AaHog1 Regulates Infective Structural Differentiation Mediated by Physicochemical Signals from Pear Fruit Cuticular Wax, Stress Response, and Alternaria alternata Pathogenicity

The high-osmolarity glycerol response kinase, Hog1, affects several cellular responses, but the precise regulatory role of the Hog1 mitogen-activated protein (MAP) kinase in the differentiation of the infective structure of Alternaria alternata induced by pear cuticular wax and hydrophobicity has not yet clarified. In this study, the AaHog1 in A. alternata was identified and functionally characterized. AaHog1 has threonine-glycine-tyrosine (TGY) phosphorylation sites. Moreover, the expression level of AaHog1 was significantly upregulated during the stages of appressorium formation of A. alternata on the fruit-wax-extract-coated GelBond hydrophobic film surface. Importantly, our results showed that the appressorium and infection hyphae formation rates were significantly reduced in ΔAaHog1 mutants. Furthermore, AaHog1 is beneficial for the growth and development, stress tolerance, virulence, and cell-wall-degrading enzyme activity of A. alternata. These findings may be useful for dissecting the AaHog1 regulatory mechanism in relation to the pathogenesis of A. alternata.