Fungi are an important source of enzymes for saccharification of plant polysaccharides and production of
biofuels. Understanding of the regulation and induction of expression of genes encoding these enzymes
is still incomplete. To explore the induction mechanism, we analysed the response of the industrially
important fungus Aspergillus niger to wheat straw, with a focus on events occurring shortly after exposure
to the substrate. RNA sequencing showed that the transcriptional response after 6 h of exposure to wheat
straw was very different from the response at 24 h of exposure to the same substrate. For example, less than
half of the genes encoding carbohydrate active enzymes that were induced after 24 h of exposure to wheat
straw, were also induced after 6 h exposure. Importantly, over a third of the genes induced after 6 h of exposure
to wheat straw were also induced during 6 h of carbon starvation, indicating that carbon starvation is
probably an important factor in the early response to wheat straw. The up-regulation of the expression of a
high number of genes encoding CAZymes that are active on plant-derived carbohydrates during early carbon
starvation suggests that these enzymes could be involved in a scouting role during starvation, releasing
inducing sugars from complex plant polysaccharides. We show, using proteomics, that carbon-starved cultures
indeed release CAZymes with predicted activity on plant polysaccharides. Analysis of the enzymatic
activity and the reaction products, indicates that these proteins are enzymes that can degrade various plant
polysaccharides to generate both known, as well as potentially new, inducers of CAZymes.
2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license