RNAi-Mediated Gene Silencing of Trcot1 Induces a Hyperbranching Phenotype in Trichoderma reesei

TrLys9 participates in fungal development and lysine biosynthesis in Trichoderma reesei

Fungi uniquely synthesize lysine through the α-aminoadipate pathway. The saccharopine reductase ScLys9 catalyzes the formation of saccharopine from ɑ-aminoadipate 6-semialdehyde, the seventh step in the lysine biosynthesis pathway in Saccharomyces cerevisiae. Here, we characterized the functions of TrLys9, an ortholog of S. cerevisiae ScLys9 in the industrial filamentous fungus Trichoderma reesei. Transcriptional level analysis indicated that TrLYS9 expression was higher in the conidial stage than in other stages. Disruption of TrLYS9 led to lysine auxotrophy. Phenotype analysis of the ΔTrlys9 mutant showed that TrLYS9 was involved in fungal development including vegetative growth, conidiation, and conidial germination and lysine biosynthesis. Cellulase production was also impaired in the ΔTrlys9 mutant due to the failure of conidial germination in liquid cellulase-inducing medium. Defects in radial growth and asexual development of the ΔTrlys9 mutant were fully recovered when exogenous lysine was added to the medium. These results imply that TrLys9 is involved in fungal development and lysine biosynthesis in T. reesei.

Fungal strain improvement for efficient cellulase production and lignocellulosic biorefinery: Current status and future prospects

Lignocellulosic biomass (LCB) has been recognized as a valuable carbon source for the sustainable production of biofuels and value-added biochemicals. Crude enzymes produced by fungal cell factories benefit economic LCB degradation. However, high enzyme production cost remains a great challenge. Filamentous fungi have been widely used to produce cellulolytic enzymes. Metabolic engineering of fungi contributes to efficient cellulase production for LCB biorefinery. Here the latest progress in utilizing fungal cell factories for cellulase production was summarized, including developing genome engineering tools to improve the efficiency of fungal cell factories, manipulating promoters, and modulating transcription factors. Multi-omics analysis of fungi contributes to identifying novel genetic elements for enhancing cellulase production. Furthermore, the importance of translation regulation of cellulase production are emphasized. Efficient development of fungal cell factories based on integrative strain engineering would benefit the overall bioconversion efficacy of LCB for sustainable bioproduction.

Current genetic strategies to investigate gene functions in Trichoderma reesei

The filamentous fungus Trichoderma reesei (teleomorph Hypocrea jecorina, Ascomycota) is a well-known lignocellulolytic enzymes-producing strain in industry. To increase the fermentation titer of lignocellulolytic enzymes, random mutagenesis and rational genetic engineering in T. reesei were carried out since it was initially found in the Solomon Islands during the Second World War. Especially the continuous exploration of the underlying regulatory network during (hemi)cellulase gene expression in the post-genome era provided various strategies to develop an efficient fungal cell factory for these enzymes' production. Meanwhile, T. reesei emerges competitiveness potential as a filamentous fungal chassis to produce proteins from other species (e.g., human albumin and interferon α-2b, SARS-CoV-2 N antigen) in virtue of the excellent expression and secretion system acquired during the studies about (hemi)cellulase production. However, all the achievements in high yield of (hemi)cellulases are impossible to finish without high-efficiency genetic strategies to analyze the proper functions of those genes involved in (hemi)cellulase gene expression or secretion. Here, we in detail summarize the current strategies employed to investigate gene functions in T. reesei. These strategies are supposed to be beneficial for extending the potential of T. reesei in prospective strain engineering.

Reduced viscosity mutants of Trichoderma reesei with improved industrial fermentation characteristics

Morphological mutants of Trichoderma reesei were isolated following chemical or insertional mutagenesis. The mutant strains were shown to have reduced viscosity under industrially relevant fermentation conditions and to have maintained high specific productivity of secreted protein. This allowed higher biomass concentration to be maintained during the production phase and, consequently, increased volumetric productivity of secreted protein. The causative mutations were traced to four individual genes (designated sfb3, ssb7, seb1, and mpg1). We showed that two of the morphological mutations could be combined in a single strain to further reduce viscosity and enable a 100% increase in volumetric productivity.

Disruption of the Trichoderma reesei gul1 gene stimulates hyphal branching and reduces broth viscosity in cellulase production

Hyphal morphology is considered to have a close relationship with the production level of secreted proteins by filamentous fungi. In this study, the gul1 gene, which encodes a putative mRNA-binding protein, was disrupted in cellulase-producing fungus Trichoderma reesei. The hyphae of Δgul1 strain produced more lateral branches than the parent strain. Under the condition for cellulase production, disruption of gul1 resulted in smaller mycelial clumps and significantly lower viscosity of fermentation broth. In addition, cellulase production was improved by 22% relative to the parent strain. Transcriptome analysis revealed that a set of genes encoding cell wall remodeling enzymes as well as hydrophobins were differentially expressed in the Δgul1 strain. The results suggest that the regulatory role of gul1 in cell morphogenesis is likely conserved in filamentous fungi. To our knowledge, this is the first report on the engineering of gul1 in an industrially important fungus.