Improvement of the transient expression system for production of recombinant proteins in plants

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.

Oral Immunization With a Plant HSP90-SAG1 Fusion Protein Produced in Tobacco Elicits Strong Immune Responses and Reduces Cyst Number and Clinical Signs of Toxoplasmosis in Mice

Plant 90kDa heat shock protein (HSP90) is a potent adjuvant that increases both humoral and cellular immune responses to diverse proteins and peptides. In this study, we explored whether Arabidopsis thaliana HSP90 (AtHsp81.2) can improve the immune effects of a Toxoplasma gondii surface antigen 1 (SAG1). We designed two constructs containing the sequence of mature antigen (SAG1m), from aa77 to aa322, and B- and T-cell antigenic epitope-containing SAG1HC, from aa221 to aa319 fused to AtHsp81.2 sequence. When comparing the transient expression in Nicotiana tabacum X-27-8 leaves, which overexpress the suppressor helper component protease HC-Pro-tobacco etch virus (TEV), to that in N. benthamiana leaves, co-agroinfiltrated with the suppressor p19, optimal conditions included 6-week-old N. benthamiana plants, 7-day time to harvest, Agrobacterium tumefaciens cultures with an OD600nm of 0.6 for binary vectors and LED lights. While AtHsp81.2-SAG1m fusion protein was undetectable by Western blot in any of the evaluated conditions, AtHsp81.2-SAG1HC was expressed as intact fusion protein, yielding up to 90μg/g of fresh weight. Besides, the AtHsp81.2-SAG1HC mRNA was strongly expressed compared to the endogenous Nicotiana tabacum elongation factor-alpha (NtEFα) gene, whereas the AtHsp81.2-SAG1m mRNA was almost undetectable. Finally, mice were orally immunized with AtHsp81.2-SAG1HC-infiltrated fresh leaves (plAtHsp81.2-SAG1HC group), recombinant AtHsp81.2-SAG1HC purified from infiltrated leaves (rAtHsp81.2-SAG1HC group), non-infiltrated fresh leaves (control group), or phosphate-buffered saline (PBS group). Serum samples from plAtHsp81.2-SAG1HC-immunized mice had significantly higher levels of IgGt, IgG2a, and IgG2b anti-SAG1HC antibodies than serum from rAtHsp81.2-SAG1HC, control, and PBS groups. The number of cysts per brain in the plAtHsp81.2-SAG1HC-immunized mice was significantly reduced, and the parasite load in brain tissue was also lower in this group compared with the remaining groups. In an immunoblot assay, plant-expressed AtHsp81.2-SAG1HC was shown to react with antibodies present in sera from T. gondii-infected people. Therefore, the plant expression of a T. gondii antigen fused to the non-pathogenic adjuvant and carrier plant HSP90 as formulations against T. gondii can improve the vaccine efficacy, and plant extract can be directly used for vaccination without the need to purify the protein, making this platform a suitable and powerful biotechnological system for immunogenic antigen expression against toxoplasmosis.

The Major Peanut Allergen Ara h 2 Produced in Nicotiana benthamiana Contains Hydroxyprolines and Is a Viable Alternative to the E. Coli Product in Allergy Diagnosis

Peanut allergy is a potentially life-threatening disease that is mediated by allergen-specific immunoglobulin E (IgE) antibodies. The major peanut allergen Ara h 2, a 2S albumin seed storage protein, is one of the most dangerous and potent plant allergens. Ara h 2 is posttranslationally modified to harbor four disulfide bridges and three hydroxyprolines. These hydroxyproline residues are required for optimal IgE-binding to the DPYSPOHS motifs representing an immunodominant IgE epitope. So far, recombinant Ara h 2 has been produced in Escherichia coli, Lactococcus lactis, Trichoplusia ni insect cell, and Chlamydomonas reinhardtii chloroplast expression systems, which were all incapable of proline hydroxylation. However, molecular diagnosis of peanut allergy is performed using either natural or E. coli-produced major peanut allergens. As IgE from the majority of patients is directed to Ara h 2, it is of great importance that the recombinant Ara h 2 harbors all of its eukaryotic posttranslational modifications. We produced hydroxyproline-containing and correctly folded Ara h 2 in the endoplasmic reticulum of leaf cells of Nicotiana benthamiana plants, using the plant virus-based magnICON® transient expression system with a yield of 200 mg/kg fresh biomass. To compare prokaryotic with eukaryotic expression methods, Ara h 2 was expressed in E. coli together with the disulfide-bond isomerase DsbC and thus harbored disulfide bridges but no hydroxyprolines. The recombinant allergens from N. benthamiana and E. coli were characterized and compared to the natural Ara h 2 isolated from roasted peanuts. Natural Ara h 2 outperformed both recombinant proteins in IgE-binding and activation of basophils via IgE cross-linking, the latter indicating the potency of the allergen. Interestingly, significantly more efficient IgE cross-linking by the N. benthamiana-produced allergen was observed in comparison to the one induced by the E. coli product. Ara h 2 from N. benthamiana plants displayed a higher similarity to the natural allergen in terms of basophil activation due to the presence of hydroxyproline residues, supporting so far published data on their contribution to the immunodominant IgE epitope. Our study advocates the use of N. benthamiana plants instead of prokaryotic expression hosts for the production of the major peanut allergen Ara h 2.

Specific methylation of (11R)-carlactonoic acid by an Arabidopsis SABATH methyltransferase

An Arabidopsis S-adenosyl-L-methionine-dependent methyltransferase belonging to the SABATH family catalyzes the specific carboxymethylation of (11R)-carlactonoic acid. Methyl carlactonoate (MeCLA), found in Arabidopsis (Arabidopsis thaliana) as a non-canonical strigolactone (SL), may be a biosynthetic intermediate of various non-canonical SLs and biologically active as a plant hormone. MeCLA is formed from carlactonoic acid (CLA), but the methyltransferases (MTs) converting CLA to MeCLA remain unclear. Previous studies have demonstrated that the carboxymethylation of acidic plant hormones is catalyzed by the same protein family, the SABATH family (Wang et al. in Evol Bioinform 15:117693431986086. https://doi.org/10.1177/1176934319860864 , 2019). In the present study, we focused on the At4g36470 gene, an Arabidopsis SABATH MT gene co-expressed with the MAX1 gene responsible for CLA formation for biochemical characterization. The recombinant At4g36470 protein expressed in Escherichia coli exhibited exclusive activity against naturally occurring (11R)-CLA among the substrates, including CLA enantiomers and a variety of acidic plant hormones. The apparent K_m value for (11R)-CLA was 1.46 μM, which was relatively smaller than that of the other Arabidopsis SABATH MTs responsible for the carboxymethylation of acidic plant hormones. The strict substrate specificity and high affinity of At4g36470 suggested it is an (11R)-CLA MT. We also confirmed the function of the identified gene by reconstructing MeCLA biosynthesis using transient expression in Nicotiana benthamiana. Phylogenetic analysis demonstrated that At4g36470 and its orthologs in non-canonical SL-producing plants cluster together in an exclusive clade, suggesting that the SABATH MTs of this clade may be involved in the carboxymethylation of CLA and the biosynthesis of non-canonical SLs.

Transient protein expression systems in plants and their applications

The production of recombinant proteins is important in academic research to identify protein functions. Moreover, recombinant enzymes are used in the food and chemical industries, and high-quality proteins are required for diagnostic, therapeutic, and pharmaceutical applications. Though many recombinant proteins are produced by microbial or mammalian cell-based expression systems, plants have been promoted as alternative, cost-effective, scalable, safe, and sustainable expression systems. The development and improvement of transient expression systems have significantly reduced the period of protein production and increased the yield of recombinant proteins in plants. In this review, we consider the importance of plant-based expression systems for recombinant protein production and as genetic engineering tools.

