Abstract
Potassium, a widely accepted macronutrient, is vital for many physiological processes such as regulation of cell volume, maintenance of intracellular pH, synthesis of proteins and activation of enzymes in filamentous fungi. Another cation, calcium, plays an essential role in many signaling processes from lower to higher eukaryotes. Imbalance in the intracellular ionic levels of potassium or calcium causes adverse effects on cell growth, morphology and development, and eventually death. Previous studies on the adaptation of Aspergillus nidulans to salt and osmotic stress conditions have revealed the role of SltA, a C2H2 zinc finger transcription factor in cation homeostasis. SltA is highly conserved in the Ascomycota phylum with no identifiable homolog in S. cerevisiae and other yeast-like fungi, and prevents toxicity by the cations Na+, K+, Li+, Cs+ and Mg2+, but not by Ca2+. However its role in morphology and biosynthesis of natural products such as mycotoxins remained unknown. This study shows the first characterization of the role of calcium and SltA fungal homologs in morphogenesis using the model system A. nidulans. Addition of potassium to sltA deletion mutants resulted in decreased levels of sterigmatocystin production. A similar phenotype was observed for both types of mutants in veA1 and veA + genetic background. Expression of the sterigmatocystin genes aflR and stcU was strongly reduced in sltA deletion mutant when K+ was added. Additionally, increased concentrations of K+ drastically reduced sexual and asexual development, as well as radial growth in deletion sltA colonies. This reduction was accompanied by lower expression of the morphology related genes nsdD, steA and brlA. Interestingly, addition of calcium was able to stimulate asexual and sexual development and remediate the deletion sltA phenotype, including defects in morphology and toxin production.
Highlights
Filamentous fungi inhabit a wide range of both indoor and outdoor environments, which includes soil, plants, animals and human hosts
In previous studies carried out by Spielvogel et al, 2008 [4] on sltA mutant in veA1 genetic background only a slight reduction of growth was observed on Cove’s minimal medium with ammonium tartrate as nitrogen source but strong sensitivity was detected to elevated concentrations of mono and divalent cations (Figure S1) with the exception of calcium [4]
Normal expression of nsdD and steA requires a sltA wild-type allele. We examined whether this defect in the first stages of sexual development was caused by changes in the expression of nsdD and steA genes, necessary for sexual development in A. nidulans [33,34]. nsdD expression was reduced in the ∆sltA strain (RSS1.6P) as compared to the corresponding control strain (Figure 5B)
Summary
Filamentous fungi inhabit a wide range of both indoor and outdoor environments, which includes soil, plants, animals and human hosts. They are capable of growing in extreme environmental niches such as salterns, hot springs, deserts, deep sea sediments, bird excreta, aquatic habitats, mine drainages and in the crevices of rocks. Potassium is a very important nutrient required for many physiological processes such as regulation of cell volume, maintenance of intracellular pH, synthesis of proteins and activation of enzymes in several organisms such as plants, animals, bacteria and filamentous fungi. Under hypertonic ambient conditions, high-salt concentrations cause cells to lose water when cations enter the cell. Na+ is highly toxic and is usually maintained in low levels, while K+ is accumulated at high concentrations in the cell. For more than five decades, yeast cells have been employed as model
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