MAPK Cascades Mediating Biocontrol Activity of Trichoderma brevicrassum Strain TC967

Trichoderma brevicrassum strain TC967 is a novel biological control agent (BCA) against the plant pathogen Rhizoctonia solani and promotes plant growth. Mitogen-activated protein kinase (MAPK) cascades are involved in a variety of physiological functions of Trichoderma, but functions of each MAPK in regulating biocontrol have not been characterized in a single Trichoderma strain. In this study, we assembled and annotated the genome of strain TC967 and identified its three MAPK gene sequences. Functions of Fus3-, Slt2-, and Hog1-MAPK in strain TC967 were dissected. All three MAPKs were involved in hyphal growth. Hog1-MAPK was essential for conidiation and tolerance to saline/osmotic stress. Both Fus3- and Slt2-MAPK deletion mutants reduced cell-wall integrity (CWI) and increased the activities of chitinase and protease. The growth of Rhizoctoniasolani was further inhibited by volatile organic compounds (VOCs) and secondary metabolites produced by Fus3- and Slt2-MAPK deletion mutants, respectively. Biocontrol assays demonstrated that Fus3- and Slt2-MAPK deletion mutants were considerably more effective in disease control than the wild-type strain. RNA-seq analysis revealed that MAPK collectively played a major role in regulating biocontrol-related gene expressions, including the genes in charge of secondary metabolism, fungal cell wall-degrading enzymes (FCWDEs), and small secreted cysteine-rich proteins (SSCPs).