High-level expression of thermostable cellulolytic enzymes in tobacco transplastomic plants and their use in hydrolysis of an industrially pretreated Arundo donax L. biomass

Harnessing the potential of chloroplast-derived expression elements for enhanced production of cellulases in Escherichia coli

Thermophilic cellulases can play a crucial part in the efficient breakdown of cellulose-a major component of lignocellulosic plant biomass, however, their commercial production needs simple and robust biomanufacturing biosystems. In this study, two cellulases (β-glucosidase and endoglucanase) were heterologously expressed in Escherichia coli under a chloroplast-derived constitutive promoter and expression-enhancing terminator. The genes encoding the cellulases were sourced from a thermophilic bacterium Thermotoga maritima to exploit their industrially needed thermotolerance potential. The codon-optimized gene sequences were synthesized and placed under a tobacco chloroplast 16S rRNA promoter (Prrn), along with the 5' UTR (untranslated region) from gene 10 of phage T7 (T7g10). A six-residue long histidine tag (His6-tag) was attached to the N-terminus for protein detection. A high-level of expression of β-glucosidase and endoglucanase in E. coli was recorded from the chloroplast promoter and terminator. Furthermore, the activity assays confirmed that the recombinant enzymes maintained their activity at elevated temperatures. Thermostability analysis showed that recombinant enzymes retained their thermotolerance even after being expressed in a non-native host. Where, β-glucosidase and endoglucanase showed their optimum activities at 90 °C and 100 °C, respectively. Examination of the 3D structures of T. maritima cellulases revealed differential ionic interactions contributing to this high degree of thermotolerance. The study highlights the feasibility of producing thermostable versions of recombinant enzymes in E. coli at high levels. Our finding underscores the potential of this approach to meet industrial demands for efficient enzyme production employing E. coli as a robust biomanufacturing platform.

Exploring recent progress of molecular farming for therapeutic and recombinant molecules in plant systems

An excellent technique for producing pharmaceuticals called "molecular farming" enables the industrial mass production of useful recombinant proteins in genetically modified organisms. Protein-based pharmaceuticals are rising in significance because of a variety of factors, including their bioreactivity, precision, safety, and efficacy rate. Heterologous expression methods for the manufacturing of pharmaceutical products have been previously employed using yeast, bacteria, and animal cells. However, the high cost of mammalian cell system, and production, the chance for product complexity, and contamination, and the hurdles of scaling up to commercial production are the limitations of these traditional expression methods. Plants have been raised as a hopeful replacement system for the expression of biopharmaceutical products due to their potential benefits, which include low production costs, simplicity in scaling up to commercial manufacturing levels, and a lower threat of mammalian toxin contaminations and virus infections. Since plants are widely utilized as a source of therapeutic chemicals, molecular farming offers a unique way to produce molecular medicines such as recombinant antibodies, enzymes, growth factors, plasma proteins, and vaccines whose molecular basis for use in therapy is well established. Biopharming provides more economical and extensive pharmaceutical drug supplies, including vaccines for contagious diseases and pharmaceutical proteins for the treatment of conditions like heart disease and cancer. To assess its technical viability and the efficacy resulting from the adoption of molecular farming products, the following review explores the various methods and methodologies that are currently employed to create commercially valuable molecules in plant systems.

Advances in Subcellular Accumulation Design for Recombinant Protein Production in Tobacco

Plants and their use as bioreactors for the generation of recombinant proteins have become one of the hottest topics in the field of Plant Biotechnology and Plant Synthetic Biology. Plant bioreactors offer superior engineering potential compared to other types, particularly in the realm of subcellular accumulation strategies for increasing production yield and quality. This review explores established and emerging strategies for subcellular accumulation of recombinant proteins in tobacco bioreactors, highlighting recent advancements in the field. Additionally, the review provides reference to the crucial initial step of selecting an optimal subcellular localization for the target protein, a design that substantially impacts production outcomes.

Production of biologically active human basic fibroblast growth factor (hFGFb) using Nicotiana tabacum transplastomic plants

MAIN CONCLUSION

We generated transplastomic tobacco lines that stably express a human Basic Fibroblast Growth Factor (hFGFb) in their chloroplasts stroma and purified a biologically active recombinant hFGFb. MAIN: The use of plants as biofactories presents as an attractive technology with the potential to efficiently produce high-value human recombinant proteins in a cost-effective manner. Plastid genome transformation stands out for its possibility to accumulate recombinant proteins at elevated levels. Of particular interest are recombinant growth factors, given their applications in animal cell culture and regenerative medicine. In this study, we produced recombinant human Fibroblast Growth Factor (rhFGFb), a crucial protein required for animal cell culture, in tobacco chloroplasts. We successfully generated two independent transplastomic lines that are homoplasmic and accumulate rhFGFb in their leaves. Furthermore, the produced rhFGFb demonstrated its biological activity by inducing proliferation in HEK293T cell lines. These results collectively underscore plastid genome transformation as a promising plant-based bioreactor for rhFGFb production.

Enzyme-Based Biostimulants Influence Physiological and Biochemical Responses of Lactuca sativa L

Biostimulants (BSs) are natural materials (i.e., organic or inorganic compounds, and/or microorganisms) having beneficial effects on plant growth and productivity, and able to improve resilience/tolerance to biotic and abiotic stresses. Therefore, they represent an innovative alternative to the phyto- and agrochemicals, being environmentally friendly and a valuable tool to cope with extreme climate conditions. The objective of this study was to investigate the effects of several biomolecules (i.e., Xylanase, β-Glucosidase, Chitinase, and Tramesan), alone or in combinations, on lettuce plant growth and quality. With this aim, the influence of these biomolecules on biomass, pigment content, and antioxidant properties in treated plants were investigated. Our results showed that Xylanase and, to a lesser extent, β-Glucosidase, have potentially biostimulant activity for lettuce cultivation, positively influencing carotenoids, total polyphenols, and ascorbic acid contents; similar effects were found with respect to antioxidative properties. Furthermore, the effect of the more promising molecules (Xylanase and β-Glucosidase) was also evaluated in kiwifruit cultured cells to test their putative role as sustainable input for plant cell biofactories. The absence of phytotoxic effects of both molecules at low doses (0.1 and 0.01 µM), and the significantly enhanced cell biomass growth, indicates a positive impact on kiwifruit cells.

