Characterization of the developmental regulator FlbE in Aspergillus fumigatus and Aspergillus nidulans

Unveiling the functions of the Lim-domain binding protein MaPtaB in Metarhizium acridum

BACKGROUND

The Lim-domain binding protein PtaB, a homolog of Mfg1, governs conidiation and biofilm formation in several fungi. PtaB includes a conserved Lim-binding domain and two predicted nuclear localization sequences at its C terminus, and is co-regulated with the transcription factor Som1 downstream of the cyclic AMP-dependent protein kinase A (cAMP/PKA) pathway. However, the function of PtaB in entomopathogenic fungi remain poorly understood.

RESULTS

Inactivation of PtaB in Metarhizium acridum resulted in delayed conidial germination, reduced conidial yield and increased sensitivities to cell wall disruptors, ultraviolet B irradiation and heat shock. In addition, the fungal virulence was significantly decreased after deletion of MaPtaB because of impairments in appressorium formation, cuticle penetration and evasion of insect immune responses in M. acridum. The MaPtaB-deletion and MaSom1-deletion strains showed similar phenotypes supporting that MaSom1/MaPtaB complex controls M. acridum normal conidiation and pathogenic progress. Upon loss of MaPtaB or MaSom1, the fungal sporulation mode in M. acridium shifted from microcycle conidiation to normal conidiation on SYA, a microcycle conidiation medium. Transcriptional analysis showed that more differentially expression genes were identified in MaSom1 RNA sequencing, and MaSom1 and MaPtaB may regulate the expression of genes for conidiation, nutrient metabolism and the cell cycle to control conidiation pattern shift.

CONCLUSION

These data corroborate a complex control function for MaPtaB as an important central factor interacting with MaSom1 in the cAMP/PKA pathway, which links stress tolerance, conidiation and virulence in the entomopathogenic fungus M. acridum. © 2024 Society of Chemical Industry.

Comprehensive Insights into the Remarkable Function and Regulatory Mechanism of FluG during Asexual Development in Beauveria bassiana

Asexual development is the main propagation and transmission mode of Beauveria bassiana and the basis of its pathogenicity. The regulation mechanism of conidiation and the key gene resources for utilization are key links to improving the conidia yield and quality of Beauveria bassiana. Their clarification may promote the industrialization of fungal pesticides. Here, we compared the regulation of morphology, resistance to external stress, virulence, and nutrient utilization capacity between the upstream developmental regulatory gene fluG and the key genes brlA, abaA, and wetA in the central growth and development pathway. The results showed that the ΔbrlA and ΔabaA mutants completely lost the capacity to conidiate and that the ΔwetA mutant had seriously reduced conidiation capacity. Although the deletion of fluG did not reduce the conidiation ability as much as deletions of brlA, abaA, and wetA, it significantly reduced the fungal response to external stress, virulence, and nutrient utilization, while the deletion of the three other genes had little effect. Via transcriptome analysis and screening the yeast nuclear system library, we found that the differentially expressed genes in the ΔfluG mutants were concentrated in the signaling pathways of ABC transporters, propionate metabolism, tryptophan metabolism, DNA replication, mismatch repair, and fatty acid metabolism. FluG directly acted on 40 proteins that were involved in various signaling pathways such as metabolism, oxidative stress, and cell homeostasis. The analysis indicated that the regulatory function of fluG was mainly involved in DNA replication, cell homeostasis, fungal growth and metabolism, and the response to external stress. Our results revealed the biological function of fluG in asexual development and the responses to several environmental stresses as well as its influence on the asexual development regulatory network in B. bassiana.

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.

The role of the Flb protein family in the life cycle of Aspergillus niger

Genes flbA-E are involved in sporulation and vegetative growth in Aspergillus nidulans. Inactivation of either of these genes results in a fluffy phenotype with delayed or even abolished sporulation. Previously, a non-sporulating phenotype was obtained by inactivating flbA in Aspergillus niger, which was accompanied by lysis, thinner cell walls, and an increased secretome complexity. Here, we further studied the role of the flb genes of A. niger. Strains ΔflbA, ΔflbB and ΔflbE showed increased biomass formation, while inactivation of flbA-D reduced, or even abolished, formation of conidia. Strain ΔflbA was more sensitive to H2O2, DTT, and the cell wall integrity stress compounds SDS and Congo Red (CR). Also, ΔflbC was more sensitive to SDS, while ΔflbB, ΔflbD, and ΔflbE were more sensitive to CR. On the other hand, inactivation of flbE increased resistance to H2O2. Enzyme secretion was impacted when the Δflb strains were grown on xylose. Strain ΔflbE showed reduced xylanase, cellulase and amylase secretion. On the other hand, amylase secretion at the periphery of the ΔflbA colony was reduced but not in its center, while secretion of this enzyme was increased in the center of the ΔflbB colony but not at its periphery. Inactivation of flbC and flbD also impacted zonal cellulase and amylase activity. Together, the Flb protein family of A. niger function in biomass formation, sporulation, stress response, and protein secretion.

The Forkhead Gene fkhB is Necessary for Proper Development in Aspergillus nidulans

The forkhead domain genes are important for development and morphogenesis in fungi. Six forkhead genes fkhA-fkhF have been found in the genome of the model filamentous Ascomycete Aspergillus nidulans. To identify the fkh gene(s) associated with fungal development, we examined mRNA levels of these six genes and found that the level of fkhB and fkhD mRNA was significantly elevated during asexual development and in conidia. To investigate the roles of FkhB and FkhD, we generated fkhB and fkhD deletion mutants and complemented strains and investigated their phenotypes. The deletion of fkhB, but not fkhD, affected fungal growth and both sexual and asexual development. The fkhB deletion mutant exhibited decreased colony size with distinctly pigmented (reddish) asexual spores and a significantly lower number of conidia compared with these features in the wild type (WT), although the level of sterigmatocystin was unaffected by the absence of fkhB. Furthermore, the fkhB deletion mutant produced sexual fruiting bodies (cleistothecia) smaller than those of WT, implying that the fkhB gene is involved in both asexual and sexual development. In addition, fkhB deletion reduced fungal tolerance to heat stress and decreased trehalose accumulation in conidia. Overall, these results suggest that fkhB plays a key role in proper fungal growth, development, and conidial stress tolerance in A. nidulans.

Regulators of the Asexual Life Cycle of Aspergillus nidulans

The genus Aspergillus, one of the most abundant airborne fungi, is classified into hundreds of species that affect humans, animals, and plants. Among these, Aspergillus nidulans, as a key model organism, has been extensively studied to understand the mechanisms governing growth and development, physiology, and gene regulation in fungi. A. nidulans primarily reproduces by forming millions of asexual spores known as conidia. The asexual life cycle of A. nidulans can be simply divided into growth and asexual development (conidiation). After a certain period of vegetative growth, some vegetative cells (hyphae) develop into specialized asexual structures called conidiophores. Each A. nidulans conidiophore is composed of a foot cell, stalk, vesicle, metulae, phialides, and 12,000 conidia. This vegetative-to-developmental transition requires the activity of various regulators including FLB proteins, BrlA, and AbaA. Asymmetric repetitive mitotic cell division of phialides results in the formation of immature conidia. Subsequent conidial maturation requires multiple regulators such as WetA, VosA, and VelB. Matured conidia maintain cellular integrity and long-term viability against various stresses and desiccation. Under appropriate conditions, the resting conidia germinate and form new colonies, and this process is governed by a myriad of regulators, such as CreA and SocA. To date, a plethora of regulators for each asexual developmental stage have been identified and investigated. This review summarizes our current understanding of the regulators of conidial formation, maturation, dormancy, and germination in A. nidulans.

