Variation in the Subcellular Localization and Protein Folding Activity among Arabidopsis thaliana Homologs of Protein Disulfide Isomerase

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.

Comparative transcriptomics analysis at the key stage of maize ear development dissect heterosis

Important traits related to maize (Zea mays L.) grain yield, such as kernel row number, ear length, kernel number per row, are determined during the development of female inflorescence. There is a significant positive correlation between yield component and the activity of inflorescence meristem (IM). To find the key stage of heterosis in the development of the ear, immature ears (from the IM stage until the end of the floral meristem [FM] stage) of Yudan888 and its parent lines were sampled to assay phenotype and for comparative transcriptomics analysis. The immature ear length of Yudan888 at the IM stage fitted an additive (mid-parental) model, but it showed high parental dominance at the spikelet-pair meristem (SPM) stage. Comparative analysis of transcriptomes suggested significant differences between additive and nonadditive expression patterns for different developmental stages. The number of distinct maternal or paternal genes (DMP) (genes expressed only in one parental line and their hybrid but silenced in another line) was greater than ABF1 (genes expressed in both parental lines but silenced in hybrid) at each stage. Gene Ontology (GO) enrichment suggested that the cell redox homeostasis genes with overdominance expression patterns in hybrids have an important contribution to heterosis. According to our research, an ear length heterosis network was established. The discovery of the inflection point for ear length heterosis allows us for inferring that the transition state of IM to SPM may be the starting point of ear length heterosis. These findings improved the understanding of maize ear length heterosis.

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.

Endoplasmic Reticulum Stress and Reactive Oxygen Species in Plants

The endoplasmic reticulum (ER) is a key compartment responsible for protein processing and folding, and it also participates in many signal transduction and metabolic processes. Reactive oxygen species (ROS) are important signaling messengers involved in the redox equilibrium and stress response. A number of abiotic and biotic stresses can trigger the accumulation of unfolded or misfolded proteins and lead to ER stress. In recent years, a number of studies have reported that redox metabolism and ROS are closely related to ER stress. ER stress can benefit ROS generation and even cause oxidative burden in plants, finally leading to oxidative stress depending on the degree of ER stress. Moreover, ER stress activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-mediated ROS signaling, increases antioxidant defense mechanisms, and alters the glutathione (GSH) redox state. Meanwhile, the accumulation of ROS plays a special role in inducing the ER stress response. Given these factors, plants have evolved a series of complex regulatory mechanisms to interact with ROS in response to ER stress. In this review, we summarize the perceptions and responses of plant ER stress and oxidative protein folding in the ER. In addition, we analyze the production and signaling of ROS under ER stress in detail in order to provide a theoretical basis for reducing ER stress to improve the crop survival rate in agricultural applications.

Coordinated regulation of plant immunity by poly(ADP-ribosyl)ation and K63-linked ubiquitination

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational modification reversibly catalyzed by poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolases (PARGs) and plays a key role in multiple cellular processes. The molecular mechanisms by which PARylation regulates innate immunity remain largely unknown in eukaryotes. Here we show that Arabidopsis UBC13A and UBC13B, the major drivers of lysine 63 (K63)-linked polyubiquitination, directly interact with PARPs/PARGs. Activation of pathogen-associated molecular pattern (PAMP)-triggered immunity promotes these interactions and enhances PARylation of UBC13. Both parp1 parp2 and ubc13a ubc13b mutants are compromised in immune responses with increased accumulation of total pathogenesis-related (PR) proteins but decreased accumulation of secreted PR proteins. Protein disulfide-isomerases (PDIs), essential components of endoplasmic reticulum quality control (ERQC) that ensure proper folding and maturation of proteins destined for secretion, complex with PARPs/PARGs and are PARylated upon PAMP perception. Significantly, PARylation of UBC13 regulates K63-linked ubiquitination of PDIs, which may further promote their disulfide isomerase activities for correct protein folding and subsequent secretion. Taken together, these results indicate that plant immunity is coordinately regulated by PARylation and K63-linked ubiquitination.

