Boron in Plant Biology

Abstract: 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 stimulated by the discovery that B is essential for animals, a discovery that will not only encourage the participation of a wider cadre of scientists but will refocus the efforts of plant biologists toward a determination of roles for B outside the plant cell wall. Determination of the function of B in biology and of the mechanisms of B uptake in biological systems, is essential to our understanding and management of B deficiency and toxicity in plants and animals in both agricultural and natural environments. Through an analysis of existing data and the development of new hypotheses, this review aims to provide a vision of the future of research into the biology of boron.