Abstract
Fungi utilize a phosphorelay system coupled to a MAP kinase module for sensing and processing environmental signals. In Aspergillus nidulans, response regulator SskA transmits osmotic and oxidative stress signals to the stress MAPK (SAPK) SakA. Using a genetic approach together with GFP tagging and molecular bifluorescence we show that SakA and ATF/CREB transcription factor AtfA define a general stress-signalling pathway that plays differential roles in oxidative stress responses during growth and development. AtfA is permanently localized in the nucleus, while SakA accumulates in the nucleus in response to oxidative or osmotic stress signals or during normal spore development, where it physically interacts with AtfA. AtfA is required for expression of several genes, the conidial accumulation of SakA and the viability of conidia. Furthermore, SakA is active (phosphorylated) in asexual spores, remaining phosphorylated in dormant conidia and becoming dephosphorylated during germination. SakA phosphorylation in spores depends on certain (SskA) but not other (SrrA and NikA) components of the phosphorelay system. Constitutive phosphorylation of SakA induced by the fungicide fludioxonil prevents both, germ tube formation and nuclear division. Similarly, Neurospora crassa SakA orthologue OS-2 is phosphorylated in intact conidia and gets dephosphorylated during germination. We propose that SakA-AtfA interaction regulates gene expression during stress and conidiophore development and that SAPK phosphorylation is a conserved mechanism to regulate transitions between non-growing (spore) and growing (mycelia) states.