The unfolded protein response is triggered by a plant viral movement protein

bZIP60 and Bax inhibitor 1 contribute IRE1-dependent and independent roles to potexvirus infection

IRE1, BI-1, and bZIP60 monitor compatible plant-potexvirus interactions though recognition of the viral TGB3 protein. This study was undertaken to elucidate the roles of three IRE1 isoforms, the bZIP60U and bZIP60S, and BI-1 roles in genetic reprogramming of cells during potexvirus infection. Experiments were performed using Arabidopsis thaliana knockout lines and Plantago asiatica mosaic virus infectious clone tagged with the green fluorescent protein gene (PlAMV-GFP). There were more PlAMV-GFP infection foci in ire1a/b, ire1c, bzip60, and bi-1 knockout than wild-type (WT) plants. Cell-to-cell movement and systemic RNA levels were greater bzip60 and bi-1 than in WT plants. Overall, these data indicate an increased susceptibility to virus infection. Transgenic overexpression of AtIRE1b or StbZIP60 in ire1a/b or bzip60 mutant background reduced virus infection foci, while StbZIP60 expression influences virus movement. Transgenic overexpression of StbZIP60 also confers endoplasmic reticulum (ER) stress resistance following tunicamycin treatment. We also show bZIP60U and TGB3 interact at the ER. This is the first demonstration of a potato bZIP transcription factor complementing genetic defects in Arabidopsis. Evidence indicates that the three IRE1 isoforms regulate the initial stages of virus replication and gene expression, while bZIP60 and BI-1 contribute separately to virus cell-to-cell and systemic movement.

Improving transient protein expression in agroinfiltrated Nicotiana benthamiana

Agroinfiltration of Nicotiana benthamiana is routinely used in plant science and molecular pharming to transiently express proteins of interest. Here, we discuss four phenomena that should be avoided to improve transient expression. Immune responses can be avoided by depleting immune receptors and employing pathogen-derived effectors; transcript degradation by using silencing inhibitors or RNA interference machinery mutants; endoplasmic reticulum stress by co-expressing chaperones; and protein degradation can be avoided with subcellular targeting, protease mutants and co-expressing protease inhibitors. We summarise the reported increased yields for various recombinant proteins achieved with these approaches and highlight remaining challenges to further improve the efficiency of this versatile protein expression platform.

Basic leucine zipper transcription activators - tools to improve production and quality of human erythropoietin in Nicotiana benthamiana

Human erythropoietin (hEPO) is one of the most in-demand biopharmaceuticals, however, its production is challenging. When produced in a plant expression system, hEPO results in extensive plant tissue damage and low expression. It is demonstrated that the modulation of the plant protein synthesis machinery enhances hEPO production. Co-expression of basic leucine zipper transcription factors with hEPO prevents plant tissue damage, boosts expression, and increases hEPO solubility. bZIP28 co-expression up-regulates genes associated with the unfolded protein response, indicating that the plant tissue damage caused by hEPO expression is due to the native protein folding machinery being overwhelmed and that this can be overcome by co-expressing bZIP28.

Dynamics of ER stress-induced gene regulation in plants

Endoplasmic reticulum (ER) stress is a potentially lethal condition that is induced by the abnormal accumulation of unfolded or misfolded secretory proteins in the ER. In eukaryotes, ER stress is managed by the unfolded protein response (UPR) through a tightly regulated, yet highly dynamic, reprogramming of gene transcription. Although the core principles of the UPR are similar across eukaryotes, unique features of the plant UPR reflect the adaptability of plants to their ever-changing environments and the need to balance the demands of growth and development with the response to environmental stressors. The past decades have seen notable progress in understanding the mechanisms underlying ER stress sensing and signalling transduction pathways, implicating the UPR in the effects of physiological and induced ER stress on plant growth and crop yield. Facilitated by sequencing technologies and advances in genetic and genomic resources, recent efforts have driven the discovery of transcriptional regulators and elucidated the mechanisms that mediate the dynamic and precise gene regulation in response to ER stress at the systems level.

Integrated transmission electron microscopy and proteomic analyses reveal the cytoarchitectural response to cucumber mosaic virus infection in tobacco

Cucumber mosaic virus (CMV) causes huge economic losses to agriculture every year; thus, understanding the mechanism of plant resistance to CMV is imperative. In this study, an integrated analysis of transmission electron microscopy (TEM) observations and proteomic results was used to identify cytoarchitectural differences in Nicotiana tabacum cv. NC82 (susceptible) and cv. Taiyan 8 (T.T.8; resistant) following infection with CMV. The TEM observations showed that the structure of the chloroplasts and mitochondria was severely damaged at the late stage of infection in NC82. Moreover, the chloroplast stroma and mitochondrial cristae were reduced and disaggregated. However, in T.T.8, organelle structure remained largely intact Selective autophagy predominated in T.T.8, whereas non-selective autophagy dominated in NC82, resembling cellular disorder. Proteomic analysis of T.T.8 revealed differentially expressed proteins (DEPs) mostly associated with photosynthesis, respiration, reactive oxygen species (ROS) scavenging, and cellular autophagy. Biochemical analyses revealed that ROS-related catalase, autophagy-related disulfide isomerase, and jasmonic acid and antioxidant secondary metabolite synthesis-related 4-coumarate:CoA ligase (Nt4CL) exhibited different trends and significant differences in expression in the two cultivars after CMV inoculation. Furthermore, mutant phenotyping verified that reduced Nt4CL expression impaired resistance in T.T.8. The identified DEPs are crucial for maintaining intracellular homeostatic balance and likely contribute to the mechanism of CMV resistance in tobacco. These findings increase our understanding of plant cytological mechanisms conferring resistance to CMV infection.

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.

Sugarcane streak mosaic virus P1 protein inhibits unfolded protein response through direct suppression of bZIP60U splicing

The unfolded protein response (UPR) is a cell-designated strategy that maintains the balance of protein folding in the endoplasmic reticulum (ER). UPR features a network of signal transduction pathways that reprogram the transcription, mRNA translation, and protein post-translational modification to relieve the ER stresses from unfolded/misfolded proteins. Infection with plant viruses can induce the UPR, and activated UPR often promotes plant viral infections in turn. However, the mechanism used by plant viruses to balance UPR and achieve robust infection remain largely unknown. In this study, P1SCSMV was identified as a virus-encoded RNA silencing suppressor (VSR). Heterologous overexpression of P1SCSMV via potato virus X (PVX) was found lead to programmed cell death (PCD) in Nicotiana benthamiana. Furthermore, P1SCSMV was also found to inhibit the PVX infection-triggered UPR by downregulating UPR-related genes and directly induced the distortion and collapse of the ER polygonal meshes on PVX-P1SCSMV infected N. benthamiana. Moreover, self-interaction, VSR activity, UPR inhibition, and cell death phenotype of P1SCSMV were also found to be dependent on its bipartite nuclear localization signal (NLS) (251RKRKLFPRIPLK262). P1SCSMV was found to directly bind to the stem-loop region of NbbZIP60U via its NLS and inhibit the UPR pathways, ultimately resulting in a PCD phenotype in PVX-P1SCSMV infected N. benthamiana leaves. This study also revealed the balancing role of potyviruses encoded P1SCSMV in the UPR pathway to achieve robust viral infection. This may represent a novel virulence strategy for plant viruses.

