Expression of thermostable microbial cellulases in the chloroplasts of nicotine-free tobacco

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.

Transformation of the Plastid Genome in Tobacco: The Model System for Chloroplast Genome Engineering

The protocol we report here is based on biolistic delivery of transforming DNA to tobacco leaves, selection of transplastomic clones by spectinomycin or kanamycin resistance and regeneration of plants with uniformly transformed plastid genomes. Because the plastid genome of Nicotiana tabacum derives from Nicotiana sylvestris, and the two genomes are highly conserved, vectors developed for N. tabacum can be used in N. sylvestris. The tissue culture responses of N. tabacum cv. Petit Havana and N. sylvestris accession TW137 are similar. Plastid transformation in a subset of N. tabacum cultivars and in Nicotiana benthamiana requires adjustment of the tissue culture protocol. We describe updated vectors targeting insertions in the unique and repeated regions of the plastid genome, vectors suitable for regulated gene expression by the engineered PPR10 RNA binding protein as well as systems for marker gene excision.

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.

Proteolytic analysis of Trichoderma reesei in celluase-inducing condition reveals a role for trichodermapepsin (TrAsP) in cellulase production

Filamentous fungi produce a variety of proteases with significant biotechnological potential and show diverse substrate specificities. Proteolytic analysis of the industrial enzyme producer Trichoderma reesei has been sparse. Therefore, we determined the substrate specificity of T. reesei secretome and its main protease Trichodermapepsin (TrAsP) up to P1 position using FRETS-25Xaa-libraries. The role of TrAsP was analyzed using T. reesei QM9414 and the deletant QM∆trasp in Avicel. We observed higher activities of CMCase, Avicelase, and Xylanase in QM∆t rasp compared to that of QM9414. Saccharification rate of cellulosic biomass also increased when using secretome of QM∆trasp but the effect was not significant due to the absence of difference in BGL activity compared to QM9414. Higher TrAsP was produced when monosaccharides were used as a carbon source compared to cellulase inducers such as Avicel and α-sophorose. These results elucidate the relationship between TrAsP and cellulase production in T. reesei and suggest a physiological role for TrAsP.

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.

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.

Chloroplasts: state of research and practical applications of plastome sequencing

This review presents origins, structure and expression of chloroplast genomes. It also describes their sequencing, analysis and modification, focusing on potential practical uses and biggest challenges of chloroplast genome modification. During the evolution of eukaryotes, cyanobacteria are believed to have merged with host heterotrophic cell. Afterward, most of cyanobacterial genes from cyanobacteria were transferred to cell nucleus or lost in the process of endosymbiosis. As a result of these changes, a primary plastid was established. Nowadays, plastid genome (plastome) is almost always circular, has a size of 100-200 kbp (120-160 in land plants), and harbors 100-120 highly conserved unique genes. Plastids have their own gene expression system, which is similar to one of their cyanobacterial ancestors. Two different polymerases, plastid-derived PEP and nucleus-derived NEP, participate in transcription. Translation is similar to the one observed in cyanobacteria, but it also utilizes protein translation factors and positive regulatory mRNA elements absent from bacteria. Plastoms play an important role in genetic transformation. Transgenes are introduced into them either via gene gun (in undamaged tissues) or polyethylene glycol treatment (when protoplasts are targeted). Antibiotic resistance markers are the most common tool used for selection of transformed plants. In recent years, plastome transformation emerged as a promising alternative to nuclear transformation because of (1) high yield of target protein, (2) removing the risk of outcrossing with weeds, (3) lack of silencing mechanisms, and (4) ability to engineer the entire metabolic pathways rather than single gene traits. Currently, the main directions of such research regard: developing efficient enzyme, vaccine antigen, and biopharmaceutical protein production methods in plant cells and improving crops by increasing their resistance to a wide array of biotic and abiotic stresses. Because of that, the detailed knowledge of plastome structure and mechanism of functioning started to play a major role.

Chloroplast genomes: diversity, evolution, and applications in genetic engineering

Chloroplasts play a crucial role in sustaining life on earth. The availability of over 800 sequenced chloroplast genomes from a variety of land plants has enhanced our understanding of chloroplast biology, intracellular gene transfer, conservation, diversity, and the genetic basis by which chloroplast transgenes can be engineered to enhance plant agronomic traits or to produce high-value agricultural or biomedical products. In this review, we discuss the impact of chloroplast genome sequences on understanding the origins of economically important cultivated species and changes that have taken place during domestication. We also discuss the potential biotechnological applications of chloroplast genomes.

Strategies for the production of cell wall-deconstructing enzymes in lignocellulosic biomass and their utilization for biofuel production

Microbial cell wall-deconstructing enzymes are widely used in the food, wine, pulp and paper, textile, and detergent industries and will be heavily utilized by cellulosic biorefineries in the production of fuels and chemicals. Due to their ability to use freely available solar energy, genetically engineered bioenergy crops provide an attractive alternative to microbial bioreactors for the production of cell wall-deconstructing enzymes. This review article summarizes the efforts made within the last decade on the production of cell wall-deconstructing enzymes in planta for use in the deconstruction of lignocellulosic biomass. A number of strategies have been employed to increase enzyme yields and limit negative impacts on plant growth and development including targeting heterologous enzymes into specific subcellular compartments using signal peptides, using tissue-specific or inducible promoters to limit the expression of enzymes to certain portions of the plant or certain times, and fusion of amplification sequences upstream of the coding region to enhance expression. We also summarize methods that have been used to access and maintain activity of plant-generated enzymes when used in conjunction with thermochemical pretreatments for the production of lignocellulosic biofuels.

