Resistance Genes Against Lettuce Downy Mildew

Downy mildew (DM), caused by Bremia lactucae, is the most important foliar disease of lettuce worldwide, which decreases the quality of the marketable portion of the crop. The use of resistant varieties carrying dominant genes (Dm genes) is the most effective method for controlling this disease; however, pathogen variability has led to the rapid defeat of individual Dm genes. The known Dm genes are located in major resistance clusters (MRC) containing genes encoding nucleotide binding-leucine rich repeat (NBS-LRR). Many resistance genes have been introgressed from wild species by repeated backcrosses to cultivated lettuce resulting in numerous near-isogenic lines (NILs). Whole genome sequencing of donor and recurrent genotypes and their NILs was used to identify the chromosome segments introgressed from the wild species. This has located the candidate chromosomal positions for resistance genes as well as additional segments. Resistance gene enrichment sequencing (RenSeq) allowed the identification of numerous NBS-LRR sequences from these NILs and from several lettuce cultivars carrying known Dm genes. This, in combination with phenotypic data from these lines has allowed the identification of several candidate genes for known and new resistances against Bremia lactucae. Knowledge of resistance genes at the sequence level provides the basis for engineering pathogen resistance varieties by using genome editing through CRISPR-cas9 system and breeding for resistance to multiple isolates of Bremia lactucae. This will lead to the opportunity for more durable resistance to lettuce DM.