RG-I accounts for 7-14% of the primary wall (157) and 20-33% of pectin (194). Unlike HG and RG-II, RG-I has a backbone of up to 100 repeats of the disaccharide [^4)-a — D-GalpA-(1^2)-a-L-Rhap-(1^] (2, 157, 261, 262, 361, 362). The GalA residues in RG-I maybe O-acetylated at C-3 or C-2 (157, 199, 288-290). The average molecular weight of sycamore RG-I has been reported to be 105-106 Da (157). Between 20 and 80% of the rhamnosyl residues are substituted at C-4, and sometimes at C-3, with side chains composed mostly of arabinosyl and/or galactosyl residues (2, 157, 264), referred to as galactans (157, 264, 278), arabinans (214, 264, 290), and arabinogalactans (2, 157, 190, 214, 264). These side chains may range in size from 1 to 50 or more glycosyl residues (2, 157, 290). A large amount of immunocytochemical evidence based on antibodies against specific RG-I carbohydrate epitopes (160) indicates that the precise structure of the side chains of RG-I varies in a cell type and development-specific manner (214, 363). Representative side chains or portions of side chains that have been identified in RG-I have been previously summarized in (192).

The RG-I galactans may contain only galactosyl residues or may contain other neutral glycosyl residues (157) or acidic residues such as GalA (190), GlcA (157,190, 278), or 4- O — methylGlcA (278). Some galactans also have p-1,6-branching (190). As mentioned above, the size and linkages in the galactan side branches of RG-I vary depending upon the species (157).

RG-I also contains side chains of individual or multiple L-arabinofuranosyl (Araf) residues or chains of 1,5-linked-a-L-Ara f substituted at O-3 and occasionally at O-2 with additional Ara f residues (190, 290, 364). Such side chains are referred to as arabinans.

Some RG-I side chains contain both arabinosyl and galactosyl residues. These side chains are referred to as arabinogalactans that have been divided into Type I and Type II arabino­galactans. Type I arabinogalactans contain a 1^4-linked p-D-galactan backbone while the Type II arabinogalactans contain a 1^3-linked p-D-galactan backbone and are often highly branched (2, 157, 190, 214). Type II arabinogalactans maybe associated with glucurono- mannoglycans (190). Some studies suggest that mannose may be a component in some pectins, probably as a side branch to RG-I (190), however, the structural role of mannose in pectin has not been clearly demonstrated and therefore mannose is not discussed as a component in pectin here. Some of the Type II arabinogalactan is associated with arabino — galactan proteins (AGPs) (365-368), hydroxyproline-rich proteins located at the plasma membrane, cell wall, or in media surrounding suspension-cultured cells (366, 367, 369, 370). It is not always clear whether specific Type II arabinogalactan structures isolated from wall extracts are associated with RG-I, AGPs, or both. However, multiple lines of evidence show that some Type II arabinogalactan is associated with RG-I. This includes the cross reactivity of the antibody CCRC-M7 with both RG-I and arabinogalactan proteins (371). CCRC-M7 recognizes a trimer or larger of p-(1,6)-Gal carrying one or more Ara residues (372). Pectic polysaccharides from the Chenopodiaceae family including spinach (Spinacia oleracea) and sugar beet (Beta vulgaris) are esterified with phenolics such as ferulic acid (157, 291, 373), on galactose and arabinose residues that are likely substituents in the side branches of RG-I (157, 291, 292, 374).