Transgenic Plant-Produced Hydrolytic Enzymes and the Potential of Insect Gut-Derived Hydrolases for Biofuels

Various perennial C4 grass species have tremendous potential for use as lignocellulosic biofuel feedstocks. Currently available grasses require costly pre-treatment and exogenous hydrolytic enzyme application to break down complex cell wall polymers into sugars that can then be fermented into ethanol. It has long been hypothesized that engineered feedstock production of cell wall degrading (CWD) enzymes would be an efficient production platform for of exogenous hydrolytic enzymes. Most research has focused on plant overexpression of CWD enzyme-coding genes from free-living bacteria and fungi that naturally break down plant cell walls. Recently, it has been found that insect digestive tracts harbor novel sources of lignocellulolytic biocatalysts that might be exploited for biofuel production. These CWD enzyme genes can be located in the insect genomes or in symbiotic microbes. When CWD genes are transformed into plants, negative pleiotropic effects are possible such as unintended cell wall digestion. The use of codon optimization along with organelle and tissue specific targeting improves CWD enzyme yields. The literature teaches several important lessons on strategic deployment of CWD genes in transgenic plants, which is the focus of this review.

Comparison of Thermobifida fusca Cellulases Expressed in Escherichia coli and Nicotiana tabacum Indicates Advantages of the Plant System for the Expression of Bacterial Cellulases

The economic conversion of lignocellulosic biomass to biofuels requires in addition to pretreatment techniques access to large quantities of inexpensive cellulases to be competitive with established first generation processes. A solution to this problem could be achieved by plant based expression of these enzymes. We expressed the complete set of six cellulases and an additional β-glucosidase expressed from Thermobifida fusca in the bacterium Escherichia coli and in tobacco plants (Nicotiana tabacum). This was done to determine whether functional enzyme expression was feasible in these organisms. In extracts of recombinant E. coli cells, five of the proteins were detected by western blot analysis, but exocellulases E3 and E6 were undetectable. In the plant-based expression system we were able to detect all six cellulases but not the β-glucosidase even though activity was detectable. When E. coli was used as the expression system, endocellulase E2 was active, while endocellulases E1 and E5 showed only residual activity. The remaining cellulases appeared completely inactive against the model substrates azo-carboxymethyl-cellulose (Azo-CMC) and 4-methylumbelliferyl-cellobioside (4-MUC). Only the β-glucosidase showed high activity against 4-MUC. In contrast, all the plant-derived enzymes were active against the respective model substrates. Our data indicate that some enzymes of bacterial origin are more active and more efficiently expressed in plants than in a bacterial host.

The Engineered Chloroplast Genome Just Got Smarter

Chloroplasts are known to sustain life on earth by providing food, fuel, and oxygen through the process of photosynthesis. However, the chloroplast genome has also been smartly engineered to confer valuable agronomic traits and/or serve as bioreactors for the production of industrial enzymes, biopharmaceuticals, bioproducts, or vaccines. The recent breakthrough in hyperexpression of biopharmaceuticals in edible leaves has facilitated progression to clinical studies by major pharmaceutical companies. This review critically evaluates progress in developing new tools to enhance or simplify expression of targeted genes in chloroplasts. These tools hold the promise to further the development of novel fuels and products, enhance the photosynthetic process, and increase our understanding of retrograde signaling and cellular processes.

Production by Tobacco Transplastomic Plants of Recombinant Fungal and Bacterial Cell-Wall Degrading Enzymes to Be Used for Cellulosic Biomass Saccharification

Biofuels from renewable plant biomass are gaining momentum due to climate change related to atmospheric CO2 increase. However, the production cost of enzymes required for cellulosic biomass saccharification is a major limiting step in this process. Low-cost production of large amounts of recombinant enzymes by transgenic plants was proposed as an alternative to the conventional microbial based fermentation. A number of studies have shown that chloroplast-based gene expression offers several advantages over nuclear transformation due to efficient transcription and translation systems and high copy number of the transgene. In this study, we expressed in tobacco chloroplasts microbial genes encoding five cellulases and a polygalacturonase. Leaf extracts containing the recombinant enzymes showed the ability to degrade various cell-wall components under different conditions, singly and in combinations. In addition, our group also tested a previously described thermostable xylanase in combination with a cellulase and a polygalacturonase to study the cumulative effect on the depolymerization of a complex plant substrate. Our results demonstrate the feasibility of using transplastomic tobacco leaf extracts to convert cell-wall polysaccharides into reducing sugars, fulfilling a major prerequisite of large scale availability of a variety of cell-wall degrading enzymes for biofuel industry.

