A RACK1 family protein regulates pathogenicity of Peronophythora litchii 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 homologue identified in P. litchii, and named PlRACK1. The protein 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, and we found that it was involved in mycelial growth, cell wall integrity, ROS metabolism, laccase activity, and pathogenicity of P. litchii. PlMAPK1 interacted with RACK1, and they 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 provide insights into the direct interactions between RACK1, MAPKs, and MKP, and their functions in growth, development, and pathogenesis in a plant pathogenic oomycete. PlRACK1 played a critical role in response to ROS and pathogenesis of P. litchii.PlMAPK1 interacted with PlRACK1, and they jointly regulated sporangiophore branching.PlMKP1 interacted with PlMAPK1/PlMAPK2 and was involved in pathogenesis.PlBglX, identified by transcriptome analysis of PlRACK1 mutant, is a virulence factor. PlRACK1 played a critical role in response to ROS and pathogenesis of P. litchii. PlMAPK1 interacted with PlRACK1, and they jointly regulated sporangiophore branching. PlMKP1 interacted with PlMAPK1/PlMAPK2 and was involved in pathogenesis. PlBglX, identified by transcriptome analysis of PlRACK1 mutant, is a virulence factor.