Production of Human Acid-Alpha Glucosidase With a Paucimannose Structure by Glycoengineered Arabidopsis Cell Culture

Plant cell cultures have emerged as a promising platform for the production of biopharmaceutics due to their cost-effectiveness, safety, ability to control the cultivation, and secrete products into culture medium. However, the use of this platform is hindered by the generation of plant-specific N-glycans, the inability to produce essential N-glycans for cellular delivery of biopharmaceutics, and low productivity. In this study, an alternative acid-alpha glucosidase (GAA) for enzyme replacement therapy of Pompe disease was produced in a glycoengineered Arabidopsis alg3 cell culture. The N-glycan composition of the GAA consisted of a predominantly paucimannosidic structure, Man3GlcNAc2 (M3), without the plant-specific N-glycans. Supplementing the culture medium with NaCl to a final concentration of 50 mM successfully increased GAA production by 3.8-fold. GAA from an NaCl-supplemented culture showed a similar N-glycan profile, indicating that the NaCl supplementation did not affect N-glycosylation. The results of this study highlight the feasibility of using a glycoengineered plant cell culture to produce recombinant proteins for which M3 or mannose receptor-mediated delivery is desired.

Multi-Redox Active Carbons and Hydrocarbons: Control of their Redox Properties and Potential Applications

Precise control over redox properties is essential for high-performance organic electronic devices such as organic batteries, electrochromic devices, and information storage devices. In this context, multi-redox active carbons and hydrocarbons, represented as C_x H_y molecules (x≥1, y≥0), are highly sought after, because they can switch between multiple redox states. Herein, we outline the redox properties of C_x H_y molecules as solutes and adsorbed species. Furthermore, the limitations of evaluating their redox properties and the possible solutions are summarized. Additionally, the theoretical capacity (mAh/g) and gravimetric energy density (Wh/kg) of secondary batteries were estimated based on the redox properties of 185 C_x H_y molecules, which have primarily been reported in the last decade. Among them, seven C_x H_y molecules were found to have the potential to surpass the energy density of LiNi_0.6 Mn_0.2 Co_0.2 O₂ /graphite batteries. The use of C_x H_y molecules in multielectrochromic devices and multi-bit memory is also explained. We believe that this review will encourage further utilization of C_x H_y molecules thereby promoting its applications in organic electronic devices.

An elevated anxiety level among prepubertal autistic boys with non-treatment-seeking functional gastrointestinal disorders: A case-control study

Children with autism commonly suffer from comorbid functional gastrointestinal disorders (FGID) and anxiety. The raised prevalence of both conditions in autism suggests complex reciprocal relationships, which are seldom explored in non-treatment-seeking FGID. The relationships between subtypes of FGID and anxiety are also unclear. This study recruited boys with autism and age-matched typically developing (TD) boys, aged 4-11 years, who were not actively seeking help for gastrointestinal problems. Their parents completed the Rome IV Diagnostic Questionnaires for Pediatric FGID. Four groups of children with and without autism/FGID were identified and compared on their anxiety level using the Spence children's anxiety scale. In 69 boys with autism and 69 age-matched TD boys, FGID were identified in 22 and 16 boys, respectively. ANCOVA demonstrated a significant interaction effect of autism and FGID on anxiety (F[1, 129] = 5.43, p = 0.021), while conditional logistic regression identified an interaction effect of autism and anxiety on the odds of FGID (OR 1.038, 95% CI 1.002-1.075, p = 0.038). Explorative post hoc analysis showed higher anxiety in functional nausea and vomiting disorder (p = 0.033) and functional abdominal pain disorder (p = 0.029) among boys with autism than TD boys with the same respective subtypes of FGID. In summary, among prepubertal boys with autism, the presence of FGID that are non-treatment-seeking in nature, has a significantly stronger association with higher levels of anxiety than TD boys. The strength of association may be more prominent in subtypes of FGID. Possible pathomechanisms including the underlying microbiota spectra and inflammatory paths should be explored in future studies. LAY SUMMARY: Anxiety and gastrointestinal problems are common symptoms in autism. Given that gut health could be linked to emotions, their association in young boys with autism was studied. The presence of nausea vomiting, or abdominal pain were associated with raised anxiety among boys with autism, yet this was not observed in typically developing boys. This suggests that anxiety among autistic children could be partly explained by the presence of FGID.

Prevention of necrosis caused by transient expression in Nicotiana benthamiana by application of ascorbic acid

Application of high concentrations of sodium ascorbate suppresses necrosis caused by the expression of recombinant proteins in Nicotiana benthamiana, resulting in an increase in protein accumulation.

Transient expression of recombinant proteins in plants

Transient protein expression in plant cells is less time consuming than the production of whole transgenic plants. For transient expression, agroinfiltration is a simple and effective method to deliver transgenes into plant cells. After an Agrobacterium infection, recombinant proteins can be produced in plant cells from 3 to 10days. To increase protein yield, a deconstructed viral vector has been used. This chapter provides a detailed description of the transient expression of recombinant proteins in a well-developed host strain of Nicotiana benthamiana. This study also describes the necessary steps for the extraction of soluble proteins from agroinfiltrated leaves.

Plant transporters involved in combating boron toxicity: beyond 3D structures

Membrane transporters control the movement and distribution of solutes, including the disposal or compartmentation of toxic substances that accumulate in plants under adverse environmental conditions. In this minireview, in the light of the approaching 100th anniversary of unveiling the significance of boron to plants (K. Warington, 1923; Ann. Bot.37, 629) we discuss the current state of the knowledge on boron transport systems that plants utilise to combat boron toxicity. These transport proteins include: (i) nodulin-26-like intrinsic protein-types of aquaporins, and (ii) anionic efflux (borate) solute carriers. We describe the recent progress made on the structure–function relationships of these transport proteins and point out that this progress is integral to quantitative considerations of the transporter's roles in tissue boron homeostasis. Newly acquired knowledge at the molecular level has informed on the transport mechanics and conformational states of boron transport systems that can explain their impact on cell biology and whole plant physiology. We expect that this information will form the basis for engineering transporters with optimised features to alleviate boron toxicity tolerance in plants exposed to suboptimal soil conditions for sustained food production.

An engineered extraplastidial pathway for carotenoid biofortification of leaves

Carotenoids are lipophilic plastidial isoprenoids highly valued as nutrients and natural pigments. A correct balance of chlorophylls and carotenoids is required for photosynthesis and therefore highly regulated, making carotenoid enrichment of green tissues challenging. Here we show that leaf carotenoid levels can be boosted through engineering their biosynthesis outside the chloroplast. Transient expression experiments in Nicotiana benthamiana leaves indicated that high extraplastidial production of carotenoids requires an enhanced supply of their isoprenoid precursors in the cytosol, which was achieved using a deregulated form of the main rate-determining enzyme of the mevalonic acid (MVA) pathway. Constructs encoding bacterial enzymes were used to convert these MVA-derived precursors into carotenoid biosynthetic intermediates that do not normally accumulate in leaves, such as phytoene and lycopene. Cytosolic versions of these enzymes produced extraplastidial carotenoids at levels similar to those of total endogenous (i.e. chloroplast) carotenoids. Strategies to enhance the development of endomembrane structures and lipid bodies as potential extraplastidial carotenoid storage systems were not successful to further increase carotenoid contents. Phytoene was found to be more bioaccessible when accumulated outside plastids, whereas lycopene formed cytosolic crystalloids very similar to those found in the chromoplasts of ripe tomatoes. This extraplastidial production of phytoene and lycopene led to an increased antioxidant capacity of leaves. Finally, we demonstrate that our system can be adapted for the biofortification of leafy vegetables such as lettuce.