A novel and fully scalable Agrobacterium spray-based process for manufacturing cellulases and other cost-sensitive proteins in plants

Transient transfection of plants by vacuum infiltration of Agrobacterium vectors represents the state of the art in plant-based protein manufacturing; however, the complexity and cost of this approach restrict it to pharmaceutical proteins. We demonstrated that simple spraying of Nicotiana plants with Agrobacterium vectors in the presence of a surfactant can substitute for vacuum inoculation. When the T-DNA of Agrobacterium encodes viral replicons capable of cell-to-cell movement, up to 90% of the leaf cells can be transfected and express a recombinant protein at levels up to 50% of total soluble protein. This simple, fast and indefinitely scalable process was successfully applied to produce cellulases, one of the most volume- and cost-sensitive biotechnology products. We demonstrate here for the first time that representatives of all hydrolase classes necessary for cellulosic biomass decomposition can be expressed at high levels, stored as silage without significant loss of activity and then used directly as enzyme additives. This process enables production of cellulases, and other potential high-volume products such as noncaloric sweetener thaumatin and antiviral protein griffithsin, at commodity agricultural prices and could find broad applicability in the large-scale production of many other cost-sensitive proteins.

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

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

Engineering plastid genomes: methods, tools, and applications in basic research and biotechnology

The small bacterial-type genome of the plastid (chloroplast) can be engineered by genetic transformation, generating cells and plants with transgenic plastid genomes, also referred to as transplastomic plants. The transformation process relies on homologous recombination, thereby facilitating the site-specific alteration of endogenous plastid genes as well as the precisely targeted insertion of foreign genes into the plastid DNA. The technology has been used extensively to analyze chloroplast gene functions and study plastid gene expression at all levels in vivo. Over the years, a large toolbox has been assembled that is now nearly comparable to the techniques available for plant nuclear transformation and that has enabled new applications of transplastomic technology in basic and applied research. This review describes the state of the art in engineering the plastid genomes of algae and land plants (Embryophyta). It provides an overview of the existing tools for plastid genome engineering, discusses current technological limitations, and highlights selected applications that demonstrate the immense potential of chloroplast transformation in several key areas of plant biotechnology.

Protein maturation and proteolysis in plant plastids, mitochondria, and peroxisomes

Plastids, mitochondria, and peroxisomes are key organelles with dynamic proteomes in photosynthetic eukaryotes. Their biogenesis and activity must be coordinated and require intraorganellar protein maturation, degradation, and recycling. The three organelles together are predicted to contain ∼200 presequence peptidases, proteases, aminopeptidases, and specific protease chaperones/adaptors, but the substrates and substrate selection mechanisms are poorly understood. Similarly, lifetime determinants of organellar proteins, such as N-end degrons and tagging systems, have not been identified, but the substrate recognition mechanisms likely share similarities between organelles. Novel degradomics tools for systematic analysis of protein lifetime and proteolysis could define such protease-substrate relationships, degrons, and protein lifetime. Intraorganellar proteolysis is complemented by autophagy of whole organelles or selected organellar content, as well as by cytosolic protein ubiquitination and degradation by the proteasome. This review summarizes (putative) plant organellar protease functions and substrate-protease relationships. Examples illustrate key proteolytic events.

Plant biotechnology for lignocellulosic biofuel production

Lignocelluloses from plant cell walls are attractive resources for sustainable biofuel production. However, conversion of lignocellulose to biofuel is more expensive than other current technologies, due to the costs of chemical pretreatment and enzyme hydrolysis for cell wall deconstruction. Recalcitrance of cell walls to deconstruction has been reduced in many plant species by modifying plant cell walls through biotechnology. These results have been achieved by reducing lignin content and altering its composition and structure. Reduction of recalcitrance has also been achieved by manipulating hemicellulose biosynthesis and by overexpression of bacterial enzymes in plants to disrupt linkages in the lignin-carbohydrate complexes. These modified plants often have improved saccharification yield and higher ethanol production. Cell wall-degrading (CWD) enzymes from bacteria and fungi have been expressed at high levels in plants to increase the efficiency of saccharification compared with exogenous addition of cellulolytic enzymes. In planta expression of heat-stable CWD enzymes from bacterial thermophiles has made autohydrolysis possible. Transgenic plants can be engineered to reduce recalcitrance without any yield penalty, indicating that successful cell wall modification can be achieved without impacting cell wall integrity or plant development. A more complete understanding of cell wall formation and structure should greatly improve lignocellulosic feedstocks and reduce the cost of biofuel production.

Production and characterization of in planta transiently produced polygalacturanase from Aspergillus niger and its fusions with hydrophobin or ELP tags

BACKGROUND

Pectinases play an important role in plant cell wall deconstruction and have potential in diverse industries such as food, wine, animal feed, textile, paper, fuel, and others. The demand for such enzymes is increasing exponentially, as are the efforts to improve their production and to implement their use in several industrial processes. The goal of this study was to examine the potential of producing polygalacturonase I from Aspergillus niger in plants and to investigate the effects of subcellular compartmentalization and protein fusions on its accumulation and activity.

RESULTS

Polygalacturonase I from Aspergillus niger (AnPGI) was transiently produced in Nicotiana benthamiana by targeting it to five different cellular compartments: apoplast, endoplasmic reticulum (ER), vacuole, chloroplast and cytosol. Accumulation levels of 2.5%, 3.0%, and 1.9% of total soluble protein (TSP) were observed in the apoplast, ER, and vacuole, respectively, and specific activity was significantly higher in vacuole-targeted AnPGI compared to the same enzyme targeted to the ER or apoplast. No accumulation was found for AnPGI when targeted to the chloroplast or cytosol. Analysis of AnPGI fused with elastin-like polypeptide (ELP) revealed a significant increase in the protein accumulation level, especially when targeted to the vacuole where the protein doubles its accumulation to 3.6% of TSP, while the hydrophobin (HFBI) fusion impaired AnPGI accumulation and both tags impaired activity, albeit to different extents. The recombinant protein showed activity against polygalacturonic acid with optimum conditions at pH 5.0 and temperature from 30 to 50°C, depending on its fusion. In vivo analysis of reducing sugar content revealed a higher release of reducing sugars in plant tissue expressing recombinant AnPGI compared to wild type N. benthamiana leaves.

