On-site cellulase production and efficient saccharification of corn stover employing cbh2 overexpressing Trichoderma reesei with novel induction system

The combination of continuous-pulse feeding hydrolysates with soybean hulls induction during fed-batch fermentation of Trichoderma reesei b5 significantly elevated the cellulase production and its degradation ability on lignocellulosic biomass

To date, the high cost of cellulase is still a major factor limiting industrial production of lignocellulose bioconversion. This study developed a novel strategy combining continuous-pulse feeding glucose with soybean hulls (SBH) induction to efficiently elevate cellulase production by Trichoderma reesei b5 and enhance hydrolysis performance of cellulase on lignocellulose. In this strategy, the sugar concentration in the fermentation system was strictly controlled to meet the sugar requirements of strain for enzyme production without inhibition. Using SBH instead of microcrystalline cellulose effectively enhanced cellulase secretion as well as improved the enzyme system compositions, causing the improvement of hydrolysis performance. Using this strategy, filter-paperase (FPase) activity in the fermentation broth was more than ten-folds higher than that in batch fermentation (6.0 U/mL vs. 75.9 U/mL), and achieved the highest β-glucosidase activity reported in the literature to date (905 U/mL). This study also confirmed the feasibility of using on-site produced enzymatic hydrolysates instead of glucose as feeding medium. It provides a feasible process for large-scale production of low-cost and efficient cellulase, which is of great significance for the construction of "sugar platforms" based on lignocellulose and industrialization of lignocellulosic bioconversion.

High-level production of Aspergillus niger prolyl endopeptidase from agricultural residue and its application in beer brewing

BACKGROUND

Prolyl endopeptidase from Aspergillus niger (AN-PEP) is a prominent serine proteinase with various potential applications in the food and pharmaceutical industries. However, the availability of efficient and low-cost AN-PEP remains a challenge owing to its low yield and high fermentation cost.

RESULTS

Here, AN-PEP was recombinantly expressed in Trichoderma reesei (rAN-PEP) under the control of the cbh1 promoter and its secretion signal. After 4 days of shaking flask cultivation with the model cellulose Avicel PH101 as the sole carbon source, the extracellular prolyl endopeptidase activity reached up to 16.148 U/mL, which is the highest titer reported to date and the secretion of the enzyme is faster in T. reesei than in other eukaryotic expression systems including A. niger and Komagataella phaffii. Most importantly, when cultivated on the low-cost agricultural residue corn cob, the recombinant strain was found to secret a remarkable amount of rAN-PEP (37.125 U/mL) that is twice the activity under the pure cellulose condition. Furthermore, treatment with rAN-PEP during beer brewing lowered the content of gluten below the ELISA kit detection limit (< 10 mg/kg) and thereby, reduced turbidity, which would be beneficial for improving the non-biological stability of beer.

CONCLUSION

Our research provides a promising approach for industrial production of AN-PEP and other enzymes (proteins) from renewable lignocellulosic biomass, which provides a new idea with relevant researchers for the utilization of agricultural residues.

Focus and Insights into the Synthetic Biology-Mediated Chassis of Economically Important Fungi for the Production of High-Value Metabolites

Substantial progress has been achieved and knowledge gaps addressed in synthetic biology-mediated engineering of biological organisms to produce high-value metabolites. Bio-based products from fungi are extensively explored in the present era, attributed to their emerging importance in the industrial sector, healthcare, and food applications. The edible group of fungi and multiple fungal strains defines attractive biological resources for high-value metabolites comprising food additives, pigments, dyes, industrial chemicals, and antibiotics, including other compounds. In this direction, synthetic biology-mediated genetic chassis of fungal strains to enhance/add value to novel chemical entities of biological origin is opening new avenues in fungal biotechnology. While substantial success has been achieved in the genetic manipulation of economically viable fungi (including Saccharomyces cerevisiae) in the production of metabolites of socio-economic relevance, knowledge gaps/obstacles in fungal biology and engineering need to be remedied for complete exploitation of valuable fungal strains. Herein, the thematic article discusses the novel attributes of bio-based products from fungi and the creation of high-value engineered fungal strains to promote yield, bio-functionality, and value-addition of the metabolites of socio-economic value. Efforts have been made to discuss the existing limitations in fungal chassis and how the advances in synthetic biology provide a plausible solution.

