Borate-Rhamnogalacturonan II Bonding Reinforced by Ca(2+) 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 Ca(2+). The complex formed in the presence of Ca(2+) was more stable than that without Ca(2+). A naturally occurring boron (B)-RG-II complex isolated from radish (Raphanus sativus L. cv Aokubi-daikon) root contained equimolar amounts of Ca(2+) and B. Removal of the Ca(2+) by trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid induced cleavage of the complex into monomeric RG-II. These data suggest that Ca(2+) 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 Ca(2+) but only 13% of the B and 22% of the pectic polysaccharides. The remaining Ca(2+) 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 Ca(2+)(,) 78% of B, and 49% of pectic polysaccharides. These results suggest that not only Ca(2+) but also borate and Ca(2+) cross-linking in the RG-II region retain so-called chelator-soluble pectic polysaccharides in cell walls.