Characterization of the asexual developmental genes brlA and wetA in Monascus ruber M7

Effects of MrwetA on Sexual Reproduction and Secondary Metabolism of Monascus ruber M7 Based on Transcriptome Analysis

wetA, one of the conidiation center regulatory genes in many filamentous fungi, plays an important role in promoting asexual spores (conidia) maturation. Our recent research has found that knocking out or overexpressing MrwetA (a homolog of wetA) in Monascus ruber M7 does not affect the development of its asexual spores like other fungi, but both repress the development of its sexual spores (ascospores). However, the mechanism remains unclear. In this study, the function of MrwetA on sexual reproduction and secondary metabolism in M. ruber M7 was confirmed by a complementary experiment. Moreover, the regulatory roles of MrwetA in modulating the expression of genes involved in sexual reproduction, meiosis, and biosynthesis of Monascus pigment and citrinin were analyzed based on the transcriptional data. These results not only contribute to clarifying the regulation of the reproduction and secondary metabolism of Monascus spp., but also to enriching the regulation molecular mechanism of reproduction in filamentous fungi.

Strategies for the Enhancement of Secondary Metabolite Production via Biosynthesis Gene Cluster Regulation in Aspergillus oryzae

The filamentous fungus Aspergillus oryzae (A. oryzae) has been extensively used for the biosynthesis of numerous secondary metabolites with significant applications in agriculture and food and medical industries, among others. However, the identification and functional prediction of metabolites through genome mining in A. oryzae are hindered by the complex regulatory mechanisms of secondary metabolite biosynthesis and the inactivity of most of the biosynthetic gene clusters involved. The global regulatory factors, pathway-specific regulatory factors, epigenetics, and environmental signals significantly impact the production of secondary metabolites, indicating that appropriate gene-level modulations are expected to promote the biosynthesis of secondary metabolites in A. oryzae. This review mainly focuses on illuminating the molecular regulatory mechanisms for the activation of potentially unexpressed pathways, possibly revealing the effects of transcriptional, epigenetic, and environmental signal regulation. By gaining a comprehensive understanding of the regulatory mechanisms of secondary metabolite biosynthesis, strategies can be developed to enhance the production and utilization of these metabolites, and potential functions can be fully exploited.

Histone lysine methyltransferases MpDot1 and MpSet9 are involved in the production of lovastatin and MonAzPs by histone crosstalk modification

Increasing data suggested that histone methylation modification plays an important role in regulating biosynthesis of secondary metabolites (SMs). Monascus spp. have been applied to produce hypolipidemic drug lovastatin (also called monacolin K, MK) and edible Monascus-type azaphilone pigments (MonAzPs). However, little is known about how histone methylation regulates MK and MonAzPs. In this study, we constructed H3K9 methyltransferase deletion strain ΔMpDot1 and H4K20 methyltransferase deletion strain ΔMpSet9 using Monascus pilosus MS-1 as the parent. The result showed that deletion of MpDot1 reduced the production of MK and MonAzPs, and deletion of MpSet9 increased MonAzPs production. Real-time quantitative PCR (RT-qPCR) showed inactivation of mpdot1 and mpset9 disturbed the expression of genes responsible for the biosynthesis of MK and MonAzPs. Western blot suggested that deletion of MpDot1 reduced H3K79me and H4K16ac, and deletion of MpSet9 decreased H4K20me3 and increased H4pan acetylation. Chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) showed ΔMpDot1 strain and ΔMpSet9 strain reduced the enrichment of H3K79me2 and H4K20me3 in the promoter regions of key genes for MK and MonAzPs biosynthesis, respectively. These results suggested that MpDot1 and MpSet9 affected the synthesis of SMs by regulating gene transcription and histone crosstalk, providing alternative approach for regulation of lovastatin and MonAzPs.

