Expression of thermostable bacterial beta-glucosidase (BglB) in transgenic tobacco plants

Exploring the diversity of β-glucosidase: Classification, catalytic mechanism, molecular characteristics, kinetic models, and applications

High-value chemicals and energy-related products can be produced from biomass. Biorefinery technology offers a sustainable and cost-effective method for this high-value conversion. β-glucosidase is one of the key enzymes in biorefinery processes, catalyzing the production of glucose from aryl-glycosides and cello-oligosaccharides via the hydrolysis of β-glycosidic bonds. Although β-glucosidase plays a critical catalytic role in the utilization of cellulosic biomass, its efficacy is often limited by substrate or product inhibitions, low thermostability, and/or insufficient catalytic activity. To provide a detailed overview of β-glucosidases and their benefits in certain desired applications, we collected and summarized extensive information from literature and public databases, covering β-glucosidases in different glycosidase hydrolase families and biological kingdoms. These β-glucosidases show differences in amino acid sequence, which are translated into varying degrees of the molecular properties critical in enzymatic applications. This review describes studies on the diversity of β-glucosidases related to the classification, catalytic mechanisms, key molecular characteristics, kinetics models, and applications, and highlights several β-glucosidases displaying high stability, activity, and resistance to glucose inhibition suitable for desired biotechnological applications.

Enhanced Biomass Yield of and Saccharification in Transgenic Tobacco Over-Expressing β-Glucosidase

Here, we report an increase in biomass yield and saccharification in transgenic tobacco plants (Nicotiana tabacumL.) overexpressing thermostable β-glucosidase from Thermotoga maritima, BglB, targeted to the chloroplasts and vacuoles. The transgenic tobacco plants showed phenotypic characteristics that were significantly different from those of the wild-type plants. The biomass yield and life cycle (from germination to flowering and harvest) of the transgenic tobacco plants overexpressing BglB were 52% higher and 36% shorter than those of the wild-type tobacco plants, respectively, indicating a change in the genome transcription levels in the transgenic tobacco plants. Saccharification in biomass samples from the transgenic tobacco plants was 92% higher than that in biomass samples from the wild-type tobacco plants. The transgenic tobacco plants required a total investment (US$/year) corresponding to 52.9% of that required for the wild-type tobacco plants, but the total biomass yield (kg/year) of the transgenic tobacco plants was 43% higher than that of the wild-type tobacco plants. This approach could be applied to other plants to increase biomass yields and overproduce β-glucosidase for lignocellulose conversion.

In planta expression of hyperthermophilic enzymes as a strategy for accelerated lignocellulosic digestion

Conversion of lignocellulosic biomass to biofuels and biomaterials suffers from high production costs associated with biomass pretreatment and enzymatic hydrolysis. In-planta expression of lignocellulose-digesting enzymes is a promising approach to reduce these cost elements. However, this approach faces a number of challenges, including auto-hydrolysis of developing cell walls, plant growth and yield penalties, low expression levels and the limited stability of expressed enzymes at the high temperatures generally used for biomass processing to release fermentable sugars. To overcome these challenges we expressed codon-optimized recombinant hyperthermophilic endoglucanase (EG) and xylanase (Xyn) genes in A. thaliana. Transgenic Arabidopsis lines expressing EG and Xyn enzymes at high levels without any obvious plant growth or yield penalties were selected for further analysis. The highest enzyme activities were observed in the dry stems of transgenic lines, indicating that the enzymes were not degraded during stem senescence and storage. Biomass from transgenic lines exhibited improved saccharification efficiency relative to WT control plants. We conclude that the expression of hyperthermophilic enzymes in plants is a promising approach for combining pretreatment and enzymatic hydrolysis processes in lignocellulosic digestion. This study provides a valid foundation for further studies involving in planta co-expression of core and accessory lignocellulose-digesting enzymes.

