Establishment of a new method to quantitatively evaluate hyphal fusion ability in Aspergillus oryzae

Analysis of genomic characteristics and their influence on metabolism in Aspergillus luchuensis albino mutants using genome sequencing

Black Aspergillus luchuensis and its white albino mutant are essential fungi for making alcoholic beverages in Japan. A large number of industrial strains have been created using novel isolation or gene/genome mutation techniques. Such mutations influence metabolic and phenotypic characteristics in industrial strains, but few comparative studies of inter-strain mutation have been conducted. We carried out comparative genome analyses of 8 industrial strains of A. luchuensis and A. kawachii IFO 4308, the latter being the first albino strain to be isolated. Phylogenetic analysis based on 8938 concatenated genes exposed the diversity of black koji strains and uniformity among albino industrial strains, suggesting that passaged industrial albino strains have more genetic mutations compared with strain IFO 4308 and black koji strains. Comparative analysis showed that the albino strains had mutations in genes not only for conidial pigmentation but also in those that encode N-terminal acetyltransferase A and annexin XIV-like protein. The results also suggest that some mutations may have emerged through subculturing of albino strains. For example, mutations in the genes for isocitrate lyase and sugar transporters were observed only in industrial albino strains. This implies that selective pressure for increasing enzyme activity or secondary metabolites may have influenced the mutation of genes associated with environmental stress responses in A. luchuensis albino strains. Our study clarifies hitherto unknown genetic and metabolic characteristics of A. luchuensis industrial strains and provides potential applications for comparative genome analysis for breeding koji strains.

Novel Fus3- and Ste12-interacting protein FsiA activates cell fusion-related genes in both Ste12-dependent and -independent manners in Ascomycete filamentous fungi

Filamentous fungal cells, unlike yeasts, fuse during vegetative growth. The orthologs of mitogen-activated protein (MAP) kinase Fus3 and transcription factor Ste12 are commonly involved in the regulation of cell fusion. However, the specific regulatory mechanisms underlying cell fusion in filamentous fungi have not been revealed. In the present study, we identified the novel protein FsiA as an AoFus3- and AoSte12-interacting protein in the filamentous fungus Aspergillus oryzae. The expression of AonosA and cell fusion-related genes decreased upon fsiA deletion and increased with fsiA overexpression, indicating that FsiA is a positive regulator of cell fusion. In addition, the induction of cell fusion-related genes by fsiA overexpression was also observed in the Aoste12 deletion mutant, indicating that FsiA can induce the cell fusion-related genes in an AoSte12-independent manner. Surprisingly, the fsiA and Aoste12 double deletion mutant exhibited higher cell fusion efficiency and increased mRNA levels of the cell fusion-related genes as compared to the fsiA single deletion mutant, which revealed that AoSte12 represses the cell fusion-related genes in the fsiA deletion mutant. Taken together, our data demonstrate that FsiA activates the cell fusion-related genes by suppressing the negative function of AoSte12 as well as by an AoSte12-independent mechanism.

BiFC-based visualisation system reveals cell fusion morphology and heterokaryon incompatibility in the filamentous fungus Aspergillus oryzae

Aspergillus oryzae is an industrially important filamentous fungus used for Japanese traditional food fermentation and heterologous protein production. Although cell fusion is important for heterokaryon formation and sexual/parasexual reproduction required for cross breeding, knowledge on cell fusion and heterokaryon incompatibility in A. oryzae is limited because of low cell fusion frequency. Therefore, we aimed to develop a BiFC system to specifically visualise fused cells and facilitate the analysis of cell fusion in A. oryzae. The cell fusion ability and morphology of 15 A. oryzae strains were investigated using heterodimerising proteins LZA and LZB fused with split green fluorescence protein. Morphological investigation of fused cells revealed that cell fusion occurred mainly as conidial anastomosis during the early growth stage. Self-fusion abilities were detected in most industrial A. oryzae strains, but only a few strain pairs showed non-self fusion. Protoplast fusion assay demonstrated that almost all the pairs capable of non-self fusion were capable of heterokaryon formation and vice versa, thus providing the first evidence of heterokaryon incompatibility in A. oryzae. The BiFC system developed in this study provides an effective method in studying morphology of fused cells and heterokaryon incompatibility in the filamentous fungal species with low cell fusion efficiency.

A Cellular Fusion Cascade Regulated by LaeA Is Required for Sclerotial Development in Aspergillus flavus

Aspergillus flavus is a saprophytic soil fungus that poses a serious threat worldwide as it contaminates many food and feed crops with the carcinogenic mycotoxin called aflatoxin. This pathogen persists as sclerotia in the soil which enables fungal survival in harsh environmental conditions. Sclerotia formation by A. flavus depends on successful cell communication and hyphal fusion events. Loss of LaeA, a conserved developmental regulator in fungi, abolishes sclerotia formation in this species whereas overexpression (OE) of laeA results in enhanced sclerotia production. Here we demonstrate that sclerotia loss and inability to form heterokaryons in A. flavusΔlaeA is mediated by homologs of the Neurospora crassa ham (hyphal anastomosis) genes termed hamE-I in A. flavus. LaeA positively regulates ham gene expression and deletion of hamF, G, H, or I phenocopies ΔlaeA as demonstrated by heterokaryon and sclerotia loss and reduced aflatoxin synthesis and virulence of these mutants. Deletion of hamE showed a less severe phenotype. hamE-I homologs are positively regulated by the clock controlled transcription factor ADV-1 in N. crassa. Similarly, the ADV-1 homolog NosA regulates hamE-I expression in A. flavus, is required for sclerotial development and is itself positively regulated by LaeA. We speculate that a putative LaeA>NosA>fusion cascade underlies the previously described circadian clock regulation of sclerotia production in A. flavus.