Boron Induces Hyperpolarization of Sunflower Root Cell Membranes and Increases Membrane Permeability to K+

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Abstract
Although many studies have alluded to a role for boron (B) in membrane function, there is little evidence for a direct effect of B on the plasmalemma of higher plant cells. These studies were conducted to demonstrate, by electrophysiological techniques, a direct effect of B on the membrane potential (Em) of sunflower (Helianthus annuus [L.], cv Mammoth Grey Stripe) root tip cells and to determine if the response to B occurs rapidly enough to account for the previously observed effects of B on ion uptake. By inserting a glass microelectrode into an individual cell in the root tip, the Em of the cell was determined in basal salt medium (BSM), pH 6.0. The perfusion solution surrounding the root tissue was then changed to BSM + 50 micromolar H3BO3, pH 6.0. The exposure to B induced a significant plasmalemma hyperpolarization in sunflower root cells within 20 minutes. After just 3 minutes of exposure to B, the change in Em was already significantly different from the negligible change in Em observed over time in root cells never exposed to B. Membrane hyperpolarization could be caused by a stimulation of the proton pump or by a change in the conductance of one or more permeable ions. Since B has been shown to affect K+ uptake by plants, the electrophysiological techniques described above were used to determine if B has an effect on membrane permeability to K+, and could thereby lead to an increased diffusion potential. When sunflower root tips were pretreated in 50 micromolar B for 2 hours, cell membranes exhibited a significantly greater depolarization with each 10-fold increase in external [K+] than minus-B cells. Subsequent studies demonstrated that the depolarization due to increased external [K+] was also significantly greater when tissue was exposed to B at the same time as the 10-fold increase in [K+], indicating that the effect of B on K+ permeability was immediate. Analysis of sunflower root tips demonstrated that treatment in 50 micromolar B caused a significantly greater accumulation of K+ after 48 hours. The B-induced increase in K+ uptake may cause a subsequent stimulation of the H+-ATPase (proton pump) and lead to the observed hyperpolarization of root cell membranes. Alternatively, B may stimulate the proton pump, with the subsequent hyperpolarization resulting in an increased driving force for K+ influx.