Involvement of Calcium/Calmodulin Signaling in Cercosporin Toxin Biosynthesis by Cercospora nicotianae

Abstract

ABSTRACT Cercosporin is a non-host-selective, perylenequinone toxin produced by many phytopathogenic Cercospora species. The involvement of Ca 2+ /calmodulin (CaM) signaling in cercosporin biosynthesis was investigated by using pharmacological inhibitors. The results suggest that maintaining endogenous Ca 2+ homeostasis is required for cercosporin biosynthesis in Cercospora nicotianae . The addition of excess Ca 2+ to the medium slightly increased fungal growth but resulted in a reduction in cercosporin production. The addition of Ca 2+ chelators [EGTA and 1,2-bis(2-aminophenoxy)ethane- N , N , N ′, N ′-tetraacetic acid] also reduced cercosporin production. Ca 2+ channel blockers exhibited a strong inhibition of cercosporin production only at higher concentrations (>2 mM). Cercosporin production was reduced greatly by Ca 2+ ionophores (A23187 and ionomycin) and internal Ca 2+ 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 Ca 2+ /CaM inhibitors were partially or completely reversed by the addition of external Ca 2+ . As assessed with Fluo-3/AM (a fluorescent Ca 2+ indicator), the Ca 2+ content in the cytoplasm decreased significantly when fungal cultures were grown in a medium containing Ca 2+ /CaM antagonists, confirming the specificity of those Ca 2+ /CaM antagonists in C . nicotianae . Taken together, the results suggest that Ca 2+ /CaM signal transduction may play a pivotal role in cercosporin biosynthesis in C . nicotianae .