The genome of Lactuca saligna, a wild relative of lettuce, provides insight into non-host resistance to the downy mildew Bremia lactucae

Lactuca saligna L. is a wild relative of cultivated lettuce (Lactuca sativa L.), with which it is partially interfertile. Hybrid progeny suffer from hybrid incompatibility (HI), resulting in reduced fertility and distorted transmission ratios. Lactuca saligna displays broad-spectrum resistance against lettuce downy mildew caused by Bremia lactucae Regel and is considered a non-host species. This phenomenon of resistance in L. saligna is called non-host resistance (NHR). One possible mechanism behind this NHR is through the plant-pathogen interaction triggered by pathogen recognition receptors, including nucleotide-binding leucine-rich repeat (NLR) proteins and receptor-like kinases (RLKs). We report a chromosome-level genome assembly of L. saligna (accession CGN05327), leading to the identification of two large paracentric inversions (>50 Mb) between L. saligna and L. sativa. Genome-wide searches delineated the major resistance clusters as regions enriched in NLRs and RLKs. Three of the enriched regions co-locate with previously identified NHR intervals. RNA-seq analysis of Bremia-infected lettuce identified several differentially expressed RLKs in NHR regions. Three tandem wall-associated kinase-encoding genes (WAKs) in the NHR8 interval display particularly high expression changes at an early stage of infection. We propose RLKs as strong candidates for determinants of the NHR phenotype of L. saligna.