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.

The Role of Plant Transcription Factors in the Fight against Plant Viruses

Plants are vulnerable to the challenges of unstable environments and pathogen infections due to their immobility. Among various stress conditions, viral infection is a major threat that causes significant crop loss. In response to viral infection, plants undergo complex molecular and physiological changes, which trigger defense and morphogenic pathways. Transcription factors (TFs), and their interactions with cofactors and cis-regulatory genomic elements, are essential for plant defense mechanisms. The transcriptional regulation by TFs is crucial in establishing plant defense and associated activities during viral infections. Therefore, identifying and characterizing the critical genes involved in the responses of plants against virus stress is essential for the development of transgenic plants that exhibit enhanced tolerance or resistance. This article reviews the current understanding of the transcriptional control of plant defenses, with a special focus on NAC, MYB, WRKY, bZIP, and AP2/ERF TFs. The review provides an update on the latest advances in understanding how plant TFs regulate defense genes expression during viral infection.

A viral movement protein co-opts endoplasmic reticulum luminal-binding protein and calreticulin to promote intracellular movement

Intracellular movement is an important step for the initial spread of virus in plants during infection. This process requires virus-encoded movement proteins (MPs) and their interaction with host factors. Despite the large number of known host factors involved in the movement of different viruses, little is known about host proteins that interact with one of the MPs encoded by potexviruses, the triple-gene-block protein 3 (TGBp3). The main obstacle lies in the relatively low expression level of potexviral TGBp3 in hosts and the weak or transient nature of interactions. Here, we used TurboID-based proximity labeling to identify the network of proteins directly or indirectly interacting with the TGBp3 of a potexvirus, Bamboo mosaic virus (BaMV). Endoplasmic reticulum (ER) luminal-binding protein 4 and calreticulin 3 of Nicotiana benthamiana (NbBiP4 and NbCRT3, respectively) associated with the functional TGBp3-containing BaMV movement complexes, but not the movement-defective mutant, TGBp3M. Fluorescent microscopy revealed that TGBp3 colocalizes with NbBiP4 or NbCRT3 and the complexes move together along ER networks to cell periphery in N. benthamiana. Loss- and gain-of-function experiments revealed that NbBiP4 or NbCRT3 is required for the efficient spread and accumulation of BaMV in infected leaves. In addition, overexpression of NbBiP4 or NbCRT3 enhanced the targeting of BaMV TGBp1 to plasmodesmata (PD), indicating that NbBiP4 and NbCRT3 interact with TGBp3 to promote the intracellular transport of virion cargo to PD that facilitates virus cell-to-cell movement. Our findings revealed additional roles for NbBiP4 and NbCRT3 in BaMV intracellular movement through ER networks or ER-derived vesicles to PD, which enhances the spread of BaMV in N. benthamiana.

Binding immunoglobulin 2 functions as a proviral factor for potyvirus infections in Nicotiana benthamiana

Infection of viruses from the genera Bromovirus, Potyvirus, and Potexvirus in Nicotiana benthamiana induces significant up-regulation of the genes that encode the HSP70 family, including binding immunoglobulin protein 2 (BiP2). Three up-regulated genes were knocked down and infection assays with these knockdown lines demonstrated the importance of the BiP2 gene for potyvirus infection but not for infection by the other tested viruses. Distinct symptoms of cucumber mosaic virus (CMV) and potato virus X (PVX) were observed in the BiP2 knockdown line at 10 days postagroinfiltration. Interestingly, following inoculation with either soybean mosaic virus (SMV) or pepper mottle virus (PepMoV) co-expressing green fluorescent protein (GFP), neither crinkle symptoms nor GFP signals were observed in the BiP2 knockdown line. Subsequent reverse transcription-quantitative PCR analysis demonstrated that knockdown of BiP2 resulted in a significant decrease of SMV and PepMoV RNA accumulation but not PVX or CMV RNA accumulation. Further yeast two-hybrid and co-immunoprecipitation analyses validated the interaction between BiP2 and nuclear inclusion protein b (NIb) of SMV. Together, our findings suggest the crucial role of BiP2 as a proviral host factor necessary for potyvirus infection. The interaction between BiP2 and NIb may be the critical factor determining susceptibility in N. benthamiana, but further studies are needed to elucidate the underlying mechanism.

The V2 Protein from the Geminivirus Tomato Yellow Leaf Curl Virus Largely Associates to the Endoplasmic Reticulum and Promotes the Accumulation of the Viral C4 Protein in a Silencing Suppression-Independent Manner

Viruses are strict intracellular parasites that rely on the proteins encoded in their genomes for the effective manipulation of the infected cell that ultimately enables a successful infection. Viral proteins have to be produced during the cell invasion and takeover in sufficient amounts and in a timely manner. Silencing suppressor proteins evolved by plant viruses can boost the production of viral proteins; although, additional mechanisms for the regulation of viral protein production likely exist. The strongest silencing suppressor encoded by the geminivirus tomato yellow leaf curl virus (TYLCV) is V2: V2 suppresses both post-transcriptional and transcriptional gene silencing (PTGS and TGS), activities that are associated with its localization in punctate cytoplasmic structures and in the nucleus, respectively. However, V2 has been previously described to largely localize in the endoplasmic reticulum (ER), although the biological relevance of this distribution remains mysterious. Here, we confirm the association of V2 to the ER in Nicotiana benthamiana and assess the silencing suppression activity-independent impact of V2 on protein accumulation. Our results indicate that V2 has no obvious influence on the localization of ER-synthesized receptor-like kinases (RLKs) or ER quality control (ERQC)/ER-associated degradation (ERAD), but dramatically enhances the accumulation of the viral C4 protein, which is co-translationally myristoylated, possibly in proximity to the ER. By using the previously described V2_C84S/86S mutant, in which the silencing suppression activity is abolished, we uncouple RNA silencing from the observed effect. Therefore, this work uncovers a novel function of V2, independent of its capacity to suppress silencing, in the promotion of the accumulation of another crucial viral protein.

Molecular Mechanisms of Plant Responses to Salt Stress

Saline-alkali soils pose an increasingly serious global threat to plant growth and productivity. Much progress has been made in elucidating how plants adapt to salt stress by modulating ion homeostasis. Understanding the molecular mechanisms that affect salt tolerance and devising strategies to develop/breed salt-resilient crops have been the primary goals of plant salt stress signaling research over the past few decades. In this review, we reflect on recent major advances in our understanding of the cellular and physiological mechanisms underlying plant responses to salt stress, especially those involving temporally and spatially defined changes in signal perception, decoding, and transduction in specific organelles or cells.

