The Unfolded Protein Response Supports Plant Development and Defense as well as Responses to Abiotic Stress

Protein disulfide isomerase-9 interacts with the lumenal region of the transmembrane endoplasmic reticulum stress sensor kinase, IRE1, to modulate the unfolded protein response in Arabidopsis

Environmental stressors disrupt secretory protein folding and proteostasis in the endoplasmic reticulum (ER), leading to ER stress. The unfolded protein response (UPR) senses ER stress and restores proteostasis by increasing the expression of ER-resident protein folding chaperones, such as protein disulfide isomerases (PDIs). In plants, the transmembrane ER stress sensor kinase, IRE1, activates the UPR by unconventionally splicing the mRNA encoding the bZIP60 transcription factor, triggering UPR gene transcription. The induced PDIs catalyze disulfide-based polypeptide folding to restore the folding capacity in the ER; however, the substrates with which PDIs interact are largely unknown. Here, we demonstrate that the Arabidopsis PDI-M subfamily member, PDI9, modulates the UPR through interaction with IRE1. This PDI9-IRE1 interaction was largely dependent on Cys63 in the first dithiol redox active domain of PDI9, and Cys233 and Cys107 in the ER lumenal domain of IRE1A and IRE1B, respectively. In vitro and in vivo, PDI9 coimmunoprecipitated with IRE1A and IRE1B. Moreover, the PDI9:RFP and Green Fluorescence Protein (GFP):IRE1 fusions exhibited strong interactions as measured by fluorescence lifetime imaging microscopy-fluorescence resonance energy transfer (FLIM-FRET) when coexpressed in mesophyll protoplasts. The UPR-responsive PDI9 promoter:mCherry reporter and the UPR-dependent splicing of the bZIP60 intron from the mRNA of the 35S::bZIP60-intron:GFP reporter were both significantly induced in the pdi9 mutants, indicating a derepression and hyperactivation of UPR. The inductions of both reporters were substantially attenuated in the ire1a-ire1b mutant. We propose a model in which PDI9 modulates the UPR through two competing activities: secretory protein folding and via interaction with IRE1 to maintain proteostasis in plants.

A long non-coding RNA associated with H3K7me3 methylation negatively regulates OsZIP16 transcription under cadmium stress

Cadmium (Cd) is a toxic environmental pollutant, posing a high risk to crop production and human health. However, the genetic mechanisms for regulation of Cd accumulation in crops are poorly understood. In this study, we functionally identified a novel long non-coding RNA (lncRNA, TCONS_00502780) that repressed a locus encoding an uncharacterized metal transporter ZIP16 (ZRT/IRT-like Protein) in rice. LncRNA-OsZIP16 (L16) is resident in the antisense strand of OsZIP16. Both L16 and OsZIP16 were transcriptionally expressed during the life cycle, but under Cd stress the L16 transcription was repressed, whereas the OsZIP16 expression was upregulated. OsZIP16 is localized to the endoplasmic reticulum. Knocking out OsZIP16 by CRISPR-Cas9 (C16) resulted in Cd sensitivity, manifested by reduced plant growth and intense cellular damage with a slightly higher Cd translocation from roots to shoots, suggesting that OsZIP16 expression is required for rice growth and development under Cd stress. Conversely, OsZIP16 constitutive overexpression (OE16) lines displayed a growth phenotype compatible to the wide-type with lower Cd translocation ratio from roots to shoots. L16 knock-down lines by RNA interference (L16-R) showed a similar phenotype to the OE16 lines, while the L16 overexpression (L16-OE) lines were phenotypically similar to the C16 lines. The OsZIP16 transcription was upregulated in the L16-R lines but downregulated in the L16-OE lines. Using an antibody against the trimethylation of histone H3 lysine 27 (H3K27me3) followed by chromatin immunoprecipitation (ChIP), we found the reduced H3K27me3 methylation marks surrounding the OsZIP16 gene under Cd stress. Further examination of H3K27me3 in the L16-R lines revealed that the methylation levels were also significantly lower than those in WT. Taken together, these data suggest that the L16 could negatively regulate the OsZIP16 transcriptional expression through an epigenetic mechanism for rice adaption to Cd stress.

Hexosamine biosynthesis and related pathways, protein N-glycosylation and O-GlcNAcylation: their interconnection and role in plants

N-Acetylglucosamine (GlcNAc), a fundamental amino sugar moiety, is essential for protein glycosylation, glycolipid, GPI-anchor protein, and cell wall components. Uridine diphosphate-GlcNAc (UDP-GlcNAc), an active form of GlcNAc, is synthesized through the hexosamine biosynthesis pathway (HBP). Although HBP is highly conserved across organisms, the enzymes involved perform subtly distinct functions among microbes, mammals, and plants. A complete block of HBP normally causes lethality in any life form, reflecting the pivotal role of HBP in the normal growth and development of organisms. Although HBP is mainly composed of four biochemical reactions, HBP is exquisitely regulated to maintain the homeostasis of UDP-GlcNAc content. As HBP utilizes substrates including fructose-6-P, glutamine, acetyl-CoA, and UTP, endogenous nutrient/energy metabolites may be integrated to better suit internal growth and development, and external environmental stimuli. Although the genes encoding HBP enzymes are well characterized in microbes and mammals, they were less understood in higher plants in the past. As the HBP-related genes/enzymes have largely been characterized in higher plants in recent years, in this review we update the latest advances in the functions of the HBP-related genes in higher plants. In addition, HBP's salvage pathway and GlcNAc-mediated two major co- or post-translational modifications, N-glycosylation and O-GlcNAcylation, are also included in this review. Further knowledge on the function of HBP and its product conjugates, and the mechanisms underlying their response to deleterious environments might provide an alternative strategy for agricultural biofortification in the future.

Coordinated recruitment of conserved defense-signaling pathways in PVYO-Infected Nicotiana benthamiana

Potato virus Y (PVY) is an aphid-transmitted potyvirus that affects economically important solanaceous species. In this study, the phenomena and mechanisms following infection with PVY were investigated in tobacco (Nicotiana benthamiana). In tobacco plants, infection with a mild strain of PVY (PVYO) induced stunted growth in the first two leaves at the shoot apex starting 7 days post-infection (dpi), and mosaic symptoms began to appear on newly developing young leaves at 14 dpi. Using enzyme-linked immunosorbent assay and ultrastructure analysis, we confirmed that viral particles accumulated only in the upper developing leaves of infected plants. We analyzed reactive oxygen species (ROS) generation in leaves from the bottom to the top of the plants to investigate whether delayed symptom development in leaves was associated with a defense response to the virus. In addition, the ultrastructural analysis confirmed the increase of ATG4 and ATG8, which are autophagy markers by endoplasmic reticulum (ER) stress, and the expression of genes involved in viral RNA suppression. Overall, our results suggested that viral RNA silencing and induced autophagy may play a role in the inhibition of viral symptom development in host plants in response to PVYO infection.

