Aspergillus nidulans gfdB, Encoding the Hyperosmotic Stress Protein Glycerol-3-phosphate Dehydrogenase, Disrupts Osmoadaptation in Aspergillus wentii

The genome of the osmophilic Aspergillus wentii, unlike that of the osmotolerant Aspergillus nidulans, contains only the gfdA, but not the gfdB, glycerol 3-phosphate dehydrogenase gene. Here, we studied transcriptomic changes of A. nidulans (reference strain and ΔgfdB gene deletion mutant) and A. wentii (reference strain and An-gfdB expressing mutant) elicited by high osmolarity. A. nidulans showed a canonic hyperosmotic stress response characterized by the upregulation of the trehalose and glycerol metabolism genes (including gfdB), as well as the genes of the high-osmolarity glycerol (HOG) map kinase pathway. The deletion of gfdB caused only negligible alterations in the transcriptome, suggesting that the glycerol metabolism was flexible enough to compensate for the missing GfdB activity in this species. A. wentii responded differently to increased osmolarity than did A. nidulans, e.g., the bulk upregulation of the glycerol and trehalose metabolism genes, along with the HOG pathway genes, was not detected. The expression of An-gfdB in A. wentii did not abolish osmophily, but it reduced growth and caused much bigger alterations in the transcriptome than did the missing gfdB gene in A. nidulans. Flexible glycerol metabolism and hence, two differently regulated gfd genes, may be more beneficial for osmotolerant (living under changing osmolarity) than for osmophilic (living under constantly high osmolarity) species.