HwHog1 kinase activity is crucial for survival of Hortaea werneckii in extremely hyperosmolar environments

• Hortaea werneckii HOG pathway is robust and includes two redundant kinases HwHog1A/B.
• HwHog1A/B activity is crucial for Hortaea colony growth at ⩾3.0 M osmolytes.
• HwHog1 activated by ⩾3.0 M NaCl, but not KCl and sorbitol, induces gene transcription.
• HOG pathway senses osmolyte type and is vital for extreme osmotolerance of Hortaea.
• Hortaea survival at <3.0 M osmolytes is not dependent on HOG or calcineurin pathways.

Although suggested, the involvement of the HOG pathway in adaptation processes in extremely halotolerant fungus Hortaea werneckii has never been specifically demonstrated. Here, we show that the H. werneckii HOG pathway is very robust, and that it includes two functionally redundant MAPK homologues, HwHog1A and HwHog1B, that show osmolyte-type-dependent phosphorylation. Inhibition of HwHog1 kinase activity with the ATP analogue BPTIP restricts H. werneckii colony growth at 3.0 M NaCl, KCl and sorbitol, most likely due to restricted cell division. On the other hand, HwHog1-regulated transcription of a selected group of genes (HwSTL1, HwGUT2, HwOPI3, HwGDH1, HwUGP1, HwGPD1) is an osmolyte-specific process that is important for induction of gene transcription with high NaCl, for regulation of specific genes with high sorbitol, and has no role in KCl stressed cells. Survival of H. werneckii at moderate NaCl and KCl concentrations is not dependent on HwHog1 activity or the calcineurin pathway, and thus alternative mechanisms must exist. The HOG pathway described here is vital for the extreme osmotolerance of H. werneckii, and its regulation shows important differences from the homologue pathways characterised in other mesophilic and halotolerant fungi.