Effect of leaf position and days post-infiltration on transient expression of colorectal cancer vaccine candidate proteins GA733-Fc and GA733-FcK in Nicotiana benthamiana plant

Immunization with thetumor-associated antigen GA733 glycoprotein, which is highly expressed in colorectal cancer, is considered to be a promising strategy for cancer prevention and treatment. We cloned a fusion gene of GA733 and immunoglobulin Fc fragment (GA733-Fc), and that of GA733-Fc and an endoplasmic reticulum retention motif (GA733-FcK) into the Cowpea mosaic virus (CPMV)-based transient plant expression vector, pEAQ-HT. Agrobacterium tumefaciens (LBA4404) transformed with the vectors pEAQ-HT-GA733-Fc and pEAQ-HT-GA733-FcK was infiltrated into the leaves of Nicotiana benthamiana plants. To optimize harvesting of leaf to express therapeutic glycoproteins both spatially and temporally, protein expression levels at various leaf positions (top, middle, and base) and days post-infiltration (dpi) were investigated. The GA733-Fc and GA733-FcK genes were detected in leaves at 1–10 dpi using PCR. As assessed by western blot, GA733-Fc and GA733-FcK were expressed at the highest levels in the top leaf position at 5 dpi, and GA733-FcK was expressed more than GA733-Fc. The proteins were successfully purified from infiltrated N. benthamiana leaves using protein A affinity chromatography. ELISA verified that an anti-GA733 antibody recognized both purified proteins. Thus, a functional GA733-Fc colorectal cancer vaccine protein can be transiently expressed using a CPMV virus-based vector, with an optimized expression time and leaf position post-infiltration.

Efficient base editing in tomato using a highly expressed transient system

KEY MESSAGE

Base editing in tomatoes was achieved by transient expression. The Solanaceae plants, particularly the tomato (Solanum lycopersicum), is of huge economic value worldwide. The tomato is a unique model plant for studying the functions of genes related to fruit ripening. Deeper understanding of tomatoes is of great importance for both plant research and the economy. Genome editing technology, such as CRISPR/Cas9, has been used for functional genetic research. However, some challenges, such as low transformation efficiency, remain with this technology. Moreover, the foreign Cas9 and gRNA expression cassettes must be removed to obtain null-segregants In this study, we used a high-level transient expression system to improve the base editing technology. A high-level transient expression system has been established previously using geminiviral replication and a double terminator. The pBYR2HS vector was used for this transient expression system. nCas9-CDA and sgRNA-SlHWS were introduced into this vector, and the protein and RNA were then transiently expressed in tomato tissues by agroinfiltration. The homozygous mutant produced by base editing was obtained in the next generation with an efficiency of about 18%. nCas9-free next-generation plants were 71%. All the homozygous base-edited plants in next generation are nCas9-free. These findings show that the high-level transient expression system is useful for base editing in tomatoes.

SARS-CoV-2 Antigens Expressed in Plants Detect Antibody Responses in COVID-19 Patients

Background: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has swept the world and poses a significant global threat to lives and livelihoods, with 115 million confirmed cases and at least 2.5 million deaths from Coronavirus disease 2019 (COVID-19) in the first year of the pandemic. Developing tools to measure seroprevalence and understand protective immunity to SARS-CoV-2 is a priority. We aimed to develop a serological assay using plant-derived recombinant viral proteins, which represent important tools in less-resourced settings. Methods: We established an indirect ELISA using the S1 and receptor-binding domain (RBD) portions of the spike protein from SARS-CoV-2, expressed in Nicotiana benthamiana. We measured antibody responses in sera from South African patients (n = 77) who had tested positive by PCR for SARS-CoV-2. Samples were taken a median of 6 weeks after the diagnosis, and the majority of participants had mild and moderate COVID-19 disease. In addition, we tested the reactivity of pre-pandemic plasma (n = 58) and compared the performance of our in-house ELISA with a commercial assay. We also determined whether our assay could detect SARS-CoV-2-specific IgG and IgA in saliva. Results: We demonstrate that SARS-CoV-2-specific immunoglobulins are readily detectable using recombinant plant-derived viral proteins, in patients who tested positive for SARS-CoV-2 by PCR. Reactivity to S1 and RBD was detected in 51 (66%) and 48 (62%) of participants, respectively. Notably, we detected 100% of samples identified as having S1-specific antibodies by a validated, high sensitivity commercial ELISA, and optical density (OD) values were strongly and significantly correlated between the two assays. For the pre-pandemic plasma, 1/58 (1.7%) of samples were positive, indicating a high specificity for SARS-CoV-2 in our ELISA. SARS-CoV-2-specific IgG correlated significantly with IgA and IgM responses. Endpoint titers of S1- and RBD-specific immunoglobulins ranged from 1:50 to 1:3,200. S1-specific IgG and IgA were found in saliva samples from convalescent volunteers. Conclusion: We demonstrate that recombinant SARS-CoV-2 proteins produced in plants enable robust detection of SARS-CoV-2 humoral responses. This assay can be used for seroepidemiological studies and to measure the strength and durability of antibody responses to SARS-CoV-2 in infected patients in our setting.

Biological drug therapy for ocular angiogenesis: Anti-VEGF agents and novel strategies based on nanotechnology

Currently, biological drug therapy for ocular angiogenesis treatment is based on the administration of anti‐VEGF agents via intravitreal route. The molecules approved with this purpose for ocular use include pegaptanib, ranibizumab, and aflibercept, whereas bevacizumab is commonly off‐label used in the clinical practice. The schedule dosage involves repeated intravitreal injections of anti‐VEGF agents to achieve and maintain effective concentrations in retina and choroids, which are administrated as solutions form. In this review article, we describe the features of different anti‐VEGF agents, major challenges for their ocular delivery and the nanoparticles in development as delivery system of them. In this way, several polymeric and lipid nanoparticles are explored to load anti‐VEGF agents with the aim of achieving sustained drug release and thus, minimize the number of intravitreal injections required. The main challenges were focused in the loading the molecules that maintain their bioactivity after their release from nanoparticulate system, followed the evaluation of them through studies of formulation stability, pharmacokinetic, and efficacy in in vitro and in vivo models. The analysis was based on the information published in peer‐reviewed published papers relevant to anti‐VEGF treatments and nanoparticles developed as ocular anti‐VEGF delivery system.

P25 Gene Knockout Contributes to Human Epidermal Growth Factor Production in Transgenic Silkworms

Transgenic silkworm expression systems have been applied for producing various recombinant proteins. Knocking out or downregulating an endogenous silk protein is considered a viable strategy for improving the ability of transgenic expression systems to produce exogenous proteins. Here, we report the expression of human epidermal growth factor (hEGF) in a P25 gene knockout silkworm. The hEGF gene regulated by the P25 gene promoter was integrated into a silkworm's genome. Five transgenic positive silkworm lineages were generated with different insertion sites on silkworm chromosomes and the ability to synthesize and secrete proteins into cocoons. Then, a cross-strategy was used to produce transgenic silkworms with a P25 gene knockout background. The results of the protein analysis showed that the loss of an endogenous P25 protein can increase the hEGF production to about 2.2-fold more than normal silkworms. Compared to those of transgenic silkworms with wild type (non-knockout) background, the morphology and secondary structure of cocoon silks were barely changed in transgenic silkworms with a P25 gene knockout background, indicating their similar physical properties of cocoon silks. In conclusion, P25 gene knockout silkworms may become an efficient bioreactor for the production of exogenous proteins and a promising tool for producing various protein-containing silk biomaterials.

Plant molecular farming for the production of valuable proteins - Critical evaluation of achievements and future challenges

Recombinant proteins play an important role in many areas of our lives. For example, recombinant enzymes are used in the food and chemical industries and as high-quality proteins for research, diagnostic and therapeutic applications. The production of recombinant proteins is still dominated by expression systems based on microbes and mammalian cells, although the manufacturing of recombinant proteins in plants - known as molecular farming - has been promoted as an alternative, cost-efficient strategy for three decades. Several molecular farming products have reached the market, but the number of success stories has been limited by industrial inertia driven by perceptions of low productivity, the high cost of downstream processing, and regulatory hurdles that create barriers to translation. Here, we discuss the technical and economic factors required for the successful commercialization of molecular farming, and consider potential future directions to enable the broader application of production platforms based on plants.

