UDP-glucose:glycoprotein glucosyltransferase (UGGT) is a presumed folding sensor of protein quality control in the endoplasmic reticulum (ER). Previous biochemical studies with nonphysiological substrates revealed that UGGT can glucosylate nonnative glycoproteins by recognizing subtle folding defects; however, its physiological function remains undefined. Here, we show that mutations in the Arabidopsis EBS1 gene suppressed the growth defects of a brassinosteroid (BR) receptor mutant, bri1-9, in an allele-specific manner by restoring its BR sensitivity. Using a map-based cloning strategy, we discovered that EBS1 encodes the Arabidopsis homolog of UGGT. We demonstrated that bri1-9 is retained in the ER through interactions with several ER chaperones and that ebs1 mutations significantly reduce the stringency of the retention-based ER quality control, allowing export of the structurally imperfect yet biochemically competent bri1-9 to the cell surface for BR perception. Thus, our discovery provides genetic support for a physiological role of UGGT in high-fidelity ER quality control.