A role for calcium in appressorium induction in Phytophthora palmivora

Appressorium formation in vitro by Phytophthora palmivora was strongly infl uenced by Ca2+ ions: treatment of zoospores with an Inhibitor of stretch-ac tivated ion channels (Gd3+), an inorganic Ca2+ channel blocker (Nd3+), or E GTA inhibited appressorium formation. The Ca2+ ionophore A23187 had a posit ive effect on this process which points to an important role of increased C a2+ levels for appressorium formation. The latter is known to start about 6 0 min after incubation of zoospores. This early preparatory phase correspon ds to the time period found to be sensitive to an inhibition of Ca2+ channe ls by Nd3+. The calmodulin antagonist TFP inhibited appressorium formation, pointing to an involvement of the Ca2+-calmodulin complex in this process. Our experiments do not point to a role of cAMP in appressorium induction. E xogenously applied cAMP and some compounds (NaF, 2-deoxyadenosine and IBMX) known to elevate endogenous levels of cAMP did not induce appressorium for mation.