The plant-specific transcription factor gene NAC103 is induced by bZIP60 through a new cis-regulatory element to modulate the unfolded protein response in Arabidopsis

The unfolded protein response (UPR) plays important roles in plant development and plant-pathogen interactions, as well as in plant adaptation to adverse environmental stresses. Previously bZIP28 and bZIP60 have been identified as important UPR regulators for mitigating the endoplasmic reticulum (ER) stress in Arabidopsis thaliana. Here we report the biological function of NAC103 in a novel transcriptional regulatory cascade, connecting bZIP60 to the UPR downstream genes in Arabidopsis. Expression of NAC103 was induced by ER stress, and was completely abolished in the bZIP60 null mutant. A new ER stress-responsive cis-element UPRE-III (TCATCG) on the NAC103 promoter was identified, and trans-activation of UPRE-III by bZIP60 was confirmed in both yeast cells and Arabidopsis protoplasts. The direct binding of bZIP60 to UPRE-III-containing DNA was also demonstrated in an electrophoretic mobility shift assay. NAC103 formed homodimers in yeast two-hybrid and bimolecular fluorescence complementation assays. It had transcriptional activation activity and was localized in the nucleus. Over-expression of NAC103 had pleiotropic effects on plant growth, and induced expression of several UPR downstream genes in Arabidopsis under normal growth conditions. The activation of UPR gene promoters by NAC103 was also confirmed in effector/reporter protoplast assays. Thus, our study demonstrates a transcriptional regulatory cascade in which NAC103 relays ER stress signals from bZIP60 to UPR downstream genes through a newly identified ER stress cis-element (UPRE-III) and transcriptional activation activity of its encoded protein NAC103.