Targeted genome editing of plants and plant cells for biomanufacturing

Plants have provided humans with useful products since antiquity, but in the last 30 years they have also been developed as production platforms for small molecules and recombinant proteins. This initially niche area has blossomed with the growth of the global bioeconomy, and now includes chemical building blocks, polymers and renewable energy. All these applications can be described as “plant molecular farming” (PMF). Despite its potential to increase the sustainability of biologics manufacturing, PMF has yet to be embraced broadly by industry. This reflects a combination of regulatory uncertainty, limited information on process cost structures, and the absence of trained staff and suitable manufacturing capacity. However, the limited adaptation of plants and plant cells to the requirements of industry-scale manufacturing is an equally important hurdle. For example, the targeted genetic manipulation of yeast has been common practice since the 1980s, whereas reliable site-directed mutagenesis in most plants has only become available with the advent of CRISPR/Cas9 and similar genome editing technologies since around 2010. Here we summarize the applications of new genetic engineering technologies to improve plants as biomanufacturing platforms. We start by identifying current bottlenecks in manufacturing, then illustrate the progress that has already been made and discuss the potential for improvement at the molecular, cellular and organism levels. We discuss the effects of metabolic optimization, adaptation of the endomembrane system, modified glycosylation profiles, programmable growth and senescence, protease inactivation, and the expression of enzymes that promote biodegradation. We outline strategies to achieve these modifications by targeted gene modification, considering case-by-case examples of individual improvements and the combined modifications needed to generate a new general-purpose “chassis” for PMF.

A Pipeline towards the Biochemical Characterization of the Arabidopsis GT14 Family

Glycosyltransferases (GTs) catalyze the synthesis of glycosidic linkages and are essential in the biosynthesis of glycans, glycoconjugates (glycolipids and glycoproteins), and glycosides. Plant genomes generally encode many more GTs than animal genomes due to the synthesis of a cell wall and a wide variety of glycosylated secondary metabolites. The Arabidopsis thaliana genome is predicted to encode over 573 GTs that are currently classified into 42 diverse families. The biochemical functions of most of these GTs are still unknown. In this study, we updated the JBEI Arabidopsis GT clone collection by cloning an additional 105 GT cDNAs, 508 in total (89%), into Gateway-compatible vectors for downstream characterization. We further established a functional analysis pipeline using transient expression in tobacco (Nicotiana benthamiana) followed by enzymatic assays, fractionation of enzymatic products by reversed-phase HPLC (RP-HPLC) and characterization by mass spectrometry (MS). Using the GT14 family as an exemplar, we outline a strategy for identifying effective substrates of GT enzymes. By addition of UDP-GlcA as donor and the synthetic acceptors galactose-nitrobenzodiazole (Gal-NBD), β-1,6-galactotetraose (β-1,6-Gal₄) and β-1,3-galactopentose (β-1,3-Gal₅) to microsomes expressing individual GT14 enzymes, we verified the β-glucuronosyltransferase (GlcAT) activity of three members of this family (AtGlcAT14A, B, and E). In addition, a new family member (AT4G27480, 248) was shown to possess significantly higher activity than other GT14 enzymes. Our data indicate a likely role in arabinogalactan-protein (AGP) biosynthesis for these GT14 members. Together, the updated Arabidopsis GT clone collection and the biochemical analysis pipeline present an efficient means to identify and characterize novel GT catalytic activities.

Gene expression of PLAT and ATS3 proteins increases plant resistance to insects

Genes of the PLAT protein family, including PLAT and ATS3 subfamilies of higher plants and homologs of liverwort, are involved in plant defense against insects. Laticifer cells in plants contain large amounts of anti-microbe or anti-insect proteins and are involved in plant defense against biotic stresses. We previously found that PLAT proteins accumulate in laticifers of fig tree (Ficus carica) at comparable levels to those of chitinases, and the transcript level of ATS3, another PLAT domain-containing protein, is highest in the transcriptome of laticifers of Euphorbia tirucalli. In this study, we investigated whether the PLAT domain-containing proteins are involved in defense against insects. Larvae of the lepidopteran Spodoptera litura showed retarded growth when fed with Nicotiana benthamiana leaves expressing F. carica PLAT or E. tirucalli ATS3 genes, introduced by agroinfiltration using expression vector pBYR2HS. Transcriptome analysis of these leaves indicated that ethylene and jasmonate signaling were activated, leading to increased expression of genes for PR-1, β-1,3-glucanase, PR5 and trypsin inhibitors, suggesting an indirect mechanism of PLAT- and ATS3-induced resistance in the host plant. Direct cytotoxicity of PLAT and ATS3 to insects was also possible because heterologous expression of the corresponding genes in Drosophila melanogaster caused apoptosis-mediated cell death in this insect. Larval growth retardation of S. litura occurred when they were fed radish sprouts, a good host for agroinfiltration, expressing any of nine homologous genes of dicotyledon Arabidopsis thaliana, monocotyledon Brachypodium distachyon, conifer Picea sitchensis and liverwort Marchantia polymorpha. Of these nine genes, the heterologous expression of A. thaliana AT5G62200 and AT5G62210 caused significant increases in larval death. These results indicated that the PLAT protein family has largely conserved anti-insect activity in the plant kingdom (249 words).

Production of Gloeophyllum trabeum Endoglucanase Cel12A in Nicotiana benthamiana for Cellulose Degradation

Lignocellulosic biomass from plants has been used as a biofuel source and the potent acidic endoglucanase GtCel12A has been isolated from Gloeophyllum trabeum, a filamentous fungus. In this study, we established a plant-based platform for the production of active GtCel12A fused to family 3 cellulose-binding module (CBM3). We used the signal sequence of binding immunoglobulin protein (BiP) and the endoplasmic reticulum (ER) retention signal for the accumulation of the produced GtCel12A in the ER. To achieve enhanced enzyme expression, we incorporated the M-domain of the human receptor-type tyrosine-protein phosphatase C into the construct. In addition, to enable the removal of N-terminal domains that are not necessary after protein expression, we further incorporated the cleavage site of Brachypodium distachyon small ubiquitin-like modifier. The GtCel12A-CBM3 fusion protein produced in the leaves of Nicotiana benthamiana exhibited not only high solubility but also efficient endoglucanase activity on the carboxymethyl cellulose substrate as determined by 3,5-dinitrosalicylic acid assay. The endoglucanase activity of GtCel12A-CBM3 was maintained even when immobilized on microcrystalline cellulose beads. Taken together, these results indicate that GtCel12A endoglucanase produced in plants might be used to provide monomeric sugars from lignocellulosic biomass for bioethanol production.

SARS-CoV-2 Antigens Expressed in Plants Detect Antibody Responses in COVID-19 Patients

Background: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has swept the world and poses a significant global threat to lives and livelihoods, with 115 million confirmed cases and at least 2.5 million deaths from Coronavirus disease 2019 (COVID-19) in the first year of the pandemic. Developing tools to measure seroprevalence and understand protective immunity to SARS-CoV-2 is a priority. We aimed to develop a serological assay using plant-derived recombinant viral proteins, which represent important tools in less-resourced settings. Methods: We established an indirect ELISA using the S1 and receptor-binding domain (RBD) portions of the spike protein from SARS-CoV-2, expressed in Nicotiana benthamiana. We measured antibody responses in sera from South African patients (n = 77) who had tested positive by PCR for SARS-CoV-2. Samples were taken a median of 6 weeks after the diagnosis, and the majority of participants had mild and moderate COVID-19 disease. In addition, we tested the reactivity of pre-pandemic plasma (n = 58) and compared the performance of our in-house ELISA with a commercial assay. We also determined whether our assay could detect SARS-CoV-2-specific IgG and IgA in saliva. Results: We demonstrate that SARS-CoV-2-specific immunoglobulins are readily detectable using recombinant plant-derived viral proteins, in patients who tested positive for SARS-CoV-2 by PCR. Reactivity to S1 and RBD was detected in 51 (66%) and 48 (62%) of participants, respectively. Notably, we detected 100% of samples identified as having S1-specific antibodies by a validated, high sensitivity commercial ELISA, and optical density (OD) values were strongly and significantly correlated between the two assays. For the pre-pandemic plasma, 1/58 (1.7%) of samples were positive, indicating a high specificity for SARS-CoV-2 in our ELISA. SARS-CoV-2-specific IgG correlated significantly with IgA and IgM responses. Endpoint titers of S1- and RBD-specific immunoglobulins ranged from 1:50 to 1:3,200. S1-specific IgG and IgA were found in saliva samples from convalescent volunteers. Conclusion: We demonstrate that recombinant SARS-CoV-2 proteins produced in plants enable robust detection of SARS-CoV-2 humoral responses. This assay can be used for seroepidemiological studies and to measure the strength and durability of antibody responses to SARS-CoV-2 in infected patients in our setting.

