Differential contribution of Arabidopsis chitin receptor complex components to defense signaling and ubiquitination‐dependent endocytotic removal from the plasma membrane

In Arabidopsis, the enzymatically active lysin motif-containing receptor-like kinase (LysM-RLK) CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) and the pseudokinases LYSIN MOTIF-CONTAINING RECEPTOR-LIKE KINASE 5 (LYK5) and LYK4 are the core components of the canonical chitin receptor complex. CERK1 dimerizes and autophosphorylates upon chitin binding, resulting in activation of chitin signaling. In this study, we clarified and further elucidated the individual contributions of LYK4 and LYK5 to chitin-dependent signaling using mutant (combination)s and stably transformed Arabidopsis plants expressing fluorescence-tagged LYK5 and LYK4 variants from their endogenous promoters. Our analyses revealed that LYK5 interacts with CERK1 upon chitin treatment, independently of LYK4 and vice versa. We show that chitin-induced autophosphorylation of CERK1 is predominantly dependent on LYK5, whereas chitin-triggered ROS generation is almost exclusively mediated by LYK4. This suggests specific signaling functions of these two co-receptor proteins apart from their redundant function in mitogen-activated protein kinase (MAPK) signaling and transcriptional reprogramming. Moreover, we demonstrate that LYK5 is subject to chitin-induced and CERK1-dependent ubiquitination, which serves as a signal for chitin-induced internalization of LYK5. Our experiments provide evidence that a combination of phosphorylation and ubiquitination events controls LYK5 removal from the plasma membrane via endocytosis, which likely contributes to receptor complex desensitization.