The pore size of non-graminaceous plant cell walls is rapidly decreased byborate ester cross-linking of the pectic polysaccharide rhamnogalacturonanII

The walls of suspension-cultured Chenopodium album L. cells grown continual ly for more than 1 year on B-deficient medium contained monomeric rhamnogal acturonan II (mRG-II) but not the berate ester cross-linked RG II dimer (dR G-II-B). The walls of these cells had an increased size limit for dextran p ermeation, which is a measure of wall pore size. Adding boric acid to growi ng B-deficient cells resulted in B binding to the wall, the formation of dR G-II-B from mRG-II, and a reduction in wall pore size within an min. The wa ll pore size of denatured B-grown cells was increased by treatment at pH le ss than or equal to 2.0 or by treatment with Ca2+-chelating agents. The aci d-mediated increase in wall pore size was prevented by boric acid alone at pH 2.0 and by boric acid together with Ca2+, but not by Na+ or Mg2+ ions at pH 1.5. The Ca2+-chelator-mediated increase in pore size was partially red uced by boric acid. Our results suggest that B-mediated cross-linking of RG -II in the walls of living plant cells generates a pectin network with a de creased size exclusion limit for polymers. The formation, stability, and po ssible functions of a berate ester cross-linked pectic network in the prima ry walls of nongraminaceous plant cells are discussed.