Truncation of a protein disulfide isomerase, PDIL2-1, delays embryo sac maturation and disrupts pollen tube guidance in Arabidopsis thaliana

Integrated Transcriptome and Proteome Analysis Reveals the Regulatory Mechanism of Root Growth by Protein Disulfide Isomerase in Arabidopsis

Protein disulfide isomerase (PDI, EC 5.3.4.1) is a thiol-disulfide oxidoreductase that plays a crucial role in catalyzing the oxidation and rearrangement of disulfides in substrate proteins. In plants, PDI is primarily involved in regulating seed germination and development, facilitating the oxidative folding of storage proteins in the endosperm, and also contributing to the formation of pollen. However, the role of PDI in root growth has not been previously studied. This research investigated the impact of PDI gene deficiency in plants by using 16F16 [2-(2-Chloroacetyl)-2,3,4,9-tetrahydro-1-methyl-1H-pyrido[3,4-b]indole-1-carboxylic acid methyl ester], a small-molecule inhibitor of PDI, to remove functional redundancy. The results showed that the growth of Arabidopsis roots was significantly inhibited when treated with 16F16. To further investigate the effects of 16F16 treatment, we conducted expression profiling of treated roots using RNA sequencing and a Tandem Mass Tag (TMT)-based quantitative proteomics approach at both the transcriptomic and proteomic levels. Our analysis revealed 994 differentially expressed genes (DEGs) at the transcript level, which were predominantly enriched in pathways associated with "phenylpropane biosynthesis", "plant hormone signal transduction", "plant-pathogen interaction" and "starch and sucrose metabolism" pathways. Additionally, we identified 120 differentially expressed proteins (DEPs) at the protein level. These proteins were mainly enriched in pathways such as "phenylpropanoid biosynthesis", "photosynthesis", "biosynthesis of various plant secondary metabolites", and "biosynthesis of secondary metabolites" pathways. The comprehensive transcriptome and proteome analyses revealed a regulatory network for root shortening in Arabidopsis seedlings under 16F16 treatment, mainly involving phenylpropane biosynthesis and plant hormone signal transduction pathways. This study enhances our understanding of the significant role of PDIs in Arabidopsis root growth and provides insights into the regulatory mechanisms of root shortening following 16F16 treatment.

Expression Characterization of AtPDI11 and Functional Analysis of AtPDI11 D Domain in Oxidative Protein Folding

The formation and isomerization of disulfide bonds mediated by protein disulfide isomerase (PDI) in the endoplasmic reticulum (ER) is of fundamental importance in eukaryotes. Canonical PDI structure comprises four domains with the order of a-b-b′-a′. In Arabidopsis thaliana, the PDI-S subgroup contains only one member, AtPDI11, with an a-a′-D organization, which has no orthologs in mammals or yeast. However, the expression pattern of AtPDI11 and the functioning mechanism of AtPDI11 D domain are currently unclear. In this work, we found that PDI-S is evolutionarily conserved between land plants and algal organisms. AtPDI11 is expressed in various tissues and its induction by ER stress is disrupted in bzip28/60 and ire1a/b mutants that are null mutants of key components in the unfolded protein response (UPR) signal transduction pathway, suggesting that the induction of AtPDI11 by ER stress is mediated by the UPR signaling pathway. Furthermore, enzymatic activity assays and genetic evidence showed that the D domain is crucially important for the activities of AtPDI11. Overall, this work will help to further understand the working mechanism of AtPDI11 in catalyzing disulfide formation in plants.

Arabidopsis PDI11 interacts with lectin molecular chaperons calreticulin 1 and 2 through its D domain

The endoplasmic reticulum (ER) is equipped with protein disulfide isomerases (PDIs), molecular chaperons, and other folding enzymes to ensure that newly synthesized proteins in the ER are properly folded. Molecular chaperons and PDIs can form complex to promote protein folding in the ER of mammalian cells. In plants, many PDIs associate with each other and function cooperatively in oxidative protein folding. As a plant unique protein disulfide isomerase, Arabidopsis thaliana PDI11 (AtPDI11) demonstrates oxidative protein folding activities and works synergistically with AtPDI2/5. However, whether AtPDI11 associates with molecular chaperons or AtPDIs in catalyzing disulfide formation remained unknown. Here, we find that AtPDI11 interacts with ER resident lectin chaperones calreticulin 1 (CRT1) and CRT2. Furthermore, the D domain, but not the a or a' domain of AtPDI11 provides the biding sites for its interaction with CRT1/2. Moreover, the P domain of CRT1 is responsible for its interaction with AtPDI11. Our work implies that Arabidopsis CRT1/2 may specifically recruit AtPDI11 to assist the folding of glycoproteins in the ER.

Advances in proteome-wide analysis of plant lysine acetylation

Lysine acetylation (LysAc) is a conserved and important post-translational modification (PTM) that plays a key role in plant physiological and metabolic processes. Based on advances in Lys-acetylated protein immunoenrichment and mass-spectrometric technology, LysAc proteomics studies have been performed in many species. Such studies have made substantial contributions to our understanding of plant LysAc, revealing that Lys-acetylated histones and nonhistones are involved in a broad spectrum of plant cellular processes. Here, we present an extensive overview of recent research on plant Lys-acetylproteomes. We provide in-depth insights into the characteristics of plant LysAc modifications and the mechanisms by which LysAc participates in cellular processes and regulates metabolism and physiology during plant growth and development. First, we summarize the characteristics of LysAc, including the properties of Lys-acetylated sites, the motifs that flank Lys-acetylated lysines, and the dynamic alterations in LysAc among different tissues and developmental stages. We also outline a map of Lys-acetylated proteins in the Calvin-Benson cycle and central carbon metabolism-related pathways. We then introduce some examples of the regulation of plant growth, development, and biotic and abiotic stress responses by LysAc. We discuss the interaction between LysAc and N^α-terminal acetylation and the crosstalk between LysAc and other PTMs, including phosphorylation and succinylation. Finally, we propose recommendations for future studies in the field. We conclude that LysAc of proteins plays an important role in the regulation of the plant life cycle.

