Rab GDP-dissociation inhibitor gdiA is an essential gene required for cell wall chitin deposition in Aspergillus niger

The G-protein alpha subunit AaGA1 positively regulates vegetative growth, appressorium-like formation, and pathogenicity in Alternaria alternata

AIMS

The Gα subunit is a major component of heterotrimeric G proteins, which play a crucial role in the development and pathogenicity of several model fungi. However, its detailed function in the causal agent of pear black spot (Alternaria alternata) is unclear. Our aim was to understand the characteristics and functions of AaGA1 in A. alternata.

METHODS AND RESULTS

AaGA1 was cloned from A. alternata in this study, which encodes 353 amino acids and has a "G-alpha" domain. Mutant ΔAaGA1 resulted in reduced vegetative growth, conidiation, and spore germination. Especially, mutant ΔAaGA1 produced only fewer conidia on the V8A medium, and spore formation-related genes AbaA, BrlA, and WetA were significantly downregulated. More tolerance against cell wall-inhibiting agents was observed after the deletion of AaGA1. Moreover, AaGA1 deletion led to a significant reduction in melanin and toxin production. Interestingly, deletion of AaGA1 resulted in defective appressorium-like formations, complete loss of the ability to penetrate cellophane, and decreased infection on non-wound inoculated tobacco leaves. Cell wall-degrading enzyme-related genes PME, CL, Cut2, and LC were significantly downregulated in mutant ΔAaGA1 mutant, significantly reducing virulence on wound-inoculated pear fruits.

CONCLUSIONS

The G protein alpha subunit AaGA1 is indispensable for fungal development, appressorium-like formations, and pathogenicity in A. alternata.

FgVAC1 is an Essential Gene Required for Golgi-to-Vacuole Transport and Fungal Development in Fusarium graminearum

Fusarium graminearum is an important plant pathogen that causes head blight in cereal crops such as wheat, barley, and rice worldwide. In this study, we identified and functionally characterized FgVAC1, an essential gene in F. graminearum that encodes a Rab5 effector involved in membrane tethering functions. The essentiality of FgVAC1 was confirmed through a conditional promoter replacement strategy using the zearalenone-inducible promoter (PZEAR). Cytological analyses revealed that FgVac1 colocalizes with FgRab51 on early endosomes and regulates the proper transport of the vacuolar hydrolase FgCpy1 to the vacuole. Suppression of FgVAC1 led to inhibited vegetative growth, reduced asexual and sexual reproduction, decreased deoxynivalenol (DON) biosynthesis, and diminished pathogenicity. Our findings highlight the significant role of FgVac1 in vacuolar protein sorting, fungal development, and plant infection in F. graminearum.

Amino sugars influence Aspergillus fumigatus cell wall polysaccharide biosynthesis, and biofilm formation through interfering galactosaminogalactan deacetylation

Aspergillus fumigatus is a ubiquitous fungal pathogen responsible for a significant number of deaths annually due to invasive aspergillosis infection. While the utilization of diverse carbon sources, including amino sugars, has been explored in other fungi, its impact on A. fumigatus remains uncharted territory. In this study, we investigated A. fumigatus responses to glucose (Glc), glucosamine (GlcN) and N-acetylglucosamine (GlcNAc) as carbon sources. GlcN inhibited growth, reduced sporulation and delayed germination, while GlcNAc had no such effects. Both amino sugars induced alterations in cell wall composition, leading to a reduction in glucan and galactomannan levels while increasing chitin and mannan content, rendering A. fumigatus susceptible to cell wall stress and osmotic stress. GlcN repressed biofilm formation via downregulation of galactosaminogalactan (GAG) cluster genes, notably agd3, which encodes a GAG-specific deacetylase. Moreover, GlcN increased biofilm susceptibility to echinocandins, suggesting its potential for enhancing the effectiveness of antifungal treatments. This study sheds light on the multifaceted effects of amino sugars on A. fumigatus, encompassing growth, cell wall biosynthesis, and biofilm formation, offering promising avenues for innovative aspergillosis treatment strategies.

