Calcium Regulated Appresorium Formation of the Entomopathogenic Fungus Zoophthora Radicans

Abstract

The fungusZoophthora radicans (Zygomycetes: EntomophthoraThe fungusZoophthora radicans (Zygomycetes: Entomophthorales) requires external Ca2+ for appressorium formation but not for conidial germination. The number of appressoria formed depends on the Ca2+ concentration of the medium. At low [Ca2+] (100 pM) nuclear division and germ tube growth are significantly reduced compared to higher Ca2+ concentrations (10 and 1,000 μM). By contrast, neither external K+ nor external Cl− is needed for germination or appressorium formation. Treatment of conidia with a Ca2+-antagonist, Nd3+, and a Ca2+-channel blocker, nifedipine, inhibits appressorium formation, showing that a Ca2+ influx is required for appressorium formation. Furthermore, the partial yet saturating inhibition by nifedipine and complete inhibition by Nd3+ indicates that at least two kinds of Ca2+ channels are involved in appressorium formation. A contribution of intracellular Ca2+ to the signal transduction chain for the formation of appressoria is demonstrated by the inhibitory effect of the intracellular Ca2+ antagonist TMB-8. The calmodulin antagonists R24571, TFP, W-7, and W-5 inhibit appressorium formation at concentrations which have no effect on germination. The data presented in this paper are consistent with the hypothesis that a Ca2+/calmodulin system is involved in regulating appressorium formation. However, since the direct effects of the drugs were not specifically tested on their proposed binding sites, we leave room for alternative hypotheses that have yet to be formulated.les) requires external Ca2+ for appressorium formation but not for conidial germination. The number of appressoria formed depends on the Ca2+ concentration of the medium. At low [Ca2+] (100 pM) nuclear division and germ tube growth are significantly reduced compared to higher Ca2+ concentrations (10 and 1,000 μM). By contrast, neither external K+ nor external Cl− is needed for germination or appressorium formation. Treatment of conidia with a Ca2+-antagonist, Nd3+, and a Ca2+-channel blocker, nifedipine, inhibits appressorium formation, showing that a Ca2+ influx is required for appressorium formation. Furthermore, the partial yet saturating inhibition by nifedipine and complete inhibition by Nd3+ indicates that at least two kinds of Ca2+ channels are involved in appressorium formation. A contribution of intracellular Ca2+ to the signal transduction chain for the formation of appressoria is demonstrated by the inhibitory effect of the intracellular Ca2+ antagonist TMB-8. The calmodulin antagonists R24571, TFP, W-7, and W-5 inhibit appressorium formation at concentrations which have no effect on germination. The data presented in this paper are consistent with the hypothesis that a Ca2+/calmodulin system is involved in regulating appressorium formation. However, since the direct effects of the drugs were not specifically tested on their proposed binding sites, we leave room for alternative hypotheses that have yet to be formulated