Overexpression of flbA, an early regulator of Aspergillus asexual sporulation, leads to activation of brlA and premature initiation of development

Alternaria solani core effector Aex59 is a new member of the Alt a 1 protein family and is recognized as a PAMP

Early blight caused by Alternaria solani is a destructive disease in potato production. Here, through systematically screening of an effector protein pool consisting of 115 small cysteine-containing candidate Aex (Alternariaextracellular proteins) in A. solani, we identified a core effector protein named Aex59, a pathogen-associated molecular pattern (PAMP) molecule. Aex59 is uniquely present in the Ascomycota of fungi and can activate defense responses in multiple plants. Targeted gene disruption showed that Aex59 is a virulence factor and participates in spore development. Perception of Aex59 in Nicotiana benthamiana does not depend on the receptor-like kinases Brassinosteroid-associated kinase1 (BAK1) and Suppressor of BIR1-1 (SOBIR1), which are required for multiple pattern recognition receptors (PRR) pathways. Sequence analysis revealed that Aex59 is a new member of the Alt a 1 protein family and is a potential molecular marker capable of aiding in the classification of the fungi Alternaria spp.

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.

Multifunctionality of AsCFEM6 and AsCFEM12 effectors from the potato early blight pathogen Alternaria solani

Plant pathogens secrete fungal-specific common in several fungal extracellular membrane (CFEM) effectors to manipulate host immunity and contribute to their virulence. Little is known about effectors and their functions in Alternaria solani, the necrotrophic fungal pathogen causing potato early blight. To identify candidate CFEM effector genes, we mined A. solani genome databases. This led to the identification of 12 genes encoding CFEM proteins (termed AsCFEM1-AsCFEM12) and 6 of them were confirmed to be putative secreted effectors. In planta expression revealed that AsCFEM6 and AsCFEM12 have elicitor function that triggers plant defense response including cell death in different botanical families. Targeted gene disruption of AsCFEM6 and AsCFEM12 resulted in a change in spore development, significant reduction of virulence on potato and eggplant susceptible cultivars, increased resistance to fungicide stress, variation in iron acquisition and utilization, and the involvement in 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis pathway. Using maximum likelihood method, we found that positive selection likely caused the polymorphism within AsCFEM6 and AsCFEM12 homologs in different Alternaria spp. Site-directed mutagenesis analysis indicated that positive selection sites within their CFEM domains are required for cell death induction in Nicotiana benthamiana and are critical for response to abiotic stress in yeast. These results demonstrate that AsCFEM effectors possess additional functions beyond their roles in host plant immune response and pathogen virulence.

Establishment of RNA Interference Genetic Transformation System and Functional Analysis of FlbA Gene in Leptographium qinlingensis

Leptographium qinlingensis is a pathogenic fungus of Pinus armandii that is epidemic in the Qinling Mountains. However, an effective gene interference strategy is needed to characterize the pathogenic genes in this fungus on a functional level. Using the RNA silencing vector pSilent-1 as a template, we established an RNA interference genetic transformation system mediated by Agrobacterium tumefaciens GV3101, which is suitable for the gene study for Leptographium qinlingensis by homologous recombination and strain interference system screening. The LqFlbA gene was silenced using the RNA interference approach described above, and the resulting transformants displayed various levels of silencing with a gene silencing effectiveness ranging from 41.8% to 91.4%. The LqFlbA-RNAi mutant displayed altered colony morphology, sluggish mycelium growth, and diminished pathogenicity toward the host P. armandii in comparison to the wild type. The results indicate that this method provides a useful reverse genetic system for studying the gene function of L. qinlingensis, and that LqFlbA plays a crucial role in the growth, development, and pathogenicity of L. qinlingensis.

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.

Regulation of Conidiogenesis in Aspergillus flavus

Aspergillus flavus is a representative fungal species in the Aspergillus section Flavi and has been used as a model system to gain insights into fungal development and toxin production. A. flavus has several adverse effects on humans, including the production of the most carcinogenic mycotoxin aflatoxins and causing aspergillosis in immune-compromised patients. In addition, A. flavus infection of crops results in economic losses due to yield loss and aflatoxin contamination. A. flavus is a saprophytic fungus that disperses in the ecosystem mainly by producing asexual spores (conidia), which also provide long-term survival in the harsh environmental conditions. Conidia are composed of the rodlet layer, cell wall, and melanin and are produced from an asexual specialized structure called the conidiophore. The production of conidiophores is tightly regulated by various regulators, including the central regulatory cascade composed of BrlA-AbaA-WetA, the fungi-specific velvet regulators, upstream regulators, and developmental repressors. In this review, we summarize the findings of a series of recent studies related to asexual development in A. flavus and provide insights for a better understanding of other fungal species in the section Flavi.

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.

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.

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.

