GARP-retromer-mediated recycling of a chitin synthase coordinates polarized growth, mycotoxin biosynthesis and virulence in Fusarium graminearum

The endosome-to-trans-Golgi network (TGN) retrograde trafficking pathway is critical for polarized growth, pathogenicity, and deoxynivalenol (DON) biosynthesis in filamentous fungi, yet its key cargoes and transport mechanisms remain poorly understood. Here, we identify that the class III chitin synthase FgChs3b is likely an important cargo that requires the conserved GARP-retromer-mediated endosome-to-TGN pathway for its proper recycling and polarized distribution in the wheat head blight pathogen Fusarium graminearum . FgChs3b dynamically localizes to endosomes, the TGN, the plasma membrane and the Spitzenkörper, and its polarized delivery to the hyphal apex strictly depends on an intact GARP–retromer complex. Disruption of either GARP or retromer impairs FgChs3b recycling, resulting in its mis-sorting to the vacuole and depletion from Spitzenkörper-associated growth sites. Previously considered essential, FgCHS3B has remained largely uncharacterized. We report the first successful generation of FgCHS3B deletion mutants and show that FgChs3b is indispensable for polarized hyphal growth, conidiation, pathogenicity, and DON biosynthesis. Notably, FgChs3b is fungus-specific, defining a GARP–retromer–FgChs3b trafficking axis unique to filamentous fungi. These findings provide mechanistic insights into vesicle trafficking in fungal cell biology and reveal a potential selective target for the development of safe and effective antifungal strategies.