ENDOMEMBRANE PROTEIN 70 (EMP70) proteins constitute a 12-member superfamily in Arabidopsis thaliana， and are the most abundant protein species in plant Golgi proteomes. However， the physiological functions of EMPs in plants remain largely unknown. Here we have demonstrated that two AtEMP12 T-DNA insertion mutants are sensitive to ER (endoplasmic reticulum) stress as induced by tunicamycin and dithiothreitol treatments. Interestingly， the unfolded protein response (UPR) is constitutively activated in the knockout mutant emp12-1 under normal growth conditions， suggesting that the activation is a result of insufficient chaperones in the ER to aid protein folding. Indeed， we have further shown that BiP is secreted into the apoplast in emp12-1， while the K/HDEL receptor ERD2a， which regulates BiP trafficking， is exclusively localized in the ER in emp12-1， instead of its known ER-Golgi dual-localization. Given an enhanced retrograde transport of ERD2a， along with less dimerized receptor formed in the absence of EMP12， ERD2a may be prematurely returned to the ER without its bound ligands. Therefore， we propose that EMP12 may act as a novel regulator of the K/HDEL receptor to ensure an effective retrograde transport of K/HDEL ligands.