BRs are perceived by a BRI1 and BAK1 containing receptor kinase complex, which triggers a phosphorylation-dependent signal transduction cascade that ultimately leads to de-phosphorylation and activation of the BES1/BZR1 family of transcription factors, which, together with different types of bHLH transcription factors, control BR target gene expression. Although BRs were discovered in the 1970s only, the biosynthesis, signal transduction and functions of BRs are well characterized today. This rapid progress has been made possible by the application of multiple strategies for elucidating BR action including forward genetic approaches facilitated by the use of BR biosynthesis inhibitors. The use of chemical inhibitors of enzyme function is a powerful tool to alter metabolic pathways or signal transduction cascades in cellular organisms. Their most prominent applications are as pharmaceuticals for the treatment of diseases and as pesticides and herbicides in agriculture. In recent years chemical inhibitors have also become invaluable tools for research, applied in ��chemical biology�� to the study and manipulation of biological systems. Chemical inhibitors, which target BR biosynthesis known to date are brassinazole, Brz2001, Brz220 and propiconazole. So far only the molecular targets of Brz and Brz220 have been identified. Both triazoles inhibit the activity of the cytochrome P450 DWF4, an enzyme that catalyzes a ratelimiting step of BR biosynthesis, by binding to its prosthetic haem group. Sterol biosynthesis inhibitors active in plants have also been characterized although their modes of action have remained largely elusive. They include compounds such as the herbicide LAB 170250F, which impairs sterol synthesis by acting on cytochrome P450s that catalyze obtusifoliol-14- demethylation. In this study we identify voriconazole and related triazoles, used as antifungal therapeutic drugs for the treatment of Aspergillus sp. and R428 inquirer Candida sp. infections as potent inhibitors of BRdependant sterol biosynthesis in plants. Voriconazole acts at mM concentrations, is incorporated by plants within a few hours, decreases sterol and BR contents and severely impairs growth of both monocotyledonous and dicotyledonous plant species, with one notable exception: the woodland strawberry Fragaria vesca. F. vesca was employed as a model to elucidate modes of voriconazole toxicity in plants. In an approach to PD325901 MEK inhibitor assess the ability of pharmaceuticals to alter BR homeostasis of plants we found that fluconazole, a triazole used as an antifungal therapeutic drug, induced phenotypes indicative of BR deficiency in Arabidopsis thaliana.