Three Combined Quantitative Trait Loci from Nonhost Lactuca saligna Are Sufficient to Provide Complete Resistance of Lettuce Against Bremia lactucae

Abstract

The nonhost resistance of wild lettuce (Lactuca saligna) to downy mildew (Bremia lactucae) is based on at least 15 quantitative trait loci (QTL), each effective at one or more plant developmental stages. We used QTL pyramiding (stacking) to determine how many of these QTL from L. saligna are sufficient to impart complete resistance towards B. lactucae to cultivated lettuce, L. sativa. The alleles of four of the most promising QTL, rbq4, rbq5, rbq6+11, and rbq7 are effective at both the young and adult plant stages. Lines with these four QTL in all possible combinations were generated by crossing the respective backcross inbred lines (BIL). Using the 11 resulting lines (combiBIL), we determined that combinations of three QTL, rbq4, rbq5, and rbq6+11, led to increased levels of resistance; however, one QTL, rbq7, did not add to the resistance level when combined with the other QTL. One line, tripleBIL268, which contains the three QTL rbq4, rbq5, and rbq6+11, was completely resistant to B. lactucae at the young plant stage. This suggests that these three QTL are sufficient to confer the complete resistance of the nonhost L. saligna and any additional QTL in L. saligna are redundant. Histological analysis of B. lactucae infection in L. saligna, the BIL, and the combiBIL 48 h after inoculation revealed different microscopical phenotypes of resistance. The QTL differed with respect to the stage of the infection process with which they interfered.

Nonhost wild lettuce as a donor for resistance to downy mildew in cultivated lettuce

“…However, HR responses are also histologically observed in susceptible lettuce and potato cultivars, and are associated with both host and nonhost resistance in potato (Vleeshouwers et al, 2000;Zhang et al, 2009b). Therefore, the presence of responsiveness to an effector in a nonhost does not necessarily mean that this contributes to NHR.…”

“…Microscopic observations on L. saligna inoculated with B. lactucae indicated that NHR is mainly based on prehyphal or prehaustorial resistance. Only malformed hypha-like structures and no haustoria were observed (Lebeda and Reinink 1994;Zhang et al 2009b).…”

“…Per mapping population independent detached leaf assays were conducted on 5-8-wk-old adult plants as previously described in Jeuken & Lindhout (2002) Zhang et al, 2009b;den Boer et al, 2014), and L. sativa cv Olof and L. sativa cv Cobham Green (susceptible). Infection severity level (ISL) was scored visually as the percentage of leaf area covered with sporangiophores between 10 and 14 days after inoculation.…”

“…Histological analysis suggested that germinated conidia in L. saligna are arrested before normal hyphae formation (Lebeda et al 2008;Zhang et al 2009b). Previous genetic studies on progenies from L. saligna CGN05271 crossed with L. sativa cv Olof identified three minor QTLs in F2 populations at adult plant stage (Jeuken and Lindhout 2002).…”

“…Since 1976, several studies indicate that L. saligna accessions are totally disease-free (Netzer et al, 1976;Norwood et al, 1981;Lebeda, 1986;Gustafsson, 1989;Bonnier et al, 1992;Petrželová et al, 2011;Van Treuren et al, 2011). Histology studies suggested that germinated conidia in L. saligna were arrested before normal hyphae formation (Lebeda et al, 2008;Zhang et al, 2009b).…”