Propeptide of a precursor to a plant vacuolar protein required for vacuolar targeting

Toxin Homology Domain in Plant Type 2 Prolyl 4-Hydroxylases Acts as a Golgi Localization Domain

Prolyl 4-hydroxylase (P4H) generates hydroxyproline residues in proteins. Two classes of P4H have been found in plants. Type 1 P4H has a signal anchor at the N-terminus, while type 2 P4H has both an N-terminal signal peptide and a C-terminal toxin homology domain (Tox1 domain) with six conserved cysteine residues. We analyzed the localization of tobacco type 2 P4H (NtP4H2.2) in tobacco BY-2 cells. Cell fractionation studies, immunostaining of cells, and GFP fusion study indicated that NtP4H2.2 localizes predominantly to the Golgi apparatus and is a peripheral membrane protein associated with the luminal side of organelles. Expression of the GFP-Tox1 domains of NtP4H2.2 and another tobacco type 2 P4H NtP4H2.1 in BY-2 cells and Arabidopsis epidermal cells indicated that these proteins were targeted to the Golgi. The Tox1 domains from Arabidopsis and rice type 2 P4Hs also directed GFP to the Golgi in tobacco BY-2 cells. The Tox1 domain of NtP4H2.2 increased the membrane association of GFP, and mutation of the cysteine residues in this domain abolished Golgi localization. Furthermore, the catalytic domain of NtP4H2.2 also directed GFP to the Golgi. Thus, the Tox1 domains of plant P4Hs are the Golgi localization domains, and tobacco P4H2.2 localizes to the Golgi by the action of both this domain and the catalytic domain.

Comprehensive analysis of the UDP-glucuronate decarboxylase (UXS) gene family in tobacco and functional characterization of NtUXS16 in Golgi apparatus in Arabidopsis

BACKGROUND

UDP-glucuronate decarboxylase (also named UXS) converts UDP-glucuronic acid (UDP-GlcA) to UDP-xylose (UDP-Xyl) by decarboxylation of the C6-carboxylic acid of glucuronic acid. UDP-Xyl is an important sugar donor that is required for the synthesis of plant cell wall polysaccharides.

RESULTS

In this study, we first carried out the genome-wide identification of NtUXS genes in tobacco. A total of 17 NtUXS genes were identified, which could be divided into two groups (Group I and II), and the Group II UXSs can be further divided into two subgroups (Group IIa and IIb). Furthermore, the protein structures, intrachromosomal distributions and gene structures were thoroughly analyzed. To experimentally verify the subcellular localization of NtUXS16 protein, we transformed tobacco BY-2 cells with NtUXS16 fused to the monomeric red fluorescence protein (mRFP) at the C terminus under the control of the cauliflower mosaic virus (CaMV) 35S promoter. The fluorescent signals of NtUXS16-mRFP were localized to the medial-Golgi apparatus. Contrary to previous predictions, protease digestion analysis revealed that NtUXS16 is not a type II membrane protein. Overexpression of NtUXS16 in Arabidopsis seedling in darkness led to a significant increase in hypocotyl length and a reduction in root length compared with the wild type. In summary, these results suggest Golgi apparatus localized-NtUXS16 plays an important role in hypocotyl and root growth in the dark.

CONCLUSION

Our findings facilitate our understanding of the novel functions of NtUXS16 and provide insights for further exploration of the biological roles of NtUXS genes in tobacco.

Nanobody-based VSR7 tracing shows clathrin-dependent TGN to Golgi recycling

Receptor-mediated transport of soluble proteins is nature's key to empowering eukaryotic cells to access a plethora of macromolecules, either by direct accumulation or as products from resulting biochemical pathways. The transport efficiency of these mechanisms results from the receptor's capability to capture, transport, and release ligands on the one hand and the cycling ability that allows for performing multiple rounds of ligand transport on the other. However, the plant VACUOLAR SORTING RECEPTOR (VSR) protein family is diverse, and their ligand-specificity and bidirectional trafficking routes and transport mechanisms remain highly controversial. Here we employ nanobody-epitope interaction-based molecular tools to assess the function of the VSR 7 in vivo. We demonstrate the specificity of the VSR7 for sequence-specific vacuolar sorting signals, and we trace its anterograde transport and retrograde recycling route. VSR7 localizes at the cis-Golgi apparatus at steady state conditions and transports ligands downstream to release them in the trans-Golgi network/early endosome (TGN/EE) before undergoing clathrin-dependent recycling from the TGN/EE back to the cis-Golgi.

Secreted arabinogalactan protein from salt-adapted tobacco BY-2 cells appears to be glycosylphosphatidyl inositol-anchored and associated with lipophilic moieties

Arabinogalactan proteins (AGPs) are plant extracellular proteoglycans associated with the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. This moiety is thought to be cleaved by phospholipase for secretion. Salt-adapted tobacco BY-2 cells were reported to secrete large amounts of AGPs into the medium. To investigate this mechanism, we expressed a fusion protein of tobacco sweet potato sporamin and AGP (SPO-AGP) in BY-2 cells and analyzed its fate after salt-adapting the cells. A two-phase separation analysis using Triton X-114 indicated that a significant proportion of SPO-AGP in the medium was recovered in the detergent phase, suggesting that this protein is GPI-anchored. Differential ultracentrifugation and a gradient density fractionation implicated extracellular vesicles or particles with SPO-AGP in the medium. Endogenous AGP secreted from salt-adapted and nontransgenic BY-2 cells behaved similarly to SPO-AGP. These results suggest that a part of the secreted AGPs from salt-adapted tobacco BY-2 cells are GPI-anchored and associated with particles or vesicles.

Glycosylphosphatidylinositol-anchoring is required for the proper transport and extensive glycosylation of a classical arabinogalactan protein precursor in tobacco BY-2 cells

Many precursors of plant arabinogalactan proteins (AGPs) contain a C-terminal glycosylphosphatidylinositol (GPI)-anchoring signal. Using NtAGP1, a classical tobacco AGP, as a model, and green fluorescent protein (GFP) and sweet potato sporamin (SPO) as tags, we analyzed the localization and modification of AGP and its mutant without GPI-anchoring signal (AGPΔC) in tobacco BY-2 cells. The NtAGP1 fusion proteins migrated as large smear on SDS-polyacrylamide gel, and these proteins also localized preferentially to the plasma membrane. In contrast, fusions of AGPΔC with GFP and SPO yielded several forms: The largest were secreted, whereas others were recovered in the endomembrane organelles, including vacuoles. Comparison of the glycan structures of the microsomal SPO-AGP and the secreted SPO-AGPΔC using antibodies against the glycan epitopes of AGP indicated that the glycan structures of these proteins are different. These observations indicate that GPI-anchoring is required for the proper transport and glycosylation of the AGP precursor.

Fusing the 3'UTR of seed storage protein genes leads to massive recombinant protein accumulation in seeds

The demand for recombinant proteins is rising dramatically, and effective production systems are currently being developed. The production of recombinant proteins in plants is a promising approach due to its low cost and low risk of contamination of the proteins with endotoxins or infectious agents from the culture serum. Plant seeds primarily accumulate seed storage proteins (SSPs), which are transcribed and translated from a few genes; therefore, the mechanism underlying SSP accumulation has been studied to help devise ways to increase recombinant protein production. We found that the 3'UTR of SSP genes are essential for SSP accumulation and can be used in the production of recombinant proteins in Arabidopsis. Fusion of the 3'UTR of SSP genes to the 3' ends of DNA sequences encoding recombinant proteins enables massive accumulation of recombinant proteins with enzymatic activity in Arabidopsis seeds. This method is also applicable to the production of human Interferon Lambda-3 (IFN-lambda 3), a candidate biopharmaceutical compound against hepatitis C infection. Considering the low cost and ease of protein production in Arabidopsis, as well as the rapid growth of this plant, our method is useful for large-scale preparation of recombinant proteins for both academic research and biopharmaceutical production.

Structural insights into how vacuolar sorting receptors recognize the sorting determinants of seed storage proteins

Seeds such as rice and soybean are major food staples in the human diet. During seed development, storage proteins are deposited in a specialized organelle called the protein storage vacuole and are mobilized to provide nutrients during germination. Storage proteins are transported as cargoes via specific protein–protein interactions with the vacuolar sorting receptors. Supported by structural and mutagenesis studies, our work provides insights into how the sequence-specific information, or the vacuolar sorting determinant, on the storage proteins is recognized by the vacuolar sorting receptors for their targeting to the vacuoles. Insights gained into the rules of receptor–cargo recognition will be useful in engineering recombinant proteins for biotechnological applications of the protein storage vacuoles in seeds.