Two regulators of G-protein signaling (RGS) proteins FlbA1 and FlbA2 differentially regulate fumonisin B1 biosynthesis in Fusarium verticillioides

Fumonisins are a group of mycotoxins produced by maize pathogen Fusarium verticillioides that pose health concerns to humans and animals. Yet we still lack a clear understanding of the mechanism of fumonisins regulation during pathogenesis. The heterotrimeric G protein complex, which consists of canonical subunits and various regulators of G-protein signaling (RGS) proteins, plays an important role in transducing signals under environmental stress. Earlier studies demonstrated that Gα and Gβ subunits are positive regulators of fumonisin B1 (FB1) biosynthesis and that two RGS genes, FvFlbA1 and FvFlbA2, were highly upregulated in Gβ deletion mutant ∆Fvgbb1. Notably, FvFlbA2 has a negative role in FB1 regulation. While many fungi contain a single copy of FlbA, F. verticillioides harbors two putative FvFlbA paralogs, FvFlbA1 and FvFlbA2. In this study, we further characterized functional roles of FvFlbA1 and FvFlbA2. While ∆FvflbA1 deletion mutant exhibited no significant defects, ∆FvflbA2 and ∆FvflbA2/A1 mutants showed thinner aerial hyphal growth while promoting FB1 production. FvFlbA2 is required for proper expression of key conidia regulation genes, including putative FvBRLA, FvWETA, and FvABAA, while suppressing FUM21, FUM1, and FUM8 expression. Split luciferase assays determined that FvFlbA paralogs interact with key heterotrimeric G protein components, which in turn will lead altered G-protein-mediated signaling pathways that regulate FB1 production and asexual development in F. verticillioides.

Heterotrimeric G-protein signalers and RGSs in Aspergillus fumigatus

The heterotrimeric G-protein (G-protein) signaling pathway is one of the most important signaling pathways that transmit external signals into the inside of the cell, triggering appropriate biological responses. The external signals are sensed by various G-protein-coupled receptors (GPCRs) and transmitted into G-proteins consisting of the α, β, and γ subunits. Regulators of G-protein signaling (RGSs) are the key controllers of G-protein signaling pathways. GPCRs, G-proteins, and RGSs are the primary upstream components of the G-protein signaling pathway, and they are highly conserved in most filamentous fungi, playing diverse roles in biological processes. Recent studies characterized the G-protein signaling components in the opportunistic pathogenic fungus Aspergillus fumigatus. In this review, we have summarized the characteristics and functions of GPCRs, G-proteins, and RGSs, and their regulatory roles in governing fungal growth, asexual development, germination, stress tolerance, and virulence in A. fumigatus.

Smt3, a homologue of yeast SUMO, contributes to asexual development, environmental adaptation, and host infection of a filamentous entomopathogen

Small ubiquitin-like modifiers (SUMOs) act as the modifiers that regulate several important eukaryotic cell events during sumoylation, but little is known about the functions of SUMO or sumoylation in filamentous entomopathogens. Here, we report the important roles of a single SUMO-encoding gene, smt3, in Beauveria bassiana, a filamentous fungal insect pathogen that serves as a main source of wide-spectrum fungal insecticides. The deletion of smt3 led to significant growth defects on the minimal media with different carbon and nitrogen sources, an obvious reduction (45.7 %) in aerial conidiation during optimal cultivation, and increasing sensitivities to metal ions, oxidation, cell wall perturbation, and the fungicide carbendazim during conidial germination and/or colony growth. Compared with the wild-type, the percentage of germination of conidia stored at 4 °C decreased by 83.9 %, and virulence to Galleria mellonella via normal infection was delayed by 24.6 %. However, conidial thermotolerance increased slightly by 11.4 % in Δsmt3. These findings concurred with the repressed transcripts of some phenotype-related genes and decreased activities of antioxidant enzymes. Taken together, smt3 or sumoylation plays vital roles in the asexual development, environmental adaptation, and pathogenicity 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.

Involvement of a G Protein Regulatory Circuit in Alternative Oxidase Production in Neurospora crassa

The Neurospora crassa nuclear aod-1 gene encodes an alternative oxidase that functions in mitochondria. The enzyme provides a branch from the standard electron transport chain by transferring electrons directly from ubiquinol to oxygen. In standard laboratory strains, aod-1 is transcribed at very low levels under normal growth conditions. However, if the standard electron transport chain is disrupted, aod-1 mRNA expression is induced and the AOD1 protein is produced. We previously identified a strain of N. crassa, that produces high levels of aod-1 transcript under non-inducing conditions. Here we have crossed this strain to a standard lab strain and determined the genomic sequences of the parents and several progeny. Analysis of the sequence data and the levels of aod-1 mRNA in uninduced cultures revealed that a frameshift mutation in the flbA gene results in the high uninduced expression of aod-1 The flbA gene encodes a regulator of G protein signaling that decreases the activity of the Gα subunit of heterotrimeric G proteins. Our data suggest that strains with a functional flbA gene prevent uninduced expression of aod-1 by inactivating a G protein signaling pathway, and that this pathway is activated in cells grown under conditions that induce aod-1 Induced cells with a deletion of the gene encoding the Gα protein still have a partial increase in aod-1 mRNA levels, suggesting a second pathway for inducing transcription of the gene in N. crassa We also present evidence that a translational control mechanism prevents production of AOD1 protein in uninduced cultures.

