A RACK1 family protein regulates pathogenicity of oomycete by acting as a scaffold for MAPK signal modules

Litchi downy blight caused by Peronophythora litchii is the most destructive disease of litchi (Litchi chinensis). RACK1 (Receptor for activated C kinase 1) is a group of scaffold proteins mainly involved in the regulation of various signaling pathways by interacting with signal transduction proteins and affecting the activity of these proteins. In this study, a RACK1 homologous protein was identified in P. litchii, named PlRACK1, which was found to interact with the mitogen-activated protein kinases, PlMAPK1 and PlMAPK2. CRISPR/Cas9-mediated genome editing technology was used to knock out PlRACK1. We found that PlRACK1 was involved in mycelial growth, cell wall integrity, ROS metabolism, laccase activity, and pathogenicity of P. litchii. PlMAPK1 interacted with RACK1 and jointly regulated sporangiophore branching of P. litchii. Transcriptome analysis showed that P. litchii MAPK Phosphatase 1 (PlMKP1) and beta-glucoside (PlBglX) were regulated by PlRACK1, both of which were also required for the pathogenicity of P. litchii. As well, PlMKP1 also interacted with PlMAPK1 and PlMAPK2. These results reveal for the first time the direct interactions between RACK1, MAPKs and MKP, and their functions in growth, development and pathogenesis in a plant pathogenic oomycete.