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.

Role of AtYap1 in the reactive oxygen species regulation of lovastatin production in Aspergillus terreus

Lovastatin has great medical and economic importance, and its production in Aspergillus terreus is positively regulated at transcriptional level, by reactive oxygen species (ROS) generated during idiophase. To investigate the role of the transcription factor Yap1 in the regulation of lovastatin biosynthesis by ROS, an orthologue of yap1 was identified in A. terreus TUB F-514 and knocked down (silenced) by RNAi. Results confirmed that the selected knockdown strain (Siyap1) showed decreased yap1 expression in both culture systems (submerged and solid-state fermentation). Transformants showed higher sensitivity to oxidative stress. Interestingly, knockdown mutant showed higher ROS levels in idiophase and an important increase in lovastatin production in submerged and solid-state fermentations: 60 and 70% increase, respectively. Furthermore, sporulation also increased by 600%. This suggested that AtYap1 was functioning as a negative regulator of the biosynthetic genes, and that lack of AtYap1 in the mutants would be derepressing these genes and could explain increased production. However, we have shown that lovastatin production is proportional to ROS levels, so ROS increase in the mutants alone could also be the cause of production increase. In this work, when ROS levels were decreased with antioxidant, to the levels shown by the parental strain, the lovastatin production and kinetics were similar to the ones of the parental strain. This means that AtYap1 does not regulate lovastatin biosynthetic genes, and that production increase observed in the knockdown strain was an indirect effect caused by ROS increase. This conclusion is compared with studies on other secondary metabolites produced by other fungal species. KEY POINTS: • ROS regulates lovastatin biosynthesis at transcriptional level, in solid-state, and in submerged fermentations. • ATyap1 knockdown mutants showed important lovastatin production increases (60 and 70%) and higher ROS levels. • When ROS were decreased in the silenced mutant to the parental strain's level, lovastatin kinetics were identical to the parental strain's.

The Putative C2H2 Transcription Factor VadH Governs Development, Osmotic Stress Response, and Sterigmatocystin Production in Aspergillus nidulans

The VosA-VelB hetero-dimeric complex plays a pivotal role in regulating development and secondary metabolism in Aspergillus nidulans. In this work, we characterize a new VosA/VelB-activated gene called vadH, which is predicted to encode a 457-amino acid length protein containing four adjacent C₂H₂ zinc-finger domains. Mutational inactivation of vosA or velB led to reduced mRNA levels of vadH throughout the lifecycle, suggesting that VosA and VelB have a positive regulatory effect on the expression of vadH. The deletion of vadH resulted in decreased asexual development (conidiation) but elevated production of sexual fruiting bodies (cleistothecia), indicating that VadH balances asexual and sexual development in A. nidulans. Moreover, the vadH deletion mutant exhibited elevated susceptibility to hyperosmotic stress compared to wild type and showed elevated production of the mycotoxin sterigmatocystin (ST). Genome-wide expression analyses employing RNA-Seq have revealed that VadH is likely involved in regulating more genes and biological pathways in the developmental stages than those in the vegetative growth stage. The brlA, abaA, and wetA genes of the central regulatory pathway for conidiation are downregulated significantly in the vadH null mutant during asexual development. VadH also participates in regulating the genes, mat2, ppgA and lsdA, etc., related to sexual development, and some of the genes in the ST biosynthetic gene cluster. In summary, VadH is a putative transcription factor with four C₂H₂ finger domains and is involved in regulating asexual/sexual development, osmotic stress response, and ST production in A. nidulans.

Roles of BrlA and AbaA in Mediating Asexual and Insect Pathogenic Lifecycles of Metarhizium robertsii

BrlA and AbaA are key activators of the central developmental pathway (CDP) that controls asexual development in Aspergillus but their roles remain insufficiently understood in hypocerealean insect pathogens. Here, regulatory roles of BrlA and AbaA orthologs in Metarhizium robertsii (Clavicipitaceae) were characterized for comparison to those elucidated previously in Beauveria bassiana (Cordycipitaceae) at phenotypic and transcriptomic levels. Time-course transcription profiles of brlA, abaA, and the other CDP activator gene wetA revealed that they were not so sequentially activated in M. robertsii as learned in Aspergillus. Aerial conidiation essential for fungal infection and dispersal, submerged blastospore production mimicking yeast-like budding proliferation in insect hemocoel, and insect pathogenicity via cuticular penetration were all abolished as a consequence of brlA or abaA disruption, which had little impact on normal hyphal growth. The disruptants were severely compromised in virulence via cuticle-bypassing infection (intrahemocoel injection) and differentially impaired in cellular tolerance to oxidative and cell wall-perturbing stresses. The ΔbrlA and ΔabaA mutant shad 255 and 233 dysregulated genes (up/down ratios: 52:203 and 101:122) respectively, including 108 genes co-dysregulated. These counts were small compared with 1513 and 2869 dysregulated genes (up/down ratios: 707:806 and 1513:1356) identified in ΔbrlA and ΔabaA mutants of B. bassiana. Results revealed not only conserved roles for BrlA and AbaA in asexual developmental control but also their indispensable roles in fungal adaptation to the insect-pathogenic lifecycle and host habitats. Intriguingly, BrlA- or AbaA-controlled gene expression networks are largely different between the two insect pathogens, in which similar phenotypes were compromised in the absence of either brlA or abaA.

The function of a conidia specific transcription factor CsgA in Aspergillus nidulans

Aspergillus spp. mainly reproduce asexually via asexual spores called conidia. In this study, we identified CsgA, a conidia-specific Zn₂Cys₆ transcription factor containing the GAL4-like zinc-finger domain, and characterized the roles of CsgA in the model organism Aspergillus nidulans. In A. nidulans, the ΔcsgA strain produced abnormal conidiophores and exhibited increased conidial production. The deletion of csgA resulted in impaired production of sexual fruiting bodies (cleistothecia) and lower mutA expression levels. Overexpression of csgA led to decreased conidia production but increased cleistothecia production, suggesting that CsgA is essential for proper asexual and sexual development in A. nidulans. In conidia, the deletion of csgA resulted in increased trehalose content, higher spore viability, and increased tolerance to thermal and oxidative stresses. Transcriptomic analysis revealed that the loss of csgA affects the expression of genes related to conidia germination, DNA repair, and secondary metabolite biosynthesis. Further analysis revealed that the ΔcsgA strain exhibited delayed conidial germination and abnormal germ tube length. Additionally, the production of sterigmatocystin increased in the ΔcsgA conidia compared to that in the controls. Overall, these results suggest that CsgA is crucial for proper fungal development, spore viability, conidial germination, and sterigmatocystin production in A. nidulans.