The MaCreA Gene Regulates Normal Conidiation and Microcycle Conidiation in Metarhizium acridum

As a C₂H₂ type zinc finger transcription factor, CreA is the key in Carbon Catabolism Repression (CCR) pathway, which negatively regulates the genes in carbon sources utilization. As conidiation in filamentous fungi is affected by nutritional conditions, CreA may contribute to fungal conidiation, which has been well studied in filamentous fungi, especially Aspergillus spp., but researches on entomopathogenic fungi are not enough. In this study, we found a homologous gene MaCreA in Metarhizium acridum, and the MaCreA deletion strain showed delayed conidiation, significant decrease in conidial yield, and 96.88% lower conidial production, when compared with the wild-type strain, and the normal conidiation and microcycle conidiation pattern shift was blocked. RT-qPCR showed that the transcription levels of the genes FlbD and LaeA (related to asexual development) were significantly altered, and those of most of the conidiation-related genes were higher in ΔMaCreA strain. The results of RNA-Seq revealed that MaCreA regulated the two conidiation patterns by mediating genes related to cell cycle, cell division, cell wall, and cell polarity. In conclusion, CreA, as a core regulatory gene in conidiation, provides new insight into the mechanism of conidiation in entomopathogenic fungi.

Functional studies of aegerolysin and MACPF-like proteins in Aspergillus niger

Proteins of the aegerolysin family have a high abundance in Fungi. Due to their specific binding to membrane lipids, and their membrane-permeabilization potential in concert with protein partner(s) belonging to a membrane-attack-complex/perforin (MACPF) superfamily, they were proposed as useful tools in different biotechnological and biomedical applications. In this work, we performed functional studies on expression of the genes encoding aegerolysin and MACPF-like proteins in Aspergillus niger. Our results suggest the sporulation process being crucial for strong induction of the expression of all these genes. However, deletion of either of the aegerolysin genes did not influence the growth, development, sporulation efficiency and phenotype of the mutants, indicating that aegerolysins are not key factors in the sporulation process. In all our expression studies we noticed a strong correlation in the expression of one aegerolysin and MACPF-like gene. Aegerolysins were confirmed to be secreted from the fungus. We also showed the specific interaction of a recombinant A. niger aegerolysin with an invertebrate-specific membrane sphingolipid. Moreover, using this protein labelled with mCherry we successfully stained insect cells membranes containing this particular sphingolipid. Our combined results suggest, that aegerolysins in this species, and probably also in other aspergilli, could be involved in defence against predators.

Rewiring of transcriptional networks as a major event leading to the diversity of asexual multicellularity in fungi

Complex multicellularity (CM) is characterized by the generation of three-dimensional structures that follow a genetically controlled program. CM emerged at least five times in evolution, one of them in fungi. There are two types of CM programs in fungi, leading, respectively, to the formation of sexual or asexual spores. Asexual spores foment the spread of mycoses, as they are the main vehicle for dispersion. In spite of this key dependence, there is great morphological diversity of asexual multicellular structures in fungi. To advance the understanding of the mechanisms that control initiation and progression of asexual CM and how they can lead to such a remarkable morphological diversification, we studied 503 fungal proteomes, representing all phyla and subphyla, and most known classes. Conservation analyses of 33 regulators of asexual development suggest stepwise emergence of transcription factors. While velvet proteins constitute one of the most ancient systems, the central regulator BrlA emerged late in evolution (with the class Eurotiomycetes). Some factors, such as MoConX4, seem to be species-specific. These observations suggest that the emergence and evolution of transcriptional regulators rewire transcriptional networks. This process could reach the species level, resulting in a vast diversity of morphologies.

Fungal spores: Highly variable and stress-resistant vehicles for distribution and spoilage

This review highlights the variability of fungal spores with respect to cell type, mode of formation and stress resistance. The function of spores is to disperse fungi to new areas and to get them through difficult periods. This also makes them important vehicles for food contamination. Formation of spores is a complex process that is regulated by the cooperation of different transcription factors. The discussion of the biology of spore formation, with the genus Aspergillus as an example, points to possible novel ways to eradicate fungal spore production in food. Fungi can produce different types of spores, sexual and asexually, within the same colony. The absence or presence of sexual spore formation has led to a dual nomenclature for fungi. Molecular techniques have led to a revision of this nomenclature. A number of fungal species form sexual spores, which are exceptionally stress-resistant and survive pasteurization and other treatments. A meta-analysis is provided of numerous D-values of heat-resistant ascospores generated during the years. The relevance of fungal spores for food microbiology has been discussed.

