High-level transient expression of recombinant protein in lettuce

Adding value to banana farming: Antibody production in post-harvest leaves

Banana, a globally popular fruit, is widely cultivated in tropical and sub-tropical regions. After fruit harvest, remaining banana plant materials are low-value byproducts, mostly composted or used as fibre or for food packaging. As an aim to potentially increase farmer income, this study explored underutilised banana biomass as a novel plant tissue for production of a high-value product. Protein scFvTG130 used in this study, is an anti-toxoplasma single chain variable fragment antibody that can be used in diagnostics and neutralising the Toxoplasma gondii pathogen. Using detached banana leaves, we investigated the factors influencing the efficacy of a transient expression system using reporter genes and recombinant protein, scFvTG130. Transient expression was optimal at 2 days after detached banana leaves were vacuum infiltrated at 0.08 MPa vacuum pressure for a duration of 3 min with 0.01% (v/v) Tween20 using Agrobacterium strain GV3101 harbouring disarmed virus-based vector pIR-GFPscFvTG130. The highest concentration of anti-toxoplasma scFvTG130 antibody obtained using detached banana leaves was 22.8 µg/g fresh leaf tissue. This first study using detached banana leaf tissue for the transient expression of a recombinant protein, successfully demonstrated anti-toxoplasma scFvTG130 antibody expression, supporting the potential application for other related proteins using an underutilised detached banana leaf tissue.

Recent development of oral vaccines (Review)

Oral immunization can elicit an effective immune response and immune tolerance to specific antigens. When compared with the traditional injection route, delivering antigens via the gastrointestinal mucosa offers superior immune effects and compliance, as well as simplicity and convenience, making it a more optimal route for immunization. At present, various oral vaccine delivery systems exist. Certain modified bacteria, such as Salmonella, Escherichia coli and particularly Lactobacillus, are considered promising carriers for oral vaccines. These carriers can significantly enhance immunization efficiency by actively replicating in the intestinal tract following oral administration. The present review provided a discussion of the main mechanisms of oral immunity and the research progress made in the field of oral vaccines. Additionally, it introduced the advantages and disadvantages of the currently more commonly administered injectable COVID-19 vaccines, alongside the latest advancements in this area. Furthermore, recent developments in oral vaccines are summarized, and their potential benefits and side effects are discussed.

Effects of different light conditions on transient expression and biomass in Nicotiana benthamiana leaves

In the process of the production of recombinant proteins by using an Agrobacterium-mediated transient gene expression system, the effectiveness of the control of light conditions pre- and post-agroinfiltration on efficiency of transient expression is worth being evaluated. In this study, Nicotiana benthamiana plants were used as a bioreactor to investigate the effects of different light conditions pre- and post-agroinfiltration on the transient expression of green fluorescent protein (GFP). The results showed that the plants grown under light condition for 5 weeks had the highest level of transient expression among those grown for 4-8 weeks. In the pre-agroinfiltration, the level of transient expression of GFP was obviously decreased by the increase in light intensity or by the shortening of the photoperiod. Although the shortening of the photoperiod post-agroinfiltration also decreased the level of transient expression, moderate light intensity post-agroinfiltration was needed for higher level of transient expression efficiency. However, there was no strong correlation between the transient expression efficiency and plant growth. The results suggested that light condition was an important factor affecting the level of transient expression in plants. Hence, light conditions should be optimized to obtain higher productivity of recombinant protein from transient expression systems.

Risk assessment and bioburden evaluation of Agrobacterium tumefaciens-mediated transient protein expression in plants using the CaMV35S promoter

Large-scale transient expression of recombinant proteins in plants is increasingly used and requires the multi-liter cultivation of Agrobacterium tumefaciens transformed with an expression vector, which is often cloned in Escherichia coli first. Depending on the promoter, unintentional activity can occur in both bacteria, which could pose a safety risk to the environment and operators if the protein is toxic. To assess the risk associated with transient expression, we first tested expression vectors containing the CaMV35S promoter known to be active in plants and bacteria, along with controls to measure the accumulation of the corresponding recombinant proteins. We found that, in both bacteria, even the stable model protein DsRed accumulated at levels near the detection limit of the sandwich ELISA (3.8 µg L^-1). Higher levels were detected in short cultivations (< 12 h) but never exceeded 10 µg L^-1. We determined the abundance of A. tumefaciens throughout the process, including infiltration. We detected few bacteria in the clarified extract and found none after blanching. Finally, we combined protein accumulation and bacterial abundance data with the known effects of toxic proteins to estimate critical exposures for operators. We found that unintended toxin production in bacteria is negligible. Furthermore, the intravenous uptake of multiple milliliters of fermentation broth or infiltration suspension would be required to reach acute toxicity even when handling the most toxic products (LD₅₀ ~ 1 ng kg^-1). The unintentional uptake of such quantities is unlikely and we therefore regard transient expression as safe in terms of the bacterial handling procedure.

