CRISPR/Cas9-Based Genome Editing in the Filamentous Fungus Fusarium fujikuroi and Its Application in Strain Engineering for Gibberellic Acid Production

A user-friendly CRISPR/Cas9 system for mutagenesis of Neurospora crassa

As a widely used eukaryotic model organism, Neurospora crassa offers advantages in genetic studies due to its diverse biology and rapid growth. Traditional genetic manipulation methods, such as homologous recombination, require a considerable amount of time and effort. In this study, we present an easy-to-use CRIPSR/Cas9 system for N. crassa, in which the cas9 sequence is incorporated into the fungal genome and naked guide RNA is introduced via electroporation. Our approach eliminates the need for constructing multiple vectors, speeding up the mutagenesis process. Using cyclosporin-resistant-1 (csr-1) as a selectable marker gene, we achieved 100% editing efficiency under selection conditions. Furthermore, we successfully edited the non-selectable gene N-acylethanolamine amidohydrolase-2 (naa-2), demonstrating the versatility of the system. Combining gRNAs targeting csr-1 and naa-2 simultaneously increased the probability of finding mutants carrying the non-selectable mutation. The system is not only user-friendly but also effective, providing a rapid and efficient method for generating loss-of-function mutants in N. crassa compared to traditional methods.

Advances in CRISPR-Cas systems for fungal infections

Fungi contain a wide range of bioactive secondary metabolites (SMs) that have numerous applications in various fields, including agriculture, medicine, human health, and more. It is common for genes responsible for the production of secondary metabolites (SMs) to form biosynthetic gene clusters (BGCs). The identification and analysis of numerous unexplored gene clusters (BGCs) and their corresponding substances (SMs) has been significantly facilitated by the recent advancements in genomic and genetic technologies. Nevertheless, the exploration of secondary metabolites with commercial value is impeded by a variety of challenges. The emergence of modern CRISPR/Cas technologies has brought about a paradigm shift in fungal genetic engineering, significantly streamlining the process of discovering new bioactive compounds. This study begins with an examination of fungal biosynthetic gene clusters (BGCs) and their interconnections with the secondary metabolites (SMs) they generate. Following that, a brief summary of the conventional methods employed in fungal genetic engineering is provided. This study explores various sophisticated CRISPR/Cas-based methodologies and their utilization in examining the synthesis of secondary metabolites (SMs) in fungi. The chapter provides an in-depth analysis of the limitations and obstacles encountered in CRISPR/Cas-based systems when applied to fungal genetic engineering. It also proposes promising avenues for future research to optimize the efficiency of these systems.

Expression of microbial lipase in filamentous fungus Aspergillus niger: a review

Lipase has high economic importance and is widely used in biodiesel, food, detergents, cosmetics, and pharmaceutical industries. The rapid development of synthetic biology and system biology has not only paved the way for comprehensively understanding the efficient operation mechanism of Aspergillus niger cell factories but also introduced a new technological system for creating and optimizing high-efficiency A. niger cell factories. In this review, all relevant data on microbial lipase enzyme sources and general properties are gathered and updated. The relationship between A. niger strain morphology and protein production is discussed. The safety of A. niger strain is investigated to ensure product safety. The biotechnologies and factors influencing lipase expression in A. niger are summarized. This review focuses on various strategies to improve lipase expression in A. niger. The summary of these methods and the application of the gene editing technology CRISPR/Cas9 system can further improve the efficiency of constructing the engineered lipase-producing A. niger.

Development of a marker recyclable CRISPR/Cas9 system for scarless and multigene editing in Fusarium fujikuroi

Iterative metabolic engineering of Fusarium fujikuroi has traditionally been hampered by its low homologous recombination efficiency and scarcity of genetic markers. Thus, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas9) system has emerged as a promising tool for precise genome editing in this organism. Some integrated CRISPR/Cas9 strategies have been used to engineer F. fujikuroi to improve GA3 production capabilities, but low editing efficiency and possible genomic instability became the major obstacle. Herein, we developed a marker recyclable CRISPR/Cas9 system for scarless and multigene editing in F. fujikuroi. This system, based on an autonomously replicating sequence, demonstrated the capability of a single plasmid harboring all editing components to achieve 100%, 75%, and 37.5% editing efficiency for single, double, and triple gene targets, respectively. Remarkably, even with a reduction in homologous arms to 50 bp, we achieved a 12.5% gene editing efficiency. By employing this system, we successfully achieved multicopy integration of the truncated 3-hydroxy-3-methyl glutaryl coenzyme A reductase gene (tHMGR), leading to enhanced GA3 production. A key advantage of our plasmid-based gene editing approach was the ability to recycle selective markers through a simplified protoplast preparation and recovery process, which eliminated the need for additional genetic markers. These findings demonstrated that the single-plasmid CRISPR/Cas9 system enables rapid and precise multiple gene deletions/integrations, laying a solid foundation for future metabolic engineering efforts aimed at industrial GA3 production.

Metabolomics Combined with Physiology and Transcriptomics Reveal the Response of Samsoniella hepiali to Key Metabolic Pathways and Its Degradation Mechanism during Subculture

During the subculture of filamentous fungi, obvious signs of degradation occur which affect the growth and development of the strain, change the content of metabolites, and interfere with gene expression. However, the specific molecular mechanism of filamentous fungi degradation is still unclear. In this study, a filamentous fungus Samsoniella hepiali was used as the research object, and it was continuously subcultured. The results showed that when the strain was subcultured to the F8 generation, the strain began to show signs of degradation, which was manifested by affecting the apparent morphology, reducing the growth rate and sporulation, and destroying the antioxidant system. Further transcriptome and metabolomics analyses were performed, and the results showed differentially expressed genes (DEGs) and differentially accumulated metabolites (DAMs) that were mainly enriched in four metabolic pathways: ABC transporters; fatty acid degradation; alanine, aspartate, and glutamate metabolism; and purine metabolism. Many of the metabolites that were significantly enriched in different pathways may mainly be regulated by genes belonging to proteins and enzymes, such as Abcd3, Ass1, and Pgm1. At the same time, in the process of subculture, many genes and metabolites that can induce apoptosis and senescence continue to accumulate, causing cell damage and consuming a lot of energy, which ultimately leads to the inhibition of mycelial growth. In summary, this study clarified the response of S. hepiali strains to key metabolic pathways during subculture and some reasons for the degradation of strains.

Development of an Efficient CRISPR/Cas9 System in Fusarium verticillioides and Its Application in Reducing Mycotoxin Contamination

Fusarium verticillioides (F. verticillioides) is a globally recognized and highly impactful fungal pathogen of maize, causing yield losses and producing harmful mycotoxins that pose a threat to human and animal health. However, the genetic tools available for studying this crucial fungus are currently limited in comparison to other important fungal pathogens. To address this, an efficient CRISPR/Cas9 genome editing system based on an autonomously replicating plasmid with an AMA1 sequence was established in this study. First, gene disruption of pyrG and pyrE via nonhomologous end-joining (NHEJ) pathway was successfully achieved, with efficiency ranging from 66 to 100%. Second, precise gene deletions were achieved with remarkable efficiency using a dual sgRNA expression strategy. Third, the developed genome editing system can be applied to generate designer chromosomes in F. verticillioides, as evidenced by the deletion of a crucial 38 kb fragment required for fumonisin biosynthesis. Fourth, the pyrG recycling system has been established and successfully applied in F. verticillioides. Lastly, the developed ΔFUM1 and ΔFUM mutants can serve as biocontrol agents to reduce the fumonisin B1 (FB1) contamination produced by the toxigenic strain. Taken together, these significant advancements in genetic manipulation and biocontrol strategies provide valuable tools for studying and mitigating the impact of F. verticillioides on maize crops.

