Fine Mapping Of The Powdery Mildew Resistance Gene Pm And Downy Mildew Resistance Major QTL Dm5.2 In A Cucumis Hystrix Introgression Line Of Cucumber

Cucumber（Cucumis sativus L.）is an important economic crop of the Cucurbitaceae Melon genus,which is widely cultivated all over the world.Due to the narrow genetic basis and low intraspecific variation rate,cucumber lacks important resistance genes.In actual production,it is susceptible to various diseases.Powdery mildew and downy mildew are the two main leaf diseases in cucumbers,which pose a huge threat to the yield and quality of cucumbers.The breeding of powdery mildew and downy mildew resistant varieties is the most economical and environmental protection method in cucumber resistance breeding.The discovery of powdery mildew and downy mildew resistance genes and related molecular markers in cucumber is of great significance for the improvement of cucumber varieties resistant to powdery mildew and downy mildew and cucumber production.Endogenous defense enzymes play an important role in cucumber resistance to pathogen infection,and the determination of endogenous defense enzymes in Cucumber after pathogen infection has a certain reference value for explaining cucumber disease resistance from a physiological point of view.In this study,based on the predecessors’preliminary location of the Cucumis hystrix introgression line of cucumber IL52 powdery mildew resistance gene pm and downy mildew resistance major QTL dm5.2,F₂ population and major downy mildew resistance QTL were constructed by a Cucumis hystrix introgression line of cucumber IL52 which is resistant to powdery mildew and downy mildew and cucumber inbred line CCMC which is susceptible to powdery mildew and downy mildew.The secondary segregation population constructed for the main effect QTL dm5.2 for downy mildew resistance was used to fine mapping dm5.2.The main research contents and results are as follows:1.Fine mapping and candidate gene prediction of powdery mildew resistance gene pm in IL52:1)In the autumn of 2019,a F₂ population with 610 individual plants was constructed by crossing the resistant parent IL52 with the susceptible parent CCMC.The resistance phenotype of the population was identified by natural disease in the field.The sequencing of naturally occurring leaf pathogens in the field showed that the leaf disease was powdery mildew,and the pathogen was Podosphaera Xanthii,which was the dominant pathogen of cucumber powdery mildew.The segregation ratio of resistant and susceptible plants was 1:3,which indicated that pm was a recessive trait controlled by a single gene in IL52.2)Using molecular markers in the initial mapping region of powdery mildew resistance gene pm and four pairs of polymorphic marker primers developed based on the results of parent re-sequencing data,the powdery mildew resistance gene pm was mapped to the 52kb region on chromosome 5 with flanking markers of 20SSR3 and SNP9.All the genes within52kb were analyzed and seven candidate genes were predicted based on the re-sequencing data,namely Csa5G623430、Csa5G623440、Csa5G623450、Csa5G623460、Csa5G623470、Csa5G623480、Csa5G623490.The SNP/In Del mutation occurred in the coding region of Csa5G623430、Csa5G623480 and Csa5G623490,while a non-synonymous SNP mutation occurred in Csa5G623490.q RT-PCR analysis showed that only Csa5G623430 was differentially expressed in both parents at different time points after inoculation.2.Fine mapping of the major QTL dm5.2 for downy mildew resistance1)The RIL-20 individual plants in the F₆ RILs population constructed by IL52 and CCMC were screened by the dm5.2 foreground marker and other QTL background markers to obtain a secondary segregating population of single-fragment substitution lines containing only dm5.2.Using the parental re-sequencing data,9 pairs of encryption markers were developed in the target segment,and the recombinant individual plants were screened through these encryption markers.Finally,dm5.2 was located in the 349kb interval on chromosome 5,and the flanking markers were 17ID42 and 17ID73.Through parental re-sequencing and cucumber reference genome information,6 candidate genes were obtained in the fine mapping interval,namely Csa5G608180.1、Csa5G608540.1、Csa5G608560.1、Csa5G609610.1、Csa5G609720.1、Csa5G609730.1.2)The contents of peroxidase（POD）,superoxide dismutase（SOD）,malondialdehyde（MDA）and hydrogen peroxide（H₂O₂）in IL52 and CCMC inoculated with downy mildew were determined on 0,1,2,3,5 and 7 days respectively.The results showed that:The content of the four resistance related substances increased,decreased and then increased after inoculating with downy mildew,indicating that the four resistance related substances were involved in the resistance process.This provides a reference for early resistance identification of cucumber downy mildew.