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Characterization of the Electrogenicity of Soybean (Glycine max L.) Roots ¹

ATP Dependence and Effect of ATPase Inhibitors

Section of Plant Biology, Division of Biological Sciences, Cornell University, Ithaca, New York 14853

The ATP-dependence of the membrane potential of soybean (Glycine max L. cv Williams '79) roots was determined by correlating the decrease in ATP levels with the decrease in the membrane potential caused by carbonyl cyanide m-chlorophenylhydrazone (CCCP) or cyanide. The effects of the ATPase inhibitors, vanadate and fluoride, were also examined in an attempt to establish the role of a plasma membrane localized electrogenic proton pumping ATPase in generating the membrane potential.

The membrane potential of intact seedling roots bathed in artificial pond water was –167 ± 27 millivolts (n = 130); the ATP level was 161 ± 27 nanomoles per gram fresh weight (n = 102) when measured using rapid acid homogenization of the intact roots. Depolarization to the diffusion potential (–100 millivolts) occurred at about 90 nanomoles ATP per gram fresh weight, 50 to 70% of the control ATP level.

Cyanide-induced ATP loss and membrane potential depolarization occurred at similar rates, but CCCP-induced ATP loss was slower than the rate of depolarization. CCCP may initially depolarize the membrane potential by increasing the proton permeability of the plasma membrane and short-circuiting the putative electrogenic proton pump. Vanadate and NaF caused a relatively slight depolarization of the membrane potential. Both inhibited microsomal ATPase activity substantially at the concentrations causing the depolarization.