Fig. 6

Comprehensive panel of the ER stress response in soybean. To enter the ER lumen, secretory proteins are translated by ER-associated polysomes, and the nascent secretory peptide is co-translationally transported to the ER through the Sec61 translocation complex (1). In the ER, the pre-assembly oligosaccharide core (Glc3Man9GlcNAc2; N-glycan) is transferred (2) from the ER-localized dolichyl pyrophosphate (Dol-PP) to the nascent polypeptide by oligosaccharyltransferase (OST). Processing or trimming of the N-glycan begins in the ER with the sequential removal of the more external glucose residues by glucosidase I (3) and glucosidase II (4). The monoglycosylated glucan-peptide is targeted to the calnexin/calreticulin system containing the protein disulfide isomerase (PDI) accessory protein for proper folding (5). Folded proteins are released from this N-glycan-dependent quality control mechanism through hydrolysis of the third glucose residue by GluII. Properly folded proteins leave the ER. Unfolded proteins may be re-glycosylated by UDP-glucose:glycoprotein glucosyltransferase (UGGT) to re-enter the CNX/CRT-mediated folding cycle (6). The removal of glucose residues and transient re-addition of the innermost glucose during protein folding contribute to the ER retention time of a given glycoprotein. Failure to achieve the proper conformation within a defined period of time is a signal for exclusion of the glycoprotein from the CNX/CRT folding cycle by the sequential removal of two α1,2-mannose residues by MNS3 and MNS4/MNS5 (7). The removal of these residues exposes an α1,6-mannose, which targets the glycoprotein to the ERAD pathway. EBS6 and EBS5 recruit unfolded glycoproteins to redirect them to the membrane-associated ERAD complex for ubiquitination and retrotranslocation to the cytosol, where they are targeted to the proteasome (9). Ubiquitinated ERAD substrates are directed from the ER to the proteasome via the trimeric complex cdc48/Ufd1/Npl4 (9). ER stress induces the accumulation of unfolded proteins in the lumen and activates the UPR pathway (10). BiP-mediated dissociation 0of the UPR transducers GmbZIP37/38 (AtbZIP17/28) (11) allows for the mobilization of these receptors to the Golgi (14), where they are proteolytically cleaved by S1P and S2P (15), releasing the N-terminal bZIP domains as functional TFs that are then translocated to the nucleus (15), where they activate ER stress-responsive promoters (16). In the other arm of the UPR (12), under ER stress, GmIRE1 dimerizes to activate its ribonuclease activity, which promotes unconventional splicing of the GmbZIP68 (AtbZIP60) mRNA, generating an active TF (GmbZIP68S) lacking the transmembrane segment. GmbZIP68S (AtbZIP60S) moves to the nucleus to induce the expression of molecular chaperones, ERAD components, GmbZIP68 (AtbZIP60), GmNAC021 (AtNAC062) and GmNAC103 (AtNAC089). Evidence indicates that GmbZIP37/38 and GmbZIP68S may act in concert as heterodimers to activate ER stress-responsive genes. GmbZIP68S (AtbZIP60S) also induces expression of the TF GmNAC020 (AtNAC103) to further amplify the ER stress response (19). As a plasma membrane component of the ER stress response (17), membrane-tethered GmNAC021 (AtNAC62) also undergoes regulated intramembrane proteolysis (RIP) for release into the nucleus as a positive regulator of ER stress-responsive genes. If the UPR is not capable of restoring ER homeostasis under prolonged and severe stress, then PCD responses are activated for the regulated disposal of abnormal cells. ER stress-induced proteolysis of ER membrane-tethered GmNAC103 (AtNAC089) exemplifies an ER stress-induced plant-specific PCD response (20). RIP-mediated translocation of GmNAC103 to the nucleus allows for the induction of PCD-associated gene expression, promoting DNA fragmentation and an increase in caspase-3/7-like activity. A distinct ER stress-induced PCD response in soybean integrates an osmotic stress signal into a full response (22). The combination of ER stress and osmotic stress fully induces the expression of the TF GmERD15 (22) to activate the expression of the membrane-associated protein DCD/NRP-B (23). Induction of DCD/NRP-B activates a signaling cascade that culminates with the induction of the GmNAC081 and GmNAC030 TFs (24), which form heterodimers to fully transactivate the vacuolar processing enzyme (VPE) promoter (25). VPE exhibits caspase-1-like activity and induces plant-specific PCD, mediated by collapse of the vacuole (26)