The thiol-disulfide exchange activity of AtPDI1 is involved in the response to abiotic stresses

BACKGROUND

Arabidopsis protein disulfide isomerase 1 (AtPDI1) has been demonstrated to have disulfide isomerase activity and to be involved in the stress response. However, whether the anti-stress function is directly related to the activities of thiol-disulfide exchange remains to be elucidated.

RESULTS

In the present study, encoding sequences of AtPDI1 of wild-type (WT) and double-cysteine-mutants were transformed into an AtPDI1 knockdown Arabidopsis line (pdi), and homozygous transgenic plants named pdi-AtPDI1, pdi-AtPDI1_m1 and pdi-AtPDI1_m2 were obtained. Compared with the WT and pdi-AtPDI1, the respective germination ratios of pdi-AtPDI1_m1 and pdi-AtPDI1_m2 were significantly lower under abiotic stresses and exogenous ABA treatment, whereas the highest germination rate was obtained with AtPDI1 overexpression in the WT (WT- AtPDI1). The root length among different lines was consistent with the germination rate; a higher germination rate was observed with a longer root length. When seedlings were treated with salt, drought, cold and high temperature stresses, pdi-AtPDI1_m1, pdi-AtPDI1_m2 and pdi displayed lower survival rates than WT and AtPDI1 overexpression plants. The transcriptional levels of ABA-responsive genes and genes encoding ROS-quenching enzymes were lower in pdi-AtPDI1_m1 and pdi-AtPDI1_m2 than in pdi-AtPDI1.

CONCLUSION

Taken together, these results clearly suggest that the anti-stress function of AtPDI1 is directly related to the activity of disulfide isomerase.

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

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

Morphological observation and protein expression of fertile and abortive ovules in Castanea mollissima

Chinese chestnuts (Castanea mollissima Blume.) contain 12-18 ovules in one ovary, but only one ovule develops into a seed, indicating a high ovule abortion rate. In this study, the Chinese chestnut 'Huaihuang' was used to explore the possible mechanisms of ovule abortion with respect to morphology and proteomics. The morphology and microstructure of abortive ovules were found to be considerably different from those of fertile ovules at 20 days after anthesis (20 DAA). The fertile ovules had completely formed tissues, such as the embryo sac, embryo and endosperm. By contrast, in the abortive ovules, there were no embryo sacs, and wide spaces between the integuments were observed, with few nucelli. Fluorescence labelling of the nuclei and transmission electron microscopy (TEM) observations showed that cells of abortive ovules were abnormally shaped and had thickened cell walls, folded cell membranes, condensed cytoplasm, ruptured nuclear membranes, degraded nucleoli and reduced mitochondria. The iTRAQ (isobaric tag for relative and absolute quantitation) results showed that in the abortive ovules, low levels of soluble protein with small molecular weights were found, and most of differently expressed proteins (DEPs) were related to protein synthesis, accumulation of active oxygen free radical, energy synthesis and so on. These DEPs might be associated with abnormal ovules formation.

The Endoplasmic Reticulum Role in the Plant Response to Abiotic Stress

The endoplasmic reticulum (ER) is the organelle where one third of the proteins of a cell are synthetized. Several of these proteins participate in the signaling and response of cells, tissues, or from the organism to the environment. To secure the proper synthesis and folding of these proteins, or the disposal of unfolded or misfolded proteins, the ER has different mechanisms that interact and regulate each other. These mechanisms are known as the ER quality control (ERQC), ER-associated degradation (ERAD) and the unfolded protein response (UPR), all three participants of the maintenance of ER protein homeostasis or proteostasis. Given the importance of the client proteins of these ER mechanisms in the plant response to the environment, it is expected that changes or alterations on their components have an impact on the plant response to environmental cues or stresses. In this mini review, we focus on the impact of the alteration of components of ERQC, ERAD and UPR in the plant response to abiotic stresses such as drought, heat, osmotic, salt and irradiation. Also, we summarize findings from recent publications looking for a connection between these processes and their possible client(s) proteins. From this, we observed that a clear connection has been established between the ERAD and UPR mechanisms, but evidence that connects ERQC components to these both processes or their possible client(s) proteins is still lacking. As a proposal, we suggest the use of proteomics approaches to uncover the identity of these proteins and their connection with ER proteostasis.