PtaB, a lim-domain binding protein in Aspergillus fumigatus regulates biofilm formation and conidiation through distinct pathways

The exopolysaccharide galactosaminogalactan (GAG) plays an important role in mediating adhesion, biofilm formation, and virulence in the pathogenic fungus Aspergillus fumigatus. The developmental modifiers MedA, StuA, and SomA regulate GAG biosynthesis, but the mechanisms underlying this regulation are poorly understood. PtaB is a lim-domain binding protein that interacts with the transcription factor SomA and is required for normal conidiation and biofilm formation. Disruption of ptaB resulted in impaired GAG production and conidiation in association with a markedly reduced expression of GAG biosynthetic genes (uge3 and agd3), developmental regulators (medA and stuA), and genes involved in the core conidiation pathway. Overexpression of medA and dual overexpression of uge3 and agd3 in the ΔptaB mutant increased biofilm formation but not conidiation, whereas overexpression of core conidiation genes rescued conidiation but not biofilm formation. Overexpression of stuA modestly increased both conidiation and biofilm formation. Analysis of ptaB truncation mutants revealed that overexpression of the lim-domain binding region restored conidiation but not biofilm formation, suggesting that ptaB may govern these processes by interacting with different partners. These studies establish that PtaB governs GAG biosynthesis at the level of substrate availability and polymer deacetylation and that PtaB-mediated biofilm formation and conidiation are largely independent pathways.

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.

Electrospun cellulose acetate composites containing supported metal nanoparticles for antifungal membranes

Electrospun cellulose acetate composites containing silver and copper nanoparticles supported in sepiolite and mesoporous silica were prepared and tested as fungistatic membranes against the fungus Aspergillus niger. The nanoparticles were in the 3-50nm range for sepiolite supported materials and limited by the size of mesopores (5-8nm) in the case of mesoporous silica. Sepiolite and silica were well dispersed within the fibers, with larger aggregates in the micrometer range, and allowed a controlled release of metals to create a fungistatic environment. The effect was assessed using digital image analysis to evaluate fungal growth rate and fluorescence readings using a viability stain. The results showed that silver and copper nanomaterials significantly impaired the growth of fungi when the spores were incubated either in direct contact with particles or included in cellulose acetate composite membranes. The fungistatic effect took place on germinating spores before hyphae growth conidiophore formation. After 24h the cultures were separated from fungistatic materials and showed growth impairment only due to the prior exposure. Growth reduction was important for all the particles and membranes with respect to non-exposed controls. The effect of copper and silver loaded materials was not significantly different from each other with average reductions around 70% for bare particles and 50% for membranes. Copper on sepiolite was particularly efficient with a decrease of metabolic activity of up to 80% with respect to controls. Copper materials induced rapid maturation and conidiation with fungi splitting in sets of subcolonies. Metal-loaded nanomaterials acted as reservoirs for the controlled release of metals. The amount of silver or copper released daily by composite membranes represented roughly 1% of their total load of metals. Supported nanomaterials encapsulated in nanofibers allow formulating active membranes with high antifungal performance at the same time minimizing the risk of nanoparticle release into the environment.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

The Accordant Trend of Both Parameters (rgs Expression and cAMP Content) Follows the Pattern of Development of Fruiting Body in Volvariella volvacea

The formation of fruiting body in Volvariella volvacea is affected by endogenous genes and environmental factors. However, its regulation at a molecular level is still poorly understood. To study the genes involved in the formation of fruiting body, we cloned a new regulator of the G protein signaling (RGS) encoding gene (rgs) from V. volvacea. Phylogenetic analysis showed that RGS in V. volvacea and other basidiomycete RGS proteins from Schizophyllum commune and Coprinus cinereus belong to the same clade. In addition, we assayed intracellular cAMP content in the three developmental stages (mycelium, fruiting body primordia, and button). We also found that the expression of rgs was highly positively correlated to the content of intracellular cAMP during fruiting body formation. The conserved protein sequences and expression of rgs, together with high concent of cAMP at primordia tissue, suggested that rgs gene and cAMP may play a crucial role in fruiting body formation in V. volvacea.

Systems approaches to predict the functions of glycoside hydrolases during the life cycle of Aspergillus niger using developmental mutants ∆brlA and ∆flbA

BACKGROUND

The filamentous fungus Aspergillus niger encounters carbon starvation in nature as well as during industrial fermentations. In response, regulatory networks initiate and control autolysis and sporulation. Carbohydrate-active enzymes play an important role in these processes, for example by modifying cell walls during spore cell wall biogenesis or in cell wall degradation connected to autolysis.

