Influences of genetically perturbing synthesis of the typical yellow pigment on conidiation, cell wall integrity, stress tolerance, and cellulase production in Trichoderma reesei

Small GTPase Rab7 is involved in stress adaptation to carbon starvation to ensure the induced cellulase biosynthesis in Trichoderma reesei

BACKGROUND

The saprophytic filamentous fungus Trichoderma reesei represents one of the most prolific cellulase producers. The bulk production of lignocellulolytic enzymes by T. reesei not only relies on the efficient transcription of cellulase genes but also their efficient secretion after being translated. However, little attention has been paid to the functional roles of the involved secretory pathway in the high-level production of cellulases in T. reesei. Rab GTPases are key regulators in coordinating various vesicle trafficking associated with the eukaryotic secretory pathway. Specifically, Rab7 is a representative GTPase regulating the transition of the early endosome to the late endosome followed by its fusion to the vacuole as well as homotypic vacuole fusion. Although crosstalk between the endosomal/vacuolar pathway and the secretion pathway has been reported, the functional role of Rab7 in cellulase production in T. reesei remains unknown.

RESULTS

A TrRab7 was identified and characterized in T. reesei. TrRab7 was shown to play important roles in T. reesei vegetative growth and vacuole morphology. Whereas knock-down of Trrab7 significantly compromised the induced production of T. reesei cellulases, overexpression of the key transcriptional activator, Xyr1, restored the production of cellulases in the Trrab7 knock-down strain (Ptcu-rab7KD) on glucose, indicating that the observed defective cellulase biosynthesis results from the compromised cellulase gene transcription. Down-regulation of Trrab7 was also found to make T. reesei more sensitive to various stresses including carbon starvation. Interestingly, overexpression of Snf1, a serine/threonine protein kinase known as an energetic sensor, partially restored the cellulase production of Ptcu-rab7KD on Avicel, implicating that TrRab7 is involved in an energetic adaptation to carbon starvation which contributes to the successful cellulase gene expression when T. reesei is transferred from glucose to cellulose.

CONCLUSIONS

TrRab7 was shown to play important roles in T. reesei development and a stress response to carbon starvation resulting from nutrient shift. This adaptation may allow T. reesei to successfully initiate the inducing process leading to efficient cellulase production. The present study provides useful insights into the functional involvement of the endosomal/vacuolar pathway in T. reesei development and hydrolytic enzyme production.

Co-inoculation of Soybean Seedling with Trichoderma asperellum and Irpex laceratus Promotes the Absorption of Nitrogen and Phosphorus

Soybean are one of the main oil crops in the world. The study demonstrated that co-inoculation with Trichoderma asperellum (Sordariomycetes, Hypocreomycetidae) and Irpex laceratus (Basidiomycota, Polyporales) isolated from Kosteletzkya virginica can promote the growth of soybean seedlings. The two fungi were found to produce various enzymes, including cellulase, amylase, laccase, protease, and urease. Upon inoculation, T. asperellum mainly colonized within the phloem of the roots in soybean seedlings, while I. laceratus mainly in the xylem and phloem of the roots. Physiological parameters, such as plant height, root length, and fresh weight, were significantly increased in soybean seedlings co-inoculated with T. asperellum and I. laceratus. Moreover, the expression of key genes related to N and P absorption and metabolism was also increased, leading to improved N and P utilization efficiency in soybean seedlings. These results indicate that the two fungi may have complementary roles in promoting plant growth, co-inoculation with T. asperellum and I. laceratus can enhance the growth and nutrient uptake of soybean. These findings suggest that T. asperellum and I. laceratus have the potential to be used as bio-fertilizers to improve soybean growth and yield.

C-terminus of serine-arginine protein kinase-like protein, SrpkF, is involved in conidiophore formation and hyphal growth under salt stress in Aspergillus aculeatus

The serine-arginine protein kinase-like protein, SrpkF, was identified as a regulator for the cellulose-responsive induction of cellulase genes in Aspergillus aculeatus. To analyze various aspects of SrpkF function, we examined the growth of the control strain (MR12); C-terminus deletion mutant, which produced SrpkF1-327 (ΔCsrpkF); whole gene-deletion mutant of srpkF (ΔsrpkF), srpkF overexpressing strain (OEsprkF); and the complemented strain (srpkF+) under various stress conditions. All test strains grew normally on minimal medium under control, high salt (1.5 M KCl), and high osmolality (2.0 M sorbitol and 1.0 M sucrose). However, only ΔCsrpkF showed reduced conidiation on 1.0 M NaCl media. Conidiation of ΔCsrpkF on 1.0 M NaCl media was reduced to 12% compared with that of srpkF+. Further, when OEsprkF and ΔCsrpkF were pre-cultured under salt stress conditions, germination under salt stress conditions was enhanced in both strains. By contrast, deletion of srpkF did not affect hyphal growth and conidiation under the same conditions. We then quantified the transcripts of the regulators involved in the central asexual conidiation pathway in A. aculeatus. The findings revealed that the expression of brlA, abaA, wetA, and vosA was reduced in ΔCsrpkF under salt stress. These data suggest that in A. aculeatus, SrpkF regulates conidiophore development. The C-terminus of SrpkF seems to be important for regulating SrpkF function in response to culture conditions such as salt stress.