FluG and FluG-like FlrA Coregulate Manifold Gene Sets Vital for Fungal Insect-Pathogenic Lifestyle but Not Involved in Asexual Development

The central developmental pathway (CDP) activator gene brlA is activated by the upstream genes fluG and flbA–flbE in Aspergillus nidulans. Increasing evidences of fungal genome divergence make it necessary to clarify whether such genetic principles fit Pezizomycotina. Previously, fluG disruption resulted in limited conidiation defect and little effect on the expression of brlA and flbA–flbE in Beauveria bassiana possessing the other FluG-like regulator FlrA. Here, single-disruption (SD) mutants of flrA and double-disruption (DD) mutants of flrA and fluG were analyzed to clarify whether FlrA and FluG are upstream regulators of key CDP genes. Despite similar subcellular localization, no protein-protein interaction was detected between FlrA and FluG, suggesting mutual independence. Three flrA SD mutants showed phenotypes similar to those previously described for ΔfluG, including limited conidiation defect, facilitated blastospore production, impaired spore quality, blocked host infection, delayed proliferation in vivo, attenuated virulence, and increased sensitivities to multiple stresses. Three DD mutants resembled the SD mutants in all phenotypes except more compromised pathogenicity and tolerance to heat shock- or calcofluor white-induced stress. No CDP gene appeared in 1,622 and 2,234 genes dysregulated in the ΔflrA and ΔfluG mutants, respectively. The majority (up/down ratio: 540:875) of those dysregulated genes were co-upregulated or co-downregulated at similar levels in the two mutants. These findings unravel novel roles for flrA and fluG in coregulating manifold gene sets vital for fungal adaptation to insect-pathogenic lifestyle and environment but not involved in CDP activation.

IMPORTANCE FluG is a core regulator upstream of central developmental pathway (CDP) in Aspergillus nidulans but multiple FluG-like regulators (FLRs) remain functionally uncharacterized in ascomycetes. Our previous study revealed no role for FluG in the CDP activation and an existence of sole FLR (FlrA) in an insect-pathogenic fungus. This study reveals a similarity of FlrA to FluG in domain architecture and subcellular localization. Experimental data from analyses of targeted single- and double-gene knockout mutants demonstrate similar roles of FrlA and FluG in stress tolerance and infection cycle but no role of either in CDP activation. Transcriptomic analyses reveal that FlrA and FluG coregulate a large number of same genes at similar levels. However, the regulated genes include no key CDP gene. These findings uncover that FlrA and FluG play similar roles in the fungal adaptation to insect-pathogenic lifestyle and environment but no role in the activation of CDP.

MaNsdD regulates conidiation negatively by inhibiting the AbaA expression required for normal conidiation in Metarhizium acridum

Conidiation necessary for filamentous fungal survival and dispersal, proceeds in two fashions, namely normal conidiation through conidiophores differentiated from hyphae, and microcycle conidiation through conidial budding. Normal conidiation has been well studied whereas mechanisms underlying microcycle conidiation are still largely unknown. Here, we report that a gene (MaNsdD) homologous to NsdD in Aspergillus nidulans serves as a suppressor of normal conidiation but a positive regulator of hyphal development in Metarhizium acridum. Disruption of MaNsdD (ΔMaNsdD) resulted in microcycle conidiation and significantly descended in conidial resistance to heat while improved to UV irradiation. Transcriptomic analysis revealed that many genes involved in conidiation, cell division and cell wall formation were differentially expressed in ΔMaNsdD, and likely associated with the conidiation process. We found that a gene (MaAbaA) homologous to the core asexual development regulator AbaA in A. nidulans, was negatively controlled by MaNsdD. Disruption of MaAbaA led to the abolition of the conidiation process of M. acridum. These findings unravel a novel regulatory mechanism of microcycle conidiation, and add a knowledge to the asexual conidiation pathway of filamentous fungi.

Differential Roles of Five Fluffy Genes (flbA–flbE) in the Lifecycle In Vitro and In Vivo of the Insect–Pathogenic Fungus Beauveria bassiana

The fluffy genes flbA–flbE are well-known players in the upstream developmental activation pathway that activates the key gene brlA of central developmental pathway (CDP) to initiate conidiation in Aspergillus nidulans. Here, we report insignificant roles of their orthologs in radial growth of Beauveria bassiana under normal culture conditions and different stresses although flbA and flbD were involved in respective responses to heat shock and H₂O₂. Aerial conidiation level was lowered in the deletion mutants of flbB and flbE (~15%) less than of flbA and flbC (~30%), in which the key CDP genes brlA and abaA were repressed consistently during normal incubation. The CDP-controlled blastospore production in submerged cultures mimicking insect hemolymph was abolished in the flbA mutant with brlA and abaA being sharply repressed, and decreased by 55% in the flbC mutant with only abaA being downregulated. The fungal virulence against a model insect was attenuated in the absence of flbA more than of flbC irrespective of normal cuticle infection or cuticle-bypassing infection (intrahemocoel injection). These findings unravel more important role of flbA than of flbC, but null roles of flbB/D/E, in B. bassiana’s insect–pathogenic lifecycle and a scenario distinctive from that in A.nidulans.

The Zinc Finger Transcription Factor BbCmr1 Regulates Conidium Maturation in Beauveria bassiana

The entomopathogenic fungus Beauveria bassiana is a typical filamentous fungus and has been used for pest biocontrol. Conidia are the main active agents of fungal pesticides; however, we know little about conidial developmental mechanisms and less about maturation mechanisms. We found that a Zn₂Cys₆ transcription factor of B. bassiana (named BbCmr1) was mainly expressed in late-stage conidia and was involved in conidium maturation regulation. Deletion of Bbcmr1 impaired the conidial cell wall and resulted in a lower conidial germination rate under UV (UV), heat shock, H₂O₂, Congo red (CR) and SDS stresses compared to the wild type. Transcription levels of the genes associated with conidial wall components and trehalose synthase were significantly reduced in the ΔBbcmr1 mutant. Further analysis found that BbCmr1 functions by upregulating BbWetA, a well-known transcription factor in the central development of BrlA-AbaA-WetA. The expression of Bbcmr1 was positively regulated by BbBrlA. These results indicated that BbCmr1 played important roles in conidium maturation by interacting with the central development pathway, which provided insight into the conidial development networks in B. bassiana. IMPORTANCE Conidium maturation is a pivotal event in conidial development and affects fungal survival ability under various biotic/abiotic stresses. Although many transcription factors have been reported to regulate conidial development, we know little about the molecular mechanism of conidium maturation. Here, we demonstrated that the transcription factor BbCmr1 of B. bassiana was involved in conidium maturation, regulating cell wall structure, the expression of cell wall-related proteins, and trehalose synthesis. BbCmr1 orchestrated conidium maturation by interplaying with the central development pathway BrlA-AbaA-WetA. BbBrlA positively regulated the expression of Bbcmr1, and the latter positively regulated BbwetA expression, which forms a regulatory network mediating conidial development. This finding was critical to understand the molecular regulatory networks of conidial development in B. bassiana and provided avenues to engineer insect fungal pathogens with high-quality conidia.