Constitutive overexpression of cellobiohydrolase 2 in Trichoderma reesei reveals its ability to initiate cellulose degradation

Cellulose degradation results from the synergistic effect of different enzymes, but which enzyme is involved in the initial stage of cellulose degradation is still not well understood. Cellobiohydrolase 2 (CBH2) attached to the conidial surface is possibly associated with the initial stage. However, its specific mechanism is still incompletely known. This study explored the potential role of CBH2 in initiating cellulose degradation using a constitutive overexpression strategy. First, the CBH2-overexpression Trichoderma reesei strains Qgc2-5 and Qrc2-40 were constructed using the constitutive promoters Pgpd1 and PrpS30, respectively. It was found that cbh2 was expressed at a high level under the glucose conditions and was significantly higher than that of the parental strain QM9414 at the early stage of 29 h when cellulose was used as the carbon source. Particularly, the constitutive overexpression of cbh2 caused the strong expression of major cellulase-encoding genes (cbh1, eg1, and eg2) and the rapid decomposition of cellulosic material. Meanwhile, the scanning electron microscope showed that the groove-like structure of the cellulose surface was eroded seriously owing to CBH2 overexpression, which caused the cellulose surface to be smooth. These results showed that the overexpression of CHB2 caused the major cellulase enzymes to be expressed and contributed to cellulose degradation, showing the potential role of CBH2 in the initial stage of the cellulose hydrolytic process.

Engineering Trichoderma reesei for the hyperproduction of cellulose induced protein 1 (Cip1) on a sophorose-containing inducer to efficiently saccharify alkali-pretreated corn stover

Trichoderma reesei was induced to produce cellulase by a combination of glucose and β-disaccharide; however, lower levels of auxiliary proteins for degrading lignocellulosic biomass were detected by iTRAQ analysis compared with cellulose as an inducer, especially cellulose induced protein 1 (CIP1). In this study, A pdc1 promoter-driven overexpression of the endogenous Trcip1 gene was observed in T. reesei Rut C30, and the Trcip1 transcription levels of the two transformants, T. reesei OE-cip1-1 and OE-cip1-2, demonstrated 31.2- and 164.6-fold increases, respectively, but there was no significant change in cellobiohydrolase, endoglucanase and filter paper activity at 48 h. The crude enzyme was then used to hydrolyze corn stover. For T. reesei OE-cip1-1 and OE-cip1-2, the hydrolysis efficiency increased by 25.0 and 28.6% with a solid loading of 5% at 2 h, respectively. Simultaneously, 85.5 and 85.2 g/L glucose were released using a cellulase cocktail at high solid loading (20%), and these glucose release rates were significantly greater than that of T. reesei Rut C30 cellulase (77.4 g/L) at 120 h. Furthermore, scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that the enhanced hydrolysis efficiency was primarily triggered by the decrease in the crystallinity of lignocellulose, and the fiber structure had varying degrees of loosening and disintegration.

Tailoring the expression of Xyr1 leads to efficient production of lignocellulolytic enzymes in Trichoderma reesei for improved saccharification of corncob residues

BACKGROUND

The filamentous fungus Trichoderma reesei is extensively used for the industrial-scale cellulase production. It has been well known that the transcription factor Xyr1 plays an important role in the regulatory network controlling cellulase gene expression. However, the role of Xyr1 in the regulation of cellulase expression has not been comprehensively elucidated, which hinders further improvement of lignocellulolytic enzyme production.

RESULTS

Here, the expression dosage of xyr1 was tailored in T. reesei by differentially overexpressing the xyr1 gene under the control of three strong promoters (Pegl2, Pcbh1, and Pcdna1), and the transcript abundance of xyr1 was elevated 5.8-, 12.6-, and 47.2-fold, respectively. We found expression of cellulase genes was significantly increased in the Pegl2-driven xyr1 overexpression strain QE2X, whereas relatively low in the Pcbh1- and Pcdna1-driven overexpression strains. We also found that the Pegl2-driven overexpression of xyr1 caused a more significant opening of chromatin in the core promoter region of the prominent cellulase genes. Furthermore, the cellulase activity showed a 3.2-fold increase in the strain QE2X, while insignificant improvement in the Pcbh1- and Pcdna1-driven strains. Finally, the saccharification efficiency toward acid-pretreated corncob residues containing high-content lignin by the crude enzyme from QE2X was increased by 57.2% compared to that from the parental strain. Moreover, LC-MS/MS and RT-qPCR analysis revealed that expression of accessory proteins (Cip1, Cip2, Swo1, and LPMOs) was greatly improved in QE2X, which partly explained the promoting effect of the Pegl2-driven overexpression on enzymatic hydrolysis of lignocellulose biomass.

CONCLUSIONS

Our results underpin that the precise tailoring expression of xyr1 is essential for highly efficient cellulase synthesis, which provide new insights into the role of Xyr1 in regulating cellulase expression in T. reesei. Moreover, these results also provides a prospective strategy for strain improvement to enhance the lignocellulolytic enzyme production for use in biorefinery applications.

