Rhamnogalacturonan-II, a Pectic Polysaccharide in the Walls of Growing Plant Cell, Forms a Dimer That Is Covalently Cross-linked by a Borate Ester

Abstract

Rhamnogalacturonan II (RG-II) is a structurally complex pectic polysaccharide present in the walls of growing plant cells. We now report that RG-II, released by endopolygalacturonase treatment of the walls of suspension-cultured sycamore cells and etiolated pea stems, exists mainly as a dimer that is cross-linked by a borate ester. The borate ester is completely hydrolyzed at room temperature within 30 min at pH 1, partially hydrolyzed between pH 2 and 4, and stable above pH 4. The dimer is formed in vitro between pH 2.4 and 6. 2 by treating monomeric RG-II (0.5 mM) with boric acid (1.2 mM); the dimer formed after 24 h at pH 3.4 and 5.0 accounts for approximately 30 and approximately 5%, respectively, of the RG-II. In contrast, the dimer accounts for approximately 80 and approximately 54% of the RG-II when the monomer is treated for 24 h at pH 3.4 and 5.0, respectively, with boric acid and 0.5 m Sr2+, Pb2+, or Ba2+. The amount of dimer formed at pH 3.4 or 5.0 is not increased by addition of 0.5 mM Ca2+, Cd2+, Cu2+, Mg2+, Ni2+, and Zn2+. Steric considerations appear to regulate dimer formation since those divalent cations that enhance dimer formation have an ionic radius >1.1 A. Our data suggest that the borate ester is located on C-2 and C-3 of two of the four 3'-linked apiosyl residues of dimeric RG-II. Our results, taken together with the results of two previous studies (Kobayashi, M., Matoh, T., and Azuma, J.-I. (1996) Plant Physiol. 110, 1017-1020; Ishii, T., and Matsunaga, T. (1996) Carbohydr. Res. 284, 1-9) provide substantial evidence that this plant cell wall pectic polysaccharide is covalently cross-linked.

Functional Characterization of UDP-apiose Synthases from Bryophytes and Green Algae Provides Insight into the Appearance of Apiose-containing Glycans during Plant Evolution

“…Apiose links two side chains (A and B) to the galacturonan backbone of RG-II. The apiosyl residues of side chain A in two RG-II molecules are cross-linked by a borate diester to form the RG-II dimer (7)(8)(9)(10). At least 90% of the RG-II in primary walls exists as a dimer (11), and a reduction in the extent of RG-II cross-linking typically results in the formation of abnormal cell walls (12).…”

Loss of Highly Branched Arabinans and Debranching of Rhamnogalacturonan I Accompany Loss of Firm Texture and Cell Separation during Prolonged Storage of Apple

“…Although the HGs are generally unbranched, xylosyl residues may be attached at the O-3 positions of the GalA residues to form xylogalacturonans (Schols et al, 1995). Four complex side chains may be added in a precise configuration to form RG IIs (O'Neill et al, 1996). RG IIs may dimerize through formation of boron di-diesters, which promote normal shoot development O'Neill et al, 2001) and contributes to the tensile strength of the wall (Ryden et al, 2003).…”

The Gene Expression and Enzyme Activity of Plant 3-Deoxy-D-Manno-2-Octulosonic Acid-8-Phosphate Synthase Are Preferentially Associated with Cell Division in a Cell Cycle-Dependent Manner

“…Despite a complex structure, RG-II is evolutionarily conserved in the plant kingdom because it is present in the primary cell wall of all higher plants predominantly in the form of a dimer that is crosslinked by a borate di-ester (dRG-II-B) between two apiosyl residues (O'Neill et al, 1996;Ishii et al, 1999). Nevertheless the precise function of RG-II remains to be fully established.…”

“…Eleven different glycosyl residues are present in RG-II. Among these residues are the seldom-observed sugars: aceric acid, apiose, 3-deoxy-d-lyxo-heptulosonic acid, and 3-deoxy-d-manno-octulosonic acid (Kdo) (O'Neill et al, 1996;Pérez et al, 2000). It was long believed that Kdo was synthesized only by Gram-negative bacteria as a component of the lipopolysaccharides present in the outer leaflet of the outer membrane (Rick, 1987).…”

“…The cell plate itself is formed by the fusion of Golgi-derived vesicles transported by the phragmoplast microtubules during cytokinesis, the final stage of cell division (Cosgrove, 1997;Verma, 2002). Hence, assuming that Kdo is solely found in the RG-II pectic fraction of the plant cell wall (York et al, 1985a;O'Neill et al, 1996;Pérez et al, 2000), it is expected that its synthesis may be associated to the formation of the cell plate and putatively to cell cycle progression especially during mitosis. Nevertheless, the coupling to the cell cycle of gene expression and enzymatic activities involved in cell wall pectic polysaccharides are still poorly documented.…”