The phytohormone auxin is a key regulator of plant growth and development. Molecular studies in Arabidopsis have shown that auxin perception and signaling is mediated via TIR1/AFB–Aux/IAA co-receptors that assemble as part of the SCFTIR1/AFB E3 ubiquitin-ligase complex and direct the auxin-regulated degradation of Aux/IAA transcriptional repressors. Despite the importance of auxin signaling, little is known about the functional regulation of the TIR1/AFB receptor family. Here we show that TIR1 can oligomerize in planta via a set of spatially clustered amino acid residues. While none of the residues identified reside in the interaction interface of the TIR1-Aux/IAA degron, they nonetheless regulate the binding of TIR1 to Aux/IAA substrate proteins and their subsequent degradation in vivo as an essential aspect of auxin signaling. We propose oligomerization of TIR1 as a novel regulatory mechanism in the regulation of auxin-mediated plant patterning and development., The phytohormone auxin plays a diverse and critical role in plant growth and development. In Arabidopsis, the F-box protein TIR1 is a component of an E3 SCF ubiquitin-ligase complex that serves as the auxin receptor and directs the ubiquitin-dependent degradation of repressors of auxin signaling known as the Aux/IAA proteins. Little is known regarding the stoichiometry of the SKP1, CUL1, RBX1 and TIR1 subunits that comprise the SCFTIR1 complex. Here, we show that TIR1 is capable of oligomerization in planta. We also show that mutant alleles that abolish TIR1 oligomerization impair the degradation of known SCFTIR1 substrates and fail to restore auxin signaling and response in tir1 loss-of-function genetic backgrounds. The results suggest that TIR1 homo-oligomerization is an important aspect of the regulation of SCFTIR1 function and auxin signaling.
Dezfulian, Mohammad H.; Jalili, Espanta; Roberto, Don Karl A.; Moss, Britney L.; Khoo, Kerry; Nemhauser, Jennifer L.; and Crosby, William L., "Oligomerization of SCFTIR1 Is Essential for Aux/IAA Degradation and Auxin Signaling in Arabidopsis" (2016). PLoS Genetics, 12, 9.