bZIP factors of the Unfolded Protein Response interact with PIF4 to promote thermomorphogenesis

When Arabidopsis plants are exposed to warm temperatures (e.g., 29 °C), they undergo adaptive growth known as thermomorphogenesis. This process is primarily regulated by the phytochrome B (phyB)-PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) module; however, the potential involvement of additional signaling pathways remains underexplored. Here, we show that warmth triggers endoplasmic reticulum (ER) stress, activating both arms of the Unfolded Protein Response (UPR). Three UPR-associated bZIP transcription factors, bZIP17, bZIP28 and bZIP60, promote hypocotyl growth under warmth in a PIF4-dependent manner. Active bZIP factors form complexes with PIF4 in the nucleus, where they bind to promoter regions of PIF4 and other growth-related genes to enhance their expression. In parallel, bZIPs overexpression counteract the inhibitory effect of phyB on PIF4 stability, thereby reinforcing thermomorphogenic growth. Together, our findings define a regulatory axis that links ER stress, the UPR and thermomorphogenesis, clarifying how plants coordinate physiological and environmental cues to adapt to warming conditions.