Enhancers and core promoter elements are essential for the activity of a cryptic gene activation sequence from tobacco, tCUP

Plant production of a virus-like particle-based vaccine candidate against porcine reproductive and respiratory syndrome

Porcine reproductive and respiratory syndrome (PRRS) is a disease leading to spontaneous abortions and stillbirths in sows and lowered life quality and expectancy in growing pigs. PRRS is prevalent worldwide and has significant economic impacts to swine industries around the globe. Co-expression of the two most abundant proteins in the viral envelope, the matrix protein (M) and glycosylated protein 5 (GP5), can produce a neutralizing immune response for the virus providing a potentially effective subunit vaccine against the disease, but these proteins are difficult to express. The goal of this research was to display antigenic portions of the M and GP5 proteins on the surface of tobacco mosaic virus-like particles. A modified tobacco mosaic virus coat protein (TMVc) was transiently expressed in Nicotiana benthamiana leaves and targeted to three subcellular compartments along the secretory pathway to introduce glycosylation patterns important for M-GP5 epitope immunogenicity. We found that accumulation levels in the apoplast were similar to the ER and the vacuole. Because glycans present on plant apoplastic proteins are closest to those present on PRRSV proteins, a TMVc-M-GP5 fusion construct was targeted to the apoplast and accumulated at over 0.5 mg/g of plant fresh weight. TMVc virus-like particles self-assembled in plant cells and surface-displayed the M-GP5 epitope, as visualized by transmission electron microscopy and immunogold localization. These promising findings lay the foundation for immunogenicity and protective-immunity studies in animals to examine the efficacy of this vaccine candidate as a measure to control PRRS.

CRISPR-Mediated Engineering across the Central Dogma in Plant Biology for Basic Research and Crop Improvement

The central dogma (CD) of molecular biology is the transfer of genetic information from DNA to RNA to protein. Major CD processes governing genetic flow include the cell cycle, DNA replication, chromosome packaging, epigenetic changes, transcription, posttranscriptional alterations, translation, and posttranslational modifications. The CD processes are tightly regulated in plants to maintain genetic integrity throughout the life cycle and to pass genetic materials to next generation. Engineering of various CD processes involved in gene regulation will accelerate crop improvement to feed the growing world population. CRISPR technology enables programmable editing of CD processes to alter DNA, RNA, or protein, which would have been impossible in the past. Here, an overview of recent advancements in CRISPR tool development and CRISPR-based CD modulations that expedite basic and applied plant research is provided. Furthermore, CRISPR applications in major thriving areas of research, such as gene discovery (allele mining and cryptic gene activation), introgression (de novo domestication and haploid induction), and application of desired traits beneficial to farmers or consumers (biotic/abiotic stress-resilient crops, plant cell factories, and delayed senescence), are described. Finally, the global regulatory policies, challenges, and prospects for CRISPR-mediated crop improvement are discussed.

Spatiotemporal activities of Douglas-fir BiP Pro1 promoter in transgenic potato

The PmBiPPro1 promoter of the luminal binding protein (BiP) from Douglas-fir is fully functional in transgenic potato, responsive to wounding, and has high transcriptional activity in tubers. A predefined pattern and level of transgene expression targeted to specific tissues or organs and at a particular developmental stage is a pre-requisite for the successful development of plants with desired traits. Here, we evaluated the transcriptional activity of the PmBiPPro1 promoter of the luminal binding protein (BiP) from Douglas-fir, by expressing reporter β-D-glucuronidase (GUS) gene constructs containing three different PmBiPPro1 promoter versions (2258 bp, 1259 bp, and 278 bp) in transgenic potato. In conifers, this promoter regulates the endoplasmic reticulum (ER) molecular chaperon of the HSP70 stress-related protein family and is essential for proper functioning of the ER. Stable expression analysis demonstrated that two of three PmBiPPro1 promoter versions (PmBiPPro1-1 and PmBiPPro1-3) were fully functional in the heterologous host, exhibited high transcriptional activities in the leaves of unstressed potatoes, and were responsive to wounding. Deletion analysis showed that the positive cis-active regulatory elements necessary for higher level expression resided within the - 1243 to - 261 region, whereas negative cis-active elements encompassed nucleotides - 2242 to - 1243. Histochemical staining revealed high level of GUS activities in tissues associated with a high rate of cell division and secretory activities. Most importantly, the PmBiPPro1 promoters, especially the full-length version, had activity in microtubers at a level that was much higher than in any other potato organ or tissue. The - 2242 to - 1243 bp region likely contains important cis element(s) that interact with tuber-specific transcription factors required for promoter activation in the storage organs. The organ-specific activity of the PmBiPPro1 promoters may be useful for targeted expression of heterologous molecules in potato tubers.

In-solution antibody harvesting with a plant-produced hydrophobin-Protein A fusion

Purification is a bottleneck and a major cost factor in the production of antibodies. We set out to engineer a bifunctional fusion protein from two building blocks, Protein A and a hydrophobin, aiming at low-cost and scalable antibody capturing in solutions. Immunoglobulin-binding Protein A is widely used in affinity-based purification. The hydrophobin fusion tag, on the other hand, has been shown to enable purification by two-phase separation. Protein A was fused to two different hydrophobin tags, HFBI or II, and expressed transiently in Nicotiana benthamiana. The hydrophobins enhanced accumulation up to 35-fold, yielding up to 25% of total soluble protein. Both fused and nonfused Protein A accumulated in protein bodies. Hence, the increased yield could not be attributed to HFB-induced protein body formation. We also demonstrated production of HFBI-Protein A fusion protein in tobacco BY-2 suspension cells in 30 l scale, with a yield of 35 mg/l. Efficient partitioning to the surfactant phase confirmed that the fusion proteins retained the amphipathic properties of the hydrophobin block. The reversible antibody-binding capacity of the Protein A block was similar to the nonfused Protein A. The best-performing fusion protein was tested in capturing antibodies from hybridoma culture supernatant with two-phase separation. The fusion protein was able to carry target antibodies to the surfactant phase and subsequently release them back to the aqueous phase after a change in pH. This report demonstrates the potential of hydrophobin fusion proteins for novel applications, such as harvesting antibodies in solutions.