A Poplar Rust Effector Protein Associates with Protein Disulfide Isomerase and Enhances Plant Susceptibility

Melampsora larici-populina (Mlp), the causal agent of Populus leaf rust, secretes an array of effectors into the host through the haustorium to gain nutrients and suppress immunity. The precise mechanisms by which these effectors promote virulence remain unclear. To address this question, we developed a transgenic Arabidopsis line expressing a candidate effector, Mlp124357. Constitutive expression of the effector increased plant susceptibility to pathogens. A GxxxG motif present in Mlp124357 is required for its subcellular localization at the vacuolar membrane of the plant cell, as replacement of the glycine residues with alanines led to the delocalization of Mlp124357 to the nucleus and cytoplasm. We used immunoprecipitation and mass spectrometry (MS) to identify Mlp124357 interaction partners. Only one of the putative interaction partners knock-out line caused delocalization of the effector, indicating that Arabidopsis protein disulfide isomerase-11 (AtPDI-11) is required for the effector localization. This interaction was further confirmed by a complementation test, a yeast-two hybrid assay and a molecular modeling experiment. Moreover, localization results and infection assays suggest that AtPDI-11 act as a helper for Mlp124357. In summary, our findings established that one of Mlp effectors resides at the vacuole surface and modulates plant susceptibility.

The Arabidopsis Protein Disulfide Isomerase Subfamily M Isoform, PDI9, Localizes to the Endoplasmic Reticulum and Influences Pollen Viability and Proper Formation of the Pollen Exine During Heat Stress

Plants adapt to heat via thermotolerance pathways in which the activation of protein folding chaperones is essential. In eukaryotes, protein disulfide isomerases (PDIs) facilitate the folding of nascent and misfolded proteins in the secretory pathway by catalyzing the formation and isomerization of disulfide bonds and serving as molecular chaperones. In Arabidopsis, several members of the PDI family are upregulated in response to chemical inducers of the unfolded protein response (UPR), including both members of the non-classical PDI-M subfamily, PDI9 and PDI10. Unlike classical PDIs, which have two catalytic thioredoxin (TRX) domains separated by two non-catalytic TRX-fold domains, PDI-M isoforms are orthologs of mammalian P5/PDIA6 and possess two tandem catalytic domains. Here, PDI9 accumulation was found to be upregulated in pollen in response to heat stress. Histochemical staining of plants harboring the PDI9 and PDI10 promoters fused to the gusA gene indicated they were actively expressed in the anthers of flowers, specifically in the pollen and tapetum. Immunoelectron microscopy revealed that PDI9 localized to the endoplasmic reticulum in root and pollen cells. transfer DNA (T-DNA) insertional mutations in the PDI9 gene disrupted pollen viability and development in plants exposed to heat stress. In particular, the pollen grains of pdi9 mutants exhibited disruptions in the reticulated pattern of the exine and an increased adhesion of pollen grains. Pollen in the pdi10 single mutant did not display similar heat-associated defects, but pdi9 pdi10 double mutants (DMs) completely lost exine reticulation. Interestingly, overexpression of PDI9 partially led to heat-associated defects in the exine. We conclude that PDI9 plays an important role in pollen thermotolerance and exine biogenesis. Its role fits the mechanistic theory of proteostasis in which an ideal balance of PDI isoforms is required in the endoplasmic reticulum (ER) for normal exine formation in plants subjected to heat stress.

