Arabidopsis PROTEASOME REGULATOR1 is required for auxin-mediated suppression of proteasome activity and regulates auxin signalling

TLDR

Auxin signals through the TIR1 receptor family to trigger degradation of Aux/IAA repressors, but how the proteasome is regulated in this pathway remains unclear. The study proposes that auxin modulates proteasome activity via PTRE1 to fine‑tune Aux/IAA repressor homeostasis and thereby adjust auxin responses. PTRE1 acts as a positive regulator of the 26S proteasome; ptre1 mutants are resistant to auxin‑mediated proteasome suppression, exhibit reduced Aux/IAA degradation, display auxin‑related phenotypes, and auxin changes PTRE1 subcellular localization, indicating a regulatory mechanism.

Abstract

The plant hormone auxin is perceived by the nuclear F-box protein TIR1 receptor family and regulates gene expression through degradation of Aux/IAA transcriptional repressors. Several studies have revealed the importance of the proteasome in auxin signalling, but details on how the proteolytic machinery is regulated and how this relates to degradation of Aux/IAA proteins remains unclear. Here we show that an Arabidopsis homologue of the proteasome inhibitor PI31, which we name PROTEASOME REGULATOR1 (PTRE1), is a positive regulator of the 26S proteasome. Loss-of-function ptre1 mutants are insensitive to auxin-mediated suppression of proteasome activity, show diminished auxin-induced degradation of Aux/IAA proteins and display auxin-related phenotypes. We found that auxin alters the subcellular localization of PTRE1, suggesting this may be part of the mechanism by which it reduces proteasome activity. Based on these results, we propose that auxin regulates proteasome activity via PTRE1 to fine-tune the homoeostasis of Aux/IAA repressor proteins thus modifying auxin activity.