Comparative Transcriptomic Analysis of MAPK-Mediated Regulation of Pathogenicity, Stress Responses, and Development in Cytospora chrysosperma

Abstract

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction pathways that mediate cellular responses to various biotic and abiotic signals in plant-pathogenic fungi. Generally, there are three MAPKs in filamentous pathogenic fungi: Pmk1/Fus3/Kss1, Hog1, and Stl2. Our previous studies have shown that CcPmk1 is a core regulator of fungal pathogenicity in Cytospora chrysosperma, the causal agent of canker disease in a wide range of woody plants. Here, we identified and functionally characterized the other two MAPK genes ( CcHog1 and CcSlt2) and then compared the transcriptional differences among these three MAPKs in C. chrysosperma. We found that the MAPKs shared convergent and distinct roles in fungal development, stress responses, and virulence. For example, CcHog1, CcSlt2, and CcPmk1 were all involved in conidiation and response to stresses, including hyperosmotic pressure, cell wall inhibition agents, and H2O2, but only CcPmk1 and CcSlt2 were required for hyphal growth and fungal pathogenicity. Transcriptomic analysis showed that numerous hyperosmosis- and cell wall-related genes significantly reduced their expression levels in Δ CcHog1 and Δ CcSlt2, respectively. Interestingly, RNA- and ribosome-related processes were significantly enriched in the upregulated genes of Δ CcSlt2, whereas they were significantly enriched in the downregulated genes of Δ CcPmk1. Moreover, two secondary metabolite gene clusters were significantly downregulated in Δ CcPmk1, Δ CcSlt2, and/or Δ CcHog1. Importantly, some virulence-associated genes were significantly downregulated in Δ CcPmk1 and/or Δ CcSlt2, such as candidate effector genes. Collectively, these results suggest that the similar and distinct phenotypes of each MAPK deletion mutant may result from the transcriptional regulation of a series of common or specific downstream genes, which provides a better understanding of the regulation network of MAPKs in C. chrysosperma.