| Boron is an essential micronutrient for higher plants. The mechanism of B uptake remains a controversial subject, and there is evidence supporting both active and passive uptake. B is taken up by the plant roots as undissociated boric acid (H3BO3) and the permeability coefficient of boric acid has not been experimentally determined across any artificial or plant membranes. In the present study we determined the permeability coefficient across artificial lipid bilayers and through the plant membranes. The experimentally determined permeability coefficient of boric acid in liposomes made out of phosphatidylcholine was 4.9 * 10-6 cm sec-1. Several factors were shown to affect the permeability of boric acid including the presence of sterols (cholesterol), phospholipid head group, length of the fatty acyl chain, pH of the medium and the presence of B complexing compounds.; The influence on B uptake of HgCl2, phloretin and DIDS, substances known to inhibit aquaporin-mediated urea transport, was examined. These inhibitors caused significant inhibition of B uptake by squash plants. The addition of other nonelectrolytes of size similar to boric acid, inhibited B uptake to various degrees. The kinetics of B uptake was severely affected by HgCl 2 but remained linear in both cases (HgCl2 treatment and control).; The experimentally determined permeability coefficient was 5.2 * 10 -8 cm sec-1 3 * 10-7 cm sec-1 , 2.4 * 10-8 cm sec-1 in microsomal vesicles, plasma membrane and vesicles without a plasma membrane fraction respectively, isolated from squash roots. Boric acid and water permeation were inhibited by 33% and 70% respectively with HgCl2 and this inhibition was reversed when 2-mercaptoethanol was added to the uptake solution. The energy of activation for B transport into plasma membrane vesicles (Ea) was found to be 10.2 kcal mole-1 which indicates that B transport is at least partially a channel mediated process.; Heterologous expression of plant aquaporins (PIP1, PIP3, NLM1) and the GlpF in Xenopus oocytes showed that PIP1 increased the transport of boric acid by 30% while the other aquaporins tested (NML1, PIP3) and the GlpF did not show any detectable transport of boric acid.; These data indicate that there are possibly two mechanisms of B uptake passive through lipid bilayers and channels mediated through aquaporins or other Hg sensitive channels. |