Production of Genistein in Amaranthus tricolor var. tristis and Spinacia oleracea by Expression of Glycine max Isoflavone Synthase

Isoflavonoids, the diverse group of secondary metabolites derived from the phenylpropanoid pathway, are distributed predominantly in leguminous plants. It has received considerable attention in recent days due to its health promoting benefits and is known to prevent certain diseases in humans. These isoflavonoids are synthesized from flavonoid intermediates of phenylpropanoid pathway by the enzyme isoflavone synthase. Metabolic engineering of isoflavonoid biosynthesis in non-legume crop plants could offer the health benefits of these compounds in diverse plant species further contributing for crop improvement. The transient expression of heterologous genes in the host is considered as an alternative to stable expression, that can provide a rapid way of studying the pathway engineering for metabolite production and could also act as a production platform for nutraceuticals and biopharmaceuticals. In this study, isoflavone genistein was produced in Amaranthus tricolor var. tristis and Spinacia oleracea by transiently expressing Glycine max isoflavone synthase (GmIFS). The GmIFS gene was cloned in plant expression vector pEarleyGate 102 HA and pEAQ-HT-DEST 3 and transformed into plants by agroinfiltration. The presence of transgene in the agroinfiltrated leaves was confirmed by semiquantitative reverse-transcription polymerase chain reaction. The flavonoid substrate naringenin and isoflavonoid genistein were quantified using high performance liquid chromatography in both wild-type and infiltrated leaf samples of both the plants. The naringenin content varied in the range of 65.5-338.5 nM/g fresh weight, while the accumulation of genistein was observed with varying concentrations from 113 to 182.6 nM/g fresh weight in the agroinfiltrated leaf samples of both A. tricolor var. tristis and S. oleracea. These results indicate that the transient expression of GmIFS gene has led to the synthesis of isoflavonoid genistein in A. tricolor var. tristis and S. oleracea providing an insight that stable expression of this gene could enrich the nutraceutical content in the crop plants. To the best of our knowledge, this is the first report on transient expression of GmIFS gene for the production of genistein in A. tricolor var. tristis and S. oleracea.

Agroinfiltration Mediated Scalable Transient Gene Expression in Genome Edited Crop Plants

Agrobacterium-mediated transformation is one of the most commonly used genetic transformation method that involves transfer of foreign genes into target plants. Agroinfiltration, an Agrobacterium-based transient approach and the breakthrough discovery of CRISPR/Cas9 holds trending stature to perform targeted and efficient genome editing (GE). The predominant feature of agroinfiltration is the abolishment of Transfer-DNA (T-DNA) integration event to ensure fewer biosafety and regulatory issues besides showcasing the capability to perform transcription and translation efficiently, hence providing a large picture through pilot-scale experiment via transient approach. The direct delivery of recombinant agrobacteria through this approach carrying CRISPR/Cas cassette to knockout the expression of the target gene in the intercellular tissue spaces by physical or vacuum infiltration can simplify the targeted site modification. This review aims to provide information on Agrobacterium-mediated transformation and implementation of agroinfiltration with GE to widen the horizon of targeted genome editing before a stable genome editing approach. This will ease the screening of numerous functions of genes in different plant species with wider applicability in future.

Molecular pharming to support human life on the moon, mars, and beyond

Space missions have always assumed that the risk of spacecraft malfunction far outweighs the risk of human system failure. This assumption breaks down for longer duration exploration missions and exposes vulnerabilities in space medical systems. Space agencies can no longer reduce the majority of the human health and performance risks through crew members selection process and emergency re-supply or evacuation. No mature medical solutions exist to address this risk. With recent advances in biotechnology, there is promise for lessening this risk by augmenting a space pharmacy with a biologically-based space foundry for the on-demand manufacturing of high-value medical products. Here we review the challenges and opportunities of molecular pharming, the production of pharmaceuticals in plants, as the basis of a space medical foundry to close the risk gap in current space medical systems. Plants have long been considered to be an important life support object in space and can now also be viewed as programmable factories in space. Advances in molecular pharming-based space foundries will have widespread applications in promoting simple and accessible pharmaceutical manufacturing on Earth.

From Farm to Finger Prick-A Perspective on How Plants Can Help in the Fight Against COVID-19

As a consequence of the COVID-19 pandemic crisis, farmers across the country are plowing under their fields and laying off workers. Plant biomass has been shown by the DARPA “Blue Angel” project in 2010 to be an efficient way to rapidly make vaccines and diagnostics. This technology could pivot some areas of agriculture toward biomedical products to aid in the COVID-19 pandemic response.