CRISPR-Cas-mediated unfolded protein response control for enhancing plant stress resistance

Plants consistently encounter environmental stresses that negatively affect their growth and development. To mitigate these challenges, plants have developed a range of adaptive strategies, including the unfolded protein response (UPR), which enables them to manage endoplasmic reticulum (ER) stress resulting from various adverse conditions. The CRISPR-Cas system has emerged as a powerful tool for plant biotechnology, with the potential to improve plant tolerance and resistance to biotic and abiotic stresses, as well as enhance crop productivity and quality by targeting specific genes, including those related to the UPR. This review highlights recent advancements in UPR signaling pathways and CRISPR-Cas technology, with a particular focus on the use of CRISPR-Cas in studying plant UPR. We also explore prospective applications of CRISPR-Cas in engineering UPR-related genes for crop improvement. The integration of CRISPR-Cas technology into plant biotechnology holds the promise to revolutionize agriculture by producing crops with enhanced resistance to environmental stresses, increased productivity, and improved quality traits.

The Endoplasmic Reticulum Is a Key Battleground between Phytoplasma Aggression and Host Plant Defense

Phytoplasmas are intracellular plant pathogens that heavily rely on host cell nutrients for survival and propagation due to their limited ability to synthesize essential substrates. The endoplasmic reticulum (ER), which plays a vital role in various cellular processes, including lipid and protein biosynthesis, is an attractive target for numerous intracellular pathogens to exploit. This study investigated the impact of potato purple top (PPT) phytoplasma infection on the ER in tomato plants. Abnormal accumulation of ER-resident proteins, disrupted ER network structures, and formation of protein aggregates in the phloem were observed using confocal microscopy and transmission electron microscopy, indicating a phytoplasma-infection-induced disturbance in ER homeostasis. The colocalization of phytoplasmas with the accumulated ER-resident proteins suggests an association between ER stress, unfolded protein response (UPR) induction, and phytoplasma infection and colonization, with the ER stress response likely contributing to the host plant's defense mechanisms. Quantitative real-time PCR revealed a negative correlation between ER stress/UPR activation and PPT phytoplasma titer, implying the involvement of UPR in curbing phytoplasma proliferation. Inducing ER stress and activating the UPR pathway effectively decreased phytoplasma titer, while suppressing the ER-resident protein, binding immunoglobulin protein (BiP) increased phytoplasma titer. These results highlight the ER as an intracellular battleground where phytoplasmas exploit host components for survival and multiplication, while host plants deploy defense mechanisms to counteract the invasion. Understanding the intricate interactions between phytoplasmas and plant hosts at the subcellular level, particularly within the ER, provides valuable insights for developing new strategies to control phytoplasma diseases.

A sword or a buffet: plant endomembrane system in viral infections

The plant endomembrane system is an elaborate collection of membrane-bound compartments that perform distinct tasks in plant growth and development, and in responses to abiotic and biotic stresses. Most plant viruses are positive-strand RNA viruses that remodel the host endomembrane system to establish intricate replication compartments. Their fundamental role is to create optimal conditions for viral replication, and to protect replication complexes and the cell-to-cell movement machinery from host defenses. In addition to the intracellular antiviral defense, represented mainly by RNA interference and effector-triggered immunity, recent findings indicate that plant antiviral immunity also includes membrane-localized receptor-like kinases that detect viral molecular patterns and trigger immune responses, which are similar to those observed for bacterial and fungal pathogens. Another recently identified part of plant antiviral defenses is executed by selective autophagy that mediates a specific degradation of viral proteins, resulting in an infection arrest. In a perpetual tug-of-war, certain host autophagy components may be exploited by viral proteins to support or protect an effective viral replication. In this review, we present recent advances in the understanding of the molecular interplay between viral components and plant endomembrane-associated pathways.

Parallel evolution of two AIM24 protein subfamilies and their conserved functions in ER stress tolerance in land plants

Despite decades of efforts in genome sequencing and functional characterization, some important protein families remain poorly understood. In this study, we report the classification, evolution, and functions of the largely uncharacterized AIM24 protein family in plants, including the identification of a novel subfamily. We show that two AIM24 subfamilies (AIM24-A and AIM24-B) are commonly distributed in major plant groups. These two subfamilies not only have modest sequence similarities and different gene structures but also are of independent bacterial ancestry. We performed comparative functional investigations on the two AIM24 subfamilies using three model plants: the moss Physcomitrium patens, the liverwort Marchantia polymorpha, and the flowering plant Arabidopsis thaliana. Intriguingly, despite their significant differences in sequence and gene structure, both AIM24 subfamilies are involved in ER stress tolerance and the unfolded protein response (UPR). In addition, transformation of the AIM24-A gene from P. patens into the AIM24-B null mutant of A. thaliana could at least partially rescue ER stress tolerance and the UPR. We also discuss the role of AIM24 genes in plant development and other cellular activities. This study provides a unique example of parallel evolution in molecular functions and can serve as a foundation for further investigation of the AIM24 family in plants.

The conformation of glutenin polymers in wheat grain: some genetic and environmental factors associated with this important characteristic

In a previous study we used asymmetric-flow field-flow fractionation to determine the polymer mass (Mw), gyration radius (Rw) and the polydispersity index of glutenin polymers (GPs) in wheat (Triticum aestivum). Here, using the same multi-location trials (4 years, 11 locations, and 192 cultivars), we report the factors that are associated with the conformation (Conf) of the polymers, which is the slope of Log(Rw) versus a function of Log(Mw). We found that Conf varied between 0.285 and 0.740, it had low broad-sense heritability (H2=16.8), and it was significantly influenced by the temperature occurring over the last month of grain filling. Higher temperatures were found to increase Rw and the compactness and sphericity of GPs. Alleles for both high- and low-molecular-weight glutenin subunits had a significant influence on the Conf value. Assuming a Gaussian distribution for Mw, the number of polymers present in wheat grains was computed for different kernel weights and protein concentrations, and it was found to exceed 1012 GPs per grain. Using atomic force microscopy and cryo-TEM, images of GPs were obtained for the first time. Under higher average temperature, GPs became larger and more spherical and consequently less prone to rapid hydrolysis. We propose some orientations that could be aimed at potentially reducing the impact of numerous GPs on people suffering from non-celiac gluten sensitivity.

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.

