Activation of the Transducers of Unfolded Protein Response in Plants

Investigating the effects of tauroursodeoxycholic acid (TUDCA) in mitigating endoplasmic reticulum stress and cellular responses in Pak choi

The accumulation of misfolded proteins in the endoplasmic reticulum (ER) within plant cells due to unfavourable conditions leads to ER stress. This activates interconnected pathways involving reactive oxygen species (ROS) and unfolded protein response (UPR), which play vital roles in regulating ER stress. The aim of this study is to investigate the underlying mechanisms of tunicamycin (TM) induced ER stress and explore the potential therapeutic applications of tauroursodeoxycholic acid (TUDCA) in mitigating cellular responses to ER stress in Pak choi (Brassica campestris subsp. chinensis). The study revealed that ER stress in Pak choi leads to detrimental effects on plant morphology, ROS levels, cellular membrane integrity, and the antioxidant defence system. However, treatment with TUDCA in TM-induced ER stressed Pak choi improved morphological indices, pigment contents, ROS accumulation, cellular membrane integrity, and antioxidant defence system restoration. Additionally, TUDCA also modulates the transcription levels of ER stress sensors genes, ER chaperone genes, and ER-associated degradation (ERAD) genes during ER stress in Pak choi. Furthermore, TUDCA has demonstrated its ability to alleviate ER stress, stabilize the UPR, reduce oxidative stress, prevent apoptosis, and positively influence plant growth and development. These results collectively comprehend TUDCA as a promising agent for mitigating ER stress-induced damage in Pak choi plants and provide valuable insights for further research and potential applications in crop protection and stress management.

The role of salicylic acid on glutathione metabolism under endoplasmic reticulum stress in tomato

The endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) are highly dependent on phytohormones such as salicylic acid (SA). In this study, the effect of SA supplementation and the lack of endogenous SA on glutathione metabolism were investigated under ER stress in wild-type (WT) and transgenic SA-deficient NahG tomato (Solanum lycopersicum L.) plants. The expression of the UPR marker gene SlBiP was dependent on SA levels and remained lower in NahG plants. Exogenous application of the chemical chaperone 4-phenylbutyrate (PBA) also reduced tunicamycin (Tm)-induced SlBiP transcript accumulation. At the same time, Tm-induced superoxide and hydrogen peroxide production were independent of SA, whereas the accumulation of reduced form of glutathione (GSH) and the oxidised glutathione (GSSG) was regulated by SA. Tm increased the activity of glutathione reductase (GR; EC 1.6.4.2) independently of SA, but the activities of dehydroascorbate reductase (DHAR; EC 1.8.5.1) and glutathione S-transferases (GSTs; EC 2.5.1.18) were increased by Tm in a SA-dependent manner. SlGR2, SlGGT and SlGSTT2 expression was activated in a SA-dependent way upon Tm. Although expression of SlGSH1, SlGSTF2, SlGSTU5 and SlGTT3 did not change upon Tm treatment in leaves, SlGR1 and SlDHAR2 transcription decreased. PBA significantly increased the expression of SlGR1, SlGR2, SlGSTT2, and SlGSTT3, which contributed to the amelioration of Tm-induced ER stress based on the changes in lipid peroxidation and cell viability. Malondialdehyde accumulation and electrolyte leakage were significantly higher in WT as compared to NahG tomato leaves under ER stress, further confirming the key role of SA in this process.

Modifications in Ultrastructural Characteristics and Redox Status of Plants under Environmental Stress: A Review

The rate of global environmental change is unprecedented, with climate change causing an increase in the oscillation and intensification of various abiotic stress factors that have negative impacts on crop production. This issue has become an alarming global concern, especially for countries already facing the threat of food insecurity. Abiotic stressors, such as drought, salinity, extreme temperatures, and metal (nanoparticle) toxicities, are recognized as major constraints in agriculture, and are closely associated with the crop yield penalty and losses in food supply. In order to combat abiotic stress, it is important to understand how plant organs adapt to changing conditions, as this can help produce more stress-resistant or stress-tolerant plants. The investigation of plant tissue ultrastructure and subcellular components can provide valuable insights into plant responses to abiotic stress-related stimuli. In particular, the columella cells (statocytes) of the root cap exhibit a unique architecture that is easily recognizable under a transmission electron microscope, making them a useful experimental model for ultrastructural observations. In combination with the assessment of plant oxidative/antioxidative status, both approaches can shed more light on the cellular and molecular mechanisms involved in plant adaptation to environmental cues. This review summarizes life-threatening factors of the changing environment that lead to stress-related damage to plants, with an emphasis on their subcellular components. Additionally, selected plant responses to such conditions in the context of their ability to adapt and survive in a challenging environment are also described.

The sieve-element endoplasmic reticulum: A focal point of phytoplasma-host plant interaction?

The rough endoplasmic reticulum (r-ER) is of paramount importance for adaptive responses to biotic stresses due to an increased demand for de novo synthesis of immunity-related proteins and signaling components. In nucleate cells, disturbance of r-ER integrity and functionality leads to the "unfolded protein response" (UPR), which is an important component of innate plant immune signalling. In contrast to an abundance of reports on r-ER responses to biotic challenges, sieve-element endoplasmic reticulum (SE-ER) responses to phytoplasma infection have not been investigated. We found that morphological SE-ER changes, associated with phytoplasma infection, are accompanied by differential expression of genes encoding proteins involved in shaping and anchoring the reticulum. Phytoplasma infection also triggers an increased release of bZIP signals from the (SE-ER)/r-ER and consequent differential expression of UPR-related genes. The modified expression patterns seem to reflect a trade-off between survival of host cells, needed for the phytoplasmic biotrophic lifestyle, and phytoplasmas. Specialized plasmodesmata between sieve element and companion cell may provide a corridor for transfer of phytoplasma effectors inducing UPR-related gene expression in companion cells.