New Insights of Transcriptional Regulator AflR in Aspergillus flavus Physiology

Aspergillus flavusaflR, a gene encoding a Zn(II)₂Cys₆ DNA-binding domain, is an important transcriptional regulator of the aflatoxin biosynthesis gene cluster. Our previous results of Gene ontology (GO) analysis for the binding sites of AflR in A. flavus suggest that AflR may play an integrative regulatory role. In this study the ΔaflR and overexpression (OE) strains based on the well-established double-crossover recombinational technique were constructed to investigate the integrative function of the aflR gene in A. flavus. The disruption of aflR severely affected the aflatoxin biosynthetic pathway, resulting in a significant decrease in aflatoxin production. The aflatoxin B₁ (AFB₁) of the ΔaflR strain was 180 ng/mL and aflatoxin B₂ (AFB₂) was 2.95 ng/mL on YES medium for 5 days, which was 1/1,000 of that produced by the wild-type strain (WT). In addition, the ΔaflR strain produced relatively sparse conidia and a very small number of sclerotia. On the seventh day, the sclerotia yield on each plate of the WT and OE strains exceeded 1,000, while the sclerotial formation of the ΔaflR strain was not detected until 14 days. However, the biosynthesis of cyclopiazonic acid (CPA) was not affected by aflR gene disruption. Transcriptomic analysis of the ΔaflR strain grown on potato dextrose agar (PDA) plates at 0 h, 24 h, and 72 h showed that expression of clustering genes involved in the biosynthesis of aflatoxin was significantly downregulated. Meanwhile, the ΔaflR strain compared with the WT strain showed significant expression differences in genes involved in spore germination, sclerotial development, and carbohydrate metabolism compared to the WT. The results demonstrated that the A. flavusaflR gene also played a positive role in the fungal growth and development in addition to aflatoxin biosynthesis.

IMPORTANCE Past studies of the A. flavusaflR gene and its orthologues in related Aspergillus species were solely focused on their roles in secondary metabolism. In this study, we used the ΔaflR and OE strains to demonstrate the role of aflR in growth and development of A. flavus. For the first time, we confirmed that the ΔaflR strain also was defective in production of conidia and sclerotia, asexual propagules of A. flavus. Our transcriptomic analysis further showed that genes involved in spore germination, sclerotial development, aflatoxin biosynssssthesis, and carbohydrate metabolism exhibited significant differences in the ΔaflR strain compared with the WT strain. Our study indicates that AflR not only plays an important role in regulating aflatoxin synthesis but also in playing a positive role in the conidial formation and sclerotial development in A. flavus. This study reveals the critical and positive role of the aflR gene in fungal growth and development, and provides a theoretical basis for the genetic studies of other aspergilli.

Construction of Light-Responsive Gene Regulatory Network for Growth, Development and Secondary Metabolite Production in Cordyceps militaris

Cordyceps militaris is an edible fungus that produces many beneficial compounds, including cordycepin and carotenoid. In many fungi, growth, development and secondary metabolite production are controlled by crosstalk between light-signaling pathways and other regulatory cascades. However, little is known about the gene regulation upon light exposure in C. militaris. This study aims to construct a gene regulatory network (GRN) that responds to light in C. militaris. First, a genome-scale GRN was built based on transcription factor (TF)-target gene interactions predicted from the Regulatory Sequence Analysis Tools (RSAT). Then, a light-responsive GRN was extracted by integrating the transcriptomic data onto the genome-scale GRN. The light-responsive network contains 2689 genes and 6837 interactions. From the network, five TFs, Snf21 (CCM_04586), an AT-hook DNA-binding motif TF (CCM_08536), a homeobox TF (CCM_07504), a forkhead box protein L2 (CCM_02646) and a heat shock factor Hsf1 (CCM_05142), were identified as key regulators that co-regulate a large group of growth and developmental genes. The identified regulatory network and expression profiles from our analysis suggested how light may induce the growth and development of C. militaris into a sexual cycle. The light-mediated regulation also couples fungal development with cordycepin and carotenoid production. This study leads to an enhanced understanding of the light-responsive regulation of growth, development and secondary metabolite production in the fungi.

The putative sensor histidine kinase VadJ coordinates development and sterigmatocystin production in Aspergillus nidulans

The VosA-VelB heterocomplex governs expression of several genes associated with fungal development and secondary metabolism. In this study, we have investigated the functions of one of the VosA-VelB-activated developmental genes vadJ in development and production of the mycotoxin sterigmatocystin in the model fungus Aspergillus nidulans. The vadJ gene is predicted to encode a 957-amino acid length protein containing a highly conserved sensor histidine kinase domain. The deletion of vosA or velB resulted in decreased mRNA levels of vadJ throughout the life cycle, suggesting that VosA and VelB are necessary for proper expression of vadJ. Nullifying vadJ led to highly restricted colony growth, lowered formation of asexual spores, and about two-fold reduction in conidial viability. Conversely, the deletion of vadJ resulted in elevated production of sexual fruiting bodies and sterigmatocystin. These suggest that VadJ is necessary for proper coordination of asexual and sexual development, and sterigmatocystin production. In accordance with this idea, the deletion of vadJ led to elevated mRNA levels of the two key sexual developmental activators esdC and nsdD. In summary, the putative sensor histidine kinase VadJ represses sexual development and sterigmatocystin production, but activates asexual development in A. nidulans.

Unveiling the Functions of the VosA-VelB Target Gene vidD in Aspergillus nidulans

The velvet regulators VosA and VelB are primarily involved in spore maturation and dormancy. Previous studies found that the VosA-VelB hetero-complex coordinates certain target genes that are related to fungal differentiation and conidial maturation in Aspergillus nidulans. Here, we characterized the VosA/VelB-inhibited developmental gene vidD in A. nidulans. Phenotypic analyses demonstrated that the vidD deleted mutant exhibited defect fungal growth, a reduced number of conidia, and delayed formation of sexual fruiting bodies. The deletion of vidD decreased the amount of conidial trehalose, increased the sensitivity against heat stress, and reduced the conidial viability. Moreover, the absence of vidD resulted in increased production of sterigmatocystin. Together, these results show that VidD is required for proper fungal growth, development, and sterigmatocystin production in A. nidulans.

A Homeodomain-Containing Transcriptional Factor PoHtf1 Regulated the Development and Cellulase Expression in Penicillium oxalicum

Homeodomain-containing transcription factors (Htfs) play important roles in animals, fungi, and plants during some developmental processes. Here, a homeodomain-containing transcription factor PoHtf1 was functionally characterized in the cellulase-producing fungi Penicillium oxalicum 114-2. PoHtf1 was shown to participate in colony growth and conidiation through regulating the expression of its downstream transcription factor BrlA, the key regulator of conidiation in P. oxalicum 114-2. Additionally, PoHtf1 inhibited the expression of the major cellulase genes by coordinated regulation of cellulolytic regulators CreA, AmyR, ClrB, and XlnR. Furthermore, transcriptome analysis showed that PoHtf1 participated in the secondary metabolism including the pathway synthesizing conidial yellow pigment. These data show that PoHtf1 mediates the complex transcriptional-regulatory network cascade between developmental processes and cellulolytic gene expression in P. oxalicum 114-2. Our results should assist the development of strategies for the metabolic engineering of mutants for applications in the enzymatic hydrolysis for biochemical production.

