Conserved and divergent roles of the HOG1 kinase of Alternaria longipes in mycelial and conidial development, multi-stress responses, melanin production and pathogenicity

Abstract

The protein kinase HOG1 in high osmolarity glycerol pathway is one of mitogen-activated protein kinases and plays an important role in fungi cellular responses to external stress and stimuli. In this study, AlHOG1, a yeast HOG1 homolog, was isolated from tobacco pathogenic fungus Alternaria longipes and its disrupted mutants were generated using homologous recombination. Compared with the wild-type strain, the mutants grew slower under Minimal-Media, produced less conidia on Potato-Dextrose-Agar and showed a retardation phenotype in conidial germination rate. Colony color observation indicated that AlHOG1 most likely negatively regulated melanin production under nutrients-deficient or osmotic stress. The same action mode of AlHOG1 on hyphal growth in response to certain carbon or nitrogen sources was speculated. Additionally, the disrupted strains were sensitive to osmotic and oxidative stress, and showed increased resistance to dicarboximide, phenylpyrrole and carbendazim fungicides. Meanwhile, results from the comparative analysis between the AlHOG1- and the AlHK1 (encoding a group III histidine kinase)-disrupted mutants showed that AlHOG1 mediated-pathway was partly regulated by the upstream AlHK1 kinase for cellular resistance to salts osmotic stress and that AlHK1 was a primary regulator for cellular sugar and fungicides stress adaptation and likely modulated both AlHOG1-mediated pathway and other unknown mechanisms. Noteworthily, AlHOG1 but not AlHK1 was confirmed to be associated with high concentration glycerol adaptation, which is the first report to our knowledge. Overall, our results indicated that AlHOG1 was involved in multiple physiological processes in A. longipes, including mycelial and conidial development, melanin production, nutrients sensing and multi-stress responses.