In planta production and characterization of a hyperthermostable GH10 xylanase in transgenic sugarcane

Sugarcane (Saccharum sp. hybrids) is one of the most efficient and sustainable feedstocks for commercial production of fuel ethanol. Recent efforts focus on the integration of first and second generation bioethanol conversion technologies for sugarcane to increase biofuel yields. This integrated process will utilize both the cell wall bound sugars of the abundant lignocellulosic sugarcane residues in addition to the sucrose from stem internodes. Enzymatic hydrolysis of lignocellulosic biomass into its component sugars requires significant amounts of cell wall degrading enzymes. In planta production of xylanases has the potential to reduce costs associated with enzymatic hydrolysis but has been reported to compromise plant growth and development. To address this problem, we expressed a hyperthermostable GH10 xylanase, xyl10B in transgenic sugarcane which displays optimal catalytic activity at 105 °C and only residual catalytic activity at temperatures below 70 °C. Transgene integration and expression in sugarcane were confirmed by Southern blot, RT-PCR, ELISA and western blot following biolistic co-transfer of minimal expression cassettes of xyl10B and the selectable neomycin phosphotransferase II. Xylanase activity was detected in 17 transgenic lines with a fluorogenic xylanase activity assay. Up to 1.2% of the total soluble protein fraction of vegetative progenies with integration of chloroplast targeted expression represented the recombinant Xyl10B protein. Xyl10B activity was stable in vegetative progenies. Tissues retained 75% of the xylanase activity after drying of leaves at 35 °C and a 2 month storage period. Transgenic sugarcane plants producing Xyl10B did not differ from non-transgenic sugarcane in growth and development under greenhouse conditions. Sugarcane xylan and bagasse were used as substrate for enzymatic hydrolysis with the in planta produced Xyl10B. TLC and HPLC analysis of hydrolysis products confirmed the superior catalytic activity and stability of the in planta produced Xyl10B with xylobiose as a prominent degradation product. These findings will contribute to advancing consolidated processing of lignocellulosic sugarcane biomass.

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.

Phenotypic Changes in Transgenic Tobacco Plants Overexpressing Vacuole-Targeted Thermotoga maritima BglB Related to Elevated Levels of Liberated Hormones

The hyperthermostable β-glucosidase BglB of Thermotoga maritima was modified by adding a short C-terminal tetrapeptide (AFVY, which transports phaseolin to the vacuole, to its C-terminal sequence). The modified β-glucosidase BglB was transformed into tobacco (Nicotiana tabacum L.) plants. We observed a range of significant phenotypic changes in the transgenic plants compared to the wild-type (WT) plants. The transgenic plants had faster stem growth, earlier flowering, enhanced root systems development, an increased biomass biosynthesis rate, and higher salt stress tolerance in young plants compared to WT. In addition, programed cell death was enhanced in mature plants. Furthermore, the C-terminal AFVY tetrapeptide efficiently sorted T. maritima BglB into the vacuole, which was maintained in an active form and could perform its glycoside hydrolysis function on hormone conjugates, leading to elevated hormone [abscisic acid (ABA), indole 3-acetic acid (IAA), and cytokinin] levels that likely contributed to the phenotypic changes in the transgenic plants. The elevation of cytokinin led to upregulation of the transcription factor WUSCHELL, a homeodomain factor that regulates the development, division, and reproduction of stem cells in the shoot apical meristems. Elevation of IAA led to enhanced root development, and the elevation of ABA contributed to enhanced tolerance to salt stress and programed cell death. These results suggest that overexpressing vacuole-targeted T. maritima BglB may have several advantages for molecular farming technology to improve multiple targets, including enhanced production of the β-glucosidase BglB, increased biomass, and shortened developmental stages, that could play pivotal roles in bioenergy and biofuel production.

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.

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.

Improved recombinant cellulase expression in chloroplast of tobacco through promoter engineering and 5' amplification promoting sequence

Economical production of bioethanol from lignocellulosic biomass still faces many technical limitations. Cost-effective production of fermentable sugars is still not practical for large-scale production of bioethanol due to high costs of lignocellulolytic enzymes. Therefore, plant molecular farming, where plants are used as bioreactors, was developed for the mass production of cell wall degrading enzymes that will help reduce costs. Subcellular targeting is also potentially more suitable for the accumulation of recombinant cellulases. Herein, we generated transgenic tobacco plants (Nicotiana tabacum cv. SR1) that accumulated Thermotoga maritima BglB cellulase, which was driven by the alfalfa RbcsK-1A promoter and contained a small subunit of the rubisco complex transit peptide. The generated transformants possessed high specific BglB activity and did not show any abnormal phenotypes. Furthermore, we genetically engineered the RbcsK-1A promoter (MRbcsK-1A) and fused the amplification promoting sequence (aps) to MRbcsK-1A promoter to obtain high expression of BglB in transgenic plants. AMRsB plant lines with aps-MRbcsK-1A promoter showed the highest specific activity of BglB, and the accumulated BglB protein represented up to 9.3 % of total soluble protein. When BglB was expressed in Arabidopsis and tobacco plants, the maximal production capacity of recombinant BglB was 0.59 and 1.42 mg/g wet weight, respectively. These results suggests that suitable recombinant expression of cellulases in subcellular compartments such as chloroplasts will contribute to the cost-effective production of enzymes, and will serve as the solid foundation for the future commercialization of bioethanol production via plant molecular farming.

