The Ubiquitin-Specific Protease TNI/UBP14 Functions in Ubiquitin Recycling and Affects Auxin Response

The ubiquitin-mediated proteasomal pathway regulates diverse cellular processes in plants by rapidly degrading target proteins, including the repressors of hormone signaling. Though ubiquitin proteases play a key role in this process by cleaving polyubiquitin chains to monomers, their function has not been studied in detail by mutational analysis. Here, we show that mutation in TARANI/UBIQUITIN-SPECIFIC PROTEASE14 (TNI/UBP14) leads to reduced auxin response and widespread auxin-related phenotypic defects in Arabidopsis (Arabidopsis thaliana). In a tni partial loss-of-function mutant that was originally isolated based on altered leaf shape, activity of the auxin-responsive reporters DR5::GUS, DR5::nYFP, and IAA2::GUS was reduced. Genetic interaction studies suggest that TNI is involved in auxin signaling and acts alongside TIR1, ARF7, and AUX1 Map-based cloning identified TNI as UBP14 Inefficient splicing of the mutant TNI transcript resulted in the formation of an inactive UBP14 protein, which led to accumulation of polyubiquitin chains and excess polyubiquitinated proteins in the mutant. In addition to the reduced auxin response, increased levels of DII:VENUS, IAA18:GUS, and HS::AXR3-NT:GUS were also observed in tni, perhaps due to inefficient polyubiquitin hydrolysis and proteasome-mediated degradation. Together, our study identifies a function for TNI/UBP14 in the auxin response through ubiquitin recycling.