Production of Biopharmaceuticals in Nicotiana benthamiana-Axillary Stem Growth as a Key Determinant of Total Protein Yield

The biosynthesis of trillin 6'-O-glucoside: A low-abundance yet pharmacologically active polyphyllin from Paris polyphylla

Natural products from medicinal plants serve as an invaluable resource for drug discovery and development. However, low-abundance natural products are often understudied due to the challenges of obtaining sufficient quantities for pharmacological testing in cells or animals. Additionally, their complex stereochemistry and functional groups make chemical synthesis and purification difficult. In this study, we showcased the power of biosynthetic approaches to explore these underexplored compounds, using the low-abundance polyphyllin trillin 6'-O-glucoside from Paris polyphylla as an example. We identified two trillin 6'-O-glucosyltransferases required for its biosynthesis and successfully reconstructed the entire pathway in Nicotiana benthamiana. We demonstrated that trillin 6'-O-glucoside exhibits anti-bacterial activity comparable to major polyphyllins like polyphyllins I, II, and VII. Notably, it also showed much lower hemolytic activity, a common side effect of those major polyphyllins. Together, our study underscores the advantages of employing biosynthetic approaches to explore natural products that exist in low or trace abundances yet possess equally important pharmacological activities.

Rhizobium rhizogenes A4-derived strains mediate hyper-efficient transient gene expression in Nicotiana benthamiana and other solanaceous plants

Agroinfiltration, a method utilizing agrobacteria to transfer DNA into plant cells, is widely used for transient gene expression in plants. Besides the commonly used Agrobacterium strains, Rhizobium rhizogenes can also introduce foreign DNA into host plants for gene expression. While many R. rhizogenes strains have been known for inducing hairy root symptoms, their use for transient expression has not been fully explored. Here, we showed that R. rhizogenes A4 outperformed all other tested agrobacterial strains in agroinfiltration experiments on leaves of Nicotiana benthamiana and other solanaceous plants. By conducting an agroinfiltration screening in N. benthamiana leaves using various agrobacterial strains carrying the RUBY reporter gene cassette, we discovered that A4 mediates the strongest and fastest transient expression. Utilizing the genomic information, we developed a collection of disarmed and modified strains derived from A4. By performing vacuum infiltration assays, we demonstrated that these A4-derived strains efficiently transiently transform 6-week-old N. benthamiana leaves, showing less sensitivity to the age of plants compared to the laboratory strain GV3101. Furthermore, we performed agroinfiltration using AS109, an A4-derived disarmed strain, on the leaves of tomato, pepper, and eggplant. Remarkably, AS109 mediated transient gene expression on tested solanaceous plants more effectively than all the tested commonly used agrobacterial strains. This discovery paves the way for establishing R. rhizogenes A4-derived strains as a new option for enhancing transient expression in N. benthamiana and facilitating the functional study of plant genes in other solanaceous species.

Molecular changes in agroinfiltrated leaves of Nicotiana benthamiana expressing suppressor of silencing P19 and coronavirus-like particles

The production of coronavirus disease 2019 vaccines can be achieved by transient expression of the spike (S) protein of severe acute respiratory syndrome coronavirus 2 in agroinfiltrated leaves of Nicotiana benthamiana. Relying on bacterial vector Agrobacterium tumefaciens, this process is favoured by co-expression of viral silencing suppressor P19. Upon expression, the S protein enters the cell secretory pathway, before being trafficked to the plasma membrane where formation of coronavirus-like particles (CoVLPs) occurs. We previously characterized the effects of influenza virus hemagglutinin forming VLPs through similar processes. However, leaf samples were only collected after 6 days of expression, and it is unknown whether influenza VLPs (HA-VLPs) and CoVLPs induce similar responses. Here, time course sampling was used to profile responses of N. benthamiana leaf cells expressing P19 only, or P19 and the S protein. The latter triggered early but transient activation of the unfolded protein response and waves of transcription factor genes involved in immunity. Accordingly, defence genes were induced with different expression kinetics, including those promoting lignification, terpene biosynthesis, and oxidative stress. Cross-talk between stress hormone pathways also occurred, including repression of jasmonic acid biosynthesis genes after agroinfiltration, and dampening of salicylic acid responses upon S protein accumulation. Overall, HA-VLP- and CoVLP-induced responses broadly overlapped, suggesting nanoparticle production to have the most effects on plant immunity, regardless of the virus surface proteins expressed. Taking advantage of RNAseq inferences, we finally show the co-expression of Kunitz trypsin inhibitors to reduce CoVLP-induced defence and leaf symptoms, with no adverse effect on plant productivity.