Systematic bioprospection for cellulolytic actinomycetes in the Chihuahuan Desert: isolation and enzymatic profiling

The quest for microbial cellulases has intensified as a response to global challenges in biofuel production. The efficient deconstruction of lignocellulosic biomass holds promise for generating valuable products in various industries such as food, textile, and detergents. This article presents a systematic bioprospection aimed at isolating actinomycetes with exceptional cellulose deconstruction capabilities. Our methodology explored the biodiverse oligotrophic region of Cuatro Cienegas, Coahuila, within the Chihuahuan Desert. Among the evaluated actinomycetes collection, 78% exhibited cellulolytic activity. Through a meticulous screening process based on enzymatic index evaluation, we identified a highly cellulolytic Streptomyces strain for further investigation. Submerged fermentation of this strain revealed an endoglucanase enzymatic activity of 149 U/mg. Genomic analysis of strain Streptomyces sp. STCH565-A revealed unique configurations of carbohydrate-active enzyme (CAZyme) genes, underscoring its potential for lignocellulosic bioconversion applications. These findings not only highlight the significance of the Chihuahuan Desert as a rich source of cellulolytic microorganisms but also offer insights into the systematic exploration and selection of high-performing cellulolytic microorganisms for application in diverse environmental contexts. In conclusion, our bioprospecting study lays a foundation for harnessing the cellulolytic potential of actinomycetes from the Chihuahuan Desert, with implications for advancing cellulose deconstruction processes in various industries. The findings can serve as a blueprint for future bioprospecting efforts in different regions, facilitating the targeted discovery of microorganisms with exceptional cellulosic deconstruction capabilities.

Hyper-expression of GFP-fused active hFGF21 in tobacco chloroplasts

Human fibroblast growth factor 21 (hFGF21) is a promising candidate for metabolic diseases. In this study, a tobacco chloroplast transformation vector, pWYP21406, was constructed that consisted of codon-optimized encoding gene hFGF21 fused with GFP at its 5' terminal; it was driven by the promoter of plastid rRNA operon (Prrn) and terminated by the terminator of plastid rps16 gene (Trps16). Spectinomycin-resistant gene (aadA) was the marker and placed in the same cistron between hFGF21 and the terminator Trps16. Transplastomic plants were generated by the biolistic bombardment method and proven to be homoplastic by Southern blotting analysis. The expression of GFP was detected under ultraviolet light and a laser confocal microscope. The expression of GFP-hFGF21 was confirmed by immunoblotting and quantified by enzyme-linked immunosorbnent assay (ELISA). The accumulation of GFP-hFGF21 was confirmed to be 12.44 ± 0.45% of the total soluble protein (i.e., 1.9232 ± 0.0673 g kg^-1 of fresh weight). GFP-hFGF21 promoted the proliferation of hepatoma cell line HepG2, inducing the expression of glucose transporter 1 in hepatoma HepG2 cells and improving glucose uptake. These results suggested that a chloroplast expression is a promising approach for the production of bioactive recombinant hFGF21.

Successful production of human epidermal growth factor in tobacco chloroplasts in a biologically active conformation

Human epidermal growth factor (hEGF) is an important therapeutic compound with multiple applications particularly in pharmaceutical industry. Human EGF has already been expressed in different expression systems, however, the production of hEGF with bioactivity in chloroplasts has not been successful so far. In this study, we expressed a 6 × His-tagged hEGF in tobacco chloroplasts in its native conformation for the potential of large-scale production of hEGF for industrial applications. Several transplastomic plant lines were obtained, which were screened by PCR (polymerase chain reaction) using primers specific to selectable gene aadA, hEGF- and GFP-coding sequences that were included in the chloroplast expression vector. The selected lines were confirmed to be homoplasmic by PCR verification and Southern blot analysis. Immunoblotting assays of homoplasmic lines using antibodies raised against hEGF confirmed the accumulation of hEGF in transplastomic plants and the ELISA results demonstrated the expression levels of hEGF were between 0.124% and 0.165% of the total soluble proteins (TSP), namely, 23.16-25.77 ng/g of the fresh weight. In terms of activity, the data from cell proliferation and elongation assays showed that the tobacco-derived recombinant hEGF was as bioactive as its commercial counterpart. To our knowledge, this is the first report of recombinant production of hEGF with native bioactivity form in the chloroplast stroma. Overall, our results demonstrate the potential of higher plant chloroplasts for the production of a human therapeutic, hEGF, in an active conformation.