Engineering drought and salinity tolerance traits in crops through CRISPR-mediated genome editing: Targets, tools, challenges, and perspectives

Prolonged periods of drought triggered by climate change hamper plant growth and cause substantial agricultural yield losses every year. In addition to drought, salinity is one of the major abiotic stresses that severely affect crop health and agricultural production. Plant responses to drought and salinity involve multiple processes that operate in a spatiotemporal manner, such as stress sensing, perception, epigenetic modifications, transcription, post-transcriptional processing, translation, and post-translational changes. Consequently, drought and salinity stress tolerance are polygenic traits influenced by genome-environment interactions. One of the ideal solutions to these challenges is the development of high-yielding crop varieties with enhanced stress tolerance, together with improved agricultural practices. Recently, genome-editing technologies, especially clustered regularly interspaced short palindromic repeats (CRISPR) tools, have been effectively applied to elucidate how plants deal with drought and saline environments. In this work, we aim to portray that the combined use of CRISPR-based genome engineering tools and modern genomic-assisted breeding approaches are gaining momentum in identifying genetic determinants of complex traits for crop improvement. This review provides a synopsis of plant responses to drought and salinity stresses at the morphological, physiological, and molecular levels. We also highlight recent advances in CRISPR-based tools and their use in understanding the multi-level nature of plant adaptations to drought and salinity stress. Integrating CRISPR tools with modern breeding approaches is ideal for identifying genetic factors that regulate plant stress-response pathways and for the introgression of beneficial traits to develop stress-resilient crops.

Deciphering the mechanism of anhydrobiosis in the entomopathogenic nematode Heterorhabditis indica through comparative transcriptomics

The entomopathogenic nematode, Heterorhabditis indica, is a popular biocontrol agent of high commercial significance. It possesses tremendous genetic architecture to survive desiccation stress by undergoing anhydrobiosis to increase its lifespan-an attribute exploited in the formulation technology. The comparative transcriptome of unstressed and anhydrobiotic H. indica revealed several previously concealed metabolic events crucial for adapting towards the moisture stress. During the induction of anhydrobiosis in the infective juveniles (IJ), 1584 transcripts were upregulated and 340 downregulated. As a strategy towards anhydrobiotic survival, the IJ showed activation of several genes critical to antioxidant defense, detoxification pathways, signal transduction, unfolded protein response and molecular chaperones and ubiquitin-proteasome system. Differential expression of several genes involved in gluconeogenesis - β-oxidation of fatty acids, glyoxylate pathway; glyceroneogenesis; fatty acid biosynthesis; amino-acid metabolism - shikimate pathway, sachharopine pathway, kyneurine pathway, lysine biosynthesis; one-carbon metabolism-polyamine pathway, transsulfuration pathway, folate cycle, methionine cycle, nucleotide biosynthesis; mevalonate pathway; and glyceraldehyde-3-phosphate dehydrogenase were also observed. We report the role of shikimate pathway, sachharopine pathway and glyceroneogenesis in anhydrobiotes, and seven classes of repeat proteins, specifically in H. indica for the first time. These results provide insights into anhydrobiotic survival strategies which can be utilized to strengthen the development of novel formulations with enhanced and sustained shelf-life.

The plant endoplasmic reticulum UPRome: A repository and pathway browser for genes involved in signaling networks linked to the endoplasmic reticulum

The endoplasmic reticulum (ER) houses sensors that respond to environmental stress and underly plants' adaptative responses. These sensors transduce signals that lead to changes in nuclear gene expression. The ER to nuclear signaling pathways are primarily attributed to the unfolded protein response (UPR) and are also integrated with a wide range of development, hormone, immune, and stress signaling pathways. Understanding the role of the UPR in signaling network mechanisms that associate with particular phenotypes is crucially important. While UPR-associated genes are the subject of ongoing investigations in a few model plant systems, most remain poorly annotated, hindering the identification of candidates across plant species. This open-source curated database provides a centralized resource of peer reviewed knowledge of ER to nuclear signaling pathways for the plant community. We provide a UPRome interactive viewer for users to navigate through the pathways and to access annotated information. The plant ER UPRome website is located at http://uprome.tamu.edu. We welcome contributions from the researchers studying the ER UPR to incorporate additional genes into the database through the "contact us" page.

Role of ethylene in ER stress and the unfolded protein response in tomato (Solanum lycopersicum L.) plants

The unfolded protein response (UPR) plays a significant role in the maintenance of cellular homeostasis under endoplasmic reticulum (ER) stress, which is highly dependent on the regulation of defense-related phytohormones. In this study, the role of ethylene (ET) in ER stress and UPR was investigated in the leaves of intact tomato (Solanum lycopersicum) plants. Exogenous application of the ET precursor 1-aminocyclopropane-1-carboxylic acid not only resulted in higher ET emission from leaves but also increased the expression of the UPR marker gene SlBiP and the transcript levels of the ER stress sensor SlIRE1, as well as the levels of SlbZIP60, after 24 h in tomato leaves. Using ET receptor Never ripe (Nr) mutants, a significant role of ET in tunicamycin (Tm)-induced ER stress sensing and signaling was confirmed based on the changes in the expression levels of SlIRE1b and SlBiP. Furthermore, the analysis of other defense-related phytohormones showed that the Tm-induced ET can affect positively the levels of and response to salicylic acid. Additionally, it was found that nitric oxide production and lipid peroxidation, as well as the electrolyte leakage induced by Tm, is regulated by ET, whereas the levels of H₂O₂ and proteolytic activity seemed to be independent of ET under ER stress in the leaves of tomato plants.