Functions of PUF Family RNA-Binding Proteins in Aspergillus nidulans

RNA-binding proteins are involved in RNA metabolism and posttranscriptional regulation of various fundamental biological processes. The PUF family of RNA-binding proteins is highly conserved in eukaryotes, and its members regulate gene expression, mitochondrial biogenesis, and RNA processing. However, their biological functions in Aspergillus species remain mostly unknown in filamentous fungi. Here we have characterized the puf genes in the model organism Aspergillus nidulans. We generated deletion mutant strains for the five putative puf genes present in the A. nidulans genome and investigated their developmental phenotypes. Deletion of pufA or pufE affected fungal growth and asexual development. pufA mutants exhibited decreased production of asexual spores and reduced mRNA expression of genes regulating asexual development. The pufE deletion reduced colony growth, increased formation of asexual spores, and delayed production of sexual fruiting bodies. In addition, the absence of pufE reduced both sterigmatocystin production and the mRNA levels of genes in the sterigmatocystin cluster. Finally, pufE deletion mutants showed reduced trehalose production and lower resistance to thermal stress. Overall, these results demonstrate that PufA and PufE play roles in the development and sterigmatocystin metabolism in A. nidulans.

Distinctive role of fluG in the adaptation of Beauveria bassiana to insect-pathogenic lifecycle and environmental stresses

The upstream developmental activation (UDA) pathway comprises three fluG-cored cascades (fluG-flbA, fluG-flbE/B/D and fluG-flbC) that activate the key gene brlA of central developmental pathway (CDP) to initiate conidiation in aspergilli. However, the core role of fluG remains poorly understood in other fungi. Here, we report distinctive role of fluG in the insect-pathogenic lifecycle of Beauveria bassiana. Disruption of fluG resulted in limited conidiation defect, which was mitigated with incubation time and associated with time-course up-regulation/down-regulation of all flb and CDP genes and another fluG-like gene (BBA_06309). In ΔfluG, increased sensitivities to various stresses correlated with repression of corresponding stress-responsive genes. Its virulence through normal cuticle infection was attenuated greatly due to blocked secretion of cuticle-degrading enzymes and delayed formation of hyphal bodies (blastospores) to accelerate proliferation in vivo and host death. In submerged ΔfluG cultures mimicking insect haemolymph, largely increased blastospore production concurred with drastic up-regulation of the CDP genes brlA and abaA, which was associated with earlier up-regulation of most flb genes in the cultures. Our results unveil an essentiality of fluG for fungal adaptation to insect-pathogenic lifecycle and suggest the other fluG-like gene to act as an alternative player in the UDA pathway of B. bassiana.

Velvet activated McrA plays a key role in cellular and metabolic development in Aspergillus nidulans

McrA is a key transcription factor that functions as a global repressor of fungal secondary metabolism in Aspergillus species. Here, we report that mcrA is one of the VosA-VelB target genes and McrA governs the cellular and metabolic development in Aspergillus nidulans. The deletion of mcrA resulted in a reduced number of conidia and decreased mRNA levels of brlA, the key asexual developmental activator. In addition, the absence of mcrA led to a loss of long-term viability of asexual spores (conidia), which is likely associated with the lack of conidial trehalose and increased β-(1,3)-glucan levels in conidia. In supporting its repressive role, the mcrA deletion mutant conidia contain more amounts of sterigmatocystin and an unknown metabolite than the wild type conidia. While overexpression of mcrA caused the fluffy-autolytic phenotype coupled with accelerated cell death, deletion of mcrA did not fully suppress the developmental defects caused by the lack of the regulator of G-protein signaling protein FlbA. On the contrary to the cellular development, sterigmatocystin production was restored in the ΔflbA ΔmcrA double mutant, and overexpression of mcrA completely blocked the production of sterigmatocystin. Overall, McrA plays a multiple role in governing growth, development, spore viability, and secondary metabolism in A. nidulans.

Homeobox proteins are essential for fungal differentiation and secondary metabolism in Aspergillus nidulans

The homeobox domain-containing transcription factors play an important role in the growth, development, and secondary metabolism in fungi and other eukaryotes. In this study, we characterized the roles of the genes coding for homeobox-type proteins in the model organism Aspergillus nidulans. To examine their roles in A. nidulans, the deletion mutant strains for each gene coding for homeobox-type protein were generated, and their phenotypes were examined. Phenotypic analyses revealed that two homeobox proteins, HbxA and HbxB, were required for conidia production. Deletion of hbxA caused abnormal conidiophore production, decreased the number of conidia in both light and dark conditions, and decreased the size of cleistothecia structures. Overexpressing hbxA enhanced the production of asexual spores and formation of conidiophore under the liquid submerged conditions. The hbxB deletion mutant strains exhibited decreased asexual spore production but increased cleistothecia production. The absence of hbxB decreased the trehalose content in asexual spores and increased their sensitivity against thermal and oxidative stresses. The ΔhbxA strains produced more sterigmatocystin, which was decreased in the ΔhbxB strain. Overall, our results show that HbxA and HbxB play crucial roles in the differentiation and secondary metabolism of the fungus A. nidulans.

The role of the VosA-repressed dnjA gene in development and metabolism in Aspergillus species

The DnaJ family of proteins (or J-proteins) are molecular chaperones that govern protein folding, degradation, and translocation in many organisms. Although J-proteins play key roles in eukaryotic and prokaryotic biology, the role of J-proteins in Aspergillus species is currently unknown. In this study, we characterized the dnjA gene, which encodes a putative DnaJ protein, in two Aspergillus species: Aspergillus nidulans and Aspergillus flavus. Expression of the dnjA gene is inhibited by the velvet regulator VosA, which plays a pivotal role in spore survival and metabolism in Aspergillus. The deletion of dnjA decreased the number of asexual spores (conidia), produced abnormal conidiophores, and reduced sexual fruiting bodies (cleistothecia) or sclerotia. In addition, the absence of dnjA caused increased sterigmatocystin or aflatoxin production in A. nidulans and A. flavus, respectively. These results suggest that DnjA plays a conserved role in asexual and sexual development and mycotoxin production in Aspergillus species. However, DnjA also plays a species-specific role; AniDnjA but not AflDnjA, affects conidial viability, trehalose contents, and thermal tolerance of conidia. In plant virulence assay, the infection ability of the ΔAfldnjA mutant decreased in the kernels, suggesting that DnjA plays a crucial role in the pathogenicity of A. flavus. Taken together, these results demonstrate that DnjA is multifunctional in Aspergillus species; it is involved in diverse biological processes, including fungal differentiation and secondary metabolism.

Characterizing the role of Zn cluster family transcription factor ZcfA in governing development in two Aspergillus species

Filamentous fungi reproduce asexually or sexually, and the processes of asexual and sexual development are tightly regulated by a variety of transcription factors. In this study, we characterized a Zn2Cys6 transcription factor in two Aspergillus species, A. nidulans (AN5859) and A. flavus (AFLA_046870). AN5859 encodes a Zn2Cys6 transcription factor, called ZcfA. In A. nidulans, ΔzcfA mutants exhibit decreased fungal growth, a reduction in cleistothecia production, and increased asexual reproduction. Overexpression of zcfA results in increased conidial production, suggesting that ZcfA is required for proper asexual and sexual development in A. nidulans. In conidia, deletion of zcfA causes decreased trehalose levels and decreased spore viability but increased thermal sensitivity. In A. flavus, the deletion of the zcfA homolog AFLA_046870 causes increased conidial production but decreased sclerotia production; these effects are similar to those of zcfA deletion in A. nidulans development. Overall, these results demonstrate that ZcfA is essential for maintaining a balance between asexual and sexual development and that some roles of ZcfA are conserved in Aspergillus spp.

