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Sporulation is an essential part of the life cycle of the industrially important filamentous fungus Aspergillus niger. The formation of conidiophores, spore-bearing structures, requires remodelling of the fungal cell wall, as demonstrated by the differences in carbohydrate composition of cell walls of vegetative mycelium and spores. Glycoside hydrolases that are involved in this process have so far remained unidentified. Using transcriptome analysis, we have identified genes encoding putative cell-wall-modifying proteins with enhanced expression in sporulating aerial mycelium compared to vegetative mycelium. Among the most strongly induced genes were those encoding a protein consisting of a putative chitin binding module (CBM14) and the chitinolytic enzymes NagA, CfcI and CtcB. Reporter studies showed that the N-acetyl-β-hexosaminidase gene nagA was expressed both in vegetative hyphae and in aerial structures (aerial hyphae, conidiophores and conidia) upon starvation. In contrast, promoter activities of the chitinase genes ctcB and cfcI were specifically localized in the conidiophores and conidia. CtcB is an endo-chitinase and CfcI releases monomers from chitin oligosaccharides: together these enzymes have the potential to degrade chitin of the fungal cell wall. Inactivation of both the cfcI and ctcB genes affected neither radial growth rate, nor formation and germination of spores. The amount of chitin in the spore walls of a ΔcfcIΔctcB double deletion strain, however, was significantly increased compared with the wild-type, thus indicating that CfcI and CtcB indeed modify the A. niger cell walls during sporulation. These novel insights in the sporulation process in aspergilli are of strong scientific relevance, and also may aid industrial strain engineering.

Chitinases CtcB and CfcI modify the cell wall in sporulating aerial mycelium of Aspergillus niger