CONCLUSION

Our results demonstrate that subcellular compartmentalization of enzymes has an impact on both the target protein accumulation and its activity, especially in the case of proteins that undergo post-translational modifications, and should be taken into consideration when protein production strategies are designed. Using plants to produce heterologous enzymes for the degradation of a key component of the plant cell wall could reduce the cost of biomass pretreatment for the production of cellulosic biofuels.

Recombinant hyperthermophilic enzyme expression in plants: a novel approach for lignocellulose digestion

Plant biomass, as an abundant renewable carbon source, is a promising alternative to fossil fuels. However, the enzymes most commonly used for depolymerization of lignocellulosic biomass are expensive, and the development of cost-effective alternative conversion technologies would be desirable. One possible option is the heterologous expression of genes encoding lignocellulose-digesting enzymes in plant tissues. To overcome simultaneously issues of toxicity and incompatibility with high-temperature steam explosion processes, the use of heterologous genes encoding hyperthermophilic enzymes may be an attractive alternative. This approach could reduce the need for exogenous enzyme additions prior to fermentation, reducing the cost of the complete processing operation. This review highlights recent advances and future prospects for using hyperthermophilic enzymes in the biofuels industry.

Plastid transformation in Nicotiana tabacum and Nicotiana sylvestris by biolistic DNA delivery to leaves

The protocol we report here is based on biolistic delivery of the transforming DNA to tobacco leaves, selection of transplastomic clones by spectinomycin resistance and regeneration of plants with uniformly transformed plastid genomes. Because the plastid genome of Nicotiana tabacum derives from Nicotiana sylvestris, and the two genomes are highly conserved, vectors developed for N. tabacum can be used in N. sylvestris. Also, the tissue culture responses of N. tabacum cv. Petit Havana and N. sylvestris accession TW137 are similar, allowing plastid engineering protocols developed for N. tabacum to be directly applied to N. sylvestris. However, the tissue culture protocol is applicable only in a subset of N. tabacum cultivars. Here we highlight differences between the protocols for the two species. We describe updated vectors targeting insertions in the unique and repeated regions of the plastid genome as well as systems for marker excision. The simpler genetics of the diploid N. sylvestris, as opposed to the allotetraploid N. tabacum, make it an attractive model for plastid transformation.

Challenges and advances in the heterologous expression of cellulolytic enzymes: a review

Second generation biofuel development is increasingly reliant on the recombinant expression of cellulases. Designing or identifying successful expression systems is thus of preeminent importance to industrial progress in the field. Recombinant production of cellulases has been performed using a wide range of expression systems in bacteria, yeasts and plants. In a number of these systems, particularly when using bacteria and plants, significant challenges have been experienced in expressing full-length proteins or proteins at high yield. Further difficulties have been encountered in designing recombinant systems for surface-display of cellulases and for use in consolidated bioprocessing in bacteria and yeast. For establishing cellulase expression in plants, various strategies are utilized to overcome problems, such as the auto-hydrolysis of developing plant cell walls. In this review, we investigate the major challenges, as well as the major advances made to date in the recombinant expression of cellulases across the commonly used bacterial, plant and yeast systems. We review some of the critical aspects to be considered for industrial-scale cellulase production.

Genetic engineering of the chloroplast: novel tools and new applications

The plastid genome represents an attractive target of genetic engineering in crop plants. Plastid transgenes often give high expression levels, can be stacked in operons and are largely excluded from pollen transmission. Recent research has greatly expanded our toolbox for plastid genome engineering and many new proof-of-principle applications have highlighted the enormous potential of the transplastomic technology in both crop improvement and the development of plants as bioreactors for the sustainable and cost-effective production of biopharmaceuticals, enzymes and raw materials for the chemical industry. This review describes recent technological advances with plastid transformation in seed plants. It focuses on novel tools for plastid genome engineering and transgene expression and summarizes progress with harnessing the potential of plastid transformation in biotechnology.

Optimization of transplastomic production of hemicellulases in tobacco: effects of expression cassette configuration and tobacco cultivar used as production platform on recombinant protein yields

BACKGROUND

Chloroplast transformation in tobacco has been used extensively to produce recombinant proteins and enzymes. Chloroplast expression cassettes can be designed with different configurations of the cis-acting elements that govern foreign gene expression. With the aim to optimize production of recombinant hemicellulases in transplastomic tobacco, we developed a set of cassettes that incorporate elements known to facilitate protein expression in chloroplasts and examined expression and accumulation of a bacterial xylanase XynA. Biomass production is another important factor in achieving sustainable and high-volume production of cellulolytic enzymes. Therefore, we compared productivity of two tobacco cultivars - a low-alkaloid and a high-biomass - as transplastomic expression platforms.