In Arabidopsis, vacuolar sorting receptor isoform 1 (VSR1) sorts 12S globulins to the protein storage vacuoles during seed development. Vacuolar sorting is mediated by specific protein–protein interactions between VSR1 and the vacuolar sorting determinant located at the C terminus (ctVSD) on the cargo proteins. Here, we determined the crystal structure of the protease-associated domain of VSR1 (VSR1-PA) in complex with the C-terminal pentapeptide (₄₆₈RVAAA₄₇₂) of cruciferin 1, an isoform of 12S globulins. The ₄₆₈RVA₄₇₀ motif forms a parallel β-sheet with the switch III residues (₁₂₇TMD₁₂₉) of VSR1-PA, and the ₄₇₁AA₄₇₂ motif docks to a cradle formed by the cargo-binding loop (₉₅RGDCYF₁₀₀), making a hydrophobic interaction with Tyr99. The C-terminal carboxyl group of the ctVSD is recognized by forming salt bridges with Arg95. The C-terminal sequences of cruciferin 1 and vicilin-like storage protein 22 were sufficient to redirect the secretory red fluorescent protein (spRFP) to the vacuoles in Arabidopsis protoplasts. Adding a proline residue to the C terminus of the ctVSD and R95M substitution of VSR1 disrupted receptor–cargo interactions in vitro and led to increased secretion of spRFP in Arabidopsis protoplasts. How VSR1-PA recognizes ctVSDs of other storage proteins was modeled. The last three residues of ctVSD prefer hydrophobic residues because they form a hydrophobic cluster with Tyr99 of VSR1-PA. Due to charge–charge interactions, conserved acidic residues, Asp129 and Glu132, around the cargo-binding site should prefer basic residues over acidic ones in the ctVSD. The structural insights gained may be useful in targeting recombinant proteins to the protein storage vacuoles in seeds.

Understanding the evolution of endosymbiotic organelles based on the targeting sequences of organellar proteins

Organellogenesis, a key aspect of eukaryotic cell evolution, critically depends on the successful establishment of organellar protein import mechanisms. Phylogenetic analysis revealed that the evolution of the two endosymbiotic organelles, the mitochondrion and the chloroplast, is thought to have occurred at time periods far from each other. Despite this, chloroplasts and mitochondria have highly similar protein import mechanisms. This raises intriguing questions such as what underlies such similarity in the import mechanisms and how these similar mechanisms have evolved. In this review, we summarise the recent findings regarding sorting and specific targeting of these organellar proteins. Based on these findings, we propose possible evolutionary scenarios regarding how the signal sequences of chloroplasts and mitochondrial proteins ended up having such relationship.

In vivo light-sheet microscopy resolves localisation patterns of FSD1, a superoxide dismutase with function in root development and osmoprotection

Superoxide dismutases (SODs) are enzymes detoxifying superoxide to hydrogen peroxide while temporal developmental expression and subcellular localisation are linked to their functions. Therefore, we aimed here to reveal in vivo developmental expression, subcellular, tissue- and organ-specific localisation of iron superoxide dismutase 1 (FSD1) in Arabidopsis using light-sheet and Airyscan confocal microscopy. FSD1-GFP temporarily accumulated at the site of endosperm rupture during seed germination. In emerged roots, it showed the highest abundance in cells of the lateral root cap, columella, and endodermis/cortex initials. The largest subcellular pool of FSD1-GFP was localised in the plastid stroma, while it was also located in the nuclei and cytosol. The majority of the nuclear FSD1-GFP is immobile as revealed by fluorescence recovery after photobleaching. We found that fsd1 knockout mutants exhibit reduced lateral root number and this phenotype was reverted by genetic complementation. Mutant analysis also revealed a requirement for FSD1 in seed germination during salt stress. Salt stress tolerance was coupled with the accumulation of FSD1-GFP in Hechtian strands and superoxide removal. It is likely that the plastidic pool is required for acquiring oxidative stress tolerance in Arabidopsis. This study suggests new developmental and osmoprotective functions of SODs in plants.

The Presence of the Hairy-Root-Disease-Inducing (Ri) Plasmid in Wheat Endophytic Rhizobia Explains a Pathogen Reservoir Function of Healthy Resistant Plants

A large number of strains in the Rhizobium radiobacter species complex (biovar 1 Agrobacterium) have been known as causative pathogens for crown gall and hairy root diseases. Strains within this complex were also found as endophytes in many plant species with no symptoms. The aim of this study was to reveal the endophyte variation of this complex and how these endophytic strains differ from pathogenic strains. In this study, we devised a simple but effective screening method by exploiting the high resolution power of mass spectrometry. We screened endophyte isolates from young wheat and barley plants, which are resistant to the diseases, and identified seven isolates from wheat as members of the R. radiobacter species complex. Through further analyses, we assigned five strains to the genomovar (genomic group) G1 and two strains to G7 in R. radiobacter Notably, these two genomovar groups harbor many known pathogenic strains. In fact, the two G7 endophyte strains showed pathogenicity on tobacco, as well as the virulence prerequisites, including a 200-kbp Ri plasmid. All five G1 strains possessed a 500-kbp plasmid, which is present in well-known crown gall pathogens. These data strongly suggest that healthy wheat plants are reservoirs for pathogenic strains of R. radiobacterIMPORTANCE Crown gall and hairy root diseases exhibit very wide host-plant ranges that cover gymnosperm and dicot plants. The Rhizobium radiobacter species complex harbors causative agents of the two diseases. Recently, endophyte isolates from many plant species have been assigned to this species complex. We isolated seven endophyte strains belonging to the species complex from wheat plants and revealed their genomovar affiliations and plasmid profile. The significance of this study is the finding of the genomovar correlation between the endophytes and the known pathogens, the presence of a virulence ability in two of the seven endophyte strains, and the high ratio of the pathogenic strains in the endophyte strains. This study therefore provides convincing evidence that could unravel the mechanism that maintains pathogenic agents of this species and sporadically delivers them to susceptible plants.

Sucrose starvation induces the degradation of proteins in trans-Golgi network and secretory vesicle cluster in tobacco BY-2 cells

Endomembrane transport system begins at the endoplasmic reticulum (ER), continues to the Golgi apparatus and subsequent compartment called trans-Golgi network (TGN). We found that SUT2, a tobacco sucrose-transporter ortholog and was localized in the TGN, decreased significantly under a sucrose-starvation condition. The tobacco SNARE protein SYP41, localized in the TGN and secretory vesicle cluster (SVC), also decreased under the starvation. Similarly, the SCAMP2-RFP fusion protein, which is localized in TGN, SVC, and plasma membrane (PM), was distributed solely in the PM under the starvation. Under the same starvation condition, protein secretion was not arrested but pectin deposition to cell wall was suppressed. These data indicated that the protein composition in TGN and existence of the SVC are regulated by sugar availability. Furthermore, our findings as well as the involvement of SVC in pectin secretion suggested that synthesis and transport of pectin are regulated by the level of extracellular sugars.

ABBREVIATIONS

ER: endoplasmic reticulum; GI-TGN: Golgi-released independent TGN; GFP: green fluorescent protein; mRFP: monomeric red fluorescent protein; P4H1.1: prolyl 4-hydroxylase 1.1; PM: plasma membrane; SCAMP2: secretory carrier membrane protein 2; SUT2: sucrose transporter 2; SVC: secretory vesicle cluster; SYP41: syntaxin of plant 41; TGN: trans-Golgi network; YFP: yellow fluorescent protein.