Protein body formation in leaves of Nicotiana benthamiana: a concentration-dependent mechanism influenced by the presence of fusion tags

Protein bodies (PBs) are endoplasmic reticulum (ER) derived organelles originally found in seeds whose function is to accumulate seed storage proteins. It has been shown that PB formation is not limited to seeds and green fluorescent protein (GFP) fused to either elastin-like polypeptide (ELP) or hydrophobin (HFBI) fusion tags induce the formation of PBs in leaves of N. benthamiana. In this study, we compared the ELP- and HFBI-induced PBs and showed that ELP-induced PBs are larger than HFBI-induced PBs. The size of ELP- and HFBI-induced PBs increased over time along with the accumulation levels of their fused protein. Our results show that PB formation is a concentration-dependent mechanism in which proteins accumulating at levels higher than 0.2% of total soluble protein are capable of inducing PBs in vivo. Our results show that the presence of fusion tags is not necessary for the formation of PBs, but affects the distribution pattern and size of PBs. This was confirmed by PBs induced by fluorescent proteins as well as fungal xylanases. We noticed that in the process of PB formation, secretory and ER-resident molecules are passively sequestered into the lumen of PBs. We propose to use this property of PBs as a tool to increase the accumulation levels of erythropoietin and human interleukin-10 by co-expression with PB-inducing proteins.

Identification and use of the sugarcane bacilliform virus enhancer in transgenic maize

BACKGROUND

Transcriptional enhancers are able to increase transcription from heterologous promoters when placed upstream, downstream and in either orientation, relative to the promoter. Transcriptional enhancers have been used to enhance expression of specific promoters in transgenic plants and in activation tagging studies to help elucidate gene function.

RESULTS

A transcriptional enhancer from the Sugarcane Bacilliform Virus - Ireng Maleng isolate (SCBV-IM) that can cause increased transcription when integrated into the the genome near maize genes has been identified. In transgenic maize, the SCBV-IM promoter was shown to be comparable in strength to the maize ubiquitin 1 promoter in young leaf and root tissues. The promoter was dissected to identify sequences that confer high activity in transient assays. Enhancer sequences were identified and shown to increase the activity of a heterologous truncated promoter. These enhancer sequences were shown to be more active when arrayed in 4 copy arrays than in 1 or 2 copy arrays. When the enhancer array was transformed into maize plants it caused an increase in accumulation of transcripts of genes near the site of integration in the genome.

CONCLUSIONS

The SCBV-IM enhancer can activate transcription upstream or downstream of genes and in either orientation. It may be a useful tool to activate enhance from specific promoters or in activation tagging.

Temporal and spatial control of gene expression in horticultural crops

Biotechnology provides plant breeders an additional tool to improve various traits desired by growers and consumers of horticultural crops. It also provides genetic solutions to major problems affecting horticultural crops and can be a means for rapid improvement of a cultivar. With the availability of a number of horticultural genome sequences, it has become relatively easier to utilize these resources to identify DNA sequences for both basic and applied research. Promoters play a key role in plant gene expression and the regulation of gene expression. In recent years, rapid progress has been made on the isolation and evaluation of plant-derived promoters and their use in horticultural crops, as more and more species become amenable to genetic transformation. Our understanding of the tools and techniques of horticultural plant biotechnology has now evolved from a discovery phase to an implementation phase. The availability of a large number of promoters derived from horticultural plants opens up the field for utilization of native sequences and improving crops using precision breeding. In this review, we look at the temporal and spatial control of gene expression in horticultural crops and the usage of a variety of promoters either isolated from horticultural crops or used in horticultural crop improvement.

Production and characterization of in planta transiently produced polygalacturanase from Aspergillus niger and its fusions with hydrophobin or ELP tags

BACKGROUND

Pectinases play an important role in plant cell wall deconstruction and have potential in diverse industries such as food, wine, animal feed, textile, paper, fuel, and others. The demand for such enzymes is increasing exponentially, as are the efforts to improve their production and to implement their use in several industrial processes. The goal of this study was to examine the potential of producing polygalacturonase I from Aspergillus niger in plants and to investigate the effects of subcellular compartmentalization and protein fusions on its accumulation and activity.

RESULTS

Polygalacturonase I from Aspergillus niger (AnPGI) was transiently produced in Nicotiana benthamiana by targeting it to five different cellular compartments: apoplast, endoplasmic reticulum (ER), vacuole, chloroplast and cytosol. Accumulation levels of 2.5%, 3.0%, and 1.9% of total soluble protein (TSP) were observed in the apoplast, ER, and vacuole, respectively, and specific activity was significantly higher in vacuole-targeted AnPGI compared to the same enzyme targeted to the ER or apoplast. No accumulation was found for AnPGI when targeted to the chloroplast or cytosol. Analysis of AnPGI fused with elastin-like polypeptide (ELP) revealed a significant increase in the protein accumulation level, especially when targeted to the vacuole where the protein doubles its accumulation to 3.6% of TSP, while the hydrophobin (HFBI) fusion impaired AnPGI accumulation and both tags impaired activity, albeit to different extents. The recombinant protein showed activity against polygalacturonic acid with optimum conditions at pH 5.0 and temperature from 30 to 50°C, depending on its fusion. In vivo analysis of reducing sugar content revealed a higher release of reducing sugars in plant tissue expressing recombinant AnPGI compared to wild type N. benthamiana leaves.

