Receptor trafficking couples intracellular auxin perception to rapid signaling

Abstract

Rapid auxin responses are triggered by Auxin-Binding Protein 1 (ABP1) signaling; however, the coupling of hormone perception to signaling activation is unclear. ABP1, a cell surface auxin receptor, is localized to endoplasmic reticulum (ER), raising a fundamental question of where auxin is perceived and how signaling-competent receptors are mobilized to cell surface. Herein, we established a dark-induced auxin synthesis system in Arabidopsis hypocotyl; abp1 mutants showed pronounced elongation. Auxin dose-dependently stimulates LORELEI-like glycosyl-phosphatidylinositol-anchored protein 1 (GPI-AP, LLG1) interaction with ABP1, driving their outward trafficking from ER. However, this complex dissociated in acidic apoplast, facilitating H+-ATPase-mediated auxin signaling. Our findings reveal how intracellular auxin perception is converted into rapid cell-surface signaling, establishing a mechanistic framework for fast auxin responses and uncovering a GPI-AP-mediated trafficking mechanism for ER-retained receptors.