Heat Stress-Mediated Constraints in Maize (Zea mays) Production: Challenges and Solutions

An increase in temperature and extreme heat stress is responsible for the global reduction in maize yield. Heat stress affects the integrity of the plasma membrane functioning of mitochondria and chloroplast, which further results in the over-accumulation of reactive oxygen species. The activation of a signal cascade subsequently induces the transcription of heat shock proteins. The denaturation and accumulation of misfolded or unfolded proteins generate cell toxicity, leading to death. Therefore, developing maize cultivars with significant heat tolerance is urgently required. Despite the explored molecular mechanism underlying heat stress response in some plant species, the precise genetic engineering of maize is required to develop high heat-tolerant varieties. Several agronomic management practices, such as soil and nutrient management, plantation rate, timing, crop rotation, and irrigation, are beneficial along with the advanced molecular strategies to counter the elevated heat stress experienced by maize. This review summarizes heat stress sensing, induction of signaling cascade, symptoms, heat stress-related genes, the molecular feature of maize response, and approaches used in developing heat-tolerant maize varieties.

A plant receptor domain with functional analogies to animal malectin disables ER stress responses upon infection

Malectins from the oligosaccharyltransferase (OST) complex in the endoplasmic reticulum (ER) of animal cells are involved in ER quality control and contribute to the Unfolded Protein Response (UPR). Malectins are not found in plant cells, but malectin-like domains (MLDs) are constituents of many membrane-bound receptors. In Arabidopsis thaliana, the MLD-containing receptor IOS1 promotes successful infection by filamentous plant pathogens. We show that the MLD of its exodomain retains IOS1 in the ER of plant cells and attenuates the infection-induced UPR. Expression of the MLD in the ios1-1 knockout background is sufficient to complement infection-related phenotypes of the mutant, such as increased UPR and reduced disease susceptibility. IOS1 interacts with the ER membrane-associated ribophorin HAP6 from the OST complex, and hap6 mutants show decreased pathogen-responsive UPR and increased disease susceptibility. Altogether, this study revealed a previously uncharacterized role of a plant receptor domain in the regulation of ER stress during infection.

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The Unfolded Protein Response (UPR) in plants impairs downy mildew infection

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The pathogen exploits a molecular mechanism of the host cell to promote disease

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The extracellular domain of the receptor IOS1 attenuates the pathogen-induced UPR

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IOS1 interacts with the ribophorin HAP6 in the ER to fine-tune the UPR

Endoplasmic Reticulum Stress and Unfolded Protein Response Signaling in Plants

Plants are sensitive to a variety of stresses that cause various diseases throughout their life cycle. However, they have the ability to cope with these stresses using different defense mechanisms. The endoplasmic reticulum (ER) is an important subcellular organelle, primarily recognized as a checkpoint for protein folding. It plays an essential role in ensuring the proper folding and maturation of newly secreted and transmembrane proteins. Different processes are activated when around one-third of newly synthesized proteins enter the ER in the eukaryote cells, such as glycosylation, folding, and/or the assembling of these proteins into protein complexes. However, protein folding in the ER is an error-prone process whereby various stresses easily interfere, leading to the accumulation of unfolded/misfolded proteins and causing ER stress. The unfolded protein response (UPR) is a process that involves sensing ER stress. Many strategies have been developed to reduce ER stress, such as UPR, ER-associated degradation (ERAD), and autophagy. Here, we discuss the ER, ER stress, UPR signaling and various strategies for reducing ER stress in plants. In addition, the UPR signaling in plant development and different stresses have been discussed.

Metabolomic profiles exhibit the influence of endoplasmic reticulum stress on sorghum seedling growth over time

Environmental stresses disturb the endoplasmic reticulum (ER) protein folding. However, primary metabolic responses induced by ER stress remain unclear. Thus, we investigated the morphophysiological and metabolomic changes under ER stress, induced by dithiothreitol (DTT) and tunicamycin (TM) treatments in sorghum seedlings from 24 to 96 h. The ER stress caused lipid peroxidation and increased the expression of SbBiP1, SbPDI, and SbIRE1. The development impairment was more pronounced in roots than in shoots as distinct metabolomic profiles were observed. DTT decreased root length, lateral roots, and root hair, while TM decreased mainly the root length. At 24 h, under ER stresses, the glutamic acid and o-acetyl-serine were biomarkers in the shoots. While homoserine, pyroglutamic acid, and phosphoric acid were candidates for roots. At the latest time (96 h), kestose and galactinol were key metabolites for shoots under DTT and TM, respectively. In roots, palatinose, trehalose, and alanine were common markers for DTT and TM late exposure. The accumulation of sugars such as arabinose and kestose occurred mainly in roots in the presence of DTT at a later time, which also inhibited glycolysis and the tricarboxylic acid cycle (TCA). Amino acid metabolism was induced, which also contributed TCA components decreasing, such as succinate in shoots and citrate in roots. Thus, our study may provide new insights into primary metabolism modulated by ER stress and seedling development.

Cell Death Signaling From Endoplasmic Reticulum Stress: Plant-Specific and Conserved Features

The endoplasmic reticulum (ER) stress response is triggered by any condition that disrupts protein folding and promotes the accumulation of unfolded proteins in the lumen of the organelle. In eukaryotic cells, the evolutionarily conserved unfolded protein response is activated to clear unfolded proteins and restore ER homeostasis. The recovery from ER stress is accomplished by decreasing protein translation and loading into the organelle, increasing the ER protein processing capacity and ER-associated protein degradation activity. However, if the ER stress persists and cannot be reversed, the chronically prolonged stress leads to cellular dysfunction that activates cell death signaling as an ultimate attempt to survive. Accumulating evidence implicates ER stress-induced cell death signaling pathways as significant contributors for stress adaptation in plants, making modulators of ER stress pathways potentially attractive targets for stress tolerance engineering. Here, we summarize recent advances in understanding plant-specific molecular mechanisms that elicit cell death signaling from ER stress. We also highlight the conserved features of ER stress-induced cell death signaling in plants shared by eukaryotic cells.

