The callus formation capacity of strawberry leaf explants is modulated by DNA methylation

Shoot regeneration from leaf tissue requires the de-differentiation of cells from a highly differentiated state into an actively dividing state, but it remains unclear how this physiological transition occurs and is regulated, especially at the epigenetic level. Here, we characterized the DNA methylome represented by 5-methylcytosine (5mC) in leaf and callus tissue derived from leaf explants of woodland strawberry, Fragaria vesca. We detected an overall increase in DNA methylation and distinct 5mC enrichment patterns in the CG, CHG, and CHH sequence contexts in genes and transposable elements. Our analyses revealed an intricate relationship between DNA methylation and gene expression level in leaves or leaf-derived callus. However, when considering the genes involved in callus formation and shoot regeneration, e.g. FvePLT3/7, FveWIND3, FveWIND4, FveLOG4 and FveIAA14, their dynamic transcription levels were associated with differentially methylated regions located in the promoters or gene bodies, indicating a regulatory role of DNA methylation in the transcriptional regulation of pluripotency acquisition in strawberry. Furthermore, application of the DNA methyltransferase inhibitor 5'-azacytidine (5'-Aza) hampered both callus formation and shoot regeneration from the leaf explants. We further showed that 5'-Aza downregulated the expression of genes involved in cell wall integrity, such as expansin, pectin lyase, and pectin methylesterase genes, suggesting an essential role of cell wall metabolism during callus formation. This study reveals the contribution of DNA methylation to callus formation capacity and will provide a basis for developing a strategy to improve shoot regeneration for basic and applied research applications.