Advanced Microscopy Reveals Complex Developmental and Subcellular Localization Patterns of ANNEXIN 1 in Arabidopsis

Annexin 1 (ANN1) is the most abundant member of the evolutionary conserved multigene protein superfamily of annexins in plants. Generally, annexins participate in diverse cellular processes, such as cell growth, differentiation, vesicle trafficking, and stress responses. The expression of annexins is developmentally regulated, and it is sensitive to the external environment. ANN1 is expressed in almost all Arabidopsis tissues, while the most abundant is in the root, root hairs, and in the hypocotyl epidermal cells. Annexins were also occasionally proposed to associate with cytoskeleton and vesicles, but they were never developmentally localized at the subcellular level in diverse plant tissues and organs. Using advanced light-sheet fluorescence microscopy (LSFM), we followed the developmental and subcellular localization of GFP-tagged ANN1 in post-embryonic Arabidopsis organs. By contrast to conventional microscopy, LSFM allowed long-term imaging of ANN1-GFP in Arabidopsis plants at near-environmental conditions without affecting plant viability. We studied developmental regulation of ANN1-GFP expression and localization in growing Arabidopsis roots: strong accumulation was found in the root cap and epidermal cells (preferentially in elongating trichoblasts), but it was depleted in dividing cells localized in deeper layers of the root meristem. During root hair development, ANN1-GFP accumulated at the tips of emerging and growing root hairs, which was accompanied by decreased abundance in the trichoblasts. In aerial plant parts, ANN1-GFP was localized mainly in the cortical cytoplasm of trichomes and epidermal cells of hypocotyls, cotyledons, true leaves, and their petioles. At the subcellular level, ANN1-GFP was enriched at the plasma membrane (PM) and vesicles of non-dividing cells and in mitotic and cytokinetic microtubular arrays of dividing cells. Additionally, an independent immunolocalization method confirmed ANN1-GFP association with mitotic and cytokinetic microtubules (PPBs and phragmoplasts) in dividing cells of the lateral root cap. Lattice LSFM revealed subcellular accumulation of ANN1-GFP around the nuclear envelope of elongating trichoblasts. Massive relocation and accumulation of ANN1-GFP at the PM and in Hechtian strands and reticulum in plasmolyzed cells suggest a possible osmoprotective role of ANN1-GFP during plasmolysis/deplasmolysis cycle. This study shows complex developmental and subcellular localization patterns of ANN1 in living Arabidopsis plants.

High expression of GUS activities in sweet potato storage roots by sucrose-inducible minimal promoter

We developed transgenic sweet potato with Spo^min (sucrose-inducible minimal promoter)-GUS gene-fused constructs. Induced GUS activities by Spo^min were higher than those by CaMV 35S promoter. We developed transgenic sweet potato (Ipomoea batatas L. Lam. cv. Kokei no. 14) plants with Spo^min (sucrose-inducible minimal promoter)-GUS gene-fused constructs with signal peptides for sorting to cytosol, apoplast and ER, and we analyzed the GUS expression pattern of cut tissue after sucrose treatment. Induced GUS activities by Spo^min were several hundred times higher than those by the CaMV 35S promoter. Also, GUS activities in storage roots induced with a Spo^min-cytosol-GUS construct were higher than those with either Spo^min-apoplast or -ER-GUS constructs. The induced GUS activities by Spo^min were higher in storage roots without sucrose treatment than those with sucrose treatment. Chilling (4 °C) storage roots with Spo^min constructs for 4 weeks produced higher GUS activities than in storage roots stored at 25 °C for 4 weeks. The calculated maximum GUS content in the storage roots was up to about 224.2 μg/g fresh weight. The chilling treatment increased the free sucrose content in the storage roots, and this increase in endogenous sugar levels induced increased GUS activities in the storage roots. Therefore, Spo^min appears to be a useful promoter to develop protein production systems using sweet potato variety Kokei no. 14 storage roots by postharvest treatment.

Using Photoconvertible and Extractable Fluorescent Proteins to Study Autophagy in Plants

Several methodologies have been employed to understand the kinetics of induced autophagic degradation in plants, but most of them are not capable of distinguishing the autophagic cargo proteins before and after induction of autophagy in cells. Here, we designed a mass photoconverter that allowed us to simultaneously monitor protein synthesis and degradation in tobacco BY-2 cells using a photoconvertible fluorescence marker protein, Kikume Green Red (KikGR). An example of a new protocol for the analysis of autophagy progression using a fusion protein of cytochrome b5 and KikGR under phosphate starvation is described. The other example described is the analysis of the proliferation of Golgi apparatus in tobacco BY-2 cells using the fusion protein of a prolyl 4-hydroxylase NtP4H1.1 and monomeric KikGR. A detailed protocol on key analysis, as well as tips and notes for experiments using KikGR proteins, are described.

Gene expression and promoter analysis of a novel tomato aldo-keto reductase in response to environmental stresses

The functional role of an uncharacterized tomato (Solanum lycopersicum) aldo-keto reductase 4B, denoted as SlAKR4B, was investigated. The gene expression of tomato SlAKR4B was detected at a high level in the senescent leaves and the ripening fruits of tomato. Although d-galacturonic acid reductase activities tended to be higher in tomato SlAKR4B-overexpressing transgenic tobacco BY-2 cell lines than those in control cell lines, SlAKR4B gene expression was not well correlated with l-ascorbic acid content among the cell lines. The analysis of the transgenic cell lines showed that tomato SlAKR4B has enzyme activities toward d-galacturonic acid as well as glyceraldehyde and glyoxal, suggesting that the SlAKR4B gene encodes a functional enzyme in tomato. Gene expression of SlAKR4B was induced by NaCl, H2O2, and plant hormones such as salicylic acid and jasmonic acid, suggesting that SlAKR4B is involved in the stress response. The transient expression assay using protoplasts showed the promoter activity of the SlAKR4B gene was as high as that of the cauliflower mosaic virus 35S promoter. Also, the promoter region of the SlAKR4B gene was suggested to contain cis-element(s) for abiotic stress-inducible expression.

Membrane topology of Golgi-localized probable S-adenosylmethionine-dependent methyltransferase in tobacco (Nicotiana tabacum) BY-2 cells

S-adenosylmethionine (SAM)-dependent methyltransferases (MTases) transfer methyl groups to substrates. In this study, a novel putative tobacco SAM-MTase termed Golgi-localized methyl transferase 1 (GLMT1) has been characterized. GLMT1 is comprised of 611 amino acids with short N-terminal region, putative transmembrane region, and C-terminal SAM-MTase domain. Expression of monomeric red fluorescence protein (mRFP)-tagged protein in tobacco BY-2 cell indicated that GLMT1 is a Golgi-localized protein. Analysis of the membrane topology by protease digestion suggested that both C-terminal catalytic region and N-terminal region seem to be located to the cytosolic side of the Golgi apparatus. Therefore, GLMT1 might have a different function than the previously studied SAM-MTases in plants.

A comparative study of proteomic differences between pencil and storage roots of sweetpotato (Ipomoea batatas (L.) Lam.)

Fibrous roots of sweetpotato (Ipomoea batatas (L.) Lam.) usually develop into both pencil and storage roots. To understand protein function in root development, a proteomic analysis was conducted on the pencil and storage roots of the light orange-fleshed sweetpotato cultivar, Yulmi. Two-dimensional gel electrophoresis showed that expression of 30 protein spots differed between pencil and storage roots: 15 proteins were up-regulated or expressed in pencil roots and 15 in storage roots. Differentially expressed proteins spots were investigated using matrix-assisted laser desorption/ionization time of flight mass spectrometry, and 10 proteins from pencil roots were identified as binding protein isoform A, catechol oxidase, peroxidases, ascorbate peroxidase, endochitinase, flavanone 3-hydroxylase and unknown proteins. Of the proteins up-regulated in, or restricted to, storage roots, 13 proteins were identified as protein disulfide isomerase, anionic peroxidase, putative ripening protein, sporamin B, sporamin A and sporamin A precursor. An analysis of enzyme activity revealed that catechol oxidase and peroxidase as the first and last enzymes of the lignin biosynthesis pathway, and ascorbate peroxidase had higher activities in pencil than in storage roots. The total concentration of phenolic compounds was also far higher in pencil than in storage roots, and lignin accumulated only in pencil roots. These results provide important insight into sweetpotato proteomics, and imply that lignin biosynthesis and stress-related proteins are up-regulated or uniquely expressed in pencil roots. The results indicate that the reduction of carbon flow toward phenylpropanoid biosynthesis and its delivery to carbohydrate metabolism is a major event in storage root formation.

