Comparison of transient protein expression in tobacco leaves and plant suspension culture

Metabolic Engineering Strategies of Industrial Hemp (Cannabis sativa L.): A Brief Review of the Advances and Challenges

Industrial hemp (Cannabis sativa L.) is a diploid (2n = 20), dioecious plant that is grown for fiber, seed, and oil. Recently, there has been a renewed interest in this crop because of its panoply of cannabinoids, terpenes, and other phenolic compounds. Specifically, hemp contains terpenophenolic compounds such as cannabidiol (CBD) and cannabigerol (CBG), which act on cannabinoid receptors and positively regulate various human metabolic, immunological, and physiological functions. CBD and CBG have an effect on the cytokine metabolism, which has led to the examination of cannabinoids on the treatment of viral diseases, including COVID-19. Based on genomic, transcriptomic, and metabolomic studies, several synthetic pathways of hemp secondary metabolite production have been elucidated. Nevertheless, there are few reports on hemp metabolic engineering despite obvious impact on scientific and industrial sectors. In this article, recent status and current perspectives on hemp metabolic engineering are reviewed. Three distinct approaches to expedite phytochemical yield are discussed. Special emphasis has been placed on transgenic and transient gene delivery systems, which are critical for successful metabolic engineering of hemp. The advent of new tools in synthetic biology, particularly the CRISPR/Cas systems, enables environment-friendly metabolic engineering to increase the production of desirable hemp phytochemicals while eliminating the psychoactive compounds, such as tetrahydrocannabinol (THC).

Development of a minigenome cassette for Lettuce necrotic yellows virus: A first step in rescuing a plant cytorhabdovirus

Rhabdoviruses are enveloped negative-sense RNA viruses that have numerous biotechnological applications. However, recovering plant rhabdoviruses from cDNA remains difficult due to technical difficulties such as the need for concurrent in planta expression of the viral genome together with the viral nucleoprotein (N), phosphoprotein (P) and RNA-dependent RNA polymerase (L) and viral genome instability in E. coli. Here, we developed a negative-sense minigenome cassette for Lettuce necrotic yellows virus (LNYV). We introduced introns into the unstable viral ORF and employed Agrobacterium tumefaciens to co-infiltrate Nicotiana with the genes for the N, P, and L proteins together with the minigenome cassette. The minigenome cassette included the Discosoma sp. red fluorescent protein gene (DsRed) cloned in the negative-sense between the viral trailer and leader sequences which were placed between hammerhead and hepatitis delta ribozymes. In planta DsRed expression was demonstrated by western blotting while the appropriate splicing of introduced introns was confirmed by sequencing of RT-PCR product.

Transient Recombinant Protein Production in Glycoengineered Nicotiana benthamiana Cell Suspension Culture

Transient recombinant protein production is a promising alternative to stable transgenic systems, particularly for emergency situations in which rapid production of novel therapeutics is needed. In plants, Agrobacterium tumefaciens can be used as a gene delivery vector for transient expression. A potential barrier for plant-based production of human therapeutics is that different glycosylation patterns are found on plant and mammalian proteins. Since glycosylation can affect the efficacy, safety and stability of a therapeutic protein, methods to control glycan structures and distributions in plant-based systems would be beneficial. In these studies, we performed Agrobacterium-mediated transient expression in glycoengineered plant cell suspension cultures. To reduce the presence of plant-specific glycans on the product, we generated and characterized cell suspension cultures from β-1,2-xylosyltransferase and α-1,3-fucosyltransferase knockdown Nicotiana benthamiana. An anthrax decoy fusion protein was transiently produced in these glycoengineered plant cell suspension cultures through co-culture with genetically engineered Agrobacterium. The mass ratio of Agrobacterium to plant cells used was shown to impact recombinant protein expression levels. N-glycosylation analysis on the anthrax decoy fusion protein produced in glycoengineered N. benthamiana showed a dramatic reduction in plant-specific N-glycans. Overall, the results presented here demonstrate the feasibility of a simple, rapid and scalable process for transient production of recombinant proteins without plant-specific glycans in a glycoengineered plant cell culture host.