Arabidopsis NPF2.13 functions as a critical transporter of bacterial natural compound tunicamycin in plant-microbe interaction

Metabolites including antibiotics, enzymes, and volatiles produced by plant-associated bacteria are key factors in plant-microbiota interaction that regulates various plant biological processes. There should be crucial mediators responsible for their entry into host plants. However, less is known about the identities of these plant transporters. We report that the Arabidopsis Nitrate Transporter1 (NRT1)/NPF protein NPF2.13 functions in plant uptake of tunicamycin (TM), a natural antibiotic produced by several Streptomyces spp., which inhibits protein N-glycosylation. Loss of NPF2.13 function resulted in enhanced TM tolerance, whereas NPF2.13 overexpression led to TM hypersensitivity. Transport assays confirmed that NPF2.13 is a H⁺ /TM symporter and the transport is not affected by other substrates like nitrate. NPF2.13 exclusively showed TM transport activity among tested NPFs. Tunicamycin uptake from TM-producing Streptomyces upregulated the expression of nitrate-related genes including NPF2.13. Moreover, nitrate allocation to younger leaves was promoted by TM in host plants. Tunicamycin could also benefit plant defense against the pathogen. Notably, the TM effects were significantly repressed in npf2.13 mutant. Overall, this study identifies NPF2.13 protein as an important TM transporter in plant-microbe interaction and provides insights into multiple facets of NPF proteins in modulating plant nutrition and defense by transporting exterior bacterial metabolites.

Development of a GFP biosensor reporter for the unfolded protein response-signaling pathway in plants: incorporation of the bZIP60 intron into the GFP gene

The ability to measure the activation of the unfolded protein response (UPR) in plants is important when they are exposed to stressful environments. To this end, we developed a unique and versatile biosensor-reporter system to indicate the activation of UPR in living plant cells. The small cytoplasmically spliced intron from the bZIP60 locus was incorporated into the 5' end of the GFP gene, creating the 35S::bZIP60 intron:GFP construct. When this construct is transiently expressed in Arabidopsis protoplasts, the presence of the bZIP60 intron prevents GFP mRNA from being translated under non-UPR conditions. However, when UPR is activated, the IRE1 kinase/ribonuclease splices this intron from the GFP mRNA and its translation proceeds, generating GFP fluorescence. We demonstrated the utility of the system in Arabidopsis leaf protoplasts treated with DTT, which is a chemical inducer of UPR, followed by visualization and quantification using confocal microscopy. The 35S::bZIP60 intron:GFP construct was also expressed in protoplasts from an overexpressor line containing the coding sequence for the UPR-induced, protein folding chaperone, protein disulfide isomerase-9 (PDI9). PDI9 also influences the strength of the UPR signaling pathway. Protoplasts from WT and PDI9 overexpressor plants treated with DTT exhibited significantly higher GFP fluorescence relative to untreated protoplasts, indicating that the bZIP60 intron was spliced from the GFP mRNA in response to activation of UPR. RT-PCR further confirmed the higher induction of PDI9 and bZIP60 (total and spliced) mRNA levels in DTT-treated protoplasts relative to controls. This system can be adapted for monitoring crop stress and for basic studies dissecting the UPR signaling pathway.

RNA-seq Gene Profiling Reveals Transcriptional Changes in the Late Phase during Compatible Interaction between a Korean Soybean Cultivar (Glycine max cv. Kwangan) and Pseudomonas syringae pv. syringae B728a

Soybean (Glycine max (L) Merr.) provides plant-derived proteins, soy vegetable oils, and various beneficial metabolites to humans and livestock. The importance of soybean is highly underlined, especially when carbon-negative sustainable agriculture is noticeable. However, many diseases by pests and pathogens threaten sustainable soybean production. Therefore, understanding molecular interaction between diverse cultivated varieties and pathogens is essential to developing disease-resistant soybean plants. Here, we established a pathosystem of the Korean domestic cultivar Kwangan against Pseudomonas syringae pv. syringae B728a. This bacterial strain caused apparent disease symptoms and grew well in trifoliate leaves of soybean plants. To examine the disease susceptibility of the cultivar, we analyzed transcriptional changes in soybean leaves on day 5 after P. syringae pv. syringae B728a infection. About 8,900 and 7,780 differentially expressed genes (DEGs) were identified in this study, and significant proportions of DEGs were engaged in various primary and secondary metabolisms. On the other hand, soybean orthologs to well-known plant immune-related genes, especially in plant hormone signal transduction, mitogen-activated protein kinase signaling, and plant-pathogen interaction, were mainly reduced in transcript levels at 5 days post inoculation. These findings present the feature of the compatible interaction between cultivar Kwangan and P. syringae pv. syringae B728a, as a hemibiotroph, at the late infection phase. Collectively, we propose that P. syringae pv. syringae B728a successfully inhibits plant immune response in susceptible plants and deregulates host metabolic processes for their colonization and proliferation, whereas host plants employ diverse metabolites to protect themselves against infection with the hemibiotrophic pathogen at the late infection phase.

Comprehensive Transcriptome Analysis Reveals Genome-Wide Changes Associated with Endoplasmic Reticulum (ER) Stress in Potato (Solanum tuberosum L.)

We treated potato (Solanum tuberosum L.) plantlets with TM and performed gene expression studies to identify genome-wide changes associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). An extensive network of responses was identified, including chromatin remodeling, transcriptional reprogramming, as well as changes in the structural components of the endomembrane network system. Limited genome-wide changes in alternative RNA splicing patterns of protein-coding transcripts were also discovered. Significant changes in RNA metabolism, components of the translation machinery, as well as factors involved in protein folding and maturation occurred, which included a broader set of genes than expected based on Arabidopsis research. Antioxidant defenses and oxygen metabolic enzymes are differentially regulated, which is expected of cells that may be experiencing oxidative stress or adapting to protect proteins from oxidation. Surges in protein kinase expression indicated early signal transduction events. This study shows early genomic responses including an array of differentially expressed genes that have not been reported in Arabidopsis. These data describe novel ER stress responses in a solanaceous host.

Transcriptome Analysis Reveals Contrasting Plant Responses of Sorghum bicolor upon Colonization by Two Formae Speciales of Sporisorium reilianum

The biotrophic fungus Sporisorium reilianum exists in two host-adapted formae speciales that cause head smut in maize (S. reilianum f. sp. zeae; SRZ) and sorghum (S. reilianum f. sp. reilianum; SRS). In sorghum, the spread of SRZ is limited to the leaves. To understand the plant responses to each forma specialis, we determined the transcriptome of sorghum leaves inoculated either with SRS or SRZ. Fungal inoculation led to gene expression rather than suppression in sorghum. SRZ induced a much greater number of genes than SRS. Each forma specialis induced a distinct set of plant genes. The SRZ-induced genes were involved in plant defense mainly at the plasma membrane and were associated with the Molecular Function Gene Ontology terms chitin binding, abscisic acid binding, protein phosphatase inhibitor activity, terpene synthase activity, chitinase activity, transmembrane transporter activity and signaling receptor activity. Specifically, we found an upregulation of the genes involved in phospholipid degradation and sphingolipid biosynthesis, suggesting that the lipid content of the plant plasma membrane may contribute to preventing the systemic spread of SRZ. In contrast, the colonization of sorghum with SRS increased the expression of the genes involved in the detoxification of cellular oxidants and in the unfolded protein response at the endoplasmic reticulum, as well as of the genes modifying the cuticle wax and lipid composition through the generation of alkanes and phytosterols. These results identified plant compartments that may have a function in resistance against SRZ (plasma membrane) and susceptibility towards SRS (endoplasmic reticulum) that need more attention in the future.

