Targeting pathogen sterols: Defence and counterdefence?

Sterols, a class of lipids found in the cellular membranes of all eukaryotes, have vital roles in regulating membrane fluidity and permeability. While animal cells contain cholesterol in their membranes, plant cells contain phytosterols such as campesterol, sitosterol, and stigmasterol. The sterol ergosterol, discovered more than a century ago from the ergot fungus Claviceps purpurea [1], is a common component of many plant and human pathogenic fungi and is required for fungal growth. Fungal plant pathogens—in particular, the smut (Ustilaginomycotina subphyla) and the powdery mildew (Pezizomycotina subphyla) fungi—contain ergosterol in their membranes, while the rust fungi (Pucciniomycotina subphyla) synthesise slightly different forms of sterols [2]. In contrast to most pathogenic fungi, the oomycete pathogens from the order Peronosporales, such as Phytophthora and Pythium, are sterol auxotrophs. Therefore, these organisms acquire sterols externally, most likely from host membranes during pathogenesis [3].