Plastids: The Green Frontiers for Vaccine Production

Cell-Penetrating Peptides for Use in Development of Transgenic Plants

Genetically modified plants and crops can contribute to remarkable increase in global food supply, with improved yield and resistance to plant diseases or insect pests. The development of biotechnology introducing exogenous nucleic acids in transgenic plants is important for plant health management. Different genetic engineering methods for DNA delivery, such as biolistic methods, Agrobacterium tumefaciens-mediated transformation, and other physicochemical methods have been developed to improve translocation across the plasma membrane and cell wall in plants. Recently, the peptide-based gene delivery system, mediated by cell-penetrating peptides (CPPs), has been regarded as a promising non-viral tool for efficient and stable gene transfection into both animal and plant cells. CPPs are short peptides with diverse sequences and functionalities, capable of agitating plasma membrane and entering cells. Here, we highlight recent research and ideas on diverse types of CPPs, which have been applied in DNA delivery in plants. Various basic, amphipathic, cyclic, and branched CPPs were designed, and modifications of functional groups were performed to enhance DNA interaction and stabilization in transgenesis. CPPs were able to carry cargoes in either a covalent or noncovalent manner and to internalize CPP/cargo complexes into cells by either direct membrane translocation or endocytosis. Importantly, subcellular targets of CPP-mediated nucleic acid delivery were reviewed. CPPs offer transfection strategies and influence transgene expression at subcellular localizations, such as in plastids, mitochondria, and the nucleus. In summary, the technology of CPP-mediated gene delivery provides a potent and useful tool to genetically modified plants and crops of the future.

The first report describes features of the chloroplast genome of Withania frutescens

Genomic studies not only help researcher not only to identify genomic features in organisms, but also facilitate understanding of evolutionary relationships. Species in the Withania genus have medicinal benefits, and one of them is Withania frutescens, which is used to treat various diseases. This report investigates the nucleotides and genic features of chloroplast genome of Withania frutescens and trying to clarify the evolutionary relationship with Withania sp and family Solanaceae. We found that the total size of Withania frutescens chloroplast genome was 153.771 kb (the smallest chloroplast genome in genus Withania). A large single-copy region (91.285 kb), a small single-copy region (18.373 kb) form the genomic region, and are distinct from each other by a large inverted repeat (22.056 kb). 137 chloroplast genes are found including 4 rRNAs, 38 tRNAs and 83 protein-coding genes. The Withania frutescens chloroplast genome as well as four closest relatives was compared for features such as structure, nucleotide composition, simple sequence repeats (SSRs) and codon bias. Compared to other Withania species, Withania frutescens has unique characteristics. It has the smallest chloroplast genome of any Withania species, isoleucine is the major amino acid, and tryptophan is the minor, In addition, there are no ycf3 and ycf4 genes, fourth, there are only fifteen replicative genes, while in most other species there are more. Using fast minimum evolution and neighbor joining, we have reconstructed the trees to confirm the relationship with other Solanacaea species. The Withania frutescens chloroplast genome is submitted under accession no. ON153173.

Design and assembly of plant-based COVID-19 candidate vaccines: reсent development and future prospects

An outbreak of a new variant of the coronavirus infection, known as COVID-19, occurred at the end of 2019 in China, in the city of Wuhan. It was caused by the SARS-CoV-2 virus. This variant of the virus is characterized by a high degree of variability and, as the current situation with its spread across different regions of the globe shows, it can lead to a progressive spread of infection among the human population and become the cause of a pandemic. The world scientific community is making tremendous efforts to develop means of protection,prevention and treatment of this disease based on modern advances in molecular biology, immunology and vaccinology. This review provides information on the current state of research in the field of vaccine development against COVID-19 with an emphasis on the role of plants in solving this complex problem. Although plants have long been used by mankind as sources of various medicinal substances, in a pandemic, plant expression systems become attractive as biofactories or bioreactors for the production of artificially created protein molecules that include protective antigens against viral infection. The design and creation of such artificial molecules underlies the development of recombinant subunit vaccines aimed at a rapid response against the spread of infections with a high degree of variability. The review presents the state of research covering a period of just over two years, i. e. since the emergence of the new outbreak of coronavirus infection. The authors tried to emphasize the importance of rapid response of research groups from various scientific fields towards the use of existing developments to create means of protection against various pathogens. With two plant expression systems – stable and transient – as examples, the development of work on the creation of recombinant subunit vaccines against COVID-19 in various laboratories and commercial companies is shown. The authors emphasize that plant expression systems have promise for the development of not only protective means under conditions of rapid response (subunit vaccines), but also therapeutic agents in the form of monoclonal antibodies against COVID-19 synthesized in plant cells.