Using Agrobacterium tumefaciens to Assemble Multi-step Metabolic Pathways in Nicotiana benthamiana

Within the realm of the natural world, plants emerge as prolific producers of diverse bioactive compounds with pharmaceutical, nutritional, and industrial applications. However, many of these compounds are scarce with low concentrations and specific distributions among species, prompting the exploration of methods for producing them in plant biofactories. Typically, pathways comprising several enzymatic steps need to be engineered in plant hosts to produce the desired product of interest from available metabolic precursors. Transient expression systems, specifically agroinfiltration of Nicotiana benthamiana leaves with Agrobacterium tumefaciens, is a potent and cost-effective method for testing synthetic gene combinations. Here, we present a protocol to produce metabolites through a multi-step pathway, exemplifying the assembly of a carotenoid synthesis pathway within the plant cell cytosol. The approach showcases the efficiency and simplicity of agroinfiltration-mediated transient expression systems in reconstructing metabolic pathways, offering a valuable and sustainable alternative to stably transformed lines.

Characterization of a plant-derived monoclonal antibody targeting extracellular enveloped virions of Monkeypox virus

In 2022, the global outbreak of monkeypox virus (MPXV) with increased human-to-human transmission triggered urgent public health interventions. Plant-derived monoclonal antibodies (mAbs) are being explored as potential therapeutic strategies due to their diverse mechanisms of antiviral activity. MPXV produces two key infectious particles: the mature virion (MV) and the extracellular enveloped virion (EV), both essential for infection and spread. Effective therapies must target both to halt replication and transmission. Our prior research demonstrated the development of a potent neutralizing mAb against MPXV MV. This study focuses on developing a plant-derived mAb targeting MPXV EV, which is critical for viral dissemination within the host and generally resistant to antibody neutralization. Our findings reveal that the mAb (H2) can be robustly produced in Nicotiana benthamiana plants via transient expression. The plant-made H2 mAb effectively targets MPXV EV by binding specifically to the A35 MPXV antigen. Importantly, H2 mAb shows notable neutralizing activity against the infectious MPXV EV particle. This investigation is the first to report the development of a plant-derived anti-EV mAb for MPXV prevention and treatment, as well as the first demonstration of anti-MPXV EV activity by an mAb across any production platform. It highlights the potential of plant-produced mAbs as therapeutics for emerging infectious diseases, including the MPXV outbreak.

Synergetic light and cytokinin treatments mitigate the recombinant protein yield depression induced by high-density cultivation of hydroponically-grown Nicotiana benthamiana

Plant molecular farming is currently operating a transition from soil-based cultures toward hydroponic systems. In this study, we designed a whole-plant NFT (nutrient film technique) platform for the transient expression of influenza virus-like particles harboring hemagglutinin H1 proteins in Nicotiana benthamiana. In particular, we examined the effects of plant density during the post-infiltration expression phase on plant growth and H1 yield in relation to the daily light integral (DLI) received by the crop and the exogenous application of 6-BAP cytokinin (CK). We expected from previous work that high DLI and CK treatments would stimulate the development of highly productive leaves on axillary (secondary) stems and thereby improve the H1 yield at the whole-plant scale. Increasing plant density from 35.7 to 61 plants m-2 during the post-infiltration phase significantly decreased the proportion of axillary leaf biomass by 30% and H1 yield per plant by 39%, resulting in no additional yield gain on a whole-crop area basis. Adding CK to the recirculated nutrient solution decreased the harvested leaf biomass by 31% and did not enhance the relative proportion of S leaves of the plants as previously reported with foliar CK application. There was a 36% increase in H1 yield when doubling the DLI from 14 to 28 mol m-2 s-1, and up to 71% yield gain when combining such an increase in DLI with the hydroponic CK treatment. Contrary to our expectations, leaves located on the main stem, particularly those from the upper half of the plant (i.e., eighth leaf and above), contributed about 80% of total H1 yield. Our study highlights the significantly different phenotype (~30% less secondary leaf biomass) and divergent responses to light and CK treatments of NFT-grown N. benthamiana plants compared to previous studies conducted on potted plants.

