Identification of a cis-element that mediates multiple pathways of the endoplasmic reticulum stress response in rice

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) lumen leads to ER stress. Intracellular signalling pathways are activated to alleviate the stress. The ER stress sensor IRE1 induces the active form of key transcription factors, such as XBP1 in mammals and bZIP50 in Oryza sativa (rice), by mediating the unconventional splicing of their mRNAs. Although the characterization of cis-elements that are recognized by these transcription factors is essential for understanding ER stress responses, such cis-elements remain unidentified in plants. Here, a cis-element named pUPRE-II was identified from promoters of bZIP50-dependent genes using chromatin immunoprecipitation assays and electrophoretic mobility shift assays. The sequence of pUPRE-II (e.g., 5'-GATGACGCGTAC-3' in the OsSAR1 promoter) was found to be flexible and not identical with that of mUPRE, a cis-element that preferentially interacts with mammalian XBP1. Unexpectedly, the transcription factor bZIP60, another ER stress sensor in rice, and a counterpart of mammalian ATF6, also showed strong binding affinity for pUPRE-II without assistance from co-factors. Reporter assays indicated that pUPRE-II significantly contributes to gene expression mediated by bZIP50 or bZIP60 in rice. Although both bZIP50 and bZIP60 bound to pUPRE-II, these transcription factors showed distinct requirements for transcriptional activation. This study provides a missing link between ER stress sensors and stress-responsive genes in rice. Furthermore, the characteristics of pUPRE-II highlight the uniqueness of ER stress-responsive transcription in plants.