Plastidial Expression of 3β-Hydroxysteroid Dehydrogenase and Progesterone 5β-Reductase Genes Confer Enhanced Salt Tolerance in Tobacco

The short-chain dehydrogenase/reductase (SDR) gene family is widely distributed in all kingdoms of life. The SDR genes, 3β-hydroxysteroid dehydrogenase (3β-HSD) and progesterone 5-β-reductases (P5βR1, P5βR2) play a crucial role in cardenolide biosynthesis pathway in the Digitalis species. However, their role in plant stress, especially in salinity stress management, remains unexplored. In the present study, transplastomic tobacco plants were developed by inserting the 3β-HSD, P5βR1 and P5βR2 genes. The integration of transgenes in plastomes, copy number and transgene expression at transcript and protein level in transplastomic plants were confirmed by PCR, end-to-end PCR, qRT-PCR and Western blot analysis, respectively. Subcellular localization analysis showed that 3β-HSD and P5βR1 are cytoplasmic, and P5βR2 is tonoplast-localized. Transplastomic lines showed enhanced growth in terms of biomass and chlorophyll content compared to wild type (WT) under 300 mM salt stress. Under salt stress, transplastomic lines remained greener without negative impact on shoot or root growth compared to the WT. The salt-tolerant transplastomic lines exhibited enhanced levels of a series of metabolites (sucrose, glutamate, glutamine and proline) under control and NaCl stress. Furthermore, a lower Na+/K+ ratio in transplastomic lines was also observed. The salt tolerance, mediated by plastidial expression of the 3β-HSD, P5βR1 and P5βR2 genes, could be due to the involvement in the upregulation of nitrogen assimilation, osmolytes as well as lower Na+/K+ ratio. Taken together, the plastid-based expression of the SDR genes leading to enhanced salt tolerance, which opens a window for developing saline-tolerant plants via plastid genetic engineering.

Gene introduction approaches in chloroplast transformation and its applications

BACKGROUND

Chloroplast is a type of plastid that is believed to be originated from ancestral cyanobacteria. Chloroplast besides being a major component for photosynthesis, also takes part in another major plant metabolism, making it one of the major components of plants.

MAIN BODY

Chloroplast transformation is an alternative and better genetic engineering approach compared to the nuclear transformation that has been widely applied in plant genetic engineering. Chloroplast transformation has exhibited various positive effects as compared to nuclear transformation. This is a more preferred technique by researchers. To carry out chloroplast transformation, the vector design must be performed, and a selectable marker needs to be incorporated before the chloroplast could uptake the construct. The common way of introducing a gene into the host, which is the chloroplast, involves the biolistic, PEG-mediated, carbon nanotubes carriers, UV-laser microbeam, and Agrobacterium-mediated transformation approaches. Apart from discussing the processes involved in introducing the gene into the chloroplast, this review also focuses on the various applications brought about by chloroplast transformation, particularly in the field of agriculture and environmental science.

CONCLUSION

Chloroplast transformation has shown a lot of advantages and proven to be a better alternative compared to nuclear genome transformation. Further studies must be conducted to uncover new knowledge regarding chloroplast transformation as well as to discover its additional applications in the fields of biotechnology.