Microtubule Dynamics Plays a Vital Role in Plant Adaptation and Tolerance to Salt Stress

Although recent studies suggest that the plant cytoskeleton is associated with plant stress responses, such as salt, cold, and drought, the molecular mechanism underlying microtubule function in plant salt stress response remains unclear. We performed a comparative proteomic analysis between control suspension-cultured cells (A0) and salt-adapted cells (A120) established from Arabidopsis root callus to investigate plant adaptation mechanisms to long-term salt stress. We identified 50 differentially expressed proteins (45 up- and 5 down-regulated proteins) in A120 cells compared with A0 cells. Gene ontology enrichment and protein network analyses indicated that differentially expressed proteins in A120 cells were strongly associated with cell structure-associated clusters, including cytoskeleton and cell wall biogenesis. Gene expression analysis revealed that expressions of cytoskeleton-related genes, such as FBA8, TUB3, TUB4, TUB7, TUB9, and ACT7, and a cell wall biogenesis-related gene, CCoAOMT1, were induced in salt-adapted A120 cells. Moreover, the loss-of-function mutant of Arabidopsis TUB9 gene, tub9, showed a hypersensitive phenotype to salt stress. Consistent overexpression of Arabidopsis TUB9 gene in rice transgenic plants enhanced tolerance to salt stress. Our results suggest that microtubules play crucial roles in plant adaptation and tolerance to salt stress. The modulation of microtubule-related gene expression can be an effective strategy for developing salt-tolerant crops.

iTRAQ-based quantitative proteomic analysis reveals NtGNL1-dependent regulatory network underlying endosome trafficking for pollen tube polar growth

Endosome trafficking has been reported to play an essential role in pollen tube polar growth and NtGNL1 (Nicotiana tabacum GNOM-LIKE 1) regulates the polar growth through endosome trafficking. However, the regulation network and detailed molecular mechanisms underlying endosome trafficking remain unclear. Here, comparative proteomic analysis was carried out to survey the overall effect of NtGNL1 on pollen tube polar growth and NtGNL1-dependent endosome trafficking. With multiple comparative systems (RNAi, Wild type, and BFA or wortmannin treatments), 481 distinct proteins were identified including 43 common DEPs (differentially expressed proteins), of which 16 significant DEPs were common among RNAi, BFA, and wortmannin treated pollen tubes, indicating their close relation to the endosome trafficking. GO annotation indicates that the vesicle trafficking of gnl1HE pollen tubes differs from that of the BFA and wortmannin treated pollen tubes in the COPII-coated vesicle budding process. KEGG pathway analysis suggests that the Pentose phosphate pathway is critical for the NtGNL1-dependent endosome trafficking. Yeast two-hybrid further confirmed that the NtGNL1-Sec7 domain interacted strongly with VPS32.2, TCTP, PIS2, and PDIL2-1, suggesting that the core functional region of NtGNL1 is the Sec7 domain. Therefore, NtGNL1 likely functions via its Sec7 binding with these proteins to affect endosome trafficking. Our results provide a clear outline of proteins involving in NtGNL1-dependent endosome trafficking and valuable clues for understanding the regulatory mechanism of NtGNL1 guided pollen tube polar growth.

Genome-Wide Identification and Expression Profiling of the PDI Gene Family Reveals Their Probable Involvement in Abiotic Stress Tolerance in Tomato (Solanum Lycopersicum L.)

Protein disulfide isomerases (PDI) and PDI-like proteins catalyze the formation and isomerization of protein disulfide bonds in the endoplasmic reticulum and prevent the buildup of misfolded proteins under abiotic stress conditions. In the present study, we conducted the first comprehensive genome-wide exploration of the PDI gene family in tomato (Solanum lycopersicum L.). We identified 19 tomato PDI genes that were unevenly distributed on 8 of the 12 tomato chromosomes, with segmental duplications detected for 3 paralogous gene pairs. Expression profiling of the PDI genes revealed that most of them were differentially expressed across different organs and developmental stages of the fruit. Furthermore, most of the PDI genes were highly induced by heat, salt, and abscisic acid (ABA) treatments, while relatively few of the genes were induced by cold and nutrient and water deficit (NWD) stresses. The predominant expression of SlPDI1-1, SlPDI1-3, SlPDI1-4, SlPDI2-1, SlPDI4-1, and SlPDI5-1 in response to abiotic stress and ABA treatment suggested they play regulatory roles in abiotic stress tolerance in tomato in an ABA-dependent manner. Our results provide new insight into the structure and function of PDI genes and will be helpful for the selection of candidate genes involved in fruit development and abiotic stress tolerance in tomato.

Genome-wide characterization of the PDI gene family in Medicago truncatula and their roles in response to endoplasmic reticulum stress

Protein disulfide isomerases (PDIs) are pivotal protein folding catalysts in the endoplasmic reticulum (ER) through formation of disulfide bond, isomerization, and inhibition of misfolded protein aggregation. When protein folding capacity is overwhelmed by the demands during transitions between growth phases or under environmental changes, the accumulation of unfolded or misfolded proteins in the ER triggers ER stress. However, little is known about the PDI gene family in the model legume Medicago truncatula, especially the responses to ER stress. Therefore, we identified 17 putative PDI genes from the genome of M. truncatula and present their gene and protein structures, phylogenetic relationships, chromosomal distributions, and synteny analysis with the orthologs in four other eudicot species, including Arabidopsis thaliana, Glycine max, Brassica rapa, and Vitis vinifera. Moreover, expression profiles derived from transcriptome data showed distinct expression patterns of MtPDI genes among plant organs, while real-time quantitative PCR analysis and data from the proteome revealed the potential roles of MtPDI genes in response to ER stress. Our study provides a foundation for further investigations of the biological roles of PDI genes in Medicago, especially their roles in response to ER stress.

Sequence analysis of the Petunia inflata S-locus region containing 17 S-Locus F-Box genes and the S-RNase gene involved in self-incompatibility

Self-incompatibility in Petunia is controlled by the polymorphic S-locus, which contains S-RNase encoding the pistil determinant and 16-20 S-locus F-box (SLF) genes collectively encoding the pollen determinant. Here we sequenced and assembled approximately 3.1 Mb of the S₂ -haplotype of the S-locus in Petunia inflata using bacterial artificial chromosome clones collectively containing all 17 SLF genes, SLFLike1, and S-RNase. Two SLF pseudogenes and 28 potential protein-coding genes were identified, 20 of which were also found at the S-loci of both the S_6a -haplotype of P. inflata and the S_N -haplotype of self-compatible Petunia axillaris, but not in the S-locus remnants of self-compatible potato (Solanum tuberosum) and tomato (Solanum lycopersicum). Comparative analyses of S-locus sequences of these three S-haplotypes revealed potential genetic exchange in the flanking regions of SLF genes, resulting in highly similar flanking regions between different types of SLF and between alleles of the same type of SLF of different S-haplotypes. The high degree of sequence similarity in the flanking regions could often be explained by the presence of similar long terminal repeat retroelements, which were enriched at the S-loci of all three S-haplotypes and in the flanking regions of all S-locus genes examined. We also found evidence of the association of transposable elements with SLF pseudogenes. Based on the hypothesis that SLF genes were derived by retrotransposition, we identified 10 F-box genes as putative SLF parent genes. Our results shed light on the importance of non-coding sequences in the evolution of the S-locus, and on possible evolutionary mechanisms of generation, proliferation, and deletion of SLF genes.

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.