RESULTS

Four different cassettes expressing XynA produced various mutant phenotypes of the transplastomic plants, affected their growth rate and resulted in different accumulation levels of the XynA enzyme. The most productive cassette was identified and used further to express XynA and two additional fungal xylanases, Xyn10A and Xyn11B, in a high-biomass tobacco cultivar. The high biomass cultivar allowed for a 60% increase in XynA production per plant. Accumulation of the fungal enzymes reached more than 10-fold higher levels than the bacterial enzyme, constituting up to 6% of the total soluble protein in the leaf tissue. Use of a well-characterized translational enhancer with the selected expression cassette revealed inconsistent effects on accumulation of the recombinant xylanases. Additionally, differences in the enzymatic activity of crude plant extracts measured in leaves of different age suggest presence of a specific xylanase inhibitor in the green leaf tissue.

CONCLUSION

Our results demonstrate the pivotal importance of the expression cassette design and appropriate tobacco cultivar for high-level transplastomic production of recombinant proteins.

Expression of fungal cutinase and swollenin in tobacco chloroplasts reveals novel enzyme functions and/or substrates

In order to produce low-cost biomass hydrolyzing enzymes, transplastomic lines were generated that expressed cutinase or swollenin within chloroplasts. While swollenin expressing plants were homoplasmic, cutinase transplastomic lines remained heteroplasmic. Both transplastomic lines showed interesting modifications in their phenotype, chloroplast structure, and functions. Ultrastructural analysis of chloroplasts from cutinase- and swollenin-expressing plants did not show typical lens shape and granal stacks. But, their thylakoid membranes showed unique scroll like structures and chloroplast envelope displayed protrusions, stretching into the cytoplasm. Unusual honeycomb structures typically observed in etioplasts were observed in mature chloroplasts expressing swollenin. Treatment of cotton fiber with chloroplast-derived swollenin showed enlarged segments and the intertwined inner fibers were irreversibly unwound and fully opened up due to expansin activity of swollenin, causing disruption of hydrogen bonds in cellulose fibers. Cutinase transplastomic plants showed esterase and lipase activity, while swollenin transplastomic lines lacked such enzyme activities. Higher plants contain two major galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), in their chloroplast thylakoid membranes that play distinct roles in their structural organization. Surprisingly, purified cutinase effectively hydrolyzed DGDG to MGDG, showing alpha galactosidase activity. Such hydrolysis resulted in unstacking of granal thylakoids in chloroplasts and other structural changes. These results demonstrate DGDG as novel substrate and function for cutinase. Both MGDG and DGDG were reduced up to 47.7% and 39.7% in cutinase and 68.5% and 67.5% in swollenin expressing plants. Novel properties and functions of both enzymes reported here for the first time should lead to better understanding and enhanced biomass hydrolysis.

Production of a subunit vaccine candidate against porcine post-weaning diarrhea in high-biomass transplastomic tobacco

Post-weaning diarrhea (PWD) in piglets is a major problem in piggeries worldwide and results in severe economic losses. Infection with Enterotoxigenic Escherichia coli (ETEC) is the key culprit for the PWD disease. F4 fimbriae of ETEC are highly stable proteinaceous polymers, mainly composed of the major structural subunit FaeG, with a capacity to evoke mucosal immune responses, thus demonstrating a potential to act as an oral vaccine against ETEC-induced porcine PWD. In this study we used a transplastomic approach in tobacco to produce a recombinant variant of the FaeG protein, rFaeG(ntd/dsc), engineered for expression as a stable monomer by N-terminal deletion and donor strand-complementation (ntd/dsc). The generated transplastomic tobacco plants accumulated up to 2.0 g rFaeG(ntd/dsc) per 1 kg fresh leaf tissue (more than 1% of dry leaf tissue) and showed normal phenotype indistinguishable from wild type untransformed plants. We determined that chloroplast-produced rFaeG(ntd/dsc) protein retained the key properties of an oral vaccine, i.e. binding to porcine intestinal F4 receptors (F4R), and inhibition of the F4-possessing (F4+) ETEC attachment to F4R. Additionally, the plant biomass matrix was shown to delay degradation of the chloroplast-produced rFaeG(ntd/dsc) in gastrointestinal conditions, demonstrating a potential to function as a shelter-vehicle for vaccine delivery. These results suggest that transplastomic plants expressing the rFaeG(ntd/dsc) protein could be used for production and, possibly, delivery of an oral vaccine against porcine F4+ ETEC infections. Our findings therefore present a feasible approach for developing an oral vaccination strategy against porcine PWD.

Accumulation of recombinant cellobiohydrolase and endoglucanase in the leaves of mature transgenic sugar cane

A major strategic goal in making ethanol from lignocellulosic biomass a cost-competitive liquid transport fuel is to reduce the cost of production of cellulolytic enzymes that hydrolyse lignocellulosic substrates to fermentable sugars. Current production systems for these enzymes, namely microbes, are not economic. One way to substantially reduce production costs is to express cellulolytic enzymes in plants at levels that are high enough to hydrolyse lignocellulosic biomass. Sugar cane fibre (bagasse) is the most promising lignocellulosic feedstock for conversion to ethanol in the tropics and subtropics. Cellulolytic enzyme production in sugar cane will have a substantial impact on the economics of lignocellulosic ethanol production from bagasse. We therefore generated transgenic sugar cane accumulating three cellulolytic enzymes, fungal cellobiohydrolase I (CBH I), CBH II and bacterial endoglucanase (EG), in leaves using the maize PepC promoter as an alternative to maize Ubi1 for controlling transgene expression. Different subcellular targeting signals were shown to have a substantial impact on the accumulation of these enzymes; the CBHs and EG accumulated to higher levels when fused to a vacuolar-sorting determinant than to an endoplasmic reticulum-retention signal, while EG was produced in the largest amounts when fused to a chloroplast-targeting signal. These results are the first demonstration of the expression and accumulation of recombinant CBH I, CBH II and EG in sugar cane and represent a significant first step towards the optimization of cellulolytic enzyme expression in sugar cane for the economic production of lignocellulosic ethanol.