Identification and Functional Analysis of a Protein Disulfide Isomerase (AtPDI1) in Arabidopsis thaliana

Protein disulfide isomerase (PDI) catalyzes the conversion of thiol-disulfide and plays an important role in various physiological events in animals. A PDI (OaPDI) from a tropical plant was detailed studied and it was found to be involved in response of biotic stress (Gruber et al., 2007). However, the activities of PDI related to physiological functions in plants are poorly understood. In the present study, a homolog of human PDI in Arabidopsis (AtPDI1), encoded by the gene (At3g54960), was characterized. The recombinant AtPDI1 protein had disulfide isomerase activity in vitro and two pairs of conservative cysteines in catalytic domains play a crucial role in the PDI activities. Expression of AtPDI1 in Escherichia coli significantly enhanced stress tolerance of cells and the mutations of critical cysteines almost lose this function. In plants, AtPDI1 was strongly induced by abiotic stresses and exogenous abscisic acid. An ArabidopsisAtPDI1 knockdown mutant (pdi1) and overexpression lines of transgenic plants obtained by this investigation were used to further examine the function of AtPDI1. The mutant line was more sensitive to stresses than the wild-type, while overexpressing AtPDI1 increased tolerance of seedlings to abiotic stresses, with a higher germination ratio and longer length of roots than the wild-type. Our results suggested AtPDI1 played roles in anti-stresses in Arabidopsis, which relate to the activities of PDI.

Interplay between the unfolded protein response and reactive oxygen species: a dynamic duo

Secretory proteins undergo modifications such as glycosylation and disulphide bond formation before proper folding, and move to their final destination via the endomembrane system. Accumulation of unfolded proteins in the endoplasmic reticulum (ER) due to suboptimal environmental conditions triggers a response called the unfolded protein response (UPR), which induces a set of genes that elevate protein folding capacity in the ER. This review aims to establish a connection among ER stress, UPR, and reactive oxygen species (ROS), which remains an unexplored topic in plants. For this, we focused on mechanisms of ROS production originating from ER stress, the interaction between ER stress and overall ROS signalling process in the cell, and the interaction of ER stress with other organellar ROS signalling pathways such as of the mitochondria and chloroplasts. The roles of the UPR during plant hormone signalling and abiotic and biotic stress responses are also discussed in connection with redox and ROS signalling.

Conserved Residues Lys57 and Lys401 of Protein Disulfide Isomerase Maintain an Active Site Conformation for Optimal Activity: Implications for Post-Translational Regulation

Despite its study since the 1960's, very little is known about the post-translational regulation of the multiple catalytic activities performed by protein disulfide isomerase (PDI), the primary protein folding catalyst of the cell. This work identifies a functional role for the highly conserved CxxC-flanking residues Lys⁵⁷ and Lys⁴⁰¹ of human PDI in vitro. Mutagenesis studies have revealed these residues as modulating the oxidoreductase activity of PDI in a pH-dependent manner. Non-conservative amino acid substitutions resulted in enzyme variants upwards of 7-fold less efficient. This attenuated activity was found to translate into a 2-fold reduction of the rate of electron shuttling between PDI and the intraluminal endoplasmic reticulum oxidase, ERO1α, suggesting a functional significance to oxidative protein folding. In light of this, the possibility of lysine acetylation at residues Lys⁵⁷ and Lys⁴⁰¹ was assessed by in vitro treatment using acetylsalicylic acid (aspirin). A total of 28 acetyllysine residues were identified, including acLys⁵⁷ and acLys⁴⁰¹. The kinetic behavior of the acetylated protein form nearly mimicked that obtained with a K57/401Q double substitution variant providing an indication that acetylation of the active site-flanking lysine residues can act to reversibly modulate PDI activity.

A Non-Classical Member of the Protein Disulfide Isomerase Family, PDI7 of Arabidopsis thaliana, Localizes to the cis-Golgi and Endoplasmic Reticulum Membranes