ATG8-Interacting Motif: Evolution and Function in Selective Autophagy of Targeting Biological Processes

Autophagy is an evolutionarily conserved vacuolar process functioning in the degradation of cellular components for reuse. In plants, autophagy is generally activated upon stress and its regulation is executed by numbers of AuTophaGy-related genes (ATGs), of which the ATG8 plays a dual role in both biogenesis of autophagosomes and recruitment of ATG8-interacting motif (AIM) anchored selective autophagy receptors (SARs). Such motif is either termed as AIM or ubiquitin-interacting motif (UIM), corresponding to the LC3-interacting region (LIR)/AIM docking site (LDS) or the UIM docking site (UDS) of ATG8, respectively. To date, dozens of AIM or UIM containing SARs have been characterized. However, the knowledge of these motifs is still obscured. In this review, we intend to summarize the current understanding of SAR proteins and discuss the conservation and diversification of the AIMs/UIMs, expectantly providing new insights into the evolution of them in various biological processes in plants.

Stress response proteins NRP1 and NRP2 are pro-survival factors that inhibit cell death during ER stress

Environmental stresses cause an increased number of unfolded or misfolded proteins to accumulate in the endoplasmic reticulum (ER), resulting in ER stress. To restore ER homeostasis and survive, plants initiate an orchestrated signaling pathway known as the unfolded protein response (UPR). Asparagine-rich protein (NRP) 1 and NRP2, two homologous proteins harboring a Development and Cell Death domain, are associated with various stress responses in Arabidopsis (Arabidopsis thaliana), but the relevant molecular mechanism remains obscure. Here, we show that NRP1 and NRP2 act as key pro-survival factors during the ER stress response and that they inhibit cell death. Loss-of-function of NRP1 and NRP2 results in decreased tolerance to the ER stress inducer tunicamycin (TM), accelerating cell death. NRP2 is constitutively expressed while NRP1 is induced in plants under ER stress. In Arabidopsis, basic leucine zipper protein (bZIP) 28 and bZIP60 are important transcription factors in the UPR that activates the expression of many ER stress-related genes. Notably, under ER stress, bZIP60 activates NRP1 by directly binding to the UPRE-I element in the NRP1 promoter. These findings reveal a pro-survival strategy in plants wherein the bZIP60-NRPs cascade suppresses cell death signal transmission, improving survival under adverse conditions.

Differential protein abundance of vetiver grass in response to acid mine drainage

Acid mine drainage (AMD) is an acidic and metalliferous discharge that imposes oxidative stress on living things through bioaccumulation and physical exposure. The abandoned Tab-Simco mining site of Southern Illinois generates highly acidic AMD with elevated sulfate (SO₄ ^2- ) and various metals. Vetiver grass (Chrysopogon zizanioides) is effective for the remediation of Tab-Simco AMD at both mesocosm and microcosm levels over extended periods. In this study, we conducted a proteomic investigation of vetiver shoots under short and long-term exposure to AMD. Our objective was to decipher the physiological responses of vetiver to the combined abiotic stresses of AMD (metal and low pH). Differential regulation was observed for longer-term (56 days) exposure to AMD, which resulted in 17 upregulated and nine downregulated proteins, whereas shorter-term (7 days) exposure led to 14 upregulated and 14 downregulated proteins. There were significant changes to photosynthesis, including upregulation of electron transport chain proteins for light-dependent reactions after 56 days, whereas differential regulation of enzymes relating to C₄ carbon fixation was observed after 7 days. Significant changes in amino acid and nitrogen metabolism, including upregulation of ethylene and flavonoid biosynthesis, along with plant response to nitrogen starvation, were observed. Short-term changes also included upregulation of glutathione reductase and methionine sulfoxide reductase, whereas longer-term changes included changes in protein misfolding and ER-associated protein degradation for stress management and acclimation.

Effect of an Endoplasmic Reticulum Retention Signal Tagged to Human Anti-Rabies mAb SO57 on Its Expression in Arabidopsis and Plant Growth

Transgenic Arabidopsis thaliana expressing an anti-rabies monoclonal antibody (mAb), SO57, was obtained using Agrobacterium-mediated floral dip transformation. The endoplasmic reticulum (ER) retention signal Lys-Asp-Glu-Leu (KDEL) was tagged to the C-terminus of the anti-rabies mAb heavy chain to localize the mAb to the ER and enhance its accumulation. When the inaccurately folded proteins accumulated in the ER exceed its storage capacity, it results in stress that can affect plant development and growth. We generated T1 transformants and obtained homozygous T3 seeds from transgenic Arabidopsis to investigate the effect of KDEL on plant growth. The germination rate did not significantly differ between plants expressing mAb SO57 without KDEL (SO plant) and mAb SO57 with KDEL (SOK plant). The primary roots of SOK agar media grown plants were slightly shorter than those of SO plants. Transcriptomic analysis showed that expression of all 11 ER stress-related genes were not significantly changed in SOK plants relative to SO plants. SOK plants showed approximately three-fold higher mAb expression levels than those of SO plants. Consequently, the purified mAb amount per unit of SOK plant biomass was approximately three times higher than that of SO plants. A neutralization assay revealed that both plants exhibited efficient rapid fluorescent focus inhibition test values against the rabies virus relative to commercially available human rabies immunoglobulins. KDEL did not upregulate ER stress-related genes; therefore, the enhanced production of the mAb did not affect plant growth. Thus, KDEL fusion is recommended for enhancing mAb production in plant systems.

