Biosynthesis of Pectin

Abstract

The plant cell wall is composed of polysaccharides and proteins. In addition, some cells have walls impregnated with lignin. In all cases, the polysaccharides constitute the major part of the wall. The wall polysaccharides are often classified into cellulose, hemicelluloses, and pectin, and these three types are represented in almost all cell walls in varying proportions. Cellulose is the main load-bearing structure and is composed of β-1,4-linked glucan chains organized in more or less crystalline microfibrils. Hemicelluloses include several different polymers, chiefly xylans, xyloglucans, and (gluco)mannans, which are characterized by having a backbone of β-1,4-linked sugars with an equatorial linkage configuration (Scheller and Ulvskov, 2010). Pectin is the third group of polysaccharides, characterized by relatively high extractability using acid or chelators and a high content of GalUA. Together, the hemicelluloses and pectins constitute the matrix in which cellulose microfibrils are embedded. The interactions between the different polysaccharides ensure the strong yet dynamic and flexible properties of the cell wall. Various pectic polysaccharides can be detected in the cell wall, including homogalacturonan (HG), xylogalacturonan (XGA), apiogalacturonan, rhamnogalacturonan I (RGI), and rhamnogalacturonan II (RGII; Fig. 1). In Arabidopsis (Arabidopsis thaliana) leaves, the cell wall contains approximately 50% (w/w) pectin but the content varies depending on environment, tissue, and species (Zablackis et al., 1995). The ratio between HG, XGA, RGI, and RGII is also variable, but typically HG is the most abundant polysaccharide, constituting about 65% of the pectin, while RGI constitutes 20% to 35% (Mohnen, 2008). XGA and RGII are minor components, each constituting less than 10% (Zandleven et al., 2007; Mohnen, 2008). The different pectic polysaccharides are not separate molecules but covalently linked domains. Unbranched homopolymer chains of α-1,4-linked d-GalUA are described as HG. The backbone of GalUA residues can be substituted at various positions with other sugar moieties, including Xyl (XGA) and apiofuranose (apiogalacturonan). In XGA, a single Xyl is attached to the O-3 position of some GalUA residues. Additional Xyl residues can be attached to the first Xyl with β-1,4 linkage (Zandleven et al., 2006). XGA is particularly abundant in reproductive tissues but is also present in other tissues, such as Arabidopsis leaves (Zandleven et al., 2007). In aquatic angiosperms such as Lemna and Spirodela, the galacturonan backbone is decorated with d-apiofuranose at O-2 or O-3 (Hart and Kindel, 1970; Ovodov et al., 1971; Longland et al., 1989). Schematic structure of pectin. Pectin consists of four different types of polysaccharides, and their structures are shown. Kdo, 3-Deoxy-d-manno-2-octulosonic acid; DHA, 3-deoxy-d-lyxo-2-heptulosaric acid. HG and RGI are much more abundant than the other components (see text). Besides the simple substitutions in XGA and apiogalacturonan, clusters of complex side chains are also attached onto the O-2 or O-3 position in the galacturonan backbone to form RGII (Fig. 1). These side chains are composed of 12 types of glycosyl residues linked together by at least 22 different glycosidic bonds. Some of the glycosyl residues and glycosidic linkages found in RGII side chains are rare and considered unique in plant polysaccharides (e.g. 2-O-methyl-l-Fuc, l-aceric acid, and α-1,3- xylofuranose). Despite its complexity, the structure of RGII is highly conserved among vascular plants (Matsunaga et al., 2004; O'Neill et al., 2004). Frequently, RGII exists as a dimer mediated by borate ion attached to the A branch. Boron is an essential micronutrient for plants, most likely due to an important role of RGII dimerization in ensuring the integrity of the cell wall (O'Neill et al., 2001). Thus, although RGII is a minor component of pectin, it seems to have an essential role, and that may be the main reason why very few mutants with altered RGII have been identified, although many genes must be involved in RGII biosynthesis. RGI is the only type of pectin not built upon pure galacturonan backbones. Instead, it is a branched polymer with a backbone of disaccharide (α-1,4-d-GalA-α-1,2-l-Rha) repeats. The Rha residues in the backbone can be substituted with β-1,4-galactan, branched arabinan, and/or arabinogalactan side chains. Unlike RGII, the structure of the side chains of RGI can vary greatly among plants. Even a model suggesting that HG and its substituted variants are side chains of RGI was proposed (Vincken et al., 2003). However, the discovery of RGI backbone oligomers with HG and XGA as nonreducing end extensions suggests otherwise (Nakamura et al., 2002; Coenen et al., 2008). Galacturonic acid in pure galacturonan backbones is further modified by methyl esterification at the C-6 carboxyl position and/or by O-acetylation at the O-2 or O-3 position, whereas, in RGI, only the O-3 acet