Figure 5.

Arms race in adjusting the zero-current voltages of the background conductance. (A) Relative phosphate (H/P) and sugar (H/C) fluxes as a function of the zero-current voltages of the plant's (${\text{V}}_{\text{p}}^0$) and fungus' (${\text{V}}_{\text{f}}^0$) background conductances. ${\text{V}}_{\text{p}}^0$ and ${\text{V}}_{\text{f}}^0$ are strongly influenced by the activity of H⁺-ATPases; a higher pump activity drives the values to more negative voltages. If the plant unilaterally invests more pump energy, ${\text{V}}_{\text{p}}^0$ gets more negative and the plant benefits from a higher P-influx while reducing the C-efflux. In its extreme the plant could even achieve a C-flux from the fungus to the plant (negative values for H/C-flux). The gain of the plant is at the cost of the fungus. In return, the fungus can also invest pump energy and drive ${\text{V}}_{\text{f}}^0$ more negative with positive consequences for its C/P-balance but at the cost of the plant. The arms race between plant and fungus (1 → 2 → 3 → 4 → …) as equal partners ends in a draw at which ΔV₀ = ${\text{V}}_{\text{f}}^0$ − ${\text{V}}_{\text{p}}^0$ = 0 with negative ${\text{V}}_{\text{p}}^0$ and ${\text{V}}_{\text{f}}^0$ values. The dot indicates the condition chosen for the other simulations in this study (${\text{V}}_{\text{p}}^0$ = ${\text{V}}_{\text{f}}^0$ = −75 mV). (B,C) Apoplastic carbon/sugar (B, [C]) and phosphate (C, [P]) concentrations at equilibrium of the transporter network as a function of ${\text{V}}_{\text{p}}^0$ and ${\text{V}}_{\text{f}}^0$. During the arms race between plant and fungus both concentrations decline (black and gray curves).