Identification of Essential Genes for the Establishment of Spray-Induced Gene Silencing-Based Disease Control in Fusarium graminearum

As resistance to chemical fungicides continues to increase inFusarium graminearum, there is a growing need to develop novel disease control strategies. To discover essential genes that could serve as new disease control targets, we selected essential gene candidates that had failed to be deleted in previous studies. Thirteen genes were confirmed to be essential, either by constructing conditional promoter replacement mutants or by employing a clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9)-mediated editing strategy. We synthesized double-stranded RNAs (dsRNAs) targeting these essential genes and analyzed their protective effects in plants using a spray-induced gene silencing (SIGS) method. When dsRNAs targeting Fg10360, Fg13150, and Fg06123 were applied to detached barley leaves prior to fungal inoculation, disease lesions were greatly reduced. Our findings provide evidence of the potential of essential genes identified by a SIGS method to be effective targets for the control of fungal diseases.

ChsA, a Class Ⅱ Chitin Synthase, Contributes to Asexual Conidiation, Mycelial Morphology, Cell Wall Integrity, and the Production of Enzymes and Organic Acids in Aspergillus niger

Chitin synthases (CHSs) are vital enzymes for the synthesis of chitin and play important and differential roles in fungal development, cell wall integrity, environmental adaptation, virulence, and metabolism in fungi. However, except for ChsC, a class III CHS, little is known about the functions of CHSs in Aspergillus niger, an important fungus that is widely applied in the fermentation industry and food processing, as well as a spoilage fungus of food and a human pathogen. This study showed the important functions of ChsA, a class II CHS, in A. niger using multi-phenotypic and transcriptional analyses under various conditions. The deletion of chsA led to severe defects in conidiation on different media and resulted in the formation of smaller and less compact pellets with less septa in hyphal cells during submerged fermentation. Compared with the WT, the ΔchsA mutants exhibited less chitin content, reduced growth under the stresses of cell wall-disturbing and oxidative agents, more released protoplasts, a thicker conidial wall, decreased production of amylases, pectinases, cellulases, and malic acid, and increased citric acid production. However, ΔchsA mutants displayed insignificant changes in their sensitivity to osmotic agents and infection ability on apple. These findings concurred with the alteration in the transcript levels and enzymatic activities of some phenotype-related genes. Conclusively, ChsA is important for cell wall integrity and mycelial morphology, and acts as a positive regulator of conidiation, cellular responses to oxidative stresses, and the production of malic acid and some enzymes, but negatively regulates the citric acid production in A. niger.

Protein homeostasis, regulation of energy production and activation of DNA damage-repair pathways are involved in the heat stress response of Pseudogymnoascus spp

Proteome changes can be used as an instrument to measure the effects of climate change, predict the possible future state of an ecosystem and the direction in which is headed. In this study, proteomic and GO functional enrichment analysis of six Pseudogymnoascus spp. isolated from various global biogeographical regions were carried out to determine their response to heat stress. In total, 2,122 proteins were identified with high confidence. Comparative quantitative analysis showed that changes in proteome profiles varied greatly between isolates from different biogeographical regions. Although the identities of the proteins that changed varied between the different regions, the functions they governed were similar. Gene Ontology analysis showed enrichment of proteins involved in multiple protective mechanisms, including the modulation of protein homeostasis, regulation of energy production, and activation of DNA damage and repair pathways. Our proteomic analysis did not show any clear relationship between protein changes and the strains' biogeographical origins. This article is protected by copyright. All rights reserved.

