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...

B deficiency results in a rapid inhibition of plant growth, and yet the form and function of B in plants remains unclear. In this paper we provide evidence that B is chemically localized and strudurally important in the cell wall of plants. The localization and chemical fractionation of B was followed in squash plants (Curcurbifa pepo 1.) and cultured tobacco cells (Nicofiana fabacum) grown in Breplete or B-deficient medium. As squash plants and cultured tobacco cells became deficient, an increasingly large proportion of cellular B was found to be localized in the cell wall. Cytoplasmic B concentrations were reduced to essentially zero as plants became deficient, whereas cell wall B concentration remained ator above 10 pg B/g cell wall dry weight in all experiments. Chemical and enzymic fractionation studies suggest that the majority of cell B is associated with pectins within the cell wall. Physical analysis of Bdeficient tissue indicates that cell wall plastic extensibility is greatly reduced under B deficiency, and anatomical observations indicate that B deficiency impairs normal cell elongation in growing plant tissue. In plants in which B deficiency had inhibited all plant growth, tissues remained green and did not show any additional visible symptoms for at least 1 week with no additional 6. This occurred even though cytoplasmic B had been reduced to extremely low levels (~0 . 2 pg/g). This suggests that B in these species is largely associated with the cell wall and that any cytoplasmic role for B is satisfied by very low concentrations of B. The localization of B in the cell wall, its association with cell wall pectins, and the contingent effects of B on cell wall extensibility suggest that B plays a critical, although poorly defined, role in the cell wall strudure of higher plants.

B is an essential micronutrient for higher plants and its deficiency results in the rapid inhibition of plant growth. The uptake, transport, and function of B in plants appears to be dependent on the formation of B complexes. B uptake, for example, is a passive, nonmetabolic process determined in part by the formation of nonexchangeable B complexes within the cytoplasm and cell wall .When B is present at low to adequate concentrations, the Historically, B has been considered a phloem-immobile element in plants (Oertli and Richardson, 1970). The occurrence of B-deficiency symptoms in young, growing tissue also indicates that B is not readily retranslocated within the plant. Recently, however, it has been demonstrated that B is phloem-mobile in species that translocate significant amounts of sorbitol in the phloem . Based upon these results and in vitro evidence it was proposed that the mobility of B in these species is mediated by the formation of 8-sorbitol complexes . Subsequently, we have found that B is also phloemmobile in species that translocate significant amounts of mannitol or dulcitol, further suggesting that B complexes with sorbitol, mannitol, or dulcitol may mediate the phloem mobility of B.The identification of B complexes is central to an improved understanding of B physiology. In this report we describe the isolation and characterization of soluble B complexes from phloem sap or of nectar from phloem-fed extrafloral nectaries. The physiological significance of these results is discussed. MATERIALS A N D M E T H O D S majority of-cellular B (>95%) is associated with cell wallPhloem mobility of foliar-applied isotopic B was demonpectins where it may be critica1 for normal cell wall expanstrated in celery (Apium gruveolens L.) according to the sion . Recently, B was found to be methodology of Brown and Hu (1996). Celery was grown present as a B-rhamnogalacturonan complex within plant in 11-L pots filled with a perlitesuper soil mixture (2:1, cell walls (Ishii and Matsunaga, 1996; Kobayashi et al., v/v). Each pot contained 2 plants and there were 16 repli-1996; ONeill et al., 1996), and the B requirement of a cate plants. The plants were grown in a greenhouse with particular plant species has been shown to correlate with day/night temperatures of 27/17"C. Plants were fed the pectin content of the cell wall (Matoh et al., 1993; weekly with one-half-strength complete Hoagland solution al., 1996). (Hoagland and Arnon, 1950). After 2 months of growth three leaflets from leaf number 4 (counting from bottom) on eight replicate plants were immersed for 10 s in 50 mM[lOB]-enriched (95.91 % IOB) boric acid solution with 0.05% (v/v) surfactant L-77 (Loveland Industries, Inc., Greeley, CO). The leaves were then gently shaken and blotted to Abbreviations: DHB, 2,5-dihydroxybenzoic acid; MALDI-ETMS,

Spatial imaging of cadmium (Cd) in the hyperaccumulator Sedum alfredii was investigated in vivo by laser ablation inductively coupled plasma mass spectrometry and x-ray microfluorescence imaging. Preferential Cd accumulation in the pith and cortex was observed in stems of the Cd hyperaccumulating ecotype (HE), whereas Cd was restricted to the vascular bundles in its contrasting nonhyperaccumulating ecotype. Cd concentrations of up to 15,000 mg g 21 were measured in the pith cells, which was many fold higher than the concentrations in the stem epidermis and vascular bundles in the HE plants. In the leaves of the HE, Cd was mainly localized to the mesophyll and vascular cells rather than the epidermis. The distribution pattern of Cd in both stems and leaves of the HE was very similar to calcium but not zinc, irrespective of Cd exposure levels. Extended x-ray absorption fine structure spectroscopy analysis showed that Cd in the stems and leaves of the HE was mainly associated with oxygen ligands, and a larger proportion (about 70% in leaves and 47% in stems) of Cd was bound with malic acid, which was the major organic acid in the shoots of the plants. These results indicate that a majority of Cd in HE accumulates in the parenchyma cells, especially in stems, and is likely associated with calcium pathways and bound with organic acid (malate), which is indicative of a critical role of vacuolar sequestration of Cd in the HE S. alfredii.

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