Involvement of an SRF-MADS protein McmA in regulation of extracellular enzyme production and asexual/sexual development in Aspergillus nidulans

Mutations in Podospora anserina MCM1 and VelC Trigger Spontaneous Development of Barren Fruiting Bodies

The ascomycete Podospora anserina is a heterothallic filamentous fungus found mainly on herbivore dung. It is commonly used in laboratories as a model system, and its complete life cycle lasting eight days is well mastered in vitro. The main objective of our team is to understand better the global process of fruiting body development, named perithecia, induced normally in this species by fertilization. Three allelic mutants, named pfd3, pfd9, and pfd23 (for "promoting fruiting body development") obtained by UV mutagenesis, were selected in view of their abilities to promote barren perithecium development without fertilization. By complete genome sequencing of pfd3 and pfd9, and mutant complementation, we identified point mutations in the mcm1 gene as responsible for spontaneous perithecium development. MCM1 proteins are MADS box transcription factors that control diverse developmental processes in plants, metazoans, and fungi. We also identified using the same methods a mutation in the VelC gene as responsible for spontaneous perithecium development in the vacua mutant. The VelC protein belongs to the velvet family of regulators involved in the control of development and secondary metabolite production. A key role of MCM1 and VelC in coordinating the development of P. anserina perithecia with gamete formation and fertilization is highlighted.

Novel Transcription Factor CXRD Regulates Cellulase and Xylanase Biosynthesis in Penicillium oxalicum under Solid-State Fermentation

Penicillium oxalicum produces an integrated, extracellular cellulase and xylanase system, strictly regulated by several transcription factors. However, the understanding of the regulatory mechanism of cellulase and xylanase biosynthesis in P. oxalicum is limited, particularly under solid-state fermentation (SSF) conditions. In our study, deletion of a novel gene, cxrD (cellulolytic and xylanolytic regulator D), resulted in 49.3 to 2,230% enhanced production of cellulase and xylanase, except for 75.0% less xylanase at 2 days, compared with the P. oxalicum parental strain, when cultured on solid medium containing wheat bran plus rice straw for 2 to 4 days after transfer from glucose. In addition, the deletion of cxrD delayed conidiospore formation, leading to 45.1 to 81.8% reduced asexual spore production and altered mycelial accumulation to various extents. Comparative transcriptomics and real-time quantitative reverse transcription-PCR found that CXRD dynamically regulated the expression of major cellulase and xylanase genes and conidiation-regulatory gene brlA under SSF. In vitro electrophoretic mobility shift assays demonstrated that CXRD bound to the promoter regions of these genes. The core DNA sequence 5'-CYGTSW-3' was identified to be specifically bound by CXRD. These findings will contribute to understanding the molecular mechanism of negative regulation of fungal cellulase and xylanase biosynthesis under SSF. IMPORTANCE Application of plant cell wall-degrading enzymes (CWDEs) as catalysts in biorefining of lignocellulosic biomass into bioproducts and biofuels reduces both chemical waste production and carbon footprint. The filamentous fungus Penicillium oxalicum can secrete integrated CWDEs, with potential for industrial application. Solid-state fermentation (SSF), simulating the natural habitat of soil fungi, such as P. oxalicum, is used for CWDE production, but a limited understanding of CWDE biosynthesis hampers the improvement of CWDE yields through synthetic biology. Here, we identified a novel transcription factor CXRD, which negatively regulates the biosynthesis of cellulase and xylanase in P. oxalicum under SSF, providing a potential target for genetic engineering to improve CWDE production.

The Role of Chromatin and Transcriptional Control in the Formation of Sexual Fruiting Bodies in Fungi

Fungal fruiting bodies are complex, three-dimensional structures that arise from a less complex vegetative mycelium. Their formation requires the coordinated action of many genes and their gene products, and fruiting body formation is accompanied by major changes in the transcriptome. In recent years, numerous transcription factor genes as well as chromatin modifier genes that play a role in fruiting body morphogenesis were identified, and through research on several model organisms, the underlying regulatory networks that integrate chromatin structure, gene expression, and cell differentiation are becoming clearer. This review gives a summary of the current state of research on the role of transcriptional control and chromatin structure in fruiting body development. In the first part, insights from transcriptomics analyses are described, with a focus on comparative transcriptomics. In the second part, examples of more detailed functional characterizations of the role of chromatin modifiers and/or transcription factors in several model organisms (Neurospora crassa, Aspergillus nidulans, Sordaria macrospora, Coprinopsis cinerea, and Schizophyllum commune) that have led to a better understanding of regulatory networks at the level of chromatin structure and transcription are discussed.

