Mining Of The Resistance Gene For Powdery Mildew In Cucumber

Powdery mildew is one of the most important diseases for cucumber worldwide. It can cause serious losses for cucumber production. Studies on cucumber powdery mildew got more and more attention at home and abroad. But because of the difference between the genetic background of experiment materials, physiological strains and the methods used for the identification of the resistence, researchers had drawn various conclusions. The information about gene mapping of the resistance and the molecular markers linked to the resistance were not good enough to satisfy the marker assisted selection(MAS) breeding. In this study, F2 and F2:3 population, derived from cucumber lines PI200815(high resistance to powdery mildew) and 931(a selected inbred line from the cultived variety “xintaimici” with high susceptible to powdery mildew), were used as plant materials to analyze the inheritance and detect quantitative trait loci(QTL) for the resistance. The artificial inoculation identification method and SSR molecular marker technology were employed in the present study. At the same time, taking the advantage of the whole-genome re-sequenced core germplasm, we conducted Genome-Wide Association Study(GWAS) to detect the powdery mildew resistence gene in the whole genome. The main results were summarized as following:(1)We conducted the resistance identification for P1、P2、F1 、F2 and F2:3 in three seasons. The results showed that the disease index(DI) in F2 population had continuroius characteristics and fit the nomal distribution. It was concluded the powdery mildew resistence in PI200815 is a quantitative trait controlled by several genes.(2)2112 pairs of SSR primers, developed by using cucumber whole genome sequence, were used to screen the polymorphism between the two parental lines. There were 129 primers generating polymorphism amplicons. Using F2 segregation population, a genetic map with 129 markers was constructed. It consisted of 8 linkage groups corresponding to 7 chromosomes of cucumber. The total length of the map is 857.9cM with an average interval of 6.65 cM.(3) QTL analysis of the resistance to powdery mildew was conducted using the genetic map and the identification results in three seasons. Two QTLs, named pmQTL1.1 and pmQTL6.1, were detected and located on chromosome 1(Chr.1) and Chr.6 of cucumber, respectively. The pmQTL1.1 was detected in the all three seasons and considered as a steady major QTL with the flanking markers of SSR03860 and SSR14445. It accounted for phenotypic variances of 37.9% with logarithm of odds(LOD) score of 15.33. The pmQTL6.1 was detected only in one season with the LOD of 3.31 and phenotypic variances of 7%. It was considered as an unstable minor QTL.(4)According to the genome sequence information, the physical distance of the flanking markers of pmQTL1.1 was 689 kb containing 95 candidate genes. Among the candidate genes, there were two NBS-like resistant proteins, one protein related to programmed cell death and two stress regulated proteins.(5)We conducted GWAS for the resistance gene using the re-sequenced core germplasm. Six steady singals were detected: pmG1.1, pmG2.1, pmG4.1, pmG5.1, pmG5.2 and pmG5.3. Three loci, pmG2.1, pmG5.2 and pmG5.3, were detected repeatedly. And pmG1.1, pmG5.2,pmG5.3 were consistent with the previous QTL mapping results.