Conserved Roles of MonA in Fungal Growth and Development in Aspergillus Species

MonA is a subunit of a guanine nucleotide exchange factor that is important for vacuole passing and autophagy processes in eukaryotes. In this study, we characterized the function of MonA, an orthologue of Saccharomyces cerevisiae Mon1, in the model fungus Aspergillus nidulans and a toxigenic fungus A. flavus. In A. nidulans, the absence of AnimonA led to decreased fungal growth, reduced asexual reproduction, and defective cleistothecia production. In addition, AnimonA deletion mutants exhibited decreased spore viability, had reduced trehalose contents in conidia, and were sensitive to thermal stress. In A. flavus, deletion of AflmonA caused decreased fungal growth and defective production of asexual spores and sclerotia structures. Moreover, the absence of monA affected vacuole morphology in both species. Taken together, these results indicate that MonA plays conserved roles in controlling fungal growth, development and vacuole morphology in A. nidulans and A. flavus.

A Comprehensive Analysis of the Small GTPases Ypt7 Involved in the Regulation of Fungal Development and Secondary Metabolism in Monascus ruber M7

Ypts (yeast protein transports),also called as ras-associated binding GTPases (Rab), are the largest group of the small GTPases family, which have been extensively studied in model eukaryotic cells and play a pivotal role in membane trafficking, while this study showed potential regulation role of Ypts in fungi. One of Ypts, Ypt7 may be involved in fungal development and secondary metabolism, but the exact mechanism still exists a controversy. In current study, the functions of a Monascus ypt7 homologous gene (mrypt7) from Monascus ruber M7 was investigated by combination of gene-deletion (Δmrypt7), overexpression (M7::PtrpC-mrypt7) and transcriptome analysis. Results showed that the radial growth rate of Δmrypt7 was significantly slower than M. ruber M7, little conidia and ascospores can be observed in Δmrypt7, but the yield of intracellular secondary metabolites was dramatically increased. Simultaneously, the mrypt7 overexpression strain possessed similar capacity for sporulation and secondary metabolism observed in M. ruber M7. Transcriptome results further illustrated that mrypt7 could coordinate with numerous genes involved in the vegetative growth, conidiogenesis, secondary metabolism biosynthesis and transportation of M. ruber M7. Combined with the similar effect of Ypt7 homologs on other fungi, we propose that Ypt7 works more like a global regulatory factor in fungi. To our knowledge, it is the first time to investigate Ypt7 functions in Monascus. It could also improve the understanding of Ypt7 functions in fungi.

Antifungal Peptides of the AFP Family Revisited: Are These Cannibal Toxins?

The emergence and spread of pathogenic fungi resistant to currently used antifungal drugs represents a serious challenge for medicine and agriculture. The use of smart antimicrobials, so-called “dirty drugs” which affect multiple cellular targets, is one strategy to prevent resistance. Of special interest is the exploitation of the AFP family of antimicrobial peptides, which include its founding member AFP from Aspergillus giganteus. This latter is a highly potent inhibitor of chitin synthesis and affects plasma membrane integrity in many human and plant pathogenic fungi. A transcriptomic meta-analysis of the afp-encoding genes in A. giganteus and A. niger predicts a role for these proteins during asexual sporulation, autophagy, and nutrient recycling, suggesting that AFPs are molecules important for the survival of A. niger and A. giganteus under nutrient limitation. In this review, we discuss parallels which exist between AFPs and bacterial cannibal toxins and provide arguments that the primary function of AFPs could be to kill genetically identical siblings. We hope that this review inspires computational and experimental biologists studying alternative explanations for the nature and function of antimicrobial peptides beyond the general assumption that they are mere defense molecules to fight competitors.

The role of VosA/VelB-activated developmental gene vadA in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans primarily reproduces by forming asexual spores called conidia, the integrity of which is governed by the NF-κB type velvet regulators VosA and VelB. The VosA-VelB hetero-complex regulates the expression of spore-specific structural and regulatory genes during conidiogenesis. Here, we characterize one of the VosA/VelB-activated developmental genes, called vadA, the expression of which in conidia requires activity of both VosA and VelB. VadA (AN5709) is predicted to be a 532-amino acid length fungal-specific protein with a highly conserved domain of unknown function (DUF) at the N-terminus. This DUF was found to be conserved in many Ascomycota and some Glomeromycota species, suggesting a potential evolutionarily conserved function of this domain in fungi. Deletion studies of vadA indicate that VadA is required for proper downregulation of brlA, fksA, and rodA, and for proper expression of tpsA and orlA during sporogenesis. Moreover, vadA null mutant conidia exhibit decreased trehalose content, but increased β(1,3)-glucan levels, lower viability, and reduced tolerance to oxidative stress. We further demonstrate that the vadA null mutant shows increased production of the mycotoxin sterigmatocystin. In summary, VadA is a dual-function novel regulator that controls development and secondary metabolism, and participates in bridging differentiation and viability of newly formed conidia in A. nidulans.

Differential Control of Asexual Development and Sterigmatocystin Biosynthesis by a Novel Regulator in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans primarily reproduces by forming asexual spores called conidia and produces the mycotoxin sterigmatocystin (ST), the penultimate precursor of aflatoxins. It has been known that asexual development and ST production are tightly co-regulated by various regulatory inputs. Here, we report that the novel regulator AslA with a C₂H₂ domain oppositely regulates development and ST biosynthesis. Nullifying aslA resulted in defective conidiation and reduced expression of brlA encoding a key activator of asexual development, which indicates that AslA functions as an upstream activator of brlA expression. aslA deletion additionally caused enhanced ST production and expression of aflR encoding a transcriptional activator for ST biosynthetic genes, suggesting that AslA functions as an upstream negative regulator of aflR. Cellular and molecular studies showed that AslA has a trans-activation domain and is localized in the nuclei of vegetative and developing cells but not in spores, indicating that AslA is likely a transcription factor. Introduction of the aslA homologs from distantly-related aspergilli complemented the defects caused by aslA null mutation in A. nidulans, implying a functional conservancy of AslA. We propose that AslA is a novel regulator that may act at the split control point of the developmental and metabolic pathways.

Diversity, Application, and Synthetic Biology of Industrially Important Aspergillus Fungi

The filamentous fungal genus Aspergillus consists of over 340 officially recognized species. A handful of these Aspergillus fungi are predominantly used for food fermentation and large-scale production of enzymes, organic acids, and bioactive compounds. These industrially important Aspergilli primarily belong to the two major Aspergillus sections, Nigri and Flavi. Aspergillus oryzae (section Flavi) is the most commonly used mold for the fermentation of soybeans, rice, grains, and potatoes. Aspergillus niger (section Nigri) is used in the industrial production of various enzymes and organic acids, including 99% (1.4 million tons per year) of citric acid produced worldwide. Better understanding of the genomes and the signaling mechanisms of key Aspergillus species can help identify novel approaches to enhance these commercially significant strains. This review summarizes the diversity, current applications, key products, and synthetic biology of Aspergillus fungi commonly used in industry.