How vacuolar sorting receptor proteins interact with their cargo proteins: crystal structures of apo and cargo-bound forms of the protease-associated domain from an Arabidopsis vacuolar sorting receptor

In plant cells, soluble proteins are directed to vacuoles because they contain vacuolar sorting determinants (VSDs) that are recognized by vacuolar sorting receptors (VSR). To understand how a VSR recognizes its cargo, we present the crystal structures of the protease-associated domain of VSR isoform 1 from Arabidopsis thaliana (VSR1PA) alone and complexed with a cognate peptide containing the barley (Hordeum vulgare) aleurain VSD sequence of 1ADSNPIRPVT10. The crystal structures show that VSR1PA binds the sequence, Ala-Asp-Ser, preceding the NPIR motif. A conserved cargo binding loop, with a consensus sequence of 95RGxCxF100, forms a cradle that accommodates the cargo-peptide. In particular, Arg-95 forms a hydrogen bond to the Ser-3 position of the VSD, and the essential role of Arg-95 and Ser-3 in receptor-cargo interaction was supported by a mutagenesis study. Cargo binding induces conformational changes that are propagated from the cargo binding loop to the C terminus via conserved residues in switch I-IV regions. The resulting 180° swivel motion of the C-terminal tail is stabilized by a hydrogen bond between Glu-24 and His-181. A mutagenesis study showed that these two residues are essential for cargo interaction and trafficking. Based on our structural and functional studies, we present a model of how VSRs recognize their cargos.

Wide-range high-resolution transmission electron microscopy reveals morphological and distributional changes of endomembrane compartments during log to stationary transition of growth phase in tobacco BY-2 cells

Rapid growth of plant cells by cell division and expansion requires an endomembrane trafficking system. The endomembrane compartments, such as the Golgi stacks, endosome and vesicles, are important in the synthesis and trafficking of cell wall materials during cell elongation. However, changes in the morphology, distribution and number of these compartments during the different stages of cell proliferation and differentiation have not yet been clarified. In this study, we examined these changes at the ultrastructural level in tobacco Bright yellow 2 (BY-2) cells during the log and stationary phases of growth. We analyzed images of the BY-2 cells prepared by the high-pressure freezing/freeze substitution technique with the aid of an auto-acquisition transmission electron microscope system. We quantified the distribution of secretory and endosomal compartments in longitudinal sections of whole cells by using wide-range gigapixel-class images obtained by merging thousands of transmission electron micrographs. During the log phase, all Golgi stacks were composed of several thick cisternae. Approximately 20 vesicle clusters (VCs), including the trans-Golgi network and secretory vesicle cluster, were observed throughout the cell. In the stationary-phase cells, Golgi stacks were thin with small cisternae, and only a few VCs were observed. Nearly the same number of multivesicular body and small high-density vesicles were observed in both the stationary and log phases. Results from electron microscopy and live fluorescence imaging indicate that the morphology and distribution of secretory-related compartments dramatically change when cells transition from log to stationary phases of growth.

Identification of Novel Peptidyl Serine α-Galactosyltransferase Gene Family in Plants

In plants, serine residues in extensin, a cell wall protein, are glycosylated with O-linked galactose. However, the enzyme that is involved in the galactosylation of serine had not yet been identified. To identify the peptidyl serine O-α-galactosyltransferase (SGT), we chose Chlamydomonas reinhardtii as a model. We established an assay system for SGT activity using C. reinhardtii and Arabidopsis thaliana cell extracts. SGT protein was partially purified from cell extracts of C. reinhardtii and analyzed by tandem mass spectrometry to determine its amino acid sequence. The sequence matched the open reading frame XP_001696927 in the C. reinhardtii proteome database, and a corresponding DNA fragment encoding 748 amino acids (BAL63043) was cloned from a C. reinhardtii cDNA library. The 748-amino acid protein (CrSGT1) was produced using a yeast expression system, and the SGT activity was examined. Hydroxylation of proline residues adjacent to a serine in acceptor peptides was required for SGT activity. Genes for proteins containing conserved domains were found in various plant genomes, including A. thaliana and Nicotiana tabacum. The AtSGT1 and NtSGT1 proteins also showed SGT activity when expressed in yeast. In addition, knock-out lines of AtSGT1 and knockdown lines of NtSGT1 showed no or reduced SGT activity. The SGT1 sequence, which contains a conserved DXD motif and a C-terminal membrane spanning region, is the first example of a glycosyltransferase with type I membrane protein topology, and it showed no homology with known glycosyltransferases, indicating that SGT1 belongs to a novel glycosyltransferase gene family existing only in the plant kingdom.

Delivering of proteins to the plant vacuole--an update

Trafficking of soluble cargo to the vacuole is far from being a closed issue as it can occur by different routes and involve different intermediates. The textbook view of proteins being sorted at the post-Golgi level to the lytic vacuole via the pre-vacuole or to the protein storage vacuole mediated by dense vesicles is now challenged as novel routes are being disclosed and vacuoles with intermediate characteristics described. The identification of Vacuolar Sorting Determinants is a key signature to understand protein trafficking to the vacuole. Despite the long established vacuolar signals, some others have been described in the last few years, with different properties that can be specific for some cells or some types of vacuoles. There are also reports of proteins having two different vacuolar signals and their significance is questionable: a way to increase the efficiency of the sorting or different sorting depending on the protein roles in a specific context? Along came the idea of differential vacuolar sorting, suggesting a possible specialization of the trafficking pathways according to the type of cell and specific needs. In this review, we show the recent advances in the field and focus on different aspects of protein trafficking to the vacuoles.

ER and vacuoles: never been closer

The endoplasmic reticulum (ER) represents the gateway for intracellular trafficking of membrane proteins, soluble cargoes and lipids. In all eukaryotes, the best described mechanism of exiting the ER is via COPII-coated vesicles, which transport both membrane proteins and soluble cargoes to the cis-Golgi. The vacuole, together with the plasma membrane, is the most distal point of the secretory pathway, and many vacuolar proteins are transported from the ER through intermediate compartments. However, past results and recent findings demonstrate the presence of alternative transport routes from the ER towards the tonoplast, which are independent of Golgi- and post-Golgi trafficking. Moreover, the transport mechanism of the vacuolar proton pumps VHA-a3 and AVP1 challenges the current model of vacuole biogenesis, pointing to the endoplasmic reticulum for being the main membrane source for the biogenesis of the plant lytic compartment. This review gives an overview of the current knowledge on the transport routes towards the vacuole and discusses the possible mechanism of vacuole biogenesis in plants.

Polarized localization and borate-dependent degradation of the Arabidopsis borate transporter BOR1 in tobacco BY-2 cells

In Arabidopsis the borate transporter BOR1, which is located in the plasma membrane, is degraded in the presence of excess boron by an endocytosis-mediated mechanism. A similar mechanism was suggested in rice as excess boron decreased rice borate transporter levels, although in this case whether the decrease was dependent on an increase in degradation or a decrease in protein synthesis was not elucidated. To address whether the borate-dependent degradation mechanism is conserved among plant cells, we analyzed the fate of GFP-tagged BOR1 (BOR1-GFP) in transformed tobacco BY-2 cells. Cells expressing BOR1-GFP displayed GFP fluorescence at the plasma membrane, especially at the membrane between two attached cells. The plasma membrane signal was abolished when cells were incubated in medium with a high concentration of borate (3 to 5 mM). This decrease in BOR1-GFP signal was mediated by a specific degradation of the protein after internalization by endocytosis from the plasma membrane. Pharmacological analysis indicated that the decrease in BOR1-GFP largely depends on the increase in degradation rate and that the degradation was mediated by a tyrosine-motif and the actin cytoskeleton. Tyr mutants of BOR1-GFP, which has been shown to inhibit borate-dependent degradation in Arabidopsis root cells, did not show borate-dependent endocytosis in tobacco BY-2 cells. These findings indicate that the borate-dependent degradation machinery of the borate transporter is conserved among plant species.

Increase methylmercury accumulation in Arabidopsis thaliana expressing bacterial broad-spectrum mercury transporter MerE

The bacterial merE gene derived from the Tn21 mer operon encodes a broad-spectrum mercury transporter that governs the transport of methylmercury and mercuric ions across bacterial cytoplasmic membranes, and this gene is a potential molecular tool for improving the efficiency of methylmercury phytoremediation. A transgenic Arabidopsis engineered to express MerE was constructed and the impact of expression of MerE on methylmercury accumulation was evaluated. The subcellular localization of transiently expressed GFP-tagged MerE was examined in Arabidopsis suspension-cultured cells. The GFP-MerE was found to localize to the plasma membrane and cytosol. The transgenic Arabidopsis expressing MerE accumulated significantly more methymercury and mercuric ions into plants than the wild-type Arabidopsis did. The transgenic plants expressing MerE was significantly more resistant to mercuric ions, but only showed more resistant to methylmercury compared with the wild type Arabidopsis. These results demonstrated that expression of the bacterial mercury transporter MerE promoted the transport and accumulation of methylmercury in transgenic Arabidopsis, which may be a useful method for improving plants to facilitate the phytoremediation of methylmercury pollution.