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.

THESEUS1 is involved in tunicamycin-induced root growth inhibition, ectopic lignin deposition, and cell wall damage-induced unfolded protein response

Endoplasmic reticulum (ER) stress activates unfolded protein responses (UPRs), such as promoting protein folding under the control of specific gene expression. Our previous study showed that ER stress induced by ER stress inducers such as tunicamycin (Tm), an inhibitor of N-linked glycan synthesis, causes ectopic lignin deposition in Arabidopsis roots, but the relationship between UPR and ectopic lignin deposition remains unclear. The receptor-like kinase THESEUS1 (THE1) has been shown to sense cell wall damage (CWD) induced in Arabidopsis by cellulose synthase inhibitors such as isoxaben (ISO) and to activate ectopic lignin deposition. In this study, we assessed the involvement of THE1 in ectopic lignin deposition caused by the ER stress inducer Tm. The loss-of-function mutation of THE1, the1-3, suppressed Tm-induced root growth inhibition and ectopic lignin deposition, revealing that THE1 is involved in root growth defects and ectopic lignin deposition caused by ER stress. Similarly, ISO treatment induced ectopic lignin deposition as well as the expression of the UPR marker genes binding protein 3 (BiP3) and ER-localized DnaJ 3b (ERdj3b). Conversely, in the the1-3 mutant, ISO-induced ectopic lignin deposition and the expression of BiP3 and ERdj3b were suppressed. These results showed that THE1 is involved in not only root growth inhibition and ectopic lignin deposition caused by ER stress but also CWD-induced UPR.

A rice protein modulates endoplasmic reticulum homeostasis and coordinates with a transcription factor to initiate blast disease resistance

Endoplasmic reticulum (ER) homeostasis is essential for plants to manage responses under environmental stress. Plant immune activation requires the ER, but how ER homeostasis is associated with plant immune activation is largely unexplored. Here we find that transcription of an HVA22 family gene, OsHLP1 (HVA22-like protein 1), is induced by Magnaporthe oryzae infection. Overexpression of OsHLP1 significantly enhances blast disease resistance but impairs ER morphology in rice (Oryza sativa), resulting in enhanced sensitivity to ER stress. OsHLP1 interacts with the NAC (NAM, ATAF, and CUC) transcription factor OsNTL6 at the ER. OsNTL6 localizes to the ER and is relocated to the nucleus after cleavage of the transmembrane domain. OsHLP1 suppresses OsNTL6 protein accumulation, whereas OsNTL6 counteracts OsHLP1 by alleviating sensitivity to ER stress and decreasing disease resistance in OsHLP1 overexpression plants. These findings unravel a mechanism whereby OsHLP1 promotes disease resistance by compromising ER homeostasis when plants are infected by pathogens.

Interference of Arabidopsis N-Acetylglucosamine-1-P Uridylyltransferase Expression Impairs Protein N-Glycosylation and Induces ABA-Mediated Salt Sensitivity During Seed Germination and Early Seedling Development

N-acetylglucosamine (GlcNAc) is the fundamental amino sugar moiety that is essential for protein glycosylation. UDP-GlcNAc, an active form of GlcNAc, is synthesized through the hexosamine biosynthetic pathway (HBP). Arabidopsis N-acetylglucosamine-1-P uridylyltransferases (GlcNAc1pUTs), encoded by GlcNA.UTs, catalyze the last step in the HBP pathway, but their biochemical and molecular functions are less clear. In this study, the GlcNA.UT1 expression was knocked down by the double-stranded RNA interference (dsRNAi) in the glcna.ut2 null mutant background. The RNAi transgenic plants, which are referred to as iU1, displayed the reduced UDP-GlcNAc biosynthesis, altered protein N-glycosylation and induced an unfolded protein response under salt-stressed conditions. Moreover, the iU1 transgenic plants displayed sterility and salt hypersensitivity, including delay of both seed germination and early seedling establishment, which is associated with the induction of ABA biosynthesis and signaling. These salt hypersensitive phenotypes can be rescued by exogenous fluridone, an inhibitor of ABA biosynthesis, and by introducing an ABA-deficient mutant allele nced3 into iU1 transgenic plants. Transcriptomic analyses further supported the upregulated genes that were involved in ABA biosynthesis and signaling networks, and response to salt stress in iU1 plants. Collectively, these data indicated that GlcNAc1pUTs are essential for UDP-GlcNAc biosynthesis, protein N-glycosylation, fertility, and the response of plants to salt stress through ABA signaling pathways during seed germination and early seedling development.

Changes in leaf ecophysiological traits and proteome profile provide new insights into variability of salt response in the succulent halophyte Cakile maritima

Natural variability of stress tolerance in halophytic plants is of significance both ecologically and in view of identifying molecular traits for salt tolerance in plants. Using ecophysiological and proteomic analyses, we address these phenomena in two Tunisian accessions of the oilseed halophyte, Cakile maritima Scop., thriving on arid and semi-arid Mediterranean bioclimatic stages (Djerba and Raoued, respectively), with a special emphasis on the leaves. Changes in biomass, photosynthetic gas exchange and pigment concentrations in C. maritima plants treated with three salinity levels (0, 100 and 300mM NaCl) were monitored for 1month. Comparative two-dimensional gel electrophoresis (2-DE) revealed 94 and 56 proteins of differential abundance in Raoued and Djerba accessions, respectively. These salinity-responsive proteins were mainly related to photosynthesis and oxidative phosphorylation (OXPHOS). Although Djerba accession showed a lower biomass productivity, it showed a slightly higher CO2 assimilation rate than Raoued accession when salt-treated. Photosynthesis impairment in both accessions under salinity was also suggested by the lower abundance of proteins involved in Calvin cycle and electron transfer. A significant increase of protein spots involved in the OXPHOS system was found in Djerba accession, suggesting an increase in mitochondrial respiration for increased ATP production under saline conditions, whereas a lesser pronounced trend was observed for Raoued accession. The latter showed in addition higher abundance of proteins involved in photorespiration. Salt-challenged plants of Djerba also likely developed mechanisms for scavenging ROS in leaves as shown by the increase in superoxide dismutase and thioredoxin, while an opposite trend was found in Raoued.