Synergistic effects of 2A-mediated polyproteins on the production of lignocellulose degradation enzymes in tobacco plants

Cost-effective bioethanol production requires a supply of various low-cost enzymes that can hydrolyse lignocellulosic materials consisting of multiple polymers. Because plant-based enzyme expression systems offer low-cost and large-scale production, this study simultaneously expressed β-glucosidase (BglB), xylanase (XylII), exoglucanase (E3), and endoglucanase (Cel5A) in tobacco plants, which were individually fused with chloroplast-targeting transit peptides and linked via the 2A self-cleaving oligopeptideex from foot-and-mouth disease virus (FMDV) as follows: [RsBglB-2A-RaCel5A], [RsXylII-2A-RaCel5A], and [RsE3-2A-RaCel5A]. The enzymes were targeted to chloroplasts in tobacco cells and their activities were confirmed. Similarly to the results of a transient assay using Arabidopsis thaliana protoplasts, when XylII was placed upstream of the 2A sequence, the [RsXylII-2A-RaCel5A] transgenic tobacco plant had a more positive influence on expression of the protein placed downstream. The [RsBglB-2A-RaCel5A] and [RsE3-2A-RaCel5A] transgenic lines displayed higher activities towards carboxylmethylcellulose (CMC) compared to those in the [RsXylII-2A-RaCel5A] transgenic line. This higher activity was attributable to the synergistic effects of the different cellulases used. The [RsBglB-2A-RaCel5A] lines exhibited greater efficiency (35-74% increase) of CMC hydrolysis when the exoglucanase CBHII was added. Among the various exoglucanases, E3 showed higher activity with the crude extract of the [RsBglB-2A-RaCel5A] transgenic line. Transgenic expression of 2A-mediated multiple enzymes induced synergistic effects and led to more efficient hydrolysis of lignocellulosic materials for bioethanol production.

Characteristics of bifunctional acidic endoglucanase (Cel5B) from Gloeophyllum trabeum

The endoglucanase (Cel5B) from the filamentous fungus Gloeophyllum trabeum was cloned and expressed without a signal peptide, and alanine residue 22 converted to glutamine in Pichia pastoris GS115. The DNA sequence of Cel5B had an open reading frame of 1,077 bp, encoding a protein of 359 amino acid residues with a molecular weight of 47 kDa. On the basis of sequence similarity, Cel5B displayed active site residues at Glu-175 and Glu-287. Both residues lost full hydrolytic activity when replaced with alanine through point mutation. The purified recombinant Cel5B showed very high specific activity, about 80- to 1,000-fold and 13- to 70-fold in comparison with other endoglucanases and cellobiohydrolase, on carboxymethylcellulose and filter paper, respectively, at pH 3.5 and 55°C. Cel5B displayed bifunctional characteristics under acidic conditions. The kinetic properties of the enzyme determined using a Lineweaver-Burk plot indicated that Cel5B is a catalytically efficient cellulolytic enzyme. These results suggest that Cel5B has high bifunctional endo- and exoglucanase activity under acidic conditions and is a good candidate for bioconversion of lignocellulose.

Post-synthetic modification of plant cell walls by expression of microbial hydrolases in the apoplast

The systematic creation of defined cell wall modifications in the model plant Arabidopsis thaliana by expression of microbial hydrolases with known specific activities is a promising approach to examine the impacts of cell wall composition and structure on both plant fitness and cell wall recalcitrance. Moreover, this approach allows the direct evaluation in living plants of hydrolase specificity, which can differ from in vitro specificity. To express genes encoding microbial hydrolases in A. thaliana, and target the hydrolases to the apoplast compartment, we constructed an expression cassette composed of the Cauliflower Mosaic Virus 35S RNA promoter, the A. thaliana β-expansin signal peptide, and the fluorescent marker protein YFP. Using this construct we successfully introduced into Colombia-0 plants three Aspergillus nidulans hydrolases, β-xylosidase/α-arabinosidase, feruloyl esterase, acetylxylan esterase, and a Xanthomonas oryzae putative a-L: -arabinofuranosidase. Fusion with YFP permitted quick and easy screening of transformants, detection of apoplastic localization, and protein size confirmation. Compared to wild-type Col-0, all transgenic lines showed a significant increase in the corresponding hydrolytic activity in the apoplast and changes in cell wall composition. Examination of hydrolytic activity in the transgenic plants also showed, for the first time, that the X. oryzae gene indeed encoded an enzyme with α-L: -arabinofuranosidase activity. None of the transgenic plants showed a visible phenotype; however, the induced compositional changes increased the degradability of biomass from plants expressing feruloyl esterase and β-xylosidase/α-arabinosidase. Our results demonstrate the viability of creating a set of transgenic A. thaliana plants with modified cell walls to use as a toolset for investigation of how cell wall composition contributes to recalcitrance and affects plant fitness.