Tobacco Plastid Transformation as Production Platform of Lytic Polysaccharide MonoOxygenase Auxiliary Enzymes

Plant biomass is the most abundant renewable resource in nature. In a circular economy perspective, the implementation of its bioconversion into fermentable sugars is of great relevance. Lytic Polysaccharide MonoOxygenases (LPMOs) are accessory enzymes able to break recalcitrant polysaccharides, boosting biomass conversion and subsequently reducing costs. Among them, auxiliary activity of family 9 (AA9) acts on cellulose in synergism with traditional cellulolytic enzymes. Here, we report for the first time, the production of the AA9 LPMOs from the mesophilic Trichoderma reesei (TrAA9B) and the thermophilic Thermoascus aurantiacus (TaAA9B) microorganisms in tobacco by plastid transformation with the aim to test this technology as cheap and sustainable manufacture platform. In order to optimize recombinant protein accumulation, two different N-terminal regulatory sequences were used: 5' untranslated region (5'-UTR) from T7g10 gene (DC41 and DC51 plants), and 5' translation control region (5'-TCR), containing the 5'-UTR and the first 14 amino acids (Downstream Box, DB) of the plastid atpB gene (DC40 and DC50 plants). Protein yields ranged between 0.5 and 5% of total soluble proteins (TSP). The phenotype was unaltered in all transplastomic plants, except for the DC50 line accumulating AA9 LPMO at the highest level, that showed retarded growth and a mild pale green phenotype. Oxidase activity was spectrophotometrically assayed and resulted higher for the recombinant proteins without the N-terminal fusion (DC41 and DC51), with a 3.9- and 3.4-fold increase compared to the fused proteins.

Plastid Transformation: New Challenges in the Circular Economy Era

In a circular economy era the transition towards renewable and sustainable materials is very urgent. The development of bio-based solutions, that can ensure technological circularity in many priority areas (e.g., agriculture, biotechnology, ecology, green industry, etc.), is very strategic. The agricultural and fishing industry wastes represent important feedstocks that require the development of sustainable and environmentally-friendly industrial processes to produce and recover biofuels, chemicals and bioactive molecules. In this context, the replacement, in industrial processes, of chemicals with enzyme-based catalysts assures great benefits to humans and the environment. In this review, we describe the potentiality of the plastid transformation technology as a sustainable and cheap platform for the production of recombinant industrial enzymes, summarize the current knowledge on the technology, and display examples of cellulolytic enzymes already produced. Further, we illustrate several types of bacterial auxiliary and chitinases/chitin deacetylases enzymes with high biotechnological value that could be manufactured by plastid transformation.

Plant synthetic biology innovations for biofuels and bioproducts

Plant-based biosynthesis of fuels, chemicals, and materials promotes environmental sustainability, which includes decreases in greenhouse gas emissions, water pollution, and loss of biodiversity. Advances in plant synthetic biology (synbio) should improve precision and efficacy of genetic engineering for sustainability. Applicable synbio innovations include genome editing, gene circuit design, synthetic promoter development, gene stacking technologies, and the design of environmental sensors. Moreover, recent advancements in developing spatially resolved and single-cell omics contribute to the discovery and characterization of cell-type-specific mechanisms and spatiotemporal gene regulations in distinct plant tissues for the expression of cell- and tissue-specific genes, resulting in improved bioproduction. This review highlights recent plant synbio progress and new single-cell molecular profiling towards sustainable biofuel and biomaterial production.

Functional peptide-mediated plastid transformation in tobacco, rice, and kenaf

In plant engineering, plastid transformation is more advantageous than nuclear transformation because it results in high levels of protein expression from multiple genome copies per cell and is unaffected by gene silencing. The common plastid transformation methods are biolistic bombardment that requires special instruments and PEG-mediated transformation that is only applicable to protoplast cells. Here, we aimed to establish a new plastid transformation method in tobacco, rice, and kenaf using a biocompatible fusion peptide as a carrier to deliver DNA into plastids. We used a fusion peptide, KH-AtOEP34, comprising a polycationic DNA-binding peptide (KH) and a plastid-targeting peptide (AtOEP34) to successfully deliver and integrate construct DNA into plastid DNA (ptDNA) via homologous recombination. We obtained transformants in each species using selection with spectinomycin/streptomycin and the corresponding resistance gene aadA. The constructs remained in ptDNA for several months after introduction even under non-selective condition. The transformants normally flowered and are fertile in most cases. The offspring of the transformants (the T₁ generation) retained the integrated construct DNA in their ptDNA, as indicated by PCR and DNA blotting, and expressed GFP in plastids from the integrated construct DNA. In summary, we successfully used the fusion peptide method for integration of foreign DNA in tobacco, rice, and kenaf ptDNA, and the integrated DNA was transmitted to the next generations. Whereas optimization is necessary to obtain homoplasmic plastid transformants that enable stable heterologous expression of genes, the plastid transformation method shown here is a novel nanomaterial-based approach distinct from the conventional methods, and we propose that this easy method could be used to target a wide variety of plants.

Advances in plastid transformation for metabolic engineering in higher plants

The plastid (chloroplast) genome of higher plants is an appealing target for metabolic engineering via genetic transformation. Although the bacterial-type plastid genome is small compared with the nuclear genome, it can accommodate large quantities of foreign genes that precisely integrate through homologous recombination. Engineering complex metabolic pathways in plants often requires simultaneous and concerted expression of multiple transgenes, the possibility of stacking several transgenes in synthetic operons makes the transplastomic approach amazing. The potential for extraordinarily high-level transgene expression, absence of epigenetic gene silencing and transgene containment due to the exclusion of plastids from pollen transmission in most angiosperm species further add to the attractiveness of plastid transformation technology. This minireview describes recent advances in expanding the toolboxes for plastid genome engineering, and highlights selected high-value metabolites produced using transplastomic plants, including artemisinin, astaxanthin and paclitaxel.

Improving environmental stress resilience in crops by genome editing: insights from extremophile plants

In basic and applied sciences, genome editing has become an indispensable tool, especially the versatile and adaptable CRISPR/Cas9 system. Using CRISPR/Cas9 in plants has enabled modifications of many valuable traits, including environmental stress tolerance, an essential aspect when it comes to ensuring food security under climate change pressure. The CRISPR toolbox enables faster and more precise plant breeding by facilitating: multiplex gene editing, gene pyramiding, and de novo domestication. In this paper, we discuss the most recent advances in CRISPR/Cas9 and alternative CRISPR-based systems, along with the technical challenges that remain to be overcome. A revision of the latest proof-of-concept and functional characterization studies has indeed provided more insight into the quantitative traits affecting crop yield and stress tolerance. Additionally, we focus on the applications of CRISPR/Cas9 technology in regard to extremophile plants, due to their significance on: industrial, ecological and economic levels. These still unexplored genetic resources could provide the means to harden our crops against the threat of climate change, thus ensuring food security over the next century.

