The mitogen-activated protein kinase gene CcPmk1 is required for fungal growth, cell wall integrity and pathogenicity in Cytospora chrysosperma

Cytospora chrysosperma, the causal agent of canker disease in a wide range of woody plants, results in significant annual economic and ecological losses. Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction pathways that play a crucial role in mediating cellular responses to environmental and host signals in plant pathogenic fungi. In this study, we identified an ortholog of the Fus3/Kssl-related MAPK gene, CcPmk1, and characterized its functions in C. chrysosperma. The expression of CcPmk1 was highly induced by inoculation on poplar twigs, and targeted deletion of CcPmk1 resulted in the loss of pathogenicity, indicating that CcPmk1 is an important regulator of virulence. In addition, CcPmk1 deletion mutants (Delta CcPmk1) displayed reduced growth and conidiation, decreased fungal biomass production and hyperbranching. Furthermore, our results indicated that CcPmk1 deletion mutants exhibited hypersensitivity to cell wall inhibitors and cell wall degrading enzymes. Correspondingly, the transcription of cell wall biosynthesis-related genes in the Delta CcPmk1 strain was downregulated compared to that in the wild-type strain. Moreover, we found that CcPmkl could positively regulate the expression of several candidate effector encoding genes which were highly induced in planta. Hence, we hypothesized that CcPmk1 regulates the expression of a series of effectors to promote virulence. Overall, we concluded that the functions of CcPmk1 extend to fungal development, cell wall integrity and pathogenicity in C. chrysosperma.