Regulator of G Protein Signaling Contributes to the Development and Aflatoxin Biosynthesis in Aspergillus flavus through the Regulation of Gα Activity

Glucose and HODEs regulate Aspergillus ochraceus quorum sensing through the GprC-AcyA pathway

Aspergillus ochraceus is the traditional ochratoxin A (OTA)-producing fungus with density-dependent behaviors, which is known as quorum sensing (QS) that is mediated by signaling molecules. Individual cells trend to adapt environmental changes in a "whole" flora through communications, allowing fungus to occupy an important ecological niche. Signals perception, transmission, and feedback are all rely on a signal network that constituted by membrane receptors and intracellular effectors. However, the interference of density information in signal transduction, which regulates most life activities of Aspergillus, have yet to be elucidated. Here we show that the G protein-coupled receptor (GPCR) to cAMP pathway is responsible for transmitting density information, and regulates the key point in life cycle of A. ochraceus. Firstly, the quorum sensing phenomenon of A. ochraceus is confirmed, and identified the density threshold is 103 spores/mL, which represents the low density that produces the most OTA in a series quorum density. Moreover, the GprC that classified as sugar sensor, and intracellular adenylate cyclase (AcyA)-cAMP-PKA pathway that in response to ligands glucose and HODEs are verified. Furthermore, GprC and AcyA regulate the primary metabolism as well as secondary metabolism, and further affects the growth of A. ochraceus during the entire life cycle. These studies highlight a crucial G protein signaling pathway for cell communication that is mediated by carbohydrate and oxylipins, and clarified a comprehensive effect of fungal development, which include the direct gene regulation and indirect substrate or energy supply. Our work revealed more signal molecules that mediated density information and connected effects on important adaptive behaviors of Aspergillus ochraceus, hoping to achieve comprehensive prevention and control of mycotoxin pollution from interrupting cell communication.

Class VI G protein-coupled receptors in Aspergillus oryzae regulate sclerotia formation through GTPase-activating activity

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in eukaryotes that sense and transduce extracellular signals into cells. In Aspergillus oryzae, 16 canonical GPCR genes are identified and classified into nine classes based on the sequence similarity and proposed functions. Class VI GPCRs (AoGprK-1, AoGprK-2, and AoGprR in A. oryzae), unlike other GPCRs, feature a unique hybrid structure containing both the seven transmembrane (7-TM) and regulator of G-protein signaling (RGS) domains, which is not found in animal GPCRs. We report here that the mutants with double or triple deletion of class VI GPCR genes produced significantly increased number of sclerotia compared to the control strain when grown on agar plates. Interestingly, complementation analysis demonstrated that the expression of the RGS domain without the 7-TM domain is sufficient to restore the phenotype. In line with this, among the three Gα subunits in A. oryzae, AoGpaA, AoGpaB, and AoGanA, forced expression of GTPase-deficient mutants of either AoGpaA or AoGpaB caused an increase in the number of sclerotia formed, suggesting that RGS domains of class VI GPCRs are the negative regulators of these two GTPases. Finally, we measured the expression of velvet complex genes and sclerotia formation-related genes and found that the expression of velB was significantly increased in the multiple gene deletion mutants. Taken together, these results demonstrate that class VI GPCRs negatively regulate sclerotia formation through their GTPase-activating activity in the RGS domains. KEY POINTS: • Class VI GPCRs in A. oryzae regulate sclerotia formation in A. oryzae • RGS function of class VI GPCRs is responsible for regulation of sclerotia formation • Loss of class VI GPCRs resulted in increased expression of sclerotia-related genes.

Regulation of Fungal Morphogenesis and Pathogenicity of Aspergillus flavus by Hexokinase AfHxk1 through Its Domain Hexokinase_2