Improved Protoplast Production Protocol for Fungal Transformations Mediated by CRISPR/Cas9 in Botrytis cinerea Non-Sporulating Isolates

Botrytis cinerea is a necrotrophic fungus that causes considerable economic losses in commercial crops. Fungi of the genus Botrytis exhibit great morphological and genetic variability, ranging from non-sporogenic and non-infective isolates to highly virulent sporogenic ones. There is growing interest in the different isolates in terms of their methodological applications aimed at gaining a deeper understanding of the biology of these fungal species for more efficient control of the infections they cause. This article describes an improvement in the protoplast production protocol from non-sporogenic isolates, resulting in viable protoplasts with regenerating capacity. The method improvements consist of a two-day incubation period with mycelium plugs and orbital shaking. Special mention is made of our preference for the VinoTaste Pro enzyme in the KC buffer as a replacement for Glucanex, as it enhances the efficacy of protoplast isolation in B459 and B371 isolates. The methodology described here has proven to be very useful for biotechnological applications such as genetic transformations mediated by the CRISPR/Cas9 tool.

Efficient gene editing in the slow-growing, non-sporulating, melanized, endophytic fungus Berkleasmium sp. Dzf12 using a CRISPR/Cas9 system

The endophytic fungus Berkleasmium sp. Dzf12 that was isolated from Dioscorea zingiberensis, is a proficient producer of palmarumycins, which are intriguing polyketides of the spirobisnaphthalene class. These compounds displayed a wide range of bioactivities, including antibacterial, antifungal, and cytotoxic activities. However, conventional genetic manipulation of Berkleasmium sp. Dzf12 is difficult and inefficient, partially due to the slow-growing, non-sporulating, and highly pigmented behavior of this fungus. Herein, we developed a CRISPR/Cas9 system suitable for gene editing in Berkleasmium sp. Dzf12. The protoplast preparation was optimized, and the expression of Cas9 in Berkleasmium sp. Dzf12 was validated. To assess the gene disruption efficiency, a putative 1, 3, 6, 8-tetrahydroxynaphthalene synthase encoding gene, bdpks, involved in 1,8-dihydroxynaphthalene (DHN)-melanin biosynthesis, was selected as the target for gene disruption. Various endogenous sgRNA promoters were tested, and different strategies to express sgRNA were compared, resulting in the construction of an optimal system using the U6 snRNA-1 promoter as the sgRNA promoter. Successful disruption of bdpks led to a complete abolishment of the production of spirobisnaphthalenes and melanin. This work establishes a useful gene targeting disruption system for exploration of gene functions in Berkleasmium sp. Dzf12, and also provides an example for developing an efficient CRISPR/Cas9 system to the fungi that are difficult to manipulate using conventional genetic tools.

Utilization of CRISPR-Cas genome editing technology in filamentous fungi: function and advancement potentiality

Filamentous fungi play a crucial role in environmental pollution control, protein secretion, and the production of active secondary metabolites. The evolution of gene editing technology has significantly improved the study of filamentous fungi, which in the past was laborious and time-consuming. But recently, CRISPR-Cas systems, which utilize small guide RNA (sgRNA) to mediate clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (Cas), have demonstrated considerable promise in research and application for filamentous fungi. The principle, function, and classification of CRISPR-Cas, along with its application strategies and research progress in filamentous fungi, will all be covered in the review. Additionally, we will go over general matters to take into account when editing a genome with the CRISPR-Cas system, including the creation of vectors, different transformation methodologies, multiple editing approaches, CRISPR-mediated transcriptional activation (CRISPRa) or interference (CRISPRi), base editors (BEs), and Prime editors (PEs).

Establishment of a selectable marker recycling system for iterative gene editing in Fusarium fujikuroi

Gibberellic acid (GA3) is a vital plant growth hormone widely used in agriculture. Currently, GA3 production relies on liquid fermentation by the filamentous fungus Fusarium fujikuroi. However, the lack of an effective selection marker recycling system hampers the application of metabolic engineering technology in F. fujikuroi, as multiple-gene editing and positive-strain screening still rely on a limited number of antibiotics. In this study, we developed a strategy using pyr4-blaster and CRISPR/Cas9 tools for recycling orotidine-5'-phosphate decarboxylase (Pyr4) selection markers. We demonstrated the effectiveness of this method for iterative gene integration and large gene-cluster deletion. We also successfully improved GA3 titers by overexpressing geranylgeranyl pyrophosphate synthase and truncated 3-hydroxy-3-methyl glutaryl coenzyme A reductase, which rewired the GA3 biosynthesis pathway. These results highlight the efficiency of our established system in recycling selection markers during iterative gene editing events. Moreover, the selection marker recycling system lays the foundation for further research on metabolic engineering for GA3 industrial production.

Expansion of YALIcloneHR toolkit for Yarrowia lipolytica combined with Golden Gate and CRISPR technology

Metabolic Engineering of yeast is a critical approach to improving the production capacity of cell factories. To obtain genetically stable recombinant strains, the exogenous DNA is preferred to be integrated into the genome. Previously, we developed a Golden Gate toolkit YALIcloneNHEJ, which could be used as an efficient modular cloning toolkit for the random integration of multigene pathways through the innate non-homologous end-joining repair mechanisms of Yarrowia lipolytica. We expanded the toolkit by designing additional building blocks of homologous arms and using CRISPR technology. The reconstructed toolkit was thus entitled YALIcloneHR and designed for gene-specific knockout and integration. To verify the effectiveness of the system, the gene PEX10 was selected as the target for the knockout. This system was subsequently applied for the arachidonic acid production, and the reconstructed strain can accumulate 4.8% of arachidonic acid. The toolkit will expand gene editing technology in Y. lipolytica, which would help produce other chemicals derived from acetyl-CoA in the future.

Improving the productivity of gibberellic acid by combining small-molecule compounds-based targeting technology and transcriptomics analysis in Fusarium fujikuroi

Gibberellic acid (GA3), produced industrially by Fusarium fujikuroi, stands as a crucial plant growth regulator extensively employed in the agriculture filed while limited understanding of the global metabolic network hinders researchers from conducting rapid targeted modifications. In this study, a small-molecule compounds-based targeting technology was developed to increase GA3 production. Firstly, various small molecules were used to target key nodes of different pathways and the result displayed that supplement of terbinafine improved significantly GA3 accumulation, which reached to 1.08 g/L. Subsequently, lipid and squalene biosynthesis pathway were identified as the key pathways influencing GA3 biosynthesis by transcriptomic analysis. Thus, the strategies including in vivo metabolic engineering modification and in vitro supplementation of lipid substrates were adopted, both contributed to an enhanced GA3 yield. Finally, the engineered strain demonstrated the ability to achieve a GA3 yield of 3.24 g/L in 5 L bioreactor when utilizing WCO as carbon source and feed.