Members of the protein disulfide isomerase (PDI)-C subfamily are chimeric proteins containing the thioredoxin (Trx) domain of PDIs, and the conserved N- and C-terminal Pfam domains of Erv41p/Erv46p-type cargo receptors. They are unique to plants and chromalveolates. The Arabidopsis genome encodes three PDI-C isoforms: PDI7, PDI12 and PDI13. Here we demonstrate that PDI7 is a 65 kDa integral membrane glycoprotein expressed throughout many Arabidopsis tissues. Using a PDI7-specific antibody, we show through immunoelectron microscopy that PDI7 localizes to the endoplasmic reticulum (ER) and Golgi membranes in wild-type root tip cells, and was also detected in vesicles. Tomographic modeling of the Golgi revealed that PDI7 was confined to the cis-Golgi, and accumulated primarily at the cis-most cisterna. Shoot apical meristem cells from transgenic plants overexpressing PDI7 exhibited a dramatic increase in anti-PDI7 labeling at the cis-Golgi. When N- or C-terminal fusions between PDI7 and the green fluorescent protein variant, GFP(S65T), were expressed in mesophyll protoplasts, the fusions co-localized with the ER marker, ER-mCherry. However, when GFP(S65T) was positioned internally within PDI7 (PDI7-GFPint), the fusion strongly co-localized with the cis-Golgi marker, mCherry-SYP31, and faintly labeled the ER. In contrast to the Golgi-resident fusion protein (Man49-mCherry), PDI7-GFPint did not redistribute to the ER after brefeldin A treatment. Protease protection experiments indicated that the Trx domain of PDI7 is located within the ER/Golgi lumen. We propose a model where PDI-C isoforms function as cargo receptors for proteins containing exposed cysteine residues, cycling them from the Golgi back to the ER.

Dual-targeting of Arabidopsis DMP1 isoforms to the tonoplast and the plasma membrane

The reports of dual-targeted proteins in plants have steadily increased over the past years. The vast majority of these proteins are soluble proteins distributed between compartments of the non-secretory pathway, predominantly chloroplasts and mitochondria. In contrast, dual-targeted transmembrane proteins, especially of the secretory pathway, are rare and the mechanisms leading to their differential targeting remain largely unknown. Here, we report dual-targeting of the Arabidopsis DUF679 Membrane Protein 1 (DMP1) to the tonoplast (TP) and the plasma membrane (PM). In Arabidopsis and tobacco two equally abundant DMP1 isoforms are synthesized by alternative translation initiation: a full length protein, DMP1.1, and a truncated one, DMP1.2, which lacks the N-terminal 19 amino acids including a TP-targeting dileucine motif. Accumulation of DMP1.1 and DMP1.2 in the TP and the PM, respectively, is Brefeldin A-sensitive, indicating transit via the Golgi. However, DMP1.2 interacts with DMP1.1, leading to extensive rerouting of DMP1.2 to the TP and “eclipsed” localization of DMP1.2 in the PM where it is barely visible by confocal laser scanning microscopy but clearly detectable by membrane fractionation. It is demonstrated that eGFP fusion to either DMP1 terminus can cause mistargeting artifacts: C-terminal fusion to DMP1.1 or DMP1.2 results in altered ER export and N-terminal fusion to DMP1.1 causes mistargeting to the PM, presumably by masking of the TP targeting signal. These results illustrate how the interplay of alternative translation initiation, presence or absence of targeting information and rerouting due to protein-protein interaction determines the ultimate distribution of a transmembrane protein between two membranes.

Arabidopsis protein disulfide isomerase-8 is a type I endoplasmic reticulum transmembrane protein with thiol-disulfide oxidase activity

BACKGROUND

In eukaryotes, classical protein disulfide isomerases (PDIs) facilitate the oxidative folding of nascent secretory proteins in the endoplasmic reticulum by catalyzing the formation, breakage, and rearrangement of disulfide bonds. Terrestrial plants encode six structurally distinct subfamilies of PDIs. The novel PDI-B subfamily is unique to terrestrial plants, and in Arabidopsis is represented by a single member, PDI8. Unlike classical PDIs, which lack transmembrane domains (TMDs), PDI8 is unique in that it has a C-terminal TMD and a single N-terminal thioredoxin domain (instead of two). No PDI8 isoforms have been experimentally characterized to date. Here we describe the characterization of the membrane orientation, expression, sub-cellular localization, and biochemical function of this novel member of the PDI family.