Efficient Plant Production of Recombinant NS1 Protein for Diagnosis of Dengue

Dengue fever is endemic in more than 120 countries, which account for 3.9 billion people at risk of infection worldwide. The absence of a vaccine with effective protection against the four serotypes of this virus makes differential molecular diagnosis the key step for the correct treatment of the disease. Rapid and efficient diagnosis prevents progression to a more severe stage of this disease. Currently, the limiting factor in the manufacture of dengue (DENV) diagnostic kits is the lack of large-scale production of the non-structural 1 (NS1) protein (antigen) to be used in the capture of antibodies from the blood serum of infected patients. In this work, we use plant biotechnology and genetic engineering as tools for the study of protein production for research and commercial purposes. Gene transfer, integration and expression in plants is a valid strategy for obtaining large-scale and low-cost heterologous protein production. The authors produced NS1 protein of the dengue virus serotype 2 (NS1DENV2) in the Arabidopsis thaliana plant. Transgenic plants obtained by genetic transformation expressed the recombinant protein that was purified and characterized for diagnostic use. The yield was 203 μg of the recombinant protein per gram of fresh leaf. By in situ immunolocalization, transgenic protein was observed within the plant tissue, located in aggregates bodies. These antigens showed high sensitivity and specificity to both IgM (84.29% and 91.43%, respectively) and IgG (83.08% and 87.69%, respectively). The study goes a step further to validate the use of plants as a strategy for obtaining large-scale and efficient protein production to be used in dengue virus diagnostic tests.

Transient Gene Expression is an Effective Experimental Tool for the Research into the Fine Mechanisms of Plant Gene Function: Advantages, Limitations, and Solutions

A large data array on plant gene expression accumulated thanks to comparative omic studies directs the efforts of researchers to the specific or fine effects of the target gene functions and, as a consequence, elaboration of relatively simple and concurrently effective approaches allowing for the insight into the physiological role of gene products. Numerous studies have convincingly demonstrated the efficacy of transient expression strategy for characterization of the plant gene functions. The review goals are (i) to consider the advantages and limitations of different plant systems and methods of transient expression used to find out the role of gene products; (ii) to summarize the current data on the use of the transient expression approaches for the insight into fine mechanisms underlying the gene function; and (iii) to outline the accomplishments in efficient transient expression of plant genes. In general, the review discusses the main and critical steps in each of the methods of transient gene expression in plants; areas of their application; main results obtained using plant objects; their contribution to our knowledge about the fine mechanisms of the plant gene functions underlying plant growth and development; and clarification of the mechanisms regulating complex metabolic pathways.

RAP Tag and PMab-2 Antibody: A Tagging System for Detecting and Purifying Proteins in Plant Cells

An affinity tag system requires both high affinity and specificity. The RAP tag epitope DMVNPGLEDRIE, derived from rat podoplanin (PDPN), is specifically recognized by PMab-2 monoclonal antibodies in rats. Here, we demonstrated that high levels of PMab-2 can be produced in Nicotiana benthamiana and plant-derived PMab-2 possesses similar activity to CHO-derived PMab-2, and the RAP tag presents a useful tagging system for detecting and purifying proteins from plant cells. The heavy chain of PMab-2 fused with KDEL, an endoplasmic reticulum retention sequence, and the light chain of the antibody were introduced into N. benthamiana by agroinfiltration. The expression of PMab-2 peaked 4 days after agroinfiltration, and approximately 0.3 mg/g fresh weight of the antibody was accumulated. After purification, the plant-derived PMab-2 successfully recognized rat PDPN expressed in CHO-K1 cells and exhibited almost the same binding activity as CHO-derived PMab-2. The RAP-tagged proteins expressed in plant cells were specifically recognized by PMab-2. These results indicate that PMab-2 can accumulate at high levels in N. benthamiana and is easily purified and that the RAP tagging system presents a useful tool for detecting and purifying proteins of interest in plant cells.

Transient protein expression in tobacco BY-2 plant cell packs using single and multi-cassette replicating vectors

KEY MESSAGE

This is the first evidence that replicating vectors can be successfully used for transient protein expression in BY-2 plant cell packs. Transient recombinant protein expression in plants and recently also plant cell cultures are of increasing interest due to the speed, safety and scalability of the process. Currently, studies are focussing on the design of plant virus-derived vectors to achieve higher amounts of transiently expressed proteins in these systems. Here we designed and tested replicating single and multi-cassette vectors that combine elements for enhanced replication and hypertranslation, and assessed their ability to express and particularly co-express proteins by Agrobacterium-mediated transient expression in tobacco BY-2 plant cell packs. Substantial yields of green and red fluorescent proteins of up to ~ 700 ng/g fresh mass were detected in the plant cells along with position-dependent expression. This is the first evidence of the ability of replicating vectors to transiently express proteins in BY-2 plant cell packs.

Production of bioactive human IFN-γ protein by agroinfiltration in tobacco

In animals, interferon-γ (IFN-γ) is known as a cytokine involved in antiviral and anticancer activities with a higher biochemical activity in contrast to other IFNs. To produce recombinant human IFN-γ (hIFN-γ) protein in tobacco, factors influencing gene delivery were first evaluated for higher efficiency of transient expression by fluorometric measurement of GUS activity. Higher levels of transient expression were observed in leaves of Nicotiana tabacum cv. Samsun infiltrated with GV3101 strain (optical density equal to 1.0 at 600 nm) under treatment of 200 μM AS at 4 days post agroinfiltration (dpa). The Samsun cv. proved to be amenable with 1.4- and 1.5-fold higher levels of transient expression than Xanthi and N. benthamiana, respectively. In addition, the GV3101 remained the best strain for use in transient assays without any necrotic response in tobacco. The levels of transient hIFN-γ expression were also estimated in the Samsun cv. infiltrated with different Agrobacterium tumefaciens strains carrying various expression constructs. Higher levels of accumulation were obtained with targeting the hIFN-γ protein to endoplasmic reticulum (ER) or apoplastic space than those expressed into cytoplasm. Moreover, antiviral bioassay revealed that recombinant hIFN-γ protein produced in tobacco is biologically active and protects the Vero cells from infection generated by vesicular stomatitis virus (VSV).

The key role of terminators on the expression and post-transcriptional gene silencing of transgenes

Transgenes have become essential to modern biology, being an important tool in functional genomic studies and also in the development of biotechnological products. One of the major challenges in the generation of transgenic lines concerns the expression of transgenes, which, compared to endogenes, are particularly susceptible to silencing mediated by small RNAs (sRNAs). Several reasons have been put forward to explain why transgenes often trigger the production of sRNAs, such as the high level of expression induced by commonly used strong constitutive promoters, the lack of introns, and features resembling viral and other exogenous sequences. However, the relative contributions of the different genomic elements with respect to protecting genes from the silencing machinery and their molecular mechanisms remain unclear. Here, we present the results of a mutagenesis screen conceived to identify features involved in the protection of endogenes against becoming a template for the production of sRNAs. Interestingly, all of the recovered mutants had alterations in genes with proposed function in transcription termination, suggesting a central role of terminators in this process. Indeed, using a GFP reporter system, we show that, among different genetic elements tested, the terminator sequence had the greatest effect on transgene-derived sRNA accumulation and that a well-defined poly(A) site might be especially important. Finally, we describe an unexpected mechanism, where transgenes containing certain intron/terminator combinations lead to an increase in the production of sRNAs, which appears to interfere with splicing.

