| Xinjiang wild apple(Malus sieversii)is the largest wild germplasm resources in Tianshan wild fruit forest,which has rich genetic diversity,strong cold resistance,insect resistance,drought resistance and other excellent traits.The spread and spread of Agrilus mali Mats to a sharp decrease in the area of wild apple community in Xinjiang,which destroyed the germplasm resources of wild apple in Xinjiang and affected the protection of natural ecological system and the breeding of cultivated apple.Studying the structural characteristics and related secondary metabolites changes of Xinjiang wild apple after being damaged by pests is helpful for breeding and improving varieties with stronger resistance.In order to determine which metabolic processes are involved in the defense response of Xinjiang wild apple against Agrilus mali Mats,paraffin sections were made to observe the internal anatomical differences between sensitive and resistant strains.At the same time,extensive targeted metabolomics technology was used to analyze the metabolic differences between resistant and sensitive strains,and RT-qPCR was used to focus on the expression of key synthetic genes and key enzyme activities in phenylpropanoid and flavonoid metabolic pathways.The results are as follows:(1)Under natural conditions,the anatomical structure of xylem in branches of Malus sieversii was analyzed.The vessel density had a great influence on insect resistance,and the vessel density of insect-resistant plants was lower than that of sensitive plants,which were 136.2 number/μm2 and 163.1 number/μm2,respectively.The thickness of xylem and width of xylem ray had little effect on insect resistance of Xinjiang wild apple.From the bark anatomical structure of branches of Xinjiang wild apple,the cortex thickness and phloem thickness of resistant and susceptible plants of Xinjiang wild apple were significantly different,and the phloem thickness of resistant plants was lower than that of sensitive plants,which were 194.9 μm and 432.3 μm,respectively.The cortical thickness of insect-resistant plants was 1 282.9 μm and 1037.7 μm higher than that of sensitive plants,respectively.There was no significant difference in periderm thickness.(2)The sensitive and resistant lines of Malus sieversii were analyzed by extensive targeted metabolomics.A total of 518 metabolites were detected,while the flavonoids and phenolic acids were more,which were 83 and 80,respectively.A total of 123 metabolites with significant differences were identified in the sensitive and resistant lines,among which 80 metabolites in the resistant line were higher than those in the sensitive line,accounting for 65.04%;43 metabolites were low,accounting for 34.96%.Pathway enrichment analysis of differential metabolites was carried out by KEGG database,and it was found that differential metabolites were enriched significantly in these four metabolic pathways,namely,secondary metabolic biosynthetic pathway,phenylpropanoid biosynthetic pathway,purine metabolic pathway and tryptophan metabolic pathway.The differential metabolites included in these four metabolic pathways were 14(38.89%),5(13.89%),4(11.11%)and 2(5.56%),respectively.(3)In this study,it was found that the contents of JA,JA-ILE,and H2JA in resistant lines increased to varying degrees,and the JA-ILE content was significantly different between sensitive lines and resistant lines.Correlation analysis between gene expression and enzyme activity of phenylpropanoid biosynthesis pathway and flavonoid biosynthesis pathway showed that their variation trends were basically consistent in susceptible and resistant strains.In resistant lines,the expression of LAR,CHS,C4H,ANS and F3 ’,5’H genes related to phenylpropanoid metabolism and flavonoid metabolism increased significantly,and the activity of these five enzymes increased significantly,while the expression of β-glu gene and enzyme activity decreased. |
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