Signaling from the endoplasmic reticulum activates brassinosteroid signaling and promotes acclimation to stress in Arabidopsis

The ability to acclimate to stresses enables plants to grow and develop under adverse environmental conditions. Regulated intramembrane proteolysis (RIP) triggered by endoplasmic reticulum (ER) stress mediates some forms of stress signaling. Brassinosteroids (BRs) have been implicated in plant adaptation to stress, but no mechanisms for activation have been discovered. Here, we reveal a connection between ER stress signaling and BR-mediated growth and stress acclimation. Arabidopsis transcription factors bZIP17 and bZIP28 were translocated from the ER through the Golgi, where they were proteolytically cleaved by site 2 protease and released to translocate into the nucleus. Stresses, including heat and inhibition of protein glycosylation, increased translocation of these two bZIPs to the nucleus. These nuclear-localized bZIPs not only activated ER chaperone genes but also activated BR signaling, which was required for stress acclimation and growth. Thus, these bZIPs link ER stress and BR signaling, which may be a mechanism by which plant growth and stress responses can be integrated.