An in vivo expression system for the identification of cargo proteins of vacuolar sorting receptors in Arabidopsis culture cells

Vacuolar sorting receptors (VSRs) are type I integral membrane family proteins that in plant cells are thought to recognize cargo proteins at the late Golgi or trans-Golgi network (TGN) for vacuolar transport via the pre-vacuolar compartment (PVC). However, little is known about VSR cargo proteins in plants. Here we developed and tested an in vivo expression system for the identification of VSR cargos which is based on the premise that the expressed N-terminus of VSRs will be secreted into the culture medium along with their corresponding cargo proteins. Indeed, transgenic Arabidopsis culture cell lines expressing VSR N-terminal binding domains (VSRNTs) were shown to secrete truncated VSRs (BP80NT, AtVSR1NT and AtVSR4NT) with attached cargo molecules into the culture medium. Putative cargo proteins were identified through mass spectrometry. Several identified cargo proteins were confirmed by localization studies and interaction analysis with VSRs. The screening strategy described here should be applicable to all VSRs and will help identify and study cargo proteins for individual VSR proteins. This method should be useful for both cargo identification and protein-protein interaction in vivo.

Evidence that proliferation of golgi apparatus depends on both de novo generation from the endoplasmic reticulum and formation from pre-existing stacks during the growth of tobacco BY-2 cells

In higher plants, the numbers of cytoplasmic-distributed Golgi stacks differ based on function, age and cell type. It has not been clarified how the numbers are controlled, whether all the Golgi apparatus in a cell function equally and whether the increase in Golgi number is a result of the de novo formation from the endoplasmic reticulum (ER) or fission of pre-existing stacks. A tobacco prolyl 4-hydroxylase (NtP4H1.1), which is a cis-Golgi-localizing type II membrane protein, was tagged with a photoconvertible fluorescent protein, mKikGR (monomeric Kikume green red), and expressed in tobacco bright yellow 2 (BY-2) cells. Transformed cells were exposed to purple light to convert the fluorescence from green to red. A time-course analysis after the conversion revealed a progressive increase in green puncta and a decrease in the red puncta. From 3 to 6 h, we observed red, yellow and green fluorescent puncta corresponding to pre-existing Golgi; Golgi containing both pre-existing and newly synthesized protein; and newly synthesized Golgi. Analysis of the number and fluorescence of Golgi at different phases of the cell cycle suggested that an increase in Golgi number with both division and de novo synthesis occurred concomitantly with DNA replication. Investigation with different inhibitors suggested that the formation of new Golgi and the generation of Golgi containing both pre-existing and newly synthesized protein are mediated by different machineries. These results and modeling based on quantified results indicate that the Golgi apparatuses in tobacco BY-2 cells are not uniform and suggest that both de novo synthesis from the ER and Golgi division contribute almost equally to the increase in proliferating cells.

Development of the gateway recycling cloning system for multiple linking of expression cassettes in a defined order, and direction on gateway compatible binary vectors

We developed the Gateway recycling cloning system, which allows multiple linking of expression cassettes by multiple rounds of the Gateway LR reaction. Employing this system, the recycling donor vector pRED419 was subjected to the first LR reaction with an attR1-attR2 type destination vector. Then conversion vector pCON was subjected to an LR reaction to restore the attR1-attR2 site on the destination vector for the next cloning cycle. By repetition of these two simple steps, we linked four expression cassettes of a reporter gene in Gateway binary vector pGWB1, introduced the constructs into tobacco BY-2 cells, and observed the expression of transgenes.

Mechanisms and concepts paving the way towards a complete transport cycle of plant vacuolar sorting receptors

Delivery of proteins to the lytic vacuole in plants is a complex cascade of selective interactions that specifically excludes residents of the endoplasmic reticulum and secreted proteins. Vacuolar transport must be highly efficient to avoid mistargeting of hydrolytic enzymes to locations where they could be harmful. While plant vacuolar sorting signals have been well described for two decades, it is only during the last 5 years that a critical mass of data was gathered that begins to reveal how vacuolar sorting receptors (VSRs) may complete a full transport cycle. Yet, the field is far from reaching a consensus regarding the organelles that could be involved in vacuolar sorting, their potential biogenesis, and the ultimate recycling of membranes and protein machinery that maintain this pathway. This review will highlight the important landmarks in our understanding of VSR function and compare recent transport models that have been proposed so that an emerging picture of plant vacuolar sorting mechanisms can be drawn.

A vacuolar β-glucosidase homolog that possesses glucose-conjugated abscisic acid hydrolyzing activity plays an important role in osmotic stress responses in Arabidopsis

The phytohormone abscisic acid (ABA) plays a critical role in various physiological processes, including adaptation to abiotic stresses. In Arabidopsis thaliana, ABA levels are increased both through de novo biosynthesis and via β-glucosidase homolog1 (BG1)-mediated hydrolysis of Glc-conjugated ABA (ABA-GE). However, it is not known how many different β-glucosidase proteins produce ABA from ABA-GE and how the multiple ABA production pathways are coordinated to increase ABA levels. Here, we report that a previously undiscovered β-glucosidase homolog, BG2, produced ABA by hydrolyzing ABA-GE and plays a role in osmotic stress response. BG2 localized to the vacuole as a high molecular weight complex and accumulated to high levels under dehydration stress. BG2 hydrolyzed ABA-GE to ABA in vitro. In addition, BG2 increased ABA levels in protoplasts upon application of exogenous ABA-GE. Overexpression of BG2 rescued the bg1 mutant phenotype, as observed for the overexpression of NCED3 in bg1 mutants. Multiple Arabidopsis bg2 alleles with a T-DNA insertion in BG2 were more sensitive to dehydration and NaCl stress, whereas BG2 overexpression resulted in enhanced resistance to dehydration and NaCl stress. Based on these observations, we propose that, in addition to the de novo biosynthesis, ABA is produced in multiple organelles by organelle-specific β-glucosidases in response to abiotic stresses.

Secretory pathway research: the more experimental systems the better

Transient gene expression, in plant protoplasts or specific plant tissues, is a key technique in plant molecular cell biology, aimed at exploring gene products and their modifications to examine functional subdomains, their interactions with other biomolecules, and their subcellular localization. Here, we highlight some of the major advantages and potential pitfalls of the most commonly used transient gene expression models and illustrate how ectopic expression and the use of dominant mutants can provide insights into protein function.

Expression of the bacterial heavy metal transporter MerC fused with a plant SNARE, SYP121, in Arabidopsis thaliana increases cadmium accumulation and tolerance

The bacterial merC gene from the Tn21-encoded mer operon is a potential molecular tool for improving the efficiency of metal phytoremediation. Arabidopsis SNARE molecules, including SYP111, SYP121, and AtVAM3 (SYP22), were attached to the C-terminus of MerC to target the protein to various organelles. The subcellular localization of transiently expressed GFP-fused MerC-SYP111, MerC-SYP121, and MerC-AtVAM3 was examined in Arabidopsis suspension-cultured cells. We found that GFP-MerC-SYP111 and GFP-MerC-SYP121 localized to the plasma membrane, whereas GFP-AtVAM3 localized to the vacuolar membranes. These results demonstrate that SYP111/SYP121 and AtVAM3 target foreign molecules to the plasma membrane and vacuolar membrane, respectively. To enhance the efficiency and potential of plants to sequester and accumulate cadmium from contaminated sites, transgenic Arabidopsis plants expressing MerC, MerC-SYP111, MerC-SYP121, or MerC-AtVAM3 were generated. The transgenic plants that expressed MerC, MerC-SYP121, or MerC-AtVAM3 appeared to be normal, whereas the transgenic that expressed MerC-SYP111 exhibited severe growth defects. The transgenic plants expressing merC-SYP121 were more resistant to cadmium than the wild type and accumulated significantly more cadmium. Thus, the expression of MerC-SYP121 in the plant plasma membrane may provide an ecologically compatible approach for the phytoremediation of cadmium pollution.

