Update on boron in higher plants -: Uptake, primary translocation and compartmentation

This review focuses on the uptake and primary translocation of boron (B), as well as on the subcellular compartmentation of B and its role in cell walls of higher plants. B uptake occurs via passive diffusion across the lipid bilayer, facilitated transport through major intrinsic proteins (MIPs), and energy-dependent transport through a high affinity uptake system. Whereas the first two represent passive uptake systems, which are constitutively present, the latter is induced by low B supply and is able to establish a concentration gradient for B between the root symplasm and the external medium. At high B supply, a substantial retention of B can be observed at xylem loading, and passive processes are most likely responsible for that. At low B supply, another energy-dependent high affinity transport system for B seems to be induced which establishes an additional concentration gradient between root symplasm and the xylem. The possible significance of all these processes at various B supplies is discussed. The role of soluble B complexes in uptake and primary translocation of B has been evaluated, but the few data available do not allow comprehensive conclusions to be drawn. In any case, there are no indications that soluble B complexes play a major role in either uptake or primary translocation of B. The subcellular compartmentation of B still remains a matter of controversy, but it is unequivocally clear that B is present in all subcellular compartments (apoplasm, cell wall, cytosol and vacuole). The relative distribution of B between these is dependent on plant species and experimental conditions and may vary greatly. Recent results on the well-established role of B in cell walls are summarized and their physiological significance discussed.