RESULTS

In this study, we used developmental mutants (ΔflbA and ΔbrlA) which are characterized by an aconidial phenotype when grown on a plate, but also in bioreactor-controlled submerged cultivations during carbon starvation. By comparing the transcriptomes, proteomes, enzyme activities and the fungal cell wall compositions of a wild type A. niger strain and these developmental mutants during carbon starvation, a global overview of the function of carbohydrate-active enzymes is provided. Seven genes encoding carbohydrate-active enzymes, including cfcA, were expressed during starvation in all strains; they may encode enzymes involved in cell wall recycling. Genes expressed in the wild-type during starvation, but not in the developmental mutants are likely involved in conidiogenesis. Eighteen of such genes were identified, including characterized sporulation-specific chitinases and An15g02350, member of the recently identified carbohydrate-active enzyme family AA11. Eight of the eighteen genes were also expressed, independent of FlbA or BrlA, in vegetative mycelium, indicating that they also have a role during vegetative growth. The ΔflbA strain had a reduced specific growth rate, an increased chitin content of the cell wall and specific expression of genes that are induced in response to cell wall stress, indicating that integrity of the cell wall of strain ΔflbA is reduced.

CONCLUSION

The combination of the developmental mutants ΔflbA and ΔbrlA resulted in the identification of enzymes involved in cell wall recycling and sporulation-specific cell wall modification, which contributes to understanding cell wall remodeling mechanisms during development.

The global regulator FfSge1 is required for expression of secondary metabolite gene clusters but not for pathogenicity in Fusarium fujikuroi

The plant pathogenic fungus Fusarium fujikuroi is the causal agent of bakanae disease on rice due to its ability to produce gibberellins. Besides these phytohormones, F. fujikuroi is able to produce several other secondary metabolites (SMs). Although much progress has been made in the field of secondary metabolism, the transcriptional regulation of SM biosynthesis is complex and still incompletely understood. Environmental conditions, global as well as pathway-specific regulators and chromatin remodelling have been shown to play major roles. Here, the role of FfSge1, a homologue of the morphological switch regulators Wor1 and Ryp1 in Candida albicans and Histoplasma capsulatum, respectively, is explored with emphasis on secondary metabolism. FfSge1 is not required for formation of conidia and pathogenicity but is involved in vegetative growth. Transcriptome analysis of the mutant Δffsge1 compared with the wild type, as well as comparative chemical analysis between the wild type, Δffsge1 and OE:FfSGE1, revealed that FfSge1 functions as a global activator of secondary metabolism in F. fujikuroi. Double mutants of FfSGE1 and other SM regulatory genes brought insights into the hierarchical regulation of secondary metabolism. In addition, FfSge1 is also required for expression of a yet uncharacterized SM gene cluster containing a non-canonical non-ribosomal peptide synthetase.

Insights into Monascus biology at the genetic level

The genus of Monascus was nominated by van Tieghem in 1884, but its fermented product-red mold rice (RMR), namely red yeast rice, has been used as folk medicines, food colorants, and fermentation starters for more than thousands of years in oriental countries. Nowadays, RMR is widely developed as food supplements around the world due to its functional compounds such as monacolin K (MK, also called lovastatin) and γ-aminobutyric acid. But the usage of RMR also incurs controversy resulting from contamination of citrinin (a kind of mycotoxin) produced by some Monascus strains. In the past decade, it has made great progress to Monascus spp. at the genetic level with the application of molecular biology techniques to restrain the citrinin production and increase the yields of MK and pigment in RMR, as well as aid Monascus classification and phylogenesis. Up to now, hundreds of papers about Monascus molecular biology (MMB) have been published in the international primary journals. However, to our knowledge, there is no MMB review issued until now. In this review, current understanding of Monascus spp. from the view of molecular biology will be covered and insights into research areas that need to be further investigated will also be discussed.

The sugarcane defense protein SUGARWIN2 causes cell death in Colletotrichum falcatum but not in non-pathogenic fungi