Comparative acetylomic analysis reveals differentially acetylated proteins regulating anther and pollen development in kenaf cytoplasmic male sterility line

Cytoplasmic male sterility (CMS) is widely used in plant breeding and represents a perfect model to understand cyto-nuclear interactions and pollen development research. Lysine acetylation in proteins is a dynamic and reversible posttranslational modification (PTM) that plays an important roles in diverse cell processes and signaling. However, studies addressing acetylation PTM regarding to anther and pollen development in CMS background are largely lacking. To reveal the possible mechanism of kenaf (Hibiscus cannabinus L.) CMS and pollen development, we performed a label-free-based comparative acetylome analysis in kenaf anther of a CMS line and wild-type (Wt). Using whole transcriptome unigenes of kenaf as the reference genome, we identified a total of 1204 Kac (lysin acetylation) sites on 1110 peptides corresponding to 672 unique proteins. Futher analysis showed 56 out of 672 proteins were differentially acetylated between CMS and Wt line, with 13 and 43 of those characterized up- and downregulated, respectively. Thirty-eight and 82 proteins were detected distinctively acetylated in CMS and Wt lines, respectively. And evaluation of the acetylomic and proteomic results indicated that the most significantly acetylated proteins were not associated with abundant changes at the protein level. Bioinformatics analysis demonstrated that many of these proteins were involved in various biological processes which may play key roles in pollen development, inculding tricarboxylic acid (TCA) cycle and energy metabolism, protein folding, protein metabolism, cell signaling, gene expression regulation. Taken together, our results provide insight into the CMS molecular mechanism and pollen development in kenaf from a protein acetylation perspective.

Oxidative protein folding in the plant endoplasmic reticulum

For most of the proteins synthesized in the endoplasmic reticulum (ER), disulfide bond formation accompanies protein folding in a process called oxidative folding. Oxidative folding is catalyzed by a number of enzymes, including the family of protein disulfide isomerases (PDIs), as well as other proteins that supply oxidizing equivalents to PDI family proteins, like ER oxidoreductin 1 (Ero1). Oxidative protein folding in the ER is a basic vital function, and understanding its molecular mechanism is critical for the application of plants as protein production tools. Here, I review the recent research and progress related to the enzymes involved in oxidative folding in the plant ER. Firstly, nine groups of plant PDI family proteins are introduced. Next, the enzymatic properties of plant Ero1 are described. Finally, the cooperative folding by multiple PDI family proteins and Ero1 is described.

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.

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.

Genome-wide characterization and expression profiling of PDI family gene reveals function as abiotic and biotic stress tolerance in Chinese cabbage (Brassica rapa ssp. pekinensis)

Background

Protein disulfide isomerase (PDI) and PDI-like proteins contain thioredoxin domains that catalyze protein disulfide bond, inhibit aggregation of misfolded proteins, and function in isomerization during protein folding in endoplasmic reticulum and responses during abiotic stresses.Chinese cabbage is widely recognized as an economically important, nutritious vegetable, but its yield is severely hampered by various biotic and abiotic stresses. Because of, it is prime need to identify those genes whose are responsible for biotic and abiotic stress tolerance. PDI family genes are among of them.

Results

We have identified 32 PDI genes from the Br135K microarray dataset, NCBI and BRAD database, and in silico characterized their sequences. Expression profiling of those genes was performed using cDNA of plant samples imposed to abiotic stresses; cold, salt, drought and ABA (Abscisic Acid) and biotic stress; Fusarium oxysporum f. sp. conglutinans infection. The Chinese cabbage PDI genes were clustered in eleven groups in phylogeny. Among them, 15 PDI genes were ubiquitously expressed in various organs, while 24 PDI genes were up-regulated under salt and drought stress. By contrast, cold and ABA stress responsive gene number were ten and nine, respectively. In case of F. oxysporum f. sp. conglutinans infection 14 BrPDI genes were highly up-regulated. Interestingly, BrPDI1–1 gene was identified as putative candidate against abiotic (salt and drought) and biotic stresses, BrPDI5–2 gene for ABA stress, and BrPDI1–4, 6–1 and 9–2 were putative candidate genes for both cold and chilling injury stresses.

Conclusions

Our findings help to elucidate the involvement of PDI genes in stress responses, and they lay the foundation for functional genomics in future studies and molecular breeding of Brassica rapa crops. The stress-responsive PDI genes could be potential resources for molecular breeding of Brassica crops resistant to biotic and abiotic stresses.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4277-2) contains supplementary material, which is available to authorized users.

Identification of differentially regulated maize proteins conditioning Sugarcane mosaic virus systemic infection

Sugarcane mosaic virus (SCMV) is the most important cause of maize dwarf mosaic disease. To identify maize genes responsive to SCMV infection and that may be involved in pathogenesis, a comparative proteomic analysis was performed using the first and second systemically infected leaves (termed 1 SL and 2 SL, respectively). Seventy-one differentially expressed proteins were identified in 1 SL and 2 SL upon SCMV infection. Among them, eight proteins showed the same changing patterns in both 1 SL and 2 SL. Functional annotations of regulated proteins and measurement of photosynthetic activity revealed that photosynthesis was more inhibited and defensive gene expression more pronounced in 1 SL than in 2 SL. Knockdown of regulated proteins in both 1 SL and 2 SL by a brome mosaic virus-based gene silencing vector in maize indicated that protein disulfide isomerase-like and phosphoglycerate kinase were required for optimal SCMV replication. By contrast, knockdown of polyamine oxidase (ZmPAO) significantly increased SCMV accumulation, implying that ZmPAO activity might contribute to resistance or tolerance. The results suggest that combining comparative proteomic analyses of different tissues and virus-induced gene silencing is an efficient way to identify host proteins supporting virus replication or enhancing resistance to virus infection.

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.

Proteomic analysis of the mature Brassica stigma reveals proteins with diverse roles in vegetative and reproductive development

The stigma, the specialized apex of the Brassicaceae gynoecium, plays a role in pollen capture, discrimination, hydration, germination, and guidance. Despite this crucial role in reproduction, the global proteome underlying Brassicaceae stigma development and function remains largely unknown. As a contribution towards the characterization of the Brassicaceae dry stigma global proteome, more than 2500 Brassica napus mature stigma proteins were identified using three different gel-based proteomics approaches. Most stigma proteins participated in Metabolic Processes, Responses to Stimulus or Stress, Cellular or Developmental Processes, and Transport. The stigma was found to express a wide variety of proteins with demonstrated roles in cellular and organ development including proteins known to be involved in cellular expansion and morphogenesis, embryo development, as well as gynoecium and stigma development. Comparisons to a corresponding proteome from a very morphologically different Poaceae dry stigma showed a very similar distribution of proteins among different functional categories, but also revealed evident distinctions in protein composition especially in glucosinolate and carotenoid metabolism, photosynthesis, and self-incompatibility. To our knowledge, this study reports the largest Brassicaceae stigma protein dataset described to date.

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.

