Velvet proteins are conserved transcriptional and epigenetic regulators involved in fungal development, secondary metabolism, stress responses and pathogenicity. In A. nidulans, the core family includes VeA, VelB, VelC and VosA. The trimeric VelB–VeA–LaeA complex is particularly important: in darkness, nuclear VeA links VelB with the methyltransferase LaeA, coordinating sexual development and secondary-metabolite production, whereas reduced nuclear localization of VeA under light is associated with altered complex assembly and a shift toward asexual development.

Velvet proteins contain a conserved velvet domain of approximately 200 amino acids that resembles the Rel homology domain of mammalian NF-κB transcription factors. A recent architectural model divides this domain into an approximately 30-amino-acid N-terminal DNA-binding region and an approximately 100-amino-acid C-terminal dimerization region. Although the dimerization region had been characterized in earlier work, the sequence and functional requirements of the DNA-binding region were largely unresolved. The study used VelB as a model because it has the shortest sequence and high cross-species conservation among the four A. nidulans velvet homologs, and because loss of VelB produces distinct developmental phenotypes.

Researchers aligned 4,999 putative fungal velvet domains and then individually replaced 15 conserved VelB DNA-binding-region residues with alanine. Comparison of the resulting colonies with wild-type, velB-deletion and velB-complemented controls identified three substitutions with the strongest phenotypic effects: R71A, R80A and R81A. Under light, wild-type and complemented strains produced green conidia, while all three arginine mutants produced gray-green conidia. Under sealed dark conditions, the R80A and R81A mutants failed to form cleistothecia, the sexual fruiting bodies, whereas the R71A mutant produced only 13.3% of the wild-type cleistothecial yield.

The mutants also showed a marked loss of long-term conidial viability. In assays performed on solid minimal medium, conidia from wild-type and complemented strains retained approximately 50% viability after 14 days, compared with roughly 15% for the three point mutants, a statistically significant reduction. Unlike the velB deletion strain, which completely lost conidial viability after 14 days, the point mutants retained partial viability. Liquid chromatography–mass spectrometry further showed disrupted secondary-metabolite profiles that partly overlapped with those of the deletion strain. Emericellin, epishamixanthone, F9775A/B and shamixanthone were attenuated or absent, whereas asperthecin, emericellamide C, emericellamide E and terrequinone A were significantly increased; austinoneol A remained stable across the tested strains.

Structural modeling provided a possible mechanistic explanation for the observed defects. Using AlphaFold 3 and the crystal structure of the A. nidulans VosA–VelB heterodimer as a reference, the researchers modeled wild-type VelB and the R71A, R80A and R81A variants and calculated per-residue electrostatic potentials. R71 occupied the strongest electropositive region, with a potential of +2.391 kcal/(mol·e), and its alanine substitution produced the largest absolute decrease, 1.229 kcal/(mol·e). All three mutations lowered the protein’s mean, minimum and maximum calculated Coulomb potential and altered the electrostatic profile at nearly every modeled residue. These are model-based calculations, rather than direct measurements of DNA binding.

Across the 4,999 velvet-domain sequences, the corresponding positions were highly conserved: arginine occurred at position 71 in 85% of sequences and lysine in 4%; at position 80, the respective frequencies were 91% and 2%; and at position 81, they were 84% and 9%. A domain-swap experiment tested functional compatibility by replacing the native VelB DNA-binding region with the region from the velvet protein of Capsaspora owczarzaki. This chimeric strain only partly rescued the deletion phenotype, with approximately 23% conidial viability and 34% cleistothecial abundance relative to wild type. The Capsaspora region lacked the arginine corresponding to VelB R71.

Adding that arginine to the Capsaspora domain substantially improved the chimeric strain’s measured phenotypes. Fourteen-day conidial viability rose to 91% of the wild-type level and cleistothecial formation to 86%, compared with 23% and 34% for the unmodified chimera. Western blotting showed that the chimeric proteins were expressed at normal levels comparable to the velB cDNA–GFP control, although the experiment did not directly measure DNA occupancy or exclude every possible contribution from expression or localization. The study therefore supports a conserved requirement for R71, R80 and R81 in VelB-mediated regulation, while its conclusions are bounded by the use of one fungal model and computational rather than direct DNA-binding measurements.