Chloroplast Engineering: Fundamental Insights and Its Application in Amelioration of Environmental Stress

Chloroplasts are specialized organelle that are responsible for converting light energy to chemical energy, thereby driving the carbon dioxide fixation. Apart from photosynthesis, chloroplast is the site for essential cellular processes that determine the plant adaptation to changing environment. Owing to the presence of their own expression system, it provides an optimum platform for engineering valued traits as well as site for synthesis of bio-compounds. Advancements in technology have further enhanced the scope of using chloroplast as a multifaceted tool for the biotechnologist to develop stress-tolerant plants and ameliorate environmental stress. Focusing on chloroplast biotechnology, this review discusses the advances in chloroplast engineering and its application in enhancing plant adaptation and resistance to environmental stress and the development of new bioproducts and processes. This is accomplished through analysis of its biogenesis and physiological processes, highlighting the chloroplast engineering and recent developments in chloroplast biotechnology. In the first part of the review, the evolution and principles of structural organization and physiology of chloroplast are discussed. In the second part, the chief methods and mechanisms involved in chloroplast transformation are analyzed. The last part represents an updated analysis of the application of chloroplast engineering in crop improvement and bioproduction of industrial and health compounds.

Compendium on Food Crop Plants as a Platform for Pharmaceutical Protein Production

Tremendous advances in crop biotechnology related to the availability of molecular tools and methods developed for transformation and regeneration of specific plant species have been observed. As a consequence, the interest in plant molecular farming aimed at producing the desired therapeutic proteins has significantly increased. Since the middle of the 1980s, recombinant pharmaceuticals have transformed the treatment of many serious diseases and nowadays are used in all branches of medicine. The available systems of the synthesis include wild-type or modified mammalian cells, plants or plant cell cultures, insects, yeast, fungi, or bacteria. Undeniable benefits such as well-characterised breeding conditions, safety, and relatively low costs of production make plants an attractive yet competitive platform for biopharmaceutical production. Some of the vegetable plants that have edible tubers, fruits, leaves, or seeds may be desirable as inexpensive bioreactors because these organs can provide edible vaccines and thus omit the purification step of the final product. Some crucial facts in the development of plant-made pharmaceuticals are presented here in brief. Although crop systems do not require more strictly dedicated optimization of methodologies at any stages of the of biopharmaceutical production process, here we recall the complete framework of such a project, along with theoretical background. Thus, a brief review of the advantages and disadvantages of different systems, the principles for the selection of cis elements for the expression cassettes, and available methods of plant transformation, through to the protein recovery and purification stage, are all presented here. We also outline the achievements in the production of biopharmaceuticals in economically important crop plants and provide examples of their clinical trials and commercialization.

Production of the sheep pox virus structural protein SPPV117 in tobacco chloroplasts

OBJECTIVE

A chloroplast transgenic approach was assessed in order to produce a structural protein SPPV117 of sheep pox virus in Nicotiana tabacum for the future development of a plant-based subunit vaccine against sheep pox.

RESULTS

Two DNA constructs containing SPPV117 coding sequence under the control of chloroplast promoter and terminator of psbA gene or rrn promoter and rbcL terminator were designed and inserted into the chloroplast genome by a biolistic method. The transgenic plants were selected via PCR analysis. Northern and Western blot analysis showed expression of the transgene at transcriptional and translational levels, respectively. The recombinant protein accumulated to about 0.3% and 0.9% of total soluble protein in leaves when expressed from psbA and rrn promoter, respectively. Plant-produced SPPV117 protein was purified using metal affinity chromatography and the protein yield was 19.67  ±  1.25 µg g-1 (FW). The serum of a sheep infected with the virus recognised the chloroplast-produced protein indicating that the protein retains its antigenic properties.

CONCLUSIONS

These results demonstrate that chloroplasts are a suitable system for the production of a candidate subunit vaccine against sheep pox.

Optimal secretion of thermostable Beta-glucosidase in Bacillus subtilis by signal peptide optimization

Commercial applications of β-glucosidase (BGL) demands its purity and availability on a large scale. In the present study, we aim to optimize the expression and secretion of a thermostable BGL from Pyrococcus furiosus (PfuBGL) in B. subtilis strain RIK1285. Initial studies with base strain BV002 harboring aprE signal peptide (aprESP) showed PfuBGL yield of 0.743 ± 0.19 pNP U/ml only. A library of 173 different homologous SPs from B. subtilis 168 genome was fused with target PfuBGL gene (PF0073) in pBE-S vector and extracellularly expressed in RIK1285 strain to identify optimal SP for PfuBGL secretion. High-throughput screening of the resulting SP library for BGL activity with a synthetic substrate followed by systematic scaling of the clones yielded a gene construct with CitHSP reporting a sixteen fold enhancement of PfuBGL secretion in comparison to base strain. Batch fermentation (7.5 L scale) PfuBGL yield of the BV003 strain with CitHSP-PF0073 fusion was observed to be 12.08 ± 0.21 pNP U/ml with specific activity of 35.52 ± 0.53 U/mg. Thus, the study represents report on the secretory expression of thermostable PfuBGL using B. subtilis as a host organism and demonstrating its high potential for industrial production of any protein/enzyme.