Negative regulation and developmental competence in Aspergillus

Asexual development (conidiation) in the filamentous fungus Aspergillus nidulans is governed by orchestrated gene expression. The three key negative regulators of conidiation SfgA, VosA, and NsdD act at different control point in the developmental genetic cascade. Here, we have revealed that NsdD is a key repressor affecting the quantity of asexual spores in Aspergillus. Moreover, nullifying both nsdD and vosA results in abundant formation of the development specific structure conidiophores even at 12 h of liquid culture, and near constitutive activation of conidiation, indicating that acquisition of developmental competence involves the removal of negative regulation exerted by both NsdD and VosA. NsdD's role in repressing conidiation is conserved in other aspergilli, as deleting nsdD causes enhanced and precocious activation of conidiation in Aspergillus fumigatus or Aspergillus flavus. In vivo NsdD-DNA interaction analyses identify three NsdD binding regions in the promoter of the essential activator of conidiation brlA, indicating a direct repressive role of NsdD in conidiation. Importantly, loss of flbC or flbD encoding upstream activators of brlA in the absence of nsdD results in delayed activation of brlA, suggesting distinct positive roles of FlbC and FlbD in conidiation. A genetic model depicting regulation of conidiation in A. nidulans is presented.

Characterization of the aodA, dnmA, mnSOD and pimA genes in Aspergillus nidulans

Mitochondria play key roles in cellular energy generation and lifespan of most eukaryotes. To understand the functions of four nuclear-encoded genes predicted to be related to the maintenance of mitochondrial morphology and function in Aspergillus nidulans, systematic characterization was carried out. The deletion and overexpression mutants of aodA, dnmA, mnSOD and pimA encoding alternative oxidase, dynamin related protein, manganese superoxide dismutase and Lon protease, respectively, were generated and examined for their growth, stress tolerances, respiration, autolysis, cell death, sterigmatocystin production, hyphal morphology and size, and mitochondrial superoxide production as well as development. Overall, genetic manipulation of these genes had less effect on cellular physiology and ageing in A. nidulans than that of their homologs in another fungus Podospora anserina with a well-characterized senescence. The observed interspecial phenotypic differences can be explained by the dissimilar intrinsic stabilities of the mitochondrial genomes in A. nidulans and P. anserina. Furthermore, the marginally altered phenotypes observed in A. nidulans mutants indicate the presence of effective compensatory mechanisms for the complex networks of mitochondrial defense and quality control. Importantly, these findings can be useful for developing novel platforms for heterologous protein production, or on new biocontrol and bioremediation technologies based on Aspergillus species.

Resistance is not futile: gliotoxin biosynthesis, functionality and utility

Gliotoxin biosynthesis is encoded by the gli gene cluster in Aspergillus fumigatus. The biosynthesis of gliotoxin is influenced by a suite of transcriptionally-active regulatory proteins and a bis-thiomethyltransferase. A self-protection system against gliotoxin is present in A. fumigatus. Several additional metabolites are also produced via the gliotoxin biosynthetic pathway. Moreover, the biosynthesis of unrelated natural products appears to be influenced either by gliotoxin or by the activity of specific reactions within the biosynthetic pathway. The activity of gliotoxin against animal cells and fungi, often mediated by interference with redox homeostasis or protein modification, is revealing new metabolic interactions within eukaryotic systems. Nature has provided a most useful natural product with which to reveal some of its many molecular secrets.

Genetic control of asexual development in aspergillus fumigatus

Aspergillus fumigatus is one of the most common fungi found in the environment. It is an opportunistic human pathogen causing invasive pulmonary aspergillosis with a high mortality rate in immunocompromised patients. Conidia, the asexual spores, serve as the main dispersal and infection agent allowing entrance of the fungus into the host through the respiratory tract. Therefore, understanding the asexual developmental process that gives rise to the conidia is of great interest to the scientific community and is currently the focus of an immense load of research being conducted. We have been studying the genetic basis that controls asexual development and gliotoxin biosynthesis in A. fumigatus. In this review, we discuss the genetic regulatory system that dictates conidiation in this important fungus by covering the roles of crucial genetic factors from the upstream heterotrimeric G-protein signaling components to the more specific downstream central activators of the conidiation pathway. In addition, other key asexual regulators including the velvet regulators, the Flb proteins and their associated regulatory factors are discussed.

Co-ordination between BrlA regulation and secretion of the oxidoreductase FmqD directs selective accumulation of fumiquinazoline C to conidial tissues in Aspergillus fumigatus

Aerial spores, crucial for propagation and dispersal of the Kingdom Fungi, are commonly the initial inoculum of pathogenic fungi. Natural products (secondary metabolites) have been correlated with fungal spore development and enhanced virulence in the human pathogen Aspergillus fumigatus but mechanisms for metabolite deposition in the spore are unknown. Metabolomic profiling of A. fumigatus deletion mutants of fumiquinazoline (Fq) cluster genes reveal that the first two products of the Fq cluster, FqF and FqA, are produced to comparable levels in all fungal tissues but the final enzymatically derived product, FqC, predominantly accumulates in the fungal spore. Loss of the sporulation-specific transcription factor, BrlA, yields a strain unable to produce FqA or FqC. Fluorescence microscopy showed FmqD, the oxidoreductase required to generate FqC, was secreted via the Golgi apparatus to the cell wall in an actin-dependent manner. In contrast, all other members of the Fq pathway including the putative transporter, FmqE - which had no effect on Fq biosynthesis - were internal to the hyphae. The co-ordination of BrlA-mediated tissue specificity with FmqD secretion to the cell wall presents a previously undescribed mechanism to direct localization of specific secondary metabolites to spores of the differentiating fungus.

VelC positively controls sexual development in Aspergillus nidulans

Fungal development and secondary metabolism is intimately associated via activities of the fungi-specific velvet family proteins including VeA, VosA, VelB and VelC. Among these, VelC has not been characterized in Aspergillus nidulans. In this study, we characterize the role of VelC in asexual and sexual development in A. nidulans. The velC mRNA specifically accumulates during the early phase of sexual development. The deletion of velC leads to increased number of conidia and reduced production of sexual fruiting bodies (cleistothecia). In the velC deletion mutant, mRNA levels of the brlA, abaA, wetA and vosA genes that control sequential activation of asexual sporulation increase. Overexpression of velC causes increased formation of cleistothecia. These results suggest that VelC functions as a positive regulator of sexual development. VelC is one of the five proteins that physically interact with VosA in yeast two-hybrid and GST pull down analyses. The ΔvelC ΔvosA double mutant produced fewer cleistothecia and behaved similar to the ΔvosA mutant, suggesting that VosA is epistatic to VelC in sexual development, and that VelC might mediate control of sex through interacting with VosA at specific life stages for sexual fruiting.

