1e:T4de,The walls of suspension-cultured Chenopodium album L. cells grown continually for more than 1 year on B-deficient medium contained monomeric rhamnogalacturonan II (mRG-II) but not the borate ester cross-linked RG II dimer (dRG-II-B). The walls of these cells had an increased size limit for dextran permeation, which is a measure of wall pore size. Adding boric acid to growing B-deficient cells resulted in B binding to the wall, the formation of dRG-II-B from mRG-II, and a reduction in wall pore size within 10 min. The wall pore size of denatured B-grown cells was increased by treatment at pH 2+, La³⁺, and Ca²⁺ increase dimer formationin vitro in a concentration- and pH-dependent manner. Pb²⁺ is the most effective cation. The dimer accounts for 55% of the RG-II when the monomer (0.5 mm) is treated for 5 min at pH 3.5 with boric acid (1 mm) and Pb²⁺ (0.5 mm); at pH 5 the rate of conversion is somewhat slower. Hg²⁺ does not increase the rate of dimer formation. A cation's charge density and its ability to form a coordination complex with RG-II, in addition to steric factors, may regulate the rate and stability of dimer formation in vitro. Our data provide evidence that the structure of RG-II itself determines which apiofuranosyl residues are esterified with borate and that in the presence of boric acid and certain cations, two RG-II monomers self-assemble to form a dimer.

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