The interest of biologists in boron (B) has largely been focused on its role in plants for which B was established as essential in 1923 (Warington, 1923[296]). Evidence that B has a biological role in other organisms was first indicated by the establishment of essentiality of B for diatoms (Smyth and Dugger, 1981[296]) and cyanobacteria (Bonilla et al., 1990[296]; Garcia‐Gonzalez et al., 1991[296]; Bonilla et al., 1997[296]). Recently, B was shown to stimulate growth in yeast (Bennett et al., 1999[296]) and to be essential for zebrafish (Danio rerio) (Eckhert and Rowe, 1999[296]; Rowe and Eckhert, 1999[296]) and possibly for trout (Oncorhynchus mykiss) (Eckhert, 1998[296]; Rowe et al., 1998[296]), frogs (Xenopus laevis) (Fort et al., 1998[296]) and mouse (Lanoue et al., 2000[296]). There is also preliminary evidence to suggest that B has at least a beneficial role in humans (Nielsen, 2000[296]). While research into the role of B in plants has been ongoing for 80 years it has only been in the past 5 years that the first function of B in plants has been defined. Boron is now known to be essential for cell wall structure and function, likely through its role as a stabilizer of the cell wall pectic network and subsequent regulation of cell wall pore size. A role for B in plant cell walls, however, is inadequate to explain all of the effects of B deficiency seen in plants. The suggestion that B plays a broader role in biology is supported by the discovery that B is essential for animals where a cellulose‐rich cell wall is not present. Careful consideration of the physical and chemical properties of B in biological systems, and of the experimental data from both plants and animals suggests that B plays a critical role in membrane structure and hence function. Verification of B association with membranes would represent an important advance in modern biology. For several decades there has been uncertainty as to the mechanisms of B uptake and transport within plants. This uncertainty has been driven by a lack of adequate methodology to measure membrane fluxes of B at physiologically relevant concentrations. Recent experimentation provides the first direct measurement of membrane permeability of B and illustrates that passive B permeation contributes sufficient B at adequate levels of B supply, but would be inadequate at conditions of marginal B supply. The hypothesis that an active, carrier mediated process is involved in B uptake at low B supply is supported by research demonstrating that B uptake can be stimulated by B deprivation, that uptake rates follow a Michaelis‐Menton kinetics, and can be inhibited by application of metabolic inhibitors. Since the mechanisms of element uptake are generally conserved between species, an understanding of the processes of B uptake is relevant to studies in both plants and animals. The study of B in plant biology has progressed markedly in the last decade and we are clearly on the cusp of additional, significant discoveries. Research in this field will be greatly stimulat...

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.

The B pools in the roots and the characteristics of B uptake and its loading into the xylem were investi-gated in sunflower (Helianthus annuus L.) plants precultured with high (100 M) or low (1 M) 11 B supply. In order to study B fluxes and their dependence on root metabolic activity, short-term treatments with differential 10 B supply in combination with metabolic inhibition treatments (50 M 2,4-dinitrophenol; root zone temperature of 7˚C) or with no further treatment (control) were carried out. Subsequently, xylem exudate was collected, and roots were harvested and fractionated into two B pools that differed in their water-solubility as well as in their exchangeability. The exchange or release of 11 B initially present during the 3 h treatment was maximal at 18% in the cell wall pool, whilst it was up to 94% in the symplasmic pool. All observed alterations in the cell wall-bound B can be explained by passive processes. Control plants precultured with high B supply showed a linear response of the 10 B concentrations in the root cell sap and in the xylem exudate to the differential short-term 10B supply, and this was not affected by the metabolic inhibition treatments. In the control plants precultured with low B supply, the response of the 10 B concentrations in the root cell sap and xylem exudate to the differential short-term 10 B supply appeared to be a com-bination of a saturable and a linear component. The metabolic inhibition treatments turned off the saturable compo-nent and the response became linear. In summary, the results suggest that B uptake into the root symplasm, as well as xylem loading, are performed by two transport mechanisms, with the linear components representing B transport by passive diffusion. The saturable components may represent unknown carrier- or channel-mediated transport of B, which is dependent on metabolic energy.

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