Full components conversion of lignocellulose via a closed-circuit biorefinery process on a pilot scale

This study developed a closed-circuit biorefinery process for full conversion of lignocellulose into ethanol, biogas and organic fertilizer with zero waste on a pilot scale. In the process, subcritical water pretreatment could effectively break the structure of wheat straw (WS), and ethanol was obtained from pretreated wheat straw (PWS) using two batches of simultaneous saccharification and fermentation (SSF). The pretreatment and ethanol fermentation wastes were reused for biogas and organic fertilizer production by anaerobic digestion (AD), whereas the pretreatment and ethanol conversion efficiency were reduced when supernatant after AD was recovered for next batch pretreatment. The yields of ethanol (0.08-0.09 g/g), biogas (0.05-0.10 L/g) and organic fertilizer (0.55-0.79 g/g) were demonstrated through mass balance. Furthermore, the hidden problems were exposed on pilot-scale conversion process, and several strategies were provided for optimizing the biorefinery process in the future.

Thermophilic Geobacillus WSUCF1 Secretome for Saccharification of Ammonia Fiber Expansion and Extractive Ammonia Pretreated Corn Stover

A thermophilic Geobacillus bacterial strain, WSUCF1 contains different carbohydrate-active enzymes (CAZymes) capable of hydrolyzing hemicellulose in lignocellulosic biomass. We used proteomic, genomic, and bioinformatic tools, and genomic data to analyze the relative abundance of cellulolytic, hemicellulolytic, and lignin modifying enzymes present in the secretomes. Results showed that CAZyme profiles of secretomes varied based on the substrate type and complexity, composition, and pretreatment conditions. The enzyme activity of secretomes also changed depending on the substrate used. The secretomes were used in combination with commercial and purified enzymes to carry out saccharification of ammonia fiber expansion (AFEX)-pretreated corn stover and extractive ammonia (EA)-pretreated corn stover. When WSUCF1 bacterial secretome produced at different conditions was combined with a small percentage of commercial enzymes, we observed efficient saccharification of EA-CS, and the results were comparable to using a commercial enzyme cocktail (87% glucan and 70% xylan conversion). It also opens the possibility of producing CAZymes in a biorefinery using inexpensive substrates, such as AFEX-pretreated corn stover and Avicel, and eliminates expensive enzyme processing steps that are used in enzyme manufacturing. Implementing in-house enzyme production is expected to significantly reduce the cost of enzymes and biofuel processing cost.

Regulation of β-Disaccharide Accumulation by β-Glucosidase Inhibitors to Enhance Cellulase Production in Trichoderma reesei

Trichoderma reesei is a high-yield producer of cellulase for applications in lignocellulosic biomass conversion, but its cellulase production requires induction. A mixture of glucose and β-disaccharide has been demonstrated to achieve high-level cellulase production. However, as inducers, β-disaccharides are prone to be hydrolyzed by β-glucosidase (BGL) during fermentation, therefore β-disaccharides need to be supplemented through feeding to overcome this problem. Here, miglitol, an α-glucosidase inhibitor, was investigated as a BGL inhibitor, and exhibited an IC50 value of 2.93 μg/mL. The cellulase titer was more than two-fold when miglitol was added to the fermentation medium of T. reesei. This method was similar to the prokaryotic expression system using unmetabolized isopropyl-β-D-thiogalactopyranoside (IPTG) as the inducer instead of lactose to continuously induce gene expression. However, cellulase activity was not enhanced with BGL inhibition when lactose or cellulose was used as an inducer, which demonstrated that the transglycosidase activity of BGL is important for the inducible activity of lactose and cellulose. This novel method demonstrates potential in stimulating cellulase production and provides a promising system for T. reesei protein expression.

Morphologically favorable mutant of Trichoderma reesei for low viscosity cellulase production

Metabolite production by filamentous fungi hampered because of high viscosity generated during growth. Low viscosity fermentation by mold is one of the preferred ways of large scale enzymes production. Cellulolytic enzymes play a key role during the process of lignocellulosic biomass conversion. In this study a mutant RC-23-1 was isolated through mutagenesis (diethyl sulfate followed by UV) of T. reesei RUT-C30. RC-23-1 not only gave higher cellulase production but also generated lower viscosity during enzyme production. Viscosity of mutant growth was more than three times lower than parent strain. RC-23-1 shows unique, yeast like colony morphology on solid media and small pellet like growth in liquid media. This mutant did not spread like mold on solid media. This mutant produces cellulases constitutively when grown in sugars. Using only glucose, the cellulase production was 4.1 FPU/ml. Among polysaccharides (avicel, xylan and pectin), avicel gave maximum of 6.2 FPU/ml and pretreated biomass (rice straw, wheat straw and sugarcane bagasse) produced 5.1-5.8 FPU/ml. At 7L scale reactor, fed-batch process was designed for cellulase production using different carbon and nitrogen sources. Maximum yield of cellulases was 182 FPU/g of lactose consumed was observed in fed-batch process. The produced enzyme used for hydrolysis of acid pretreated rice straw (20% solid loading) and maximum of 60 % glucan conversion was observed. RC-23-1 mutant is good candidate for large scale cellulase production and could be a model strain to study mold to yeast-like transformation. This article is protected by copyright. All rights reserved.

