CERK1 is required for chitin-triggered reactive oxygen species generation in melon and is broadly conserved in cucurbits.

CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1), originally identified in Arabidopsis thaliana, encodes a pattern recognition receptor that perceives the fungal cell wall component called chitin to activate immune responses, including the production of reactive oxygen species (ROS) against fungal pathogens. Functional CERK1 orthologs have been identified in plants, such as tomato, rice, and wheat. However, the knowledge of chitin-triggered immunity in Cucurbitaceae plants is currently limited. This study revealed that chitin triggers ROS generation in melon (Cucumis melo) and cucumber (Cucumis sativus), indicating that chitin is recognized by cucurbits. A subsequent homology search using the Arabidopsis CERK1 sequence identified CERK1 ortholog candidates of melon (CmCERK1) and cucumber (CsCERK1). Virus-induced gene silencing of CmCERK1 severely reduced chitin-triggered ROS generation in melon, indicating that CmCERK1 is essential for chitin recognition and the subsequent immune response. Genomic PCR of CmCERK1 and ROS assay upon chitin treatment in multiple melon commercial cultivars also showed that functional CmCERK1 is conserved in all the tested cultivars. Further analysis of the available genomes of various cucurbit plants suggested that CERK1 is broadly conserved in cucurbit plants.