Enhanced HIV SOSIP Envelope yields in plants through transient co-expression of peptidyl-prolyl isomerase B and calreticulin chaperones and ER targeting

High yield production of recombinant HIV SOSIP envelope (Env) trimers has proven elusive as numerous disulfide bonds, proteolytic cleavage and extensive glycosylation pose high demands on the host cell machinery and stress imposed by accumulation of misfolded proteins may ultimately lead to cellular toxicity. The present study utilized the Nicotiana benthamiana/p19 (N.b./p19) transient plant system to assess co-expression of two ER master regulators and 5 chaperones, crucial in the folding process, to enhance yields of three Env SOSIPs, single chain BG505 SOSIP.664 gp140, CH505TF.6R.SOSIP.664.v4.1 and CH848-10.17-DT9. Phenotypic changes in leaves induced by SOSIP expression were employed to rapidly identify chaperone-assisted improvement in health and expression. Up to 15-fold increases were obtained by co-infiltration of peptidylprolvl isomerase (PPI) and calreticulin (CRT) which were further enhanced by addition of the ER-retrieval KDEL tags to the SOSIP genes; levels depending on individual SOSIP type, day of harvest and chaperone gene dosage. Results are consistent with reducing SOSIP misfolding and cellular stress due to increased exposure to the plant host cell's calnexin/calreticulin network and accelerating the rate-limiting cis-trans isomerization of Xaa-Pro peptide bonds respectively. Plant transient co-expression facilitates rapid identification of host cell factors and will be translatable to other complex glycoproteins and mammalian expression systems.

Regulation of heat shock proteins 70 and their role in plant immunity

Heat shock proteins 70 (HSP70s) are steadily gaining more attention in the field of plant biotic interactions. Though their regulation and activity in plants are much less well characterized than are those of their counterparts in mammals, accumulating evidence indicates that the role of HSP70-mediated defense mechanisms in plant cells is indispensable. In this review, we summarize current knowledge of HSP70 post-translational control in plants. We comment on the phytohormonal regulation of HSP70 expression and protein abundance, and identify a prominent role for cytokinin in HSP70 control. We outline HSP70s' subcellular localizations, chaperone activity, and chaperone-mediated protein degradation. We focus on the role of HSP70s in plant pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and discuss the contribution of different HSP70 subfamilies to plant defense against pathogens.

Endoplasmic Reticulum Stress of Gut Enterocyte and Intestinal Diseases

The endoplasmic reticulum, a vast reticular membranous network from the nuclear envelope to the plasma membrane responsible for the synthesis, maturation, and trafficking of a wide range of proteins, is considerably sensitive to changes in its luminal homeostasis. The loss of ER luminal homeostasis leads to abnormalities referred to as endoplasmic reticulum (ER) stress. Thus, the cell activates an adaptive response known as the unfolded protein response (UPR), a mechanism to stabilize ER homeostasis under severe environmental conditions. ER stress has recently been postulated as a disease research breakthrough due to its significant role in multiple vital cellular functions. This has caused numerous reports that ER stress-induced cell dysfunction has been implicated as an essential contributor to the occurrence and development of many diseases, resulting in them targeting the relief of ER stress. This review aims to outline the multiple molecular mechanisms of ER stress that can elucidate ER as an expansive, membrane-enclosed organelle playing a crucial role in numerous cellular functions with evident changes of several cells encountering ER stress. Alongside, we mainly focused on the therapeutic potential of ER stress inhibition in gastrointestinal diseases such as inflammatory bowel disease (IBD) and colorectal cancer. To conclude, we reviewed advanced research and highlighted future treatment strategies of ER stress-associated conditions.

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.

Autophagy Inhibits Intercellular Transport of Citrus Leaf Blotch Virus by Targeting Viral Movement Protein

Autophagy is an evolutionarily conserved cellular-degradation mechanism implicated in antiviral defense in plants. Studies have shown that autophagy suppresses virus accumulation in cells; however, it has not been reported to specifically inhibit viral spread in plants. This study demonstrated that infection with citrus leaf blotch virus (CLBV; genus Citrivirus, family Betaflexiviridae) activated autophagy in Nicotiana benthamiana plants as indicated by the increase of autophagosome formation. Impairment of autophagy through silencing of N. benthamiana autophagy-related gene 5 (NbATG5) and NbATG7 enhanced cell-to-cell and systemic movement of CLBV; however, it did not affect CLBV accumulation when the systemic infection had been fully established. Treatment using an autophagy inhibitor or silencing of NbATG5 and NbATG7 revealed that transiently expressed movement protein (MP), but not coat protein, of CLBV was targeted by selective autophagy for degradation. Moreover, we identified that CLBV MP directly interacted with NbATG8C1 and NbATG8i, the isoforms of autophagy-related protein 8 (ATG8), which are key factors that usually bind cargo receptors for selective autophagy. Our results present a novel example in which autophagy specifically targets a viral MP to limit the intercellular spread of the virus in plants.

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.

Gimme shelter: three-dimensional architecture of the endoplasmic reticulum, the replication site of grapevine Pinot gris virus

Grapevine leaf mottling and deformation is a novel grapevine disease that has been associated with grapevine Pinot gris virus (GPGV). The virus was observed exclusively inside membrane-bound structures in the bundle sheath cells of the infected grapevines. As reported widely in the literature, many positive-sense single-stranded RNA viruses modify host-cell membranes to form a variety of deformed organelles, which shelter viral genome replication from host antiviral compounds. Morphologically, the GPGV-associated membranous structures resemble the deformed endoplasmic reticulum described in other virus-host interactions. In this study we investigated the GPGV-induced membranous structures observed in the bundle sheath cells of infected plants. The upregulation of different ER stress-related genes was evidenced by RT-qPCR assays, further confirming the involvement of the ER in grapevine/GPGV interaction. Specific labelling of the membranous structures with an antibody against luminal-binding protein identified them as ER. Double-stranded RNA molecules, which are considered intermediates of viral replication, were localised exclusively in the ER-derived structures and indicated that GPGV exploited this organelle to replicate itself in a shelter niche. Novel analyses using focussed ion-beam scanning electron microscopy (FIB-SEM) were performed in grapevine leaf tissues to detail the three-dimensional organisation of the ER-derived structures and their remodelling due to virus replication.

Resistance induction based on the understanding of molecular interactions between plant viruses and host plants

BACKGROUND

Viral diseases cause significant damage to crop yield and quality. While fungi- and bacteria-induced diseases can be controlled by pesticides, no effective approaches are available to control viruses with chemicals as they use the cellular functions of their host for their infection cycle. The conventional method of viral disease control is to use the inherent resistance of plants through breeding. However, the genetic sources of viral resistance are often limited. Recently, genome editing technology enabled the publication of multiple attempts to artificially induce new resistance types by manipulating host factors necessary for viral infection.

MAIN BODY

In this review, we first outline the two major (R gene-mediated and RNA silencing) viral resistance mechanisms in plants. We also explain the phenomenon of mutations of host factors to function as recessive resistance genes, taking the eIF4E genes as examples. We then focus on a new type of virus resistance that has been repeatedly reported recently due to the widespread use of genome editing technology in plants, facilitating the specific knockdown of host factors. Here, we show that (1) an in-frame mutation of host factors necessary to confer viral resistance, sometimes resulting in resistance to different viruses and that (2) certain host factors exhibit antiviral resistance and viral-supporting (proviral) properties.