GlSwi6 Positively Regulates Cellulase and Xylanase Activities through Intracellular Ca2+ Signaling in Ganoderma lucidum

Ganoderma lucidum is a white-rot fungus that produces a range of lignocellulolytic enzymes to decompose lignin and cellulose. The mitogen-activated protein kinase (MAPK) pathway has been implicated in xylanases and cellulases production. As the downstream transcription factor of Slt2-MAPK, the function of Swi6 in G. lucidum has not been fully studied. In this study, the transcription factor GlSwi6 in G. lucidum was characterized and shown to significantly positively regulate cellulases and xylanases production. Knockdown of the GlSwi6 gene decreased the activities of cellulases and xylanases by approximately 31%~38% and 54%~60% compared with those of the wild-type (WT) strain, respectively. Besides, GlSwi6 can be alternatively spliced into two isoforms, GlSwi6A and GlSwi6B, and overexpression of GlSwi6B increased the activities of cellulase and xylanase by approximately 50% and 60%, respectively. Further study indicates that the existence of GlSwi6B significantly increased the concentration of cytosolic Ca²⁺. Our study indicated that GlSwi6 promotes the activities of cellulase and xylanase by regulating the Ca²⁺ signaling. These results connected the GlSwi6 and Ca²⁺ signaling in the regulation of cellulose degradation, and provide an insight for further improvement of cellulase or xylanase activities in G. lucidum as well as other fungi.

Engineering interventions in industrial filamentous fungal cell factories for biomass valorization

Filamentous fungi possess versatile capabilities for synthesizing a variety of valuable bio compounds, including enzymes, organic acids and small molecule secondary metabolites. The advancements of genetic and metabolic engineering techniques and the availability of sequenced genomes discovered their potential as expression hosts for recombinant protein production. Remarkably, plant-biomass degrading filamentous fungi show the unique capability to decompose lignocellulose, an extremely recalcitrant biopolymer. The basic biochemical approaches have motivated several industrial processes for lignocellulose biomass valorisation into fermentable sugars and other biochemical for biofuels, biomolecules, and biomaterials. The review gives insight into current trends in engineering filamentous fungi for enzymes, fuels, and chemicals from lignocellulose biomass. This review describes the variety of enzymes and compounds that filamentous fungi produce, engineering of filamentous fungi for biomass valorisation with a special focus on lignocellulolytic enzymes and other bulk chemicals.

Thermostable Cellulases / Xylanases From Thermophilic and Hyperthermophilic Microorganisms: Current Perspective

The bioconversion of lignocellulose into monosaccharides is critical for ensuring the continual manufacturing of biofuels and value-added bioproducts. Enzymatic degradation, which has a high yield, low energy consumption, and enhanced selectivity, could be the most efficient and environmentally friendly technique for converting complex lignocellulose polymers to fermentable monosaccharides, and it is expected to make cellulases and xylanases the most demanded industrial enzymes. The widespread nature of thermophilic microorganisms allows them to proliferate on a variety of substrates and release substantial quantities of cellulases and xylanases, which makes them a great source of thermostable enzymes. The most significant breakthrough of lignocellulolytic enzymes lies in lignocellulose-deconstruction by enzymatic depolymerization of holocellulose into simple monosaccharides. However, commercially valuable thermostable cellulases and xylanases are challenging to produce in high enough quantities. Thus, the present review aims at giving an overview of the most recent thermostable cellulases and xylanases isolated from thermophilic and hyperthermophilic microbes. The emphasis is on recent advancements in manufacturing these enzymes in other mesophilic host and enhancement of catalytic activity as well as thermostability of thermophilic cellulases and xylanases, using genetic engineering as a promising and efficient technology for its economic production. Additionally, the biotechnological applications of thermostable cellulases and xylanases of thermophiles were also discussed.

Current perspective on production and applications of microbial cellulases: a review

The potential of cellulolytic enzymes has been widely studied and explored for bioconversion processes and plays a key role in various industrial applications. Cellulase, a key enzyme for cellulose-rich waste feedstock-based biorefinery, has increasing demand in various industries, e.g., paper and pulp, juice clarification, etc. Also, there has been constant progress in developing new strategies to enhance its production, such as the application of waste feedstock as the substrate for the production of individual or enzyme cocktails, process parameters control, and genetic manipulations for enzyme production with enhanced yield, efficiency, and specificity. Further, an insight into immobilization techniques has also been presented for improved reusability of cellulase, a critical factor that controls the cost of the enzyme at an industrial scale. In addition, the review also gives an insight into the status of the significant application of cellulase in the industrial sector, with its techno-economic analysis for future applications. The present review gives a complete overview of current perspectives on the production of microbial cellulases as a promising tool to develop a sustainable and greener concept for industrial applications.