CONCLUSION

Our results demonstrate that subcellular compartmentalization of enzymes has an impact on both the target protein accumulation and its activity, especially in the case of proteins that undergo post-translational modifications, and should be taken into consideration when protein production strategies are designed. Using plants to produce heterologous enzymes for the degradation of a key component of the plant cell wall could reduce the cost of biomass pretreatment for the production of cellulosic biofuels.

Structure-function relationships in hydrophobins: probing the role of charged side chains

Hydrophobins are small fungal proteins that are amphiphilic and have a strong tendency to assemble at interfaces. By taking advantage of this property, hydrophobins have been used for a number of applications: as affinity tags in protein purification, for protein immobilization, such as in foam stabilizers, and as dispersion agents for insoluble drug molecules. Here, we used site-directed mutagenesis to gain an understanding of the molecular basis of their properties. We especially focused on the role of charged amino acids in the structure of hydrophobins. For this purpose, fusion proteins consisting of Trichoderma reesei hydrophobin I (HFBI) and the green fluorescent protein (GFP) that contained various combinations of substitutions of charged amino acids (D30, K32, D40, D43, R45, K50) in the HFBI structure were produced. The effects of the introduced mutations on binding, oligomerization, and partitioning were characterized in an aqueous two-phase system. It was found that some substitutions caused better surface binding and reduced oligomerization, while some showed the opposite effects. However, all mutations decreased partitioning in surfactant systems, indicating that the different functions are not directly correlated and that partitioning is dependent on finely tuned properties of hydrophobins. This work shows that not all functions in self-assembly are connected in a predictable way and that a simple surfactant model for hydrophobin function is insufficient.

High-level production of human interleukin-10 fusions in tobacco cell suspension cultures

The production of pharmaceutical proteins in plants has made much progress in recent years with the development of transient expression systems, transplastomic technology and humanizing glycosylation patterns in plants. However, the first therapeutic proteins approved for administration to humans and animals were made in plant cell suspensions for reasons of containment, rapid scale-up and lack of toxic contaminants. In this study, we have investigated the production of human interleukin-10 (IL-10) in tobacco BY-2 cell suspension and evaluated the effect of an elastin-like polypeptide tag (ELP) and a green fluorescent protein (GFP) tag on IL-10 accumulation. We report the highest accumulation levels of hIL-10 obtained with any stable plant expression system using the ELP fusion strategy. Although IL-10-ELP has cytokine activity, its activity is reduced compared to unfused IL-10, likely caused by interference of ELP with folding of IL-10. Green fluorescent protein has no effect on IL-10 accumulation, but examining the trafficking of IL-10-GFP over the cell culture cycle revealed fluorescence in the vacuole during the stationary phase of the culture growth cycle. Analysis of isolated vacuoles indicated that GFP alone is found in vacuoles, while the full-size fusion remains in the whole-cell extract. This indicates that GFP is cleaved off prior to its trafficking to the vacuole. On the other hand, IL-10-GFP-ELP remains mostly in the ER and accumulates to high levels. Protein bodies were observed at the end of the culture cycle and are thought to arise as a consequence of high levels of accumulation in the ER.

Preservation and faithful expression of transgene via artificial seeds in alfalfa

Proper preservation of transgenes and transgenic materials is important for wider use of transgenic technology in plants. Here, we report stable preservation and faithful expression of a transgene via artificial seed technology in alfalfa. DNA constructs containing the uid reporter gene coding for β-glucuronidase (GUS) driven by a 35S promoter or a tCUP promoter were introduced into alfalfa via Agrobacterium-mediated genetic transformation. Somatic embryos were subsequently induced from transgenic alfalfa plants via in vitro technology. These embryos were treated with abscisic acid to induce desiccation tolerance and were subjected to a water loss process. After the desiccation procedure, the water content in dried embryos, or called artificial seeds, was about 12–15% which was equivalent to that in true seeds. Upon water rehydration, the dried somatic embryos showed high degrees of viability and exhibited normal germination. Full plants were subsequently developed and recovered in a greenhouse. The progeny plants developed from artificial seeds showed GUS enzyme activity and the GUS expression level was comparable to that of plants developed from somatic embryos without the desiccation process. Polymerase chain reaction analysis indicated that the transgene was well retained in the plants and Southern blot analysis showed that the transgene was stably integrated in plant genome. The research showed that the transgene and the new trait can be well preserved in artificial seeds and the progeny developed. The research provides a new method for transgenic germplasm preservation in different plant species.

Protein body formation in stable transgenic tobacco expressing elastin-like polypeptide and hydrophobin fusion proteins

BACKGROUND

Plants are recognized as an efficient and inexpensive system to produce valuable recombinant proteins. Two different strategies have been commonly used for the expression of recombinant proteins in plants: transient expression mediated by Agrobacterium; or stable transformation of the plant genome. However, the use of plants as bioreactors still faces two main limitations: low accumulation levels of some recombinant proteins and lack of efficient purification methods. Elastin-like polypeptide (ELP), hydrophobin I (HFBI) and Zera® are three fusion partners found to increase the accumulation levels of recombinant proteins and induce the formation of protein bodies (PBs) in leaves when targeted to the endoplasmic reticulum (ER) in transient expression assays. In this study the effects of ELP and HFBI fusion tags on recombinant protein accumulation levels and PB formation was examined in stable transgenic Nicotiana tabacum.

