Seminars

The phytohormone auxin is a key developmental signal in plants. Auxin acts as a ‘molecular glue’ that promotes the interaction between its co-receptors, the TIR1/AFB1-3 F-box proteins and the AUX/IAA repressor proteins. Auxin-targeted ubiquitination and degradation of AUX/IAAs releases the interacting transcription factors referred to as AUXIN RESPONSE FACTORS (ARFs), allowing them to regulate the expression of their gene targets. However, the mechanism that mediates the dissociation of ARF proteins from TIR1−AUX/IAA complexes is unclear. This study reports that phosphorylation of ARF7 and ARF19 via the glycogen synthase kinase 3 (GSK3)-like kinase BIN2 results in the disassociation of these ARF proteins from their AUX/IAA repressor complex. BIN2 directly interacted with ARF7 and ARF19, and induces their phosphorylation in planta. BIN2 phosphorylation of ARF7 decreased the binding affinity of ARF7 for AUX/IAAs, leading to the activation of auxin signaling. Genetic and pharmacological studies demonstrated that BIN2-mediated regulation of ARF7 and ARF19 activity is important for lateral root primordia development. In summary, this study reveals a new regulatory module in auxin signaling involving the auxin perception-independent dissociation of ARF and AUX/IAA proteins via BIN2-mediated phosphorylation.