The plant hormone auxin is a multi-functional and central co-ordinator of plant growth and development. Given its complexity, research to uncover its regulatory mechanisms are of interest. Here, Jürgen Kleine-Vehn, Elena Feraru and colleagues investigate the mechanisms by which levels of PIN-LIKES (PILS) proteins, responsible for auxin transport into the endoplasmic reticulum (ER), are regulated. Using a forward genetic screen approach in Arabidopsis seedlings, they identify gloomy and shiny pils 1 (gasp1) as a novel E3 ubiquitin ligase responsible for perturbing PILS protein abundance. Here, gasp1 mutants show significantly reduced levels of auxin signalling, which can also be achieved by PILS overexpression. Unexpectedly, however, PILS protein levels were decreased in gasp1 mutants, and their overexpression did not exacerbate the reduced auxin signalling seen. In addition, using alterations in auxin levels, the authors show that PILS protein abundance is regulated by auxin signalling. Together, these findings suggest the existence of a regulatory feedback loop whereby GASP1-reliant auxin signalling controls PILS protein abundance and concomitantly attenuates auxin signalling by ER sequestration. This study therefore highlights an auxin feedback on its own signalling output, which could have broad implications in our understanding of auxin signalling regulation in plant development and growth.