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. 