predominant linkage types are indicated in the text. Underneath the diagrams with the monosaccharide building units, the symbol for the polymeric structure of that polysaccharide is indicated. These symbols will be used in the rest of the paper. The first structural elements of pectin to be discussed have a backbone of 1,4-linked α-d-GalpA residues and are referred to as galacturonans. Three types of galacturonan can be distinguished in the plant cell wall: the unsubstituted homogalacturonan (HG), rhamnogalacturonan II (RG-II; see below), and xylogalacturonan (XGA). The GalA residues of HG can be methyl-esterified at C-6 and carry acetyl groups on O-2 and O-3. XGA is a branched galacturonan with β-d-Xylp-(1→3) side chains (Schols et al., 1995; Visser and Voragen, 1996). The degree of xylosylation can vary between 25% (watermelon [Citrullis vulgaris]) and 75% (apple [Malus domestica]; Schols et al., 1995; Visser and Voragen, 1996). The GalA residues of XGA can be methyl-esterified as in HG (Schols et al., 1995). The methyl-esterification of HG, in particular, has drawn the attention of many research groups, because it determines to a large extent the industrial applicability of pectin. Not only the amount of methyl-esterification is important, but also the distribution of methyl groups on the HG backbone. Blocks of more than 10 unesterified GalA residues generally yield pectin molecules, which are sensitive to Ca2 +-cross-linking (Daas et al., 2001). HGs can contain clusters of four different (heterooligomeric) side chains with very peculiar sugar residues (such as Api, AceA, Dha, and Kdo). These side chains, together with the approximately nine galacturonyl residues to which they are connected, are referred to as RG-II (O'Neill et al., 2001; Ridley et al., 2001). The name RG-II is somewhat misleading, because it suggests that this structure contains a rhamnogalacturonan backbone just like rhamnogalacturonan I (RG-I). However, the Rha residues are much less abundant in RG-II than in RG-I, and they are present in the side chains of RG-II instead of in the backbone. Although the structure of RG-II is highly conserved in plants, new structural details are continuously being added to this complex element (Ridley et al., 2001). The distribution of the four side chains in RG-II, as well as the distribution of RG-II in HG, still remain to be established. It is not known whether XGA can harbor RG-II elements. Another constituent polysaccharide of pectin, RG-I, is composed of a repeating disaccharide unit [→2)-α-l-Rhap-(1→ 4)-α-d-GalpA-(1→]n, where n can be larger than 100 (McNeil et al., 1980; Visser and Voragen, 1996). The galacturonyl residues can carry acetyl groups on O-2 and O-3. The rhamnosyl residues can be substituted at O-4 with neutral sugars (McNeil et al., 1980; Lau et al., 1987). The proportion of branched Rha residues generally varies from approximately 20% to approximately 80% depending on the source of the polysaccharide (Visser and Voragen, 1996), although essentially unbranched RG-I molecules have also been reported in the literature (Penfield et al., 2001). The side chains can be single unit [β-d-Galp-(1→4)] but also polymeric, such as arabinogalactan I (AG-I) and arabinan (50 glycosyl residues or more). AG-I is composed of a 1,4-linked β-d-Galp backbone; α-l-Araf residues can be attached to the O-3 of the galactosyl residues (Ridley et al., 2001). The arabinans consist of a 1,5-linked α-l-Araf backbone, which can be substituted with α-l-Araf-(1→2)-, α-l-Araf-(1→3)-, and/or α-l-Araf-(1→3)-α-l-Araf-(1→3)-side chains (Ridley et al., 2001). Complexes of RG-I, AG-I, and arabinan are often referred to as pectic hairy regions (HR), in which AG-I and arabinan are the “hairs.” The abundance of HR and the amount/nature of the hairs can differ considerably from species to species. Arabinogalactan II (AG-II), is mainly associated with proteins (arabinogalactan proteins or AGPs), and it is still unclear whether this polysaccharide is part of the pectin complex. Pectin and AG-II often seem to co-extr