Arabidopsis Hairy Roots Producing High Level of Active Human Gastric Lipase

Arabidopsis hairy roots were used to produce human gastric lipase. When treated with 2,4-D, the hairy roots developed into thick organs that produced more protein than untreated roots. This was first assessed using green fluorescent protein-producing root lines from which the protein diffused into the culture medium. When growing hairy roots which express the human gastric lipase gene, very little lipase was found in the medium. Incubating the roots in a low pH buffer resulted in lipase diffusion into the buffer, avoiding the need for grinding. The activity of the enzyme on 4-methylumbellireryl-oleate and on tributyrin was determined. Approximately 6000 units of enzyme were recovered per gram of root. The enzyme was also extracted from freeze-dried roots before and after a 2-month storage period at room temperature. This work demonstrates the relevance of Arabidopsis hairy roots for the production of human gastric lipase.

Design of a Type-1 Diabetes Vaccine Candidate Using Edible Plants Expressing a Major Autoantigen

Type-1 diabetes (T1D) is a metabolic disease involving the autoimmune destruction of insulin-producing pancreatic beta cells. It is often diagnosed by the detection of autoantibodies, typically those recognizing insulin itself or the 65-kDa isoform of glutamic acid decarboxylase (GAD65). Oral insulin can be used to induce systemic immunological tolerance and thus prevent or delay the onset of T1D, suggesting that combination treatments with other autoantigens such as GAD65 could be even more successful. GAD65 has induced oral tolerance and prevented T1D in preclinical studies but it is difficult to produce in sufficient quantities for clinical testing. Here we combined edible plant systems, namely spinach (Spinacia oleracea cv Industra) and red beet (Beta vulgaris cv Moulin Rouge), with the magnICON^® expression system to develop a safe, cost-effective and environmentally sustainable platform for the large-scale production of GAD65. The superior red beet platform was extensively characterized in terms of recombinant protein yields and bioequivalence to wild-type plants, and the product was tested for its ability to resist simulated gastric digestion. Our results indicate that red beet plants are suitable for the production of a candidate oral vaccine based on GAD65 for the future preclinical and clinical testing of T1D immunotherapy approaches.

Growth kinetics and scale-up of Agrobacterium tumefaciens

Production of recombinant proteins in plants through Agrobacterium-mediated transient expression is a promising method of producing human therapeutic proteins, vaccines, and commercial enzymes. This process has been shown to be viable at a large scale and involves growing large quantities of wild-type plants and infiltrating the leaf tissue with a suspension of Agrobacterium tumefaciens bearing the genes of interest. This study examined one of the steps in this process that had not yet been optimized: the scale-up of Agrobacterium production to sufficient volumes for large-scale plant infiltration. Production of Agrobacterium strain C58C1 pTFS40 was scaled up from shake flasks (50-100 mL) to benchtop (5 L) scale with three types of media: Lysogeny broth (LB), yeast extract peptone (YEP) media, and a sucrose-based defined media. The maximum specific growth rate (μ max) of the strain in the three types of media was 0.46 ± 0.04 h-1 in LB media, 0.43 ± 0.03 h-1 in YEP media, and 0.27 ± 0.01 h-1 in defined media. The maximum biomass concentration reached at this scale was 2.0 ± 0.1, 2.8 ± 0.1, and 2.6 ± 0.1 g dry cell weight (DCW)/L for the three media types. Production was successfully scaled up to a 100-L working volume reactor with YEP media, using k L a as the scale-up parameter.

Media development for large scale Agrobacterium tumefaciens culture

A chemically defined media was developed for growing Agrobacterium tumefaciens at large scale for commercial production of recombinant proteins by transient expression in plants. Design of experiments was used to identify major and secondary effects of ten media components: sucrose, ammonium sulfate ((NH₄ )₂ SO₄ ), magnesium sulfate heptahydrate (MgSO₄ *7H₂ O), calcium chloride dihydrate (CaCl₂ *2H₂ O), iron (II) sulfate heptahydrate (FeSO₄ *7H₂ O), manganese (II) sulfate monohydrate (MnSO₄ *H₂ O), zinc sulfate heptahydrate (ZnSO₄ *7H₂ O), sodium chloride (NaCl), potassium chloride (KCl) and a sodium/potassium phosphate buffer (Na₂ HPO₄ /KH₂ PO₄ ). Calcium and zinc were found to have no detectable impact on biomass concentration or transient expression level, and concentrations of the other components that maximized final biomass concentration were determined. The maximum specific growth rate of Agrobacterium strain C58C1 pTFS40 in this media was 0.33 ± 0.01 h^-1 and the final biomass concentration after 26 h of batch growth in shake flasks was 2.6 g dry cell weight/L. Transient expression levels of the reporter protein GUS following infiltration of a recombinant Agrobacterium strain C58C1 into N. benthamiana were comparable when the strain was grown in the defined media, Lysogeny Broth (LB) media, or yeast extract-peptone (YEP) media. In LB and YEP media, free amino acid concentration was measured at three points over the course of batch growth of Agrobacterium strain C58C1 pTFS40; results indicated that l-serine and l-asparagine were depleted from the media first, followed by l-alanine and l-glutamic acid. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1218-1225, 2017.