Interaction of VvbZIP60s and VvHSP83 in response to high-temperature stress in grapes

The occurrence of frequent, extreme high temperatures affects agriculture and causes irreversible damage during the ripening period of grapes. Breeding high-temperature-tolerant varieties of grapes is the main way to deal with this challenge, thus necessitating research on the regulatory mechanism of high-temperature tolerance. Extreme high temperature causes the mismatch of proteins in the endoplasmic reticulum in plant cells and initiates the unfolded protein response (UPR). The transcription factor bZIP60 participates in the UPR process. In the present study, VvbZIP60 and VvbZIP60s (unconventional splicing of VvbZIP60) were cloned and expressed in a transgenic system to verify heat tolerance. VvbZIP60s was found to be a key gene in adapting to heat stress. VvbZIP60s/60u interacted with VvHSP83 as observed in two yeast hybrids, with bimolecular fluorescence complementation and pull-down assays. VvHSP83 is also a key gene for plants to adapt to heat stress by participating in the renaturation and degradation of denatured proteins under adversity, causing plants to resist high temperatures. This study provides a basis for analyzing the mechanism of high-temperature tolerance in grapes.

Comparing Early Transcriptomic Responses of 18 Soybean (Glycine max) Genotypes to Iron Stress

Iron deficiency chlorosis (IDC) is an abiotic stress that negatively affects soybean (Glycine max [L.] Merr.) production. Much of our knowledge of IDC stress responses is derived from model plant species. Gene expression, quantitative trait loci (QTL) mapping, and genome-wide association studies (GWAS) performed in soybean suggest that stress response differences exist between model and crop species. Our current understanding of the molecular response to IDC in soybeans is largely derived from gene expression studies using near-isogenic lines differing in iron efficiency. To improve iron efficiency in soybeans and other crops, we need to expand gene expression studies to include the diversity present in germplasm collections. Therefore, we collected 216 purified RNA samples (18 genotypes, two tissue types [leaves and roots], two iron treatments [sufficient and deficient], three replicates) and used RNA sequencing to examine the expression differences of 18 diverse soybean genotypes in response to iron deficiency. We found a rapid response to iron deficiency across genotypes, most responding within 60 min of stress. There was little evidence of an overlap of specific differentially expressed genes, and comparisons of gene ontology terms and transcription factor families suggest the utilization of different pathways in the stress response. These initial findings suggest an untapped genetic potential within the soybean germplasm collection that could be used for the continued improvement of iron efficiency in soybean.

Constitutive Photomorphogenic 1 Enhances ER Stress Tolerance in Arabidopsis

Interaction between light signaling and stress response has been recently reported in plants. Here, we investigated the role of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a key regulator of light signaling, in endoplasmic reticulum (ER) stress response in Arabidopsis. The cop1-4 mutant Arabidopsis plants were highly sensitive to ER stress induced by treatment with tunicarmycin (Tm). Interestingly, the abundance of nuclear-localized COP1 increased under ER stress conditions. Complementation of cop1-4 mutant plants with the wild-type or variant types of COP1 revealed that the nuclear localization and dimerization of COP1 are essential for its function in plant ER stress response. Moreover, the protein amount of ELONGATED HYPOCOTYL 5 (HY5), which inhibits bZIP28 to activate the unfolded protein response (UPR), decreased under ER stress conditions in a COP1-dependent manner. Accordingly, the binding of bZIP28 to the BIP3 promoter was reduced in cop1-4 plants and increased in hy5 plants compared with the wild type. Furthermore, introduction of the hy5 mutant locus into the cop1-4 mutant background rescued its ER stress-sensitive phenotype. Altogether, our results suggest that COP1, a negative regulator of light signaling, positively controls ER stress response by partially degrading HY5 in the nucleus.

Phytophthora infection signals-induced translocation of NAC089 is required for endoplasmic reticulum stress response-mediated plant immunity

Plants deploy various immune receptors to recognize pathogen-derived extracellular signals and subsequently activate the downstream defense response. Recently, increasing evidence indicates that the endoplasmic reticulum (ER) plays a part in the plant defense response, known as ER stress-mediated immunity (ERSI), that halts pathogen infection. However, the mechanism for the ER stress response to signals of pathogen infection remains unclear. Here, we characterized the ER stress response regulator NAC089, which was previously reported to positively regulate programed cell death (PCD), functioning as an ERSI regulator. NAC089 translocated from the ER to the nucleus via the Golgi in response to Phytophthora capsici culture filtrate (CF), which is a mixture of pathogen-associated molecular patterns (PAMPs). Plasma membrane localized co-receptor BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1 (BAK1) was required for the CF-mediated translocation of NAC089. The nuclear localization of NAC089, determined by the NAC domain, was essential for immune activation and PCD. Furthermore, NAC089 positively contributed to host resistance against the oomycete pathogen P. capsici and the bacteria pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. We also proved that NAC089-mediated immunity is conserved in Nicotiana benthamiana. Together, we found that PAMP signaling induces the activation of ER stress in plants, and that NAC089 is required for ERSI and plant resistance against pathogens.

Plant defence mechanisms against mycotoxin Fumonisin B1

Fumonisin B1 (FB1) is the most harmful mycotoxin which prevails in several crops and affects the growth and yield as well. Hence, keeping the alarming consequences of FB1 under consideration, there is still a need to seek other more reliable approaches and scientific knowledge for FB1-induced cell death and a comprehensive understanding of the mechanisms of plant defence strategies. FB1-induced disturbance in sphingolipid metabolism initiates programmed cell death (PCD) through various modes such as the elevated generation of reactive oxygen species, lipid peroxidation, cytochrome c release from the mitochondria, and activation of specific proteases and nucleases causing DNA fragmentation. There is a close interaction between sphingolipids and defence phytohormones in response to FB1 exposure regulating PCD and defence. In this review, the model plant Arabidopsis and various crops have been presented with different levels of susceptibility and resistivity exposed to various concentration of FB1. In addition to this, regulation of PCD and defence mechanisms have been also demonstrated at the physiological, biochemical and molecular levels to help the understanding of the role and function of FB1-inducible molecules and genes and their expressions in plants against pathogen attacks which could provide molecular and biochemical markers for the detection of toxin exposure.