An AMA1/MSP119 Adjuvanted Malaria Transplastomic Plant-Based Vaccine Induces Immune Responses in Test Animals

Malaria is a tropical human disease, caused by protozoan parasites, wherein a significant number of the world's population is at risk. Annually, more than 219 million new cases are reported. Although there are prevention treatments, there are no highly and widely effective licensed anti-malarial vaccines available for use. Opportunities for utilization of plant-based vaccines as novel platforms for developing safe, reliable, and affordable treatments offer promise for developing such a vaccine against malaria. In this study, a Malchloroplast candidate vaccine was designed, composed of segments of AMA1 and MSP1 proteins, two epitopes of Plasmodium falciparum, along with a GK1 peptide from Taenia solium as adjuvant, and this was expressed in tobacco chloroplasts. Transplastomic tobacco lines were generated using biolistic transformation, and these were confirmed to carry the synthetic gene construct. Expression of the synthetic GK1 peptide was confirmed using RT-PCR and Western blots. Furthermore, the GK1 peptide was detected by HPLC at levels of up to 6 µg g-1 dry weight of tobacco leaf tissue. The plant-derived Malchloroplast candidate vaccine was subsequently tested in BALB/c female mice following subcutaneous administration, and was found to elicit specific humoral responses. Furthermore, components of this candidate vaccine were recognized by antibodies in Plasmodium falciparum malaria patients and were immunogenic in test mice. Thus, this study provided a 'proof of concept' for a promising plant-based candidate subunit vaccine against malaria.

Independent translation of ORFs in dicistronic operons, synthetic building blocks for polycistronic chloroplast gene expression

We designed a dicistronic plastid marker system that relies on the plastid's ability to translate polycistronic mRNAs. The identification of transplastomic clones is based on selection for antibiotic resistance encoded in the first open reading frame (ORF) and accumulation of the reporter gene product in tobacco chloroplasts encoded in the second ORF. The antibiotic resistance gene may encode spectinomycin or kanamycin resistance based on the expression of aadA or neo genes, respectively. The reporter gene used in the study is the green fluorescent protein (GFP). The mRNA level depends on the 5'-untranslated region of the first ORF. The protein output depends on the strengths of the ribosome binding, and is proportional with the level of translatable mRNA. Because the dicistronic mRNA is not processed, we could show that protein output from the second ORF is independent from the first ORF. High-level GFP accumulation from the second ORF facilitates identification of transplastomic events under ultraviolet light. Expression of multiple proteins from an unprocessed mRNA is an experimental design that enables predictable protein output from polycistronic mRNAs, expanding the toolkit of plant synthetic biology.

Comparative Plastomics of Ashwagandha (Withania, Solanaceae) and Identification of Mutational Hotspots for Barcoding Medicinal Plants

Within the family Solanaceae, Withania is a small genus belonging to the Solanoideae subfamily. Here, we report the de novo assembled chloroplast genome sequences of W. coagulans, W. adpressa, and W. riebeckii. The length of these genomes ranged from 154,162 to 154,364 base pairs (bp). These genomes contained a pair of inverted repeats (IRa and IRb) ranging from 25,029 to 25,071 bp that were separated by a large single-copy (LSC) region of 85,635-85,765 bp and a small single-copy (SSC) region of 18,457-18,469 bp. We analyzed the structural organization, gene content and order, guanine-cytosine content, codon usage, RNA-editing sites, microsatellites, oligonucleotide and tandem repeats, and substitutions of Withania plastomes, which revealed high similarities among the species. Comparative analysis among the Withania species also highlighted 10 divergent hotspots that could potentially be used for molecular marker development, phylogenetic analysis, and species identification. Furthermore, our analyses showed that even three mutational hotspots (rps4-trnT, trnM-atpE, and rps15) were sufficient to discriminate the Withania species included in current study.

