Autophagy promotes survival in aging submerged cultures of the filamentous fungus Aspergillus niger.

Autophagy is a well-conserved catabolic process constitutively active in eukaryotes that is involved in maintaining cellular homeostasis by the targeting of cytoplasmic content and organelles to vacuoles. Autophagy is strongly induced by the limitation of nutrients including carbon, nitrogen, and oxygen and is clearly associated with cell death. It has been demonstrated that the accumulation of empty hyphal compartments and cryptic growth in carbon-starved submerged cultures of the filamentous fungus Aspergillus niger is accompanied by a joint transcriptional induction of autophagy genes. This study examines the role of autophagy by deleting the atg1, atg8, and atg17 orthologs in A. niger and phenotypically analyzing the deletion mutants in surface and submerged cultures. The results indicate that atg1 and atg8 are essential for efficient autophagy, whereas deletion of atg17 has little to no effect on autophagy in A. niger. Depending on the kind of oxidative stress confronted with, autophagy deficiency renders A. niger either more resistant (menadione) or more sensitive (H2O2) to oxidative stress. Fluorescence microscopy showed that mitochondrial turnover upon carbon depletion in submerged cultures is severely blocked in autophagy-impaired A. niger mutants. Furthermore, automated image analysis demonstrated that autophagy promotes survival in maintained carbon-starved cultures of A. niger. Taken together, the results suggest that besides its function in nutrient recycling, autophagy plays important roles in physiological adaptation by organelle turnover and protection against cell death upon carbon depletion in submerged cultures.

The molecular and genetic basis of conidial pigmentation in Aspergillus niger.

A characteristic hallmark of Aspergillus niger is the formation of black conidiospores. We have identified four loci involved in spore pigmentation of A. niger by using a combined genomic and classical complementation approach. First, we characterized a newly isolated color mutant, colA, which lacked pigmentation resulting in white or colorless conidia. Pigmentation of the colA mutant was restored by a gene (An12g03950) which encodes a putative 4'phosphopantetheinyl transferase protein (PptA). 4'Phosphopantetheinyl transferase activity is required for the activation of Polyketide Synthases (PKSs) and/or Non-Ribosomal Peptide Synthases (NRPSs). The loci whose mutation resulted in fawn, olive, and brown color phenotypes were identified by complementation. The fawn phenotype was complemented by a PKS protein (FwnA, An09g05730), the ovlA mutant by An14g05350 (OlvA) and the brnA mutant by An14g05370 (BrnA), the respective homologs of alb1/pksP, ayg1 and abr1 in A. fumigatus. Targeted disruption of the pptA, fwnA, olvA and brnA genes confirmed the complementation results. Disruption of the pptA gene abolished synthesis of all polyketides and non-ribosomal peptides, while the naphtho-γ-pyrone subclass of polyketides were specifically dependent on fwnA, and funalenone on fwnA, olvA and brnA. Thus, secondary metabolite profiling of the color mutants revealed a close relationship between polyketide synthesis and conidial pigmentation in A. niger.