AoMedA has a complex regulatory relationship with AoBrlA, AoAbaA, and AoWetA in conidiation, trap formation, and secondary metabolism in the nematode-trapping fungus Arthrobotrys oligospora

The asexual sporulation of filamentous fungi is an important mechanism for their reproduction, survival, and pathogenicity. In Aspergillus and several filamentous fungi, BrlA, AbaA, and WetA are the key elements of a central regulatory pathway controlling conidiation, and MedA is a developmental modifier that regulates temporal expression of central regulatory genes; however, their roles are largely unknown in nematode-trapping (NT) fungi. Arthrobotrys oligospora is a representative NT fungus, which can capture nematodes by producing adhesive networks (traps). Here, we characterized the function of AoMedA and three central developmental regulators (AoBrlA, AoAbaA, and AoWetA) in A. oligospora by gene disruption, phenotypic comparison, and multi-omics analyses, as these regulators are required for conidiation and play divergent roles in mycelial development, trap formation, lipid droplet accumulation, vacuole assembly, and secondary metabolism. A combined analysis of phenotypic traits and transcriptome showed that AoMedA and AoWetA are involved in the regulation of peroxisome, endocytosis, and autophagy. Moreover, yeast one-hybrid analysis showed that AoBrlA can regulate AoMedA, AoAbaA, and AoWetA, whereas AoMedA and AoAbaA can regulate AoWetA. Our results highlight the important roles of AoMedA, AoBrlA, AoAbaA, and AoWetA in conidiation, mycelia development, trap formation, and pathogenicity of A. oligospora and provide a basis for elucidating the relationship between conidiation and trap formation of NT fungi. IMPORTANCE Conidiation is the most common reproductive mode for many filamentous fungi and plays an essential role in the pathogenicity of fungal pathogens. Nematode-trapping (NT) fungi are a special group of filamentous fungi owing to their innate abilities to capture and digest nematodes by producing traps (trapping devices). Sporulation plays an important role in the growth and reproduction of NT fungi, and conidia are the basic components of biocontrol reagents for controlling diseases caused by plant-parasitic nematodes. Arthrobotrys oligospora is a well-known NT fungus and is a routinely used model fungus for probing the interaction between fungi and nematodes. In this study, the functions of four key regulators (AoMedA, AoBrlA, AoAbaA, and AoWetA) involved in conidiation were characterized in A. oligospora. A complex interaction between AoMedA and three central regulators was noted; these regulators are required for conidiation and trap formation and play a pleiotropic role in multiple intracellular activities. Our study first revealed the role of AoMedA and three central regulators in conidiation, trap formation, and pathogenicity of A. oligospora, which contributed to elucidating the regulatory mechanism of conidiation in NT fungi and helped in developing effective reagents for biocontrol of nematodes.

Effect of Exogenous and Endogenous Ectoine on Monascus Development, Metabolism, and Pigment Stability

Monascus, a key player in fermented food production, is known for generating Monascus pigments (MPs) and monacolin K (MK), possessing bioactive properties. However, the limited stability of MPs and mycotoxin citrinin (CTN) constrain the Monascus industry. Extremolytes like ectoine, derived from bacteria, exhibit cytoprotective potential. Here, we investigated the impact of ectoine on Monascus purpureus ATCC 16365, emphasizing development and secondary metabolism. Exogenous 5 mM ectoine supplementation substantially increased the yields of MPs and MK (105%-150%) and reduced CTN production. Ectoine influenced mycelial growth, spore development, and gene expression in Monascus. Remarkably, ectoine biosynthesis was achieved in Monascus, showing comparable effects to exogenous addition. Notably, endogenous ectoine effectively enhanced the stability of MPs under diverse stress conditions. Our findings propose an innovative strategy for augmenting the production and stability of bioactive compounds while reducing CTN levels, advancing the Monascus industry.

