Involved in endoplasmic reticulum (ER) stress response. Senses unfolded proteins in the lumen of the ER via its N-terminal domain which leads to enzyme auto-activation. The active endoribonuclease domain splices bZIP50 mRNA to generate a new C-terminus, converting it into a potent unfolded-protein response (UPR) transcriptional activator, which then induces transcription of UPR target genes, such as luminal-binding protein (BiP) chaperones. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimer; disulfide-linked. Dimer formation is driven by hydrophobic interactions within the N-terminal luminal domains and stabilized by disulfide bridges (By similarity). Expressed in roots, nodes, internodes, leaf sheaths, leaf blades, young ears and mature ears. Autophosphorylated. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.