Tailoring in fungi for next generation cellulase production with special reference to CRISPR/CAS system

Cellulose is the utmost plenteous source of biopolymer in our earth, and fungi are the most efficient and ubiquitous organism in degrading the cellulosic biomass by synthesizing cellulases. Tailoring through genetic manipulation has played a substantial role in constructing novel fungal strains towards improved cellulase production of desired traits. However, the traditional methods of genetic manipulation of fungi are time-consuming and tedious. With the availability of the full-genome sequences of several industrially relevant filamentous fungi, CRISPR-CAS (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) technology has come into the focus for the proficient development of manipulated strains of filamentous fungi. This review summarizes the mode of action of cellulases, transcription level regulation for cellulase expression, various traditional strategies of genetic manipulation with CRISPR-CAS technology to develop modified fungal strains for a preferred level of cellulase production, and the futuristic trend in this arena of research.

Comparative transcriptome analysis of Trichoderma reesei reveals different gene regulatory networks induced by synthetic mixtures of glucose and β-disaccharide

The mixture of glucose and β-disaccharide (MGD) synthesized by transglycosylation of glucose as a low-cost soluble carbon source can efficiently induce cellulase production in Trichoderma reesei, which holds potential for the biorefining of lignocellulosic biomass. However, it is not yet fully understood how MGD induces T. reesei cellulase. In this study, transcriptomic analyses were conducted to investigate the molecular basis of MGD for lignocellulose-degrading enzyme production of T. reesei Rut C30 compared with that on lactose. Particular attention was paid to CAZymes, transcription factors, transporters and other protein processing pathways related to lignocellulose degradation. As a result, MGD can elicit transcription of GH5-, GH6- and GH7-encoding cellulases that is up to 1.4-fold higher than that induced by lactose, but GH11- and GH74-encoding xylanases are downregulated by 1.7- and 4.4-fold, respectively. Gene expression profiles suggest that the transcription activators xyr1 and vib1 are significantly upregulated and that the mitogen-activated protein kinase pathway is strengthened compared to the case of lactose induction. In addition, hac1-encoding UPR-specific transcription factors are significantly upregulated by MGD, which may be enhanced due to proper folding and processing of nascent proteins. These findings provide a theoretical basis for further understanding the characterization of efficient cellulase production using MGD as an inducer in T. reesei and offer potential strategies for strain improvement.

Rational engineering of the Trichoderma reesei RUT-C30 strain into an industrially relevant platform for cellulase production

BACKGROUND

The path for the development of hypersecreting strains of Trichoderma reesei capable of producing industrially relevant enzyme titers remains elusive despite over 70 years of research and industrial utilization. Herein, we describe the rational engineering of the publicly available T. reesei RUT-C30 strain and a customized process for cellulase production based on agroindustrial by-products.

RESULTS

A CRISPR/Cas9 system was used to introduce six genetic modifications in RUT-C30. Implemented changes included the constitutive expression of a mutated allele of the cellulase master regulator XYR1, the expression of two heterologous enzymes, the β-glucosidase CEL3A from Talaromyces emersonii and the invertase SUC1 from Aspergillus niger, and the deletion of genes encoding the cellulase repressor ACE1 and the extracellular proteases SLP1 and PEP1. These alterations resulted in a remarkable increase of protein secretion rates by RUT-C30 and amended its well described β-glucosidase deficiency while enabling the utilization of sucrose and eliminating the requirement of inducing sugars for enzyme production. With a developed sugarcane molasses-based bioprocess, the engineered strain reached an extracellular protein titer of 80.6 g L^-1 (0.24 g L^-1 h^-1), which is the highest experimentally supported titer so far reported for T. reesei. The produced enzyme cocktail displayed increased levels of cellulase and hemicellulase activities, with particularly large increments being observed for the specific activities of β-glucosidase (72-fold) and xylanase (42-fold). Notably, it also exhibited a saccharification efficiency similar to that of a commercially available cellulase preparation in the deconstruction of industrially pretreated sugarcane straw.

CONCLUSION

This work demonstrates the rational steps for the development of a cellulase hyperproducing strain from a well-characterized genetic background available in the public domain, the RUT-C30, associated with an industrially relevant bioprocess, paving new perspectives for Trichoderma research on cellulase production.