Elucidating the functional role of heat stress transcription factor A6b (TaHsfA6b) in linking heat stress response and the unfolded protein response in wheat

TaHsfA6b-4D relocalizes intracellularly upon heat stress and play a significant role in linking the heat stress response to unfolded-protein response so as to maintain cellular homeostasis. Heat stress transcription factors (Hsfs) play a crucial role in protecting the plants against heat stress (HS). In case of wheat, TaHsfA6b-4D (earlier known as TaHsfA2d) has been identified as a seed preferential transcription factor and its role has been shown in various abiotic stresses such as heat, salt and drought stress. In the present study, a homeologue of TaHsfA6b gene (TaHsfA6b-4A) was identified and was found to be transcriptionally inactive but it localized to the nucleus. Interestingly, TaHsfA6b-4D localized to the endoplasmic reticulum-Golgi complex and peroxisomes under non-stress conditions, but was observed to accumulate in the nucleus upon HS. The expression of TaHsfA6b-4D was upregulated by dithiothreitol (DTT), which is a known ER stress inducer. Consistent with this, Arabidopsis transgenic plants overexpressing TaHsfA6b-4D performed better on DTT containing media, which further corroborated with the increased expression of ER stress marker genes in these transgenic plants in comparison to the wild type plants. Thus, these studies together suggest that TaHsfA6b-4D may relocalize intracellularly upon heat stress and may play a significant role in linking the unfolded-protein response with heat stress response so as to maintain protein homeostasis inside the cell under heat stress.

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.

Identification of Potential Cytokinin Responsive Key Genes in Rice Treated With Trans-Zeatin Through Systems Biology Approach

Rice is an important staple food grain consumed by most of the population around the world. With climate and environmental changes, rice has undergone a tremendous stress state which has impacted crop production and productivity. Plant growth hormones are essential component that controls the overall outcome of the growth and development of the plant. Cytokinin is a hormone that plays an important role in plant immunity and defense systems. Trans-zeatin is an active form of cytokinin that can affect plant growth which is mediated by a multi-step two-component phosphorelay system that has different roles in various developmental stages. Systems biology is an approach for pathway analysis to trans-zeatin treated rice that could provide a deep understanding of different molecules associated with them. In this study, we have used a weighted gene co-expression network analysis method to identify the functional modules and hub genes involved in the cytokinin pathway. We have identified nine functional modules comprising of different hub genes which contribute to the cytokinin signaling route. The biological significance of these identified hub genes has been tested by applying well-proven statistical techniques to establish the association with the experimentally validated QTLs and annotated by the DAVID server. The establishment of key genes in different pathways has been confirmed. These results will be useful to design new stress-resistant cultivars which can provide sustainable yield in stress-specific conditions.

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.

Identification of the Complex Interplay Between Nematode-Related lncRNAs and Their Target Genes in Glycine max L

Soybean (Glycine max) is a major plant protein source and oilseed crop. However, plant-parasitic nematodes (PPNs) affect its annual yield. In the current study, in order to better understand the regulation of defense mechanism against PPNs in soybean, we investigated the role of long non-coding RNAs (lncRNAs) in response to two nematode species, Heterodera glycines (SCN: soybean cyst nematode) and Rotylenchulus reniformis (reniform). To this end, two publicly available RNA-seq data sets (SCN data set and RAD: reniform-associated data set) were employed to discover the lncRNAome profile of soybean under SCN and reniform infection, respectively. Upon identification of unannotated transcripts in these data sets, a seven-step pipeline was utilized to sieve these transcripts, which ended up in 384 and 283 potential lncRNAs in SCN data set and RAD, respectively. These transcripts were then used to predict cis and trans nematode-related targets in soybean genome. Computational prediction of target genes function, some of which were also among differentially expressed genes, revealed the involvement of putative nematode-responsive genes as well as enrichment of multiple stress responses in both data sets. Finally, 15 and six lncRNAs were proposed to be involved in microRNA-mediated regulation of gene expression in soybean in response to SNC and reniform infection, respectively. Collectively, this study provides a novel insight into the signaling and regulatory network of soybean-pathogen interactions and opens a new window for further research.

Two γ-zeins induce the unfolded protein response

The rapid, massive synthesis of storage proteins that occurs during seed development stresses endoplasmic reticulum (ER) homeostasis, which activates the ER unfolded protein response (UPR). However, how different storage proteins contribute to UPR is not clear. We analyzed vegetative tissues of transgenic Arabidopsis (Arabidopsis thaliana) plants constitutively expressing the common bean (Phaseolus vulgaris) soluble vacuolar storage protein PHASEOLIN (PHSL) or maize (Zea mays) prolamins (27-kDa γ-zein or 16-kDa γ-zein) that participate in forming insoluble protein bodies in the ER. We show that 16-kDa γ-zein significantly activates the INOSITOL REQUIRING ENZYME1/BASIC LEUCINE ZIPPER 60 (bZIP60) UPR branch-but not the bZIP28 branch or autophagy-leading to induction of major UPR-controlled genes that encode folding helpers that function inside the ER. Protein blot analysis of IMMUNOGLOBULIN-BINDING PROTEIN (BIP) 1 and 2, BIP3, GLUCOSE REGULATED PROTEIN 94 (GRP94), and ER-localized DNAJ family 3A (ERDJ3A) polypeptides confirmed their higher accumulation in the plant expressing 16-kDa γ-zein. Expression of 27-kDa γ-zein significantly induced only BIP3 and ERDJ3A transcription even though an increase in GRP94 and BIP1/2 polypeptides also occurred in this plant. These results indicate a significant but weaker effect of 27-kDa γ-zein compared to 16-kDa γ-zein, which corresponds with the higher availability of 16-kDa γ-zein for BIP binding, and indicates subtle protein-specific modulations of plant UPR. None of the analyzed genes was significantly induced by PHSL or by a mutated, soluble form of 27-kDa γ-zein that traffics along the secretory pathway. Such variability in UPR induction may have influenced the evolution of storage proteins with different tissue and subcellular localization.

Time of the day prioritizes the pool of translating mRNAs in response to heat stress

The circadian clock helps organisms to anticipate and coordinate gene regulatory responses to changes in environmental stimuli. Under growth limiting temperatures, the time of the day modulates the accumulation of polyadenylated mRNAs. In response to heat stress, plants will conserve energy and selectively translate mRNAs. How the clock and/or the time of the day regulates polyadenylated mRNAs bound by ribosomes in response to heat stress is unknown. In-depth analysis of Arabidopsis thaliana translating mRNAs found that the time of the day gates the response of approximately one-third of the circadian-regulated heat-responsive translatome. Specifically, the time of the day and heat stress interact to prioritize the pool of mRNAs in cue to be translated. For a subset of mRNAs, we observed a stronger gated response during the day, and preferentially before the peak of expression. We propose previously overlooked transcription factors (TFs) as regulatory nodes and show that the clock plays a role in the temperature response for select TFs. When the stress was removed, the redefined priorities for translation recovered within 1 h, though slower recovery was observed for abiotic stress regulators. Through hierarchical network connections between clock genes and prioritized TFs, our work provides a framework to target key nodes underlying heat stress tolerance throughout the day.