Assessment of the immunogenicity of residual host cell protein impurities of OsrHSA

Human serum albumin (HSA) is the most abundant protein in human plasma and is widely used at high doses for treating various diseases. Recombinant HSA is an alternative approach to plasma-derived HSA, providing increased safety and an unlimited supply. However, the safety of the residual host cell proteins (HCPs) co-purified with Oryza sativa HSA (OsrHSA) remains to be determined. An animal system was used to assess the immunogenicity of OsrHSA and its residual HCPs. Low immunogenicity and immunotoxicity of the residual HCPs at a dose of 25 μg/kg, equivalent to 25 times the clinical dosage of HSA, were observed. An anti-drug-antibody (ADA) analysis revealed that anti-HSA, anti-OsrHSA or anti-HCP antibodies developed with a low frequency in pHSA and OsrHSA treatments, but the titers were as low as 1.0–2.0. Furthermore, the titer and the incidence of the specific antibodies were not significantly different between the pHSA and OsrHSA groups, indicating that OsrHSA presents similar immunogenicity to that of pHSA. More importantly, no cytokines were stimulated after the administration of OsrHSA and the residual HCPs, suggesting that there was no risk of a cytokine storm. These results demonstrated that the residual HCPs from OsrHSA have low immunogenicity, indicating that the rice endosperm is one of the best hosts for plant molecular pharming.

Leaf-Encapsulated Vaccines: Agroinfiltration and Transient Expression of the Antigen Staphylococcal Endotoxin B in Radish Leaves

Transgene introgression is a major concern associated with transgenic plant-based vaccines. Agroinfiltration can be used to selectively transform nonreproductive organs and avoid introgression. Here, we introduce a new vaccine modality in which Staphylococcal enterotoxin B (SEB) genes are agroinfiltrated into radishes (Raphanw sativus L.), resulting in transient expression and accumulation of SEB in planta. This approach can simultaneously express multiple antigens in a single leaf. Furthermore, the potential of high-throughput vaccine production was demonstrated by simultaneously agroinfiltrating multiple radish leaves using a multichannel pipette. The expression of SEB was detectable in two leaf cell types (epidermal and guard cells) in agroinfiltrated leaves. ICR mice intranasally immunized with homogenized leaves agroinfiltrated with SEB elicited detectable antibody to SEB and displayed protection against SEB-induced interferon-gamma (IFN-γ) production. The concept of encapsulating antigens in leaves rather than purifying them for immunization may facilitate rapid vaccine production during an epidemic disease.

Engineering analysis of the high-density heterotrophic cultivation of mung bean sprouts

This study investigated the heterotrophic growth behavior of mung beans cultivated in an individual bed under water supply. The fresh weight of mung beans in the bed was estimated, and changes in temperature, and oxygen and carbon dioxide concentrations were recorded during the cultivation period. The specific growth rate, oxygen uptake rate, and carbon dioxide evolution rate, based on the fresh weight in the bed, were calculated. Growth under heterotrophic cultivation can be classified into the following three stages. Reductions in specific oxygen uptake rate, specific carbon dioxide evolution rate, and specific energy production rate corresponded to that of specific growth rate. Indicators of biological activity related to oxygen and carbon dioxide were evaluated quantitatively for beds under high-density heterotrophic cultivation. Moreover, the results obtained from this study successfully demonstrate that there is a relationship between the growth of mung beans and indicators of biological activity.

Transient Expression Systems in Plants: Potentialities and Constraints

Plants have been used from old to extract and isolate by different means the products of interest that they store. In recent years new techniques have emerged that allow the use of plants as factories to overexpress transiently and often efficiently, specific genes of interest, either endogenous or foreign, in their native form or modified. These techniques allow and facilitate the targeted purification of gene products for research and commercial purposes without resorting to lengthy, time-consuming and sometimes challenging plant stable transformations, while avoiding some of their associated regulatory constraints. In this chapter we describe the main strategies available for the transient expression of gene sequences and their encoded products in plants. We discuss biological issues affecting transient expression, including resistance responses elicited by the plant against sequences that it recognizes naturally as foreign, and ways to neutralize them. We also discuss the relative advantages of each expression strategy as well as their inherent drawbacks and technical limitations, and how to partially prevent or overcome them, whenever possible.

