Figure 5Relative fluxes of phosphates (Pi) and sugars (C) across the fungal plasma membrane. Without additional Pi-source in the apoplast (value 0 at the x-axis, black dots) there is a constant flux of phosphate via the proton-coupled phosphate (H/P) transporter from the fungus to the apoplast and a constant flux of sugar from the apoplast to the fungus. For better comparison these fluxes were normalized to 1 and -1, respectively, and all other fluxes were calculated relative to these control values. With increasing ATP-release and Pi-production the Pi-efflux gets smaller and is zero at a relative Pi-production rate of 1 (grey triangles). At higher ATP-release and Pi-production rates the fungus imports Pi, i.e. the transport direction of phosphate has been inverted in comparison to the control condition (white square). The C-flux via the proton-coupled sugar (H/C) transporter is not affected by the additional Pi-source (horizontal grey line). Relative fluxes of Pi and C across the plant plasma membrane. Without additional Pi-source in the apoplast (value 0 at the x-axis, black dots) there is a constant flux of sugar via the H/C transporter from the plant to the apoplast (and thereafter to the fungus) and a constant flux of phosphate (coming from the fungus) via the H/P transporter from the apoplast to the plant. With increasing ATP-release and Pi-production the Pi-influx via the H/P transporter increases while the C-flux via the H/C transporter is unaffected by the additional Pi-source (horizontal grey line). Phosphate fluxes across the plant plasma membrane. Besides the Pi-uptake via the H/P transporter (B, black line, for clarity not shown in C), the plant loses Pi due to the ATP-release (light grey line). The difference between Pi-uptake and Pi-loss is the net Pi-balance of the plant (dark grey line). Without ATP-release (value 0 at the x-axis, black dots) the plant gains Pi. At a relative ATP-release and Pi-production of 1 (grey triangles) the plant release as much phosphate as it takes up, while at higher ATP-release values, the plant loses Pi. Schematic representation of the fluxes for three scenarios: (i) no ATP-release and Pi-production (left, black dots in A-C), (ii) moderate ATP-release, Adenosine (A) and Pi-production (middle, grey triangles in A-C), and (iii) high ATP-release, A and Pi-production (right, white squares in A-C). If there is no ATP-release (left), there is a constant flux of Pi from the fungus via the apoplast to the plant and a constant flux of sugars in the inverse direction. The energy for these fluxes is provided by the phosphate gradient between fungus and plant and the sugar gradient between plant and fungus. At a moderate ATP-release and Pi-production rate (middle, value 1 on the x-axis, grey triangles in A-C), there is no Pi-flux from the fungus to the apoplast anymore. The unchanged uptake of sugars is now energized by the proton pump of the fungus. In this condition, the plant retrieves the cleaved Pi originating from the ATP-release via the H/P cotransporter, a transport which is energized by both, the sugar gradient and the proton pump. At a high ATP-release and Pi-production rate (right, value 2 on the x-axis, white squares in A-C), there is a larger Pi-uptake by the plant and a Pi-uptake by the fungus. Both processes are energized by larger activities of the proton pumps.