CCK1, a PMK1-type MAP kinase is required for hyphal growth, pigmentation, conidiation, enzyme activity, osmotic stress response, and pathogenicity in Corynespora cassiicola

Abstract

Corynespora cassiicola is a fungal pathogen causing Corynespora Leaf Fall disease (CLF) in rubber trees (Hevea brasiliensis). CLF has great economic impact in rubber-producing countries. However, the molecular mechanisms involved in the pathogenicity of C. cassiicola remain poorly understood. Mitogen-activated protein kinase (MAPK) is one of the major signaling pathways responsible for regulation of fungal development and virulence in many plant fungal pathogens. The C. cassiicola cck1 gene encodes a MAPK that is homologous to the rice pathogen Magnaporthe grisea MAPK Pmk1 protein. In this study, targeted cck1 gene disruption and complementation experiments were carried out in C. cassiicola. Compared to the wild-type and CΔcck1 complement, the Δcck1 mutant had more pigmentation and exhibited dramatically reduced aerial mycelial mass and mycelial growth rates, along with the inability to produce conidia or conidiophores. In addition, the Δcck1 hyphae were darker in color and had less branches and septae with visibly condensed protoplasm. The Δcck1 mutant was more resistant to hyperosmotic stress, and was able to produce greater amount of laccase and lesser cellulase in vitro; however, the production of laccase was delayed. Importantly, Δcck1 mycelia and crude toxin were less virulent upon inoculation of rubber tree leaves. These findings indicate that the C. cassiicola CCK1 MAP kinase is a versatile regulator involved in nearly every aspect of plant fungal pathogenicity including mycelial growth, development, differentiation, pigmentation, conidiation, production of extracellular degradation enzymes, osmoregulation, and virulence.