Multiple biological functions of sporamin related to stress tolerance in sweet potato (Ipomoea batatas Lam)

The initial investigation of the nature of the proteins in the tuber of sweet potato (Ipomoea batatas Lam.) revealed a globulin-designated "ipomoein," which was reported by Jones and Gersdorff, (1931). Later, "ipomoein" was renamed "sporamin" and was found to be a major storage protein that accounted for over 80% of the total protein in the tuberous root. To date, sporamin has been studied by a series of biochemical and molecular approaches. The first purification of sporamin into two major fractions, A and B, was successfully completed in 1985. Several characteristics of the protein, such as the diversification of the nucleotide sequences in the gene family, the protein structure, the biological functions of storage, defense, inhibitory activity and ROS scavenging, were identified. In the past decade, sporamin was classified as a Kunitz-type trypsin inhibitor, and its insect-resistance capability has been examined in transgenic tobacco and cauliflower plants, indicating the multiple functions of this protein has evolved to facilitate the growth and development of sweet potato. Sporamin is constitutively expressed in the tuberous root and is not normally expressed in the stem or leaves. However, this protein is expressed systemically in response to wounding and other abiotic stresses. These dual expression patterns at the transcriptional level revealed that the complex regulatory mechanism of sporamin was modulated by environmental stresses. The versatile functions of sporamin make this storage protein a good research model to study molecular evolution, regulatory mechanisms and physiological functions in plants. This review summarizes and discusses recent approaches and future perspectives in agricultural biotechnology.

Subcellular targeting and biosynthesis of cyclotides in plant cells

PREMISE OF THE STUDY

The cyclotide kalata B1 is found in the leaves of Oldenlandia affinis and is a potent insecticidal and nematocidal molecule. This peptide is cleaved from a precursor protein, Oak1, and ligation of the N- and C-termini occurs to form a continuous peptide backbone. The subcellular location of the excision and cyclization reactions is unknown, and there is debate as to which enzyme catalyzes the event. To determine where in the plant cell Oak1 is processed, we prepared constructs encoding GFP (green fluorescent protein) linked to the cyclotide precursor Oak1.

METHODS

The GFP constructs were transiently expressed in the leaves of Nicotiana benthamiana, and GFP fluorescence was observed in living cells using confocal microscopy. A Fei Mao (FM) styryl dye was infiltrated into whole leaves that were still growing and expressing GFP constructs, enabling the plasma membrane and the tonoplast to be highlighted for visualization of the vacuole in living cells.

KEY RESULTS

The full length Oak1 precursor directed GFP to the vacuole, suggesting that excision and cyclization of the cyclotide domain occurs in the vacuole where the cyclotides are then stored. The N-terminal propeptide and N-terminal repeat of Oak1 were both sufficient to target GFP to the vacuole, although the C-terminal propeptide, which is essential for cyclization, was not a targeting signal.

CONCLUSIONS

The vacuolar location of cyclotides supports our hypothesis that the vacuolar processing enzyme, asparaginyl endoproteinase, has a pivotal role in excision and cyclization from cyclotide precursors.

N-terminal vacuolar sorting signal at the mouse antibody alters the N-linked glycosylation pattern in suspension-cultured tobacco BY2 cells

Recombinant DNA technology enables the use of plants as the host for the production of pharmaceutical proteins, such as antibodies. The glycosylation of recombinant proteins plays physiological and biological roles. However, because glycosylation in plants is different from that in human cells, the development of glycoengineering is required. In plant cells, glycan structures are shown to correlate with the localization of the recombinant protein produced. In this study, the vacuolar sorting signal (VSS) of sporamin was fused to the heavy (H) and light (L) chains of a mouse monoclonal antibody (mAb), and the mAb was produced in suspension-cultured tobacco BY2 cells. The sugar chain structures were determined by high-performance liquid chromatography, exoglycosidase digestion, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Typical plant glycans with α1,3-fucosylation and/or β1,2-xylosylation derived from mAb with the VSS-fused H-chain (mIgG1000) and mAb with the VSS-fused H- and L-chain (mIgG1010) occupied the large amount of the total N-glycans, 72.1% and 85.0%, respectively, such as those derived from mAb without VSS (mIgG0000), 74.6% (Fujiyama et al., J. Biosci. Bioeng., 101, 212-218, 2006). In contrast, the typical plant glycan structure Man₃FucXylGlcNAc₂ particularly in vacuoles accounted for 37.8% of the total sugar chains derived from mIgG1000 and 58.5% of those derived from mIgG1010 compared with 24.3% of those derived from mIgG0000. These results suggest that the sporamin signal peptide fused to mAb acts as a VSS and leads to the increase in the amount of Man₃FucXylGlcNAc₂, which is the main N-glycan structure in vacuoles.

Identification of sorting motifs of AtβFruct4 for trafficking from the ER to the vacuole through the Golgi and PVC

Although much is known about the molecular mechanisms involved in transporting soluble proteins to the central vacuole, the mechanisms governing the trafficking of membrane proteins remain largely unknown. In this study, we investigated the mechanism involved in targeting the membrane protein, AtβFructosidase 4 (AtβFruct4), to the central vacuole in protoplasts. AtβFruct4 as a green fluorescent protein (GFP) fusion protein was transported as a membrane protein during transit from the endoplasmic reticulum (ER) through the Golgi apparatus and the prevacuolar compartment (PVC). The N-terminal cytosolic domain of AtβFruct4 was sufficient for transport from the ER to the central vacuole and contained sequence motifs required for trafficking. The sequence motifs, LL and PI, were found to be critical for ER exit, while the EEE and LCPYTRL sequence motifs played roles in trafficking primarily from the trans Golgi network (TGN) to the PVC and from the PVC to the central vacuole, respectively. In addition, actin filaments and AtRabF2a, a Rab GTPase, played critical roles in vacuolar trafficking at the TGN and PVC, respectively. On the basis of these results, we propose that the vacuolar trafficking of AtβFruct4 depends on multiple sequence motifs located at the N-terminal cytoplasmic domain that function as exit and/or sorting signals in different stages during the trafficking process.

Construction of a Der p2-transgenic plant for the alleviation of airway inflammation

In clinical therapy, the amount of antigen administered to achieve oral tolerance for allergic diseases is large, and the cost is a major consideration. In this study, we used tobacco plants to develop a large-scale protein production system for allergen-specific immunotherapy, and we investigated the mechanisms of oral tolerance induced by a transgenic plant-derived antigen. We used plants (tobacco leaves) transgenic for the Dermatophagoides pteronyssinus 2 (Der p2) antigen to produce Der p2. Mice received total protein extract from Der p2 orally once per day over 6 days (days 0-2 and days 6-8). Mice were also sensitized and challenged with yeast-derived recombinant Der p2 (rDer p2), after which the mice were examined for airway hyper-responsiveness and airway inflammation. After sensitization and challenge with rDer p2, mice that were fed with total protein extracted from transgenic plants showed decreases in serum Der p2-specific IgE and IgG1 titers, decreased IL-5 and eotaxin levels in bronchial alveolar lavage fluid, and eosinophil infiltration in the airway. In addition, hyper-responsiveness was also decreased in mice that were fed with total protein extracted from transgenic plants, and CD4(+)CD25(+)Foxp3(+) regulatory T cells were significantly increased in mediastinal and mesenteric lymph nodes. Furthermore, splenocytes isolated from transgenic plant protein-fed mice exhibited decreased proliferation and increased IL-10 secretion after stimulation with rDer p2. The data here suggest that allergen-expressing transgenic plants could be used for therapeutic purposes for allergic diseases.

Activity of the C-terminal-dependent vacuolar sorting signal of horseradish peroxidase C1a is enhanced by its secondary structure

Plant class III peroxidase (PRX) catalyzes the oxidation and oxidative polymerization of a variety of phenolic compounds while reducing hydrogen peroxide. PRX proteins are classified into apoplast type and vacuole type based on the absence or the presence of C-terminal propeptides, which probably function as vacuolar sorting signals (VSSs). In this study, in order to improve our understanding of vacuole-type PRX, we analyzed regulatory mechanisms of vacuolar sorting of a model vacuole-type PRX, the C1a isozyme of horseradish (Armoracia rusticana) (HRP C1a). Using cultured transgenic tobacco cells and protoplasts derived from horseradish leaves, we characterized HRP C1a's VSS, which is a 15 amino acid C-terminal propeptide (C15). We found that the C-terminal hexapeptide of C15 (C6), which is well conserved among vacuole-type PRX proteins, forms the core of the C-terminal-dependent VSS. We also found that the function of C6 is enhanced by the remaining N-terminal part of C15 which probably folds into an amphiphilic α-helix.

