Relationship of Transplasmalemma Redox Activity to Proton and Solute Transport by Roots of Zea mays

Abstract

Transplasmalemma redox activity, monitored in the presence of exogenous ferricyanide stimulates net H⁺ excretion and inhibits the uptake of K⁺ and α-aminoisobutyric acid by freshly cut or washed, apical and subapical root segments of corn (Zea mays L. cv “Seneca Chief”). H⁺ excretion is seen only following a lag of about 5 minutes after ferricyanide addition, even though the reduction of ferricyanide occurs before 5 minutes and continues linearly. Once detected, the enhanced rate of H⁺ excretion is retarded by the ATPase inhibitors N,N′-dicyclohexylcarbodiimide, diethylstilbestrol, and vanadate. A model is presented in which plasmalemma redox activity in the presence of ferricyanide involves the transport only of electrons across the plasmalemma, resulting in a depolarization of the membrane potential and activation of an H⁺-ATPase. Such a model implies that this class of redox activity does not provide an additional and independent pathway for H⁺ transport, but that the activity may be an important regulator of H⁺ excretion. The 90% inhibition of K⁺ (⁸⁶Rb⁺) uptake within 2 minutes after ferricyanide addition can be contrasted with the 5 to 15% inhibition of uptake of α-aminoisobutyric acid. The possibility exists that a portion of the K⁺ and most of the α-aminoisobutyric acid uptake inhibitions are related to the ferricyanide-induced depolarization of the membrane potential, but that the redox state of some component of the K⁺ uptake system may also regulate K⁺ fluxes.