Auxin promotes GPI-anchored protein-mediated trafficking of ABP1 to enable cell-surface auxin signaling

Cell-surface auxin signaling is essential for rapid cellular and growth responses to auxin. Auxin-Binding Protein 1 (ABP1) has long been proposed as an extracellular auxin receptor; however, it is predominantly retained in the endoplasmic reticulum (ER), posing a longstanding paradox. Whether ABP1 senses auxin within the ER and how the ER-retained ABP1 reaches the apoplast remain unclear. Here, we establish a dark-induced, auxin-triggered hypocotyl elongation system in Arabidopsis, in which abp1 single mutants exhibit a pronounced phenotype. we show that dark-induced auxin promotes LLG1 expression and its dose-dependent interaction with ABP1, driving ABP1 trafficking from the ER to the plasma membrane (PM) through a LORELEI-like GPI-anchored protein 1 (LLG1)-based mechanism. Upon reaching the apoplast, ABP1 dissociates from LLG1 under acidic conditions, enabling activation of AHAs-mediated cell-surface signaling. Together, our results establish ABP1 as an ER-localized auxin receptor, refine the molecular framework of cell-surface auxin signaling, and uncover a GPI-AP-mediated trafficking mechanism for ER-retained receptors.