The commonplace observation that plants are immune to most potential pathogens is known as nonhost resistance (NHR). The genetic basis of NHR is poorly understood. Inheritance studies of NHR require crosses of nonhost species with a host, but these crosses are usually unsuccessful. The plant-pathosystem of lettuce and downy mildew, Bremia lactucae, provides a rare opportunity to study the inheritance of NHR, because the nonhost wild lettuce species Lactuca saligna is sufficiently cross-compatible with the cultivated host Lactuca sativa. Our previous studies on NHR in one L. saligna accession led to the hypothesis that multi-locus epistatic interactions might explain NHR. Here, we studied NHR at the species level in nine accessions. Besides the commonly used approach of studying a target trait from a wild donor species in a cultivar genetic background, we also explored the opposite, complementary approach of cultivar introgression in a wild species background. This bidirectional approach encompassed (1) nonhost into host introgression: identification of L. saligna derived chromosome regions that were overrepresented in highly resistant BC1 plants (F1 × L. sativa), (2) host into nonhost introgression: identification of L. sativa derived chromosome regions that were overrepresented in BC1 inbred lines (F1 × L. saligna) with relatively high infection levels. We demonstrated that NHR is based on resistance factors from L. saligna and the genetic dose for NHR differs between accessions. NHR seemed explained by combinations of epistatic genes on three or four chromosome segments, of which one chromosome segment was validated by the host into nonhost approach.
Figure 1: Boxplot of average infection severity level (%, at seedling stage) per Lactuca species based on public data of the Centre for Genetic Resources, the Netherlands (CGN) supplemented with our own data on three L. saligna accessions. The band inside the box is the median. Red numbers and diamonds indicate the species average. Averages are based on all tested interactions per accession tested with 10–23 Bl: races and for L. saligna also include the three additional accessions tested with only three races. Number of accessions included per species: L. sativa (n = 780), L. serriola (n = 490), L. virosa (n = 45), and L. saligna (n = 54). Green square (our data): L. saligna 275-5 average infection severity level to three Bl: races (21, 24 and 29). Blue triangle (our data): L. saligna CGN11341 and CGN15726 average infection severity level to three Bl: races (21, 24 and 29) (color figure online)