CONCLUSION

A detailed understanding of the host factor functions would enable the development of strategies for the induction of a new type of viral resistance, taking into account the provision of a broad resistance spectrum and the suppression of the appearance of resistance-breaking strains.

Double-faced role of Bcl-2-associated athanogene 7 in plant-Phytophthora interaction

Due to their sessile nature, plants must respond to various environmental assaults in a coordinated manner. The endoplasmic reticulum is a central hub for plant responses to various stresses. We previously showed that Phytophthora utilizes effector PsAvh262-mediated binding immunoglobulin protein (BiP) accumulation for suppressing endoplasmic reticulum stress-triggered cell death. As a BiP binding partner, Bcl-2-associated athanogene 7 (BAG7) plays a crucial role in the maintenance of the unfolded protein response, but little is known about its role in plant immunity. In this work, we reveal a double-faced role of BAG7 in Arabidopsis-Phytophthora interaction in which it regulates endoplasmic reticulum stress-mediated immunity oppositely in different cellular compartments. In detail, it acts as a susceptibility factor in the endoplasmic reticulum, but plays a resistance role in the nucleus against Phytophthora. Phytophthora infection triggers the endoplasmic reticulum-to-nucleus translocation of BAG7, the same as abiotic heat stress; however, this process can be prevented by PsAvh262-mediated BiP accumulation. Moreover, the immunoglobulin/albumin-binding domain in PsAvh262 is essential for both pathogen virulence and BiP accumulation. Taken together, our study uncovers a double-faced role of BAG7; Phytophthora advances its colonization in planta by utilizing an effector to detain BAG7 in the endoplasmic reticulum.

Citrus miraculin-like protein hijacks a viral movement-related p33 protein and induces cellular oxidative stress in defence against Citrus tristeza virus

To defend against pathogens, plants have developed a complex immune system, which recognizes the pathogen effectors and mounts defence responses. In this study, the p33 protein of Citrus tristeza virus (CTV), a viral membrane-associated effector, was used as a molecular bait to explore virus interactions with host immunity. We discovered that Citrus macrophylla miraculin-like protein 2 (CmMLP2), a member of the soybean Kunitz-type trypsin inhibitor family, targets the viral p33 protein. The expression of CmMLP2 was up-regulated by p33 in the citrus phloem-associated cells. Knock-down of the MLP2 expression in citrus plants resulted in a higher virus accumulation, while the overexpression of CmMLP2 reduced the infectivity of CTV in the plant hosts. Further investigation revealed that, on the one hand, binding of CmMLP2 interrupts the cellular distribution of p33 whose proper function is necessary for the effective virus movement throughout the host. On the other hand, the ability of CmMLP2 to reorganize the endomembrane system, amalgamating the endoplasmic reticulum and the Golgi apparatus, induces cellular stress and accumulation of the reactive oxygen species, which inhibits the replication of CTV. Altogether, our data suggest that CmMLP2 employs a two-way strategy in defence against CTV infection.

Transcriptional Regulatory Networks Associate with Early Stages of Potato Virus X Infection of Solanum tuberosum

Potato virus X (PVX) belongs to genus Potexvirus. This study characterizes the cellular transcriptome responses to PVX infection in Russet potato at 2 and 3 days post infection (dpi). Among the 1242 differentially expressed genes (DEGs), 268 genes were upregulated, and 37 genes were downregulated at 2 dpi while 677 genes were upregulated, and 265 genes were downregulated at 3 dpi. DEGs related to signal transduction, stress response, and redox processes. Key stress related transcription factors were identified. Twenty-five pathogen resistance gene analogs linked to effector triggered immunity or pathogen-associated molecular pattern (PAMP)-triggered immunity were identified. Comparative analysis with Arabidopsis unfolded protein response (UPR) induced DEGs revealed genes associated with UPR and plasmodesmata transport that are likely needed to establish infection. In conclusion, this study provides an insight on major transcriptional regulatory networked involved in early response to PVX infection and establishment.

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.

The unfolded protein response plays dual roles in rice stripe virus infection through fine-tuning the movement protein accumulation

The movement of plant viruses is a complex process that requires support by the virus-encoded movement protein and multiple host factors. The unfolded protein response (UPR) plays important roles in plant virus infection, while how UPR regulates viral infection remains to be elucidated. Here, we show that rice stripe virus (RSV) elicits the UPR in Nicotiana benthamiana. The RSV-induced UPR activates the host autophagy pathway by which the RSV-encoded movement protein, NSvc4, is targeted for autophagic degradation. As a counteract, we revealed that NSvc4 hijacks UPR-activated type-I J-domain proteins, NbMIP1s, to protect itself from autophagic degradation. Unexpectedly, we found NbMIP1 stabilizes NSvc4 in a non-canonical HSP70-independent manner. Silencing NbMIP1 family genes in N. benthamiana, delays RSV infection, while over-expressing NbMIP1.4b promotes viral cell-to-cell movement. Moreover, OsDjA5, the homologue of NbMIP1 family in rice, behaves in a similar manner toward facilitating RSV infection. This study exemplifies an arms race between RSV and the host plant, and reveals the dual roles of the UPR in RSV infection though fine-tuning the accumulation of viral movement protein.

Insight into the bZIP Gene Family in Solanum tuberosum: Genome and Transcriptome Analysis to Understand the Roles of Gene Diversification in Spatiotemporal Gene Expression and Function

The basic region-leucine zipper (bZIP) transcription factors (TFs) form homodimers and heterodimers via the coil–coil region. The bZIP dimerization network influences gene expression across plant development and in response to a range of environmental stresses. The recent release of the most comprehensive potato reference genome was used to identify 80 StbZIP genes and to characterize their gene structure, phylogenetic relationships, and gene expression profiles. The StbZIP genes have undergone 22 segmental and one tandem duplication events. Ka/Ks analysis suggested that most duplications experienced purifying selection. Amino acid sequence alignments and phylogenetic comparisons made with the Arabidopsis bZIP family were used to assign the StbZIP genes to functional groups based on the Arabidopsis orthologs. The patterns of introns and exons were conserved within the assigned functional groups which are supportive of the phylogeny and evidence of a common progenitor. Inspection of the leucine repeat heptads within the bZIP domains identified a pattern of attractive pairs favoring homodimerization, and repulsive pairs favoring heterodimerization. These patterns of attractive and repulsive heptads were similar within each functional group for Arabidopsis and S. tuberosum orthologs. High-throughput RNA-seq data indicated the most highly expressed and repressed genes that might play significant roles in tissue growth and development, abiotic stress response, and response to pathogens including Potato virus X. These data provide useful information for further functional analysis of the StbZIP gene family and their potential applications in crop improvement.