Comparative analysis of the Squamosa Promoter Binding-Like (SPL) gene family in Nicotiana benthamiana and Nicotiana tabacum

SQUAMOSA PROMOTER BINDING-LIKE (SPL) proteins constitute a large family of transcription factors known to play key roles in growth and developmental processes, including juvenile-to-adult and vegetative-to-reproductive phase transitions. This makes SPLs interesting targets for precision breeding in plants of the Nicotiana genus used as e.g. recombinant biofactories. We report the identification of 49 SPL genes in Nicotiana tabacum cv. K326 and 43 SPL genes in Nicotiana benthamiana LAB strain, which were classified into eight phylogenetic groups according to the SPL classification in Arabidopsis. Exon-intron gene structure and DNA-binding domains were highly conserved between homeologues and orthologues. Thirty of the NbSPL genes and 33 of the NtSPL genes were found to be possible targets of microRNA 156. The expression of SPL genes in leaves was analysed by RNA-seq at three different stages, revealing that genes not under miR156 control were in general constitutively expressed at high levels, whereas miR156-regulated genes showed lower expression, often developmentally regulated. We selected the N. benthamiana SPL13_1a gene as target for a CRISPR/Cas9 knock-out experiment. We show here that a full knock-out in this single gene leads to a significant delay in flowering time, a trait that could be exploited to increase biomass for recombinant protein production.

A Monoclonal Antibody Produced in Glycoengineered Plants Potently Neutralizes Monkeypox Virus

The 2022 global outbreaks of monkeypox virus (MPXV) and increased human-to-human transmission calls for the urgent development of countermeasures to protect people who cannot benefit from vaccination. Here, we describe the development of glycovariants of 7D11, a neutralizing monoclonal IgG antibody (mAb) directed against the L1 transmembrane protein of the related vaccinia virus, in a plant-based system as a potential therapeutic against the current MPVX outbreak. Our results indicated that 7D11 mAb quickly accumulates to high levels within a week after gene introduction to plants. Plant-produced 7D11 mAb assembled correctly into the tetrameric IgG structure and can be easily purified to homogeneity. 7D11 mAb exhibited a largely homogeneous N-glycosylation profile, with or without plant-specific xylose and fucose residues, depending on the expression host, namely wild-type or glycoengineered plants. Plant-made 7D11 retained specific binding to its antigen and displayed a strong neutralization activity against MPXV, as least as potent as the reported activity against vaccinia virus. Our study highlights the utility of anti-L1 mAbs as MPXV therapeutics, and the use of glycoengineered plants to develop mAb glycovariants for potentially enhancing the efficacy of mAbs to combat ever-emerging/re-emerging viral diseases.