Plants as Factories for Human Pharmaceuticals: Applications and Challenges

Plant molecular farming (PMF), defined as the practice of using plants to produce human therapeutic proteins, has received worldwide interest. PMF has grown and advanced considerably over the past two decades. A number of therapeutic proteins have been produced in plants, some of which have been through pre-clinical or clinical trials and are close to commercialization. Plants have the potential to mass-produce pharmaceutical products with less cost than traditional methods. Tobacco-derived antibodies have been tested and used to combat the Ebola outbreak in Africa. Genetically engineered immunoadhesin (DPP4-Fc) produced in green plants has been shown to be able to bind to MERS-CoV (Middle East Respiratory Syndrome), preventing the virus from infecting lung cells. Biosafety concerns (such as pollen contamination and immunogenicity of plant-specific glycans) and costly downstream extraction and purification requirements, however, have hampered PMF production from moving from the laboratory to industrial application. In this review, the challenges and opportunities of PMF are discussed. Topics addressed include; transformation and expression systems, plant bioreactors, safety concerns, and various opportunities to produce topical applications and health supplements.

Expression of peach sucrose transporters in heterologous systems points out their different physiological role

Sucrose is the major phloem-translocated component in a number of economically important plant species. The comprehension of the mechanisms involved in sucrose transport in peach fruit appears particularly relevant, since the accumulation of this sugar, during ripening, is crucial for the growth and quality of the fruit. Here, we report the functional characterisation and subcellular localisation of three sucrose transporters (PpSUT1, PpSUT2, PpSUT4) in peach, and we formulate novel hypotheses about their role in accumulation of sugar. We provide evidence, about the capability of both PpSUT1 and PpSUT4, expressed in mutant yeast strains to transport sucrose. The functionality of PpSUT1 at the plasma membrane, and of PpSUT4 at the tonoplast, has been demonstrated. On the other hand, the functionality of PpSUT2 was not confirmed: this protein is unable to complement two sucrose uptake-deficient mutant yeast strains. Our results corroborate the hypotheses that PpSUT1 partakes in phloem loading in leaves, and PpSUT4 sustains cell metabolism by regulating sucrose efflux from the vacuole.

A procedure for the transient expression of genes by agroinfiltration above the permissive threshold to study temperature-sensitive processes in plant-pathogen interactions

Localized expression of genes in plants from T-DNAs delivered into plant cells by Agrobacterium tumefaciens is an important tool in plant research. The technique, known as agroinfiltration, provides fast, efficient ways to transiently express or silence a desired gene without resorting to the time-consuming, challenging stable transformation of the host, the use of less efficient means of delivery, such as bombardment, or the use of viral vectors, which multiply and spread within the host causing physiological alterations themselves. A drawback of the agroinfiltration technique is its temperature dependence: early studies have shown that temperatures above 29 °C are nonpermissive to tumour induction by the bacterium as a result of failure in pilus formation. However, research in plant sciences is interested in studying processes at these temperatures, above the 25 °C experimental standard, common to many host-environment and host-pathogen interactions in nature, and agroinfiltration is an excellent tool for this purpose. Here, we measured the efficiency of agroinfiltration for the expression of reporter genes in plants from T-DNAs at the nonpermissive temperature of 30 °C, either transiently or as part of viral amplicons, and envisaged procedures that allow and optimize its use for gene expression at this temperature. We applied this technical advance to assess the performance at 30 °C of two viral suppressors of silencing in agropatch assays [Potato virus Y helper component proteinase (HCPro) and Cucumber mosaic virus 2b protein] and, within the context of infection by a Potato virus X (PVX) vector, also assessed indirectly their effect on the overall response of the host Nicotiana benthamiana to the virus.

