Regulators of the Asexual Life Cycle of Aspergillus nidulans

STRIPAK, a fundamental signaling hub of eukaryotic development

SUMMARYThe striatin-interacting phosphatase and kinase (STRIPAK) complex is involved in the regulation of many developmental processes in eukaryotic microorganisms and all animals, including humans. STRIPAK is a component of protein phosphatase 2A (PP2A), a highly conserved serine-threonine phosphatase composed of catalytic subunits (PP2Ac), a scaffolding subunit (PP2AA) and various substrate-directing B regulatory subunits. In particular, the B''' regulatory subunit called striatin has evoked major interest over the last 20 years. Studies in fungal systems have contributed substantially to our current knowledge about STRIPAK composition, assembly, and cellular localization, as well as its regulatory role in autophagy and the morphology of fungal development. STRIPAK represents a signaling hub with many kinases and thus integrates upstream and downstream information from many conserved eukaryotic signaling pathways. A profound understanding of STRIPAK's regulatory role in fungi opens the gateway to understanding the multifarious functions carried out by STRIPAK in higher eukaryotes, including its contribution to malignant cell growth.

Transcriptomic and genetic analysis reveals a Zn2Cys6 transcription factor specifically required for conidiation in submerged cultures of Thermothelomyces thermophilus

Filamentous fungi are important producers of enzymes and secondary metabolites. The industrial thermophilic species, Thermothelomyces thermophilus, is closely related to the model fungus, Neurospora crassa. A critical aspect of the filamentous fungal life cycle is the production of asexual spores (conidia), which are regulated by various stimuli, including nutrient availability. Several species of fungi, including T. thermophilus, produce conidia under submerged fermentation conditions, which can be detrimental to product yields. In this study, transcriptional profiling of T. thermophilus was used to map changes during asexual development in submerged cultures, which revealed commonalities of regulation between T. thermophilus and N. crassa. We further identified a transcription factor, res1, whose deletion resulted in a complete loss of conidia production under fermentation conditions, but which did not affect conidiation on plates. Under fermentation conditions, the ∆res1 deletion strain showed increased biomass production relative to the wild-type strain, indicating that the manipulation of res1 in T. thermophilus has the potential to increase productivity in industrial settings. Overexpression of res1 caused a severe growth defect and early conidia production on both plates and in submerged cultures, indicating res1 overexpression can bypass regulatory aspects associated with conidiation on plates. Using chromatin-immunoprecipitation sequencing, we identified 35 target genes of Res1, including known conidiation regulators identified in N. crassa, revealing common and divergent aspects of asexual reproduction in these two species.IMPORTANCEFilamentous fungi, such as Thermothelomyces thermophilus, are important industrial species and have been harnessed in the Biotechnology industry for the production of industrially relevant chemicals and proteins. However, under fermentation conditions, some filamentous fungi will undergo a switch from mycelial growth to asexual development. In this study, we use transcriptional profiling of asexual development in T. thermophilus and identify a transcription factor that specifically regulates the developmental switch to the production of unwanted asexual propagules under fermentation conditions, thus altering secreted protein production. Mutations in this transcription factor Res1 result in the loss of asexual development in submerged cultures but do not affect asexual sporulation when exposed to air. The identification of stage-specific developmental regulation of asexual spore production and comparative analyses of conidiation in filamentous ascomycete species have the potential to further manipulate these species for industrial advantage.

Regulation of Conidiation and Aflatoxin B1 Biosynthesis by a Blue Light Sensor LreA in Aspergillus flavus

Conidia are important for the dispersal of Aspergillus flavus, which usually generates aflatoxin B1 (AFB1) and poses a threat to the safety of agricultural food. The development of conidia is usually susceptible to changes in environmental conditions, such as nutritional status and light. However, how the light signal is involved in the conidiation in A. flavus is still unknown. In this study, LreA was identified to respond to blue light and mediate the promotion of conidiation in A. flavus, which is related to the central development pathway. At the same time, blue light inhibited the biosynthesis of AFB1, which was mediated by LreA and attributed to the transcriptional regulation of aflR and aflS expression. Our findings disclosed the function and mechanism of the blue light sensor LreA in regulating conidiation and AFB1 biosynthesis, which is beneficial for the prevention and control of A. flavus and mycotoxins.

