Auxin is a small molecule morphogen that bridges SCFTIR1/AFB-AUX/IAA co-receptor 
interactions leading to ubiquitylation and proteasome-dependent degradation of 
AUX/IAA transcriptional repressors. Here, we systematically dissect auxin 
sensing by SCFTIR1-IAA6 and SCFTIR1-IAA19 co-receptor complexes, and assess 
IAA6/IAA19 ubiquitylation in vitro and IAA6/IAA19 degradation in vivo. We show 
that TIR1-IAA19 and TIR1-IAA6 have distinct auxin affinities that correlate with 
ubiquitylation and turnover dynamics of the AUX/IAA. We establish a system to 
track AUX/IAA ubiquitylation in IAA6 and IAA19 in vitro and show that it occurs 
in flexible hotspots in degron-flanking regions adorned with specific Lys 
residues. We propose that this signature is exploited during auxin-mediated 
SCFTIR1-AUX/IAA interactions. We present evidence for an evolving AUX/IAA 
repertoire, typified by the IAA6/IAA19 ohnologues, that discriminates the range 
of auxin concentrations found in plants. We postulate that the intrinsic 
flexibility of AUX/IAAs might bias their ubiquitylation and destruction kinetics 
enabling specific auxin responses.