Agrobacterium infection and plant defense-transformation success hangs by a thread

The value of Agrobacterium tumefaciens for plant molecular biologists cannot be appreciated enough. This soil-borne pathogen has the unique capability to transfer DNA (T-DNA) into plant systems. Gene transfer involves both bacterial and host factors, and it is the orchestration of these factors that determines the success of transformation. Some plant species readily accept integration of foreign DNA, while others are recalcitrant. The timing and intensity of the microbially activated host defense repertoire sets the switch to "yes" or "no." This repertoire is comprised of the specific induction of mitogen-activated protein kinases (MAPKs), defense gene expression, production of reactive oxygen species (ROS) and hormonal adjustments. Agrobacterium tumefaciens abuses components of the host immunity system it mimics plant protein functions and manipulates hormone levels to bypass or override plant defenses. A better understanding of the ongoing molecular battle between agrobacteria and attacked hosts paves the way toward developing transformation protocols for recalcitrant plant species. This review highlights recent findings in agrobacterial transformation research conducted in diverse plant species. Efficiency-limiting factors, both of plant and bacterial origin, are summarized and discussed in a thought-provoking manner.

Tropaeolum tops tobacco - simple and efficient transgene expression in the order Brassicales

Transient expression systems are valuable tools in molecular biology. Agrobacterial infiltration of leaves is well-established in tobacco, but has led to limited success in the model plant Arabidopsis thaliana. An efficient expression system combining the advantages of Arabidopsis (well-characterised) and the simplicity of leaf infiltration is desirable. Here, I describe Agrobacterium tumefaciens-mediated transformation of Tropaeolummajus (nasturtium, order Brassicales) as a remarkably simple, cheap and highly efficient transient expression system. It provides the Arabidopsis community with a tool to study subcellular localisation, protein-protein interactions and reporter gene activities (e.g. luciferase, β-glucuronidase) in a genetic background that is closely related to their primary model organism. Unlike Arabidopsis, Tropaeolum is capable of engaging in endomycorrhizal associations and is therefore relevant also to symbiosis research. RNAi-based approaches are more likely to succeed than in the distantly-related Nicotiana transformation system. Tropaeolummajus was voted the "medicinal plant of the year 2013". Conquering this plant for genetic manipulations harbours potential for biotechnological and pharmacological applications.

Recent advances towards development and commercialization of plant cell culture processes for the synthesis of biomolecules

Plant cell culture systems were initially explored for use in commercial synthesis of several high-value secondary metabolites, allowing for sustainable production that was not limited by the low yields associated with natural harvest or the high cost associated with complex chemical synthesis. Although there have been some commercial successes, most notably paclitaxel production from Taxus sp., process limitations exist with regards to low product yields and inherent production variability. A variety of strategies are being developed to overcome these limitations including elicitation, in situ product removal and metabolic engineering with single genes and transcription factors. Recently, the plant cell culture production platform has been extended to pharmaceutically active heterologous proteins. Plant systems are beneficial because they are able to produce complex proteins that are properly glycosylated, folded and assembled without the risk of contamination by toxins that are associated with mammalian or microbial production systems. Additionally, plant cell culture isolates transgenic material from the environment, allows for more controllable conditions over field-grown crops and promotes secretion of proteins to the medium, reducing downstream purification costs. Despite these benefits, the increase in cost of heterologous protein synthesis in plant cell culture as opposed to field-grown crops is significant and therefore processes must be optimized with regard to maximizing secretion and enhancing protein stability in the cell culture media. This review discusses recent advancements in plant cell culture processing technology, focusing on progress towards overcoming the problems associated with commercialization of these production systems and highlighting recent commercial successes.

Recycling of Solanum sucrose transporters expressed in yeast, tobacco, and in mature phloem sieve elements

The plant sucrose transporter SUT1 (from Solanum tuberosum, S. lycopersicum, or Zea mays) exhibits redox-dependent dimerization and targeting if heterologously expressed in S. cerevisiae (Krügel et al., 2008). It was also shown that SUT1 is present in motile vesicles when expressed in tobacco cells and that its targeting to the plasma membrane is reversible. StSUT1 is internalized in the presence of brefeldin A (BFA) in yeast, plant cells, and in mature sieve elements as confirmed by immunolocalization. These results were confirmed here and the dynamics of intracellular SUT1 localization were further elucidated. Inhibitor studies revealed that vesicle movement of SUT1 is actin-dependent. BFA-mediated effects might indicate that anterograde vesicle movement is possible even in mature sieve elements, and could involve components of the cytoskeleton that were previously thought to be absent in SEs. Our results are in contradiction to this old dogma of plant physiology and the potential of mature sieve elements should therefore be re-evaluated. In addition, SUT1 internalization was found to be dependent on the plasma membrane lipid composition. SUT1 belongs to the detergent-resistant membrane (DRM) fraction in planta and is targeted to membrane raft-like microdomains when expressed in yeast (Krügel et al., 2008). Here, SUT1-GFP expression in different yeast mutants, which were unable to perform endocytosis and/or raft formation, revealed a strong link between SUT1 raft localization, the sterol composition and membrane potential of the yeast plasma membrane, and the capacity of the SUT1 protein to be internalized by endocytosis. The results provide new insight into the regulation of sucrose transport and the mechanism of endocytosis in plant cells.