NsdD is a key repressor of asexual development in Aspergillus nidulans

Asexual development (conidiation) of the filamentous fungus Aspergillus nidulans occurs via balanced activities of multiple positive and negative regulators. For instance, FluG (+) and SfgA (-) govern upstream regulation of the developmental switch, and BrlA (+) and VosA (-) control the progression and completion of conidiation. To identify negative regulators of conidiation downstream of FluG-SfgA, we carried out multicopy genetic screens using sfgA deletion strains. After visually screening >100,000 colonies, we isolated 61 transformants exhibiting reduced conidiation. Responsible genes were identified as AN3152 (nsdD), AN7507, AN2009, AN1652, AN5833, and AN9141. Importantly, nsdD, a key activator of sexual reproduction, was present in 10 independent transformants. Furthermore, deletion, overexpression, and double-mutant analyses of individual genes have led to the conclusion that, of the six genes, only nsdD functions in the FluG-activated conidiation pathway. The deletion of nsdD bypassed the need for fluG and flbA∼flbE, but not brlA or abaA, in conidiation, and partially restored production of the mycotoxin sterigmatocystin (ST) in the ΔfluG, ΔflbA, and ΔflbB mutants, suggesting that NsdD is positioned between FLBs and BrlA in A. nidulans. Nullifying nsdD caused formation of conidiophores in liquid submerged cultures, where wild-type strains do not develop. Moreover, the removal of both nsdD and vosA resulted in even more abundant development of conidiophores in liquid submerged cultures and high-level accumulation of brlA messenger (m)RNA even at 16 hr of vegetative growth. Collectively, NsdD is a key negative regulator of conidiation and likely exerts its repressive role via downregulating brlA.

Penicillium decumbens BrlA extensively regulates secondary metabolism and functionally associates with the expression of cellulase genes

Penicillium decumbens has been used in the industrial production of lignocellulolytic enzymes in China for more than 15 years. Conidiation is essential for most industrial fungi because conidia are used as starters in the first step of fermentation. To investigate the mechanism of conidiation in P. decumbens, we generated mutants defective in two central regulators of conidiation, FluG and BrlA. Deletion of fluG resulted in neither "fluffy" phenotype nor alteration in conidiation, indicating possible different upstream mechanisms activating brlA between P. decumbens and Aspergillus nidulans. Deletion of brlA completely blocked conidiation. Further investigation of brlA expression in different media (nutrient-rich or nutrient-poor) and different culture states (liquid or solid) showed that brlA expression is required but not sufficient for conidiation. The brlA deletion strain exhibited altered hyphal morphology with more branches. Genome-wide expression profiling identified BrlA-dependent genes in P. decumbens, including genes previously reported to be involved in conidiation as well as previously reported chitin synthase genes and acid protease gene (pepB). The expression levels of seven secondary metabolism gene clusters (from a total of 28 clusters) were drastically regulated in the brlA deletion strain, including a downregulated cluster putatively involved in the biosynthesis of the mycotoxins roquefortine C and meleagrin. In addition, the expression levels of most cellulase genes were upregulated in the brlA deletion strain detected by real-time quantitative PCR. The brlA deletion strain also exhibited an 89.1 % increase in cellulase activity compared with the wild-type strain. The results showed that BrlA in P. decumbens not only has a key role in regulating conidiation, but it also regulates secondary metabolism extensively as well as the expression of cellulase genes.

Oxygen and an extracellular phase transition independently control central regulatory genes and conidiogenesis in Aspergillus fumigatus

Conidiogenesis is the primary process for asexual reproduction in filamentous fungi. As the conidia resulting from the conidiogenesis process are primarily disseminated via air currents and/or water, an outstanding question has been how fungi recognize aerial environments suitable for conidial development. In this study, we documented the somewhat complex development of the conidia-bearing structures, termed conidiophores, from several Aspergillus species in a subsurface (gel-phase) layer of solid media. A subset of the isolates studied was able to develop conidiophores in a gel-phase environment, but exposure to the aeriform environment was required for the terminal developmental transition from phialide cells to conidia. The remaining Aspergilli could not initiate the conidiogenesis process until they were exposed to the aeriform environment. Our observations of conidiophore development in high or low oxygen conditions in both aeriform and gel-phase environments revealed that oxygen and the aeriform state are positive environmental factors for inducing conidiogenesis in most of the aspergilli tested in this study. Transcriptional analysis using A. fumigatus strain AF293 confined to either the aeriform or gel-phase environments revealed that expression of a key regulatory gene for conidiophore development (AfubrlA) is facilitated by oxygen while expression of another regulatory gene controlling conidia formation from phialides (AfuabaA) was repressed regardless of oxygen levels in the gel-embedded environment. Furthermore, by comparing the developmental behavior of conidiation-defective mutants lacking genes controlling various regulatory checkpoints throughout the conidiogenesis pathway, we propose that this aerial response by the fungus requires both oxygen and the phase transition (solid to aeriform), with these environmental signals integrating into the upstream regulatory pathway and central regulatory pathway of conidiogenesis, respectively. Our findings provide not only novel insight into how fungi respond to an aerial environment to trigger development for airborne conidia production but also the relationship between environmental factors and conidiogenesis regulation in aspergilli.

Gβ-like CpcB plays a crucial role for growth and development of Aspergillus nidulans and Aspergillus fumigatus

Growth, development, virulence and secondary metabolism in fungi are governed by heterotrimeric G proteins (G proteins). A Gβ-like protein called Gib2 has been shown to function as an atypical Gβ in Gpa1-cAMP signaling in Cryptococcus neoformans. We found that the previously reported CpcB (cross pathway control B) protein is the ortholog of Gib2 in Aspergillus nidulans and Aspergillus fumigatus. In this report, we further characterize the roles of CpcB in governing growth, development and toxigenesis in the two aspergilli. The deletion of cpcB results in severely impaired cellular growth, delayed spore germination, and defective asexual sporulation (conidiation) in both aspergilli. Moreover, CpcB is necessary for proper expression of the key developmental activator brlA during initiation and progression of conidiation in A. nidulans and A. fumigatus. Somewhat in accordance with the previous study, the absence of cpcB results in the formation of fewer, but not micro-, cleistothecia in A. nidulans in the presence of wild type veA, an essential activator of sexual development. However, the cpcB deletion mutant cleistothecia contain no ascospores, validating that CpcB is required for progression and completion of sexual fruiting including ascosporogenesis. Furthermore, unlike the canonical GβSfaD, CpcB is not needed for the biosynthesis of the mycotoxin sterigmatocystin (ST) as the cpcB null mutant produced reduced amount of ST with unaltered STC gene expression. However, in A. fumigatus, the deletion of cpcB results in the blockage of gliotoxin (GT) production. Further genetic analyses in A. nidulans indicate that CpcB may play a central role in vegetative growth, which might be independent of FadA- and GanB-mediated signaling. A speculative model summarizing the roles of CpcB in conjunction with SfaD in A. nidulans is presented.

Development in Aspergillus

The genus Aspergillus represents a diverse group of fungi that are among the most abundant fungi in the world. Germination of a spore can lead to a vegetative mycelium that colonizes a substrate. The hyphae within the mycelium are highly heterogeneous with respect to gene expression, growth, and secretion. Aspergilli can reproduce both asexually and sexually. To this end, conidiophores and ascocarps are produced that form conidia and ascospores, respectively. This review describes the molecular mechanisms underlying growth and development of Aspergillus.