Production of hyperthermostable GH10 xylanase Xyl10B from Thermotoga maritima in transplastomic plants enables complete hydrolysis of methylglucuronoxylan to fermentable sugars for biofuel production

Overcoming the recalcitrance in lignocellulosic biomass for efficient hydrolysis of the polysaccharides cellulose and hemicellulose to fermentable sugars is a research priority for the transition from a fossilfuel-based economy to a renewable carbohydrate economy. Methylglucuronoxylans (MeGXn) are the major components of hemicellulose in woody biofuel crops. Here, we describe efficient production of the GH10 xylanase Xyl10B from Thermotoga maritima in transplastomic plants and demonstrate exceptional stability and catalytic activities of the in planta produced enzyme. Fully expanded leaves from homotransplastomic plants contained enzymatically active Xyl10B at a level of 11-15% of their total soluble protein. Transplastomic plants and their seed progeny were morphologically indistinguishable from non-transgenic plants. Catalytic activity of in planta produced Xyl10B was detected with poplar, sweetgum and birchwood xylan substrates following incubation between 40 and 90 °C and was also stable in dry and stored leaves. Optimal yields of Xyl10B were obtained from dry leaves if crude protein extraction was performed at 85 °C. The transplastomic plant derived Xyl10B showed exceptional catalytic activity and enabled the complete hydrolysis of MeGXn to fermentable sugars with the help of a single accessory enzyme (α-glucuronidase) as revealed by the sugar release assay. Even without this accessory enzyme, the majority of MeGXn was hydrolyzed by the transplastomic plant-derived Xyl10B to fermentable xylose and xylobiose.

High-level expression of a suite of thermostable cell wall-degrading enzymes from the chloroplast genome

The biological conversion of plant biomass into fermentable sugars is key to the efficient production of biofuels and other renewable chemicals from plants. As up to more than 90% of the dry weight of higher plants is fixed in the cell wall, this will require the low-cost production of large amounts of cell wall-degrading enzymes. Transgenic plants can potentially provide an unbeatably cheap production platform for industrial enzymes. Transgene expression from the plastid genome is particularly attractive, due to high-level foreign protein accumulation in chloroplasts, absence of epigenetic gene silencing and improved transgene containment. Here, we have explored the potential of transplastomic plants to produce large amounts of thermostable cell wall-degrading enzymes from the bacterium Thermobifida fusca. We show that a set of four enzymes that are required for efficient degradation of cellulose (and the hemicellulose xyloglucan) could be expressed successfully in transplastomic tobacco plants. However, overexpression of the enzymes (to between approximately 5 and 40% of the plant's total soluble protein) resulted in pigment-deficient mutant phenotypes. We demonstrate that the chloroplast-produced cellulolytic enzymes are highly active. Although further optimization is needed, our data indicate that transgenic plastids offer great potential for the production of enzyme cocktails for the bioconversion of cellulosic biomass.

Chloroplast-derived enzyme cocktails hydrolyse lignocellulosic biomass and release fermentable sugars

It is widely recognized that biofuel production from lignocellulosic materials is limited by inadequate technology to efficiently and economically release fermentable sugars from the complex multi-polymeric raw materials. Therefore, endoglucanases, exoglucanase, pectate lyases, cutinase, swollenin, xylanase, acetyl xylan esterase, beta glucosidase and lipase genes from bacteria or fungi were expressed in Escherichia coli or tobacco chloroplasts. A PCR-based method was used to clone genes without introns from Trichoderma reesei genomic DNA. Homoplasmic transplastomic lines showed normal phenotype and were fertile. Based on observed expression levels, up to 49, 64 and 10, 751 million units of pectate lyases or endoglucanase can be produced annually, per acre of tobacco. Plant production cost of endoglucanase is 3100-fold, and pectate lyase is 1057 or 1480-fold lower than the same recombinant enzymes sold commercially, produced via fermentation. Chloroplast-derived enzymes had higher temperature stability and wider pH optima than enzymes expressed in E. coli. Plant crude-extracts showed higher enzyme activity than E. coli with increasing protein concentration, demonstrating their direct utility without purification. Addition of E. coli extracts to the chloroplast-derived enzymes significantly decreased their activity. Chloroplast-derived crude-extract enzyme cocktails yielded more (up to 3625%) glucose from filter paper, pine wood or citrus peel than commercial cocktails. Furthermore, pectate lyase transplastomic plants showed enhanced resistance to Erwina soft rot. This is the first report of using plant-derived enzyme cocktails for production of fermentable sugars from lignocellulosic biomass. Limitations of higher cost and lower production capacity of fermentation systems are addressed by chloroplast-derived enzyme cocktails.