Plants respond to pathogens and insect attacks by inducing and accumulating a large set of defense-related proteins. Two homologues of a barley wound-inducible protein (BARWIN) have been characterized in sugarcane, SUGARWIN1 and SUGARWIN2 (sugarcane wound-inducible proteins). Induction of SUGARWINs occurs in response to Diatraea saccharalis damage but not to pathogen infection. In addition, the protein itself does not show any effect on insect development; instead, it has antimicrobial activities toward Fusarium verticillioides, an opportunistic fungus that usually occurs after D. saccharalis borer attacks on sugarcane. In this study, we sought to evaluate the specificity of SUGARWIN2 to better understand its mechanism of action against phytopathogens and the associations between fungi and insects that affect plants. We used Colletotrichum falcatum, a fungus that causes red rot disease in sugarcane fields infested by D. saccharalis, and Ceratocystis paradoxa, which causes pineapple disease in sugarcane. We also tested whether SUGARWIN2 is able to cause cell death in Aspergillus nidulans, a fungus that does not infect sugarcane, and in the model yeast Saccharomyces cerevisiae, which is used for bioethanol production. Recombinant SUGARWIN2 altered C. falcatum morphology by increasing vacuolization, points of fractures and a leak of intracellular material, leading to germling apoptosis. In C. paradoxa, SUGARWIN2 showed increased vacuolization in hyphae but did not kill the fungi. Neither the non-pathogenic fungus A. nidulans nor the yeast S. cerevisiae was affected by recombinant SUGARWIN2, suggesting that the protein is specific to sugarcane opportunistic fungal pathogens.

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.

Cytokinesis-required Cdc14 is a signaling hub of asexual development and multi-stress tolerance in Beauveria bassiana

A dual-specificity, paralogue-free Cdc14 phosphatase was located in the nuclei of Beauveria bassiana (filamentous entomopathogen) and functionally characterized. Inactivation of cdc14 caused defective cytokinesis due to multinucleate cells formed in Δcdc14 and 89% decrease of blastospore production, followed by slower growth and a loss of ≥ 96% conidial yield under normal conditions. These defects coincided well with drastic down-regulation of 25 genes required for mitosis and conidiation. Moreover, Δcdc14 became hypersensitive to oxidative, osmotic, and cell wall and mitosis perturbing stresses, and lost 41−70% of conidial thermotolerance, UV-B resistance and virulence, accompanied with transcriptional down-regualtion of various signaling factors and stress-responsive effectors and depressed phosphorylation signals of Hog1 and Slt2 in high-osmolarity glycerol and cell-wall integrity pathways. All changes were well restored by rescuing cdc14. Our findings indicate that Cdc14 vital for the fungal cytokinesis acts as a signaling hub in regulating not only asexual development but multi-stress responses and virulence.

The N-terminus region of the putative C2H2 transcription factor Ada1 harbors a species-specific activation motif that regulates asexual reproduction in Fusarium verticillioides

Fusarium verticillioides is an important plant pathogenic fungus causing maize ear and stalk rots. In addition, the fungus is directly associated with fumonisin contamination of food and feeds. Here, we report the functional characterization of Ada1, a putative Cys2-His2 zinc finger transcription factor with a high level of similarity to Aspergillus nidulans FlbC, which is required for the activation of the key regulator of conidiation brlA. ADA1 is predicted to encode a protein with two DNA binding motifs at the C terminus and a putative activator domain at the N terminus region. Deletion of the flbC gene in A. nidulans results in "fluffy" cotton-like colonies, with a defect in transition from vegetative growth to asexual development. In this study we show that Ada1 plays a key role in asexual development in F. verticillioides. Conidia production was significantly reduced in the knockout mutant (Δada1), in which aberrant conidia and conidiophores were also observed. We identified genes that are predicted to be downstream of ADA1, based on A. nidulans conidiation signaling pathway. Among them, the deletion of stuA homologue, FvSTUA, resulted in near absence of conidia production. To further investigate the functional conservation of this transcription factor, we complemented the Δada1 strain with A. nidulans flbC, F. verticillioides ADA1, and chimeric constructs. A. nidulans flbC failed to restore conidia production similar to the wild-type level. However, the Ada1N-terminal domain, which contains a putative activator, fused to A. nidulans FlbC C-terminal motif successfully complemented the Δada1 mutant. Taken together, Ada1 is an important transcriptional regulator of asexual development in F. verticillioides and that the N-terminus domain is critical for proper function of this transcription factor.

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.

Heterogeneity in the mycelium: implications for the use of fungi as cell factories

Fungi are widely used as cell factories for the production of pharmaceutical compounds, enzymes and metabolites. Fungi form colonies that consist of a network of hyphae. During the last two decades it has become clear that fungal colonies within a liquid culture are heterogeneous in size and gene expression. Heterogeneity in growth, secretion, and RNA composition can even be found between and within zones of colonies. These findings imply that productivity in a bioreactor may be increased by reducing the heterogeneity within the culture. The results also imply that molecular mechanisms underlying productivity of fungi in bioreactors should not be studied at the culture level but at the level of micro-colony populations or even at zonal or hyphal level.

