Based On Transcriptome And Metabonomics Analysis Of MeJA-mediated Defense Mechanism Of Malus Crabapple In Response To O₃ Stress

With the acceleration of industrialization and urbanization in China,the emissions of industrial exhaust gas and traffic exhaust are increasing.Ozone（O₃）pollution has become a type of invisible killer that seriously threatens the survival of animals and plants.Spraying plant hormone on the leaves of plants is one of the common effective measures to reduce the damage of O₃ to plants.As a common plant hormone,methyl jasmonate（MeJA）not only regulates the growth and development of plants,but also responds to and transmits external stimulation signals,stimulates plants to make a series of defense responses,so as to improve the ability of plants to resist stress.Ornamental crabapple,as the main tree species for Landscape Application in the world,is also a rare fine ornamental flower and apple rootstock in temperate regions of the northern hemisphere.It is of great significance to understand its stress resistance mechanism for promoting modern landscape construction and improving the quality of ornamental fruit trees in China.Therefore,this experiment used three-month-old’Hongjiu’crabapple seedlings as the test material,used an open-top air chamber to simulate the O₃ environment,and taking the signal cross between O₃ and MeJA as the entry point,comprehensively using of physiology,transcriptomics,metabolomics and other technical means to explore the mechanism of MeJA-mediated the regulatory network of Malus crabapple in response to O₃ stress.In order to provide a new perspective for defense mechanism of plants to cope with O₃ pollution,and lay a foundation for cultivating stress-resistant landscaping tree species.The main findings are as follows:1.After O₃ stress,the leaves of’Hongjiu’had obvious scorched spots,and the damage area was as high as 2/3.Exogenous MeJA can significantly improve the damage of O₃ to the leaves of Malus crabapple.As the MeJA concentration increased,the spots gradually disappeared,and the leaf color changed from green to purple.2.Under O₃ stress,the chlorophyll content of’Hongjiu’leaves decreased with the prolongation of O₃ treatment time,while the anthocyanin content and SOD,POD activity increased first and then decreased,and reached the peak 12 hours after O₃ stress,but MDA content continued to increase.Spraying MeJA reduced the chlorophyll degradation caused by O₃ stress,at the same time increased the content of anthocyanins,activated the enzyme activities of SOD and POD,and reduced the content of MDA.With the increase of MeJA concentration,the effect is enhanced,and the effect of detoxifying O₃ was best when the concentration was 200μmol·L^-1.3.The results of transcriptome data analysis showed that 13814 differentially expressed genes were found under O₃ stress,among which the expression of McWRKY75 up-regulated expression was about 800 times,it was a key transcription factor to resist O₃ stress.And it was positively correlated with flavonoid related structural genes（McC4H,McDFR,McANR）,while the ethylene transcription factors McERF09 and McERF109-like have O₃ response specificity.It was found for the first time that phospholipase A was the most direct phospholipase in response to O₃.Compared with the untreated samples,494 differential genes were found in the pre-treatment of MeJA,of which 316 belonged to the overlapping genes caused by the stimulation signals of MeJA and O₃.4.Metabolomic analysis results showed that the phenylpropane,flavonoid and anthocyanin were the main metabolic pathways of crabapple to resist O₃ stress,especially naringin chalcone and geranium,which were the metabolites of flavonoid synthesis,play a key role in the response of crabapple to O₃ stress.Exogenous MeJA can enhance the tolerance of crabapple to O₃ stress by promoting sucrose synthesis,amino acid metabolism and flavonoid metabolism.Neohesperidin and luteolin,the synthetic metabolites of flavonoids,are the main overlapping metabolites of cross signals of O₃ and MeJA.5.Combined analysis of differential genes and differential metabolites revealed that 61genes regulate 21 metabolites in the phenylpropane,flavonoid,and anthocyanin metabolism pathways,and 167 genes regulate 10 metabolites in the amino acid metabolism pathway.