Molecular Farming of Pembrolizumab and Nivolumab

Immune checkpoint inhibitors (ICIs) are a class of immunotherapy agents capable of alleviating the immunosuppressive effects exerted by tumorigenic cells. The programmed cell death protein 1 (PD-1)/programmed death-ligand 1 (PD-L1) immune checkpoint is one of the most ubiquitous checkpoints utilized by tumorigenic cells for immune evasion by inducing apoptosis and inhibiting the proliferation and cytokine production of T lymphocytes. Currently, the most frequently used ICIs targeting the PD-1/PD-L1 checkpoint include monoclonal antibodies (mAbs) pembrolizumab and nivolumab that bind to PD-1 on T lymphocytes and inhibit interaction with PD-L1 on tumorigenic cells. However, pembrolizumab and nivolumab are costly, and thus their accessibility is limited in low- and middle-income countries (LMICs). Therefore, it is essential to develop novel biomanufacturing platforms capable of reducing the cost of these two therapies. Molecular farming is one such platform utilizing plants for mAb production, and it has been demonstrated to be a rapid, low-cost, and scalable platform that can be potentially implemented in LMICs to diminish the exorbitant prices, ultimately leading to a significant reduction in cancer-related mortalities within these countries.

Rip it, stitch it, click it: A Chemist's guide to VLP manipulation

Viruses are some of nature's most ubiquitous self-assembled molecular containers. Evolutionary pressures have created some incredibly robust, thermally, and enzymatically resistant carriers to transport delicate genetic information safely. Virus-like particles (VLPs) are human-engineered non-infectious systems that inherit the parent virus' ability to self-assemble under controlled conditions while being non-infectious. VLPs and plant-based viral nanoparticles are becoming increasingly popular in medicine as their self-assembly properties are exploitable for applications ranging from diagnostic tools to targeted drug delivery. Understanding the basic structure and principles underlying the assembly of higher-order structures has allowed researchers to disassemble (rip it), reassemble (stitch it), and functionalize (click it) these systems on demand. This review focuses on the current toolbox of strategies developed to manipulate these systems by ripping, stitching, and clicking to create new technologies in the biomedical space.

Effect of the At-CDC27a gene on Nicotiana benthamiana phenotype and accumulation of recombinant proteins

In this study the anaphase promoting complex subunit CDC27a from Arabidopsis thaliana was introduced in the genome of Nicotiana benthamiana by Agrobacterium tumefaciens. The presence of the At-CDC27a gene facilitates plant biomass production. Compared to wild type N. benthamiana the leaf mass fraction of the best performing transgenic line At-CDC27a-29 was increased up to 154%. The positive effect of the At-CDC27a expression on leaf biomass accumulation was accompanied by an enlarged total leaf area. Furthermore, the ectopic expression of the At-CDC27a also affected cellular conditions for the production of foreign proteins delivered by the TRBO vector. In comparison to the non-transgenic control, the protein accumulation in the At-CDC27a-29 plant host increased up to 146% for GFP and up to 181% for scFv-TM43-E10. Collectively, the modified N. benthamiana plants developed in this study might be useful to improve the yield of recombinant proteins per biomass unit in closed facilities.

Transient proteolysis reduction of Nicotiana benthamiana-produced CAP256 broadly neutralizing antibodies using CRISPR/Cas9

The hypersensitive response is elicited by Agrobacterium infiltration of Nicotiana benthamiana, including the induction and accumulation of pathogenesis-related proteins, such as proteases. This includes the induction of the expression of several cysteine proteases from the C1 (papain-like cysteine protease) and C13 (legumain-like cysteine protease) families. This study demonstrates the role of cysteine proteases: NbVPE-1a, NbVPE-1b, and NbCysP6 in the proteolytic degradation of Nicotiana benthamiana (glycosylation mutant ΔXTFT)-produced anti-human immunodeficiency virus broadly neutralizing antibody, CAP256-VRC26.25. Three putative cysteine protease cleavage sites were identified in the fragment crystallizable region. We further demonstrate the transient coexpression of CAP256-VRC26.25 with CRISPR/Cas9-mediated genome editing vectors targeting the NbVPE-1a, NbVPE-1b, and NbCysP6 genes which resulted in a decrease in CAP256-VRC26.25 degradation. No differences in structural features were observed between the human embryonic kidney 293 (HEK293)-produced and ΔXTFT broadly neutralizing antibodies produced with and without the coexpression of genome-editing vectors. Furthermore, despite the presence of proteolytically degraded fragments of plant-produced CAP256-VRC26.25 without the coexpression of genome editing vectors, no influence on the in vitro functional activity was detected. Collectively, we demonstrate an innovative in planta strategy for improving the quality of the CAP256 antibodies through the transient expression of the CRISPR/Cas9 vectors.

