THE ROLE OF THE PLASMA-MEMBRANE IN DIFFERENTIATION OF PHYTOPHTHORA-PALMIVORA ZOOSPORES

Sealed plasma membrane vesicles of about 90% purity were isolated from zoospores and mycelium of Phytophthora palmivora by chemical lysis an d mechanical disruption, followed by differential and sucrose density gradient centrifugation. Both ATPase activity and proton transport (ac ridine orange quenching) showed similar properties: sensitivity to van adate and calcium, insensitivity to oligomycin and molybdate, pH optim um 6.5, K-m 3.6-3.8 mM for the substrate Mg:ATP = 1:1. These are chara cteristic properties of fungal and plant E(1)E(2)-type H+-pumping ATPa se, suggesting that plasma membrane ATPase is involved directly in act ive proton transport across the membrane. Zoospores loaded with the fl uorescent compound BCECF showed that internal pH rose during encystmen t in high (1-5 mM) but not low (0.1-0.5 mM) external calcium concentra tions; pectin enhanced a pH rise only at high external calcium concent rations. Plasma membrane ATPase may play a vital part in controlling i nternal pH and providing an electrochemical gradient for calcium uptak e, but high external concentrations of calcium lead to an uncontrolled rise in internal pH, due in part to inhibition of the proton pump by high internal concentrations of calcium. However, in normal physiologi cal concentrations of calcium, the main function of calcium in zoospor e encystment is as a secondary messenger within the zoospore, rather t han as an inhibitor of ATPase.