Defect in Root-Shoot Translocation of Boron in Arabidopsis thaliana Mutant bor 1-1

Compartmental analysis of B in wild type and bor 1-1 mutant plants of Arabidopsis thaliana were carried out to elucidate reports that the mutant bor 1-1 has a defect(s) in absorption andlor translocation of B, which results in reduced B contents in its shoot (Noguchi et al., 1997b). The plants were grown hydroponically and B contents in water soluble fractions (cell sap) and in water insoluble residues (WIR) were determined. At low B supply (3 μmol · L-1), in shoots of bor 1-1 mutant plants, concentrations of water soluble B (B in cell sap) were more than 20-fold lower than in wild type plants. The difference in B contents was less distinct in WIRs. At higher B supply (30 and 100 μmol · L-1), there were only marginal or no differences in B contents of both fractions between the mutant and wild type plants. Consequently, the reduced B contents in shoots of the mutant plants at low B supply were due mainly to reduced B contents in the water soluble fractions (cell sap). The results suggest that the bor 1-1 mutation has little or no effect on the binding of B in the cell wall, since B in WIRs mainly represents cell wall bound B. In wild type plants, concentrations of B in xylem exudates were about 3.4-fold higher than those in root cell saps at low B supply (3 μmol · L-1), whereas B concentrations were similar in both fractions when B was sufficiently supplied (30 μmol · L-1), suggesting that A. thaliana plants possess mechanisms to concentrate B in the process of root-to-shoot transport only at low B supply. In bor 1-1 mutant plants, concentrations of B in shoots, xylem exudates and root cell saps were about 25, 30 and 65%, respectively, of those in wild type plants at low B supply, whereas B concentrations were similar in both genotypes under B sufficient conditions. These data suggest that the major defect in the bor 1-1 mutant is at the B concentrating mechanism in the root-to-shoot transport of B that functions only at low B supply.