Retarded Embryo Development 1 (RED1) regulates embryo development, seed maturation and plant growth in Arabidopsis

Plant seeds accumulate large amounts of protein and carbohydrate as storage reserves during maturation. Thus, understanding the genetic control of embryo and seed development may provide bioengineering tools for yield improvement. In this study, we report the identification of Retarded Embryo Development 1 (RED1) gene in Arabidopsis, whose two independent T-DNA insertion mutant lines, SALK_085642 (red1-1) and SALK_022583 (red1-2), show a retarded embryo development phenotype. The embryogenesis process ceases at the late heart stage in red1-1 and at the bent-cotyledon stage in red1-2, respectively, resulting in seed abortion in both lines. The retarded embryo development and seed abortion phenotypes reverted to normal when RED1 complementation constructs were introduced into mutant plants. Small red1-2 homozygous plants can be successfully rescued by culturing immature seeds, indicating that seed abortion likely results from compromised tolerance to the desiccation process associated with seed maturation. Consistent with this observation, red1-2 seeds accumulate less protein, and the expression of two late embryo development reporter transgenes, LEA::GUS and β-conglycinin::GUS, was significantly weak and started relatively late in the red1-2 mutant lines compared to the wild type. The RED1 gene encodes a plant specific novel protein that is localized in the nucleus. These results indicate that RED1 plays important roles in embryo development, seed maturation and plant growth.

A disulphide isomerase gene (PDI-V) from Haynaldia villosa contributes to powdery mildew resistance in common wheat

In this study, we report the contribution of a PDI-like gene from wheat wild relative Haynaldia villosa in combating powdery mildew. PDI-V protein contains two conserved thioredoxin (TRX) active domains (a and a′) and an inactive domain (b). PDI-V interacted with E3 ligase CMPG1-V protein, which is a positive regulator of powdery mildew response. PDI-V was mono-ubiquitinated by CMPG1-V without degradation being detected. PDI-V was located on H. villosa chromosome 5V and encoded for a protein located in the endoplasmic reticulum. Bgt infection in leaves of H. villosa induced PDI-V expression. Virus induced gene silencing of PDIs in a T. durum-H. villosa amphiploid compromised the resistance. Single cell transient over-expression of PDI-V or a truncated version containing the active TXR domain a decreased the haustorial index in moderately susceptible wheat cultivar Yangmai 158. Stable transgenic lines over-expressing PDI-V in Yangmai 158 displayed improved powdery mildew resistance at both the seedling and adult stages. By contrast over-expression of point-mutated PDI-V^C57A did not increase the level of resistance in Yangmai 158. The above results indicate a pivotal role of PDI-V in powdery mildew resistance and showed that conserved TRX domain a is critical for its function.

The Diversity of the Pollen Tube Pathway in Plants: Toward an Increasing Control by the Sporophyte

Plants, unlike animals, alternate multicellular diploid, and haploid generations in their life cycle. While this is widespread all along the plant kingdom, the size and autonomy of the diploid sporophyte and the haploid gametophyte generations vary along evolution. Vascular plants show an evolutionary trend toward a reduction of the gametophyte, reflected both in size and lifespan, together with an increasing dependence from the sporophyte. This has resulted in an overlooking of the importance of the gametophytic phase in the evolution of higher plants. This reliance on the sporophyte is most notorious along the pollen tube journey, where the male gametophytes have to travel a long way inside the sporophyte to reach the female gametophyte. Along evolution, there is a change in the scenery of the pollen tube pathway that favors pollen competition and selection. This trend, toward apparently making complicated what could be simple, appears to be related to an increasing control of the sporophyte over the gametophyte with implications for understanding plant evolution.

Phylogenetic characterization and promoter expression analysis of a novel hybrid protein disulfide isomerase/cargo receptor subfamily unique to plants and chromalveolates

Protein disulfide isomerases (PDIs) play critical roles in protein folding by catalyzing the formation and rearrangement of disulfide bonds in nascent secretory proteins. There are six distinct PDI subfamilies in terrestrial plants. A unique feature of PDI-C subfamily members is their homology to the yeast retrograde (Golgi-to-endoplasmic reticulum) cargo receptor proteins, Erv41p and Erv46p. Here, we demonstrate that plant Erv41p/Erv46p-like proteins are divided into three subfamilies: ERV-A, ERV-B and PDI-C, which all possess the N-proximal and C-proximal conserved domains of yeast Erv41p and Erv46p. However, in PDI-C isoforms, these domains are separated by a thioredoxin domain. The distribution of PDI-C isoforms among eukaryotes indicates that the PDI-C subfamily likely arose through an ancient exon-shuffling event that occurred before the divergence of plants from stramenopiles and rhizarians. Arabidopsis has three PDI-C genes: PDI7, PDI12, and PDI13. PDI12- and PDI13-promoter: β-glucuronidase (GUS) gene fusions are co-expressed in pollen and stipules, while PDI7 is distinctly expressed in the style, hydathodes, and leaf vasculature. The PDI-C thioredoxin domain active site motif CxxS is evolutionarily conserved among land plants. Whereas PDI12 and PDI13 retain the CxxS motif, PDI7 has a CxxC motif similar to classical PDIs. We hypothesize that PDI12 and PDI13 maintain the ancestral roles of PDI-C in Arabidopsis, while PDI7 has undergone neofunctionalization. The unusual PDI/cargo receptor hybrid arrangement in PDI-C isoforms has no counterpart in animals or yeast, and predicts the need for pairing redox functions with cargo receptor processes during protein trafficking in plants and other PDI-C containing organisms.

Peptide signaling in pollen tube guidance

Fertilization is an important life event for sexually reproductive plants. Part of this process involves precise regulation of a series of complicated cell-cell communications between male and female tissues. Through genetic and omics approaches, many genes and proteins involved in this process have been identified. Here we review our current understanding of signaling components during fertilization. We will especially focus on LURE peptides and related signaling events that are required for micropylar pollen tube guidance. We will also summarize signaling events required for termination of micropylar pollen tube guidance after fertilization.

The Plant Ovule Secretome: A Different View toward Pollen-Pistil Interactions

During plant sexual reproduction, continuous exchange of signals between the pollen and the pistil (stigma, style, and ovary) plays important roles in pollen recognition and selection, establishing breeding barriers and, ultimately, leading to optimal seed set. After navigating through the stigma and the style, pollen tubes (PTs) reach their final destination, the ovule. This ultimate step is also regulated by numerous signals emanating from the embryo sac (ES) of the ovule. These signals encompass a wide variety of molecules, but species-specificity of the pollen-ovule interaction relies mainly on secreted proteins and their receptors. Isolation of candidate genes involved in pollen-pistil interactions has mainly relied on transcriptomic approaches, overlooking potential post-transcriptional regulation. To address this issue, ovule exudates were collected from the wild potato species Solanum chacoense using a tissue-free gravity-extraction method (tf-GEM). Combined RNA-seq and mass spectrometry-based proteomics led to the identification of 305 secreted proteins, of which 58% were ovule-specific. Comparative analyses using mature ovules (attracting PTs) and immature ovules (not attracting PTs) revealed that the last maturation step of ES development affected almost half of the ovule secretome. Of 128 upregulated proteins in anthesis stage, 106 were not regulated at the mRNA level, emphasizing the importance of post-transcriptional regulation in reproductive development.