Review on Emerging Therapeutic Strategies for Managing Cardiovascular Disease

Cardiovascular disease (CVD) remains a foremost global health concern, necessitating ongoing exploration of innovative therapeutic strategies. This review surveys the latest developments in cardiovascular therapeutics, offering a comprehensive overview of emerging approaches poised to transform disease management. The examination begins by elucidating the current epidemiological landscape of CVD and the economic challenges it poses to healthcare systems. It proceeds to scrutinize the limitations of traditional therapies, emphasizing the need for progressive interventions. The core focus is on novel pharmacological interventions, including advancements in drug development, targeted therapies, and repurposing existing medications. The burgeoning field of gene therapy and its potential in addressing genetic predispositions to cardiovascular disorders are explored, alongside the integration of artificial intelligence and machine learning in risk assessment and treatment optimization. Non-pharmacological interventions take center stage, with an exploration of digital health technologies, wearable devices, and telemedicine as transformative tools in CVD management. Regenerative medicine and stem cell therapies, offering promises of tissue repair and functional recovery, are investigated for their potential impact on cardiac health. This review also delves into the interplay of lifestyle modifications, diet, exercise, and behavioral changes, emphasizing their pivotal role in cardiovascular health and disease prevention. As precision medicine gains prominence, this synthesis of emerging therapeutic modalities aims to guide clinicians and researchers in navigating the dynamic landscape of cardiovascular disease management, fostering a collective effort to alleviate the global burden of CVD and promote a healthier future.

Improving Gibberellin GA3 Production with the Construction of a Genome-Scale Metabolic Model of Fusarium fujikuroi

Liquid fermentation is the primary method for GA3 production usingFusarium fujikuroi. However, production capacity is limited due to unknown metabolic pathways. To address this, we constructed a genome-scale metabolic model (iCY1235) with 1753 reactions, 1979 metabolites, and 1235 genes to understand the GA3 regulation mechanisms. The model was validated by analyzing growth rates under different glucose uptake rates and identifying essential genes. We used the model to optimize fermentation conditions, including carbon sources and dissolved oxygen. Through the OptForce algorithm, we identified 20 reactions as targets. Overexpressing FFUJ_02053 and FFUJ_14337 resulted in a 37.5 and 75% increase in GA3 titers, respectively. These targets enhance carbon flux toward GA3 production. Our model holds promise for guiding the metabolic engineering of F. fujikuroi to achieve targeted overproduction. In summary, our study utilizes the iCY1235 model to understand GA3 regulation, optimize fermentation conditions, and identify specific targets for enhancing GA3 production through metabolic engineering.

Understanding the Fermentation Potentiality For Gibberellic Acid (GA3) Production Using Fungi

Gibberellins represent an important group of potent phytohormones, growth-promoting, closely related diterpenoid acids biologically derived from tetracyclic diterpenoid hydrocarbon. Among these, gibberellic acid (GA3) has received the greatest attention. GA3 is a highly valued plant growth regulator which has various applications in agriculture. It is extensively used for beneficial effects including stem elongation, elimination of dormancy, sex expression, seed germination, flowering, and fruit senescence. Along with plants, many microbes are also producing GA3 as their secondary metabolite, and among these, fungi are reported to produce a higher amount of GA3. Fermentation technology based on submerged fermentation and solid-state fermentation for the production of GA3 has been used with its merits and demerits using Fusarium moniliforme fungus in the industry. Several mathematical models and optimization tools were also designed for enhancing the fermentative yield by researchers. The detailed analysis is essential to understand all the fermentation aspects, various unit parameters, process operation approaches, reduction in cost, and assessment of the possible uses of these models in the production of GA3 for higher yield. Recently, exclusive research is executed to lower down the production cost of GA3 approaching various strategies.

Strategies for the Development of Industrial Fungal Producing Strains

The use of microorganisms in industry has enabled the (over)production of various compounds (e.g., primary and secondary metabolites, proteins and enzymes) that are relevant for the production of antibiotics, food, beverages, cosmetics, chemicals and biofuels, among others. Industrial strains are commonly obtained by conventional (non-GMO) strain improvement strategies and random screening and selection. However, recombinant DNA technology has made it possible to improve microbial strains by adding, deleting or modifying specific genes. Techniques such as genetic engineering and genome editing are contributing to the development of industrial production strains. Nevertheless, there is still significant room for further strain improvement. In this review, we will focus on classical and recent methods, tools and technologies used for the development of fungal production strains with the potential to be applied at an industrial scale. Additionally, the use of functional genomics, transcriptomics, proteomics and metabolomics together with the implementation of genetic manipulation techniques and expression tools will be discussed.

Establishment of a genome editing tool using CRISPR-Cas9 ribonucleoprotein complexes in the non-model plant pathogen Sphaerulina musiva

CRISPR-Cas9 is a versatile genome editing system widely used since 2013 to introduce site-specific modifications into the genomes of model and non-model species. This technology is used in various applications, from gene knock-outs, knock-ins, and over-expressions to more precise changes, such as the introduction of nucleotides at a targeted locus. CRISPR-Cas9 has been demonstrated to be easy to establish in new species and highly efficient and specific compared to previous gene editing strategies such as Zinc finger nucleases and transcription activator-like effector nucleases. Grand challenges for emerging CRISPR-Cas9 tools in filamentous fungi are developing efficient transformation methods for non-model organisms. In this paper, we have leveraged the establishment of CRISPR-Cas9 genome editing tool that relies on Cas9/sgRNA ribonucleoprotein complexes (RNPs) in the model species Trichoderma reesei and developed the first protocol to efficiently transform the non-model species, Sphaerulina musiva. This fungal pathogen constitutes a real threat to the genus Populus, a foundational bioenergy crop used for biofuel production. Herein, we highlight the general considerations to design sgRNAs and their computational validation. We also describe the use of isolated protoplasts to deliver the CRISPR-Cas9 RNP components in both species and the screening for targeted genome editing events. The development of engineering tools in S. musiva can be used for studying genes involved in diverse processes such as secondary metabolism, establishment, and pathogenicity, among many others, but also for developing genetic mitigation approaches. The approach described here provides guidance for potential development of transformation systems in other non-model spore-bearing ascomycetes.

Application progress of CRISPR/Cas9 genome-editing technology in edible fungi

Edible fungi are not only delicious but are also rich in nutritional and medicinal value, which is highly sought after by consumers. As the edible fungi industry continues to rapidly advance worldwide, particularly in China, the cultivation of superior and innovative edible fungi strains has become increasingly pivotal. Nevertheless, conventional breeding techniques for edible fungi can be arduous and time-consuming. CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated nuclease 9) is a powerful tool for molecular breeding due to its ability to mediate high-efficiency and high-precision genome modification, which has been successfully applied to many kinds of edible fungi. In this review, we briefly summarized the working mechanism of the CRISPR/Cas9 system and highlighted the application progress of CRISPR/Cas9-mediated genome-editing technology in edible fungi, including Agaricus bisporus, Ganoderma lucidum, Flammulina filiformis, Ustilago maydis, Pleurotus eryngii, Pleurotus ostreatus, Coprinopsis cinerea, Schizophyllum commune, Cordyceps militaris, and Shiraia bambusicola. Additionally, we discussed the limitations and challenges encountered using CRISPR/Cas9 technology in edible fungi and provided potential solutions. Finally, the applications of CRISPR/Cas9 system for molecular breeding of edible fungi in the future are explored.