Mature Luffa Leaves (Luffa cylindrica L.) as a Tool for Gene Expression Analysis by Agroinfiltration

We exploited the potential of cucurbits for ectopic gene expression. Agroinfiltration is a simple and commonly used method to obtain transient expression of foreign genes in plants. In contrast to in vitro transformation techniques, agroinfiltration can be used for genetic modification of mature plant tissues. Although the cucurbits are commonly used as model plants for molecular biology and biotechnology studies, to date there are no literature sources on the possibility of transient gene expression in mature cucurbit tissues. Our research has shown that mature leaves of Luffa cylindrica L. (luffa), in contrast to other cucurbit species, can be successfully transiently transformed with Agrobacterium tumefaciens. We efficiently transformed luffa leaves with a reporter gene encoding β-glucuronidase (GUS). The GUS activity in transiently transformed leaf tissues was detected within 24 h after the infiltration with bacteria. Additionally, we have shown that the activity of a transiently expressed the GUS gene can be monitored directly in the EDTA-exudates collected from the cut petioles of the agroinfiltrated leaves. The results suggest that luffa leaves can be useful as a plant expression system for studies of physiological and biochemical processes in cucurbits.

Removal of bacterial suspension water occupying the intercellular space of detached leaves after agroinfiltration improves the yield of recombinant hemagglutinin in a Nicotiana benthamiana transient gene expression system

The use of detached leaves instead of whole plants provides an alternative means for recombinant protein production based on Agrobacterium tumefaciens-mediated transient gene overexpression. However, the process for high-level protein production in detached leaves has not yet been established. In this study, we focused on leaf handling and maintenance conditions immediately after infiltration with Agrobacterium suspension (agroinfiltration) to improve recombinant protein expression in detached Nicotiana benthamiana leaves. We demonstrated that the residual water of bacterial suspension in detached leaves had significant impact on the yield of recombinant influenza hemagglutinin (HA). Immediately after agroinfiltration, detached leaves were stored in a dehumidified chamber to allow bacterial suspension water occupying intercellular space to be removed by transpiration. We varied the duration of this water removal treatment from 0.7 to 4.4 h, which resulted in leaf fresh weights ranging from 0.94 to 1.28 g g(-1) relative to weights measured just before agroinfiltration. We used these relative fresh weights (RFWs) as an indicator of the amount of residual water. The detached leaves were then incubated in humidified chambers for 6 days. We found that the presence of residual water significantly decreased HA yield, with a clear inverse correlation observed between HA yield and RFW. We next compared HA yields in detached leaves with those obtained from intact leaves by whole-plant expression performed at the same time. The maximum HA yield obtained from a detached leaf with a RFW of approximately 1.0, namely, 800 μg gFW(-1), was comparable to the mean HA yield of 846 μg gFW(-1) generated in intact leaves. Our results indicate the necessity of removing bacterial suspension water from agroinfiltrated detached leaves in transient overexpression systems and point to a critical factor enabling the detached-leaf system as a viable recombinant protein factory.

Environment Control to Improve Recombinant Protein Yields in Plants Based on Agrobacterium-Mediated Transient Gene Expression

Agrobacterium-mediated transient expression systems enable plants to produce a wide range of recombinant proteins on a rapid timescale. To achieve economically feasible upstream production and downstream processing, two yield parameters should be considered: (1) recombinant protein content per unit biomass and (2) recombinant protein productivity per unit area-time at the end of the upstream production. Because environmental factors in the upstream production have impacts on these parameters, environment control is important to maximize the recombinant protein yield. In this review, we summarize the effects of pre- and postinoculation environmental factors in the upstream production on the yield parameters and discuss the basic concept of environment control for plant-based transient expression systems. Preinoculation environmental factors associated with planting density, light quality, and nutrient supply affect plant characteristics, such as biomass and morphology, which in turn affect recombinant protein content and productivity. Accordingly, environment control for such plant characteristics has significant implications to achieve a high yield. On the other hand, postinoculation environmental factors, such as temperature, light intensity, and humidity, have been shown to affect recombinant protein content. Considering that recombinant protein production in Agrobacterium-mediated transient expression systems is a result of a series of complex biological events starting from T-DNA transfer from Agrobacterium tumefaciens to protein biosynthesis and accumulation in leaf tissue, we propose that dynamic environment control during the postinoculation process, i.e., changing environmental conditions at an appropriate timing for each event, may be a promising approach to obtain a high yield. Detailed descriptions of plant growth conditions and careful examination of environmental effects will significantly contribute to our knowledge to stably obtain high recombinant protein content and productivity, thus enhancing the utility of plant-based transient expression systems as recombinant protein factories.