Expression and characterization of protein disulfide isomerase family proteins in bread wheat

BACKGROUND

The major wheat seed proteins are storage proteins that are synthesized in the rough endoplasmic reticulum (ER) of starchy endosperm cells. Many of these proteins have intra- and intermolecular disulfide bonds. In eukaryotes, the formation of most intramolecular disulfide bonds in the ER is thought to be catalyzed by protein disulfide isomerase (PDI) family proteins. The cDNAs that encode eight groups of bread wheat (Triticum aestivum L.) PDI family proteins have been cloned, and their expression levels in developing wheat grains have been determined. The purpose of the present study was to characterize the enzymatic properties of the wheat PDI family proteins and clarify their expression patterns in wheat caryopses.

RESULTS

PDI family cDNAs, which are categorized into group I (TaPDIL1Aα, TaPDIL1Aβ, TaPDIL1Aγ, TaPDIL1Aδ, and TaPDIL1B), group II (TaPDIL2), group III (TaPDIL3A), group IV (TaPDIL4D), and group V (TaPDIL5A), were cloned. The expression levels of recombinant TaPDIL1Aα, TaPDIL1B, TaPDIL2, TaPDIL3A, TaPDIL4D, and TaPDIL5A in Escherichia coli were established from the cloned cDNAs. All recombinant proteins were expressed in soluble forms and purified. Aside from TaPDIL3A, the recombinant proteins exhibited oxidative refolding activity on reduced and denatured ribonuclease A. Five groups of PDI family proteins were distributed throughout wheat caryopses, and expression levels of these proteins were higher during grain filling than in the late stage of maturing. Localization of these proteins in the ER was confirmed by fluorescent immunostaining of the immature caryopses. In mature grains, the five groups of PDI family proteins remained in the aleurone cells and the protein matrix of the starchy endosperm.

CONCLUSIONS

High expression of PDI family proteins during grain filling in the starchy endosperm suggest that these proteins play an important role in forming intramolecular disulfide bonds in seed storage proteins. In addition, these PDI family proteins that remain in the aleurone layers of mature grains likely assist in folding newly synthesized hydrolytic enzymes during germination.

Proteomic analysis of MG132-treated germinating pollen reveals expression signatures associated with proteasome inhibition

Chemical inhibition of the proteasome has been previously found to effectively impair pollen germination and tube growth in vitro. However, the mediators of these effects at the molecular level are unknown. By performing 2DE proteomic analysis, 24 differentially expressed protein spots, representing 14 unique candidate proteins, were identified in the pollen of kiwifruit (Actinidia deliciosa) germinated in the presence of the MG132 proteasome inhibitor. qPCR analysis revealed that 11 of these proteins are not up-regulated at the mRNA level, but are most likely stabilized by proteasome inhibition. These differentially expressed proteins are predicted to function in various pathways including energy and lipid metabolism, cell wall synthesis, protein synthesis/degradation and stress responses. In line with this evidence, the MG132-induced changes in the proteome were accompanied by an increase in ATP and ROS content and by an alteration in fatty acid composition.

Morphological and proteomic analysis reveal the role of pistil under pollination in Liriodendron chinense (Hemsl.) Sarg

Pollination is an important physiological process during which interaction between pollen and pistil occurs. This interaction could determine whether or not fertilization will occur and hence the ratio of plant seed setting. Liriodendron chinense (Hemsl.) Sarg. (L. chinense) exhibits a distinct phenomenon where seed setting ratio is not more than 10% in natural environment. To explore the origin of this phenomenon, we conducted a comparative morphological and proteomic analysis on L. chinense pistils upon pollination. The morphological analysis showed that pollen grows well in vitro, but much slower on pistil or nutrient medium containing pistil extract. Proteomic analysis showed that 493 proteins had changed the expression after pollination. Among them, 468 and 51 proteins were identified by isobaric tags for relative and absolute quantitation and two-dimensional gel electrophoresis respectively, and 26 proteins were common in the two methods. After proteins functional categorization, 66 differentially expressed proteins that are involved in reproduction process were found. Further analysis showed that among the reproductive process related proteins, protein disulfide-isomerase A6 and four embryo-defective proteins showed closer relations with the low seed setting phenomenon. The results indicated that the element from pistil might be the main reason leading to low seed setting in L. chinense, which will provide new insights in the mechanisms underlying L. chinense reproduction process.

Knockdown of the Arabidopsis thaliana chloroplast protein disulfide isomerase 6 results in reduced levels of photoinhibition and increased D1 synthesis in high light

A chloroplast protein disulfide isomerase (PDI) was previously proposed to regulate translation of the unicellular green alga Chlamydomonas reinhardtii chloroplast psbA mRNA, encoding the D1 protein, in response to light. Here we show that AtPDI6, one of 13 Arabidopsis thaliana PDI genes, also plays a role in the chloroplast. We found that AtPDI6 is targeted and localized to the chloroplast. Interestingly, AtPDI6 knockdown plants displayed higher resistance to photoinhibition than wild-type plants when exposed to a tenfold increase in light intensity. The AtPDI6 knockdown plants also displayed a higher rate of D1 synthesis under a similar light intensity. The increased resistance to photoinhibition may not be rationalized by changes in antenna or non-photochemical quenching. Thus, the increased D1 synthesis rate, which may result in a larger proportion of active D1 under light stress, may led to the decrease in photoinhibition. These results suggest that, although the D1 synthesis rates observed in wild-type plants under high light intensities are elevated, repair can potentially occur faster. The findings implicate AtPDI6 as an attenuator of D1 synthesis, modulating photoinhibition in a light-regulated manner.

