Abstract
The plant hormone auxin is a master regulator in plant growth and development. Indole 3-acetic acid (IAA), predominant naturally occurring auxin regulates the cell division and elongation leading to auxin-mediated developmental process. Auxin is perceived in F-box protein, TIR1/AFB auxin receptor, component of the E3 ubiquitin-ligase complex SCF^<TIR1>, and enhance the degradation of Aux/IAA repressor in the ubiquitin-proteasome pathway. Thus, auxin-dependant turnover of Aux/IAA regulate the gene expression. In this study, we describe a series of novel auxin antagonists for the modulation of the TIR1-Aux/IAA interaction. Based on the structure of TIR1-auixn complex, we identified the potent auxin antagonist, a-(phenylethyl-2-oxo)-IAA as a lead compound for TIR1/AFB receptors by in silico virtual screening, and designed auxinole as most potent auxin antagonist specific to TIR1. Auxinole bind into TIR1 receptor to block the formation of TIR1-IAA-Aux/IAA complex, and inhibited auxin-responsive gene expression. Molecular docking study demonstrated that auxinole would strongly interact with Phe82 residue of TIR1 that is crucial residue for Aux/IAA recognition. Auxinole also competitively inhibited various auxin responses in plant, such as lateral root and root hair formation, hypocotyl elongation and tropic response to gravity. Our works not only substantiates the useful chemical tools for plant biology, but also demonstrate a new class of inhibitor for the protein-protein interaction induced by small molecule that is common mechanism in other plant hormones perception, such as jasmonate, gibberellin, and abscisic acid.