Thermostable enzymes as biocatalysts in the biofuel industry

Lignocellulose is the most abundant carbohydrate source in nature and represents an ideal renewable energy source. Thermostable enzymes that hydrolyze lignocellulose to its component sugars have significant advantages for improving the conversion rate of biomass over their mesophilic counterparts. We review here the recent literature on the development and use of thermostable enzymes for the depolymerization of lignocellulosic feedstocks for biofuel production. Furthermore, we discuss the protein structure, mechanisms of thermostability, and specific strategies that can be used to improve the thermal stability of lignocellulosic biocatalysts.

A hepatitis C virus core polypeptide expressed in chloroplasts detects anti-core antibodies in infected human sera

Hepatitis C virus (HCV) is a major disease agent affecting approximately 3% of the world's population. Expression in plant chloroplasts enables low-cost production of the conserved HCV core protein used in diagnostic tests to combat virus spread in developing countries with high infection rates. The bactericidal activity of the 21 kDa precore protein hinders cloning the core gene in plastid expression cassettes, which are active in bacteria due to the similarities between bacterial and plastid promoters and ribosome binding sites. This was overcome by using a topology-dependent expression cassette containing tandem rrn and psbA plastid promoters, whose activity was shown to be dependent on temperature. The viral core gene and a codon-optimised gene encoding a C-terminal truncated 16 kDa core polypeptide were expressed in tobacco chloroplasts. The codon-optimised gene increased monocistronic core mRNA levels by at least 2-fold and core polypeptides by over 5-fold, relative to the native viral gene. Expression of the 16 kDa core polypeptide was stable in leaves of different ages. Anti-core antibodies in HCV-infected human sera were detected by the 16 kDa core polypeptide in total leaf protein fractionated on Western blots providing a first step towards developing a chloroplast-based HCV diagnostic method.

Potato virus X coat protein fusion to human papillomavirus 16 E7 oncoprotein enhance antigen stability and accumulation in tobacco chloroplast

Cervical cancer linked to infection with human papillomavirus (HPV) is the third cause of cancer-related death in women. As the virus cannot be propagated in culture, vaccines have been based on recombinant antigens with inherited high-cost production. In a search of alternative cheap production system, E7 HPV type 16 protein, an attractive candidate for anticancer vaccine development, was engineered to be expressed in tobacco chloroplast. In addition, E7 coding sequence was fused to potato virus X coat protein (CP) to compare expression level. Results show that E7CP transcript accumulation reached lower levels than non-fused E7. However, antigen expression levels were higher for fusion protein indicating that CP stabilizes E7 peptide in the chloroplast stroma. These results support viability of transplastomic plants for antigen production and the relevance of improving recombinant peptide stability for certain transgenes to enhance protein accumulation in this organelle.

Expression of Acidothermus cellulolyticus E1 endo-beta-1,4-glucanase catalytic domain in transplastomic tobacco

As part of an effort to develop transgenic plants as a system for the production of lignocellulose-degrading enzymes, we evaluated the production of the endo-beta-1,4-glucanase E1 catalytic domain (E1cd) of Acidothermus cellulolyticus in transplastomic tobacco. In an attempt to increase the translation efficiency of the E1cd cassette, various lengths of the N-terminus of the psbA gene product were fused to the E1cd protein. The psbA gene of the plastid genome encodes the D1 polypeptide of photosystem II and is known to encode an efficiently translated mRNA. Experiments in an Escherichia coli expression system indicated that the fusion of short (10-22 amino acid) segments of D1 to E1cd resulted in modest increases in E1cd abundance and were compatible with E1cd activity. Plastid expression cassettes encoding unmodified E1cd and a 10-amino-acid D1 fusion (10nE1cd) were used to generate transplastomic tobacco plants. Expression of the E1cd open reading frame in transplastomic tobacco resulted in very low levels of the enzyme. The transplastomic plants accumulated a high level of E1cd mRNA, however, indicating that post-transcriptional processes were probably limiting the production of recombinant protein. The accumulation of 10nE1cd in transplastomic tobacco was approximately 200-fold higher than that of unmodified E1cd, yielding 10nE1cd in excess of 12% of total soluble protein in the extracts of the lower leaves. Most importantly, the active recombinant enzyme was recovered very easily and efficiently from dried plant material and constituted as much as 0.3% of the dry weight of leaf tissue.

Simultaneous cloning and expression of two cellulase genes from Bacillus subtilis newly isolated from Golden Takin (Budorcas taxicolor Bedfordi)

A bacterial strain with high cellulase activity was isolated of feces sample of Golden Takin (Budorcas taxicolor Bedfordi). The bacterium was classified and designated Bacillus subtilis LN by morphological and 16SrDNA gene sequence analysis. Two putative cellulase genes, CelL15 and CelL73, were simultaneously cloned from the isolated strain by PCR. The putative gene CelL15 consisted of an open reading frame (ORF) of 1470 nucleotides and encoded a protein of 490 amino acids with a molecular weight of 54kDa. The CelL73 gene consisted of an open reading frame (ORF) of 741 nucleotides and encoded a protein of 247 amino acids with a molecular weight of 27kDa. Both genes were purified and cloned into pET-28a for expression in Escherichia coli BL21 (DE3). The ability of E. coli to degrade cellulose was enhanced when the two recombinants were cultured together.