Interrogation of the cell wall integrity pathway in Aspergillus niger identifies a putative negative regulator of transcription involved in chitin deposition

Post-fermentation fungal biomass waste provides a viable source for chitin. Cell wall chitin of filamentous fungi, and in particular its de-N-acetylated derivative chitosan, has a wide range of commercial applications. Although the cell wall of filamentous fungi comprises 10–30% chitin, these yields are too low for cost-effective production. Therefore, we aimed to identify the genes involved in increased chitin deposition by screening a collection of UV-derived cell wall mutants in Aspergillus niger. This screen revealed a mutant strain (RD15.4#55) that showed a 30–40% increase in cell wall chitin compared to the wild type. In addition to the cell wall chitin phenotype, this strain also exhibited sensitivity to SDS and produces an unknown yellow pigment. Genome sequencing combined with classical genetic linkage analysis identified two mutated genes on chromosome VII that were linked with the mutant phenotype. Single gene knockouts and subsequent complementation analysis revealed that an 8 bp deletion in NRRL3_09595 is solely responsible for the associated phenotypes of RD15.4#55. The mutated gene, which was named cwcA (cell wall chitin A), encodes an orthologue of Saccharomyces cerevisiae Bypass of ESS1 (BYE1), a negative regulator of transcription elongation. We propose that this conserved fungal protein is involved in preventing cell wall integrity signaling under non-inducing conditions, where loss of function results in constitutive activation of the cell wall stress response pathway, and consequently leads to increased chitin content in the mutant cell wall.

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An Aspergillus niger UV-mutant with increased cell wall chitin was characterized.

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Causative mutation was identified in a single gene, named cell wall chitin A (cwcA).

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CwcA is orthologous to yeast Bye1p and exists as a single copy gene.

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Three relevant domains are found in both CwcA and Bye1p: PHD, TFIIS and SPOC.

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CwcA acts as negative regulator of CWI signaling.

Dsk2 involves in conidiation, multi-stress tolerance and thermal adaptation in Beauveria bassiana

Dsk2 is a nuclear-enriched ubiquitin-like polyubiquitin-binding protein that regulates protein degradation in yeast but has not been explored yet in filamentous fungi, such as Beauveria bassiana. Here, we report Beauveria bassiana Dsk2 located both in the nucleus and in cytoplasm of hyphal cells. Deletion of Dsk2 resulted in mild growth defect on scant media with various carbon/nitrogen sources and dramatic attenuation in conidiation capability at optimal condition. Compared to the wild-type, ΔDsk2 strains are much more sensitive to high osmotic and oxidative pressure during vegetative growth. Meanwhile, the mutant strains showed an increased chemical tolerance to Congo red and calcofluor white, two cell wall perturbing agents. The transcriptional changes of genes involved in central development, superoxide dismutase and chitin synthesis pathway indicate that Dsk2 acts as a multi-functional regulator in adapting to environmental changes. Importantly, Dsk2 negatively regulated the ability of thermal resistance in B. bassiana, which makes it a potential target gene for constructing engineering anti-thermal strains in the circumstance of global warming. Altogether, our finding highlights novel roles of Dsk2 involved in the asexual cycle, multi-stress tolerance and pest control potential of B. bassiana.

Genetic Characterization of Mutations Related to Conidiophore Stalk Length Development in Aspergillus niger Laboratory Strain N402

Aspergillus niger is an important filamentous fungus in industrial biotechnology for the production of citric acid and enzymes. In the late 1980s, the A. niger N400/NRRL3 strain was selected for both fundamental and applied studies in relation to several processes including gluconic acid and protein production. To facilitate handling of A. niger, the N400 wild-type strain was UV mutagenized in two consecutive rounds to generate N401 and N402. N402 was used as a reference laboratory strain and exhibits the phenotypes with reduced conidiophore stalk length and reduced radial growth. The conidiophore stalk length and radial growth of A. niger strain N400 were determined and compared to N401 and N402. The length of N400 conidiophore stalks (2.52 ± 0.40 mm) was reduced in N401 and N402 to 0.66 ± 0.14 mm and 0.34 ± 0.06 mm, respectively. Whereas N400 reached a colony diameter of 6.7 ± 0.2 cm after 7 days, N401 and N402 displayed reduced radial growth phenotype (4.3 ± 0.1 and 4.1 ± 0.1, respectively). To identify the mutations (dubbed cspA and cspB) responsible for the phenotypes of N401 and N402, the genomes were sequenced and compared to the N400 genome sequence. A parasexual cross was performed between N400 and N402 derivatives to isolate segregants which allowed cosegregation analysis of single nucleotide polymorphisms and insertions and deletions among the segregants. The shorter conidiophore stalk and reduced radial growth in N401 (cspA) was found to be caused by a 9-kb deletion on chromosome III and was further narrowed down to a truncation of NRRL3_03857 which encodes a kinesin-like protein homologous to the A. nidulans UncA protein. The mutation responsible for the further shortening of conidiophore stalks in N402 (cspB) was found to be caused by a missense mutation on chromosome V in a hitherto unstudied C2H2 transcription factor encoded by the gene NRRL3_06646. The importance of these two genes in relation to conidiophore stalk length and radial growth was confirmed by single and double gene deletion studies. The mutations in the laboratory strain N402 should be taken into consideration when studying phenotypes in the N402 background.

