Stresses leading to the accumulation of misfolded proteins in the endoplasmic reticulum (ER) elicit a highly conserved ER stress response in plants called the unfolded protein response (UPR). While the response itself is well documented in plants, the components of the signaling pathway are less well known. We have identified three membrane-associated basic domain/leucine zipper (bZIP) factors in Arabidopsis thaliana that are candidates for ER stress sensors/transducers. One of these factors, bZIP28, an ER-resident transcription factor, is activated in response to treatment by tunicamycin (TM), an agent that blocks N-linked protein glycosylation. Following TM treatment, bZIP28 is processed, releasing its N-terminal, cytoplasm-facing domain, which is translocated to the nucleus. Expression of a truncated form of bZIP28, containing only the cytoplasmic domain of the protein, upregulated the expression of ER stress response genes in the absence of stress conditions. Thus, bZIP28 serves as a sensor/transducer in Arabidopsis to mediate ER stress responses related to UPR.