Molecular characterization and expression profiling of the protein disulfide isomerase gene family in Brachypodium distachyon L

Protein disulfide isomerases (PDI) are involved in catalyzing protein disulfide bonding and isomerization in the endoplasmic reticulum and functions as a chaperone to inhibit the aggregation of misfolded proteins. Brachypodium distachyon is a widely used model plant for temperate grass species such as wheat and barley. In this work, we report the first molecular characterization, phylogenies, and expression profiles of PDI and PDI-like (PDIL) genes in B. distachyon in different tissues under various abiotic stresses. Eleven PDI and PDIL genes in the B. distachyon genome by in silico identification were evenly distributed across all five chromosomes. The plant PDI family has three conserved motifs that are involved in catalyzing protein disulfide bonding and isomerization, but a different exon/intron structural organization showed a high degree of structural differentiation. Two pairs of genes (BdPDIL4-1 and BdPDIL4-2; BdPDIL7-1 and BdPDIL7-2) contained segmental duplications, indicating each pair originated from one progenitor. Promoter analysis showed that Brachypodium PDI family members contained important cis-acting regulatory elements involved in seed storage protein synthesis and diverse stress response. All Brachypodium PDI genes investigated were ubiquitously expressed in different organs, but differentiation in expression levels among different genes and organs was clear. BdPDIL1-1 and BdPDIL5-1 were expressed abundantly in developing grains, suggesting that they have important roles in synthesis and accumulation of seed storage proteins. Diverse treatments (drought, salt, ABA, and H₂O₂) induced up- and down-regulated expression of Brachypodium PDI genes in seedling leaves. Interestingly, BdPDIL1-1 displayed significantly up-regulated expression following all abiotic stress treatments, indicating that it could be involved in multiple stress responses. Our results provide new insights into the structural and functional characteristics of the plant PDI gene family.

Anatomy and transcript profiling of gynoecium development in female sterile Brassica napus mediated by one alien chromosome from Orychophragmus violaceus

BACKGROUND

The gynoecium is one of the most complex organs of angiosperms specialized for seed production and dispersal, but only several genes important for ovule or embryo sac development were identified by using female sterile mutants. The female sterility in oilseed rape (Brassica napus) was before found to be related with one alien chromosome from another crucifer Orychophragmus violaceus. Herein, the developmental anatomy and comparative transcript profiling (RNA-seq) for the female sterility were performed to reveal the genes and possible metabolic pathways behind the formation of the damaged gynoecium.

RESULTS

The ovules in the female sterile Brassica napus with two copies of the alien chromosomes (S1) initiated only one short integument primordium which underwent no further development and the female gametophyte development was blocked after the tetrad stage but before megagametogenesis initiation. Using Brassica_ 95k_ unigene as the reference genome, a total of 28,065 and 27,653 unigenes were identified to be transcribed in S1 and donor B. napus (H3), respectively. Further comparison of the transcript abundance between S1 and H3 revealed that 4540 unigenes showed more than two fold expression differences. Gene ontology and pathway enrichment analysis of the Differentially Expressed Genes (DEGs) showed that a number of important genes and metabolism pathways were involved in the development of gynoecium, embryo sac, ovule, integuments as well as the interactions between pollen and pistil.

CONCLUSIONS

DEGs for the ovule development were detected to function in the metabolism pathways regulating brassinosteroid (BR) biosynthesis, adaxial/abaxial axis specification, auxin transport and signaling. A model was proposed to show the possible roles and interactions of these pathways for the sterile gynoecium development. The results provided new information for the molecular mechanisms behind the gynoecium development at early stage in B. napus.

Cell type-specific transcriptome of Brassicaceae stigmatic papilla cells from a combination of laser microdissection and RNA sequencing

Pollination is an early and critical step in plant reproduction, leading to successful fertilization. It consists of many sequential processes, including adhesion of pollen grains onto the surface of stigmatic papilla cells, foot formation to strengthen pollen-stigma interaction, pollen hydration and germination, and pollen tube elongation and penetration. We have focused on an examination of the expressed genes in papilla cells, to increase understanding of the molecular systems of pollination. From three representative species of Brassicaceae (Arabidopsis thaliana, A. halleri and Brassica rapa), stigmatic papilla cells were isolated precisely by laser microdissection, and cell type-specific gene expression in papilla cells was determined by RNA sequencing. As a result, 17,240, 19,260 and 21,026 unigenes were defined in papilla cells of A. thaliana, A. halleri and B. rapa, respectively, and, among these, 12,311 genes were common to all three species. Among the17,240 genes predicted in A. thaliana, one-third were papilla specific while approximately half of the genes were detected in all tissues examined. Bioinformatics analysis revealed that genes related to a wide range of reproduction and development functions are expressed in papilla cells, particularly metabolism, transcription and membrane-mediated information exchange. These results reflect the conserved features of general cellular function and also the specific reproductive role of papilla cells, highlighting a complex cellular system regulated by a diverse range of molecules in these cells. This study provides fundamental biological knowledge to dissect the molecular mechanisms of pollination in papilla cells and will shed light on our understanding of plant reproduction mechanisms.

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

Protein disulfide isomerases (PDIs) catalyze the formation, breakage, and rearrangement of disulfide bonds to properly fold nascent polypeptides within the endoplasmic reticulum (ER). Classical animal and yeast PDIs possess two catalytic thioredoxin-like domains (a, a') and two non-catalytic domains (b, b'), in the order a-b-b'-a'. The model plant, Arabidopsis thaliana, encodes 12 PDI-like proteins, six of which possess the classical PDI domain arrangement (AtPDI1 through AtPDI6). Three additional AtPDIs (AtPDI9, AtPDI10, AtPDI11) possess two thioredoxin domains, but without intervening b-b' domains. C-terminal green fluorescent protein (GFP) fusions to each of the nine dual-thioredoxin PDI homologs localized predominantly to the ER lumen when transiently expressed in protoplasts. Additionally, expression of AtPDI9:GFP-KDEL and AtPDI10: GFP-KDDL was associated with the formation of ER bodies. AtPDI9, AtPDI10, and AtPDI11 mediated the oxidative folding of alkaline phosphatase when heterologously expressed in the Escherichia coli protein folding mutant, dsbA-. However, only three classical AtPDIs (AtPDI2, AtPDI5, AtPDI6) functionally complemented dsbA-. Interestingly, chemical inducers of the ER unfolded protein response were previously shown to upregulate most of the AtPDIs that complemented dsbA-. The results indicate that Arabidopsis PDIs differ in their localization and protein folding activities to fulfill distinct molecular functions in the ER.

Oxidative protein-folding systems in plant cells

Plants are unique among eukaryotes in having evolved organelles: the protein storage vacuole, protein body, and chloroplast. Disulfide transfer pathways that function in the endoplasmic reticulum (ER) and chloroplasts of plants play critical roles in the development of protein storage organelles and the biogenesis of chloroplasts, respectively. Disulfide bond formation requires the cooperative function of disulfide-generating enzymes (e.g., ER oxidoreductase 1), which generate disulfide bonds de novo, and disulfide carrier proteins (e.g., protein disulfide isomerase), which transfer disulfides to substrates by means of thiol-disulfide exchange reactions. Selective molecular communication between disulfide-generating enzymes and disulfide carrier proteins, which reflects the molecular and structural diversity of disulfide carrier proteins, is key to the efficient transfer of disulfides to specific sets of substrates. This review focuses on recent advances in our understanding of the mechanisms and functions of the various disulfide transfer pathways involved in oxidative protein folding in the ER, chloroplasts, and mitochondria of plants.

