Endoplasmic reticulum‐mediated unfolded protein response is an integral part of singlet oxygen signalling in plants

Singlet oxygen (¹O₂) is a by‐product of photosynthesis that triggers a signalling pathway leading to stress acclimation or to cell death. By analyzing gene expressions in a ¹O₂‐overproducing Arabidopsis mutant (ch1) under different light regimes, we show here that the ¹O₂ signalling pathway involves the endoplasmic reticulum (ER)‐mediated unfolded protein response (UPR). ch1 plants in low light exhibited a moderate activation of UPR genes, in particular bZIP60, and low concentrations of the UPR‐inducer tunicamycin enhanced tolerance to photooxidative stress, together suggesting a role for UPR in plant acclimation to low ¹O₂ levels. Exposure of ch1 to high light stress ultimately leading to cell death resulted in a marked upregulation of the two UPR branches (bZIP60/IRE1 and bZIP28/bZIP17). Accordingly, mutational suppression of bZIP60 and bZIP28 increased plant phototolerance, and a strong UPR activation by high tunicamycin concentrations promoted high light‐induced cell death. Conversely, light acclimation of ch1 to ¹O₂ stress put a limitation in the high light‐induced expression of UPR genes, except for the gene encoding the BIP3 chaperone, which was selectively upregulated. BIP3 deletion enhanced Arabidopsis photosensitivity while plants treated with a chemical chaperone exhibited enhanced phototolerance. In conclusion, ¹O₂ induces the ER‐mediated UPR response that fulfils a dual role in high light stress: a moderate UPR, with selective induction of BIP3, is part of the acclimatory response to ¹O₂, and a strong activation of the whole UPR is associated with cell death.     

Rice stripe virus activates the bZIP17/28 branch of the unfolded protein response signalling pathway to promote viral infection

The unfolded protein response (UPR) plays important roles in plant virus infection. Our previous study has proved that rice stripe virus (RSV) infection elicits host UPR. However, the mechanism on how the UPR is triggered upon RSV infection remains obscure. Here, we show that the bZIP17/28 branch of the UPR signalling pathway is activated upon RSV infection in Nicotiana benthamiana. We found that membrane-associated proteins NSvc2 and NSvc4 encoded by RSV are responsible for the activation of the bZIP17/28 branch. Ectopic expression of NSvc2 or NSvc4 in plant leaves induced the proteolytic processing of NbbZIP17/28 and up-regulated the expression of UPR-related genes. Silencing NbbZIP17/28 significantly inhibited RSV infection. We show that RSV can specifically elicit the UPR through the bZIP17/28 branch, thus promoting virus infection of N. benthamiana plants.     

Inter‐regulation of the unfolded protein response and auxin signaling

The unfolded protein response (UPR) is a signaling network triggered by overload of protein‐folding demand in the endoplasmic reticulum (ER), a condition termed ER stress. The UPR is critical for growth and development; nonetheless, connections between the UPR and other cellular regulatory processes remain largely unknown. Here, we identify a link between the UPR and the phytohormone auxin, a master regulator of plant physiology. We show that ER stress triggers down‐regulation of auxin receptors and transporters in Arabidopsis thaliana. We also demonstrate that an Arabidopsis mutant of a conserved ER stress sensor IRE1 exhibits defects in the auxin response and levels. These data not only support that the plant IRE1 is required for auxin homeostasis, they also reveal a species‐specific feature of IRE1 in multicellular eukaryotes. Furthermore, by establishing that UPR activation is reduced in mutants of ER‐localized auxin transporters, including PIN5, we define a long‐neglected biological significance of ER‐based auxin regulation. We further examine the functional relationship of IRE1 and PIN5 by showing that an ire1 pin5 triple mutant enhances defects of UPR activation and auxin homeostasis in ire1 or pin5. Our results imply that the plant UPR has evolved a hormone‐dependent strategy for coordinating ER function with physiological processes.     

