Plants rely on cell surface immune receptors to detect microbial patterns and initiate effective defense responses. Although the Asteraceae family is one of the largest and economically important plant groups, little information is available about its pattern-triggered immunity signaling. Cultivated lettuce ( Lactuca sativa L.) recognizes a 24-amino acid peptide (nlp24) from necrosis- and ethylene-inducing peptide 1-like proteins (NLPs) found in bacteria, fungi, and oomycetes. Here, we perform an extensive characterization of nlp24-induced immune responses in lettuce and identify the LETTUCE nlp24 RECEPTOR (LNR) as the leucine-rich repeat receptor-like kinase mediating its recognition. Remarkably, nlp24 recognition in lettuce and subsequent activation of defenses strongly depend on the conserved heptapeptide motif (GHRHDWE). Structural modeling-guided mutagenesis experiments suggest that residues in the nlp24 heptapeptide interact with a hydrophobic pocket in the LNR solenoid structure. Divergent ligand specificities and the absence of sequence homology between LNR and Arabidopsis nlp24-recognizing receptor indicate that the NLP recognition in lettuce and Arabidopsis emerged independently, through convergent evolution. Our phylogenetic analysis shows that LNR is closely related to Arabidopsis MIK2 (MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2), but belongs to a distinct, Asteraceae-specific monophyletic subgroup that has undergone a significant expansion in Lactuca . Our findings provide insights into the mechanisms of pattern-triggered immunity in lettuce and the fast evolution of its immune receptor repertoire. On the translational side, our findings open opportunities for the crop defense improvement via interfamily transfer.
Significance statement
Understanding pathogen recognition in crops is key to improving disease resistance. Our study identified the cell surface immune receptor in cultivated lettuce that senses the nlp24 pattern derived from secreted proteins (NLPs) of prokaryotic and eukaryotic microbial pathogens. Unlike the previously characterized receptor from Arabidopsis (RLP23), the lettuce nlp24 receptor (LNR) detects the deeply conserved heptapeptide motif of NLP proteins that is required for host cell lysis. Transfer of the LNR receptor to a Solanaceous species conferred quantitative resistance to the oomycete pathogen Phytophthora capsici . Our findings advance the understanding of pattern-triggered immunity in a major leafy crop and highlight LNR as a promising receptor for broad-spectrum plant resistance engineering.