, Volume 152, Issue 5, pp 430-436

Properties of the phosphate and phosphite transport systems of Phytophthora palmivora

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Germlings of Phytophthora palmivora possess at least two systems for the uptake of inorganic phosphate (Pi). The first is synthesized on germination in medium containing 50 μM Pi and has a Km of approx. 30 μM (Vmax=7–9 nmol Pi/h·106 cells). The second is synthesized under conditions of Pi-deprivation and has a higher affinity for Pi (Km=1–2 μM), but a lower Vmax (0.5–2 nmol Pi/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 Pi in the medium. The Km of the lower affinity system is 3 mM (Vmax=20 nmol phosphite/h·106 cells) and that of the higher affinity system is 0.6 mM (Vmax=12 nmol/h·106 cells). Pi and phosphite are competitive inhibitors for each other's transport in both systems. However, whereas mM concentrations of phosphite are necessary to inhibit Pi transport, only μM concentrations of Pi are required to inhibit phosphite transport. A third system of uptake for Pi also exists, since when phosphate-deprived cells are presented with mM concentrations of Pi, 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.