Deletion of flbA results in increased secretome complexity and reduced secretion heterogeneity in colonies of Aspergillus niger

Aspergillus niger is a cell factory for the production of enzymes. This fungus secretes proteins in the central part and at the periphery of the colony. The sporulating zone of the colony overlapped with the nonsecreting subperipheral zone, indicating that sporulation inhibits protein secretion. Indeed, strain ΔflbA that is affected early in the sporulation program secreted proteins throughout the colony. In contrast, the ΔbrlA strain that initiates but not completes sporulation did not show altered spatial secretion. The secretome of 5 concentric zones of xylose-grown ΔflbA colonies was assessed by quantitative proteomics. In total 138 proteins with a signal sequence for secretion were identified in the medium of ΔflbA colonies. Of these, 18 proteins had never been reported to be part of the secretome of A. niger, while 101 proteins had previously not been identified in the culture medium of xylose-grown wild type colonies. Taken together, inactivation of flbA results in spatial changes in secretion and in a more complex secretome. The latter may be explained by the fact that strain ΔflbA has a thinner cell wall compared to the wild type, enabling efficient release of proteins. These results are of interest to improve A. niger as a cell factory.

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.

Insight into the transcriptional regulation of Msn2 required for conidiation, multi-stress responses and virulence of two entomopathogenic fungi

Msn2/4 transcription factors in some fungi have null effects on virulence and cellular stress responses. Here we found that the transcriptional regulation of Msn2 orthologs is vital for the conidiation, virulence and multi-stress responses of Beauveria bassiana (Bb) and Metarhizium robertsii (Mr), which lack Msn4 orthologs. Compared to wild-type and complemented strains of each fungus with all similar phenotypes, ΔBbmsn2 and ΔMrmsn2 showed remarkable defects in conidial yield (∼40% decrease) and virulence (∼25% decrease). Both delta mutants lost 20-65% of their tolerances to hyperosmolarity, oxidation, carbendazim, cell wall perturbing and high temperature at 34 °C during colony growth. Their conidia were also significantly (18-41%) less tolerant to oxidation, hyperosmolarity, wet-heat stress at 45 °C and UV-B irradiation. Accompanied with the defective phenotypes, several conidiation- and virulence-associated genes were greatly repressed in ΔBbmsn2 and ΔMrmsn2. Moreover, differentially expressed genes in the transcriptomes of ΔBbmsn2 versus wild type were ∼3% more under oxidative stress, but ∼12% fewer under heat shock, than those in the ΔMrmsn2 counterparts. Many stress-responsive effector genes and cellular signaling factors were remarkably downregulated. Taken together, the two entomopathogens could have evolved somewhat distinct stress-responsive mechanisms finely tuned by Msn2, highlighting the biological significance of Msn2 orthologs for filamentous fungi.

P-type calcium ATPase functions as a core regulator of Beauveria bassiana growth, conidiation and responses to multiple stressful stimuli through cross-talk with signalling networks

P-type Ca(2+) -ATPase (Pmr1) is a core element in calcium-calcineurin pathway and evidence for its cross-talk with other signalling pathways in filamentous fungi is of scarcity. Here, we characterized the striking functions of a Golgi Ca(2+) -ATPase (Bbpmr1) in Beauveria bassiana (fungal entomopathogen) by multi-phenotypic and transcriptional analyses under normal and stressful conditions. Bbpmr1 inactivation caused severe defects in nutritional uptake, growth, conidiation and germination under normal conditions, drastic reductions in cell tolerances to oxidative, hyperosmotic, cell wall disturbing and fungicidal stresses and toxic metal ions during colony growth and/or conidial germination, and half loss of the fungal biocontrol potential represented by conidial virulence, thermotolerance and UV-B resistance. Accompanied with the multi-phenotypic defects, four important genes associated with asexual development were repressed by ≥ 75% in ΔBbpmr1 versus wild type, and all or most of stress-responsive genes encoding 14 cascaded proteins in MAPK pathways, two Ras GTPases, two protein kinases, Ssk1-type response regulator, TOR signalling protein, and many downstream enzymes and proteins were greatly downregulated in ΔBbpmr1 under the chemical stresses. Conclusively, Bbpmr1 regulates positively fundamental aspects on B. bassiana biology and environmental adaptation through wide cross-talk with cellular signalling networks including MAPK cascades and those upstream or independent of the cascades.

The putative guanine nucleotide exchange factor RicA mediates upstream signaling for growth and development in Aspergillus

Heterotrimeric G proteins (G proteins) govern growth, development, and secondary metabolism in various fungi. Here, we characterized ricA, which encodes a putative GDP/GTP exchange factor for G proteins in the model fungus Aspergillus nidulans and the opportunistic human pathogen Aspergillus fumigatus. In both species, ricA mRNA accumulates during vegetative growth and early developmental phases, but it is not present in spores. The deletion of ricA results in severely impaired colony growth and the total (for A. nidulans) or near (for A. fumigatus) absence of asexual sporulation (conidiation). The overexpression (OE) of the A. fumigatus ricA gene (AfricA) restores growth and conidiation in the ΔAnricA mutant to some extent, indicating partial conservation of RicA function in Aspergillus. A series of double mutant analyses revealed that the removal of RgsA (an RGS protein of the GanB Gα subunit), but not sfgA, flbA, rgsB, or rgsC, restored vegetative growth and conidiation in ΔAnricA. Furthermore, we found that RicA can physically interact with GanB in yeast and in vitro. Moreover, the presence of two copies or OE of pkaA suppresses the profound defects caused by ΔAnricA, indicating that RicA-mediated growth and developmental signaling is primarily through GanB and PkaA in A. nidulans. Despite the lack of conidiation, brlA and vosA mRNAs accumulated to normal levels in the ΔricA mutant. In addition, mutants overexpressing fluG or brlA (OEfluG or OEbrlA) failed to restore development in the ΔAnricA mutant. These findings suggest that the commencement of asexual development requires unknown RicA-mediated signaling input in A. nidulans.