A Review of Virus-Like Particle-Based SARS-CoV-2 Vaccines in Clinical Trial Phases

The Coronavirus disease 2019 (COVID-19) pandemic has affected more than 269 million worldwide, with more than five million deaths as of early December 2021. The main concerns in this pandemic include the asymptomatic nature of COVID-19, leading to the infection of many healthy people, the infectious nature of the pathogen, and its high spreading rate. The disease features have highlighted the importance of controlling this pandemic via vaccines. There has been a worldwide race to produce better, more protective, and efficacious vaccines. Simultaneously, different new variants of the virus are emerging. Therefore, there is a concern about the efficacy of the vaccines against new variants. The platform used for COVID-19 vaccine development needs to be flexible enough to enable the manufacturer to react suitably to new virus variants. We performed a comprehensive search in the online databases of PubMed, Scopus, Google Scholar, clinicaltrials.gov, WHO, ICTRP, and Cochrane until December 10th, 2021. There are 331 candidate vaccines in clinical development, with 194 in the preclinical stage and 137 in different clinical phases. Eleven platforms have been used for the development of COVID-19 vaccines, including inactivated/live attenuated virus, protein subunit, virus-like particle (VLP), non-replicating/replicating viral vectors (VVnr or VVr), VVr or VVnr plus antigen-presenting cell, bacterial antigen-spore expression vector, DNA, and RNA. The VLP-based vaccine platform is a safe, highly immunogenic, and flexible platform for developing vaccines. This review focuses on VLP-based vaccine platforms and explicitly discusses the six VLP-based COVID-19 vaccines in clinical trial phases.

Production of Potyvirus-Derived Nanoparticles Decorated with a Nanobody in Biofactory Plants

Viral nanoparticles (VNPs) have recently attracted attention for their use as building blocks for novel materials to support a range of functions of potential interest in nanotechnology and medicine. Viral capsids are ideal for presenting small epitopes by inserting them at an appropriate site on the selected coat protein (CP). VNPs presenting antibodies on their surfaces are considered highly promising tools for therapeutic and diagnostic purposes. Due to their size, nanobodies are an interesting alternative to classic antibodies for surface presentation. Nanobodies are the variable domains of heavy-chain (VHH) antibodies from animals belonging to the family Camelidae, which have several properties that make them attractive therapeutic molecules, such as their small size, simple structure, and high affinity and specificity. In this work, we have produced genetically encoded VNPs derived from two different potyviruses—the largest group of RNA viruses that infect plants—decorated with nanobodies. We have created a VNP derived from zucchini yellow mosaic virus (ZYMV) decorated with a nanobody against the green fluorescent protein (GFP) in zucchini (Cucurbita pepo) plants. As reported for other viruses, the expression of ZYMV-derived VNPs decorated with this nanobody was only made possible by including a picornavirus 2A splicing peptide between the fused proteins, which resulted in a mixed population of unmodified and decorated CPs. We have also produced tobacco etch virus (TEV)-derived VNPs in Nicotiana benthamiana plants decorated with the same nanobody against GFP. Strikingly, in this case, VNPs could be assembled by direct fusion of the nanobody to the viral CP with no 2A splicing involved, likely resulting in fully decorated VNPs. For both expression systems, correct assembly and purification of the recombinant VNPs was confirmed by transmission electron microscope; the functionality of the CP-fused nanobody was assessed by western blot and binding assays. In sum, here we report the production of genetically encoded plant-derived VNPs decorated with a nanobody. This system may be an attractive alternative for the sustainable production in plants of nanobody-containing nanomaterials for diagnostic and therapeutic purposes.