Mitigation of deleterious phenotypes in chloroplast-engineered plants accumulating high levels of foreign proteins

BACKGROUND

The global demand for functional proteins is extensive, diverse, and constantly increasing. Medicine, agriculture, and industrial manufacturing all rely on high-quality proteins as major active components or process additives. Historically, these demands have been met by microbial bioreactors that are expensive to operate and maintain, prone to contamination, and relatively inflexible to changing market demands. Well-established crop cultivation techniques coupled with new advancements in genetic engineering may offer a cheaper and more versatile protein production platform. Chloroplast-engineered plants, like tobacco, have the potential to produce large quantities of high-value proteins, but often result in engineered plants with mutant phenotypes. This technology needs to be fine-tuned for commercial applications to maximize target protein yield while maintaining robust plant growth.

RESULTS

Here, we show that a previously developed Nicotiana tabacum line, TetC-cel6A, can produce an industrial cellulase at levels of up to 28% of total soluble protein (TSP) with a slight dwarf phenotype but no loss in biomass. In seedlings, the dwarf phenotype is recovered by exogenous application of gibberellic acid. We also demonstrate that accumulating foreign protein represents an added burden to the plants' metabolism that can make them more sensitive to limiting growth conditions such as low nitrogen. The biomass of nitrogen-limited TetC-cel6A plants was found to be as much as 40% lower than wildtype (WT) tobacco, although heterologous cellulase production was not greatly reduced compared to well-fertilized TetC-cel6A plants. Furthermore, cultivation at elevated carbon dioxide (1600 ppm CO₂) restored biomass accumulation in TetC-cel6A plants to that of WT, while also increasing total heterologous protein yield (mg Cel6A plant^-1) by 50-70%.

CONCLUSIONS

The work reported here demonstrates that well-fertilized tobacco plants have a substantial degree of flexibility in protein metabolism and can accommodate considerable levels of some recombinant proteins without exhibiting deleterious mutant phenotypes. Furthermore, we show that the alterations to protein expression triggered by growth at elevated CO₂ can help rebalance endogenous protein expression and/or increase foreign protein production in chloroplast-engineered tobacco.

Mass Production of Virus-Like Particles Using Chloroplast Genetic Engineering for Highly Immunogenic Oral Vaccine Against Fish Disease

Nervous necrosis virus (NNV) is the causative agent of viral nervous necrosis (VNN), which is one of the most serious fish diseases leading to mass mortality in a wide range of fish species worldwide. Although a few injectable inactivated vaccines are commercially available, there is a need for more labor-saving, cost-effective, and fish-friendly immunization methods. The use of transgenic plants expressing pathogen-derived recombinant antigens as edible vaccines is an ideal way to meet these requirements. In this study, chloroplast genetic engineering was successfully utilized to overexpress the red-spotted grouper NNV capsid protein (RGNNV-CP). The RGNNV-CP accumulated at high levels in all young, mature, and old senescent leaves of transplastomic tobacco plants (averaging approximately 3 mg/g leaf fresh weight). The RGNNV-CP efficiently self-assembled into virus-like particles (RGNNV-VLPs) in the chloroplast stroma of the transgenic lines, which could be readily observed by in situ transmission electron microscopy. Furthermore, intraperitoneal injection and oral administration of the crudely purified protein extract containing chloroplast-derived RGNNV-VLPs provided the sevenband grouper fish with sufficient protection against RGNNV challenge, and its immunogenicity was comparable to that of a commercial injectable vaccine. These findings indicate that chloroplast-derived VLP vaccines may play a promising role in the prevention of various diseases, not only in fish but also in other animals, including humans.

A microalgal-based preparation with synergistic cellulolytic and detoxifying action towards chemical-treated lignocellulose

High-temperature bioconversion of lignocellulose into fermentable sugars has drawn attention for efficient production of renewable chemicals and biofuels, because competing microbial activities are inhibited at elevated temperatures and thermostable cell wall degrading enzymes are superior to mesophilic enzymes. Here, we report on the development of a platform to produce four different thermostable cell wall degrading enzymes in the chloroplast of Chlamydomonas reinhardtii. The enzyme blend was composed of the cellobiohydrolase CBM3GH5 from C. saccharolyticus, the β-glucosidase celB from P. furiosus, the endoglucanase B and the endoxylanase XynA from T. neapolitana. In addition, transplastomic microalgae were engineered for the expression of phosphite dehydrogenase D from Pseudomonas stutzeri, allowing for growth in non-axenic media by selective phosphite nutrition. The cellulolytic blend composed of the glycoside hydrolase (GH) domain GH12/GH5/GH1 allowed the conversion of alkaline-treated lignocellulose into glucose with efficiencies ranging from 14% to 17% upon 48h of reaction and an enzyme loading of 0.05% (w/w). Hydrolysates from treated cellulosic materials with extracts of transgenic microalgae boosted both the biogas production by methanogenic bacteria and the mixotrophic growth of the oleaginous microalga Chlorella vulgaris. Notably, microalgal treatment suppressed the detrimental effect of inhibitory by-products released from the alkaline treatment of biomass, thus allowing for efficient assimilation of lignocellulose-derived sugars by C. vulgaris under mixotrophic growth.

