The Medicago truncatula LYR4 intracellular domain serves as a scaffold in immunity signaling independent of its phosphorylation activity

Plants perceive and respond to chitin derived from fungal cell walls through lysine motif (LysM) receptor kinases. In the model legume Medicago truncatula , CERK1 and LYR4 represent the LysM receptor pair important for chitin-triggered immunity signaling. Here, we show that both the active kinase receptor CERK1 and the pseudokinase receptor LYR4 contribute to immunity signaling, leading to the production of reactive oxygen species (ROS). We determine the crystal structure of the LYR4 core intracellular domain with a bound nucleotide analog in the active site. Biochemical characterization shows that LYR4 binds ATP and has both autophosphorylation as well as transphosphorylation activity towards CERK1. However, in planta experiments demonstrate that the phosphorylation ability is not necessary for the function of LYR4 in chitin-triggered ROS production, but that the presence of its intracellular domain is indispensable. Together, we show that in chitin-triggered immunity the intracellular domain of LYR4 serves as a signaling scaffold independent of its catalytic activity.