Molecular farming - The slope of enlightenment

Molecular farming can be defined as the use of plants to produce recombinant protein products. The technology is now >30 years old. The early promise of molecular farming was based on three perceived advantages: the low costs of growing plants, the immense scalability of agricultural production, and the inherent safety of plants as hosts for the production of pharmaceuticals. This resulted in a glut of research publications in which diverse proteins were expressed in equally diverse plant-based systems, and numerous companies were founded hoping to commercialize the new technology. There was a moderate degree of success for companies producing non-pharmaceutical proteins, but in the pharmaceutical sector the anticipation raised by promising early research was soon met by the cold hard reality of industrial pragmatism. Plants did not have a track record of success in pharmaceutical protein manufacturing, lacked a regulatory framework, and did not perform as well as established industry platforms. Negative attitudes towards genetically modified plants added to the mix. By the early 2000s, major industry players started to lose interest and pharmaceutical molecular farming fell from a peak of expectation into a trough of disillusionment, just as predicted by the Gartner hype cycle. But many of the pioneers of molecular farming have refocused their activities and have worked to address the limitations that hampered the first generation of technologies. The field has now consolidated around a smaller number of better-characterized platforms and has started to develop standardized methods and best practices, mirroring the evolution of more mature industry sectors. Likewise, attention has turned from proof-of-principle studies to realistic techno-economic modeling to capture significant niche markets, replicating the success of the industrial molecular farming sector. Here we argue that these recent developments signify that pharmaceutical molecular farming is now climbing the slope of enlightenment and will soon emerge as a mature technology.

Expression of ESAT-6 antigen from Mycobacterium tuberculosis in broccoli: An edible plant

Tuberculosis (TB) is one of the major infectious diseases caused by Mycobacterium tuberculosis. The development of an effective and economical vaccine for controlling TB is essential especially for developing countries. Edible plants can serve as biofactories to produce vaccine antigens. In this study, 6 kDa early secretory antigenic target (ESAT-6) of M. tuberculosis was expressed in Brassica oleracea var. italica via Agrobacterium-mediated transformation to facilitate oral delivery of antigen. ESAT-6 gene was cloned using Gateway® cloning strategy. Transformation and presence of transgene was confirmed through PCR. Expression level of transgene was calculated via quantitative real-time PCR (qRT-PCR) and the maximum integrated transgene number was two. Maximum amount of total soluble fraction of ESAT-6 was evaluated by immunoblotting, estimated to accumulate up to 0.5% of total soluble protein. The recombinant ESAT-6 protein was further purified and detected using silver staining and Western blotting. ESAT-6 protein induced humoral immune response in mice immunized orally and subcutaneously. The expression of M. tuberculosis antigen in edible plants could aid in the development of cost-effective and oral delivery of an antigen-based subunit vaccine against TB. To the best our knowledge, it is the first report of expression of a vaccine antigen in broccoli.

Chloroplast-based inducible expression of ESAT-6 antigen for development of a plant-based vaccine against tuberculosis

Mycobacterium tuberculosis causes tuberculosis in humans. The major disease burden of tuberculosis lies in developing countries. Lack of an effective vaccine for adults is one of the major hurdles for controlling this deadly disease. In the present study, 6 kDa early secretory antigenic target (ESAT-6) of M. tuberculosis was inducibly expressed in chloroplasts of Nicotiana tabacum. The expression of ESAT-6 in chloroplasts was controlled by T7 promoter that was activated by nuclear-generated signal peptide. Tobacco plants, containing nuclear component, were transformed via biolistic bombardment with pEXP-T7-ESAT-6 obtained by Gateway® cloning. Transformation and homoplasmic status of transplastomic plants was confirmed by polymerase chain reaction and Southern blotting. Plants were induced for protein expression by spraying with 5% ethanol for 1 day, 3 days, 7 days and 10 days. ESAT-6 protein was detected by immunoblot analysis and maximum protein was obtained for 10 days induced plants that was estimated to accumulate up to 1.2% of total soluble fraction of protein. Transplastomic plants showed completely normal morphology. Transplastomic and untransformed plants became slightly chlorotic upon prolonged exposure to ethanol until 10 days. Taken together, this data could help in the development of an antigen-based subunit vaccine against tuberculosis.