Rapid Combinatorial Coexpression of Biosynthetic Genes by Transient Expression in the Plant Host Nicotiana benthamiana

Metabolic engineering of heterologous hosts requires coexpression of multiple genes, often more than ten for a single pathway. Traditional approaches to create genetic constructs for this purpose are highly inflexible and suffer from very low throughput. In this book chapter, we describe a powerful method to overcome this bottleneck, namely, combinatorial co-expression in the Australian tobacco plant Nicotiana benthamiana. This system is based on Agrobacterium tumefaciens-mediated transient gene expression, often called agroinfiltration. Herein, instead of creating complex multigenic constructs, coexpression is achieved by simply mixing different plasmid-bearing Agrobacterium strains without the need for different selection markers. We present a practical guide for coexpressing multiple biosynthetic genes followed by GC-MS analysis, using production of the plant triterpene β-amyrin as an example. Our chapter provides a guideline to harness the potential of this versatile expression system in the natural product community for studying and engineering metabolic pathways.

AgroLux: bioluminescent Agrobacterium to improve molecular pharming and study plant immunity

Agroinfiltration in Nicotiana benthamiana is widely used to transiently express heterologous proteins in plants. However, the state of Agrobacterium itself is not well studied in agroinfiltrated tissues, despite frequent studies of immunity genes conducted through agroinfiltration. Here, we generated a bioluminescent strain of Agrobacterium tumefaciens GV3101 to monitor the luminescence of Agrobacterium during agroinfiltration. By integrating a single copy of the lux operon into the genome, we generated a stable 'AgroLux' strain, which is bioluminescent without affecting Agrobacterium growth in vitro and in planta. To illustrate its versatility, we used AgroLux to demonstrate that high light intensity post infiltration suppresses both Agrobacterium luminescence and protein expression. We also discovered that AgroLux can detect Avr/Cf-induced immune responses before tissue collapse, establishing a robust and rapid quantitative assay for the hypersensitive response (HR). Thus, AgroLux provides a non-destructive, versatile and easy-to-use imaging tool to monitor both Agrobacterium and plant responses.

Biosynthesis and synthetic biology of psychoactive natural products

Psychoactive natural products play an integral role in the modern world. The tremendous structural complexity displayed by such molecules confers diverse biological activities of significant medicinal value and sociocultural impact. Accordingly, in the last two centuries, immense effort has been devoted towards establishing how plants, animals, and fungi synthesize complex natural products from simple metabolic precursors. The recent explosion of genomics data and molecular biology tools has enabled the identification of genes encoding proteins that catalyze individual biosynthetic steps. Once fully elucidated, the "biosynthetic pathways" are often comparable to organic syntheses in elegance and yield. Additionally, the discovery of biosynthetic enzymes provides powerful catalysts which may be repurposed for synthetic biology applications, or implemented with chemoenzymatic synthetic approaches. In this review, we discuss the progress that has been made toward biosynthetic pathway elucidation amongst four classes of psychoactive natural products: hallucinogens, stimulants, cannabinoids, and opioids. Compounds of diverse biosynthetic origin - terpene, amino acid, polyketide - are identified, and notable mechanisms of key scaffold transforming steps are highlighted. We also provide a description of subsequent applications of the biosynthetic machinery, with an emphasis placed on the synthetic biology and metabolic engineering strategies enabling heterologous production.

Current updates and research on plant-based vaccines for coronavirus disease 2019

The primary outbreak of severe acute respiratory syndrome coronavirus 2, causing pneumonia-like symptoms in patients named coronavirus disease 2019 (COVID-19) had evolved into a global pandemic. COVID-19 has surpassed Middle East respiratory syndrome and severe acute respiratory syndrome in terms of rate and scale causing more than one million deaths. Development of an effective vaccine to fight against the spread of COVID-19 is the main goal of many countries around the world and plant-based vaccines are one of the available methods in vaccine developments. Plant-based vaccine has gained its reputation among researchers for its known effective manufacturing process and cost effectiveness. Many companies around the world are participating in the race to develop an effective vaccine by using the plant system. This review discusses different approaches used as well as highlights the challenges faced by various companies and research groups in developing the plant-based COVID-19 vaccine.