A classical swine fever virus E2 fusion protein produced in plants elicits a neutralizing humoral immune response in mice and pigs

Classical swine fever (CSF) is one of the most important viral diseases of swine worldwide. Although live or attenuated virus vaccines have been used to control CSFV, it is difficult to distinguish vaccinated pigs from infected pigs; this leads to restrictions on import and export. Subunit vaccines based on the CSFV E2 glycoprotein have been developed using baculovirus or insect cell systems, but some weaknesses remain. Here, we describe production of an E2 recombinant protein using a Nicotiana benthamiana plant expression system. To do this, we took advantage of the ability of the swine Fc domain to increase solubility and stability of the fusion protein and to strengthen immune responses in target animals. N. benthamiana expressed high amounts of pFc2-fused E2 proteins, which were isolated and purified by affinity chromatography to yield a high pure recombinant protein in a cost-effective manner. Native-polyacrylamide gel electrophoresis and size exclusion chromatography confirmed that the pmE2:pFc2 fusion exists as a multimer rather than as a dimer. Injection of recombinant pmE2 protein into mice or piglets generated anti-pmE2 antibodies with efficient neutralizing activity against CSFV. These results suggest that a purified recombinant E2 protein produced in N. benthamiana generates high titers of neutralizing antibodies in vivo; as such, the protein could be developed as a subunit vaccine against CSFV.

Heterologous expression of biologically active Mambalgin-1 peptide as a new potential anticancer, using a PVX-based viral vector in Nicotiana benthamiana

Mambalgin-1 is a peptide that acts as a potent analgesic through inhibiting acid-sensing ion channels (ASIC) in nerve cells. Research has shown that ASIC channels are involved in the proliferation and growth of cancer cells; therefore, Mambalgin-1 can be a potential anti-cancer by inhibiting these channels. In the present study, the Nicotiana benthamiana codon optimized Mambalgin-1 gene was synthesized and cloned in PVX (potato virus X) viral vector. The two cultures of Agrobacterium containing Mambalgin-1 and P19 silencing suppressor genes were co-agroinfiltrated into N. benthamiana leaves. Five days post infiltration, the production of recombinant Mambalgin-1 was determined by western blotting. For biological activity, MTT (3(4, 5-dimethylthiazole-2-yl)-2, 5-diphenyltetrazolium bromide) was analyzed for the cytotoxicity recombinant Mambalgin-1 from the transformed plants on nervous (SH-SY5Y) and breast (MCF7) cancer cells. The results showed that the plants expressing open reading frame of Mambalgin-1 showed recombinant 7.4 kDa proteins in the entire plant extract. In the MTT test, it was found that Mambalgin-1 had cytotoxic effects on SH-SY5Y cancer cells, yet no effects on MCF7 cancer cells were observed. According to the results, the expression of the biologically active recombinant Mambalgin-1 in the transformed plant leaves was confirmed and Mambalgin-1 can also have anti-cancer (inhibition of ASIC channels) potential along with its already known analgesic effect.

Genome editing in PDS genes of tomatoes by non-selection method and of Nicotiana benthamiana by one single guide RNA to edit two orthologs

The CRISPR/Cas9 system is widely used for targeted mutagenesis in many organisms including plants. For application of this system, tissue culture methods need to be established. In this study, detailed methods for introduction of mutations in tomato and Nicotiana benthamiana plants using the CRISPR/Cas9 system are described. The methods include tissue culture protocols for tomato and N. benthamiana. We also demonstrate the methodology to generate Cas9-free genome edited tomato plants and use of one single guide RNA (sgRNA) to edit two orthologs in N. benthamiana. The examples of editing the PHYTOENE DESATURASE (PDS) genes in these plants are also provided. The Cas9-free tomato line was obtained when tomato plants were cultured on a non-selective medium after transformation with the CRISPR/Cas9 system. Two orthologs of PDS in N. benthamiana were mutated using a sgRNA, because these orthologs contain the same nucleotide sequences with PAM motif. These mutations were inherited to the next generation. The mutations in the PDS genes resulted in an albino phenotype in tomato and N. benthamiana plants. These results demonstrate that the non-selective method is one of the ways to obtain Cas9-free genome editing in tomato plants and that the two orthologs can be edited by one sgRNA in N. benthamiana.

Insights Into Genetic and Molecular Elements for Transgenic Crop Development

Climate change and the exploration of new areas of cultivation have impacted the yields of several economically important crops worldwide. Both conventional plant breeding based on planned crosses between parents with specific traits and genetic engineering to develop new biotechnological tools (NBTs) have allowed the development of elite cultivars with new features of agronomic interest. The use of these NBTs in the search for agricultural solutions has gained prominence in recent years due to their rapid generation of elite cultivars that meet the needs of crop producers, and the efficiency of these NBTs is closely related to the optimization or best use of their elements. Currently, several genetic engineering techniques are used in synthetic biotechnology to successfully improve desirable traits or remove undesirable traits in crops. However, the features, drawbacks, and advantages of each technique are still not well understood, and thus, these methods have not been fully exploited. Here, we provide a brief overview of the plant genetic engineering platforms that have been used for proof of concept and agronomic trait improvement, review the major elements and processes of synthetic biotechnology, and, finally, present the major NBTs used to improve agronomic traits in socioeconomically important crops.

Radish sprouts as an efficient and rapidly available host for an agroinfiltration-based transient gene expression system

Agroinfiltration, the infiltration of plants with Agrobacterium harboring a plasmid that contains a specific gene, is used to transiently express a gene in a heterologous organism. Using the "Tsukuba system", greater amounts of target protein accumulate compared with usual expression plasmids. Reported host plants, including Nicotiana benthamiana, a common plant for agroinfiltration, need several weeks after sowing to grow enough for infection. To shorten the culture period and, thereby, improve target protein production, we tested sprouts as host plants. Sprouts were grown in the dark to encourage elongation so that vacuum infiltration becomes easier, and this was followed by a few days of exposure to illumination before infection with pBYR2HS-EGFP, the EGFP expression plasmid of the Tsukuba system. Among six tested species of Fabaceae and Brassicaceae, radish showed the highest transient expression. Among six tested radish cultivars, Kaiware, Hakata, and Banryoku provided the best results. Culturing for 5 day, including 1 day of imbibition and 1 to 2 day of exposure to illumination resulted in EGFP fluorescence in 80% of the cotyledon area. Thus, a remarkable amount of EGFP was obtained only 8 day after seed imbibition. The EGFP amount in Kaiware cotyledons was comparable with Rubisco at ∼0.7 mg/g fresh weight. Kaiware sold in supermarkets could also be used, but resulted in lower expression levels.

Expression of Single Chain Variable Fragment (scFv) Molecules in Plants: A Comprehensive Update

Single chain variable fragments (scFvs) are generated by joining together the variable heavy and light chain of a monoclonal antibody (mAb) via a peptide linker. They offer some advantages over the parental mAb such as low molecular weight, heterologous production, multimeric form, and multivalency. The scFvs were produced against more than 50 antigens till date using 10 different plant species as the expression system. There were considerable improvements in the expression and purification strategies of scFv in the last 24 years. With the growing demand of scFv in therapeutic and diagnostic fields, its biosynthesis needs to be increased. The easiness in development, maintenance, and multiplication of transgenic plants make them an attractive expression platform for scFv production. The review intends to provide comprehensive information about the use of plant expression system to produce scFv. The developments, advantages, pitfalls, and possible prospects of improvement for the exploitation of plants in the industrial level are discussed.