Scale-up of Agrobacterium-mediated transient protein expression in bioreactor-grown Nicotiana glutinosa plant cell suspension culture

The reporter gene beta-glucuronidase was transiently expressed in a 51-L bioreactor-grown plant cell suspension culture of Nicotiana glutinosa at a yield of approximately 1.1 mg through co-culture with an auxotrophic strain of Agrobacterium tumefaciens. The three order of magnitude scale-up involved the investigation of factors contributing to transient expression including the timing of Agrobacterium inoculation relative to the plant cell growth phase, plant tissue culture hormonal triggers and plant cell cycle synchronization. The co-culture process was simplified to facilitate implementation in a pilot-scale bioreactor. At the shake flask scale it was determined that elevated concentrations of oxygen in the headspace were detrimental to transient expression levels and the addition of acetosyringone to the co-culture had a negligible effect. The bacterial preparation process was also streamlined, permitting the direct transfer of the Agrobacterium culture from a bench-scale fermentor to the pilot-scale plant cell culture bioreactor. Increasing expression levels and overcoming batch-to-batch variability despite extensive procedure systemization remain the major technical hurdles.

Improving containment strategies in biopharming

This review examines the challenges of segregating biopharmed crops expressing pharmaceutical or veterinary agents from mainstream crops, particularly those destined for food or feed use. The strategy of using major food crops as production vehicles for the expression of pharmaceutical or veterinary agents is critically analysed in the light of several recent episodes of contamination of the human food chain by non-approved crop varieties. Commercially viable strategies to limit or avoid biopharming intrusion into the human food chain require the more rigorous segregation of food and non-food varieties of the same crop species via a range of either physical or biological methods. Even more secure segregation is possible by the use of non-food crops, non-crop plants or in vitro plant cultures as production platforms for biopharming. Such platforms already under development range from outdoor-grown Nicotiana spp. to glasshouse-grown Arabidopsis, lotus and moss. Amongst the more effective methods for biocontainment are the plastid expression of transgenes, inducible and transient expression systems, and physical containment of plants or cell cultures. In the current atmosphere of heightened concerns over food safety and biosecurity, the future of biopharming may be largely determined by the extent to which the sector is able to maintain public confidence via a more considered approach to containment and security of its plant production systems.

A simplified method for the analysis of transcription factor-promoter interactions that allows high-throughput data generation

Transient expression systems are intensively used to study the transactivation potential of transcription factors and to confirm target promoters. Here we present a novel system based on the high-efficiency transformation of cultured Arabidopsis thaliana cells by agrobacteria. To demonstrate the potential of this system, we compared it with a commonly used protoplast transfection assay, and studied the regulation of phenylpropanoid biosynthetic pathway genes by various transcription factors. Both systems led to comparable results on the regulation of the promoters tested. However, the agrobacterium-mediated co-transformation assay needs significantly less time, requires only mixing of cultured plant cells with agrobacteria, is less labour-intensive and allows handling of multiple assays in parallel, making it suitable for medium- to high-throughput analyses. In addition, the binary vectors used are the same for both cell-based assays and stable plant transformations.

