Recognition of lettuce downy mildew effector BLR38 in Lactuca serriola LS102 requires two unlinked loci

Plant-pathogenic oomycetes secrete effector proteins to suppress host immune responses. Resistance proteins may recognize effectors and activate immunity, which is often associated with a hypersensitive response (HR). Transient expression of effectors in plant germplasm and screening for HR has proven to be a powerful tool in the identification of new resistance genes. In this study, 14 effectors from the lettuce downy mildew Bremia lactucae race Bl:24 were screened for HR induction in over 150 lettuce accessions. Three effectors—BLN06, BLR38 and BLR40—were recognized in specific lettuce lines. The recognition of effector BLR38 in Lactuca serriola LS102 did not co-segregate with resistance against race Bl:24, but was linked to resistance against multiple other B. lactucae races. Two unlinked loci are both required for effector recognition and are located near known major resistance clusters. Gene dosage affects the intensity of the BLR38-triggered HR, but is of minor importance for disease resistance.