At-CycD2 Enhances Accumulation of Above-Ground Biomass and Recombinant Proteins in Transgenic Nicotiana benthamiana Plants

Transient expression in Nicotiana benthamiana holds great potential for recombinant protein manufacturing due to its advantages in terms of speed and yield compared to stably transformed plants. To continue improving the quantity of recombinant proteins the plant host will need to be modified at both plant and cellular levels. In attempt to increase leaf mass fraction, we transformed N. benthamiana with the At-CycD2 gene, a positive regulator of the cell cycle. Phenotypic characterization of the T₁ progeny plants revealed their accelerated above-ground biomass accumulation and enhanced rate of leaf initiation. In comparison to non-transgenic control the best performing line At-CycD2-15 provided 143 and 140% higher leaf and stem biomass fractions, respectively. The leaf area enlargement of the At-CycD2-15 genotype was associated with the increase of epidermal cell number compensated by slightly reduced cell size. The production capacity of the At-CycD2-15 transgenic line was superior to that of the non-transgenic N. benthamiana. The accumulation of transiently expressed GFP and scFv-TM43-E10 proteins per unit biomass was increased by 138.5 and 156.7%, respectively, compared to the wild type. With these results we demonstrate the potential of cell cycle regulator gene At-CycD2 to modulate both plant phenotype and intracellular environment of N. benthamiana for enhanced recombinant protein yield.

Plant Platforms for Efficient Heterologous Protein Production

Production of recombinant proteins is primarily established in cultures of mammalian, insect and bacterial cells. Concurrently, concept of using plants to produce high-value pharmaceuticals such as vaccines, antibodies, and dietary proteins have received worldwide attention. Newer technologies for plant transformation such as plastid engineering, agroinfiltration, magnifection, and deconstructed viral vectors have been used to enhance the protein production in plants along with the inherent advantage of speed, scale, and cost of production in plant systems. Production of therapeutic proteins in plants has now a more pragmatic approach when several plant-produced vaccines and antibodies successfully completed Phase I clinical trials in humans and were further scheduled for regulatory approvals to manufacture clinical grade products on a large scale which are safe, efficacious, and meet the quality standards. The main thrust of this review is to summarize the data accumulated over the last two decades and recent development and achievements of the plant derived therapeutics. It also attempts to discuss different strategies employed to increase the production so as to make plants more competitive with the established production systems in this industry.

On the verge of the market - Plant factories for the automated and standardized production of biopharmaceuticals

The market for biopharmaceuticals is dominated by recombinant proteins and is driven mainly by the development of vaccines and antibodies. Manufacturing predominantly relies on fermentation-based production platforms, which have limited scalability and suffer from high upstream process costs. As an alternative, the production of recombinant proteins in whole plants (plant molecular farming) provides a scalable and cost efficient upstream process because each plant functions as a self-contained bioreactor, avoiding costs associated with single-use devices and cleaning-in-place. Despite many proof-of-concept studies and the approval of a few products as medical devices, the only approved pharmaceutical proteins manufactured in whole plants have been authorized under emergency protocols. The absence of approvals under standard clinical development pathways in part reflects the lack of standardized process equipment and unit operations, leading to industry inertia based on familiarity with fermenter systems. Here we discuss the upstream production steps of plant molecular farming by transient expression in intact plants, including seeding, plant cultivation, infiltration with Agrobacterium tumefaciens, post-infiltration incubation, and harvesting. We focus on cultivation techniques because they strongly affect the subsequent steps and overall process design. We compare the benefits and drawbacks of open field, greenhouse and vertical farm strategies in terms of upfront investment costs, batch reproducibility, and decoupling from environmental impacts. We consider process automation, monitoring and adaptive process design in the context of Industry 4.0, which can boost process efficiency and batch-to-batch uniformity to improve regulatory compliance. Finally, we discuss the costs-benefit aspects of the different cultivation systems.

