| Summary: | DNA methylation, an important epigenetic modification, regulates the expression of genes and is therefore involved in the transitions between floral developmental stages in flowering plants. To explore whether DNA methylation plays different roles in the floral development of individual male and female dioecious plants, we injected 5-azacytidine (5-azaC), a DNA methylation inhibitor, into the trunks of female and male basket willow (<i>Salix viminalis</i> L.) trees before flower bud initiation. As expected, 5-azaC decreased the level of DNA methylation in the leaves of both male and female trees during floral development; however, it increased DNA methylation in the leaves of male trees at the flower transition stage. Furthermore, 5-azaC increased the number, length and diameter of flower buds in the female trees but decreased these parameters in the male trees. The 5-azaC treatment also decreased the contents of soluble sugars, starch and reducing sugars in the leaves of the female plants, while increasing them in the male plants at the flower transition stage; however, this situation was largely reversed at the flower development stage. In addition, 5-azaC treatment decreased the contents of auxin indoleacetic acid (IAA) in both male and female trees at the flower transition stage. These results indicate that hypomethylation in leaves at the flower transition stage promotes the initiation of flowering and subsequent floral growth in <i>Salix viminalis</i>, suggesting that DNA methylation plays a similar role in vegetative⁻reproductive transition and early floral development. Furthermore, methylation changes during the vegetative⁻reproductive transition and floral development were closely associated with the biosynthesis, metabolism and transportation of carbohydrates and IAA. These results provide insight into the epigenetic regulation of carbohydrate accumulation.
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