More players in the plant unfolded response.

The endoplasmic reticulum (ER) is the major source of molecules that directly interact with the cell’s external environment. This occurs through the secretory pathway, which starts from the ER and regulates macromolecular traffic to the cell surface and inner hydrolytic compartments. It is thus no wonder that the ER functions as a major, very efficient station that integrates signaling from the environment and takes decisions on appropriate responses. The most promiscuous ER signaling mechanism is the unfolded protein response (UPR) (1). A major UPR sensing system appears to be conserved in all eukaryotes and monitors the workload of the ER in the folding and assembly of newly synthesized secretory proteins. This workload can be increased when environmental conditions negatively affect protein folding or when developmental response programs require higher synthesis of secretory proteins. In these situations, ER resident transmembrane sensors start a cascade of various events that eventually alters the expression of many genes, mainly stimulating the synthesis of ER folding helpers and at the same time repressing general synthesis of passenger secretory proteins (1). When the response is not sufficient, unresolved UPR induces programmed cell death (PCD). In PNAS, two studies (2, 3) now provide important progress in characterizing the activity of upstream and downstream players of plant UPR and their relationships with developmental programs as well as PCD. Inositol requiring enzyme1 (IRE1), the only UPR sensor identified in budding yeast, is a transmembrane ER protein present also in plants and metazoans. Its luminal domain interacts with the major ER chaperone immunoglobulin binding protein (BiP) and thus senses the amount of resting BiP (4). When ER workload increases, BiP is displaced, IRE1 oligomerizes, authophosphorylates, and activates its ribonuclease cytosolic domain, starting a series of events that is not exactly the same in all eukaryotes but always leads to major gene expression alterations. Additional, kingdom-specific UPR transmembrane sensors exist, probably reflecting the diverse strategies available to unicellular eukaryotes, metazoans or plants: PKR-like ER kinase (PERK) and the activating transcription factor 6 (ATF6) in metazoans and the basic leucine zipper (bZIP) transcription factors bZIP28 and bZIP17 in plants (1). All of them are BiP interactors. The major substrate of plant IRE1 is the transcription factor bZIP60. This is normally an integral ER transmembrane protein, but IRE1 ribonuclease activity removes an intron, causing a frameshift leading to the synthesis of an alternative, soluble bZIP60 form that can thus migrate into the nucleus (5, 6). BiP release from bZIP28 and bZIP17 instead induces their traffic from the ER to the Golgi apparatus, followed by removal of the transmembrane domains by Golgi proteases (7–9). This mechanism is similar to the one that activates ATF6, whereas metazoan IRE1 acts slightly differently from the plant homolog: its substrate mRNA encodes a soluble but inactive transcription factor activated upon IRE1-mediated intron removal (1). There are two IRE1 genes in Arabidopsis: IRE1a and IRE1b. Single KOs do not have growth defects, but the double ire1a ire1b mutant is defective in root growth also in the absence of imposed UPR stress, indicating that the sensors play a role during normal growth of a highly secreting tissue (10). This double mutant is also more sensitive to tunicamycin, an inhibitor of protein N-glycosylation and the most widely used UPR inducer (10). Deng et al. (2) stacked KO mutations of IRE1 and other known Arabidopsis UPR sensors and analyzed seedling survival, as well as root elongation in the absence of stress or on UPR (in this case induced by the reducing agent DTT). The triple mutant bzip28, ire1a ire1b is lethal. The phenotype of bzip28 ire1b is similar to that of ire1a ire1b, Protein folding