Senses unfolded proteins in the lumen of the endoplasmic reticulum via its N-terminal domain which leads to enzyme auto-activation. The active endoribonuclease domain splices bZIP60 mRNA to generate a new C-terminus, converting it into a potent unfolded-protein response transcriptional activator which then induces transcription of UPR target genes. Involved in organ growth regulation. Plays a role in plant immunity and abiotic stress responses. Required for ER stress-induced autophagy. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] The kinase domain is activated by trans-autophosphorylation. Kinase activity is required for activation of the endoribonuclease domain (By similarity). Homodimer; disulfide-linked. Dimer formation is driven by hydrophobic interactions within the N-terminal luminal domains and stabilized by disulfide bridges (By similarity). A number of isoforms are produced. According to EST sequences. Ubiquitous. Detected in the apical meristem, at leaf margins where vascular bundles end, in the anthers before pollen is formed and in the ovules at a very early stage of development. There is no expression in more mature embryos. Also strongly expressed in the cotyledons immediately after germination but not later on. By ER stress inducer tunicamycin, by salicylic acid (SA) and by bacterial pathogen infection. Autophosphorylated. No visible phenotype. Ire1a and ire1b double mutant is more sensitive to the ER stress inducer tunicamycin than the wild-type and is enable to give rise to the spliced bZIP60 mRNA form (PubMed:22355548). Ire1a and ire1b double mutant displays short roots and a ER stress-sensitive phenotype (PubMed:21914012). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.