Isolation of sealed plasma membrane vesicles from Phytophthora megasperma f. sp. glycinea: II. Partial characterization of Ca²⁺ transport and glyceollin effects

Calcium uptake was examined in sealed plasma membrane vesicles isolated from the plant pathogenic fungus, Phytophthora megasperma f. sp. glycinea. Calcium uptake was ATP-dependent and by the addition of various ionophores in the presence of ATP, it was shown that Ca²⁺ transport was mediated by a nH⁺ Ca²⁺ antiport. Further evidence for this antiport mechanism included Ca²⁺ uptake driven by an imposed pH gradient and the observation that calcium could dissipate a steady-state pH gradient across the vesicle membrane. Transport mediated by the nH⁺ Ca²⁺ antiport was optimal at pH 7.0, and demonstrated saturation kinetics for Ca²⁺ with a K_m of about 7 μm. Glyceollin, a soybean phytoalexin, was found to inhibit Ca²⁺ transport consistent with its ability to increase H⁺ conductance. In the presence of glyceollin, calcium leakage from Phytophthora membrane vesicles also increased. This study provides basic information about calcium transport in a plant pathogenic fungus as well as demonstrating a possible mode of action of a phytoalexin.