Establishment of a CRISPR/Cas9-Mediated Efficient Knockout System of Trichoderma hamatum T21 and Pigment Synthesis PKS Gene Knockout

Trichoderma hamatum is a filamentous fungus that serves as a biological control agent for multiple phytopathogens and as an important resource promising for fungicides. However, the lack of adequate knockout technologies has hindered gene function and biocontrol mechanism research of this species. This study obtained a genome assembly of T. hamatum T21, with a 41.4 Mb genome sequence comprising 8170 genes. Based on genomic information, we established a CRISPR/Cas9 system with dual sgRNAs targets and dual screening markers. CRISPR/Cas9 plasmid and donor DNA recombinant plasmid were constructed for disruption of the Thpyr4 and Thpks1 genes. The result indicates the consistency between phenotypic characterization and molecular identification of the knockout strains. The knockout efficiencies of Thpyr4 and Thpks1 were 100% and 89.1%, respectively. Moreover, sequencing revealed fragment deletions between dual sgRNA target sites or GFP gene insertions presented in knockout strains. The situations were caused by different DNA repair mechanisms, nonhomologous end joining (NHEJ), and homologous recombination (HR). Overall, we have successfully constructed an efficient and convenient CRISPR/Cas9 system in T. hamatum for the first time, which has important scientific significance and application value for studies on functional genomics of Trichoderma and other filamentous fungi.

Gibberellic acid overproduction in Fusarium fujikuroi using regulatory modification and transcription analysis

Gibberellic acid (GA₃), one of the natural diterpenoids produced by Fusarium fujikuroi, serves as an important phytohormone in agriculture for promoting plant growth. Presently, the metabolic engineering strategies for increasing the production of GA₃ are progressing slowly, which seriously restricted the advancing of the cost-effective industrial production of GA₃. In this study, an industrial strain with high-yield GA₃ of F. fujikuroi was constructed by metabolic modification, coupling with transcriptome analysis and promoter engineering. The over-expression of AreA and Lae1, two positive factors in the regulatory network, generated an initial producing strain with GA₃ production of 2.78 g L^-1. Compared with a large abundance of transcript enrichments in the GA₃ synthetic gene cluster discovered by the comparative transcriptome analysis, geranylgeranyl pyrophosphate synthase 2 (Ggs2), and cytochrome P450-3 genes, two key genes that respectively participated in the initial and final step of biosynthesis, were identified to be downregulated when the highest GA₃ productivity was obtained. Employing with a nitrogen-responsive bidirectional promoter, the two rate-limiting genes were dynamically upregulated, and therefore, the production of GA₃ was increased to 3.02 g L^-1. Furthermore, the top 20 upregulated genes were characterized in GA₃ over-production, and their distributions in chromosomes suggested potential genomic regions with a high transcriptional level for further strain development. The construction of a GA₃ high-yield-producing strain was successfully achieved, and insights into the enriched functional transcripts provided novel strain development targets of F. fujikuroi, offering an efficient microbial development platform for industrial GA₃ production. KEY POINTS: • Global regulatory modification was achieved in F. fujikuroi for GA₃ overproduction. • Comparative transcriptome analysis revealed bottlenecks in GA specific-pathway. • A dynamically nitrogen-regulated bidirectional promoter was cloned and employed.

Advances and Challenges in CRISPR/Cas-Based Fungal Genome Engineering for Secondary Metabolite Production: A Review

Fungi represent an important source of bioactive secondary metabolites (SMs), which have wide applications in many fields, including medicine, agriculture, human health, and many other industries. The genes involved in SM biosynthesis are usually clustered adjacent to each other into a region known as a biosynthetic gene cluster (BGC). The recent advent of a diversity of genetic and genomic technologies has facilitated the identification of many cryptic or uncharacterized BGCs and their associated SMs. However, there are still many challenges that hamper the broader exploration of industrially important secondary metabolites. The recent advanced CRISPR/Cas system has revolutionized fungal genetic engineering and enabled the discovery of novel bioactive compounds. In this review, we firstly introduce fungal BGCs and their relationships with associated SMs, followed by a brief summary of the conventional strategies for fungal genetic engineering. Next, we introduce a range of state-of-the-art CRISPR/Cas-based tools that have been developed and review recent applications of these methods in fungi for research on the biosynthesis of SMs. Finally, the challenges and limitations of these CRISPR/Cas-based systems are discussed and directions for future research are proposed in order to expand their applications and improve efficiency for fungal genetic engineering.

Synthetic peptides for the precise transportation of proteins of interests to selectable subcellular areas

Proteins, as gifts from nature, provide structure, sequence, and function templates for designing biomaterials. As first reported here, one group of proteins called reflectins and derived peptides were found to present distinct intracellular distribution preferences. Taking their conserved motifs and flexible linkers as Lego bricks, a series of reflectin-derivates were designed and expressed in cells. The selective intracellular localization property leaned on an RMs (canonical conserved reflectin motifs)-replication-determined manner, suggesting that these linkers and motifs were constructional fragments and ready-to-use building blocks for synthetic design and construction. A precise spatiotemporal application demo was constructed in the work by integrating RLNto2 (as one representative of a synthetic peptide derived from RfA1) into the Tet-on system to effectively transport cargo peptides into nuclei at selective time points. Further, the intracellular localization of RfA1 derivatives was spatiotemporally controllable with a CRY2/CIB1 system. At last, the functional homogeneities of either motifs or linkers were verified, which made them standardized building blocks for synthetic biology. In summary, the work provides a modularized, orthotropic, and well-characterized synthetic-peptide warehouse for precisely regulating the nucleocytoplasmic localization of proteins.

The expanding CRISPR toolbox for natural product discovery and engineering in filamentous fungi

Covering: up to May 2022Fungal genetics has transformed natural product research by enabling the elucidation of cryptic metabolites and biosynthetic steps. The enhanced capability to add, subtract, modulate, and rewrite genes via CRISPR/Cas technologies has opened up avenues for the manipulation of biosynthetic gene clusters across diverse filamentous fungi. This review discusses the innovative and diverse strategies for fungal natural product discovery and engineering made possible by CRISPR/Cas-based tools. We also provide a guide into multiple angles of CRISPR/Cas experiment design, and discuss current gaps in genetic tool development for filamentous fungi and the promising opportunities for natural product research.

Identification of neutral genome integration sites with high expression and high integration efficiency in Fusarium venenatum TB01

CRISPR/Cas9-mediated homology-directed recombination is an efficient method to express target genes. Based on the above method, providing ideal neutral integration sites can ensure the reliable, stable, and high expression of target genes. In this study, we obtained a fluorescent transformant with neutral integration and high expression of the GFP expression cassette from the constructed GFP expression library and named strain FS. The integration site mapped at 4886 bp upstream of the gene FVRRES_00686 was identified in strain FS based on a Y-shaped adaptor-dependent extension, and the sequence containing 600 bp upstream and downstream of this site was selected as the candidate region for designing sgRNAs (Sites) for CRISPR/Cas9-mediated homology-directed recombination. PCR analysis showed that the integration efficiency of CRISPR/Cas9-mediated integration of target genes in designed sites reached 100%. Further expression stability and applicability analysis revealed that the integration of the target gene into the above designed sites can be stably inherited and expressed and has no negative effect on the growth of F. venenatum TB01. These results indicate the above designed neutral sites have the potential to accelerate the development of F. venenatum TB01 through overexpression of target genes in metabolic engineering.