Optimization of cell suspension culture of transformed and untransformed lettuce for the enhanced production of secondary metabolites and their pharmaceutical evaluation

In vitro suspension culture techniques are cost effective for large-scale production of secondary metabolites. In the present study, firstly, suspension cultures of untransformed Lactuca sativa were prepared using different hormonal combinations and were subjected to different pH, temperature and salt concentrations. Maximum biomass was obtained for suspensions supplemented with 1.5 mg/L BAP and 0.1 mg/L NAA, at pH 5.8, temperature 28 °C and 0 mM NaCl concentration. Using these parameters, suspensions were produced for rol ABC- and rol C-transformed lines of L. sativa. All the transgenic lines showed prominent increase in fresh weight (FW) and dry weight (DW) with maximum values for rol ABC2 line producing 169.8 mg/mL FW and 25.3 mg/mL DW. The exudates of transformed and untransformed plants were tested for the antioxidant activity and in vivo assays on rats. Maximum phenolic content (261 μg/mL) and flavonoid content (637.6 μg/mL) were obtained for rol C1 transgenic line. Total antioxidant capacity was found maximum (1451.7 μg/mL) for untransformed lettuce, whereas rol C1 showed maximum total reducing power activity (637.6 μg/mL). In DPPH assay, maximum activity (104.7 μg/mL) was shown by rol ABC3 line. In rats analgesic assay, maximum activity (74.9%) was shown by rol C2. Line rol C1 showed maximum anti-inflammatory activity (69.2%) and maximum antidepressant activity (minimum immobility time of 55 s). Maximum anticoagulant activity was observed for rol ABC2 with maximum clotting time of 130 s. The present study could help in using lettuce suspension culture as platform for the enhanced production of important metabolites.

Vaccine and antibody production in plants: developments and computational tools

Plants as bioreactors have been widely used to express efficient vaccine antigens against viral, bacterial and protozoan infections. To date, many different plant-based expression systems have been analyzed, with a growing preference for transient expression systems. Antibody expression in diverse plant species for therapeutic applications is well known, and this review provides an overview of various aspects of plant-based biopharmaceutical production. Here, we highlight conventional and gene expression technologies in plants along with some illustrative examples. In addition, the portfolio of products that are being produced and how they relate to the success of this field are discussed. Stable and transient gene expression in plants, agrofiltration and virus infection vectors are also reviewed. Further, the present report draws attention to antibody epitope prediction using computational tools, one of the crucial steps of vaccine design. Finally, regulatory issues, biosafety and public perception of this technology are also discussed.

Engineered PPR proteins as inducible switches to activate the expression of chloroplast transgenes

The engineering of plant genomes presents exciting opportunities to modify agronomic traits and to produce high-value products in plants. Expression of foreign proteins from transgenes in the chloroplast genome offers advantages that include the capacity for prodigious protein output, the lack of transgene silencing and the ability to express multicomponent pathways from polycistronic mRNA. However, there remains a need for robust methods to regulate plastid transgene expression. We designed orthogonal activators that boost the expression of chloroplast transgenes harbouring cognate cis-elements. Our system exploits the programmable RNA sequence specificity of pentatricopeptide repeat proteins and their native functions as activators of chloroplast gene expression. When expressed from nuclear transgenes, the engineered proteins stimulate the expression of plastid transgenes by up to ~40-fold, with maximal protein abundance approaching that of Rubisco. This strategy provides a means to regulate and optimize the expression of foreign genes in chloroplasts and to avoid deleterious effects of their products on plant growth.

Alternative Methods of Vaccine Delivery: An Overview of Edible and Intradermal Vaccines

Vaccines are recognized worldwide as one of the most important tools for combating infectious diseases. Despite the tremendous value conferred by currently available vaccines toward public health, the implementation of additional vaccine platforms is also of key importance. In fact, currently available vaccines possess shortcomings, such as inefficient triggering of a cell-mediated immune response and the lack of protective mucosal immunity. In this regard, recent work has been focused on vaccine delivery systems, as an alternative to injectable vaccines, to increase antigen stability and improve overall immunogenicity. In particular, novel strategies based on edible or intradermal vaccine formulations have been demonstrated to trigger both a systemic and mucosal immune response. These novel vaccination delivery systems offer several advantages over the injectable preparations including self-administration, reduced cost, stability, and elimination of a cold chain. In this review, the latest findings and accomplishments regarding edible and intradermal vaccines are described in the context of the system used for immunogen expression, their molecular features and capacity to induce a protective systemic and mucosal response.