The influence of feedstock characteristics on enzyme production in Trichoderma reesei: a review on productivity, gene regulation and secretion profiles

Biorefineries, designed for the production of lignocellulose-based chemicals and fuels, are receiving increasing attention from the public, governments, and industries. A major obstacle for biorefineries to advance to commercial scale is the high cost of the enzymes required to derive the fermentable sugars from the feedstock used. As summarized in this review, techno-economic studies suggest co-localization and integration of enzyme manufacturing with the cellulosic biorefinery as the most promising alternative to alleviate this problem. Thus, cultivation of Trichoderma reesei, the principal producer of lignocellulolytic enzymes, on the lignocellulosic biomass processed on-site can reduce the cost of enzyme manufacturing. Further, due to a complex gene regulation machinery, the fungus can adjust the gene expression of the lignocellulolytic enzymes towards the characteristics of the feedstock, increasing the hydrolytic efficiency of the produced enzyme cocktail. Despite extensive research over decades, the underlying regulatory mechanisms are not fully elucidated. One aspect that has received relatively little attention in literature is the influence the characteristics of a lignocellulosic substrate, i.e., its chemical and physical composition, has on the produced enzyme mixture. Considering that the fungus is dependent on efficient enzymatic degradation of the lignocellulose for continuous supply of carbon and energy, a relationship between feedstock characteristics and secretome composition can be expected. The aim of this review was to systematically collect, appraise, and aggregate data and integrate results from studies analyzing enzyme production by T. reesei on insoluble cellulosic model substrates and lignocellulosic biomass. The results show that there is a direct effect of the substrate's complexity (rated by structure, composition of the lignin-carbohydrate complex, and recalcitrance in enzymatic saccharification) on enzyme titers and the composition of specific activities in the secretome. It further shows that process-related factors, such as substrate loading and cultivation set-up, are direct targets for increasing enzyme yields. The literature on transcriptome and secretome composition further supports the proposed influence of substrate-related factors on the expression of lignocellulolytic enzymes. This review provides insights into the interrelation between the characteristics of the substrate and the enzyme production by T. reesei, which may help to advance integrated enzyme manufacturing of substrate-specific enzymes cocktails at scale.

Techniques for enhancing the tolerance of industrial microbes to abiotic stresses: A review

The diversity of stress responses and survival strategies evolved by microorganism enables them to survive and reproduce in a multitude of harsh environments, whereas the discovery of the underlying resistance genes or mechanisms laid the foundation for the directional enhancement of microbial tolerance to abiotic stresses encountered in industrial applications. Many biological techniques have been developed for improving the stress resistance of industrial microorganisms, which greatly benefited the bacteria on which industrial production is based. This review introduces the main techniques for enhancing the resistance of microorganisms to abiotic stresses, including evolutionary engineering, metabolic engineering, and process engineering, developed in recent years. In addition, we also discuss problems that are still present in this area and offer directions for future research.

Improvements of thermophilic enzymes: From genetic modifications to applications

Thermozymes (from thermophiles or hyperthermophiles) offer obvious advantages due to their excellent thermostability, broad pH adaptation, and hydrolysis ability, resulting in diverse industrial applications including food, paper, and textile processing, biofuel production. However, natural thermozymes with low yield and poor adaptability severely hinder their large-scale applications. Extensive studies demonstrated that using genetic modifications such as directed evolution, semi-rational design, and rational design, expression regulations and chemical modifications effectively improved enzyme's yield, thermostability and catalytic efficiency. However, mechanism-based techniques for thermozymes improvements and applications need more attention. In this review, stabilizing mechanisms of thermozymes are summarized for thermozymes improvements, and these improved thermozymes eventually have large-scale industrial applications.

Engineering of filamentous fungi for efficient conversion of lignocellulose: Tools, recent advances and prospects

Filamentous fungi, as the main producers of lignocellulolytic enzymes in industry, need to be engineered to improve the economy of large-scale lignocellulose conversion. Investigation of the cellular processes involved in lignocellulolytic enzyme production, as well as optimization of enzyme mixtures for higher hydrolysis efficiency, have provided effective targets for the engineering of lignocellulolytic fungi. Recently, the development of efficient genetic manipulation systems in several lignocellulolytic fungi opens up the possibility of systems engineering of these strains. Here, we review the recent progresses made in the engineering of lignocellulolytic fungi and highlight the research gaps in this area.