Progress and Challenges in Elucidating the Functional Role of Effectors in the Soybean-Phytophthora sojae Interaction

Phytophthora sojae, the agent responsible for stem and root rot, is one of the most damaging plant pathogens of soybean. To establish a compatible-interaction, P. sojae secretes a wide array of effector proteins into the host cell. These effectors have been shown to act either in the apoplastic area or the cytoplasm of the cell to manipulate the host cellular processes in favor of the development of the pathogen. Deciphering effector-plant interactions is important for understanding the role of P. sojae effectors in disease progression and developing approaches to prevent infection. Here, we review the subcellular localization, the host proteins, and the processes associated with P. sojae effectors. We also discuss the emerging topic of effectors in the context of effector-resistance genes interaction, as well as model systems and recent developments in resources and techniques that may provide a better understanding of the soybean-P. sojae interaction.

Editing Aspergillus terreus using the CRISPR-Cas9 system

CRISPR-Cas9 technology has been utilized in different organisms for targeted mutagenesis, offering a fast, precise and cheap approach to speed up molecular breeding and study of gene function. Until now, many researchers have established the demonstration of applying the CRISPR/Cas9 system to various fungal model species. However, there are very few guidelines available for CRISPR/Cas9 genome editing in Aspergillus terreus. In this study, we present CRISPR/Cas9 genome editing in A. terreus. To optimize the guide ribonucleic acid (gRNA) expression, we constructed a modified single-guide ribonucleic acid (sgRNA)/Cas9 expression plasmid. By co-transforming an sgRNA/Cas9 expression plasmid along with maker-free donor deoxyribonucleic acid (DNA), we precisely disrupted the lovB and lovR genes, respectively, and created targeted gene insertion (lovF gene) and iterative gene editing in A. terreus (lovF and lovR genes). Furthermore, co-delivering two sgRNA/Cas9 expression plasmids resulted in precise gene deletion (with donor DNA) in the ku70 and pyrG genes, respectively, and efficient removal of the DNA between the two gRNA targeting sites (no donor DNA) in the pyrG gene. Our results showed that the CRISPR/Cas9 system is a powerful tool for precise genome editing in A. terreus, and our approach provides a great potential for manipulating targeted genes and contributions to gene functional study of A. terreus.

Multivariate modular metabolic engineering for enhanced gibberellic acid biosynthesis in Fusarium fujikuroi

Gibberellic acid (GA3) is one of natural phytohormones, widely used in agriculture and downstream fields. Qualified for the nature productivity, Fusarium fujikuroi was currently employed for the industrial biotransformation from agriculture residues into GA3. Herein, Multivariate modular metabolic engineering (MMME) was assigned to reconstitute the metabolic balance in F. fujikuroi for enhancing GA3 production. Three modules including precursor pool, cluster-specific channel and P450-mediated oxidation in GA3 biosynthetic pathway were defined and optimized separately. The enhancement of both precursor pool and cluster-specific channel pushed metabolic flux transfer into the GA3-specific pathway. Moreover, both introduction of Vitreoscilla hemoglobin and reinforcement of NADPH-dependent cytochrome P450 reductase facilitated oxidation cofactor transfer and subsequently boosted mycelium growth and GA3 biosynthesis. Integration of three modules in the engineered strain accumulated 2.89 g/L GA3 in shake flask via submerged fermentation, presenting a promising modular metabolic engineering model for efficient microbial transformation in agro-industrial application.

Analysis of FR901379 Biosynthetic Genes in Coleophoma empetri by Clustered Regularly Interspaced Short Palindromic Repeats/Cas9-Based Genomic Manipulation

The compound FR901379, a sulfated echinocandin produced by the filamentous fungus Coleophoma empetri F-11899, is an important intermediate for the synthesis of the antifungal drug micafungin. In this study, we established an efficient clustered regularly interspaced short palindromic repeats/Cas9-based gene editing tool for the industrial production strain C. empetri SIPI1284. With this method, the efficiency of gene mutagenesis in the target locus is up to 84%, which enables the rapid gene disruption for the analysis of FR901379 biosynthetic genes. Next, we verified the putative functional genes of the FR901379 biosynthetic gene cluster via gene disruption and gene complementation in vivo. These core functional genes included the nonribosomal peptide synthetase gene (CEnrps), the fatty-acyl-AMP ligase gene (CEligase) responsible for the formation of the activated form of palmitic acid and its transfer to CEnrps, four nonheme mononuclear iron oxygenase genes (CEoxy1, CEoxy2, CEoxy3, and CEoxy4) responsible for the synthesis of nonproteinogenic amino acids, l-homotyrosine biosynthesis genes (CEhtyA-D), two cytochrome P450 enzyme genes (CEp450-1 and CEp450-2), and a transcription regulator gene (CEhyp). In addition, by screening the whole genome, we identified two unknown genes (CEp450-3 and CEsul) responsible for the sulfonyloxy group of FR901379, which were separated from the core FR901379 biosynthetic cluster. Furthermore, during gene disruptions in the research, we obtained a series of FR901379 analogues and elucidated the relationship between the groups and antifungal activities.

Recent advances in metabolic regulation and bioengineering of gibberellic acid biosynthesis in Fusarium fujikuroi

The plant growth hormone gibberellic acid (GA₃), as one of the representative secondary metabolites, is widely used in agriculture, horticulture and brewing industry. GA₃ is detected in both plants and several fungi with the ability to stimulate plant growth. Currently, the main mode of industrial production of GA₃ is depended on the microbial fermentation via long-period submerged fermentation using Fusarium fujikuroi as the only producing strain, qualified for its natural productivity. However, the demand of large-sale industrialization of GA₃ was still restricted by the low productivity. The biosynthetic route of GA₃ in F. fujikuroi is now well-defined. Furthermore, the multi-level regulation mechanisms involved in the whole network of GA₃ production have also been gradually unveiled by the past two decades based on the identification and characterization of several global regulators and their mutual functions. Combined with the quick development of genetic manipulation techniques, the rational modification of producing strain F. fujikuroi development become practical for higher productivity achievement. Herein, we review the latest advances in the molecular regulation of GA₃ biosynthesis in F. fujikuroi and conclude a comprehensive network involving nitrogen depression, global regulator, histone modification and G protein signaling pathway. Correspondingly, the bioengineering strategies covering conventional random mutation, genetic manipulating platform development, metabolic edition and fermentation optimization were also systematically proposed.