The Cryptochrome CryA Regulates Lipid Droplet Accumulation, Conidiation, and Trap Formation via Responses to Light in Arthrobotrys oligospora

Light is a key environmental factor affecting conidiation in filamentous fungi. The cryptochrome/photolyase CryA, a blue-light receptor, is involved in fungal development. In the present study, a homologous CryA (AoCryA) was identified from the widely occurring nematode-trapping (NT) fungus Arthrobotrys oligospora, and its roles in the mycelial growth and development of A. oligospora were characterized using gene knockout, phenotypic comparison, staining technique, and metabolome analysis. The inactivation of AocryA caused a substantial decrease in spore yields in dark conditions but did not affect spore yields in the wild-type (WT) and ∆AocryA mutant strains in light conditions. Corresponding to the decrease in spore production, the transcription of sporulation-related genes was also significantly downregulated in dark conditions. Contrarily, the ∆AocryA mutants showed a substantial increase in trap formation in dark conditions, while the trap production and nematode-trapping abilities of the WT and mutant strains significantly decreased in light conditions. In addition, lipid droplet accumulation increased in the ∆AocryA mutant in dark conditions, and the mutants showed an increased tolerance to sorbitol, while light contributed to the synthesis of carotenoids. Finally, AoCryA was found to affect secondary metabolic processes. These results reveal, for the first time, the function of a homologous cryptochrome in NT fungi.

Rad6 Regulates Conidiation by Affecting the Biotin Metabolism in Beauveria bassiana

Rad6 is a canonical ubiquitin-conjugating enzyme known for its role in regulating chromosome-related cellular processes in yeast and has been proven to have multiple functions in Beauveria bassiana, including insect-pathogenic lifestyle, UV damage repair, and conidiation. However, previous studies have only reported the key role of Rad6 in regulating conidial production in a nutrient-rich medium, without any deep mechanism analyses. In this study, we found that the disruption of Rad6 leads to a profound reduction in conidial production, irrespective of whether the fungus is cultivated in nutrient-rich or nutrient-poor environments. The absence of rad6 exerts a suppressive effect on the transcription of essential genes in the central developmental pathway, namely, brlA, abaA, and wetA, resulting in a direct downregulation of conidiation capacity. Additionally, mutant strains exhibited a more pronounced decline in both conidial generation and hyphal development when cultured in nutrient-rich conditions. This observation correlates with the downregulation of the central developmental pathway (CDP) downstream gene vosA and the upregulation of flaA in nutrient-rich cultures. Moreover, single-transcriptomics analyses indicated that irregularities in biotin metabolism, DNA repair, and tryptophan metabolism are the underlying factors contributing to the reduced conidial production. Comprehensive dual transcriptomics analyses pinpointed abnormal biotin metabolism as the primary cause of conidial production decline. Subsequently, we successfully restored conidial production in the Rad6 mutant strain through the supplementation of biotin, further confirming the transcriptomic evidence. Altogether, our findings underscore the pivotal role of Rad6 in influencing biotin metabolism, subsequently impacting the expression of CDP genes and ultimately shaping the asexual life cycle of B. bassiana.

Mutual regulation of novel transcription factors RsrD and RsrE positively modulates the production of raw-starch-degrading enzyme in Penicillium oxalicum

Filamentous fungi can produce raw-starch-degrading enzyme, however, regulation of production of raw-starch-degrading enzyme remains poorly understood thus far. Here, two novel transcription factors raw-starch-degrading enzyme regulator D (RsrD) and raw-starch-degrading enzyme regulator E (RsrE) were identified to participate in the production of raw-starch-degrading enzyme in Penicillium oxalicum. Individual knockout of rsrD and rsrE in the parental strain Δku70 resulted in 31.1%-92.9% reduced activity of raw-starch-degrading enzyme when cultivated in the presence of commercial starch from corn. RsrD and RsrE contained a basic leucine zipper and a Zn2Cys6-type DNA-binding domain, respectively, but with unknown functions. RsrD and RsrE dynamically regulated the expression of genes encoding major amylases over time, including raw-starch-degrading glucoamylase gene PoxGA15A and α-amylase gene amy13A. Interestingly, RsrD and RsrE regulated each other at transcriptional level, through binding to their own promoter regions; nevertheless, both failed to bind to the promoter regions of PoxGA15A and amy13A, as well as the known regulatory genes for regulation of amylase gene expression. RsrD appears to play an epistatic role in the module RsrD-RsrE on regulation of amylase gene expression. This study reveals a novel regulatory pathway of fungal production of raw-starch-degrading enzyme.IMPORTANCETo survive via combating with complex extracellular environment, filamentous fungi can secrete plant polysaccharide-degrading enzymes that can efficiently hydrolyze plant polysaccharide into glucose or other mono- and disaccharides, for their nutrients. Among the plant polysaccharide-degrading enzymes, raw-starch-degrading enzymes directly degrade and convert hetero-polymeric starch into glucose and oligosaccharides below starch gelatinization temperature, which can be applied in industrial biorefinery to save cost. However, the regulatory mechanism of production of raw-starch-degrading enzyme in fungi remains unknown thus far. Here, we showed that two novel transcription factors raw-starch-degrading enzyme regulator D (RsrD) and raw-starch-degrading enzyme regulator E (RsrE) positively regulate the production of raw-starch-degrading enzyme by Penicillium oxalicum. RsrD and RsrE indirectly control the expression of genes encoding enzymes with amylase activity but directly regulate each other at transcriptional level. These findings expand diversity of gene expression regulation in fungi.

