DNA hypomethylation mediates flower opening and senescence in sweet osmanthus through auxin and ethylene responsive pathways

Recent studies have shown that DNA methylation is critical for fruit ripening and leaf senescence, but its effect on flowering is unclear. This study investigated the dynamic changes in DNA methylation and its regulatory role on flower opening and senescence of Osmanthus fragrans 'Liuyejingui' by whole-genome bisulfite sequencing and transcriptome sequencing at six developmental stages (unopened floral bud to late flower senescence, S1 to S6). In total 3222 differentially methylated regions (DMRs)were identified in senescent flowers, of which 89 percent were hypomethylated (hypo-DMRs), and the number increased form S1 to S6, suggesting that DNA demethylation is associated with the progression of flowering. Analysis of hypo-DMR-associated up-regulated differentially expressed genes revealed that small auxin up-regulated RNAs (OfSAURs)were the most highly significantly enriched from S3 to S5, and ethylene-responsive transcription factors (OfERFs)were strongly activated in senescent flowers. These results suggested that DNA hypomethylation was important for flower expansion and flower senescence through the auxin and ethylene response pathways. Spray application of a DNA methylation inhibitor to unopened floral buds triggered a premature opening and senescence phenotype by promoting endogenous ethylene production and carotenoid accumulation, and indicated that DNA demethylation mediated flower opening and senescence by activating the expression of OfSAURs and OfERFs. An epigenetic regulatory three-component is proposed, in which flower opening and senescence are regulated by DNA demethylation, mediating the expression of specific gene families, together with the plant hormones auxin and ethylene, to transition the flower into an opening- and senescence-competent state.