High-level production of single chain monellin mutants with enhanced sweetness and stability in tobacco chloroplasts

MAIN CONCLUSION

Plastid-based MNEI protein mutants retain the structure, stability and sweetness of their bacterial counterparts, confirming the attractiveness of the plastid transformation technology for high-yield production of recombinant proteins. The prevalence of obesity and diabetes has dramatically increased the industrial demand for the development and use of alternatives to sugar and traditional sweeteners. Sweet proteins, such as MNEI, a single chain derivative of monellin, are the most promising candidates for industrial applications. In this work, we describe the use of tobacco chloroplasts as a stable plant expression platform to produce three MNEI protein mutants with improved taste profile and stability. All plant-based proteins were correctly expressed in tobacco chloroplasts, purified and subjected to in-depth chemical and sensory analyses. Recombinant MNEI mutants showed a protein yield ranging from 5% to more than 50% of total soluble proteins, which, to date, represents the highest accumulation level of MNEI mutants in plants. Comparative analyses demonstrated the high similarity, in terms of structure, stability and function, of the proteins produced in plant chloroplasts and bacteria. The high yield and the extreme sweetness perceived for the plant-derived proteins prove that plastid transformation technology is a safe, stable and cost-effective production platform for low-calorie sweeteners, with an estimated production of up to 25-30 mg of pure protein/plant.

Platforms for Plant-Based Protein Production

Plant molecular farming depends on a diversity of plant systems for production of useful recombinant proteins. These proteins include protein biopolymers, industrial proteins and enzymes, and therapeutic proteins. Plant production systems include microalgae, cells, hairy roots, moss, and whole plants with both stable and transient expression. Production processes involve a narrowing diversity of bioreactors for cell, hairy root, microalgae, and moss cultivation. For whole plants, both field and automated greenhouse cultivation methods are used with products expressed and produced either in leaves or seeds. Many successful expression systems now exist for a variety of different products with a list of increasingly successful commercialized products. This chapter provides an overview and examples of the current state of plant-based production systems for different types of recombinant proteins.

Transplastomic plants yield a multicomponent vaccine against cysticercosis

Low cost vaccines against cysticercosis are needed to fight this parasitosis, especially in developing countries. Herein polycistron arrangements were designed to accomplish the simultaneous expression of multiple protective antigens from Taenia solium in the plant cell as an attractive biofactory and delivery vehicle of vaccines. Transplastomic plants carrying synthetic polycistrons were able to simultaneously express the KETc1, KETc7, KETc12, GK1, and TSOL18/HP6-Tsol antigens; which retained their antigenicity and ability to induce humoral responses in BALB/c mice. These clones may be useful for the production of low-cost cysticercosis vaccine prototypes.

Structure of Plastid Genomes of Photosynthetic Eukaryotes

This review presents current views on the plastid genomes of higher plants and summarizes data on the size, structural organization, gene content, and other features of plastid DNAs. Special emphasis is placed on the properties of organization of land plant plastid genomes (nucleoids) that distinguish them from bacterial genomes. The prospects of genetic engineering of chloroplast genomes are discussed.

Transformation of Lettuce with rol ABC Genes: Extracts Show Enhanced Antioxidant, Analgesic, Anti-Inflammatory, Antidepressant, and Anticoagulant Activities in Rats

