Epigenomic Regulatory Mechanism of Flowering in Apple Demonstrated in Two Varieties with Contrasted Flowering Behaviors

Abstract Background: Flowering is the necessary condition and yield basis for woody fruits in their life cycle. Although there has been considerable interest in the regulatory mechanisms underlying floral induction and flowering, the associated epigenetic modifications remain relatively uncharacterized. Results: We identified the genome-wide of DNA methylation changes and the transcriptional responses in axillary bud of ‘Qinguan’ (QA) and ‘Fuji’ (FA) varieties with contrasted flowering behaviors. The DNA methylations were19.35%, 62.96% and 17.68% for FA, and 19.64%, 62.49% and 17.86% for QA in the CG, CHG and CHH contexts, respectively. Number of hypermethylated or hypomethylated DMRs in different regions were contributed to significantly up/downregulated gene expression. DNA methylation can positively or negatively regulate gene expression based on the CG, CHG and CHH contexts in different regions. Additionally, the huge differences in transcription of MIKCc-Type MADS-box genes, and multiple flowering genes in multiple flowering pathways (i.e., light, age, GA and sugar) by changing DNA methylation, contributed to contrasted flowering behaviors in both QA and FA. Specifically, the floral meristem identify genes (i.e., FT, LEAFY, AP1 and SOC1) were significantly higher expression in QA than FA, but the floral repressor (i.e., SVP, AGL15, and AGL18) had an opposite result. Significant differences in multiple hormone levels were due to DEGs and their DMRs in their synthesis pathways, leading to both contrasted axillary bud outgrowth and flowering behaviors. Conclusions: The whole-genome bisulfite sequencing (BS) libraries of QA and FA with diverse flowering capabilities have been constructed for finding whole-genome cytosine methylation profiles. The RNA sequencing of QA and FA and diverse flowering capabilities have been combined together to identify the gene expression patterns and the correlation with their methylation states so that we can better understand the epigenetic regulation mechanisms of floral induction and formation in apple.