Deletion of the Aspergillus niger Pro-Protein Processing Protease Gene kexB Results in a pH-Dependent Morphological Transition during Submerged Cultivations and Increases Cell Wall Chitin Content

There is a growing interest in the use of post-fermentation mycelial waste to obtain cell wall chitin as an added-value product. In the pursuit to identify suitable production strains that can be used for post-fermentation cell wall harvesting, we turned to an Aspergillus niger strain in which the kexB gene was deleted. Previous work has shown that the deletion of kexB causes hyper-branching and thicker cell walls, traits that may be beneficial for the reduction in fermentation viscosity and lysis. Hyper-branching of ∆kexB was previously found to be pH-dependent on solid medium at pH 6.0, but was absent at pH 5.0. This phenotype was reported to be less pronounced during submerged growth. Here, we show a series of controlled batch cultivations at a pH range of 5, 5.5, and 6 to examine the pellet phenotype of ΔkexB in liquid medium. Morphological analysis showed that ΔkexB formed wild type-like pellets at pH 5.0, whereas the hyper-branching ΔkexB phenotype was found at pH 6.0. The transition of phenotypic plasticity was found in cultivations at pH 5.5, seen as an intermediate phenotype. Analyzing the cell walls of ΔkexB from these controlled pH-conditions showed an increase in chitin content compared to the wild type across all three pH values. Surprisingly, the increase in chitin content was found to be irrespective of the hyper-branching morphology. Evidence for alterations in cell wall make-up are corroborated by transcriptional analysis that showed a significant cell wall stress response in addition to the upregulation of genes encoding other unrelated cell wall biosynthetic genes.

A seven-membered cell wall related transglycosylase gene family in Aspergillus niger is relevant for cell wall integrity in cell wall mutants with reduced α-glucan or galactomannan

Chitin is an important fungal cell wall component that is cross-linked to β-glucan for structural integrity. Acquisition of chitin to glucan cross-links has previously been shown to be performed by transglycosylation enzymes in Saccharomyces cerevisiae, called Congo Red hypersensitive (Crh) enzymes. Here, we characterized the impact of deleting all seven members of the crh gene family (crhA-G) in Aspergillus niger on cell wall integrity, cell wall composition and genome-wide gene expression. In this study, we show that the seven-fold crh knockout strain shows slightly compact growth on plates, but no increased sensitivity to cell wall perturbing compounds. Additionally, we found that the cell wall composition of this knockout strain was virtually identical to that of the wild type. In congruence with these data, genome-wide expression analysis revealed very limited changes in gene expression and no signs of activation of the cell wall integrity response pathway. However, deleting the entire crh gene family in cell wall mutants that are deficient in either galactofuranose or α-glucan, mainly α-1,3-glucan, resulted in a synthetic growth defect and an increased sensitivity towards Congo Red compared to the parental strains, respectively. Altogether, these results indicate that loss of the crh gene family in A. niger does not trigger the cell wall integrity response, but does play an important role in ensuring cell wall integrity in mutant strains with reduced galactofuranose or α-glucan.