Lettuce is an edible crop that is well known for dietary and antioxidant benefits. The present study was conducted to investigate the effects of rol ABC genes on antioxidant and medicinal potential of lettuce by Agrobacterium-mediated transformation. Transgene integration and expression was confirmed through PCR and real-time RT-PCR, respectively. The transformed plants showed 91-102 % increase in total phenolic contents and 53-65 % increase in total flavonoid contents compared to untransformed plants. Total antioxidant capacity and total reducing power increased up to 112 and 133 % in transformed plants, respectively. Results of DPPH assay showed maximum 51 % increase, and lipid peroxidation assay exhibited 20 % increase in antioxidant activity of transformed plants compared to controls. Different in vivo assays were carried out in rats. The transgenic plants showed up to 80 % inhibition in both hot plate analgesic assay and carrageenan-induced hind paw edema test, while untransformed plants showed only 45 % inhibition. Antidepressant and anticoagulant potential of transformed plants was also significantly enhanced compared to untransformed plants. Taken together, the present work highlights the use of rol genes to enhance the secondary metabolite production in lettuce and improve its analgesic, anti-inflammatory, antidepressant, and anticoagulatory properties.

Expression and functional evaluation of biopharmaceuticals made in plant chloroplasts

After approval of the first plant-made biopharmaceutical by FDA for human use, many protein drugs are now in clinical development. Within the last decade, significant advances have been made in expression of heterologous complex/large proteins in chloroplasts of edible plants using codon optimized human or viral genes. Furthermore, advances in quantification enable determination of in-planta drug dosage. Oral delivery of plastid-made biopharmaceuticals (PMB) is affordable because it eliminates prohibitively expensive fermentation, purification processes addressing major challenges of short shelf-life after cold storage. In this review, we discuss recent advances in PMBs against metabolic, inherited or infectious diseases, and also mechanisms of post-translational modifications (PTM) in order to increase our understanding of functional PMBs.

Production of Phloroglucinol, a Platform Chemical, in Arabidopsis using a Bacterial Gene

Phloroglucinol (1,3,5-trihydroxybenzene; PG) and its derivatives are phenolic compounds that are used for various industrial applications. Current methods to synthesize PG are not sustainable due to the requirement for carbon-based precursors and co-production of toxic byproducts. Here, we describe a more sustainable production of PG using plants expressing a native bacterial or a codon-optimized synthetic PhlD targeted to either the cytosol or chloroplasts. Transgenic lines were analyzed for the production of PG using gas and liquid chromatography coupled to mass spectroscopy. Phloroglucinol was produced in all transgenic lines and the line with the highest PhlD transcript level showed the most accumulation of PG. Over 80% of the produced PG was glycosylated to phlorin. Arabidopsis leaves have the machinery to glycosylate PG to form phlorin, which can be hydrolyzed enzymatically to produce PG. Furthermore, the metabolic profile of plants with PhlD in either the cytosol or chloroplasts was altered. Our results provide evidence that plants can be engineered to produce PG using a bacterial gene. Phytoproduction of PG using a bacterial gene paves the way for further genetic manipulations to enhance the level of PG with implications for the commercial production of this important platform chemical in plants.

Vaccination via Chloroplast Genetics: Affordable Protein Drugs for the Prevention and Treatment of Inherited or Infectious Human Diseases

Plastid-made biopharmaceuticals treat major metabolic or genetic disorders, including Alzheimer's, diabetes, hypertension, hemophilia, and retinopathy. Booster vaccines made in chloroplasts prevent global infectious diseases, such as tuberculosis, malaria, cholera, and polio, and biological threats, such as anthrax and plague. Recent advances in this field include commercial-scale production of human therapeutic proteins in FDA-approved cGMP facilities, development of tags to deliver protein drugs to targeted human cells or tissues, methods to deliver precise doses, and long-term stability of protein drugs at ambient temperature, maintaining their efficacy. Codon optimization utilizing valuable information from sequenced chloroplast genomes enhanced expression of eukaryotic human or viral genes in chloroplasts and offered unique insights into translation in chloroplasts. Support from major biopharmaceutical companies, development of hydroponic production systems, and evaluation by regulatory agencies, including the CDC, FDA, and USDA, augur well for advancing this novel concept to the clinic and revolutionizing affordable healthcare.

