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

The SUMO gene MrSmt3 is involved in SUMOylation, conidiation and stress response in Metarhizium robertsii

Smt3, as a small ubiquitin-like modifier (SUMO), play an essential role in the regulation of protein SUMOylation, and thus this process can affect various important biological functions. Here, we investigated the roles of MrSmt3 (yeast SUMO/Smt3 homologs) in the entomopathogenic fungus Metarhizium robertsii. Our results of subcellular localization assays demonstrated that MrSmt3 was present in the cytoplasm and nucleus, whereas MrSmt3 was largely localized in the nucleus during oxidative stress. Importantly, disruption of MrSmt3 significantly decreased the level of protein SUMOylation under heat stress. Deletion of MrSmt3 led to a significant decrease in conidial production, and increased sensitivity to various stresses, including heat, oxidative, and cell wall-disturbing agents. However, bioassays of direct injection and topical inoculation demonstrated that deletion of MrSmt3 did not affect fungal virulence. Furthermore, RNA-seq analysis identified 1,484 differentially expressed genes (DEGs) of the WT and ΔMrSmt3 during conidiation, including 971 down-regulated DEGs and 513 up-regulated DEGs, and further analysis showed that the expression level of several classical conidiation-associated genes, such as transcription factor AbaA (MAA_00694), transcription factor bZIP (MAA_00888) and transcription factor Ste12 (MAA_10450), was down-regulated in the ΔMrSmt3 mutant. Specifically, the major downregulated DEGs were mainly associated with a variety of metabolic regulatory processes including metabolic process, organic substance metabolic process and primary metabolic process. Collectively, our findings highlight the important roles of the SUMO gene MrSmt3 in modulating SUMOylation, conidiation and stress response in M. robertsii.

The novel spore-specific regulator SscA controls Aspergillus conidiogenesis

IMPORTANCE

Filamentous fungi produce myriads of asexual spores, which are the main reproductive particles that act as infectious or allergenic agents. Although the serial of asexual sporogenesis is coordinated by various genetic regulators, there remain uncharacterized transcription factors in Aspergillus. To understand the underlying mechanism of spore formation, integrity, and viability, we have performed comparative transcriptomic analyses on three Aspergillus species and found a spore-specific transcription factor, SscA. SscA has a major role in conidial formation, maturation and dormancy, and germination in Aspergillus nidulans. Functional studies indicate that SscA coordinates conidial wall integrity, amino acid production, and secondary metabolism in A. nidulans conidia. Furthermore, the roles of SscA are conserved in other Aspergillus species. Our findings that the SscA has broad functions in Aspergillus conidia will help to understand the conidiogenesis of Aspergillus species.

Elucidating the Obligate Nature and Biological Capacity of an Invasive Fungal Corn Pathogen

Tar spot is a devasting corn disease caused by the obligate fungal pathogen Phyllachora maydis. Since its initial identification in the United States in 2015, P. maydis has become an increasing threat to corn production. Despite this, P. maydis has remained largely understudied at the molecular level, due to difficulties surrounding its obligate lifestyle. Here, we generated a significantly improved P. maydis nuclear and mitochondrial genome, using a combination of long- and short-read technologies, and also provide the first transcriptomic analysis of primary tar spot lesions. Our results show that P. maydis is deficient in inorganic nitrogen utilization, is likely heterothallic, and encodes for significantly more protein-coding genes, including secreted enzymes and effectors, than previous determined. Furthermore, our expression analysis suggests that, following primary tar spot lesion formation, P. maydis might reroute carbon flux away from DNA replication and cell division pathways and towards pathways previously implicated in having significant roles in pathogenicity, such as autophagy and secretion. Together, our results identified several highly expressed unique secreted factors that likely contribute to host recognition and subsequent infection, greatly increasing our knowledge of the biological capacity of P. maydis, which have much broader implications for mitigating tar spot of corn. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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.