RESULTS

The accumulation of recombinant protein and PB formation was evaluated in two cultivars of Nicotiana tabacum transformed with green fluorescent protein (GFP) fused to ELP or HFBI, both targeted and retrieved to the ER. The ELP and HFBI tags increased the accumulation of the recombinant protein and induced the formation of PBs in leaves of stable transgenic plants from both cultivars. Furthermore, these tags induced the formation of PBs in a concentration-dependent manner, where a specific level of recombinant protein accumulation was required for PBs to appear. Moreover, agro-infiltration of plants accumulating low levels of recombinant protein with p19, a suppressor of post-transcriptional gene silencing (PTGS), increased accumulation levels in four independent transgenic lines, suggesting that PTGS might have caused the low accumulation levels in these plants.

CONCLUSION

The use of ELP and HFBI tags as fusion partners in stable transgenic plants of tobacco is feasible and promising. In a constitutive environment, these tags increase the accumulation levels of the recombinant protein and induce the formation of PBs regardless of the cultivar used. However, a specific level of recombinant protein accumulation needs to be reached for PBs to form.

Enhanced transgene expression in rice following selection controlled by weak promoters

BACKGROUND

Techniques that enable high levels of transgene expression in plants are attractive for the commercial production of plant-made recombinant pharmaceutical proteins or other gene transfer related strategies. The conventional way to increase the yield of desired transgenic products is to use strong promoters to control the expression of the transgene. Although many such promoters have been identified and characterized, the increase obtainable from a single promoter is ultimately limited to a certain extent.

RESULTS

In this study, we report a method to magnify the effect of a single promoter by using a weak promoter-based selection system in transgenic rice. tCUP1, a fragment derived from the tobacco cryptic promoter (tCUP), was tested for its activity in rice by fusion to both a β-glucuronidase (GUS) reporter and a hygromycin phosphotransferase (HPT) selectable marker. The tCUP1 promoter allowed the recovery of transformed rice plants and conferred tissue specific expression of the GUS reporter, but was much weaker than the CaMV 35S promoter in driving a selectable marker for growth of resistant calli. However, in the resistant calli and regenerated transgenic plants selected by the use of tCUP1, the constitutive expression of green fluorescent protein (GFP) was dramatically increased as a result of the additive effect of multiple T-DNA insertions. The correlation between attenuated selection by a weak promoter and elevation of copy number and foreign gene expression was confirmed by using another relatively weak promoter from nopaline synthase (Nos).

CONCLUSIONS

The use of weak promoter derived selectable markers leads to a high T-DNA copy number and then greatly increases the expression of the foreign gene. The method described here provides an effective approach to robustly enhance the expression of heterogenous transgenes through copy number manipulation in rice.

A Novel Moderate Constitutive Promoter Derived from Poplar (Populus tomentosa Carrière)

A novel sequence that functions as a promoter element for moderate constitutive expression of transgenes, designated as the PtMCP promoter, was isolated from the woody perennial Populus tomentosa. The PtMCP promoter was fused to the GUS reporter gene to characterize its expression pattern in different species. In stable Arabidopsis transformants, transcripts of the GUS reporter gene could be detected by RT-PCR in the root, stem, leaf, flower and silique. Further histochemical and fluorometric GUS activity assays demonstrated that the promoter could direct transgene expression in all tissues and organs, including roots, stems, rosette leaves, cauline leaves and flowers of seedlings and maturing plants. Its constitutive expression pattern was similar to that of the CaMV35S promoter, but the level of GUS activity was significantly lower than in CaMV35S promoter::GUS plants. We also characterized the promoter through transient expression in transgenic tobacco and observed similar expression patterns. Histochemical GUS staining and quantitative analysis detected GUS activity in all tissues and organs of tobacco, including roots, stems, leaves, flower buds and flowers, but GUS activity in PtMCP promoter::GUS plants was significantly lower than in CaMV35S promoter::GUS plants. Our results suggested that the PtMCP promoter from poplar is a constitutive promoter with moderate activity and that its function is presumably conserved in different species. Therefore, the PtMCP promoter may provide a practical choice to direct moderate level constitutive expression of transgenes and could be a valuable new tool in plant genetic engineering.

The production and characterization of a new active lipase from Acremonium alcalophilum using a plant bioreactor

BACKGROUND

Microorganisms are the most proficient decomposers in nature, using secreted enzymes in the hydrolysis of lignocellulose. As such, they present the most abundant source for discovery of new enzymes. Acremonium alcalophilum is the only known cellulolytic fungus that thrives in alkaline conditions and can be cultured readily in the laboratory. Its optimal conditions for growth are 30°C and pH 9.0-9.2. The genome sequence of Acremonium alcalophilum has revealed a large number of genes encoding biomass-degrading enzymes. Among these enzymes, lipases are interesting because of several industrial applications including biofuels, detergent, food processing and textile industries.

