Characterization of phosphonate uptake in two Phytophthora spp. and its inhibition by phosphate
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The uptake of the phosphonate ion, the active breakdown product in plant tissues of the systemic anti-Oomycete compound Fosetyl-Al (aluminium tris-Oethylphosphonate), was investigated in two Phytophthora spp. of differential sensitivity. Uptake was due to the simultaneous operation of two transport systems, one of low affinity (high Km) and one of high affinity (low Km). The relative contribution of each transport system varied with the external concentration of phosphonate, suggesting that phosphonate was a potent regulator of both systems. Phosphate was a partial competitive inhibitor with respect to phosphonate. Phosphate competed with phosphonate for uptake with a Ki of 105 μM for P. cryptogea and 68 μM for P. citrophthora. Uptake was sensitive to pH, showing a maximum at pH 5.0 to 5.5. P. cryptogea was more efficient in phosphonate uptake, although it was less sensitive to inhibition by phosphonate in vitro, than P. citrophthora. This implied that the selective activity of phosphonate was not due to differential rates of uptake of this oxyanion. These results were discussed in relation to the mode of action of phosphonate towards Oomycetes.
Key wordsPhosphonate Uptake Phosphate Inhibition Phytophthora cryptogea Phytophthora citrophthora
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