Subject of Research: Experimental dissection of conserved arginine residues in the Aspergillus nidulans VelB DNA-binding region

Article Title: Functional dissection of velvet DNA-binding region in Aspergillus nidulans VelB uncovers an essential conserved arginine cluster

Article References: Chen, W., Köhler, A. M., Deng, M., Zhu, H., & Braus, G. H. (2026). Functional dissection of velvet DNA-binding region in Aspergillus nidulans VelB uncovers an essential conserved arginine cluster. IMA Fungus, 17, Article e204179. https://doi.org/10.3897/imafungus.17.204179

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.204179

Keywords: Aspergillus nidulans, VelB, velvet proteins, DNA-binding region, arginine cluster, fungal development, sexual reproduction, spore viability, secondary metabolism, electrostatic potential, Functional, dissection

The central insight of the review is that nutrient limitation operates in two interconnected modes. On one hand, starvation is a metabolic stress that passively restricts growth by depriving cells of the energy and building blocks needed for proliferation. On the other hand, and more strikingly, nutrient scarcity functions as information: it tells the fungus that continued local growth is becoming unprofitable and that dispersal, dormancy, or recombination may offer better future returns. This adaptive duality, the authors argue, has driven the evolution of precise sensing pathways that transform an environmental deficit into an anticipatory cue for fitness realignment. The result is a developmental window in which fungi can commit to reproduction, but only if they retain enough physiological competence to complete the costly program.

The clearest illustration comes from budding yeast, Saccharomyces cerevisiae, which enters meiotic sporulation only under a specific combination of conditions: the cells must be diploid with both mating types, starved of nitrogen, depleted of fermentable glucose, and supplied with a non-fermentable carbon source such as acetate. Each element of this recipe is physiologically coherent. Nitrogen limitation reduces the value of mitotic division, glucose depletion lifts repression of respiratory and meiotic programs, and acetate fuels meiosis and spore-wall construction. Under nutrient-rich conditions, the conserved cAMP-PKA and TOR signaling pathways promote growth and actively suppress meiotic genes; when conditions turn unfavorable, that repression is relieved and the master meiotic regulators IME1 and IME2 are induced. Sporulation, in other words, is a conditional developmental program, not a nonspecific starvation collapse.

Fission yeast, Schizosaccharomyces pombe, achieves a similar outcome through a different regulatory architecture. Nitrogen starvation is the principal cue that triggers sexual differentiation: compatible mating types arrest in the G1 phase of the cell cycle, mate, fuse their nuclei, enter meiosis, and produce stress-resistant spores. The pathway runs through the transcription factor Ste11, which induces mating and meiotic genes once growth-promoting cAMP-PKA and TOR signals subside. Commitment is then controlled by a molecular switch involving the Pat1 kinase and the RNA-binding protein Mei2. During vegetative growth, Pat1 phosphorylates Mei2 and marks it for destruction; upon nitrogen starvation and successful conjugation, the inhibitor Mei3 is expressed, Pat1 is inactivated, and the cell becomes irreversibly committed to meiosis. The comparison between the two yeasts supports a modular model in which conserved nutrient-sensing pathways feed into species-specific reproductive circuits.

Filamentous fungi add further layers of complexity. In Aspergillus nidulans, the best-defined genetic model, asexual development depends on the BrlA-AbaA-WetA transcriptional cascade, with BrlA initiating conidiophore construction, AbaA directing phialide differentiation, and WetA ensuring spore maturation and long-term viability. Upstream regulators such as FluG and the Flb proteins connect colony state to this central cascade, while light and the Velvet complex bias the outcome: light generally favors asexual conidiation, whereas darkness promotes sexual development through the nuclear accumulation of the VeA protein. In Neurospora crassa, the White Collar Complex and the FRQ-based circadian clock generate rhythmic conidiation, timing spore production to predictable daily cycles. In plant pathogens such as Magnaporthe oryzae and Fusarium graminearum, nutrient limitation intertwines with host-derived stresses, and the resulting spores, conidia in rice blast or airborne ascospores in Fusarium head blight, are the engines of epidemic spread.

