Boron nutrition of cultured tobacco BY-2 cells .2. Characterization of the boron-polysaccharide complex

In cultured tobacco BY-2 cells, more than 90% of the cellular boron (B) occurs in the cell wall and a negligible amount of B is detected in the membrane fraction. Nearly 80% of the cell wall B binds to rhamnogalacturonan II (RG-II) to form a borate-dimeric RG-II complex. Monomeric RG-II is not detected in the cell wall, but it is detected in the extracellular polysaccharides, The complex is reconstituted spontaneously in vitro simply by mixing monomeric RG-II and boric acid at pH 4. Germanic acid, which partially substitutes for B in the growth of the B-deprived plants, also induces dimerization of RG-II, These results suggests that B may fulfill its essential function as forming the B-RG-II complex in cell walls.