Transient Protein Expression by Agroinfiltration in Lettuce

Current systems of recombinant protein production include bacterial, insect, and mammalian cell culture. However, these platforms are expensive to build and operate at commercial scales and/or have limited abilities to produce complex proteins. In recent years, plant-based expression systems have become top candidates for the production of recombinant proteins as they are highly scalable, robust, safe, and can produce complex proteins due to having a eukaryotic endomembrane system. Newly developed "deconstructed" viral vectors delivered via Agrobacterium tumefaciens (agroinfiltration) have enabled robust plant-based production of proteins with a wide range of applications. The leafy Lactuca sativa (lettuce) plant with its strong foundation in agriculture is an excellent host for pharmaceutical protein production. Here, we describe a method for agroinfiltration of lettuce that can rapidly produce high levels of recombinant proteins in a matter of days and has the potential to be scaled up to an agricultural level.

Influence of Vacuum Cooling on Escherichia coli O157:H7 Infiltration in Fresh Leafy Greens via a Multiphoton-Imaging Approach

Microbial pathogen infiltration in fresh leafy greens is a significant food safety risk factor. In various postharvest operations, vacuum cooling is a critical process for maintaining the quality of fresh produce. The overall goal of this study was to evaluate the risk of vacuum cooling-induced infiltration of Escherichia coli O157:H7 into lettuce using multiphoton microscopy. Multiphoton imaging was chosen as the method to locate E. coli O157:H7 within an intact lettuce leaf due to its high spatial resolution, low background fluorescence, and near-infrared (NIR) excitation source compared to those of conventional confocal microscopy. The variables vacuum cooling, surface moisture, and leaf side were evaluated in a three-way factorial study with E. coli O157:H7 on lettuce. A total of 188 image stacks were collected. The images were analyzed for E. coli O157:H7 association with stomata and E. coli O157:H7 infiltration. The quantitative imaging data were statistically analyzed using analysis of variance (ANOVA). The results indicate that the low-moisture condition led to an increased risk of microbial association with stomata (P < 0.05). Additionally, the interaction between vacuum cooling levels and moisture levels led to an increased risk of infiltration (P < 0.05). This study also demonstrates the potential of multiphoton imaging for improving sensitivity and resolution of imaging-based measurements of microbial interactions with intact leaf structures, including infiltration.

Agrobacterium tumefaciens mediated transient expression of plant cell wall-degrading enzymes in detached sunflower leaves

For biofuel applications, synthetic endoglucanase E1 and xylanase (Xyn10A) derived from Acidothermus cellulolyticus were transiently expressed in detached whole sunflower (Helianthus annuus L.) leaves using vacuum infiltration. Three different expression systems were tested, including the constitutive CaMV 35S-driven, CMVar (Cucumber mosaic virus advanced replicating), and TRBO (Tobacco mosaic virus RNA-Based Overexpression Vector) systems. For 6-day leaf incubations, codon-optimized E1 and xylanase driven by the CaMV 35S promoter were successfully expressed in sunflower leaves. The two viral expression vectors, CMVar and TRBO, were not successful although we found high expression in Nicotiana benthamiana leaves previously for other recombinant proteins. To further enhance transient expression, we demonstrated two novel methods: using the plant hormone methyl jasmonic acid in the agroinfiltration buffer and two-phase optimization of the leaf incubation temperature. When methyl jasmonic acid was added to Agrobacterium tumefaciens cell suspensions and infiltrated into plant leaves, the functional enzyme production increased 4.6-fold. Production also increased up to 4.2-fold when the leaf incubation temperature was elevated above the typical temperature, 20C, to 30C in the late incubation phase, presumably due to enhanced rate of protein synthesis in plant cells. Finally, we demonstrated co-expression of E1 and xylanase in detached sunflower leaves. To our knowledge, this is the first report of (co)expression of heterologous plant cell wall-degrading enzymes in sunflower.