Challenges and perspectives in commercializing plastid transformation technology

Plastid transformation has emerged as an alternative platform to generate transgenic plants. Attractive features of this technology include specific integration of transgenes-either individually or as operons-into the plastid genome through homologous recombination, the potential for high-level protein expression, and transgene containment because of the maternal inheritance of plastids. Several issues associated with nuclear transformation such as gene silencing, variable gene expression due to the Mendelian laws of inheritance, and epigenetic regulation have not been observed in the plastid genome. Plastid transformation has been successfully used for the production of therapeutics, vaccines, antigens, and commercial enzymes, and for engineering various agronomic traits including resistance to biotic and abiotic stresses. However, these demonstrations have usually focused on model systems such as tobacco, and the technology per se has not yet reached the market. Technical factors limiting this technology include the lack of efficient protocols for the transformation of cereals, poor transgene expression in non-green plastids, a limited number of selection markers, and the lengthy procedures required to recover fully segregated plants. This article discusses the technology of transforming the plastid genome, the positive and negative features compared with nuclear transformation, and the current challenges that need to be addressed for successful commercialization.

The Last Ten Years of Advancements in Plant-Derived Recombinant Vaccines against Hepatitis B

Disease prevention through vaccination is considered to be the greatest contribution to public health over the past century. Every year more than 100 million children are vaccinated with the standard World Health Organization (WHO)-recommended vaccines including hepatitis B (HepB). HepB is the most serious type of liver infection caused by the hepatitis B virus (HBV), however, it can be prevented by currently available recombinant vaccine, which has an excellent record of safety and effectiveness. To date, recombinant vaccines are produced in many systems of bacteria, yeast, insect, and mammalian and plant cells. Among these platforms, the use of plant cells has received considerable attention in terms of intrinsic safety, scalability, and appropriate modification of target proteins. Research groups worldwide have attempted to develop more efficacious plant-derived vaccines for over 30 diseases, most frequently HepB and influenza. More inspiring, approximately 12 plant-made antigens have already been tested in clinical trials, with successful outcomes. In this study, the latest information from the last 10 years on plant-derived antigens, especially hepatitis B surface antigen, approaches are reviewed and breakthroughs regarding the weak points are also discussed.

Oral Delivery of Protein Drugs Bioencapsulated in Plant Cells

Plants cells are now approved by the FDA for cost-effective production of protein drugs (PDs) in large-scale current Good Manufacturing Practice (cGMP) hydroponic growth facilities. In lyophilized plant cells, PDs are stable at ambient temperature for several years, maintaining their folding and efficacy. Upon oral delivery, PDs bioencapsulated in plant cells are protected in the stomach from acids and enzymes but are subsequently released into the gut lumen by microbes that digest the plant cell wall. The large mucosal area of the human intestine offers an ideal system for oral drug delivery. When tags (receptor-binding proteins or cell-penetrating peptides) are fused to PDs, they efficiently cross the intestinal epithelium and are delivered to the circulatory or immune system. Unique tags to deliver PDs to human immune or nonimmune cells have been developed recently. After crossing the epithelium, ubiquitous proteases cleave off tags at engineered sites. PDs are also delivered to the brain or retina by crossing the blood–brain or retinal barriers. This review highlights recent advances in PD delivery to treat Alzheimer's disease, diabetes, hypertension, Gaucher's or ocular diseases, as well as the development of affordable drugs by eliminating prohibitively expensive purification, cold chain and sterile delivery.

Need of cost-effective vaccines in developing countries: What plant biotechnology can offer?

To treat current infectious diseases, different therapies are used that include drugs or vaccines or both. Currently, the world is facing an increasing problem of drug resistance from many pathogenic microorganisms. In majority of cases, when vaccines are used, formulations consist of live attenuated microorganisms. This poses an additional risk of infection in immunocompromised patients and people suffering from malnutrition in developing countries. Therefore, there is need to improve drug therapy as well as to develop next generation vaccines, in particular against infectious diseases with highest mortality rates. For patients in developing countries, costs related to treatments are one of the major hurdles to reduce the disease burden. In many cases, use of prophylactic vaccines can help to control the incidence of infectious diseases. In the present review, we describe some infectious diseases with high impact on health of people in low and middle income countries. We discuss the prospects of plants as alternative platform for the development of next-generation subunit vaccines that can be a cost-effective source for mass immunization of people in developing countries.