Divergent DNA methylation signatures of juvenile seedlings grafts and adult apple trees
Plants are continuously exposed to environmental perturbations. Outcrossing annual plants can adapt rapidly to these changes via sexual mating and DNA mutations. However, perennial and clonally reproducing plants may have developed particular mechanisms allowing them to adapt to these changes and transmit this information to their offspring. It has been proposed that the mechanisms allowing this plasticity of response could come in the form of epigenetic marks that would evolve throughout a plants lifetime and modulate gene expression. To study these mechanisms, we used apple (Malus domestica) as a model perennial and clonally propagated plant. First, we investigated the DNA methylation patterns of mature trees compared to juvenile seedlings. While we did not observe a drastic genome-wide change in DNA methylation levels, we found clear changes in DNA methylation patterns localized in regions enriched in genes involved in photosynthesis. Transcriptomic analysis showed that genes involved in this pathway were overexpressed in seedlings. Secondly, we compared global DNA methylation of a newly grafted plant to its mother tree to assess if acquired epigenomic marks were transmitted via grafting. We identified clear changes, albeit showing weaker DNA methylation differences. Our results show that a majority of DNA methylation patterns from the tree are transmitted to newly grafted plants albeit with specific local differences. Both the epigenomic and transcriptomic data indicate that grafted plants are at an intermediate phase between an adult tree and seedling and inherit part of the epigenomic history of their mother tree.