ThHog1 controls the hyperosmotic stress response in Trichoderma harzianum

Trichoderma harzianum is a widespread mycoparasitic fungus, able to successfully colonize a wide range of substrates under different environmental conditions. Transcript profiling revealed a subset of genes induced in T. harzianum under hyperosmotic shock. The hog1 gene, a homologue of the MAPK HOG1 gene that controls the hyperosmotic stress response in Saccharomyces cerevisiae, was characterized. T. harzianum hog1 complemented the hog1Δ mutation in S. cerevisiae, but showed different features to yeast alleles: improved osmoresistance by expression of the hog1 allele and a lack of lethality when the hog1 F315S allele was overexpressed. ThHog1 protein was phosphorylated in T. harzianum under different stress conditions such as hyperosmotic or oxidative stress, among others. By using a ThHog1-GFP fusion, the protein was shown to be localized in nuclei under these stress conditions. Two mutant strains of T. harzianum were constructed: one carrying the hog1 F315S allele, and a knockdown hog1-silenced strain. The silenced strain was highly sensitive to osmotic stress, and showed intermediate levels of resistance against oxidative stress, indicating that the main role of ThHog1 protein is in the hyperosmotic stress response. Stress cross-resistance experiments showed evidences of a secondary role of ThHog1 in oxidative stress. The strain carrying the hog1 F315S allele was highly resistant to the calcineurin inhibitor cyclosporin A, which suggests the existence of links between the two pathways. The two mutant strains showed a strongly reduced antagonistic activity against the plant pathogens Phoma betae and Colletotrichum acutatum, which points to a role of ThHog1 protein in fungus–fungus interactions.