Pollen development in cotton (Gossypium hirsutum) is highly sensitive to heat exposure during the tetrad stage

The development of gametes in plants is acutely susceptible to heatwaves as brief as a few days, adversely affecting pollen maturation and reproductive success. Pollen in cotton (Gossypium hirsutum) was differentially affected when tetrad and binucleate stages were exposed to heat, revealing new insights into the interaction between heat and pollen development. Squares were tagged and exposed to 36/25°C (day/night, moderate heat) or 40/30°C (day/night, extreme heat) for 5 days. Mature pollen grains and leaves were collected for physiological and proteomic responses. While photosynthetic competence was not compromised even at 40°C, leaf tissues became leakier. In contrast, pollen grains were markedly smaller after the tetrad stage was exposed to 40°C and boll production was reduced by 65%. Sugar levels in pollen grains were elevated after exposure to heat, eliminating carbohydrate deficits as a likely cause of poor reproductive capacity. Proteomic analysis of pure pollen samples revealed a particularly high abundance of 70-kDa heat shock (Hsp70s) and cytoskeletal proteins. While short-term bursts of heat had a minor impact on leaves, male gametophyte development was profoundly damaged. Cotton acclimates to maxima of 36°C at both the vegetative and reproductive stages but 5-days exposure to 40°C significantly impairs reproductive development.

Heat stress response mechanisms in pollen development

Being rooted in place, plants are faced with the challenge of responding to unfavourable local conditions. One such condition, heat stress, contributes massively to crop losses globally. Heatwaves are predicted to increase, and it is of vital importance to generate crops that are tolerant to not only heat stress but also to several other abiotic stresses (e.g. drought stress, salinity stress) to ensure that global food security is protected. A better understanding of the molecular mechanisms that underlie the temperature stress response in pollen will be a significant step towards developing effective breeding strategies for high and stable production in crop plants. While most studies have focused on the vegetative phase of plant growth to understand heat stress tolerance, it is the reproductive phase that requires more attention as it is more sensitive to elevated temperatures. Every phase of reproductive development is affected by environmental challenges, including pollen and ovule development, pollen tube growth, male-female cross-talk, fertilization, and embryo development. In this review we summarize how pollen is affected by heat stress and the molecular mechanisms employed during the stress period, as revealed by classical and -omics experiments.

Sweet Modifications Modulate Plant Development

Plant development represents a continuous process in which the plant undergoes morphological, (epi)genetic and metabolic changes. Starting from pollination, seed maturation and germination, the plant continues to grow and develops specialized organs to survive, thrive and generate offspring. The development of plants and the interplay with its environment are highly linked to glycosylation of proteins and lipids as well as metabolism and signaling of sugars. Although the involvement of these protein modifications and sugars is well-studied, there is still a long road ahead to profoundly comprehend their nature, significance, importance for plant development and the interplay with stress responses. This review, approached from the plants' perspective, aims to focus on some key findings highlighting the importance of glycosylation and sugar signaling for plant development.

Tomato bZIP60 mRNA undergoes splicing in endoplasmic reticulum stress and in response to environmental stresses

Environmental stresses activate endoplasmic reticulum (ER) stress response pathways, collectively known as the unfolded protein response (UPR). IRE1/bZIP60 pathway is the most conserved of all UPR pathways from yeast to plants. Transcription factor bZIP60 is activated by the cytoplasmic splicing of its mRNA by Inositol Requiring Enzyme1 (IRE1) protein. bZIP60 mRNA has a typical stem-loop structure that is required for its splicing by IRE1 ribonuclease. We identified the tomato bZIP60 (SlbZIP60) and secondary structure prediction showed that it has the conserved stem-loop structure. Further, we demonstrate that SlbZIP60 is spliced upon treatment with an ER stress-inducing agent, tunicamycin. Tunicamycin also upregulated the expression of SlbZIP60. Finally, we show that SlbZIP60 undergo physiologically activated splicing in certain tissues of the plant and respond to environmental stresses, heat, and virus infection. This study will help for a deeper understanding of ER stress pathways and how they contribute to the stress tolerance of tomato, one of the important vegetable crops, cultivated under varied environmental 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.

Arabidopsis bZIP18 and bZIP52 Accumulate in Nuclei Following Heat Stress where They Regulate the Expression of a Similar Set of Genes

Heat stress (HS) is a major abiotic stress that negatively impacts crop yields across the globe. Plants respond to elevated temperatures by changing gene expression, mediated by transcription factors (TFs) functioning to enhance HS tolerance. The involvement of Group I bZIP TFs in the heat stress response (HSR) is not known. In this study, bZIP18 and bZIP52 were investigated for their possible role in the HSR. Localization experiments revealed their nuclear accumulation following heat stress, which was found to be triggered by dephosphorylation. Both TFs were found to possess two motifs containing serine residues that are candidates for phosphorylation. These motifs are recognized by 14–3–3 proteins, and bZIP18 and bZIP52 were found to bind 14–3–3 ε, the interaction of which sequesters them to the cytoplasm. Mutation of both residues abolished 14–3–3 ε interaction and led to a strict nuclear localization for both TFs. RNA-seq analysis revealed coordinated downregulation of several metabolic pathways including energy metabolism and translation, and upregulation of numerous lncRNAs in particular. These results support the idea that bZIP18 and bZIP52 are sequestered to the cytoplasm under control conditions, and that heat stress leads to their re-localization to nuclei, where they jointly regulate gene expression.

A temporal hierarchy underpins the transcription factor-DNA interactome of the maize UPR

Adverse environmental conditions reduce crop productivity and often increase the load of unfolded or misfolded proteins in the endoplasmic reticulum (ER). This potentially lethal condition, known as ER stress, is buffered by the unfolded protein response (UPR), a set of signaling pathways designed to either recover ER functionality or ignite programmed cell death. Despite the biological significance of the UPR to the life of the organism, the regulatory transcriptional landscape underpinning ER stress management is largely unmapped, especially in crops. To fill this significant knowledge gap, we performed a large-scale systems-level analysis of the protein-DNA interaction (PDI) network in maize (Zea mays). Using 23 promoter fragments of six UPR marker genes in a high-throughput enhanced yeast one-hybrid assay, we identified a highly interconnected network of 262 transcription factors (TFs) associated with significant biological traits and 831 PDIs underlying the UPR. We established a temporal hierarchy of TF binding to gene promoters within the same family as well as across different families of TFs. Cistrome analysis revealed the dynamic activities of a variety of cis-regulatory elements (CREs) in ER stress-responsive gene promoters. By integrating the cistrome results into a TF network analysis, we mapped a subnetwork of TFs associated with a CRE that may contribute to UPR management. Finally, we validated the role of a predicted network hub gene using the Arabidopsis system. The PDIs, TF networks, and CREs identified in our work are foundational resources for understanding transcription-regulatory mechanisms in the stress responses and crop improvement.

