Figure 3.

Rapid equilibration of the nutrient uptake system. (A) Time course of equilibration of the membrane voltages at the fungal plasma membrane (left) and the plant plasma membrane (right). (B) Time course of equilibration of the apoplastic phosphate (left) or sugar concentrations (right). (C) Time course of equilibration of proton-couplesd phosphate (I_H/P, left) and sugar fluxes (I_H/C, right) across the plant and fungal plasma membrane. Irrespective of the starting conditions (apoplastic P and C concentrations and voltages at the fungal and plant plasma membranes) the system reaches quickly an equilibrium that depends on the P and C concentrations in the cytosols of plant and fungus and the relative settings of the transporters. The equilibration processes for two different starting conditions [1, thicker lines, gray in (B), dark blue or orange in (A,C), and 2, thinner lines, black in (B), light blue and brown in (A,C)] are shown. Please note that the equilibration time depends on the expression level of the transporters. In this study, a rather low expression level was chosen. The overall increase of the expression level would result in faster equilibration. In equilibrium there is still a continuous flux of phosphate from the fungus to the apoplast (C, left, positive current) and from the apoplast to the plant (C, left, negative current). Similarly, there is a constant flux of sugar from the plant via the apoplast to the fungus (C, right).