The Promoter Toolbox for Recombinant Gene Expression in Trichoderma reesei

The ascomycete Trichoderma reesei is one of the main fungal producers of cellulases and xylanases based on its high production capacity. Its enzymes are applied in food, feed, and textile industry or in lignocellulose hydrolysis in biofuel and biorefinery industry. Over the last years, the demand to expand the molecular toolbox for T. reesei to facilitate genetic engineering and improve the production of heterologous proteins grew. An important instrument to modify the expression of key genes are promoters to initiate and control their transcription. To date, the most commonly used promoter for T. reesei is the strong inducible promoter of the main cellobiohydrolase cel7a. Beside this one, there is a number of alternative inducible promoters derived from other cellulase- and xylanase encoding genes and a few constitutive promoters. With the advances in genomics and transcriptomics the identification of new constitutive and tunable promoters with different expression strength was simplified. In this review, we will discuss new developments in the field of promoters and compare their advantages and disadvantages. Synthetic expression systems constitute a new option to control gene expression and build up complex gene circuits. Therefore, we will address common structural features of promoters and describe options for promoter engineering and synthetic design of promoters. The availability of well-characterized gene expression control tools is essential for the analysis of gene function, detection of bottlenecks in gene networks and yield increase for biotechnology applications.

Proteomic researches for lignocellulose-degrading enzymes: A mini-review

Protective action of lignin/hemicellulose networks and crystalline structures of embedded cellulose render lignocellulose material resistant to external enzymatic attack. To eliminate this bottleneck, research has been conducted in which advanced proteomic techniques are applied to identify effective commercial hydrolytic enzymes. This mini-review summarizes researches on lignocellulose-degrading enzymes, the mechanisms of the responses of various lignocellulose-degrading strains and microbial communities to various carbon sources and various biomass substrates, post-translational modifications of lignocellulose-degrading enzymes, new lignocellulose-degrading strains, new lignocellulose-degrading enzymes and a new method of secretome analysis. The challenges in the practical use of enzymatic hydrolysis process to realize lignocellulose biorefineries are discussed, along with the prospects for the same.

Metabolic Engineering of Fungal Strains for Efficient Production of Cellulolytic Enzymes

Filamentous fungi are widely used for production of cellulase and other cellulolytic enzymes. Metabolic engineering of filamentous fungal strains has been applied to improve enzyme production, and rapid progress has been made in the recent years. In this chapter, genetic tools and methods to develop superior enzyme producers are summarized, which includes establishment of genetic modification systems, selection and redesign of promoters, and metabolic engineering using either native transcription factors or artificial ones. In addition, enhancement of cellulase production through morphology engineering was also discussed. Emerging tools including CRISPR-Cas9-based genome editing and synthetic biology are highlighted, which are speeding up mechanisms elucidation and strain development, and will further facilitate economic cellulolytic enzyme production.

Isolation of new cellulase and xylanase producing strains and application to lignocellulosic biomasses hydrolysis and succinic acid production

The enzymatic extracellular mixtures of two new microorganisms - Streptomyces flavogriseus AE64X and AE63X - isolated from Eucalyptus camaldulensis and Populus nigra and producing cellulase and xylanase, were characterized and applied to hydrolysis of pretreated Arundo donax, Populus nigra and Panicum virgatum (10% w/v) replacing the commercial enzymes Accelerase 1500 and Accelerase XY (5.4 and 145 U/g of pretreated biomass, respectively). It is worth of noting that the newly developed extracellular enzymatic mixtures, without any purification step and at the same dosage, presented saccharification yields that are higher (86% for S. flavogriseus AE64X) than those of commercial enzymes (81%). Moreover, these enzymatic mixes allowed us to hydrolyse both cellulose and xylan within the different lignocellulose biomasses substituting both the cellulase and xylanase of commercial source. The produced sugars were also fermentable by Basfia succiniciproducens BPP7 into succinic acid with high yield.

Enhancing bioethanol production from water hyacinth by new combined pretreatment methods

This study investigated the possibility of enhancing bioethanol production by combined pretreatment methods for water hyacinth. Three different kinds of pretreatment methods, including microbial pretreatment, microbial combined dilute acid pretreatment, and microbial combined dilute alkaline pretreatment, were investigated for water hyacinth degradation. The results showed that microbial combined dilute acid pretreatment is the most effective method, resulting in the highest cellulose content (39.4 ± 2.8%) and reducing sugars production (430.66 mg·g^-1). Scanning Electron Microscopy and Fourier Transform Infrared Spectrometer analysis indicated that the basic tissue of water hyacinth was significantly destroyed. Compared to the other previously reported pretreatment methods for water hyacinth, which did not append additional cellulase and microbes for hydrolysis process, the microbial combined dilute acid pretreatment of our research could achieve the highest reducing sugars. Moreover, the production of bioethanol could achieve 1.40 g·L^-1 after fermentation, which could provide an extremely promising way for utilization of water hyacinth.

Enzymatic saccharification of lignocellulosic biorefinery: Research focuses

To realize lignocellulosic biorefinery is of global interest, with enzymatic saccharification presenting an essential stage to convert polymeric sugars to mono-sugars for fermentation use. This mini-review summarizes qualitatively the research focuses discussed the review articles presented in the past 22 months and other relevant papers. The research focuses on pretreatment with improved efficiency, enhanced enzyme production with high yields and high extreme tolerance, feasible combined saccharification and fermentation processes, detailed mechanisms corresponding to the enzymatic saccharification in lignocellulosic biorefinery, and the costs are discussed.

