<i>CERK1</i> is required for chitin-triggered reactive oxygen species generation in melon and is broadly conserved in cucurbits

CHITIN ELICITOR RECEPTOR KINASE 1 (<i>CERK1</i>), originally identified in <i>Arabidopsis thaliana</i>, 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 <i>CERK1</i> 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 (<i>Cucumis melo</i>) and cucumber (<i>Cucumis sativus</i>), 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 <i>CmCERK1</i> severely reduced chitin-triggered ROS generation in melon, indicating that <i>CmCERK1</i> is essential for chitin recognition and the subsequent immune response. Genomic PCR of <i>CmCERK1</i> and ROS assay upon chitin treatment in multiple melon commercial cultivars also showed that functional <i>CmCERK1</i> is conserved in all the tested cultivars. Further analysis of the available genomes of various cucurbit plants suggested that <i>CERK1</i> is broadly conserved in cucurbit plants.