Human fungal pathogens reveal the clinical stakes of these switches. In Candida albicans, the white-opaque epigenetic switch has long been considered a prerequisite for mating, but recent work shows that glucose depletion can bypass it entirely, rendering white cells mating-competent without the switch. Phosphate limitation, acting through the PHO pathway, can similarly induce the opaque state even in otherwise mating-incompetent cells. In Cryptococcus neoformans, whose sexual reproduction produces the basidiospores that serve as primary infectious propagules, all fourteen core autophagy genes are required for meiotic progression and spore formation, and the heme activator protein complex links iron homeostasis directly to sexual development by repressing the pheromone-responsive Cpk1 MAPK pathway. These findings underscore that nutrient sensing and reproductive commitment are deeply entangled even under host-imposed selection.

Perhaps the most striking recent discovery concerns a post-transcriptional layer of control. In the class Sordariomycetes, sexual development is accompanied by extensive adenosine-to-inosine mRNA editing, mediated not by the animal-style ADAR enzymes but by a fungal-specific Tad2-Tad3-Ame1 complex. Because inosine is read as guanosine during translation, this editing generates transcript-level changes that alter codons without touching the genome. The editing occurs almost exclusively during fruiting-body development and ascospore formation, and experimental studies in Fusarium and Neurospora show that it contributes to perithecium development, meiosis, and spore maturation through targets such as Dbf2, Mus81, and Spo11. Crucially, by restricting reproduction-beneficial protein variants to the sexual phase, editing allows fungi to preserve vegetative stress resilience, such as Mus81-dependent heat tolerance, while still meeting the distinct genetic demands of meiosis, elegantly resolving the antagonistic pleiotropy between survival and reproduction.

Why should a fungus pay the steep cost of sex when clonal spores can disperse and survive just as well? The review evaluates two evolutionary frameworks. Fitness-associated sex theory proposes that low-fitness individuals benefit from recombination because sex allows alleles to escape maladapted genetic backgrounds. Experimental work in Aspergillus nidulans supports this: sexual reproduction is associated with low-fitness conditions, and sublethal fungicide stress increases outcrossing, with recombinant offspring showing improved performance under stress. The abandon-ship framework extends the logic, treating dispersal, dormancy, and sex as alternative escape routes from a deteriorating situation: escape in space through conidia, escape in time through chlamydospores and sclerotia, and escape in genetic identity through recombination. Both models predict that the beneficiaries of stress-induced sex may be the recombinant offspring rather than the stressed parent, a distinction that demands careful measurement of parental condition, propagule quality, and descendant performance.

The practical implications are substantial. In industrial biotechnology, a rational two-phase strategy, first building biomass under favorable conditions and then applying controlled developmental cues, can maximize yields of viable, stress-tolerant spores for biocontrol agents, inoculants, and fermentation starters, with mechanistic markers such as conidiation-regulator expression guiding the timing of the shift. In agriculture and medicine, anti-sporulation interventions range from highly specific targets such as the BrlA and WetA transcription factors to broader signaling nodes and environmental management of light, humidity, and crop residues. Yet the authors caution that conserved regulators carry risks for beneficial fungi and that strong selective pressure under field conditions can drive pathogens toward altered sporulation kinetics or cryptic alternative pathways. The review closes with a call for causal, ecologically grounded models that link environmental perception, molecular regulation, reproductive output, and fitness consequences, moving the field from describing stress-associated reproduction to predicting and ultimately manipulating fungal life-history decisions.

Subject of Research: Stress-driven sporulation and sexual development in fungi

Article Title: When growth becomes risky: stress-driven sporulation and sexual development in fungi

Article References: Yang, J., Wang, Q., & Liu, H. (2026). When growth becomes risky: stress-driven sporulation and sexual development in fungi. Stress Biology, 6(1), Article 55. https://doi.org/10.1007/s44154-026-00333-1

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00333-1

Keywords: fungal development, sporulation, sexual reproduction, nutrient limitation, stress signaling, cAMP-PKA, TOR signaling, HOG MAPK, Velvet complex, A-to-I RNA editing, fitness-associated sex, disease management