Histone Acetyltransferase Rtt109 Regulates Development, Morphogenesis, and Citrinin Biosynthesis in Monascus purpureus

Histone acetyltransferase (HAT) has been reported to be pivotal for various physiological processes in many fungi. However, the functions that HAT Rtt109 perform in edible fungi Monascus and the underlying mechanism remains unclear. Here, we identified the rtt109 gene in Monascus, constructed the rtt109 knockout strain (Δrtt109) and its complementary strain (Δrtt109:com) by CRISPR/Cas9 methods, and functionally characterized the roles that Rtt109 play in Monascus. Deletion of rtt109 significantly reduced conidia formation and colony growth, whereas, it increased the yield of Monascus pigments (MPs) and citrinin (CTN). Further real-time quantitative PCR (RT-qPCR) analysis indicated that Rtt109 remarkably affected the transcriptional expression of key genes related to development, morphogenesis, and secondary metabolism of Monascus. Together, our results revealed the critical roles of HAT Rtt109 in Monascus, and enriched our current knowledge of the development and regulation of secondary metabolism in fungi, throwing light on restraining or eliminating citrinin in the development and industrial applications of Monascus.

Essential Role of WetA, but No Role of VosA, in Asexual Development, Conidial Maturation and Insect Pathogenicity of Metarhizium robertsii

Conidial maturation, which is crucial for conidial quality, is controlled by the asexual development activator WetA and the downstream, velvety protein VosA in Aspergillus. Their orthologs have proved functional in conidial quality control of Beauveria bassiana, as seen in Aspergillus, but are functionally unexplored, in Metarhizium robertsii, another hypocrealean insect pathogen. Here, WetA and VosA prove essential and nonessential for M. robertsii's life cycle, respectively. Disruption of wetA increased hyphal sensitivity to oxidative stress and Congo red-induced cell wall stress, but had little impact on radial growth. The ΔwetA mutant was severely compromised in conidiation capacity and conidial quality, which was featured by slower germination, decreased UV resistance, reduced hydrophobicity, and deformed hydrophobin rodlet bundles that were assembled onto conidial coat. The mutant's virulence was greatly attenuated via normal infection due to a blockage of infection-required cellular processes. All examined phenotypes were unaffected for the ΔvosA mutant. Intriguingly, mannitol was much less accumulated in the 7- and 15-day-old cultures of ΔwetA and ΔvosA than of control strains, while accumulated trehalose was not detectable at all, revealing little a link of intracellular polyol accumulation to conidial maturation. Transcriptomic analysis revealed differential regulation of 160 genes (up/down ratio: 72:88) in ΔwetA. These genes were mostly involved in cellular component, biological process, and molecular function but rarely associated with asexual development. Conclusively, WetA plays a relatively conserved role in M. robertsii's spore surface structure, and also a differentiated role in some other cellular processes associated with conidial maturation. VosA is functionally redundant in M. robertsii unlike its ortholog in B. bassiana. IMPORTANCE WetA and VosA regulate conidiation and conidial maturation required for the life cycle of Beauveria bassiana, like they do in Aspergillus, but remain functionally unexplored in Metarhizium robertsii, another hypocrealean pathogen considered to have evolved insect pathogenicity ~130 million years later than B. bassiana. This study reveals a similar role of WetA ortholog in asexual development, conidial maturation, and insect pathogenicity, and also its distinctive role in mediating some other conidial maturation-related cellular events, but has functional redundancy of VosA in M. robertsii. The maturation process vital for conidial quality proves dependent on a role of WetA in spore wall assembly but is independent of its role in intracellular polyol accumulation. Transcriptomic analysis reveals a link of WetA to 160 genes involved in cellular component, biological process, and molecular function. Our study unveils that M. robertsii WetA or VosA is functionally differential or different from those learned in B. bassiana and other ascomycetes.

Recent Advances in Chitin Biosynthesis Associated with the Morphology and Secondary Metabolite Synthesis of Filamentous Fungi in Submerged Fermentation

Metabolites produced by filamentous fungi are used extensively in the food and drug industries. With the development of the morphological engineering of filamentous fungi, numerous biotechnologies have been applied to alter the morphology of fungal mycelia and enhance the yields and productivity of target metabolites during submerged fermentation. Disruption of chitin biosynthesis can modify the cell growth and mycelial morphology of filamentous fungi and regulate the biosynthesis of metabolites during submerged fermentation. In this review, we present a comprehensive coverage of the categories and structures of the enzyme chitin synthase, chitin biosynthetic pathways, and the association between chitin biosynthesis and cell growth and metabolism in filamentous fungi. Through this review, we hope to increase awareness of the metabolic engineering of filamentous fungal morphology, provide insights into the molecular mechanisms of morphological control via chitin biosynthesis, and describe strategies for the application of morphological engineering to enhance the production of target metabolites in filamentous fungi during submerged fermentation.

