Gibbestatin B inhibits the GA-induced expression of alpha-amylase expression in cereal seeds

Genetic variation in plant CYP51s confers resistance against voriconazole, a novel inhibitor of brassinosteroid-dependent sterol biosynthesis

Brassinosteroids (BRs) are plant steroid hormones with structural similarity to mammalian sex steroids and ecdysteroids from insects. The BRs are synthesized from sterols and are essential regulators of cell division, cell elongation and cell differentiation. In this work we show that voriconazole, an antifungal therapeutic drug used in human and veterinary medicine, severely impairs plant growth by inhibiting sterol-14α-demethylation and thereby interfering with BR production. The plant growth regulatory properties of voriconazole and related triazoles were identified in a screen for compounds with the ability to alter BR homeostasis. Voriconazole suppressed growth of the model plant Arabidopsis thaliana and of a wide range of both monocotyledonous and dicotyledonous plants. We uncover that voriconazole toxicity in plants is a result of a deficiency in BRs that stems from an inhibition of the cytochrome P450 CYP51, which catalyzes a step of BR-dependent sterol biosynthesis. Interestingly, we found that the woodland strawberry Fragaria vesca, a member of the Rosaceae, is naturally voriconazole resistant and that this resistance is conferred by the specific CYP51 variant of F. vesca. The potential of voriconazole as a novel tool for plant research is discussed.

Yokonolide B, a novel inhibitor of auxin action, blocks degradation of AUX/IAA factors

Yokonolide B (YkB; also known as A82548A), a spiroketal-macrolide, was isolated from Streptomyces diastatochromogenes B59 in a screen for inhibitors of beta-glucoronidase expression under the control of an auxin-responsive promoter in Arabidopsis. YkB inhibits the expression of auxin-inducible genes as shown using native and synthetic auxin promoters as well as using expression profiling of 8300 Arabidopsis gene probes but does not affect expression of an abscisic acid- and a gibberellin A3-inducible gene. The mechanism of action of YkB is to block AUX/IAA protein degradation; however, YkB is not a general proteasome inhibitor. YkB blocks auxin-dependent cell division and auxin-regulated epinastic growth mediated by auxin-binding protein 1. Gain of function mutants such as shy2-2, slr1, and axr2-1 encoding AUX/IAA transcriptional repressors and loss of function mutants encoding components of the ubiquitin-proteolytic pathway such as axr1-3 and tir1-1, which display increased AUX/IAAs protein stability, are less sensitive to YkB, although axr1 and tir1 mutants were sensitive to MG132, a general proteasome inhibitor, consistent with a site of action downstream of AXR1 and TIR. YkB-treated seedlings displayed similar phenotypes as dominant AUX/IAA mutants. Taken together, these results indicate that YkB acts to block AUX/IAA protein degradation upstream of AXR and TIR, links a shared element upstream of AUX/IAA protein stability to auxin-induced cell division/elongation and to auxin-binding protein 1, and provides a new tool to dissect auxin signal transduction.