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). 