Agrobacterium-mediated viral vector-amplified transient gene expression in Nicotiana glutinosa plant tissue culture

A viral vector based on the bean yellow dwarf virus was investigated for its potential to increase transient gene expression. An intron-containing GUS reporter gene and the cis-acting viral regulatory elements were incorporated in the viral vector and could be complemented by the viral replication-associated proteins provided on a secondary vector. All vectors were delivered to Nicotiana glutinosa plant cell suspension or hairy root cultures via auxotrophic Agrobacterium tumefaciens. Cell culture generated greater yield of reporter gene expression than did root culture, as a result of the limitation imposed on roots to express the protein only in surface tissue containing actively dividing cells. Reporter gene expression increased for cell culture when the reporter gene construct was co-delivered with the construct supplying both viral replication associated proteins (REP and REPA); gene expression decreased when the construct supplying only the viral REP protein was co-delivered. Reporter protein expression increased from 0.091% for the reporter construct alone to 0.22% total soluble protein (% TSP) when the viral Rep-supplying vector was co-delivered with the reporter gene construct. Reporter protein was generated 3 days after the initiation of bacterial co-culture, providing for rapid generation of heterologous protein in cell culture.

Transient β-glucuronidase expression in lily (Lilium longflorum L.) pollen via wounding-assisted Agrobacterium-mediated transformation

A pollen-based transient expression system has been developed. Lily pollen grains, wounded by vigorous shaking in the presence of aluminum oxide particles, were transformed by infiltration with Agrobacterium tumefaciens LBA4404 cells harboring the beta-glucuronidase (GUS) gene construct, pBI121. In histochemical and fluorometric GUS analysis, the wounding processes allowed efficient transformation and, in cDNA blot hybridization, GUS mRNA synthesis was clearly detected. Lily pollen with appropriate wounds, therefore, can be used conveniently for the rapid production of recombinant proteins.

Production of plum pox virus HC-Pro functionally active for aphid transmission in a transient-expression system

Potyviruses are non-persistently transmitted by aphid vectors with the assistance of a viral accessory factor known as helper component (HC-Pro), a multifunctional protein that is also involved in many other essential processes during the virus infection cycle. A transient Agrobacterium-mediated expression system was used to produce Plum pox virus (PPV) HC-Pro in Nicotiana benthamiana leaves from constructs that incorporated the 5' region of the genome, yielding high levels of HC-Pro in agroinfiltrated leaves. The expressed PPV HC-Pro was able to assist aphid transmission of purified virus particles in a sequential feeding assay, and to complement transmission-defective variants of the virus. Also, HC-Pro of a second potyvirus, Tobacco etch virus (TEV), was expressed and found to be functional for aphid transmission. These results show that this transient system can be useful for production of functionally active HC-Pro in potyviruses, and the possible uses of this approach to study the mechanism of transmission are discussed.

An in vivo, luciferase-based, Agrobacterium-infiltration assay system: implications for post-transcriptional gene silencing

An in vivo assay system for analyzing transient luciferase expression in tobacco leaves infused with Agrobacterium tumefaciens is described. The system makes use of A. tumefaciens harboring T-DNA vectors containing either an intron-containing firefly (Photinus pyralis) luciferase (EC 1.13.12.7) gene or an intron-containing sea pansy (Renilla reniformis) luciferase (EC 1.13.12.5) gene. Single or mixed Agrobacterium lines were infiltrated into leaf tissues (Nicotiana tabacum or Nicotiana benthamiana) through stomatal openings and leaf disks from infused areas floated on reaction buffers specific to each enzyme. Photons emitted were then measured to determine reporter gene activity. Parameters affecting assay reliability and sensitivity were tested, including: buffer composition; bacterial density; infusion location; reaction kinetics; and environmental factors (light and temperature). The resulting in vivo assay system generates results comparable to those obtained using a commercially available in vitro dual-luciferase(R) reporter gene assay, and reports relative expression levels, as well as induction characteristics, analogous to those obtained using leaf tissue from stably transformed plants harboring the same promoter::gene constructs. Light and temperature were observed to markedly impact transient reporter activities. Co-expression of viral suppressors of post-transcriptional gene silencing (PTGS), HcPro, p19 and AC2, confirms the occurrence of PTGS within infused zones, and provides a convenient mechanism for PTGS analysis. The in vivo transient assay was used to examine the effect on PTGS of factors such as: promoter strength; incubation temperature and double-stranded RNA production. Results from these assays provide insight into the mechanism(s) used by plants to trigger and maintain PTGS.