Innovation in plant-based transient protein expression for infectious disease prevention and preparedness

Addressing new challenges in global health and biosecurity requires responsive and accessible platforms for the manufacture of preventative or therapeutic interventions. Transient protein expression in plants has evolved into a technology that offers a unique combination of rapid expression, inherent scalability, and flexibility in gene stacking with the capability to produce complex proteins and protein assemblies. Technical developments that have driven the progress of transient expression in plants include advanced expression systems, protein engineering and synthetic biology approaches to transiently, or stably, modify host plants. The plasticity of transient expression in plants, speed of scalability and relatively low capital costs, highlight the great potential of this technology in the future of human and animal health.

pH Gradient Mitigation in the Leaf Cell Secretory Pathway Attenuates the Defense Response of Nicotiana benthamiana to Agroinfiltration

Partial neutralization of the Golgi lumen pH by the ectopic expression of influenza virus M2 proton channel is useful to stabilize acid-labile recombinant proteins in plant cells, but the impact of pH gradient mitigation on host cellular functions has not been investigated. Here, we assessed the unintended effects of M2 expression on the leaf proteome of Nicotiana benthamiana infiltrated with the bacterial gene vector Agrobacterium tumefaciens. An isobaric tags for relative and absolute quantification quantitative proteomics procedure was followed to compare the leaf proteomes of plants agroinfiltrated with either an "empty" vector or an M2-encoding vector. Leaves infiltrated with the empty vector had a low soluble protein content compared to noninfiltrated control leaves, associated with increased levels of stress-related proteins but decreased levels of photosynthesis-associated proteins. M2 expression partly compromised these effects of agroinfiltration to restore soluble protein content in the leaf tissue, associated with restored levels of photosynthesis-associated proteins and reduced levels of stress-related proteins in the apoplast. These data illustrate the cell-wide influence of the Golgi lumen pH homeostasis on the leaf proteome of N. benthamiana responding to microbial challenge. They also underline the relevance of assessing the eventual unintended effects of accessory proteins used to modulate specific cellular or metabolic functions in plant protein biofactories.

Seasonal Weather Changes Affect the Yield and Quality of Recombinant Proteins Produced in Transgenic Tobacco Plants in a Greenhouse Setting

Transgenic plants have the potential to produce recombinant proteins on an agricultural scale, with yields of several tons per year. The cost-effectiveness of transgenic plants increases if simple cultivation facilities such as greenhouses can be used for production. In such a setting, we expressed a novel affinity ligand based on the fluorescent protein DsRed, which we used as a carrier for the linear epitope ELDKWA from the HIV-neutralizing antibody 2F5. The DsRed-2F5-epitope (DFE) fusion protein was produced in 12 consecutive batches of transgenic tobacco (Nicotiana tabacum) plants over the course of 2 years and was purified using a combination of blanching and immobilized metal-ion affinity chromatography (IMAC). The average purity after IMAC was 57 ± 26% (n = 24) in terms of total soluble protein, but the average yield of pure DFE (12 mg kg^-1) showed substantial variation (± 97 mg kg^-1, n = 24) which correlated with seasonal changes. Specifically, we found that temperature peaks (>28°C) and intense illuminance (>45 klx h^-1) were associated with lower DFE yields after purification, reflecting the loss of the epitope-containing C-terminus in up to 90% of the product. Whereas the weather factors were of limited use to predict product yields of individual harvests conducted for each batch (spaced by 1 week), the average batch yields were well approximated by simple linear regression models using two independent variables for prediction (illuminance and plant age). Interestingly, accumulation levels determined by fluorescence analysis were not affected by weather conditions but positively correlated with plant age, suggesting that the product was still expressed at high levels, but the extreme conditions affected its stability, albeit still preserving the fluorophore function. The efficient production of intact recombinant proteins in plants may therefore require adequate climate control and shading in greenhouses or even cultivation in fully controlled indoor farms.