The Role of Endoplasmic Reticulum Stress Response in Pollen Development and Heat Stress Tolerance

Endoplasmic reticulum (ER) stress is defined by a protracted disruption in protein folding and accumulation of unfolded or misfolded proteins in the ER. This accumulation of unfolded proteins can result from excessive demands on the protein folding machinery triggered by environmental and cellular stresses such as nutrient deficiencies, oxidative stress, pathogens, and heat. The cell responds to ER stress by activating a protective pathway termed unfolded protein response (UPR), which comprises cellular mechanisms targeted to maintain cellular homeostasis by increasing the ER's protein folding capacity. The UPR is especially significant for plants as being sessile requires them to adapt to multiple environmental stresses. While multiple stresses trigger the UPR at the vegetative stage, it appears to be active constitutively in the anthers of unstressed plants. Transcriptome analysis reveals significant upregulation of ER stress-related transcripts in diploid meiocytes and haploid microspores. Interestingly, several ER stress-related genes are specifically upregulated in the sperm cells. The analysis of gene knockout mutants in Arabidopsis has revealed that defects in ER stress response lead to the failure of normal pollen development and enhanced susceptibility of male gametophyte to heat stress conditions. In this mini-review, we provide an overview of the role of ER stress and UPR in pollen development and its protective roles in maintaining male fertility under heat stress conditions.

Arabidopsis Defense against the Pathogenic Fungus Drechslera gigantea Is Dependent on the Integrity of the Unfolded Protein Response

Drechslera gigantea Heald & Wolf is a worldwide-spread necrotrophic fungus closely related to the Bipolaris genus, well-known because many member species provoke severe diseases in cereal crops and studied because they produce sesterpenoid phytoxins named ophiobolins which possess interesting biological properties. The unfolded protein response (UPR) is a conserved mechanism protecting eukaryotic cells from the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER). In plants, consolidated evidence supports the role of UPR in the tolerance to abiotic stress, whereas much less information is available concerning the induction of ER stress by pathogen infection and consequent UPR elicitation as part of the defense response. In this study, the infection process of D. gigantea in Arabidopsis thaliana wild type and UPR-defective bzip28 bzip60 double mutant plants was comparatively investigated, with the aim to address the role of UPR in the expression of resistance to the fungal pathogen. The results of confocal microscopy, as well as of qRT-PCR transcript level analysis of UPR genes, proteomics, microRNAs expression profile and HPLC-based hormone analyses demonstrated that ophiobolin produced by the fungus during infection compromised ER integrity and that impairment of the IRE1/bZIP60 pathway of UPR hampered the full expression of resistance, thereby enhancing plant susceptibility to the pathogen.

Potato virus X-mediated constitutive expression of Plutella xylostella PxSDF2L1 gene in Nicotiana benthamiana confers resistance to Phytophthora parasitica var. nicotianae

BACKGROUND

The Plutella xylostella PxSDF2L1 gene was previously reported to enhance insect resistance to pathogen at high basal transcription rate. PxSDF2L1 shows similitude with the stromal cell-derived factor 2 (SDF2), an ER stress-induced chaperon protein that is highly conserved throughout animals and plants. The precise biological function of SDF2 is not clear, but its expression is required for innate immunity in plants. Here, we investigate whether a continuous expression of PxSDF2L1 in Nicotiana benthamiana can similarly confer resistance to plant pathogen, particularly, the black shank Phytophthora parasitica var. nicotianae.

RESULTS

The N. benthamiana plants were inoculated with agrobacteria transformed with a PVX-based binary vector carrying the PxSDF2L1 gene; similar agroinoculation experiments with a PVX vector carrying the GFP gene were used for controls. In pot trials, agroinfected N. benthamiana plants constitutively expressing PxSDF2L1 showed a significant reduction of stem disease symptoms caused by the inoculation with P. parasitica, compared with controls.

CONCLUSIONS

We confirm a role of PxSDF2L1 in resistance to black shank, with a potential application to engineering active resistance against this oomycete in the commercial N. tabacum species and propose its evaluation in other crop families and plant pathogens.

CRISPR-Mediated Engineering across the Central Dogma in Plant Biology for Basic Research and Crop Improvement

The central dogma (CD) of molecular biology is the transfer of genetic information from DNA to RNA to protein. Major CD processes governing genetic flow include the cell cycle, DNA replication, chromosome packaging, epigenetic changes, transcription, posttranscriptional alterations, translation, and posttranslational modifications. The CD processes are tightly regulated in plants to maintain genetic integrity throughout the life cycle and to pass genetic materials to next generation. Engineering of various CD processes involved in gene regulation will accelerate crop improvement to feed the growing world population. CRISPR technology enables programmable editing of CD processes to alter DNA, RNA, or protein, which would have been impossible in the past. Here, an overview of recent advancements in CRISPR tool development and CRISPR-based CD modulations that expedite basic and applied plant research is provided. Furthermore, CRISPR applications in major thriving areas of research, such as gene discovery (allele mining and cryptic gene activation), introgression (de novo domestication and haploid induction), and application of desired traits beneficial to farmers or consumers (biotic/abiotic stress-resilient crops, plant cell factories, and delayed senescence), are described. Finally, the global regulatory policies, challenges, and prospects for CRISPR-mediated crop improvement are discussed.

Tanned or Sunburned: How Excessive Light Triggers Plant Cell Death

Plants often encounter light intensities exceeding the capacity of photosynthesis (excessive light) mainly due to biotic and abiotic factors, which lower CO₂ fixation and reduce light energy sinks. Under excessive light, the photosynthetic electron transport chain generates damaging molecules, hence leading to photooxidative stress and eventually to cell death. In this review, we summarize the mechanisms linking the excessive absorption of light energy in chloroplasts to programmed cell death in plant leaves. We highlight the importance of reactive carbonyl species generated by lipid photooxidation, their detoxification, and the integrating role of the endoplasmic reticulum in the adoption of phototolerance or cell-death pathways. Finally, we invite the scientific community to standardize the conditions of excessive light treatments.