Probing natural variation of IRE1 expression and endoplasmic reticulum stress responses in Arabidopsis accessions

The environmental effects shape genetic changes in the individuals within plant populations, which in turn contribute to the enhanced genetic diversity of the population as a whole. Thus, individuals within the same species can acquire and accumulate genetic differences in their genomes depending on their local environment and evolutionary history. IRE1 is a universal endoplasmic reticulum (ER) stress sensor that activates an evolutionarily conserved signalling cascade in response to biotic and abiotic stresses. Here, we selected nine different Arabidopsis accessions along with the reference ecotype Columbia-0, based on their geographical origins and differential endogenous IRE1 expression under steady-state conditions to investigate the natural variation of ER stress responses. We cloned and analysed selected upstream regulatory regions of IRE1a and IRE1b, which revealed differential levels of their inducibility. We also subjected these accessions to an array of biotic and abiotic stresses including heat, ER stress-inducing chemical tunicamycin, phytohormone salicylic acid, and pathogen infection. We measured IRE1-mediated splicing of its evolutionarily conserved downstream client as well as transcript accumulation of ER-resident chaperones and co-chaperones. Collectively, our results illustrate the expression polymorphism of a major plant stress receptor and its relationship with molecular and physiological ER stress sensitivity.

NbALY916 is involved in potato virus X P25-triggered cell death in Nicotiana benthamiana

Systemic necrosis often occurs during viral infection of plants and is thought mainly to be the result of long-term stress induced by viral infection. Potato virus X (PVX) encodes the P25 pathogenicity factor that triggers a necrotic reaction during PVX-potato virus Ysynergistic coinfection. In this study, we discovered that NbALY916, a multifunctional nuclear protein, could interact with P25. When NbALY916 expression was reduced by tobacco rattle virus (TRV)-based virus-induced gene silencing, the accumulation of P25 was increased, which would be expected to cause more severe necrosis. However, silencing of NbALY916 reduced the extent of cell death caused by P25. Furthermore, we found that overexpression of NbALY916 increased the accumulation of H2 O2 and triggered more extensive cell death when coexpressed with P25, even though accumulation of P25 was itself reduced by the increased expression of NbALY916. Furthermore, transient expression of P25 specifically induced the expression of NbALY916 mRNA, but not the mRNAs of three other ALYs in Nicotiana benthamiana. In addition, we showed that silencing of NbALY916 or transient overexpression of NbALY916 affected the infection of PVX in N. benthamiana. Our results reveal that NbALY916 has an antiviral role that, in the case of PVX, operates by inducing the accumulation of H2 O2 and mediating the degradation of P25.

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.

RNA silencing-related genes contribute to tolerance of infection with potato virus X and Y in a susceptible tomato plant

BACKGROUND

In plants, the RNA silencing system functions as an antiviral defense mechanism following its induction with virus-derived double-stranded RNAs. This occurs through the action of RNA silencing components, including Dicer-like (DCL) nucleases, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDR). Plants encode multiple AGOs, DCLs, and RDRs. The functions of these components have been mainly examined in Arabidopsis thaliana and Nicotiana benthamiana. In this study, we investigated the roles of DCL2, DCL4, AGO2, AGO3 and RDR6 in tomato responses to viral infection. For this purpose, we used transgenic tomato plants (Solanum lycopersicum cv. Moneymaker), in which the expression of these genes were suppressed by double-stranded RNA-mediated RNA silencing.

METHODS

We previously created multiple DCL (i.e., DCL2 and DCL4) (hpDCL2.4) and RDR6 (hpRDR6) knockdown transgenic tomato plants and here additionally did multiple AGO (i.e., AGO2 and AGO3) knockdown plants (hpAGO2.3), in which double-stranded RNAs cognate to these genes were expressed to induce RNA silencing to them. Potato virus X (PVX) and Y (PVY) were inoculated onto these transgenic tomato plants, and the reactions of these plants to the viruses were investigated. In addition to observation of symptoms, viral coat protein and genomic RNA were detected by western and northern blotting and reverse transcription-polymerase chain reaction (RT-PCR). Host mRNA levels were investigated by quantitative RT-PCR.

RESULTS

Following inoculation with PVX, hpDCL2.4 plants developed a more severe systemic mosaic with leaf curling compared with the other inoculated plants. Systemic necrosis was also observed in hpAGO2.3 plants. Despite the difference in the severity of symptoms, the accumulation of PVX coat protein (CP) and genomic RNA in the uninoculated upper leaves was not obviously different among hpDCL2.4, hpRDR6, and hpAGO2.3 plants and the empty vector-transformed plants. Moneymaker tomato plants were asymptomatic after infection with PVY. However, hpDCL2.4 plants inoculated with PVY developed symptoms, including leaf curling. Consistently, PVY CP was detected in the uninoculated symptomatic upper leaves of hpDCL2.4 plants through western blotting. Of note, PVY CP was rarely detected in other asymptomatic transgenic or wild-type plants. However, PVY was detected in the uninoculated upper leaves of all the inoculated plants using reverse transcription-polymerase chain reactions. These findings indicated that PVY systemically infected asymptomatic Moneymaker tomato plants at a low level (i.e., no detection of CP via western blotting).

CONCLUSION

Our results indicate that the tomato cultivar Moneymaker is susceptible to PVX and shows mild mosaic symptoms, whereas it is tolerant and asymptomatic to systemic PVY infection with a low virus titer. In contrast, in hpDCL2.4 plants, PVX-induced symptoms became more severe and PVY infection caused symptoms. These results indicate that DCL2, DCL4, or both contribute to tolerance to infection with PVX and PVY. PVY CP and genomic RNA accumulated to a greater extent in DCL2.4-knockdown plants. Hence, the contribution of these DCLs to tolerance to infection with PVY is at least partly attributed to their roles in anti-viral RNA silencing, which controls the multiplication of PVY in tomato plants. The necrotic symptoms observed in the PVX-infected hpAGO2.3 plants suggest that AGO2, AGO3 or both are also distinctly involved in tolerance to infection with PVX.

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.

Genome-wide identification and characterization of Solanum tuberosum BiP genes reveal the role of the promoter architecture in BiP gene diversity

The endoplasmic reticulum (ER) immunoglobulin binding proteins (BiPs) are molecular chaperones involved in normal protein maturation and refolding malformed proteins through the unfolded protein response (UPR). Plant BiPs belong to a multi-gene family contributing to development, immunity, and responses to environmental stresses. This study identified three BiP homologs in the Solanum tuberosum (potato) genome using phylogenetic, amino acid sequence, 3-D protein modeling, and gene structure analysis. These analyses revealed that StBiP1 and StBiP2 grouped with AtBiP2, whereas StBiP3 grouped with AtBiP3. While the protein sequences and folding structures are highly similar, these StBiPs are distinguishable by their expression patterns in different tissues and in response to environmental stressors such as treatment with heat, chemicals, or virus elicitors of UPR. Ab initio promoter analysis revealed that potato and Arabidopsis BiP1 and BiP2 promoters were highly enriched with cis-regulatory elements (CREs) linked to developmental processes, whereas BiP3 promoters were enriched with stress related CREs. The frequency and linear distribution of these CREs produced two phylogenetic branches that further resolve the groups identified through gene phylogeny and exon/intron phase analysis. These data reveal that the CRE architecture of BiP promoters potentially define their spatio-temporal expression patterns under developmental and stress related cues.

