Abstract
Intracellular localization of boron (B) was examined and it was found that B both in a water-soluble and water-insoluble form occurs mainly in cell walls. Almost all the cell-wall bound B is in a form of a B-dimeric rhamnogalacturonan II (RG-II) complex, in which B cross-links two RG-II chains through borate-diol ester bonding. The B-RG-II complex ubiquitously occurs in higher plant cell walls. In B-deficient cells, cell walls swelled and monomeric RG-II was detected. The calcium ion is also a native constituent of the B-RG-II complex, and this ion works to strengthen the borate-diol ester bonding. Sequential extraction of Ca2+ from cell walls revealed that only a limited portion of the cell-wall Ca2+ is responsible for retaining the pectic polysaccharides in cell walls. The Ca2+ responsible is that associated with the RG-II region. These results suggest that cross-linking by B and Ca2+ at the RG-II region anchors the pectic network into cell walls and arranges the pectic chains to form appropriate coordinate bonding with Ca2+ in polygalacturonic acid regions.
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