Attachment of Agrobacterium tumefaciens to leaf tissue in response to infiltration conditions

Transient expression of recombinant proteins in plant tissues following Agrobacterium-mediated gene transfer is a promising technique for rapid protein production. However, transformation rates and transient expression levels can be sub-optimal depending on process conditions. Attachment of Agrobacterium tumefaciens to plant cells is an early, critical step in the gene transfer pathway. Bacterial attachment levels and patterns may influence transformation and, by extension, transient expression. In this study, attachment of A. tumefaciens to lettuce leaf tissue was investigated in response to varying infiltration conditions, including bacterial density, surfactant concentration, and applied vacuum level. Bacterial density was found to most influence attachment levels for the levels tested (10(8) , 10(9) , and 10(10) CFU/mL), with the relationship between bacterial density and attachment levels following a saturation trend. Surfactant levels tested (Break-Thru S240: 1, 10, 100, and 1,000 µL/L) also had a significant positive effect on bacterial attachment while vacuum level (5, 25, and 45 kPa) did not significantly affect attachment in areas exposed to bacteria. In planta transgene transient expression levels were measured following infiltration with 10(8) , 10(9) , and 10(10) CFU/mL bacterial suspension. Notably, the highest attachment level tested led to a decrease in transient expression, suggesting a potential link between bacterial attachment levels and downstream phenomena that may induce gene silencing. These results illustrate that attachment can be controlled by adjusting infiltration conditions and that attachment levels can impact transgene transient expression in leaf tissue.

Agrobacterium tumefaciens-mediated transient transformation of Arabidopsis thaliana leaves

Transient assays provide a convenient alternative to stable transformation. Compared to the generation of stably transformed plants, agroinfiltration is more rapid, and samples can be analyzed a few days after inoculation. Nevertheless, at difference of tobacco and other plant species, Arabidopsis thaliana remains recalcitrant to routine transient assays. In this chapter, we describe a transient expression assay using simple infiltration of intact Arabidopsis leaves with Agrobacterium tumefaciens carrying a plasmid expressing a reporter fluorescent protein. In this protocol, Agrobacterium aggressiveness was increased by a prolonged treatment in an induction medium deficient in nutrients and containing acetosyringone. Besides, Arabidopsis plants were cultivated in intermediate photoperiod (12 h light-12 h dark) to promote leaf growth.

Predictive models for transient protein expression in tobacco (Nicotiana tabacum L.) can optimize process time, yield, and downstream costs

The transient expression of recombinant biopharmaceutical proteins in plants can suffer inter-batch variation, which is considered a major drawback under the strict regulatory demands imposed by current good manufacturing practice (cGMP). However, we have achieved transient expression of the monoclonal antibody 2G12 and the fluorescent marker protein DsRed in tobacco leaves with ∼ 15% intra-batch coefficients of variation, which is within the range reported for transgenic plants. We developed models for the transient expression of both proteins that predicted quantitative expression levels based on five parameters: The OD(600 nm) of Agrobacterium tumefaciens (from 0.13 to 2.00), post-inoculation incubation temperature (15-30°C), plant age (harvest at 40 or 47 days after seeding), leaf age, and position within the leaf. The expression models were combined with a model of plant biomass distribution and extraction, generating a yield model for each target protein that could predict the amount of protein in specific leaf parts, individual leaves, groups of leaves, and whole plants. When the yield model was combined with a cost function for the production process, we were able to perform calculations to optimize process time, yield, or downstream costs. We illustrate this procedure by transferring the cost function from a production process using transgenic plants to a hypothetical process for the transient expression of 2G12. Our models allow the economic evaluation of new plant-based production processes and provide greater insight into the parameters that affect transient protein expression in plants.

Rapid, in situ detection of Agrobacterium tumefaciens attachment to leaf tissue

Attachment of the plant pathogen Agrobacterium tumefaciens to host plant cells is an early and necessary step in plant transformation and agroinfiltration processes. However, bacterial attachment behavior is not well understood in complex plant tissues. Here we developed an imaging-based method to observe and quantify A. tumefaciens attached to leaf tissue in situ. Fluorescent labeling of bacteria with nucleic acid, protein, and vital dyes was investigated as a rapid alternative to generating recombinant strains expressing fluorescent proteins. Syto 16 green fluorescent nucleic acid stain was found to yield the greatest signal intensity in stained bacteria without affecting viability or infectivity. Stained bacteria retained the stain and were detectable over 72 h. To demonstrate in situ detection of attached bacteria, confocal fluorescent microscopy was used to image A. tumefaciens in sections of lettuce leaf tissue following vacuum-infiltration with labeled bacteria. Bacterial signals were associated with plant cell surfaces, suggesting detection of bacteria attached to plant cells. Bacterial attachment to specific leaf tissues was in agreement with known leaf tissue competencies for transformation with Agrobacterium. Levels of bacteria attached to leaf cells were quantified over time post-infiltration. Signals from stained bacteria were stable over the first 24 h following infiltration but decreased in intensity as bacteria multiplied in planta. Nucleic acid staining of A. tumefaciens followed by confocal microscopy of infected leaf tissue offers a rapid, in situ method for evaluating attachment of A. tumefaciens' to plant expression hosts and a tool to facilitate management of transient expression processes via agroinfiltration.