The dual-targeted purple acid phosphatase isozyme AtPAP26 is essential for efficient acclimation of Arabidopsis to nutritional phosphate deprivation

Induction of intracellular and secreted acid phosphatases (APases) is a widespread response of orthophosphate (Pi)-starved (-Pi) plants. APases catalyze Pi hydrolysis from a broad range of phosphomonoesters at an acidic pH. The largest class of nonspecific plant APases is comprised of the purple APases (PAPs). Although the biochemical properties, subcellular location, and expression of several plant PAPs have been described, their physiological functions have not been fully resolved. Recent biochemical studies indicated that AtPAP26, one of 29 PAPs encoded by the Arabidopsis (Arabidopsis thaliana) genome, is the predominant intracellular APase, as well as a major secreted APase isozyme up-regulated by -Pi Arabidopsis. An atpap26 T-DNA insertion mutant lacking AtPAP26 transcripts and 55-kD immunoreactive AtPAP26 polypeptides exhibited: (1) 9- and 5-fold lower shoot and root APase activity, respectively, which did not change in response to Pi starvation, (2) a 40% decrease in secreted APase activity during Pi deprivation, (3) 35% and 50% reductions in free and total Pi concentration, respectively, as well as 5-fold higher anthocyanin levels in shoots of soil-grown -Pi plants, and (4) impaired shoot and root development when subjected to Pi deficiency. By contrast, no deleterious influence of AtPAP26 loss of function occurred under Pi-replete conditions, or during nitrogen or potassium-limited growth, or oxidative stress. Transient expression of AtPAP26-mCherry in Arabidopsis suspension cells verified that AtPAP26 is targeted to the cell vacuole. Our results confirm that AtPAP26 is a principal contributor to Pi stress-inducible APase activity, and that it plays an important role in the Pi metabolism of -Pi Arabidopsis.

Functional analysis of an Arabidopsis thaliana abiotic stress-inducible facilitated diffusion transporter for monosaccharides

Sugars play indispensable roles in biological reactions and are distributed into various tissues or organelles via transporters in plants. Under abiotic stress conditions, plants accumulate sugars as a means to increase stress tolerance. Here, we report an abiotic stress-inducible transporter for monosaccharides from Arabidopsis thaliana that is termed ESL1 (ERD six-like 1). Expression of ESL1 was induced under drought and high salinity conditions and with exogenous application of abscisic acid. Promoter analyses using beta-glucuronidase and green fluorescent protein reporters revealed that ESL1 is mainly expressed in pericycle and xylem parenchyma cells. The fluorescence of ESL1-green fluorescent protein-fused protein was detected at tonoplast in transgenic Arabidopsis plants and tobacco BY-2 cells. Furthermore, alanine-scanning mutagenesis revealed that an N-terminal LXXXLL motif in ESL1 was essential for its localization at the tonoplast. Transgenic BY-2 cells expressing mutated ESL1, which was localized at the plasma membrane, showed an uptake ability for monosaccharides. Moreover, the value of K(m) for glucose uptake activity of mutated ESL1 in the transgenic BY-2 cells was extraordinarily high, and the transport activity was independent from a proton gradient. These results indicate that ESL1 is a low affinity facilitated diffusion transporter. Finally, we detected that vacuolar invertase activity was increased under abiotic stress conditions, and the expression patterns of vacuolar invertase genes were similar to that of ESL1. Under abiotic stress conditions, ESL1 might function coordinately with the vacuolar invertase to regulate osmotic pressure by affecting the accumulation of sugar in plant cells.

AtGRP5, a vacuole-located glycine-rich protein involved in cell elongation

Although several glycine-rich protein (GRP) genes were isolated and characterized, very little is known about their function. The primary structure of AtGRP5 from Arabidopsis thaliana has a signal peptide followed by a region with high glycine content. In this work, green fluorescent protein fusions were obtained in order to characterize the sub-cellular localization of the AtGRP5 protein. The results indicated that this protein is the first described vacuolar GRP. Sense, antisense and RNAi transgenic A. thaliana plants were generated and analyzed phenotypically. Plants overexpressing AtGRP5 showed longer roots and an enhanced elongation of the inflorescence axis, while antisense and RNAi plants demonstrated the opposite phenotype. The analysis of a knockout T-DNA line corroborates the phenotypes obtained with the antisense and RNAi plants. Altogether, these results suggest that this vacuolar GRP could be involved in organ growth by promoting cell elongation processes.

A mobile secretory vesicle cluster involved in mass transport from the Golgi to the plant cell exterior

Secretory proteins and extracellular glycans are transported to the extracellular space during cell growth. These materials are carried in secretory vesicles generated at the trans-Golgi network (TGN). Analysis of the mammalian post-Golgi secretory pathway demonstrated the movement of separated secretory vesicles in the cell. Using secretory carrier membrane protein 2 (SCAMP2) as a marker for secretory vesicles and tobacco (Nicotiana tabacum) BY-2 cell as a model cell, we characterized the transport machinery in plant cells. A combination of analyses, including electron microscopy of quick-frozen cells and four-dimensional analysis of cells expressing fluorescent-tagged SCAMP2, enabled the identification of a clustered structure of secretory vesicles generated from TGN that moves in the cell and eventually fuses with plasma membrane. This structure was termed the secretory vesicle cluster (SVC). The SVC was also found in Arabidopsis thaliana and rice (Oryza sativa) cells and moved to the cell plate in dividing tobacco cells. Thus, the SVC is a motile structure involved in mass transport from the Golgi to the plasma membrane and cell plate in plant cells.

Phosphorylation modification of wheat lectin VER2 is associated with vernalization-induced O-GlcNAc signaling and intracellular motility

BACKGROUND

O-linked beta-N-acetylglucosamine (O-GlcNAc) modification of proteins mediates stress response and cellular motility in animal cells. The plant lectin concanavalin A can increase nuclear O-GlcNAc levels and decrease cytoplasmic O-GlcNAc levels in T lymphocytes. However, the functions of O-GlcNAc signaling in plants, as well as the relation between plant lectins and O-GlcNAc in response to environmental stimuli are largely undefined.

METHODOLOGY/PRINCIPAL FINDINGS

We describe a jacalin-like lectin VER2 in wheat that shows N-acetylglucosamine and galactose specificity. Immunocytochemical localization showed VER2 expression induced predominantly at potential nuclear structures in shoot tips and young leaves and weakly in cytoplasm in response to vernalization. In contrast, under devernalization (continuous stimulation with a higher temperature after vernalization), VER2 signals appeared predominantly in cytoplasm. 2-D electrophoresis, together with western blot analysis, showed phosphorylation modification of VER2 under vernalization. Immunoblot assay with O-GlcNAc-specific antibody revealed that vernalization increased O-GlcNAc modification of proteins at the global level. An O-GlcNAc-modified protein co-immunoprecipitated with VER2 in vernalized wheat plants but not in devernalized materials. The dynamic of VER2 was observed in transgenic Arabidopsis overexpressing the VER2-GFP fusion protein. Overexpressed VER2 accelerated nuclear migration. Immunogold labeling and indirect immunofluoresence colocalization assay indicated that VER2-GFP was targeted to the secretory pathway.

CONCLUSIONS/SIGNIFICANCE

O-GlcNAc signaling is involved in the vernalization response in wheat, and phosphorylation is necessary for the lectin VER2 involving O-GlcNAc signaling during vernalization. Our findings open the way to studies of O-GlcNAc protein modification in response to environmental signals in plants.

Chimeric fluorescent fusion proteins to monitor autophagy in plants

Autophagy is induced under nutrient-deficient conditions in both growing tobacco BY-2 cultured cells as well as Arabidopsis and others intact plants. The fluorescent protein-tagged structural protein for autophagosomes, the Atg8 protein, allows nondestructive detection of autophagy induction in plant cells and tissues by fluorescence microscopy. Using this technique, the general operation of autophagy in growing root cells has been observed. A synthetic cargo protein for autophagy consisting of cytochrome b5 and the red fluorescence protein, DsRed, allows for the quantitative assay of autophagy in tobacco cells. This chapter describes methods for detecting autophagy in these plant cells using fluorescent protein fusions in situ with light microscopy, as well as quantification of autophagy.

