Mol. Cell https://doi.org/10.1016/j.molcel.2019.06.044 (2019)

Auxin is a phytohormone that regulates root development and growth through transcriptional regulation, mediated by the members of the auxin response factor (ARF) family. ARF7 and ARF19 proteins exhibit elevated auxin transcriptional responsiveness near the growing root tip but are diminished in mature roots. Powers et al. observed that ARF7/19 proteins were organized as cytoplasmic protein complexes or condensates made up of higher order oligomers in mature root cells but were present as nuclear-localized monomers at the growing root tip. Formation of the ARF19 complex required the intrinsically disordered MR domain and the PB1 domain, which is necessary for mediating endogenous ARF–ARF interactions. The formation of these oligomeric complexes corresponded to diminished auxin transcriptional activity. Mutation of a critical lysine (K962) in the PB1 domain reduced ARF19 complex formation, resulting in constitutive nuclear localization and enhanced auxin-mediated transcription. Although future studies will be needed to reveal how the ARF7/19 condensate is resolved into a monomer in high auxin-responsive tissues, the findings from Powers et al. reveal a new regulatory mechanism that controls auxin responsiveness.