We examined concentrations of boron (B) and dimerization of rhamnogalacturonan II (RG‐II), a B‐binding polysaccharide, in the cell wall of a low‐B sensitive mutant of Arabidopsis thaliana, bor1‐1, to investigate possible effects of the bor1‐1 mutation on the biochemical form of pectins in the cell wall. In the bor1‐1 mutant, B concentrations in the cell wall from shoots were lower than those in the wild type at low B supply, whereas they were similar at sufficient B supply. The amount of B present as borate ester of the RG‐II dimer (dRG‐II‐B) in the bor1‐1 mutant was lower than that in the wild type at low B supply. In the wild type, about 90 % of RG‐II was present as dRG‐II‐B, both, at low and sufficient B supply. In the bor1‐1 mutant, about 60 % of RG‐II was in its monomeric form (mRG‐II) at low B supply, whereas more than 85 % of it was present as dRG‐II‐B at sufficient B supply. However, similar as the wild type, mRG‐II derived from the bor1‐1 mutant was able to form dRG‐II‐B in vitro in the presence of borate and lead. Sugar composition of cell wall fractions was similar in both genotypes. These results suggest that the polysaccharide composition in the cell wall was not strongly affected by the bor1‐1 mutation. The observed difference in dimerization of RG‐II at low B supply is most likely due to a reduced B concentration in the shoots of the bor1‐1 mutant.
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Noguchi et al. (2003) studied this question.