CRISPR-Mediated Base Editing: From Precise Point Mutation to Genome-Wide Engineering in Nonmodel Microbes

Nonmodel microbes with unique and diverse metabolisms have become rising stars in synthetic biology; however, the lack of efficient gene engineering techniques still hinders their development. Recently, the use of base editors has emerged as a versatile method for gene engineering in a wide range of organisms including nonmodel microbes. This method is a fusion of impaired CRISPR/Cas9 nuclease and base deaminase, enabling the precise point mutation at the target without inducing homologous recombination. This review updates the latest advancement of base editors in microbes, including the conclusion of all microbes that have been researched by base editors, the introduction of newly developed base editors, and their applications. We provide a list that comprehensively concludes specific applications of BEs in nonmodel microbes, which play important roles in industrial, agricultural, and clinical fields. We also present some microbes in which BEs have not been fully established, in the hope that they are explored further and so that other microbial species can achieve arbitrary base conversions. The current obstacles facing BEs and solutions are put forward. Lastly, the highly efficient BEs and other developed versions for genome-wide reprogramming of cells are discussed, showing great potential for future engineering of nonmodel microbes.

YALIcloneNHEJ: An Efficient Modular Cloning Toolkit for NHEJ Integration of Multigene Pathway and Terpenoid Production in Yarrowia lipolytica

Non-homologous end-joining (NHEJ)-mediated random integration in Yarrowia lipolytica has been demonstrated to be an effective strategy for screening hyperproducer strains. However, there was no multigene assembly method applied for NHEJ integration, which made it challenging to construct and integrate metabolic pathways. In this study, a Golden Gate modular cloning system (YALIcloneNHEJ) was established to develop a robust DNA assembly platform in Y. lipolytica. By optimizing key factors, including the amounts of ligase and the reaction cycles, the assembly efficiency of 4, 7, and 10 fragments reached up to 90, 75, and 50%, respectively. This YALIcloneNHEJ system was subsequently applied for the overproduction of the sesquiterpene (-)-α-bisabolol by constructing a biosynthesis route and enhancing the flux in the mevalonate pathway. The resulting strain produced 4.4 g/L (-)-α-bisabolol, the highest titer reported in yeast to date. Our study expands the toolbox of metabolic engineering and is expected to enable a highly efficient production of various terpenoids.

Recent advances in the development of Aspergillus for protein production

Aspergillus had been widely used in the industrial production of recombinant proteins. In addition to the safety and broad substrate utilization spectrum, its efficient post-translational modification and strong protein secretion capacity have significant advantages for developing an excellent protein-producing cell factory in industrial production. However, the difficulties in genetic manipulation of Aspergillus and varying expression levels of different heterologous proteins hampered its further development and application. Recently, the development of CRISPR genome editing and high-throughput screening platforms has facilitated the Aspergillus development of a wide range of modifications and applications. Meanwhile, multi-omics analysis and multiplexed genetic engineering have promoted effective knowledge mining. This paper provides a comprehensive and updated review of these advances, including high-throughput screening, genome editing, protein expression modules, and fermentation optimization. It also highlights and discusses the latest significant progress, aiming to provide a practical guide for implementing Aspergillus as an efficient protein-producing cell factory.

CRISPR-based metabolic engineering in non-model microorganisms

Non-model microorganisms possess unique and versatile metabolic characteristics, offering great opportunities as cell factories for biosynthesis of target products. However, lack of efficient genetic tools for pathway engineering represents a big challenge to unlock the full production potential of these microbes. Over the past years, CRISPR systems have been extensively developed and applied to domesticate non-model microorganisms. In this paper, we summarize the current significant advances in designing and constructing CRISPR-mediated genetic modification systems in non-model microorganisms, such as bacteria, fungi and cyanobacteria. We particularly put emphasis on reviewing some successful implementations in metabolic pathway engineering via CRISPR-based genome editing tools. Moreover, the current barriers and future perspectives on improving the editing efficiency of CRISPR systems in non-model microorganisms are also discussed.

CRISPR/Cas9 mediated editing of the Quorn fungus Fusarium venenatum A3/5 by transient expression of Cas9 and sgRNAs targeting endogenous marker gene PKS12

Background

Gene editing using CRISPR/Cas9 is a widely used tool for precise gene modification, modulating gene expression and introducing novel proteins, and its use has been reported in various filamentous fungi including the genus Fusarium. The aim of this study was to optimise gene editing efficiency using AMA1 replicator vectors for transient expression of CRISPR constituents in Fusarium venenatum (A3/5), used commercially in the production of mycoprotein (Quorn™).

Results

We present evidence of CRISPR/Cas9 mediated gene editing in Fusarium venenatum, by targeting the endogenous visible marker gene PKS12, which encodes a polyketide synthase responsible for the synthesis of the pigment aurofusarin. Constructs for expression of single guide RNAs (sgRNAs) were cloned into an AMA1 replicator vector incorporating a construct for constitutive expression of cas9 codon-optimised for Aspergillus niger or F. venenatum. Vectors were maintained under selection for transient expression of sgRNAs and cas9 in transformed protoplasts. 100% gene editing efficiency of protoplast-derived isolates was obtained using A. niger cas9 when sgRNA transcription was regulated by the F. venenatum 5SrRNA promoter. In comparison, expression of sgRNAs using a PgdpA-ribozyme construct was much less effective, generating mutant phenotypes in 0–40% of isolates. Viable isolates were not obtained from protoplasts transformed with an AMA1 vector expressing cas9 codon-optimised for F. venenatum.

Conclusions

Using an AMA1 replicator vector for transient expression of A. niger cas9 and sgRNAs transcribed from the native 5SrRNA promoter, we demonstrate efficient gene editing of an endogenous marker gene in F. venenatum, resulting in knockout of gene function and a visible mutant phenotype in 100% of isolates. This establishes a platform for further development of CRISPR/Cas technology in F. venenatum for use as a research tool, for understanding the controls of secondary metabolism and hyphal development and validating prototypes of strains produced using traditional methods for strain improvement.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40694-021-00121-8.

CRISPR/Cas9-mediated genome editing directed by a 5S rRNA-tRNAGly hybrid promoter in the thermophilic filamentous fungus Humicola insolens

BACKGROUND

Humicola insolens is a filamentous fungus with high potential of producing neutral and heat- and alkali-resistant cellulase. However, the genetic engineering tools, particularly the genome-editing tool, are scarce, hindering the study of cellulase expression regulation in this organism.

RESULTS

Herein, a CRISPR/Cas9 genome-editing system was established in H. insolens based on a hybrid 5S rRNA-tRNA^Gly promoter. This system is superior to the HDV (hepatitis delta virus) system in genome editing, allowing highly efficient single gene destruction in H. insolens with rates of deletion up to 84.1% (37/44). With this system, a putative pigment synthesis gene pks and the transcription factor xyr1 gene were disrupted with high efficiency. Moreover, the extracellular protein concentration and cellulase activity largely decreased when xyr1 was deleted, demonstrating for the first time that Xyr1 plays an important role in cellulase expression regulation.

CONCLUSIONS

The established CRISPR/Cas9 system is a powerful genetic operation tool for H. insolens, which will accelerate studies on the regulation mechanism of cellulase expression and engineering of H. insolens for higher cellulase production.