The MYB-like protein MylA contributes to conidiogenesis and conidial germination in Aspergillus nidulans

Myeloblastosis (MYB)-like proteins are a family of highly conserved transcription factors in animals, plants, and fungi and are involved in the regulation of mRNA expression of genes. In this study, we identified and characterized one MYB-like protein in the model organism Aspergillus nidulans. We screened the mRNA levels of genes encoding MYB-like proteins containing two MYB repeats in conidia and found that the mRNA levels of four genes including flbD, cicD, and two uncharacterized genes, were high in conidia. To investigate the roles of two uncharacterized genes, AN4618 and AN10944, deletion mutants for each gene were generated. Our results revealed that AN4618 was required for fungal development. Therefore, we further investigated the role of AN4618, named as mylA, encoding the MYB-like protein containing two MYB repeats. Functional studies revealed that MylA was essential for normal fungal growth and development. Phenotypic and transcriptomic analyses demonstrated that deletion of mylA affected stress tolerance, cell wall integrity, and long-term viability in A. nidulans conidia. In addition, the germination rate of the mylA deletion mutant conidia was decreased compared with that of the wild-type conidia. Overall, this study suggests that MylA is critical for appropriate development, conidial maturation, dormancy, and germination in A. nidulans.

Coordination of two regulators SscA and VosA in Aspergillus nidulans conidia

Airborne fungal spores are a major cause of fungal diseases in humans, animals, and plants as well as contamination of foods. Previous studies found a variety of regulators including VosA, VelB, WetA, and SscA for sporogenesis and the long-term viability in Aspergillus nidulans. To gain a mechanistic understanding of the complex regulatory mechanisms in asexual spores, here, we focused on the relationship between VosA and SscA using comparative transcriptomic analysis and phenotypic studies. The ΔsscA ΔvosA double-mutant conidia have lower spore viability and stress tolerance compared to the ΔsscA or ΔvosA single mutant conidia. Deletion of sscA or vosA affects chitin levels and mRNA levels of chitin biosynthetic genes in conidia. In addition, SscA and VosA are required for the dormant state of conidia and conidial germination by modulating the mRNA levels of the cytoskeleton and development-associated genes. Overall, these results suggest that SscA and VosA play interdependent roles in governing spore maturation, dormancy, and germination in A. nidulans.

Pleiotropic functions of SscA on the asexual spore of the human pathogenic fungus Aspergillus fumigatus

Asexual spores, called conidia, are key reproductive fungal particles that enable survival in harsh environmental conditions or host systems. The conidia can infect humans, animals, and plants to cause various fungal diseases. Transcription factors, including VosA, WetA, and SscA, have key roles in conidia formation and long-term survival in Aspergillus nidulans. Herein, we report the pleiotropic functions of SscA in the conidia of the human pathogen A. fumigatus. The deletion of sscA increased conidia formation despite decreased fungal growth. Absence of sscA impaired long-term survival and reduced spore resistance to various stresses, including heat, UV, and oxidation. Transcriptomic analyses showed that SscA involved the mRNA expression of cell wall organisation-related genes. Importantly, the sscA deletion mutant conidia contained an increased amount of β-glucan and chitin compared to wild type conidia. In addition, conidial gliotoxin production was decreased in the sscA deletion strain. Overall, SscA has pleiotropic roles in conidia formation, maturation and dormancy and mycotoxin production in A. fumigatus.

The novel spore-specific regulator SscA controls Aspergillus conidiogenesis

IMPORTANCE

Filamentous fungi produce myriads of asexual spores, which are the main reproductive particles that act as infectious or allergenic agents. Although the serial of asexual sporogenesis is coordinated by various genetic regulators, there remain uncharacterized transcription factors in Aspergillus. To understand the underlying mechanism of spore formation, integrity, and viability, we have performed comparative transcriptomic analyses on three Aspergillus species and found a spore-specific transcription factor, SscA. SscA has a major role in conidial formation, maturation and dormancy, and germination in Aspergillus nidulans. Functional studies indicate that SscA coordinates conidial wall integrity, amino acid production, and secondary metabolism in A. nidulans conidia. Furthermore, the roles of SscA are conserved in other Aspergillus species. Our findings that the SscA has broad functions in Aspergillus conidia will help to understand the conidiogenesis of Aspergillus species.