Expressions of thermostable bacterial cellulases in tobacco plant

An economical method for the conversion of lignocellulosic biomass is to use plants as bioreactors for cellulases production. Two bacterial thermostable cellulases (E2 and E3) and a E3-E2 fusion form were expressed in tobacco, driven by a double 35S promoter and 5' TEV-UTL. The enzymes were targeted to the apoplast and cytosol via 5' signal peptides and 3' retention signal peptides, respectively, and all showed functional activities. All transgenic plants exhibited normal growth compared to wild type. Transgenic plants that expressed apoplast-localized E2 had the highest average activity, about 1.5 and 3 times higher than those expressed ER-localized and cytosolic E2, respectively. Effect of subcellular compartment localization was due primarily to post-transcriptional modification, since mRNA abundances were similar despite the range of cellulase activities obtained. The recombinant cellulases exhibited good thermostability below 65 °C. After storing for 3 days at -20 and 28 °C, the enzymes lost nearly 20 and 80% of activity, respectively. The results suggested a potential application for heterologous expression of cellulases in plant for biomass conversion.

In planta differential targeting analysis of Thermotoga maritima Cel5A and CBM6-engineered Cel5A for autohydrolysis

The heterologous expression of glycosyl hydrolases in bioenergy crops can improve the lignocellulosic conversion process for ethanol production. We attempted to obtain high-level expression of an intact Thermotoga maritima endoglucanase, Cel5A, and CBM6-engineered Cel5A in transgenic tobacco plants for the mass production and autohydrolysis of endoglucanase. Cel5A expression was targeted to different subcellular compartments, namely, the cytosol, apoplast, and chloroplast, using the native form of the pathogenesis-related protein 1a (PR1a) and Rubisco activase (RA) transit peptides. Cel5A transgenic tobacco plants with the chloroplast transit peptide showed the highest average endoglucanase activity and protein accumulation up to 4.5% total soluble protein. Cel5A-CBM6 was targeted to the chloroplast and accumulated up to 5.2% total soluble protein. In terms of the direct conversion of plant tissue into free sugar, the Cel5A-CBM6 transgenic plant was 33% more efficient than the Cel5A transgenic plant. The protein stability of Cel5A and Cel5A-CBM6 in lyophilized leaf material is an additional advantage in the bioconversion process.

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.

An efficient downstream box fusion allows high-level accumulation of active bacterial beta-glucosidase in tobacco chloroplasts

Production of enzymes for lignocellulose hydrolysis in planta has been proposed as a lower-cost alternative to microbial production, with plastid transformation as a preferred method due to high foreign protein yields. An important regulator of chloroplast protein production is the downstream box (DB) region, located immediately downstream of the start codon. Protein accumulation can vary over several orders of magnitude by altering the DB region. Experiments in bacteria have suggested that these differences in protein accumulation may result from changes in translation efficiency, though the precise mechanism of DB function is not known. In this study, three DB regions were fused to the bglC ORF encoding a β-glucosidase from the thermophilic bacterium Thermobifida fusca and inserted into the tobacco (Nicotiana tabacum) plastid genome. More than a two order of magnitude of difference in BglC protein accumulation was observed, dependent on the identity of the DB fusion. Differential transcript accumulation explained some the observed differences in protein accumulation, but in addition, less 3' degradation of bglC transcripts was observed in transgenic plants that accumulated the most BglC enzyme. Chloroplast-produced BglC was active against both pure cellobiose and against tobacco lignocellulose. These experiments demonstrate the potential utility of transplastomic plants as a vehicle for heterologous β-glucosidase production for the cellulosic ethanol industry.