SNARE Protein AoSec22 Orchestrates Mycelial Growth, Vacuole Assembly, Trap Formation, Stress Response, and Secondary Metabolism in Arthrobotrys oligospora

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) facilitate intracellular vesicle trafficking and membrane fusion in eukaryotes and play a vital role in fungal growth, development, and pathogenicity. However, the functions of SNAREs are still largely unknown in nematode-trapping fungi. Arthrobotrys oligospora is a representative species of nematode-trapping fungi that can produce adhesive networks (traps) for nematode predation. In this study, we characterized AoSec22 in A. oligospora, a homolog of the yeast SNARE protein Sec22. Deletion of Aosec22 resulted in remarkable reductions in mycelial growth, the number of nuclei, conidia yield, and trap formation, especially for traps that failed to develop mature three-dimensional networks. Further, absence of Aosec22 impaired fatty acid utilization, autophagy, and stress tolerance; in addition, the vacuoles became small and fragmented in the hyphal cells of the ∆Aosec22 mutant, and large vacuoles failed to form. The reduced sporulation capacity correlated with the transcriptional repression of several sporulation-related genes, and the impaired accumulation of lipid droplets is in line with the transcriptional repression of several genes involved in fatty acid oxidation. Moreover, absence of Aosec22 remarkably impaired secondary metabolism, resulting in 4717 and 1230 compounds upregulated and downregulated in the ∆Aosec22 mutant, respectively. Collectively, our data highlighted that the SNARE protein AoSec22 plays a pleiotropic role in mycelial growth and development, vacuole assembly, lipid metabolism, stress response, and secondary metabolism; in particular, it is required for the proper development of traps in A. oligospora.

Disruption of the Chitin Biosynthetic Pathway Results in Significant Changes in the Cell Growth Phenotypes and Biosynthesis of Secondary Metabolites of Monascus purpureus

In this study, the gene monascus-5162 from Monascus purpureus LQ-6, identified as chitin synthase gene VI (chs6), was knocked out to disrupt the chitin biosynthetic pathway and regulate the biosynthesis of Monascus pigments (MPs) and citrinin. The results showed that the aerial hyphae on a solid medium were short and sparse after the deletion of chs6 in M. purpureus LQ-6, significantly reducing the germination percentage of active spores to approximately 22%, but the colony diameter was almost unaffected. Additionally, the deletion of chs6 changed the mycelial morphology of M. purpureus LQ-6 during submerged fermentation and increased its sensitivity to environmental factors. MP and citrinin biosynthesis was dramatically inhibited in the recombinant strain. Furthermore, comparative transcriptome analysis revealed that the pathways related to spore development and growth, including the MAPK signaling pathway, chitin biosynthetic pathway, and regulatory factors LaeA and WetA genes, were significantly downregulated in the early phase of fermentation. The mRNA expression levels of genes in the cluster of secondary metabolites were significantly downregulated, especially those related to citrinin biosynthesis. This is the first detailed study to reveal that chs6 plays a vital role in regulating the cell growth and secondary metabolism of the Monascus genus.

Inactivation of MrSir2 in Monascus ruber Influenced the Developmental Process and the Production of Monascus Azaphilone Pigments

Monascus species are the producers of Monascus azaphilone pigments (MonAzPs) and lipid-lowering component Monacolin K, which have been widely used as food colorant and health products. In this study, silent information regulator 2 (Sir2) homolog (MrSir2) was characterized, and its impacts on the development and MonAzPs production of Monascus ruber were evaluated. Enzyme activity test in vitro showed that MrSir2 was an NAD⁺-dependent histone deacetylase. Compared to WT, Δmrsir2 strain accumulated more acetylated lysine residues of histone H3 subunit during its vegetative growth phase, and it exhibited accelerated mycelial aging, more spores, increased resistance to oxidative stress, and more MonAzPs production. RNA-Seq-based transcriptome analysis revealed that MrSir2 mainly regulated the gene expression in macromolecular metabolism such as carbohydrates, proteins, and nucleotides, as well as genes encoding cell wall synthesis and cell membrane component, indicating that MrSir2 probably facilitates the metabolic transition from the primary growth phase to the mycelial aging. Taken together, MrSir2 mainly targets H3 subunit at the vegetative growth phase and affects the development of M. ruber and MonAzPs production.