Expression of a Hyperthermophilic Cellobiohydrolase in Transgenic Nicotiana tabacum by Protein Storage Vacuole Targeting

Plant expression of microbial Cell Wall Degrading Enzymes (CWDEs) is a valuable strategy to produce industrial enzymes at affordable cost. Unfortunately, the constitutive expression of CWDEs may affect plant fitness to variable extents, including developmental alterations, sterility and even lethality. In order to explore novel strategies for expressing CWDEs in crops, the cellobiohydrolase CBM3GH5, from the hyperthermophilic bacterium Caldicellulosiruptor saccharolyticus, was constitutively expressed in N. tabacum by targeting the enzyme both to the apoplast and to the protein storage vacuole. The apoplast targeting failed to isolate plants expressing the recombinant enzyme despite a large number of transformants being screened. On the opposite side, the targeting of the cellobiohydrolase to the protein storage vacuole led to several transgenic lines expressing CBM3GH5, with an enzyme yield of up to 0.08 mg g DW-1 (1.67 Units g DW-1) in the mature leaf tissue. The analysis of CBM3GH5 activity revealed that the enzyme accumulated in different plant organs in a developmental-dependent manner, with the highest abundance in mature leaves and roots, followed by seeds, stems and leaf ribs. Notably, both leaves and stems from transgenic plants were characterized by an improved temperature-dependent saccharification profile.

Overexpression of a multifunctional β-glucosidase gene from thermophilic archaeon Sulfolobus solfataricus in transgenic tobacco could facilitate glucose release and its use as a reporter

The β-glucosidase, which hydrolyzes the β(1-4) glucosidic linkage of disaccharides, oligosaccharides and glucose-substituted molecules, has been used in many biotechnological applications. The current commercial source of β-glucosidase is mainly microbial fermentation. Plants have been developed as bioreactors to produce various kinds of proteins including β-glucosidase because of the potential low cost. Sulfolobus solfataricus is a thermoacidophilic archaeon that can grow optimally at high temperature, around 80 °C, and pH 2-4. We overexpressed the β-glucosidase gene from S. solfataricus in transgenic tobacco via Agrobacteria-mediated transformation. Three transgenic tobacco lines with β-glucosidase gene expression driven by the rbcS promoter were obtained, and the recombinant proteins were accumulated in chloroplasts, endoplasmic reticulum and vacuoles up to 1%, 0.6% and 0.3% of total soluble protein, respectively. By stacking the transgenes via crossing distinct transgenic events, the level of β-glucosidase in plants could further increase. The plant-expressed β-glucosidase had optimal activity at 80 °C and pH 5-6. In addition, the plant-expressed β-glucosidase showed high thermostability; on heat pre-treatment at 80 °C for 2 h, approximately 70% residual activity remained. Furthermore, wind-dried leaf tissues of transgenic plants showed good stability in short-term storage at room temperature, with β-glucosidase activity of about 80% still remaining after 1 week of storage as compared with fresh leaf. Furthermore, we demonstrated the possibility of using the archaebacterial β-glucosidase gene as a reporter in plants based on alternative β-galactosidase activity.

Plastome engineering in vegetable crops: current status and future prospects

Plastome (plastid genome) engineering has grown up and got smarter for the transgene expression. Plastid transformation has profound benefits over nuclear transformation, includes a higher level of transgene expression, integration via homologous recombination, transgene containment, lack of gene silencing, and position effect. Substantial and fruitful progress has been achieved in plastome engineering of vegetable crops through the use of improved regeneration/selection procedures, plastid transformation vectors with efficient promoters, and 3^/, 5^/regulatory sequences. Plastid transformation technology developed for vegetable crops being used as a platform for the production of industrially important proteins and some of the genes of agronomic importance has been stably integrated and expressed in plastome. Although great progress has been accomplished in the plastid transformation of vegetable crops, still it is restricted to few species because of the unavailability of whole plastome sequencing. In this review, the author focus on the technology, progress, and advancements in plastid transformation of vegetable plants such as lettuce, tomato, potato, cabbage, cauliflower, eggplant, carrot, soybean, and bitter melon are reviewed. The conclusions, future prospects, and expansion of plastid transformation technology to other vegetable crops for genetic improvement and production of edible vaccines are proposed.

Plastid Transformation: How Does it Work? Can it Be Applied to Crops? What Can it Offer?

In recent years, plant genetic engineering has advanced agriculture in terms of crop improvement, stress and disease resistance, and pharmaceutical biosynthesis. Cells from land plants and algae contain three organelles that harbor DNA: the nucleus, plastid, and mitochondria. Although the most common approach for many plant species is the introduction of foreign DNA into the nucleus (nuclear transformation) via Agrobacterium- or biolistics-mediated delivery of transgenes, plastid transformation offers an alternative means for plant transformation. Since there are many copies of the chloroplast genome in each cell, higher levels of protein accumulation can often be achieved from transgenes inserted in the chloroplast genome compared to the nuclear genome. Chloroplasts are therefore becoming attractive hosts for the introduction of new agronomic traits, as well as for the biosynthesis of high-value pharmaceuticals, biomaterials and industrial enzymes. This review provides a comprehensive historical and biological perspective on plastid transformation, with a focus on current and emerging approaches such as the use of single-walled carbon nanotubes (SWNTs) as DNA delivery vehicles, overexpressing morphogenic regulators to enhance regeneration ability, applying genome editing techniques to accelerate double-stranded break formation, and reconsidering protoplasts as a viable material for plastid genome engineering, even in transformation-recalcitrant species.

Bulaon CJ, Phoolcharoen W. Plant Molecular Farming: A Viable Platform for Recombinant Biopharmaceutical Production

The demand for recombinant proteins in terms of quality, quantity, and diversity is increasing steadily, which is attracting global attention for the development of new recombinant protein production technologies and the engineering of conventional established expression systems based on bacteria or mammalian cell cultures. Since the advancements of plant genetic engineering in the 1980s, plants have been used for the production of economically valuable, biologically active non-native proteins or biopharmaceuticals, the concept termed as plant molecular farming (PMF). PMF is considered as a cost-effective technology that has grown and advanced tremendously over the past two decades. The development and improvement of the transient expression system has significantly reduced the protein production timeline and greatly improved the protein yield in plants. The major factors that drive the plant-based platform towards potential competitors for the conventional expression system are cost-effectiveness, scalability, flexibility, versatility, and robustness of the system. Many biopharmaceuticals including recombinant vaccine antigens, monoclonal antibodies, and other commercially viable proteins are produced in plants, some of which are in the pre-clinical and clinical pipeline. In this review, we consider the importance of a plant- based production system for recombinant protein production, and its potential to produce biopharmaceuticals is discussed.