Optimization of cellulolytic enzyme components through engineering Trichoderma reesei and on-site fermentation using the soluble inducer for cellulosic ethanol production from corn stover

BACKGROUND

Cellulolytic enzymes produced by Trichoderma reesei are widely studied for biomass bioconversion, and enzymatic components vary depending on different inducers. In our previous studies, a mixture of glucose and disaccharide (MGD) was developed and used to induce cellulase production. However, the enzymatic profile induced by MGD is still not defined, and further optimization of the enzyme cocktail is also required for efficient ethanol production from lignocellulosic biomass.

RESULTS

In this study, cellulolytic enzymes produced by T. reesei Rut C30 using MGD and alkali-pretreated corn stover (APCS) as inducer were compared. Cellular secretome in response to each inducer was analyzed, which revealed a similar enzyme profile. However, significant difference in the content of cellulases and xylanase was detected. Although MGD induction enhanced β-glucosidase production, its activity was still not sufficient for biomass hydrolysis. To overcome such a disadvantage, aabgl1 encoding β-glucosidase in Aspergillus aculeatus was heterologously expressed in T. reesei Rut C30 under the control of the pdc1 promoter. The recombinant T. reesei PB-3 strain showed an improved β-glucosidase activity of 310 CBU/mL in the fed-batch fermentation, 71-folds higher than that produced by the parent strain. Meanwhile, cellulase activity of 50 FPU/mL was detected. Subsequently, the crude enzyme was applied for hydrolyzing corn stover with a solid loading of 20% through separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation, respectively, for ethanol production. Better performance was observed in the SHF process, through which a total of 119.9 g/L glucose was released within 12 h for concomitant ethanol production of 54.2 g/L.

CONCLUSIONS

The similar profile of cellulolytic enzymes was detected under the induction of MGD and APCS, but higher amount of cellulases was present in the crude enzyme induced by MGD. However, β-glucosidase activity induced by MGD was not sufficient for hydrolyzing lignocellulosic biomass. High titers of cellulases and β-glucosidase were achieved simultaneously by heterologous expression of aabgl1 in T. reesei and fed-batch fermentation through feeding MGD. We demonstrated that on-site cellulase production by T. reesei PB-3 has a potential for efficient biomass saccharification and ethanol production from lignocellulosic biomass.

A closed-loop strategy for endoglucanase production using sugarcane bagasse liquefied by a home-made enzymatic cocktail

Use of the same lignocellulosic biomass as feedstock for enzymes and ethanol production has been suggested as a lower cost option in future biorefineries. Here, we propose a closed-loop strategy to produce the cellulolytic enzymes required for biomass hydrolysis using sugarcane bagasse liquefied by a home-made enzymatic cocktail as carbon source and inducer. The fed-batch liquefaction conditions were firstly evaluated using commercial enzymes. Subsequently, the effects of different liquefied materials and solids loadings on endoglucanase production by Aspergillus niger cultivated in submerged fermentation were investigated. The liquefied bagasse produced using the home-made cocktail was more favorable for endoglucanase production, resulting in improvement up to 17%, compared to bagasse liquefied by commercial enzymes. The results indicated that liquefied bagasse produced by home-made enzymatic cocktail could provide a cost-effective carbon source and inducer for cellulolytic enzyme production, and could contribute to closing loops within the biorefinery, thus reducing costs and minimizing waste.

Improvement of cellulolytic enzyme production and performance by rational designing expression regulatory network and enzyme system composition

Filamentous fungi are considered as the most efficient producers expressing lignocellulose-degrading enzymes. Penicillium oxalicum strains possess extraordinary fungal lignocellulolytic enzyme systems and can efficiently utilize plant biomass. In recent years, the regulatory aspects of production of hydrolytic enzymes by P. oxalicum have been well established. This review aims to discuss the recent developments for the production of lignocellulolytic enzymes by P. oxalicum. The main cellulolytic transcription factors mediating the complex transcriptional-regulatory network are highlighted. The genome-wide identification of cellulolytic transcription factors, the cascade regulation network for cellulolytic gene expression, and the synergistic and dose-controlled regulation by cellulolytic regulators are discussed. A cellulase regulatory network sensitive to inducers in intracellular environments, the cross-talk of regulation of lignocellulose-degrading enzyme and amylase, and accessory enzymes are also demonstrated. Finally, strategies for the metabolic engineering of P. oxalicum, which show promising applications in the enzymatic hydrolysis for biochemical production, are established.