Germination of Aspergillus niger conidia

Aspergillus niger is a black-spored filamentous fungus that forms asexual spores called conidospores (‘conidia’). Germination of conidia, leading to the formation of hyphae, is initiated by conidial swelling and mobilisation of endogenous carbon and energy stores, followed by polarisation and emergence of a hyphal germ tube. These morphological and biochemical changes which define the model of germination have been studied with the aim of understanding how conidia sense and utilise different soluble carbon sources for germination. Microscopy and flow cytometry were used to track the morphological changes and results showed that the germination of A. niger conidia was quicker and more homogenous in rich media than in minimal media. The germination of conidia was also shown to be quicker in the presence of D-glucose than D-xylose. In the absence of a carbohydrate, no visual indicators of germination were evident. Added to this, the metabolism of internal storage compounds was shown to only occur in the presence of a suitable carbon source. Specific environmental carbon sources may therefore serve as triggers of germination, i.e. to initiate the catabolism of stores such as D-trehalose and the swelling of conidia. Studies carried out using D-glucose analogues identified the structural features of sugars that trigger or support conidial germination. These studies showed that the arrangement of atoms on carbons 3 and 4, on the pyranose ring structure of D-glucose, are essential to serve as a trigger of germination. The trigger step preceeds, and is separate from, the energy generation step that supports the continued outgrowth. Transcriptomic studies found that the most significant changes were associated with the breaking of dormancy. The data also revealed that fermentative metabolism present at the early stages of spore germination is rapidly replaced by respiratory metabolism