Pectic Polysaccharides Pectins are polysaccharides that contain a high proportion of 1,4-linked α-D-galactosyluronic acid residues (Ridley et al. 2001). These polysaccharides are typically solublized by treating walls with aqueous buffers, chelating agents, and dilute mineral acids. Pectins are also solubilized by treating walls with homogeneous endoglycanses (e.g. endopolygalacturonase). All primary walls contain three pectic polysaccharides - homogalacturonan (HG), rhamnogalacturonan I (RG-I), and rhamnogalacturonan II (RG-II) (O'Neill et al 1990).

Homogalacturonan Homogalacturonan (HGA) is composed of 1,4 linked α-D-galactosyluronic acid residues (GalpA). Some of the carboxyl groups are methyl esterified. The GalpA residues may also be O-acetylated at C2 and C3. HGA with a low degree of methyl esterification readily forms ordered structures (gels) in the presence of calcium ions. Five monoclonal antibodies (JIM5, JIM7, LM7, PAM1, and 2F2) that recognize portions of homogalacturonan are currently available (Willats and Knox, 2003). JIM5 and JIM7 recognizes different patterns of methyl esterification on HG; the JIM5 epitope contains low or no methyl esters, whereas the JIM7 epitope is more heavily methyl esterified. PAM1 binds to block-wise de-esterified HG, while LM7 binds to non-block-wise de-esterified HG. The 2F4 monoclonal antibody is believed to recognize non-esterified HGA that is cross-linked by calcium. Back to Top

Rhamnogalacturonan I Rhamnogalacturonans are a family of closely related polysaccharides that have a backbone composed of the repeating disaccharide 4)-α-D-GalpA-(1,2)-α-L-Rhap-(1,. Between 20 and 80% of the Rhap residues are, depending on plant source and method of isolation, substituted at C4 with neutral and acidic oligosaccharides (Ridley et al., 2001). These oligosaccharides predominantly contain branched and linear α-L-Araf and β-D-Galp residues (see figure below).

Four monoclonal antibodies (LM5, LM6, CCRC-M2, and CCRC-M7) recognize different portions of the RG-I molecule are available (Willats and Knox, 2003). LM5 and LM6 recognize 1,4-linked β-galactan and 1,5-linked α-arabinan, respectively. CCRC-M7 recognizes arabinosylated 1,6-linked β-galactan. A fourth monoclonal antibody CCRC-M2 is believed to recognize a α-L-fucosylated epitope on RG-I, although this has not yet been demonstrated unequivocally. Back to Top

Rhamnogalacturonan Backbone The mucilage that surrounds Arabidopsis seeds is a polysaccharide composed of a rhamnogalacturonan backbone that has no or very few oligosaccharide side chains (Penfield et al 2001). No monoclonal antibodies that recognize this structure are currently available. Back to Top

Xylogalacturonan The walls of specific plant tissue including soybean and pea seeds, apple fruit, carrot callus, and pine pollen contain xyloglacturonans (O'Neill et al. 1990). This polysaacharide has a backbone composed of 1,4-linked α-D-GalpA residues. Some of the GalpA residues are substituted at C3 with β-Xylp residues (see figure). One monclonal antibody (LM8) is available that recognizes this polysaccharide. Back to Top

Rhamnogalacturonan II The generation of monoclonal antibodies that recognize RG-II is described at the CCRC Plant Cell Wall web site.

References O'Neill et al (1990). The pectic polysaccharides of primary cell walls. In Methods in Plant Biochemistry, (PM Dey ed), Academic Press, pp. 415-451. Ridley et al. (2001). Pectins: structure, biosynthesis, and oligogalacturonide-related signalling. Phytochemistry 57: 929-967. Penfield et al (2001). MYB61 is required for mucilage deposition and extrusion in the Arabidopsis seed coat. Plant Cell 13: 2777-2791. Willats and Knox (2003). Molecules in context: probes for cell wall analysis. In The Plant Cell Wall (JKC Rose ed), Blackwell, pp. 92-110. Back to Top

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A National Science Foundation-funded (Grant No DBI-0421683) research project at The Complex Carbohydrate Research Center of The University of Georgia