ARF-Aux/IAA interactions through domain III/IV are not strictly required for auxin-responsive gene expression

Auxin response factors (ARFs), together with auxin/indole acetic acid proteins (Aux/IAAs), are transcription factors that play key roles in regulating auxin-responsive transcription in plants. Current models for auxin signaling predict that auxin response is dependent on ARF-Aux/IAA interactions mediated by the related protein-protein interaction domain (i.e., referred to as the CTD) found in the ARF and Aux/IAA C-terminal regions. When auxin concentrations in a cell are low, ARF activators residing on the promoters of auxin response genes are thought to be inactive because of the association with dominant Aux/IAA repressors. When auxin concentrations are elevated, the Aux/IAA repressors are recruited to auxin receptors and degraded via the ubiquitin-proteasome pathway. Destruction of the Aux/IAA repressors allows the ARF activators to function in derepressing/activating auxin response genes. While this auxin signaling pathway is simple and attractive, it is unclear whether auxin-regulated gene expression is solely dependent on ARF-Aux/IAA interactions. Here we show that auxin can affect the expression of auxin response genes in a manner that is independent of the ARF activator CTD.