BIP and the unfolded protein response are important for potyvirus and potexvirus infection

Plant potexvirus and potyvirus infection can trigger endoplasmic reticulum (ER) stress. ER stress signaling increases the expression of cytoprotective ER-chaperones, especially the BiP chaperones which contribute to pro-survival functions when plants are subjected to infection. The inositol requiring enzyme (IRE1) is one ER stress sensor that is activated to splice the bZIP60 mRNA which produces a truncated transcription factor that activates gene expression in the nucleus. The IRE1/bZIP60 pathway is associated with restricting potyvirus and potexvirus infection. Recent data also identified the IRE1-independent UPR pathways led by bZIP28 and bZIP17 contribute to potexvirus and potyvirus infection. These three bZIP pathways recognize cis-regulatory elements in the BiP promoters to enhance gene expression. BiP is part of a negative feedback loop that regulates the activities of the ER stress transducers IRE1, bZIP28, and bZIP17 to block their activation. We discuss a model in which bZIP60 and bZIP17 synergistically induce BiP and other genes restricting Plantago asiatica mosaic virus (PlAMV; a potexvirus) infection while bZIP60 and bZIP28 independently induce genes supporting PlAMV infection. Regarding Turnip mosiac virus (TuMV, a potyvirus) infection, bZIP60 and bZIP28 serve to repress local and systemic infection. Finally, tauroursodeoxycholic acid treatments were used to demonstrate that the protein folding capacity significantly influences PlAMV accumulation.

Multiple ER-to-nucleus stress signaling pathways are activated during Plantago asiatica mosaic virus and Turnip mosaic virus infection in Arabidopsis thaliana

Pathogens and other adverse environmental conditions can trigger endoplasmic reticulum (ER) stress. ER stress signaling increases the expression of cytoprotective ER-chaperones. The inositol-requiring enzyme (IRE1) is one ER stress sensor that is activated to splice the bZIP60 mRNA that produces a truncated transcription factor that activates gene expression in the nucleus. The IRE1/bZIP60 pathway is associated with restricting potyvirus and potexvirus infection. This study shows that the Plantago asiatica mosaic virus (PlAMV) triple gene block 3 (TGB3) and the Turnip mosaic virus (TuMV) 6K2 proteins activate alternative transcription pathways involving the bZIP17, bZIP28, BAG7, NAC089 and NAC103 factors in Arabidopsis thaliana. Using the corresponding knockout mutant lines, we show that bZIP17, bZIP60, BAG7 and NAC089 are factors in reducing PlAMV infection, whereas bZIP28 and bZIP60 are factors in reducing TuMV infection. We propose a model in which bZIP60 and bZIP17 synergistically induce genes restricting PlAMV infection, while bZIP60 and bZIP28 independently induce genes supporting PlAMV infection. Regarding TuMV-green fluorescent protein (GFP) infection, bZIP60 and bZIP28 serve to repress local and systemic infection. Finally, tauroursodeoxycholic acid treatments were used to demonstrate that the protein folding capacity significantly influences PlAMV accumulation.

Salt stress induces endoplasmic reticulum stress-responsive genes in a grapevine rootstock

Grapevines, although adapted to occasional drought or salt stress, are relatively sensitive to growth- and yield-limiting salinity stress. To understand the molecular mechanisms of salt tolerance and endoplasmic reticulum (ER) stress and identify genes commonly regulated by both stresses in grapevine, we investigated transcript profiles in leaves of the salt-tolerant grapevine rootstock 1616C under salt- and ER-stress. Among 1643 differentially expressed transcripts at 6 h post-treatment in leaves, 29 were unique to ER stress, 378 were unique to salt stress, and 16 were common to both stresses. At 24 h post-treatment, 243 transcripts were unique to ER stress, 1150 were unique to salt stress, and 168 were common to both stresses. GO term analysis identified genes in categories including ‘oxidative stress’, ‘protein folding’, ‘transmembrane transport’, ‘protein phosphorylation’, ‘lipid transport’, ‘proteolysis’, ‘photosynthesis’, and ‘regulation of transcription’. The expression of genes encoding transporters, transcription factors, and proteins involved in hormone biosynthesis increased in response to both ER and salt stresses. KEGG pathway analysis of differentially expressed genes for both ER and salt stress were divided into four main categories including; carbohydrate metabolism, amino acid metabolism, signal transduction and lipid metabolism. Differential expression of several genes was confirmed by qRT-PCR analysis, which validated our microarray results. We identified transcripts for genes that might be involved in salt tolerance and also many genes differentially expressed under both ER and salt stresses. Our results could provide new insights into the mechanisms of salt tolerance and ER stress in plants and should be useful for genetic improvement of salt tolerance in grapevine.

Effects of Jasmonic Acid in ER Stress and Unfolded Protein Response in Tomato Plants

Endoplasmic reticulum (ER) stress elicits a protective mechanism called unfolded protein response (UPR) to maintain cellular homeostasis, which can be regulated by defence hormones. In this study, the physiological role of jasmonic acid (JA) in ER stress and UPR signalling has been investigated in intact leaves of tomato plants. Exogenous JA treatments not only induced the transcript accumulation of UPR marker gene SlBiP but also elevated transcript levels of SlIRE1 and SlbZIP60. By the application of JA signalling mutant jai1 plants, the role of JA in ER stress sensing and signalling was further investigated. Treatment with tunicamycin (Tm), the inhibitor of N-glycosylation of secreted glycoproteins, increased the transcript levels of SlBiP. Interestingly, SlIRE1a and SlIRE1b were significantly lower in jai1. In contrast, the transcript accumulation of Bax Inhibitor-1 (SlBI1) and SlbZIP60 was higher in jai1. To evaluate how a chemical chaperone modulates Tm-induced ER stress, plants were treated with sodium 4-phenylbutyrate, which also decreased the Tm-induced increase in SlBiP, SlIRE1a, and SlBI1 transcripts. In addition, it was found that changes in hydrogen peroxide content, proteasomal activity, and lipid peroxidation induced by Tm is regulated by JA, while nitric oxide was not involved in ER stress and UPR signalling in leaves of tomato.