Industrial Use of Cell Wall Degrading Enzymes: The Fine Line Between Production Strategy and Economic Feasibility

Cell Wall Degrading Enzymes (CWDEs) are a heterogeneous group of enzymes including glycosyl-hydrolases, oxidoreductases, lyases, and esterases. Microbes with degrading activities toward plant cell wall polysaccharides are the most relevant source of CWDEs for industrial applications. These organisms secrete a wide array of CWDEs in amounts strictly necessary for their own sustenance, nonetheless the production of CWDEs from wild type microbes can be increased at large-scale by using optimized fermentation strategies. In the last decades, advances in genetic engineering allowed the expression of recombinant CWDEs also in lab-domesticated organisms such as E. coli, yeasts and plants, dramatically increasing the available options for the large-scale production of CWDEs. The optimization of a CWDE-producing biofactory is a hard challenge that biotechnologists tackle by testing different expression strategies and expression-hosts. Although both the yield and production costs are critical factors to produce biomolecules at industrial scale, these parameters are often disregarded in basic research. This review presents the main characteristics and industrial applications of CWDEs directed toward the cell wall of plants, bacteria, fungi and microalgae. Different biofactories for CWDE expression are compared in order to highlight strengths and weaknesses of each production system and how these aspects impact the final enzyme cost and, consequently, the economic feasibility of using CWDEs for industrial applications.

Functional Improvement of Human Cardiotrophin 1 Produced in Tobacco Chloroplasts by Co-expression with Plastid Thioredoxin m

Human cardiotrophin 1 (CT1), a cytokine with excellent therapeutic potential, was previously expressed in tobacco chloroplasts. However, the growth conditions required to reach the highest expression levels resulted in an impairment of its bioactivity. In the present study, we have examined new strategies to modulate the expression of this recombinant protein in chloroplasts so as to enhance its production and bioactivity. In particular, we assessed the effect of both the fusion and co-expression of Trx m with CT1 on the production of a functional CT1 by using plastid transformation. Our data revealed that the Trx m fusion strategy was useful to increase the expression levels of CT1 inside the chloroplasts, although CT1 bioactivity was significantly impaired, and this was likely due to steric hindrance between both proteins. By contrast, the expression of functional CT1 was increased when co-expressed with Trx m, because we demonstrated that recombinant CT1 was functionally active during an in vitro signaling assay. While Trx m/CT1 co-expression did not increase the amount of CT1 in young leaves, our results revealed an increase in CT1 protein stability as the leaves aged in this genotype, which also improved the recombinant protein's overall production. This strategy might be useful to produce other functional biopharmaceuticals in chloroplasts.

Molecular, biochemical, and proteomic analyses of transplastomic tobacco plants expressing an endoglucanase support chloroplast-based molecular farming for industrial scale production of enzymes

The successful production of recombinant enzymes by tobacco transplastomic plants must maintain compatibility of the heterologous enzyme with chloroplast metabolism and its long-time enzyme stability. Based on previous reports, it has been taken for granted that following biolistic-transformation, homoplasticity could be obtained from the initially heteroplastic state following successive rounds of selection in the presence of the selection agent. However, several studies indicated that this procedure does not always ensure the complete elimination of unmodified wild-type plastomes. The present study demonstrates that CelK1 transplastomic plants, which were photosyntetically as active as untransformed ones, remain heteroplastomic even after repeated selection steps and that this state does not impair the relatively high-level production of the recombinant enzyme. In fact, even in the heteroplastomic state, the recombinant protein represented about 6% of the total soluble proteins (TSP). Moreover, our data also show that, while the recombinant endoglucanase undergoes phosphorylation, this post-translation modification does not have any significant impact on the enzymatic activity. Biomass storage might be required whenever the enzyme extraction process could not be performed immediately following the harvest of tobacco mature plants. In this respect, we have observed that enzyme activity in the detached leaves stored at 4 °C is maintained up to 20 weeks without significant loss of activity. These findings may have major implications in the future of chloroplast genetic engineering-based molecular farming to produce industrial enzymes in transplastomic plants.

Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes

: Energy demand is constantly growing, and, nowadays, fossil fuels still play a dominant role in global energy production, despite their negative effects on air pollution and the emission of greenhouse gases, which are the main contributors to global warming. An alternative clean source of energy is represented by the lignocellulose fraction of plant cell walls, the most abundant carbon source on Earth. To obtain biofuels, lignocellulose must be efficiently converted into fermentable sugars. In this regard, the exploitation of cell wall lytic enzymes (CWLEs) produced by lignocellulolytic fungi and bacteria may be considered as an eco-friendly alternative. These organisms evolved to produce a variety of highly specific CWLEs, even if in low amounts. For an industrial use, both the identification of novel CWLEs and the optimization of sustainable CWLE-expressing biofactories are crucial. In this review, we focus on recently reported advances in the heterologous expression of CWLEs from microbial and plant expression systems as well as some of their industrial applications, including the production of biofuels from agricultural feedstock and of value-added compounds from waste materials. Moreover, since heterologous expression of CWLEs may be toxic to plant hosts, genetic strategies aimed in converting such a deleterious effect into a beneficial trait are discussed.

