Physiological Response Of Seed-used Pumpkin (Cucurbita Peop L.) To Powdery Mildew Stress And Identification Of VQ Gene Family Related To Disease Resistance

In this study,powdery mildew disease-resistant inbred line F2 and susceptible inbred line M3 of seed-used pumpkin（Cucurbita peop L.）were used as test materials,and the physiological race 2US of powdery mildew was inoculated in an artificial climate chamber.The dynamic development of powdery mildew resistant strain F2 and susceptible strain M3 were observed,and the changes in reactive oxygen species accumulation and protective enzyme activities in leaves were observed.Transcriptome analysis was performed in Cucurbita peop L.leaves infected powdery mildew at different times.Differential genes in transcriptome were screened and VQ gene was found to be differentially expressed under powdery mildew stress.The genome-wide VQ family was identified by using the published genome Cucurbita peop L.and its expression pattern was analyzed.The results of this study laid a foundation for the screening of powdery mildew resistance and the breeding of powdery mildew resistance.The main results were as follows:1.Compared with M3,pathogen growth on leaves of resistant strain F2 was slow and difficult to infect leaves after inoculation with powdery mildew.The content of H₂O₂and O^2-in leaves of F2 resistant strain increased rapidly at the early stage of infection and then decreased gradually,while the content of H₂O₂and O^2-in leaves of F2 susceptible strain increased slowly at the early stage of infection and remained high after reaching the maximum value,and the content of MDA in leaves of F2 susceptible strain was always higher than that of F2 resistant strain.At the same time,histochemical staining showed that the leaf staining of the resistant strain was faster than that of the susceptible strain,and then the staining area decreased and tended to a lower level.2.The activities of antioxidant enzymes CAT,SOD,POD,PAL,and PPO in leaves of F2 and M3 were significantly increased after inoculation with powdery mildew,but the activities and the increase of the activities in F2 were higher than those in M3.The results showed that powdery mildew of F2 leaves developed slowly and was difficult to be infected.Powdery mildew spots were small on the leaves after mycelium formation.Compared with susceptible strains,reactive oxygen species（ROS）content of resistant strains increased rapidly at the beginning of powdery mildew infection.With the increase of ROS content,the antioxidant enzyme activity of resistant strains increased rapidly to prevent oxidative stress.Compared with the susceptible strain,PAL and PPO activity of the resistant strain reached the peak 2d and 4D earlier,respectively,thus activating the defense mechanism earlier to help the leaf tissue resist the infection of pathogens.3.A total of 3819 differentially expressed genes were identified by transcriptome analysis.Compared with 0h,2291 differentially expressed genes were generated after 12h infection,of which 825 were up-regulated and 1466 were down-regulated.1528differentially expressed genes were generated after 24h infection,of which 337 were up-regulated and 1191 were down-regulated.The results showed that the C.pepo seedlings had a positive response to powdery mildew infection at the gene level.Bioinformatics analysis showed that the differential genes caused by powdery mildew infection were mainly related to phenylpropane metabolism,plant hormone signal transduction and reactive oxygen species metabolism,and the number of genes responding to 12h was more than that responding to 24h.4.VQ protein is a plant specific-protein,which plays an important role in plant growth and development and biotic and abiotic stress response.This study was the first to systematically analyze the VQ gene family of C.pepo and to understand its expression patterns under different stimuli.A total of 44 VQ genes were identified and divided into 8groups（I-VIII）by phylogenetic analysis.Two genes,Cp VQ1 and Cp VQ2,were not located on chromosomes,and other Cp VQ genes were randomly distributed on chromosomes except chromosomes 15 and 18.It is noteworthy that the main event driving the expansion of the VQ gene family is chromosome fragment replication.According to q RT-PCR analysis,the VQ genes was expressed in different tissues,and the VQ gene was regulated differently under various abiotic stresses and powdery mildew stresses,indicating that the VQ gene plays an important role in plant stress response.