Schematic illustration of signaling pathways leading to osmotic and oxidative stress resistance and fungicide sensitivity in A. alternata.

A. alternata deploys the HSK1- and the SKN7-mediated phosphorelay signaling pathways between histidine (His) and asparate (Asp) to cope with sugar-induced osmotic stress. SSK1 plays a minor role in sugar-induced osmotic stress resistance. However, SSK1 and HOG1 mediated by unknown kinase sensors confer resistance to oxidative and salt-induced osmotic stress. Low-level H2O2 produced by NADPH oxidases (NOX) is proposed to activate SSK1, HOG1, YAP1, and SKN7, all implicated in ROS resistance. HSK1, SSK1, HOG1, SKN7, but not YAP1 and NOX are involved in fungicide sensitivity.