Genome-wide identification and characterization of Solanum tuberosum BiP genes reveal the role of the promoter architecture in BiP gene diversity

The endoplasmic reticulum (ER) immunoglobulin binding proteins (BiPs) are molecular chaperones involved in normal protein maturation and refolding malformed proteins through the unfolded protein response (UPR). Plant BiPs belong to a multi-gene family contributing to development, immunity, and responses to environmental stresses. This study identified three BiP homologs in the Solanum tuberosum (potato) genome using phylogenetic, amino acid sequence, 3-D protein modeling, and gene structure analysis. These analyses revealed that StBiP1 and StBiP2 grouped with AtBiP2, whereas StBiP3 grouped with AtBiP3. While the protein sequences and folding structures are highly similar, these StBiPs are distinguishable by their expression patterns in different tissues and in response to environmental stressors such as treatment with heat, chemicals, or virus elicitors of UPR. Ab initio promoter analysis revealed that potato and Arabidopsis BiP1 and BiP2 promoters were highly enriched with cis-regulatory elements (CREs) linked to developmental processes, whereas BiP3 promoters were enriched with stress related CREs. The frequency and linear distribution of these CREs produced two phylogenetic branches that further resolve the groups identified through gene phylogeny and exon/intron phase analysis. These data reveal that the CRE architecture of BiP promoters potentially define their spatio-temporal expression patterns under developmental and stress related cues.

Identification of a Novel NtLRR-RLK and Biological Pathways That Contribute to Tolerance of TMV in Nicotiana tabacum

Tobacco mosaic virus (TMV) infection can causes serious damage to tobacco crops. To explore the approach of preventing TMV infection of plants, two tobacco cultivars with different resistances to TMV were used to analyze transcription profiling before and after TMV infection. The involvement of biological pathways differed between the tolerant variety (Yuyan8) and the susceptible variety (NC89). In particular, the plant-virus interaction pathway was rapidly activated in Yuyan8, and specific resistance genes were enriched. Liquid chromatography tandem mass spectrometry analysis detected large quantities of antiviral substances in the tolerant Yuyan8. A novel Nicotiana tabacum leucine-rich repeat receptor kinase (NtLRR-RLK) gene was identified as being methylated and this was verified using bisulfite sequencing. Transient expression of TMV-green fluorescent protein in pRNAi-NtLRR-RLK transgenic plants confirmed that NtLRR-RLK was important for susceptibility to TMV. The specific protein interaction map generated from our study revealed that levels of BIP1, E3 ubiquitin ligase, and LRR-RLK were significantly elevated, and all were represented at node positions in the protein interaction map. The same expression tendency of these proteins was also found in pRNAi-NtLRR-RLK transgenic plants at 24 h after TMV inoculation. These data suggested that specific genes in the infection process can activate the immune signal cascade through different resistance genes, and the integration of signal pathways could produce resistance to the virus. These results contribute to the overall understanding of the molecular basis of plant resistance to TMV and in the long term could identify new strategies for prevention and control virus infection.

Transient expression and purification of β-caryophyllene synthase in Nicotiana benthamiana to produce β-caryophyllene in vitro

The sesquiterpene β-caryophyllene is an ubiquitous component in many plants that has commercially been used as an aroma in cosmetics and perfumes. Recent studies have shown its potential use as a therapeutic agent and biofuel. Currently, β-caryophyllene is isolated from large amounts of plant material. Molecular farming based on the Nicotiana benthamiana transient expression system may be used for a more sustainable production of β-caryophyllene. In this study, a full-length cDNA of a new duplicated β-caryophyllene synthase from Artemisia annua (AaCPS1) was isolated and functionally characterized. In order to produce β-caryophyllene in vitro, the AaCPS1 was cloned into a plant viral-based vector pEAQ-HT. Subsequently, the plasmid was transferred into the Agrobacterium and agroinfiltrated into N. benthamiana leaves. The AaCPS1 expression was analyzed by quantitative PCR at different time points after agroinfiltration. The highest level of transcripts was observed at 9 days post infiltration (dpi). The AaCPS1 protein was extracted from the leaves at 9 dpi and purified by cobalt–nitrilotriacetate (Co-NTA) affinity chromatography using histidine tag with a yield of 89 mg kg⁻¹ fresh weight of leaves. The protein expression of AaCPS1 was also confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analyses. AaCPS1 protein uses farnesyl diphosphate (FPP) as a substrate to produce β-caryophyllene. Product identification and determination of the activity of purified AaCPS1 were done by gas chromatography–mass spectrometry (GC–MS). GC–MS results revealed that the AaCPS1 produced maximum 26.5 ± 1 mg of β-caryophyllene per kilogram fresh weight of leaves after assaying with FPP for 6 h. Using AaCPS1 as a proof of concept, we demonstrate that N. benthamiana can be considered as an expression system for production of plant proteins that catalyze the formation of valuable chemicals for industrial applications.

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.

Porcine parvovirus replication is suppressed by activation of the PERK signaling pathway and endoplasmic reticulum stress-mediated apoptosis

Endoplasmic reticulum (ER) stress is associated with numerous mammalian diseases, especially viral diseases. Porcine parvovirus (PPV) is the causative agent of reproductive failure in swine. Here, we observed that the PPV infection of porcine kidney 15 and porcine testis cells resulted in the activation of ER stress sensors mediated by protein kinase R-like ER kinase (PERK), but not inositol-requiring enzyme 1 and activating transcription factor 6 (ATF6). ER stress activation obviously blocked PPV replication. Depletion of proteins, such as PERK, eukaryotic initiation factor 2, and ATF4, by small interfering RNA significantly enhanced PPV replication. Moreover, the pro-apoptotic factor C/EBP homologous protein was identified a key factor in the inhibition of PPV replication. These data demonstrate that PPV infection activates ER stress through the PERK signaling pathway and that ER stress inhibits further PPV replication by promoting apoptosis.

AtERO1 and AtERO2 Exhibit Differences in Catalyzing Oxidative Protein Folding in the Endoplasmic Reticulum

Disulfide bonds are essential for the folding of the eukaryotic secretory and membrane proteins in the endoplasmic reticulum (ER), and ER oxidoreductin-1 (Ero1) and its homologs are the major disulfide donors that supply oxidizing equivalents in the ER. Although Ero1 homologs in yeast (Saccharomyces cerevisiae) and mammals have been extensively studied, the mechanisms of plant Ero1 functions are far less understood. Here, we found that both Arabidopsis (Arabidopsis thaliana) ERO1 and its homolog AtERO2 are required for oxidative protein folding in the ER. The outer active site, the inner active site, and a long-range noncatalytic disulfide bond are required for AtERO1's function. Interestingly, AtERO1 and AtERO2 also exhibit significant differences. The ero1 plants are more sensitive to reductive stress than the ero2 plants. In vivo, both AtERO1 and AtERO2 have two distinct oxidized isoforms (Ox1 and Ox2), which are determined by the formation or breakage of the putative regulatory disulfide. AtERO1 is mainly present in the Ox1 redox state, while more AtERO2 exists in the Ox2 state. Furthermore, AtERO1 showed much stronger oxidative protein-folding activity than AtERO2 in vitro. Taken together, both AtERO1 and AtERO2 are required to regulate efficient and faithful oxidative protein folding in the ER, but AtERO1 may serves as the primary sulfhydryl oxidase relative to AtERO2.

