Properties of the phosphate and phosphite transport systems of Phytophthora palmivora

Germlings of Phytophthora palmivora possess at least two systems for the uptake of inorganic phosphate (P_i). The first is synthesized on germination in medium containing 50 M P_i and has a K_m of approx. 30 M (V_max=7–9 nmol P_i/h·10⁶ cells). The second is synthesized under conditions of P_i-deprivation and has a higher affinity for P_i (K_m=1–2 M), but a lower V_max (0.5–2 nmol P_i/h·10⁶ cells). The fungicide phosphite likewise enters the germlings via two different transport systems, the synthesis of which also depends on the concentration of P_i in the medium. The K_m of the lower affinity system is 3 mM (V_max=20 nmol phosphite/h·10⁶ cells) and that of the higher affinity system is 0.6 mM (V_max=12 nmol/h·10⁶ cells). P_i and phosphite are competitive inhibitors for each other's transport in both systems. However, whereas mM concentrations of phosphite are necessary to inhibit P_i transport, only M concentrations of P_i are required to inhibit phosphite transport. A third system of uptake for P_i also exists, since when phosphate-deprived cells are presented with mM concentrations of P_i, they transport the anion at a very high rate (around 100 nmol/h·10⁶ cells). High rates of transport of phosphite are also observed when these cells are presented with mM concentrations of this anion.