Drosophila attack inhibits hyphal regeneration and defense mechanisms activation for the fungus Trichoderma atroviride

The capability to respond to wounding is a process shared by organisms of different kingdoms that can result in the regeneration of whole-body parts or lost structures or organs. Filamentous fungi constitute a rich food source that ensures survival and reproduction of their predators and are therefore continuously exposed to mechanical damage. Nevertheless, our understanding of how fungi respond to wounding and predators is scarce. Fungi like plants and animals respond to injury recognizing Damage- and Microbe-Associated Molecular Patterns (DAMPs/MAMPs) that activate Ca²⁺ and Mitogen-Activated Protein Kinase dependent signaling for the activation of defense mechanisms. During herbivory, plants, in addition to activating pathways related to injury, activate specific responses to combat their predators. Using a transcriptional approach, we studied the capacity of the filamentous fungus Trichoderma atroviride to activate specific responses to injury and attack by different arthropods. Attack by Drosophila melanogaster inhibited the transcriptional activation of genes required for hyphal regeneration, and the fungal innate immune and chemical defense responses. We also provide mechanistic insight of this inhibition involving components of the D. melanogaster salivary glands that repress the expression of a set of genes and block hyphal regeneration.

A mitogen-activated protein kinase PoxMK1 mediates regulation of the production of plant-biomass-degrading enzymes, vegetative growth, and pigment biosynthesis in Penicillium oxalicum

Mitogen-activated protein kinase (MAPK) cascades are broadly conserved and play essential roles in multiple cellular processes, including fungal development, pathogenicity, and secondary metabolism. Their function, however, also exhibits species and strain specificity. Penicillium oxalicum secretes plant-biomass-degrading enzymes (PBDEs) that contribute to the carbon cycle in the natural environment and to utilization of lignocellulose in industrial processes. However, knowledge of the MAPK pathway in P. oxalicum has been relatively limited. In this study, comparative transcriptomic analysis of P. oxalicum, cultured on different carbon sources, found ten putative kinase genes with significantly modified transcriptional levels. Six of these putative kinase genes were knocked out in the parental strain ∆PoxKu70, and deletion of the gene, Fus3/Kss1-like PoxMK1 (POX00158), resulted in the largest reduction (91.1%) in filter paper cellulase production. Further tests revealed that the mutant ∆PoxMK1 lost 37.1 to 92.2% of PBDE production, under both submerged- and solid-state fermentation conditions, compared with ∆PoxKu70. In addition, the mutant ∆PoxMK1 had reduced vegetative growth and increased pigment biosynthesis. Comparative transcriptomic analysis showed that PoxMK1 deletion from P. oxalicum downregulated the expression of major PBDE genes and known regulatory genes such as PoxClrB and PoxCxrB, whereas the transcription of pigment biosynthesis-related genes was upregulated. Comparative phosphoproteomic analysis revealed that PoxMK1 deletion considerably modified phosphorylation of key transcription- and signal transduction-associated proteins, including transcription factors Mcm1 and Atf1, RNA polymerase II subunits Rpb1 and Rpb9, MAPK-associated Hog1 and Ste7, and cyclin-dependent kinase Kin28. These findings provide novel insights into understanding signal transduction and regulation of PBDE gene expression in fungi.Key points• PoxMK1 is involved in expression of PBDE- and pigment synthesis-related genes.• PoxMK1 is required for vegetative growth of P. oxalicum.• PoxMK1 is involved in phosphorylation of key TFs, kinases, and RNA polymerase II.

Regulators of plant biomass degradation in ascomycetous fungi

Fungi play a major role in the global carbon cycle because of their ability to utilize plant biomass (polysaccharides, proteins, and lignin) as carbon source. Due to the complexity and heterogenic composition of plant biomass, fungi need to produce a broad range of degrading enzymes, matching the composition of (part of) the prevalent substrate. This process is dependent on a network of regulators that not only control the extracellular enzymes that degrade the biomass, but also the metabolic pathways needed to metabolize the resulting monomers. This review will summarize the current knowledge on regulation of plant biomass utilization in fungi and compare the differences between fungal species, focusing in particular on the presence or absence of the regulators involved in this process.

McmA-dependent and -independent regulatory systems governing expression of ClrB-regulated cellulase and hemicellulase genes in Aspergillus nidulans

Fungal cellulolytic and hemicellulolytic enzymes are promising tools for industrial hydrolysis of cellulosic biomass; however, the regulatory network underlying their production is not well understood. The recent discovery of the transcriptional activators ClrB and McmA in Aspergillus nidulans implied a novel regulatory mechanism driven by their interaction, experimental evidence for which was obtained from transcriptional and DNA-binding analyses in this study. It was found that ClrB was essential for induced expression of all the genes examined in this study, while McmA dependency of their expression was gene-dependent. DNA-binding studies revealed McmA assisted in the recruitment of ClrB to the cellulose-responsive element (CeRE) in the promoters of eglA and eglB, expression of which was significantly reduced in the mcmA mutant. The CCG triplet within the CeRE served as the recognition sequence for the ClrB monomer. In contrast, ClrB did not require McmA for binding as a homodimer to the CGGN₈ CCG sequences in the promoter of mndB, expression of which was affected less in the mcmA mutant than in all other examined genes. Thus, there are two types of ClrB-mediated regulation: McmA-assisted and McmA-independent. This novel McmA-ClrB synergistic system provides new insights into the complex regulatory network involved in cellulase and hemicellulase production.