Vacuolar sorting behaviors of 11S globulins in plant cells

Plant seed cells amass storage proteins that are synthesized on the endoplasmic reticulumn (ER) and then transported to protein storage vacuoles (PSVs). Many dicotyledonous seeds contain 11S globulin (11S) as a major storage protein. We investigated the accumulation behaviors of pea and pumpkin 11S during seed maturation and compared them with soybean 11S biogenesis (Mori et al., 2004). The accumulation of pea 11S in seeds was very similar to that of soybean 11S at all the development stages we examined, whereas pumpkin 11S condensed in the ER. The determinant of accumulation behavior might be the surface hydrophobicity of 11S. Further, we examined the accumulation of 11Ss in tobacco BY-2 cells to analyze behavior in the same environment. 11Ss expressed in BY2 cells were all observed in precursor form (pro11S). Pro11S with high surface hydrophobicity might be transported to vacuoles in a multivesicular body-mediated pathway when the expression level remains low.

Expression of modified gene encoding functional human alpha-1-antitrypsin protein in transgenic tomato plants

Transgenic plants offer promising alternative for large scale, sustainable production of safe, functional, recombinant proteins of therapeutic and industrial importance. Here, we report the expression of biologically active human alpha-1-antitrypsin in transgenic tomato plants. The 1,182 bp cDNA sequence of human AAT was strategically designed, modified and synthesized to adopt codon usage pattern of dicot plants, elimination of mRNA destabilizing sequences and modifications around 5' and 3' flanking regions of the gene to achieve high-level regulated expression in dicot plants. The native signal peptide sequence was substituted with modified signal peptide sequence of tobacco (Nicotiana tabacum) pathogenesis related protein PR1a, sweet potato (Ipomoea batatas) sporamineA and with dicot-preferred native signal peptide sequence of AAT gene. A dicot preferred translation initiation context sequence, 38 bp alfalfa mosaic virus untranslated region were incorporated at 5' while an endoplasmic reticulum retention signal (KDEL) was incorporated at 3' end of the gene. The modified gene was synthesized by PCR based method using overlapping oligonucleotides. Tomato plants were genetically engineered by nuclear transformation with Agrobacterium tumefaciens harbouring three different constructs pPAK, pSAK and pNAK having modified AAT gene with different signal peptide sequences under the control of CaMV35S duplicated enhancer promoter. Promising transgenic plants expressing recombinant AAT protein upto 1.55% of total soluble leaf protein has been developed and characterized. Plant-expressed recombinant AAT protein with molecular mass of around approximately 50 kDa was biologically active, showing high specific activity and efficient inhibition of elastase activity. The enzymatic deglycosylation established proper glycosylation of the plant-expressed recombinant AAT protein in contrast to unglycosylated rAAT expressed in E. coli ( approximately 45 kDa). Our results demonstrate feasibility for high-level expression of biologically active, glycosylated human alpha-1-antitrypsin in transgenic tomato plants.

Characterization of a tobacco TPK-type K+ channel as a novel tonoplast K+ channel using yeast tonoplasts

The tonoplast K(+) membrane transport system plays a crucial role in maintaining K(+) homeostasis in plant cells. Here, we isolated cDNAs encoding a two-pore K(+) channel (NtTPK1) from Nicotiana tabacum cv. SR1 and cultured BY-2 tobacco cells. Two of the four variants of NtTPK1 contained VHG and GHG instead of the GYG signature sequence in the second pore region. All four products were functional when expressed in the Escherichia coli cell membrane, and NtTPK1 was targeted to the tonoplast in tobacco cells. Two of the three promoter sequences isolated from N. tabacum cv. SR1 were active, and expression from these was increased approximately 2-fold by salt stress or high osmotic shock. To determine the properties of NtTPK1, we enlarged mutant yeast cells with inactivated endogenous tonoplast channels and prepared tonoplasts suitable for patch clamp recording allowing the NtTPK1-related channel conductance to be distinguished from the small endogenous currents. NtTPK1 exhibited strong selectivity for K(+) over Na(+). NtTPK1 activity was sensitive to spermidine and spermine, which were shown to be present in tobacco cells. NtTPK1 was active in the absence of Ca(2+), but a cytosolic concentration of 45 microM Ca(2+) resulted in a 2-fold increase in the amplitude of the K(+) current. Acidification of the cytosol to pH 5.5 also markedly increased NtTPK1-mediated K(+) currents. These results show that NtTPK1 is a novel tonoplast K(+) channel belonging to a different group from the previously characterized vacuolar channels SV, FV, and VK.

Transgenic plants for animal health: plant-made vaccine antigens for animal infectious disease control

A variety of plant species have been genetically modified to accumulate vaccine antigens for human and animal health and the first vaccine candidates are approaching the market. The regulatory burden for animal vaccines is less than that for human use and this has attracted the attention of researchers and companies, and investment in plant-made vaccines for animal infectious disease control is increasing. The dosage cost of vaccines for animal infectious diseases must be kept to a minimum, especially for non-lethal diseases that diminish animal welfare and growth, so efficient and economic production, storage and delivery are critical for commercialization. It has become clear that transgenic plants are an economic and efficient alternative to fermentation for large-scale production of vaccine antigens. The oral delivery of plant-made vaccines is particularly attractive since the expensive purification step can be avoided further reducing the cost per dose. This review covers the current status of plant-produced vaccines for the prevention of disease in animals and focuses on barriers to the development of such products and methods to overcome them.

Development of series of gateway binary vectors, pGWBs, for realizing efficient construction of fusion genes for plant transformation

We developed a new series of binary vectors useful for Gateway cloning to facilitate transgenic experiments in plant biotechnology. The new system, Gateway Binary Vectors (pGWBs) realized efficient cloning, constitutive expression using the cauliflower mosaic virus (CaMV) 35S promoter and the construction of fusion genes by simple clonase reaction with an entry clone. The reporters employable in this system are beta-glucuronidase (GUS), synthetic green fluorescent protein with S65T mutation (sGFP), luciferase (LUC), enhanced yellow fluorescent protein (EYFP), and enhanced cyan fluorescent protein (ECFP). The tags available are 6xHis, FLAG, 3xHA, 4xMyc, 10xMyc, GST, T7-epitope, and tandem affinity purification (TAP). In total, 13 kinds of reporter or tag were arranged and were almost applicable to both N- and C-fusions. The pGWBs could be used for many purposes, such as promoter::reporter analysis, observation of subcellular localization by the expression of proteins fused to a reporter or tag, and analysis of protein-protein interaction by copurification and immunodetection experiments. The pGWBs were constructed with modified pBI101 containing a CaMV35S promoter-driven hygromycin phosphotransferase (HPT) gene as the second selection marker. We also constructed pGWBs with the marker HPT driven by the nopaline synthase promoter. By using the pGWB system, the expression of tagged proteins, and the localization of GFP-fused proteins were easily analyzed. Moreover, tissue-specific and inducible gene expression using a promoter was also monitored with pGWBs. It is expected that, the pGWB system will serve as a powerful tool for plasmid construction in plant research.

Comprehensive analysis of glucan elicitor-regulated gene expression in tobacco BY-2 cells reveals a novel MYB transcription factor involved in the regulation of phenylpropanoid metabolism

We previously demonstrated that a beta-1,3-, 1,6-oligoglucan (AaGlucan) from the fungus Alternaria alternata 102 shows strong elicitor activity in tobacco BY-2 cells. We have used cDNA microarray analysis to monitor global changes in gene expression in tobacco cells treated with this A. alternata fraction or with laminarin. In total, we identified 265 genes that were induced 1 h after treatment with an AaGlucan-enriched fraction or laminarin. Among them, we characterized in detail a novel tobacco R2R3 MYB-type transcription factor homolog (NtMYBGR1) and two DC1 domain-containing genes (NtDC1A and NtDC1B). Microarray data, together with overexpression and metabolic analyses, indicated that NtMYBGR1, but not the NtDC1 proteins, primarily targets the phenylpropanoid synthesis-related genes PAL and 4CL. These results suggest that NtMYBGR1 specifically regulates defense responses in BY-2 cells by enhancing phenylpropanoid metabolism in response to AaGlucan and laminarin elicitors.