The oxidative protein folding machinery in plant cells

Formation of intra-molecular disulfides and concomitant oxidative protein folding is essential for stability and catalytic function of many soluble and membrane-bound proteins in the endomembrane system, the mitochondrial inter-membrane space and the thylakoid lumen. Disulfide generation from free cysteines in nascent polypeptide chains is generally a catalysed process for which distinct pathways exist in all compartments. A high degree of similarities between highly diverse eukaryotic and bacterial systems for generation of protein disulfides indicates functional conservation of key processes throughout evolution. However, while many aspects about molecular function of enzymatic systems promoting disulfide formation have been demonstrated for bacterial and non-plant eukaryotic organisms, it is now clear that the plant machinery for oxidative protein folding displays distinct details, suggesting that the different pathways have been adapted to plant-specific requirements in terms of compartmentation, molecular function and regulation. Here, we aim to evaluate biological diversity by comparing the plant systems for oxidative protein folding to the respective systems from non-plant eukaryotes.

Male-female crosstalk during pollen germination, tube growth and guidance, and double fertilization

Sperm cells of flowering plants are non-motile and thus require transportation to the egg apparatus via the pollen tube to execute double fertilization. During its journey, the pollen tube interacts with various sporophytic cell types that support its growth and guide it towards the surface of the ovule. The final steps of tube guidance and sperm delivery are controlled by the cells of the female gametophyte. During fertilization, cell-cell communication events take place to achieve and maximize reproductive success. Additional layers of crosstalk exist, including self-recognition and specialized processes to prevent self-fertilization and consequent inbreeding. In this review, we focus on intercellular communication between the pollen grain/pollen tube including the sperm cells with the various sporophytic maternal tissues and the cells of the female gametophyte. Polymorphic-secreted peptides and small proteins, especially those belonging to various subclasses of small cysteine-rich proteins (CRPs), reactive oxygen species (ROS)/NO signaling, and the second messenger Ca(2+), play center stage in most of these processes.

Isolation and proteomic characterization of the Arabidopsis Golgi defines functional and novel components involved in plant cell wall biosynthesis

The plant Golgi plays a pivotal role in the biosynthesis of cell wall matrix polysaccharides, protein glycosylation, and vesicle trafficking. Golgi-localized proteins have become prospective targets for reengineering cell wall biosynthetic pathways for the efficient production of biofuels from plant cell walls. However, proteomic characterization of the Golgi has so far been limited, owing to the technical challenges inherent in Golgi purification. In this study, a combination of density centrifugation and surface charge separation techniques have allowed the reproducible isolation of Golgi membranes from Arabidopsis (Arabidopsis thaliana) at sufficiently high purity levels for in-depth proteomic analysis. Quantitative proteomic analysis, immunoblotting, enzyme activity assays, and electron microscopy all confirm high purity levels. A composition analysis indicated that approximately 19% of proteins were likely derived from contaminating compartments and ribosomes. The localization of 13 newly assigned proteins to the Golgi using transient fluorescent markers further validated the proteome. A collection of 371 proteins consistently identified in all replicates has been proposed to represent the Golgi proteome, marking an appreciable advancement in numbers of Golgi-localized proteins. A significant proportion of proteins likely involved in matrix polysaccharide biosynthesis were identified. The potential within this proteome for advances in understanding Golgi processes has been demonstrated by the identification and functional characterization of the first plant Golgi-resident nucleoside diphosphatase, using a yeast complementation assay. Overall, these data show key proteins involved in primary cell wall synthesis and include a mixture of well-characterized and unknown proteins whose biological roles and importance as targets for future research can now be realized.

Emerging role of ER quality control in plant cell signal perception

The endoplasmic reticulum quality control (ER-QC) is a conserved mechanism in surveillance of secreted signaling factors during cell-to-cell communication in eukaryotes. Recent data show that the ER-QC plays important roles in diverse cell-to-cell signaling processes during immune response, vegetative and reproductive development in plants. Pollen tube guidance is a precisely guided cell-cell communication process between the male and female gametophytes during plant reproduction. Recently, the female signal has been identified as small secreted peptides, but how the pollen tube responds to this signal is still unclear. In this review, we intend to summarize the role of ER-QC in plants and discuss the recent advances regarding our understanding of the mechanism of pollen tube response to the female signals.

Mutation in SUMO E3 ligase, SIZ1, disrupts the mature female gametophyte in Arabidopsis

Female gametophyte is the multicellular haploid structure that can produce embryo and endosperm after fertilization, which has become an attractive model system for investigating molecular mechanisms in nuclei migration, cell specification, cell-to-cell communication and many other processes. Previous reports found that the small ubiquitin-like modifier (SUMO) E3 ligase, SIZ1, participated in many processes depending on particular target substrates and suppression of salicylic acid (SA) accumulation. Here, we report that SIZ1 mediates the reproductive process. SIZ1 showed enhanced expression in female organs, but was not detected in the anther or pollen. A defect in the siz1-2 maternal source resulted in reduced seed-set regardless of high SA concentration within the plant. Moreover, aniline blue staining and scanning electron microscopy revealed that funicular and micropylar pollen tube guidance was arrested in siz1-2 plants. Some of the embryo sacs of ovules in siz1-2 were also disrupted quickly after stage FG7. There was no significant affects of the siz1-2 mutation on expression of genes involved in female gametophyte development- or pollen tube guidance in ovaries. Together, our results suggest that SIZ1 sustains the stability and normal function of the mature female gametophyte which is necessary for pollen tube guidance.

The potato sucrose transporter StSUT1 interacts with a DRM-associated protein disulfide isomerase

Organization of proteins into complexes is crucial for many cellular functions. Recently, the SUT1 protein was shown to form homodimeric complexes, to be associated with lipid raft-like microdomains in yeast as well as in plants and to undergo endocytosis in response to brefeldin A. We therefore aimed to identify SUT1-interacting proteins that might be involved in dimerization, endocytosis, or targeting of SUT1 to raft-like microdomains. Therefore, we identified potato membrane proteins, which are associated with the detergent-resistant membrane (DRM) fraction. Among the proteins identified, we clearly confirmed StSUT1 as part of DRM in potato source leaves. We used the yeast two-hybrid split ubiquitin system (SUS) to systematically screen for interaction between the sucrose transporter StSUT1 and other membrane-associated or soluble proteins in vivo. The SUS screen was followed by immunoprecipitation using affinity-purified StSUT1-specific peptide antibodies and mass spectrometric analysis of co-precipitated proteins. A large overlap was observed between the StSUT1-interacting proteins identified in the co-immunoprecipitation and the detergent-resistant membrane fraction. One of the SUT1-interacting proteins, a protein disulfide isomerase (PDI), interacts also with other sucrose transporter proteins. A potential role of the PDI as escort protein is discussed.