Coordinate expression of multiple proteins in plant cells by exploiting endogenous kex2p-like protease activity

Simultaneous expression of multiple proteins in plants finds ample applications. Here, we examined the biotechnological application of native kex2p-like protease activity in plants for coordinate expression of multiple secretory proteins from a single transgene encoding a cleavable polyprotein precursor. We expressed a secretory red fluorescent protein (DsRed) or human cytokine (GMCSF), fused to a downstream green fluorescent protein (GFP) by a linker containing putative recognition sites of the kex2p-like protease in tobacco cells and referred to them as RKG and GKG cells, respectively. Our analyses showed that GFP is cleaved off the fusion proteins and secreted into the media by both RKG and GKG cells. The cleaved GFP product displayed the expected fluorescence characteristics. Using GFP immunoprecipitation and fluorescence analysis, the cleaved DsRed product in the RKG cells was found to be functional as well. However, DsRed was not detected in the RKG culture medium, possibly due to its tetramer formation. Cleaved and biologically active GMCSF could also be detected in GKG cell extracts, but secreted GMCSF was found to be only at a low level, likely because of instability of GMCSF protein in the medium. Processing of polyprotein precursors was observed to be similarly effective in tobacco leaf, stem and root tissues. Importantly, we also demonstrated that, via agroinfiltration, polyprotein precursors can be efficiently processed in plant species other than tobacco. Collectively, our results demonstrate the utility of native kex2p-like protease activity for the expression of multiple secretory proteins in plant cells using cleavable polyprotein precursors containing kex2p linker(s).

Functional analysis of the DAT gene promoter using transient Catharanthus roseus and stable Nicotiana tabacum transformation systems

The Catharanthus roseus DAT gene encodes the enzyme acetyl-CoA:deacetylvindoline-4-O-acetyltransferase involved in the last step of the indole alkaloid pathway leading to vindoline. This gene is characterized by specific cell type expression in idioblasts and laticifers. To understand the specific transcriptional regulation mechanism(s) of DAT, several DAT promoter GUS constructs were cloned into pCAMBIA1305.1. Agroinfiltration of different explant types of C. roseus resulted in organ-specific accumulation of GUS, albeit at various levels. Heterologous accumulation of GUS in transgenic tobacco revealed both general and non-specific expression with the exception of a stomata-specific expression when 2.3 kb of the DAT promoter was coupled with a portion of the DAT ORF. These results suggest that in addition to the 2.3 kb upstream of the DAT transcriptional start site, additional cis-acting elements may be responsible for the specific spatial expression of DAT in vivo. Furthermore, hairy roots transformed with DAT promoter GUS constructs demonstrated GUS expression in root tissues (visualized through GUS enzyme activity), even though DAT is repressed in non-transformed roots.

Highly efficient Agrobacterium-mediated transformation of banana cv. Rasthali (AAB) via sonication and vacuum infiltration

A reproducible and efficient transformation method was developed for the banana cv. Rasthali (AAB) via Agrobacterium-mediated genetic transformation of suckers. Three-month-old banana suckers were used as explant and three Agrobacterium tumefaciens strains (EHA105, EHA101, and LBA4404) harboring the binary vector pCAMBIA1301 were used in the co-cultivation. The banana suckers were sonicated and vacuum infiltered with each of the three A. tumefaciens strains and co-cultivated in the medium containing different concentrations of acetosyringone for 3 days. The transformed shoots were selected in 30 mg/l hygromycin-containing selection medium and rooted in rooting medium containing 1 mg/l IBA and 30 mg/l hygromycin. The presence and integration of the hpt II and gus genes into the banana genome were confirmed by GUS histochemical assay, polymerase chain reaction, and southern hybridization. Among the different combinations tested, high transformation efficiency (39.4 ± 0.5% GUS positive shoots) was obtained when suckers were sonicated and vacuum infiltered for 6 min with A. tumefaciens EHA105 in presence of 50 μM acetosyringone followed by co-cultivation in 50 μM acetosyringone-containing medium for 3 days. These results suggest that an efficient Agrobacterium-mediated transformation protocol for stable integration of foreign genes into banana has been developed and that this transformation system could be useful for future studies on transferring economically important genes into banana.

Review article: commercialization of whole-plant systems for biomanufacturing of protein products: evolution and prospects

Technology for enabling plants to biomanufacture nonnative proteins in commercially significant quantities has been available for just over 20 years. During that time, the agricultural world has witnessed rapid commercialization and widespread adoption of transgenic crops enhanced for agronomic performance (herbicide-tolerance, insect-resistance), while plant-made pharmaceuticals (PMPs) and plant-made industrial products (PMIPs) have been limited to experimental and small-scale commercial production. This difference in the rate of commercial implementation likely reflects the very different business-development challenges associated with 'product' technologies compared with 'enabling' ('platform') technologies. However, considerable progress has been made in advancing and refining plant-based production of proteins, both technologically and in regard to identifying optimal business prospects. This review summarizes these developments, contrasting today's technologies and prospective applications with those of the industry's formative years, and suggesting how the PM(I)P industry's evolution has generated a very positive outlook for the 'plant-made' paradigm.