High-level bacterial cellulase accumulation in chloroplast-transformed tobacco mediated by downstream box fusions

The Thermobifida fusca cel6A gene encoding an endoglucanase was fused to three different downstream box (DB) regions to generate cel6A genes with 14 amino acid fusions. The DB-Cel6A fusions were inserted into the tobacco (Nicotiana tabacum cv. Samsun) chloroplast genome for protein expression. Accumulation of Cel6A protein in transformed tobacco leaves varied over approximately two orders of magnitude, dependent on the identity of the DB region fused to the cel6A open reading frame (ORF). Additionally, the DB region fused to the cel6A ORF affected the accumulation of Cel6A protein in aging leaves, with the most effective DB regions allowing for high level accumulation of Cel6A protein in young, mature, and old leaves, while Cel6A protein accumulation decreased with leaf age when less effective DB regions were fused to the cel6A ORF. In the most highly expressed DB-Cel6A construct, enzymatically active Cel6A protein accumulated at up to 10.7% of total soluble leaf protein (%TSP). The strategy used for high-level endoglucanase expression may be useful for expression of other cellulolytic enzymes in chloroplasts, ultimately leading to cost-effective heterologous enzyme production for cellulosic ethanol using transplastomic plants.

Plastid transformation of high-biomass tobacco variety Maryland Mammoth for production of human immunodeficiency virus type 1 (HIV-1) p24 antigen

Chloroplast transformation of the high-biomass tobacco variety Maryland Mammoth has been assessed as a production platform for the human immunodeficiency virus type 1 (HIV-1) p24 antigen. Maryland Mammoth offers the prospect of higher yields of intact functional protein per unit floor area of contained glasshouse per unit time prior to flowering. Two different transformation constructs, pZSJH1p24 (for the insertion of a native p24 cDNA between the rbcL and accD genes) and pZF5 (for the insertion of a chloroplast-codon-optimized p24 gene between trnfM and trnG) were examined for the production of p24. Plants generated with construct pZSJH1p24 exhibited a normal green phenotype, but p24 protein accumulated only in the youngest leaves (up to approximately 350 microg/g fresh weight or approximately 2.5% total soluble protein) and was undetectable in mature leaves. In contrast, some of the plants generated with pZF5 exhibited a yellow phenotype (pZF5-yellow) with detectable p24 accumulation (up to approximately 450 microg/g fresh weight or approximately 4.5% total soluble protein) in all leaves, regardless of age. Total protein in pZF5-yellow leaves was reduced by approximately 40%. The pZF5-yellow phenotype was associated with recombination between native and introduced direct repeat sequences of the rbcL 3' untransformed region in the plastid genome. Chloroplast-expressed p24 was recognized by a conformation-dependent monoclonal antibody to p24, and p24 protein could be purified from pZF5-yellow leaves using a simple procedure, involving ammonium sulphate precipitation and ion-exchange chromatography, without the use of an affinity tag. The purified p24 was shown to be full length with no modifications, such as glycosylation or phosphorylation, using N-terminal sequencing and mass spectrometry.

Heterologous expression of glycosyl hydrolases in planta: a new departure for biofuels

The concept of expressing non-plant glycosyl hydrolase genes in plant tissue is nearly two decades old, yet relatively little work in this field has been reported. However, resurgent interest in technologies aimed at enabling processes that convert biomass to sugars and fuels has turned attention toward this intuitive solution. There are several challenges facing researchers in this field, including the development of better and more specifically targeted delivery systems for hydrolytic genes, the successful folding and post-translational modification of heterologous proteins and the development of cost-effective process strategies utilizing these transformed plants. The integration of these concepts, from the improvement of biomass production and conversion characteristics to the heterologous production of glycosyl hydrolases in a high yielding bioenergy crop, holds considerable promise for improving the lignocellulosic conversion of biomass to ethanol and subsequently to fuels.

A guide to choosing vectors for transformation of the plastid genome of higher plants

Plastid transformation, originally developed in tobacco (Nicotiana tabacum), has recently been extended to a number of crop species enabling in vivo probing of plastid function and biotechnological applications. In this article we report new plastid vectors that enable insertion of transgenes in the inverted repeat region of the plastome between the trnV and 3'rps12 or trnI and trnA genes. Efficient recovery of transplastomic clones is ensured by selection for spectinomycin (aadA) or kanamycin (neo) resistance genes. Expression of marker genes can be verified using commercial antibodies that detect the accumulation of neomycin phosphotranseferase II, the neo gene product, or the C-terminal c-myc tag of aminoglycoside-3''-adenylytransferase, encoded by the aadA gene. Aminoglycoside-3''-adenylytransferase, the spectinomycin inactivating enzyme, is translationally fused with green fluorescent protein in two vectors so that transplastomic clones can be selected by spectinomycin resistance and visually identified by fluorescence in ultraviolet light. The marker genes in the new vectors are flanked by target sites for Cre or Int, the P1 and phiC31 phage site-specific recombinases. When uniform transformation of all plastid genomes is obtained, the marker genes can be excised by Cre or Int expressed from a nuclear gene. Choice of expression signals for the gene of interest, complications caused by the presence of plastid DNA sequences recognized by Cre, and loss of transgenes by homologous recombination via duplicated sequences are also discussed to facilitate a rational choice from among the existing vectors and to aid with new target-specific vector designs.