RESULTS

We identified a lipA gene in the genome sequence of Acremonium alcalophilum, encoding a protein with a predicted lipase domain with weak sequence identity to characterized enzymes. Unusually, the predicted lipase displays ≈ 30% amino acid sequence identity to both feruloyl esterase and lipase of Aspergillus niger. LipA, when transiently produced in Nicotiana benthamiana, accumulated to over 9% of total soluble protein. Plant-produced recombinant LipA is active towards p-nitrophenol esters of various carbon chain lengths with peak activity on medium-chain fatty acid (C8). The enzyme is also highly active on xylose tetra-acetate and oat spelt xylan. These results suggests that LipA is a novel lipolytic enzyme that possesses both lipase and acetylxylan esterase activity. We determined that LipA is a glycoprotein with pH and temperature optima at 8.0 and 40°C, respectively.

CONCLUSION

Besides being the first heterologous expression and characterization of a gene coding for a lipase from A. alcalophilum, this report shows that LipA is very versatile exhibiting both acetylxylan esterase and lipase activities potentially useful for diverse industry sectors, and that tobacco is a suitable bioreactor for producing fungal proteins.

Hydrophobin fusions for high-level transient protein expression and purification in Nicotiana benthamiana

Insufficient accumulation levels of recombinant proteins in plants and the lack of efficient purification methods for recovering these valuable proteins have hindered the development of plant biotechnology applications. Hydrophobins are small and surface-active proteins derived from filamentous fungi that can be easily purified by a surfactant-based aqueous two-phase system. In this study, the hydrophobin HFBI sequence from Trichoderma reesei was fused to green fluorescent protein (GFP) and transiently expressed in Nicotiana benthamiana plants by Agrobacterium tumefaciens infiltration. The HFBI fusion significantly enhanced the accumulation of GFP, with the concentration of the fusion protein reaching 51% of total soluble protein, while also delaying necrosis of the infiltrated leaves. Furthermore, the endoplasmic reticulum-targeted GFP-HFBI fusion induced the formation of large novel protein bodies. A simple and scalable surfactant-based aqueous two-phase system was optimized to recover the HFBI fusion proteins from leaf extracts. The single-step phase separation was able to selectively recover up to 91% of the GFP-HFBI up to concentrations of 10 mg mL(-1). HFBI fusions increased the expression levels of plant-made recombinant proteins while also providing a simple means for their subsequent purification. This hydrophobin fusion technology, when combined with the speed and posttranslational modification capabilities of plants, enhances the value of transient plant-based expression systems.

The sweet potato ADP-glucose pyrophosphorylase gene (ibAGP1) promoter confers high-level expression of the GUS reporter gene in the potato tuber

Molecular farming refers to the process of creating bioengineered plants with the capability of producing potentially valuable products, such as drugs, vaccines, and chemicals. We have investigated the potential of the sweet potato ADP-glucose pyrophosphorylase gene (ibAGP1) promoter and its transit peptide (TP) as an expression system for the mass production of foreign proteins in potato. The ibAGP1 promoter and its TP sequence were transformed into potato along with beta-glucuronidase (GUS) as a reporter gene, and GUS activity was subsequently analyzed in the transgenic potato plants. In tuber tissues, GUS activity in transgenic plants carrying only the ibAGP1 promoter (ibAGP1::GUS) increased up to 15.6-fold compared with that of transgenic plants carrying only the CaMV35S promoter (CaMV35S::GUS). GUS activity in transgenic plants was further enhanced by the addition of the sweetpotato TP to the recombinant vector (ibAGP1::TP::GUS), with tuber tissues showing a 26-fold increase in activity compared with that in the CaMV35S::GUS-transgenic lines. In leaf tissues, the levels of GUS activity found in ibAGP1::GUS-transgenic lines were similar to those in CaMV35S::GUS-lines, but they were significantly enhanced in ibAGP1::TP::GUS-lines. GUS activity gradually increased with increasing tuber diameter in ibAGP1::GUS-transgenic plants, reaching a maximum level when the tuber was 35 mm in diameter. In contrast, extremely elevated levels of GUS activity - up to about 10-fold higher than that found in CaMV35S::GUS-lines - were found in ibAGP1::TP::GUS-transgenic lines at a much earlier stage of tuber development (diameter 4 mm), and these higher levels were maintained throughout the entire tuber developmental stage. These results suggest that the sweetpotato ibAGP1 promoter and its TP are a potentially strong foreign gene expression system that can be used for molecular farming in potato plants.

Induction of protein body formation in plant leaves by elastin-like polypeptide fusions

BACKGROUND

Elastin-like polypeptides are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the simple non-chromatographic purification of recombinant proteins. In addition, elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, thus addressing the major limitation of plant-based expression systems, which is a low production yield. This study's main objectives were to determine the general utility of elastin-like polypeptide protein fusions in various intracellular compartments and to elucidate elastin-like polypeptide's mechanism of action for increasing recombinant protein accumulation in the endoplasmic reticulum of plants.

RESULTS

The effect of elastin-like polypeptide fusions on the accumulation of green fluorescent protein targeted to the cytoplasm, chloroplasts, apoplast, and endoplasmic reticulum was evaluated. The endoplasmic reticulum was the only intracellular compartment in which an elastin-like polypeptide tag was shown to significantly enhance recombinant protein accumulation. Interestingly, endoplasmic reticulum-targeted elastin-like polypeptide fusions induced the formation of a novel type of protein body, which may be responsible for elastin-like polypeptide's positive effect on recombinant protein accumulation by excluding the heterologous protein from normal physiological turnover. Although expressed in the leaves of plants, these novel protein bodies appeared similar in size and morphology to the prolamin-based protein bodies naturally found in plant seeds. The elastin-like polypeptide-induced protein bodies were highly mobile organelles, exhibiting various dynamic patterns of movement throughout the cells, which were dependent on intact actin microfilaments and a functional actomyosin motility system.