CRISPR/Cas 9-Based Editing in the Production of Bioactive Molecules

Plants, fungi, and bacteria synthesize a wide range of secondary metabolites that exhibit diverse biological activities. These bioactives, due to their potential benefits in research and therapeutics, have gained immense industrial importance. There is a need to synthesize these bioactives at significantly higher concentrations using cost-effective measures to be economically viable. However, the broader study of industrially important secondary metabolites has been hindered, thus, far due to a shortage of reliable, comparatively easy, and highly effective gene manipulation techniques. With the advent of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas), there is a revolution in the field of genetic engineering. CRISPR/Cas system, due to its simplicity and ease of use. This has widened its application in plant breeding, strain improvement, and engineering the metabolic pathways involved in the biochemical synthesis of industrially valuable bioactive. This review briefly introduces the CRISPR/Cas9 system and summarizes the applications of CRISPR/Cas9-mediated editing tools for the production of plant and fungal-derived bioactives.

Versatile Applications of the CRISPR/Cas Toolkit in Plant Pathology and Disease Management

New tools and advanced technologies have played key roles in facilitating basic research in plant pathology and practical approaches for disease management and crop health. Recently, the CRISPR/Cas (clustered regularly interspersed short palindromic repeats/CRISPR-associated) system has emerged as a powerful and versatile tool for genome editing and other molecular applications. This review aims to introduce and highlight the CRISPR/Cas toolkit and its current and future impact on plant pathology and disease management. We will cover the rapidly expanding horizon of various CRISPR/Cas applications in the basic study of plant-pathogen interactions, genome engineering of plant disease resistance, and molecular diagnosis of diverse pathogens. Using the citrus greening disease as an example, various CRISPR/Cas-enabled strategies are presented to precisely edit the host genome for disease resistance, to rapidly detect the pathogen for disease management, and to potentially use gene drive for insect population control. At the cutting edge of nucleic acid manipulation and detection, the CRISPR/Cas toolkit will accelerate plant breeding and reshape crop production and disease management as we face the challenges of 21st century agriculture.

Genetic engineering of yeast, filamentous fungi and bacteria for terpene production and applications in food industry

Terpenes are a major class of natural aromatic compounds in grapes and wines to offer the characteristic flavor and aroma, serving as important quality traits of wine products. Saccharomyces cerevisiae represents an excellent cell factory platform for large-scale bio-based terpene production. This review describes the biosynthetic pathways of terpenes in different organisms. The metabolic engineering of S. cerevisiae for promoting terpene biosynthesis and the alternative microbial engineering platforms including filamentous fungi and Escherichia coli are also elaborated. Additionally, the potential applications of the terpene products from engineered microorganisms in food and beverage industries are also discussed. This review provides comprehensive information for an innovative supply way of terpene via microbial cell factory, which could facilitate the development and application of this technique at the industrial scale.

Perspectives for epigenetic editing in crops

Site-specific nucleases (SSNs) have drawn much attention in plant biotechnology due to their ability to drive precision mutagenesis, gene targeting or allele replacement. However, when devoid of its nuclease activity, the underlying DNA-binding activity of SSNs can be used to bring other protein functional domains close to specific genomic sites, thus expanding further the range of applications of the technology. In particular, the addition of functional domains encoding epigenetic effectors and chromatin modifiers to the CRISPR/Cas ribonucleoprotein complex opens the possibility to introduce targeted epigenomic modifications in plants in an easily programmable manner. Here we examine some of the most important agronomic traits known to be controlled epigenetically and review the best studied epigenetic catalytic effectors in plants, such as DNA methylases/demethylases or histone acetylases/deacetylases and their associated marks. We also review the most efficient strategies developed to date to functionalize Cas proteins with both catalytic and non-catalytic epigenetic effectors, and the ability of these domains to influence the expression of endogenous genes in a regulatable manner. Based on these new technical developments, we discuss the possibilities offered by epigenetic editing tools in plant biotechnology and their implications in crop breeding.

Novel CRISPR-Cas Systems: An Updated Review of the Current Achievements, Applications, and Future Research Perspectives

According to Darwin's theory, endless evolution leads to a revolution. One such example is the Clustered Regularly Interspaced Palindromic Repeats (CRISPR)-Cas system, an adaptive immunity system in most archaea and many bacteria. Gene editing technology possesses a crucial potential to dramatically impact miscellaneous areas of life, and CRISPR-Cas represents the most suitable strategy. The system has ignited a revolution in the field of genetic engineering. The ease, precision, affordability of this system is akin to a Midas touch for researchers editing genomes. Undoubtedly, the applications of this system are endless. The CRISPR-Cas system is extensively employed in the treatment of infectious and genetic diseases, in metabolic disorders, in curing cancer, in developing sustainable methods for fuel production and chemicals, in improving the quality and quantity of food crops, and thus in catering to global food demands. Future applications of CRISPR-Cas will provide benefits for everyone and will save countless lives. The technology is evolving rapidly; therefore, an overview of continuous improvement is important. In this review, we aim to elucidate the current state of the CRISPR-Cas revolution in a tailor-made format from its discovery to exciting breakthroughs at the application level and further upcoming trends related to opportunities and challenges including ethical concerns.

Tailored biosynthesis of gibberellin plant hormones in yeast

The application of small amounts of natural plant growth hormones, such as gibberellins (GAs), can increase the productivity and quality of many vegetable and fruit crops. However, gibberellin growth hormones usage is limited by the high cost of their production, which is currently based on fermentation of a natural fungal producer Fusarium fujikuroi that produces a mix of several GAs. We explored the potential of the oleaginous yeast Yarrowia lipolytica to produce specific profiles of GAs. Firstly, the production of the GA-precursor ent-kaurenoic acid (KA) at 3.75 mg/L was achieved by expression of biosynthetic enzymes from the plant Arabidopsis thaliana and upregulation of the mevalonate (MVA) pathway.

We then built a GA₄-producing strain by extending the GA-biosynthetic pathway and upregulating the MVA-pathway further, resulting in 17.29 mg/L GA₄. Additional expression of the F. fujikoroi GA-biosynthetic enzymes resulted in the production of GA₇ (trace amounts) and GA₃ (2.93 mg/L). Lastly, through protein engineering and the expression of additional KA-biosynthetic genes, we increased the GA₃-production 4.4-fold resulting in 12.81 mg/L. The developed system presents a promising resource for the recombinant production of specific gibberellins, identifying bottlenecks in GA biosynthesis, and discovering new GA biosynthetic genes.

Classification

Biological Sciences, Applied Biological Sciences.

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A complete biosynthetic pathway towards gibberellins was reconstructed in a microbial host

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The pathway towards ent-kaurenoic acid consisted of Arabidopsis thaliana enzymes

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The pathway from ent-kaurenoic acid to gibberellins GA₃, GA₄ and GA₇ consisted of Fusarium fujikuroi enzymes

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Y. lipolytica expressed 14 heterologous genes for gibberellins biosynthesis and had 5 genome edits for improved mevalonate flux

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The strains produced up to 12 mg/L of GA₃ and up to 17 mg/L GA₄ in small-scale cultivations

Applications of CRISPR/Cas9 in the Synthesis of Secondary Metabolites in Filamentous Fungi

Filamentous fungi possess the capacity to produce a wide array of secondary metabolites with diverse biological activities and structures, such as lovastatin and swainsonine. With the advent of the post-genomic era, increasing amounts of cryptic or uncharacterized secondary metabolite biosynthetic gene clusters are continually being discovered. However, owing to the longstanding lack of versatile, comparatively simple, and highly efficient genetic manipulation techniques, the broader exploration of industrially important secondary metabolites has been hampered thus far. With the emergence of CRISPR/Cas9-based genome editing technology, this dilemma may be alleviated, as this advanced technique has revolutionized genetic research and enabled the exploitation and discovery of new bioactive compounds from filamentous fungi. In this review, we introduce the CRISPR/Cas9 system in detail and summarize the latest applications of CRISPR/Cas9-mediated genome editing in filamentous fungi. We also briefly introduce the specific applications of the CRISPR/Cas9 system and CRISPRa in the improvement of secondary metabolite contents and discovery of novel biologically active compounds in filamentous fungi, with specific examples noted. Additionally, we highlight and discuss some of the challenges and deficiencies of using the CRISPR/Cas9-based genome editing technology in research on the biosynthesis of secondary metabolites as well as future application of CRISPR/Cas9 strategy in filamentous fungi are highlighted and discussed.