Evaluation of Disease Severity and Global Transcriptome Response Induced by Citrus bark cracking viroid, Hop latent viroid, and Their Co-Infection in Hop (Humulus lupulus L.)

Viroids are small non-capsidated, single-stranded, covalently-closed circular noncoding RNA replicons of 239-401 nucleotides that exploit host factors for their replication, and some cause disease in several economically important crop plants, while others appear to be benign. The proposed mechanisms of viroid pathogenesis include direct interaction of the genomic viroid RNA with host factors and post-transcriptional or transcriptional gene silencing via viroid-derived small RNAs (vd-sRNAs) generated by the host defensive machinery. Humulus lupulus (hop) plants are hosts to several viroids among which Hop latent viroid (HLVd) and Citrus bark cracking viroid (CBCVd) are attractive model systems for the study of viroid-host interactions due to the symptomless infection of the former and severe symptoms induced by the latter in this indicator host. To better understand their interactions with hop plant, a comparative transcriptomic analysis based on RNA sequencing (RNA-seq) was performed to reveal the transcriptional alterations induced as a result of single HLVd and CBCVd infection in hop. Additionally, the effect of HLVd on the aggressiveness of CBCVd that underlies severe stunting in hop in a mixed infection was studied by transcriptomic analysis. Our analysis revealed that CBCVd infection resulted in dynamic changes in the activity of genes as compared to single HLVd infection and their mixed infection. The differentially expressed genes that are involved in defense, phytohormone signaling, photosynthesis and chloroplasts, RNA regulation, processing and binding; protein metabolism and modification; and other mechanisms were more modulated in the CBCVd infection of hop. Nevertheless, Gene Ontology (GO) classification and pathway enrichment analysis showed that the expression of genes involved in the proteolysis mechanism is more active in a mixed infection as compared to a single one, suggesting co-infecting viroids may result in interference with host factors more prominently. Collectively, our results provide a deep transcriptome of hop and insight into complex single HLVd, CBCVd, and their coinfection in hop-plant interactions.

The Tug-of-War between Plants and Viruses: Great Progress and Many Remaining Questions

Plants are persistently challenged by various phytopathogens. To protect themselves, plants have evolved multilayered surveillance against all pathogens. For intracellular parasitic viruses, plants have developed innate immunity, RNA silencing, translation repression, ubiquitination-mediated and autophagy-mediated protein degradation, and other dominant resistance gene-mediated defenses. Plant viruses have also acquired diverse strategies to suppress and even exploit host defense machinery to ensure their survival. A better understanding of the defense and counter-defense between plants and viruses will obviously benefit from the development of efficient and broad-spectrum virus resistance for sustainable agriculture. In this review, we summarize the cutting edge of knowledge concerning the defense and counter-defense between plants and viruses, and highlight the unexploited areas that are especially worth investigating in the near future.

Cell death triggered by the P25 protein in Potato virus X-associated synergisms results from endoplasmic reticulum stress in Nicotiana benthamiana

The synergistic interaction of Potato virus X (PVX) with a number of potyviruses results in systemic necrosis in Nicotiana spp. Previous investigations have indicated that the viral suppressor of RNA silencing (VSR) protein P25 of PVX triggers systemic necrosis in PVX-associated synergisms in a threshold-dependent manner. However, little is still known about the cellular processes that lead to this necrosis, and whether the VSR activity of P25 is involved in its elicitation. Here, we show that transient expression of P25 in the presence of VSRs from different viruses, including the helper component-proteinase (HC-Pro) of potyviruses, induces endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), which ultimately lead to ER collapse. However, the host RNA silencing pathway was dispensable for the elicitation of cell death by P25. Confocal microscopy studies in leaf patches co-expressing P25 and HC-Pro showed dramatic alterations in ER membrane structures, which correlated with the up-regulation of bZIP60 and several ER-resident chaperones, including the ER luminal binding protein (BiP). Overexpression of BiP alleviated the cell death induced by the potexviral P25 protein when expressed together with VSRs derived from different viruses. Conversely, silencing of the UPR master regulator, bZIP60, led to an increase in cell death elicited by the P25/HC-Pro combination as well as by PVX-associated synergism. In addition to its role as a negative regulator of P25-induced cell death, UPR partially restricted PVX infection. Thus, systemic necrosis caused by PVX-associated synergistic infections is probably the effect of an unmitigated ER stress following the overaccumulation of a viral protein, P25, with ER remodelling activity.

Oxidative protein folding: state-of-the-art and current avenues of research in plants

Contents Summary 1230 I. Introduction 1230 II. Formation and isomerization of disulfides in the ER and the Golgi apparatus 1231 III. The disulfide relay in the mitochondrial intermembrane space: why are plants different? 1236 IV. Disulfide bond formation on luminal proteins in thylakoids 1240 V. Conclusion 1242 Acknowledgements 1242 References 1242 SUMMARY: Disulfide bonds are post-translational modifications crucial for the structure and function of thousands of proteins. Their formation and isomerization, referred to as oxidative folding, require specific protein machineries found in oxidizing subcellular compartments, namely the endoplasmic reticulum and the associated endomembrane system, the intermembrane space of mitochondria and the thylakoid lumen of chloroplasts. At least one protein component is required for transferring electrons from substrate proteins to an acceptor that is usually molecular oxygen. For oxidation reactions, incoming reduced substrates are oxidized by thiol-oxidoreductase proteins (or domains in case of chimeric proteins), which are usually themselves oxidized by a single thiol oxidase, the enzyme generating disulfide bonds de novo. By contrast, the description of the molecular actors and pathways involved in proofreading and isomerization of misfolded proteins, which require a tightly controlled redox balance, lags behind. Herein we provide a general overview of the knowledge acquired on the systems responsible for oxidative protein folding in photosynthetic organisms, highlighting their particularities compared to other eukaryotes. Current research challenges are discussed including the importance and specificity of these oxidation systems in the context of the existence of reducing systems in the same compartments.