CONCLUSION

An endoplasmic reticulum-targeted elastin-like polypeptide fusion approach provides an effective strategy for depositing large amounts of concentrated heterologous protein within the limited space of the cell via storage in stable protein bodies. Furthermore, encapsulation of recombinant proteins into physiologically inert organelles can function to insulate the protein from normal cellular mechanisms, thus limiting unnecessary stress to the host cell. Since elastin-like polypeptide is a mammalian-derived protein, this study demonstrates that plant seed-specific factors are not required for the formation of protein bodies in vegetative plant tissues, suggesting that the endoplasmic reticulum possesses an intrinsic ability to form protein body-like accretions in eukaryotic cells when overexpressing particular proteins.

Optimization of elastin-like polypeptide fusions for expression and purification of recombinant proteins in plants

The demand for recombinant proteins for medical and industrial use is expanding rapidly and plants are now recognized as an efficient, inexpensive means of production. Although the accumulation of recombinant proteins in transgenic plants can be low, we have previously demonstrated that fusions with an elastin-like polypeptide (ELP) tag can significantly enhance the production yield of a range of different recombinant proteins in plant leaves. ELPs are biopolymers with a repeating pentapeptide sequence (VGVPG)(n) that are valuable for bioseparation, acting as thermally responsive tags for the non-chromatographic purification of recombinant proteins. To determine the optimal ELP size for the accumulation of recombinant proteins and their subsequent purification, various ELP tags were fused to green fluorescent protein, interleukin-10, erythropoietin and a single chain antibody fragment and then transiently expressed in tobacco leaves. Our results indicated that ELP tags with 30 pentapeptide repeats provided the best compromise between the positive effects of small ELP tags (n = 5-40) on recombinant protein accumulation and the beneficial effects of larger ELP tags (n = 80-160) on recombinant protein recovery during inverse transition cycling (ITC) purification. In addition, the C-terminal orientation of ELP fusion tags produced higher levels of target proteins, relative to N-terminal ELP fusions. Importantly, the ELP tags had no adverse effect on the receptor binding affinity of erythropoietin, demonstrating the inert nature of these tags. The use of ELP fusion tags provides an approach for enhancing the production of recombinant proteins in plants, while simultaneously assisting in their purification.

Auto-excision of selectable marker genes from transgenic tobacco via a stress inducible FLP/FRT site-specific recombination system

Antibiotic resistance marker genes are powerful selection tools for use in plant transformation processes. However, once transformation is accomplished, the presence of these resistance genes is no longer necessary and can even be undesirable. We herein describe the successful excision of antibiotic resistance genes from transgenic plants via the use of an oxidative stress-inducible FLP gene. FLP encodes a recombinase that can eliminate FLP and hpt selection genes flanked by two FRT sites. During a transformation procedure in tobacco, transformants were obtained by selection on hygromycin media. Regenerants of the initial transformants were screened for selective marker excision in hydrogen peroxide supplemented media and both the FLP and hpt genes were found to have been eliminated. About 13-41% of regenerated shoots on hydrogen peroxide media were marker-free. This auto-excision system, mediated by the oxidative stress-inducible FLP/FRT system to eliminate a selectable marker gene can be very readily adopted and used to efficiently generate marker-free transgenic plants.

Plant recombinant erythropoietin attenuates inflammatory kidney cell injury

Human erythropoietin (EPO) is a pleiotropic cytokine with remarkable tissue-protective activities in addition to its well-established role in red blood cell production. Unfortunately, conventional mammalian cell cultures are unlikely to meet the anticipated market demands for recombinant EPO because of limited capacity and high production costs. Plant expression systems may address these limitations to enable practical, cost-effective delivery of EPO in tissue injury prevention therapeutics. In this study, we produced human EPO in tobacco and demonstrated that plant-derived EPO had tissue-protective activity. Our results indicated that targeting to the endoplasmic reticulum (ER) provided the highest accumulation levels of EPO, with a yield approaching 0.05% of total soluble protein in tobacco leaves. The codon optimization of the human EPO gene for plant expression had no clear advantage; furthermore, the human EPO signal peptide performed better than a tobacco signal peptide. In addition, we found that glycosylation was essential for the stability of plant recombinant EPO, whereas the presence of an elastin-like polypeptide fusion had a limited positive impact on the level of EPO accumulation. Confocal microscopy showed that apoplast and ER-targeted EPO were correctly localized, and N-glycan analysis demonstrated that complex plant glycans existed on apoplast-targeted EPO, but not on ER-targeted EPO. Importantly, plant-derived EPO had enhanced receptor-binding affinity and was able to protect kidney epithelial cells from cytokine-induced death in vitro. These findings demonstrate that tobacco plants may be an attractive alternative for the production of large amounts of biologically active EPO.

pORE: a modular binary vector series suited for both monocot and dicot plant transformation

We present a series of 14 binary vectors suitable for Agrobacterium-mediated transformation of dicotyledonous plants and adaptable for biolistic transformation of monocotyledonous plants. The vector size has been minimized by eliminating all non-essential elements from the vector backbone and T-DNA regions while maintaining the ability to replicate independently. The smallest of the vector series is 6.3 kb and possesses an extensive multiple cloning site with 21 unique restriction endonuclease sites that are compatible with common cloning, protein expression, yeast two-hybrid and other binary vectors. The T-DNA region was engineered using a synthetic designer oligonucleotide resulting in an entirely modular system whereby any vector element can be independently exchanged. The high copy number ColE1 origin of replication has been included to enhance plasmid yield in Escherichia coli. FRT recombination sites flank the selectable marker cassette regions and allow for in planta excision by FLP recombinase. The pORE series consists of three basic types; an 'open' set for general plant transformation, a 'reporter' set for promoter analysis and an 'expression' set for constitutive expression of transgenes. The sets comprise various combinations of promoters (P (HPL), P (ENTCUP2) and P (TAPADH)), selectable markers (nptII and pat) and reporter genes (gusA and smgfp).

