Membrane-associated compartmentalization of zearalenone biosynthetic enzymes and Syn2-associated zearalenone homeostasis in Fusarium graminearum.
Subcellular compartmentalization has attracted increasing attention in fungal secondary metabolism, particularly in the biosynthesis and trafficking of mycotoxins. However, the subcellular site of zearalenone (ZEA) biosynthesis and the mechanisms underlying its export in Fusarium graminearum remain poorly understood. ZEA is a polyketide mycotoxin that poses a serious threat to food safety through contamination of cereal grains and induces severe estrogenic effects in mammals. Its biosynthesis is governed by a dedicated biosynthetic gene cluster consisting of PKS4, PKS13, ZEB1, and ZEB2. In this study, we investigated the subcellular organization of the ZEA biosynthetic machinery and found that key biosynthetic enzymes accumulated in punctate structures that overlapped with small CMAC-positive vacuolar structures and were closely associated with FM4-64-labeled membranes. Furthermore, our results suggest that the syntaxin-like t-SNARE protein Syn2 contributes to extracellular ZEA accumulation and intracellular toxin homeostasis. Disruption of SYN2 abolished visible ZEA crystal formation on the hyphal surface and was associated with increased intracellular ZEA retention. This intracellular accumulation was accompanied by strong induction of the ZEA biosynthetic gene cluster and reduced cellular viability. Moreover, deletion of ZEB2 in the Δsyn2 background abolished ZEA production and restored cell viability, supporting an association between Zeb2-dependent ZEA biosynthesis and the cytotoxic phenotype of the Δsyn2 mutant. Together, our findings suggest a potential link between membrane-associated organization of ZEA biosynthetic enzymes, Syn2-associated ZEA distribution, intracellular toxin homeostasis, and fungal viability. Further studies will be required to define the precise mechanisms underlying ZEA transport and compartment function.