CRISPR/Cas9-mediated genome editing in Penicillium oxalicum and Trichoderma reesei using 5S rRNA promoter-driven guide RNAs

OBJECTIVE

To construct convenient CRISPR/Cas9-mediated genome editing systems in industrial enzyme-producing fungi Penicillium oxalicum and Trichoderma reesei.

RESULTS

Employing the 5S rRNA promoter from Aspergillus niger for guide RNA expression, the β-glucosidase gene bgl2 in P. oxalicum was deleted using a donor DNA carrying 40-bp homology arms or a donor containing no selectable marker gene. Using a markerless donor DNA as editing template, precise replacement of a small region was achieved in the creA gene. In T. reesei, the A. niger 5S rRNA promoter was less efficient than that in P. oxalicum when used for gene editing. Using a native 5S rRNA promoter, stop codons were introduced into the lae1 coding region using a markerless donor DNA with an editing efficiency of 36.67%.

CONCLUSIONS

Efficient genome editing systems were developed in filamentous fungi P. oxalicum and T. reesei by using heterologous or native 5S rRNA promoters for guide RNA expression.

Genome-based engineering of ligninolytic enzymes in fungi

Background

Many fungi grow as saprobic organisms and obtain nutrients from a wide range of dead organic materials. Among saprobes, fungal species that grow on wood or in polluted environments have evolved prolific mechanisms for the production of degrading compounds, such as ligninolytic enzymes. These enzymes include arrays of intense redox-potential oxidoreductase, such as laccase, catalase, and peroxidases. The ability to produce ligninolytic enzymes makes a variety of fungal species suitable for application in many industries, including the production of biofuels and antibiotics, bioremediation, and biomedical application as biosensors. However, fungal ligninolytic enzymes are produced naturally in small quantities that may not meet the industrial or market demands. Over the last decade, combined synthetic biology and computational designs have yielded significant results in enhancing the synthesis of natural compounds in fungi.

Main body of the abstract

In this review, we gave insights into different protein engineering methods, including rational, semi-rational, and directed evolution approaches that have been employed to enhance the production of some important ligninolytic enzymes in fungi. We described the role of metabolic pathway engineering to optimize the synthesis of chemical compounds of interest in various fields. We highlighted synthetic biology novel techniques for biosynthetic gene cluster (BGC) activation in fungo and heterologous reconstruction of BGC in microbial cells. We also discussed in detail some recombinant ligninolytic enzymes that have been successfully enhanced and expressed in different heterologous hosts. Finally, we described recent advance in CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-Cas (CRISPR associated) protein systems as the most promising biotechnology for large-scale production of ligninolytic enzymes.

Short conclusion

Aggregation, expression, and regulation of ligninolytic enzymes in fungi require very complex procedures with many interfering factors. Synthetic and computational biology strategies, as explained in this review, are powerful tools that can be combined to solve these puzzles. These integrated strategies can lead to the production of enzymes with special abilities, such as wide substrate specifications, thermo-stability, tolerance to long time storage, and stability in different substrate conditions, such as pH and nutrients.

An advanced genetic toolkit for exploring the biology of the rock-inhabiting black fungus Knufia petricola

Microcolonial black fungi are a group of ascomycetes that exhibit high stress tolerance, yeast-like growth and constitutive melanin formation. They dominate a range of hostile natural and man-made environments, from desert rocks and salterns to dishwashers, roofs and solar panels. Due to their slow growth and a lack of genetic tools, the underlying mechanisms of black fungi’s phenotypic traits have remained largely unexplored. We chose to address this gap by genetically engineering the rock-inhabiting fungus Knufia petricola (Eurotiomycetes, Chaetothyriales), a species that exhibits all characteristics of black fungi. A cell biological approach was taken by generating K. petricola strains expressing green or red fluorescent protein variants. By applying: (1) traditional gene replacement; (2) gene editing and replacement via plasmid-based or ribonucleoprotein (RNP)-based CRISPR/Cas9, and (3) silencing by RNA interference (RNAi), we constructed mutants in the pathways leading to melanin, carotenoids, uracil and adenine. Stable single and double mutants were generated with homologous recombination (HR) rates up to 100%. Efficient, partially cloning-free strategies to mutate multiple genes with or without resistance cassettes were developed. This state-of-the-art genetic toolkit, together with the annotated genome sequence of strain A95, firmly established K. petricola as a model for exploring microcolonial black fungi.

CRISPR/Cas9-based genome engineering: A new breakthrough in the genetic manipulation of filamentous fungi

Filamentous fungi have several industrial, environmental, and medical applications. However, they are rarely utilized owing to the limited availability of full-genome sequences and genetic manipulation tools. Since the recent discovery of the full-genome sequences for certain industrially important filamentous fungi, CRISPR/Cas9 technology has drawn attention for the efficient development of engineered strains of filamentous fungi. CRISPR/Cas9 genome editing has been successfully applied to diverse filamentous fungi. In this review, we briefly discuss the use of common genetic transformation techniques as well as CRISPR/Cas9-based systems in filamentous fungi. Furthermore, we describe potential limitations and challenges in the practical application of genome engineering of filamentous fungi. Finally, we provide suggestions and highlight future research prospects in the area.

Modular engineering of Shiraia bambusicola for hypocrellin production through an efficient CRISPR system

Shiraia bambusicola exhibits an excellent capability to produce high-value pharmacological drugs, such as hypocrellin. However, less effective molecular tools hamper the processes to discover or exploit these metabolites. To address this issue, the more effective CRISPR/Cas9 system was constructed by optimizing the sgRNA transcription elements and disrupting the endogenous non-homologous end-joining pathway. These tactics prompted the gene-targeting frequency of 100% and simultaneously multiplex genome editing in S. bambusicola. This optimal CRISPR system encouraged us to rewire the entire hypocrellin flux and improve the yield by orchestrating the substrate pool supply, the central hypocrellin pathway, and the antioxidant system. Thus, 8632 mg/L hypocrellin was obtained, resulting in a 12-fold increase than that of the wild-type strain. This engineered S. bambusicola can still endure oxidative stresses from higher target metabolites and sustain an excellent biological activity. This study provides a whole conception to establish the more efficient genome-editing system. Higher conserved transcription elements for sgRNA expressions inspire us to adopt this system for gene modifications of other filamentous fungi. The rational and global biosystems outline will offer guidance to modulate metabolite productivity in other filamentous fungi.