Mrada3 is required for sexual reproduction and secondary metabolite production in industrial fungi Monascus strain

AIMS

Monascus spp. are valuable industrial fungi for producing beneficial compounds. Since sporulation is often coupled with the production of secondary metabolites, the current study was performed to investigate how Mrada3 regulated asexual and sexual development and the production of edible pigments and mycotoxin.

METHODS AND RESULTS

The functional characteristics of Mrada3 were identified by gene deletion and overexpression in Monascus ruber M7 (the wild-type, WT). The results revealed that the ΔMrada3 strain aborted sexual development, but it produced many more conidia than WT. RNA-Seq data showed the deletion of Mrada3 altered the expression levels of partial genes involved in sexual and asexual development. In addition, the deletion of Mrada3 also resulted in slower growth, lower pigment production, and increased citrinin yield at the late period. For the Mrada3-overexpressed strain, the number of ascospores and pigment content were significantly higher than those of WT, but citrinin was slightly lower than that of WT.

CONCLUSIONS

The Mrada3 gene plays a vital role in the sporulation development and secondary metabolism of Monascus species.

SIGNIFICANCE AND IMPACT OF THE STUDY

Mrada3 is first identified as an essential regulator for sexual development in Monascus species, enriching the regulatory knowledge of sexual development in filamentous fungi.

Effect of γ-Heptalactone on the Morphology and Production of Monascus Pigments and Monacolin K in Monascus purpureus

Monascus is used widely in Asian countries and produces various biologically active metabolites, such as Monascus pigments (MPs) and monacolin K (MK). In this study, the effect of γ-heptalactone on secondary metabolites and mycelial growth during Monascus purpureus M1 fermentation was investigated. After the addition of 50 μM γ-heptalactone, the yields of MPs (yellow, orange, and red) reached maxima, increased by 115.70, 141.52, and 100.88%, respectively. The 25 μM γ-heptalactone groups showed the highest yield of MK was increased by 62.38% compared with that of the control. Gene expression analysis showed that the relative expression levels of MPs synthesis genes (MpPKS5, MpFasA2, mppB, mppC, mppD, mppG, mpp7, and mppR1/R2) were significantly upregulated after γ-heptalactone treatment. The relative expression levels of MK synthesis genes (mokA, mokC, mokE, mokH, and mokI) were significantly affected. The mycelium samples treated with γ-heptalactone exhibited more folds and swelling than that in the samples of the control group. This study confirmed that the addition of γ-heptalactone has the potential to induce yields of MPs and MK, and promote the expression of biosynthesis genes, which may be related to the transformation of mycelial morphology in M. purpureus.

Inactivation of mrpigH Gene in Monascus ruber M7 Results in Increased Monascus Pigments and Decreased Citrinin with mrpyrG Selection Marker

Monascus pigments (MPs) have been used as food colorants for several centuries in Asian countries and are currently used around the world via Asian catering. The MPs biosynthetic pathway has been well-illustrated; however, the functions of a few genes including mrpigH in the MPs gene cluster of M. ruber M7 are still unclear. In the current study, mrpigH was disrupted in Δmrlig4ΔmrpyrG, a highly efficient gene modification system, using mrpyrG as a selection marker, and ΔmrpigHΔmrlig4ΔmrpyrG::mrpyrG and ΔmrpigHΔmrlig4ΔmrpyrG have been obtained. Subsequently, their morphologies, biomasses, MPs and citrinin (CIT) production were analyzed, respectively. These results have revealed that the deletion of mrpigH has significant effects on the morphology and growth of M. ruber M7. Moreover, compared with M. ruber M7, the yields of MPs and CIT were drastically increased and decreased in mrpigH mutants, respectively.