Evidence for channel mediated transport of boric acid in squash (Cucurbita pepo)

Boron (B) is taken up by plant roots as undissociated boric acid which is a non-electrolyte of similar size to urea and other non-electrolytes. In animal systems, non-electrolytes are transported across biological membranes through aquaporins or through non-aquaporin channels. In artificial lipids boric acid is known to diffuse directly through the lipid bilayer at a rate that is determined by lipid composition. A possible role for channel proteins in in-vitro B uptake is suggested by recent work in which B uptake into isolated membrane vesicles was inhibited by channel blockers and by demonstration that the expression of the plant channel protein PIP1 in Xenopus oocytes increases boric acid uptake by 30%. This study examines whether B transport is a channel-mediated process in intact plants. In the presence of the channel inhibitors HgCl2, phloretin, and DIDS, B uptake by squash plants was reduced by 40-90% by HgCl2 (as HgCl2 varied from 50 muM to 1 mM), 44% by phloretin (250 muM) and 58% by DIDS (250 muM). The effect of Hg ions on B uptake was reversed by 2-mercaptoethanol. The addition of other non-electrolytes in size ranges similar to boric acid inhibited B uptake to various degrees. Addition of urea resulted in 54% decrease in B uptake, while, acetamide, formamide, thiourea and glycerol inhibited uptake by 50, 35, 53 and 44%, respectively. The effect of HgCl2 on B uptake was greater at high B concentrations than at low B concentrations. These data and information from in-vivo studies suggest two possible mechanisms of B uptake: passive diffusion through lipid bilayers and channel-mediated transport.