UPR signaling at the nexus of plant viral, bacterial, and fungal defenses

In recent years there have been significant advances in our understanding of the ER stress responses in plants that are associated with virus infection, as well as bacterial and fungal diseases. In plants, ER stress induced by virus infection includes several signaling pathways that include the unfolded protein response (UPR) to promote the expression of chaperone proteins for proper protein folding. Understanding how facets of ER stress signaling broadly engage in pathogen responses, as well as those that are specific to virus infection is important to distinguishing features essential for broad cellular defenses and processes that may be specifically linked to viral infectivity and disease.

Quantitative Proteomic Analysis of ER Stress Response Reveals both Common and Specific Features in Two Contrasting Ecotypes of Arabidopsis thaliana

Accumulation of unfolded and misfolded proteins in endoplasmic reticulum (ER) elicits a well-conserved response called the unfolded protein response (UPR), which triggers the upregulation of downstream genes involved in protein folding, vesicle trafficking, and ER-associated degradation (ERAD). Although dynamic transcriptomic responses and the underlying major transcriptional regulators in ER stress response in Arabidopsis have been well established, the proteome changes induced by ER stress have not been reported in Arabidopsis. In the current study, we found that the Arabidopsis Landsberg erecta (Ler) ecotype was more sensitive to ER stress than the Columbia (Col) ecotype. Quantitative mass spectrometry analysis with Tandem Mass Tag (TMT) isobaric labeling showed that, in total, 7439 and 7035 proteins were identified from Col and Ler seedlings, with 88 and 113 differentially regulated (FC > 1.3 or <0.7, p < 0.05) proteins by ER stress in Col and Ler, respectively. Among them, 40 proteins were commonly upregulated in Col and Ler, among which 10 were not upregulated in bzip28 bzip60 double mutant (Col background) plants. Of the 19 specifically upregulated proteins in Col, as compared with that in Ler, components in ERAD, N-glycosylation, vesicle trafficking, and molecular chaperones were represented. Quantitative RT-PCR showed that transcripts of eight out of 19 proteins were not upregulated (FC > 1.3 or <0.7, p < 0.05) by ER stress in Col ecotype, while transcripts of 11 out of 19 proteins were upregulated by ER stress in both ecotypes with no obvious differences in fold change between Col and Ler. Our results experimentally demonstrated the robust ER stress response at the proteome level in plants and revealed differentially regulated proteins that may contribute to the differed ER stress sensitivity between Col and Ler ecotypes in Arabidopsis.

ER-Phagy and Its Role in ER Homeostasis in Plants

The endoplasmic reticulum (ER) is the largest continuous membrane-bound cellular organelle and plays a central role in the biosynthesis of lipids and proteins and their distribution to other organelles. Autophagy is a conserved process that is required for recycling unwanted cellular components. Recent studies have implicated the ER as a membrane source for the formation of autophagosomes, vesicles that transport material to the vacuole during autophagy. When unfolded proteins accumulate in the ER and/or the ER lipid bilayer is disrupted, a condition known as ER stress results. During ER stress, ER membranes can also be engulfed through autophagy in a process termed ER-phagy. An interplay between ER stress responses and autophagy thus maintains the functions of the ER to allow cellular survival. In this review, we discuss recent progress in understanding ER-phagy in plants, including identification of regulatory factors and selective autophagy receptors. We also identify key unanswered questions in plant ER-phagy for future study.

Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold stress is a significant environmental factor that negatively affects plant growth and development in particular when it occurs during the growth phase. Plants have evolved means to protect themselves from damage caused by chilling or freezing temperatures and some plant species, in particular those from temperate geographical zones, can increase their basal level of freezing tolerance in a process termed cold acclimation. Cold acclimation improves plant survival, but also represses growth, since it inhibits activity of the growth-promoting hormones gibberellins (GAs). In addition to GAs, the steroid hormones brassinosteroids (BRs) also take part in growth promotion and cold stress signaling; however, in contrast to Gas, BRs can improve cold stress tolerance with fewer trade-offs in terms of growth and yields. Here we summarize our current understanding of the roles of BRs in cold stress responses with a focus on freezing tolerance and cold acclimation pathways.

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.

Loss of the Acetyltransferase NAA50 Induces Endoplasmic Reticulum Stress and Immune Responses and Suppresses Growth

Stress signaling in plants is carefully regulated to ensure proper development and reproductive fitness. Overactive defense signaling can result in dwarfism as well as developmental defects. In addition to requiring a substantial amount of energy, plant stress responses place a burden upon the cellular machinery, which can result in the accumulation of misfolded proteins and endoplasmic reticulum (ER) stress. Negative regulators of stress signaling, such as ENHANCED DISEASE RESISTANCE 1 (EDR1), ensure that stress responses are properly suspended when they are not needed, thereby conserving energy for growth and development. Here, we describe the role of an uncharacterized N-terminal acetyltransferase, NAA50, in the regulation of plant development and stress responses in Arabidopsis (Arabidopsis thaliana). Our results demonstrate that NAA50, an interactor of EDR1, plays an important role in regulating the tradeoff between plant growth and defense. Plants lacking NAA50 display severe developmental defects as well as induced stress responses. Reduction of NAA50 expression results in arrested stem and root growth as well as senescence. Furthermore, our results demonstrate that the loss of NAA50 results in constitutive ER stress signaling, indicating that NAA50 may be required for the suppression of ER stress. This work establishes NAA50 as essential for plant development and the suppression of stress responses, potentially through the regulation of ER stress.

Molecular genetic analyses of abiotic stress responses during plant reproductive development

Plant responses to abiotic stresses during vegetative growth have been extensively studied for many years. Daily environmental fluctuations can have dramatic effects on plant vegetative growth at multiple levels, resulting in molecular, cellular, physiological, and morphological changes. Plants are even more sensitive to environmental changes during reproductive stages. However, much less is known about how plants respond to abiotic stresses during reproduction. Fortunately, recent advances in this field have begun to provide clues about these important processes, which promise further understanding and a potential contribution to maximize crop yield under adverse environments. Here we summarize information from several plants, focusing on the possible mechanisms that plants use to cope with different types of abiotic stresses during reproductive development, and present a tentative molecular portrait of plant acclimation during reproductive stages. Additionally, we discuss strategies that plants use to balance between survival and productivity, with some comparison among different plants that have adapted to distinct environments.