Heat treatment alleviates the growth and photosynthetic impairment of transplastomic plants expressing Leishmania infantum Hsp83-Toxoplasma gondii SAG1 fusion protein

Previously, we showed that transplastomic tobacco plants expressing the LiHsp83-SAG1 fusion protein displayed a chlorotic phenotype and growth retardation, while plants expressing the SAG1 and GRA4 antigens alone did not. We conducted a comprehensive examination of the metabolic and photosynthetic parameters that could be affecting the normal growth of LiHsp83-SAG1 plants in order to understand the origin of these pleiotropic effects. These plants presented all photosynthetic pigments and parameters related to PSII efficiency significantly diminished. However, the expression of CHLI, RSSU and LHCa/b genes did not show significant differences between LiHsp83-SAG1 and control plants. Total protein, starch, and soluble sugar contents were also greatly reduced in LiHsp83-SAG1 plants. Since Hsp90 s are constitutively expressed at much higher concentrations at high temperatures, we tested if the fitness of LiHsp83-SAG1 over-expressing LiHsp83 would improve after heat treatment. LiHsp83-SAG1 plants showed an important alleviation of their phenotype and an evident recovery of the PSII function. As far as we know, this is the first report where it is demonstrated that a transplastomic line performs much better at higher temperatures. Finally, we detected that LiHsp83-SAG1 protein could be binding to key photosynthesis-related proteins at 37 °C. Our results suggest that the excess of this molecular chaperone could benefit the plant in a possible heat shock and prevent the expected denaturation of proteins. However, the LiHsp83-SAG1 protein content was weakly decreased in heat-treated plants. Therefore, we cannot rule out that the alleviation observed at 37 °C may be partially due to a reduction of the levels of the recombinant protein.

Green Biotechnology: A Brief Update on Plastid Genome Engineering

Plant genetic engineering has become an inevitable tool in the molecular breeding of crops. Significant progress has been made in the generation of novel plastid transformation vectors and optimized transformation protocols. There are several advantages of plastid genome engineering over conventional nuclear transformation. Some of the advantages include multigene engineering by expression of biosynthetic pathway genes as operons, extremely high-level expression of protein accumulation, lack of transgene silencing, etc. Transgene containment owing to maternal inheritance is another important advantage of plastid genome engineering. Chloroplast genome modification usually results in alteration of several thousand plastid genome copies in a cell. Several therapeutic proteins, edible vaccines, antimicrobial peptides, and industrially important enzymes have been successfully expressed in chloroplasts so far. Here, we critically recapitulate the latest developments in plastid genome engineering. Latest advancements in plastid genome sequencing are briefed. In addition, advancement of extending the toolbox for plastid engineering for selected applications in the area of molecular farming and production of industrially important enzyme is briefed.

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.

Technological advances and applications of hydrolytic enzymes for valorization of lignocellulosic biomass

Hydrolytic enzymes are indispensable tools in the production of various foodstuffs, drugs, and consumables owing to their applications in almost every industrial process nowadays. One of the foremost areas of interest involving the use of hydrolytic enzymes is in the transformation of lignocellulosic biomass into value added products. However, limitations of the processes due to inadequate enzyme activity and stability with a narrow range of pH and temperature optima often limit their effective usage. The innovative technologies, involving manipulation of enzyme activity and stability through mutagenesis, genetic engineering and metagenomics lead to a major leap in all the fields using hydrolytic enzymes. This article provides recent advancement towards the isolation and use of microbes for lignocellulosic biomass utilisation, microbes producing the hydrolytic enzymes, the modern age technologies used to manipulate and enhance the hydrolytic enzyme activity and the applications of such enzymes in value added products development from lignocellulosic biomass.

Carbohydrate active enzyme domains from extreme thermophiles: components of a modular toolbox for lignocellulose degradation

Lignocellulosic biomass is a promising feedstock for the manufacture of biodegradable and renewable bioproducts. However, the complex lignocellulosic polymeric structure of woody tissue is difficult to access without extensive industrial pre-treatment. Enzyme processing of partly depolymerised biomass is an established technology, and there is evidence that high temperature (extremely thermophilic) lignocellulose degrading enzymes [carbohydrate active enzymes (CAZymes)] may enhance processing efficiency. However, wild-type thermophilic CAZymes will not necessarily be functionally optimal under industrial pre-treatment conditions. With recent advances in synthetic biology, it is now potentially possible to build CAZyme constructs from individual protein domains, tailored to the conditions of specific industrial processes. In this review, we identify a 'toolbox' of thermostable CAZyme domains from extremely thermophilic organisms and highlight recent advances in CAZyme engineering which will allow for the rational design of CAZymes tailored to specific aspects of lignocellulose digestion.

Recent achievements obtained by chloroplast transformation

Chloroplasts play a great role for sustained wellbeing of life on the planet. They have the power and raw materials that can be used as sophisticated biological factories. They are rich in energy as they have lots of pigment-protein complexes capable of collecting sunlight, in sugar produced by photosynthesis and in minerals imported from the plant cell. Chloroplast genome transformation offers multiple advantages over nuclear genome which among others, include: integration of the transgene via homologus recombination that enables to eliminate gene silencing and position effect, higher level of transgene expression resulting into higher accumulations of foreign proteins, and significant reduction in environmental dispersion of the transgene due to maternal inheritance which helps to minimize the major critic of plant genetic engineering. Chloroplast genetic engineering has made fruit full progresses in the development of plants resistance to various stresses, phytoremediation of toxic metals, and production of vaccine antigens, biopharmaceuticals, biofuels, biomaterials and industrial enzymes. Although successful results have been achieved, there are still difficulties impeding full potential exploitation and expansion of chloroplast transformation technology to economical plants. These include, lack of species specific regulatory sequences, problem of selection and shoot regeneration, and massive expression of foreign genes resulting in phenotypic alterations of transplastomic plants. The aim of this review is to critically recapitulate the latest development of chloroplast transformation with special focus on the different traits of economic interest.

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.