As a filamentous pathogenic fungus with high-yield of aflatoxin B1, Aspergillus flavus is commonly found in various agricultural products. It is crucial to develop effective strategies aimed at the prevention of the contamination of A. flavus and aflatoxin. Hexokinase AfHxk1 is a critical enzyme in fungal glucose metabolism. However, the role of AfHxk1 in A. flavus development, aflatoxin biosynthesis, and virulence has not yet been explored. In this study, afHxk1 gene deletion mutant (ΔafHxk1), complementary strain (Com-afHxk1), and the domain deletion strains (afHxk1ΔD1 and afHxk1ΔD2) were constructed by homologous recombination. Phenotype study and RT-qPCR revealed that AfHxk1 upregulates mycelium growth and spore and sclerotia formation, but downregulates AFB1 biosynthesis through related classical signaling pathways. Invading models and environmental stress analysis revealed that through involvement in carbon source utilization, conidia germination, and the sensitivity response of A. flavus to a series of environmental stresses, AfHxk1 deeply participates in the regulation of pathogenicity of A. flavus to crop kernels and Galleria mellonella larvae. The construction of domain deletion strains, afHxk1ΔD1 and afHxk1ΔD2, further revealed that AfHxk1 regulates the morphogenesis, mycotoxin biosynthesis, and the fungal pathogenicity mainly through its domain, Hexokinase_2. The results of this study revealed the biological role of AfHxk1 in Aspergillus spp., and might provide a novel potential target for the early control of the contamination of A. flavus.

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.

Phosducin-like protein PoPlp1 impacts cellulase and amylase expression and development in Penicillium oxalicum via the G protein-cAMP signaling pathway

In this study, a phosducin-like protein, PoPlp1, was identified and functionally studied in the cellulase-producing strain Penicillium oxalicum 114-2. PoPlp1 was proven to participate in several biological processes, including mycelium development, conidiation, and expression of cellulases and amylases. With deletion of Poplp1, morphology and development varied significantly in ΔPoplp1. Colony growth, glucose utilization, and the hydrolysis capability of starch and cellulose were limited, whereas conidiation was enhanced. Based on detection of the levels of expression of transcription factors involved in asexual development, we conjectured that PoPlp1 is involved in conidiation via the major factor BrlA. We explored the effect of PoPlp1 on cellulase and amylase expression and observed that cellulase and amylase activity and major gene transcription levels were all dramatically reduced in ΔPoplp1. Deletion of PoPlp1 caused a decrease in intracellular cAMP levels, and the cellulase gene expression level of ΔPoplp1 was restored to a certain extent through external addition of cAMP. These findings demonstrate that PoPlp1 may affect cellulase and amylase expression by regulating cAMP concentration. To comprehensively explore the mechanism of PoPlp1 in regulating multiple biological processes, we performed a comparative transcriptomic analysis between strains P. oxalicum 114-2 and ΔPoplp1. The major cellulase and amylase genes were all downregulated, congrent with the results of real-time quantitative polymerase chain reaction analysis. The genes involved in the G protein-cAMP signaling pathway, including several G-protein-coupled receptors, one regulator of G protein signaling, and two cAMP phosphodiesterases, were disrupted by deletion of PoPlp1. These results confirm the positive function of PoPlp1 in the G protein-cAMP signaling pathway. This functional analysis of PoPlp1 will be very beneficial for further study of the regulatory mechanisms of cellulase expression and other biological processes in P. oxalicum 114-2 via the G protein-cAMP signaling pathway.

Genomic and Phenotypic Trait Variation of the Opportunistic Human Pathogen Aspergillus flavus and Its Close Relatives

Fungal diseases affect millions of humans annually, yet fungal pathogens remain understudied. The mold Aspergillus flavus can cause both aspergillosis and fungal keratitis infections, but closely related species are not considered clinically relevant. To study the evolution of A. flavus pathogenicity, we examined genomic and phenotypic traits of two strains of A. flavus and three closely related species, Aspergillus arachidicola (two strains), Aspergillus parasiticus (two strains), and Aspergillus nomiae (one strain). We identified >3,000 orthologous proteins unique to A. flavus, including seven biosynthetic gene clusters present in A. flavus strains and absent in the three nonpathogens. We characterized secondary metabolite production for all seven strains under two clinically relevant conditions, temperature and salt concentration. Temperature impacted metabolite production in all species, whereas salinity did not affect production of any species. Strains of the same species produced different metabolites. Growth under stress conditions revealed additional heterogeneity within species. Using the invertebrate fungal disease model Galleria mellonella, we found virulence of strains of the same species varied widely; A. flavus strains were not more virulent than strains of the nonpathogens. In a murine model of fungal keratitis, we observed significantly lower disease severity and corneal thickness for A. arachidicola compared to other species at 48 h postinfection, but not at 72 h. Our work identifies variations in key phenotypic, chemical, and genomic attributes between A. flavus and its nonpathogenic relatives and reveals extensive strain heterogeneity in virulence that does not correspond to the currently established clinical relevance of these species. IMPORTANCE Aspergillus flavus is a filamentous fungus that causes opportunistic human infections, such as aspergillosis and fungal keratitis, but its close relatives are considered nonpathogenic. To begin understanding how this difference in pathogenicity evolved, we characterized variation in infection-relevant genomic, chemical, and phenotypic traits between strains of A. flavus and its relatives. We found extensive variation (or strain heterogeneity) within the pathogenic A. flavus as well as within its close relatives, suggesting that strain-level differences may play a major role in the ability of these fungi to cause disease. Surprisingly, we also found that the virulence of strains from species not considered to be pathogens was similar to that of A. flavus in both invertebrate and murine models of disease. These results contrast with previous studies on Aspergillus fumigatus, another major pathogen in the genus, for which significant differences in infection-relevant chemical and phenotypic traits are observed between closely related pathogenic and nonpathogenic species.

