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). 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) and cyanobacteria (Bonilla et al., 1990; Garcia-Gonzalez et al., 1991; Bonilla et al., 1997). Recently, B was shown to stimulate growth in yeast (Bennett et al., 1999) and to be essential for zebrafish (DANIO RERIO) (Eckhert and Rowe, 1999; Rowe and Eckhert, 1999) and possibly for trout (ONCORHYNCHUS MYKISS) (Eckhert, 1998; Rowe et al., 1998), frogs (XENOPUS LAEVIS) (Fort et al., 1998) and mouse (Lanoue et al., 2000). There is also preliminary evidence to suggest that B has at least a beneficial role in humans (Nielsen, 2000). 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