A strong constitutive gene expression system derived from ibAGP1 promoter and its transit peptide

To develop a strong constitutive gene expression system, the activities of ibAGP1 promoter and its transit peptide were investigated using transgenic Arabidopsis and a GUS reporter gene. The ibAGP1 promoter directed GUS expression in almost entire tissues including rosette leaf, inflorescence stem, inflorescence, cauline leaf and root, suggesting that the ibAGP1 promoter is a constitutive promoter. GUS expression mediated by ibAGP1 promoter was weaker than that by CaMV35S promoter in all tissue types, but when GUS protein was targeted to plastids with the aid of the ibAGP1 transit peptide, GUS levels increased to higher levels in lamina, petiole and cauline leaf compared to those produced by CaMV35S promoter. The enhancing effect of ibAGP1 transit peptide on the accumulation of foreign protein was tissue-specific; accumulation was high in lamina and inflorescence, but low in root and primary inflorescence stem. The transit peptide effect in the leaves was maintained highly regardless of developmental stages of plants. The ibAGP1 promoter and its transit peptide also directed strong GUS gene expression in transiently expressed tobacco leaves. These results suggest that the ibAGP1 promoter and its transit peptide are a strong constitutive foreign gene expression system for transgenesis of dicot plants.

Elastin-like polypeptide fusions enhance the accumulation of recombinant proteins in tobacco leaves

The production of recombinant proteins in plants is an active area of research and many different high-value proteins have now been produced in plants. Tobacco leaves have many advantages for recombinant protein production particularly since they allow field production without seeds, flowers or pollen and therefore provide for contained production. Despite these biosafety advantages recombinant protein accumulation in leaves still needs to be improved. Elastin-like polypeptides are repeats of the amino acids "VPGXG" that undergo a temperature dependant phase transition and have utility in the purification of recombinant proteins but can also enhance the accumulation of recombinant proteins they are fused to. We have used a 11.3 kDa elastin-like polypeptide as a fusion partner for three different target proteins, human interleukin-10, murine interleukin-4 and the native major ampullate spidroin protein 2 gene from the spider Nephila clavipes. In both transient analyses and stable transformants the concentrations of the fusion proteins were at least an order of magnitude higher for all of the fusion proteins when compared to the target protein alone. Therefore, fusions with a small ELP tag can be used to significantly enhance the accumulation of a range of different recombinant proteins in plant leaves.

Analysis and use of the tobacco eIF4A-10 promoter elements for transgene expression

The eIF4A gene family codes for proteins which unwind secondary structures of mRNA during translational initiation. The tobacco eIF4A-10 promoter is one of a few of constitutive promoters found in plants. Research was conducted to identify the proximal promoter elements and to evaluate the potential application of the promoter for regulating transgene expression in a range of crop plants. A large intron (892 bp) in the leader sequence was found to be dispensable for constitutive promoter activity and did not contribute to the overall performance of the promoter. Deletion analysis showed that the upstream region between -151 bp and -73bp relative to the transcriptional start site was essential for the high level of expression and the constitutive activity. The data indicated that the elements in this region may coordinate and compensate each other for the high levels of promoter expression. The downstream leader sequence also contained a strong quantitative enhancer element that was essential for the full activity of the eIF4A-10 promoter. The eIF-4A10 promoter was found to be active in a wide range of plant species and tissues indicating that it will be useful for the constitutive expression of transgenes in plants.

Characterization of a strong, constitutive mung bean (Vigna radiata L.) promoter with a complex mode of regulation in planta

We report the cloning and characterization in tobacco and Arabidopsis of a Vigna radiata L. (mung bean) promoter that controls the expression of VR-ACS1, an auxin-inducible ACC synthase gene. The VR-ACS1 promoter exhibits a very unusual behavior when studied in plants different from its original host, mung bean. GUS and luciferase in situ assays of transgenic plants containing VR-ACS1 promoter fusions show strong constitutive reporter gene expression throughout tobacco and Arabidopsis development. In vitro quantitative analyses show that transgenic plants harboring VR-ACS1 promoter-reporter constructs have on average 4-6 fold higher protein and activity levels of both reporter genes than plants transformed with comparable CaMV 35S promoter fusions. Similar transcript levels are present in VR-ACS1 and CaMV 35S promoter lines, suggesting that the high levels of gene product observed for the VR-ACS1 promoter are the combined result of transcriptional and translational activation. All tested deletion constructs retaining the core promoter region can drive strong constitutive promoter activity in transgenic plants. This is in contrast to mung bean, where expression of the native VR-ACS1 gene is almost undetectable in plants grown under normal conditions, but is rapidly and highly induced by a variety of stimuli. The constitutive behavior of the VR-ACS1 promoter in heterologous hosts is surprising, suggesting that the control mechanisms active in mung bean are impaired in tobacco and Arabidopsis. The 'aberrant' behavior of the VR-ACS1 promoter is further emphasized by its failure to respond to auxin and cycloheximide in heterologous hosts. VR-ACS1 promoter regulatory mechanisms seem to be different from all previously characterized auxin-inducible promoters.