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.

MAPkinases regulate secondary metabolism, sexual development and light dependent cellulase regulation in Trichoderma reesei

The filamentous fungus Trichoderma reesei is a prolific producer of plant cell wall degrading enzymes, which are regulated in response to diverse environmental signals for optimal adaptation, but also produces a wide array of secondary metabolites. Available carbon source and light are the strongest cues currently known to impact secreted enzyme levels and an interplay with regulation of secondary metabolism became increasingly obvious in recent years. While cellulase regulation is already known to be modulated by different mitogen activated protein kinase (MAPK) pathways, the relevance of the light signal, which is transmitted by this pathway in other fungi as well, is still unknown in T. reesei as are interconnections to secondary metabolism and chemical communication under mating conditions. Here we show that MAPkinases differentially influence cellulase regulation in light and darkness and that the Hog1 homologue TMK3, but not TMK1 or TMK2 are required for the chemotropic response to glucose in T. reesei. Additionally, MAPkinases regulate production of specific secondary metabolites including trichodimerol and bisorbibutenolid, a bioactive compound with cytostatic effect on cancer cells and deterrent effect on larvae, under conditions facilitating mating, which reflects a defect in chemical communication. Strains lacking either of the MAPkinases become female sterile, indicating the conservation of the role of MAPkinases in sexual fertility also in T. reesei. In summary, our findings substantiate the previously detected interconnection of cellulase regulation with regulation of secondary metabolism as well as the involvement of MAPkinases in light dependent gene regulation of cellulase and secondary metabolite genes in fungi.

Identification of a Bidirectional Promoter from Trichoderma reesei and Its Application in Dual Gene Expression

The cellulolytic filamentous fungus Trichoderma reesei has a strong capability in protein synthesis and secretion and is increasingly used as a fungal chassis for the production of heterologous proteins or secondary metabolites. However, bidirectional promoters that would significantly facilitate multiple genes’ expression have not been characterized in T. reesei. Herein, we show that a 767-bp intergenic region between two polyketide synthase encoding genes that were involved in the biosynthesis of the typical yellow pigment served as a bidirectional promoter in T. reesei. This region was shown to be able to drive the simultaneous expression of two fluorescence reporter genes when fused to each end. Quantitative RT-PCR analysis demonstrated that the driving strength of this bidirectional promoter from each direction reached about half of that of the commonly used promoter PgpdA. Moreover, the co-expression of two cellulase genes driven by this bidirectional promoter enabled T. reesei to produce cellulases on glucose and improved the total cellulase activities with cellulose Avicel as the carbon source. Our work identified the first bidirectional promoter in T. reesei, which would facilitate gene co-expression and find applications in synthetic biology using fungal systems.

A histone H3K9 methyltransferase Dim5 mediates repression of sorbicillinoid biosynthesis in Trichoderma reesei

Sorbicillinoids (also termed yellow pigment) are derived from either marine or terrestrial fungi, exhibit various biological activities and therefore show potential as commercial products for human or animal health. The cellulolytic filamentous fungus Trichoderma reesei is capable to biosynthesize sorbicillinoids, but the underlying regulatory mechanism is not yet completely clear. Herein, we identified a histone H3 lysine 9 (H3K9) methyltransferase, Dim5, in T. reesei. TrDIM5 deletion caused an impaired vegetative growth as well as conidiation, whereas the ∆Trdim5 strain displayed a remarkable increase in sorbicillinoid production. Post TrDIM5 deletion, the transcription of sorbicillinoid biosynthesis‐related (SOR) genes was significantly upregulated with a more open chromatin structure. Intriguingly, hardly any expression changes occurred amongst those genes located on both flanks of the SOR gene cluster. In addition, the assays provided evidence that H3K9 triple methylation (H3K9me3) modification acted as a repressive marker at the SOR gene cluster and thus directly mediated the repression of sorbicillinoid biosynthesis. Transcription factor Ypr1 activated the SOR gene cluster by antagonizing TrDim5‐mediated repression and therefore contributed to forming a relatively more open local chromatin environment, which further facilitated its binding and SOR gene expression. The results of this study will contribute to understanding the intricate regulatory network in sorbicillinoid biosynthesis and facilitate the endowment of T. reesei with preferred features for sorbicillinoid production by genetic engineering.