RESULTS

Histochemical staining of plants harboring a PDI8 promoter:β-glucuronidase (GUS) fusion revealed that the PDI8 promoter is highly active in young, expanding leaves, the guard cells of cotyledons, and in the vasculature of several organs, including roots, leaves, cotyledons, and flowers. Immunoelectron microscopy studies using a PDI8-specific antibody on root and shoot apical cells revealed that PDI8 localizes to the endoplasmic reticulum (ER). Transient expression of two PDI8 fusions to green fluorescent protein (spGFP-PDI8 and PDI8-GFP-KKED) in leaf mesophyll protoplasts also resulted in labeling of the ER. Protease-protection immunoblot analysis indicated that PDI8 is a type I membrane protein, with its catalytic domain facing the ER lumen. The lumenal portion of PDI8 was able to functionally complement the loss of the prokaryotic protein foldase, disulfide oxidase (DsbA), as demonstrated by the reconstitution of periplasmic alkaline phosphatase in Escherichia coli.

CONCLUSION

The results indicate that PDI8 is a type I transmembrane protein with its catalytic domain facing the lumen of the ER and functions in the oxidation of cysteines to produce disulfide bonds. It likely plays a role in folding newly-synthesized secretory proteins as they translocate across the ER membrane into the lumen. These foundational results open the door to identifying the substrates of PDI8 to enable a more thorough understanding of its function in plants.

Transcriptional Dynamics Driving MAMP-Triggered Immunity and Pathogen Effector-Mediated Immunosuppression in Arabidopsis Leaves Following Infection with Pseudomonas syringae pv tomato DC3000

Transcriptional reprogramming is integral to effective plant defense. Pathogen effectors act transcriptionally and posttranscriptionally to suppress defense responses. A major challenge to understanding disease and defense responses is discriminating between transcriptional reprogramming associated with microbial-associated molecular pattern (MAMP)-triggered immunity (MTI) and that orchestrated by effectors. A high-resolution time course of genome-wide expression changes following challenge with Pseudomonas syringae pv tomato DC3000 and the nonpathogenic mutant strain DC3000hrpA- allowed us to establish causal links between the activities of pathogen effectors and suppression of MTI and infer with high confidence a range of processes specifically targeted by effectors. Analysis of this information-rich data set with a range of computational tools provided insights into the earliest transcriptional events triggered by effector delivery, regulatory mechanisms recruited, and biological processes targeted. We show that the majority of genes contributing to disease or defense are induced within 6 h postinfection, significantly before pathogen multiplication. Suppression of chloroplast-associated genes is a rapid MAMP-triggered defense response, and suppression of genes involved in chromatin assembly and induction of ubiquitin-related genes coincide with pathogen-induced abscisic acid accumulation. Specific combinations of promoter motifs are engaged in fine-tuning the MTI response and active transcriptional suppression at specific promoter configurations by P. syringae.

Dual protein trafficking to secretory and non-secretory cell compartments: clear or double vision?

Approximately 18% of Arabidopsis thaliana proteins encode a signal peptide for translocation to the endoplasmic reticulum (ER), the gateway of the eukaryotic secretory pathway. However, it was recently discovered that some ER proteins can undergo both co-translational import into the ER/secretory pathway and trafficking to compartments outside of the secretory pathway. This phenomenon is observed among members of the protein disulfide isomerase (PDI) family, which are traditionally regarded as ER enzymes involved in protein folding. Although classical PDIs possess an N-terminal signal peptide and a C-terminal ER retention signal, some also dual localize to secretory and non-secretory compartments, including mammalian PDI ERp57, Chlamydomonas reinhardtii PDI RB60, and A. thaliana AtPDI2. ERp57 is present in both the ER and nucleus where it influences gene transcription. RB60 localizes to the ER and chloroplast where it modulates the redox state of polyadenylate-binding protein RB47. AtPDI2, which interacts with transcription factor MEE8, localizes to the ER-secretory pathway and the nucleus. A model proposing secretory trafficking of AtPDI2 and nuclear co-translocation of an AtPDI2-MEE8 complex illustrates the diversity of dual targeting mechanisms, the multifunctional roles of some PDIs, and the potential co-translocation of other proteins to multiple subcellular compartments.