Cell cycle arrest mediated by WEE1 is involved in the unfolded protein response in plants

Activation of the unfolded protein response (UPR) in mammalian cells leads to cell cycle arrest at the G1 phase (Thomas et al., J Biol Chem 288:7606–7617, 2013). However, how UPR signaling affects cell cycle arrest remains largely unknown in plants. Here, we examined UPR and endoreduplication in Col-0, wee1, and ER stress sensing-deficient ire1a&b plants during DNA replication and ER stress conditions. We found that WEE1, an essential negative regulator of the cell cycle, is involved in the maintenance of ER homeostasis during genotoxic stress and the ER stress hypersensitivity of ire1a&b is alleviated by loss-of-function mutation in WEE1. WEE1-mediated cell cycle arrest was required for IRE1–bZIP60 pathway activation during ER stress. In contrast, loss-of-function mutation in WEE1 caused increased expression of UPR-related genes during DNA replication stress. WEE1 and IRE1 were required for endoreduplication during DNA replication stress and ER stress, respectively. Taken together, these findings suggest that cell cycle regulation is associated with UPR activation in different manners during ER stress and DNA replication stress in Arabidopsis.     

The Effects of Melatonin on Transcriptional Profile of Unfolded Protein Response Genes Under Endoplasmic Reticulum Stress in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

When the load of secretory pathway is increased or folding capacity in the endoplasmic reticulum (ER) is insufficient, unfolded proteins might accumulate in ER lumen causing a phenomenon called ER stress. During ER stress, normal cell functions are suppressed and unfolded protein response (UPR) is induced. Studies in animal systems suggest that melatonin alleviates the detrimental effects of ER stress; however, there is no study in plants in this respect. Hence, in this study, we investigated the possible role of melatonin on alleviation of ER stress in model plant Arabidopsis thaliana. Tunicamycin (Tm) was used to specifically induce ER stress. Melatonin treatment (10 and 25 μM but not 1 μM) increased root growth under Tm treatment, but it did not reach control levels. ER stress induced the expressions of ER stress sensor/transducer genes, ER chaperones and folding helper genes, ER-associated degradation (ERAD) genes, and ER stress-associated apoptosis genes in roots and shoots (a total of 16 genes). Among them, the expressions of ER stress sensor/transducer bZIP17, bZIP28, IRE1A, IRE1B, ERAD-related SEL1, and apoptosis genes AGB1 were decreased back to control levels with 25 μM melatonin under ER stress in roots. Moreover, Tm?+?melatonin treatments decreased the expressions of these genes when compared to only Tm-treated plants. Downregulation of UPR components with increased concentrations of melatonin under Tm treatment demonstrated that melatonin alleviated the detrimental effects of ER stress.     

AtIRE1A/AtIRE1B and AGB1 independently control two essential unfolded protein response pathways in Arabidopsis

The endoplasmic reticulum (ER) has the ability to maintain the balance between demand for and synthesis of secretory proteins. To ensure protein‐folding homeostasis in the ER, cells invoke signaling pathways known as the unfolded protein response (UPR). To initiate UPR, yeasts largely rely on a conserved sensor, IRE1. In metazoans, there are at least three independent UPR signalling pathways. Some UPR transducers have been identified in plants, but no genetic interaction among them has yet been examined. The Arabidopsis genome encodes two IRE1 sequence homologs, AtIRE1A and AtIRE1B. Here we provide evidence that AtIRE1A and AtIRE1B have overlapping functions that are essential for the plant UPR. A double mutant of AtIRE1A and AtIRE1B, atire1a atire1b, showed reduced ER stress tolerance and a compromised UPR activation phenotype. We have also established that Arabidopsis AGB1, a subunit of the ubiquitous heterotrimeric GTP‐binding protein family, and AtIRE1A/AtIRE1B independently control two essential plant UPR pathways. By demonstrating that atire1a atire1b has a short root phenotype that is enhanced by an agb1 loss‐of‐function mutation, we have identified a role for UPR transducers in organ growth regulation.