Identification Of Resistance To Acidovorax Citrulli Among Different Melon Accessions And Detection Of QTLs For Bacterial Fruit Blotch And Downy Mildew Resistance In Melon

Melon（Cucumis melo L）has been a major economic fruit crop widely cultivated in Xinjiang for a long history.Xinjiang melon,in particular,enjoys great reputation worldwide because of its unique favor and high sugar content.However,the frequently-occurring bacterial fruit blotch（BFB）and downy mildew（DM）would cause serious wilt or death to melon plants and thus largely limit the melon production.Melon has a rich germplasm resource.Some varieties are resistant to BFB and DM diseases but have poor agronomic characterisitcs.Thus,it is urgent to explore the resistance genes of these resistant varieties and improve the existing melon varieties using resistance genes and molecular marker technology.On this basis,melon BFB and DM were studied.The main contents and results are as follows:（1）A comprehensive survey and sample collection was performed during the growing seasons between 2014 and 2016 in Changji and Shihezi,Xinjiang.After isolation,the samples were treated by pathogenicity identification and alignment analysis for 16S r DNA sequencing.Totally,65 bacterial strains were isolated and purified from melon samples.The 16S r DNA sequence alignment identiflied 27 pathogenic isolates with 99%homology with Acidovorax.citrulli.The pathogenic bacteria collected in different years had the same 16S r DNA sequence and all were highly pathogenic.The isolates from different collection points were clustered in the same phylogenetic tree and homologous to the control strains xjgb-wy6 and Acidovorax citrulli XJ-4.（2）A stable resistance identification platform for BFB at the melon seedling stage was established by improving the traditional greenhouse spray inoculation method.The simulated field method of artificial inoculation ensures the consistency and repeatability of melon diseases.This method also objectively reflects the performance of plant resistance in the field by comparing the disease evaluation in different melon seedling materials with simulated field,natural disease in field and artificial inoculation.（3）Different melon varieties were screened in terms of resistance to Aac.Between 2014and 2017,the BFB resistance among 132 melon materials was identified using five different pathogenic environments（indoor detached leaves,indoor seedling spray inoculation,natural field investigation,simulated field seedling spray inoculation,plot block seedling spray inoculation）.The reactions to BFB were significantly different among the melon lines.,but the majority of Xinjiang local varieties and commodity accessions were extremely susceptible to BFB.Nevertheless,16 resistant varieties were identified,including PMR5,PMR6,MR-1,Vedrantis Nantais,Nantais Oblong;Ano2,GM（imported varieties）;Y8,Y9,Y13（thin-skinned melon）;Y17,Y18（wild varieties）;ANK-9,X25M,PHLL and XF2-2（melon inbred line）.Among them,Y8,Y9,MR-1 and Ano2 exhibited stable resistance under all five pathogenic conditions;Vedrantis and Nantais Oblong showed more stable resistance in indoor artificial inoculation and lower resistance in field.（4）The inheritance of stable resistance among melon varieties was studied.Fourteen segregated populations,constructed by 4 resistant parents（Y8,Y9,MR-1,Nantais Oblong）crossed with different susceptible parents,did not obey the single-gene genetic separation ratios.The resistance of Y8 was controlled by multiple genes.SSR marker combined with BSA method identified that BFB resistance related loci may be located in LG4 and LG5 in melon Y8.The separation of Y9 was roughly the same as that of Y8,which indicate the inheritance of incomplete dominance and multi-gene control.The resistance of MR-1 was controlled by multiple resistant genes.LG1,LG2,LG5 and LG7 were highly correlated with the BFB resistance.Nantais Oblong showed an in significant resistance gene effect and its combinations with different susceptible parents were recessive and multi-gene controlled.（5）The quantitative trait locus（QTL）for BFB resistance was analyzed by combining traditional genetic mapping with parental sequencing in the high resistant Ano2 line.RIL₈population was constructed using the resistant line Ano2（P₁）and the susceptible line K413（P₂）.Primary QTL mapping was performed with the genetic linkage map established with RIL₈.And then,fine mapping was performed in BC₁P₁ and F₂ population.bfb QTL10.1 was located in LG10 between chr10-0356 and chr10-0365,with physical distance of 94.5 kb.The region contained 16 genes from MELO3C011954T1 to MELO3C011970T1.（6）With the Downy mildew resistant line MR-1,the QTLs for DM resistance were located and the candidate genes were predicted.The DM-resistant QTLs in MR-1 were located by using traditional genetic mapping and QTL-seq.Resistance-related QTLs were detected on LG3,LG4,LG5 and LG9 linkage groups.It was found DM-QTL4.1,a major DM resistant QTL,could be detected repeatedly in different mapping methods,years and mapping groups.DM-QTL4.1 was flanked by chr4-2791 and GCM336 with a 11 kb region,which contained three genes of MELO3C009804T1,MELO3C009805T1 and MELO3C009806T1.Among them,MELO3C009806T1 encoded the disease resistance protein RGA4,leucine repeat sequence（LRR）and protein kinase domain（NB）,and was involved in the signal transduction mechanism of disease resistance.MELO3C009804T1 encoded the ethylene responsive transcription AP2-like factor involved in plant response to stress.We speculate MELO3C009804T1 and MELO3C009806T1 may be associated with the DM resistance in MR-1.Four molecular markers closely linked to the resistant loci were also identified,including chr4-2791 and GCM336 linked to DM-QTL4.1,Chr5-1353 and chr5-1488 linked to DM-QTL5.1,with genetic distances of 0.5,0.6,1.9 and 0.1 c M,respectively.These markers will be used for molecular marker-assisted selection of resistant melon lines,and future research will be directed at identifying and cloning the resistance gene.