DNA Methylation Changes In Response To Sucrose And Chilling Stress And The Transcript Levels Of Differentially Methylated Genes In Cucumbers

Chilling is a common adverse factor in vegetable cultivation process, which causes the reduction of vegetable productivity and quality seriously. Cucumber is one of the main cultivated vegetables in our country which is usually effected by chilling stress when cultivated in open field and greenhouse. So it is of great significance to investigate the impaction of chilling stress on cucumber and to explore the way to alleviate it. This paper reveals the mechanism of chilling stress alleviated by exogenous sucrose on cucumber seedlings in the macro, micro, physiological and molecular levels, which offers a basis for the application of exogenous sucrose in alleviating chilling stress of cucumbers and provide references for the study of the mechanisms of other exogenous substances alleviating plant stresses. Furthermore, the paper applies the quantitative real time PCR to determine the transcript expressions of the differential methylation genes in cucumber leaves and uses the BSP technology to study the methylation levels in CG islands of some genes, which provides a theoretical basis for the application of the selected genes in the genetic engineering of cucumber resistance and lays the foundation for elucidating the mechanism of cucumber in response to abiotic stress. The main findings are as follows:(1) Effects of sucrose alleviating chilling stress on cucumber seedling growth, antioxidant enzymes and other related indexesIn this study, chilling stress led to the growth of (Cucumis sativus L) cv. Jinchun no.4decreased significantly, while sucrose pretreatment markedly enhanced the growth indexes of cucumber seedlings. Therefore, chilling stress seriously inhibits the growth of cucumber seedlings, while the pretreatment of exogenous sucrose alleviates the growth inhibition induced by chilling stress.In this study, as compared to control, chilling stress improved the contents of MDA and H2O2and the formation of O2-in the second cucumber leaves. On the other hands, after pretreatment with sucrose, the activities of antioxidant enzyme including SOD, GPX, GSH-Px, APX, MDHAR, DHAR and GR, and the contents of the antioxidants (AsA and GSH), proline and soluble sugars, and the activities of SAI and NI were all significantly enhanced, which further decreased the contents of MDA and H2O2and the formation of O2- in cucumber leaves. Therefore, we conclude that sucrose pretreatment enhances the activities of antioxidant enzymes in cucumber leaves, decreases the ROS levels, and reduces the degree of membrane lipid peroxidation, thus alleviating the damage of the chilling stress on cucumbers.(2) Effect of sucrose on genomic DNA methylation of cucumber under chilling stressAccording to the MSAP results, chilling stress effects the level of genomic DNA methylation of the cucumber leaves, and sucrose pretreatment further changes the DNA methylation.(3) The transcript expression of the differential methylation genes in cucumber leaves and the changes of methylation in CG islandsThe transcript levels of the differential methylation genes are different in the different treatments which are induced by CA under chilling stress and by H2O2under heat stress, and the CG islands are also different in the selected differential methylation genes. Therefore, the DNA methylation maybe participate in the alleviation effect on the stresses of CA and H2O2.