Effect of dichlorophenolindophenol, dichlorophenolindophenol-sulfonate, and cytochromec on redox capacity and simultaneous net H⁺/K⁺ fluxes in aeroponically grown seedling roots of sunflower (Helianthus annuus L.): New evidence for a plasma membrane CN⁻-resistant redox chain

Summary

Excised roots from axenically grown sunflower seedlings reduced or oxidized exogenously added 2,6-dichlorophenolindophenol (DCIP), DCIP-sulfonate (DCIP-S), and cytochromec, and affected simultaneous H⁺/K⁺ net fluxes. Experiments were performed with nonpretreated “living” and CN⁻-pretreated “poisoned” roots (control and CN⁻-roots). CN⁻-roots showed no H⁺/K⁺ net flux activity but still affected the redox state of the compounds tested. The hydrophobic electron acceptor DCIP decreased the rate of H⁺ efflux in control roots with extension of the maximum rate and optimal pH ranges, then the total net H⁺ efflux (∝H⁺) equalled that of the roots without DCIP. The simultaneously measured K⁺ influx rate was first inhibited, then inverted into efflux, and finally influx recovered to low rates. This effect could not be due to uptake of the negatively charged DCIP, but due to the lower H⁺ efflux and the transmembrane electron efflux caused by DCIP, which would depolarize the membrane and open outward K⁺ channels. The different H⁺ efflux kinetics characteristics, together with the small but significant DCIP reduction by CN⁻-roots were taken as evidence that an alternative CN⁻-resistant redox chain in the plasma membrane was involved in DCIP reduction. The hydrophilic electron acceptor DCIP-S enhanced both H⁺ and K⁺ flux rates by control roots. DCIP-S was not reduced, but slightly oxidized by control roots, after a lag, while CN⁻-roots did not significantly oxidize or reduce DCIP-S. Perhaps the hydrophobic DCIP could have access to and drain electrons from an intermediate carrier deep inside the membrane, to which the hydrophilic DCIP-S could not penetrate. Also cytochromec enhanced ∝H⁺ and ∝K⁺, consistent with the involvement of the CN⁻-resistant redox chain. Control roots did not reduce but oxidize cytochromec after a 15 min lag, and CN⁻-roots doubled the rate of cytochromec oxidation without any lag. NADH in the medium spontaneously reduced cytochromec, but control or CN⁻-roots oxidized cytochromec, despite of the presence of NADH. In this case CN⁻-roots were less efficient, while control roots doubled the rate of cytochromec oxidation by CN⁻-roots, after a 10 min lag in which cytochromec was reduced at the same rate as the medium plus NADH did. CN⁻-roots seemed to have a fully activated CN⁻-resistant branch. The described effects on K⁺ flux were consistent with the current hypothesis that redox compounds changed the electric membrane potential (de- or hyperpolarization), which induces the opening of voltage-gated in- or outward K⁺ channels.