Agrobacterium-mediated transient gene expression and silencing: a rapid tool for functional gene assay in potato

Potato is the third most important food crop worldwide. However, genetic and genomic research of potato has lagged behind other major crops due to the autopolyploidy and highly heterozygous nature associated with the potato genome. Reliable and technically undemanding techniques are not available for functional gene assays in potato. Here we report the development of a transient gene expression and silencing system in potato. Gene expression or RNAi-based gene silencing constructs were delivered into potato leaf cells using Agrobacterium-mediated infiltration. Agroinfiltration of various gene constructs consistently resulted in potato cell transformation and spread of the transgenic cells around infiltration zones. The efficiency of agroinfiltration was affected by potato genotypes, concentration of Agrobacterium, and plant growth conditions. We demonstrated that the agroinfiltration-based transient gene expression can be used to detect potato proteins in sub-cellular compartments in living cells. We established a double agroinfiltration procedure that allows to test whether a specific gene is associated with potato late blight resistance pathway mediated by the resistance gene RB. This procedure provides a powerful approach for high throughput functional assay for a large number of candidate genes in potato late blight resistance.

A model of Agrobacterium tumefaciens vacuum infiltration into harvested leaf tissue and subsequent in planta transgene transient expression

Agrobacterium-mediated gene transfer, or agroinfiltration, can be a highly efficient method for transforming and inducing transient transgene expression in plant tissue. The technique uses the innate DNA secretion pathway of Agrobacterium tumefaciens to vector a particular plasmid-encoded segment of DNA from the bacteria to plant cells. Vacuum is often applied to plant tissue submerged in a suspension of A. tumefaciens to improve agroinfiltration. However, the effects of vacuum application on agroinfiltration and in planta transient transgene expression have not been well quantified. Here we show that vacuum application and release act to drive A. tumefaciens suspension into the interior of leaf tissue. Moreover, the amount of suspension that enters leaves can be predicted based on the vacuum intensity and duration. Furthermore, we show that transient expression levels of an agroinfiltrated reporter gene vary in response to the amount of A. tumefaciens vacuum infiltrated into leaf tissue, suggesting that vacuum infiltration conditions can be tailored to achieve optimal transient transgene expression levels after agroinfiltration.

Transient transformation of plants

Transient expression in plants is a valuable tool for many aspects of functional genomics and promoter testing. It can be used both to over-express and to silence candidate genes. It is also scaleable and provides a viable alternative to microbial fermentation and animal cell culture for the production of recombinant proteins. It does not depend on chromosomal integration of heterologous DNA so is a relatively facile procedure and can lead to high levels of transgene expression. Recombinant DNA can be introduced into plant cells via physical methods, via Agrobacterium or via viral vectors.

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.

Transient co-expression of post-transcriptional gene silencing suppressors and beta-glucuronidase in harvested lettuce leaf tissue does not improve recombinant protein accumulation in planta

Agrobacterium-mediated gene transfer was used to co-express three virus-derived post-transcriptional gene silencing (PTGS) suppressors, P19 from tomato bushy stunt virus and two species of helper component proteinase (HcPro) from tobacco etch virus (TEV) and turnip mosaic virus, with beta-glucuronidase (GUS) in harvested lettuce leaf tissue to investigate whether GUS accumulation increases in the presence of PTGS suppressors. Co-expression incubations were 3-5 days at 4 and 22 degrees C. GUS activity and leaf viability were measured after incubation. Co-expression of PTGS suppressors did not elevate GUS expression levels. Under certain incubation conditions, co-expression of TEV HcPro significantly lowered transient GUS expression and was detrimental to leaf viability, suggesting that expression of PTGS silencers may have a negative effect on transient expression levels that outweighs any effects of PTGS suppression in harvested leaf tissues.

Comparison of several Nicotiana species as hosts for high-scale Agrobacterium-mediated transient expression

Agrobacterium-mediated transient expression may be regarded as a promising method for inexpensive large-scale production of recombinant proteins. We optimized the protocol of transient expression in Nicotiana benthamiana and compared six Australian species of Nicotiana as hosts for transient expression. The transient expression of GFP under 35S CaMV promoter was observed in all species tested, although the GFP content in leaves of N. benthamiana, N. exigua, and N. excelsior was significantly higher (3.8, 3.7, and 2.0% TSP, respectively). Usage of viral-based expression system resulted in considerable increase of GFP accumulation in N. excelsior and N. benthamiana (63.5 and 16.2% TSP, respectively). We displayed that N. excelsior has the best characteristics in regard to biomass yield as well as GFP accumulation level for both types of the expression cassettes tested.