Characterization of Proteins Involved in Chloroplast Targeting Disturbed by Rice Stripe Virus by Novel Protoplast⁻Chloroplast Proteomics

Rice stripe virus (RSV) is one of the most devastating viral pathogens in rice and can also cause the general chlorosis symptom in Nicotiana benthamiana plants. The chloroplast changes associated with chlorosis symptom suggest that RSV interrupts normal chloroplast functions. Although the change of proteins of the whole cell or inside the chloroplast in response to RSV infection have been revealed by proteomics, the mechanisms resulted in chloroplast-related symptoms and the crucial factors remain to be elucidated. RSV infection caused the malformation of chloroplast structure and a global reduction of chloroplast membrane protein complexes in N. benthamiana plants. Here, both the protoplast proteome and the chloroplast proteome were acquired simultaneously upon RSV infection, and the proteins in each fraction were analyzed. In the protoplasts, 1128 proteins were identified, among which 494 proteins presented significant changes during RSV; meanwhile, 659 proteins were identified from the chloroplasts, and 279 of these chloroplast proteins presented significant change. According to the label-free LC–MS/MS data, 66 nucleus-encoded chloroplast-related proteins (ChRPs), which only reduced in chloroplast but not in the whole protoplast, were identified, indicating that these nuclear-encoded ChRPswere not transported to chloroplasts during RSV infection. Gene ontology (GO) enrichment analysis confirmed that RSV infection changed the biological process of protein targeting to chloroplast, where 3 crucial ChRPs (K4CSN4, K4CR23, and K4BXN9) were involved in the regulation of protein targeting into chloroplast. In addition to these 3 proteins, 41 among the 63 candidate proteins were characterized to have chloroplast transit peptides. These results indicated that RSV infection changed the biological process of protein targeting into chloroplast and the location of ChRPs through crucial protein factors, which illuminated a new layer of RSV–host interaction that might contribute to the symptom development.

Endoplasmic Reticulum Plays a Critical Role in Integrating Signals Generated by Both Biotic and Abiotic Stress in Plants

Most studies of environmental adaptations in plants have focused on either biotic or abiotic stress factors in an attempt to understand the defense mechanisms of plants against individual stresses. However, in the natural ecosystem, plants are simultaneously exposed to multiple stresses. Stress-tolerant crops developed in translational studies based on a single stress often fail to exhibit the expected traits in the field. To adapt to abiotic stress, recent studies have identified the need for interactions of plants with various microorganisms. These findings highlight the need to understand the multifaceted interactions of plants with biotic and abiotic stress factors. The endoplasmic reticulum (ER) is an organelle that links various stress responses. To gain insight into the molecular integration of biotic and abiotic stress responses in the ER, we focused on the interactions of plants with RNA viruses. This interaction points toward the relevance of ER in viral pathogenicity as well as plant responses. In this mini review, we explore the molecular crosstalk between biotic and abiotic stress signaling through the ER by elaborating ER-mediated signaling in response to RNA viruses and abiotic stresses. Additionally, we summarize the results of a recent study on phytohormones that induce ER-mediated stress response. These studies will facilitate the development of multi-stress-tolerant transgenic crops in the future.

TabZIP74 Acts as a Positive Regulator in Wheat Stripe Rust Resistance and Involves Root Development by mRNA Splicing

Basic leucine zipper (bZIP) membrane-bound transcription factors (MTFs) play important roles in regulating plant growth and development, abiotic stress responses, and disease resistance. Most bZIP MTFs are key components of signaling pathways in endoplasmic reticulum (ER) stress responses. In this study, a full-length cDNA sequence encoding bZIP MTF, designated TabZIP74, was isolated from a cDNA library of wheat near-isogenic lines of Taichung29*6/Yr10 inoculated with an incompatible race CYR32 of Puccinia striiformis f. sp. tritici (Pst). Phylogenic analysis showed that TabZIP74 is highly homologous to ZmbZIP60 in maize and OsbZIP74 in rice. The mRNA of TabZIP74 was predicted to form a secondary structure with two kissing hairpin loops that could be spliced, causing an open reading frame shift immediately before the hydrophobic region to produce a new TabZIP74 protein without the transmembrane domain. Pst infection and the abiotic polyethylene glycol (PEG) and abscisic acid (ABA) treatments lead to TabZIP74 mRNA splicing in wheat seedling leaves, while both spliced and unspliced forms in roots were detected. In the confocal microscopic examination, TabZIP74 is mobilized in the nucleus from the membrane of tobacco epidermal cells in response to wounding. Knocking down TabZIP74 with barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) enhanced wheat seedling susceptibility to stripe rust and decreased drought tolerance and lateral roots of silenced plants. These findings demonstrate that TabZIP74 mRNA is induced to splice when stressed by biotic and abiotic factors, acts as a critically positive regulator for wheat stripe rust resistance and drought tolerance, and is necessary for lateral root development.

Exploring the Diversity of Mechanisms Associated With Plant Tolerance to Virus Infection

Tolerance is defined as an interaction in which viruses accumulate to some degree without causing significant loss of vigor or fitness to their hosts. Tolerance can be described as a stable equilibrium between the virus and its host, an interaction in which each partner not only accommodate trade-offs for survival but also receive some benefits (e.g., protection of the plant against super-infection by virulent viruses; virus invasion of meristem tissues allowing vertical transmission). This equilibrium, which would be associated with little selective pressure for the emergence of severe viral strains, is common in wild ecosystems and has important implications for the management of viral diseases in the field. Plant viruses are obligatory intracellular parasites that divert the host cellular machinery to complete their infection cycle. Highjacking/modification of plant factors can affect plant vigor and fitness. In addition, the toxic effects of viral proteins and the deployment of plant defense responses contribute to the induction of symptoms ranging in severity from tissue discoloration to malformation or tissue necrosis. The impact of viral infection is also influenced by the virulence of the specific virus strain (or strains for mixed infections), the host genotype and environmental conditions. Although plant resistance mechanisms that restrict virus accumulation or movement have received much attention, molecular mechanisms associated with tolerance are less well-understood. We review the experimental evidence that supports the concept that tolerance can be achieved by reaching the proper balance between plant defense responses and virus counter-defenses. We also discuss plant translation repression mechanisms, plant protein degradation or modification pathways and viral self-attenuation strategies that regulate the accumulation or activity of viral proteins to mitigate their impact on the host. Finally, we discuss current progress and future opportunities toward the application of various tolerance mechanisms in the field.

SENSITIVE TO SALT1, An Endoplasmic Reticulum-Localized Chaperone, Positively Regulates Salt Resistance

Salt stress seriously affects plant growth and development. Through genetic screening, we identified and characterized an Arabidopsis (Arabidopsis thaliana) sensitive to salt1 (ses1) mutant. SES1 was ubiquitously expressed and induced by salt treatment. The salt-sensitive phenotype of ses1 was due neither to the overaccumulation of Na⁺ nor to the suppression of salt tolerance-associated genes. SES1 encoded an uncharacterized endoplasmic reticulum (ER)-localized protein. Coinciding with its subcellular distribution, ses1 exhibited overactivation of unfolded protein response genes and was largely influenced by severe ER stress. Biochemical evidence revealed that SES1 functions as an important molecular chaperone to alleviate salt-induced ER stress. Furthermore, the ER stress sensor basic leucine zipper factor17 transactivated SES1 by binding directly to its promoter region. These results provide insights into salt stress responses and ER homeostasis and shed light on the mechanism by which SES1 modulates salt resistance.