Analysis Of Physiological Factors And Gene Expression Profile Of Cucumber In Resistance To Downy Mildew And Mapping Of Resistance Genes

Cucumber（Cucumis sativus L.）is one of the primary vegetable crops cultivated in open fields and facilities in our country.Downy mildew（DM）,caused by Pseudoperonospora cubensis,is a main disease in cucumber production.It can drop the cucumber production greatly and causing serious economic loss.The excessive use of chemical agents will pollute the environment and bring pesticide-resistant strains.Therefore,the cultivation of downy mildew resistant varieties is an environmental-friendly and effective method to control cucumber diseases.So,finding the resistance pathway for cucumber downy mildew and locating the main resistance genes of cucumber to downy mildew are important for cucumber molecular breeding.Our laboratory has downy mildew resistance inbred line 51（IL51）and susceptibility inbred line 53（IL53）,which are ideal materials to study the resistance pathway for downy mildew in cucumber,and to locate the resistance gene.In this paper,the physiological mechanism of resistance,transcriptome,and resequencing results were analyzed,a potential cucumber defense signaling pathway was constructed and several candidate genes in the cucumber defense mechanism were identified.These results are helpful to the molecular breeding of cucumber resistance to downy mildew and provide a theoretical basis for cucumber cultivation and management.The main results are as follows:1.Study on the physiological mechanism of cucumber downy mildew resistance.The germination and infection rate of Pseudoperonospora cubensis on the leaf surface of IL51 were decreased,which were found by trypan orchid staining.Meanwhile,in IL51,the content of H₂O₂ in vivo increased due to the increase of SOD（superoxide dismutase activity）and the decrease of peroxidase activity.Phloroglucinol staining and the quantitative determination indicated that the lignin content in IL51 increased with the increase of infection time.2.Transcriptome analysis.Transcriptome analysis,which used resistance and susceptibility pools in F₂ population with IL51 and IL53 as parents,showed that a series of differentially expressed genes（DEGs）within and between resistant and susceptible pools throughout a time course of infection.DEGs were involved in multiple functions of defense response:PAMP recognition,signal transduction,ROS and lignin accumulation,and transcription regulators.Finally,we predicted a potential molecular mechanism of SAR caused by PTI for cucumber resistance to DM.3.Genetic analysis.Two phenotypic identification results of F₂ generation plants showed that only one gene caused the resistance difference of IL51 and IL53 in the early stage of infection by Pseudomycete cupressis.In contrast,the variation in resistance traits was controlled by multiple genes in the late stage of infection.4.Genome resequencing analysis.Through joint phenotypic analysis of the results of two times of genome resequencing,a total of 36 target regions were identified,in which 297 genes had SNP mutations.Based on the results of resequencing and SNP association analysis,57 specific Indel markers were designed,which can be used to locate subsequent resistance genes.5.Joint analysis of genome resequencing and transcriptome.Transcriptome analysis of 297 potential target genes showed that 105 genes were significantly differentially expressed in susceptible and resistant populations over time of infection.Among them,Csa1G088440,Csa6G445780,and Csa6G446290 were only detected in the resistant pools,and no expression was detected in the susceptible pools,while Csa1G414240 and Csa4G551640 had opposite expression trends.The abnormal expression trend of 5 genes may be the cause of the differences in IL 51 and IL53 traits.