Genetics and Genomics of Lettuce-Pathogen Interactions

Over 800 candidate genes, putatively involved in pathogen recognition and defense responses, have been identified in lettuce. The majority of these disease resistance genes are clustered in the genome and co-segregate  with clusters of disease resistant phenotypes. An RNAi approach was employed to identify gene families involved in determining different disease resistant specificities. A subset of candidate genes, primarily NB-LRR encoding genes,  which were selected for functional analysis based on both genetic and phylogenetic information. RNAi tester-stocks for 27 different genes have been generated and tested for a total of 23 resistance phenotypes. So far, 16 disease resistance specificities in four different clusters, Dm3, Dm5/8, Dm7 and Dm13, have been abrogated by different RNAi lines. Members of the same gene family can detect both oomycete and bacterial effectors. A dexamethasone-inducible RNAi vector was used for the functional analysis of genes involved in signaling pathways in lettuce, because constitutive silencing of the EDS1 and NDR1 homologs was previously shown to be lethal in lettuce. Contrary to observations based on studies in other species the LsNDR1 gene is involved in the signaling triggered by a TIR-NBS-LRR encoding gene family.  Furthermore LsNDR1 is not only required for elicitation of defense responses but also for developmental processes.