The PHD finger of Arabidopsis SIZ1 recognizes trimethylated histone H3K4 mediating SIZ1 function and abiotic stress response

Arabidopsis SIZ1 encodes a SUMO E3 ligase to regulate abiotic and biotic stress responses. Among SIZ1 or mammalian PIAS orthologs, plant SIZ1 proteins contain the plant homeodomain (PHD) finger, a C₄HC₃ zinc finger. Here, we investigated the importance of PHD of Arabidopsis SIZ1. The Pro_SIZ1::SIZ1(ΔPHD):GFP was unable to complement growth retardation, ABA hypersensitivity, and the cold-sensitive phenotype of the siz1 mutant, but Pro_SIZ1::SIZ1:GFP could. Substitution of C162S in the PHD finger was unable to complement the siz1 mutation. Tri-methylated histone H3K4 (H3K4me3) was recognized by PHD, not by PHD(C162S). WRKY70 was up-regulated in the siz1-2 mutant and H3K4me3 accumulated at high levels in the WRKY70 promoter. PHD interacts with ATX, which mediates methylation of histone, probably leading to suppression of ATX’s function. These results suggest that the PHD finger of SIZ1 is important for recognition of the histone code and is required for SIZ1 function and transcriptional suppression.

Kenji Miura et al. investigate the role of the plant homeodomain (PHD) finger of the Arabidopsis SIZ1 protein. They show that the PHD finger is involved in hormone response and temperature sensitivity, and plays an important role in H3K4 methylation, thereby affecting recognition of histone code and transcriptional suppression.

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.

Pilot Production of SARS-CoV-2 Related Proteins in Plants: A Proof of Concept for Rapid Repurposing of Indoor Farms Into Biomanufacturing Facilities

The current CoVid-19 crisis is revealing the strengths and the weaknesses of the world's capacity to respond to a global health crisis. A critical weakness has resulted from the excessive centralization of the current biomanufacturing capacities, a matter of great concern, if not a source of nationalistic tensions. On the positive side, scientific data and information have been shared at an unprecedented speed fuelled by the preprint phenomena, and this has considerably strengthened our ability to develop new technology-based solutions. In this work, we explore how, in a context of rapid exchange of scientific information, plant biofactories can serve as a rapid and easily adaptable solution for local manufacturing of bioreagents, more specifically recombinant antibodies. For this purpose, we tested our ability to produce, in the framework of an academic lab and in a matter of weeks, milligram amounts of six different recombinant monoclonal antibodies against SARS-CoV-2 in Nicotiana benthamiana. For the design of the antibodies, we took advantage, among other data sources, of the DNA sequence information made rapidly available by other groups in preprint publications. mAbs were engineered as single-chain fragments fused to a human gamma Fc and transiently expressed using a viral vector. In parallel, we also produced the recombinant SARS-CoV-2 N protein and the receptor binding domain (RBD) of the Spike protein in planta and used them to test the binding specificity of the recombinant mAbs. Finally, for two of the antibodies, we assayed a simple scale-up production protocol based on the extraction of apoplastic fluid. Our results indicate that gram amounts of anti-SARS-CoV-2 antibodies could be easily produced in little more than 6 weeks in repurposed greenhouses with little infrastructure requirements using N. benthamiana as production platform. Similar procedures could be easily deployed to produce diagnostic reagents and, eventually, could be adapted for rapid therapeutic responses.

The Emergency Response Capacity of Plant-Based Biopharmaceutical Manufacturing-What It Is and What It Could Be

Several epidemic and pandemic diseases have emerged over the last 20 years with increasing reach and severity. The current COVID-19 pandemic has affected most of the world's population, causing millions of infections, hundreds of thousands of deaths, and economic disruption on a vast scale. The increasing number of casualties underlines an urgent need for the rapid delivery of therapeutics, prophylactics such as vaccines, and diagnostic reagents. Here, we review the potential of molecular farming in plants from a manufacturing perspective, focusing on the speed, capacity, safety, and potential costs of transient expression systems. We highlight current limitations in terms of the regulatory framework, as well as future opportunities to establish plant molecular farming as a global, de-centralized emergency response platform for the rapid production of biopharmaceuticals. The implications of public health emergencies on process design and costs, regulatory approval, and production speed and scale compared to conventional manufacturing platforms based on mammalian cell culture are discussed as a forward-looking strategy for future pandemic responses.

High-Yield Production of the Major Birch Pollen Allergen Bet v 1 With Allergen Immunogenicity in Nicotiana benthamiana

Type I allergy is an immunological disorder triggered by allergens and causes significant health problems. The major allergen of birch pollen is Bet v 1, which belongs to the pathogen-related protein 10 (PR-10) family. Here, we established a rapid and robust method for the production of Bet v 1 in Nicotiana benthamiana leaves, with binding activity to allergic patients' IgE. The Bet v 1 allergen was expressed in N. benthamiana using a strong agroinfiltration-based transient protein expression system, which consists of a deconstructed geminiviral vector system with a double terminator. Five days post-infiltration, the allergen concentration in N. benthamiana leaves was 1.2 mg/g of fresh mass, being this the maximum yield of Bet v 1 in plants reported up to now. A part of plant-derived Bet v 1 was glycosylated. Bet v 1 purified from N. benthamiana or Brevibacillus brevis was used to carry out enzyme-linked immunoassays; both recombinant allergens were found to have comparable binding properties to the IgE of allergic patients. These results suggest that our plant expression system allows rapid and robust production of the allergen, which keeps the immunogenicity.

Plant 3' Regulatory Regions From mRNA-Encoding Genes and Their Uses to Modulate Expression

Molecular biotechnology has made it possible to explore the potential of plants for different purposes. The 3' regulatory regions have a great diversity of cis-regulatory elements directly involved in polyadenylation, stability, transport and mRNA translation, essential to achieve the desired levels of gene expression. A complex interaction between the cleavage and polyadenylation molecular complex and cis-elements determine the polyadenylation site, which may result in the choice of non-canonical sites, resulting in alternative polyadenylation events, involved in the regulation of more than 80% of the genes expressed in plants. In addition, after transcription, a wide array of RNA-binding proteins interacts with cis-acting elements located mainly in the 3' untranslated region, determining the fate of mRNAs in eukaryotic cells. Although a small number of 3' regulatory regions have been identified and validated so far, many studies have shown that plant 3' regulatory regions have a higher potential to regulate gene expression in plants compared to widely used 3' regulatory regions, such as NOS and OCS from Agrobacterium tumefaciens and 35S from cauliflower mosaic virus. In this review, we discuss the role of 3' regulatory regions in gene expression, and the superior potential that plant 3' regulatory regions have compared to NOS, OCS and 35S 3' regulatory regions.

The Whys and Wherefores of Transitivity in Plants

Transitivity in plants is a mechanism that produces secondary small interfering RNAs (siRNAs) from a transcript targeted by primary small RNAs (sRNAs). It expands the silencing signal to additional sequences of the transcript. The process requires RNA-dependent RNA polymerases (RDRs), which convert single-stranded RNA targets into a double-stranded (ds) RNA, the precursor of siRNAs and is critical for effective and amplified responses to virus infection. It is also important for the production of endogenous secondary siRNAs, such as phased siRNAs (phasiRNAs), which regulate several genes involved in development and adaptation. Transitivity on endogenous transcripts is very specific, utilizing special primary sRNAs, such as miRNAs with unique features, and particular ARGONAUTEs. In contrast, transitivity on transgene and virus (exogenous) transcripts is more generic. This dichotomy of responses implies the existence of a mechanism that differentiates self from non-self targets. In this work, we examine the possible mechanistic process behind the dichotomy and the intriguing counter-intuitive directionality of transitive sequence-spread in plants.