Regulator of G Protein Signaling Proteins Control Growth, Development and Cellulase Production in Neurospora crassa

Heterotrimeric (αβγ) G protein signaling pathways are critical environmental sensing systems found in eukaryotic cells. Exchange of GDP for GTP on the Gα subunit leads to its activation. In contrast, GTP hydrolysis on the Gα is accelerated by Regulator of G protein Signaling (RGS) proteins, resulting in a return to the GDP-bound, inactive state. Here, we analyzed growth, development and extracellular cellulase production in strains with knockout mutations in the seven identified RGS genes (rgs-1 to rgs-7) in the filamentous fungus, Neurospora crassa. We compared phenotypes to those of strains with either knockout mutations or expressing predicted constitutively activated, GTPase-deficient alleles for each of the three Gα subunit genes (gna-1^Q204L, gna-2^Q205L or gna-3^Q208L). Our data revealed that six RGS mutants have taller aerial hyphae than wild type and all seven mutants exhibit reduced asexual sporulation, phenotypes shared with strains expressing the gna-1^Q204L or gna-3^Q208L allele. In contrast, Δrgs-1 and Δrgs-3 were the only RGS mutants with a slower growth rate phenotype, a defect in common with gna-1^Q204L strains. With respect to female sexual development, Δrgs-1 possessed defects most similar to gna-3^Q208L strains, while those of Δrgs-2 mutants resembled strains expressing the gna-1^Q204L allele. Finally, we observed that four of the seven RGS mutants had significantly different extracellular cellulase levels relative to wild type. Of interest, the Δrgs-2 mutant had no detectable activity, similar to the gna-3^Q208L strain. In contrast, the Δrgs-1 and Δrgs-4 mutants and gna-1^Q204L and gna-2^Q205L strains exhibited significantly higher cellulase activity than wild type. With the exception of sexual development, our results demonstrate the greatest number of genetic interactions between rgs-1 and gna-1 and rgs-2 and gna-3 in N. crassa.

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.

Phylogeny of Regulators of G-Protein Signaling Genes in Leptographium qinlingensis and Expression Levels of Three RGSs in Response to Different Terpenoids

Leptographium qinlingensis is a bark beetle-vectored pine pathogen in the Chinese white pine beetle (Dendroctonus armandi) epidemic in Northwest China. L. qinlingensis colonizes pines despite the trees’ massive oleoresin terpenoid defenses. Regulators of G-protein signaling (RGS) proteins modulate heterotrimeric G-protein signaling negatively and play multiple roles in the growth, asexual development, and pathogenicity of fungi. In this study, we have identified three L. qinlingensis RGS genes, and the phylogenetic analysis shows the highest homology with the regulators of G-protein signaling proteins sequence from Ophiostoma piceae and Grosmannia clavigera. The expression profiles of three RGSs in the mycelium of L. qinlingensis treated with six different terpenoids were detected, as well as their growth rates. Under six terpenoid treatments, the growth and reproduction in L. qinlingensis were significantly inhibited, and the growth inflection day was delayed from 8 days to 12–13 days. By analyzing the expression level of three RGS genes of L. qinlingensis with different treatments, results indicate that LqFlbA plays a crucial role in controlling fungal growth, and both LqRax1 and LqRgsA are involved in overcoming the host chemical resistances and successful colonization.