Borate-Rhamnogalacturonan II Bonding Reinforced by Ca2+ Retains Pectic Polysaccharides in Higher-Plant Cell Walls

The extent of in vitro formation of the borate-dimeric-rhamnogalacturonan II (RG-II) complex was stimulated by Ca²⁺. The complex formed in the presence of Ca²⁺ was more stable than that without Ca²⁺. A naturally occurring boron (B)-RG-II complex isolated from radish (Raphanus sativus L. cv Aokubi-daikon) root contained equimolar amounts of Ca²⁺ and B. Removal of the Ca²⁺ by trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid induced cleavage of the complex into monomeric RG-II. These data suggest that Ca²⁺ is a normal component of the B-RG-II complex. Washing the crude cell walls of radish roots with a 1.5% (w/v) sodium dodecyl sulfate solution, pH 6.5, released 98% of the tissue Ca²⁺ but only 13% of the B and 22% of the pectic polysaccharides. The remaining Ca²⁺ was associated with RG-II. Extraction of the sodium dodecyl sulfate-washed cell walls with 50 mm trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, pH 6.5, removed the remaining Ca²⁺_, 78% of B, and 49% of pectic polysaccharides. These results suggest that not only Ca²⁺ but also borate and Ca²⁺ cross-linking in the RG-II region retain so-called chelator-soluble pectic polysaccharides in cell walls.Boron (B) is an essential element for higher plant growth, although its primary function is not known (Loomis and Durst, 1992). Determining the sites of B in cells is required to identify its function. In cultured tobacco cells more than 80% of cellular B is in the cell wall (Matoh et al., 1993), whereas the membrane fraction (Kobayashi et al., 1997) and protoplasts (Matoh et al., 1992) do not contain a significant amount of B. In radish (Raphanus sativus L. cv Aokubi-daikon) root cell walls, B cross-links two RG-II regions of pectic polysaccharides through a borate-diol ester (Kobayashi et al., 1995, 1996). The association of B with RG-II has been confirmed in sugar beet (Ishii and Matsunaga, 1996), bamboo (Kaneko et al., 1997), sycamore and pea (O''Neill et al., 1996), and red wine (Pellerin et al., 1996). In cultured tobacco cells the B associated with RG-II accounts for about 80% of the cell wall B (Kobayashi et al., 1997) and RG-II may be the exclusive carrier of B in higher plant cell walls (Matoh et al., 1996). Germanic acid, which partly substitutes for B in the growth of the B-deprived plants (Skok, 1957), also cross-links two RG-II chains (Kobayashi et al., 1997). These results suggest that the physiological role of B is to cross-link cell wall pectic polysaccharides in the RG-II region and thereby form a pectic network.It is believed that in the cell wall pectic polysaccharides are cross-linked with Ca²⁺, which binds to carboxyl groups of the polygalacturonic acid regions (Jarvis, 1984). Thus, the ability of B and Ca²⁺ to cross-link cell wall pectic polysaccharides needs to be evaluated. In this report we describe the B-RG-II complex of radish root and the role of B-RG-II and Ca²⁺ in the formation of a pectic network.