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.

Heterologous Expression of Secondary Metabolite Genes in Trichoderma reesei for Waste Valorization

Trichoderma reesei (Hypocrea jecorina) was developed as a microbial cell factory for the heterologous expression of fungal secondary metabolites. This was achieved by inactivation of sorbicillinoid biosynthesis and construction of vectors for the rapid cloning and expression of heterologous fungal biosynthetic genes. Two types of megasynth(et)ases were used to test the strain and vectors, namely a non-reducing polyketide synthase (nr-PKS, aspks1) from Acremonium strictum and a hybrid highly-reducing PKS non-ribosomal peptide synthetase (hr-PKS-NRPS, tenS + tenC) from Beauveria bassiana. The resulting engineered T. reesei strains were able to produce the expected natural products 3-methylorcinaldehyde and pretenellin A on waste materials including potato, orange, banana and kiwi peels and barley straw. Developing T. reesei as a heterologous host for secondary metabolite production represents a new method for waste valorization by the direct conversion of waste biomass into secondary metabolites.

From induction to secretion: a complicated route for cellulase production in Trichoderma reesei

The filamentous fungus Trichoderma reesei has been widely used for cellulase production that has extensive applications in green and sustainable development. Increasing costs and depletion of fossil fuels provoke the demand for hyper-cellulase production in this cellulolytic fungus. To better manipulate T. reesei for enhanced cellulase production and to lower the cost for large-scale fermentation, it is wise to have a comprehensive understanding of the crucial factors and complicated biological network of cellulase production that could provide new perspectives for further exploration and modification. In this review, we summarize recent progress and give an overview of the cellular process of cellulase production in T. reesei, including the carbon source-dependent cellulase induction, complicated transcriptional regulation network, and efficient protein assembly and trafficking. Among that, the key factors involved in cellulase production were emphasized, shedding light on potential perspectives for further engineering.

Dual Regulatory Role of Chromatin Remodeler ISW1 in Coordinating Cellulase and Secondary Metabolite Biosynthesis in Trichoderma reesei

The saprophytic filamentous fungus Trichoderma reesei represents one of the most prolific cellulase producers isolated from nature. T. reesei also produces a typical yellow pigment identified as sorbicillinoids during cultivation. Here, we identified an evolutionarily conserved histone remodeling factor, ISW1, in T. reesei that simultaneously participates in regulating cellulase and the yellow pigment biosynthesis. Trisw1 deletion almost abolished vegetable growth, asexual spore formation, and cellulase gene expression. However, its absence significantly enhanced the production of the yellow pigment. The observed dual regulatory role of TrISW1 was dependent on its ATPase activity. We demonstrated that Trisw1 disruption elevated the transcription of ypr1 coding for the transcriptional activator of sor genes encoding the polyketide synthases catalyzing the biosynthesis of sorbicillinoids but compromised that of xyr1 encoding the key transcriptional activator of cellulase genes. Discrete T. reesei homologous ISW1 accessory factors were also found to exert differential effects on the expression of these two types of genes. Further analyses showed that TrISW1 was recruited to cellulase gene promoters, and its absence interfered with loss of histone H4 at the cbh1 and eg1 promoters upon cellulose induction. To the contrary, Trisw1 deletion facilitated loss of H4 at the sor locus. These data indicate that TrISW1 represents an important chromatin remodeler with a dual role in coordinating the cellulolytic response and biosynthesis of the major secondary metabolite in T. reesei. IMPORTANCE Microorganisms, including Trichoderma reesei, constantly face the challenge to outcompete other species to ensure efficient colonization in their natural habitat. They achieve this usually by adopting two alternative strategies by either maintaining fast growth on limited nutrient resources or producing a versatile array of secondary metabolites to fight against competitors. These two strategies, however, have to be subtly controlled to balance the assignment of and thus make the best use of cellular resources. Here, we identified a chromatin remodeling factor, TrISW1, with a dual role in coordinating the cellulolytic response and biosynthesis of the major secondary metabolite in T. reesei. The data also provide a novel insight into how T. reesei takes advantage of a chromatin remodeler to exquisitely balance two different adaptive strategies to ensure an efficient allocation of cellular resources to achieve efficient colonization in a specific environment.