Combined NaCl and DTT diminish harmful ER-stress effects in the sorghum seedlings CSF 20 variety

Plants have developed mechanisms to avoid harmful effects of Na⁺ accumulation, such as the signaling pathway of carrier proteins Na⁺/H⁺ (NHX) and salt overly sensitive (SOS). Besides, endoplasmic reticulum (ER) could integrate plant cell response. Thus, we aimed to understand the effects of ER homeostasis impairment, and its relationship to salt stress during early stages of the Sorghum bicolor CSF 20 a salt-tolerant variety. Three days old seedlings were challenged with NaCl (0, 50, 75 and 100 mM), dithiothreitol (DTT) at 0, 2.5, 5.0 10.0 mM, and the combined NaCl and DTT treatments. Tunicamycin (TUN) was also used as a second inducer of ER stress in a quantitative PCR, to corroborate with DTT's results. There was no significant change in growth parameters under NaCl treatments. Nevertheless, seedling length, mass and Na⁺ content were decreased as DTT concentration was increased. Under combined NaCl and DTT treatments, shoot length and fresh and dry masses were maintained at control levels. On the other hand, the levels of Na⁺ were decreased, in comparison to NaCl treatment. Genes analyzed by qPCR revealed that NaCl was able to induce all of them, except for SbbZIP60, however it was induced under combined stresses. In conclusion, the results indicated that S. bicolor seedlings of CSF 20 variety were tolerant to salt and sensible to ER stress. The combination of both stresses restored the ER homeostasis promoting a decrease of Na⁺ content via the membrane transporters SbNHX1, SbSOS1, and SbPDI ER-chaperone and the ER sensor SbbZIP60.

A high-throughput screening method to identify proteins involved in unfolded protein response of the endoplasmic reticulum in plants

BACKGROUND

The unfolded protein response (UPR) is a highly conserved process in eukaryotic organisms that plays a crucial role in adaptation and development. While the most ubiquitous components of this pathway have been characterized, current efforts are focused on identifying and characterizing other UPR factors that play a role in specific conditions, such as developmental changes, abiotic cues, and biotic interactions. Considering the central role of protein secretion in plant pathogen interactions, there has also been a recent focus on understanding how pathogens manipulate their host's UPR to facilitate infection.

RESULTS

We developed a high-throughput screening assay to identify proteins that interfere with UPR signaling in planta. A set of 35 genes from a library of secreted proteins from the maize pathogen Ustilago maydis were transiently co-expressed with a reporter construct that upregulates enhanced yellow fluorescent protein (eYFP) expression upon UPR stress in Nicotiana benthamiana plants. After UPR stress induction, leaf discs were placed in 96 well plates and eYFP expression was measured. This allowed us to identify a previously undescribed fungal protein that inhibits plant UPR signaling, which was then confirmed using the classical but more laborious qRT-PCR method.

CONCLUSIONS

We have established a rapid and reliable fluorescence-based method to identify heterologously expressed proteins involved in UPR stress in plants. This system can be used for initial screens with libraries of proteins and potentially other molecules to identify candidates for further validation and characterization.

The Multifaceted Roles of Plant Hormone Salicylic Acid in Endoplasmic Reticulum Stress and Unfolded Protein Response

Different abiotic and biotic stresses lead to the accumulation of unfolded and misfolded proteins in the endoplasmic reticulum (ER), resulting in ER stress. In response to ER stress, cells activate various cytoprotective responses, enhancing chaperon synthesis, protein folding capacity, and degradation of misfolded proteins. These responses of plants are called the unfolded protein response (UPR). ER stress signaling and UPR can be regulated by salicylic acid (SA), but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in ER stress and UPR is summarized in model plants and crops to gain a better understanding of SA-regulated processes at the physiological, biochemical, and molecular levels.

Endoplasmic reticulum stress-induced accumulation of VAMP721/722 requires CALRETICULIN 1 and CALRETICULIN 2 in Arabidopsis

Excessive demand for translation and protein folding in the endoplasmic reticulum (ER) can cause ER stress in plants. Here, we show that CALRETICULIN 1 (CRT1) and CRT2 are critical components in the accumulation of VESICLE-ASSOCIATED MEMBRANE PROTEIN 721 (VAMP721) and VAMP722 during ER stress responses. We show that CRT2 interacts with VAMP722 and that CRT1/2 post-translationally maintain elevated VAMP721/722 levels under ER stress. The greater growth inhibition in VAMP721/722-deficient plants, induced by tunicamycin, suggests that plants under ER stress maintain physiological homeostasis, at least in part, by regulating VAMP721/722 levels, as VAMP721/722 are known to participate in various biological processes.

Protein hyperproduction in fungi by design

The secretion of enzymes used by fungi to digest their environment has been exploited by humans for centuries for food and beverage production. More than a century after the first biotechnology patent, we know that the enzyme cocktails secreted by these amazing organisms have tremendous use across a number of industrial processes. Secreting the maximum titer of enzymes is critical to the economic feasibility of these processes. Traditional mutagenesis and screening approaches have generated the vast majority of strains used by industry for the production of enzymes. Until the emergence of economical next generation DNA sequencing platforms, the majority of the genes mutated in these screens remained uncharacterized at the sequence level. In addition, mutagenesis comes with a cost to an organism’s fitness, making tractable rational strain design approaches an attractive alternative. As an alternative to traditional mutagenesis and screening, controlled manipulation of multiple genes involved in processes that impact the ability of a fungus to sense its environment, regulate transcription of enzyme-encoding genes, and efficiently secrete these proteins will allow for rational design of improved fungal protein production strains.