Involvement of calcium/calmodulin signaling in cercosporin toxin biosynthesis by Cercospora nicotianae

Cercosporin is a non-host-selective, perylenequinone toxin produced by many phytopathogenic Cercospora species. The involvement of Ca2+ /calmodulin (CaM) signaling in cercosporin biosynthesis was investigated by using pharmacological inhibitors. The results suggest that maintaining endogenous Ca2+ homeostasis is required for cercosporin biosynthesis in Cercospora nicotianae. The addition of excess Ca2+ to the medium slightly increased fungal growth but resulted in a reduction in cercosporin production. The addition of Ca2+ chelators [EGTA and 1,2-bis(2-aminophenoxy)ethane-NNN',N'-tetraacetic acid] also reduced cercosporin production. Ca2+ channel blockers exhibited a strong inhibition of cercosporin production only at higher concentrations (>2 mM). Cercosporin production was reduced greatly by Ca2+ ionophores (A23187 and ionomycin) and internal Ca2+ blocker [3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester]. Phospholipase C inhibitors (lithium, U73122, and neomycin) led to a concentration-dependent inhibition of cercosporin biosynthesis. Furthermore, the addition of CaM inhibitors (compound 48/80, trifluoperazine, W-7, and chlorpromazine) also markedly reduced cercosporin production. In contrast to W-7, W-5, with less specificity for CaM, led to only minor inhibition of cercosporin production. The inhibitory effects of Ca2+/CaM inhibitors were partially or completely reversed by the addition of external Ca2+. As assessed with Fluo-3/AM (a fluorescent Ca2+ indicator), the Ca2+ content in the cytoplasm decreased significantly when fungal cultures were grown in a medium containing Ca2+/CaM antagonists, confirming the specificity of those Ca2+/CaM antagonists in C. nicotianae. Taken together, the results suggest that Ca2+/CaM signal transduction may play a pivotal role in cercosporin biosynthesis in C.nicotianae.