Submerged Conidiation and Product Formation by Aspergillus niger at Low Specific Growth Rates Are Affected in Aerial Developmental Mutants

Abstract

Exposure to an aerial environment or severe nutrient limitation induces asexual differentiation in filamentous fungi. Submerged cultivation of Aspergillus niger in carbon-and energy-limited retentostat cultures both induces and fuels conidiation. Physiological and transcriptomic analyses have revealed that this differentiation strongly affects product formation. Since conidiation is inherent in the aerial environment, we hypothesized that product formation near zero growth can be influenced by affecting differentiation or development of aerial hyphae in general. To investigate this idea, three developmental mutants (⌬fwnA, scl-1, and scl-2 mutants) that have no apparent vegetative growth defects were cultured in maltose-limited retentostat cultures. The secondary-metabolite profile of the wild-type strain defined flavasperone, aurasperone B, tensidol B, and two so far uncharacterized compounds as associated with conidium formation, while fumonisins B 2 , B 4 , and B 6 were characteristic of early response to nutrient limitation by the vegetative mycelium. The developmental mutants responded differently to the severe substrate limitation, which resulted in distinct profiles of growth and product formation. fwnA encodes the polyketide synthase responsible for melanin biosynthesis during aerial differentiation, and we show that conidial melanin synthesis in submerged retentostat cultures and aurasperone B production are fwnA dependent. The scl-1 and scl-2 strains are two UV mutants generated in the ⌬fwnA background that displayed reduced asexual conidiation and formed sclerotium-like structures on agar plates. The reduced conidiation phenotypes of the scl-1 and scl-2 strains are reflected in the retentostat cultivation and are accompanied by elimination or severely reduced accumulation of secondary metabolites and distinctly enhanced accumulation of extracellular protein. This investigation shows that submerged conidiation and product formation of a mitosporic fungus cultured at low specific growth rates can be fundamentally affected by interfering with the genetic program for differentiation of aerial hyphae, opening new perspectives for tailoring industrial performance.The black-spored ascomycete Aspergillus niger is a ubiquitous saprophyte in nature and occurs as a common spoilage mold on stored food and feed (15,26,30). Its successful occupation of these ecological niches results at least partially from an ability to secrete a wide array of substrate-degrading enzymes, which, like organic acid biosynthesis, is exploited in industrial production (32, 34). Also contributing to its economic importance are less desirable traits, such as conditional production of toxic secondary metabolites or mycotoxins, like ochratoxin A and fumonisins (30,36). Secondary-metabolite accumulation in the genus Aspergillus and its relatives is most diverse and abundant under subaerial growth conditions on solid media that allow differentiation or development of vegetative hyphae into conidia, sclerotia, or cleistothecia (2, 45).Despit...

Physiological and Transcriptional Responses of Different Industrial Microbes at Near-Zero Specific Growth Rates

“…By integration of data from transcriptome analysis and metabolic modeling, L. plantarum was predicted to produce plant growthstimulating metabolites, including indole compounds and phenylacetate, at (near-)zero growth rates (6). Short-term retentostat cultivation elicited significant metabolic changes in A. niger, including the production of the secondary metabolites flavasperone, aurasperone B, tensidol B, and fumonisins B 2 , B 4 , and B 6 (54). These results show that in some but not all (two of the four analyzed here) species, (near-)zero growth elicits the production of secondary metabolites.…”

The Transcriptional Repressor TupA in Aspergillus niger Is Involved in Controlling Gene Expression Related to Cell Wall Biosynthesis, Development, and Nitrogen Source Availability

“…We noticed that these two genes are very highly expressed during sclerotia formation in the A. niger sclA-1 mutant strain [73], based on RNA analysis extracted from sclerotia (Jørgenson and Ram, unpublished results). In several filamentous fungi, Tup1 functions as a global repressor which regulates genes associated with morphological differentiation, sexual and asexual reproduction, and pathogenicity [31]–[33].…”

The global regulator LaeA controls production of citric acid and endoglucanases inAspergillus carbonarius

“…All three isolates produced naphtho-γ-pyrones and pyranonigrin A, but the relative amounts of a polar naphtho-γ-pyrone-related compound (BMS 192548) was higher in the laeA knockout than in the WT and laeA-overexpressed strains. The naphtho-γ-pyrones are related to the black pigment in Aspergillus section Nigri conidia [19,30], but BMS 192548 may also be secreted into the medium as a yellow pigment if overproduced as in the laeA knockout.…”