T-DNA tagging and characterization of a cryptic root-specific promoter in Arabidopsis

From a T-DNA tagged Arabidopsis population, a line, M-57 showing GUS (beta-glucuronidase) expression in the vascular regions of young roots was identified. Southern analysis revealed presence of a single T-DNA insert. Using inverse PCR, the plant sequence flanking the T-DNA insertion was cloned. The insertion was identified to be in the intergenic area between loci At4G13940 and At4G13930, coding for SAHH (S-Adenosyl-l-Homocysteine Hydrolase) and SHMT (Serine Hydroxy Methyl Transferase) genes, respectively. A 452-bp fragment immediately upstream of the T-DNA insertion when cloned and mobilized as a GUS fusion was capable of driving a similar root-specific expression of reporter gene in transgenic Arabidopsis plants and their progenies. This cryptic promoter element does not show the presence of any known root-specific promoter element.

Inducible excision of selectable marker gene from transgenic plants by the cre/lox site-specific recombination system

In a plant transformation process, it is necessary to use marker genes that allow the selection of regenerated transgenic plants. However, selectable marker genes are generally superfluous once an intact transgenic plant has been established. Furthermore, they may cause regulatory difficulties for approving transgenic crop release and commercialization. We constructed a binary expression vector with the Cre/lox system with a view to eliminating a marker gene from transgenic plants conveniently. In the vector, recombinase gene cre under the control of heat shock promoter and selectable marker gene nptII under the control of CaMV35S promoter were placed between two lox P sites in direct orientation, while the gene of interest was inserted outside of the lox P sites. By using this vector, both cre and nptII genes were eliminated from most of the regenerated plants of primary transformed tobacco through heat shock treatment, while the gene of interest was retained and stably inherited. This auto-excision strategy, mediated by the Cre/lox system and subjected to heat shock treatment to eliminate a selectable marker gene, is easy to adopt and provides a promising approach to generate marker-free transgenic plants.

Repression of gene expression by Arabidopsis HD2 histone deacetylases

The four HD2 proteins of Arabidopsis thaliana (AtHD2A-D) belong to a unique class of histone deacetylases that is plant specific. Previously, we have demonstrated that one of the members, AtHD2A, can mediate transcriptional repression when targeted to the promoter of a reporter gene. Here, we report that AtHD2B and AtHD2C can also repress gene expression. AtHD2A and AtHD2C differ from AtHD2B and AtHD2D in the composition of their structural domains. Our data show that both structural types play a role in the repression of gene transcription. We demonstrate that AtHD2A can mediate gene repression through interactions with transcription factors in plants. By fusing AtHD2A with the DNA-binding domain of the plant transcriptional factor Pti4, the expression of a GCC box containing reporter gene was repressed. We also demonstrated repression of a GUS gene with GAL4 enhancers using transgenic plants that expressed a GAL4/AtHD2A fusion gene. Furthermore, the expression of the GAL4/AtHD2A protein using the seed-specific napin promoter (NAP2) and the constitutive tCUP promoter demonstrated that repression of transgenes could be achieved in a tissue-specific or unrestricted manner. Targeting of HD2 proteins to specific promoters using transcription factor DNA-binding domains may therefore provide a new technology for silencing target genes and pathways in plants as well as for assessing the function of unknown transcription factors.

The new RENT family of repetitive elements in Nicotiana species harbors gene regulatory elements related to the tCUP cryptic promoter

The tCUP cryptic constitutive promoter was discovered in the tobacco genome by T-DNA (transfer DNA) tagging with a promoterless GUS-nos gene. Here, we show that the portion of the tCUP sequence containing a variety of cryptic gene regulatory elements is related to a new family of moderately repetitive sequences (10(2) copies), the RENT (repetitive element from Nicotiana tabacum) family. The RENT family is found only in certain Nicotiana species. Five RENT elements were cloned and sequenced. The RENT elements are a minimum of 5 kb in length and share 80-90% sequence similarity throughout their length. The 5' termini are the same in the isolated RENT family members and are characterized by a conserved border sequence (TGTTGA(T or C)ACCCAATTTT(T or C)). The 3' ends of RENT sequence similarity vary in location and sequence. The tCUP cryptic promoter originated from a unique truncated RENT element that interrupts a phytochelatin synthase-like gene that may have undergone rearrangements prior to or resulting from T-DNA insertion. No evidence was found for expressed coding regions within the RENT elements; however, like the cryptic gene regulatory elements within the tCUP sequence, the isolated RENT elements possess promoter activity and translational enhancer activity.

Functional analysis of tomato Pti4 in Arabidopsis

Pti4 is a tomato (Lycopersicon esculentum) transcription factor that belongs to the ERF (ethylene-responsive element binding factor) family of proteins. It interacts with the Pto kinase in tomato, which confers resistance to the Pseudomonas syringae pv tomato pathogen that causes bacterial speck disease. To study the function of Pti4, transgenic Arabidopsis plants were generated that expressed tomato Pti4 driven by the strong constitutive promoters, cauliflower mosaic virus 35S and tCUP. Global gene expression analysis by Affimetric GeneChip indicated that expression of Pti4 in transgenic Arabidopsis plants induced the expression of GCC box-containing PR genes. We also demonstrated that Pti4 enhanced GCC box-mediated transcription of a reporter gene. The data suggests that tomato Pti4 could act as a transcriptional activator to regulate expression of GCC box-containing genes. Furthermore, we show that the expression of tomato Pti4 in transgenic Arabidopsis plants produced a phenotype similar to that seen in plants treated with ethylene, thus providing evidence that the Pti4 gene is involved in the regulation of a subset of ethylene-responsive genes containing the GCC box.