FlbD, a Myb transcription factor of Aspergillus nidulans, is uniquely involved in both asexual and sexual differentiation

In the fungus Aspergillus nidulans, inactivation of the flbA to -E, fluG, fluF, and tmpA genes results in similar phenotypes, characterized by a delay in conidiophore and asexual spore production. flbB to -D encode transcription factors needed for proper expression of the brlA gene, which is essential for asexual development. However, recent evidence indicates that FlbB and FlbE also have nontranscriptional functions. Here we show that fluF1 is an allele of flbD which results in an R47P substitution. Amino acids C46 and R47 are highly conserved in FlbD and many other Myb proteins, and C46 has been proposed to mediate redox regulation. Comparison of ΔflbD and flbD(R47P) mutants uncovered a new and specific role for flbD during sexual development. While flbD(R47P) mutants retain partial function during conidiation, both ΔflbD and flbD(R47P) mutants are unable to develop the peridium, a specialized external tissue that differentiates during fruiting body formation and ends up surrounding the sexual spores. This function, unique among other fluffy genes, does not affect the viability of the naked ascospores produced by mutant strains. Notably, ascospore development in these mutants is still dependent on the NADPH oxidase NoxA. We generated R47K, C46D, C46S, and C46A mutant alleles and evaluated their effects on asexual and sexual development. Conidiation defects were most severe in ΔflbD mutants and stronger in R47P, C46D, and C46S strains than in R47K strains. In contrast, mutants carrying the flbD(C46A) allele exhibited conidiation defects in liquid culture only under nitrogen starvation conditions. The R47K, R47P, C46D, and C46S mutants failed to develop any peridial tissue, while the flbD(C46A) strain showed normal peridium development and increased cleistothecium formation. Our results show that FlbD regulates both asexual and sexual differentiation, suggesting that both processes require FlbD DNA binding activity and that FlbD is involved in the response to nitrogen starvation.

Structure-function analysis of Rgs1 in Magnaporthe oryzae: role of DEP domains in subcellular targeting

BACKGROUND

Rgs1, a prototypical Regulator of G protein Signaling, negatively modulates the cyclic AMP pathway thereby influencing various aspects of asexual development and pathogenesis in the rice-blast fungus Magnaporthe oryzae. Rgs1 possesses tandem DEP motifs (termed DEP-A and DEP-B; for Dishevelled, Egl-10, Pleckstrin) at the N-terminus, and a Gα-GTP interacting RGS catalytic core domain at the C-terminus. In this study, we focused on gaining further insights into the mechanisms of Rgs1 regulation and subcellular localization by characterizing the role(s) of the individual domains and the full-length protein during asexual development and pathogenesis in Magnaporthe.

METHODOLOGY/PRINCIPAL FINDINGS

Utilizing western blot analysis and specific antisera against the N- and C-terminal halves of Rgs1, we identify and report the in vivo endoproteolytic processing/cleavage of full-length Rgs1 that yields an N-terminal DEP and a RGS core domain. Independent expression of the resultant DEP-DEP half (N-Rgs1) or RGS core (C-Rgs1) fragments, failed to complement the rgs1Δ defects in colony morphology, aerial hyphal growth, surface hydrophobicity, conidiation, appressorium formation and infection. Interestingly, the full-length Rgs1-mCherry, as well as the tagged N-terminal DEP domains (individually or in conjunction) localized to distinct punctate vesicular structures in the cytosol, while the catalytic RGS core motif was predominantly vacuolar.

CONCLUSIONS/SIGNIFICANCE

Based on our data from sequence alignments, immuno-blot and microscopic analysis, we propose that the post-translational proteolytic processing of Rgs1 and the vacuolar sequestration of the catalytic RGS domain represents an important means of down regulating Rgs1 function and thus forming an additional and alternative means of regulating G protein signaling in Magnaporthe. We further hypothesize the prevalence of analogous mechanisms functioning in other filamentous fungi. Furthermore, we conclusively assign a specific vesicular/membrane targeting function for the N-terminal DEP domains of Rgs1 in the rice-blast fungus.