Characterization of DNA Methylation Variations During Fruit Development and Ripening of Sweet Orange

DNA methylation is a type of epigenetic modification involved in many biological processes. Recent studies have shown that DNA methylation plays important roles in regulating fruit development and ripening. In the present study, sweet oranges at six developmental stages were investigated to understand the possible roles of DNA methylation during fruit development and ripening. Analysis of genomic DNA methylation levels in different fruit tissues and developmental stages indicated that methylation levels were not static during fruit development and ripening. To explore the possible contributions of methylation-related genes in fruit development and ripening process, the full-length coding sequences corresponding to three types of DNA methyltransferase families (CsMET1, CsCMT3, and CsDRM1), a chromatin-remodeling gene (CsDDM1), and a demethylation gene (CsDME1) were isolated. The expression profiles of these methylation-related genes were analyzed in vegetative tissues (seedlings and leaves) and reproductive tissues (three stages of flowering and six stages of fruit development). The results showed that CsDRM1 was highly expressed in seedlings, leaves, and flowers compared with other genes. During fruit development and ripening, CsMET1, CsCMT1, CsCMT2, CsDRM1, and CsDMEs showed higher expression in the peel than in the flesh. However, global DNA methylation level changes could not be explained by the expression of a single gene, suggesting that the methylation and demethylation system may cooperatively contribute to the final global DNA methylation pattern.