Physiological And Biochemical Differences Analysis Of Resistance And Susceptible Melon Lines In Response To Podosphaera Xanthii Infection

Melon（Cucumis melo L.）is an annual herbaceous plant belonging to the Cucurbitaceae.It is an important fruit-type vegetable crop widely cultivated in temperate to tropical regions.Powdery mildew is one of the main fungal diseases in the production of melons,which seriously harms the yield and quality of melons and causes significant economic losses.Screening of melon powdery mildew resistant germplasm resources and breeding new melon varieties resistant to powdery mildew are important means to solve the problem of powdery mildew.However,the research foundation of melon powdery mildew resistance mechanism is weak,and the related physiological and biochemical mechanism of resistance generation is not clear.Therefore,this study selected 11melon materials on the basis of previous work,and identified the resistance of each melon by inoculating powdery mildew in the field.The physiological and biochemical mechanisms of different resistant melon materials against Podosphaera xanthii infection were studied by means of microscopic observation,physiological staining,determination of physiological indicators,and expression analysis of ROS metabolism-related genes.The results are as follows:Observed through the paraffin section and scanning electron microscope of the leaf structure of different melon materials.Analyze the relationship between the physiological structure of melon and powdery mildew resistance.The results showed that the tightness of the structure of melon leaves and the arrangement of epidermal cells were significantly related to powdery mildew resistance.If the leaf structure is tight and the epidermal cell arrangement is compact,the melon material is resistant to disease,if the leaf structure is loose and the epidermal cell arrangement is loose,the melon material is susceptible to disease.Observe the infection process of powdery mildew bacteria on different melon materials by staining powdery mildew bacteria,revealing the difference in disease resistance of different melons.It was found that there were two types of powdery mildew resistance in melon materials.The first type of resistance exhibits the characteristics that germs cannot invade.Melon materials M4-118,M4-119,M4-120,M4-121 and M4-123 belong to the first type of resistance.The second type of resistance showed that the bacteria could not spread.Melon materials M4-122 and M4-125 belonged to the second type of resistance.DAB staining was used to observe H₂O₂ accumulation in melon leaves and trypan blue staining to observe cell death.It was found that the second type of resistance had H₂O₂ accumulation and leaf cell death in the leaves 48h after inoculation.This was not found in the first type of resistance.The measurement results of H₂O₂ content of each material at 0h,24h,48h and 72h after inoculation showed that the H₂O₂ content of the second resistant type material increased significantly at 48h after inoculation.It is speculated that the resistance production and the cell death induced by H₂O₂ accumulation related.However,the H₂O₂ content of the first type of resistance material did not change significantly after inoculation.It is speculated that the reason for its resistance is that the cell structure prevents the invasion of pathogens.The activity of ROS metabolism related enzyme in different melon materials after inoculation was determined and the change rule of ROS metabolism related enzyme activity in different melon materials was not consistent.The results indicated that various ROS metabolism related enzyme respond to the invasion of powdery mildew in melon.The differentially expressed ROS metabolism related enzyme genes were selected to determine their expression patterns in melon materials with different resistance types according to the analysis results of laboratory transcriptome sequencing data.It was found that the ROS metabolism related enzyme genes in the second resistant type melon material had the same expression pattern,while the expression of each gene in the first resistant type melon material had no obvious change.The disease-resistant mechanism of the first type of disease-resistant material may be more complex,regulated by many factors,and further research is needed.In summary,this study found that there are two types of powdery mildew resistance in melons,and clarified the reasons for the appearance of the two types of disease resistance at the physiological and biochemical level.The results provided a research basis for screening melon powdery mildew resistance germplasm resources,and provided germplasm resources for mapping and cloning of melon powdery mildew resistance genes.It also promoted the research on the response mechanism of melon to powdery mildew invasion at the physiological and biochemical level.