Modes‐of‐action of antifungal compounds: Stressors and (target‐site‐specific) toxins, toxicants, or toxin–stressors

Abstract Fungi and antifungal compounds are relevant to the United Nation's Sustainable Development Goals. However, the modes‐of‐action of antifungals—whether they are naturally occurring substances or anthropogenic fungicides—are often unknown or are misallocated in terms of their mechanistic category. Here, we consider the most effective approaches to identifying whether antifungal substances are cellular stressors, toxins/toxicants (that are target‐site‐specific), or have a hybrid mode‐of‐action as toxin–stressors (that induce cellular stress yet are target‐site‐specific). This newly described ‘toxin–stressor’ category includes some photosensitisers that target the cell membrane and, once activated by light or ultraviolet radiation, cause oxidative damage. We provide a glossary of terms and a diagrammatic representation of diverse types of stressors, toxic substances, and toxin–stressors, a classification that is pertinent to inhibitory substances not only for fungi but for all types of cellular life. A decision‐tree approach can also be used to help differentiate toxic substances from cellular stressors (Curr Opin Biotechnol 2015 33: 228–259). For compounds that target specific sites in the cell, we evaluate the relative merits of using metabolite analyses, chemical genetics, chemoproteomics, transcriptomics, and the target‐based drug‐discovery approach (based on that used in pharmaceutical research), focusing on both ascomycete models and the less‐studied basidiomycete fungi. Chemical genetic methods to elucidate modes‐of‐action currently have limited application for fungi where molecular tools are not yet available; we discuss ways to circumvent this bottleneck. We also discuss ecologically commonplace scenarios in which multiple substances act to limit the functionality of the fungal cell and a number of as‐yet‐unresolved questions about the modes‐of‐action of antifungal compounds pertaining to the Sustainable Development Goals.