Oxidative protein folding: selective pressure for prolamin evolution in rice

During seed development, endosperm cells of highly productive cereals, including rice, synthesize disulfide-rich proteins in large amounts and deposit them into storage organelles. Disulfide bond formation involves electron transfer and generates H(2)O(2) as a by-product. To ensure proper development and maturation of seeds, the endosperm cells must supply large amounts of oxidizing equivalents to dithiols in nascent proteins in a controlled manner. This review compares multiple oxidative protein folding systems in yeast, cultured human cells, and rice endosperm. We discuss possible roles of ERO1, other sulfhydryl oxidases, and the protein disulfide isomerase family in the formation of disulfide bonds in storage proteins and the development of protein bodies. Rice prolamins, encoded by a multigene family, are divided into Cys-rich and Cys-depleted subgroups. We discuss the potential importance of disulfide bond formation in the evolution of the prolamin family in japonica rice.

Protein disulfide isomerase-2 of Arabidopsis mediates protein folding and localizes to both the secretory pathway and nucleus, where it interacts with maternal effect embryo arrest factor

Protein disulfide isomerase (PDI) is a thiodisulfide oxidoreductase that catalyzes the formation, reduction and rearrangement of disulfide bonds in proteins of eukaryotes. The classical PDI has a signal peptide, two CXXC-containing thioredoxin catalytic sites (a,a'), two noncatalytic thioredoxin fold domains (b,b'), an acidic domain (c) and a C-terminal endoplasmic reticulum (ER) retention signal. Although PDI resides in the ER where it mediates the folding of nascent polypeptides of the secretory pathway, we recently showed that PDI5 of Arabidopsis thaliana chaperones and inhibits cysteine proteases during trafficking to vacuoles prior to programmed cell death of the endothelium in developing seeds. Here we describe Arabidopsis PDI2, which shares a primary structure similar to that of classical PDI. Recombinant PDI2 is imported into ER-derived microsomes and complements the E. coli protein-folding mutant, dsbA. PDI2 interacted with proteins in both the ER and nucleus, including ER-resident protein folding chaperone, BiP1, and nuclear embryo transcription factor, MEE8. The PDI2-MEE8 interaction was confirmed to occur in vitro and in vivo. Transient expression of PDI2-GFP fusions in mesophyll protoplasts resulted in labeling of the ER, nucleus and vacuole. PDI2 is expressed in multiple tissues, with relatively high expression in seeds and root tips. Immunoelectron microscopy with GFP- and PDI2-specific antisera on transgenic seeds (PDI2-GFP) and wild type roots demonstrated that PDI2 was found in the secretory pathway (ER, Golgi, vacuole, cell wall) and the nuclei. Our results indicate that PDI2 mediates protein folding in the ER and has new functional roles in the nucleus.

Attraction of tip-growing pollen tubes by the female gametophyte

Pollen tube guidance is the mechanism whereby the direction of pollen tube growth is controlled by female cells of the pistil. Some key genes and molecules have recently been identified as being involved in pollen tube guidance. In this review article, we discuss the molecular basis of pollen tube guidance, especially in Arabidopsis thaliana, by summarizing recent progress in various plant species. Attractant molecules and receptors for gametophytic pollen tube guidance are the focus of this article.

Distinct roles of protein disulfide isomerase and P5 sulfhydryl oxidoreductases in multiple pathways for oxidation of structurally diverse storage proteins in rice

In the rice (Oryza sativa) endosperm, storage proteins are synthesized on the rough endoplasmic reticulum (ER), in which prolamins are sorted to protein bodies (PBs) called type-I PB (PB-I). Protein disulfide isomerase (PDI) family oxidoreductase PDIL2;3, an ortholog of human P5, contains a conserved structural disulfide in the redox-inactive thioredoxin-like (TRX) domain and was efficiently targeted to the surface of PB-I in a redox active site-dependent manner, whereas PDIL1;1, an ortholog of human PDI, was localized in the ER lumen. Complementation analyses using PDIL1;1 knockout esp2 mutant indicated that the a and a' TRX domains of PDIL1;1 exhibited similar redox activities and that PDIL2;3 was unable to perform the PDIL1;1 functions. PDIL2;3 knockdown inhibited the accumulation of Cys-rich 10-kD prolamin (crP10) in the core of PB-I. Conversely, crP10 knockdown dispersed PDIL2;3 into the ER lumen. Glutathione S-transferase-PDIL2;3 formed a stable tetramer when it was expressed in Escherichia coli, and the recombinant PDIL2;3 tetramer facilitated α-globulin(C79F) mutant protein to form nonnative intermolecular disulfide bonds in vitro. These results indicate that PDIL2;3 and PDIL1;1 are not functionally redundant in sulfhydryl oxidations of structurally diverse storage proteins and play distinct roles in PB development. We discuss PDIL2;3-dependent and PDIL2;3-independent oxidation pathways that sustain disulfide bonds of crP10 in PB-I.

Analysis of ER stress in developing rice endosperm accumulating beta-amyloid peptide

The common neurodegenerative disorder known as Alzheimer's disease is characterized by cerebral neuritic plaques of amyloid beta (Abeta) peptide. Plaque formation is related to the highly aggregative property of this peptide, because it polymerizes to form insoluble plaques or fibrils causing neurotoxicity. Here, we expressed Abeta peptide as a new causing agent to endoplasmic reticulum (ER) stress to study ER stress occurred in plant. When the dimer of Abeta(1-42) peptide was expressed in maturing seed under the control of the 2.3-kb glutelin GluB-1 promoter containing its signal peptide, a maximum of about 8 mug peptide per grain accumulated and was deposited at the periphery of distorted ER-derived PB-I protein bodies. Synthesis of Abeta peptide in the ER lumen severely inhibited the synthesis and deposition of seed storage proteins, resulting in the generation of many small and abnormally appearing PB bodies. This ultrastructural change was accounted for by ER stress leading to the accumulation of aggregated Abeta peptide in the ER lumen and a coordinated increase in ER-resident molecular chaperones such as BiPs and PDIs in Abeta-expressing plants. Microarray analysis also confirmed that expression of several BiPs, PDIs and OsbZIP60 containing putative transmembrane domains was affected by the ER stress response. Abeta-expressing transgenic rice kernels exhibited an opaque and shrunken phenotype. When grain phenotype and expression levels were compared among transgenic rice grains expressing several different recombinant peptides, such detrimental effects on grain phenotype were correlated with the expressed peptide causing ER stress rather than expression levels.

Female gametophyte-controlled pollen tube guidance

During the evolution of flowering plants, their sperm cells have lost mobility and are transported from the stigma to the female gametophyte via the pollen tube to achieve double fertilization. Pollen tube growth and guidance is largely governed by the maternal sporophytic tissues of the stigma, style and ovule. However, the last phase of the pollen tube path is under female gametophyte control and is expected to require extensive cell-cell communication events between both gametophytes. Until recently, little was known about the molecules produced by the female gametophyte that are involved in this process. In the present paper, we review the most recent development in this field and focus on the role of secreted candidate signalling ligands.