Summary: Auxin plays a pivotal role in plant development by activating AUXIN RESPONSE FACTORs (ARFs). Under low auxin levels, ARF activity is inhibited by interacting with Aux/IAAs. Aux/IAAs are degraded when the cellular auxin concentration increases, causing the release of ARF inhibition. Here, we show that levels of the ARF5/MONOPTEROS (MP) protein are regulated in a cell-type-specific and isoform-dependent manner. We find that the stability of MP isoforms is differentially controlled depending on the auxin level. The canonical MP isoform is degraded by the proteasome in root tissues with low auxin levels. While auxin sharpens the MP localization domain in roots, it does not do so in ovules or embryos. Our research highlights a mechanism for providing spatial control of auxin signaling capacity. Together with recent advances in understanding the tissue-specific expression and post-transcriptional modification of auxin signaling components, these results provide insights into understanding how auxin can elicit so many distinct responses.