In Vivo Removal of N-Terminal Fusion Domains From Recombinant Target Proteins Produced in Nicotiana benthamiana

Plants show great potential for producing recombinant proteins in a cost-effective manner. Many strategies have therefore been employed to express high levels of recombinant proteins in plants. Although foreign domains are fused to target proteins for high expression or as an affinity tag for purification, the retention of foreign domains on a target protein may be undesirable, especially for biomedical purposes. Thus, their removal is often crucial at a certain time point after translation. Here, we developed a new strategy to produce target proteins without foreign domains. This involved in vivo removal of foreign domains fused to the N-terminus by the small ubiquitin-related modifier (SUMO) domain/SUMO-specific protease system. This strategy was tested successfully by generating a recombinant gene, BiP:p38:bdSUMO : His:hLIF, that produced human leukemia inhibitory factor (hLIF) fused to p38, a coat protein of the Turnip crinkle virus; the inclusion of p38 increased levels of protein expression. The recombinant protein was expressed at high levels in the leaf tissue of Nicotiana benthamiana. Coexpression of bdSENP1, a SUMO-specific protease, proteolytically released His:hLIF from the full-length recombinant protein in the endoplasmic reticulum of N. benthamiana leaf cells. His:hLIF was purified from leaf extracts via Ni²⁺-NTA affinity purification resulting in a yield of 32.49 mg/kg, and the N-terminal 5-residues were verified by amino acid sequencing. Plant-produced His:hLIF was able to maintain the pluripotency of mouse embryonic stem cells. This technique thus provides a novel method of removing foreign domains from a target protein in planta.

Production of bacteriophage-encoded endolysin, LysP11, in Nicotiana benthamiana and its activity as a potent antimicrobial agent against Erysipelothrix rhusiopathiae

KEY MESSAGE

We produced a biologically active phage-encoded endolysin, LysP11, in N. benthamiana. Plant-produced LysP11 exhibited robust antimicrobial activity against E. rhusiopathiae, and C-terminal domain of LysP11 bound specifically to E. rhusiopathiae. Bacterial resistance to antibiotics, a serious issue in terms of global public health, is one of the leading causes of death today. Thus, new antimicrobial agents are needed to combat pathogens. Recent research suggests that bacteriophages and endolysins derived from bacteriophages are potential alternatives to traditional antibiotics. Here, we examined the antimicrobial activity of LysP11, which is encoded by Propionibacterium phage P1.1 and comprises an N-terminal amidase-2 domain and a C-terminal domain with no homology to other bacteriophage endolysins. LysP11 was produced in Nicotiana benthamiana (N. benthamiana) using an Agrobacterium-mediated transient expression strategy. LysP11 was purified on microcrystalline cellulose-binding resin after attachment of the Clostridium thermocellum-derived family 3 cellulose-binding domain as an affinity tag. The affinity tag was removed using the small ubiquitin-related modifier (SUMO) domain and SUMO-specific protease. Plant-produced LysP11 showed strong antimicrobial activity toward Erysipelothrix rhusiopathiae (E. rhusiopathiae), mediated via lysis of the cell wall. Lytic activity was optimal at pH 8.0-9.0 (37 °C) and increased at higher concentrations of NaCl up to 400 mM. Furthermore, the C-terminal domain of LysP11 bound specifically to the E. rhusiopathiae cell wall. Based on these results, we propose that LysP11 is a potential candidate antimicrobial agent against E. rhusiopathiae.

Agroinfiltration-based efficient transient protein expression in leguminous plants

Transient protein expression is an effective tool to rapidly unravel novel gene functions, such as transcriptional activity of promoters and sub-cellular localization of proteins. However, transient expression is not applicable to some species and varieties because of insufficient expression levels. We recently developed one of the strongest agroinfiltration-based transient protein expression systems for plant cells, termed 'Tsukuba system.' About 4 mg/g fresh weight of protein expression in Nicotiana benthamiana was obtained using this system. The vector pBYR2HS, which contains a geminiviral replication system and a double terminator, can be used in various plant species and varieties, including lettuces, eggplants, tomatoes, hot peppers, and orchids. In this study, we assessed the applicability of the Tsukuba system to several species of legumes, including Lotus japonicus, soybean Glycine max, and common bean Phaseolus vulgaris. The GFP protein was transiently expressed in the seedpods of all examined legume species, however, protein expression in leaves was observed only in P. vulgaris. Taken together, our system is an effective tool to examine gene function rapidly in several legume species based on transient protein expression.

Efficient transient protein expression in tomato cultivars and wild species using agroinfiltration-mediated high expression system

The new transient protein expression system using the pBYR2HS vector is applicable to several tomato cultivars and wild species with high level of protein expression. Innovation and improvement of effective tools for transient protein expression in plant cells is critical for the development of plant biotechnology. We have created the new transient protein expression system using the pBYR2HS vector that led to about 4 mg/g fresh weight of protein expression in Nicotiana benthamiana. In this study, we validated the adaptability of this transient protein expression system by agroinfiltration to leaves and fruits of several tomato cultivars and wild species. Although the GFP protein was transiently expressed in the leaves and fruits of all tomato cultivars and wild species, we observed species-specific differences in protein expression. In particular, GFP protein expression was higher in the leaves and fruits of Micro-Tom, Solanum pimpinellifolium (0043) and S. pimpinellifolium (0049-w1) than in those of cultivars and wild species. Furthermore, Agrobacterium with GABA transaminase enhanced transient expression in tomato fruits of Micro-Tom. Taken together with these results, our system is applicable to several tomato cultivars and species as well as a model tomato, even though characteristics are often different among tomato cultivars or species. Thus, the system is an effective, simple, and valuable tool to achieve rapid transgene expression to examine gene function in tomato plant cells.

Development of Recombinant Protein-Based Vaccine Against Classical Swine Fever Virus in Pigs Using Transgenic Nicotiana benthamiana

Classical swine fever virus (CSFV) is highly contagious, and fatal to infected pigs. Vaccines against CSFV have been developed from attenuated or modified live viruses. These vaccines are effective for immunization of animals, but they are associated with problems such as the accidental spreading of viruses to animals in the field, and with barriers to trade following vaccination. Here, we report the generation of transgenic Nicotiana benthamiana plants for large-scale, cost-effective production of E2 fusion protein for use as a recombinant vaccine against CSFV in pigs. Transgenic N. benthamiana plants harboring an intergenic, single-copy insertion of a chimeric gene encoding E2 fusion protein had high levels of transgene expression. For large-scale production of E2 fusion protein from leaf tissues, we developed a protein-purification protocol consisting of cellulose-binding domain (CBD)-cellulose-based affinity purification and size-exclusion gel-filtration chromatography. E2 fusion proteins showed high immunogenicity in piglets and provided protection against CSFV challenge. The CBD in the E2 fusion protein was also highly immunogenic. These results suggest that plant-produced recombinant E2 fusion proteins can be developed into cost-effective vaccines against CSFV, with the CBD as a marker antigen to differentiate between vaccination and natural infection.

Critical Analysis of the Commercial Potential of Plants for the Production of Recombinant Proteins

Over the last three decades, the expression of recombinant proteins in plants and plant cells has been promoted as an alternative cost-effective production platform. However, the market is still dominated by prokaryotic and mammalian expression systems, the former offering high production capacity at a low cost, and the latter favored for the production of complex biopharmaceutical products. Although plant systems are now gaining widespread acceptance as a platform for the larger-scale production of recombinant proteins, there is still resistance to commercial uptake. This partly reflects the relatively low yields achieved in plants, as well as inconsistent product quality and difficulties with larger-scale downstream processing. Furthermore, there are only a few cases in which plants have demonstrated economic advantages compared to established and approved commercial processes, so industry is reluctant to switch to plant-based production. Nevertheless, some plant-derived proteins for research or cosmetic/pharmaceutical applications have reached the market, showing that plants can excel as a competitive production platform in some niche areas. Here, we discuss the strengths of plant expression systems for specific applications, but mainly address the bottlenecks that must be overcome before plants can compete with conventional systems, enabling the future commercial utilization of plants for the production of valuable proteins.