The Alteration Of DNA Methylation In Cucumis Sativus When Exogenous5-aminolevulinic Acid And Ferulic Acid Mitigate Dehydration Stress And Methyl Jasminate Alleviates Chilling Stress

Dhydration and chilling stress are common stress factors in vegetable cultivation process.Dhydration stress occurs in many places of the world, and chilling stress impacts thecultivation of many vegetables. Hence, it is of great significance to investigate the impactionof dehydration and chilling stress on plants and to explore the way to alleviate stresses. Thepaper reported the mechanism of dehydration stress mitigated by exogenous5-aminolevulinicacid （ALA） and ferulic acid （FA）, as well as chilling stress mitigated by methyl jasmonate（MeJA）. We investigated the exogenous ALA, FA and MeJA effect on the plant growthparameters, leaves ultrastructure, antioxidant enzymes activities and the changes of genomicDNA methylation of dehydration and chilling stressed cucumbers. Then the DNAmethylation different fragments were separated and further cloned, sequenced, and sequenceanalyzed. This paper reveals the mechanism of exogenous ALA and FA alleviate thecucumber dehydration stress and exogenous the MeJA relief cucumber chilling stress in themacro, micro, physiological and molecular levels, which provide a basis for exogenous ALA,FA or MeJA-resisting cultivation of cucumbers and provide references for the study ofmechanisms of other exogenous substances to alleviate plant stresses. The findings are asfollows:（1） Effects of exogenous ALA and FA alleviate dehydration stress and exogenous MeJArelieve chilling stress on cucumber seedling growth parameters, antioxidant enzymes andrelated indexesIn this study, dehydration and chilling stress led to the growth indexes of （Cucumissativus L） cv. Yuexiu no.3and cv. Jinchun no.4decreased significantly, while exogenousALA, FA or MeJA treatment before dehydration or chilling stress significantly increased thegrowth indexes in cucumber seedlings. Therefore, dehydration and chilling stress inhibits the growth of cucumber seedlings, while exogenous ALA, FA or MeJA treatment alleviates thegrowth inhibition.Compared to control, dehydration and chilling stress both increased levels of MDA and（or） ROS (endogenous H₂O₂and O₂^.-) in cucumber leaves （the second true leaves） in thispaper. However, after pretreatment with ALA, FA or MeJA, the activities of antioxidantenzyme including SOD, CAT, GPX, GSH-Px, APX, DHAR, MDHAR and GR and theactivities of antioxidants （AsA and GSH） and proline and soluble sugars were significantlyincreased. Hence, the levels of MDA and ROS in cucumber leaves were decreased. Weconcluded that exogenous ALA, FA or MeJA pretreatment increases antioxidant enzymesand non-enzymatic antioxidants activities in cucumber leaves and decreases the ROS levels,thus reducing the damage of dehydration or chilling stress on leaf membrane system.（2） Effects of exogenous ALA, FA or MeJA on DNA methylation of cucumber genomeunder dehydration or chilling stressAccording to the MSAP results, we found that dehydration or chilling stress has littleeffect on the level of methylation of the cucumber leaves genome, while exogenous ALA, FAor MeJA pretreatment was able to improve the demethylation probability of the cucumberleaves genome. After dehydration and chilling stress, the total methylation frequency ofcucumber leaves genome was improved. However, exogenous ALA, FA or MeJA treatmentbefore dehydration or chilling stress declined the total methylation frequency of cucumberleaves genome. Sequence analysis of DNA methylation differences fragments showed thatDNA methylation is mainly distributed in the coding region of the genome, and has a highdegree of homology with the cucumber genome. The methylation trend of genomic DNA inthe second true leaves, the first true leaves, roots, stems of cucumber seedlings exhibittissue-specific expression. Bisulfite sequencing PCR results show us that ALA, FA andMeJA increase or decrease the methylation percentage of some CpG sites in the candidategenes of dehydration and chilling-stressed leaves.