COST1 regulates autophagy to control plant drought tolerance

Plants balance their competing requirements for growth and stress tolerance via a sophisticated regulatory circuitry that controls responses to the external environments. We have identified a plant-specific gene, COST1 (constitutively stressed 1), that is required for normal plant growth but negatively regulates drought resistance by influencing the autophagy pathway. An Arabidopsis thaliana cost1 mutant has decreased growth and increased drought tolerance, together with constitutive autophagy and increased expression of drought-response genes, while overexpression of COST1 confers drought hypersensitivity and reduced autophagy. The COST1 protein is degraded upon plant dehydration, and this degradation is reduced upon treatment with inhibitors of the 26S proteasome or autophagy pathways. The drought resistance of a cost1 mutant is dependent on an active autophagy pathway, but independent of other known drought signaling pathways, indicating that COST1 acts through regulation of autophagy. In addition, COST1 colocalizes to autophagosomes with the autophagosome marker ATG8e and the autophagy adaptor NBR1, and affects the level of ATG8e protein through physical interaction with ATG8e, indicating a pivotal role in direct regulation of autophagy. We propose a model in which COST1 represses autophagy under optimal conditions, thus allowing plant growth. Under drought, COST1 is degraded, enabling activation of autophagy and suppression of growth to enhance drought tolerance. Our research places COST1 as an important regulator controlling the balance between growth and stress responses via the direct regulation of autophagy.

Multilevel regulation of endoplasmic reticulum stress responses in plants: where old roads and new paths meet

The sessile lifestyle of plants requires them to cope with a multitude of stresses in situ. In response to diverse environmental and intracellular cues, plant cells respond by massive reprogramming of transcription and translation of stress response regulators, many of which rely on endoplasmic reticulum (ER) processing. This increased protein synthesis could exceed the capacity of precise protein quality control, leading to the accumulation of unfolded and/or misfolded proteins that triggers the unfolded protein response (UPR). Such cellular stress responses are multilayered and executed in different cellular compartments. Here, we will discuss the three main branches of UPR signaling in diverse eukaryotic systems, and describe various levels of ER stress response regulation that encompass transcriptional gene regulation by master transcription factors, post-transcriptional activities including cytoplasmic splicing, translational control, and multiple post-translational events such as peptide modifications and cleavage. In addition, we will discuss the roles of plant ER stress sensors in abiotic and biotic stress responses and speculate on the future prospects of engineering these signaling events for heightened stress tolerance.

The NIa-Protease Protein Encoded by the Pepper Mottle Virus Is a Pathogenicity Determinant and Releases DNA Methylation of Nicotiana benthamiana

It is well documented that the canonical function of NIa-protease (NIa-Pro) of the potyviruses is responsible for cleaving the viral polyprotein into functional proteins. Although NIa-Pro is vital for the infection cycle of potyviruses, the function of NIa-Pro in the interaction of the potyvirus host is not clear. In this study, NIa-Pro is ectopically expressed from a potato virus X (PVX) vector and infiltrates Nicotiana benthamiana wild type and 16-TGS. The pathogenicity and inhibition of host transcriptional gene silencing (TGS) are characterized. Ectopic expression of NIa-Pro from a PVX vector resulted in severe mosaic symptoms followed by a hypersensitive-like response in N. benthamiana. Furthermore, PepMoV NIa-Pro was able to reverse established TGS of a green fluorescent protein transgene by reducing methylation of promoter sequences in N. benthamiana and possessed the capacity to interfere with the global methylation of N. benthamiana. Taken together, the results of this study likely suggest that PepMoV NIa-Pro is a pathogenicity determinant and a potent suppressor of host TGS and suggest that NIa-Pro may employ novel mechanisms to suppress host antiviral defenses. To the best of our knowledge, this is the first report of a plant RNA virus modulating host TGS in a novel manner by interfering with the establishment of the methylation step of the plant DNA methylation pathway.

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.

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.

The Noncanonical Heat Shock Protein PvNod22 Is Essential for Infection Thread Progression During Rhizobial Endosymbiosis in Common Bean

In the establishment of plant-rhizobial symbiosis, the plant hosts express nodulin proteins during root nodule organogenesis. A limited number of nodulins have been characterized, and these perform essential functions in root nodule development and metabolism. Most nodulins are expressed in the nodule and at lower levels in other plant tissues. Previously, we isolated Nodulin 22 (PvNod22) from a common bean (Phaseolus vulgaris L.) cDNA library derived from Rhizobium-infected roots. PvNod22 is a noncanonical, endoplasmic reticulum (ER)-localized, small heat shock protein that confers protection against oxidative stress when overexpressed in Escherichia coli. Virus-induced gene silencing of PvNod22 resulted in necrotic lesions in the aerial organs of P. vulgaris plants cultivated under optimal conditions, activation of the ER-unfolded protein response (UPR), and, finally, plant death. Here, we examined the expression of PvNod22 in common bean plants during the establishment of rhizobial endosymbiosis and its relationship with two cellular processes associated with plant immunity, the UPR and autophagy. In the RNA interference lines, numerous infection threads stopped their progression before reaching the cortex cell layer of the root, and nodules contained fewer nitrogen-fixing bacteroids. Collectively, our results suggest that PvNod22 has a nonredundant function during legume-rhizobia symbiosis associated with infection thread elongation, likely by sustaining protein homeostasis in the ER.

A Functional Unfolded Protein Response Is Required for Normal Vegetative Development

The unfolded protein response (UPR) is activated in plants in response to endoplasmic reticulum stress and plays an important role in mitigating stress damage. Multiple factors act in the UPR, including the membrane-associated transcription factor, BASIC LEUCINE ZIPPER 17 (bZIP17), and the membrane-associated RNA splicing factor, INOSITOL REQUIRING ENZYME1 (IRE1). We have analyzed an Arabidopsis (Arabidopsis thaliana) ire1a ire1b bzip17 triple mutant, with defects in stress signaling, and found that the mutant is also impaired in vegetative plant growth under conditions without externally applied stress. This raised the possibility that the UPR functions in plant development in the same manner as it does in responding to stress. bZIP17 is mobilized to the nucleus in response to stress, and through the analysis of a mobilization-defective bZIP17 mutant, we found that to support normal plant development bZIP17 must be capable of mobilization. Likewise, through the analysis of ire1 mutants defective in either protein kinase or RNase activities, we found that both must be operative to promote normal development. These findings demonstrate that the UPR, which is associated with stress responses in plants, also functions under unstressed conditions to support normal development.