Transcription factor involved in endoplasmic reticulum (ER) stress response. Acts as ER stress sensor and activates the transcription factor BZIP50 and the chaperone BIP1. The bZIP domain is translocated into the nucleus in response to ER stress. Highly expressed in leaf blade, and at lower levels in roots, leaf sheaths, flowers and seeds. Induced by dithiothreitol- and tunicamycin-induced endoplasmic reticulum (ER) stress response. Belongs to the bZIP family.