Auxin,one of the plant hormones,is a key regulator of plant growth and development.At the cellular level,it controls different processes,such as cell expansion,division,and differentiation that are reflected by its regulatory role in a plethora of developmental mechanisms.An important feature of the auxin action is its differential distribution within tissues mediated by the polar auxin transport machinery,which can be dynamically regulated in response to inteal and exteal stimuli.Receptors at the cell surface or cell interior are needed to sense and interpret fluctuations in the auxin distribution.Until now,three auxin-binding proteins or protein complexes have been identified or implied as the auxin receptors;two are localized in the nucleus,the SKP-Cullin-F-box-Transport Inhibitor Response 1/Auxin-related F-Box (SCFTIR1/AFB) and the S-Phase Kinase-Associated Protein 2A (SKP2A),and one,the AuxinBinding Protein 1 (ABP1),occurs prsdominantly at the endoplasmic reticulum (ER) and cell surface.Early studies have demonstrated that ABP1 is involved in the rapid regulation of the membrane potential and ion fluxes at the plasma membrane (PM) and that it mediates auxin-induced cell swelling,cell elongation,and cell division.Recently identified roles of ABP1 include that activation of expression of auxin-responsive genes as well as regulation of cell morphogenesis,cytoskeleton rearrangement,shape of leaf epidermal pavement cells,and clathrin-mediated endocytosis (reviewed in Bargann and Estelle,2014).Renewed research on ABP1 has brought new insights into the complexity of ABP1 signaling and its developmental roles.