Mannitol-1-phosphate dehydrogenase, MpdA, is required for mannitol production in vegetative cells and involved in hyphal branching, heat resistance of conidia and sexual development in Aspergillus nidulans

Aspergillus nidulans produces cleistothecia as sexual reproductive organs in a process affected by genetic and external factors. To gain a deeper insight into A. nidulans sexual development, we performed comparative proteome analyses based on the wild type developmental periods. We identified sexual development-specific proteins with a more than twofold increase in production during hypoxia or the sexual period compared to the asexual period. Among the sexual development-specific proteins analyzed by gene-deletion experiments and functional assays, MpdA, a putative mannitol-1-phosphate 5-dehydrogenase, plays multiple roles in growth and differentiation of A. nidulans. The most distinct mpdA-deletion phenotype was ascosporogenesis failure. Genetic mpdA deletion resulted in small cleistothecia with no functional ascospores. Transcriptional analyses indicated that MpdA modulates the expression of key development- and meiosis-regulatory genes during sexual development. The mpdA deletion increased hyphal branching and decreased conidial heat resistance. Mannitol production in conidia showed no difference, whereas it was decreased in mycelia and sexual cultures. Addition of mannitol during vegetative growth recovered the defects in conidial heat resistance and ascospore genesis. Taken together, these results indicate that MpdA plays an important role in sexual development, hyphal branching, and conidial heat resistance in Aspergillus nidulans.

Survival factor SvfA plays multiple roles in differentiation and is essential for completion of sexual development in Aspergillus nidulans

The first member of the velvet family of proteins, VeA, regulates sexual development and secondary metabolism in the filamentous fungus Aspergillus nidulans. In our study, through comparative proteome analysis using wild type and veA-deletion strains, new putative regulators of sexual development were identified and functionally analyzed. Among these, SvfA, containing a yeast survival factor 1 domain, plays multiple roles in the growth and differentiation of A. nidulans. Deletion of the svfA gene resulted in increased sensitivity to oxidative and cold stress as in yeast. The svfA-deletion strain showed an increase in bi-polar germination and a decrease in radial growth rate. The deletion strain formed structurally abnormal conidiophores and thus produced lower amounts of conidiospores during asexual development. The svfA-deletion strain produced few Hülle cells and small cleistothecia with no ascospores, indicating the requirement of svfA for the completion of sexual development. Transcription and genetic analyses indicated that SvfA modulates the expression of key development regulatory genes. Western blot analysis revealed two forms of SvfA. The larger form showed sexual-specific and VeA-dependent production. Also, the deletion of svfA caused decreased ST (sterigmatocystin) production. We propose that SvfA is a novel central regulator of growth, differentiation and secondary metabolism in A. nidulans.

Morphological, transcriptional, and metabolic analyses of osmotic-adapted mechanisms of the halophilic Aspergillus montevidensis ZYD4 under hypersaline conditions

Halophilic fungi in hypersaline habitats require multiple cellular responses for high-salinity adaptation. However, the exact mechanisms behind these adaptation processes remain to be slightly known. The current study is aimed at elucidating the morphological, transcriptomic, and metabolomic changes of the halophilic fungus Aspergillus montevidensis ZYD4 under hypersaline conditions. Under these conditions, the fungus promoted conidia formation and suppressed cleistothecium development. Furthermore, the fungus differentially expressed genes (P < 0.0001) that controlled ion transport, amino acid transport and metabolism, soluble sugar accumulation, fatty acid β-oxidation, saturated fatty acid synthesis, electron transfer, and oxidative stress tolerance. Additionally, the hypersalinized mycelia widely accumulated metabolites, including amino acids, soluble sugars, saturated fatty acids, and other carbon- and nitrogen-containing compounds. The addition of metabolites-such as neohesperidin, biuret, aspartic acid, alanine, proline, and ornithine-significantly promoted the growth (P ≤ 0.05) and the morphological adaptations of A. montevidensis ZYD4 grown in hypersaline environments. Our study demonstrated that morphological shifts, ion equilibrium, carbon and nitrogen metabolism for solute accumulation, and energy production are vital to halophilic fungi so that they can build tolerance to high-salinity environments.